studies in physical and theoretical chemistry 84
SOLID STATE NMR OF POLYMERS
studies in physical and theoretical chemistry Recent titles in this series
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.
studies in physical and theoretical chemistry 84
SOLID STATE NMR OF POLYMERS Edited by ISAO ANDO
Department of Polymer Chemistry Tokyo Institute of Technology Ookayama, Meguro-ku, Tokyo 152 Japan and TETSUO ASAKURA
Department of Biotechnology Tokyo University of Agriculture and Technology Naka-cho, Koganei, Tokyo Japan
1998 ELSEVIER Amsterdam
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ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands
Llbrary oF Congress Cata]ogtng-tn-Pub]tcatton
Data
So]ld state NMR oF po]ymers/ edited by Isao Ando and Tetsuo Asakura. p. cm. - - (Studles In physical and theoretical chemistry ; 84) Inc]udes blb]lographtca| references and index. ISBN 0-444-82924-5 1. Po]ymers--Ana]ysls. 2. Nuc|ear magnetic resonance spectroscopy. I. Ando. I. (Isao). 1941. I I . Asakura, Tetsuo. I I I . Series. OD139.P6S65 1998 547'.7--dc21 97-53184 CIP
ISBN' 0-444-82924-5 9 1998 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U . S . A . - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands.
Preface
As materials polymers are almost always used as "solids". A structural and dynamic characterization of the polymers in question is necessary in order to understand the relations between properties and structure and, on the basis of these relations, to design new polymer materials. As is well known, the X-ray diffraction method has contributed to the structural determination of polymers with high crystallinity. However, most polymers have low crystallinity and so structural information about the noncrystalline region, which is the major component, cannot be obtained by X-ray studies. Therefore, the X-ray diffraction method has a limitation for the structural analysis of such systems. Further, it can be said that chain segments in the noncrystalline region are sometimes in a mobile state, so that the X-ray diffraction method provides no structural or dynamical information. On the other hand, the solid state NMR method provides information about the structure and dynamics of a sample irrespective of whether the region studied is crystalline or noncrystalline. Recently, high resolution NMR studies of solids have been realized by using advanced pulse and mechanical techniques, and so have provided a variety of structural and dynamic information about polymer systems. Further, it can be said that solid state NMR has provided characteristic information that cannot be obtained by other spectroscopic methods, and that it has become a very powerful means for elucidating the structure and dynamics of polymer systems. In polymer science and technology, the advanced development of various polymer materials with ideal properties and functions is desired. To achieve this, the close relationship between physical properties and molecular structure and dynamics must be clarified precisely. Therefore, powerful techniques are required for the elucidation of this relationship. One of these is solid state NMR spectroscopy. This book is divided into two parts: the basic principles of solid state NMR and its application to polymer systems in the solid state. In the former part, the principles of NMR, important NMR parameters such as chemical shifts, relaxation times, dipolar interactions, quadrupolar interactions, pulse techniques and new NMR methods are covered. In the latter part, applications of NMR
vi
PREFACE
to a variety of polymer systems in the solid state are discussed. The book is intended for graduate students and researchers in academic environments. It provides information relevant to beginners as well as those who are experts in solid state NMR applied to polymer science and technology, materials science, chemistry, biochemistry, physics, and so on. We are delighted that so many active authors, who are leaders in the field of NMR spectroscopy and polymer characterization, have contributed to this work. We hope this book will be welcomed by the widespread NMR community and that all readers, from beginner to expert, everywhere will find the details of the various techniques and applications helpful. ISAO ANDO TETSUO ASAKURA
July, 1997
Contents
Preface Introduction I. A n d o and T. Asakura
XV
I. Basic Principles
1. 2. 3. 4. 5. 6. 6.1. 6.2. 6.3.
6.4. 6.5. 6.6.
NMR Chemical Shift and Electronic Structure I. A n d o , N. Asakawa and G.A. Webb Distance Information and Dipolar Interaction H. Saito, S. Tuzi and A. Naito NMR Relaxations and Dynamics F. Horii Spin Diffusion in Solids M. Ernst and B.H. Meier NMR Imaging and Spatial Information B. Bliimich, P. Bliimler and K. Saito Multi-nuclear NMR 1H NMR B. C. Gerstein 2H NMR A.S. Ulrich and S.L. Grage 3H NMR J.P. Bloxsidge, J.R. Jones, J.C. Russell, A.P. Sharratt, T.A. Vick and D. Zhong 15N NMR T.A. Cross 170 NMR S. Kuroki 19F NMR R.K. Harris, G.A. Monti and P. Holstein
23 51 83 123 165 166 190
212 218 236 253
vii
viii
CONTENTS
II. Applications of Solid State NMR ~
Structure and Dynamics of Crystalline and Noncrystalline Phases in Polymers T. Yamanobe
~
Oriented Fibers and Polymers T. Asakura and M. Demura
,
267 307
Polyethylene and Paraffins T. Yamanobe and H. Kurosu
327
10. Polymer Blends and Miscibility A. Asano and K. Takegoshi
351
11. Polyolefins A. Aoki and T. Asakura
415
12. Polyamides I. Ando and T. Asakura
445
13. Thermoplastic Polymers and Polyimides A . K . Whittaker
469
14. Poly(ethylene terephthalate) T. Asakura and T. Ito
491
15. Crosslinked Polymers R.V. Law and D.C. Sherrington
509
16. Electrically-Conducting Polymers H. Kurosu
589
17. Inorganic Polymers T. Takayama
613
18. Fluoropolymers R.K. Harris, G.A. Monti, and P. Holstein
667
19. Hydrogen-bonded Polymers F. Horii and K. Masuda
20.
Polymer Gel Systems H. Yasunaga, M. Kobayashi and S. Matsukawa
21.
771
Polypeptides I. Ando, T. Kameda, and N. Asakawa
23.
737
Biodegradable Polymers Y. Inoue
22.
713
819
Proteins T. Asakura, M. Demura, N. Nishikawa and H. Yoshimizu
853
CONTENTS 24. 25.
Polysaccharides and Biological Systems H. Saito, S. Tuzi, and A. Naito NMR Characterization of Functionalized Polysiloxanes G.E. Maciel
ix
891 923
III. Conclusions I. A n d o and T. Asakura
985
Subject Index
987
This Page Intentionally Left Blank
Contributors
Professor Isao Ando Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Dr. Akira Aoki Plastic Laboratory, Tokuyama Co., Harimicho, Tokushima, Japan Professor Tetsuo Asakura Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan Dr. Tsushi Asano Department of Chemistry, The National Defense Academy, Hashirimizu, Yokoshuka, Japan Dr. J.P. Bloxsidge Department of Chemistry, University of Surrey, Guildford, Surrey, UK Professor Bernhard Blfimich Institut for Makromolekulare Chemie, RWTII, Worringer Weg 1, D-52056 Aachen, Germany Dr. P. Blfimer Institut ftir Makromo|ekulare Chemie, RWTII, Worringer Weg 1, D-52056 Aachen, Germany Professor Timothy A. Cross Department of Chemistry, Florida State University, Tallahassee, Florida, USA Professor Makoto Demura Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan xi
xii
CONTRIBUTORS
Dr. Matthias Ernst NSR-Center for Molecular Structure, Design and Synthesis, Laboratory of Physical Chemistry, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands Professor Bernard C. Gerstein Department of Chemistry, Iowa State University, Ames, Iowa, USA Dr. S.L. Grage Institut ftir Molekularbiologie, Friedrich-Schiller-Universitat Jena, Winzerlaerstrasse 10, 07745 Jena, Germany Professor Robin K. Harris Department of Chemistry, University of Durham, Durham, UK Dr. Peter Holstein Institut for Experimentelle Physik I, Universit~it Leipzig, Leipzig, Germany Professor Fumitaka Horii Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan Professor Yoshio Inoue Department of Biomolecular Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Dr. Takuro Ito Co. R&D Tokyo Seikan Group, Yokohama, Kanagawa, Japan Professor John R. Jones Department of Chemistry, University of Surrey, Guildford, Surrey, UK Dr. Tsunenori Kameda Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Dr. Shigeki Kuroki Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Professor hiromichi kurosu Department of Textile and Apparel Science, Nara Women University, Kitauoyahigashi-cho, Nara, Japan
CONTRIBUTORS
xiii
Dr. Robert V. Law Department of Chemistry, Imperial College of Science, Technology and Medicine, London, UK Dr. Kenji Masuda Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan Professor Gary E. Maciel Department of Chemistry, Colorado State University, Fort Collins, Colorado, USA Dr. Shingo Matsukawa Department of Food Science and Technology, Tokyo University of Fisheries, Konan, Minato-ku, Tokyo, Japan Professor Beat H. Meier Laboratory for Physical Chemistry, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands Dr. Gustavo A. Monti Department of Chemistry, University of Durham, Durham, UK Professor Akira Naito Department of Life Science, Himeji Institute of Technology, Kamigori, Hyogo, Japan Dr. Naoki Nishikawa Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan Dr. Jeremy C. Russell Biocompatibles Ltd., Brunel Science Park, Kingston Lane, Uxbridge, Middlesex, UK Professor Hajime Saito Department of Life Science, Himeji Institute of Technology, Kamigori, Japan Dr. A.P. Sharratt ICI Chemicals and Polymers, The Heath, Runcorn, Cheshire, UK Professor David C. Sherrington Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK
xiv
CONTRIBUTORS
Dr. Toshio Takayama Department of Applied Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Kanagawa, Japan Professor Kiyonori Takegoshi Department of Chemistry, Kyoto University, Kitasirakawa, Sakyo-ku, Kyoto, Japan Dr. Satoru Tuzi Department of Life Science, Himeji Institute of Technology, Kamigori, Hyogo, Japan Professor Anne S. Ulrich Institut ftir Molekularbiologie, Friedrich-Schiller-Universitat Jena, Winzerlarstrasse 10, 07708 Jena, Germany Dr. T.A. Vick Biocompatibles Ltd., Frensham House, Farnham Business Park, Farnham GU9 8QL, UK Professor Takeshi Yamanobe Department of Materials Engineering, Gunma University, Kiryu, Gunma, Japan Dr. Hidekazu Yasunaga Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Gosyokaido, Matugasaki, Sakyo-ku, Kyoto, Japan Dr. Hiroaki Yoshimizu Department of Materials Engineering, Nagoya Institute of Technology, Gokiso, Shouwa-ku, Nagoya, Japan Professor Graham A. Webb Department of Chemistry, University of Surrey, Guilford, Surrey, UK Dr. Andrew K. Whittaker Centre for Magnetic Resonance, University of Queensland, Queensland 4072, Australia Dr. Desong Zhong Department of Chemistry, University of Surrey, Guildford, Surrey, UK
Introduction
Polymers generally form a variety of primary, secondary and higher-order structures in the solid state. This comes from the characteristic fact that a polymer chain is formed from an extremely large number of bonds and has sometimes irregular configurational structure and regiostructure. Due to such structural features some regions are found to be in the crystalline state and some in the noncrystalline state. In the former region, polymer chains are aligned like crystals and, on the other hand, in the latter region, they are randomly irregular in structure with and without molecular motion. The existence of these polymer structures is closely associated with their properties. For this reason, it becomes important to carry out precisely both structural and dynamic characterizations. It has been demonstrated that solid state N M R spectroscopy provides useful information about the structure and dynamics of polymers in the bulk. At present, in polymer science, solid state N M R is recognized as one of the most powerful means for elucidating the structure and the dynamics of solid polymers in addition to X-ray diffraction. The history of solid state NMR, which has been used in polymer science, is very old. The appearance of new techniques in solid state N M R has certainly contributed to the development of polymer science and technology. From such a background, the principles of solid state N M R and its applications to structural and dynamic characterization of polymers will be described. Previously, many excellent books and periodical monographs on fundamental N M R and advanced N M R spectroscopies, have appeared. Also some excellent books of solid state N M R of polymers have appeared. Some of these books are mentioned for the convenience of readers below [1, 2].
References 1. Basic NMR books: For example (a) E.R. Andrew, Nuclear Magnetic Resonance, Oxford University Press, Oxford, 1954; (b) A. Abragham, Principles of Nuclear Magnetism, Oxford University Press, Oxford, 1961; (c) R.K. Harris, Nuclear Magnetic Resonance, Pitman, XV
xvi
INTRODUCTION
London, 1983; (d) M. Mehring, High Resolution NMR in Solids, Springer-Verlag, Berlin, 1985; (e) B.C. Gerstein and C.R. Dybowski, Transient Techniques in NMR of Solids, Academic Press, New York, 1985; (f) C.P. Slichter, Principles of Magnetic Resonance, Springer-Verlag, Berlin, 1990; (g) E.O. Stejskal and J.D. Memory, High Resolution NMR in the Solid State, Oxford University Press, Oxford, 1994. 2. Solid state NMR for Polymers: For example (a) C.A. Fyfe, Solid State NMR for Chemists, C.R.C. Press, 1983; (b) R.A. Komoroski (Ed), High Resolution NMR of Synthetic Polymers in Bulk, VCH Publishers, 1986; (c) V.J. McBriety and K.J. Packer, Nuclear Magnetic Resonance in Solid Polymers, Cambridge University Press, Cambridge, 1993; (d) K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, London, 1994; (e) G.A. Webb and I.Ando (Eds), Ann. Repts. NMR Spectroscopy (Special Issue: NMR in Polymer Science), Vol. 34, Academic Press, London, 1997.
Chapter 1
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
N M R Chemical Shift and Electronic Structure Isao Ando ~, Naoki Asakawa 2 and Graham A. Webb 3 ~Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan; 2Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuda, Yokohama, Japan, and 3Department of Chemistry, University of Surrey, Guildford, Surrey, UK
1.1
Introduction
High-resolution solid-state NMR spectroscopy, combined with quantum chemistry, provides detailed information on the structure and electronic structures of solid polymers through the observation of the NMR chemical shift [1]. In the liquid and solution states, NMR chemical shifts of polymers are often the averaged values for all of the possible conformations because of rapid interconversion by rotations about bonds. However, in the solid state, chemical shifts are often characteristic of specific conformations because of strongly restricted rotation about the bonds. The NMR chemical shift is affected by a change of the electronic structure arising from structural changes. NMR chemical shifts in the solid state provide, therefore, useful information about the electronic structure of a polymer or polymers with a fixed structure. Furthermore, in the solid state, the components of the full chemical shift tensor can often be determined. The complete chemical shift tensor provides information on the local symmetry of the electron cloud around the nucleus and so provides much more detailed knowledge of the electronic structure of the polymer compared with the average chemical shift associated with the structure. Such a situation applies to many polymers and, in order to establish the relationship between the NMR chemical shift and the electronic structure of polymers, it is necessary to use a sophisticated theoretical method which takes account of the characteristics of polymers. Some methodologies for obtaining structures and the electronic structures of polymers, both in the solution and solid state, involve a combination of the observation and calculation of NMR chemical shifts. This approach has been applied to various polymer systems. Theoretical calculations of NMR chemical shifts for polymer systems have been achieved using two main approaches. One approach is that model compounds, such as the dimer,
2
ISAO ANDO
ET AL.
trimer, etc., as a local structure of polymer chains, are used in the calculation by combining quantum chemistry and statistical mechanics. In particular, this approach has been applied to polymer systems in solution [2]. However, in solid polymer systems it should be recognized that the results of quantum chemical calculations on model compounds are not readily transferable to polymers because of differences in the electronic structure, including longrange interactions such as intrachain and interchain interactions. Electrons are constrained to a finite region of space in small molecules but this is not necessarily the case for polymers and, hence, some additional approaches are required. Another approach is to employ the tight-binding molecular orbital (TB MO) theory, which is well known in the field of solid-state physics, to describe the electronic structure of linear polymers with periodic structure within the framework of the linear combination of atomic orbitals (LCAO) approximation for the electronic eigenfunctions [3-11]. These approaches lead to the determination of the spatial structure and/or electronic structure of polymer systems including polypeptides in the solution and solid state. The essence of these two approaches are described below.
1.2 Approach using model compounds 1.2.1
The origin of NMR chemical shift
The chemical shift of an atom depends on its electronic and molecular enviroments [12]. Note that the chemical shift relative to a standard reference is expressed by 6 and the chemical shielding by o-. The chemical shielding or for atom A can be estimated by the sum of the following terms: (1.1)
OrA = O-d .qt_ o-P _+_ O " ,
where erd is the diamagnetic term, o-p is the paramagnetic term and or' is another term which comes from the magnetic anisotropy effect, polar effect and ring-current effect. For nuclei with 2p electrons, such as 13C, lSN, etc., the relative chemical shift is predominantly governed by the paramagnetic term, and for the 1H nucleus by the first and third terms in Equation (1.1). The paramagnetic term is expressed as a function of excitation energy, bond order, and electron density according to the sum-over-states (SOS) method in the simple form as follows: (rP -- - C ~ ( r - 3 ) 2 p ( t m _ E n )
-1Q ,
(1.2)
N M R C H E M I C A L SHIFT A N D E L E C T R O N I C S T R U C T U R E
3
Table 1.1. Calculated ~3C chemical shieldings of hydrocarbons by FPT I N D O method
Sample
Calculated ~ (ppm)
Experimental c (ppm)
crd
crp
CH4
57.7
-129.3
-68.0
0
0
Ethane C2H6
57.4
-136.2
-75.7
7.7
8.0
Ethylene
57.9
-230.3
- 169.3
101.3
124.9
Methane
C2H4
O'A
6 (cal)b
(~(exp)
The negative sign means deshielding. b Relative to CH4. c Relative to CH4.
where E m - E n is the singlet-singlet excitation energy from the nth occupied to the mth unoccupied orbitals, and Q is a factor including the bond order and electron density. The quantity (r-3)Zp is the spatial dimensions for a 2p electron while C is the coefficient incorporating universal constants. This term is calculated by semi-empirical MO or ab initio MO methods. The former has some features which give the substantial aspects of the chemical shift behavior associated with the spatial structure and/or the electronic structure. The diamagnetic term is estimated from the calculated electron density. Using these procedures, the chemical shielding o-; of the model compound with any specified conformation is calculated. For example, the contributions of the paramagnetic term and diamagnetic term to the relative 13C chemical shifts of small hydrocarbon molecules, such as methane, ethane and ethylene using the FPT (finite perturbation theory) with the I N D O (semiempirical MO) method, are calculated as shown in Table 1.1 together with the experimental data [13]. Note that the negative sign of the shielding constant o- indicates deshielding and, therefore, shielding variations can be compared with the observed chemical shift 6 where a positive sign denotes deshielding. This table indicates that the paramagnetic term predominantly contributes to the relative ~3C chemical shift, and the contribution of the diamagnetic term is very small. These results show that it is very important to estimate exactly the paramagnetic term for the chemical shift calculations of nuclei with 2p electrons. 1.2.2
Medium effects on N M R chemical shifts
Most MO calculations of nuclear shielding relate to the case of a molecule in a vacuum. For nuclei forming the molecular skeleton, such as 13C, and
4
ISAO ANDO ET AL.
nuclei with small shielding ranges, such as ~H, this may not be an unreasonable approximation. This is particularly true if comparison of the theoretical results is to be made with experimental data taken on a molecule dissolved in an inert solvent. More reactive atoms, especially those with lone pair electrons such as 14N, ~SN, 170 and 19F, are very likely to have their nuclear shieldings influenced by interactions with solvent molecules. Such interactions may be specific, e.g., hydrogen bonding, or nonspecific, e.g., polarizability/polarity, or perhaps a combination of both specific and nonspecific solute-solvent interactions. An empirical procedure has been developed for quantitatively unravelling the contributions made to the shielding of solute nuclei by specific and nonspecific interactions. Nonspecific solute-solvent effects on nuclear shielding may be described by MO calculations which include the influence of solvent polarity/polarizability by means of the solvent dielectric constant, e. This approach is epitomized by the use of the solvaton model together with a semi-empirical MO basis set [14-19]. This method has successfully accounted for the observed variations of 13C and 15N shielding for a number of solute molecules in solvents with various values of e. The solvaton model has been demonstrated to predict correctly both the sign and magnitide of the solute shielding variation as the value of E of the medium is changed. This approach provides a good understanding of the various solute-solvent interactions on the 13C chemical shifts of polymers such as poly(vinyl chloride) associated with the stereochemical configurations [15]. Ab initio MO calculations of the effect of solvent on the 29Si shielding of some solvated molecules have been reported by Arshadi et al. [20]. The extended basis set calculations employed the IGLO nuclear-shielding procedure coupled with a continuum solvent model in which the solute is placed inside an appropriately dimensioned cavity within a polarizable continuum with a given value of E [21]. The calculated variation in the silicon nuclear shielding, as a function of E, agrees satisfactorily with experiment. In general, the variation in the silicon shieldings of the solvated molecules is small but not negligible. Specific solute-solvent interactions, such as hydrogen bonding or protonation, may be included in the calculation of the shielding of solute nuclei by a supermolecule approach. The appropriate structure of the solute-solvent supermolecule can be obtained using molecular mechanics simulations. At the semi-empirical MO level this approach has been used successfully to describe the effects of hydrogen bonding on the nuclear shielding of small molecules. Ab initio MO calculations, using the gauge independent atomic orbital (GIAO) orocedure, has been aoolied in a studv of the effects of
NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE
Fig. 1.1. The planar conformation of any specified configuration of a vinyl polymer. polysaccharide [22]. Calculations using a 6-31G** basis set show that hydrogen bonding can result in a change in 170 nuclear shielding by up to about 70 ppm, whereas a few ppm change at most is predicted for the XH and 13C shielding on hydrogen-bond formation. 1.2.3
Applications to polymers
A polymer chain can assume an enormous number of conformations because of the various possibilities of rotation around the chain bonds, due to molecular motion [23]. Thus, the factors governing the appearance of the NMR spectra include the structures, the relative energies of the rotational isomers, the chemical shifts and spin couplings. If molecular motion in the polymer chain is extremely slow on the NMR timescale, the spectrum represents the superposition of the spectra for the various conformations. However, if the rotation around the chain bonds is very fast on the NMR timescale, the experimentally observable chemical shift for nucleus A is given as [2, 24-281 n
<SA) = 2 PiSg.
(1.3)
/=1
The numerical indices refer to the preferred conformations, and Pg and 6/ are the probability of occurrence and the chemical shift of the preferred conformation i, respectively. This indicates that the chemical shift of a given nucleus can be obtained by a combination of a quantum chemical and a statistical mechanical method as described below. The probability, Pi, of the preferred conformations in various configurations in vinyl polymers as a function of the racemic units necessary for the calculation of the averaged chemical shift (6A) is estimated by statistical mechanics for a polymer chain. For convenience, the planar conformation in any specified configuration of a vinyl polymer is shown in Fig. 1.1. The statistical weight factors are r/, 1 and r for trans (T), positive gauche (G § and negative gauche ( G - ) conformations, respectively. The statistical weight factor for G * G - is ~o (the so-called pentane effect) [23]. The statistical weight matrices for the characterization of the array of chain conformations of vinyl polymers can
6
I S A O A N D O E T AL.
be used. The purpose of calculating the probability that the pair of skeletal bonds within the kth dyad, the kth triad, etc., in a chain are in particular rotational states is that such statistical weights are used to construct statistical matrices U = U'U". In such matrices, rows are associated with rotional states about bond i--1 and columns about bond i. The statistical weight matrices in the case of pairs of bonds adjoining C H R groups are designated U". These matrices are expressed as
;t
?
U =
B
(.o
rt 1
too
for the first skeletal bond of a dyad and
7qw 1
=
=
1
too t
TI
O)
7"09
rio
oo
too 2
r/
w
~Tw
1
r
rtw
w
rw /
for the second; the former matrix, Equation (1.5), is for a meso dyad and the latter, Equation (1.6), for a racemic one. The configuration partition function Z for the entire chain can be expressed as the sum of the statistical weights for all molecular conformations of the chain consisting of n bonds or x = n/2 repeating units 9n / 2
--
1
Z = J
--.,i--,,i
(1.7)
J
i/2 = 1
where J* = (100) and J is the transpose of (111). Let/3 and y denote indices from the set T, G + and G - . The probability Pt3~k" that the pair of skeletal bonds within the kth dyad are in the rotational states/3 and 3', respectively, is the ratio of the sum of the statistical weights for all conformations. It is given by
)
P/3v:k" = z - l J *
UhUh'+l h=l
,(xl
t
tt
(UkU(/3,y)k
U'TT" t
i ~"i+ 1 J ,
i=
1
(1.8)
NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE
7
in which U" is the matrix representing the second bond of the kth dyad with all the elements, except for U", which is replaced by zero. P~g~,, is estimated by generating Monte-Carlo chains. An averaged chemical shift is then obtained using the values obtained for P~v~,, and 6i (2). Using this developed methodology, the structural behaviors of polyethylene and paraffins, vinyl polymers such as poly (vinyl alcohol) and polypropylene, etc., in the solution have been successfully elucidated on the basis of their observed spectra [2, 24-28]. Also, this can be applied to noncrystalline and crystalline phases in polymers. In the crystalline state polymer chains assume a fixed conformation. In this case, the structural information obtained from the chemical shift corresponds to the fixed conformation. The calculation of I3C chemical shifts for a dipeptide fragment (N-acetyl-N'-methyl-L-alanineamide) [Ac-L-AlaNHMe] of poly(L-alanine) and L-alanine residue containing proteins has been attempted using the FPT INDO method in order to understand and predict the13C chemical shift behavior of polypeptides associated with a secondary structure such as an a-helix,/g-sheet, etc., and the determination of secondary structure through the observation of 13C chemical shifts [29]. The 13C chemical shifts of the C~ carbon of an alanine residue in various peptides and polypeptides vary significantly depending on the conformation, which may be a right-handed a-helix, /3-sheet, or other conformation. Such sizeable displacements of the 13C chemical shifts can be characterized by variations in the electronic structures of the local conformation as defined by the dihedral angles (d~, ~). The calculated contour map for the C~ carbon is shown in Fig. 1.2. From this map, we can estimate the 13C shielding for any specified conformation. It has been demonstrated, from comparison of the experimental data and the predicted values given by this chemical shift map, that the map successfully predicts the 13C chemical shifts of alanine residues in polypeptides and proteins [30, 31]. For example, as shown in the map, the 13C chemical shift of the right-handed a-helix form appears at high frequency by 2.5 ppm compared to the /3-sheet form. This reasonably explains the experimental result. For the chemical shift calculation of the 15N nucleus, which is also popular in polymers, the same method can be used. Most recently, ab initio calculations for the NMR chemical shifts have become available for medium-size molecules because of the remarkable power capabilities of modernworkstations, personal computers and supercomputers [32]. This leads to a quantitative discussion on the chemical shift behaviors. For example, the ab initio MO calculation with the 4-31G basis set using the GIAO-CHF (gauge independent atomic orbital-coupled Hartree-Fock) method on N-acetyl-N'-methyl-L-alanineamide which is the same model molecule as the case of the above FPT INDO calculation will be
8
ISAO A N D O ET AL.
6
-180
t.
-120
-60
0
60
~. degree
120
180
Fig. 1.2. The calculated 13C chemical shift map of the C~ carbon in N-acetyl-N'-methyl-Lalanine amide obtained using the FPT INDO method. The chemical shifts were calculated at 15~ intervals for the dihedral angles (~b, ~).
introduced [33]. All of the geometrical parameters are energy-optimized. Figure 1.3(a) shows the calculated isotropic 13C chemical shift map of the C~ carbon as a function of the dihedral angles, where the positive sign means shielding. The whole trend for this map is similar to that obtained by the FPT INDO method as shown in Fig. 1.2. The isotropic shielding constants (or) for the C~ carbon are 186.4 ppm for the dihedral angles (4~, g0, which correspond to the antiparallel/3(/3A)-sheet conformation, 189.4 ppm for the right-handed a(aR)-helix, 189.6 ppm for the left-handed a(aL)-helix; on the other hand, the observed isotropic chemical shifts (~) are 21.0 ppm for the /3a-sheet, 15.5 ppm for the aR-helix and 15.9 ppm for the aL-helix [34]. Such
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NMR C H E M I C A L SHIFT A N D E L E C T R O N I C S T R U C T U R E
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ISAO A N D O ET AL.
9
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Fig. 1.3 9The dependences on the dihedral angles (4), q,), of the chemical shielding constants for the L-alanine residue Ca and Ct3 carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio basis set. The 4-31G optimized geometries for the model molecules, N-acetyl-N'-methyl-L-alanineamide, were employed: (a) the isotropic; (b) o"11; (c) o'22; (d) o'33 for the Ct3 carbon (unit in ppm); (e) the isotropic, (f) o'11; (g) o'22; and (h) o'33 for the Ca carbon (unit in ppm).
NMR C H E M I C A L SHIFT AND E L E C T R O N I C S T R U C T U R E
13
experimental chemical shift behavior is well explained by the calculated behavior. It is found that the change of the dihedral angles dominates the isotropic chemical shift behavior of the L-alanine residue C~ carbon. In order to understand the isotropic chemical shift changes, the calculations of the principal components for chemical shielding tensor have been made. The (qS, ~) dependences of 0-11, 0"22, and 0"33 (these are defined from the least shielded to the most shielded, respectively) are shown in Fig. 3(b-d). From these figures, the fact that the isotropic C~ chemical shift for the aRhelix appears at a lower frequency (15.5 ppm) than that for the /3A-sheet (21.0 ppm) is understood by the explicit differences in the value of 0"11(A0"11 ~ 9 ppm). Careful investigation of the chemical shielding tensor shows that the paramagnetic term for 0"11 dominates the total o" value. The 2p orbital for the C,, carbon, which contributes to the C ~ C ~ or-bond, is the most effective contribution to the paramagnetic term of o"11, since the magnetic dipole-coupling integral of electrons, (qS~I L~/r3 1~) (where, 13 = 1 in this case), is dominantly estimated by the 2p-orbital perpendicular to the direction of o'al. However, in the 4-31G optimized geometry, the distance between the C~ and Ct3 carbons is 1.512 A for the /3A-sheet and 1.516 A for the aR-helix. Although this bond-length difference seems to be too small to explain the behavior of the o'11 value, the fact that the typical C ~ C t 3 bond length determined by X-ray diffraction is estimated as 1.51 A for the/3A-sheet and 1.53 ]k for the aR-helix, respectively, might indicate that the C,,~Ct3 bondlength differences, which are driven by conformational changes, do control the o'11 value. Figure 1.3(e) shows the dependence of isotropic chemical shielding for the C~ carbon against the main-chain dihedral angles. From this figure, the isotropic chemical shieldings for the C~ carbon are 160.4 ppm for the/3A-sheet, 159.6 ppm for the aR-helix 159.2 ppm for the aL-helix, 161.4 ppm for the 3x-helix and 157.9 ppm for silk I and (Ala-Gly)n form II. For these calculated shielding, the observed isotropic chemical shifts are 48.7 ppm for the/3A-sheet, 53.0 ppm for the ag-helix, 50.1 ppm for the aL-helix, 49.7 ppm for the 31-heli~ and 51.5 ppm for (Ala-Gly)n form II, respectively. Although it is obvious that there exists the main-chain dihedral-angle dependence on chemical shift for the C~ carbon, it seems more complicated than that for the C~ carbon, because the orientation of the chemical shift tensor for the C~ carbon, with respect to the molecular fixed flame, is different from one (~b, qJ) to another. Additionally, because, in a case in which the L-alanine residue carbonyl- or amide-group would form the hydrogen-bond, the hydrogen-bonding structure can also affect the behavior of the chemical shift for the C~ carbon. The structure of (Ala-Gly)~ form II and silk I, needs chemical shift calculation of the hydrogen-bonding taken into
14
I S A O A N D O ET AL.
consideration and the investigation of chemical shift carbonyl-carbon will be needed. To give a further insight into the C~ chemical-shift behavior, principal values of the chemical shielding tensor were also calculated [33]. From Fig. 1.3(f-h), it is obvious that all the principal values are quite sensitive to the (q~, 0) differences. In particular, the 0"33 is the most sensitive to the (q~, 0) differences. For almost all (q~, 0)or, the principal axis of the o33 is aligned to the C ~ C ' bond but with ca. 30 ~ deviation and is also nearly perpendicular to the C ~ C ~ bond. Since it is generally known that bond extension makes shieldings decreased [32], it is predicted that the C ~ C ' bond-length differences associated with dihedral-angles variations would contribute dominantly to the 0"33 differences. The investigation of the shielding calculation procedures provides information as to where a chemical shift change comes from. Regarding the o33 for the C~ carbon, it was found that the diamagnetic contribution for the o33 dominates the changes in the total 0"33. Therefore, the most crucial factor for this behavior is changes in geometric parameters of the C, carbon moiety along the 0"33 axis for model compounds with several main-chain dihedral angles. It should be noted that the other principal values, o11 and o22, change their orientation of principal axes for one (q~, 0) to another. One of the complexities--the orientation of the chemical shift tensor--will be discussed later, and the other complexity--the hydrogen-bonding effect--seems particularly intricate because of the chemical shift for the C~ carbon, especially the principal values of the chemical shift tensor, which would be greatly affected by differences in not only the hydrogen-bond length ( R N ~ O and R H ~ O ) , but also on the hydrogen-bond angles (e.g., < N ~ O - - - C , < H ~ O = C , etc.). The hydrogen-bonding effect on the C~ chemical shift will be estimated by the solid-state NMR measurements combined with GIAOCHF chemical-shielding calculations. As mentioned above, the principal values of the chemical shift tensor give information about the three-dimensional electronic state of a molecule. However, in order to understand the behavior of the principal values, one should obtain information about the orientation of the principal axis system of a chemical shift tensor with respect to the molecular fixed frame. Figure 1.4(a-d) shows the calculated orientations of the principal axis systems of the chemical shift tensors of the L-alanine Ct3 carbons in some peptides whose L-alanine moieties have different main-chain dihedral angles, (~b, 0) = (-57.4 ~ -47.5~ (--138.8 ~ 134.7 ~ [/3A-sheet], (-66.3 ~ -24.1 ~ [31~-helix] and (-84.3 ~ 159.0 ~ [31-helix]. Figure 1.4(a-d) shows that the 0"33 component nearly lies along the C ~ C t 3 bond for all peptides considered here, and also show that O'11 is nearly perpendicular to the plane which is
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N M R CHEMICAL SHIFT A N D ELECTRONIC S T R U C T U R E
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Fig. 1.4. e , f . Fig. 1.4. Orientation of the principal axes of the calculated 13C chemical shift for the L-alanine residue C~ carbons: (a) (~b, @)= - 5 7 . 4 ~ -47.5 ~ (aR-helix)" (b) -138.8 ~ 134.7 ~ (/3A-sheet)' (c) -66.3 ~ -24.1 ~ (3Ro-helix); (d) -84.3 ~ 159.0 ~ (3~-helix), and for the C,~ carbon: (e) (qS, ~) = - 5 7 . 4 ~ - 4 7 . 5 ~ (aR-helix); (f) --138.8 ~ 134.7 ~ (/3A-sheet); (g) -66.3 ~ -24.1 ~ (3Rrhelix) 9and (h) -84.3 ~ 159.0 ~ (31-helix).
NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE
17
Table 1.2. Calculated 13C chemical shielding for the Ca- and Ct3-atoms of the L-alanine residue by the 4-31G-GIAO-CHF method.
13C chemical shielding (ppm) Ca
Ct3
Sample
triso
0"11
0"22
0"33
0"iso
0"11
0"22
0-33
dR-helix /3A-sheet 31-helix 3Ro-helix
45.52 44.73 43.62 45.71
61.93 62.02 65.79 64.74
43.69 47.53 46.46 45.04
30.93 24.64 18.91 26.37
15.72 18.74 15.94 15.84
28.16 37.06 33.80 32.47
22.14 21.70 17.97 19.03
-3.16 -2.53 -3.49 -4.00
defined by the Ct3, C~ and the N atoms in the L-alanine residue. On the other hand, 0"22 is parallel with regard to the plane. These results agree with the experimentally determined direction of 633 of the Ct3 carbon in the L-alanine amino acid by Naito et al. [35]. As shown in Table 1.2, the 611 component for the values of dihedral angles corresponding to the flA-sheet conformation is 37.06 ppm. This shows a high frequency shift of about 9 ppm compared to that for the dR-helix conformation. This result means that the 611 component dominates the high frequency shift of the isotropic chemical shift of the Ct3 carbon for the /3A-sheet conformation. Since the 611 component does not orient to a specified chemical bond, it is difficult to comprehend intuitively the chemical shift tensor behavior of the Ct3 carbon. However, it is obvious that the through-space interaction between the Ct3 methyl group and its surroundings is important to understand the 611 behavior. Figure 1.4(e-h) shows the calculated orientations of the principal axis systems of the chemical shift tensors of L-alanine C~ carbons in the peptides with respect to the molecular flame. As is seen from these figures, the calculated orientation of the principal axis system for the C~ carbon is quite different from sample to sample. For all of the dihedral angles employed in the calculations, the 633 component of the ~3C chemical shift tensor of the Lalanine C~ carbon always lies along the C , ~ C ' bond. However, for the dihedral angles, (~b, 0 ) = ( -57.4~ -47.5 ~ JaR-helix], (-66.3, - 2 4 . 1 ) [ 3 Rhelix] and ( - 8 4 . 3 ~ 159.0~ the 611 component lies along a slightly deviated direction from the C ~ C ~ bond; while, for (~b, qs)= (-138.8 ~ 134.7 ~ [~Asheet], the 622 component is along this direction. As shown in Table 1.2, the principal value which is nearly along the C ~ C t ~ bond is 47.53 ppm for the /3A-sheet form, 61.93 ppm for the aR-helix form, 64.74 ppm for the 3R0-helix form and 65.79 ppm for (~b, 0) = (-84.3~ 159.0 ~ [31-helix]. The change of the dihedral angles causes the large deviations of the chemical shift tensor element which is along the C ~ C t 3 bond. Moreover, since the 633 value depends on changes from one dihedral angle to another, it is obvious that
18
ISAO ANDO ET AL.
there exists an explicit dihedral-angle dependence of the 633. Thus, it is thought that if the carbonyl group in the L-alanine residue forms a hydrogen bond, the 633 value will probably be affected. The principal values of the chemical shift tensor of the L-alanine C~ carbon in peptides have not been measured yet, because the chemical shift anisotropy is so small that it is rather difficult to evaluate accurately. Measurement of the principal values of the chemical shift tensor for the L-alanine C~ carbon still need future work.
1.3
Approach of using infinite polymer chains
Sometimes the estimation of the electronic structures of polymer chains necessitates the inclusion of long-range and intermolecular interactions in the chemical shift calculations. To do so, it is necessary to use a sophisticated theoretical method taking account of the characteristics of polymers. In this context, the TB MO theory from the field of solid-state physics is used, in the same sense in which it is employed in the L C A O approximation in molecular quantum chemistry to describe the electronic structures of infinite polymers with a periodical structure [3-11, 36]. In a polymer chain with linearly-bonded monomer units, the potential energy of an electron varies periodically along the chain. In such a system, the wavefunction O(k) for electrons at a position r can be obtained from Bloch's theorem as follows [36, 37]: q~(k)
= N -1/2
2 exp ( i k j ) C ~ , , ( k ) ~ c h ( r
- ja),
(1.9)
where k is the wavenumber, n is the band index, v is an orbital index in the jth cell, a is the unit vector of translational symmetry, N is the total number of cells, and 1 is the number of atomic orbitals in the cell. The term, ~b(k), represents the vth atomic orbital in the jth cell and C~,,(k) the expansion coefficient. The formulae needed to calculate the shielding of polymers using the TB MO theory incorporating the SOS method have been derived as a function of k as follows:
o'd(k) = C ~] P(k)(~b,: [ r-ll ~rP(k) = C' ~
~bo,>,
(r-3)2p(Em - E n ) - l e ,
(1.10) (1.11)
m~>n
where C and C' are constants and P(k) is the bond order as a function of k.
NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE
19
In order to compare the calculated and experimental values of the nuclear shielding, it is necessary to average the calculated data over k, within the first Brillouin zone, as given by
o- =
~
k
D(k)cr(k),
(1.12)
where D(k) is the density of states, i.e., the n u m b e r of states per unit amount of energy. Using this methodology, elucidation of the conformationdependent chemical shift of various polymer chains such as polyethylene, polyacetylene, polypyrrole, polyoxymethylene and polyoxyethylene in the solid state has been performed systematically [3-11]. For example, a comparison of the 13C chemical shift tensor of cis- and t r a n s - p o l y a c e t y l e n e s in the solid state is shown in Fig. 1.5. The direction of the principal axes are indicated at the bottom. These results agree well with the experimental data. This shows that the TB M O calculation correctly predicts the origin of the chemical shift and electronic structure associated with the structure of the polymers. Furthermore, TB I N D O / S calculations have been carried out on the seven polyacetylene chains which take an orthorhombic form [9]. From these results it has been demonstrated that the chemical shift is very sensitive to intermolecular interactions and the TB M O calculation provides useful information about the band structure. The TB M O calculation of the 15N chemical shift of polypyrrole in the solid
(o)
(b) ~
I
Oxx Oyy
Ozz
I i
- 200
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1200
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20
ISAO ANDO ET AL.
state allows useful information to be extracted from the observed spectra, i.e., that the two peaks obtained are correctly assigned to the quinoid and aromatic structures [11, 38] (The quinoid structure is closely related to the electric conductivity.) A decrease in the band gap leads to a high frequency. These results on conducting polymers demonstrate that the chemical shift behavior provides information about the band gap which, in turn, is a measure of the electrical conductivity. It can be said that TB MO calculations offer useful perspectives in interpreting the results of N M R nuclear shieldings in polymers, both in terms of the structure in the solid state and in understanding the effect of intermolecular interactions on nuclear shieldings. The latter are shown to operate through the electronic structures of the polymers considered.
References
.
3. 4. 5.
.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
(a) I. Ando, "Encyclopedia of NMR". John Wiley, New York (1996) p. 176; (b) T. Yamanobe and H. Kurosu, ibid., (1996) pp. 4468; (c) I. Ando, Ann. Rept. NMR Spectrosc. 22 (1990) 205. I. Ando and T. Asakura, Ann. Rept. NMR Spectrosc. 10A (1979) 81. T. Yamanobe, R. Chujo and I. Ando, Mol. Phys. 50 (1983) 123. T. Yamanobe and I. Ando, J. Chem. Phys. 83 (1985) 3154. T. Yamanobe, I. Ando, H. Saito, R. Tabeta, A. Shoji and T. Ozaki, Chem. Phys. 99 (1985) 259. T. Yamanobe, I. Ando, H. Saito, R. Tabeta, A. Shoji and T. Ozaki, Bull. Chem. Soc. Jpn. 58 (1985) 23. T. Yamanobe, T. Sorita, T. Komoto, I. Ando and H. Saito, J. Mol. Structure 151 (1987) 191. H. Kurosu, T. Yamanobe, T. Komoto and I. Ando, Chem. Phys. 116 (1987) 391. T. Ishii, H. Kurosu, T. Yamanobe and I. Ando, J. Chem. Phys. 89 (1988) 7315. (a) H. Kurosu, T. Yamanobe and I. Ando, J. Chem. Phys. 89 (1988) 5261; (b) H. Kurosu and I. Ando, J. Mol. Structure (Theochem) 231 (1991) 231. M. Kikuchi, H. Kurosu and I. Ando, J. Mol. Structure 269 (1992) 183. I. Ando and G.A. Webb, Theory of NMR Parameters, Academic Press, New York, 1983. M. Kondo, I. Ando, R. Chujo and A. Nshioka, J. Mag. Reson. 24 (1976) 315. I. Ando, A. Nishioka and M. Kondo, J. Mag. Reson. 21 (1976) 429. I. Ando, Y. Kato, M. Kondo and A. Nishioka, Makromol. Chem. 178 (1977) 803. M. Kondo, S. Watanabe and I. Ando, Mol. Phys. 37 (1979) 1521. I. Ando and G.A. Webb, Org. Magn. Reson. 15 (1981) 111. I. Ando, "Encyclopedia of NMR". John Wiley, New York, 1996, pp. 2512. M. Witanowski, L. Stefaniak and G.A. Webb, Ann. Rept. NMR Spectrosc. 25 (1993) 42. M. Arshadi, D. Johnels, U. Edlund, C.H. Ottoss and D. Cremer, J. Am. Chem. Soc. 118 (1996) 5120. R. Bonaccorsi, P. Pola and J. Tomasi, J. Am. Chem. Soc. 106 (1984) 1945. P.J. Wilson and G.A. Webb, J. Mol. Structure, in press.
NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.
35. 36. 37. 38.
21
P.J. Flory, "Statistical Mechanics of Chain Molecules". Interscience, New York, 1969. I. Ando, A. Nishioka and T. Asakura, Makromol. Chem. 176 (1975) 411. I. Ando and A. Nishioka, Makromol. Chem. 176 (1975) 3089. I. Ando, Y. Kato and A. Nishioka, Makromol. Chem. 177 (1976) 2759. T. Asakura, I. Ando and A. Nishioka, Makromol. Chem. 176 (1975) 1151. T. Asakura, I. Ando and A. Nishioka, Makromol. Chem. 177 (1977) 1493. I. Ando, H. Saito, R. Tabeta, A. Shoji and T. Ozaki, Macromolecules 17 (1984) 457. H. Saito, R. Tabeta, A. Shoji, T. Ozaki, I. Ando and T. Miyata, Biopolymers 23 (1984) 2279. H. Saito, R. Tabeta, T. Asakura, Y. Iwanaga, A. Shoji, T. Ozaki and I. Ando, Macromolecules 17 (1984) 1405. D.B. Chesnet, Annu. Rept. NMR Spectrosc. 21 (1989) 51. N. Asakawa, H. Kurosu and I. Ando, J. Mol. Structure 323 (1994) 279. (a) H. Saito, R. Tabeta, A. Shoji, T. Ozaki and I. Ando, Macromolecules 16 (1983) 1050; (b) H. Saito, T. Tabeta, A. Shoji, T. Ozaki, I. Ando and T. Asakura, "Magnetic Resonance in Biology and Medicine", (eds) G. Govil et al. Tata McGraw Hill, New Delhi, 1985, pp. 195; (c) H. Saito and I. Ando, Ann. Rept. NMR Spectrosc. 26 (1989) 55; (d) I. Ando, T. Yamanobe and T. Asakura, Prog. NMR Spectrosc. 22 (1990) 349; (e) H. Kurosu, S. Ando, H. Yoshimizu and I. Ando, Ann. Rept. NMR Spectrosc. 28 (1994) 189. A. Naito, S. Ganapathy, K. Akasaka, and C.A. McDowell, J. Chem. Phys. 74 (1981) 3190. I. Ando, T. Yamanobe, H. Kurosu and G.A. Webb, Ann. Rept. NMR Spectrosc. 22 (1990) 205. J.J. Ladik, "Quantum Theory of Polymers as Solids". Plenum Press, New York, 1988. H. Kurosu, M. Kikuchi, I. Ando, J. Polymer Sci., Polymer Phys. 33 (1995) 769.
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Chapter 2
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Dipolar Interactions and Interatomic Distances Akira Naito, Satoru Tuzi and Hazime Sait6 Department of Life Science, Himeji Institute of Technology, Kamigori, Hyogo, Japan
2.1
Introduction
Magnetic dipole-dipole interactions (dipolar interactions) describe the through space direct coupling of two nuclear spins I and S. Because this interaction is axially symmetric and traceless along the I-S interatomic vector, it provides geometric information of molecules in the solid state. In high resolution solid-state NMR, this interaction is eliminated to yield NMR resonances by combining the high-power dipolar decoupling and magic-angle spinning techniques (CP MAS). Recently, various new NMR techniques have been developed to recouple specific dipolar interactions between isotopically labelled nuclei under the high resolution condition. In this chapter, the fundamentals and applications to obtaining interatomic distances from the recoupled dipolar interaction are described.
2.2
Description of the dipolar interaction
Magnetic dipole-dipole coupling between two nuclear spins I and S can be expressed in angular velocity units as the Hamiltonian ~is
)~is - y_~ys__h[I. S - 3(I. r)(S. r)/r2], 2~rr3
(2.1)
where r is the internuclear vector between the spins I and S. This interaction depends on the interatomic distance and the angle between the internuclear vector, and the static magnetic field as illustrated in Fig. 2.1. For the case of homonuclear two-spin system, this interaction can be given in the presence
24
A K I R A NAITO ET AL.
Ho
S
-__y
x
Fig. 2.1. Orientation of I-S internuclear vector in the polar coordinate.
of strong magnetic field I-Io by
Y(II =
y2h
[(3 cos 2 0 -
1 ) / 2 ] ( I 1 " 12 - 311zI2z)
27rr 3
(2.2)
=A+B, where
A
~__
B
_._
Y2h (3 C O S 2 27rr 3
0 --
1)IlzI2z
~/2h (3 cos 2 0 - 1)[(11+12- + I1-I2+)/4] 2~rr 3
In Equation (2.2), 11 and 12 a r e the like spins. 3/I is the gyromagnetic ratio of I nuclei, h is the Plank's constant and r denotes the length of the I1-I2 internuclear vector. 0 is the angle between the static magnetic field and r. The B-term is called the flip-flop term and causes mutual spin-exchange when the energy levels of the states are very close to each other. Two dipolar precession frequencies in the rotating frame can be obtained from Equation (2.2) as follows:
DIPOLAR
INTERACTIONS
AND ITERATOMIC
WD = Wo -- 3D(3 cos 2 0 - 1),
DISTANCES
25
(2.3)
where
O
~.
27rr 3 '
D is called the dipolar coupling constant and Wo is the resonance frequency of the observed nuclei. The heteronuclear dipolar Hamiltonian can be expressed as
~IS =
7ITsh (3 cos 2 0 - 1)IzSz 27rr 3
(2.4)
where I and S are like or unlike nuclear spins, respectively. In this case, the difference of the Zeeman interaction between the I and S nuclei is quite large and, hence, spin-exchange cannot occur. The dipolar precession frequency in this case can be obtained from Equation (2.4) as follows: 1
COo = 0% _ ~D(3 cos 2 0 - 1).
(2.5)
It is now possible to determine internuclear distances from the dipolar interaction to evaluate molecular structure.
2.3
Dipolar interaction under MAS
When samples are rotating about an axis inclined to Om from the static magnetic field in the CP MAS experiment as shown in Fig. 2.2, 0 is timedependent and the 3 cos 2 0 - i term in Equations (3) and (5) can be expressed as a function of time as follows: 1
3 COS 2 0 ( t ) - 1 = ~(3 cos 2 0m 3
+ ~ sin 3
--
1)(3
COS 2 ]3 --
20m sin 2/3 cos(a
1)
+ tOrt)
+ 5 sin 2 0 m sin 2 13cos 2(a + tOrt),
(2.6)
where a is the azimuthal angle and /3 is the polar angle defined by the internuclear vector with respect to the rotor axis. COr is the angular velocity
26
A K I R A N A I T O E T AL.
Uo
C0rt S
Fig. 2.2. Representation of I-S internuclear vector rotating about the MAS axis.
of the rotor. When 0m is the magic angle, the first term in Equation (2.6) vanishes and Equation (2.7) is given by 3 cos 20(t) - 1 = X/2 sin 2/3 cos
tort + s i n 2
19COS2(a
+ tort).
(2.7)
Two frequencies due to the I-S dipolar interaction are expressed in angular velocity units as follows" D ~OD(a, /3, t) = --+ -2--[sin2/3cos 2(a + tort)
./---- V 2
sin 2/3 cos(a + tort)].
Z
(2.8) Therefore, the dipolar interaction under the magic-angle spinning condition is a function of time. This term can be null after taking an average over the rotor period as follows"
WD = ~
=0.
WD ( t) dt
(2.9)
This fact indicates that the dipolar interaction cannot affect the lineshape of the centre peak except for the intensities of the sidebands. Because of the
D I P O L A R I N T E R A C T I O N S AND I T E R A T O M I C DISTANCES
27
above mentioned reason, the dipolar interaction was thought to be difficult to obtain under the MAS conditions.
2.4
Recoupling of the dipolar interaction
Accurate interatomic distances can be evaluated from dipolar interactions which were normally sacrificed by the high-power decoupling and magicangle spinning techniques [1, 2]. A considerable improvement has been established to recouple the dipolar interaction by either introducing rf pulses synchronized with the MAS rotor period [3] or adjusting the rotor frequency as the difference of the chemical shift values of two isotopically labelled homonuclei [4]. Rotational echo double resonance (REDOR) [1, 3] was explored to recouple the relatively weak heteronuclear dipolar interactions under the MAS condition by applying a ~r-pulse synchronously with the rotor period. Consequently, the transverse magnetization cannot be refocused completely at the end of the rotor cycle, leading to a reduction of the echo amplitude. The extent of the reduction of the echo amplitude as a function of the number of rotor periods depends on the strength of the heteronuclear dipolar interaction. This method is extensively used to determine the relatively remote interatomic distance of 2-8 ~ . When a number of isolated pairs are involved in a R E D O R dephasing, the R E D O R transformation could be useful in determining interatomic distances because it yields single peaks in the frequency domain for each heteronuclear coupling strength [5, 6]. This approach is not, however, applicable in the case where the observed nuclei in R E D O R are coupled with multiple nuclei. The rotational resonance (RR) phenomenon [4, 7] is a recoupling of the homonuclear dipolar interaction under the MAS condition. When the rotor frequency is adjusted to multiple of the difference frequency of the chemical shift values of two different resonance lines, line broadening and acceleration of the exchange rate of the longitudinal magnetization are observed. These effects depend strongly on the magnitude of the homonuclear dipolar interaction. R E D O R and RR methods have been most extensively explored, although several approaches to determine the interatomic distances in solid molecules have been proposed: T E D O R (transferred echo double resonance) [8] is a similar method used to determine heteronuclear dipolar interactions by observing the buildup of echo amplitude. The magnetization in this method is transferred from one nucleus to the other through the heteronuclear dipolar interaction. It is useful, therefore, to eliminate naturally abundant background signals. D R A M A (dipolar recovery at the magic angle) [9] is used to recouple the homonuclear dipolar interaction, which is normally
28
A K I R A N A I T O ET AL.
averaged out by MAS by applying 90~ and 90~ pulses synchronously with the rotor period and, hence, interatomic distances between the two homonuclei can be determined. Since D R A M A strongly depends on the offset of the carrier frequency, Sun et al. [10] developed M E L O D R A M A (melding of spin-locking and DRAMA) by combining D R A M A with a spin-lock technique. This technique reduced the offset effect. SEDRA (simple excitation for the dephasing of rotational echo amplitude) [11] and RFDR (rf driven dipolar recoupling) [12] are techniques used to apply a ~r-pulse synchronously with the rotor period. These techniques also apply to determine the homonuclear dipolar interaction under the MAS condition. Because these techniques are not sensitive to the MAS frequency and offset effects, they will be useful to determine the dipolar interaction using multidimensional NMR for multiple site-labelled systems [10]. However, it has not been fully evaluated how accurately interatomic distances can be determined by these methods. As an alternative approach to evaluate molecular structure based on interatomic distances, methods of determining torsion angles have been proposed using magnetization or coherence transfer through a particular bond [13-16] or spin diffusion between isotopically labelled nuclei [17]. This approach is expected to be useful for obtaining structural information through torsion angles rather than interatomic distances, although much effort for spectral simulation has to be made to analyze the two-dimensional data to determine either one torsion angle, or one pair of dihedral angles, alone.
2.5
Theoretical background of the REDOR and RR experiments
R E D O R and RR methods have been used extensively to determine the three-dimensional structure of biomolecules, because the pulse sequence and the data analysis to yield the interatomic distance are simpler compared with the other methods. It seems worthwhile to describe the formalism of the R E D O R experiment by a density operator to take into account the effect of a finite pulse length and by the three-spin system encountered on many occasions. 2.5.1
Simple description of the REDOR Experiment [1]
Transverse magnetization which precesses about the static magnetic field, due to the dipolar interaction under the MAS condition, moves back to the same direction at every rotor period because the integration of ~OD over one rotor period is zero. Consequently, the rotational echo signals are refocused at every rotor period. When a ~r-pulse is applied to the S nucleus, which is
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
29
coupled with the I nucleus in one rotor period, it plays a role to invert the precession direction of the magnetization of the observed I nucleus. Consequently, the magnetization vector of the I nucleus cannot move back to the same direction after one rotor period. Therefore, the amplitude of the echo intensity decreases. The extent of the reduction of the rotational echo amplitude yields the interatomic distances. To evaluate the R E D O R echo amplitude theoretically, one has to consider the averaging precession frequency in the presence of a ~r-pulse at the centre of the rotor period over one rotor cycle as follows:
l[Wr2
rOD(a, fl ) = +--
Zr
Jo
rOD dt -
f(r ] rOD dt
r/2
~-D ~/~ sin 213 sin a .
(2.10)
7/"
Therefore, the phase angle, by
A~(a, /3), for
the Nc rotor cycle can be given
a(I)(ff, ]~) = O)D(Or , ]3) N c T r ,
(2.11)
where Tr is the rotor period. Finally, the echo amplitude can be obtained by averaging over every orientations as follows"
sf= l
f~ ft3c~
fl)] dasin fl
(2.12)
Therefore, the normalized echo difference, AS/So, can be given by
AS/So
= (So = 1 -
sf.
se)/So (2.13)
Experimentally, R E D O R and full echo spectra are acquired for a variety of NcTr values and the respective R E D O R (Sf) and full echo (So) amplitudes are evaluated.
30
AKIRA
NAITO
ET AL.
7I
$
2
,,
!
I I
I I
I
I
I I I
I I I
' tw' I
I
! i
I |,,,
T
--tt,-
I
Po
!
Tr
Tr
2
p(Tr)
Fig. 2.3. Pulse sequence and timing chart of REDOR experiment.
Rotational echo amplitude calculated by the density operator approach 2.5.2
The R E D O R echo amplitude can be evaluated more rigourously by using density matrix operators and a pulse sequence for the R E D O R experiment shown in Fig. 2.3 [18]. The time evolution of density operator, Po, under the heteronuclear dipolar interaction during one rotor period can be considered by taking the pulse length into account. The average Hamiltonian in the rotating frame over one rotor period can be given by
= 1
Tr
[ ~ l ( t ) r + ~2(t)tw + 7~3(t)~']
D f ~sin 2/3[sin(2c~ + Ogrtw) + sin(2c~ - Ogrtw)- 2 sin 2c~] 47r L -2X/2 sin 2/3[sin(a
+
1
1
-~09rtw) + s i n ( a - ~09rtw)
+
2 sin 2a]
22
- s i n 2/3[sin(2c~
+
09rtw) + sin(2c~ 1
+ ~/2 sin 2/3[sin(a
+
09rtw)]
4Wrtw 22
2
4Ogrtw - 7r 1
2o92t2
]
-~09rtw) + sin(a - 5COrtW)]oj~7 w _-- ~r2~IzSz
DIPOLAR
INTERACTIONS
AND
ITERATOMIC
DISTANCES
31
2"n'Wrtw ]
D
4Ir
sin2/~[cos(2~ + OOrtw) + COS(2C~ -- O)rtw)]40~t~ -- ~2~IzSy
= alzSz + blzSy,
(2.14)
where the same notations as in Equations (2.4) and (2.6) are used. ~l(t), ~2(t) and 3~3(t) are the average Hamiltonians corresponding to the period shown in Fig. 2.3. Pulse length, tw, is also considered in the calculations for the analysis of the R E D O R results. The density operator, p(Tr), at Tr after evolution under the average Hamiltonian can be calculated as m
p(Tr) = exp(-i3~Tr)po exp(i~Tr),
(2.15)
where Po is considered to be Iy after the contact pulse. Then finally the transverse magnetization at Tr can be given by (Iy(Tr)) = Tr{p (Tr)Iy} m -
1 2 cos(sX/a +
b2
Tr)
.
(2.16)
Echo amplitude in the powder sample can be calculated by averaging over every orientation as follows:
sf = l f ~ f t3(Iy(Tr)) sin ~ d~ da
(2.17)
Therefore, the normalized echo difference, AS/So, can be given by Equation (2.13). When the length of tw is zero, Equation (2.16) can be simplified as follows:
(Iy(Tr))- cos(D ~/-2 sin 2/3 sin a T r ) .
(2.18)
In this case, Equation (2.17) is equivalent to Equation (2.12) in the case of N c = 1.
32
AKIRA NAITO ET AL.
2.5.3
Echo amplitude in the three-spin system [19]
It is important to consider the case where the observed nucleus (I1) is coupled with two other heteronuclei ($1 and $2). The Hamiltonian in the three-spin system can be given by ~(t) =
T I T s h [3 cos 2 01(t) - 1 ] I z l S z l 21rr 3 YI'ysh [3 2zrr 3
COS 2
02(t) -- 1]IzxSz2
(2.19)
where rl and r2 are the I1-$1 and the I1-S2 interatomic distances, respectively. 01(t) and 02(t) correspond to the angles between the magnetic field and the I1-$1 and the Ii-S2 internuclear vectors, respectively. In the molecular coordinate system, the x axis is along the I1-$1 internuclear vector, and the $1-I1-$2 plane is laid on the x-y plane. The angle between I~-S1 and I1-S2 is denoted to be sr. The coordinate system is transformed from the molecular axis system to the MAS system by applying a rotation transformation matrix R ( a , /3, y) with Euler angles a, /3, y followed by transforming from the MAS to the laboratory coordinate system by applying R(to~t, 0m, 0). Finally, cos 01(t) and cos 02(t) are calculated as follows: cos 01(t) = (cos 3/cos/3 cos a - sin y sin a)sin 0m COS tot --(sin y cos/3 cos a + cos 3' sin a)sin 0m sin tot + sin/3 cos a cos 0m and cos 02(t) = [(cos y cos/3 cosa - sin 3' sin a)cos sr sin 0m + (COS y COS/3 sin a + sin 3' cos a)sin sr sin 0m]COS cot - [ ( s i n y cos/3 cos a + cos 3' sin a)cos sr sin 0m + (sin 3' cos/3 sin a - cos 7 cos a)sin sr sin 0m]sin cot + sin/3 sin a cos 0m sin sr + sin/3 sin a cos 0m sin ~r,
(2.20)
where Om is the magic angle between the spinner axis and the static magnetic field, and co is the angular velocity of the spinner rotating about the magicangle axis. The four resonance frequencies in the system are given by
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
33
(-0D1 = (D1 - D 2 ) / 2 (-OD2-- (D1 + D2)/2 (-0D3-- -(D1 + D2)/2
r
(2.21)
-(D1 - D2)/2 9
These dipolar-transition frequencies are time-dependent and repeat the cycle in the spinning. In the R E D O R pulse sequence, a rr-pulse is applied in the centre of the rotor period. In this case, the averaged angular velocity over one rotor cycle for each resonance is given by 1 (fTr/2
O)i(~, /3, ~/, Tr) - Tr \ j o
tODidt -
)
WDi dt . r/2
(2.22)
The phase accumulation after the Nc cycle is given by A(Ioi(~ ,/3, ]/, N c , Tr) = (.oi(o~,/~, "y, Tr) NcTr.
(2.23)
Finally, the R E D O R echo amplitude after averaging over all Euler angles was calculated as
1 i=~ 1 f ~ f t 3 f S f-- 87.i2
[cosA~i(a,~,y, Tr)]dasin~d~d ~.
(2.24)
The normalized echo difference, AS/So, can be given by Equation (2.13). This relation strongly depends, not only on the dipolar couplings of Ix-S1 and I1-82, but also on the angle S l - I i - S 2 [19].
2.6
Practical aspects of the REDOR experiment
It is emphasized that accurate interatomic distances are a prerequisite to achieving the three-dimensional structure of peptides, proteins and macromolecules. A careful evaluation of the following several points is the most important step to obtaining reliable interatomic distances by the R E D O R experiment, although they have not always been taken seriously into account in the early papers. In practice, it is advisable to employ a standard sample such as [1-13C, 15N]glycine [18], whose C - N interatomic distance is determined to be 2.48 ~ by a neutron diffraction study, to check that the
34
AKIRA NAITO ET AL.
13C-REDOR
J~
___
13C-Full echo
_. '
l jL
L '
'
'
I
'
'
'
'
I
'
'
'
1 '
I
. . . .
1 I
. . . .
100
200
-
ppm
1~
oo
0
0
'
4
8
12
16
20
NcTr (ms) Fig. 2.4. 13C REDOR and full echo spectra of [I-13C, 15N]glycine as recorded by the rotor frequency of 4000 Hz and NcTr of 4 ms (top) and plots of the AS/So vs. NcTr (bottom). Solid and open circles denote the experimental points recorded using a 15N rr-pulse of 13.0 and 24.6 Ixs, respectively. Solid, broken and dotted lines are calculated using the rr-pulses of 6, 13.0 and 24.6 lxs, respectively, together with the C-N interatomic distance of 2.48 ~ [18].
i n s t r u m e n t a l conditions of a given s p e c t r o m e t e r are correct prior to the e x p e r i m e n t of a new sample. As described in Section 2.5.2, the finite pulse length m a y affect the R E D O R factor. In fact, this effect is e x p e r i m e n t a l l y o b s e r v e d and calculated using E q u a t i o n (2.17) as shown in Fig. 2.4. T h e R E D O R p a r a m e t e r , AS/So,
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
35
as measured for 20% [1-13C, 15N]Gly is plotted for the lengths of the 15N 77"pulse of 13.0 Ixs for the experiment and 24.6 Ixs (chosen to satisfy 10% rotor cycle) as a function of NcTr with the calculated lines using the 3-pulse length and finite lengths (13.0 and 24.6 Ixs) of the 7r-pulse. It turns out, however, that the finite length of the 15N 7r-pulse does not significantly affect the R E D O R effect provided that the pulse length is less than 10% of the rotor cycle at the rotor frequency of 4000 Hz. For most spectrometers, it is very difficult to be free from fluctuations of rfpower during the acquisition of R E D O R experiments. Therefore, it is very important for the rf power to be stabilized after waiting a certain time period. If not, the 7r-pulse cannot stay as the exact 7r-pulse for a long time. Consequently, the R E D O R factor is greatly decreased to yield relatively longer interatomic distances if the rf power changes. Compensation of instability of such rf power by the pulse sequence is necessary, therefore, to be free from long-term fluctuation of amplifiers, xy-4 and xy-8 pulse sequences have been developed for this purpose and an xy-8 pulse is known to be the best sequence to compensate for the fluctuation of the rf power [20]. Since the early stage of the R E D O R experiment, contributions of natural abundance nuclei have been seriously considered as the major error source for the distance measurement [21]. It appears that the observed dipolar interaction can be modified by the presence of such neighbouring naturally abundant nuclei. This effect was originally taken into account by simply calculating the AS/So value for isolated two pairs and adding proportionally to the natural abundant fraction [21]. Careful analysis of the three-spin system, however, indicates that this sort of simple addition of the fraction of the two-spin system may result in a serious overestimate of the natural abundance effect, to yield shorter distances [18]. Therefore, the most accurate way to consider the natural abundance effect is to treat the whole spin system as a three-spin system by taking into account the neighbouring carbons in addition to the labelled pair. In practice, contributions from naturally abundant nuclei can be ignored [19] for 13C R E D O R but not for 15N R E D O R , because the proportion of naturally abundant 13C nuclei is much higher than that of the 15N nuclei. Usually, 13C, and 15N-doubly labelled samples are used in the R E D O R experiment to determine the interatomic distances between the labelled nuclei. More importantly, the dipolar interaction with the labelled 15N nuclei for the neighbouring molecules should be taken into account as an additional factor contributing to the dipolar interaction of the observed pair under consideration. This contribution could be serious when the observed distance is quite remote, because there are many contributions from nearby nuclei. This effect can be removed completely by diluting the labelled sample with
36
AKIRA NAITO ET AL.
1.0
I
1
I
I
NcTr(ms) 16
0.8
J14
o 0.6 03 <1
~12
"-'---------IL_
~10 0.4
0.2
0.0 0
_.o.__----t ) 4 !
!
I
I
20
40
60
80
100
percentage of labeld peptide
Fig. 2.5. AS/So plot against various percentages of a labeled peptide in an unlabeled one. The solid lines were obtained by least square fits of the experimental points [19].
a sample of naturally abundant molecules. The sensitivity of the signals, however, has to be sacrificed if one wants to remove the effect completely as the sample was previously diluted to 1/49 [22]. Instead, it is advised to evaluate the R E D O R factors at the infinitely diluted condition by extrapolating the data by stepwise dilution of the sample (i.e., 60%, 30%, etc.) without losing sensitivity (Fig. 2.5) [19], because a linear relationship between the R E D O R factor and the dilution is ascertained by a theoretical consideration. Alternatively, the observed plots of AS/So values against the corresponding NcTr values for the sample without dilution can be fitted by a theoretical curve obtained from the dipolar interactions among three-spin systems, although the accuracy is not always improved to the level of the dilution experiment. The transverse magnetization of the R E D O R experiment decays as a function of the I H decoupling field [23, 24]. Dipolar decoupling may be strongly interfered with by molecular motion, if any, when the motional frequency is of the same order of magnitude as the decoupling field and, hence, the transverse relaxation times (T2's) are significantly shortened. In
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
37
fact, it was found that the T2 values of the carbonyl carbons in crystalline Leu-enkephalin were very short because of the presence of backbone motion [25]. This is a serious problem for the R E D O R experiment, especially for the long distance pairs, because the S/N ratio is significantly deteriorated. In this case, it is worth considering measuring the 13C R E D O R signal under a strong decoupling field to elongate the transverse relaxation times. It is also useful to measure the distances at low temperature so as to reduce the motional frequency. It is cautioned, however, that crystalline phase transition could be associated with the freezing of the solvent molecule as encountered for a variety of enkephalin samples [26, 27]. In the commercial spectrometer, the rotor is designed to allow the sample volume to be as large as possible in order to gain better sensitivity. Obviously, this arrangement causes HI inhomogeneity which results in a broad distribution of the lengths of the 90 ~ pulses. This problem is serious for the R E D O R experiment in which a number of ~r-pulses are applied. As a result, the pulse error can accumulate during the acquisition to give serious error as shown in Fig. 2.6. Particularly, the samples located at the top or bottom part of the sample rotor feel a quite weak rf field [18, 28]. This causes a great reduction of the R E D O R factor for sample which is filled all the way along in the sample rotor. This effect should be seriously taken into account prior to experiment with a commercial spectrometer. It is, therefore, strongly recommended to fill the sample only in the centre part of the coil just like a multiple pulse experiment to be able to acquire accurate interatomic distances as much as possible by the R E D O R method.
2.7
Simple description of RR experiment [7]
In contrast to the R E D O R experiment, homonuclear dipolar interactions can be recoupled in the R R experiment by adjusting the rotor frequency to be a multiple of the difference frequency of the isotropic chemical shift values of two chemically different homonuclear spins which is called rotational resonance conditions (A(.Ois o = nO)r). Under this condition, the energy level due to the sideband of one resonance becomes equal to that due to the centreband of the other and, therefore, mixing of two-spin states occurs as indicated by the dotted arrow shown in Fig. 2.7. Consequently, the broadening of the lineshape and the acceleration of the exchange rate of the longitudinal magnetization are observed. These rotational resonance phenomena are strongly dependent on the interatomic distance. One can determine, therefore, the interhomonuclear distance by the R R experiment. When the interatomic distance between the labelled nuclei is short, a characteristic lineshape due
38
AKIRA NAITO ET AL.
13C-REDOR
13C-Full echo
. . . .
I
'
'
'
'
i
. . . .
I
. . . .
200
I
. . . .
ppm
100
1.0 O3 O3 <:1
6
12
18
24
30
NcTr(ms) Fig. 2.6. 13C REDOR and full echo spectra of [1-13C]N-Ac-Pro-Gly-[15N]Phe with an NcTr of 16 ms (top) and plots of the AS/So vs NcTr (bottom). Solid circles, solid squares and open circles denote the experimental points from the samples filled in the central 50% of the total filling volume of a 5 mm o.d. rotor, and the fully packed state of 5.0 and 7.5 mm rotors, respectively. The resulting interatomic distances were determined as 4.07, 4.37 and 4.45 A, respectively [18].
to the mixing of the spin states can be observed. In contrast, w h e n the i n t e r a t o m i c distance is long, change of the signal intensity due to e x c h a n g e of the longitudinal m a g n e t i z a t i o n s can be o b s e r v e d . As s h o w n in E q u a t i o n (2.6), the h o m o n u c l e a r dipolar interaction u n d e r the M A S c o n d i t i o n has the F o u i e r c o m p o n e n t s co~~) at the f r e q u e n c y m~Or as follows:
DIPOLAR
INTERACTIONS
AND
ITERATOMIC
B
DISTANCES
39
A
Or
icxl3> ~ .... t~--->-. or
,
~
Ab3isoI ' " .... ~ .....
113a>
A B laa>
A
B
A A
Fig. 2.7. Energy level diagram in the homonuclear two-spin system under near rotational resonance condition. 2
mR(t) =
O0(B m) exp(imoort).
~
(2.25)
m = --2
The Fourier components in this equation can be further expanded by the chemical shift anisotropy as follows" 2
O.)(n) =
~
tUB'(m)"(m--n)uA
,
(2.26)
m = --2
where, CO(B '~ is the consequence of dipolar CO(B m) and chemical shift anisotropy a(A"). When the rotational resonance condition A~Oiso = nOOr is fulfilled, ~O(B ") is the Fourier component which is equal to the energy gap between two-spin states (Fig. 2.7) and, therefore, the spin-exchange, or mixing of the wavefunctions, becomes efficient. At the rotational condition, the exchange rate R can be expressed as R 2 - r2 -
41 ~o~) 12,
(2.27)
40
A K I R A NAITO ET AL.
where r = 1/T z ~ represents the zero quantum transverse magnetization rate and ] ~o(BmI implies the rotationally driven exchange rate. When R2> 0, the difference of the longitudinal two-spins is given by ?.
_
( I z - Sz>(t)= e rt/2[cosh(Rt/2) +--sinh(Rt/2)],
R
(2.28)
w h e n R 2 < 0, ?-
( I z - Sz>(t)= e-rt/2[cos(iRt/2) + - sin(iRt/2)]. iR
(2.29)
As expected, T z ~ chemical shift anisotropy and dipolar interaction are involved in the exchange rate of the longitudinal magnetization under the MAS condition. Therefore, the separation of the dipolar interaction is quite complicated compared with the R E D O R experiment.
2.8
Practical aspect of RR experiment [26]
Experimentally, homonuclear dipolar interactions can be determined by measuring the extent of the exchange rate of the longitudinal magnetization as a function of mixing times 'I" m using the pulse sequence shown in Fig. 2.8. In the pulse sequence, a selective inversion pulse is used to invert one of the two resonances followed by the mixing time. One of the advantages of the R R experiment is that it can be performed by an ordinary double-resonance spectrometer as long as the spinner speed can be controlled using a spinner frequency controller. It is advised to use n = 1 as the rotational resonance ~2
H ~ CP
Decoupling
n/2
m:2
13c CP
INV ~
'Cm--I~
Fig. 2.8. Pulse sequence of RR experiment. INV is the selective pulse to invert one resonance.
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
41
condition because the chemical shift anisotropy is strongly affected when the n value is greater than three. When a high-field spectrometer is used, chemical shift anisotropy increases proportional to the frequency used. In that case, one has to include the chemical shift anisotropy tensor for both nuclei in the analysis of the longitudinal magnetization. In the analysis of the R R experiment, one has to use the T z ~ as discussed in the previous section. This value is difficult to determine experimentally. Practically, the T z ~ value can be approximated from the single quantum relaxation time (Y2) of the two spins by the expression [29] 1 TZO
=
1 TI1
+
1
(2.30)
TI2 '
or
T2z ~ =
1
1r(vi, + vi~)
.
(2.31)
When the chemical shift difference is very small, it is difficult to perform the R R experiment, because of the overlap of the resonance lines of the dipolar coupled nuclei, leading to difficulty in the analysis of the R R data. In this case, a rotating resonance experiment in the tilted rotating flame [30] can be used because a much higher spinning speed can be adopted for the case of a smaller chemical shift difference in the system. The natural abundance background signal can also affect the apparent amount of R R magnetization exchange. Consequently, the observed magnetization exchange rate yields a smaller magnetization exchange rate than the observed one. Therefore, a rate smaller than the real one is obtained resulting in an overestimation of the interatomic distance. Incomplete proton decoupling prevents magnetization exchange between the coupled spins because of the H1 field inhomogeneity. It is advised to irradiate with a strong proton decoupling field (>80 kHz) to a small size of sample in the rf coil to prevent H1 inhomogeneity. Overall, much care has to be taken for the R R experiment to obtain accurate interatomic distances.
42
AKIRA NAITO ET AL.
2.9 Illustrative examples for the determination of three-dimensional structure based on accurately determined interatomic distances
2.9.1
Peptides and proteins
Marshall et al. [31] synthesized an emerimicine fragment (Ac-Phe-[113C]MeAZ-MeA-MeA-Val-[15N]Gly6-Leu-MeA-MeA-OBz). The 13C-15N interatomic distance of 4 residues apart was determined to be 4.07 A by the R E D O R method. It was concluded that the structure is a-helix, because the expected distances are 4.13 and 5.87 A in the cases of the a-helix and the 3~o-helix, respectively. In a similar manner, the 19F-13C interatomic distance was measured for the 19F, 13C and 15N triply-labelled fragment (19FCHzCOPhe-MeA-MeA-[1-13C]MeA-[15N]Val-Gly-Leu-MeA-MeA-OBzl) [32] and found to be 7.8 ~ by the T E D O R method [8] after transferring the magnetization from 15N to 13C. Because the T E D O R method makes it possible to eliminate background signals due to naturally abundant nuclei, quite remote interatomic distances can be determined. Hing and Schaefer [33] also tried to determine the C - N interatomic distances of an ion channel peptide Val 1[1-13C]GlyZ-[15N]Ala3-gramicidin A in a DMPC bilayer. The dipolar interaction of the peptide in the lipid bilayer showed much smaller values compared with that in the powder state, because the helix motions significantly averaged the dipolar interactions. The extent of the scaling of the dipolar interaction shows that gramicidin A consists of the dimer with a single helix. A magainin analogue in the membrane was investigated by 13C, 31p R E D O R [34]. The result indicates that the a-helical Ala19-magainin 2 amide is bound to the head group of the lipid bilayers. It is mentioned here that the data analysis described above seems to be worth considering in order to improve the accuracy of the R E D O R data. A complete three-dimensional structure can be determined by combining a variety of interatomic distances [18, 22, 35, 36]. Garbow and coworkers have synthesized three labelled peptides which were labelled at different positions. These interatomic distances were converted to the torsion angles to yield the/3-turn II structure [22, 35]. Naito et al. [18] systematically applied this technique to elucidate the three-dimensional structure of N-Ac-Pro-GlyPhe. They proposed that the carbonyl carbon of the i-1 residue, and the amino nitrogen of the i + lth residue, should be labelled with 13C and 15N, respectively. Namely, [1-13C]N-Ac-Pro-[15N]Gly-Phe (I), N-Ac-[1-13C]Pro Gly-[15N]Phe (II) and [1-13C]N-Ac-Pro-Gly-[15N]Phe (III) were synthesized and the resulting distances determined to be 3.24, 3.43 and 4.07 ~ , respectively, utilizing the R E D O R factor obtained for the infinitely diluted state to prevent errors from the contributions of the neighbouring labelled nuclei. No
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
43
correction from the contribution of the naturally abundant nuclei turned out to be necessary. Surprisingly, these distances do not agree well with the values obtained from an X-ray diffraction study [37] available at that time, showing the maximum discrepancy between them to be 0.5 A. This value seems to be much larger than the expected error in the R E D O R experiment (___0.05 A). The reason why the distances are so different is explained by the fact that the crystal (orthorhombic) used for the R E D O R experiments is different from that used in the X-ray diffraction study (monoclinic). To check the accuracy of the R E D O R experiment, an X-ray diffraction study was performed on the same crystals used for the R E D O R experiment. It is found that the distances from the new crystalline polymorph (orthorhombic) agree well within an accuracy of 0.05 A as shown in Table 2.1. Conformational maps based on the possible combinations of the torsion angles of the Pro and Gly residues are calculated as shown in Fig. 2.9. Furthermore, the difference of the chemical shifts between the C~ and C v carbons of the Pro residues A ~ was used as a constraint to determine the 0 value ( - 1 3 ~ [38]. Since the 4~ angle of the Pro residue is restricted in many instances to - 7 5 ~ which shows the minimum energy in the residue. Therefore, the torsion angles of the Pro residue are uniquely determined to be ( - 7 5 ~, -28~ Using these torsion angles, conformational maps were calculated as shown in Fig. 2.9. Finally, two pairs of torsion angles were selected as ( - 1 1 2 ~ 48 ~ and ( - 1 1 2 ~, -48~ Energy minimization by molecular mechanics yielded the structure of the/g-turn I structure, as shown in Fig. 2.10. It was found that the three-dimensional structure of this peptide was well reproduced by a molecular dynamics simulation by taking into account all of the intermolecular interactions in the crystals [18, 39]. Elucidation of the three-dimensional structure of an opioid peptide Leuenkephalin crystal, Tyr-Gly-Gly-Phe-Leu grown from M e O H / H 2 0 mixed solvent, was performed by the R E D O R method [27] alone. This seems to be an additional challenge for this technique to reveal the three-dimensional structure of more complicated systems. Six differently-labelled Leu-enkephaline molecules were synthesized following the strategy described above and the resulting interatomic distances accurately determined. It turns out, however, that the crystalline polymorph under consideration was very easily converted to another form. Therefore, it is necessary to check whether or not the six differently-labelled samples are all in the same crystalline polymorph by means of the ~3C chemical shifts. Otherwise, meaningless data can be obtained without this precaution. When the distance data are converted to yield the necessary numbers of torsion angles, a unique combination of torsion angles in the corresponding conformational map are determined by using the chemical shift data as additional constraints. A three-dimensional
4~ 4~
Table 2.1. C - N I n t e r a t o m i c distances (A) determined from R E D O R experiments as compared with those by X-ray diffraction and M D [18]. Experimental
peptides
REDOR
X-ray
Orthorhombic
Orthorhombic
Monoclinicb
3.24 + 0.05 (3.43 _+ 0.05) c 3.43 +- 0.05 (3.66 -+ 0.05) 4.07 + 0.05 (4.45 + 0.05)
3.19 3.35 3.99
3.76 3.21 3.91
I II III
>
Calculated
Labelled
Energy-minimized a
a Energy-minimized structure based on R E D O R data. b Ref. [36]. c D a t a from Ref. [19] based on fully packed 7.5 m m rotor system.
MD
>
Orthorhombic
Monoclinic
3.22 -+ 0.10 3.32 + 0.10 3.92 _+ 0.12
3.63 - 0.10 3.33 +_ 0.10 3 83 + 0.10
> 3.17 3.57 4.17
9 t'rl
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES 180
180
.
.
.
.
45
.
b
"x ~JPro 0
-180 -180
B .
.
i .
.
0
.
~Pro
180
-180 -180
.
.
.
.
0
,
180
(~Gly
Fig. 2.9. Conformation maps for the torsion angles in Pro (a) and Gly residues (b), respectively. The A and B regions were obtained from the intersections of the constraint of ~b angles of the Pro residue. The C and D regions were then obtained by a cross-section of the two types of conformation maps [18].
"!:"
'
A
"4.07
,
Fig. 2.10. Optimized conformation of N-acetyl-Pro-Gly-Phe as obtained by the minimization of energy from the initial form as deduced from the R E D O R experiment [18].
structure was thus determined as shown in Fig. 2.11. However, this structure is not the same as that previously determined by X-ray diffraction because one is dealing with a crystalline polymorph which is not fully explored. The RR method has been used to characterize the structures of fragments of amyloid [40, 41]. Griffin et al. [40] have synthesized the/3-amyloid fragment (HzN-Leu-Met-Val-GIy-Gly-Val-Val-Ile-Ala-COzH) which is the C-terminus of the /3-amyloid protein. The structure of this molecule was
46
AKIRA NAITO ET AL.
Fig. 2.11. Three-dimensional structure of Leu-enkephalin crystal determined by the R E D O R experiment [27].
determined by the 13Cm13Cinteratomic distances and the 13C chemical shift values. The a-carbon of the ith residue, and the carbonyl carbon of the i + lth residue, were doubly labelled and the interatomic distance A[ai, i + 1] observed by using a rotational resonance method. Similarly, the interatomic distances of B[i, a(i + 2)], C[i, a(i + 3)] were also determined. Since the rotational resonance signal of A does not show a dilution effect, an intermolecular contribution does not exist. On the other hand, B and C show strong intermolecular contributions from B* and C*. Therefore, it turns out that the fragment forms an antiparallel /3-sheet. Furthermore, the intermolecular contribution indicates that/3-strands consist of antiparallel/3-sheets forming hydrogen bonds with the position which is slipped from the Nterminus position. It is interesting that the information on the intermolecular contribution made it possible to reveal the assembly of the amyloid molecules. Another important application of the R E D O R and RR methods is to determine protein structure. However, it is still difficult to determine the three-dimensional structure of the whole protein molecule using these methods. Instead, Schaefer and coworkers [42-45] have determined the structure of the ligand and the binding site of the ligand protein complex. It is possible to determine the interatomic distance by the R E D O R method in
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
47
the case of the enzyme-analogue complex, because the reaction will not forward. However, it is difficult to measure the interatomic distances of the enzyme-substrate complex since they will react in a short time. Evans and coworkers [46, 47] freeze the reaction instantaneously and the structure of the intermediate state of the complex could be observed by the R E D O R method. Griffin and coworkers [48-50] have used the RR method to determine retinal configurations in various states of photointermediates in the membrane protein bacteriorhodopsin.
2.9.2
Syntheticpolymers
In contrast to biological systems, the accuracy of the measured distance is less stringent for synthetic polymers, because they do not form high quality crystals and it is not easy to perform specifically isotope label. Local packing in a glassy polycarbonate has been examined in a homogeneous mixture of 5% [Carbonyl-a3C]polycarbonate and 95% [methyl-d6]polycarbonate using R E D O R NMR. The 13C~2D distance from the carbonyl carbon of one polycarbonate chain to the average deuterium position of the closest methyl group of the nearest-neighbour polycarbonate chain is 3.8/k. These results indicate that one of the methyl groups is closer to the carbonate group than the other. It is also demonstrated that the distribution of intermolecular carbonate-isopropylidene distances in the glass is quite narrow [51]. In addition, 13C~2D distances of the average nearest-neighbour interchain ringcarbon to ring-deuterium, and ring-carbon to methyl-deuterium, were determined to be 2.6 and 3.2 A, respectively. The short ring-ring distance indicates that the phenyl groups are tightly packed. This fact is consistent with the fact that cooperative intermolecular motions are required for ring flips [52]. The existence of locally parallel chain segments were examined by means of DRAMA, C E D R A and DANTE-selective R E D O R experiments. In this system, the 13C--13C distance in pure [carbonyl-~3C]polycarbonate, and in a homogeneous blend of [carbonyl-X3C]polycarbonate-dx4 and [methyl13C]polycarbonate, were determined [53]. R E D O R was also applied to examine the structure and dynamics of interfaces of heterogeneous polymer blends. A heterogeneous blend was prepared from [carbonyl-13C]polycarbonate and poly(p-fluorostyren-co-styrene) copolymer of p-fluorostylene. The blend was formed by coprecipitation from chloroform into methanol. A fluorine dephased 13C R E D O R signal indicates that the 1 polycarbonate chain in 20 exists at the interface, suggesting that the polycarbonate phase is embedded in a continuous polystyrene matrix which is 200 A thick or 400/k in diameter [54].
48
AKIRA NAITO ET AL.
2.10
Concluding remarks
It is emphasized here that the accurate determination of the interatomic distances are a prerequisite to achieve the three-dimensional structure of peptides, proteins and macromolecules. A protocol for R E D O R and R R experiments for this purpose is described in depth from both the theoretical and practical points of view. In addition, a systematic procedure to construct the three-dimensional structure from these distance constraints is described together with a brief review of some related subjects so far reported. We believe that the measurement of accurate interatomic distances can be the most promising and valuable means to reveal the three-dimensional structure of macromolecules such as membrane proteins in the future.
References
.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
T. Gullion and J. Schaefer, Adv. Magn. Reson. 13 (1989) 57. E. Bennett, R.G. Griffin and S. Vega, NMR 33 (1994) 1. T. Gullion and J. Schaefer, J. Magn. Reson. 81 (1989) 196. D.P. Raleigh, M.H. Levitt and R.G. Griffin, Chem. Phys. Lett. 146 (1988) 71. K.T. Mueller, T.P. Javie, D.J. Aurentz and B.W. Roberts, Chem. Phys. Lett. 242 (1995) 535. T.P. Javie, G.T. Went and K.T. Mueller, J. Am. Chem. Soc. 118 (1996) 5330. M.H. Levitt, D.P. Raleigh, F. Creuzet and R.G. Griffin, J. Chem. Phys. 92 (1990) 6347. A.W. Hing, S. Vega and J. Schaefer, J. Magn. Reson. 96 (1992) 205. R. Tycko and G. Dabbagh, Chem. Phys. Lett. 173 (1990) 461. B-Q. Sun, P.R. Costa, D. Kocisko, P.T. Lansbury, Jr. and R.G. Griffin, J. Chem. Phys. 102 (1995) 702. T. Gullion and S. Vega, Chem. Phys. Lett. 194 (1992) 423. A.E. Bennett, J.H. Ok, R.G. Griffin and S. Vega, J. Chem. Phys. 96 (1992) 8624. G.J. Boender, J. Raap, S. Prytulla, H. Oschkinat and H.J.M. de Groot, Chem. Phys. Lett. 237 (1995) 502. T. Fujiwara, K. Sugase, M. Kainosho, A. Ono and H. Akutsu, J. Am. Chem. Soc. 117 (1995) 11351. K. Schmidt-Rohr,Macromolecules 29 (1996) 3975. M. Baldus, R.J. Iuliucci and B.M. Meier, J. Am. Chem. Soc. 119 (1997) 1121. D.P. Weliky and R. Tyco, J. Am. Chem. Soc. 118 (1996) 8487. A. Naito, K. Nishimura, S. Kimura, S. Tuzi, M. Aida, N. Yasuoka and H. Sait6, J. Phys. Chem. 100 (1996) 14995. A. Naito, K. Nishimura, S. Tuzi and H. Sait6, Chem. Phys. Lett. 229 (1994) 506. T. Gullion and J. Schaefer, J. Magn. Reson. 92 (1991) 439. Y. Pan, T. Gullion and J. Schaefer. J. Magn. Reson. 90 (1990) 330. J.R. Garbow and C.A. McWherter, J. Am. Chem. Soc. 115 (1993) 238. D. Suwelack, W.P. Rothwell and J.S. Waugh, J. Chem. Phys. 73 (1980) 2559. W.P. Rothwell and J.S. Waugh, J. Chem. Phys. 74 (1981) 2721.
DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES
49
25. A. Naito, A. Fukutani, M. Uitdehaag, S. Tuzi and H. Sait6, J. Mol. Struc. 441 (1998) 231. 26. M. Kamihira, A. Naito, K. Nishimura, S. Tuzi and H. Sait6, J. Phys. Chem. (in press). 27. K. Nishimura, A. Naito, C. Hashimoto, S. Tuzi and H. Sait6 (manuscript in preparation). 28. O.B. Peersen, M. Groesbeek, S. Aimoto and S.O. Smith, J. Am. Chem. Soc. 117 (1995) 7728. 29. A. Kubo and C.A. McDowell, J. Chem. Soc. (Faraday Trans 1) 84 (1988) 3713. 30. K. Takegoshi, K. Nomura and T. Terao, Chem. Phys. Lett. 232 (1995) 424. 31. G.R. Marshall, D.P. Beusen, K. Kociolek, A.S. Redlinski, M.T. Leplawy and J. Schaefer, J. Am. Chem. Soc. 112 (1990) 4963. 32. S.M. Holl, G.R. Marshall, D.P. Beusen, K. Kociolek, A.S. Redlinski, M.T. Leplway, R. Makey, S. Vega and J. Schaefer, J. Am. Chem. Soc. 114 (1992) 4830. 33. A.W. Hing and J. Schaefer, Biochemistry 32 (1993) 7593. 34. D.J. Hirsh, J. Hammer, W.L. Maloy, J. Blazyk and J. Schaefer, Biochemistry 35 (1996) 12733. 35. J.R. Garbow, M. Breslav, O. Antohi and F. Naider, Biochemistry 33 (1994) 10094. 36. R.C. Anderson, T. Gullion, J.M. Joers, M. Shepiro, E.B. Villhauer and H.P. Weber J. Am. Chem. Soc. 117 (1995) 10546. 37. S.K. Brahmachari, T.N. Bhat, V. Sudhakar, M. Vijayan, R.S. Rapaka, R.S. Bhatnagar and VS. Aranthanarayanan, J. Am. Chem. Soc. 103 (1981) 1703. 38. I.Z. von Siemion, T. Wieland bs K-H. Pook, Angew. Chem. 87 (1975) 712. 39. M. Aida, A. Naito and H. Sait6, J. Mol. Struc. Theochem 388 (1996) 187. 40. P.T. Lansbury, Jr., P.R. Costa, J.M. Griffiths, E.J. Simon, M. Auger, K.J. Halverson, D.A. Kocisko, Z.S. Hendsch, T.T. Ashburn, R.G.S. Spencer, B. Tidor and RG. Griffin, Nature Structural Biology 2 (1995) 990. 41. J.M. Griffiths, T.T. Ashburn, M. Auger, P.R. Costa, R.G. Griffin and PT. Lansbury, Jr., J. Am. Chem. Soc. 117 (1995) 3539. 42. A.W. Hing, N. Tjandra, P.F. Cottam, J. Schaefer and C. Ho, Biochemistry 33 (1994) 8651. 43. A.M. Christensen and J. Schaefer, Biochemistry 32 (1993) 2868. 44. L.M. McDowell, A. Schmidt, E.R. Cohen, D.R. Studelska and J. Schaefer, J. Mol. Biol. 256 (1996) 160. 45. L.M. McDowell, C.K. Klug, D.D. Beusen and J. Schaefer, Biochemistry 35 (1996) 5396. 46. Y. Li, R.J. Appleyard, W.A. Shuttleworth and J.N.S. Evans, J. Am. Chem. Soc. 116 (1994) 10799. 47. Y. Li, F. Krekel, C.A. Ramilo, N. Amrhein and J.N.S. Evans, FEBS Lett. 377 (1995) 208. 48. L.K. Thompsom, A.E. McDermott, J. Raap, C.M. van der Wielen, J. Lugtenburg, J. Herzfeld and R.G. Griffin, Biochemistry 31 (1992) 7931. 49. K.V. Lakshmi, M. Auger, J. Raap, J. Lugtenburg, R.G. Griffin and J. Hertzfeld J. Am. Chem. Soc. 115 (1993) 8515. 50. K.V. Lakshmi, M.R. Farrar, J. Raap, J. Lugtenburg, R.G. Griffin and J. Herzfeld, Biochemistry 33 (1994) 8854. 51. A. Schmidt, T. Kowalewski and J. Schaefer, Macromolecules 25 (1993) 1729. 52. P.L. Lee and J. Schaefer, Macromolecules 28 (1995) 1921. 53. C.A. Klug, W. Zhn, K. Tasaki and J. Schaefer, Macromolecules 30 (1997) 1734. 54. G. Tong, Y. Pan, M. Afeworki, M.D. Poliks and J. Schaefer, Macromolecules 28 (1995) 1719.
This Page Intentionally Left Blank
Chapter 3
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
NMR Relaxations and Dynamics F. Horii Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan
3.1
Introduction
NMR observations basically contain spin relaxation processes which are associated with molecular motions with different specific frequencies in a given system. For quantitative measurements to determine the compositions of the system or selective measurements of particular components with different relaxation parameters, it is essential, therefore, to understand the principle of the relaxation mechanism. When our interest is focused on molecular motions, spin relaxation parameters such as spin-lattice relaxation time T~, spin-spin relaxation time T2, and the nuclear Overhauser enhancement (NOE), are directly measured as a function of temperature or field frequency by using appropriate pulse sequences. Such temperature or frequency dependencies of the spin relaxation parameters are analyzed in terms of appropriate models to obtain detailed information of molecular motions with frequencies of 106-1012 Hz in the system. In this chapter, the basic theories and analyses for the spin relaxation parameters are described somewhat in detail. As shown in Fig. 3.1, NMR can also probe a wide range of frequencies for molecular motions which are reflected differently on various NMR parameters. In particular, spectra reflecting the chemical shift anisotropy (CSA), the C ~ H dipolar interaction, and the 2H quadrupolar interaction are sensitive to the mid-range of frequencies, which are closely associated with important properties for glassy polymers such as impact strength and gas permeability. The basic equations and analyses for the 2H and 13C CSA spectra are also described in this chapter.
3.2
Spin relaxation parameters
The total Hamiltonian of the system containing spins I is generally given by the following equation [1]:
52
F. HORII
Fig. 3.1. Frequencies of the molecular motion detectable by NMR spectroscopy.
Ht-- Hz + Ho + Ho + Hs + Hj.
(3.1)
Here, Hz is the Zeeman term, Ho is the quadrupolar interaction term for nuclei with 1 i> 1, HD is the dipolar interaction term for nuclei with I = 1/2, Hs is the electron shielding term and Hj is the J-coupling term. Spin relaxations will be induced by the time fluctuations of these interaction terms. For example, 2H spin-lattice relaxation behaviour is dominated by Ho, whereas Ho mainly determines the relaxation process of the 1H or 13C magnetization in organic materials. In some cases without significant contributions from H o and HD, the time fluctuations of Hs and Hj also induce spin relaxation; for example, the magnetization of a carbonyl carbon with a large chemical shift anisotropy relaxes due to the contribution from the time fluctuation of Hs. Nevertheless, since the main interest of polymer scientists is 13C NMR, we focus on the description of the 13C relaxation process in this chapter. 3.2.1
Basic theory
In the case of conventional organic polymers, 13C T1, T2 and NOE values are determined mainly by the time fluctuation of the magnetic dipole-dipole
NMR RELAXATIONS AND DYNAMICS
53
interaction between the ~3C and 1H nuclei. Therefore, the spin relaxation theory for the 13Cm1H two-spin system is described here in some detail [13]. 3.2.1.1 Transitions among eigenstates and spin relaxation parameters According to traditional conventions, a two-spin system consists of spins, I and S, with spin I of 1/2 and these spins are coupled with each other through the magnetic dipole-dipole interaction (hereafter, I and S correspond to 1H and 13C nuclei, respectively). When two eigenstates of the spins defined with respect to the direction of the static magnetic field (the z direction), which correspond to their energy eigenstates, are expressed as [+ ) and i - ), the corresponding eigenstates in the z direction for the two-spin system composed of I and S are described as
I+ + > , 1 + - > , 1 - +>,1
(3.2)
>,
Assuming that there are enough numbers of these two spins in the system, and that rapid transitions occur among these eigenstates as shown in Fig. 3.2, then T1 and N O E can be described in terms of transition probabilities, w0, w~ and w2, among these eigenstates as follows: 1 T 1 --
(3.3)
,
Wo + 2w~ + w2
w
I+->
--
Wo
w,
!-+>
I+ + ~ Fig. 3.2. Four eigenstates defined for the two-spin system with respect to the direction of the static magnetic field.
54
F. HORII
)/
U 1'
u
[+ -)x ~
= I-
Ho
l+
Fig. 3.3. Four states defined for the two-spin system with respect to the direction perpendicular to the static magnetic field.
NOE = 1 +
1422- 1420
TH
9 , Wo + 2w~ + w 2 ')/c
(3.4)
where yi-i and Yc are the gyromagnetic ratios of the XH and ~3C nuclei, respectively. On the other hand, similar four states can also be defined in the direction perpendicular to the static magnetic field, for example, in the x direction, as shown in Fig. 3.3. However, these states are not eigenstates but general states which can be described by linear combinations of the four eigenstates in the z direction shown in Equation (3.2). In this case, T2 is expressed by the following equation using transition probabilities, Uo, u~ and u2, among these four states as shown in Fig. 3.3.
=
1
.
(3.5)
Uo + 2u~ + H2
3.2.1.2 Transition probabilities The total Hamiltonian H t of the system is assumed to be composed of the unperturbed Hamiltonian Ho and the perturbed Hamiltonian H' as follows"
Ht = Ho + H ' .
(3.6)
NMR R E L A X A T I O N S AND DYNAMICS
55
Here, the contribution from H' will be fairly small compared to that from Ho, and the time fluctuation of H' is assumed to induce the transitions among the eigenstates determined by Ho. According to perturbation theory, the respective transition probabilities among the eigenstates, which appear in Equations (3.3)-(3.5), are given by
1 f~
Wij = --~
(n~ I H' *(t + ~) [nj)(nj l H'(t) l n~) exp(-iwq~-) dr
(3.7)
where wq = (Ej - E~)/h, and E; and Ej are the energies for the states In)) and ]nj), respectively. These states correspond to the states shown in Figs. 3.2 and 3.3. The upper bar in Equation (3.7) indicates the ensemble average for spins and the asterisk denotes the complex conjugate. In conventional polymer systems, H' corresponds to the magnetic dipoledipole interaction HD between 13C and 1H nuclei. As is well known, HD is expressed in terms of the spin operator functions Oq'S and the orientation functions Fq's of the 13C~IH internuclear vector against the static magnetic field Bo as follows: 2
HD
=
2 2 2 -3 YcYHh r
E
F_qOq,
(3.8)
q=--2
with Fo = 1 -
3n 2
F+_1 -- (l +- im)n F+_2 = (l + i m ) 2
(3.9)
~0 = IxSx- (1/4)(I+S - + I - S +) ~+1 = (-3/2)(I+S~ + IxS ) ~+2 = (-3/4)1+S + 9
(3.10)
Here 1, m and n are the direction cosines of the 13C~IH internUclear vector in the laboratory frame, as shown in Fig. 3.4, and r is the internuclear distance. The substitution of these equations into Equation (3.7) gives the following equation:
56
F. HORII
/' l
B0
H
r
xJ Fig. 3.4.
Jc r = li + mj + nk
A schematic representation of the C m H internuclear vector in the
xyz
coordinate.
wi] - ]/2T2h2r-6 s (ni l ~ ' I nj)(gt][~qlni) q ,q' x ;~~ F*q,(t + r)F_q(t) exp(-iw~ff) d r .
(3.11)
Here, it is assumed that the distance r is a constant independent of time. After the time-consuming calculations for Equations (3.3)-(3.5) and (3.11), T1, T2, and N O E can finally be expressed by the auto-correlation functions Gq('r) of the orientation functions Fq, which describe the random time fluctuation of the C ~ H vector, or by the spectral densities Jq(o)) that are the Fourier transforms of Gq(7) with frequency o) as follows: 1
NT1 1 .....
NT2
2 2 ,-2 YHYCn
~
16r 6 2
{Jo((.OH- a)c) + 18Jl(Coc) + 9J2(O)H + WE)},
2 3,2
THTC n
36r 6
{4Jo(0) + JO(WH -- we) + 18J1(o9c) + 36J1(wi-i)
+ 9J2(O)H -t- O)c)}, NOE = 1 +
(3.12)
9J2(09H + OgC) -- Jo~on + ~Oc)
(3.13) "}/H
9 . Jo(OgH- Wc) + 18Jl(WC) + 9J2(OgH + Wc) Yc
(3.14)
Here, N is the number of protons chemically bonded to a given carbon and O)H and o)c are the Larmor frequencies of the 1H and 13Cnuclei, respectively. Moreover, the correlation functions Gq(7") and the spectral densities Jq(o)) are described as follows"
NMR RELAXATIONS AND DYNAMICS
G q ( r ) = F~(t + r)Fq(t)
57 (3.15)
q = 0, 1, 2
(3.16)
Jq(o)) = I-~7 G(r) e x p ( - i w r ) d r . d-o~
3.2.1.3 Molecular motion models A large number of structural models describing the random motions of the C ~ H internuclear vector have already been proposed for the calculations of Gq(7") or Jq(o)). In this section, the equations for Jq(m) are shown for some representative structural models as well as for different levels of model-free treatments. (a) Single correlation-time model. When the C ~ H internuclear vector undergoes isotropic random motion, the following well-known equations for Jq((.o) a r e obtained:
Jq(~O) = Kq
2Tc
2 2
l+wrc
K0 = 4/5, K1 = 2/15, K2 = 8/15.
(3.17)
Here, rc is the correlation time for the molecular motion of the C ~ H vector, meaning the time (or the life time) in which the C ~ H vector stays in the same direction without any motion. Figure 3.5 shows T1, T2 and NOE as functions of rc, which are obtained using Equation (3.17). (b) Distributions of correlation times rc. When the system in question is an ensemble of spin systems with different rc values, the distribution functions P(rc) for re should be introduced. In this c a s e , Jq(r are given by
Jq(~O) = K q
2P(rc)rc drc 1 q- 022Y 2
P(rc) drc = 1.
(3.18)
For example, in the box-type distribution P(rc) is expressed as In P(rc) - (In ,c){ln(ln_~e)-
Tcs ~ T c ~ ETcs
otherwise
,
(3.19)
and then Jq(oJ) are described as a function of the average value -? of rc and the parameter 9 for the width of the distribution shown in Equation (3.19) as follows:
58
F. HORII
I00~
I
"
I
'
i
"'
10
I
"
I
'
~-!
6.3T
~
4 . 7 T ~
=]
o.11
1
I0-12 i0-~'
0 . 01 ...
101
i.
i0-7 I0-6
. i . . . . 10-:o 10-9 lO-S !
I
I'
! '
'
':
L
]"~ _-
/9.4
T
~ . 3 T
I0 -I st
_:
4.7 T " ~ " ~
I0-2 K 2
10-3
.
I .
i0-12 lO-,l
3.0
....
,
10-1o
~ .....
I
.
!
..IX.
10-9 lO'S 10-7 I0-6 j
i
~
" '
2.3T
u~
o Z
2.0-
4.7.7'
-
6.3 9.4
-
1.0 ,,, J .... ~ .... ~.. 10-12 lO-ll lO-lO 10-9
,
!
lO-S
,
-
!'i
10-7
,
" 10-6
rr Fig. 3.5 13C T1, T2 and NOE vs. ~c under different magnetic fields (Ref. [3]).
NMR RELAXATIONS AND DYNAMICS 2Kq
- 1
Jq(w) = ~ tan ~o In e
59
(.OT In(e)
1 + e ( e - 1)-2(ln e
)2 o2=2.
(3.20)
In t h e log-x 2 distribution [4], which is frequently used for polymer systems, the distribution function is expressed as P(s) ds = ~
1
r(p)
( p s ) p - l e - p s ds ,
(3.21)
where p is the parameter used to determine the width of the distribution, s is given by (3.22)
s = logb[1 + ( b - 1)Zc/'~],
where b is an adjustable parameter between 10-1000. In this case Jq(og) is 2Kq
Jq(cO) = ~ t a n co In e
- 1
O)T In(e)
2-2" 1 + e(e - 1)-2(ln E)zw z
(3.23)
(c) C o n f o r m a t i o n j u m p m o d e l s . Some models called conformation jump models were proposed to describe the segmental motion of polymers in more realistic ways. One of the representative conformation jump models is a model proposed by Hall and Helfand [5] and Weber and Helfand [6] (hereafter referred to as the H W H model). In this model, two kinds of conformations, + and - , are assumed along a polymer chain and each structural unit is assumed to undergo a single + ~ - jump motion and a co-operative +- ~-+ jump motion. If the correlation times of the single and cooperative jump motions are Zo and z~, the auto-correlation function Gq('r) is given by G q ( g ) - Kq
exp(-z/zo)exp(-z/Zl)Io(z/'r 1).
(3.24)
Here, I o ( r / r l ) is a modified Bessel function. This equation contains two kinds of correlation time and, therefore, it will correspond to the 2z model described in the following section. Dejean et al. [7], proposed another conformation jump model (referred to as the DLM model), in which a librational motion of the jump axis was introduced as the third motion. This model will correspond to the 3z model described below but the derivation of Gq('r) w a s empirically made in this case,
60
F. HORII Gq(T) = (1 -
A)exp(-z/'ro)exp(-'r/'rl)Io(z/'rl) + A exp(--'r/'rL). (3.25)
(d) Multiple correlation-time models. There are many kinds of models which describe the complicated segmental motion of polymeric chains in the solution and solid states. In generalized structural models for such motions, which are referred to as multiple-correlation-time models [8-10], the thermal fluctuation of the C ~ H internuclear vector should be described in terms of the superposition of several independent random motions. Let O1, 02, 9 9", Op_l be the rectangular coordinate systems defined in the respective motional units. Here, O I indicates the coordinate system used to describe the most local motion of the C ~ H vector and Op denotes the laboratory frame. The direction cosines in the laboratory frame, which appear in Equation (3.9), are then related to the direction cosines ll, m i, and F/1 of the C ~ H vector in the O1 coordinate system by the following equation"
t'mt t'1t =Tp...T3T2
ml
(3.26)
nl
where Tj are the 3 x 3 matrices of the orthogonal transformation from coordinate system Oj-1 to coordinate system Oj:
T~
t cos Sj cos 0j cos ~bj - sin ~. sin ~bj sin Sj cos 0j cos ~bj + cos Sj sin 4~j - s i n 0j cos ~bj - c o s Sj cos 0j sin ~bj - sin Sj sin ~bj
c~ 0J sinai 1 - s i n Sj cos 0j sin ~bj + cos Sj cos ~bj sin 0j sin 9 sin 0j sin ~bj COS Oj
(3.27)
Here, q~j, 0j and ~bj are the Euler angles that describe the Oj_ 1 coordinate system in the Oj coordinate system. The thermal fluctuation of the C ~ H internuclear vector should be expressed, therefore, by the time fluctuation of the Euler angles in each coordinate system in the multiple correlationtime models. Jq(tO) can finally be derived when the modes of molecular motions are defined in the respective coordinate systems. Woessner [11] proposed such a molecular motion model whereby p = 2, which is hereafter referred to as 2z model. In this model the C ~ H vector
NMR RELAXATIONS AND DYNAMICS
61
undergoes the diffusional rotation about the Z l axis in the O~ system, while the Zl axis independently changes in orientation by the isotropic random motion in the laboratory frame. Then Jq(tO) are given by
Jq(tO)
= Kq[A
k
2ri 1 + (.02T 2
-'[" B
2rl 27"2 ] 1 + w2r 2 + C 1 + w2r2J ,
(3.28)
with --1 1 --1 r x = r~- + rR , r2-1 = ri- 1 + 4rR 1 ,
(3.29)
and A = (3
COS 2 OR -
1)2/4,
B = 3 sin 2 0a cos 2 0 a , (3.30)
C = (3/4)sin 2 OR.
Here, rR and T I are the correlation times for the diffusional rotation and the isotropic random motion, respectively. OR is the angle between the C ~ H internuclear vector and the Zl axis. In contrast, Howarth [12] derived Jq(tO) for the 3r model corresponding to p = 3, where three independent motions are assumed to be superposed for the overall motion of the C ~ H vector as shown in Fig. 3.6. Namely, the C m H vector undergoes diffusional rotation about the Zl axis in the O1 frame, whereas the Zl axis librates within a cone whose axis is parallel to the z2 axis in the 02 frame. Moreover, the z2 axis undergoes the isotropic random reorientation in the laboratory frame. Although an empirical approximation was made in the previous calculation, we obtained the following equations by the exact mathematical derivation [8-10]:
2rl + AR(1 -- AL) 2rl 2r2 Jq(o)) = Kq ARAL 1 + o)2T2 1 + c02r2 + BRBL 1 + w2r22 +BR(1 -- BL)
273 2r4 + CRCL 1 + w2r 2 1 + w2r 2
+ CR(1
2r5 ] 1 + ~2r2
-
eL)
(3.31)
62
F. HORII Z2 ..-.
\x"-XN~_..
----~
r162 -'7/.4
,, /x
Z1
I Y2
X2 Fig. 3.6. A schematic representation of the 3r model describing the motion of the C - - H internuclear vector.
with --1 "/'1 : --1
r2
--1
r3
= ri = r{
--1 T4 =
--1
r5
TI 1
ri
= rI
1 1 1
1
+
--1 TL --1
+ rR
+ r{~
1
--1
+ rR
+ 4rR 1 + rE
1
+4rR
1
(3.32)
and ~ ( 1 + c o s 0L)2/4
AL
=
BL
- sin 2 0L(1 + c o s 0L)2/6
COS 2
CI~ = (2 + c o s 0L)2(1 -- COS 0 L ) 2 / 2 4 , AR
=
BR =
(3
COS 2 OR -
1)2/4
3 sin 2 OR cos 2 OR
(3.33)
NMR RELAXATIONS AND DYNAMICS CR = (3 sin 4 0R)/4 9
63 (3.34)
Here, 7R, 7L and '7"I are the correlation times for the diffusional rotation, libration and isotropic motion, respectively. OR and OL are halves of the vertical angles of the cones associated with the corresponding motions. The librational motion is defined here as time-dependent reorientation in which the Zl axis randomly changes in direction within the larger cone with the z2 axis as shown in Fig. 3.6. Equation (3.31) apparently differs from the Jq(0)) derived empirically but both Jq(0)) equally reduce to the following equation when ~'R ~ ~'L ~ ~'I: I 27~ J q ( w ) - Kq ARAL 1 + 092"/-2 nt- A R ( 1 -- AL) +BR
27"L 1 + 0)27 "2
2~'R +CR 2(~'R/4) -] 1 + 0)2T2R 1 + W2(7"R/4)2_]
(3.35)
This equation is expressed as a linear combination of Lorentzian contributions from the respective random motions, if the fourth term is assumed to be negligibly small. This indicates that Equation (3.35) is a model-free equation for three types of independently superposed random motions, which is in good accord with the results of the model-free treatments described in the following section, even though a specific structural model, shown in Fig. 3.6, was used for the derivation of Equation (3.35).
(e) Model-free treatments. In the polymer system, the single-correlation-time model is not valid and many kinds of models were proposed to explain the temperature or frequency dependencies of the spin relaxation parameters. Through these historical processes, it seems reasonable to introduce plural independent motions to describe the random motion of the C ~ H internuclear vector. Nevertheless, there may exist some limitation in using detailed structural models for the respective motions. In other words, it would be very important to examine the possibility of model-free treatments in which the number of independent motions and their correlation times are first considered without introducing detailed structural models for the respective motions. Here, the first- and second-order model-free treatments are described. (i) The first-order model-free treatment. The time fluctuation of the C - - H internuclear vector should be described in terms of the superposition of several independent random motions, and the autocorrelation function Gq(7)
64
F. HORII
is assumed to be a linear combination of the exponential decay contributions from the respective motions when their correlation times differ greatly from each other as follows: P
Oq('r) = Kq E Aj exp(-'r/~'cj).
(3.36)
j=l
Here, p is the number of random motions and ~'~/are the correlation times of the respective motions. Aj seem to be thc weights of the respective motions with EAj = 1, but are simply assumed to be adjustable parameters without giving any explicit physical meaning in the first-order model-free treatment [9, 13]. King and coworkers [14, 15] also derived Equation (3.36) in a more general fashion.
(ii) The second-order model-free treatment. The second-order model-free treatment will correspond to the treatment in which explicit physical meaning is given to the coefficients of the respective terms in Equation (3.36). Lipari and Szabo [16] proposed that the motion of the C ~ H vector can be expressed as the superposition of the overall isotropic motion and the anisotropic inner local motion, and that the total Gq(~') should be given by the product of the respective contributions as follows: Gq(,/') = G I ( T ) G A ( T ) , Gi(7" ) = Kq e x p ( - ~ ' / ~ - i ) , G A ( T ) = 8 2 + (1 - S 2) e x p ( - - ~ ' / ~ ' A )
9
(3.37)
S 2 is called the generalized order parameter that indicates the extent of the spatial limitation of the anisotropic motion. When T I <~ T A, Equation (3.37) reduces to the following equation corresponding to the case of p = 2 for Equation (3.36) in a mathematical form
Here,
G q ( 7 ) = Kq[S 2 exp(-'r/'ri) + (1 - S2) exp(--'r/'rA)].
(3.38)
As is clearly seen from this equation, spin relaxation parameters are expressed in terms of two kinds of correlation times and the order parameter S2 for the inner motion. However, two motions are not enough to describe fully the segmental motion of polymers as demonstrated later. To further develop this model-free treatment, we generalize Equation
NMR RELAXATIONS AND DYNAMICS
65
(3.38) by introducing p-1 independently superposed inner motions, whose correlation functions are given by the following equation [17]: GAg(r) = S2 + (1 - S2) exp(--y/rAi)
i=1,2,...,p-1.
(3.39)
Here, r Ai and S/2 are the correlation time and order parameter for the ith inner motion. When p = 3 and TAI<~TA2<~TI, the total correlation function reduces to
Gq(r)= Kq[S2S 2 21 exp(-r/ri)
+ (1 -
+ (1 - S 2) exp(--7"/'rA1)] .
$2)$21 exp(--r/rA2) (3.40)
This equation corresponds to Equation (3.35) for the 3r model as well as Equation (3.36) for p = 3 for the first-order model-free treatment. In contrast, Clore et al. [18, 19] have empirically derived the total correlation function similar to Equation (3.40) but Equation (3.40) is expressed more explicitly in terms of the respective order parameters. 3.2.2
Examples of analyses
Figures 3.7 and 3.8 shows the frequency dependencies of 13C T1 and N O E measured for the CH2 (rrr) carbon of poly(methyl methacrylate) (PMMA) in a deuterated chloroform solution at 55~ [10]. Different curves indicate the simulated results obtained by using the box-type distribution, log-g 2 distribution, 2r and 37 models described above. As is clearly seen in Fig. 3.7, all four models can satisfactorily explain the frequency dependency of T1 without almost any difference. In contrast, the best fit is obtained as shown in Fig. 3.8 only for the 3r model using the same parameters for the simulation as used for T1 in Fig. 3.7. In this case the best fit parameters were first determined for the frequency dependency of T1 by the least-squares method. When the least-squares method was first applied to the frequency dependency of NOE, almost similar best fits were also obtained for the four models. However, only the 3r model is able to explain the frequency dependency of TI using the same parameters as for NOE. It is concluded, therefore, that the 3r model is the most plausible one to explain the frequency dependencies of T1 and N O E for the PMMA solution. This suggests that at least three different types of independent motions are necessary to describe the segmental motion of a polymer chain in solution. In fact, the frequency dependencies of T1 and N O E were also well interpreted in terms of the
66
F. HORII ,
0.4-]
i
I
,
,
,
i
..... box " ' l~ . . . .
......
21;
,,
~" _,.#-,,:,,'
31:
.//4z
0.2
0
0
50 I00 frequency/MHz
Fig. 3.7. Frequency dependency of NT1 of the CH2 (rrr) carbon for PMMA/CDC13 solution at 50~ The curves are the results obtained by the least-squares method using different models for the molecular motion. (Reproduced from Ref. 10 with permission.)
first- and second-order model-free treatments considering three superposed motions (p = 3). Figures 3.9 and 3.10 show the temperature dependencies of T~ and N O E of the CH2 (rrr) of the same P M M A solution and the results (solid and broken curves) simulated by the second-order model-free treatment with p = 3 [17]. Here, the Arrhenius' equation was assumed for the respective correlation times; '7"I = ' r i 0 exp(AEI/RT) and T A / ~ - T A i 0 exp(AEA~/RT). In this case the simulated results with p = 3 are also in good accord with the experimental results, indicating the validity of the model-free treatment. Similar analyses of the temperature dependencies of the ~3C Tx were successfully performed for the rubbery components of the solid polyesters with different methylene sequences [20, 21]. These results are also well analyzed by the second-order model-free treatment with p = 3. There are a large number of the publications of the temperature dependencies of T~ and N O E analyzed by different models of molecular motions for polymers in the dis-
3p{,
67
NMR RELAXATIONS AND DYNAMICS
box \
ILl
o 2 -Z
10g-x'~
\ G.
t~tQ
\..
"~ ....... O....
"-.
3 l;
21; I--
0
i
i ,
I
5O I00 frequency/MHz
Fig. 3.8. Frequency dependency N O E of the CH2 (rrr) carbon for P M M A / C D C l s solution at 55~ The curves are the results calculated using the same parameters for the respective models as for the T1 data shown in Fig. 3.7. (Reproduced from Ref. 10 with permission.)
solved and rubbery states. More recently, the conformation jump model referred to as the DLS model is satisfactorily employed to analyze the temperature and frequency dependencies of T1 and NOE in different polymer systems [22]. In this model three independent motions are also assumed as shown in Equation (3.25).
3.3
Lineshape analyses of different resonance lines
As is clearly seen in Equation (3.1), the resonance centre of a given nucleus will shift from the frequency determined by the Zeeman term depending on
F.
68 '
HORII
I
II
I
CH2(rrr)
100 9 MHz
10 0
I
25 9 MHz
(/) 0
"I""
l-z
,
..Q
"Q
9
0
b 10-1 -
J
o.o
0
"'0
s
s
"o.. 2.5
, , ,
I
3
,
,,
I
3.5
o
s
o.--". ,,
I
4
,.
.
.
.
4.5
103T -1 / K-1 Fig. 3.9. Temperature dependencies of NT1 of the CH2 (rrr) carbon for PMMA/CDCI3 solution. The curves are the results simulated by the second-order model-free treatment.
the quadrupolar interaction, the dipolar interaction or the chemical shift term. Since these additional interaction terms also depend on the orientation of the nucleus against the static magnetic field Bo, the resonance frequency reflecting each single interaction term can be widely distributed in powdered materials where all orientations are possible. The overall resonance line, usually called a powder pattern, has a characteristic lineshape reflecting the corresponding interaction term. Although this is the case in the rigid state, some molecular motion with an appropriate order of frequency may contribute to the motional average of the lineshape. It is possible, therefore, to obtain detailed information about the molecular motion by analyzing the lineshape in terms of an appropriate model for the molecular motion. Here, 2H NMR and ~3C chemical shift anisotropy analyses are briefly described as representative examples.
NMR RELAXATIONS ,
'
I
'
I
"
'
I
190 0
MHz
92 5 M H z
--0-
0
69
AND DYNAMICS
0 O
UJ
0
~
O2 z
Q
%
0
1[
2.5
,
!
3
i
.... !
C)-,
0
,
3.5 103T -1 / K -1
%
!
4
i .....
4.5
Fig. 3.10. Temperature dependencies of N O E of the CH2 (rrr) carbon for P M M A / C D C 1 3 solution. The curves are the results simulated by the second-order model-free treatment.
3.3.1
2H
NMR analysis
In the case of 2 H NMR, the real resonance frequency mr will shift from the resonance centre ~Oo due to the quadrupolar interaction HQ as shown in the following equation [23]"
OJ r - -
OJ 0
-~
OJQ
WQ = --+ Ao(3 COS2 f f - 1 - v sin 2 ~"cos 2~).
(3.41)
Here, Ao is the constant inherent to the nucleus, ~"and ~ are the polar angles that define the direction of B0 in the principal axis frame for the field gradient tensor, and 77 is the anisotropic parameter. For example, Fig. 3.11 shows the principal axis frame XpypZp (Cp) for the phenylene deuterium in the ortho position. As seen in this figure, the Zp axis is parallel to the direction of the C--2H bond, the YP axis is perpendicular to the phenylene plane and the Xp
70
F. HORII
2
V,X
z
~'YR
R(r162 X
~
.....
Y CL
CR
R( ~:, ~',0
R(0, 0,-6 y.
YP
Xp
H
/
R(o, - ~/3, o)
/ C~
CM
Fig. 3.11. Coordinate transformations among different flames: Cp, principal axis flame; CM, molecular flame; CR, reference flame; and CL, laboratory flame.
axis is in the plane. In this section, the simulation of the 2H NMR lineshape will be described for the phenylene ring which undergoes the 180 ~ flip motion. Now, let the flip angle assume a more general value of 6. To describe Equation (3.41) in terms of 6, some coordinate transformations should be employed among the coordinates as defined in Fig. 3.11. Here, the reference frame XRYRZ R (CR) is defined as follows: the ZR axis parallel to the phenylene bonding axis, the YR axis perpendicular to the phenylene plane and XR in the phenylene plane. Moreover, the molecular frame XVLVMZM (CM) corresponds to the reference frame after it has been subjected to a rotation around the ZR axis by 6. R(c~, fl, y), which is shown above each arrow in Fig. 3.11, is the 3 • 3 matrix for the coordinate transformation between the corresponding indicated frames, where a,/3 and y are the Euler angles for each coordinate transformation.
NMR RELAXATIONS AND DYNAMICS
71
Applying these transformations successively, the following relation is finally obtained:
R(~', ~:, 0) = q~(4~, 0, R)R(0, 0, - 6 ) R ( 0 , -~r/3, 0).
(3.42)
From this relation, therefore, cos ~" and cos ~: are given by
cos ~'= (-X/3 sin 0cos 4~cos 6 - X/3 sin 0sin ~hsin ~ + cos 0)/2,
(3.43)
cos ~: = (sin 0 cos ~hcos 6 + sin 0 sin 4~sin 6 - ~
cos 0)/2(1 - cos 2 st)1/2 .
(3.44)
Finally, the resonance frequency 03 r is expressed as a function of 6 for the phenylene ring whose orientation is defined by the polar angles ~b and 0. When the phenylene ring undergoes the flip motion with a frequency of K between the sites described by 6 = 0 and 6 = 6, the resonance line I(co) will be given by using the two-site exchange theory [1] as
1
K((01- (02)2 I(w) - 2 (w - (01)2((0- (02)2 Jr- KZ[2w- ((01 "+- (-02)]2.
(3.45)
Here, (01 and (02 are the resonance frequencies for the sites corresponding to 6 - - 0 and 3 = 6, respectively. In real samples, the polar angles 4~ and 0 are widely distributed depending on the states of samples. For powdered samples, the resonance lines Ipw(~O) are expressed as
2"/T
"/T
d fo I s nOdO/4
In addition, corrections of the pulse sequence and the pulse width used should be made in actual simulations since these simulated spectra are frequently compared with the experimental spectra measured under the given conditions as shown later.
72
F. H O R I I
K,P--
0.0
0.01
~-I
'
I
-2 -1
'
I
'
0
!
1
'
I
'
'
_.c
2
I
0.
0.05
'
I
'
-2 -1
I
'
0
I
1
'
\
I
f
'
'
2
I
'
I
-2 -1
'
I
'
0
I
1
'
\
I
'
'
2
!
'
I
-2 -1
'
I
0
'
I
~
I
1
2
1
2
'
co Ao]
1.0
0.
' 1 ' 1 ' 1 ' 1 ' 1 '
-2 -1
0
/tv\
' l ' l ' l ' l ' i '
1
2
-2 -1
0
20
5.
' l ' i ' l ' l ' l '
1
2
-2 -1
0
' l ~ l ' l ' l ' l
1
2
-2 -1
0
'
Fig. 3.12. 2H N M R spectra simulated for the p h e n y l e n e ring that u n d e r g o e s the 180 ~ flip m o t i o n with the r e d u c e d f r e q u e n c y of K' ( = KAo a).
Figure 3.12 shows the 2H NMR spectra simulated for the phenylene flip motion with different K values. In this case, 77 is assumed to be 0 because the field gradient is fully axially symmetric around the aromatic CmZH bond. The reduced flip frequency K' is described by K/Ao, where Ao is 65 kHz for the 2H nucleus. For K' < 0.001, the spectrum corresponds to the Pake pattern in the rigid state, indicating the so-called slow limit. At K' = 0.01, both edges of the spectrum seem to change slightly in shape and some marked change appears around the resonance centre at K' = 0.05. When K' increases further, the sharp lines at both sides clearly change in shape and also in intensity in concomitance with the great changes around the resonance centre. Finally, the lineshape again stays constant for K' > 20, implying the first limit. Since similar changes in lineshape are simulated for other different models of molecular motions, the modes and frequencies of the molecular motions can be determined for real systems by comparing simulated spectra with experimental spectra. Figure 3.13 shows 2H NMR spectra of different components observed at room temperature for phenylene deuterated poly(ethylene terephthalate) films which were prepared by pressing the polymer melted at 280~ into a mould left at room temperature [24]. The spectra of the crystalline, the mobile glassy and the noncrystalline components, were selectively recorded by the subtraction method using differences in 2H T1, since their T1 values differed by more than one order from each other. It should be noted here
73
NMR RELAXATIONS AND DYNAMICS
Observed
Simulated
Distribution of the flip rate
crystalline
mobil
noncrystalline/
total noncrys~
tot a
360 ~o
10-2
6
-~o :360 kHz
360 ~0
I0 o
6 -l)0-360
10 2
10 4
10 6 kHz
kHz
Fig. 3.13. Observed and simulated 2H NMR spectra of different components for the phenylene
deuterated poly(ethylene terephthalate) films, measures at 25~ (Ref. 24.)
that the noncrystalline component is composed of the mobile glassy and less mobile glassy components with different T1 values. In Fig. 3.13, simulated spectra are also shown for the respective components, which were obtained by assuming that the frequencies of the phenylene flip motion are widely distributed and their distributions described by log-Gaussians for the respective components. The simulated spectra of the respective components are in good accord with the corresponding observed spectra, confirming that there exist wide distributions in flip frequencies for the respective components. The fractions of the three components were also determined by the least-squares fitting for the total spectrum using the same distributions for the respective components. Similar analyses were also possible for the sample at different temperatures and the phenylene motions of the noncrystalline components were found to depend greatly on the temperature and conditions of the sample preparation.
74
F. HORII
3.3.2
13C chemical shift anisotropy analysis
For low naturally abundant nuclei, such as the 13C nucleus, resonance lines reflecting the chemical shift anisotropy (CSA) can be observed for solid organic materials by eliminating the effects of 1H local field with use of the so-called dipolar decoupling method. However, the CSA resonance lines are usually broad and, therefore, the respective CSA lines are superposed with each other for polymers composed of different 13C species. To separately measure these CSA lines, the following different methods were proposed: 1. 2. 3. 4. 5. 6. 7. 8. 9.
X3C labelling method; off-magic-angle spinning; spinning side-band pattern multiplet analysis [25]; magic-angle hopping [26]; two-dimensional stop and go method [27]; two-dimensional switching-angle sample spinning (SASS) [28, 29]; two-dimensional switched-speed spinning [30]; two-dimensional ultraslow MAS or magic-angle turning (MAT) [31]; selective excitation SASS [32, 33].
There are some merits and demerits in each method. However, two-dimensional SASS and MAT seem to be the most convenient method to measure CSA spectra for real polymer samples. The selective excitation SASS is also very powerful in observing the CSA of a given 13C nucleus. 3.3.2.1 Basic equations for the CSA analysis by a model of molecular motion In this section, the flip motion of the phenylene around the bonding axis is used as an example for the naturally abundant 13C CSA analysis. When the local field of the 1H nuclei is averaged out to zero by the dipolar decoupling, the Hamiltonian H of the system is given by H = Ho + Hs.
(3.47)
Hs should be described in terms of the irreducible tensors A L, T k and then the secular part of Hs, which is commutable with the Zeeman term, is expressed as [34, 35]
H, = AooToo L L L L + A2oT2o 9 Here,
(3.48)
NMR RELAXATIONS AND DYNAMICS
A~o = -Tr(oL)/V3, TLo = -yBolz/V3,
75
A~o = [3~r~3 - Tr(~r~)l/X/6, Tzko = (2/X/-6)TBolz,
(3.49)
and ~r~ is the ij component of the chemical shift tensor 0 r L in the laboratory frame. Therefore, the resonance line for site j is given by (.Oj-- (.01 + (.0;,
(3.50)
where O) I ---
yBoTr(~r~)/3 = cooTr(~rq)/3,
w; = (2/X/-6)TBoA~o = (2/X/-6)woAzko.
(3.51) (3.52)
In Equation (3.51), Tr(~r~) is expressed by Tr(~rq) in the principal axis flame, because the trace does not change by the unitary transformation. On the other hand, A2Lo should be described by A2o in the principal axis frame through appropriate coordinate transformations depending on the molecular motion allowable to the system. In the case of the phenyl-ring flip motion, some coordinate transformations should be employed as shown in Fig. 3.14 [35]. First the molecular flame CM is defined as follows: the ZM axis is parallel to the single bond for the quaternary carbon of the phenyl group, the XM axis perpendicular to the phenyl plane and the YM axis in the phenyl plane. If the reference flame CR is defined as the molecular flame in the rigid state, the molecular flame located at site j can be described by the Euler angles (6j, ej, ~Tj), with respect to the reference flame. Moreover, the molecular flame at site j is correlated with the principal axis flame Cp for the ortho carbon through another set of Euler angles (ce,/3, 3/). Here, the Xp axis is parallel to the C ~ H bond in the principal axis flame, while the Zp axis is perpendicular to the phenyl plane and the yp axis is in the plane. On the other hand, the laboratory flame CL is described by the Euler angles (~b, 0, ~) within the reference flame. As a result, the total coordinate transformation matrix Rt from the principal axis flame to the laboratory flame is expressed as R, = R ( 6 , 0,
(3.53)
Here, the matrices appearing on the righthand side of this equation are the transformation matrices between the two respective coordinates as shown in Fig. 3.14. Using this Rt, ALo in Equation (3.52) is described by the corresponding A2o in the principal axis frame as follows:
76
F. HORII
Z R YR
13o
Z N, R(~, o, v)
X Y
CR
CL
R(-nj, -ej, -sj)
.," { 9
1
| t
3 2
XM
/
/
I
I
R(c~, 13,V) "
~
I
CP
CM
Fig. 3.14. Schematic representation of the coordinate transformation from the principal
axis
frame (Cp) through the molecular frame (CM) and the reference frame (CR) to the laboratory frame (CL).
A2Lo = R t A 2 0 R t I .
(3.54)
Since the unitary transformation of the spherical tensor can be described by using Wigner rotation matrices, Equation (3.54) is given by 2
ALo .--
E . p,q,r= --2
or
A2rDr2q(a,/3, . . y)D2qp(
6j,
ej,
2 r/j)Dpo(6, 0, 4'),
(3.55t
NMR RELAXATIONS
AND DYNAMICS
77
2
Ako =
E
A2rd2rq(j~)d2p(
-
-
6.j)
p,q,r= --2 x exp(-iar)exp[-i(
2 dpo(O)
y - 7qj)qlexp[-i(&
- 8j)p].
(3.56)
Here, d2.(O) are shown in Table B.1 (Ref. [34]). A2r a r e given according to their definition by A2o = (V6/2)[o-33 A2+_l
-
Tr(~rij)/3]-
(X/6/2)o'~3.
-- 0
A2__2 = (o"11 -
(3.57)
O'22)/2 9
As a result, the general equation for o~ given by Equation (3.52) can be derived but it is not explicitly described here because of the complexity. In the case of the two-site flip motion such as the 180 ~ flip motion for the phenyl group around the C ~ C single bond, it is assumed that 7/1 = 61 = el = 0 for site j = 1, and 712 = E 2 = 0 and 62 = 6 for site j = 2. Here, 6 corresponds to the flip angle. Since a = 27r/3, /3 = - ~ r / 2 , and y = 0, the resonance frequencies o)1 and o)2, for sites j = 1 and j = 2, are expressed as (-01
= o)I + O)oO''3(3 cos 2&sin 2 0 - 3 COS2 0-~- 1)/4 --(-00(O"11 -- 0"22)[ Sin2 0COS 24~ + 2X/-3sin 20sin
(3.58)
+ 3 cos 2 0 - 1]/8, (-0 2 =
O)I nt-
o)oo.~313
cos
2(4~ - 6)
s i n 2 0 --
3
C O S 2 0 "nt-
1]/4
-COo(~r11- o'22)[sin 2 0cos 2 ( & - 6) + 2V~ sin 2 0 s i n ( & - 6) + 3 cos 2 0 - 1]/8.
(3.59)
As described in the case of 2 H N M R , the resonance line I(w) is also given by Equation (3.45) for the two-site exchange model when the exchange occurs between sites 1 and 2 and these sites give the resonance frequencies COl and co2. More generally, I(co) is given by the following equation when the exchange is induced among N sites with the flip frequency of K: I(w) : (1/N)Re[L/(1 - KL)],
(3.60)
L-
(3.61)
~] [i(w - wj) J
+ NK] -1 .
78
F. HORII
Here, oJj is the resonance frequency for site ]. For powdered samples, the powder average of the Euler angles ~b and 0 should be made also for I(w) in this case by using Equation (3.46). 3.3.2.2 Experimental results Figure 3.15(a) shows a C P / M A S ~3C N M R spectrum of glassy bisphenol-A polycarbonate (BPAPC) measured at room temperature [33]. The respective resonance lines, which are assigned to the 13C nuclei constituting the struc-
(d)
(c)
(b) 7,3,6
1
4 5
1
(a)
200
100
0
ppm from Me4Si Fig. 3.15. 50 MHz ~3C NMR spectra of BPAPC films measured at 25~ (a) CP/MAS; (b) selective excitation MAS spectrum for the C5 carbon; (c) selective excitation SASS spectrum for the C5 carbon; and (d) descaled CSA spectrum for the C5 carbon (Ref. [33]).
NMR RELAXATIONS AND DYNAMICS
79
tural unit of this polymer, are well separated from each other, in particular for the aromatic CH carbons. Then, we can selectively record the line of the C5 carbon by using the D A N T E pulse sequence, as shown in Fig. 3.15(b). Moreover, when the SASS method are combined with the selective excitation method by the D A N T E pulse sequence, the CSA spectrum of the C5 carbon can be observed satisfactorily, as shown in Fig. 3.15(c). Since the spinning axis 0~ was set to 45 ~ the CSA spectrum is scaled by a factor of f~ which is described as (3 cos 2 0 ~ - 1)/2. Figure 3.15(d) shows the descaled CSA spectrum which corresponds to the CSA spectrum in the static state. Figure 3.16 shows CSA spectra of the CH carbon of B P A P C measured at
1
4 5
C5 BPAPC
~c> 105Hz /
"~-
120~
120~ I
.---
X
-.--- . - -
0~
60 ~
!
120 ~
180 ~
120 ~
180 ~
70 ~
/'
"%.
70"0 0~
25~
_j --
I
I
I
2O0
60 ~
I
I I....
i
i
|i_
100
I
....
'
I
I
-- i
0
ppm from Me4Si
0~
,W
!. . . . . . . . .
J
60 ~
120 ~
180 ~
d
Fig. 3.16. CSA spectra for the C5 carbon of BPAPC at different temperatures: - - , observed;
.... , simulated. The histograms indicate the distributions of the flip angle S for the phenylene group (Ref. [33]).
80
F. HORII
different temperatures by the selective excitation SASS [33]. These spectra were not reproduced by the two-site flip motion of the phenylene ring with a single flip frequency and a single flip angle. This fact suggests that there may be significant distributions in flip frequency and/or flip angle. In Fig. 3.16, the simulated spectra that best fit the observed CSA spectra are also shown as broken lines. According to these analytical results, the flip frequencies are more than 105 Hz, indicating the first limit, and the flip angles are widely distributed around 0 ~ and 180 ~ The distribution curves are here assumed to be Gaussians. The significant discordance between the simulated and observed spectra at higher temperatures may be due to the onset of hindered fluctuations of the flip axis. More detailed analyses of the molecular motion of the phenylene ring are in progress by using CSA spectra measured by the MAT method over a wide range of temperatures.
References Abragam A. The Principles of Nuclear Magnetism. Oxford: Oxford University Press, 1978. 2. Solomon I. Phys Rev 1955;99:559. 3. Kitamaru R. Nuclear Magnetic Resonance--Principle and Theory. -Amsterdam: Elsevier, 1990. Schaefer J. Macromolecules 1973;6:882. Hall CK, Helfand E. J Chem Phys 1982;77:3275. Weber T.A, Helfand E. J Phys Chem 1983;87:2881. Dejean de la 13atie R, Laupretre F, Monnerie L. Macromolecules 1988;21:2045. Murayama K, Herii F, Kitamaru R. Bull Inst Chem Res Kyoto Univ 1983;61:229. Horii F, Chen Y, Nakagawa M, Gabrys B, Kitamaru R. Bull Inst Chem Res Kyoto Univ 1988;66:317. 10. Horii F, Nakagawa M, Kitamaru R, Chujo R, Hatada K, Tanaka Y. Polym J 1992;24:1155. 11. Woessner DE. J Chem Phys 1962;36:1. 12. Howarth OW. J Chem Soc Faraday Trans 2 1979;75:863. 13. Horii F, Kaji H, Ishida M, Ishida H. Polym Prepr Japan 1994;43:1472. 14. King R, Jardetzky O. Chem Phys Lett 1978;55:15. 15. Ribeiro AA, King R, Restivo C, Jardetsky O. J Am Chem Soc 1980;102:4040. 16. Lipari G, Szabo A. J Am Chem Soc 1982;104:4546,4559. 17. Horii F, Kaji H, Ishida H, Ishida M. Preprints of the 34th NMR Meeting of Japan, Tsukuba, 1995;167. 18. Clore GM, Szabo A, Bax A, Kay LE, Driscoll PC, Gronenborn AM. J Am Chem Soc 1990;112:4989. 19. Clore GM, Driscoll PC, Wingfield PT, Gronenborn AM. Biochemistry 1990;29:7387. 20. Horii F, Hirai .A. Murayama, K, Kitamaru R, Suzuki T. Macromolecules 1983;16:273. 21. Murata T, Horii F, Odani H. Preprints of the 29th NMR Meeting of Japan, Kyoto, 1990;321. 22. Dais P, Spyros A. Prog Nucl Magn Reson Spectr 1995;27:555. .
NMR RELAXATIONS AND DYNAMICS
81
23. For example, Cohen MH, Reif F. In: Seitz F, Turnbull D (eds), Solid State Physics, vol. 5. New York: Academic Press 1957;321. 24. Kawaguchi T, Mamada A, Tamura S, Horii F. Polymer (in press). 25. Herzfeld J, Berger AE. J Chem Phys 1980;73:6021. 26. Bax A, Szeverenyi NM, Maciel GE. J Magn Reson 1983;52:147. 27. Zeigler RC, Wind RA, Maciel GE. J Magn Reson 1988;79:299. 28. Bax A, Szeverenyi NM, Maciel GE. J Magn Reson 1983;55:494. 29. Terao T, Fujii T, Onodera T, Saika A. Chem Phys Lett 1984;107:145. 30. Kolbert AC, de Groot HJ, Griffin RG. J Magn Reson 1989;85:60. 31. Gan Z. J Am Chem Soc 1992;114:8307. 32. Iwamiya JH, Davis MF, Maciel GE. J Magn Reson 1990;88:199. 33. Horii F, Beppu T, Takaesu N, Ishida M. Magn Reson Chem 1994,32:$30. 34. Mehring M. Principles of High Resolution NMR in Solids. New York/Berlin: SpringerVerlag, 1983. 35. Horii F, Uyeda T. Beppu T. Murata T, Odani H. Bull Inst Chem Res Kyoto Univ 1992;70:198.
This Page Intentionally Left Blank
Chapter 4
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
Spin Diffusion in Solids Matthias Ernst and Beat H. Meier NSR-Center for Molecular Structure, Design and Synthesis, Laboratory of Physical Chemistry, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands
4.1
Introduction
The term "spin diffusion" has been coined by Bloembergen [1] to characterize the polarization-exchange process in a strongly dipolar-coupled many-spin system. As pointed out by Bloembergen, this process leads to a spatial spread of polarization originating on a given spin that mimics, under certain conditions, a diffusion process. In a true diffusion process, the entropy increases monotonically. In the exact quantum description of the "spin-diffusion" process, however, the entropy is conserved and the process is, in principle, fully reversible. Today, "spin diffusion" is often used in a quite general way to describe multispin polarization-transfer processes, whether or not the process can actually be described by a diffusion equation. In this overview, we will interpret spin diffusion in this broad sense. If applied in the context of twodimensional homonuclear experiments, it becomes synonymous with "total through-space correlation spectroscopy" (TOSSY) [2], the dipolar equivalent of the liquid-state TOCSY [3] experiment. Sometimes, two forms of spin diffusion are distinguished: spatial spin diffusion and spectral spin diffusion. Spatial spin diffusion describes the flow of magnetization in space between equivalent nuclei due to a nonequilibrium distribution of the initial longitudinal magnetization [4]. Spectral spin diffusion is the flow of polarization between spins of different resonance frequency [5]. In spectral spin diffusion the elementary step, a two-spin flip-flop process caused by an operator I(I) - § Ii-I7, is not energy conserving. The energy difference has to be compensated by an external reservoir. Real systems often show aspects of both types of spin diffusion. If one is, however, interested in extracting structural information from spin-diffusion experiments, it is advantageous to suppress the frequency differences in the spindiffusion period of the experiment (see Fig. 4.1) and to generate a pure dipolar Hamiltonian which is directly related to internuclear distances. In proton spin systems, this situation is often realized by nature since the dipolar interaction is dominating the Hamiltonian. In low-y spin systems coupled to
84
MATTHIAS ERNST AND BEAT H. MEIER
a)
Preparation initial state
"Spin Diffusion"
Evolution
b)
0
Characterization final state
Mixing
Detection
~'D
~'2
Chemical
Structure
Chemical
Information
Information
Information
r
v
tl
t2
Fig. 4.1. (a) General scheme for spin-diffusion spectroscopy; and (b) a possible realization in the context of two-dimensional spectroscopy.
an abundant high-y spin species it can be achieved by various driven spindiffusion methods which we will discuss in Section 4.2.1. In these experiments it is the goal to compensate the energy difference of two spins in order to allow for spatial spin diffusion. This compensation of the energy mismatch makes the elementary flip-flop process again energy conserving and results in a purely spatial spin-diffusion process. Neglecting relaxation, spin diffusion is a stationary process for spatially homogeneous initial polarization since the homonuclear dipolar coupling commutes with the sum polarization F~ = Therefore, a spin-diffusion experiment begins with a preparation period where the spins are distinguished by some property, e.g., chemical shift, relaxation rates T~, T2 or relaxation under r.f. irradiation. Such a differentiation property is used to generate a polarization gradient over the sample. Following the preparation period, the spin-diffusion process takes place. As mentioned above, the time evolution of the spin system during the mixing period should be governed only by the dipolar interaction. Finally, the generated state must be read out. In the detection period, we must again be able to distinguish the spins (Fig. 4.1(a)). Such an experimental scheme can be realized, for example, in the context of two-dimensional exchange spectroscopy [6, 7] as shown in Fig. 4.1(b). Spin diffusion has found two important classes of applications in solidstate NMR: (i) measuring distances between spins, e.g., between domains in
EiSi~.
SPIN DIFFUSION IN SOLIDS
85
a heterogeneous polymer; and (ii) as a transport mechanism to transfer magnetization in a two-dimensional correlation experiment that establishes the relative orientation of tensorial interactions. An example for this second class of experiments is a chemical-shielding anisotropy (CSA) correlation experiment in which the relative orientation of the CSA tensors on spatially near segments of a molecule is measured [8]. From these data, the relative orientation of the molecular segments themselves can be established.
4.2 The mechanism of spin diffusion 4.2.1
Static samples
The dipolar interaction Hamiltonian is described by a sum of two-spin interactions between all possible spin pairs in the sample
~ s s = s ~s.s.
(4.1)
i<]
In high magnetic fields I the homonuclear dipolar pair interaction between two like spins i and j is given by ~s.s = d~.ff. (3SizSjz_ Si "Sj),
(4.2)
while the one between unlike spins 2 (e.g., in heteronuclear dipolar coupling) is given by the truncated form ~ s . s = 2d~.ff. sias, a.
(4.3)
The effective dipolar-coupling frequency d~.el depends on the dipolar-coupling constant d 0 and, therefore, inversely on the third power of the internuclear distance r~j do =/.to. h,/~,// 4rr r 3. q '
(4.4)
1High magnetic fields refers to the situation where the Zeeman interaction is much larger than the dipolar interaction, an assumption fulfilled to excellent approximation in NMR experiments. 2By "unlike" we refer to the situation where the difference in resonance frequency between the two spin greatly exceeds the dipolar-coupling frequency between them. Spins of different isotopes are, at the usual NMR resonance frequencies, always unlike, while spins of the same isotope can be "like" or "unlike".
86
MATTHIAS ERNST AND BEAT H. MEIER
the angle Oij between the internuclear vector rij and the external magnetic field Bo, and a scaling factor s that depends on the type of experiment performed: d~-ff = - s "dij" (3 cos 2 0 i / - 1,.] 2
(4.5)
For laboratory-frame experiments s is equal to 1, for experiments in the presence of strong on-resonance r.f. irradiation (so-called rotating-frame experiments3), s = - 1 / 2 . The dipolar interaction leads directly to geometrical information about the spin system. The Hamiltonian contains, besides the geometrical quantities Oij and rij, only constants. If the dependences on angle and distances can be disentangled in the analysis of the experimental data, direct structure determination is possible. This disentanglement is sometimes, but not always, possible. We will return to this point later. In the N M R spectrum, the dipolar interaction represents itself as a line splitting. For isolated pairs of unlike spins, the i and j spin resonances are eft , for like spins a doublet with each split into a doublet with a splitting of 9d _,.~j a splitting of 3dff f is found. For powder samples a so called "Pake" powder pattern is found instead of the doublet. Resolved splittings are only observable for fairly well isolated spin systems consisting of two or a very few spins (e.g., the proton pair of the crystal water in gypsum [9]). For extended spin systems, a broad and rather featureless resonance line is observed. Alternatively, dipolar-coupling frequencies can be determined by observing the transfer of polarization between coupled spins. Clearly, an initial density operator that describes polarization localized on a given spin, say k, (~r(0) ~ Skz) does not commute with the Hamiltonian of Equations (4.1) and (4.2) but evolves according to
~r(t) = e - i ~ t .
0"(0)" e+i~;~t.
(4.6)
3As usual, polarization-transfer experiments where the relevant quantization axis is the static magnetic field will be called laboratory-frame experiments, while experiments where the applied r.f. field provides the relevant quantization will be referred to as rotating-frame experiments. This classification should not be confused with the frame of reference where the Hamiltonian is set up and the equation of the motion is solved in. Usually, laboratory frame polarizationtransfer experiments are described in a rotating (or doubly rotating in heteronuclear cases) frame of reference that is an interaction frame with respect to the dominant Zeeman interaction. Rotating-frame experiments are usually described in a frame of reference obtained by going into a further interaction representation, this time with respect to the r.f. field.
SPIN DIFFUSION IN SOLIDS
87
For a two-spin system, the resulting time evolution is conveniently described in terms of the sum and the difference of the spin polarizations for which the well-known equations (Si z -+- Syz)(t)= (Si z qt_ Sjz)(0 ) (Siz - S j z ) ( t ) - - ( S i z
- Sjz)(0 ) 9cos(d~yr. t)
(4.7)
result, where ( A ) = Tr{o-(t)A}. Note that the weak-coupling Hamiltonian between "unlike" spins (Equation (4.3)) does not promote polarization transfer as it commutes with all operators Siz. For a many-spin system, the solution of Equation (4.6) becomes very complicated and the individual coupling frequencies d~ff cannot always be extracted from experimental data. Nevertheless, the sum polarization Ei Siz remains time invariant and is called a constant of the motion. In principle, we must describe the time evolution of an initial nonequilibrium state o'(0) = Ei ci(O)Siz as a series of rotations of the density operator in the Hilbert space of the entire spin system. At times t > 0 not only populations but also manyspin terms of the form H~,S~,zHmSmII~S~+ appear in the density operator. Of course, this time evolution is fully deterministic and reversible. The reversibility was in fact demonstrated in the "polarization-echo" experiments [10] (Fig. 4.2) where two sequential time evolutions with a scaling factor of s = 1 and s = - 1 / 2 follow each other (see Equation (4.5)). If the second period has twice the length of the first period, the time evolution under the dipolar interaction is refocused and the density operator returns to the initial density operator. In a typical sample with 102~ proton spins, a full quantum description is impractical and simplifications are needed. To this end, the polarization-transfer process in a many-spin system is modeled by a kinetic master equation for the polarizations Pi-- (S/z):
d
Pl P2
dt
Wll W21
W12 . . . WIN . . . . .
Pl P2
(4.8)
. . . . . PN .
. . . WN1 . .
....
WNN
iN
~
The transition from Equation (4.6) to (4.8) is obviously an approximation: a system of N spin 1/2 particles is now described by N instead of 2 N variables. Whether or not such an approximation is justified must be decided on a case-by-case basis. This master-equation approach is often suggested by the
88
MATTHIAS ERNST AND BEAT H. MEIER
Polarization~)~ Pathway "
I
s s
l
Y
Xlt Acquisition
tc I tsltd
tp/" Polarization Echo
c)
1 0.8
0
#
0 O0
0
0
0.6
P
#
o # #
0.4
0 0 #
0.2
#
0 # #
0 # # X # # # # # ~ # # # 8 # # n #
Z2=0
00000000 'rl-160gs
0
100 200 300 400 500 600 "1:1+I:2
ps Fig. 4.2. Polarization-echo formation. During tc carbon polarization is generated by crosspolarization. The proton polarization decays to zero during the waiting time ts. The protons directly bonded to a 13C are selectively repolarized during td. Therefore at the point in time marked by (~D in the pulse scheme of (b), spatially localized proton polarization is prepared. This is schematically represented by the two-dimensional representation in (a). This polarization undergoes, during ~-1 a spatial spin-diffusion process (in the rotating frame) and becomes delocalized (at point (2)). The following spin-diffusion process during r2 takes place in the laboratory frame where the sign of the dipolar interaction is the inverse of the sign in the
SPIN DIFFUSION IN SOLIDS
89
experimental observation that the transfer of polarization between spins is not oscillatory but smoothly approaches a quasi-equilibrium state. However, it clearly cannot explain the appearance of polarization echoes. Following the original ideas by Bloembergen [1] and Abragam [4], the rate constants Wq are usually evaluated separately for each pair of dipolarcoupled spins. One calculates the transition probability Siz ~ Sjz within the spin pair under the action of the fluctuating fields of all spins except i and j. All coherences involving "extraneous" spins are neglected. Due to the conservation of the sum polarization in these types of experiments, the diagonal elements are given by Wjj = - ~ i e j Wq. Most approaches to describe spin diffusion are based on a perturbation treatment [11]. Abragam [4] applied "Fermi's Golden Rule" while Suter and Ernst [12] used the perturbation theory in the rotating frame to obtain an estimate for the rate constant. Henrichs and Linder [13] and Kubo and McDowell [14] used the memory-function theory to model the system. The model of fluctuating local fields can also be applied in a Liouville-space description [15]. Following the last approach, we divide the total Hamiltonian of the system SS
= 7~ij + ~ l ( t ) ,
(4.9)
into a part ~ijss which contains only the dipolar coupling between the two selected spins and a second part ~ l ( t ) which contains all other terms (dipolar couplings to other spins, Zeeman terms, J-couplings). This part will be described as a time-dependent "local field". The time-dependent Hamiltonian ~ l ( t ) broadens the levels of the two-spin system and, for a dipolar-coupling constant d~.ff which is still large compared to the broadening of the levels, leads to a damping of the oscillation found in Equation (4.7). For a dipolar frequency d~ff which is small compared to the broadening of the levels of the two-spin system, the difference magnetization can be approximated by an exponentially decaying function (Siz
-
Sjz)(/) ---
(Siz
-
S j z ) ( 0 ) 9e-eWi/.
(4.10)
rotating flame. Instead of a further diffusive spread of the proton polarization, a spatial refocusing takes place at point (~) clearly demonstrating the deterministic nature of the polarization-transfer process. Detection takes place on 13C after a further selective cross-polarization during tp. The experimental data in (c) show the decay of the proton polarization if spin diffusion only takes place in the rotating frame (~'2- 0) and the appearance of an echo for T2 ~ rl/2 in the combined rotating/laboratory-frame experiment (Figure adapted from Ref. [75])).
90
MATTHIAS ERNST AND BEAT H. MEIER
1/T 2 =
j,
i::,
,
,
,
,
I
0
(~1 - ~ 2 )
-~
',
: \\ I
,
,
,
,
~21 - ~2
Fig. 4.3. Zero-quantum spectrum fq(oJ)of the spins i and j with the rotating frame resonance frequencies l)i and Oj. The center of the line does not coincide with frequency zero, but with the difference frequency of the two spins, ~-~i- ~'~j"
For a spin system with many neighbors, the latter condition is approximately fulfilled [4]. In Equation (4.10), the rate constant for the decay of the difference magnetization can be estimated as 37" : 2
7r
Wij = 2 [di- ] fij(O) = 2 [s. dij. P2(cos Oij)]2fij(O),
(4.11)
with fq(co) being the normalized zero-quantum spectrum of the two-spin system. It is important to note at this point that the zero-quantum line is centered at the difference frequency ~ i - f ~ j while the relevant intensity in Equation (4.11) is always the intensity at frequency zero (Fig. 4.3). The integral of the zero-quantum line fq(o~) is normalized to one. This implies that with increasing line width the overall intensity of the line is reduced, For proton spins the Hamiltonian of the system can be approximated quite well by the dipolar interaction alone. Compared to the size of the homonuclear dipolar Hamiltonian, all other terms are small and can be neglected. The fluctuating part of the Hamiltonian of Equation (4.9) consists mainly of
SPIN D I F F U S I O N IN SOLIDS
91
the dipolar coupling to and the dipolar coupling among all the other spins which provide a strong broadening of the energy levels of the selected twospin system. Note that the separation into a spin system (two spins) and the environment (all other spins described as the fluctuating field) is rather artificial and crude. More refined approaches have been proposed. They are, however, computationally much more demanding [16, 17]. If no chemical-shift differences existed, an abundant low-7 spin system in the absence of further spins (or in the presence of efficient heteronuclear spin decoupling) would behave in the same way as an abundant high-7 spin system. In practice, however, the chemical-shift differences tend to be comparable to and often larger than the dipolar couplings and they must be included in the system Hamiltonian 3r = 7fijss + 3r s
(4 12)
~s. = lIiSiz + DqSjz,
(4.13)
with
where ~"~i denotes the chemical shift in the rotating flame of reference. The rate constant Wq is, according to Equation (4.11), proportional to the intensity of the zero-quantum line at frequency zero. This line is centered at frequency l ) i - flj and, for a homonuclear spin system, has a width that is of the order of d~ff. The transition probabilities vanish therefore if the chemicalshift differences dominate over the dipolar couplings (see also Fig. 4.3). In this situation, the flip-flop transitions between the two S-spins are not energy conserving and the transition must be driven from the "outside" if it should become efficient. There are several ways to enhance spin diffusion in such systems. One can broaden the zero-quantum line to such an extent that the intensity at frequency zero is non vanishing for all relevant chemical-shift differences. However, due to the normalization of the zero-quantum line, a broad line implies that the intensity fq(O) is low and that the spin diffusion proceeds slowly. A broadening of the zero-quantum line can often naturally be achieved by the coupling to a strongly-coupled proton spin system in so-called proton-driven spin diffusion; while the form of the heteronuclear dipolar-coupling Hamiltonian (Equation (4.3)) prevents polarization exchange with the protons, the S-spin zero-quantum line shape is sufficiently broadened to allow S-spin polarization transfer. The broadening of the zero-quantum line can also be caused by chemical exchange ("motionally-driven spin diffusion") [15, 18]. In a different approach one decouples the protons and removes the chemical-
92
MATTHIAS ERNST AND BEAT H. MEIER
shift difference by applying r.f. pulses to the S-spins. This method is called r.f.-driven spin diffusion [15, 19]. Proton-driven spin diffusion (see also Appendix A) is the "classical" spindiffusion experiment for low abundant spins. The line width of the one- and zero-quantum lines of the S-spins are mainly determined by the heteronuclear dipolar couplings while the homonuclear I-spin dipolar coupling makes the broadening of the levels homogeneous. Suter and Ernst [12] calculated an approximate value for the zero-quantum relaxation time 1 T zo
= 4 ~ ~alSaiS tt2j f
dr
Tr{1}i,j
Tr{ei~enrIjz e-i~enr}Iiz .
(414).
Assuming that we have a mono-exponential decay of the zero-quantum line, we can calculate the intensity of the normalized zero-quantum line for a chemical-shift difference of ~"~1 -- ~"~2as (Lorentzian lineshape) 1 f~2(0) = - "
l/T2z ~
77" (~'~1- ~'~2)2 -
( 1 / T Z Q ) 2"
(4.15)
Kubo and McDowell [14] obtained a similar expression. The zero-quantum line shape can be calculated as the convolution of the two one-quantum lines
[8, 201
fij(O)
= [fi(o))*fy(--o))](0).
(4.16)
This requires that the relaxation mechanisms leading to the broadening of the two one-quantum lines and f(roj) are uncorrelated. This condition is approximately fulfilled for natural-abundance or for singly-labeled 13Csamples where the broadening of the two one-quantum lines is dominated by different I spins. With increasing zero-quantum line width the spin-diffusion rate constant increases and reaches a maximum when the width of the zero-quantum line is equal to the chemical-shift difference of the two S-spins. If the line width is increased even further, the spin-diffusion rate constant decreases slowly but continuously due to the decreased intensity of the zero-quantum line at frequency zero (see Fig. 4.3). The slow-MAS spin-diffusion experiment (S-MAS) [21] is a modified version of the proton-driven spin-diffusion experiment described above. It is applicable to situations where the chemical-shift differences in the spectrum are mainly due to the orientational dependence of the CSA tensor. In these
f(o)i)
SPIN D I F F U S I O N IN S O L I D S
93
cases, it can greatly reduce the variations of the zero-quantum line width over the different crystallites in a powder sample. To shorten the time needed to establish a quasi-equilibrium state, the sample is rotated slowly (vMAS = 10--100 Hz) about the magic angle and the mixing time is synchronized with the sample rotation. The rotation speed has to be slower than the fluctuations of the local fields but faster than the spin-diffusion rate constant. Under this condition one can rewrite Equation (4.11) and define an average rate constant 1 Wij = 2 7r. (d~ff)2 9fij(0).
(4.17)
Assuming that the chemical-shift tensors and the dipolar-coupling tensors are uncorrelated, one can average over the two factors (d~-ff)2 and fij(O) separately. One assumes that the evolution and detection periods are short compared to the rotation period of the sample and that the spins evolve at a fixed frequency during these two times. The static and the averaged zeroquantum line intensity for S-MAS for the phenyl carbon of polystyrene are shown in Fig. 4.4. It can be clearly seen, that the averaged zero-quantum intensity is almost independent of the orientation of the crystallite and quasiequilibrium is achieved faster and more uniformly over all crystallites. The most efficient way to speed up spin diffusion is the so-called r.f.driven spin-diffusion experiment [15, 19] where the chemical-shift differences are removed by r.f. irradiation. For small chemical-shift differences, r.f.driven spin diffusion can be implemented by applying a continuous-wave r.f. field to the S-spins which can theoretically be described by a transformation into a tilted rotating frame (see Appendix B). To zeroth-order average Hamiltonian theory the chemical-shift differences are removed ( l ) i - l)j = 0 for all spins i and j) and the dipolar-coupling frequencies are scaled by a factor s = - 1 / 2 . The scaled-down (or ideally vanishing) chemical-shift difference allows one to keep the zero-quantum line narrow by decoupling the protons. This results in fast spin-diffusion rates. Furthermore, the rate constants are now determined by the S-spin coupling network, and the proton spins need not be considered for the data analysis. Highly efficient spin diffusion can be achieved if the chemical-shift difference is scaled below the linewidth due to the homonuclear dipolar couplings of the carbons alone. For uniformly 13C-labeled samples, this is a few kilohertz and for natural-abundance samples typically in the order of 15 Hz. Assuming a chemical-shift difference of 10 kHz (100 ppm at 10 Tesla), this would (in natural abundance) lead to the requirement of a spin-lock field with Ogrf/(27r) -- 3 . 3 MHz to achieve the necessary scaling (see Equation
4~
7~
:Z -]
t~
7~
Fig. 4.4. Spectral factor f ~ ( 0 ) in the spin-diffusion rate constant calculated from the experimental separated-local-field spectrum of amorphous polystyrene [21, 30] for (a) a static sample, and (b) for a spectrum obtained under slow-MAS conditions. (Adapted from Ref. [21], with permission).
95
SPIN D I F F U S I O N IN SOLIDS
25
20
"~ 15 cc
"10
Nil
\ \ \ \
9
0
5
\
10
Offset
15
20
25
[kHz]
Fig. 4.5. Ratio of cross-peak to 13CH diagonal-peak intensity for r.f.-driven spin diffusion beween the 13CH and 13CH2 resonances of adamantane (powder sample) as a function of the offset of the r.f.-carrier frequency from the center of the two resonance lines. The circles represent experimental values obtained with a 30ms cw spin-lock. The squares represent experimental values obtained with a 30ms WALTZ-17 spin lock using a spin-lock pulse of a =7r/4. The dashed line is drawn as a guide to the eye. The 13C r.f.-field strength was set to 50 kHz. Figure adapted from Ref. [19].
(4.53), Appendix B). The chemical-shift range covered by a typical r.f.-field strength of 100 kHz is only 3.5 kHz which clearly illustrates that better schemes for offset compensation at lower r.f.-power levels are necessary [15]. Such an offset compensation can be achieved by using multiple-pulse sequences (see Fig. 4.5) which are designed to generate an effective Hamiltonian with the desired properties [7, 19]. There are two requirements for the use of multiple-pulse sequences to speed up the spin-diffusion process: First, the scaling factor of the homonuclear dipolar coupling should be as large as possible and, second, the effective spin lock has to be larger than the homonuclear dipolar interactions between the S-spins [19] to prevent loss of sum magnetization into dipolar order [15].
96
M A T T H I A S ERNST AND B E A T H. M E I E R
This requires the use of a multiple-pulse scheme with a nonvanishing effective r.f. field. Such a nonvanishing effective field can be added to a sequence without an effective field by either adding an additional small-angle pulse after a full cycle of the sequence, or by increasing the power of one of the pulse phases by a small amount. There have been quite a number of multiplepulse sequences developed for the liquid-state TOCSY experiment [3, 22], which provide offset compensation at low r.f. powers. However, the requirements in the present context are different from the ones in TOCSY due to the fact that the dipolar coupling is not isotropic like the J-coupling and is not, therefore, invariant under nonselective pulses. The performance of these multiple-pulse sequences can be calculated using average Hamiltonian theory [23]. It has been shown that sequences with all pulses along one axis (with positive and negative amplitudes), like WALTZ [24] or DIPSI [25], give the highest possible scaling factor for windowless pulse sequences, i.e., 1/2 for the homonuclear dipolar coupling. Sequences which use orthogonal phases, like MLEV [26], lead always to a smaller scaling factor. For WALTZ and DIPSI, the zeroth-order average Hamiltonian contains a scaled dipolar coupling (s = -1/2) (see Equation (4.54), Appendix B), while the first-order average Hamiltonian contains a scaled chemical-shift term with a scaling factor of ~rf/O)rf compared to the cw irradiation (see Equation (4.55), Appendix B). Here, ~rf is the effective spin-lock field over a full cycle. The best offset compensation would be achieved for a vanishing effective spin-lock field and the compensation of the chemical-shift differences gets less efficient by introducing an effective spin-lock axis. However, as mentioned earlier, the effective spin lock has to be chosen large enough to dominate the homonuclear dipolar-coupling term. An important point when using multiple-pulse sequences in r.f.-driven spin diffusion is the rotating-frame relaxation time, Tip, which determines the decay of the sum magnetization. For a cw spin-lock, only the spectral densities at multiples of the r.f.-field strength w,, = n 9 ( . O r f are important. Such frequencies are, for relatively low r.f. fields, produced by the fluctuations in the proton spin system. Using a strong enough spin-lock field, one can make sure that the intensity of the spectral-density function is small at the sampled frequencies. The spectral-density function for the T~o relaxation can be approximated by the normalized proton line-shape function of the spin system. For a multiple-pulse sequence, however, the spectral-density function is sampled at the Fourier frequencies of the multiple-pulse sequence [15] which are multiples of the basic frequency (O~o= 1/'rc) and typically much lower than the r.f.-field strength. The exact coefficients at the different frequencies depend on the pulse sequence used and on the strength of the effective field. The relaxation pathway is provided by the residual heteronuclear I-S
SPIN DIFFUSION IN SOLIDS
97
dipolar coupling which is scaled the same way as the Zeeman interaction but is also influenced by the I-spin homonuclear couplings. One way to improve the relaxation behavior is simultaneous irradiation of the proton spins in order to improve the heteronuclear dipolar decoupling. Continuous-wave proton irradiation with a field much stronger than the carbon r.f. field [19] showed an improved relaxation behavior but a better approach is the simultaneous application of homonuclear proton decoupling. This can be achieved by off-resonance irradiation such that the effective field is at the magic angle (Lee-Goldburg decoupling [27]) or by a pulse sequence like BLEW-12 [28]. It has been shown that BLEW-12 and WALTZ-16 multiple-pulse sequences can be applied synchronously without reintroducing the heteronuclear dipolar coupling [29]. It was found experimentally that a spin-lock pulse after each full WALTZ cycle interferes with the BLEW-12 cycle and leads to unsatisfactory relaxation behavior [30]. The best results were obtained by combining a BLEW-12 and a WALTZ-16 multiple-pulse sequence where the amplitudes of the x-pulses in the WALTZ-16 sequences were increased by 2%. This generates an effective spin-lock field while allowing the synchronous application of the WALTZ and BLEW sequences [30]. 4.2.2
Magic-anglesample spinning
It is well known that dipolar interactions are averaged to zero under fast magic-angle spinning [31, 32]. "Fast" refers to the regime where the MAS frequency greatly exceeds the size of the dipolar couplings. For low-y spins, this regime is easily realized. For protons with coupling constants of several tens of kilohertz, it is difficult to realize except in systems that show partial averaging by internal motions. Nevertheless, rotation has an effect on a proton spin system because of the nonuniform density of the protons. The proton spin system consists usually of groups of spins that are very tightly coupled (e.g., the protons of a CH2 group) but have a smaller coupling to neighboring clusters. This heterogeneity, which is usually neglected in the theoretical description, tends to become enhanced by MAS at moderate spinning speeds. In this chapter we will consider the fast-spinning case, e.g., a situation usually encountered in 13C spectroscopy under proton decoupling. We will shortly discuss three quite different situations. In rotor-driven spin diffusion, the rotor actually provides the energy for spin flip-flop transitions that would not take place in a static sample. In dipolar recoupling, we construct an effective dipolar Hamiltonian following similar design principles as used in r.f.-driven spin diffusion. In zero-angle spinning a third approach is used which involves the mechanical reorientation of the rotor axis such that, during
98
M A T T H I A S ERNST AND B E A T H. M E I E R
the spin-diffusion time, the dipolar interactions remain secular, but during evolution and detection a high-resolution MAS spectrum is obtained [33, 34]. 4.2.2.1 Rotor-driven spin diffusion (rotational resonance) In rotor-driven spin diffusion [35-39] (usually called rotational resonance when applied to distance determination in isolated two-spin pairs) the energy difference for the flip-flop process of the two involved spins is supplied by mechanical sample rotation about the magic angle. Such an exchange of energy can only take place when the isotropic chemical-shift difference is an integer multiple of the rotation frequency. The experiment is carried out under I-spin decoupling with no r.f. present on the S-spin channel. Therefore, we can eliminate all I-spin terms in the Hamiltonian and the relevant part of the now time-dependent Hamiltonian is given by =
ss(t) +
(4.18)
Typically, the chemical-shift difference is much larger than the homonuclear dipolar coupling. By transforming the Hamiltonian of Equation (4.18) into a rotating frame (see Appendix C), it can be shown that there is positive interference between the two terms leading to a nonvanishing time-average only if the condition F/
9 (.O r ~
r~ z ,
1
r~ z , 2
(4 19)
is fulfilled, i.e., if the isotropic chemical-shift difference is an integer multiple of the rotor spinning speed. Strong recoupling is always observed for n _+1, _+2 but for higher values of n, recoupling is only found if the anisotropy of the chemical-shielding tensor is significant. Proton and r.f.-driven spin diffusion are nonselective and ideally influence all carbon spin pairs in the same way. Rotor-driven spin diffusion, however, is highly selective and enhances the spin-diffusion rate constant only if the spinning speed is matched to the isotropic chemical-shift difference (Equation (4.19)). This allows one to perform selective experiments where only the spins of two selected resonances undergo polarization exchange. 4.2.2.2 Pulsed recoupling schemes As mentioned earlier, magic-angle sample spinning averages out the dipolar coupling and, therefore, slows down or suppresses spin diffusion (except for the special case of the rotational-resonance condition). However, in many applications it is crucial to use MAS in order to increase the resolution of the spectrum and to obtain well-resolved lines. The homonuclear dipolar
SPIN DIFFUSION IN SOLIDS
99
Hamiltonian in the rotating frame can be written as the product of a secondrank space tensor A2,o and a second-rank spin tensor Y2,o [40]. MAS leads to a rotation of the spatial part of the dipolar Hamiltonian about an axis which is inclined at an angle of 54.7 ~ to the static magnetic field. This causes in the fast-spinning limit, a complete averaging of the homonuclear dipolar Hamiltonian assuming that the spin part is time-independent. If we introduce, however, a time-dependence of the spin part of the Hamiltonian, the dipolar interaction no longer averages out over a cycle of the MAS rotation. This is due to interference between the two time-dependent rotations of the spin part and the space part of the Hamiltonian. It is important to design the pulse sequence so that only the homonuclear dipolar interaction is reintroduced and not other parts of the full system Hamiltonian like heteronuclear dipolar couplings or chemical-shielding tensors. In addition, it is often desirable that the sequence is broadband, i.e., that it suppresses the isotropic chemical shift. A number of pulse sequences were designed in recent years that lead to polarization transfer by an effective double-quantum Hamiltonian of the form I+Ij+ + Ii-I7 [41-46]. In the context of this review we concentrate on zero-quantum Hamiltonians of the form of Equation (4.2). Such a Hamiltonian can, in the presence of MAS, be generated by a combined rotatingframe/laboratory-frame experiment reminiscent of the magic-echo experiments of Rhim et al. [47, 48] and Schneider and Schmiedel [49]. Such a pulse scheme was first proposed by Fujiwara et al. [50] and later optimized for broadband behavior by Baldus et al. [51]. As an example, the broadband polarization-transfer spectrum of uniformly 13C-enriched calcium acetate monohydrate [2] is shown in Fig. 4.6. Due to crystallographic inequivalence, four sites (labeled 1-4 in Fig. 4.6) can be distinguished for the resonance of the carboxylic carbon. The methyl-group signals overlap. Intramolecular polarization transfer between nuclei spectrally separated by about 160 ppm (16 kHz in the magnetic field where the spectrum was recorded) are visible as well as intermolecular cross-peaks between the crystallographically distinct carboxylic groups. Due to the deterministic nature of the Hamiltonian, this type of spectra is also called total through-space correlation spectroscopy (TOSSY) [21. 4.2.2.3 Zero-anglespinning (ZAS) In zero-angle spinning (ZAS) [33], a different approach is taken to avoid the averaging of the homonuclear dipolar-coupling Hamiltonian by the mechanical sample rotation. During the evolution and the detection time of a twodimensional experiment, the sample is spun at the magic angle. This leads to an averaging of the second-rank tensor components (dipolar-coupling and chemical-shielding tensor) of the Hamiltonian. However, during the mixing
100
MATTHIAS ERNST A N D B E A T H. MEIER
1-2
* 3
4
* 1'-4'
I
I I
I
I I
O
O
00
oGb,
-
E
O..
-
O_ O
-O
%
-
~
O eq
O I I
190
I
I
I
I
180
i
'
i
.....
I
'
' I..~I-
170 30
i
'I
ppm
I
I
20
I
I
I
I
10
Fig. 4.6. Spin-diffusion spectrum (or TOSSY spectrum) of uniformly labeled calcium acetate monohydrate. Intramolecular as well as intermolecular cross-peaks are detected. The mixing time in the presence of a RIL mixing sequence with Lee-Goldburg proton decoupling was 20ms. 512 tl experiments were performed with 16 scans each. Contour levels are shown for constant intervals between 2 and 15% of the maximal signal intensity. The signals marked by a star are assigned to a second crystal form present as a contamination. (Figure adapted from Ref. [2]).
time of the spin-diffusion experiment the rotor axis is reoriented [52] such that it becomes parallel to the static magnetic field (Fig. 4.7). A rotation about the static magnetic field does not modulate the Hamiltonian and does not, therefore, lead to a scaling or averaging of the dipolar Hamiltonian. Partial recovery can also be obtained at other angles [34]. During the mixing
101
SPIN DIFFUSION IN SOLIDS
i) heterogeneous j ~ _ ,
_
.
ii) homogeneous A
mix~~-cryst~/__
-
O
I~ ~
~0 ~
0 G
0 0
@~
o O
h
a
I
@ 700 Hz
O I
Mixing" 30 ms
Fig. 4.7. R.f.-driven ZAS experiment: schematic diagram and application to a homogeneous and heterogeneous mixture of adamantane (a) and hexamethyethane (h) [76]. The appearance of cross-peaks, due to intermolecular spin diffusion between the two components, is seen clearly in the homogeneously mixed sample, while it is absent in the heterogeneous mixture, (Figure adapted from Ref. [33]).
102
MATTHIAS
ERNST AND BEAT H. MEIER
time we can now use either proton- or r.f.-driven spin diffusion to achieve fast polarization transfer while simultaneously obtaining a high-resolution spectrum in the evolution and detection periods (Fig. 4.7).
4.3
Probing long-range order with spin diffusion
4.3.1
Spin diffusion in abundant high-y spin systems
Abundant spins with a high gyromagnetic ratio, e.g., protons, lead in the absence of line-narrowing methods to unresolved spectra described by the Hamiltonian of Equations (4.1) and (4.2). The only relevant observable is the spatial distribution of the polarization, P(?; t). Using the master-equation approach (Equation (4.8)), the spatial evolution of the polarization is described by a random walk on a grid that can be approximated by a diffusion equation _
02
0 p(?; t) = 2 D ~ ~
Ot
~
Oe2
P(?" t). '
(4.20)
Equation (4.20) is written in the principal-axis system of the diffusion tensor with the cartesian components e~. The anisotropy of the diffusion constant is usually neglected [1], and Equation (4.20) can be simplified to [4] 0 -- P(x, y, z; t) = DAP(x, y, z; t). Ot
(4.21)
The diffusion constant is given by D = W"
a2 ,
(4.22)
which assumes that the spin flips occur at a direction-independent rate W between nearest neighbors only. The nearest-neighbor distance is denoted by a and we assume that the initial polarization does not change strongly over the order of the distance a (Fig. 4.8(a)). W is calculated according to Equation (4.11) and f~j(O) is dominated by the dipolar coupling to all other spins. In a dense and regular lattice, where each spin has a number of direct neighbors, the damping of the transient oscillation expected for the spin pair (i,j) is strong enough to make the Ansatz of Equation (4.11) somewhat intuitive although it can never be rigorously justified. Note that the functional
Z
9 :Z :Z 9 t-
Fig. 4.8. Schematic representation of the spin-diffusion process by a "wave-front" in (a) a compound consisting of different domains, e.g., a polymer blend; (b) a regular structure with long-range order (e.g., a crystal); and (c) a microscopically disordered compound. The resonance frequency is encoded into the density of the filling pattern and simultaneously into the direction of the long elliptical axis, symbolizing that it can be determined either by the isotropic shift or the orientation of the shift tensor. Quasi-equilibrium is reached in (a), if the "wave" has extended over a typical domain size; in (b) after the spin-diffusion "wave" has reached the next neighbors; and in (c) after the "wave" has sampled all possible orientations, leading to the typical pattern for amorphous compounds discussed below.
t~
104
M A T T H I A S ERNST AND B E A T H. M E I E R
form of Equation (4.22) is somewhat misleading because W itself is dependent on a. First of all through the distance rij = a appearing explicitly in Equation (4.11), but also through the intensity of the zero-quantum line, which is proportional to the dipolar coupling and, therefore, to a 3. The proportionality constant, ~', depends on the crystal packing and, for a cubic lattice, is in the order of ~"~ 5 9 1 0 - 2 6 m3]s. With this definition, we rewrite Equation (4.22) as D = -~ . a
(4.23)
If we assume that sr does not change appreciably for different compounds within a class of samples, we can directly link the diffusion constant of an unknown compound Dunk to the one of a reference compound Dref through their proton densities pH [53]
Ounk= (H)I/3 Punk Dref
pHf
(4.24) "
Clauss et al. [54] have determined a reference value of D = 0.8 --- 0.2 nm2/ms for a block copolymer PS-bPMMA; Douglass and Jones [55] have calculated a value of D = 0.62 nm2/ms for alkanes. 4.3.2
Spin diffusion in abundant low-y spin systems
For proton-driven spin diffusion, the most important difference to abundant high-y spin systems is that the main contribution to the zero-quantum linewidth is no longer the homonuclear dipolar coupling, but rather, the heteronuclear dipolar coupling to protons or other abundant high-y spins in the sample. Because the zero-quantum lineshape is largely independent of the size of the homonuclear dipolar couplings and, therefore, independent of the S-spin distance, the diffusion constant now scales as,
O--
a
4,
(4.25)
where the constant sc contains the zero-quantum linewidth as generated by the extraneous proton spins, sc depends on the heteronuclear S-I coupling as well as on the homonuclear I-spin coupling and may vary considerably between compounds. If it is similar, however, in a class of compounds, we may write the scaling law as
SPIN DIFFUSION IN SOLIDS /
13 C , 4 / 3
Dunk = / P u n k ] /
Dref
105
13C]
(4.26) "
\Pref /
In proton-driven spin diffusion, the diffusion constant depends much more on the S-spin density than in r.f.-driven spin diffusion. R.f.-driven spin diffusion in low-7 spin systems shows exactly the same characteristic as spin diffusion in high-,/spin systems. It is seldom, however, applied for spin diffusion over longer distances, because r.f. irradiation has to be applied during the entire spin-diffusion period which may be technically difficult.
4.3.3
Spin diffusion in dilute low-3, spin systems
Proton and r.f.-driven spin diffusion between 13C in natural abundance [19] is observable but slow as seen by the scaling law of Equation (4.26). An estimate for the single-step rate constant W, for typical organic solids and proton-driven spin diffusion, yields W ~ 0.02 s -~. Experimental findings are in the range of 1.5-0.005 s -1 [20, 56, 57]. These rates restrict proton-driven spin diffusion to samples with long ~3C TI relaxation times in the order of a minute. For a discussion of the consequences of the statistical distribution for obtaining a diffusion equations for homonuclear spin diffusion in systems with low-abundant spins (e.g., for r.f.-driven spin diffusion), we refer to the discussion by Goldman and Jacquinot [58]. It is clear that, due to the randomness of the positions occupied by magnetic isotopes, the interpretation of spin-diffusion measurements in terms of distances is complicated and only information about coarser structures (of the size of many next-neighbor distances) can be obtained. With respect to r.f.-driven spin diffusion, the remarks made in the preceding section remain valid.
Correlation of tensor interactions by spin diffusion at quasi-equilibrium 4.3.4
An important application of spin diffusion in solid-state NMR is its use as a correlation mechanism in two-dimensional NMR spectra [13, 59]. The description of a two-dimensional tensor-correlation experiment using spin diffusion as a transport mechanism follows the general scheme for twodimensional experiments (see Fig. 4.1) [7]. During the evolution time (tl), the polarization is labeled with the resonance frequency of spin k (l)k), during
106
MATTHIAS ERNST AND BEAT H. MEIER
the mixing time ("Fro) , it is transferred via the spin-diffusio n process to another spin ~, and finally during the detection time (t2), detected with the frequency (~e). We consider the case where the Hamiltonians ~1 and ~2 are both arranged to contain only chemical-shift terms. For small S-spin homonuclear couplings, heteronuclear decoupling is sufficient to achieve this goal, but in the presence of strong homonuclear couplings, homonuclear dipolar decoupling of the S-spins is required. It is also possible to correlate the shift anisotropy with dipolar tensors [60] or two dipolar tensors with each other. Usually, proton-driven spin diffusion is chosen as the mechanism for polarization transfer. The mixing time is set such that the spatial distance covered by the diffusion exceeds the symmetric unit in a compound assumed to have some (local) translational symmetry (see Fig. 4.8(b)). In practical applications of 13C spin diffusion, the zero-quantum intensity fq(0) can depend considerably on the difference in resonance frequencies and, therefore, on the crystal orientation (Fig. 4.4(a)4). Nevertheless, the resulting two-dimensional correlation spectrum becomes independent of the mixing time itself after a long enough mixing time. As mentioned earlier, slow-MAS experiments [21] can accelerate the establishment of quasi-equilibrium. The quasi-equilibrium pattern can be used to obtain the relative orientation of the chemical-shielding tensors involved. If the isotropic chemical shift is the same for all involved spins, we obtain a typical exchange pattern which is centered around the diagonal, while for different isotropic chemical shifts the exchange pattern is centered off the diagonal. In order to analyze the different patterns in the two-dimensional spectrum, it is important to simulate the spectra efficiently. Assuming that the transverse relaxation during tx and t2 can be described by convoluting the spectrum with a lineshape function, the simulation of the two-dimensional spectrum is reduced to calculating the polarization transfer between all possible sites with different resonance frequencies. The orientation of the principal-axis system of the chemical-shift tensor of spin 1 is defined by two Euler angles 0 and 4~ relative to the external static magnetic field Bo (powder average). The orientation of the principal-axis systems of the N - 1 other chemical-shift tensors are defined by N - 1 sets of three Euler angles (c~j, /3j, ),j) relative to the principal-axis system of spin 1 (Fig. 4.9). The resonance frequency of spin j in a frame rotating with the Larmor frequency is then given by
4Figure 4.4 represents experimental data from an amorphous compound without long-range order. However, a crystalline compound shows similar behavior.
107
SPIN DIFFUSION IN SOLIDS
Fig. 4.9. Transformation of chemical-shielding tensors from different sites to a common coor-
dinate system. (Adapted from Ref. [69] with permission.) 2
~"~j(0, ~)) -- 0"is o(/) "[-
2
~ ~(m2),0((]), 0, 0) s m'= --2 m= --2
~(2)
/'
(j)
m,m' \ OLj, ~] , 'Yj ) P 2,m
(4.27) (2) ,(0~, ~3, ')/) are the where ~ri~oO)is the isotropic chemical shift of spin ], ~)m,m second-rank Wigner rotation matrices, and pO2!mare the second-rank tensor components of the chemical-shielding tensor of spin j in its principal-axis system with the values pzVJo= ~ 6 ~ p2(J)+l = 0 p2~!_+2 = 1/26 0) ~70) Here 6 ~ and r/0~ denote the anisotropy and asymmetry of the CSA tensor, respectively [23]. The full two-dimensional spectrum for a single crystallite orientation (0, 4~) is then given by N
N
,~(0)1, 0)2; Tm; 0, (/~) = ~ ~ ~((.01 -- ~'~i(O, (/)))" ~((.02 -- ~'-~](0, 4))) i=1]=1
(4.28)
• [exp(Wrm)]ij ** L(wx, 092) with 6(~o) representing the Dirac delta function and L(o91,092) representing the two-dimensional line-shape function. The macroscopic disorder of the sample can be described by averaging over all possible orientation (0, ~b) (powder average):
108
MATTHIAS ERNST AND BEAT H. MEIER 7/"
sin 0 dO
'~(('01' 0)2; Tm) = G
dr/) J)((.Ol, 0)2; 7m; 0, (])).
(4.29)
In order to calculate the spectrum, the chemical-shielding tensors of all spins, as well as their relative orientation and distances, have to be known. The assumption of a quasi-equilibrium state simplifies Equation (4.28) considerably. For long mixing times Tm all elements in exp(Wrm) have equal intensity. This leads to the following signal function for a single-crystallite orientation: N ~j)qeq((.01, 0)2; 0, (f~) "--
N
i=lj=l
96(w2 - a j ( 0 , 6 t t
**
L(tOl, Wz),
(4.30)
and the full spectrum averaged over all different crystallite orientations (0, ~b) (powder average) is given by ,~qeq((.Ol, 0.)2) = G
sin OdO
d~b 5r
w2; 0, ~b).
(4.31)
This shows that the quasi-equilibrium spectra only provide information about the relative orientation of the sites of interest (i.e., the Euler angles (a j,/3j, 7j)) and not about their distance (rq) which is encoded in the spin-diffusion rate constant Wq. As a first example for the type of information one can obtain from quasiequilibrium spectra, we present the case of methanol in its crystalline a- and /3-phase [61, 62]. The a-phase is stable below 156 K while the/3-phase exists between 159 K and the melting point at 175.4 K. In both phases the methanol forms long parallel chains. Figure 4.10(a) shows the ~3C spectra obtained for the two different phases using proton-driven spin diffusion. For the a-phase the chemical-shielding tensor was found to be axially symmetric within the precision achievable due to the homogeneous linewidth. Therefore, the relative orientation of two tensors can be described by a single Euler angle /3CSA. However, it is a priori unclear how many different angles /3csn exist due to the packing in the unit cell. Therefore, the experimental spectra have to be fit with a distribution of/3CSA angles. The result of the fit is shown in Fig. 4.10(b). As expected from the crystal structure [62], two crystallographic different sites with equal populations are found. For the/3-phase the chemical-shielding tensor is not axially symmetric. In theory, it is now necessary to use all three Euler angles (0~CSA, /~CSA, '~CSA) to describe the relative
fa~
9
9 t"
Fig. 4.10. (a) 2D powder 13C NMR spin-diffusion spectrum at 145 K of a-13CH3OH,99% 13C-labeled, with a mixing time of 600 ms. The 13C frequencies are relative to TMS. (b) Distribution function for the angle /3CSA, represented by 31 independent amplitudes, yielding the best fit of the experimental spectrum in (a). The 13C CSA tensor is assumed to be axially symmetric. The CSA principal components and a uniform Gaussian broadening are determined from the spectrum (a) and used as fixed parameters in the fitting procedure. (c) 2D powder 13C NMR spin-diffusion spectrum at 168 K of I3-13CH3OH,20% 13C-labeled, with a mixing time of 12.5 s. (d) Computer simulated two-dimensional angular distribution function P(/3CSA, YCSA) obtained from the fit to the spectrum (c). (Figure adapted from Ref. [62]).
ll0
MATTHIAS ERNST AND BEAT H. MEIER
orientation of the chemical-shielding tensors. The spectrum (see Fig. 4.10(c)) shows only significant off-diagonal intensity close to the diagonal which indicates that only small angles/3CSA contribute to the observed spectrum. Therefore, it is possible to set aCSA = 0 and to restrict the fit to two angles, /3CSA and 7CSA [62]. The results of the best fit is shown in Fig. 4.10(d). These measured orientation distributions are in good agreement with the crystallographic structure of the a- and/3-phase of methanol. The application of proton-driven CSA correlation spectroscopy to aminoacid specifically carboxylic-~3C labeled spider silk [63] is shown in Fig. 4.11. Spider silk is known to consist of alanine- and glycine-rich domains [64, 65] and is known to be semicrystalline. The assignment of alanine to the (crystalline) /3-sheet domains [66] is clearly supported by the chemical-shift correlation spectrum of Fig. 4.11. Because the X3C tensors in a/3-sheet structure are almost parallel, or antiparallel, with the tensors in spatial proximity, a diagonal spin-diffusion spectrum is expected for that structure and is indeed found. In contrast, the glycine spectrum shows considerable off-diagonal intensity. Simulations have shown that the spectrum is compatible with a local 3~-helical structure [63].
4.3.5
Establishment of distance ranges in heterogeneous compounds
Proton spin diffusion can be used to measure domain sizes in polymers. Usually, these systems have a dense network of homonuclear dipolar couplings resulting in a fast spin-diffusion rate constant. As mentioned earlier, the diffusion constant in such systems is in the order of about 1 nm2/ms which allows the probing of distances up to 200 nm. In order to see spin diffusion in such systems, we have to generate an inhomogeneous distribution of magnetization over the different domains. The best results can be obtained if one of the domains can be selectively excited, while the other is saturated or inverted. We can then observe the diffusion of the magnetization from the selectively excited region into the other one. In such a spin diffusion experiment, we obtain only the "typical shortest distance" [67], since, the equilibration of the magnetization will occur predominantly via the shortest path between the two domains. It has to be a "typical" distance since a few uncharacteristically short distances will not influence the bulk of the signal significantly. A more complete discussion of ways to generate the magnetization gradients, and what types of domains can be distinguished, is found in the book by Schmidt-Rohr and Spiess [67] and also in a recent article by VanderHart and McFadden [53].
Fig. 4.11. Proton driven spin-diffusion spectra of 1-13Cglycine and 1-13C alanine-labeled Nephila madagascariensis dragline silk at T = 150 K. A mixing time of 10 s was used. The spectrum was acquired with 128 transients per data point in tl, 96 spectra have been recorded in the F1 domain. The data matrix of 96 • 128 points was zero-filled to 256 • 256. As inset, the contour plot of the same data is shown. (Figure adapted from Ref. [63]).
112
MATTHIAS ERNST AND BEAT H. MEIER
For such an experiment to work, we have to be able to distinguish the different domains during the evolution and the detection period of the twodimensional experiment. Since proton spectral resolution in typical solids is very poor, we have to use homonuclear dipolar-decoupling methods to narrow the lines sufficiently to obtain spectral resolution. The 2D spin-diffusion CRAMPS spectrum was first recorded by Caravatti et al. [68] for blends of polystyrene (PS) and polyvinyl methyl-ether (PVME). There are other methods to generate an initial nonequilibrium polarization based on differences in linewidth or relaxation times. The reader is referred to the excellent book of Schmidt-Rohr and Spiess [67] for an overview.
4.4
Probing nearest-neighbor interactions
So far, we have assumed that we observe the spin system at times much longer than the time needed to transfer the polarization from one spin to its neighbor. For a strongly disordered compound, symbolized in Fig. 4.8(c), the quasi-equilibrium spectrum obtained for long mixing times, does not contain structural information other than that the sample is fully disordered on the length-scale probed by the experiment (e.g., amorphous). In such a situation, it is necessary to investigate the short-time behavior of the spin system [30, 69]. We assume a system with N spins 1/2 with their respective polarizations given by f ' ( t ) = (pl(t), p 2 ( t ) , . . . ,pN(t)). In matrix form, Equation (4.8), reads as
d F'(t) = WP(t). dt
(4.32)
In many applications, we are not able to distinguish all the N spins in the spectrum whose polarizations appear in P(t) due to spectral overlap. For example, if we take proton spectroscopy in a typical polymer, we are often only able to distinguish aliphatic from aromatic resonances. We can take the limited resolution into account by a basis transformation T of the polarization vector P(t). If there are M distinguishable lines in the sample, we rearrange the population vector so that the M sum polarizations, S~ = ~ i ~ a Pi form a vector S. The remaining N - M components of the transformed polarization vector are assembled into a vector D.
SPIN D I F F U S I O N IN S O L I D S
113
m
Pl P2
T
.: =I l.
4.33,
PN m
m
All quantities in D are unobservable. The master equation in the new basis reads
r wSS wSD 1 d[~]=wT[~]=LwDS dt
wDDj [ ~ ]
(4.34)
where the transformed kinetic matrix W T = T W T - 1 is used. As for W itself, the conservation of the sum polarization requires that the sum over the column vectors of W ss vanish. Except for special cases, the matrices W sD and W Ds are nonzero and in the course of the polarization-transfer process, S and D will mix. For a frequency-selective preparation of the spin system, however, we have I ) ( 0 ) = 0 and in initial rate the polarization transfer is described by
S(1-m) = wSS" 7"m " S(O).
(4.35)
From this we can calculate the cross-peak intensity as N Sa,/3(Tm) = W ~ss, / 3 " r m ' - - "
na
eq , S,~,/3
(4.36)
SS defined as the sum over all contributing spins with the rate constant W~,t3 ss 1 Wot,/3 = " E E Wij, ///3 iEajE/3
(4.37)
where n~ is the number of spins in the isochromatic class a. We can now define a rate constant R~,t3 which describes the growths of the cross peak and can be calculated as R~,t3 . . .N. . 7r E
n~n~
s deft .-ij "f ij (O), 2 i~oejE~
(4.38)
114
MATTHIAS
E R N S T A N D B E A T H. M E I E R
If we assume that the intensity of the zero-quantum line at frequency zero is the same for all the contributing spin pairs, then we can rewrite Equation (4.38) as 71"
R~'t3 = 2" g~''" f~,t3(0)
(4.39)
with a single geometrical rate factor g~,b which contains all the information about the structural information and N g~,t3 = - - "
E E
d~.ff.
(4.40)
Mc~F//3 i~c~ j~/3
Note that g~,t3 contains the internuclear distances and angles between all involved nuclei. While it can easily be calculated from a model structure, it is not possible, in general, to obtain model-independent structural information. Robyr et al. [30] have applied the analysis described above to quantify the local orientational order of the phenyl-rings in amorphous atactic polystyrene 13C enriched at the aromatic carbon C1. As all labeled carbons have the same isotropic shift, the resonance frequency is solely defined by the orientation of the phenyl ring with respect to the magnetic field. The proton-driven spindiffusion quasi-equilibrium spectrum of this compound, is shown in Fig. 4.12(a). The typical pattern expected for a completely amorphous material, with each trace along the Wl or w2 direction having the same shape (but different intensity), is observed. From r.f.-driven spin-diffusion spectroscopy in the initial-rate regime with mixing times below 4 ms, the rate constants R~,t3 have been determined (see Fig. 4.12(b)). Using experimentally determined values for f~,t3(0), the spectra can, without any adjustable parameters, be directly compared with calculated spectra from model compounds (such as shown in Fig. 4.12(c)). Excellent agreement was found in polystyrene with structures obtained from atomistic simulations [30, 70, 71]. Using these models, the distribution of the Euler angle /3, between phenyl rings at a certain distance from each other, can be evaluated. The results are displayed in Fig. 4.12(d).
4.5
Appendix A: 13C spin diffusion in protonated systems
The relevant Hamiltonian for a system of low-,/spins (S) coupled to a system of high-,/spins (I) is given by
SPIN DIFFUSION IN SOLIDS
d)
e-m
c/
115
iI
r~
o
0
30
60
90
13[degrees] Fig. 4.12. (a) 2D quasi-equilibrium proton-driven spin-diffusion spectrum at 295 K of amorphous, atactic polystyrene 13C-enriched at the aromatic carbon C1. The mixing time was set to 10 s. Within this time frame, a completely disordered environment is sampled (see Fig. 4.8(c)). (b) Rate-constants for r.f.-driven spin-diffusion obtained from mixing times smaller than 4 ms from the same compound. (c) Structure of a microstructure, constructed by Rapold et al. [71] to describe amorphous atactic polystyrene. The rate constants in (b) can be well explained by a set of such microstructures. From the microstructures, in turn, the weighted distributions p( /3)/sin /3 can be extracted. The result is given in (d). (Figure adapted from Refs. [30, 70]). __ ~[0S + ~ S S + {)~IS + ~ I I + ~ r . f . ,
(4.41)
W e can t a k e a d v a n t a g e o f the fact that the s u m m a g n e t i z a t i o n and the d i f f e r e n c e m a g n e t i z a t i o n of the S-spin s u b s p a c e of the H a m i l t o n i a n c o m m u t e in the a b s e n c e of r.f. irradiation and write our H a m i l t o n i a n as ~[~S -- a l S l z
+ ~']282z-- (~'],1 + a 2 ) S(1'4) + ( a l -
a 2 ) S(2'3) ,
~r SS -- d12(381~82~ - 8182) - - d l 2 S } , 2 " 3 ) - dl21(2"3) + dr21(1"4)
(4.42) (4.43)
116
MATTHIAS ERNST AND BEAT H. MEIER IS
= --
2dlSSl zIiz + s
~ i
2 d 2IS i S2z
Iiz
i
X 2(d Is +
,_1IS'~2(1,4)1 u2i , o z xi z "Jr- s
i
~II=
2(dliIS
--
d- 2]IS .q(2,3)1" i,'Z "tz,
(4.44)
i
~
i<]
dli(3iizijz_
i112).
(4.45)
We have used the basis of the single-spin operators [72, 73] which are defined as the Pauli matrices in the corresponding subspaces of the Hamiltonian"
It)
Is)
0
1
Ir)
Is)
0
-1
Ir)
1
0
Is)
0
S(f,~) =
Ir)
1
,~,(r,s) =
2
i 2
1
0
Is}
0
0
Ir)
Is)
1
0
Ir)
0
-1
Is)
1
s':'s) : 2
(4.46)
For the evolution of the difference magnetization it is, in the absence of r.f. irradiation, sufficient to consider the evolution of the subspace spanned by the (2,3) single-transition operators [15]. We obtain, therefore, the following reduced Hamiltonian" ~ ( 2 , 3 ):_( a l .
a 2 ) 5 ( 2 , 3 )_d 1 2 o ~2(2,3) x + s 2(dlISi _ d2ISi)S(z2,3)Iiz i
+ ~ dlI(3I/zIjz- i/Tj). i<j
(4.47)
SPIN DIFFUSION IN SOLIDS
117
Because of the large number of I-spins, this Hamiltonian is difficult to solve but we can simplify the Hamiltonian by using the local-field approach (Equation (4.9)) where all the interactions with the I-spins are represented by a local fluctuating field acting on the S-spins ~(2,3) __. (~'~1 -- ~-~2) S(2'3) --Ul2Ox A ~2(2,3) -[- A ( t ) S 7 '3) "
(4.48)
The exact nature of the time-dependent local fields determines the width of the zero-quantum line and influences the rate constant W12 as can be seen from Equation (4.11).
4.6
Appendix B: R.f.-driven spin diffusion
The case of a cw irradiation of the carbons is most easily treated by transforming the Hamiltonian into a tilted frame which is quantized along the effective field. With ~ r f : O)rf(Six _[_ S2x)
(4.49)
we can transform the Hamiltonian of Equation (4.41) into the tilted frame by = R-I~R
= ~ s + ~ s s + ~IS _[_ ~II
(4.50)
where R = exp(-i(OiSly + 0252y)) and Oi = a r c t a n ( w r f / ~ i ) . We assume that the r.f.-field strength is much larger than the chemical shifts and the heteronuclear dipolar couplings. Under these assumptions we can neglect the nonsecular terms in the tilted rotating frame and obtain [15, 19]
ffC ~ \
.{~-~2 q_ a2 ~5(1,4 ) a2_ a 2 dl 2 209rf---- q- (.Orfj -~S (2'3) -~S (2'3) . 2~0rf 2
(4.51)
The performance of multiple-pulse sequences can be calculated using average Hamiltonian theory. For WALTZ and DIPSI the average Hamiltonian up to first order is given by
118
MATTHIAS ERNST AND BEAT H. MEIER ~(o) = d12 5(2,3) 2
(4.52)
~1~(1) :. ~rf a21 -~- ~-~2 S (1'4) nt- ~ r f a 2 - ~-~2 S(z2'3).
(4.53)
and
O)rf
209rf
O)rf
20)rf
This result is also correct for continuous-wave irradiation where the effective spin-lock field is ~ r f = 0)rf leading to the same result as in Equation (4.51).
4.7
Appendix C: Rotor-driven spin diffusion
Starting from the time-dependent Hamiltonian of Equation (4.18) =
s(t) +
ss(t),
(4.54)
we can again separate the Hamiltonian into the two commuting subspaces of the sum and difference polarization. With proton decoupling we can eliminate all I-spin terms in the Hamiltonian and the relevant part of the now timedependent Hamiltonian is given, for a two-spin system, by ~(2,3)(t ) = ~r
) + ~r
= (al(t) -- a2(t))S(z2'3) - dl2(t)S (2'3)
(4.55)
Typically, the chemical shift difference is much larger than the homonuclear dipolar coupling and we can transform the Hamiltonian of Equation (4.55) into a rotating frame with ~(s2'3)(t). The propagator for this transformation is
U(t) = e x p ( - i
3~(sZ'3)(t')dt '
= exp(-i~b(t)S(z2'3)),
(4.56)
where the phase factor ~b(t) contains frequency components at harmonics of the rotation frequency and can be expressed in a Fourier series as exp(-i4~(t)) = e x p ( - i ( l ) ] s ~
~)~s~
In e x p ( i n w r t ) n= --c~
.
(4.57)
119
SPIN DIFFUSION IN SOLIDS
The intensities I~ can be identified with the intensities of the nth spinning side bands of the chemical-shielding difference tensor [74]. In the timedependent interaction frame the Hamiltonian is given by = =
dl2(t)U,(t)S(x2,3)W(t) d12(t)" (S(x2'3) cos((/)(/)) + ..y
sin(oh (t))).
(4.58)
This Hamiltonian contains time-independent terms only if the condition H 90)r =- (~iso ~ 1 m r~ 2
(4.59)
is fulfilled, i.e., if the isotropic chemical-shift difference is an integer multiple of the rotor spinning speed.
Note added in proof: Very recently, the first direct determination of the spindiffusion rate constant (Equation (4.2)) was performed (W. Zhang and D.G. Cory, Phys. Rev. Lett. 80 (1998) 1324). In a CaF2 single crystal values of D - 0.71 +_ 0.05 nm2/ms and 0.53 _+ 0.03 nm2/ms were found for two different orientations of the crystal.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
16. 17.
N. Bloembergen, Physica 15 (1949) 386. M. Baldus, R.J. Iuliucci and B.H. Meier, J. Am. Chem. Soc. 119 (1997) 1121. L. Braunschweiler and R.R. Ernst, J. Magn. Reson. 53 (1983) 512. A. Abragam, The Principles of Nuclear Magnetism. Clarendon Press, Oxford, 1961. D. Suter and R.R. Ernst, Phys. Rev. D 25 (1982) 6038. J. Jeener, B.H. Meier, P. Bachmann and R.R. Ernst, J. Chem. Phys. 71 (1979) 4546. R.R. Ernst, G. Bodenhausen and A. Wokaun, Principles of Nuclear Magnetic Resonance in One and Two Dimensions. Clarendon Press, Oxford, 1987. M. Linder, P.M. Henrichs, J.M. Hewitt and D.J. Massa, J. Chem. Phys. 82 (1985) 82. G.E. Pake, J. Chem. Phys. 16 (1948) 327. S. Zhang, B.H. Meier and R.R. Ernst, Phys. Rev. Lett. 69 (1992) 2149. B.N. Provotorov, Sov. Phys. JETP 14 (1962) 1126. D. Suter and R.R. Ernst, Phys. Rev. B 32 (1985) 5608. P.M. Henrichs and M. Linder, J. Magn. Reson. 58 (1984) 458. A. Kubo and C.A. McDowell, J. Chem. Phys. 89 (1988) 63. B.H. Meier, Polarization transfer and spin diffusion in solid-state NMR, in W. S. Warren (ed), Advances in Magnetic and Optical Resonance, vol. 18, Academic Press, New York, 1994, pp. 1-116. C. Tang and J.S. Waugh, Phys. Rev. B 45 (1992) 748. R. Brtischweiler and R.R. Ernst, J. Magn. Reson. 124 (1997) 122.
120 18. 19. 20. 21. 22.
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
42. 43. 44. 45. 46. 47. 48. 49. 50.
MATTHIAS ERNST AND BEAT H. MEIER
A. Mtiller and U. Haeberlen, Chem. Phys. Lett. 248 (1996) 249. P. Robyr, B.H. Meier and R.R. Ernst, Chem. Phys. Lett. 162 (1989) 417. D.L. VanderHart, J. Magn. Reson. 72 (1987) 13. Z.H. Gan and R.R. Ernst, Chem. Phys. Lett. 253 (1996) 13. S.J. Glaser and J.J. Quant, Homonuclear and heteronuclear Hartmann-Hahn transfer in isotropic liquids, in W.S Warren (ed), Advances in Magnetic and Optical Resonance, vol. 19, 1996, pp. 59-252. U. Haeberlen, High Resolution NMR in Solids: Selective Averaging. Academic Press, New York, 1968. A.J. Shaka, J. Keeler and R. Freeman, J. Magn. Reson. 53 (1983) 313. A.J. Shaka, C.J. Lee and A. Pines, J. Magn. Reson. 77 (1988) 274. M.H. Levitt, R. Freeman and T. Frenkiel. Advances in Magnetic Resonance, vol. 11, Academic Press, New York, 1982, p. 47. M. Lee and W.I. Goldburg, Phys. Rev 140 (1965) A1261. D.P. Burum, M. Linder and R.R. Ernst, J. Magn. Reson. 44 (1981) 173. P. Caravatti, L. Braunschweiler and R.R. Ernst, Chem. Phys. Lett. 100 (1983) 305. P. Robyr, M. Tomaselli, J. Straka, C. Grob-Pisano, U. W. Suter, B.H. Meier and R.R. Ernst, Mol. Phys. 84 (1995) 995. E.R. Andrew, A. Bradbury and R.G. Eades, Nature London 182 (1958) 1659. I.J. Lowe, Phys. Rev. Lett. 2 (1959) 285. M. Tomaselli, B.H. Meier, M. Baldus, J. Eisenegger and R.R. Ernst, Chem. Phys. Lett. 225 (1994) 131. R. Tycko, J. Am. Chem. Soc. 116 (1994) 2217. M.G. Colombo, B.H. Meier and R.R. Ernst, Chem. Phys. Lett. 146 (1988) 189. D.P. Raleigh, M.H. Levitt and R.G. Griffin, Chem. Phys. Lett. 146 (1988) 71. D.P. Raleigh, F. Creuzet, D.K. Das Gupta, M.H. Levitt and R.G. Griffin, J. Am. Chem. Soc. 111 (1989)4502. F. Creuzet, A. McDermott, R. Gebhard, K. van-der Hoef, M.B. Spijker-Assink, J. Herzfeld, J. Lugtenburg, M.H. Levitt and R.G. Griffin, Science 251 (1991) 783. G. Pavlovskaya, M. Hansen, A.A. Jones and P.T. Inglefield, Macromolecules 26 (1993) 6310. M. Mehring, Principles of High Resolution NMR in Solids, 2nd edition. Springer Verlag, Berlin, 1983. A.E. Bennett, R.G. Griffin and S. Vega, Recoupling of Homo- and Heteronuclear Dipolar Interactions in Rotating Solids, vol. 33 of NMR Basic Principles and Progress, Solid-State NMR IV. Springer Verlag, Berlin, 1994. R. Tycko and G. Dabbagh, Chem. Phys. Lett. 173 (1990) 461. R. Tycko and S.O. Smith, J. Chem. Phys. 98 (1993) 932. N.C. Nielsen, H. Bildsoe, H.J. Jakobsen and M.H. Levitt, J. Chem. Phys. 101 (1994) 1805. Y.K. Lee, N.D. Kurur, M. Helmle, O.G. Johannessen, N.C. Nielsen and M.H. Levitt, Chem. Phys. Lett. 242 (1995) 304. B.Q. Sun, P.R. Costa and R.G. Griffin, J. Magn. Reson. A 112 (1995) 191. W.K. Rhim, A. Pines and J.S. Waugh, Phys. Rev. Lett. 25 (1970) 218. W.K. Rhim, A. Pines and J.S. Waugh, Phys. Rev. B 3 (1971) 684. H. Schneider and H. Schmiedel, Physics Lett. 30A (1969) 298. T. Fujiwara, A. Ramamoorthy, K. Nagayama, K. Hioka and T. Fujito, Chem. Phys. Lett. 212 (1993) 81.
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51. M. Baldus, M. Tomaselli, B.H. Meier and R.R. Ernst, Chem. Phys. Lett. 230 (1994) 329. 52. K.T. Mueller, B.Q. Sun, G.C. Chingas, J.W. Zwanziger, T. Terao and A. Pines, J. Magn. Reson. 86 (1990) 470. 53. D.L. VanderHart and G.B. McFadden, Solid State NMR 7 (1996) 45. 54. J. Clauss, K. Schmidt-Rohr and H.W. Spiess, Acta Polymer 44 (1993) 1. 55. D.C. Douglass and G.P. Jones, J. Chem. Phys. 45 (1966) 956. 56. P. Caravatti, "Zweidimensionale und selective Pulsmethoden in der hochaufl6senden Festk6rper Kernresonanzspektroskopie". Ph.D. thesis, Dissertation ETH Nr.8027, ETH Ztirich, Switzerland, 1986. 57. C.E. Bronniman, N.M. Szeverini and G.E. Maciel, J. Chem. Phys. 79 (1983) 3694. 58. M. Goldman and J.F. Jacquinot, J. Physique 43 (1982) 1049. 59. M. Linder, A. Hohner and R.R. Ernst, J. Chem. Phys. 73 (1980) 4959. 60. D.P. Weliky, G. Dabbagh and R. Tycko, J. Magn. Reson. A 104 (1993) 10. 61. R. Tycko and G. Dabbagh, J. Am. Chem. Soc. 113 (1991) 5392. 62. P. Robyr, B.H. Meier, P. Fischer and R.R. Ernst, J. Am. Chem. Soc. 116 (1994) 5315. 63. J. Ktimmerlen, J. van Beek, F. Vollrath and B.H. Meier, Macromolecules 29 (1996) 2920. 64. C.M. Mello, K. Senecal, B. Yeung, P. Voudros and D. Kaplan, in D. Kaplan, W.W. Adams, B. Farmer and C. Viney (eds) Silk Polymers--Material Science and Biotechnology, American Chemical Society, Washington DC, 1994, p. 67. 65. R.V. Lewis, Acc. Chem. Res. 25 (1992) 392. 66. A. Simmons, E. Ray and L.W. Jelinski, Macromolecules 27 (1994) 5235. 67. K. Schmidt-Rohr and H.W. Spiess, "Multidimensional Solid-State NMR and Polymers". Academic Press, London, 1994. 68. P. Caravatti, P. Neuenschwander and R.R. Ernst, Macromolecules 18 (1985) 119. 69. P. Robyr. Two-Dimensional Polarization-Transfer NMR Spectroscopy for Studying Ordered and Disordered Solids. Ph.D. thesis, ETH Ztirich. No. 10968, 1994. 70. P. Robyr, M. Tomaselli, C. Grob-Pisano, B.H. Meier, R.R. Ernst and U.W. Suter, Macromolecules 28 (1995) 5320. 71. R.F. Rapold, U.W. Suter and D.N. Theodorou, Macromol. Theory Simul. 3 (1994) 19. 72. A. Wokaun and R.R. Ernst, J. Chem. Phys. 67 (1977) 1752. 73. S. Vega, J. Chem. Phys. 68 (1978) 5518. 74. M.M. Maricq and J.S. Waugh, J. Chem. Phys. 70 (1979) 3300. 75. S. Zhang, B.H. Meier and R.R. Ernst, J. Magn. Reson. A 108 (1994). 76. P. Caravatti, J.A. Deli, G. Bodenhausen and R.R. Ernst, J. Am. Chem. Soc. 104 (1982) 5506.
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Chapter 5
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
NMR Imaging and Spatial Information B. Bltimich 1, P. Bltimler I and K. Saito 2 llnstitiit for Makromolekulare Chemie, R WTH, Worringer Weg 1, D-52056 Aachen, Germany; 2Nippon Steel Corporation, 1618 IDA, Nakahara-ku, Kawasaki 211, Japan
NMR imaging is more widely known for its applications to medical diagnostics, where the method has become an invaluable tool complementing X-ray tomography [1-3]. But in the first reports of NMR imaging, both medical [4] and materials [5] applications were considered. Though the achievable spatial resolution is limited in practice to values larger than 10 p~m3 in fluids and soft solids, it was already known at the start, that NMR provides image contrast which is fundamentally different from that of other methods [6, 7]. The contrast is determined not only by the density of the observed nucleus, but also by the numerous other parameters, which are measured in NMR spectroscopy to investigate the molecular characteristics of condensed matter. These parameters include relaxation parameters which depend on the physical properties of the sample, and chemical shifts and different couplings which are more characteristic of the sample chemistry. Also, the size and orientation dependence of different spin interactions in solids can be analyzed for contrast parameters. By mapping suitable NMR parameters over the sample volume, structures invisible to other methods may be determined. Within the limits imposed by the probe and magnet dimensions the method is nondestructive to the sample. This chapter is divided into two parts. The first introduces the principles of NMR with spatial resolution, and the second demonstrates the use of the technique in polymer science with selected examples.
5.1
Principles
In the following text, the principles of NMR with spatial resolution are presented in condensed form. More extensive presentations have been published [8-13].
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5.1.1
Spatial resolution
5.1.1.1 The single-pulse response in time-dependent magnetic field gradients The single-pulse NMR response s(t) of a single resonance signal, like the water signal, can be expressed by the free induction decay (FID) f(t)
s(t) ~ M o f ( t ) = Mo e x p { - ( 1 / T 2 - iwo)t},
(5.1)
where COo= - y B o -
O)rf,
(5.2)
is the resonance frequency in the coordinate frame rotating with the radio frequency (rf) Wrf along the axis of the polarizing magnetic field Bo and y is the magneto-gyric ratio. The amplitude of the FID is given by the thermodynamic equilibrium magnetization Mo. It can be expressed by the space integral of the spin density Mo(x, y, z),
Spatially resolved NMR is concerned with unraveling the spatial distribution of Mo(x, y, z) and measuring associated NMR parameters at individual "volume cells", or voxels at space coordinates r = (x, y, z)'. If many contingent voxels are investigated on a plane, or if a projection is investigated, one refers to NMR imaging; if individual voxels are investigated one refers to
volume-selective NMR. The spin density distribution can be resolved by different techniques. A simple approach uses surface coils for excitation or detection, so that the volume element excited, or the signal received, are localized by the position and the sensitive volume of the rf coil [14]. The standard approach in imaging exploits the possibility of introducing a space-dependence to the resonance frequency O~o in each voxel by superposing a magnetic field gradient to a homogeneous magnetic field Bo by the use of additional coils. The gradient is a Cartesian tensor with nine components related by Maxwell's equations. With the exception of tilted gradient coils in MAS imaging [15], the gradient tensor can be reduced to a gradient vector G for an approximate description of imaging experiments with good accuracy. Then the resonance frequency (Equation 5.2) becomes dependent on the space vector r = (x, y, z)',
NMR IMAGING AND SPATIAL INFORMATION
125 (5.4)
wo(r) = - y ( B o + Gr) - O.)rf,
where the gradient vector collects the derivatives of the magnetic field in the z direction with respect to the space coordinates x, y and z,
(5.5)
G = (OBz/Ox, OBz/Oy, OBz/Oz)'.
Conventional equipment is designed in such a way, that the gradient vector is constant in space, so that the magnetic field varies linearly, and the resonance frequency at a given chemical shift is directly proportional to the space coordinate. The signal of an object, like a beaker of water measured in a magnetic field gradient, can now be calculated from Equations (5.1), (5.3) and (5.4),
s(t) ~ exp{-(1/Tz - iwo)t} f f f
Mo(x,
y, z) exp{-iyGrt} dx dy dz . (5.6)
In general, the N M R spectrum of the object shows more than one resonance line, or in solids the lineshape is non-Lorentzian. Thus, the first exponential in Equation (5.6) needs to be replaced by the FID f(t) in a homogeneous magnetic field. If the spectrum changes within the sample, the FID has to be placed inside the integral. But rather than considering spatially-dependent spectral properties, which often are essential for image contrast, time-dependent gradients shall be admitted to illustrate the basic concepts of space encoding. Then the spacedependent magnetization phase -yGrt in the exponent of the integrand has to be replaced by -yGrt-
-y
G(t')
dt'r
= k(t)r,
(5.7)
where the Fourier conjugate variable to space r, the wave vector k is introduced, k(t) : - 3/
G(t')
dt'.
(5.8)
In this notation the single-pulse response in time-dependent field gradients becomes
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B. BLUMICH ET AL.
y(,)f f f Mo( ,
.
(5.9)
This equation is the starting point for the following discussion of spatial resolution.
5.1.1.2 Spatial resolution by frequency encoding Compared with Equation (5.1), Equation (5.9) has the same form, except that the thermodynamic equilibrium magnetization is considered to be spacedependent, and that a space-dependent phase modulation has been introduced as a result of applying time-dependent magnetic field gradients. If the FID f(t) were constant in Equation (5.9), the spin density could be retrieved by inverse Fourier transformation of the single-pulse response s(t), provided that the gradient modulation had been chosen in such a way that s(t) covers all values of the wave vector k, i.e., that the entire k space had been sampled. The approximation off(t) being a constant defines the limitations in spatial resolution of NMR imaging by frequency encoding. Here, the pulse response is acquired in the presence of a time-independent gradient, say G x - OB=/Ox. After the rf excitation pulse kx grows linearly with time according to Equation (5.8), kx(t)=-yGxt. Thus, k space is sampled in the direction of kx, and time t and kx are equivalent variables. If t is replaced by k, the pulse response (Equation 5.9) becomes
(5.10) Fourier transformation over k of this product of two functions produces the convolution of the Fourier transforms of each function. The Fourier transform of the FID f(kx) is the spectrum F(x), and the Fourier transform of the triple integral is a projection P(x) = f f Mo(x, y, z)dy dz of the spin density on to the x axis, where frequency and space are related by Equation (5.4). Then the Fourier transform of Equation (5.10) can be written as S(x) ~ fF(x - x')P(x') dx'.
(5.11)
A faithful representation of the spin-density projection P(x) would be obtained by this method of frequency encoding only if the spectrum F ( x - x') were a delta spike, or a narrow liquid-state NMR line.
NMR I M A G I N G AND S P A T I A L I N F O R M A T I O N
~0~
127
~O~X
Fig. 5. I. Effect of field gradients on the NMR signal for solids and liquids. The sample consists of a rectangular and a circular object. Left, homogeneous magnetic field. Right, linear magnetic field: (a) schematic drawing of the field dependence across the sample; (b) spectroscopic response for a sample with a single, narrow line in the NMR spectrum; and (c) spectroscopic response for a sample with a single, broad line in the NMR spectrum.
The mathematical treatment above is illustrated in Fig. 5.1. A circular and a square object are two proton containing samples in a homogeneous (left) and in a constant-gradient (right) field. If the samples are liquid, a narrow line is observed from both samples in a homogeneous field and a projection of the object is measured in the linear field. The signal amplitude at each resonance frequency or space coordinate is determined by the number of spins which experience the same magnetic field strength. If the spectrum in a homogeneous field is broad (bottom traces), then the convolution (Equation 5.11) of the projection of the spin density by the lineshape becomes noticeable, and the spatial features are smeared out. Thus, the minimum distance Ax which can be resolved is defined by the ratio of the width Ac0 of spectral features to the spread of resonances introduced by the gradient. In the case of a Lorentz line, it can be related to the transverse relaxation time T2 by Ax =
x
o/(vcx)= 2/(T
vCx).
(5.12)
The acquisition of projections in constant field gradients is the principle of the back-projection method [4]. Here, the spatial resolution is limited by the signal decay in a homogeneous field. But, for liquid samples, the final limit
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B. BLI]MICH ET AL.
is from translational self-diffusion during application of the magnetic-field gradients [3].
5.1.1.3 Spatial resolution by phase encoding Broadening of the projection by the NMR spectrum (see Equation (5.11)), can be avoided if the spatial information is detected indirectly in the fashion of multidimensional Fourier NMR [16]: A magnetic-field gradient Gy is switched on for an evolution time tl following an rf excitation pulse, and the signal is acquired in a detection time t2 after the gradient has been switched off. For this 2D scheme, the signal can be written as
S(tl, re) ocf(tl) f P(y) exp{iyGyytl} dy f(t2) 9
(5.13)
In this notation with time delays tl and t2, it becomes clear that there are two ways to vary ky = -yGytl: Either tl is varied in the custom of 2D NMR, or tl is kept constant and Gy is stepped in increments AGy. The first case bears no advantage compared to direct frequency encoding discussed above. As time tl proceeds, not only does ky increase, but also the free induction decay f(q) evolves and deteriorates the spatial resolution. But if tl is kept constant the evolution of the free induction decay is suppressed, and the phase change of the signal in the evolution period depends only on ky. Thus, the spatial resolution achievable for phase encoding depends only on the maximum gradient strength available. This scheme can be expanded for indirect detection in all three dimensions. It is very suitable for broadline solid-state imaging, and the method is referred to as single-point or constanttime imaging [17, 18]. Often phase and frequency encoding are combined for acquisition of 2D images (Fig. 5.2(a)). Then the gradient Gy is called the phase encoding gradient, and G~ is the frequency encoding gradient. Spatial resolution and artifacts from magnetic susceptibility and chemical shift differ in both dimensions. 5.1.1.4 Sampling k space Different imaging schemes are often discriminated against in the way k space is sampled. Two elementary 2D imaging methods are illustrated in Fig. 5.2" 2D-Fourier imaging (2DFI, Fig. 5.2(a)) and back-projection imaging (BPI, Fig. 5.2(b)). Orthogonal gradients are switched subsequently in 2DFI, and simultaneously in BPI. As a consequence, k space is sampled in orthogonal coordinates for 2DFI. When applied simultaneously, the direction of the resulting gradient changes depending on the relative strengths of the G~ and
129
NMR IMAGING AND SPATIAL INFORMATION
Rv]
.t~l~A^. . VVVVV
. . . .
Gxt2 .
.
.
Gyt 1
Gx
:
v
Gy i
9. . . . . . . . . . . . . . . .
i
;;; ;;;=~ ; ..........
~ kx
iil|l
t2 i
I,_
e
z
z
z
z z
z z
z z
z z
z
z
z
z
:
z
:
z
z :
z
z
:
z
z
=
:
:
: :
:
:
-
v
ky RF
b)
I~A^~ .... vVVv'-
. . . .
G• I
'
'
: z - _ - _
~
:-_-_----
~kx
q~= arctan ~ x Gy ] ,, , '
I
Fig. 5.2. Schemes for acquisition of 2D images: Left, excitation and detection schemes; right, maps in k space of the traces, where signal is acquired. (a) 2D Fourier imaging (2DFI). The y direction is detected indirectly by stepping the phase encoding gradient Gy in increments of 2~Gy from scan to scan. The x direction is detected directly by frequency encoding, k space is sampled in Cartesian coordinates, and the image is calculated from the data-set by 2D Fourier transformation. (b) Back-projection imaging. Usually, only frequency encoding is used and k space is sampled in cylindrical coordinates. The image is calculated by transformation of the data-set from cylindrical to Cartesian coordinates and subsequent 2D Fourier transformation.
Gy gradients. In this way k space can be sampled in cylindrical coordinates in BPI. In either case the image is obtained by Fourier transformation of the k-space data. In practice, the 2D Fourier transformation is carried out for Cartesian coordinates on a computer, so that the data-set acquired with the BPI method needs to be interpolated to a rectangular grid before Fourier transformation. Because in BPI the point density is higher near the k space origin, features with large dimensions are mapped with better signal-to-noise ratio. 5.1.1.5
Slice and volume selection
Objects are usually three-dimensional, and often 2D slices through an object need to be imaged. Magnetization from voxels defining the slice of interest can be selected in different ways. What is required is selective excitation and a field gradient. If a planar slice is to be selected, the field gradient is
130
B. BLUMICH ET AL.
Y -.-X
Fig. 5.3. Slices with magnetization from voxels forming a plane are selected by bandwidthlimited rf excitation in a gradient applied perpendicular to the plane. Repetition with different, orthogonal gradient directions selects a line or a single volume element.
applied perpendicular to the plane, so that the magnetization from all volume elements of interest is exposed to the same magnetic field strength. Due to the effect of the gradient, voxels neighboring the plane experience a different field and are, thus, characterized by different resonance frequencies. Now rf excitation is applied which is limited in bandwidth, so that only the magnetization within the slice voxels is affected. Such selective excitation can be generated by long pulses, shaped pulses, composite pulses or pulse sequences [10]. This scheme can be repeated for different gradient directions. Without repetition a slice is selected, with one repetition a line and two repetitions a voxel (Fig. 5.3). Thus, selective excitation is needed to prepare individual volume elements in an extended object for subsequent investigation by NMR spectroscopy [19]. This type of NMR with spatial resolution is called volu-
me-selective spectroscopy. Materials applications of imaging are often done without slice selection. For many studies, the sample geometry can be chosen with a suitable symmetry, so that a projection in one dimension will reveal the features in question. Because a projection corresponds to an integration along one spatial dimension, the signal-to-noise ratio of projections is usually superior to that of selected slices.
5.1.2
Special techniques
In practice, 2DFI can be used for investigations of soft materials such as elastomers or even polyethylene using suitable equipment. But as the solid-
NMR IMAGING AND SPATIAL INFORMATION
131
state linewidth becomes too large, frequency encoding has to be discarded, and even phase encoding becomes difficult as the gradient pulses must be short enough for sufficient signal to survive for detection. Thus, the use of echoes and line-narrowing techniques is a necessity for many imaging investigations of solid polymers in order to slow down the signal decay by eliminating homonuclear dipole-dipole interactions among the observed protons. Improvements in signal-to-noise ratio by signal averaging is complicated by spin-lattice relaxation times T1 which typically are long compared to T2. Therefore, the available signal in a given measurement time is rather low, extending most experiments to overnight runs. But when compared to biological samples this often is not a problem with polymers. The same applies to increasing the sample temperature to a value slightly lower than the glass transition temperature [20] to increase the motional averaging of the broadening interactions and, thus, increase the Tz/T1 ratio. Many methods for improvements of the spatial resolution in NMR imaging of solid materials have been proposed with different impact on applications in polymer science [8, 9, 11-13, 21]. For the great variety of techniques only those are reviewed in Sections 5.1.3 and 5.1.4 below, which are being applied successfully to problems in polymer science. Depending on the dominating features, the techniques can be classified into frequency and phase encoding approaches. Considering the principles of spatial resolution in standard NMR imaging shown in Fig. 5.2(a), a naive Gedankenexperiment demonstrates the basic problems for imaging of solid materials. Polyethylene ( L D P E - H D P E ) exhibits NMR linewidths of 20-60 kHz depending on the degree of crystallinity. In order to resolve spatial features of 100 txm by frequency encoding, a gradient strength of G = 4-14 T/m is needed, which increases the necessary receiver bandwidth for a 10-mm long sample to 2-6 MHz, and pulse durations for excitation need to be as short as 100-500ns. In addition to technical problems posed by achieving such extreme specifications, the signal-to-noise ratio would be lowered by a high noise level for such broad receiver bandwidths and by hardware dead times, because the signal would decay in such a strong gradient within a few 100 ns. The T2 of the polyethylene sample is of the order of 10-100 txs. Due to the rapid signal decay, the requirements for phase encoding are similarly demanding. To achieve a spatial resolution of 100 Ixm by phase encoding, strong gradient pulses (G = 5 T/m) have to be switched for an evolution time of 50 Ixs with rise times of about 1-10 Ixs. This is not yet state of the art in commercial spectrometers. But the signal decay of the solid-state resonance can be prolonged by linenarrowing through multipulse schemes or magic-angle spinning to average
132
B. BLI21MICH ET AL.
specific or all broadening interactions of the NMR spectrum except that of the space encoding gradients. Also, extremely strong gradients can be applied in combination with narrow-band excitation and detection as in stray-field imaging (STRAFI), or the lifetime of the NMR signal can be extended by echo techniques while phase encoding is applied. So far, only nuclei like 1H and 19F with strong homonuclear dipolar couplings have been considered. Dilute nuclei like 13C exhibit mostly heteronuclear dipolar interactions, which are of the order of 10 kHz and thus much smaller. They can easily be removed by dipolar decoupling in a double resonance circuit. However, the chemical shift anisotropy (typically 100200 ppm) still limits the spatial resolution and the low abundance the signalto-noise [22-24]. While it is possible to remove all spectral broadening interactions [11], the question of contrast has to be addressed. Depending on the sample features to be investigated, specific interactions may either not be removed or reintroduced by suitable spin manipulation in order to detect the sample heterogeneities in question.
5.1.3
5.1.3.1
Frequency encoding techniques for the solid state Stray-field imaging (STRAFI)
A straightforward approach to high spatial resolution in solid-state imaging is to enlarge the gradient strength. The strongest gradients in standard superconducting magnets are found in the stray field, where a position close to the edge of the super-conducting coil provides a relatively constant gradient strength over a larger diameter. For a 4.7T magnet a typical value for the gradient in the STRAFI plane is G = 40 T/m. Because this gradient cannot be switched, only frequency encoding is possible, which is limited to very small samples due to limitations in the excitation power and the receiver bandwidth [25]. This problem can be alleviated by using narrow-band excitation with pulse lengths of 10-100 txs, by which, in the presence of such a strong gradient, only a thin slice through the sample with a thickness less then 100 txm is excited. If the sample is physically moved through this sensitive plane, a projection is acquired point-by-point (Fig. 5.4) [26-28]. Due to the rapid decay of the FID in such huge gradients, echoes are a necessity to circumvent receiver dead-times and the signal-to-noise ratio can be further improved by sampling multiple echoes from echo trains [29, 30]. From the NMR point of view, repositioning of the sample can be performed quite fast, because the neighboring slices are excited in succession so that a recycle delay is unnecessary. But a mechanical limit is imposed by inertia. Because
NMR IMAGING AND SPATIAL INFORMATION
133
Fig. 5.4. Sketch of the STRAFI principle. The sample is mechanically (arrows) stepped through a sensitive plane perpendicular to the stray field gradient along the z-direction (left). The signal intensity from each consecutive slice forms a projection of the sample (right).
the direction of the gradient is fixed, the sample is also rotated in order to acquire 2D and 3D images by the back-projection algorithm. Although the sample projection is stepped in a slow, point-wise fashion, the advantages of this technique are its robustness, so that even highly paramagnetic samples [31, 32], or samples which contain metal parts, can be imaged [27], and its relative insensitivity to the linewidth, so that solids and liquids [33, 34], or even different nuclei with comparable y (e.g., 1H and 19F) can be imaged simultaneously [35]. A further important advantage is that for some magnet designs the STRAFI-plane lies outside the magnet housing, so that arbitrarily large planar samples can be investigated [36]. The disadvantages are the demanding mechanical apparatus which has to reposition the sample with very high accuracy and the problems in introducing contrast parameters other than T1 and the effective transverse relaxation time T2eff for multipulse excitation. 5.1.3.2 Oscillatinggradients Another way of realizing strong gradients is to oscillate them during the imaging sequence with a period in the order of T2, and enhance their stability and amplitude by a resonance circuit for that specific frequency [37]. Gradient echoes are generated automatically at the gradient zero-crossings, which in combination with additional rf pulses is exploited for efficient refocusing of transverse magnetization. The advantages of this method are strong gradients and convenient experimental setup. A limitation is imposed by the requirement, that the oscillation
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frequency has to be in order of 1/T2, which practically limits applications of the method to a regime from elastomers to soft thermoplastics. Furthermore, the data handling is not straightforward, because k space is scanned in a nonlinear way making interpolation schemes to Cartesian grids necessary, and the signal decay is given by T2, rather than T2, so that Bo inhomogeneities are detrimental. Phase encoding, which avoids problems with inhomogeneous Bo fields has been implemented by applying the phase-encoding gradient with twice the frequency [38, 39], and by the use of double resonance gradient circuits [40]. 5.1.3.3 Coherentaveraging by multipulse techniques Many interactions that cause line-broadening in the solid state can be averaged by manipulation of the magnetization in the rotating frame. The associated multipulse sequences are discriminated by the type of spin interaction which is averaged by them. It is useful to classify the interactions according to those which are linear in the spin variable, indicating that they do not depend on interactions with neighboring spins, and those which are bilinear in the spin variables and account for spin-spin interactions. Linear interactions are the chemical shift, magnetic susceptibility and the Zeeman interactions as well as the interaction of the spins with the external imaging gradients. From among the bilinear interactions, the dipolar and the quadrupolar couplings are of practical importance in polymers. There are multipulse sequences which remove only the linear spin interactions, for instance CPMG [41]. Others average bilinear terms, while the linear interactions are partially reduced. An example is MREV-8 [5]. Finally, there are sequences like CMG-48 [11, 42] and TREV-8 [43-47] which remove linear and bilinear interactions. They are called "time-suspension" sequences. This name gives credit to the fact that, except for relaxation, the entire time evolution of the magnetization in the rotating frame is halted. But for frequency encoding, magnetic field gradients have to be applied, and their effect must not be averaged. This can be achieved by pulsing the gradients and changing the sign of the gradient in suitable intervals [11, 42, 43, 48], or by using oscillating gradients synchronized to the sequence, so that the zerocrossing coincides with the applied rf pulses [49-53]. The use of rf-pulse sequence for removal of linear interactions in imaging has been demonstrated in cases like 13C NMR, where strong bilinear interactions are absent or can be easily removed [54, 55]. In this case, a simple CPMG-sequence is sufficient to remove linear terms like the chemical shift anisotropy by sampling the signal stroboscopically at every echo maximum while the gradients sign is inverted in every other interval. Application of multipulse sequences that remove the dipolar broadening and scale the linear
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terms have been reported from the very beginning of NMR imaging of solid samples [5]. However, the associated sc~aling of the linear terms reduces the strength of the gradient, unless gradient switching is introduced. By using time-suspension techniques, NMR images of polymers with very high spatial resolution could be obtained, because their linewidth could be reduced by almost three orders of magnitude [11, 42, 43]. A special version of that principle makes use of magic echoes [43-47] instead of solid echoes to refocus the effect of the bilinear dipole-dipole interaction. The magic-echo technique has the main advantage that in practice it is quite forgiving to phase and amplitude misadjustments of the rf excitation [46, 47, 56-58]. The benefit of frequency encoding by multipulse sequences is rapid acquisition with excellent spatial resolution even for rigid polymers. The disadvantages are relatively low sampling rates and a rather strong influence of the resonance offset on the line-narrowing efficiency, where the offset depends on the strength of the applied gradients unless pulsed or oscillating gradients are used. Elimination of most of these restrictions has demonstrated [59], but nevertheless, multipulse imaging is tedious to set up. Therefore, applications of these techniques are rare and limited to small samples. 5.1.3.4 Averaging in the laboratory frame by MAS imaging One of the technically most demanding approaches in NMR imaging is MAS synchronized imaging [15, 60-65]. Not only have the gradients to be matched in frequency and phase with the rapidly spinning turbine, but in addition attention has to be paid to the fact that rotating gradients are often produced by a system of coils which is aligned with the spinning axis. In this case the approximation of a gradient tensor by a gradient vector is no longer valid (cf. Section 5.1.1) and gradient driving functions more complicated than simple sinusoids along individual gradient coordinates need to be employed [66, 67]. Although the technical setup is quite complicated, the application of this technique is rather straightforward and gives excellent spatial resolution. Images can be obtained either by changing the phase between the rotor and the oscillating gradients to acquire different projection directions across the sample and reconstruct the image by back projection or by switching the phase of the gradients by 90 ~ while also changing their amplitude, when going from phase-to-frequency encoding. Especially when combined with crosspolarization, ~3C NMR imaging with spectroscopic information [15, 63-65] can yield unique insight into the spatial distribution of molecular orientation [68]. A principal problem of the technique is the restriction of the sample size, because it has to fit into a MAS rotor and be carefully balanced to provide high spinning speeds, while high centrifugal forces may cause sample defor-
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mations [67]. For real-life samples, the spinning speed seems to be limited to about 10 kHz, which allows one to investigate only solids with intermediate linewidths unless multipulse line-narrowing [48, 69] or sideband suppression techniques [15] are applied as well. Therefore, applications of this technique are limited to special geometries. MAS imaging schemes alternative to rotating gradients have been proposed on the basis of discreet [70] and continuous versions [71] of MAS by magic-angle hopping and turning, respectively, but no combinations with imaging have been published. 5.1.4
Phase encoding
5.1.4.1 Constant-time or single-point techniques As mentioned in Section 5.1.2, the spatial resolution in the phase-encoded dimensions depends only on the gradient strength and the length of the constant evolution time. While this suggests that infinitely good resolution can be obtained because the gradient strength can be made arbitrarily strong and the associated evolution time infinitely long in principle, the short lifetime of the solid-state NMR signal and the limited number of spins set a signalto-noise limit, and special techniques have to be applied again [17, 18, 72]. Almost all types of echoes have been exploited together with constant-time phase encoding, in particular the solid echo [51], Jeneer-Broekaert echo or spin-alignment echo [73], the magic echo [46, 47, 74], and rotary-MAS echoes [61, 75] have been used with excellent results. The advantage of using echoes is a gain in the length of the evolution time by refocusing the magnetization after a relatively long delay, which can be used for switching of gradients. Then the echo can be sampled with the gradients off, so that the signal-tonoise ratio improves and spectroscopic resolution can be obtained [56, 67, 76]. However, if the subsequent wideline spectrum has no interesting contrast features the acquisition of a single point is sufficient. The most promising echo appears to be the magic echo [46, 47] (cf. Fig. 5.5), because it refocuses the dipolar interaction also for multiple couplings and, therefore, affords the longest evolution times for abundant nuclei in solids, for instance ~'1 = 300 Ixs in Fig. 5.2(a) for T2 = 50 txs. Due to the long delay between initial excitation and formation of the echo, either strong static gradients Go (cf. Fig. 5.5) or strong rf gradients G1 can be applied for rather long times resulting in excellent spatial resolution with an experimental setup which is simple and applicable to larger samples as well. Combinations of the principal idea with elegant and fast acquisition schemes have been proposed for samples with short T1 and T2 relaxation times, for instance for samples containing high concentrations of paramagnetic ions [77]. The advantages of constant-time imaging are the easy setup, insensitivity
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to all spectroscopic distortions, the principal applicability to large samples with diameters up to 10cm [78], and the possibility of accessing contrast parameters from spatially resolved spectra. The only, but severe, disadvantage is the necessity to encode all spatial dimensions indirectly, leading to 2D, 3D and even 4D experiments.
5.1.4.2 Multiquantum imaging The principle of multiquantum imaging in terms of improved spatial resolution is the fact, that in a multiquantum state of the coherence order p, the precession frequency is p times faster than in a single quantum state. Thus, magnetization dephasing from a gradient applied during the period of multiquantum evolution in a suitable pulse sequence is p times faster or, vice versa, the effective gradient is p times stronger than for single quantum evolution. While the principle of this idea has been demonstrated relatively early [79], applications have been shown only recently. These utilize multiquantum coherences also for functionalized contrast [80-83], while accepting the improved spatial resolution as an additional benefit. The fact that multiple quantum coherences can only be measured indirectly by their influence on the amplitude and phase of a subsequently acquired single quantum signal makes this technique a phase-encoding method. The necessity to acquire the spatial information in time-consuming extra dimensions is a penalty in all phase-encoding techniques. However, in the case of double quantum imaging of quadrupolar nuclei like, e.g., 2H, the wideline information of the single quantum spectrum can be utilized for contrast, because the quadrupolar interaction usually dominates all other spectral features and is a sensitive probe for molecular dynamics and orientations [8083].
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5.1.4.3 Cutting the sample In general, it can be stated that there is no perfect sequence or technique that serves all needs in NMR imaging of solid polymers. The spatial resolution, sample size, temperature ranges, available hardware, and the desired contrast necessary for a particular investigation have to be kept in mind, when choosing a specific technique. The conservative but valid question "why not just cut the sample to the desired voxel size and run a regular spectroscopic experiment?" has to be always considered first in practice. This applies in particular if the expected spatial features can be easily separated, if the sample has no specific value, and if the quantities to be measured are not destroyed by cutting, like e.g., strain or temperature distributions. While cutting the sample appears cumbersome for planar resolution, it is quite common practice in slice selection for materials applications of NMR imaging. 5.1.5
Diffusion and flow
The methods discussed above refer to solid and soft polymers. But there is also great interest in investigations of the liquid state and the rheologic behavior of viscous, molten or dissolved polymers. NMR imaging techniques can be functionalized in such a way that their contrast is a direct measure of flow or molecular self diffusion [10]. On the other hand, diffusion, convection and flow pose a severe problem for spatial encoding because motion of the spins during the imaging sequence smears out spatial features. While diffusion defines the spatial resolution limit in liquids [10], coherent motion of spins can be compensated for [84, 85]. The motion of liquids can be included as an additional contrast feature into NMR imaging sequences or detected in extra dimensions. This knowledge is important to study restricted diffusion in pores to determine their size [86], shear rates and alignment in flowing polymers [87-90], thermally induced convection [91], technical devices involved in polymer processing like nozzles, mixers and extruders [92-94], or polymer products like dialysis membranes, pipes and filters [85, 95-97]. Excellent treatments about flow and diffusion imaging have been published [10, 981.
5.1.6 5.1.6.1
Contrast Contrast parameters
Compared to other image-forming methods, the unique feature about NMR imaging is the abundance of parameters which can be exploited for image contrast [10, 99]. These parameters are mostly of molecular nature and linked to the chemical and physical properties of the sample. Examples of molecular
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chemical parameters are the chemical shift and the indirect spin-spin coupling. Molecular physical parameters are lineshapes, relaxation times and the strength of the dipole-dipole interaction. The latter is the fundamental quantity by which distances can be probed either on a molecular level by the dipole-dipole coupling tensor or on a mesoscopic level by spin diffusion. Nonmolecular contrast parameters are the spin density, differences in magnetic susceptibility and local magnetic fields inscribed by electric currents in the sample. Transport parameters of molecular translational diffusion and flow can be probed to inquire about macroscopic heterogeneities. But these NMR parameters are useful only insofar as they serve to detect inhomogeneities which are invisible to imaging techniques less expensive than NMR. For interpretation of NMR images, it is necessary to link these parameters to material properties. This is a general topic of NMR in materials science and not particular to imaging itself. Material properties can be divided into those describing a state of matter and into those describing the change of matter. Examples for state parameters are correlation times of molecular motions, the degree of molecular orientation, the shear modulus, the viscosity, the cross-link density of elastomers, the distribution and agglomeration of filler particles, the pore-size distribution, the temperature distribution and the thermal conductivity. Parameters attributed to the change of matter are the characteristic times of physical aging describing phase transitions in microcrystals and kinetic constants of chemical reactions describing chemical aging by thermo-oxydative processes, chain scission in polymers upon heating, the kinetics of cross-linking during rubber vulcanization and curing times for thermosetting polymer materials. 5.1.6.2 Contrast optimization The contrast AM is defined as the relative difference in image intensities M of neighboring structures i and j [1], AM = [M(r,) - M(rj)]/lMmax I ,
(5.14)
where Mmax is the maximum of M(r/) and M(rj), and r~ and rj are the space coordinates of the voxels i and j under consideration. To exploit the manifold of contrast features, it is useful to optimize the imaging experiment for generation of maximum contrast. A generic scheme applicable to Fourier imaging is illustrated in Fig. 5.6. Three time-periods are distinguished, which are reminiscent of 2D NMR [16]: A filter period, a space-encoding period and a spectroscopy period. During the filter period the initial magnetization to be imaged is manipulated by rf excitation, so that only part of the magnetization survives. For instance, this may be the
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magnetization filter
space encoding
r
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magnetization of molecules with high molecular mobility, or with weak dipole-dipole interactions between abundant spins. During the space-encoding period, the gradients G are switched on for slice selection and space encoding. At this stage the spatial resolution is determined (cf. Section 5.1.1). During the spectroscopy period, a complete FID is acquired from the magnetization of the molecules selected in the filter period and with the spatial resolution inscribed during the space-encoding period. The FID transforms into a spectrum for further analysis of chemical shifts, lineshapes, moments and other parameters. For image contrast other than the often uninformative spin density, the space-encoding period needs to be combined with either a magnetization filter or with spectroscopic data acquisition or with both. Methods by which NMR spectra are measured in a systematic fashion for contingent voxels by imaging techniques are referred to as spectroscopic imaging methods. 5.1.6.3 Magnetization filters Filters are given by pulse sequences which, in most cases, generate longitudinal magnetization which differs from the thermodynamic equilibrium state in some specific way [99, 100]. Many different filters can be conceived and combined to produce parameter weights of spin density images. Basically, any pulse sequence can be considered a magnetization filter, but particularly useful ones are T1 filters by saturation or inversion recovery; T2 filters by Hahn echoes, or repetitive Hahn echoes, following the method by Carr, Purcell, Meiboom and Gill (CPMG);
141
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-
-
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T~p filters for the longitudinal relaxation time in the rotating flame by applying spin-lock fields or multisolid echoes; combinations of relaxation-time filters and multipulse excitation; chemical-shift selective excitation; selection of multiquantum coherences for signal suppression from single quantum transitions like water; coherent and incoherent forms of homo- and heteronuclear magnetization exchange and magnetization transfer; and the encoding of molecular transport from diffusion and flow.
Of particular importance for detection of chemical or physical change in polymer materials are mobility filters, which are sensitive to differences in the numbers of molecules within a given window of correlation times. Within reasonable approximation such filters are relaxation filters. Here, T1 filters are sensitive to differences in the fast motion regime while T2 and T~o filters are sensitive to the slow motion regime. Which time window is of importance can be seen from Fig. 5.7 [101]. It shows a double-logarithmic plot of the mechanical relaxation strengths HI(~') for two carbon-black filled styrenebutadiene rubber (SBR) samples as a function of the mechanical relaxation time 7. They have been measured by dynamic mechanical relaxation spectroscopy. In terms of N M R , the curves correspond to spectral densities of motion. But the spectral densities relevant to N M R are mainly those referring
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to fluctuations of magnetic dipolar fields. Thus, NMR and mechanical spectral densities are similar but not identical. The two spectral densities shown in Fig. 5.7 are for a fully aged and unaged carbon-black filled SBR. The relaxation strength for the aged material is lower in the intermediate motion regime corresponding to T~ processes and it is significantly enhanced in the slow motion regime, which is probed by T2 and T~p. Indeed, the same trend is observed by NMR relaxation measurements [101], confirming the close interrelationship of NMR and mechanical relaxation. Because the difference in spectral densities is largest for long correlation times, T2 and T lo filters are most suitable for generation of contrast in imaging of SBR. Similar observations apply for contrast in solid polymers in general. 5.1.6.4 Parameter contrast Considering for example the simultaneous effects of a T1 filter from partial saturation when using short recycle delays, to, and the signal attenuation by T2 relaxation during a Hahn echo with echo time tE, a signal weight is introduced by which the acquired signal (Equation 5.13) needs to be multiplied with, W(to, tE) = [1 -- exp{-to/Tl(r)}] exp{--tE/Tz(r)}.
(5.15)
The delays to and tE are extrinsic filter parameters which can be adjusted in the pulse program, and Tl(r) and T2(r) are intrinsic filter parameters which are specific to the object. An image acquired with one fixed set of extrinsic filter parameters is a parameter-weighted image. Here, the signal amplitude in the image is determined by the spin density and the intrinsic filter parameters of the voxels. But spin-density effects can be eliminated completely if several parameter-weighted images are acquired for different values of extrinsic filter parameters. By fitting a given relaxation function, often a simple exponential, to the signal amplitudes of each voxel for the set of images, the intrinsic filter parameters can be determined and displayed in image form. In this way, pure T1, T2 but also spin-density images can be obtained. Such images are called parameter images, because the distribution of a particular NMR parameter is shown. NMR parameter images can be translated to material property images by calibration or relationships known from theory. For example, cross-link density can be linked to the transverse relaxation decay [101-103] and the longitudinal relaxation decay in the rotating frame [104, 105]. Relaxation of transverse magnetization in cross-linked elastomers is nonexponential (Fig.
NMR I M A G I N G AND SPATIAL I N F O R M A T I O N
143
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Fig. 5.8. Top: Transverse relaxation curve calculated for a given cross-link density (diamonds) and multiparameter fits: The Gauss-Lorentz fit (broken line) agrees well at short times. The biexponential fit (continuous line) agrees well at long times. Bottom: T2 relaxation curves of the CH group for nine differently cross-linked samples of unfilled SBR samples. The effective number Ne of Kuhn segments per cross-link chain varies between 9.43 and 15.56 corresponding to vulcameter moments between 16.5 and 1 dNm. By renormalization of the time axis the master curve (top) has been obtained.
5.8, top) [103]. It can be interpreted in terms of the residual dipolar coupling which remains unaveraged by the often fast but always anisotropic motion of chain segments between cross-links [106]. At short times the decay is Gauss-like, at longer times it is exponential. The variance of the Gaussian is a measure for cross-link density [107], and it has been shown that the relative amplitude of the Gaussian is also proportional to cross-link density [101]. On the other hand, it was found that the relaxation time of the exponential T2 decay at long times is also a function of the cross-link density [102]. It turns
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out that the entire nonexponential decay can be described by a function of time over effective number Ne of Kuhn segments in a cross-link chain [103], and cross-link density can be obtained from a single-parameter fit. Because Ne normalizes time in this model, the relaxation curves from elastomers with different cross-link densities are described by a master curve as a function of t/N~ (Fig. 5.8, bottom). Because the effective number of chain segments is determined by the number of chemical and physical cross-links it depends on temperature: It decreases at higher temperature as physical cross-links become less effective and then settles at a plateau value which is determined by the chemical cross-link density.
5.2
Applications
Applications of NMR imaging outside the medical field have been published in a variety of books and journals [8-13, 21, 86, 108-115]. A rather important subject for applications of NMR imaging is polymer science, as polymers are rich in protons, the most sensitive, stable NMR nucleus, and many polymer materials are often soft, so that the homonuclear dipole-dipole interaction among protons is partially averaged by molecular motion. Thus, polymers are far more suitable to NMR imaging than for instance ceramic materials. 5.2.1
Fluids
The liquid state plays an important role in polymer science, because polymers are often produced from liquid monomers and some polymer products are later exposed to liquids, which absorb to different extents. Furthermore, an important field of research is the rheological behavior of liquid polymers like lubricants, oils or the character and dynamics of swelling. If only the distribution of the liquid is important, standard imaging sequences can be used and the signal from the more solid polymer can be usually suppressed by making use of its short Y2 relaxation time. 5.2.1.1 Liquid distribution in solids A simple but often very successful approach in imaging of solid materials is the use of liquids as contrast agents, because they can be investigated by standard techniques. Porous structures like foams, internal cracks, voids or other spatial features can easily be analyzed in this way [116]. Furthermore, the local separation of liquids in pastes depending on external forces has been demonstrated in extrusion processes of PTFE/water pastes [94]. This kind of investigation is based on the assumption that the interaction of the
NMR IMAGING AND SPATIAL INFORMATION
145
liquid with the solid can be neglected and the overall structure of the specimen is not influenced. But polymer-solvent interactions arise frequently, and are a subject of investigation in itself, because swelling and solvation have a pronounced impact on chain mobility. Therefore, NMR relaxation rates are sensitive indicators for liquid uptake and solvent interaction. Plasticizing effects of the polymer matrix, and also reduced mobility of the liquid molecules, can be observed and easily distinguished by the use of deuterated solvents. The spatial variation of the cross-link density in elastomers has been examined by this method [109, 117-123], and the solvation effect of deuterated cyclohexane on the different chemical main and side chain groups in a polystyrene matrix has been determined by chemical shift selective imaging [124]. Almost all polymers are exposed to more or less aggressive atmospheres. Quite often degradation is caused by the absorption of liquids or their vapors [125-130]. The amount, spatial distribution and interaction of small molecules with the polymer matrix is an important indicator of polymer aging as demonstrated by an epoxy/water system [131]. Another important application is the spatial distribution of polymer binders such as waxes and oils in green-state ceramics [132,133], because an inhomogeneous distribution in the prefinished state is likely to cause mechanical failure of the final product.
5.2.1.2
Time resolved swelling of polymers
The rate and characteristics of swelling play an important role in the characterization of polymers. However, in traditional swelling experiments, detection of the swelling front and investigations of its influence on the polymer dynamics are difficult, while the characterization of these features plays a significant role in distinguishing between so-called "Case I" or Fickian and "Case II" diffusion [134]. Imaging of the swelling behavior of elastomers [109, 117-123] has been demonstrated to give information superior to classical experiments, because the progress of liquids is often inhomogeneous due to variations in local cross-link density, and concentration-dependent diffusion constants have been found by analyzing the shape of the diffusion front [118, 123]. While the ingress of liquids in elastomers is diffusion controlled and thus Case I, because the polymer chains are well above the glass transition temperature Tg, liquid transport into glassy polymers is more complex [112, 135]. The swelling behavior below Tg is characterized by a sharp boundary between the remaining polymer core and the swollen region which proceeds with constant velocity. This behavior is also called relaxation controlled because the diffusion is much faster than the segmental relaxation of the polymer.
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Fig. 5.9. Different snapshots of the diffusion of acetone into a polycarbonate rod (diameter = 10.7 mm and a height of 10 mm) after (a) 1 h; (b) 2 h 50 min; (c) 19 h 29 min; (d) 21 h 25 min; (e) 28 h 12 min; (f) 46 h 47 min; and (g) 61 h 23 min. The surrounding solvent has a different T1 and was suppressed by an inversion recovery sequence. (Reproduced with permission from Ercken et al. [134].)
This is confirmed by a decrease of the T2 relaxation rate of the polymer towards the unswollen core. The swelling of a glassy polymer is illustrated in Fig. 5.9 by a series of NMR images through a polycarbonate rod after exposure to acetone. Only the swollen regions with high molecular mobility are visible in the images. The formation and the progress of a high intensity ring at the interface between swollen and unswollen regions indicates Case II diffusion. After 21 h the rod cracks due to the strain between swollen and unswollen parts with severe consequences on the diffusion geometry [134]. Furthermore, the transition of Case-II to Case-I diffusion has been observed with temperature increasing towards Tg in a PMMA/methanol system [136]. 5.2.1.3 Polymerization reactions The formation of a solid polymer from liquid monomers can be followed by a decrease of the molecular mobility and, thus, by T2. In polymer processing the design of molds, the kind of polymerization initiation by radicals, heat or light, and the thermal conductivity of the polymer and the mold play a
NMR IMAGING AND SPATIAL INFORMATION
147
significant role for the homogeneity of the resulting product. Local viscosities and reaction rates can be observed by NMR imaging of the intensity or the relaxation of the liquid content. Inhomogeneities [137] and traveling waves of the initiator [138] have been investigated during the formation of PMMA, and the spatial variation and the dynamics of a photo-induced polymerization have been followed in another example [139]. The curing of adhesives [112], and the influence of embedded fibers on the local polymerization rates of epoxy resins [140, 141], have been investigated in a similar way. A related subject is the vulcanization process of elastomers (cf. Section 5.2.2). 5.2.2
Soft matter
NMR imaging is favorably applied to investigations of soft matter, because linewidths and relaxation times are more like those in liquids than in rigid solids. A particularly suitable class of materials is elastomers, where NMR imaging is applied routinely in industry. 5.2.2.1 Cross-link densities Elastomers are cross-linked macromolecules above the glass transition temperature. The cross-link density is the fundamental engineering quantity which, for instance, determines the modulus of elasticity. Usually, it is measured during vulcanization of well-defined rubber samples in a vulcameter by the moment necessary to perform a given torsional shear of the test sample. Swelling experiments can be performed alternatively, but are problematic for filled elastomers. Such measurements are based on the assumption that the measured quantity does not vary over the sample volume. Inhomogeneous cross-link densities can be determined from the surface hardness, but volumetric resolution is achieved by conventional methods only after cutting the sample. Given the low ratio of cross-linked to uncross-linked chain segments, direct spectroscopic detection of cross-link densities is difficult [142-144], but the relaxation times Y2 [101-103] and Tip [104, 105] are good indirect probes for cross-link densities. Quantitative results can be obtained by NMR imaging from parameterizations of relaxation curves for each voxel (cf. Fig. 5.8) [101, 145-147]. 5.2.2.2 Thermal exposed 0.25-0.3
Thermo-oxidative aging aging of styrene-butadiene rubber (SBR) and natural rubber (NR) to air leads to a hard surface layer which grows in thickness up to mm with aging time [101, 148, 149]. Growth and hardness of this
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layer can be imaged nondestructively by NMR. Following this layer in the interior of the samples a narrow region of increased molecular mobility is observed in SBR and NR, which indicates chain scission. In many cases the rubber core corresponds to more or less unchanged material.
5.2.2.3 Time and space dependence of the vulcanization process The vulcanization of carbon-black filled SBR cylinders has been followed in situ by N M R imaging [150] by observing the transverse relaxation time T2 in a slightly inhomogeneous magnetic field. In a separate experiment, the relaxation rate was calibrated against the reduced vulcameter moment for different vulcanization times of a thin sample which cures homogeneously in good approximation. The vulcameter moment is known to be proportional to crosslink density. Thus, T2 could be used to follow changes in cross-link density in time and space for a larger sample during the vulcanization process. The results of the experiments are summarized in Fig. 5.10 [150]. The sample geometry are shown (top) and the NMR-contrast parameter T2 as a
Fig. 5.10. Top: sample geometry. The sample thickness is 1.4 mm. Bottom: Space dependence of the contrast parameter T~ during the vulcanization. In a rough approximation T2 is inversely proportional to cross-link density.
NMR IMAGING AND SPATIAL INFORMATION
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function of space for different curing times (bottom). Heat has been supplied from the right and dissipated to the left. Initial sample heating (5 min) leads to an increase in segmental mobility and, thus, to larger values of T2. With the onset of vulcanization (15 min) segmental mobility and T2 decrease. The vulcanization front is recognized in the traces by the regions with the steepest slopes. From effects of thermal conductivity the slope decreases with time, moves through the sample, and the vulcanization front is smeared out. Investigations of this type can provide valuable information for the optimization of vulcanization processes.
5.2.2.4 Failure of rubber products Failure of technical rubber products can be influenced by differences in crosslink density. After swelling, such differences can be detected by Tz-weighted NMR imaging in a comparatively short time (20 min). As an example, Fig. 5.11 depicts two NMR images of slices taken through a gasket, which has been damaged in an overload test and swollen in technical oil [151]. The swollen region is recognized by bright image intensities. A detailed analysis shows that the different constituents of the oil diffuse into the rubber network with different velocities. In the undamaged part of the gasket (left) brightness increases from left to right. This corresponds to a gradient in segmental mobility which is likely to account for a gradient in cross-link density. In the damaged part (right), a fracture is noticed in the middle and dark stripes which identify significant hardening of the material. Both features are likely to have arisen during the overload test.
Fig. 5.11. NMR slices through a rubber gasket swollen in technical oil. The gasket has been exposed to an overload test. Left: Undamaged region. Right: Damaged region.
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5.2.2.5 Imaging of temperature under dynamic shear conditions The properties of elastomers under dynamic rfiechanical load are essential for many applications of rubber products. Properties determined for thick samples under dynamic load are averaged quantities due to a temperature distribution across the sample extensions. The prevailing temperature field arises from a balance of heat generation by the (temperature-dependent) loss modulus, the space-dependent thermal conductivity, and the heat dissipation through the sample surfaces. An 1H-NMR probe for in situ imaging of cylindrical elastomer samples under dynamic shear and compression has been developed [152]. The sample size and mechanical deformations realized correspond to those of standard testing procedures. Typical shear rates are 1-10 Hz. For carbon-black filled SBR, T2 is directly proportional to temperature. Thus, a T2-parameter image corresponds to a temperature image of the sample. Axial parameter projections have been acquired in dynamic equilibrium at a shear rate of 10 Hz, and a pixel resolution of 0.4 • 0.4 mm 2, for SBR cylinders with carbon-black contents ranging from 0-50 phr. 1D crosssections through those projections are depicted in Fig. 5.12. An increase of temperature in the center of the sample is observed with increasing carbon-
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~
4
6
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Fig. 5.12. Temperature profiles across axial projections of carbon-black filled rubber cylinders at 10 Hz dynamic shear.
NMR IMAGING AND SPATIAL INFORMATION
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black contents which scales with the increasing loss modulus of the samples. For a composite sample an unexpected temperature distribution has been detected [152]. 5.2.2.6 N M R imaging of stress and strain The detection of stress distributions in polymer materials underlines the unique contrast features available through NMR imaging. Stress distributions were first mapped by Tz-parameter images and subsequent calibration to stress images [153]. T2 depends on strain, because strain effects the timescale and anisotropy of segmental dynamics in the intercross-link chains (cf. Section 5.1.6). The problem of a T2 analysis of stress and strain is that strain is a tensor and T2 a scalar. Thus, a tensorial property is mapped onto a scalar. This problem can be overcome by measuring anisotropic and stress-dependent NMR parameters for generation of image contrast. Spatial displacements, and the influence on the anisotropy of the molecular diffusion rates, can be used for this purpose in ductile gels [154] and biological samples. In more rigid materials interactions like the chemical shift anisotropy and the quadrupolar coupling constant can be exploited for this purpose (cf. Section 5.2.3). A tensorial NMR parameter suitable for stress and strain imaging is the strain- and orientation-dependent quadrupole splitting of deuterons. Deuterated butadiene oligomers have been incorporated into household rubberbands by swelling in order to apply spectroscopic imaging and doublequantum imaging of deuterons to detect stress and strain distributions under applied strain [82, 83]. Double quantum (DQ) imaging suppresses signals from unstrained segments and is very sensitive to small strains. A DQ filter for 2H has been designed by modification of an I N A D E Q U A T E experiment, and qualitative NMR images of stress were acquired by measuring DQ-weighted images. By correcting these images for spin density and T2, effects, it is possible to obtain semiquantitative information about the local stress of the examined material (Fig. 5.13). The results have been confirmed by "finite element matrix" calculations (FEM), which were carried out at the Institute for Polymer Processing (IKV) in Aachen. 5.2.3
Hard matter
Hard matter presents the most serious challenge to ~H NMR imaging because of line-broadening from dipolar coupling. Signal-to-noise ratio and spatial resolution are often inferior unless dedicated solid-state imaging techniques are used (cf. Section 5.1.2). But even then the achievable resolution is rarely better than ca. 100 p~m and typical measurement times are of the order of a
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t~tg. 5.13. DQ image of a rubberband with one cut corrected for spin density and relaxation effects (left). FEM calculation of stress in an identically shaped sample (right).
day. Nevertheless, despite these difficulties, interesting applications of NMR imaging to rigid polymers can be identified. 5.2.3.1 Imaging of shear bands in polycarbonate The magic-echo technique is convenient to use on many solid-state spectrometers for imaging. It has been applied in combination with phase encoding in one space direction and multimagic-echo frequency encoding in the other space direction for imaging of shear bands in a sample of cold-drawn polycarbonate [57, 155]. Two-dimensional parameter images were obtained with the transverse multisolid-echo relaxation time T2eff as a contrast parameter which is sensitive to slow molecular motion [156]. In Fig. 5.14(a), a spin density image of the investigated piece of polycarbonate is shown, while in Fig. 5.14(b) the parameter image of T2eff is depicted. The spin density image reflects the different material thickness and shape of this sample, but no information on shear bands can be extracted. However, a shear band is clearly visible in the parameter image along the diagonal of the sample from the upper-left to the lower-right corner. This is in agreement with the optical impression of the sample in a polarizing microscope with crossed polarizers. Detailed analysis of the relaxation times indicates that the position of the plastic deformation, given by the shear band, leads to a reduction of the local mobility of the polymer in addition to the change in mobility from cold drawing. This example demonstrates that phase-encoded multimagic-echo imaging is a suitable technique for detection of morphological changes in rigid polymers. This technique of space encoding can be applied in combination with many contrast filters.
153
NMR I M A G I N G AND SPATIAL INFORMATION .
Illll
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5.2.3.2 Spectroscopic imaging of molecular orientation The multimagic echo space-encoding technique has been extended to include two spatial and one spectroscopic dimension for image contrast based on chemical shift [76]. Space was encoded by a series of magic echoes with gradient pulses applied in the delays between rf pulses. Because these delays are short, the several gradient pulses were stepped for phase encoding of the spatial resolution. For good spatial resolution the effects of chemical shift and dipole-dipole interaction were eliminated by including a 180~ spin-echo refocusing pulse in the middle of the phase encoding period. The homonuclear dipole-dipole interaction has been eliminated by the use of another multimagic-echo sequence during the detection time, leaving only the chemical shift evolution of the magnetization. Thus, from the spectroscopic dimen-
154
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sion of a 19F-spectroscopic image oriented and unoriented regions in polytetrafluoroethylene (PTFE) could be discriminated by analysis of the anisotropy of the chemical shift. This approach has been used to investigate stress propagation during crack growth in PTFE. The spectroscopic images provide a quantitative measure of the molecular orientational distribution function without resorting to a moment analysis of spectra from a series of experiments with the sample oriented at different angles with respect to the Zeeman field [157]. The method lends itself for applications to a broad variety of physical systems, e.g., to spatially resolve chemically distinct regions in incompatible polymer blends. 5.2.3.3 Imaging with oscillating gradients With oscillating gradients, echo times could be reduced from 2 ms to 128 txs on a solid-state imaging system. When echo times are as short as this, many polymers can be imaged by Fourier techniques adapted to oscillating gradients [39]. This has been demonstrated by images of a children's toy Lego | brick made from ABS plastic. The transverse relaxation curve of this material displays two significant components in which the relaxation times of 90% of the signal a r e T 2 - - 7 6 0 txs and of the remaining 10% a r e T2 = 6 ms. By oscillating-gradient imaging both components can be acquired. Figure 5.15 shows three-dimensional surface-rendered images of the sample. A total of 32 averages was acquired in about 4 h on a 32 x 64 x 64-point grid. The field of view for the images is 32 x 32 x 32 mm. A worthwhile increase in signalto-noise ratio should be possible by using gradient-echo imaging with low flip-angle pulses (FLASH) [3]. 5.2.3.4 Spin diffusion imaging of polymer morphology Spin diffusion is the transport of nuclear magnetization through space, which is mediated by the dipole-dipole interaction. This phenomenon can be ex-
Fig. 5.15. Three-dimensional, surface-rendered images of a Lego| brick with dimensions 15 x 15 x 11 mm3 obtained with oscillating gradients. (a) Top view. (b) Bottom view [39]. (Reproduced with permission from Mallet et al.)
NMR IMAGING AND SPATIAL INFORMATION
155
ploited for analysis of polymer morphology [64]. To this end, a spatial nonequilibrium state of nuclear magnetization is generated in the sample by application of a magnetization filter (Section 5.1.6). In a subsequent evolution period equilibration of the magnetization across the sample is observed. Distributions of polymer morphology can be imaged by combination of nanoscale magnetization gradients in a spin-diffusion experiment with macroscopic space encoding by external field gradients [58]. This method has been tested on block copolymers and semicrystalline polymers with macroscopic distributions in domain sizes and crystallinity. Chemical shift filters, as well as mobility filters, have been used for generation of nanoscopic magnetization gradients. A practical application of this method is the analysis of treeing from electrical aging in low-density polyethylene, which is used for insulation of high-power cables. A standard test sample was subjected to over 100,000 electrical discharges until final damage occurred by high-voltage breakthrough. Significant changes in the morphological structure were observed in the region affected by electrical treeing and subsequent breakthrough. A spatial distribution of the domain sizes was derived from the fits of a sandwich model of amorphous, interracial and crystalline nanolayers to the experimental data. The results are illustrated in Fig. 5.16 by a drawing of the sample dimensions (a), and by the layer thicknesses as a
a
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Fig. 5.16. Imaging polymer morphology by spin-diffusion contrast on a sample of electrically
aged polyethylene. (a) Sample for electrical aging in needle-plate geometry and region cut out for spin diffusion imaging with one-dimensional spatial resolution. (b) Spatially resolved distribution of the domain sizes derived from fitting theoretical diffusion curves of a sandwich layer model to the experimental data. Pronounced changes in the thickness for crystalline, interracial and amorphous layers are obtained [58].
156
B. BLI]MICHET AL.
function of space across the affected region (b). In the damaged region, the size of the rigid domains decreases from values of about 13.8 to 11.2 nm. The size of the interface decreases from about 2.4 to 0.2 nm, while that of the mobile domains increases from about 2 to 5.8 nm. While this interpretation of the spatially resolved spin diffusion data is bound to the model used, this work demonstrates that spin diffusion is another valuable NMR contrast parameter by which polymer morphology can be mapped, so that changes in polymer morphology from physical aging can be investigated by NMR imaging.
5.2.3.5 Imaging of solid rocket fuels Imaging of 1H and 27A1 has been combined for investigations of the homogeneity of solid rocket fuel, which is a composite material of aluminum filler particles and other chemicals within a polymer matrix [158]. Good homogeneity is crucial as an inhomogeneous distribution of components exerts large effects on the quality and properties of the propellant. Because 1H Fourier imaging provides only part of the information, the investigations were supplemented by 27A1 NMR imaging. The linewidth of the 27A1 NMR free-induction decay of finely powdered aluminum is more than 13 kHz at 52 MHz. This is too broad for standard Fourier imaging methods, which rely on gradient switching during spin-echo times. The 27A1linewidth is dominated by direct dipole-dipole couplings between 27A1 nuclei. Thus, homonuclear multipulse decoupling by MREV-8 was used. But the results showed that the spatial resolution was somewhat limited by imperfections of line-narrowing. Solid-state 27A1imaging is promising but will require further study to identify its application potential to a variety of materials of interest.
5.2.3.6 NMR imaging of coal Coal is a sample representative of hard materials which may be also electrically conductive. A central question of interest in coal imaging is the distribution of soft and hard matter. The heterogeneity of coal can be investigated by 3D 1H NMR imaging of samples swollen in deuterated and protonated solvents. For swollen samples, Fourier and back-projection techniques can be applied. T1 and Y2 prove to be good contrast parameters in these experiments, and image contrast produced by differences in the mobility of the macromolecular network in the coal have been compared with image contrast produced by differences in solvent accessibility in the same specimen [159]. Untreated coal samples present a serious challenge to NMR imaging, but may be addressed by the stray-field techniques.
NMR IMAGING AND SPATIAL INFORMATION
5.3
157
The N M R M O U S E
The examples presented, and extensive experience with NMR imaging of polymers, show that the most important contrast parameters are relaxation times. Measurement of these NMR parameters does not require homogeneous magnetic fields because they can be measured in inhomogeneous fields by echo methods. Highly homogeneous magnetic fields are required only for determination of chemical structure by chemical shift measurements. In addition, spatial resolution benefits from inhomogeneous fields. Thus, onesided application of Bo and B1 magnetic fields to large objects can be exploited for discrimination of material properties by NMR [160]. The NMR mobile universal surface explorer (NMR MOUSE) is a novel detector developed for this purpose [161]. It consists of two permanent magnets which supply the polarizing B0 magnetic field and a surface coil inbetween the gap of the magnets for excitation and detection. By using steadystate free-precession (SSFP) sequences, modified for use in inhomogeneous fields [162], longitudinal and transverse relaxation curves can be measured. In elastomers, the transverse magnetization decay can be used to characterize cross-link density, stress and temperature. The relaxation time decreases in an approximately linear fashion with increasing cross-link density, it increases linearly with temperature, and the relaxation rate scales with the elongation /~2 A-l, where A is the ratio of lengths before and after straining. Because of strong gradients in the polarizing Bo magnetic field (10-15 T/m), translational diffusion can be measured as well and the liquid-state NMR signal is efficiently suppressed in emulsions and suspensions [163]. In impact-modified polystyrene, stress whitening could be detected [161], and even the presence of ferromagnetic objects is not necessarily an obstacle to the NMR MOUSE: Material change from weathering could be identified by changes in T2 of a 0.5-mm thick PVC coating of a 1-mm thick sheet of iron (Fig. 5.17) [164]. Methodical developments aim at increasing the sensitivity for discrimination of small changes of material properties. Thus, from an NMR point of view, pulse sequences need to be applied in inhomogeneous Bo and B1 fields which average the interactions linear in the spin operator (magnetic field gradient, chemical shift) and maintain the bilinear ones, where the most important one is the dipole-dipole interaction. In this respect, the XY-16 multiecho sequence [165] has proven to be very suitable for use with the NMR MOUSE. Scanning of depth is achieved by changing the excitation and detection frequency and lateral resolution is obtained by displacement of the scanner. Thus, spatial resolution by the NMR is achieved by means of inhomogeneous magnetic fields as well as localized detection within the sensitive volume of the surface coil.
158
B. B L U M I C H E T A L .
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-
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-
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Fig. 5.17. Application of the NMR MOUSE to measurements of transverse relaxation in PVC coats on iron sheets. Top" Geometry of the MOUSE and the sample. Bottom" Transverse relaxation curves for unaged PVC and PVC aged by weathering.
5.4
Summary
The principles of spatial resolution and contrast in NMR imaging have been presented in this chapter. An overview of selected applications of NMR to investigations of fluid systems, technical elastomers and rigid polymers has been given. The examples chosen demonstrate the potential of NMR for measurement of macroscopic properties of polymer materials. The importance of developments of NMR methods and equipment for materials science applications was underlined by example of the NMR MOUSE. A considerable part of the work presented has been topics of diploma theses, PhD theses and postdoctoral work in Mainz and Aachen. We acknowledge the contributions of G. Eidmann, C. Ftilber, L. Gasper, A. Guthausen, D. Hauck, M. Klinkenberg, S. Laukemper-Ostendorf, K. Rombach, R. Savelsberg, U. Schmitz, G. Zimmer and support by the Deutsche Forschungsgesellschaft and the Deutsche Kautschukgesellschaft.
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B. BLUMICH ET AL. B. Bltimich and P. Bltimler, Die Makromolek. Chem. 194 (1993) 2133. D.G. Cory, J.B. Miller and A.N. Garroway, Macromol. Symp. 85 (1994) 271. J.L. Koenig, Macromol. Symp. 85 (1994) 283. J.L. Koenig: Spectroscopy of Polymers, ACS Publications, Washington DC, 1992, and references therein. S.W. Sinton, A.W. Chow and J.H. Iwamiya, Macromol. Symp. 86 (1994) 283. Magnetic resonance imaging of materials, special issue, Solid State Nucl Magn. Reson. 4 (1996). D. Mtiller, Bruker Report 140 (1994) 18. S.R. Smith and J.L. Koenig, Macromolecules 24 (1991) 3496. A.G. Webb, P. Jezzard, L.D. Hall and S. Ng, Polym. Comm. 30 (1989) 363. A.G. Webb and L.D. Hall, Polym. Comm. 31 (1990) 422. R.S. Clough and J.L. Koenig, J. Polym. Sci. C27 (1989) 451. M.R. Krejsa and J.L. Koenig, Rubber Chem. Tech. 64 (1991) 635. M.A. Rana and J.L. Koenig, Macromolecules 27 (1994) 3727. M.R. Halse, H.J. Rahman and J.H. Strange, Physica B 203 (1994) 169. G. Ding, S. Mao, L. Li and C. Ye, Chinese Sci. Bull. 41 (1996) 891. S. Blackband and P. Mansfield, J. Phys. C: Solid State Phys. 19 (1986) L49 W.P. Rothwell, P.R. Holeck and J.A Kershaw, J. Polym. Sci., Polym. Lett. Ed. 22 (1984) 241. L.A. Weisenberger and J.L. Koenig, Appl. Spectrosc. 43 (1989) 1117. L A. Weisenberger and J.L. Koenig, Macromolecules 23 (1990) 2445. K.P. Hoh, B. Perry, G. Rotter, H. Ishida and J.L. Koenig, J. Adhes. 27 (1989) 245. F. Tabak and M. Corti, J. Chem. Phys. 92 (1990) 2673. C.A. Fyfe, L.H. Randall and N.E. Burlinson, Chem. Mater. 4 (1992) 267. D.G. Cory, in D.N. Grant and R.K. Harris (Eds), Enc. Magn. Reson. 2 (1996) 1226, John Wiley, New York. See also references therein. W.A Ellingson, P.S. Wong, S.L. Dieckman, J.L. Ackerman and L. Garrido, Ceram. Bull. 68 (1989) 1180. M. Ercken, P. Adriaensens, D. Vanderzande and J. Gelan, Macromolecules 28 (1995) 8541. T.H. Mareci, S. DCnstrup and A Rigamonti, J. Mol. Liquids 38 (1988) 185. M. Ercken, P. Adriaensens, G. Reggers, R. Carleer, D. Vanderzande and J. Gelan, Macromolecules 29 (1996) 5671. P. Jackson, N.J. Clayden, N.J. Walton, T.A. Carpenter, L.D. Hall and P. Jezzard, Polym. Intern. 24 (1991) 139. B.J. Balcom, T.A Carpenter and L.D. Hall, Macromolecules 25 (1992) 6818. U. Giinther and K. Albert, J. Magn. Reson. 98 (1992) 593. P. Jackson, J. Mater. Sci. 27 (1992) 1302. A. Mavrich, F. Fondeur, H. Ishida, J.L. Koenig and H.D. Wagner, J. Adhes. 46 (1994) 91. R.S. Clough and J.L. Koenig, Rubber Chem. Tech. 62 (1989) 908 W. Gronski, U. Hoffmann, G. Simon, A Wutzler and E. Straube, Rubber Chem. Technol. 65 (1992) 63. M. Mori and J.L. Koenig, Rubber Chem. Technol. 68 (1995) 551. S. Hafner, Magn. Reson. Imag. 13 (1995) 739. W. Kuhn, P. Bart, S. Hafner, G. Simon and H. Schneider, Macromolecules 27 (1994) 5773.
NMR I M A G I N G AND SPATIAL I N F O R M A T I O N
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147. W. Kuhn. P. Barth, P. Denner and R. Mfiller, Solid State Nucl. Magn. Reson. 6 (1996) 295. 148. P. Blfimler and B. Blfimich, Macromolecules 24 (1991) 2183. 149. P. Blfimler, B. Blt~mich and H. Dumler, Kautschuk + Gummi, Kunststoffe 45 (1992) 699. 150. C. Ffilber, K Unseld, V. Herrmann and B. Blfimich, Colloid & Polymer Science 274 (1996) 191. 151. A Guthausen, NMR-Bildgebung an Gummiformteilen, Diplomarbeit, Lehrstuhl ffir Makromolekulare Chemie. RWTH-Aachen, 1996. 152. D. Hauck, P. Blfimler and B. Blfimich, Macromol. Chem. Phys. 198 (1997) 2729. 153. P. Blfimler and B. Blfimich, Acta Polymerica 44 (1993) 125. 154. T.G. Reese, V.J. Wedeen and R.M. Weisskopf, J. Magn. Reson. B 112 (1996) 253. 155. F. Weigand, U. Wiesner and H.W. Spiess, Adv. Mater. 8 (1996) 481. 156. F. Weigand, S. Hafner and H.W. Spiess, J. Magn. Reson. A 100 (1996) 201. 157. D.G. Cory, J.B. Miller and A.N. Garroway, Macromol. Symp. 86 (1994) 2.59. 158. S.W. Sinton, J.H. Iwamiya, B. Ewing and G.P. Drobny, Spectroscopy 6 (1991) 42. 159. D.C. French, S.L. Dieckman and R.E. Botto, Energy & Fuels 7 (1993) 90. 160. G.A. Mazkanin, in P. HOller, V. Hauck, C.O. Rund and R.E. Green (eds), Nondestructive Characterization of Materials. Springer Verlag, Berlin, 1989. 161. G. Eidmann, R. Savelsberg, P. Blfimler and B. Blfimich, J. Magn. Reson. A 122 (1996) 104. 162. A. Guthausen, G. Zimmer, P. Blfimler and B. BliJmich, J. Magn. Reson. 130 (1998) 1. 163. G. Simmer, A. Guthausen, P. Blfimler and B. Blfimich, to be published. 164. G. Zimmer, A. Guthausen, U. Schmitz, K. Saito and B. BliJmich, Adv. Materials 9 (1997) 387. 165. T. Gullion, D.B. Baker and M.S. Conradi, J. Magn. Reson. 26 (1990) 469.
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Chapter 6
Multi-nuclear N M R
Chapter 6. I
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
1H NMR B.C. Gerstein Iowa State University, Ames, IA 50011-3111, USA
6.1.1
Introduction
A chapter on high resolution proton NMR in polymers was written by the author in 1986 [1], and by Dybowski in 1991 [2]. Subsequently, Ibbett [3] has edited a volume on NMR spectra of polymers, Bovey [4] has written a volume on the subject, as has McBrierty [5], a masterful treatment of multidimensional techniques in NMR of solid polymers has been authored by Schmidt-Rohr and Spiess [6], and the use of solid state NMR to image polymers has been summarized by Garroway [7], so the literature in the field is understandably already rich. The three areas in which, to the author's knowledge, advances have been recently made have been: (a) an understanding of the limitations of resolution of proton NMR in solid polymers; (b) the use of multiple quantum coherence among dipolar coupled protons to study polymers; and (c) developments in the use of spin diffusion to elucidate domain structures in homopolymers, and shed light on miscibilities in polymer blends. Along with a brief review of past work, each of the more recent developments will be discussed in turn. In writing this chapter, it is assumed that the prospective audience is composed of those having great familiarity with polymer science, but less acquaintance with recent developments in solid-state NMR. A basic familiarity with the pulse NMR experiment is assumed, such that the physical import of the term "a 90x pulse" is understood to be one that places the magnetization of a system initially polarized along z, the direction of the static field, along the y direction in the rotating frame such that the resulting transverse magnetization may be detected at some phase, e.g., y, as the time decay of magnetization, My(t)~ (Iy(t)), the so-called "free induction decay". In order to understand how proton NMR of polymers may be used as presented in the current chapter, it is useful to remind ourselves of some of the results of the formalism used in describing the time-dependent quantum mechanics of spin which are not so well understood by those familiar with only the rudimentary pulse NMR experiment. We do so in Section 6.1.2. In Section 6.1.3, examples are given of the uses of the mechanics outlined in Section 6.1.2 to provide information about chemical functionality, motion
1H NMR
167
and morphology via isotropic chemical shifts, multiple quantum coherence and spin diffusion. The explicit development of the time-dependent quantum mechanics necessary for the present chapter has been presented previously. The reader is referred to those sources [1, 8] as well as in Chapters 2 and 3 of the current volume for the full development. However, for the purpose of self-consistency, and for the intended audience of this chapter, a terse summary of the results necessary for clarification of later sections is presented in the hope that the basic ideas and results obtained using them, if not understandable in detail, may be seen at least as believable possibilities.
6.1.2
Review of the time-dependent quantum mechanics of spins
6.1.2.1
The density operator and average Hamiltonians
6.1.2.1.1 The density operator We generally observe the result of a measurement on an ensemble of spins, so we first ask "what do we directly observe in an N M R experiment?" The answer is always a voltage induced in an inductor by the component of a magnetic moment perpendicular to the static field which is parallel to the z axis; V(t) ~ M~,y(t). This is to say that our signal is proportional to a component of spin angular momentum which: (a) is perpendicular (transverse) to the static field direction, and (b) varies with time. The signals we detect in an N M R experiment are those from which the carrier frequency has been demodulated. This is to say that the signal is detected in the interaction frame of the Zeeman Hamiltonian, or the rotating frame. For example, with phase detection along y in the rotating frame, and after a 90~ excitation pulse, we detect a signal proportional to the expectation value of the y component of spin in the ensemble of moments under study, the free induction decay (FID), FID --- My(t) ~ (Ty(t))- Tr{fi(t)]y}.
(6.1.1)
We emphasize that Equation (6.1.1) applies to the density operator described in the rotating frame. From this formalism it is immediately obvious that we are invoking the use of the quantum statistical method, in that the density operator, fi, must be used to describe the phenomena discussed here [9]. The operator t~ is defined as the ensemble average of the operator product ]q~)(~]; /5= [~)(*l
(6.1.2a)
B.C. GERSTEIN
168
Because the internal interactions which characterize proton NMR are all small compared to the Zeeman interaction, perturbation theory to first order may be used, and 1~) is given by a superposition of orthonormal Zeeman states, ]I, k) - Ik), in an n-dimensional spin space; F/
= 2E
C lk),
(6.1.2b)
k=l
and thus, Pnm =
CnCm*
9
(6.1.3)
The bar above the product indicates an average of an ensemble of identical systems prepared all in the same specified manner. The time-dependence of the density operator, which by Equation (6.1.1) directly yields the observed experimental signal in the rotating frame, is obtained as usual by solving the time-dependent Schr6dinger equation, idl~)/dt = 7flO), (7f in units of radial frequency) which in density operator form; becomes
idfi/dt = [7f, fi].
(6.1.4)
Here, the terms in 7f, which yield the observed NMR signal, are the internal interactions which are responsible for the richness of information available from NMR, the best known being the isotropic chemical shift. When 7f is time-independent the solution of Equation (6.1.4) is
fi(t) = exp -{iTft}. fi(0), exp {iTft}.
(6.1.5)
As usual, the frequency content of a time-dependent signal is obtained via Fourier transformation [10].
6.1.2.1.2
Internal Hamiltonians
The internal interactions, symbolically designated as 7f~, which provides the wealth of information available from NMR, but which are also responsible for broadening of solid-state spectra of protons in solids may be written as a constant, CA, times a product of coordinate space and spin space operators. In terms of the coordinate space irreducible spherical-polar tensor operators; R~,m(~, q~), and the spin space irreducible spherical tensor operators Tk,m(I, m), internal Hamiltonians may be written in the form [11]
1H NMR
169
k
~A : CA ~ k
~
(-1)mR2,-m(O, &)T2,m(I, m).
(6.1.6)
m=--k
For our purposes, A designates the anisotropic chemical shift (CS), dipolar (D) interactions. The chemical shift implicitly contains the anisotropic molecular susceptibility which places a natural limit on the resolution of proton NMR of solids under conditions of high resolution as discussed by VanderHart [12]. For example, the form of the secular terms in the two-body homonuclear dipolar Hamiltonian in the frame of the Zeeman interaction is ~DII
0)19(3 COS2 012 _ 1)(~1, 1 2 - 3IzlIz2) 9
(6.1.7)
With r12 the magnitude of the internuclear vector between spins j and k, and 1~12 is the angle between the internuclear vector and the applied external field, the dipolar frequency OD = hyZ/r312. We see that in this case the terms in R2,_m(O, ~b) in Equation (6.1.6) become (3 COS2 O 1 2 - 1), and the terms in T2,m(I, m) become (I1" I a - 3I~1Iz2). 6.1.2.1.3
Average Hamiltonians [13, 14]
The NMR signal detected from an ensemble of protons after a resonant excitation, governed by Equations (6.1.1) and (6.1.5), will, in general, result in a broadened signal in the solid state associated with dephasing caused by dipolar and shielding Hamiltonians. However, it is possible to utilize rf pulses, and motion of the sample, to attenuate the effects of such broadening. Such "rotations in spin, and in coordinate space" have the effect of making the internal Hamiltonians time-dependent; ~Cint ~ ~int(/). When the interactions driving the system are time-dependent, then Equation (6.1.5) is no longer valid as a solution to Equation (6.1.4) and a more general formalism must be used [15]. The solution to this problem was supplied by Waugh [13] in one of the most useful concepts to become available to NMR spectroscopists, that of the average Hamiltonian. The technique was to use a perturbation expansion [16], in which the first term in the expansion replaces the product, (~t), in the exponential of Equation (6.1.5) by an average form, integrated over some time interval; i.e., to a form involving IZIint--(1/tc) f dt~.nt(t), where ~'nt(t) is the internal Hamiltonian in question as manipulated by the experimenter, and the integral is over a period, tc, over which the average of the internal Hamiltonian becomes the effective value which drives the time development of the system as it is observed. The formalism used to ac-
B.C. GERSTEIN
170
complish this description is outside the limitations of this chapter, but the idea behind it is quite simple. One simply arranges for some time-dependence of the system under study by either rf excitation which makes spin operators, T~,m(I,m), time-dependent [17], e.g., the use of a ~r refocusing pulse to produce a spin echo, or by motion in coordinate space, e.g., sample rotation such as magic-angle spinning (MAS) [18] or magic-angle turning (MAT) [19], such that the requisite time-averaging of the coordinate space operators, R~,m(O, q~), is accomplished. In this case, when observed at time intervals ntc, n = 1, 2 , . . . , over which the requisite averaging is accomplished, the form of Equation (6.1.5) becomes to first order fi(ntc) = exp --{/hin tntc}" ~(0). exp {/hint
ntc}.
(6.1.8)
Depending upon the information desired, different types of averaging may be used. For example, in the case of achieving high-resolution NMR of protons in solid polymers, one wishes to remove shielding anisotropy and dipolar coupling. This is to say that one wants to make both the chemical shift Hamiltonian, ~cs, and the homonuclear dipolar Hamiltonian ~DII timedependent such that over some period of time, tc their time averages are their isotropic values. The isotropic value of the chemical shift is Ocs, the value observed in the liquid state. The isotropic value of the homonuclear dipolar interaction is zero. It is important to understand, as illustrated by the author in a study of crystallinity of polyethylene [1], that in the case of homogeneous dipolar broadening caused by the coupling of many protons, the time over which this averaging must take place must be short compared to the inverse of the homogeneous dipolar line width. For example, for a dipolar linewidth of 50 kHz, the averaging period, tc, must be short compared to 20 ~s. When this averaging condition is satisfied, the time-dependent Hamiltonian, ~int in Equation (6.1.4) becomes replaced by time averages,
(1/tc) f dt ~ c s ( t ) =
O'cs, and
(l/re)f
dt ~DII(/)
-- O.
Then, when the system is observed periodically at times ntc, n = 1, 2 , . . . , it behaves as if the internal Hamiltonians in question were their time averages. The result is a liquid-like spectrum in which the broadening associated with shielding anisotropy and dipolar interactions have been severely attenuated, and the observed signal is due to isotropic chemical shifts. Ideally, residual broadening would be lifetime broadening associated with longitudinal relaxation, the time constant being Tx.
1H NMR
171
On the other hand, (vide infra) in the case of developing multiple quantum coherence of coupled groups of protons, one wishes an average Hamiltonian for the homonuclear dipolar interaction, f dt 3~Dii(t), which is not zero, but is manipulated such that the density operator develops multiple quantum coherence as time progresses, as outlined in the next section. 6.1.2.2
Single and multiple quantum coherence
One of the more recent applications of proton NMR in solid polymers has been the use of multiple quantum coherence (MQCOH), which is developed among clusters of dipolar coupled protons (vide infra) to infer details of chain dynamics [20], and (which is another way of making the same statement) local dipolar couplings [21]. In order to begin to understand the ideas behind, and information available from experiments probing M Q C O H in polymers, let us remind ourselves of the meaning of "phase coherence" in quantum mechanics. We start with the 1 simplest case, a noninteracting ensemble of spin ~ systems, and with spin basis functions that are eigen functions of the largest interaction present, the Zeeman Hamiltonian ~ z . These are [I, m) -1~,1 g) 1 ---[a), and 1~, 1 - g1) -= ll3>. A 1 spin g system will have single particle wavefunction Iw) = C~ei~la ) + Ct~ei~l/3),
(6.1.9)
where y and ~b are the phases of the two possible single-particle basis set functions [a) and [/3), and the Ck will in general be time-dependent. And in general for the kth basis state, Ik), in a many state system, the amplitude factor is Ck(t) = Ck(t)e i4~k.
(6.1.10)
A kth single-particle wavefunction in which the global phase, 6~, on the two basis functions is the same, IqJk) = eiSk(1/~/2){lak)-
i[/3k)},
(6.1.11)
is a phase-coherent superposition of the two states which differ by k = 1 in z component of angular momentum. For this special combination, which may be shown to be produced by a 7r/2 pulse along x in the rotating frame [22], we find <~k IIyk [Wk > - 2~ 9
(6.1.12)
172
B.C. G E R S T E I N 1
In an ensemble of spin ~ noninteracting systems represented by such a phasecoherent superposition, the many-particle wavefunction is a product of the single-particle states, [q,> = II~,[q~k).
(6.1.13)
Then for such an ensemble, we find the average to be (Iy)
=
(q'l]ylq')
= (1/N)l-It, e-iak(1/V'2){(akl- i(/3kl}{E iyk}eiak(1/X/2){lak{ + i[/3k)} 1 --2~
(6 91 914)
because the phase factors cancel! Another way of stating this is that p~t3 = (1/N)ZlC~klei~k[Ct3~le-i+k = C ~ C ~ 4: 0 .
(6.1.15)
Note that in general, if Yk 4: ~bk = 8k, then the phase factors do not cancel, and the above sum is a superposition of sines and cosines which will average to zero, the random phase case. The creation o f multiple-quantum coherence is thus equivalent to causing p to evolve in such a manner that for r - s > 1, Prs :J: O.
(6.1.16)
This inequality is achieved much the same way single quantum coherence 1 was achieved for an ensemble of spins ~. An ensemble of systems for which 1 . the z component of angular momentum can be >~ is exposed to some appropriate rf excitation. This excitation results in an average Hamiltonian for the two-body dipolar Hamiltonian being manipulated in such a manner that the resulting state of any system, when observed at multiples of appropriate cycle times ntc, is a superposition of states in which the difference in the z component of angular momentum is >1. The means of achieving multiple quantum coherence specifically for dipolar coupled systems has been discussed by Hwang and Gerstein [23], but will be briefly summarized here. In short, Equation (6.1.8) is of the form eABe-A, where A and B are operators. This form may be expanded as [24] eABe - z - B + [A, B] + [A, [A, B]]/2! + .
(6.1.17)
1H N M R
173
With the density operator at zero time being the equilibrium density operator in a static field, /geq~ Iz, we apply Equation (6.1.17) to Equation (6.1.8), with I2Iint ~ IZID t o find that if IZID is allowed to cause time-development of the system for times ntc, then at these times
fi(ntc) = Iz + [f-ID, Iz]ntc + (1/2!)[ISID, [[IZlD, Iz]](ntc) 2 + .
(6.1.18)
The value of n, set by the experimenter, determines how long the system evolves under IZlD. The terms in IZID in Equation (6.1.18) are simply those involving a sum over two-body products, (Equation (6.1.7)). These contain internuclear distances rq, an average over the orientational factors (3 c o s 2 l~)ij - 1), and the two-body spin operators ( i i " Ij -- 3Izfizj), as manipulated by rf-pulse sequences. Now Iz = ZkIz~, and similarly, the two body terms will involve a sum over all two-body products in the coupled systems. The first term in Equation (6.1.18) is Iz = Zkiz~, which having only diagonal elements in a Zeeman basis set, represents populations. The next and higher terms will be the two-body products of the commutators of the average dipolar Hamiltonian, IZlD, with Iz = EkIzk. For example, if the average dipolar Hamiltonian coupling spins k and j is manipulated to be proportional to the sum of products of the stepping operators (I~-If + Ik-If), then with the commutation relation [iz, I---]- -+I• the first commutator in the series will give rise to two-body double quantum terms such as T~-If. The two-off-the-diagonal-term d~,j 4: 0. Furthermore, commutators will give rise to higher orders of MQCOH, in this case of even orders 2, 4 , . . . ; M Q C O H is propagated in units of two quanta. The development of a single quantum propagator has been described by Hwang and Gerstein [23]. The development of MQCOH depends on the details of local dipolar couplings. The intensities of the M Q C O H spectra as a function of time taken to develop MQCOH, and coherence order, k, I(k) vs. k, will reflect exactly the details of these local couplings. The detected quantity in an NMR experiment is a single quantum coherence. Therefore, the detection of orders of M Q C O H greater than 1 involves first the excitation of M Q C O H from populations, Iz "~ Peq ~ ~IVIQ, under the appropriate average dipolar Hamiltonian, HD, and the subsequent reconversion of the orders of M Q C O H back into populations; P~o ~ Peq, under the negative of I2ID. An interrogation pulse then produces a single quantum coherence which is engineered (vide infra) to reflect the multiple quantum coherence which has been developed in the production period. The general idea is illustrated in Fig. 6.1.1. The initial intensity of the observed signal, (Iv(tl,~ t2 = 0)) will oscillate in a manner which reflects the M Q C O H developed during the excitation period followed by the evolution period. With the
174
B.C. GERSTEIN
90x interrogation Excitation of MQCOH from populations; P*a " Iz OMQ(to) under average dipolar Hamiltonian, HD, Av.(to)
Evolution of oscillating MQCOH
Reconversion of --"'[ ~ulse MQCOH into ~1\[ ] \
populations under- HD,Av.
OMQ~OMQ(to,t~I OMQ(t~)~ p(to ,h) ,,
t=0
nt~ (---
tl
"--) (--
nt~
~.jt'~ >t2
--)
Ineremen
O 9
9
9
9
9
Q 9
0
NL
0
9
+
Fourier Transtbrm on tl ==~1(ol)
I(c0D
to
0
Fourier Transtbrm on to :=> I(k)
I(k)
0
oa
k=l
2
[ I i 3
4
5
Fig. 6.1.1. Schemes for obtaining MQ spectra in the frequency domain, I(~o~) and in the MQ coherence number domain, I(k) (see text).
evolution period, q, set to zero, the phases, q~k, of pulses in subsequent excitation periods can be varied such that (Iy(t~ - 0, q~k, t2 = 0)) ~ ([y(q~, k)). The raw data are then points taken as a function of the phase increments during the excitation period, as indicated at the right center of Fig. 6.1.1. A Fourier transform on q~ will then yield a plot if I(k) vs. k, as shown on the righthand side of the bottom of Fig. 6.1.1 [23]. On the other hand, with no incrementation of phases during the excitation
1H NMR
175
period, the M Q C O H developed at time ntc, the end of the excitation period, can be allowed to oscillate under whatever interactions are present during the evolution time, tx. Then the detected initial intensity of the single quantum signal, (Iv(t1, t2 -- 0)), mapped as a function of tl, will be an interference pattern reflecting all orders of M Q C O H present, as indicated in the centerleft of Fig. 6.1.1. A Fourier transform on tl will then produce the more familiar plot of I(Wl) vs. Wl for the order(s) of M Q C O H which are produced and detected. One such spectrum for double quantum coherence, centered at o~1 = 0 is shown at the bottom-left of Fig. 6.1.1. Of course, if the total decay during observation time t2 is observed, the array I(tl, t2) may be Fouriertransformed to obtain the 2D plot I(cox, w2). Note that in either case, the detection of M Q C O H is inherently a pointby-point process in which the detected array of initial intensities, I(q~, k), or I(q, t2 = 0) is transformed on q~or on tl to produce the desired representation of the "multiple quantum spectrum". Finally, both phase during the excitation period, and tl during the evolution period may be simultaneously varied in a manner which enables the experimenter to detect specific orders of M Q C O H in a final representation of I(oJ1) vs. Wl to produce, e.g., "double quantum filtered" spectra (vide infra) .
6.1.3
6.1.3.1
Proton NMR in polymers
Isotropic chemical shifts; High resolution solid-state NMR
Exactly those qualities which make the proton the easiest to detect are those which lead to its relative uselessness, compared, e.g., to that of 13C, in providing information via the most common fingerprint in NMR, the isotropic chemical shift, in rigid solid polymers. This is not to say that isotropic chemical shifts of 1H are not obtainable for protons in rigid solid p91ymers; they are [25]. But to obtain such spectra, broadening associated with homogeneous homonuclear dipolar coupling, as well as with shielding anisotropy must be minimized by the use of homonuclear pulse-decoupling, combined with magic-angle spinning [26] as outlined in Section 6.1.2.1.3. In addition, as discussed by VanderHart [12], the limiting resolution associated with molecular magnetic susceptibility anisotropy, e.g., of aromatic rings, can be roughly two orders of magnitude worse than that of the system in the liquid state, even when the averaging to zero of homonuclear dipolar and anisotropic shieldings is accomplished without experimental imperfections. The limits on resolution associated with experimental imperfections in the removal of homonuclear
176
B.C. GERSTEIN
t
9
t
1
-=~-~o-800-6~o :,~o 'z'~
~T
t
1
2Do
(PPM)
.|
.
,oo
J
6oo 8oo doo
Adipic Acid C02H
f
1
lO
o
PPM
Fig. 6.1.2. Comparison of high resolution solid-state NMR of 1H in a glassy polymer containing aromatic rings with an aliphatic crystalline compound; CRAMPS spectra of isotactic polystyrene (top), and adipic acid (bottom).
dipolar coupling, via rf pulses, thus making the time average of the spin operator (ii" t j - 3I~I~j) equal to zero for each dipolar coupled pair, (i, j), has been recently discussed by Prigl and Haeberlen [27]. The conclusion is that in the absence of other broadening mechanisms, dipolar linewidths of 0.5 ppm, and perhaps 0.3 ppm, may be achievable. An example of an NMR spectrum of 1U in isotactic polystyrene, as compared with that of the nicely crystalline compound adipic acid is shown in Fig. 6.1.2. These spectra were taken under conditions in which line-broadening associated with homogeneous homonuclear dipolar coupling, as well as with shielding anisotropy was minimized by the use of homonuclear pulse decoupling, and magic-angle spinning [26].
1H NMR
177
One immediately notes that the resolution of the spectrum of the crystalline compound adipic acid is better by a factor of at least ten compared to polystyrene. Furthermore, compared to species in the liquid state, the linewidth is still a few orders of magnitude higher than that achievable by a reasonably adjusted liquid-state NMR spectrometer. The reasons for this limitation in resolution have been mentioned above. First, in the glassy polymer polystyrene, there is a dispersion of isotropic chemical shifts associated with the noncrystalline nature of the material. In addition, the anisotropic molecular magnetic susceptibility of the aromatic ring, and not the perfection of the dipolar decoupling technique, is the limiting factor for the polystyrene. The nicely crystalline adipic acid does not have a dispersion of isotropic chemical shifts associated with the lack of crystallinity. In addition, being completely aliphatic, there is not a limiting molecular susceptibility anisotropy. It suffices to note here that the above limitations on resolution exist, so that if the desirable quantity from proton NMR of polymers is the isotropic chemical shift, the limitation must be kept in mind. There is a way, however, of circumventing the problem of resolution of proton NMR in the CRAMPS experiment alone, when the material under study is a rigid solid. This path uses the fact that the range of chemical shifts of 13C is an order of magnitude higher than that of 1H. The residual broadening, in terms of ppm, is the same for 1H as for 13C in polymers. Therefore, the isotropic chemical shifts of 13C supply a much more delicate probe of chemical functionality than do those of 1H. It is thus possible to utilize a more time-consuming two-dimensional HETCOR technique [28], in which high resolution solid-state proton NMR in one dimension are correlated with the high resolution solid-state NMR of a3C in a second dimension, thus, tying the innately higher resolution of the carbon spectra to the proton resonances, as shown for poly(2,6-dimethyl-p-phynelene oxide) in Fig. 6.1.3. The limits of resolution of both carbon and hydrogen in this case, in ppm, are basically the same, and determined by the anisotropy of the local molecular magnetic susceptibility. But because the range of shielding of carbon relative to hydrogen is so large, this susceptibility anisotropy, which can be a few ppm for nuclei in aromatic rings, is relatively unimportant for carbon compared to hydrogen. On the other hand, there are circumstances when multiple pulse decoupling is not necessary to achieve a resolution of 1H in polymers comparable to that achieved by CRAMPS. This is the case when the polymer in question exhibits sufficient mobility in the solid state that the reduced dipolar coupling associated with such motion [30], when coupled with the added motion of MAS at a frequency greater than the shielding anisotropy, results in a spectrum
178
B.C. GERSTEIN
o
("'
/
3
J
_
_=
- 0
I
-~_:_~.
tt
_
-
r.~
r .,..,
- 6
-~ - ! 0
'
i
o
~
~
,
13C Chemical Shift (ppm) Fig. 6.1.3. Solid HETCOR spectrum of poly(2,6-dimethyl-p-phenylene oxide). Note that the CRAMPS spectrum for protons in this sample, shown on the left axis, is a superposition of peaks belonging to different carbon species, as indicated by the 2D separation on the horizontal axis [29].
comparable in resolution to that obtainable by CRAMPS. One such example is that of the block copolymer poly(styrene-b-methylphenylsiloxane) (PS-bPMPS) [31]. In this system, the polystyrene component is relatively rigid, but the polymethylphenylsiloxane is sufficiently mobile that under MAS, but with no pulse decoupling of the dipolar interactions, a CRAMPS-like spectrum is achieved for the PMPS fraction, as shown in Fig. 6.1.4. These results lead one to wonder if one technically easy means of achieving relatively high resolution proton NMR of polymers might not be to imbibe a deuterated solvent into the polymer which would render the material more motionally labile than would be the polymer in the absence of solvent, and then use fast MAS for narrowing. Another might be, of course, to increase motion utilizing increased temperatures, and use MAS alone. In summary, high resolution solid-state NMR of 1H in rigid polymers is achievable using averaging techniques to remove shielding anisotropies and proton-proton dipolar couplings. But the resolution is at least two orders of
1H NMR
179
PMPS
-
i0
5
PPH
Fig. 6.1.4. Room temperature N M R
0
PS
-5
of *H in (top) poly(methyl phenyl siloxane), (middle),
a polystyrene, poly(methyl phenyl siloxane) (PS-b-PMPS) block copolymer, and (bottom) polystyrene, under single-pulse excitation and magic-angle spinning (MAS). The PMPS is sufficiently mobile that in the pure polymer, and in some samples of the block copolymer, anisotropic molecular motion, when combined with MAS at 4 kHz severely attenuates both dipolar broadening and shielding anisotropy [31].
magnitude worse than in comparable liquids, and is limited by experimental artifacts in the narrowing schemes, and by molecular susceptibility anisotropy which is the limiting factor when aromatic rings are present, even when experimental artifacts are absent. In addition, in some rubber-like polymers, there exists sufficient molecular motion reducing average dipolar coupling such that this motion, when combined with MAS, results in CRAMPS-like spectra. 6.1.3.2
Applications of multiple quantum coherence
As indicated in Section 6.1.2.2, the appearance of multiple quantum coher1 ence in dipolar coupled spin 5 solids is a direct reflection of the strength of the dipolar couplings present. In the presence of molecular motion, which makes the coordinate space term (1 - 3 cos 20ij) time-dependent; (1 - 3 cos 2 Oi,j) ~ (1 - 3 COS20i,j(t)), then it is the time average of the dipolar Hamilton-
180
B.C. GERSTEIN
ian for each i, j spin pair which determines the effective dipolar coupling. The result is that the dipolar coupling is weakened by molecular motion [30], and thus the time taken to develop a given total intensity of M Q C O H in the presence of motion, will be greater than that in the absence of motion. This total intensity is directly reflected in the initial intensity of the signal (]y(ntc, t2 = 0)) observed after production, and collection of M Q C O H as outlined in Section 6.1.2.2. This fact has been used by Roy and Gleason [32] to probe phenelyne ring 7r flips in the polycarbonate of 1,1-dichloro-2-bis-(4hydroxy-phenyl) ethylene in the temperature range 195-280 K. In this work, the initial intensity, I(q~ = 0, ntc, t = 0) of the signal observed after excitation and reconversion of M Q C O H was observed as a function of production time ntc. An example of the fraction of the signal which represented total MQCOH, relative to the total signal quantum coherence available from a single-pulse experiment, f~o, when plotted as a function of temperature, and time ntc for development of MQCOH, is shown in Fig. 6.1.5, for production times ntc = 120, 180 and 240 l~s. The fractions fMo decrease from 195 to 220 K. During this period, a small fraction of rings undergoing 7r flips modulates the interchain dipolar interactions leading to a decrease in fMo. Above 240 K, fMo increases. This result is interpreted in terms of the timescale of ring flips within a given chain 0.8-
9~ ~)
0.6-
dz IQ' 180 g s
~ "~176
0.4-
afMQ' 240 gs s
"~ 0.2 190
9"
~
~.o~
.o"
.o<~O <>o.d" 9
"-
9
|
"'"
220
"-""+
"
"'"
l
"'"'+~'"'+"'+
250
"
280
Temperature (K) Fig. 6.1.5. Fractional MQ signal intensities, fMo, of protons in the polycarbonate of 1,1dichloro-2-bis-(4-hydroxy-phenyl) ethylene in the temperature range 195-280 K. The behaviour with temperature is used to infer details of phenyl rr ring flips [32].
1H NMR
181
approaching the fast-motion limit, and thus leading to a slower decay of M Q C O H , than in the absence of such motion, and to a rise in the fraction fMo. In all of the previous discussion of M Q C O H , it has been assumed that this coherence was developed using rf pulses to tailor an averaged dipolar Hamiltonian by making the spin-space portion of the dipolar Hamiltonian, T~m(I, m), time-dependent to obtain the desired average Hamilton from which to obtain the details of M Q C O H in the system under study. It has been assumed that the only time-dependence in the coordinate space portion of the dipolar Hamiltonian, RkD-m(| ~) was imposed only by thermal motion. A further development of the use of MQ N M R to probe polymers is that of experimenter-imposed time-dependence on Rt,D-m(O, d~), the coordinate space portion of ~DII, via the use of MAS [33-35]. In these experiments, with the M Q C O H being reflected in the intensity, (Iy(t~, t2)), and subsequent 2D Fourier transform to yield I(601,w2) as outlined in Section 6.1.2.2, the 2D spectrum of protons in polycarbonate was obtained. The development of spinning side-band patterns in this plot was demonstrated, as shown in Fig. 6.1.6. A theoretical understanding of the observed patterns in terms of CH~
0
_
9
9
l
"
"
"
i
"
"
"
I
~
"
"
,
"
o,
I
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I
-50 N _J._
6
~
.
5
~
.
0
3
N o
~
~
. " i
50
40
20
0
-20
-40
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Fig. 6.1.6 9Two-dimensional time-incremented double quantum experiment on polycarbonate recorded using a single cycle of multiple quantum excitation, and a spinning speed of 14.8 kHz. The t~ evolution was restricted to double quantum coherences by phase suitable cycling. The ~o~dimension is, therefore, a double quantum spectrum. The 602 dimension is a single quantum spectrum [21].
182
B.C. G E R S T E I N 1
dipolar couplings and internuclear distances for isolated spin ~ pairs has been published [36]. 6.1.3.3
Spin diffusion and domains
One of the more powerful uses made of NMR of protons in solid polymers has been the use of spin diffusion to study heterogeneities in polymers [3746]. One means of this estimation depends on the ability to establish a gradient of magnetization, m(r, t) between different types of domains, and subsequently monitor the resultant approach to equilibrium. Examples include crystalline and noncrystalline domains within a homopolymer, and block copolymers where dimensions of separated phases are controlled by block length and polymer blends. The usual diffusion equation describes the behaviour of the magnetization as it approaches equilibrium from an initial nonequilibrium state,
am(r, O/at = DV2m(r, t),
(6.1.19)
where D/cm 2 S - 1 is the diffusion constant for transmission of magnetization via spin diffusion, which in alkanes dense in protons is calculated [38] to be 6.2 x 10 -12 cm 2 s -1. The sizes of domains over which spin diffusion may be investigated by this technique is then limited by D and T1. A typical value of T1 for polymers is roughly 1 s. If in time, t, the diffusion distance is roughly (Dt) 1/2, then for polymers with densities of protons similar to alkanes, the maximum distance sampled by spin diffusion would be about 29 nm, and minimum distances sampled of order of a few tenths of a nm. For example, in polyethylene, the transverse relaxation time of the crystalline domains, T~ tt, characteristically of order of 10 I~s, can be sufficiently shorter than that of T~ ~ such that after an excitation placing the total spin magnetization in the transverse plane, phase coherence of the magnetization of the noncrystalline regions will remain well after that of the crystalline regions is lost. The classic pulse technique to accomplish this situation was the Goldman-Shen experiment [47]. Here, at some initial time, the two domains which had been previously polarized in a static field, had their magnetization placed perpendicular to the static field, e.g., via a strong 90x degree pulse. After a time, t, such that T1 >~ T~a~ > t >>T:~tt, there is only magnetization remaining in the noncrystalline domains. A strong 90_~ pulse then places magnetization of the noncrystalline domains parallel to the static polarizing field for a variable time ~T1, during which magnetization diffuses from the noncrystalline to the crystalline domains in a manner exactly equivalent to heat flowing between regions in which a temperature difference exists.
1H NMR
z (Bo)
183
z (~o) to >
t
90.~
>t z(i~
-Y
^
x~ r v e
x
90x
(-'to --)
90.x
~
Y
90x
t
--)
90x
(-90x
(--
t
--)
Fig. 6.1.7. Pulse sequence and magnetization recovery of the Goldman-Shen experiment. Magnetization in the rigid (crystalline) domain is randomized in time to > T~2a. Recovery of magnetization of the crystalline domains, R(t), due to transfer of magnetization from noncrystalline to crystalline domains is monitored at variable times t.
The resulting recovery of magnetization of the crystalline domains, ~(t), is then monitored by a 90x "interrogation pulse" which again places the magnetization in the transverse plane for observation. The basic scheme is shown in Fig. 6.1.7. The recovery of magnetization of the crystalline domains, dO(t) = 1 - [ M x a ( t ) - Mxa(t ~ oo)]/[Mxa(t- O) - Mxa(t ~ ~)],
(6.1.20)
is fit to possible solutions of Equation (6.1.19) with appropriate boundary conditions. As an example of such a fit, Fig. 6.1.8 shows a comparison between the experimental recovery curve from Goldmen-Shen experiments on a semicrystalline polypropylene film (black points), and two different models [48], one which invokes two different phases (crystalline and noncrystalline), with different sizes for the crystalline and noncrystaline domains. The second is again a two-phase model, but allows a distribution of sizes among the do-
184
B.C. GERSTEIN 1.0
.a
1
I
!
!
!
\\
0.8 - ~~ O\ \\
O.6 -
~\\
e 0.4 -
~~---~~ Xke
2P-MD
eq.(33)
\x,x,~,.r /b = 61A o.2 -
2p-2D
eq.( 13)7. b = 89A 0.0 0.00
A/ = 0..~5
\\",,.o /
\" " . ~ _ ~. 9 9
9
,
i
',
,
,
0.05
0.10
0.15
0.20
0.25
4t
0.30
( s e c 1/2)
Fig. 6.1.8. Fit of recovery curves ~(t), on semicrystalline polypropylene film (black dots) to two different models of phases and domains [48]" two-phase, two-domain, and two-phase multidomain [48].
mains. The model with a distribution of domain sizes is found to best fit the data. A variant of the Goldman-Shen sequence to generate a magnetization gradient in a polymer system containing regions that do not vastly differ in dipolar broadening, is that of using a dipolar echo sequence with cycle time, t~, arranged to produce an average dipolar Hamilton which is zero for one portion of a polymer blend, but not for another. VanderHart [46] used this technique to study chain proximity in a blend of a dominantly aromatic polymer, e.g., a rigid-rod polymer, poly(benzo-[a,d]-dithiazol-2,6-diyl-l,4phenylene), PBZT, and a dominantly aliphatic polymer, e.g., nylon 6,6. The dipolar-broadened linewidth of the proton NMR spectrum for the pure nylon 6,6 is ca. 45 kHz. That of the PBZT is ca. 24 kHz. The pulse spacing of the dipolar echo sequence could be arranged, therefore, to average to zero the dipolar Hamiltonian for the proton-poor aromatic portion of the blend--that
1H NMR
0 ii
185
0,,
-NH(CH2)6 NHC(CH2)4Cnylon 6,6
1H Multiple Pulse
1H PBZT
S
-50
O kHz
50
10 O PPM
Fig. 6.1.9. 200 MHz spectra of aH pertaining to spin diffusion experiments on a nylon 6,6 PBZT blend (VanderHart [46]). Left, magnetization gradient created by dipolar echo sequence with spacing 30 Ixs, thus, initially favoring the PBZT portion of the blend. Right, results of spin diffusion observed using CRAMPS to obtain high resolution proton NMR of the blend, and observation of magnetization transfer between the phenyl protons of the PBZT, and the methylene protons of the nylon 6,6.
containing the PBZT--while allowing the magnetization of the proton-rich portion, the nylon 6,6, to dephase. Storage of the magnetization along the direction of the static field for a variable diffusion time with interrogation after this time could then monitor the diffusion of magnetization from the PBZT to the nylon 6,6. The left side of Fig. 6.1.9 illustrates the type of results obtained. On the left are the proton NMR at 200 MHZ, resulting from generating a polarization gradient using a single dipolar echo-pulse sequence [49], [90x, z, 90y] (echo) with pulse spacing, z, of 30 txs. During diffusion times, ranging from 50 IXS to 50 ms, magnetization diffusing from the PBZT to the nylon is reflected in the increasing linewidth of the spectrum of the blend with increasing diffusion
186
B.C. G E R S T E I N
time, thus, indicating close proximity of the two polymers in the blend. A more fine-grained view of the diffusion process in this system is provided by observing spin diffusion between chemically shifted species, e.g., the phenyl protons in the PBZT, and the methylene protons in the nylon. In order to observe this effect, a technique pioneered by Ernst and coworkers [50] was used, in which CRAMPS [26] was used to provide high-resolution NMR of chemically shifted protons, and storage of the magnetization for a variable diffusion time allowed for transmission of magnetization between aromatic protons in the PBZT, and methylene protons in the nylon 6,6. The results, as a function of diffusion times between 40 ~s and 100 ms, are shown on the right side of Fig. 6.1.9. Here, Mo refers to a scaled equilibrium spectrum of the blend, and "nylon" refers to the CRAMPS spectrum of pure nylon 6,6. Again, a gradient in magnetization was created by opening a window in the symmetrized dipolar echo sequences used to average the dipolar Hamiltonians, such that the magnetization of the nylon 6,6 could decay while retaining magnetization in the PBZT component. Storage along the static field for diffusion times varying between 40 ~s to 100 ms, followed by an interrogation period during which, again CRAMPS is used to provide the high-resolution proton NMR allowed observation of the increase in magnetization of the methylenes in the nylon 6,6 component at the expense of the aromatic protons in the PBZT. For variations on these ideas, the reader is referred to VanderHart [46]. As a final example of the use of proton NMR invoking spin diffusion to study miscibility of polymer blends, the use of CRAMPS to remove proton dipolar coupling in a blend of an aromatic poly(ether-imid) (PEI), and a poly(aryl-ether-ketone) (PEEK), with detection of the magnetization of the 13C in the blend under high resolution conditions is cited [51]. Here, detailed information on the chemical composition of the phases present, as inferred from high resolution NMR of 13C, is linked to typical sizes of domains as reflected in spin diffusion of proton magnetization. The basic idea in producing a gradient in the proton magnetization of chemically shifted protons is that under the multiple-pulse sequence used to average the proton dipolar coupling Hamiltonian to zero, the effective magnetic field about which chemically shifted spins precess is in the (1, 0, 1) direction in the rotating frame. This is to say that the spin portion of the average Hamiltonian associated with the chemical shift becomes (I~ + Iz) [52]. Two chemically shifted spins, initially polarized and aligned with each other along z, but precessing about the (1, 0, 1) direction of the effective field under the homonuclear decoupling sequence, can be "caught" after a suitable time period, such that one is aligned along z, the other along x. If
1H NMR
1H CRAMZPS
187
13C CP MAS
U.H
H
. UH
/,
_ tJ .---x
.
,
I
10
.....
!
. . . .
5
ppm
!
0
. . . . . .
H! !. . . . .
200
,H
Vl t
.
,U.H! ~
UU
! . . . .
150
!.
100
,_:._
f
50
.....
!
0
ppm
Fig. 6.1.10. Comparison of 1H CRAMPS and 13C high-resolution spectra of a blend of PEI (U) and PEEK (H), under cross-polarization from 1H to 13C, before and after, a chemical shift filter (see text) selectively destroys the magnetization of protons in the aromatic region between 5 and 10 ppm [51].
the homonuclear decoupling is turned off at this time, the effective magnetic field then becomes the static field along z. The chemically shifted spin aligned along z will then be stored, while the spin initially aligned along x will dephase in the x,y plane of the rotating frame under homonuclear dipolar coupling, thus creating a magnetization gradient between the two chemically shifted species. Then cross-polarization from IH to X3C will reflect enhanced magnetization for those carbons attached to the proton spins which have been stored along z; which have not dephased under dipolar coupling. Thus, a "chemical shift filter" is applied to the cross-polarization process. With appropriate times taken to allow spin diffusion to take place between the chemically shifted proton spins which are, and are not magnetized, and subsequent crosspolarization, the intensities of the observed high-resolution carbon signals can be utilized to monitor the diffusion process. This effect is illustrated in Fig. 6.1.10.
188
B.C. GERSTEIN
Acknowledgements I am grateful to Dr Peter Cheung, to Professors Karen Gleason, Klaus Schmidt-Rohr, Hans Spiess and to Dr Dave VanderHart for supplying me with timely examples of their work on proton NMR in polymers.
References
.
.
5.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
B.C. Gerstein, Chapter 9 in R. Komorski (ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, VCH Publishers, Inc., 1986. C.R. Dybowski, Multipulse 1H and 19F techniques, in Solid State NMR of Polymers. Plenum, New York, 1991. NMR Spectroscopy of Polymers, R.N. Ibbett (ed.), Blackie Academic and Professional, New York, 1993. F.A. Bovey, NMR of Polymers. Academic Press, San Diego, 1996. V.J. McBrierty, Nuclear Magnetic Resonance in Solid Polymers. Cambridge University Press, Cambridge, U.K., 1993. K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, San Diego, 1994. A.N. Garroway, Polymer MRI, in The Encyclopedia of Nuclear Magnetic Resonance. Wiley, 1996. B.C. Gerstein and C.R. Dybowski, Transient Techniques in NMR of Solids; An Introduction to the Theory and Practice. Academic Press, Orlando, 1985. M. Goldman, Quantum Description of High Resolution NMR in Liquids. Oxford Press, 1988, Section 2.5. D.C. Champeney, Fourier Transforms and their Physical Applications. Academic Press, London, 1973. U. Haeberlen, High Resolution NMR in Solids, Selective Averaging, Supplement 1, Advances in Magnetic Resonance. New York: Academic Press, 1976. D.L. VanderHart, Magnetic susceptibility and high resolution NMR of liquids and Solids, in The Encyclopedia of Nuclear Magnetic Resonance. Wiley, 1996. U. Haeberlen and J.S. Waugh, Phys. Rev. 175 (1968) 453. J.S. Waugh, Average Hamiltonian Theory, in The Encyclopedia of Nuclear Magnetic Resonance, Wiley, 1996, and references therein. See Chapter 4 in Ref. [8]. R.M. Wilcox, Exponential operators and parameter differentiation in quantum physics, J. Math. Phys. 8 (1967) 962. B.C. Gerstein, Echos in Solids, in The Encyclopedia of NMR, Wiley, Chichester, 1996. R. Andrew, Magic angle spinning, in The Encyclopedia of NMR, Wiley, Chichester, 1996. Jian Zhi, Hu and R. Pugmire, Magic angle turning and hopping, in The Encyclopedia of NMR, Wiley, Chichester, 1996. Ajoy K. Roy and Karen K. Gleason, J. Polymer Science B32 (1994) 2235. H. Green, J.J. Titman, J. Gottwald and H.W. Spiess J. Magn. Res., Series A 114 (1995) 264. Equation 2.161 in Ref. [8].
1H NMR 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.
189
Son Jong Hwang and B.C. Gerstein, Bulletin of Magnetic Resonance 15 (1994) 213. Ref. [8], Equation (4.19). L.M. Ryan, R.E. Taylor, A.J. Paff and B.C. Gerstein, J. Chem. Phys. 72 (1980) 508. B.C. Gerstein, R.G. Pembelton, R.C. Wilson and L.M. Ryan, J. Chem. Phys. 66 (1977) 361. R. Prigl and U. Haeberlen, Advances in Magnetic and Optical Resonance, 19 (1996) 1. D.P. Burum and A. Bielecki, J. Magn. Res. 94 (1991) 645. D.P. Burum, HETCOR in Organic Solids, in The Encyclopedia of Nuclear Magnetic Resonance, Wiley, 1996. P.W. Anderson and P.R. Weiss, Rev. Mod. Phys. 25 (1953) 269. W.Z. Cai, K. Schmidt-Rohr, N. Egger, B. Gerharz and H. W. Spiess, Polymer 34 (1993) 267. A.K. Roy and K.K. Gleason, J. Polymer Science B32 (1994) 2235. H. Green, J.J. Titman, J. Gottwald and H.W. Spiess, Chem. Phys. Lett. 227 (1994) 79. H. Green, J.J. Titman, J. Gottwald and H.W. Spiess, J. Magn. Res. All4 (1995) 264. J. Gottwald, D.E. Demco, R. Graf and H.W. Spiess, Chem. Phys. Lett. 243 (1995) 314. R. Graf, D.E. Demco, J. Gottwald, S Hafner and H.W. Spiess, J. Chem. Phys. 106 (1996) 885. D.W. McCall and D.C. Douglass, Polymer 4 (1963) 433. D.C. Douglass and G.P. Jones, J. Chem. Phys. 45 (1966) 956. B. Christ and A. Peterlin, J. Polym. Sci. Part A-2 7 (1969) 1165. G.E. Wardell, V.J. McBrierty and D.C. Douglass, J. Appl. Phys. 45 (1974) 3441. A.C. Lind J. Chem. Phys. 66 (1977) 3482. R.A. Assink, Macromolecules 11 (1978) 1233. V.J. McBrierty, D.C. Douglass and T.K. Kwei, Macromolecules 11 (1978) 1265. V.J. McBrierty, Faraday Discuss. Chem. Soc. 68 (1979) 78. T.T.P. Cheung and B.C. Gerstein, J. Appl. Phys. 52 (1981) 5517. D.L. VanderHart, Macromol. Chem., Macromol. Symp. 34 (1990) 125. M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. T.T. Peter Cheung, Polymer Prep. (Am. Chem. Soc., Div. Polym. Chem.) 38 (1997) 892. B.C. Gerstein, "Echos in Solids", in The Encyclopedia of Nuclear Magnetic Resonance, Wiley, 1996. P. Caravatti, P. Neuenschwander and R. Ernst, Macromolecules 18 (1985) 119. K. Schmidt-Rohr, J. Clauss, B. Bltimich and H.W. Spiess, Magnetic Resonance in Chemistry 28 (1990) $3. See pp 185-187 in Ref. [8] for the physical picture, and Table 5.7 therein for the mathematical result.
Chapter 6.2
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
2H NMR A.S. Ulrich* and S.L. Grage Institut fiir Molekularbiologie, Friedrich-Schiller-Universitiit Jena, Winzerlaerstrasse 10, 07745 Jena, Germany
6.2.1
Introduction
This chapter provides an introduction to the characteristic N M R properties of the deuterium nucleus and presents an overview of various applications to investigate polymer structure and dynamics. Deuterium has a vanishingly low natural abundance and possesses a spin of 1, and these particular features can be turned to advantage. Selective labelling is employed to obtain local information about a specific site on the molecule. The quadrupolar interaction dominates the wideline 2 H N M R spectrum and, to a good approximation, the symmetry axis of this interaction is aligned along the C m Z H bond. Therefore, the quadrupolar lineshape and relaxation behaviour reveal the geometry and motion of the deuterated segment in a direct manner. In this chapter, we will first summarize some general 2 H N M R theory as a basis to understand the orientation-dependent spin interactions in powders as well as oriented samples. (Magic-angle spinning techniques are not discussed here.) Various cases of motionally averaged lineshapes will then be illustrated for the fast, intermediate and slow motional regimes. Different nuclear relaxation mechanisms are also described with a view to the respective motional timescales that can be addressed. Finally, we have compiled an overview of some important pulse sequences for one- and two-dimensional wideline experiments. The manipulation of the spin-1 system is visualized here in terms of a vector model based on a simplified density operator framework. For further information, the interested reader is referred to the more detailed, excellent reviews that have been published over the past two decades o n 2 H N M R of synthetic polymers and liquid crystals [1-13], as well as on biopolymers and lipid membranes [14-22]. Deuterium labels can be introduced synthetically into the polymer backbone or side-chains, or deuterated solutes can be infused into the matrix. Thus, the labelled molecular segment is analyzed in terms of its structural and dynamic properties, and the influence of environment and experimental *Corresponding author: [email protected]
2H NMR
191
conditions can be examined. 2H NMR covers an extremely wide range of motional timescales, such as side-chain flips, backbone dynamics or diffusion processes. The degree of motional averaging may be interpreted in terms of molecular order, e.g., to discriminate between crystalline or amorphous regions within the polymer. When a uniaxially oriented sample is available, such as a drawn fibre or a liquid crystal, it is possible to measure the bond angle of the deuterated segment with respect to the macroscopic axis. Thus, 2H NMR is ideally suited to address many types of questions. Some recent examples from the past couple of years, representing by no means a comprehensive list, have been concerned with polymer phase transitions [23-28], molecular miscibility and heterogeneity [29-32], backbone and side-chain flexibility [33-40], molecular structure of oriented samples and quality of alignment [41-50], self-diffusion and diffusion of solutes [51-54], or specific aspects about molecular weight or chemical structure of cross-links and end-groups [55-59]. It ought to be emphasized that biological materials such as lipid membranes and polypeptides have been studied by similar approaches as synthetic polymers, and many 2H NMR applications have benefitted from the mutual stimulus of these complementary areas. With the exception of its intrinsically low sensitivity (less than 1% compared to 1H), deuterium offers distinct advantages due to the ease of data collection, spectral analysis and the detailed information provided [1-5]. 9 Deuterium is an essentially nonperturbing probe when substituted for a proton. It can be synthetically incorporated into virtually any site of interest on a polymer, and at relatively low cost compared to ~3C. 9 In view of the dominant quadrupolar interaction of deuterium, its weak dipolar and chemical shift interactions can be neglected, which simplifies the theoretical analysis of the quadrupolar lineshapes and relaxation data (see Sections 6.2.2 and 6.2.3). 9 The quadrupole splitting of a suitably oriented sample represents the angle of the deuterated C~2H segment relative to the axis of alignment. Thus, quasi-atomic coordinates within the molecule can be determined, and the angular distribution of the sample estimated (see Section 6.2.4). 9 The 2H NMR lineshape readily reveals molecular anisotropy and motional averaging at a segmental level. It provides information about fast and intermediate dynamics, and about molecular order (see Section 6.2.5). 9 Relaxation experiments and 2D exchange spectra cover an extremely wide range of motional regimes (101~ -2 Hz), from fast methyl rotation to molecular self-diffusion. Concise information can be obtained about the type of motion, its geometry and its frequency (see Sections 6.2.5 and 6.2.6).
192
A.S. ULRICH AND S.L. GRAGE
9 In terms of spectrometer hardware, short high-power pulses are required to cover a spectral width of up to a few hundred kHz, and fast digitizing is essential for echo experiments [13, 19]. Since T~ relaxation proceeds efficiently by a quadrupolar mechanism, rapid data acquisition is possible. 1H decoupling is not necessary, unlike the case with many other nuclei. 9 To obtain quantitative information, computer lineshape simulations may be essential. Routines and algorithms are available for most evaluations, and in all figures shown below, we are using computed spectra to illustrate the characteristic features of the 2H NMR experiments.
6.2.2
General 2H NMR theory
In the following section, the transition energies between the three quantized energy levels of deuterium (spin I = 1) are derived, in order to explain the orientation-dependence of the quadrupole splitting [1-5]. The total energy of a deuterium nucleus in a static magnetic field contains contributions from the Zeeman interaction (Hz), dipolar interactions (HD), scalar coupling (Hj), the chemical shift interaction (Hcs), and the electric quadrupolar interaction (Ho). Therefore, the complete spin Hamiltonian is given by [59] H = Hz + HD + H j + Hcs + H Q .
(6.2.1)
The Zeeman Hamiltonian Hz describes the interaction between the nuclear magnetic moment, /XN, and the external magnetic field Bo. It determines the Larmor frequency COoof deuterium, which is, for example, 76.8 MHz at 11.7 T (corresponding to a 500-MHz spectrometer). The chemical shift range for deuterium, as well as its dipolar and scalar interactions, are of the order of only a few (<5) kHz at 11.7 T. Thus, they can be ignored in view of the dominant quadrupolar coupling constant, which is eZqQ/h = 167 kHz for aliphatic CmZH bonds [19, 21]. Here, e is the elementary charge and eq the largest field gradient. Q = 2.875 • 10 -27 cm 2 is the scalar quadrupole moment for the deuteron, and h is Planck's constant. Since the deuterium quadrupolar frequency is much smaller than the Zeeman frequency difference, the 2H NMR spectrum can be described by treating the quadrupolar interaction as a first-order perturbation of the Zeeman interaction [1, 19, 21, 60]. The quadrupolar nature of deuterium is due to the nonspherical charge distribution at the nucleus, caused by the presence of the neutron next to the proton. The quadrupolar Hamiltonian HQ arises from the electrostatic interaction of the nuclear quadrupole moment with the electric field gradient
2H NMR
193
(EFG) that originates from the electrons in the C~2H bond. The EFG can be expressed in terms of a traceless, symmetric second-rank tensor (V,,). In the diagonalized form, it is fully characterized by two independent components in its principal axis system, which are usually written as [21]
Vzz- eq, 77 =- ( V x x -
(6.2.2)
(6.2.3)
Vyy)/Vzz .
Here, V~, Vxx and Vyy are the principal elements in decreasing order of magnitude, which makes the asymmetry parameter 0 ~< ~/~< 1. The electric quadrupolar Hamiltonian may now be written in terms of the components I~, Iy and I~ of the spin operator I, with I_+ - I~ _+ iIy, as [19] ( e2qQ ) 1 2 i2 H ~ = \ 4 I ~ - - - 1 ) [312 - I.I + ~r/(I+ + )].
(6.2.4)
The EFG as an intramolecular property is related to the molecular frame, while measurements are performed in the laboratory frame. Thus, the principal axis system needs to be transformed into the laboratory frame, where the axis of quantization is defined by the direction of B0. This laboratory z-axis is specified in the principal axis system of the EFG tensor by the rotation angles 0 and ~b. After truncation, the quadrupolar Hamiltonian becomes ( e2qQ ) H ~ = \ 4 I ( - ~ - 1) [312 - I.I][2(3
COS2 0 -
1 1) + 5r/sin 2 0cos 2~b]. (6.2.5)
The quadrupolar interaction shifts each of the three Zeeman energy levels by an amount
Em
(
)
e2qQ [3m 2 4 I ( 2 I - 1)
_
I(I + 1)][1(3 cos 2 0 -
1
1) + 5~/sin 2 0cos 2q~]. (6.2.6)
The 2H NMR spectrum (I = 1, Am = _+1) of a single deuteron, or of a single crystal where all the C~2H bonds are aligned parallel, thus consists of a doublet separated by a quadrupole splitting
A.S. ULRICHAND S.L. GRAGE
194
3 (e2qQ)[(3
Av~ = 2
cos2 0 - 1) 1 ] 2 + ~r/sin 2 0 cos 2~b .
(6.2.7)
In the case of axial symmetry (V~ = Vvy , 77 = 0), which is a valid approximation for aliphatic and even aromatic deuterium bonds [19, 21], the quadrupole splitting reduces to
3(e2 )(
A~'o - 2
qQ
)
3 COS220 - 1 _ Co(3 COS 2 0 -- 1).
(6.2.8)
Here, we have defined a convenient scaling factor Co=3(e2qQ/h)= 125 kHz in units of the static quadrupolar coupling constant (e2qQ/h). The fundamental dependence of the quadrupole splitting A vo on the angle 0 is illustrated in Fig. 6.2.1 and extended in Section 6.2.3. The calculated value of A z,o can be positive or negative, and it becomes zero at the magic angle of 54.74 ~ where (3 cos 2 0 - 1) = 0.
6.2.3
The deuterium powder lineshape
A polymer sample usually consists of amorphous or polycrystalline regions that are randomly oriented with respect to the magnetic field. The resulting 2H N M R spectrum then has a characteristic lineshape, shown at the top of Fig. 6.2.1. The powder spectrum, or "Pake-pattern", is the spherically weighted sum over all orientations in space, i.e., over all contributions of 0 as given by Equation (6.2.8). The hatched area in Fig. 6.2.1 represents one of the two deuterium transitions, and the arrows illustrate a few selected frequencies that correspond to representative values of 0. The peaks of any powder lineshape invariably correspond to the CmZH bonds oriented at right angle to B0 (0 = 90 ~ and the lineshape does not reveal any specific angular information. Nevertheless, the simple 2H NMR powder spectrum is extremely useful to reveal molecular dynamics at a segmental level [1-5, 23-40]. Very fast motions lead to a narrowing of the powder pattern, while averaging on the intermediate timescale may give rise to more unusual lineshapes, as will be discussed in Sections 6.2.4 and 6.2.5. For axially symmetric CmZH bonds (~7 = 0), an analytical expression can be derived for the powder lineshape [21], which arises from N nuclei distributed over the surface of a sphere of radius r. The fraction dN of nuclei oriented between 0 and 0 + dO relative to the field B0 is given by
2H NMR
195
powder lineshape 0~
l
/
B0
90 ~
} 0 " o
I
-1
I
I
0
1
o
90 ~
oriented sample at tilt angle o~
~
= 40 ~
Bo C~=0~
=
J
B~l
30 ~
Bo
B o oc
=
60 ~
Ii
II
Bo
B0
| -1
-'/2
0
~/2
,
>
1
~
. . . . . -1 "~/2 0
> V2
1
Fig. 6.2.1. Top: 2H N M R powder lineshape, which arises from the spherical distribution of
C--2H bonds over all directions in space. Selected frequencies are marked by arrows for some of the bond orientations that contribute to the hatched ~'+ transition. For example, the characteristic peak corresponds to 0 - 90 ~ Bottom: In an oriented sample, the C - - 2 H bonds form a cone with an opening angle 7. When the sample axis is aligned parallel to the external field Bo (c~ = 0~ 7 can be calculated from the quadrupole splitting according to Equation (6.2.8). Positive and negative splittings cannot be distinguished, however, as shown here for the case of 7 - 40~ (left), and 7 - 73~ (right), unless the sample is tilted.
196
A.S. U L R I C H AND S.L. G R A G E
dN=(N) 47rr 2 27rr sin 0 r dO = I N sin 0 dO.
(6.2.9)
Thus, the probability density corresponds to p(O)= (sin 0)/2. Defining the reduced resonance frequency ~'___in units of Co (see Equation (6.2.8)), it is
+(3cos20 1)2
(6.2.10)
The frequencies, ~'+ and ~'_, for the two transitions (Am = _ 1) are centred about the spectral origin, andl are defined within the respective regions of 1 >i ~+ >I - ~ and - 1 ~< ~'_ ~< ~. Thus, the lineshape of the powder pattern is given by the probability function p ( ~ ' ) = p + ( s r) +p_(~'). Here, p(g)dg describes the fraction of signal between ~" and sr + ds~, which contributes an intensity p_ from the transitions ~'+_. The two probability densities p(O) and p_+(sr_+) are related to each other according to dO 1 dO p+_(~+_) = p(O) ~ = -sin 0~ = dsr___ 2 dsr___
I d cos 0
2 d~'___
.
(6.2.11)
It follows from Equations (6.2.10) and (6.2.11) that [21]
1 P---(sr+-) ~ v~/_+"-+ zg + 1"
(6.2.12)
The powder lineshape p(sr) has two characteristic singularities at frequencies sr = _+1, which correspond to bonds inclined at 0 = 90 ~ with respect to Bo. Note that the splitting of a completely rigid powder sample is equal to Co ~ 125 kHz for aliphatic bonds, and it is reduced by molecular motions (see Section 6.2.5).
6.2.4
Lineshape analysis of oriented samples
When an oriented sample is available, such as a drawn fibre, a stretched polymer film, or an anisotropic liquid crystal [1-5, 12, 41-50], it is possible to measure the orientation of the labelled C ~ 2 H bond vector with respect to the sample axis. When local orientational constraints are collected for several connected groups in a polymer, they can be combined to calculate
2H NMR
197
its three-dimensional molecular structure. This procedure is equivalent to determining not only the relative atomic coordinates of the molecule but also placing them into the reference frame given by the sample axis. Furthermore, it is straightforward to estimate the quality of alignment in a fibre, and thus to discriminate between crystalline and amorphous regions. In a uniaxially oriented sample, the C~2H bond vectors form a cone with an opening angle y around the axis of ordering. The lower part of Fig. 6.2.1 illustrates the case of two fibres, which are measured at a series of different inclinations c~ between the sample axis and the spectrometer field direction Bo. When the fibre axis is aligned parallel to Bo (a = 0~ all C~2H bonds within the sample give rise to the same quadrupole splitting (0 = y). Since this situation may be regarded as a "single crystal", the angle y can be calculated directly from the quadrupole splitting A vo. However, for the range of angles between 35 ~ and 90 ~ there are two possible solutions to Equation (6.2.8), since positive and negative values of Avo cannot be distinguished experimentally. This ambiguity is illustrated in Fig. 6.2.1, where two different intramolecular geometries (y = 40 ~ and 73 ~ give rise to the same splitting at = 0 ~ The two different bond angles can nevertheless be discriminated by measuring a tilt series at different sample inclinations, and by analyzing the more complex lineshapes. In a tilted fibre (where a # 0 ~ the orientation of the C~2H bond vector assumes a range of values for 0, which all contribute to the resulting lineshape. In analogy to the powder lineshape derived in Section 6.2.3, the equation for an oriented lineshape is given as a function of the reduced resonance frequency ~'_+ [43] p __(g'__)
X/+-2~'+-+l 3
/X/+-2(3+l_cos(c~+y)~/cos(]
X/---2•'+_ + 1
,
.
3
(6.2.13) This lineshape function possesses up to three singularities for each of the two transitions ~'+ and ~'_. The positions of these peaks depend on the value of y and vary with the sample inclination c~. Representative 2H NMR lineshapes are shown in Fig. 6.2.1 for the two formerly indistinguishable cases of y = 40 ~ and 73 ~ Differences in the lineshapes become apparent when tilting the sample away from a = 0 ~ so the two possible solutions of y can now be clearly discriminated [41-44].
198
A.S. ULRICH AND S.L. GRAGE
Experimental 2H NMR spectra are broadened significantly compared to the theoretical lineshapes shown in Fig. 6.2.1, as a result of various effects. In addition to the intrinsic linewidth, an oriented spectrum is broadened further when the molecules in the sample are not perfectly aligned. The degree of microscopic disorder in the sample can be estimated from the linewidth of the oriented spectrum at a = 0~ [12, 42, 43]. When the intrinsic linewidth is known from the powder pattern, a comparison provides the orientational distribution of the aligned sample.
6.2.5 Motionally averaged lineshapes 2H NMR is ideally suited to explore molecular motions in the polymer. Different types of motion can be discriminated on behalf of their timescale and geometry of exchange. One-dimensional quadrupole echo lineshapes (see Section 6.2.7.1) are particularly sensitive to segmental dynamics [1-6, 912], when there is either fast exchange between discrete geometries (with ~-c~ 1/Avo) or when the motion occurs on the intermediate timescale (~c ~ 1/Avo). Dynamic processes in the intermediate to slow motional limit (~-c>> 1/Az,o) are addressed by 2D exchange spectroscopy (see Section 6.2.7.4). This approach offers a detailed and model-free description of the distribution of jump-angles, and can cover an enormous range of timescales only limited by relaxation [1, 4, 61-64]. In the following section, both 1D and 2D 2H NMR lineshape analysis will be described. Any motion on the fast or intermediate timescale changes the appearance of the static powder spectrum, as shown in Fig. 6.2.2 on the left. When the orientation of the C ~ 2 H bond gets rapidly averaged around a symmetry axis (with ~-c~ 1/Az,o), the spectrum retains its axially symmetric powder lineshape but its width is narrowed by a geometric factor. Given that 6 is the new effective angle between the motional symmetry axis and B0, Equation (6.2.8) can be deconvoluted into a product of several factors, where a time average is denoted by angular brackets [19, 21]
(ApQ) =
CQ(3 COS20 -
1)= CoO cos2 (~ - 1)/(3 \
COS2 t.,t~ --
2
1).\ /
(6.2.14)
For the most simple case of fast continuous rotation around a well-defined axis,/3 represents the unique angle between the C ~ 2 H bond and the motional symmetry axis. The narrowing factor is (3 cos 2 / 3 - 1)/2, because a tensor rather than a vector is being averaged. For example, rotation of a tetrahedral
2H NMR
static ~ powder
0
199
pr
_.../----
I I ~ ] rJ~~
rotating methyl
1"1'- 0 T- 70.5~
~
rotating phenyl
rl'-0 y - 60 ~
rotating water
TI'= 0
2-site flip of phenyl
1"1'=0.6 [3= 120~
2-site flip of water
TI'---- 1
~/= 54.7 ~
13= 109.5~
[ I
-1
I
t I
0
I
I
1
Fig. 6.2.2. Left: Simulated 2H NMR lineshapes that are averaged by various characteristic segmental motions. In the case of fast rotation, y represents the angle between the rotation axis and the C--2H bond. For a two-site jump, /3 denotes the angle between the C--2H bond in the two configurations, and the effective asymmetry parameter becomes ~'~= 0. Right: Calculated 2D exchange spectra for a two-site jump with /3 = 120~ (top), and for continuous diffusion (bottom). The distribution functions P(/3) of the reorientation angle are shown, together with the contour maps of the corresponding spectra. All data are displayed on the reduced frequency scale in units of Co, and mixing times '7" m are set equal to the motional correlation time rc.
200
A.S. ULRICH AND S.L. GRAGE
methyl group around its intramolecular angle of 109.5 ~ causes a narrowing by a factor of - 1 / 3 , as seen in Fig. 6.2.2. Rotation around the magic angle, as shown for the case of a water molecule, even leads to a virtual collapse of the quadrupole splitting, but this is only due to the geometric term rather than free tumbling. Fast anisotropic fluctuations without any well-defined geometry, such as librational or diffusive motions around a symmetry axis, also lead to a narrowing of the powder pattern. However, in this case the variation of/3 over a range of values gives a more complicated time-average of (3 cos 2 / 3 - 1) that cannot be calculated analytically. Thus, it is convenient to describe the extent of averaging in terms of a segmental or local order parameter, which is defined as Sz~ = (3 COS2 / ~ - 1)/2 [19-21]. The molecular order parameter of a structural entity describes the librational disorder in a selected reference frame [65]. Any known contribution from a well-defined rotational axis needs to be factorized out according to Equation (6.2.14). Used this way, deuterium order parameters can provide much qualitative information, for example about phase transitions or phase separations in a polymer, or about the mobility profile along a chain. One-dimensional quadrupole echo 2H NMR lineshape analysis of powder samples is particularly informative when fast, discrete jumps occur between sites of well-defined geometry as, for example, in a phenyl group undergoing two-site exchange. In this case, the characteristic Pake-pattern is transformed into an axially asymmetric lineshape with an apparent asymmetry parameter r l ' # 0 (see Equation (6.2.3)) [1-8]. The asymmetric lineshapes, shown on the left in Fig. 6.2.2, can be derived by considering how the individual components of the principal EFG tensor become averaged by the discrete jumps. Within the molecular frame, and in units of V~ as defined by Equation (6.2.2), the static axially symmetric tensor consists of the components Vzz = 1, V~ = - 1/2, and Vyy = - 1/2. This traceless tensor satisfies the condition V~ + Vyy + V~ = 0 and 77 = 0. Note that the unique V~z component lies along the C m Z H bond. During the two-site jump of a 2,3,5,6deuterated phenyl ring, for example, the Vz~ component is averaged around an angle of /3= 120 ~ which causes a reduction by a factor of (3 cos2(120 ~ - 1)/2, giving - 1 / 8 . The principal component that corresponds to the plane perpendicular to the jump axis, on the other hand, remains unaffected at - 1 / 2 . Since the trace of the EFG must remain zero, the third principal component becomes 5/8. Hence, the effective asymmetry parameter of the motionally averaged tensor is calculated to be rt' = 0.6 for the phenyl ring undergoing fast two-site exchange, and the resulting spectrum is shown in Fig. 6.2.2. The lineshape calculation used here is based on Equation (6.2.7), where the quadrupolar contributions are summed up over the two rotation angles 0 and 4>. Note that for any symmetry higher than C2 (e.g. a methyl
2H N M R
201
group), even the discrete jump model leads to an axially symmetric spectrum [6]. Motions on the intermediate timescale are somewhat harder to analyze using simple quadrupole echo spectra, since the lineshape will be determined not only by the geometry and the rate of motion but also by relaxation effects. In the slow (%>> 1/Avo) or fast (rc ~ 1/Avo) limit, the echo is virtually completely refocussed, since T2 relaxation is comparatively slow. In the intermediate regime, on the other hand, much of the signal decays irreversibly during the echo delay time, and different orientations may decay with different T2 rates. Thus, the total intensity of the spectrum tends to be reduced and the lineshape can be significantly distorted [3-6, 8-11]. Both these effects depend on the length of the echo delay time, which can be varied deliberately to discriminate among different motions. Motionally averaged spectra in the intermediate regime should be interpreted with care, but with an appropriate model, experimental lineshapes have been well reproduced over a range of correlation times and temperatures [3, 6-12]. While there may exist ambiguities in the interpretation of quadrupole echo lineshapes, 2D 2H NMR exchange spectroscopy provides an essentially model-free approach to describe dynamic processes in the polymer [1-5, 6164]. An appropriate mixing time is included in the experiment, during which reorientations of the deuterated segment can occur. Such motions reveal themselves by the appearance of off-diagonal intensity, and the jump-angles are projected directly into the 2D exchange spectrum. An example is shown on the right in Fig. 6.2.2, where the discrete two-site jump of a phenyl-ring changes the relative C~2H bond orientation by/3 = 120~ Using the projection map of the elliptical pattern, the jump-angle/3 can be directly evaluated as tan(/3) = a/b from the ratio of the principal axes. For the case of continuous rotational diffusion, on the other hand, there exists a distribution of jumpangles. This leads to a rather different appearance of the 2D exchange spectrum [1-5, 61-64], characterized by parallel ridges, as illustrated in Fig. 6.2.2. In both simulated examples, the mixing time T m of the 2D exchange experiment was set equal to the motional correlation time r~, which defines the jump rate or diffusion rate. In practice, it is possible to probe a wide range of motional timescales by choosing appropriate mixing times between about 500 p~s to 20 s.
6.2.6
Nuclear spin relaxation
The previous section has been concerned with a 2H NMR lineshape analysis of motional effects, which is most informative on the intermediate timescale. Any dynamics in the very fast or ultraslow correlation time limit, however,
202
A.S. ULRICH AND S.L. GRAGE
are more suitably investigated by relaxation time measurements [1-5, 14, 18, 19, 66]. Since deuterium relaxation is dominated by an intramolecular quadrupolar mechanism, relaxation times provide information about the rate of motion of each individually labelled group. In systems as complex as polymers there invariably exists a wide range of dynamic processes, which contribute differentially to the various relaxation pathways [2, 15]. For deuterium there are five different relaxation times, involving three different spectral densities Jo(0), Jl(~oo), and J2(26Oo). After the absorption of electromagnetic radiation, the nuclear spins will relax back to their equilibrium state corresponding to a thermal Boltzmann distribution. Coherent precession of the spins generates a net component of transverse magnetization. This decays with an exponential time constant T2 as a result of the gradual dephasing of the spins, giving rise to the transverse or "spin-spin" relaxation. Strictly, the conventional T2 consists of the transverse Zeeman Tzz and the quadrupolar transverse Tzq relaxations [14, 19], the meaning of which will be explained in Section 6.2.7.
T2 = - 2
z + Y2q
= 8 --
2 Jo(0) + 2 Ja(wo) + J2(Zwo)
9
(6.2.14)
Transverse relaxation is used to detect slow motions near Jo(0) with correlation times longer than T~ > 10 -6 S. It can be measured either with a quadrupole echo (T2~ or a Carr-Purcell-Meiboom-Gill pulse train (T q-CPMG 2 ). By comparing the data from both methods, it is possible to discriminate very slow motions down to ~-r 10 -3 s, as described in Section 6.2.7. The recovery of the Zeeman polarization to its equilibrium value is characterized by the longitudinal, or "spin-lattice" relaxation time constant Txz. Spin-lattice relaxation occurs through dissipation of the excess energy of the spins to the surrounding lattice, brought about by fluctuating fields of the appropriate frequencies, i.e., close to the Larmor frequency COoand to 26Oo.
Tlz- 8
[Jl(WO) + 4J2(2Wo)] 9
The decay of double quantum coherence is described by
(6.2.15)
2H N M R
1 1(7) 8
TDo
[Jl(WO) + 2J2(2Wo)].
203
(6.2.16)
The decay of the quadrupolar polarization is given by
Tlq
--8
[3Jl(O)o)] 9
(6.2.17)
Spin-lattice relaxation is most sensitive to fast segmental fluctuations such as side-chain rotation and backbone oscillations with correlation times of around 10 -11 S < "t"c < 10 - 6 S. A combination of different relaxation measurements makes it possible to discriminate among the different spectral densities [2, 15]. For a complete description of the frequency dependence, however, a series of measurements needs to be carried out at several different field strengths.
6.2.7
Summary of 2H NMR pulse sequences
The following paragraphs provide an overview of some important 2H NMR experiments. In analogy to the intuitive picture of a spin I = 1/2 precessing in space, the behaviour of the deuterium nucleus with a spin-1 will be visualized here in a higher-dimensional coordinate system. Some excellent reviews are recommended for further reference, which cover the special properties of spin-1 in more depth [3, 14]. In order to calculate the time evolution of a spin system and its manipulation by radio frequency (rf) pulses, it is conveniently described by a density operator o-(t). This operator can be written in a basis of (21 + 1) 2 operators Pi, which are related to the angular momentum operators Ix, Iy, I z [14] (21 + 1)2-- 1
~r(t) = coPo +
~
ci(t)pi .
(6.2.18)
i=1
A convenient basis set has been suggested in the literature and will be used here [16, 68]. For a constant number of spins, Co is not a function of time and may be set equal to 1, and Po is the unity operator. Thus, for spin I = 1/2, the system is described by the three coefficients Cl(t), C2(t) and c3(t). They reflect the three components of the magnetization vector which can be readily visualized in the rotating frame as the x, y and z coordinates. For I = 1, eight instead of three coefficients are now required.
204
A.S. ULRICH AND S.L. GRAGE
Therefore, the usual picture of a vector in real space breaks down and has to be substituted by a vector in eight-dimensional space. Nevertheless, with the above choice of the basis operator set p~, the first three components c~(t), c2(t) and c3(t) can still be thought of corresponding to the respective components in real space. They are referred to as Zeeman orders. The three components C4(t), Cs(t) and c6(t) are called quadrupolar orders, and the remaining c7(t) and c8(t) are double quantum orders. Only Cl and c2 are accessible by experiment, since their changes correspond to the precessing magnetization that induces a voltage in the rf coil. In spin operator mechanism the time evolution of the density operator is given by the Liouville-von-Neumann equation, do"
dt
= - i [ H , o-].
(6.2.19)
This important equation governs, like an equation of motion, the time development of the system under a Hamiltonian H. It yields eight coupled differential equations for the coefficients ci(t). The resulting solutions for various Hamiltonians resemble rotations in an eight-dimensional space spanned by the eight nontrivial basis operators. With the convenient basis set of Pi [14, 16, 68], the time evolution under Zeeman interaction can be visualized as a precession in the Pl-P2, P5-P6 and P7-Ps planes. The axially symmetric quadrupolar interaction, on the other hand, mixes between Pl and P6 and between P2 and P5. Therefore, evolution under quadrupolar interaction does not lead to precession of "magnetization" within the x - y plane, but rotates it out of this plane into a not directly accessible order and back again. Irradiation with rf, where B1 lies along the x- or y-axis, can be represented by the Hamiltonian H1 - yBlIx or H1 = yBxIy, respectively. For a spin-l/2 system, application of a pulse of length tw rotates the magnetization around the x- or y-axis, i.e., in the P2-P3 or P3-Pl plane, by an angle of q~= yBltw. In spin-1 systems, on the other hand, the Hamiltonian H1 with B~ along the x-axis leads to precession in three planes. The first two are defined by P2-P3 and Ps-P5, and the third one is spanned by P6 and an axis along p4,7- V3/2p4 + 1/2/)7. With B1 pointing along the y-axis, the corresponding rotations takes place in the P3-Pl, Ph-P8, and P4, -7-P5 planes (p4,-7 = X/-3]2p4-1/2p7). As can be seen, the time course of a 2H NMR experiment can be determined, without too much theory, by using the picture of a precessing vector in the corresponding planes. Thus, pulse sequences can be designed or rewritten for spin-l, e.g., to obtain echoes yielding undistorted spectra,
2H NMR
205
to reveal relaxation times, or to correlate the states of motion at two different times. Some frequently used experiments are discussed below and summarized in Fig. 6.2.3. 6.2.7.1 Quadrupole echo In solid state NMR, it is impractical to obtain a spectrum with a single pulse because the FID rapidly dephases during probe ring-down over the first 10 ~s following the pulse [13]. To avoid this problem in spin-l/2 experiments, the two-pulse Hahn echo sequence may be used [69]. For spin-1 systems, the conventional Hahn echo is replaced by the quadrupole echo sequence 90y - z - 90x - t [67]. Refocussing is achieved by a pulse with half the length of what is used for spin-i/2, since the precession of spin-1 in the corresponding plane occurs at twice the frequency. A 90~ difference between the two pulses is essential for refocussing along a detectable order, and phase cycling is applied to cancel unwanted coherences. Pulse lengths should be of the order of 5 ~s or less, when large spectral widths are to be covered, and echo delay times ~-are typically a few tens of ~s. Note that if there is motion in the intermediate regime, the echo lineshape and intensity depend critically on the delay time, as discussed in Section 6.2.5. 6.2.7.2 Relaxation time measurements In the case of spin 1/2, the transverse relaxation time T2 corresponds to dephasing in the Pz-P3 plane, and the longitudinal relaxation time T1 describes magnetization recovery along the Pl axis. For spin-1 systems, on the other hand, there exist additional relaxation pathways, as described in Section 6.2.6. Transverse dephasing of magnetization is possible during several different modes of precession, and the approach towards thermal equilibrium occurs along several of the eight components with distinct timescales T1. Considering the evolution under Zeeman interaction, only P3 and P4 are invariant orders, while the components of Pl and P2, of P5 and P6, and of P7 and P8 are intermixing. Therefore, the longitudinal relaxation times describe the approach of thermal equilibrium of the P3 and P4 orders, which are referred to as Tlz (longitudinal Zeeman) and Tlq (quadrupolar), respectively [19, 70]. Dephasing of P l-P2 coherence is denoted by Tzz (transverse Zeeman), of Ps-P6 coherence by Tzq (transverse quadrupolar) and of Pv-P8 coherence by TDo (double quantum) relaxation. The longitudinal relaxation time Tlz can be measured in an analogous way as for spin-l/2 systems by an inversion recovery experiment as depicted in Fig. 6.2.3. The recovery of signal intensity is monitored as a function of incrementing the delay time ~'1 from a few p~s up to around 5 • T1. Other relaxation times corresponding to the higher orders can be revealed by trans-
206
A.S. ULRICH AND S.L. G R A G E
Fig. 6.2.3. Pulse sequences of some important 2H NMR experiments. The response of the spin-1 nucleus to pulses and precession periods is explained in the text in terms of rotations within an eight-dimensional space.
2H N M R
207
ferring the system into the appropriate coherence state, where dephasing occurs with the desired rate. Since several orders are coupled and interchange among each other during the evolution, the measured rate usually consists of contributions from several relaxation times. For example, in analogy to spin-i/2, the quadrupolar echo can be used to measure spin-spin relaxation. However, the relaxation time T2~ determined by a variation of the pulse spacing ~- is composed of Yzz and Tzq, as given by Equation (6.2.14). Two other prominent relaxation experiments are based on the JeenerBroekaert [9, 71] and the stimulated echo sequence [69, 72], which are outlined in Fig. 6.2.3. The Jeener-Broekaert experiment explores the Tlq (quadrupolar) and TDQ (double quantum) relaxation rates by transferring the initial state into a mixture of quadrupolar and double quantum orders (corresponding to P4 and P7) via a 9 0 y - ' / 1 - 45x pulse combination. The length and phase of the second pulse are optimized to transfer the magnetization into the P4 and P7 orders while, for example, the 90x pulse of the quadrupolar echo prevents such orders altogether. After an incremented evolution time z2, another 45x pulse is applied to bring the system back into an experimentally observable order. The signal after this pulse still carries the echo information from propagation during 71, which gives rise to an echo at a time ~'1 after the third pulse. Furthermore, a second, virtual echo builds up at a time z l before the third pulse. Only the former echo is directly detectable, but an added quadrupolar echo refocusses both the real and the virtual echo. The FID then carries a positive echo at z 3 - Zl and a negative echo at 73 + ~'1 following the last 90~ pulse. Since their amplitudes are functions of +[3 exp(-~'z/Taq)+ exp(--~'z/TDQ)], a variation of T 2 reveals both relaxation times as independent values [14]. In the stimulated echo experiment, also shown in Fig. 6.2.3, the second pulse transfers the system into a mixture of Zeeman and double quantum order (along P3 and Ps). Here, the relevant relaxation times are Tlz (longitudinal Zeeman) and TDO (double quantum), for which the 45x pulses of the Jeener-Broekaert sequence are replaced by 90y pulses. Again, two echos evolve at _+71 around the third pulse, and are refocussed by the fourth pulse. The two negative echo amplitudes vary as function of ~'2, with -[exp(-~'z/Tlz) w exp(--~'z/TDo)], and both Tlz and TDO can be determined as separate values [14]. 6.2.7.3 Correlation time filtering Beyond the standard quadrupole echo experiment, multipulse sequences provide an alternative and versatile approach to measure transverse relaxation. The relaxation time constant T2~ is obtained from a series of experiments with different pulse spacings ~-. Extending this sequence by n further 90~
208
A.S. U L R I C H A N D S.L. G R A G E
pulses, separated by 2~-, yields a train of echoes at times k2~', with k = 1, 2 , . . . , n. The corresponding sequence is the spin-1 version of the CarrPurcell-Meiboom-Gill experiment (q-CPMG), also referred to as MW-4 (Mansfield-Ware 4) [16, 73-75]. Each echo is refocussed again by the following pulse in the sequence, however with reduced amplitude due to transverse relaxation. Hence, within a single experiment, the relaxation time constant Tq-CPMG 2 is determined from the amplitudes as A ( k 2 ~ ' ) - A(0) exp(-k2~-/ q-CPMG T2 ). The q-CPMG relaxation time differs from T2~ Assuming that the quadrupolar interaction is relaxed through random fluctuations with a single correlation time ~c, the Tq-CPMG relaxation rate is a function of the echo delay --2 time ~- [76] l
Tq_CPMG ~ - AM2~'c 1 - - - tanh --2
.
(6.2.20)
T
Here, AM2 is the second moment of the spectrum after the effect of motional averaging has been taken into account. By appropriate selection of ~', it is possible to explore two limiting cases. Using a long z > ~c, the value of Tq-CPMG is independent of z and equal to T2~ With z < Zc, on the other 2 hand, the relaxation rate is proportional to ~.2, giving 1/Tq-CPMG('r) ~ AM2"r2/3"rr Thus, it is possible to use the q-CPMG experiment as a correlation time filter, since decreasing z suppresses more and more contributions from slow motions to the relaxation. When a motional process is dominated by a single correlation time, a series of experiments with different z reveals the value of ~'c from the slope of 1/T q-cPMG versus ~.2. 6.2.7.4 2D exchange spectroscopy Among two-dimensional experiments, wideline exchange spectroscopy plays a prominent role in 2H NMR [1, 4, 61-64]. By correlating the frequency distributions at two different times, any changes in the resonance frequency due to reorientation can be detected in the off-diagonal intensity pattern. With the aid of lineshape simulation and by comparing different mixing times, detailed conclusions about the type and rate of motion can be drawn, as illustrated in Section 6.2.5 and Fig. 6.2.2. A typical 2D exchange experiment consists of three parts, as seen in Fig. 6.2.3. First, in an evolution period tl the system evolves with its initial frequency distribution, which corresponds to the simple Pake-pattern. Subsequently, the spin state is stored throughout a mixing time tm, during which
2H NMR
209
molecular reorientations can occur, which lead to a change in the orientationdependent spectral frequencies. In the final detection period, the evolution of the spin system is switched on again. The magnetization of any spin that is frequency-encoded from before the mixing time, will now propagate with a potentially different frequency after tm. The FID is acquired in t2, and the second dimension is obtained by incrementing tl. In order to obtain a fully complex data set in the indirectly detected dimension (equivalent to quadrature detection), two separate experiments need to be carried out with differing phases of the second and third pulse. The component kept during the mixing time will then be either the sin or cos of the phase acquired during the evolution period, and the two data sets are added up later. After preparation by a 90y pulse, the quadrupolar interaction causes a precession in the Pl-P6 plane (rather than in the Pl-P2 plane as for spin-i/2). The Pl and P6 orders are then transferred into an invariant order (P3, P4, P7 or Ps) to store the system's evolutionary state during the mixing time. When acquiring the first set of data, the Pl order is transferred into P3 by a/3_y pulse, while P6 in the second experiment is transferred into P4 and P7 by a /3-x pulse. After the mixing time, the reverse transfer is achieved by a fly or fix pulse, respectively, followed by a quadrupole echo. Any undesired contributions, which add to the FID while precession occurs between the diverse orders, can be cancelled by phase cycling [64]. The choice of the pulse length/3 is not in fact critical to the experiment, but for/3 = 54.7 ~ both data sets theoretically contribute equally to the complex 2D spectrum and give the best S/N ratio. Since different relaxation pathways may lead to different contributions, in practice the two data sets are weighted empirically such that the cross-diagonal has the least intensity.
References 1. K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid-State NMR and Polymers. Academic Press, London, 1994. 2. G. Kothe and K. Mtiller, in G.R. Luckhurst and C.A. Veracini (eds.), The Molecular Dynamics of Liquid Crystals. Kluwer, Dordrecht, 1994, 481-503. 3. D.M. Rice, in R.N. Ibbett (ed.), NMR-Spectroscopy of Polymers. Blackie Academic and Professional, Glasgow, 1993, 275-307. 4. H.W. Spiess and H. Sillescu, in E.W. Fischer, R.C. Schulz, H. Sillescu (eds.), Chem. Phys. Macromol., CH, Weinheim, 1991, 313-347. 5. R.F. Colletti and L.J. Mathias, in L. Mathias (ed.), Solid State NMR of Polymers. Plenum Press, New York, 1991, 23-60. 6. R.G. Griffin, K. Beshah, R. Ebelh/iuser, T.S. Huang, E.T. Olejniczak, D.M. Rice, D.J. Siminovitch and R.J. Wittebort, in G.J. Long and F. Grandjean (eds.), The Time Domain in Surfaces and Structural Dynamics. Kluwer, Dordrecht, 1988, 81-105.
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A.S. ULRICH AND S.L. GRAGE F.A. Bovey and L.W. Jelinski, J. Phys. Chem. 89 (1985) 571. L.W. Jelinski, Ann. Rev. Mater. Sci. 15 (1985) 359. H.W. Spiess, Advances in Polymer Science 66 (1985) 23. H.W. Spiess, Colloid & Polymer Sci. 261 (1983) 193. H. Sillescu, Pure Appl. Chem. 54 (1982) 619. H.W. Spiess, in I.M. Ward (ed.), Developments in Oriented Polymers. Applied Science, London, 1982, 47-78. R. Hentschel and H.W. Spiess. J. Magn. Res. 35 (1979) 157. M. Bloom, C. Morrison, E. Sternin and J.T. Thewalt, in Pulsed Magnetic Resonance: NMR, ESR and Optics. Clarendon Press, Oxford, UK, 1992, 274-316. C. Mayer, K. Mueller, K. Weisz and G. Kothe, Liquid Cryst. 3 (1988) 797. M. Bloom, in B. Maraviglia (Ed.), Enrico Fermi International School on the Physics of Magnetic Resonance in Biology and Medicine, Societa Italiana di Fisica. Elsevier, Amsterdam, 1987. J. Seelig and P.M. MacDonald, Acc. Chem. Res. 20 (1987) 221. M. Brown and G.W. Williams, J. Biochem. Biophys. Methods 11 (1985) 71. J.H. Davis, Biochim. Biophys. Acta 737 (1983) 117. J. Seelig and A. Seelig, Q. Rev. Biophys. 13 (1980) 19. J. Seelig, Q. Rev. Biophys. 10 (1977) 353. G. Lindblom, Curr. Opin. Colloid Interface Sci. I (1996) 287. A. Martin, C. Tefehne and W. Gronski, Macromol. Rapid Commun. 17 (1996) 305. P. Alexandridis, D. Zhou and A. Khan, Langmuir 12 (1996) 2690. V. Arrighi, J.S. Higgins, L. Abis and R.A. Weiss, Polymer 37 (1996) 141. P. Alexandridis, U. Olsson and B. Lindman, J. Phys. Chem. 100 (1996) 280. R. Muzzalupo, G.A. Ranieri, D. Catalano, G. Galli and C.A. Veracini, Liquid Crystals 19 (1995) 367. K. Zhang and A. Khan, Macromol. 28 (1995) 3807. A. Tezuka, K. Takegoshi and K. Hikichi, J. Mol. Struct. 355 (1995) 1-7, 9-13. K.L. Ngai and C.M. Roland, Macromol. 28 (1995) 4033. G.-C. Chung, J.A. Kornfield and S.D. Smith, Macromol. 27 (1994) 5729. J.A. Kornfield, G.-C. Chung and S.D. Smith, Polym. Mater. Sci. Eng. 71 (1994) 207. M. Liang and F.D. Blum, Macromol. 29 (1996) 7374. A. Dardin, C. Noeffel, H.W. Spiess, R. Stadler and E.T. Samulski, Acta Polym. 46 (1995) 291. S. Okamoto, H. Furuya and A. Abe, Polym. J. 27 (1995) 746. F.C. Schilling, K.R. Amundson and P. Sozzani, Macromol. 27 (1994) 6498. T. Hiraoki, K. Tomita and A. Kogame, Polym. J. (Tokyo) 26 (1994) 766. Z. Gao, X.F., Zhong and A. Eisenberg, Macromol. 27 (1994) 794. S. Kitazawa, T. Hiraoki T. Hamada and A. Tsutsumi, Polym. J. 26 (1994) 1213. A.S. Ulrich and A. Watts, Biophys. J. 66 (1994) 1441. T. Asakura, M. Minami, R. Shimada, M. Demura, M. Osanai, T. Fujito, M. Imanari and A.S. Ulrich, Macromol. 30 (1997) 2429. A.S. Ulrich, Macromol. Symp. 101 (1996) 81. A.S. Ulrich and A. Watts, Solid State Magn. Res. 2 (1993) 21. A.S. Ulrich, I. Wallat, M.P. Heyn and A. Watts, Nature Struct. Biol. 3 (1995) 190. A.H. Simmons, C.A. Michal and L.W. Jelinski, Science 271 (1996) 84. M. Zeghal, P. Auroy and B. Deloche, Phys. Rev. Lett. 75 (1995) 2140. S. Valic, B. Deloche, Y. Gallot and A. Skoulios, Polymer 36 (1995) 3041.
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211
48. P. Fischer, C. Schmidt and H. Finkelmann, Macromol. Rapid Commun. 16 (1995) 435. 49. D.J. Schaefer, R.J. Schadt, K.H. Gardner, V. Gabara, S.R. Allen and A.D. English, Macromolecules 28 (1995) 1152. 50. S.M. Fan and G.R. Luckhurst, J. Chem. Phys. 101 (1994) 3255. 51. S. Luesse and K. Arnold, Macromol. 29 (1996) 4251. 52. J. Rault, C. Mace, P. Judeinstein and J. Courtieu, J. Macromol. Sci. - Physics B35 (1996) 115. 53. D. Radloff, C. Boeffel and H.W. Spiess, Macromol. 29 (1996) 1528. 54. E.A. Schmitt, D.R. Flanagan and R.J. Linhardt, Macromol. 27 (1994) 743. 55. A. Abe, H. Furuya, S.Y. Nam and S. Okamoto, Acta Polym. 46 (1995) 437. 56. W.S. Price, N. Kikuchi, H. Matsuda, K. Hayamizu, S. Okada and H. Nakanishi, Macromol. 28 (1995) 5363. 57. D. Pressner, C. Goeltner, H.-W. Spiess and K. Muellen, Acta Polym. 45 (1994) 188. 58. T.W.N. Bieze, J.R.C. van der Maarel, C.D. Eisenbach and J.C. Leyte, Macromol. 27 (1994) 1355. 59. D. Bucca and B. Gordon, Macromol. 27 (1994) 862. 60. C.P. Slichter, Principles of Magnetic Resonance. Harper and Row, New York, 1963, 160176. 61. H.W. Spiess, and H. Sillescu, J. Magn. Res. 42 (1981) 381. 62. S. Kaufmann, S. Wefing and H.W. Spiess, J. Chem. Phys. 93 (1990) 197. 63. S. Wefing, S. Kaufmann and H.W. Spiess, J. Chem. Phys. 89 (1988) 1234. 64. S. Wefing and H.W. Spiess, J. Chem. Phys. 89 (1988) 1219. 65. C. Schmidt, B. Bli~mich and H.W. Spiess, J. Mag. Res. 79 (1988) 269. 66. N.O. Petersen and S.I. Chan, Biochem. 16 (1977) 2657. 67. M. Bloom and E. Sternin, Biochem. 26 (1987) 2101. 68. J.H. Davis, K.R. Jeffrey, M. Bloom, M.I. Valic and T.P. Higgs, Chem. Phys. Lett. 42 (1976) 390. 69. A.J. Vega and Z. Luz, J. Chem. Phys. 86 (1987) 1803. 70. E.L. Hahn, Phys. Rev. 80 (1950) 580. 71. K.R. Jeffrey, Bull. Magn. Reson. 3 (1981) 69. 72. J. Jeener and P. Broekaert, Phys. Rev. 157 (1967) 232. 73. N.S. Sullivan, D. Esteve and M. Devoret, J. Phys. C. Solid State Phys 15 (1982) 4895. 74. H.Y. Carr and E.M. Purcell, Phys. Rev. 94 (1954) 630. 75. S. Meiboom and D. Gill, Rev. Sci. Instrum. 29 (1958) 688. 76. P. Mansfield and D. Ware, Phys. Rev. 168 (1968) 318. 77. J.S. Blicharski, Can. J. Phys. 64 (1986) 733.
Solid State NMR of Polymers, edited by I. Ando and T. Asakura
Chapter 6.3
Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
3H NMR J.P. Bloxsidge 1, J.R. Jones 1, J.C. Russell 2, A.P. Sharratt 3, T.A. Vick 2 and D. Zhong a 1Department of Chemistry, University of Surrey, Guildford, Surrey, UK: 2Biocompatibles Ltd., Frensham House, Farnham Business Park, Farnham GU 9 8QL, UK; 31CI Chemical and Polymers, The Heath, Runcorn, Cheshire, UK
A key factor in the development of high resolution 3H NMR spectroscopy [1] of tritiated compounds in solution was the use of a sealed sample assembly within the conventional NMR tube making it very unlikely that any radioactivity would be released even in the event of a tube breakage. Since then improvements in magnet design and higher fields [2, 3] have made it possible to produce satisfactory 3H NMR spectra at much lower levels of radioactivity (<1 mCi cf---20 mCi), thereby, creating new opportunities. Tritium has, despite its radioactivity, several features in common to 13C. 9 1 Both nuclei have spin-5 and natural abundance levels that are very low, exceedingly so in the case of 3H. The latter also has a much higher sensitivity to NMR detection than 13C. Over the last 20 years, 13C NMR spectroscopy of solids has been developed into a very powerful technique [4] with applications covering a wide span of interests. Therefore, the possibility exists of developing 3H NMR spectroscopy of solids in an analogous manner, providing the even more demanding safety requirements can be met. Rapid molecular reorientation in the liquid state is the main reason why the intrinsic linewidths of solution NMR spectra are so narrow. In contrast, in most solids, molecular reorientation is not sufficiently fast to average the dipolar spin interactions and consequently broad lines are obtained. These are usually narrowed by a combination of techniques--magic-angle spinning, high power proton decoupling, cross-polarisation and multiple pulse sequences of which the former is probably the most important. Low power proton decoupling has been used regularly to obtain 3H NMR spectra of liquids but the small differences in the 1H and 3H frequencies (--~6%) pose a problem for the solid state. A similar situation exists, and has been solved recently, in the case of 19F NMR spectroscopy [5-7]. The need to decouple the 1H~3H interactions can of course be avoided by employing the appropriate deuteriated compounds [8] and this is the procedure adopted in some of the current examples, although it will not always be practicable to do so. In order to produce substantial narrowing of the broad lines, expected
3H NMR Table 6.3.1.
213
3H NMR data for various tritiated compounds
Compound
Linewidth at half-height*/Hz
Sodium acetate [2H3]-sodium acetate [ZHz]-sodium succinate [o-3H]-Benzoic acid Mixture of polymer (2) and (3)
242 111 187 101 4300 423 307 172 118
(static) (2.5 kHz rotation) (5.0 kHz rotation) (10 kHz rotation) (17 kHz rotation)
* At 10 kHz rotation unless otherwise stated. sample spinning rates must be of the same order as the spread of the spectral frequencies. It is fortunate, therefore, that recent improvements in rotor design now make it possible to do this with much greater levels of confidence than hitherto. Nevertheless, with radioactive material it is necessary to ensure that in the event of a breakage the radioactivity is contained and contamination minimised. Therefore, the tritium probe was enclosed in a perspex shield and operated under a negative pressure (monitored) so that, in the event of an accident, particles would be sucked onto a commercially available air filter and thereby retained. The 3H N M R spectra of all the specifically labelled compounds, given in Table 6.3.1, show linewidths at half-height in the range 100-250 Hz. Narrowing of the lines, by a factor of ---2, is obtained through the use of the corresponding deuteriated compounds. These findings are consistent with the substantial improvement observed [8] in the resolution of 13C MAS N M R spectra of paramagnetic lanthanide acetates resulting from the replacement of the hydrogens by deuterium. Homogeneous reduction of the unsaturated phosphoryl choline containing polymer (1), using Wilkinson's catalyst and an insufficient quantity of H2:T2 mixture to reduce fully the acetylenic triple bond, gives rise to a mixture of (2) and (3). The solution 3H N M R spectrum (1H decoupled) gives rise to two sharp singlets. The corresponding solid-state spectra (Fig. 6.3.1) illustrate the dramatic narrowing in linewidths that are obtained as the spinning speeds are increased. Furthermore, at the two highest speeds there are indications of the emergence of new signals, probably associated with the polymorphic nature of the polymer mixture. Somewhat similar findings were reported in a recent study [5] of 19F N M R spectra in the solid state. The results show that it is possible to obtain 3H N M R spectra of solid tritiated compounds in a safe and routine manner. The examples chosen are
4~
o---\
o
~\
S
0"-.... p
0
1
/
S
0 0
o
I
0 I--- p
I
T/H
o
O
I
0
O
T2/H2
0
I
\
o
O.
0
IIj
O
/
I
T 2 / ~ --~ TIH
./\
01---- P "--- 0
/T/n
I
o
T/H" ~
?
/T t,
N§
/!\ (i)
(2) Scheme 1.
(3)
9
>
3H NMR
215
(b)
(c)
(d)
(e) A
(f)
(a) io
S
-
6
ppm
4
2
0
Fig. 6.3.1. 3H MAS NMR spectrum of polymer mixture (2 + 3): (a) in solution; (b) static; (c) 2.5 kHz" (d) 5.0 kHz; (e) 10 kHz; and (f) 17 kHz.
216
J.P. BLOXSIDGE ET AL.
Ca) ~3
2
:1
ppm
(b)
5
),
3
....
2 ~
'1
"
~)pm
Fig. 6.3.2. (a) 3H NMR (1H decoupled) of a solution of [G-3H] a-methyl dihydrocinnamic
acid; (b) the 3H MAS spectrum of the corresponding solid.
of specifically tritiated compounds or, in one case, a mixture of two compounds. The relatively sharp signals obtained suggest that many generally tritiated compounds can be analysed in the same way and this is confirmed by the results shown in Fig. 6.3.2 where both the solution and solid-state 3H N M R spectra of [G-3H]a-methyl dihydrocinnamic acid are presented. Exchange into the methyl group of the precursor and the product, as well as addition to the double bond, are clearly visible in both spectra. The technique, as established, employed a Bruker AC 300 spectrometer, mainly used for solution work; the MAS probe was modified for tritium observation by increasing the tuning range of the high-frequency circuitry to cover the 300-320 MHz range. Apart from the safety precautions mentioned, which were inexpensive to install, no further modifications were necessary. Typical operating conditions using the 4-mm diameter zirconia rotors (heavy walled) were as follows: 320.13 MHz frequency with a 35 ~ pulse; MAS rate
3H NMR
217
10-17 kHz; 2-s repetition rate without dipolar decoupling or cross-polarisation. The FIDs were processed with an optimal, Lorenzian-Gaussian, resolution enhancement function and referenced to external hexamethyl disiloxane. Each rotor contained between 25 and 50 mg of material, with a total radioactivity in the range 10-30 mCi. It was convenient to run the samples overnight, although spectra could be obtained over shorter periods (4-6 h); the latter could be reduced easily by using material of higher specific activity-the conditions chosen were a good compromise between safety and convenience.
Acknowledgements We are grateful to SERC (and more recently EPSRC) for supporting the tritium work at Surrey through several grants. APS thanks ICI Katalco and TAV and DZ Biocompatibles for research studentships.
References 1. E.A. Evans, D.C. Warrell, J.A. Elvidge and J.R. Jones, Handbook of Tritium NMR Spectroscopy and Applications. John Wiley, Chichester, UK, 1985. 2. J.R. Jones, in A. Townshend, S.J. Haswell, R. Macrae, H.W. Werner, P.J. Worsfold and I.D. Wilson (eds.), Encyclopedia of Analytical Science 6, 1995, 3503: Academic Press, London. 3. J.P. Bloxsidge and J.R. Jones, Spectroscopy Europe 6(5) (1994) 10. 4. E.W. Wooten, K.T. Mueller and A. Pines, Acc. Chem. Res. 25 (1992) 209. 5. S.A. Carss, R.K. Harris, P. Holstein, B.J. Say and R.A. Fletton, J. Chem. Soc., Chem. Soc., Chem. Commun. (1994) 2407. 6. D.B. Ferguson, T.R. Krawietz and J.F. Haw, J. Chem. Soc., Chem. Commun. (1995) 1795. 7. A. Nordon, R.K. Harris, L. Yeo and K.D.M. Harris, J. Chem. Soc., Chem. Commun. (1997) 961. 8. A.N. Clayton, C.M. Dobson and C.P. Grey, J. Chem. Soc., Chem. Commun. (1990) 72.
Chapter 6. 4
Solid State NMR o f Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
15N NMR T.A. Cross Department of Chemistry, Florida State University, Tallahassee, Florida, USA
6.4.1
Introduction
15N NMR has grown in importance over the last two decades as an important tool for characterizing the structure and dynamics of polymers, both synthetic and biosynthetic. Other reviews have appeared in the recent literature [1, 2]. This chapter will focus on both the characterization of structure and dynamics of biopolymers from spectra obtained without the use of magic-angle spinning. The advantages of using 15N for the structural and dynamic characterization of polymers are many. For most solid state NMR experiments, isotopic labeling is required, not only does it improve sensitivity, but it also provides selectivity. For all but the most preliminary experiments, selectivity is essential so that site specific questions can be addressed and the heterogeneity in the environment of the observed nuclear sites can be minimized. One of the advantages for 15N is that there is very low natural abundance (0.3% compared to 1% for 13C). Consequently, upon isotopic labeling, the labeled site stands out from the natural abundance, even when labeling is not particularly efficient, or when there are many sites that contribute to the natural abundance signal. Similarly, in most polymers the number of nitrogen sites is much lower than the number of carbon sites. Again, this has the result of lowering the natural abundance background signal, thereby improving selectivity. From a chemical perspective, nitrogens are chemically reactive, often involved in hydrogen bonding and, therefore, represent very interesting functional sites for a detailed characterization. Often carbon is away from the site of chemical reactivity and they rarely provide significant interactions for polymer conformation, other than covalent interactions, of course. Other common nuclei, such as 170 and ill, are difficult to study by solid-state NMR although recent advances promise much for the future. ~70 is a quadrupole 9 5 nucleus of spin-5, but recent advances with multiple quantum detection [3], double rotation (DOR) [4], and dynamic angle spinning (DAS) [5], show promise that with higher magnetic fields, considerable 170 NMR may be possible in the future. Recently, magic-angle spinning rates have increased to 40 kHz and concurrently a remarkable improvement in 1H solid-state
lSN NMR
219
NMR spectra has been realized, especially in samples that possess significant dynamics. However, at present, 15N remains for most polymeric systems, the chemically most interesting and accessible nucleus for the solid-state NMR spectroscopist. 15N has some disadvantages as well. It has a very low gyromagnetic ratio, a factor of 10 lower than 1H. Consequently, the nuclear spin energy levels are a factor of 10 closer together and the frequency for exciting these nuclei is a factor of 10 lower than 1H. This results in substantially decreased sensitivity. In addition, 15N Ta relaxation is often quite long especially in rigid polymers. Fortunately, for those polymers that have a significant population of protons, cross-polarization can be used to circumvent both these problems. The sensitivity enhancement is proportional to the ratio of the gyromagnetic ratios and ideally provides a factor of 10 in signal enhancement. Furthermore, the polarization transfer can be repeated after only waiting for 1H TI, which is often much shorter that 15N T1 relaxation times. However, full advantage of this benefit cannot always be made because in hydrated samples, especially those with salt and water present, the radio frequency (rf) pulses heat the sample. Consequently, the delay between polarization transfer times is often dictated by a sufficient waiting period to allow for sample cooling. For these particular samples, the increased relaxation times at higher magnetic field strengths do not represent a significant problem and, consequently, the sensitivity benefit associated with higher fields can be fully realized. Now that some of the advantages and disadvantages of 15N solid-state NMR have been described, the scope of this chapter will be outlined. First, several spin interactions involving 15N nuclei will be described. The molecular basis for these interactions is important for a detailed analysis of 15N NMR spectra leading to both a dynamic and structural characterization. The separation of structural and dynamic influences on the NMR spectra will be discussed. This is a very important and challenging task that represents one of the major strengths of solid-state NMR compared to electronic and vibrational spectroscopy, as well as solution-state NMR spectroscopy. One of the primary ways to separate these influences is to observe spectra at low temperature (<200 K), where all but the methyl and amine rotational motions cease. If these motions are of interest, they also need to be characterized and their influence on the spectra described. With a dynamic description in hand, a description of how high resolution structural information can be obtained and analyzed will be presented. Finally, a brief view to the future of this rapidly expanding field of study will be presented.
220
6.4.2
T.A. CROSS
Interactions
The chemical shift interaction is most frequently studied and a number of chemical shift tensors are well characterized in polymers [6-8]. For both an analysis of structure and dynamics it is important that the tensor element magnitudes and tensor element orientations relative to the molecular frame (i.e., the covalent bonds associated with nuclear sites) be known. Fig. 6.4.1 shows the orientation of the amide 15N chemical shift tensor with respect to the molecular frame as defined by two Euler angles; for amide nitrogens in a polypeptide backbone, typically aD = 0 -----5 ~ and /3D = 104 -----2 ~ [9]. Initial characterizations of chemical shift tensors are often achieved by studying the rotation patterns of model compound single crystals about orthogonal axes [7, 8]. Such patterns are complex if several molecules form the unit cell. Each molecular orientation gives rise to a single relatively sharp resonance. As the crystal is rotated, the chemical shift undergoes excursions across the full range of frequencies defined by the magnitude of the tensor elements. If the molecular frame orientation is known with respect to the crystal frame and the rotation axes are known with respect to the crystal frame, then it is possible to uniquely define the chemical shift tensor element magnitudes and orientations in the molecular frame. However, this is only possible with model compounds and in a unique experiment, Hiyama et al.
I
I
~
N-H
I
N-D
--H
Ca
z5N
C1
VN-C
Fig. 6.4.1. 15N chemical shift, 15Nm13C, 15Nm2H and 15N~lH dipolar interaction tensor orientations relative to the molecular frame of a peptide backbone amide site. Typically, 0-22 is approximately perpendicular to the peptide plane, 0-33 lies in the CINH bond angle and 0-11 completes the orthogonal coordinate system. The dipolar interactions have their unique tensorial axis aligned with the corresponding covalent bond.
15N NMR
l'
."
'
1
'l
i
9
9
9
9
221
~
i
'
i
i
'!
!
200
I00
i
'
i
i
',"
I
0
-I00
PPM
B
o~176
'"
,
i
200
.
.
.
.
i
'"
100
'
'
'"
..... i
0
.
.
.
.
I
-100
"
PPM
6.4.2. 15N chemical shift powder pattern spectra of Boc-Gly-Gly-[15N]Gly-OBz obtained by cross-polarization and 1H decoupling. (A) Crystalline phase (monoclinic). (B) Microcrystalline phase (triclinic). Dotted lines represent spectral simulations. Asymmetry, 77, for the monoclinic sample is 0.064 while 77 = 0.44 for the triclinic sample. Chemical shift reference is arbitrary. (Reprinted with permission from Hiyama et al. [10].) Fig.
[10], showed just how variable the tensor element magnitudes can be. For the same compound in two different crystal forms (Fig. 6.4.2) the asymmetry changed from 0.06 to 0.44. This raises an important concern about solid-state NMR, the tensor element magnitudes need to be characterized for the nuclear site of interest and, preferably, for the molecule of interest in the conformation of interest. Fortunately, powder pattern spectra can be recorded and the tensor element magnitudes observed directly from the spectra (Fig. 6.4.3(A)). From the rather limited database available on chemical shift tensor orientations, it appears that the amide 15N orientations are more independent of the chemical environment than the tensor element magnitudes [9]. When necessary, the tensor element orientations can be determined relative to a dipolar axis, such as the aSN~13C dipolar axis of a covalently linked pair of nuclear spins [11-13]. To a very good approximation, the unique axis of the dipolar interaction, is aligned parallel to the bond vector direction. By assuming that the unique tensor element of the amide chemical shift tensor is
222
T.A. CROSS
A
B
C
D
I
3OO
I
200
I
100 ppm
I
0
I
-IOO
Fig. 6.4.3. 15N chemical shift powder pattern spectra of labeled gramicidin A. (A) 15N~ Trpll gramicidin A--experimental data obtained with cross-polarization and 1H dipolar decoupling at 20.3 MHz for 15N. (B) spectral simulation with O'll = 36, 0 " 2 2 = 63 and 0"33 = 194 ppm relative to the resonance from a saturated solution of 15NHaNO3. (C) [13Ca]Leulo-[15N,~]Trp11 gramicidin A - - e x p e r i m e n t as in (A) displaying a combination of 15Nchemical shift anisotropy and the 15Nnl3c dipolar interaction. (D) spectral simulation with the same 0"ii values as in (B) and with aD = 0 ~ and/3D = 106 ~
in the molecular plane formed by the amide group, it is possible to define accurately the orientation of the other two axes by observing the dipolar coupled chemical shift powder patterns as shown in Fig. 6.4.3(C, D). This assumption has been upheld by each of the single crystal studies performed to date. In addition to the chemical shift and 15N~13C dipolar interactions, it is also possible to observe the 15N~1H and 15NmZH dipolar interactions. Examples of such spectra from uniformly oriented samples are shown in Fig. 6.4.4 [14, 15]. An advantage in observing such dipolar spectra is that the dipolar tensors are virtually axially symmetric, i.e., two of the tensor elements are the same.
15N NMR
223
. . . . . .
(3
,,
3;0 . . . .
"2;0
....
i
0 ....
" 5 0
.....
".....
,
.oo
,,
o
p.p.m.
ppm
Fig. 6.4.4. 15N spectra obtained at 20.3 MHz of labeled gramicidin A from different sample preparations in oriented lipid bilayers. (A) deuterium exchange of the indole proton in [15NExTrp9] gramicidin A results in the observed 2H--15N dipolar triplet centred on the observed (B) chemical shift. Dipolar triplet observed at a 15N frequency of 20.3 MHz. (B) 15N resonance prior to deuterium exchange. A two-dimensional separated local field spectrum shows a doublet for the ~SN--1H dipolar interaction. A slice through the chemical shift axis at 145 ppm is shown to the left. The analysis of the ~SN--1H doublet and the 15N--2H triplet yields the same bond orientation for this site in the gramicidin channel. (C) ~SN--I3c dipolar splitting observed from [I3C~]AIa3-[15N]Leu4 gramicidin A. Modified version of figure from Hu and Cross [15]. (Reprinted with permission.)
Moreover, for covalently linked pairs of atoms, the unique axis of the tensor is parallel to this covalent bond direction and the magnitude of the dipolar interaction can be calculated directly from a knowledge of the gyromagnetic ratios, ~'N and Tx and the internuclear separation. vii-
yN yxhr-3(27r)
-2 .
(6.4.1)
This latter number is well known for heavy atoms, but there is considerable debate for N ~ H and N ~ D pairs. Since the magnitude is dependent o n r -3, small errors in r can translate into significant errors in vii. While there are precise N ~ H and N ~ D bond lengths from neutron diffraction, these values are only consistent with the nonspinning N M R data [16]. The magic-angle spinning (MAS) studies suggest a 5% longer bond length, or a vibrational
224
T.A. CROSS
amplitude, that is not consistent with infrared spectroscopic data [17]. Despite this ambiguity, for the sorts of studies reported here, the neutron diffraction bond lengths are appropriate and the dipolar magnitudes can be calculated. However, for all of these interactions it is important to have the dynamics effectively modeled. Librational dynamics of significant amplitude can average the tensor elements. Such motions are present even in polycrystalline samples [18]. Shown in Fig. 6.4.5 are powder pattern spectra at 276 K, below the gel to liquid crystalline phase transition that quenches global dynamics, but retains local dynamics [19] and at 143 K, well below the temperature that quenches most librational motions [20]. The small, but significant, increase in the anisotropy or width of the powder pattern suggests that at room temperature librational averaging is occurring. Consequently, it is important to have the dynamics characterized. Note, that for a molecular site, it is only necessary to characterize the dynamics once and then for the range of interactions observed at that site the motionally-averaged dipolar magnitudes can be calculated.
6.4.3
Dynamics
The previous discussion is a clear example of the separation of dynamic and structural influences on NMR spectra. Before a detailed analysis of anisotropic spin interaction data can be realized, the dynamic averaging of the tensors must be appreciated. In fact, a detailed description of local dynamics can be achieved in which models of the motions are characterized by an axis fixed in the molecular flame, an amplitude for the motion and a motional model, such as diffusion within an arc or three site jumps, etc. [21]. The data in Fig. 6.4.6 shows that the averaging of the tensor above 200 K for this 15NE1 labeled indole site is anisotropic [22]. The motional axis must be close to the 022 element, since this element of the tensor is not averaged very much. In fact, the data can be very clearly fit to a motional axis consistent with the X2 torsional axis and an amplitude of ___19~ of librational diffusion in a X2 rotameric potential energy well. While such data is sufficient to define the motionally-averaged tensors and interpret the structural constraints in light of such tensors, it has not defined the frequency of the molecular motions. Dynamics are gaining increasing attention in the polymer field for characterizing the flexibility, noncrystalline domains and phase boundaries. In biological polymers, the characterization of dynamics can lead to insights about the kinetic rates for catalysis and ion conductance. One of the greatest strengths of solid-state NMR is its ability to define
15N N M R
I
. . . .
300
I
250
'
'
'
'
I
200
. . . .
I
150
'
'
"
'
i
I00 ppm
225
'
'
'
'
i
50
. . . .
i
0
'
'
'
'
I
-50
Fig. 6.4.5. 15N powder pattern spectra of [15N,~]Trp13 gramicidin A in a lipid environment as a function of temperature. (A) at 143 K all significant amplitude motions except for methyl and primary amine groups cease. Samples were fast frozen by plunging thin films into liquid propane. 0 - 1 1 --" 43.0, 0 " 2 2 --" 65.5 and 0 " 3 3 = 204.0 ppm. (B) At 276 K in this DMPC environment the global motion is eliminated, but the local motional amplitude is similar to that above the phase transition temperature of approximately 28~ O"11 = 43.0, 0 " 2 2 - - 6 5 . 0 and 0 " 3 3 = 196.0 ppm.
m o l e c u l a r m o t i o n s . It is far m o r e successful in b e c a u s e s o l u t i o n N M R d o e s n o t g e n e r a t e the d a t a t u r a l " m o d e l of the m o t i o n s as d e s c r i b e d a b o v e . a s s u m e d , or w h a t is typically d o n e is to use t h e
this t h a n s o l u t i o n N M R , n e c e s s a r y to f o r m a "strucInstead a model must be " m o d e l - f r e e " a p p r o a c h of
T.A. CROSS
226
._,2x
B
150
'2 ~..//I : r
--'--"~J //
""~"N k~k
/~ /
243K
"5-, ~'--'"~ "., ',,
o
// I'
150
'
'"'
'
i
100
""
'
" /
50
. . . .
[
. . . .
0 ppm
I
-50
. . . .
I
. . . .
-100
I
-150
-I00
. . . . . . . . . . . . . . . . . . . . . . i ! i !
i
~, . . . . . i
I ....
!
0 I0 20 30 Amplitude Around X2 (:t:~i')
Fig. 6.4.6. Detailed characterization of local dynamics in an indole side-chain. (A) 15N powder pattern spectra as a function of temperature of [15NE1--Trp,1] gramicidin A in fully hydrated lipid bilayers. Samples were fast frozen and then allowed to warm incrementally as the data set was recorded. Each spectral simulation is a repeat of the simulation of the 143-K spectrum to show the motional averaging as a function of temperature. (B) data from spectral simulations of the experimental spectra in [A] showing a significant change in two of the three tensor elements as a function of temperature dictating that the local motions are anisotropic in nature. (Modified version of figure from Hu et al. [22], reprinted with permission.)
Lipari and Szabo [23]. However, the latter approach provides only a nebulous order parameter and no clear indication of the motions in the molecular frame. In solid-state NMR, the relaxation data can be interpreted in light of an experimentally defined model [24]. As in solution NMR, it is better to obtain the relaxation data at more than one field strength. Figure 6.4.7 shows the interpretation of two sets of relaxation data obtained from a specific site, Trp9, in the polypeptide backbone of gramicidin A [24]. From powder pattern studies it has been shown that the local motions occur about an axis consistent with the C~i--C~i+I axis, and that the motion is a librational motion of about +20 ~ [21], quite similar to the indole side-chain described earlier. The data in Fig. 6.4.7 has been interpreted in light of this experimentally defined motional model. However, the field-dependent relaxation data suggests that the amplitude is much less than -+20~ in fact it is closer to a root mean square amplitude of 5 ~ However, this apparent
~SN N M R
20 v
"o
OOee 0 9 0 9 OOo
15
0
CDeo
0 0
0 O0 0
9
0
l-l~e e
0
*
Oee
D
<
5
9 ee
9
I"I o o OD 9 OOo 9
~.10 E m E
227
9 OOee 01~o
O0 I"10
DDtB~BQ
ee
oe
ee
9 ee
ee oo oo
9
9
9
9 9
9 9
L_
, I ....
-10
I ......
I ....
-9 -8 kog(~,) (s)
!,,
-7
Fig. 6.4.7. Analysis of T1 relaxation data obtained at two field strengths (dots = 4.7 T, boxes = 9.4 T) from [15N~]Trp9 gramicidin A in hydrated and oriented lipid bilayer preparations. The analysis is constrained to an experimentally defined motional model in which the peptide plane is librating about the C~--C~ axis. The 95% confidence limits are displayed for the motional amplitude and frequency of each relaxation data set. The solution set shows a small amplitude and a nanosecond timeframe for the librational motions. (Reprinted with permission from North and Cross [24].)
conflict is readily resolved. The powder pattern experiments are sensitive to all motional frequencies greater than approximately 105 Hz. The relaxation data is very sensitive to motions near the Larmor frequencies of the dipolar coupled nuclei. If even small amplitude motions occur on this nanosecond timescale it will induce efficient relaxation. Only when nanosecond motions are absent is the T1 relaxation data sensitive to the pico- or microsecond motions. Consequently, we refer to the powder pattern analysis as a linear detector of molecular motion (for frequencies > 105 Hz), whereas relaxation data is a nonlinear detector of motional frequencies. Therefore, the T~ measurements shown here resolve part of the spectral density function in defining a 5~ motion in the nanosecond timeframe. Clearly, additional motions are present either in the micro- or picosecond timeframes, to account for the rest of the amplitude observed in the powder pattern studies. Therefore, the combination of powder pattern and relaxation studies is very powerful in that the powder pattern analysis has seen the sum of the motions, and relaxation data can be used to selectively characterize given motional frequencies. Other relaxation parameters such as Tip can be used to pursue different frequency regimes. Again, solid-state NMR has been shown to be a very powerful approach for characterizing dynamics. Here the 10-ns correlation time observed in the polypeptide backbone of the gramicidin A cation channel is identical to the timescale that the electrophysiologists say the cation takes to move from one carbonyl site to the next in the channel [24]. In other words, the molecular dynamics and functional kinetics occur
228
T.A. CROSS
on the same timescale, and represents potential correlations between dynamics and cation conductance.
6.4.4
Structure
The orientational constraints shown in Fig. 6.4.4 represent very precise and accurate structural constraints. These peptide containing lipid bilayer preparations have been oriented between thin (75 txm) glass plates and hydrated to approximately 45% by weight water. In a magnetic field, the anisotropic diamagnetic susceptibility helps to form a uniformly oriented sample. In fact, orientational dispersion of as little as _+0.3~ has been documented for these bilayer preparations [22]. The observed nuclear spin interactions from such oriented samples provide a structural constraint by constraining the orientation of individual nuclear sites to a common axis, the direction of the magnetic field as well as the bilayer normal and channel axis. By having two dipolar constraints in each peptide plane, the orientation of this plane with respect to these axes is defined. By doing this for adjacent planes the relative orientation of the two planes can be defined as shown in Fig. 6.4.8 [25-28]. However, each structural constraint does not yield a unique structural solution [28], for the chemical shift, O'ob s = OVll COS 2 011 -~- 0"22 COS 2 022 -~- 0"33 COS 2 033 ,
(6.4.2)
1 -- COS 2 011 -'1- COS 2 022 -at- COS 2 033 ,
(6.4.3)
where 0ii represents the direction angles of each tensor element to the magnetic field direction. While O'ob~is the chemical shift observed from oriented samples and oii is defined as described previously. There are three angles and two equations and, therefore, the observed chemical shift does not uniquely define the orientation of the chemical shift tensor and the molecular frame with respect to the field. Hence, these NMR observables are structural constraints rather than unique determinants of structure. For the dipolar interaction analytical solutions are achieved, A bob s -- PI[N-X(3 COS 2 0 -- 1 ) ,
(6.4.4)
where 0 is the angle of the unique tensor element, i.e., the internuclear vector, with respect to the field, vfl is the interaction magnitude as described earlier and A P o b s is the observed dipolar splitting from an oriented sample. However, the sign of A/,Pob s is not known unless the magnitude of A/,Pob s is
15N NMR
229
V•Y2 +
Fig. 6.4.8. Pairs of adjacent peptide planes can be used to define the backbone torsion angles. These peptide plane orientations are defined by a pair of dipolar interactions for each plane. Combinations of these diplanes leads to the development of an initial structure. In a similar way, initial side-chain conformations can be characterized from orientational constraints in the side-chain. (Modified version of figure from Ketchem et al. [27], reprinted with permission.)
greater than vii, in which case, the sign is positive. F u r t h e r m o r e , the sign of cos 0 cannot be d e t e r m i n e d by this approach. T h e r e f o r e , there are two to four solutions for each dipolar constraint. W h e n two dipolar constraints are c o m b i n e d for determining the orientation of the peptide plane, m a n y combinations of the two constraints are not viable because of k n o w n covalent g e o m e t r y in the plane. For instance, an N - - H orientation of 10 ~ with respect to the magnetic field direction, and an N - - C orientation of 60 ~ a c c o m m o d a t e s a m a x i m u m H N C b o n d angle of 70 ~ Since the b o n d angle is much greater than this, the combination of orientational solutions is eliminated from consi-
230
T.A. CROSS
deration [28]. Furthermore, the observed 15N chemical shift is not consistent with many of the combinations. Typically, a unique peptide plane orientation can be achieved. When two adjacent planes are brought together, only four possible 4~, ~ torsional solutions result [29]. All of these conformations result in the same polypeptide fold and hydrogen bonding pattern in the gramicidin channel [27]. This represents an initial structure to which can be added the side-chains largely defined by 2H quadrupolar-derived constraints. The initial structure is remarkable from the standpoint that each constraint has an error of a few degrees; two such constraints are used per peptide plane and yet the hydrogen bonding partners are accurately defined between the i and i + 6 peptide planes in the gramicidin structure. If the errors from each constraint are added, the sum from 14 constraints would have the potential of placing this structure far from the native fold. This is the beauty of absolute constraints and also reflects the accuracy of each constraint. Because each constraint orients the molecular frame with respect to the laboratory axis frame, the errors are just as likely to cancel as they are to add. This is because each constraint, constrains the molecule to the laboratory fixed-frame of reference. However, this initial structure is less than perfect. Hydrogen bonds are as much as 1 A from ideal /3-sheet type values and, between the side-chains, there is some van der Waals overlap. A program has been devised to refine the structure against all experimental constraints plus the hydrogen-bond distances and the C H A R M M energy [30]. In addition, initial assumptions, such as fixed covalent geometry and the planarity of the peptide plane, are relaxed in the refinement. Consequently, the cotorsion angles are determined through this refinement procedure. The refinement also resolves the final structural ambiguities [29] and results in a determination of torsion angles to -+5~ (Fig. 6.4.9). This structure represents the first peptide or protein structure determined by solid-state NMR and it has been deposited with the Brookhaven Protein Data Bank ( # I M A G ) . It also represents one of the highest resolution membrane bound peptide or protein structures determined to date. Orientational constraints have been collected for a wide variety of molecular systems from synthetic polymers [32, 33] to structural proteins, such as silk [34, 35]. Orientational constraints have also been collected for retinal bound to bacteriorhodopsin [36], suggesting a host of ligand receptor systems that might be studied. Orientational constraints have been collected on other synthetic and biosynthetic polypeptides in bilayer environments, such as Magainin-2, a toxin from frog skin [37], the M2 6 from the acetylcholine receptor [38] and M2-TMP from Influenza A virus [39]. Such studies have led to a description of the orientation of a-helices relative to the bilayer. Proteins such as the fd and Pfl bacteriophage coat proteins have also been
lSN NMR
231
Fig. 6.4.9. Once an initial structure has been achieved, the hydrogen bonds are identified and a refinement against all of the experimental constraints, ideal hydrogen bonding geometry and the CHARMM energy is performed to refine the structure. In doing so, the planarity of the peptide planes is relaxed as well as the ideal covalent geometry. The resulting structure represents the first high resolution structure experimentally defined in a hydrated lipid bilayer environment. While crystallography has produced some structures of membrane proteins, there is no recognizable lipid bilayer in the crystals. Highlighted here is the helical pattern of the oxygens that are important for cation conductance in the channel pore. (Reprinted with permission from Tian et al. [31].)
232
T.A. CROSS
studied. For the coat proteins where both uniform and amino acid specific labeling have been performed, enough constraints have been collected so that initial structural models in a bilayer environment could be developed [40]. In addition, orientational constraints and structural models have been developed for these same coat proteins as components of the intact Filamentous bacteriophage [41, 42].
6.4.5
Future developments
15N solid-state NMR has great demonstrated potential for structural and dynamic characterization of polymers. Much of the work has utilized synthetically labeled polypeptides. To make this structural and dynamic approach more generally applicable, it will be necessary to use biosynthetic labeling schemes for either amino acid specific or uniform labeling. Such labeling schemes introduces new problems such as resolving numerous resonances and making site specific assignments. Progress towards these goals has been made recently with the development of a new class of multidimensional solidstate NMR experiments based on the phase shifted Lee Goldburg experiment, called PISEMA [43]. These experiments lead to excellent resolution and correlations between 15N chemical shift and 15Nm1H dipolar interactions (Fig. 6.4.10, [44]). Additional dimensions including 1H chemical shift and 15NmlSN spin diffusion have also been demonstrated. This latter use of spin diffusion will lead to site specific assignments while the improved resolution will lead to the essential spectral resolution for resolving the signals from uniformly labeled samples. The use of orientational constraints represents only one solid-state NMR approach for polymer structural characterization. In the future, a combination of constraints will be used, no doubt, for such characterizations, in particular distance constraints from R E D O R [45], or rotational resonance [46] will be helpful. Direct measurement of torsion angles using DRAWS [47], C7 [48] or rotor-synchronized 2D exchange [49] may also prove to be beneficial. Therefore, on multiple fronts the prospects for 15N solid-state NMR are promising. In the past couple of years, high resolution 3D polypeptide structure has. been obtained from samples in anisotropic environments where structures are unobtainable by other means. Recently, new methods have been developed that will permit the observation of uniformly labeled samples. The coupling of distance and orientational constraints will be an exceptionally powerful structural approach. While the methods have been primarily demonstrated here with polypeptides and proteins, studies of nucleic acids and
15N NMR
233
-9
g"
(II
A
N "iv t~
_ 6 cQ
g
9
o tD Z
-3~
2b0
|la
a
150 15N Shift (ppm)
i
100
Fig. 6.4.10. Two-dimensional 1H--15N dipolar coupling/15N chemical shift PISEMA spectrum of an oriented sample of uniformly 15N-labeled fd coat protein in phospholipid bilayers. Some 256 transients were acquired for each of 64 tl values incremented by 40.8 Vs. The crosspolarization mix time was I ms and the recycle delay was 3 s. (Reprinted with permission from Marassi et al. [44].)
synthetic polymers are also possible with demonstrated significant results in the literature [32, 33, 50, 51].
Acknowledgements This effort has been supported through the National Institutes of Health (AI23007) and the research was largely performed at the National High Magnetic Field Laboratory supported by NSF Cooperative Agreement DMR-9527035 and the State of Florida.
References 1. A. Shoji, S. Ando, S. Kuroki, I. Ando and G.A. Webb, in G. Webb (ed.), Annu. Rep. NMR Spectr. 28 (1993), 55-98. 2. T.A. Cross, in G. Webb (ed.), Annu. Rep. NMR Spectr. 29 (1994) 123-167. 3. L. Frydman and J.S. Harwood, J. Am. Chem. Soc. 117 (1995) 5367.
234
.
10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.
T.A. CROSS A. Samoson, E. Lippmaa and A. Pines, Mol. Phys. 65 (1988) 1013. B.F. Chmelka, K.T. Mueller, A. Pines, J. Stebbins, Y. Wu and J.W. Zwanziger, Nature 339 (1989) 42. Z. Gan, D.M. Grant and R.R. Ernst, Chem. Phys. Lett. 253 (1996) 13. R.E. Stark, L.W. Jelinski, D.J. Ruben, D.A. Torchia and R.G. Griffin, J. Magn. Reson. 55 (1983) 266. G.S. Harbison, L.W. Jelinski, R.E. Stark, D.A. Torchia, J. Herzfeld and R.G. Griffin, J. Magn. Reson. 60 (1984) 79. W. Mai, W. Hu, C. Wang, and T.A. Cross, Protein Science 2 (1993) 532. Y. Hiyama, C.-H. Niu, J.V. Silverton, A. Bavoso and D.A. Torchia, J. Am. Chem. Soc. 110 (1988) 2378. T.G. Oas, C.J. Hartzell, F.W. Dahlquist and G.P. Drobny, J. Am. Chem. Soc. 109 (1987) 5962. Q. Teng and T.A. Cross, J. Magn. Reson. 85 (1989) 439. Q. Teng, M. Iqbal and T.A. Cross, J. Am. Chem. Soc. 114 (1992) 5312. S.J. Opella and J.S. Waugh, J. Chem. Phys. 66 (1977) 4919. W. Hu and T.A. Cross, Biochemistry 34 (1995) 14147. P.V. LoGrasso, L.K. Nicholson and T.A. Cross, J. Am. Chem. Soc. 111 (1989) 1910. J.E. Roberts, G.S. Harbison, M.G. Munowitz, J. Herzfeld and R.G. Griffin, J. Am. Chem. Soc. 109 (1987) 4163. M.H. Frey, J.A. DiVerdi and S.J. Opella, J. Am. Chem. Soc. 107 (1985) 7311. K.-C. Lee, W. Hu and T.A. Cross, Biophys. J. 65 (1993) 1162. N.D. Lazo, W. Hu, K.-C. Lee and T.A. Cross, Biochem. Biophys. Res. Commun. 197 (1993) 904. N.D. Lazo, W. Hu and T.A. Cross, J. Magn. Reson. 107B (1995) 43. W. Hu, N.D. Lazo and T.A. Cross, Biochemistry 34 (1995) 14138. G. Lipari and A. Szabo, J. Am. Chem. Soc. 104 (1982) 4546. C.L. North and T.A. Cross, Biochemistry 34 (1995) 5883. S.J. Opella, P.L. Stewart and K.C. Valentine, Quart. Rev. Biophys. 19 (1987) 7. Q. Teng, L.K. Nicholson and T.A. Cross, J. Mol. Biol. 218 (1991) 607. R.R. Ketchem, K.-C. Lee, S. Huo and T.A. Cross, J. Biomol. NMR 8 (1996) 1. T.A. Cross, in R.R. Muccino (ed.), Proc. of the 2nd Internat. Symp. on the Synth. and A of Isotop. Lab. Compds. Elsevier, Amsterdam: 1986 247-252. J.R. Quine, M.T. Brenneman and T.A. Cross, Biophys. J. 72 (1997) 2342. R.R. Ketchem, B. Roux and T.A. Cross, in K.M. Merz and B. Roux (eds.), Membrane Structure and Dynamics. Birkhauser, Boston, 1996, 299-322. F. Tian, K.-C. Lee, W. Hu and T.A. Cross, Biochemistry 35 (1996) 11959. T. Asakura, J. H. Yeo, M. Demura, T. Ito, T. Fujito, M. Imanari, L. K. Nicholson and T. A. Cross, Macromolecules 26 (1993) 6660. J.H. Yeo, M. Demura, T. Asakura, T. Fujito, M. Imanari, L.K. Nicholson and T.A. Cross, Solid-State Nuclear Magnetic Resonance 3 (1994) 209. L.K. Nicholson, T. Asakura, M. Demura and T.A. Cross, Biopolymers 33 (1993) 847. A.H. Simmons, C.A. Michal and L.W. Jelinski, Science 271 (1996) 84. A.S. Ulrich, A. Watts, I. Wallat and M.P. Heyn, Biochemistry 33 (1994) 5370. B. Bechinger, M. Zasloff and S.J. Opella, Protein Sci. 2 (1993) 2077. S.J. Opella, J. Gesell and B. Bechinger, in R. M. Epand (ed.) The Amphipathic Helix, CRC Press, Boca Raton, 1993, 87-106. F. Kovacs and T.A. Cross, Biophys. J. 73 (1997) 2511.
15N NMR 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.
235
P.A. McDonnell, K. Shon, Y. Kim and S.J. Opella, J. Mol. Biol. 233 (1993) 447. T.A. Cross, and S.J. Opella, J. Mol. Biol. 182 (1985) 367. K. Shon, Y. Kim, L.A. Colnago and S.J. Opella, Science 252 (1991) 1303. C.H. Wu, A. Ramamoorthy and S.J. Opella, J. Magn. Reson. 109A (1994) 270. F.M. Marassi, A. Ramamoorthy and S.J. Opella, Proc. Natl. Acad. Sci. U.S.A. 94 (1997) 8551. T. Gullion and J. Schaefer, J. Magn. Reson. 81 (1989) 196. D.P. Raleigh, M.H. Levitt and R.G. Griffin, Chem. Phys. Lett. 146 (1988) 71. M.A. Mehta, D.M. Gregory, S. Kiihne, D.J. Mitchell, M.E. Hatcher, J.C. Shiels and G.P. Drobny, Solid State NMR 7 (1996) 211. X. Feng, Y.K. Lee, D. Sandstrom, M. Eden, H. Maisel, A. Sebald and M. Levitt, Chem. Phys. Lett. 257 (1996) 314. D.P. Weliky and R. Tycko, J. Am. Chem. Soc. 118 (1996) 8487. T.A. Cross, S.J. Opella, G. Stubbs, and D.L.D. Caspar, J. Mol. Biol. 170 (1983) 1037. T.M. Alam, J. Orban, and G.P. Drobny, Biochemistry 29 (1990) 9610.
Chapter 6.5 170
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
NMR
S. Kuroki Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan
6.5.1
Introduction
The oxygen atom is one of the most important atoms constituting hydrogenbonding structures in polymers such as peptides and polypeptides. Nevertheless, solid-state 170 NMR studies for polymers have not been carried out previously. This is due to the very weak sensitivity of solid-state 170 NMR measurements which comes from the two following reasons: One is that the 170 nucleus has a very low natural abundance, which is 0.037%. Another is that the 170 nuclear spin quantum number (I) is 5/2, which implies a quadrupolar nucleus, and so the 170 signal is broadened by nuclear quadrupolar effects in the solid. On the other hand, solution-state 170 NMR spectroscopy has been successfully employed to elucidate a number of structural problems in organic chemistry [1-4], because 170 signal becomes very sharp due to the removal of the quadrupolar interaction by isotropic fast reorientation in solution. For example, as the oxygen atom is directly associated with the formation of a hydrogen bond, hydrogen bonding for the carbonyl group in various compounds often results in large low frequency shifts of the carbonyl 170 NMR signal [5, 6]. From these results, solution-state 170 NMR has been established as a means for investigating structural characterizations. From such situations, it can be expected that solid-state '70 NMR provides a deep insight into understanding the hydrogen-bonding structures of solid polymers such as peptides and polypeptides. In this chapter [7], we show the ~70 NMR spectra of polyglycine form I (PG I: B-sheet structure), polyglycine form II (PG II: 3~-helix structure), glycylglycine (GlyGly) and glycylglycine nitrate (GlyGly.HNO3) in the solid state, which cover a wide range of hydrogen bond lengths, and three types of NMR parameters such as chemical shift, quadrupolar coupling constant (eZqQ/h) and asymmetric parameter (rio), and to understand the relationship between the hydrogen-bonding structure and these NMR parameters.
237
170 NMR
6.5.2
Static 170 CP spectra of ~70-1abeled peptides and polypeptides
Figure 6.5.1 shows the plots of the 170 CP NMR signal intensity for GlyGly against the contact time. Since the carbonyl oxygen atom is not bonded to a hydrogen atom, a long contact time is need. The appropriate contact time was 9 ms. From the above plots, the Toi-i and 1HTlo values were determined to be 2.5 and 30.0 ms, respectively. The Toi-i value of GlyGly is much longer than those of inorganic solids such as A 1 0 (17OH), whose Toi-i value is 0.018 ms [8]. Static 170 CP NMR spectra of GlyGly were observed at 36.6, 54.2 and 67.8 MHz as shown in Fig. 6.5.2. Each of the spectra at 36.6 and 54.2 MHz consists of two major split signals, but the spectrum at 67.8 MHz has only one major signal. Such splitting comes from the quadrupolar interaction. By computer simulation, the quadrupolar coupling constant e2qQ/h was determined to be 8.55 MHz. This value is much larger than that for the amide 14N nitrogen nuclei (1.11 MHz). It is seen that the NMR spectra strongly depend on the measurement frequency, because the quadrupolar effect depends on the measurement frequency, and the broadening of the 170 NMR signal is further increased by quadrupolar effects as the measurement frequency decreases. Therefore, it can be said that a high-frequency measurement is needed for quadrupolar nuclei such as 170. Figure 6.5.3 shows the 67.8 MHz static 170 CP NMR spectra of PG I,
1.0 0.8 0.6 "~
0.4
o.2 II 0.0 ~ 0.0
I 5.0
!
I
10.0 15.0 Contact time(ms)
! 20.0
25.0
Fig. 6.5.1. Plots of peak intensity versus cross-polarization contact time for the GlyGly.
238
s. KUROKI 1
(e) 6
(b) 54.2 MHz
(a) 36.6 MHz
PPM i500
iO00
500
0
-500
-iO00
-1500
Fig. 6.5.2. (a) 36.6, (b) 54.2 and (c) 67.8 MHz 170 CP static spectra of GlyGly in the solid state, respectively.
PG II, GlyGly and GlyGly.HNO3. To determine the 170 NMR parameters unequivocally, the computer simulations for different frequencies such as 36.6, 54.2 and 67.8 MHz were carried out. Figure 6.5.4 shows 67.8, 54.2 and 36.6 MHz static 170 CP NMR spectra of GlyGly together with theoreticallycalculated spectra. Next, Fig. 6.5.5 shows the 67.8 MHz static 170 CP NMR spectra of PG I, PG II, GlyGly and GlyGly.HNO3 together with theoreticallycalculated spectra, respectively. The 170 NMR parameters of PG I, PG II,
170 NMR
239
(b) GlyGly
(a) GIyGIy.HNOs
PPM 800
700
600
500
400
300
200
tO0
0
-100
-200
-300
Fig. 6.5.3. (a) 67.8 MHz 170 CP static NMR spectra of GlyGly.HNO3, (b) GlyGly, (c) PG I and (d) PG II, respectively.
GlyGly and GlyGly.HNO3 obtained by computer simulations are shown in Table 6.5.1. From these results, the ~11, ~22 and 633 values of the samples change from 546 to 574 ppm, 382 to 425 ppm and -132 to -101 ppm, respectively. The magnitude of the change in 170 chemical shift is much larger than those of the carbonyl 13C and amide 15N chemical shifts. Every A6 defined by A6 = ~ 3 3 - ~iso, is about 400 ppm, which is much larger than those of carbonyl 13C and amide lSN chemical shifts, which are about 100
240
s. KUROKI
c) 67.8 MHz
. 9"
:.
600
~,:
400
200
b) 54.2MHz
ppm
0
I
I
l
600
M
!
"400
|
200
0
-200 -400
ppm
a) 36.6 MHz 9
9~.,,. g
..
1000
500"
0
-SO0 -1000
ppm
Fig. 6.5.4. (a) 36.6, (b) 54.2 and (c) 67.8 MHz 170 CP static spectra of GlyGly together with theoretically-simulated spectra, respectively.
ppm, respectively. Every r/value defined by r/= ( 8 1 1 - 622)1A6 is about 0.4. These values are common to the carbonyl oxygen in peptides. Though the chemical shift tensor is axially symmetry (611 = 622) for the case of the carbonyl oxygen of c-alanine amino acid as reported by Fiat et al. [9, 10], the chemical shift tensor of the carbonyl oxygen in peptides is not axially symmetric from these results. The eZqQ/h values change from 8.30 to 8.75 MHz. These values are larger than the eZqQ/h value of c-leucine which is 8.0 MHz at 190 K [10]. The rtQ values of polypeptides, such as PG I and II are 0.26 and 0.29, and the r/o values of oligopeptides such as GlyGly and GlyGly-HNO3 are 0.45, 0.47, respectively. This difference may come from large difference in molecular packing between them. Furthermore, it is found
170
C) PG I
NMR
" 29 . .~. 9 -.." ~ .
d)
241
PG
II
..:~. :. .v',
,~.-'~'~" . ~ F'--600
200
400
a) GIyGIy.HNO3
O'
ppm
,. "k"
l
600
, 400
, 200
,I 0
b)GlyGly
~."
'7 9 "~" : ~;.."
9 9 ... ~o
/
\
:~,~,,Z~
".
"-
:
: "~-"
~
"
,,.
...
:.
""' __.__J~'~" ~ 9
,
600
,,
400
200
,
0
ppm
!
ppm
/-/--I 600
9
"
I
400
200
0
ppm
Fig. 6.5.5. (a) 67.8 MHz 170 CP static NMR spectra of GlyGly.HNO3; (b) GlyGly; (c) PG I; and (d) PG II, together with theoretically-simulated spectra, respectively.
that the principal axis of the quadrupolar tensor and the principal axis of the 170 chemical shielding tensor for the carbonyl oxygens of peptides and polypeptides are not coincident with each other. The relationship between these two principal axes is shown in Fig. 6.5.6 for the situation of PG II. Each of the above N M R parameters is influenced strongly by the electronic structures of the molecules. This seems to reflect a slight difference in their affects by the characteristic electronic distributions of the carbonyl oxygen of peptides and polypeptides.
6.5.3 The direction of the principal axes of the electric field gradient tensor and the chemical shielding tensor of the carboyl oxygen
No studies have ever tried to determine the direction of the principal axes of the electric field gradient tensor and the chemical shielding of the carbonyl
t,~ 4~ t,,9
Table 6.5.1. Determined Sample
1 7 0 N M R parameters of solid peptides containing Gly residue
e2qQ/h (MHz)
Polypeptides PG II PG I Oligopeptides GlyGly GlyGly.HNO3
Angle ma (deg)
Chemical shift tensor (ppm) "I~Q
611
(~22
633
6iso
A6b
?c
a
/~
8.30 8.55
0.29 0.26
562 574
410 425
- 108 - 101
288 299
396 400
0.38 0.37
92 100
89 91
-81 -79
8.55 8.75
0.45 0.47
546 559
382 408
- 132 - 127
265 280
397 407
0.41 0.37
94 94
90 89
-87 -81
a Angle A indicates the Euler angles between the principal axes of the quadrupolar tensor and the chemical shift tensor.
b A6 = 6iso- 633. c T~-- ( 6 1 1 - t~-22)/A6,
0
170 N M R
243
V33 (5'11
~=8 . ~..........."'" o~=92 *
...-"
0'33.~ . . . . . . . . . . . .
> Vll
.-"
0"22 ~
v22 Fig. 6.5.6. The relationship between the principal axes of the quadrupolar tensor on the
carbonyl oxygen for the situation of PG II.
oxygen from experiment. The determined direction of the principal axes of the chemical shielding of the carbonyl oxygen is shown in Fig. 6.5.7 as determined by F P T - M N D O - P M 3 method. The 0"22 component lies approximately along the C = O bond, the cr11 component is aligned in the direction perpendicular to the C - - O bond in the peptide plane and, the 0"33 which is
io'33
\
~2
.--
.-''""
C0~
Fig. 6.5. 7. The direction of the principal axes of the chemical shielding tensor of the carbonyl
oxygen employing FPT-MNDO-PM3 method.
244
s. KUROKI
V33 "~
.
.
.
.
.
.
...'"co\
V,, Fig. 6.5.8. The direction of the principal axes of the electric field gradient tensor of the carbonyl oxygen employing FPT-MNDO-PM3 method.
the most shielded component, is aligned in the direction perpendicular to the peptide plane. It is a very interesting result that the most shielded component, 0"33, is not aligned along the direction of the C = O bond, or the direction of lone pair electron which are aligned 120~ or -120 ~ from the C---O bond direction on the peptide plane. It can be said that the sp 2 hybrid property of the carbonyl bond is removed due to the double bonding property of the peptide bond. On the other hand, the direction of the principal axes of the electric field gradient tensor of the carbonyl oxygen is shown in Fig. 6.5.8 as determined by the FPT-MNDO-PM3 method. The V22 component lies approximately along the ~ O bond, the V33 component is aligned in the direction perpendicular to the C = O bond on the peptide plane and the V i i component is aligned in the direction perpendicular to the peptide plane. It can be said that the largest component, V33 , of the electric field gradient lies along the molecular chain direction. The relationship between the electric field gradient tensor and the chemical shielding employing this calculation agree with the experimental results in Fig. 6.5.6.
6.5.4 Nuclear quadrupolar coupling constant of peptides and polypeptides
(e2qQIh) of carbonyl oxygen
The geometrical parameters and hydrogen-bonding geometrical parameters [11-14] of these peptides and polypeptides are shown in Table 6.5.2. Some
Table 6.5.2. The geometrical parameters and hydrogen-bonding geometrical parameters of peptides containing Gly residue Sample
P G II PG I GlyGly GlyGly'HNO3
Geometrical parameters
Hydrogen-bonding geometrical parameters
Dihedral angle (deg)
HB length (/k)
HB angle (deg)
Ref. HB dihedral angle (deg)
~b
~p
N... O
H... O
L C--~O. 99N
L N ~ H . 99O
N~C---~O. 99H
C z O - 99H ~ N
-78.0 -149.9 157.1 165.6
145.8 146,5 151.0 148.9
2.73 2.95 2.94 3.12
1.84 2.16 1.97 2.38
159 149 157 162
146 133 157 165
-47 68 -161 3
157 -173 -145 -156
9 Z 11 12 13 14
t,~ 4~ t.~
246
s. KUROKI 9.0
I
I
I
I
8.8
8.6 i.=,.,
8.4
8.2
I
8.0
2.7
I
I
I
2.8 2.9 3.0 3.1 Hydrogen bond length RN...o(,~)
3.2
Fig. 6.5.9. Plots of the e2qQ/hagainst the hydrogen bond length. of the geometrical parameters were calculated by using the unit cell parameters and fractional coordinates as given in the literature [11-14]. Figure 6.5.9 shows the plot of the observed e2qQ/h values against the hydrogen bond length. The e2qQ/h values decrease linearly with a decrease of the hydrogen bond length (RN... o). This relationship is expressed by
e2qO/h (MHz) = 5.15 + 1.16RN... o (/k).
(6.5.1)
This change comes from a change of the q values which are the largest component of electric gradient tensor (V33). This experimental result shows that a decrease in thehydrogen bond length leads to a decrease of the electric gradient. The q value seems to be very sensitive to the change in hydrogenbonding length.
6.5.5 170 NMR chemical shifts of carbonyl oxygen of peptides and polypeptides From Table 6.5.1, there is a large difference in the chemical shift value between peptides and polypeptides. Figure 6.5.10 shows the plot of the
~70 NMR
250
I
247
I
~" 260
I
I
"-.GlyGly
O
"0.
~
~ 27o "'O.
o 280
GIyGIy.HNO~
P G II
"0
o 290
e~
o "Q)..
o 300 o tt~ la,,u
PG I'" I
310 2.7
2.8
!
I
!
2.9 3.0 3.i Hydrogen bond length RN...o(/~)
3.2
Fig. 6.5.10. Plots of the observed isotropic 170 chemical shift against the hydrogen bond length. observed isotropic 170 chemical shifts (~iso) against the hydrogen bond length (RN... o). The 6iso values in both peptides and polypeptides move to low frequency with the decrease in the hydrogen bond length (RN... o). Figure 6.5.11(a-c) show the plot of the observed principal values of the 170 chemical shifts against the hydrogen bond length (RN... o). Every principal value in both peptides and polypeptides moves to low frequency with a decrease in the hydrogen bond length (RN... o).
6.5.6
Polyalanines
Figure 6.5.12 shows static 170 CP NMR spectra of solid (Ala),,[A/I = 100] with an c~-helix form at (a) 67.8 MHz and (b) 36.6 MHz together with theoretically-simulated spectra. The spectrum at 36.6 MHz consists of two major split signals, but that at 67.8 MHz has one major signal overlapping with two other signals. Such a large variation comes from the quadrupolar interaction because the appearance of the spectrum depends on the NMR frequency. If the NMR frequency is extremely high, the influence by quadrupolar interactions may be neglected in the spectrum. By computer simulation, the obtained e2qQ/h value and chemical shift tensor components for the a-
t,J
530
'
,
a)
'
,
,
380
,
I~
I
'
-140
I
i
GlyGly
b)
C)
t t
540 -
-130 -
390 GlyGiy
""0-. ,,
_
~,GlyGly'HNO 3
.
I~-120
E 400
%
l
GlyGly-HNO;
\
~550 -
~CI, I'G H
l
GlyGly
t
"'O
~560
a ~
q.
PG II
e,I
oo410
PG II
~-110
GlyGly.HNO 3
C
% %
"~-~
%
PGI
0
%
570 -
" ",
%
420 -
PG I ~)-..
~
, 9 PG
-100 -
I
"O.
_
%
580
I
t
,
t
I
430
2.7 2.8 2.9 3.0 3.1 3.2 H y d r o g e n b o n d l e g t h (,~)
Fig. 6.5.11. Plots of the observed principal values of bond length R N . . . o.
,
2.7
I
I
2.8
2.9
Hydrogen 170
"l
I
3.0
3.1
-90
3.2
2.7
bond length(~)
chemical shift tensor (a)
t~11 ,
(b)
~
I
I
2.8
2.9
3.0
Hydrogen t~22
and (c)
633 ,
bond
,
,,
3.1
3.2
length(~)
respectively, against the hydrogen
NMR
170
(a) 6 7 . 8 M H z
249
A
wi 9.
I
,
1000
,
9
800
t
.
600
'..y
.,.
'...
~
~
~
v
~
400
200
0
-200
-400
(b) 36.6MHz
9
~ (ppm)
-600
t-. !;
t:"..
oI
I
'i
,%;
+'.,, . . , ;
,,,.
(ppm)
1 SO0
1000
SO0
0
-SO0
-1000
-1500
Fig. 6.5.12.
170 CP static N M R spectra of (L-Ala),,[A/I = 100] together with theoreticallysimulated spectra, respectively.
helix are shown in Table 6.5.3. Similarly, the static 170 NMR spectral analyses of solid (Ala),,[A/I = 5] with a /3-sheet form as the major components (Fig. 6.5.13) were carried out. The obtained NMR parameters for the /3sheet form are shown in Table 6.5.3, together with the NMR parameters obtained from PG I (/3-sheet form) and II (31-helix form). The e2qQ/hvalues for poly (L-alanine) and polyglycine with the same/3sheet form are very close to each other. This can be understood from the experimental results that the hydrogen bond lengths for both of the/3-sheet forms are very close to each other as shown from the determination of the hydrogen bond length (RN... o = 3.02 A) by the observation of the amide carbonyl 13C chemical shift [15]. The isotropic 170 chemical shift for (Ala)n appears at lower frequency than that for polyglycine. It comes from the large low frequency shift of (~11, the direction of which is perpendicular to the C ~ O bond in the amide plane. Therefore, the hydrogen bond angle for both of the/3-sheet forms may be different from each other. The a-helix form for (Ala),, has different e2qQ/h value from that of the /3-sheet form. The former is larger than the latter. Also, the chemical shift tensor components are very different from each other. Such differences come from the different hydrogen bond lengths and angles.
tO O
Table 6.5.3. Determined 170 N M R parameters of (Ala)n together with those of PGs Sample
(MHz)
Angle ma (deg)
Chemical shift tensor (ppm)
e2qQ/h 70
~11
i~22
~33
~iso
a
j~
(Ala)n a-helix /3-sheet
9.28 8.65
0.38 O.41
595 514
435 390
- 121 - 265
303 265
100 83
76 106
-80 -85
(Gly). PG II PG I
8.30 8.55
0.29 0.26
562 574
410 425
-108 -101
288 299
92 100
89 91
-81 -79
a Angle A indicates the Euler angles between the principal axes of the quadrupolar tensor and the chemical shift tensor.
0
170
NMR
(a) 67.8MHz
251
I
t'[
:
" ,v
,
~.
l v./]A' t ~t ....
t...,...1..,
1000
800
t...
600
400
!...
200
I.
. l...
0
-200
l...l(ppm
-400
)
-600
(b) 36.6MHz
9t
I
i
t .... 1 SO0
, .... 1000 .
, .... SO0
i .... 0
! ..... -SO0
I ..... -1000
~ (ppm) -1 SO0
Fig. 6.5.13. 170 CP static NMR spectra of (L-Ala),[A/I = 5] together with theoretically-simulated spectra, respectively. 6.5.6
Conclusion
From the observed carbonyl oxygen 1 7 0 N M R spectra of PG I, PG II, GlyGly and GlyGly.HNO3 in the solid state, it is found that the e2qQ/h values decrease linearly with a decrease in the hydrogen bond length. This indicates that it is possible to determine the hydrogen bond length through the observation of e2qQ/h values. It was found that there is a difference in r/o between the polypeptides and oligopeptides. This may come from large difference in the molecular packing between them. The chemical shift values in both peptides and polypeptides move to low frequency with a decrease in the hydrogen bond length. However, there is a difference in the chemical shift values between peptides and polypeptides. From these experimental findings, it is demonstrated that 1 7 0 N M R spectroscopy becomes a useful means for elucidating the hydrogen-bonding structure in solid peptides and polypeptides. Most recently, the multiple quantum MAS N M R technique was applied to obtain a high resolution solid-state 170 N M R spectrum [16-17], but it seems to need much more time before this technique becomes useful for solid-state 1 7 0 NMR.
252
s. KUROKI
References
.
.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
D.W. Boykinm (ed), 170 NMR Spectroscopy in Organic Chemistry. CRC Press, Boca Raton, FL, 1991. A.L. Baumstark and D.W. Boykin, 170 NMR spectroscopy: applications to structural problems in organic chemistry, in A.L. Baumstark (ed), Advances in Oxygenated Processes, vol. III. JAI Press, Greenwich, CT, 1991, 141. W.G. Klemperer, in J.B. Lambert and F.G. Riddell (eds), The Multinuclear Approach to NMR Spectroscopy. Kluwer, Dordrecht, 1983, 245. J.P. Kintzinger, in P. Laszlo (ed), Newly Accessible Nuclei, vol. 2. Academic Press, New York, 1983, 79. D.W. Boykin and A.L. Baumstark, New Journal of Chemistry 16 (1992) 357. D.W. Boykin and A. Kumar, J. Heterocyclic Chem. 29 (1992) 1. S. Kuroki, A. Takahashi, I. Ando, A. Shoji and T. Ozaki, J. Mol. Struct. 323 (1994) 197. T.H. Walter, G.L. Turner and E. Oldfield, J. Magn. Reson. 76 (1988) 106. R. Goc, E. Ponnusomy, J. Tritt-Goc and D. Fiat, Int. J. Pept. Protein Res. 31 (1988) 130. R. Goc, J. Tritt-Goc and D. Fiat, Bull. Magn. Reson. 11 (1989) 238. F.H.C. Crich and A. Rich, Nature 176 (1955) 780. W.T. Astbury, C.H. Dalgleish, S.E. Darmon and G.B.B.M. Sutherland, Nature 69 (1948), 596. A. Kevick, A.R. AI-Karaghouli and T.F. Koetzle, Acta. Cryst. B33 (1977) 3796. S.N. Rao and R. Parthasarathy, Acta Cryst. B29 (1973) 2379. K. Tsuchiya, A. Takahashi, N. Takeda, N. Asakawa, S. Kuroki, I. Ando, A. Shoji and T. Ozaki, J. Mol. Struct. 350 (1995) 233. G. Wu, D. Rovnyank, B. Sun and R.G. Griffin, Chem. Phys. Lett. 249 (1995) 210. P.J. Dirken, S.C. Kohn, M.E. Smith and E.R.H. van Eck, Chem. Phys. Lett. 266 (1997) 568.
Chapter 6.6
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
19F NMR Robin K. Harris 1, Gustavo A. Monti I and Peter Holstein 2 1Department of Chemistry, University of Durham, UK; and 21nstitutfar Experimentelle Physik I, Universitiit Leipzig, Germany
Since the early days of NMR, 19F has been recognised as one of the most important nuclei, and it has been widely used for solution-state studies. This is because it is present in 100% natural abundance, is second only to the 1 proton* in its resonance frequency and has a spin quantum number of 5. These properties convey very favourable sensitivity in the NMR experiment-the receptivity is 83.4% of that for 1H and 4.73 x 103 of that for 13C. Moreover, the chemical shift range for 19F is comparable to that of 13C and well over an order of magnitude greater than 1H. It follows that, of the three obvious nuclei used to examine fluoropolymers containing hydrogen (1H, 13C and 19F), fluorine is preferred. However, although there have been a number of reports of 19F (and 1H) relaxation parameters and broadline spectra of solid fluoropolymers, the use of high-resolution 19F spectra for studying such systems has been relatively limited. Indeed, until recently the total amount of high-resolution 19F NMR work on solids of all types was surprisingly small. It has been reviewed twice in the past decade [1, 2]. Paradoxically, the reasons for the relative lack of activity on 19F are two of the major advantages of the nucleus, namely the 100% natural abundance of 19F and its high magnetic moment. These imply that dipolar interactions (both homonuclear and heteronuclear) are likely to be very strong. All the early NMR work on fluoropolymers reported measurements of 19F (and 1H) relaxation and broadline spectra, using what are now thought of as "classical" NMR techniques. Such experiments are described fully in many review articles and books (e.g., Refs. [3, 4]), so they will not be discussed further here. However, one additional complication for fluoropolymers containing protons is the possibility of cross relaxation between 19F and 1H, which renders simple T1 measurements (e.g., by inversion-recovery) intrinsically not single-exponential and therefore makes results difficult to interpret. This issue has been addressed by McBrierty and Douglass [5], who studied the transient Overhauser effect for PVDF by monitoring 19F magnetisation (using a 90~ pulse) at various times subsequent to a 1H 180~ pulse. Single-spin and cross* E x c l u d i n g the r a d i o a c t i v e i s o t o p e 3H.
254
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN
relaxation terms were evaluated using exponential terms involving their sums and differences. Clearly, both depend on motional correlation at frequencies in the MHz range (i.e., at Larmor frequencies and their sums and differences). Such complications are generally absent from perfluorinated polymers. In order to obtain high-resolution 19F spectra, special techniques are obviously necessary. The most widely used is magic-angle spinning. In the case of perfluorinated polymers, homonuclear (19F, 19F) dipolar coupling will cause substantial broadening of the fluorine spectrum, and MAS will not be very effective in achieving high resolution because of spin diffusion. The resonance band will behave homogeneously, so that linewidths under MAS will vary inversely as Pr, where Pr is the sample rotation frequency [6], requiring spinning at rates greater than or equal to 10 times the static bandwidth to be fully effective (see Fig. 6.6.1). The obvious technique to use to secure homonuclear decoupling is a phase-alternated multiple-pulse sequence, with measuring points interleaved between the pulses, such as WAHUHA, MREV8 and BR24. Indeed, the 19F spectrum of a fluorinated copolymer (tetrafluoroethylene/perfluoromethyl vinyl ether) was presented as one of the earliest examples of the use of the WAHUHA sequence [7]. However, such sequences are notoriously difficult to fully implement satisfactorily, and have been far less popular in practice than the cross polarisation and high-power proton decoupling methods used for "dilute" spins such as I3C in solids. In order to achieve high-resolution, these sequences need to be combined with MAS (to form CRAMPS--Combined Rotation and Multiple Pulse Spectroscopy). Indeed, 19F NMR of a fluoropolymer (Kel-F) was used as the first example of the CRAMPS technique: Gerstein et al. [8] were able to distinguish between the CFC1 and CF2 resonances. As an alternative to CRAMPS, high-speed MAS (over 20 kHz) may be used. Again, Kel-F was one of the examples presented in the paper by Dec et al. [9] on high-speed MAS. They were able to split the CF2 signal into two. However, in practice, the higher the MAS speed within the CRAMPS protocol the better [1], so the two approaches are complementary rather than alternatives. Problems may arise if rotor periods become comparable to cycle times of the multiple-pulse sequences, but generally the former (10 kHz MAS implies a period of 100 ~s) are significantly longer than the latter (MREV8 typically has a cycle time of ca. 20 ~s). However, the large chemical shift range of 19F can also be a problem for CRAMPS, since spectral quality is very sensitive to offset of the carrier frequency from resonance. Polymers with "chemically dilute" fluorines do not have the problems associated with large (19F,19F) dipolar interactions, but they are likely to contain abundant protons so that difficulties arise from (19F,1H) dipolar
19F NMR
100
0
-100
255
-200
ppm
-300
Fig. 6.6.1. Fluorine-19 spectra at 188.29 MHz of a PTFE-perfluoromethoxyethylene copolymer to show the effect of magic-angle spinning. Top" static sample. Middle' MAS at 2.4 kHz. Bottom: MAS at 12.9 kHz.
coupling, which can be even more significant in terms of line-broadening in 19F spectra. Although such interactions are inhomogeneous (and, therefore, MAS can be effective, in principle, at modest speeds, though possibly at the cost of giving substantial spinning sidebands), the situation is complicated by the frequent existence of strong ( 1 H , 1 H ) dipolar coupling, as is also the case for high-resolution 13C spectra of many solids. In principle, such effects can be readily overcome by high-power proton decoupling, combined with MAS, as for 13C NMR. However, the decoupling powers required are very high, and the 1H and 19F r e s o n a n c e frequencies are only 6% apart, so that interference between the two rf channels is likely to be considerable and very efficient electronic filtering is required. It appears that such considerations strongly inhibited work in this area for many years. However, efficient filtering can be obtained, and recently there has been a surge of activity in high-resolution
256
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN
19F NMR of solids containing both fluorine and hydrogen, including work on polymers. Of course, polymers with high mobility at the molecular level (i.e., above Tg) may have the (H,F) interactions substantially averaged already, so that proton decoupling is not entirely essential. Thus, proton decoupling, together with molecular mobility, can substantially narrow 19F bands of solid hydrogen-containing fluoropolymers, even where the fluorine nuclei are chemically abundant. Magic-angle spinning then becomes essential if high resolution is to be achieved. However, MAS must then average the combined effects of shielding anisotropy and (F,F) dipolar interactions (assuming multiple-pulse methods are not used for detection). Since the latter are homogeneous, low MAS rates are ineffective. Resolved sidebands are only seen as speeds increase, and MAS rates in excess of at least 10 kHz are required to obtain narrow lines for a perfluorinated system, as shown in Fig. 6.6.1. Methodologies for 19F-{1H} MAS spectra of solids have been summarised (though not specifically with respect to polymers) [10]. One consequence of high-power proton decoupling during observation of 19F spectra is that resonance frequencies will be affected by the Bloch-Siegert effect [11]. This has been thoroughly investigated by Vierk6tter [12] for some nonpolymeric fluorinated compounds, and has been observed for polymeric systems [13]. The shift (expressed as a fraction) is given by: A-- (yFBxH)2/(W~" -- W~) where "yFB1H is the magnitude of the decoupling rf effective at the 19F frequency, expressed in angular frequency units, while tOE and WH are the observe and decouple angular frequencies, respectively. The 19F signals are shifted to low frequency. Such shifts amount to ca. -1.5 to - 3 . 0 ppm for spectrometers operating at 200 MHz for 1H (see Fig. 6.6.2), but become smaller at higher fields. The shift is noticeable for 19F because of the small difference (~6%) between 1H and 19F resonances. It is generally negligible for observation of other nuclei with proton decoupling. Since the effect does not depend on the actual presence of protons, but only on the decoupler frequency and power, referencing of 19F chemical shifts is unaffected if the "decoupler" is on when recording the signal for the reference compound (e.g., C6F6). Cross polarisation from protons to fluorine nuclei becomes feasible with the double-resonance probes developed for 19F-{1H}work. Figure 6.6.3 shows a CP profile as a function of contact time. This experiment offers similar possibilities for discrimination within spectra that are traditional with 13C CPMAS spectroscopy. Thus, the dipolar dephasing pulse sequence (Fig. 6.6.4) can select resonances of fluorines remote from protons or of fluorines
19F NMR
I
-60
I
-70
I
-80
I
-90
257
I
i
I
I
-100 -110 -120 -130 ppm
Fig. 6.6.2. Bloch-Siegert shift on the 19F PVDF spectrum. The signals are shifted to low
frequency (and resolution of the shoulders assigned to crystalline domains is improved) by the decoupling. The decoupler power corresponds to ca. 100 kHz. The sample is of 10 ~m film (containing both a and/3 crystalline forms as well as amorphous domains). MAS is used at 14 kHz.
in relatively mobile domains. It is advisable to incorporate a 19F ~r-pulse to give an echo, with synchronisation to the rotor period, into the sequence. However, because fluorine is a monovalent element, it usually resides on the periphery of a molecule, unlike carbon, which generally forms the molecular framework. Therefore, XH and 19F can come very close even when unattached to the same carbon atom, e.g., in gauche-oriented C F ~ C H molecular fragments. Such considerations must be borne in mind when dipolar-dephased spectra are interpreted. Variation of H ~ F contact times can also act to discriminate between fluorine nuclei in different e n v i r o n m e n t s - - i n this case the effective parameter is the local mobility of the fluorine-containing chemical group. As in the 13C case, rigid groups in hydrogen-containing fluoropolymers tend to have rapid (efficient) cross polarisation, whereas mobile groups have inefficient CP. W h e n the groups in question are in different domains, TI~ values become i m p o r t a n t - - g e n e r a l l y being longer for more
258
R. K. H A R R I S , G. A. MONTI AND P. HOL ST E IN
_
o~
9
E 0
. ,....
C~ N
o ,...=
c" C~
E
o 0,1
CF
2
9 CHF
-
I
'
0
I
'
I
5
'
10
I
u
15
I
20
contact time (ms) Fig. 6.6.3. Dependence of 19F intensity (at 188.29 MHz) on contact time for a sample of poly(trifluoroethylene). MAS rate 12.6 kHz. The plots can be fitted to a simple equation with effective TIp (incorporating both 1H and 19F relaxation) of 14 ms and cross-polarisation times of 1.3 and 0.4 ms for the CF2 and CFH fluorines, respectively.
1H DD
I I
DD
19F
CT
~
n/vr
~
n/Vr
...
ACQ
VVvvv ...... Fig. 6.6.4. Dipolar dephasing pulse sequence for discriminating in favour of amorphous (mobile) domains. [CT = contact time; DD = high-power (dipolar) decoupling; A C Q = acquisition; vr = MAS rate].
19F
NMR
259
mobile polymer regions, at least below Tg. However, TI~ values will show minima with mobility (i.e., with temperature), which will upset any generalities. Moreover, signal decay rates at long contact times will depend on both H Tlo and TVp, whereas, in the 13C case, TxCocan usually be neglected. The full equation [14] for fitting the contact-time (t) dependence must be used, therefore, for 1H ~ 19F CP, i.e., (under Hartmann-Hahn matching conditions):
S(t) =
So a+
-
a_
[ e x P ( T a - ; ) - exp( -a+t]~THF/_1
with a
= ao[1 + (1 -
b/a2) 1/21
a - (1/2)(1 + e + THF/T1Ho+ TH#TL), THF ( 1 Tip \
b =-~-n-
+
TnF~ TV,]
THF T~p
+ e ~ ,
with
NF NH
9 =--,
and THF is the characteristic CP time (assuming CP is an elementary process). There are only three variables in this equation (apart from a scaling factor) best expressed as THF, (THv/T~) and (TI_w/TFp). Strictly speaking, the above equation is only valid for small e and is not strictly speaking applicable, but more extended theory does not seem to have been greatly treated. For semicrystalline fluoropolymers containing protons, 19F relaxation times T1 and Txo can be obtained selectively by suitable post-CP treatment (e.g., by the Torchia technique [15] for T v) or by direct polarisation methods--see Fig. 6.6.5(a,b). Corresponding pre-CP preparation periods yields proton times T1 and Tlo selected via the 19F signals for crystalline and amorphous domains (Fig. 6.6.5(c,d)). In the TI experiments for both 19F and IH nuclei, it may be important to decouple the complementary nucleus during the period allowed for the relaxation in order to avoid problems from cross relaxation. The procedure is more problematic for Tip measurements since it is essential to avoid any CP during the time in question, so Hartmann-Hahn matching must be avoided. Cross- polarisation dynamics can be investigated free of proton Tip by an adaptation of the TORQUE (--T One Rho Quenching) method (see Fig. 6.6.6) introduced by Tekely et al. [16] The TORQUE experiment starts with a spin-lock period applied to the 1H spins. Subsequently, 19F magnetisation is created by a standard cross-polarisation method, with a constant value for the sum of the precontact spin-lock time and the contact time, but where the latter is varied. Using this simple procedure,
260
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN ~/2 1H DD
~2 19F
~:/2
~
I'~
ACQ
1H DD
~/2 19F
SL
~A^^,,,___
ACQ
VVVV. Fig. 6.6.5. Pulse sequences for measuring proton and fluorine relaxation times selectively for different domains in a semicrystalline polymer. (a) The Torchia method as modified for Tv. (b) Sequence for T~o. (c) (See next page) Pre-CP proton spin inversion for TP. (d) (See next page) Pre-CP proton spin-locking for TI~. For (c) acquisitions from the two component sequences add (upper sequence) and subtract (lower sequence) to give a Freeman-Hill type of measurement. For measurement of T~ by sequence 5(d) the time r is varied. However, when this sequence is used to discriminate in favour of crystalline (relatively rigid) domains, it is fixed at a value between those of T~ for the crystalline and rigid domains. [CT = contact time; SL = spin-lock time; DD = high-power (dipolar) decoupling; ACQ = acquisition.]
subtle details of cross-polarisation dynamics, h i d d e n by the effect of T~p of the p r o t o n s in the s t a n d a r d v a r i a b l e - c o n t a c t time e x p e r i m e n t , can be o b s e r v e d in d i p o l a r - c o u p l e d systems. T h e e v o l u t i o n of the 19F m a g n e t i s a t i o n with contact time u n d e r T O R Q U E for a sample of c o m m e r c i a l Viton, poly ( V D F -
19F NMR
261
~:/2 1H
CT
DD
19F
CT
I~
~A^^
VVVV,. .
.
.
.
ACQ .
.
.
.
~/2 1H
CT
I
DD
19F
~/2 1H
SL
I
CT
DD
19F
CT
I
ACQ
VVV Fig. 6.6.5(c,d). See the preceding page for the caption. co-HFP) is shown in Fig. 6.6.7. This reveals the evolution of the 19F magnetisation with more detail than the standard variable-contact time experiment (see Fig. 18.22 in Chapter 18). Figure 6.6.7 shows that the CP process is more complex (and full cross polarisation takes longer) for the CF 3 and CF
R. K. H A R R I S , G. A. MONTI AND P. H O L S T E I N
262 ~/2 IH
I
SL
I
CT
DD
r ~
IA
19F
.. VVv/~A^^v . ..........
Fig. 6.6.6. T O R Q U E method [16] adapted for the measurement of 1H ~ 19F CP rates free of complications from T~o. The total time A is kept constant while the contact time is varied. [CT = contact time" SL = spin-lock time; DD = high-power (dipolar) decoupling; A C Q = acquisition.]
1.0o
o
~ oo
o
o
o
*
O
9 9
9
A
9 n o
9
e'-
,,l.,a r'
o
0.1-
1 0.0
9149
9 / 1 9 1 4 9 [] o 9 A9 9 [] o
o... ID
to
[]
I
0,5
9 []
9
n
o
o
@
o
~
a
n
9
~
9 zl
A
A
A
a
n
[]
[]
[]
[]
0
n
[]
[]
[]
9
-75ppmCFs
o
- 9 0 ppm CFa (PVDF)
*
-110ppmCFa
[]
-118ppmCFa
n
-184ppmCF
[]
I
1,0
I
1,5
I
2,0
I
2.5
,,I
3.0
contact time (ms)
Fig. 6.6.7. Contact-time dependence of 19F signal intensity for a T O R Q U E experiment on a commercial sample of Viton, P(VDF-co-HFP). The constant time tsL + tCT = 3 ms was used throughout.
fluorines of the HFP units than for any of the C F 2 peaks. The reasons for this observation are as yet unclear but are probably connected with internal rotation about the C - - C F 3 bond. In many cases the best way of obtaining selectively 19F spectra from polymer domains that are crystalline is to combine the use of a relatively
19F NMR
263
i
b. l
0
/\ '
: I
-100
l
I
-200
l
Fig. 6.6.8. Fluorine-19 MAS spectra of a physical mixture of PTFE and PVDF (5% of the latter). (a) Direct polarisation. (b) 1H to 19F cross polarisation. The signal at 8F = --117 ppm is from PTFE whereas that at 6F = - - 8 5 ppm arises from PVDF. [Figure reproduced with permission from Ref. 17.]
H short contact time with a 'Tip filter', i.e., with a period prior to CP during which the protons are spin-locked for a fixed time ~- (Fig. 6.6.5(d)). The use H is longer for crystalline domains of this technique for this purpose assumes Tip than amorphous ones, which will not always be the case (see above). With many fluoropolymers, direct polarisation (DP) and cross polarisation from protons are complementary techniques. Unfortunately, most NMR probes contain fluoropolymer components in the rotor caps and/or stators, so that DP spectra often contain background signals. However, the components in question are frequently perfluorinated polymers such as PTFE, so that background signals are eliminated [17] by ~H ~ 19F CP (Fig. 6.6.8). Further possibilities are opened up for investigation of fluoropolymers by two-dimensional methods. Thus, solid-state 19F COSY can be used to study potential spin exchange (see Ref. 17 for a nonpolymeric example), which may arise from "chemical" exchange (rare in polymeric systems, but conceivable for internal rotation of C ~ C F 3 groups in asymmetric environments) or from spin diffusion. The latter can be of either spectral or spatial type, the former relying on overlap of the relevant bands in the spectrum. In this context, it becomes important to ensure the highest resolution situation in the ~gF spectrum during the time allowed for spin diffusion. This, in turn, suggests that proton decoupling should be employed during that time. The WISE (two-dimen-
264
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN
b
'"
'1 " " I' " ' 1 " "
-75
-100
-125
I" " I'"
-150
-175
'1
-200
(F/ppm
Fig. 6.6.9. A stacked plot for a proton-fluorine WISE experiment for a commercial Viton sample, with 19F decoupling during the evolution time tx. Spectrometer operating conditions: contact time 50 Ixs; spin rate 10 kHz; 128 points in tl with 10 txs increments; pulse power in both channels equivalent to 3 Ixs for 90 ~ pulse angle; recycle delay 3 s.
sional) experiment [18, 19] is also applicable to fluoropolymers containing protons. This yields data on proton bandshapes relevant to particular fluorine resonances (provided short contact times are used to minimise spin diffusion effects). If fluorine decoupling is employed during t l , then the proton dimension yields H,H dipolar second moments, whereas without such decoupling the second moments contain both H,F and H,H contributions [20]. Figure 6.6.9 shows a WISE stacked plot, obtained with decoupling during tl, for a commercial sample of Viton, P(VDF-co-HFP). There is no significant difference in this case in the proton linewidths associated with different 19F chemical shifts, as expected with efficient spin diffusion. In principle, a spindiffusion period incorporated in the WISE experiment enables the effects of domain size in heterogeneous systems to be studied. The DIPSHIFT twodimensional sequence [21] is also applicable, giving H,F dipolar information separately. The proton decoupling regime need not be CW. It was shown some years ago that suitably placed 180~ pulses in the proton channel can serve as well, and this has been tested [22] in the H,F case. Moreover, CRAMPS detection can be combined in principle with CP. This has been shown [23] to be
19F NMR
265
effective for the 31p-{1H} case, but has not been demonstrated yet for 19F{1H}.
It should be stressed that the use of 19F-{IH} MAS techniques for solids is in its infancy, in contrast to the 13C-{1H}situation, yet all of the methods for the latter are applicable (sometimes with small modifications) for the former, so there is great potential for development work. Of course, a major reason for turning to solid-state NMR is lack of solubility, since for soluble polymers the superior resolution of solution-state NMR makes that the preferred technique for chemical microstructure determination. The alternative in some cases is the use of molten-state NMR, and that has been employed [24] to good effect in 19F studies of PTFE copolymers. Magnetic resonance imaging techniques using 19F would seem to offer potential scope for studying solid fluoropolymer systems, and some work has been published [25]. It is suggested [26, 27] that the stray-field method (STRAFI) may be particularly suited to such studies, but as of 1997 this remains unrealised.
Acknowledgements One of us (G.A.M.) thanks CONICET (Argentina) for a post-doctoral fellowship, during the tenure of which this article was written. We are grateful to the Deutsche Akademischer Austauschdienst and the British Council for support to enable the collaborative work to occur. We also thank the U.K. EPSRC for research grant L02906.
References 1. R.K. Harris and P. Jackson, Chem. Rev. 91 (1991) 1427. 2. J.M. Miller, Prog. NMR Spectry. 28 (1996) 255. 3. V.J. McBrierty and K.J. Packer, Nuclear Magnetic Resonance in Solid Polymers. Cambridge University Press, Cambridge, 1993. 4. A.M. Kenwright and B.J. Say, in: R.N. Ibbett (Ed), NMR Spectroscopy of Polymers, Chapter 7. Chapman & Hall, London, 1993. 5. V.J. McBrierty and D.C. Douglass, Macromolecules 10 (1977) 855. 6. E. Brunner, D. Fenzke, D. Freude and H. Pfeifer, Chem. Phys. Lett. 169 (1990) 591. 7. D. Ellett, U. Haeberlen and J.S. Waugh, J. Polym. Sci. (Polym. Lett.) 7 (1969) 71. 8. B.C. Gerstein, R.G. Pembleton, R.C. Wilson and L.M. Ryan, J. Chem. Phys. 66 (1977) 361. S.F. Dec, R.A. Wind, G.E. Maciel and F.E. Anthonio, J. Magn. Reson. 70 (1986) 355. 10. S.A. Carss, U. Scheler, R.K. Harris, P. Holstein and R.A. Fletton, Magn. Reson. Chem. 34 (1996) 63. .
266 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN F. Bloch and A. Siegert, Phys. Rev. 57 (1940) 552. S.A. Vierk6tter, J. Magn. Reson. A 118 (1996) 84. R.K. Harris, P. Holstein. U. Scheler and G. Monti, unpublished work. M. Mehring, Principles of High Resolution NMR in Solids, 2nd Edition, Springer-Verlag, Berlin/New York, 1983, pp. 147-152. D.A. Torchia, J. Magn. Reson. 30 (1978) 613. P. Tekely, V. G6rardy, P. Palmas, D. Canet and A. Retournard, Solid State NMR 4 (1995) 361. R.K. Harris, S.A. Carss, R.D. Chambers, P. Holstein, A.P. Minoja and U. Scheler, Bull. Magn. Reson. 17 (1995) 37. N. Zumbulyadis, Phys. Rev. B 33 (1986) 6495. K. Schmidt-Rohr, J. Claus and H.W. Spiess, Macromolecules 25 (1992) 3273. U. Scheler and R.K. Harris, Solid State NMR 7 (1996) 11. M.G. Munowitz and R.G. Griffin, J. Chem. Phys. 76 (1982) 2848. U. Scheler and R.K. Harris, Chem. Phys. Lett. 262 (1996) 137. R.K. Harris, P. Jackson, P.J. Wilkes and P.S. Belton, J. Magn. Reson. 73 (1987) 178. S. Yonemori, Y. Jitsugiri and T. Ogawa, Nippon Kagaku Kaishi (1995) 30. Y.M. Daud and M.R. Halse, Physica B 176 (1992) 167. E.W. Randall, personal communication. E.W. Randall, Solid State NMR 8 (1997) 173.
Chapter 7
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
Structure and Dynamics of Crystalline and Noncrystalline Phases in Polymers Takeshi Yamanobe Department of Chemistry, Gunma University, Kiryu, Gunma, Japan
7.1
Introduction
Usually, polymers are used in the solid state. Physical properties of the polymer in the solid state are influenced greatly by its structure and dynamics. Since a polymer is composed of numerous atoms with many kinds of chemical bonds, the structure, dynamics and morphology are complicated. Most polymers are semicrystalline with crystalline, amorphous and interfacial phases. Conditions for crystallization of polymers affect the crystalline/amorphous ratio. In the crystalline phase, polymer chains are placed in order and X-ray diffraction produces reflections which are sharp enough to give structural parameters, such as lattice parameters and conformation of the polymer. The amorphous phase is disordered and the polymer chains in phase are not restricted by their translation and rotation and some degree of molecular motion takes place. In such a phase, X-ray diffraction cannot give sharp diffractions but broad halos. The solid-state NMR method can give information about structural parameters and dynamics of both crystalline and amorphous phases. This information can give deep insight into physical properties, functions and the molecular design of polymers. In this chapter, structural parameters and the dyn0mics of polymers obtainable from solid-state NMR are described.
7.2
Crystalline and amorphous phases
Most differences between crystalline and amorphous phases are attributed to the order and disorder of polymer chains. All polymer chains in a crystalline phase see neighbouring chains at the same distance with the same mutual orientation. In contrast, neighbouring chains in the amorphous phase exist at random distances and random mutual orientations. Therefore, there is room for molecular motion in the amorphous phase. As the molecular motion causes a conformational transition, both parameters correlate with each
268
TAKESHI YAMANOBE
Crystalline phase
$
Amorphous phase
l
5O
I
I
I
I
i
45
I
~
,
i
1
4O
~
I
11
1
35
,
i
,
,
1
3O
~
'
I
~
I
25
i
i
i
2O
,
i
,
,
!
15
,
,
PPI,4 , .... l,-
i
10
Fig. 7.1. CPMAS NMR spectrum of polyethylene.
other. As a result, the difference between the crystalline and amorphous phases appears in the relaxation times and chemical shifts. Fyfe et al. [1] and Earl and VanderHart [2] independently observed the chemical shift difference between the crystalline and amorphous phases for polyethylene. Figure 7.1 shows a spectrum of polyethylene measured by the CPMAS method. If all of the methylene units in polyethylene are identical, the NMR spectrum gives only one peak. However, a strong peak and shoulder are observed in the real spectrum, which means that there exists two inequivalent methylene units in the solid polyethylene. From measurements on polyethylenes with various crystalline/amorphous ratios, peaks at about 33 and 31 ppm are attributed to the crystalline and amorphous phases, respectively [3]. Figure 7.2 shows NMR spectra measured with the sequence (180 ~ - ~- - 90 ~ 100 s) by varying ~'. The amorphous peak recovers more rapidly than the crystalline one. It is reported that T1 for the crystalline component reaches about 1000 s. The T1 value for the crystalline phase is longer than that for the amorphous phase. Since the T1 value increases with a correlation time in the slow region of BPP theory [4], molecular motion in the amorphous phase is faster than in the crystalline phase, which supports the peak assignment. On the basis that the chemical shifts are affected by conformation through the electronic structure around the nuclei of interest, the chemical shift difference between the crystalline and amorphous phases is explained by the
C R Y S T A L L I N E A N D N O N C R Y S T A L L I N E P H A S E S IN P O L Y M E R S
269
r
5o
45
4o
35
3o
25
20
15
lO
Fig. 7.2. Recovery of peaks for the crystalline and amorphous phases of polyethylene measured by (180 ~ - ~- - 90 ~ - 100 s) with high-power decoupling.
y-gauche effect [5]. In the crystalline phase, the methylene group takes a rigid trans-zigzag conformation, while the trans-gauche transition is taking place rapidly in the amorphous phase. The methylene carbon appears at low frequency if any carbon atom three bonds away is in a gauche, rather than a trans-conformation (y-gauche effect). Taking into account the y-gauche effect, the chemical shift 6 of methylene carbons can be expressed as [2] 8 = 6 0 - 27fg,
(7.1)
where 60 is the chemical shift of methylene carbons taking the all transconformation, fg is the equilibrium fraction of gauche conformation; y is attributed to the chemical shift difference between the gauche and transconformations and is reported to be 4-6 ppm. As seen from Equation (7.1), chemical shift decreases field as the fraction of the gauche conformation increases. The observed chemical shift difference between the crystalline and
270
TAKESHI YAMANOBE
amorphous phases can be quantitatively explained by using Equation (7.1) and assuming the trans-gauche energy difference to be 500-600 cal. Therefore, the peak assignment is also supported. If a Boltzman equilibrium population is rigourously established for the 13C magnetization at every cycle of accumulation, the 13C signals have a quantitative relative intensity. However, in real measurements, techniques to enhance the signal-to-noise ratio distort peak intensities. A direct way of quantitative analysis is the conventional single pulse method with DD and MAS. In this method, repetition times are required to be more than a few thousand seconds since T c of polyethylene is more than about 1000 s for carbons in the crystalline region. On the other hand, repetition times for the CP technique depend on T~ values which are usually 1 s at room temperature because of spin diffusion. Ideally, there is a fourfold theoretical sensitivity enhancement factor for the CP method compared with the single pulse method. However, the CP enhancement factor is affected by the proton rotating frame relaxation time, T1TM, the cross-polarization time constant, TcH and the local protonproton dipolar field. Therefore, if there are several regions where molecular mobilities are different from each other, the CP enhancement factors vary with their molecular mobilities. VanderHart and Khoury [6] reported CP enhancement factors for the crystalline and amorphous regions as 3.5 and 2.4-3.0, respectively. The CP enhancement factor for the amorphous phase is dependent on sample preparation. By using these factors and the line shapes of both the crystalline and amorphous phases, the ratio of crystalline/amorphous can be determined from the NMR spectrum.
7.3
Structural parameters and chemical shift tensor
Structural parameters such as bond lengths and bond angles can be obtained directly from the solid-state NMR spectrum if dipolar couplings between nuclei can be obtained whether the sample is in the crystalline or amorphous state. Although chemical shifts contain information about structural parameters through the electronic structure of the atom of interest, the dipolar coupling is determined directly by the length between two nuclei and the angle between magnetic field and bond direction. VanderHart [7] observed the dipolar 13C~13C coupling for an ultraoriented linear polyethylene with an 11.8 draw ratio (Fig. 7.3). Since all possible 13C~13C bond orientations with respect to magnetic field are present for powder samples, the peaks for t 3 C ~ 3 C dipolar coupling are too broad to be observed. In the ultraoriented sample, the alignment of chains in crystalline regions is estimated to be within 5~ by X-ray studies. In
271
C R Y S T A L L I N E A N D N O N C R Y S T A L L I N E P H A S E S IN P O L Y M E R S
'l'l'I'l'l'i"l'~'l"l'
I ~:o,~o~ Ix128
.
'
,
' -x.,ei ,.,/'.'
,s
I T(R')--S--I
I 'I'"1' 1 ' I ' I ' 1
~.,, ' IH. I,/ /x I.I
.,k,tV v' ~
{ "
I"'1"1' l ' l ' l " l ' l '
I AT(RZ)
!
)
H.I Draw
i
I
I~ATIRz)
I
I
i
',
loz'c
I
i
l'l
{iv
,,, i
l! --
ll'l'l''l''l'
I
.~
,
i'ZATIR,)
!
/
{
I ill
Ill
illli 50
lllil
il ill 100
lllil
il illli 150
li I I li l i li I ill I ill 200 250
PPM(CS 2)
li 1 Jill
il Ili 300
Fig. 7.3. CP spectra of ultraoriented polyethylene with an 11.8 draw ratio. The draw direction
is parallel to the magnetic field [7]. The dashed lines indicate the three pairs of 13C--13C dipolar satellite positions based on R~ = 0.153 nm and ~= 112~ (a) 102~ (b) 28~ and (c) -94~ Fig. 7.3, peak positions are given by the dashed lines for 13C--13C dipolar coupling based on usually accepted structural parameters (the bond length between carbons is 0.153 nm and C ~ C ~ C bond angle 112~ As seen from the figure, these peak positions agree with those predicted from X-ray data. Information about the C - - H bond can be deduced in a similar way. Since, usually, C - - H dipolar couplings are removed by high-power decoupling in order to obtain chemical shift dominated spectra, another technique is necessary to deduce C - - H dipolar coupling. Opella and Waugh [8] used 2D separated local field (SLF) spectroscopy for highly oriented polyethylene. By this method, chemical shifts and dipolar coupling between directly bonded C - - H can be measured. Figure 7.4(a,b) shows the chemical shift and SLF spectra for the highly oriented polyethylene with its draw axis parallel to magnetic field, respectively. As seen from the figure, a sharp peak for chemical shift axis and a dipolar triplet of 1 : 2 : 1 for the dipolar coupling axis are observed. The chemical shift position corresponds to the tensor component
272
TAKESHI YAMANOBE
(a)
I
620
H 9 from
_ CH30H
I
720
I
820 L
I
- 920
_1..
1020
-~-
(b)
T t.
I0 kHz
Fig. 7.4. Chemical shift (top) and SLF (bottom) spectra for oriented polyethylene [8].
along the chain axis, t~33. The dipolar triplet of 1:2:1 means that two C ~ H bonds in CH2 are perpendicular to magnetic field and t~33 is parallel to the chain axis. From the dipolar coupling, they deduced a bond length of 0.11 nm for C ~ H . By measuring the SLF spectrum with the draw axis perpendicular to the magnetic field, they determined the principal axis of the chemical
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
273
shift tensor which coincides with the results determined for paraffins and polyethylene [9]. Two-dimensional chemical shift/dipolar coupling spectrum gives us the principal axis of chemical shift tensor even for a powder sample. Nakai et al. [10] have reported the principal axis, C ~ H distance and the H ~ C ~ H bond angle of polyethylene by comparing the observed and simulated spectra by the 2D chemical shift/dipolar coupling method (Fig. 7.5). As the 2D spectrum is influenced by the orientation of the principal axis and dipolar coupling more than is the 1D spectrum, the principal axis and the structural parameters can be determined precisely by this method alone. This method has been applied to PE [10, 11], POM [12], PEO [11] and iPP [13]. The disordered nature of the amorphous phase has prevented the direct determination of the structural parameters by the usual structural techniques such as X-ray diffraction. Solid-state NMR can give us those parameters even for amorphous samples. Yannoni and Clark [14] applied nutation NMR spectroscopy, which is specifically designed to measure interatomic distances, to determine the bond lengths in both the cis- and trans-polyacetylenes. They used polyacetylene polymerized from a mixture of 4% doubly 13C-enriched acetylene and doubly depleted acetylene. In Fig. 7.6, the observed and simulated proton decoupled ~3C nutation spectra of the cis sample are shown. The sharp peak in the centre arises from the isolated 13C nuclei in the sample. The remainder of the spectrum is a Pake-doublet arising from the 13C~3C dipolar coupling of adjacent 13C nuclei in the polyacetylene. The best fit to the observed spectrum corresponds to a distribution of bond length with 0.137 nm. It is important to note that the spectrum obtained gives us only one bond length. They conclude that the mechanism of the Ziegler-Natta reaction leaves the original carbon ~r-double bond in the resulting cis-polyacetylene. On the contrary, trans-polyacetylene produces the Pake-doublet whose doublet maximum peaks split. The carbon-carbon distances obtained are 0.144 and 0.136 nm which are assigned to the single and double bonds, respectively. As the trans-isomer is obtained by heat treatment of the cisisomer, the mechanism of the transformation involve exchanges between the single and double bonds of cis-polyacetylene. Duijvestijn et al. [15] determined the orientation of the chemical shift tensor of trans-polyacetylene, which is the same sample used by Yannoni and Clark [14] by means of a 13C CP technique in combination with dynamic nuclear polarization. They established that the tensor element 633 has its principal axis perpendicular to the molecular plane, and the 611 axis makes an angle of 43 _+ 5 ~ with respect to the single bond, and 78 + 6~ with respect to the double bond. Nakai et al. [16] have determined the orientation of the 13C chemical shift tensor of cis-polyacetylene by the 2D chemical shift/C~H
274
TAKESHI YAMANOBE
(a) _oz Chemical, Shift
c\ i -
0
\
m
j
C) o
-I 9 "1I
40
I
I.
35
30
|
_!
ppm
(b) k Hz 1
r~z Che_mical Shift
_e
-
0-
== 0
--|
-.-
-1
40
. . . .
I
l
35
30
,
ppm
Fig. 7.5. Chemical shift/13Cm1H dipolar 2D powder pattern of polyethylene" (a) observed spectrum; and (b) simulated spectrum [10].
CRYSTALLINE AND NONCRYSTALLINE .-
:. 9
9
~176 9 ~
9
~176 9
~
9 9
9 9 ~176
275
9 ~ ~
~176 9
P H A S E S IN P O L Y M E R S
.": 9
~
9
~
9 ~
~ 9
.~
-
-
9
1000
Hz
~
.. 9 . . . .
~
Fig. 7.6. Observed and simulated outation NMR spectra at 77 K of doubly labelled cis-polyacetylene [14]. dipolar coupling spectrum and concluded that ~11 of cis-polyacetylene makes an angle of 12 _+ 3 ~ with the C - - H bond toward the C = C bond in the molecular plane (Fig. 7.7). These observed principal axes of the chemical shift tensor of polyacetylenes are supported by theoretical calculations.
7.4
Crystal structure
VanderHart [17] reported the influence of crystal structure on the chemical shift for n-alkanes and PE which are in four crystallographic forms as shown in Table 7.1. It was found that the chemical shifts of the interior methylene carbon are almost constant, about 32.9 ppm from TMS, irrespective of chain length in the pseudohexagonal, orthorhombic and monoclinic forms of nparaffins and the orthorhombic form of polyethylene. The triclinic form is represented by C-20, for which this resonance is shifted 1.3 ppm high frequency. As magnetic susceptibility effects are too small to explain the shift, the origin of the 1.3 ppm shift is not obvious. The effects of crystal structure on chemical shift for a series of cyclic paraffins and PE are summarized in Table 7.2 [18]. For cyclic paraffins, up to c-C8oH16o, the crystallographic forms are triclinic as shown in Table 7.2. In the cases of c-Clo4H2o8 and cC128H256, two diffractions which correspond to the monoclinic and orthorhombic forms are observed. For C-ClonH2os, the diffraction of the monoclinic form is stronger than that of the orthorhombic form. On the other hand, for c-C128H256, the diffraction of the orthorhombic form is stronger than that of the monoclinic form. The crystallographic forms of c-C160H320, c-C200H400 and PE single crystal are orthorhombic. For cyclic paraffins up to c-C8oH16o, the 13C NMR chemical shift of the trans zigzag methylene carbon appears at
276
TAKESHI YAMANOBE
(a)
H
H
011 C
/
~"~CI2P_
0.12Onto
20 ~ H
H
trans polyacetylene
Cb) H
G11/H
0.139nm C~ ~
iC ............. C
~ ~~-C
H
H
cis polyacetylene Fig. 7. 7. Bond lengths and angles of polyacetylenes determined by NMR.
about 33.8 ppm. For C-Clo4H2o8, two peaks are observed in the NMR spectrum: one peak appears at 34.0 ppm, the other at 32.1 ppm. The average 13C NMR chemical shift is 32.8 ppm for c-C128H256, c-C160H320 and cC2ooH4oo. From the results of both X-ray diffraction and 13C NMR chemical shift, it is concluded that 13C chemical shift of the peak for the t r a n s zigzag inner methylene carbons of cyclic paraffins and PE with the orthorhombic
277
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
Table 7.1. NMR chemical shifts and crystallographic form of n-paraffins and linear polyethyl-
enes [17] Chemical shift/ppm Alkane Paraffins C- 19 C-20 C-23 C-32 Polyethylene film Drawn sample
Crystallographic form
CH3
a-CH2
fl-CH2
int-CH2 a
Pseudohexagonal Triclinic Orthorhombic Monoclinic
14.81 16.02 15.03 15.25
24.99 26.55 25.59 25.53
34.91 36.35 35.06 35.08
32.94 34.20 32.90 32.94
Orthorhombic Monoclinic
32.88 34.3 [6]
Chemical shifts are corrected after a reference, J. Chem. Phys. 84 (1986) 1196. (original chemical shift -0.72). Table 7.2. NMR chemical shifts and crystallographic forms of cyclic paraffins and polyethylene single crystal in the solid state [18]
Sample
Crystallographic form
13C NMR chemical shift of trans zigzag methylene carbon/ppm
c-C36H72 c-C4oHso
triclinic triclinic triclinic triclinic t riclinic
34.0 33.5 33.7 33.8 34.0 33.8
C-Clo4H2o8
monoclinic (orthorhombic) a
34.0 (32.1) a
c-C128H256
orthorhombic (monoclinic) a orthorhombic orthorhombic
32.8
c-C48H96 coC64H128 c-C8oH 160 Average
c-C 16oH32o
c-C2ooH4oo Average Polyethylene (single crystal)
orthorhombic
32.9 32.7 32.8 a 32.9
a Weakly observed crystallographic form is in the parentheses.
form appear at about 1 ppm, to low frequency of those with the triclinic form.
VanderHart and Khoury [6] measured the CP/MAS NMR spectra of stretched and unstretched ultrahigh molecular weight PE sheets. In the stretched sample, there exists the monoclinic form in addition to the orthorhombic form. The existence of the monoclinic form was clearly demonstrated
278
TAKESHI YAMANOBE
by a sharp peak at 34.3 ppm which is shifted by 1.4 ppm to high frequency from the orthorhombic peak. These influences of crystal structure on chemical shift are theoretically explained by using TB MO calculations [19]. The main difference between structures of the monoclinic and triclinic forms, and of the orthorhombic form is the orientation of the C ~ C ~ C plane in a trans-zigzag chain; the alltrans-zigzag plane of any specified chain in the orthorhombic form, and the triclinic and monoclinic forms is perpendicular and parallel to those of the neighbouring chains, respectively (Fig. 7.8). Taking into account the structural differences, calculations are carried out for a set of three polymer chains by varying the interchain distances R and R' from 0.3 to 0.5 nm. Figure 7.9(a,b) show the dependences of the calculated X3C shielding on R and R'. For both forms, the ~3C shielding increases and then decreases as R and R' increase. On the basis of the lattice constant, the ~3C shieldings calculated for the two models are O ' t r i = - 5 8 . 6 ppm and O r o r t h ~-- - 5 6 . 7 ppm. The calculated difference between the triclinic and orthorhombic forms is 1.9 ppm. The chemical shift difference was verified as being caused by a local difference in intermolecular interactions in the orthorhombic form, and the triclinic and monoclinic forms through the electronic structures. In Table 7.3 is shown the 13C chemical shift of polyethylene as a function of temperature [20]. The chemical shifts for the crystalline phase are independent of temperature, but those for peak A are dependent on temperature. At -120~ the chemical shifts for the crystalline and amorphous phases are about 33 ppm and 32 ppm, respectively. The peak for the crystalline phase arises from the crystalline part of the orthorhombic form. The 13C chemical shift of the amorphous peak is about 32 ppm which is in the range of the t r a n s - c o n f o r m a t i o n . The methylene carbons seem to take a t r a n s - c o n f o r m ation in the noncrystalline phase in this temperature range because of the "
!
R
i s
i
C~
(a)
'
@,,,
(b)
Fig. 7.8. The crystallographicmodels for (a) triclinic, and (b) orthorhombic forms of polyethyl-
ene [19].
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS -61
~,,
1
"
s
.
.
.
.
I
279
"
E
e',_
-59
:3.0
t..O
5.0
(a) 9
l
-59
E-58
-5?
. -56
....
10
Y . _o
j
~.0
s.o
(b) Fig. 7.9. The dependence of the calculated NMR chemical shift on the interchain distance (R and R') [19]. (a) triclinic, and (b) orthorhombic forms.
Table 7.3. NMR chemical shift of polyethylene as a function of temperature [20] Temperature/~
90 60 25 -50 -90 -120
13C NMR chemical shift/ppm Crystalline phase
Amorphous phase
32.9 33.2 33.2 33.2 33.2 33.2
31.0 30.9 31.4 32.4 32.1
280
TAKESHI YAMANOBE
freezing of molecular motion. Therefore, the difference in chemical shifts between the crystalline and the amorphous peaks for the methylene carbons in the trans-conformation can be ascribed in going from the orthorhombic form to the noncrystalline state. In other words, the chemical shift difference does not come from a conformational change, but from the difference of crystal structure.
7.5
Conformation
The chemical shift is determined by the relatively local electronic structure. One of the most important parameters which affect chemical shift is conformation. As mentioned in the section about crystalline and amorphous phases, a typical example for the conformational effect on the chemical shift is the chemical shift difference between the crystalline and amorphous phases of polyethylene. In the crystalline phase, polyethylene takes the all trans-zigzag conformation, while, in the amorphous phase, a rapid transition between the trans and gauche conformations takes place. As a result, the chemical shift of the amorphous phase is the average of the trans and gauche conformations. It is known that polyoxymethylene in the crystalline phase takes the all gauche conformation with a 9/5 helix [21]. However, the amorphous phase has a distribution of gauche and trans-conformations. Figures 7.10 and 7.11 show the ~3C CPMAS NMR spectra and powder pattern spectra of polyoxymethylene, respectively [22]. Sample A is a polyoxymethylene single crystal. To produce Sample B, Sample A is heated to 200~ and then quenched in ice water. Sample C is a melt-quenched sample of bulk polyoxymethylene and sample D is a bulk polyoxymethylene heated and cooled at a rate of 1.7~ As reported by Veeman et al. [23], polyoxymethylene contains an amorphous phase in which the 13C chemical shift is displaced to high frequency as compared with the crystalline phase. As shown in Fig. 7.10, in addition to sample B, C and D, the single crystal polyoxymethylene also contains an amorphous peak as a shoulder. In the powder patterns, there are amorphous components as a shoulder for samples B, C and D. The isotropic chemical shift and principal values of the chemical shift tensors are obtained by fitting the observed spectra with simulated spectra and summarized in Table 7.4. As shown in Table 7.4, the isotropic chemical shifts and chemical shift tensors for the crystalline phases are almost constant in all samples, which means that polyoxymethylene takes the same conformation and crystallographic form. The isotropic chemical shift and ~ value for the amorphous phase of the single crystal is displaced to high frequency by 1 and 7 ppm, respectively, compared with other samples. It is known that solution-grown
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
(a)
281
(b)
;opp~
~o~
(c)
(d) J
~__L_
lOppm 9
J
10ppm
Fig. 7.10. CPMAS NMR spectra of polyoxymethylenes [22]: (a) Sample A; (b) Sample B; (c) Sample C; and (d) Sample D.
(a)
10ppm
t
lOppm
lOppm
lOppm
Fig. 7.11. Powder pattern NMR spectra of polyoxymethylenes [22]: (a) Sample A; (b) Sample B; (c) Sample C; and (d) Sample D.
282
TAKESHI YAMANOBE
Table 7.4. Observed isotropic NMR chemical shift and chemical shift tensors of polyoxymethylene [22] Sample
A B C D
Crystalline phase
Amorphous phase
~iso
611
~22
633
6iso
611
1~22
633
88.4 88.5 88.4 88.4
108.0 107.0 107.0 107.0
84.4 86.2 86.0 86.0
72.7 72.4 72.0 72.0
91.5 89.6 89.4 89.4
100.0 94.5 93.0 92.0
91.0 91.0 91.0 91.0
83.5 83.2 84.1 85.1
-
69
E
o. o
~-- 70
-
u
71
,
_
!
60
t
120
I
180
--
/deg. Fig. 7.12. The dependence of the calculated isotropic NMR chemical shift on the dihedral angle qJ [22].
polymer single crystals form a chain-folded lamella such as polyethylene single crystals. Based on this fact, it can be assumed that the shoulder in the spectrum of sample A comes from the sharply folded structure region. Compared with the crystalline phase, isotropic chemical shifts of the amorphous phase appear at high frequency by about 1 ppm. Anisotropy of the chemical shift tensor for the amorphous phase is much less than that for the crystalline phase. Tight binding MO calculations are carried out for polyoxymethylene by changing the dihedral angle (~) (Fig. 7.12) [22]. As
283
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS Table 7.5. NMR chemical shifts a of poly(1-butene)
Form
Form I Form II Form III Amorphous
Dihedral angle
Carbon
t
g
CH3 (side chain)
CH2
ce-CH2
13-CH2
180 163 159
60 77 81
1.55 0.75 3.05 2.25 0.00
15.46 (ca. 17) 16.96
20.68 (ca. 22) 25.21
27.19 (ca. 28) 29.7
16.40
23.46
28.32
a Referred to the CH3 resonance of the amorphous form as zero. shown in Fig. 7.12, the isotropic chemical shift varies as qJ is changed. The values of 60 ~ and 180 ~ for q~correspond to the all gauche and all trans-conformations. Calculated chemical shifts for the all gauche and trans-conformations are -69.8 and -70.8 ppm, respectively, which is in fairly good agreement with the observed results. Thus, these calculations confirm that the observed chemical shift difference between the crystalline and amorphous phases arises from the conformational differences. Poly(1-butene) takes three (TG),, conformations which forms a 31-helix depending on the sample preparation [24]. Dihedral angles for T and G are summarized in Table 7.5 together with the observed chemical shifts. As seen from the table, it is clear that the chemical shift values change depending on the dihedral angles, although three forms are generally called the (TG)n conformation. A little change in dihedral angle causes significant chemical shift differences.
7.6
Side branches and end groups
The physical properties of polymers can be affected by structural irregularities present in the polymers. These structural irregularities include end groups and side branches which affect melting points, thermal stability and so on. The existence of such irregularities can be observed efficiently by solutionstate NMR, because the linewidths of solution peaks are typically 10-100 times narrower than those of solid-state NMR. However, by solid-state NMR, we can seek how the side groups and end groups are partitioned between the crystalline and amorphous phases, which directly relate to the physical properties of the polymers. VanderHart and Perez [25] investigated the fraction of irregularities such as end groups and side branches in the crystalline and amorphous phases.
284
TAKESHI YAMANOBE
The partition of end groups and side branches between the crystalline and amorphous phases in polyethylene is determined by isolating the backbone resonances corresponding to the pure crystalline and amorphous phases by assuming as follows: (1) the backbone methylene resonance profile may be used to separate the contribution from the crystalline and amorphous phases; (2) polyethylene is simply composed of two phases (crystalline and amorphous phases); (3) 13C signals obtained after CP are proportional to the spin temperature; (4) during the CP period the protons of the irregularities have polarization levels which are close to those of their neighbouring backbone methylene protons; and (5) the irregularities are evenly distributed within each phase. The determination of the pure component line shapes is accomplished by changing the preparation of the proton spin state prior to CP in ways which change the mixture of the crystalline and amorphous components in the resulting spectra. Figure 7.13 shows the resulting spectra, CP, SL20 + CP, DE5 + CP and 90 ~ + 5 s. The first experiment, "CP", is the usual CP experiment in which all of the carbons have comparable intensities. The second, "SL20 + CP", involved a 20-ms proton spin locking period prior to CP. In the SL20 + CP sequence,
.
.
.
.
--,,,__.SL.2Oo
Cp
~DE5.CP
,
"' 140
9
i
9
130
I
I
120
I
.
.
.
.
.
.
P P M'
.
.
.
.
.
.
.
.
-
'
.
,,'o'
-
3'0
"
2'0
'
Fig. 7.13. 13C MAS spectra of polyethylene measured by four pulses (CP, SL20+ CP, DE5 + CP and 90~ s from the top) [25].
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
285
signals from the crystalline phases with longer TIHo v a l u e s are enhanced. In the DE5 + CP experiment, carbons near the protons with the narrower linewidths are enhanced. The 90~ 5 s experiment is the saturation recovery method modified by adding a 3-s delay between observation and saturation in order to avoid the problem of distorted CP intensity. This method provides fully relaxed intensities for all but the crystalline backbone. In addition to the methylene peaks of the crystalline (32.9 ppm) and amorphous (31.1 ppm) phases, small peaks of irregularities are observed. Peaks for ethyl branches, methyl and vinyl ends appear at 11.3, 15 and 115 and 139 ppm, respectively. In the crystalline-rich SL20 + CP spectrum, the peak of the ethyl branch is hardly observed and the weak methyl and vinyl end groups are observed. On the contrary, peaks of all irregularities appear clearly in the DE5 + CP and 90~ 5 s spectra. Appropriate linear combinations of these spectra yield either a pure crystalline or a pure amorphous spectrum for the backbone and, on the basis of five assumptions, the defects (Fig. 7.14). Comparing the pure crystalline spectrum with the pure amorphous phase, it is clear that the concentration of all defects is greater in the amorphous region. Ethyl branches are scarcely observed in the crystalline phase. Quantitative estimation of irregularities taking into account the actual crystallinity and CP efficiency are
pure amorp
pure cryst '
140
'
130
'
120
'
-
PPIvl
410
"
:}0
'
20
'
10
Fig. 7.14. 13C MAS spectra corresponding to the pure amorphous (top) and pure crystalline
(bottom) phases of polyethylene [25].
286
TAKESHI YAMANOBE
Table 7.6. Fractions (in units of 10-3 ) of defect Species
Phase
Methyl ends Vinyl ends Ethyl branches Methyl + vinyl ends
From solution NMR data
Crystal
Amorphous
Total
0.77 -+ 0 0.11 0.42 _+0 0.05 <0.20
0.57 + 0.07
1.34 _+0.18
1.19
0.49 _+0.05
0.91 + 0.10
1.01
1.33 +_0 . 1 0
1.23-1.63 2.25
2.65 2.20
From GPC
2.35
Values in the table represent the numbers of defect carbons per 1000 carbons in the whole sample. obtained as shown in Table 7.6 together with data from ~3C solution N M R and GPC. For the methyl and vinyl end groups, data from solid-state N M R are in agreement with those from 13C solution N M R and GPC. Approximately 57% of methyl ends are in crystalline environments whereas only about 46% of vinyl ends are in the crystal.
7.7 7.7.1
Dynamics 1H pulse N M R
A convenient way to get information about the dynamics of polymers is to measure T1, Tlf, and T2 by 1H pulse N M R . Figure 7.15 shows the side chain length-dependence of the 1H Y2 value measured at 80~ for the side chain of poly(7-n-alkyl L-glutamate)s [26]. As seen from this figure, T2 is almost constant for n = 5 - 9 and suddenly becomes double for n = 10. It is easy to find the discontinuity between n = 9 and n = 10. Based on BPP theory, T2 increases as the correlation time for the motion decreases. A sudden increment of Y2 means that there is a large difference in the side chain mobility between n ~< 9 and n >i 10. These polymers become a thermotropic liquid crystal if the number of carbon atoms in the alkyl side chain is more than 10. At 80~ the polymers with more than 10 carbon atoms are in the liquid crystalline state. Low mobility of the side chain prevents the polymers of n ~< 9 to be in the thermotropic liquid crystal. The existence of the discontinuity is evidence for the solvent like nature of the side chains in the thermotropic liquid crystalline phase. Figures 7.16 and 7.17 show the temperature dependences of 1H T1 and Tlf, values for poly(y-n-octadecy L-glutamate) ( P O L G ) and poly(y-oleyl L-
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
287
400 0
300
0
~' 200
.o
o
0 0
0 0
0
000
100
0
0
o
-=
|
00
. . . . . . . . .
10
20
Number of side--chain CH= carbon atoms
Fig. 7.15. Dependence of 1H T2 of the side chain protons of poly(y-n-alkyl L-glutamate)s on the side chain length at 80~ [26].
0.6 0.4
1" Y
I--.
0.2
1' Y
o.I --
!
I
-120 - 8 0
I
-40
,,
I ....
0
|
I
40
80
Templ*C
J
120
Fig. 7.16. Temperature dependences of T1 for poly(y-n-octadecyl L-glutamate) ( 9 and poly(yoleyl L-glutamate) (@)" [3,[3 relaxation; Y,3' relaxation; Tm, melting point of the side chain crystallite [27].
glutamate) (POLLG) [27]. The T1 value of P O L G decreases from 700 to 350 ms as the temperature is increased from - 1 0 0 to -10~ This means that the molecular motion is in the slow region; i.e., ~O~'o>> 1. Above -10~ T1 increases from 350 to 550 ms as the temperature is increased from - 1 0 to 45~ This means that the molecular motion is nearly in the extreme narrowing region. However, T1 decreases from 550 to 327 ms as the temperature is
288
TAKESHI YAMANOBE
50
t
Tm I0
Y
~5 ip=,,'-"
I" u
i
....
I
-120 -80
I
-40
I
0
_
Templ'C
!
40
|
!
80
120
Fig. 7.17. Temperature dependences of Tip for poly(3'-n-octadecyl L-glutamate) (O) and
poly(3'-oleyl L-glutamate) (O)" /3,/3 relaxation; 7,3' relaxation; Tm, melting point of the side chain crystallite [27].
increased from 15 to 60~ and again increases from 327 to 403 ms as the temperature is further increased from 60 to 120~ Two distinct minima are observed for Tip as shown in Fig. 7.17. Tip decreases from 6.0 to 3.0 ms as the temperature is increased from - 1 2 0 to -80~ and increases from 3.0 to 29ms through the first minimum as the temperature is further increased. Again, the Tlo value decreases from 30 to 60~ and increases from 21 to 41 ms through the second minimum as the temperature is increased from 60 to 120~ Comparing the minima given in Figs. 7.16 and 7.17, the minimum at lower temperature depends on the observing frequency, but that at higher temperature does not. The minimum at lower temperature comes from 3' relaxation and that at high temperature from the first order melting transition. The 7 relaxation has been attributed to the local twisting motions of the terminal alkyl groups of the side chains. The activation energy, AE, for the 3' relaxation can be determined by ~'c = ro exp(-z~E/RT), where ro is the prefactor, R is the gas constant, and T is the absolute temperature. ~c can be estimated using ~'c = 1~2"true. In Fig. 7.18, log f (the frequency in Hertz) is plotted against the inverse
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
289
10
Oh
- ,
c, I
3
4
5
,, i,
,,al
6 7 T -Ix I0 3/K-'
b 8
9
Fig. 7.18. Arrhenius plots of log f in: (a) poly(7-n-octadecyl L-glutamate) (O); and (b and c) poly(y-oleyl L-glutamate) (0) against the inverse of the absolute temperature [27].
of temperature together with the data obtained from viscoelastic measurement. The activation energy of POLG is 10 kcal/mol which is in good agreement with that (11 kcal/mol) obtained from mechanical relaxation by viscoelastic measurements. For POLLG, /3 relaxation is observed in addition to 3' relaxation. In a similar way to POLG, the activation energy for/3 and 7 relaxations are 37 and 7 kcal/mol, respectively. As the activation energy for the 7 relaxation of POLLG is smaller than that for POLG, the oleyl side chains are more flexible than the n-octadecyl side chains, which is consistent with the fact that POLLG is in a liquid crystalline state over a wide range of temperatures. Since the sensitivity of IH pulse NMR is very high and IH TI values for usual polymers are less than 1 s due to the spin diffusion, rapid measurements with short repetition times are possible. This gives us the real time measurement of nonequilibrium phenomena such as crystallization in the polymer. The crystallization process of polymers has been studied by an optical microscope, dilatometry and X-ray diffraction. These methods only gives static information about the crystallization process. The pulse NMR measurements provide both the fraction and the molecular mobility of each phase. Figures 7.19 and 7.20 show the temperature change of the fractions and T2 values of crystalline, interfacial and amorphous components for poly(e-caprolactone) [28]. From these behaviours of T2, the crystallization process can be divided into four regimes as shown in Figs. 7.19 and 7.20. In the Range I, no variation
290
TAKESHI YAMANOBE 1 O0 90
"=
..
-
_
..,
I
O OO01~
~
9
I
a
9
I
i I 9 I
I I I
II~~IIII,
I
so _
i
o
" --
70
,
.
L
_
G0
IV
m
,~ ,
',
!
i
ill
i
Aw~jaJl,
~,6,
i "J
"-" O
2L
40
, I I
30
'
I I
',
I
~ ~ n n
, , ~ - ,9 ,
, 0
,,,
J ....
I
,
,c~
.,~1
,
.
', I
ll';',,..!
TIME
I
,03
,
, I
, , ,,1
lo 4
(s)
Fig. 7.19. Temperature change of the fraction of the amounts of crystalline (&), interfacial (11) and amorphous (O) phases obtained by T2 measurements [28].
-
: 1~ 2
... 153
I
54
I
I
i
"
F"
-
i o I I
I i I I
I i I
I I ! ~
i i I I
I i i
I i I
, II,
"
atonal
I
,
I
_
n
I
~
~ ~ o e
I I I , I
I
-
" - - ' . -
I I
'J,
,,'
I
~ll, , . . . ~
L ......
L
I
I
,
!
t ~-17 ~
.,.,,
IV
! I
I
'
--
~5
, =6
III J
- - - - . 4 : -
" .-" .
. N
'
,
I
I
I
i
.
,,
....
I
,d
Fig. 7.20. Temperature changes of the ous (0) phases [28].
t
.
,~
....
,c~
TIME (s) T2
I
.....
~tdlJ~liMl&
AA
A
I
I
J
,
,l
....
I
,o4
values of crystalline (A), interfacial (m) and amorph-
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
291
of T2s observed and only the amorphous component exists. The Range I corresponds to the induction time for the primary nucleation. In Range II, the nucleation of crystallization takes place and the successive growth by the secondary nucleation follows. In this range, first, the intermediate phase appears and then the crystalline phase appears with a decrement of the amorphous phase. T2 of the amorphous phase is almost constant in this range. Although some kind of orientational order is formed in the amorphous phase which becomes the interfacial phase, the molecular mobility of most parts of the amorphous phase is unchanged. In the Range III, both the fraction and Y2 of the amorphous phase decreases and the fraction of the crystalline and interfacial phases increase. Yzs of the crystalline and interfacial phases are almost constant. In this range, the main part of the crystallization occurs. In the Range IV, only the T2 for the amorphous phase decreases with time gradually. Other parameters are almost constant. Namely, only in the amorphous phase, the slight change of the structure takes place. This range seems to correspond to the so-called secondary crystallization. Thus, it is possible to investigate the crystallization dynamics by pulse NMR.
7.7.2
CPMAS NMR
In the high resolution measurements by CPMAS NMR, the information about the molecular mobility and dynamics are involved in the intensity and the linewidth of each peak. Figure 7.21(a) shows the VT CPMAS spectra of polyethylene at temperatures between -120 and 90 ~ [20]. The spectra at temperatures from - 5 0 to -120~ are expanded in Fig. 7.21(b). At room temperature, the spectrum consists of peaks for crystalline and amorphous phases. As the temperature is increased from 25 to 90~ the intensity of peak A is enhanced. This increment of amorphous peaks arises from two effects. One is that the amount of the amorphous phase increases. The other is the increment of the CP efficiency for the amorphous phase which means that molecular mobility is increased by temperature. Both factors enhance the peak intensity of the amorphous phase. Another interesting feature is the progressive broadening of the amorphous peak as the temperature is decreased (Fig. 7.21(b)). As seen from Fig. 7.21(b), the peak at -50~ is symmetrical and consists of only one component. At - 9 0 and -120~ the peaks have shoulders at about 32 ppm. Taking into account the behaviour of amorphous peak at high temperature, the peak intensity for the amorphous phase has a minimum at about -50~ The observed full linewidth can be written as [29]
292
TAKESHI YAMANOBE (a)
A
I
9
-50"C
-90"C
....6o 5'0,.o 3o 2o",o"~ ..... CHEMICAL SHIF T / P P M
(b)
3"5
9
9
',
i
30
CHEMICAL SHIFT/PPM
Fig. 7.21. (a) CPMAS NMR spectra of melt-quenched polyethylene measured as a function of temperature, and (b) expanded CPMAS NMR spectra at temperatures from -50 to -120~ [20]. = (TrT20) -1 + (TTT2c) -1 + ('rrT2m) -1 ,
(7.2)
where the first term represents the intrinsic linewidth accounting for static line broadening mechanisms such as an inhomogeneous static field. The
C R Y S T A L L I N E AND N O N C R Y S T A L L I N E PHASES IN POLYMERS
293
second term is the broadening factor due to the chemical shift distribution caused by a variety of local conformations which is temperature independent under Tg and is averaged out by the molecular motion above Tg. The third term describes the broadening arising from the 13C--1H dipolar interaction. This interaction is removed by high-power decoupling above and below Tg. When the reorientation rate of nuclei in the amorphous phase becomes near to the frequency corresponding to the amplitude of the proton decoupling field, the motion in the amorphous phase reduces the efficiency of the decoupling and leads to a maximum linewidth [30]. From this point of view, the methylene carbons in the amorphous phase are undergoing transition between the trans- and gauche-conformations at frequency of about 60 kHz. A similar phenomenon is observed for poly(7-n-alkyl L-glutamate)s. The mobility of the n-alkyl side chain of the polymer is faster than those of the main chain. The temperature dependence of the CPMAS spectra for the polymer clearly shows the difference of the mobility. Figure 7.22 shows the temperature dependence of the CPMAS spectra for poly(7-n-hexyl Lglutamate) from -100 to 80~ Peaks from 10 to 40 ppm correspond to those for the n-alkyl side chain. The peak at 65 ppm is for the methylene carbon directly bonded to oxygen in the side chain and that at 172 ppm is carbonyl carbon in the side chain. CO (amide), 176 ppm, and Ca, 57 ppm, correspond to the main chain carbons. The side chains of this polymer do not form crystalline phases because of their shortness. Below -60~ all peaks are broad because of the inhomogeneity of the structure. At -40~ the side chain peaks are resolved and decrease their intensity compared with those from the main chain. At -20~ the side chain peaks almost disappear and only terminal methyl and main chain peaks appear in the spectrum. This disappearance of the side chain peaks means that the local motion of the side chain occurs at a frequency corresponding to the amplitude of the proton decoupling field. Above 0~ the side chain peaks reappeared with a much narrower linewidth like those of the solution spectrum rather than those below -40~ Inhomogeneity of structure is averaged out by local motion. On the contrary, main chain peaks disappear above 25~ Molecular motion of the side chain induces molecular motion of the main chain. The disappearance of the main chain peaks means that the molecular motion of the main chain is taking place at a frequency comparable with the decoupling frequency. Since the side chain peaks disappear and reappear at - 4 0 and 0~ respectively, the side chains have achieved a considerable gain in configurational freedom in a small temperature range. Above 25~ the main chain peaks never reappear within the temperature range observed, which means that the mobility of the side and main chains is very different. Thus, peak intensity contains information about the molecular motion.
294
TAKESHI
YAMANOBE
: r u ? uff U
i
e~
o v
o o
v
~o t9
o o
o
[
80 ~ 60 ~ 40 ~ 25 ~ 0~ -20 ~
1
-40 ~ -60 ~ -80 ~
^^
A
____.JV~~j
_ oo oc PPM [
200
~
.... '
'
'
'
i
150
'
"
'
'
i
100
~
'
~
'
i
50
"~
'
'
'
l
0
'
Fig. 7.22. C P M A S N M R spectra of p o l y ( y - n - h e x y l L-glutamate) as a function of t e m p e r a t u r e from - 1 0 0 to 80~
T1 measurements by high resolution NMR provide information about molecular motion of each nucleus of the polymer. Based on the ~3C~IH dipolar relaxation mechanism, T1 is expressed as follows"
1 _- h 272y2 1_~ [J(WH -- Wc) + 3J(coc) + 6J(wH + Wc)], T1 10 r6n
(7.3)
where O)n and O)c are the 1H and 13C resonance frequencies, rcH the inter-
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
295
nuclear distance and J(6o) the spectral density which has a relation with the autocorrelation function as follows"
J(co) = f + ~ G(t) exp(icot) dt.
(7.4)
G(t) is dependent on the mode of molecular motion. Since T1 is determined by the molecular motion through G(t), it is difficult to obtain information about the molecular motion directly from Ta. Usually, the mode of molecular motion is obtained by fitting the observed Ta value with simulation under the assumption of a motional model. Figure 7.23 shows the dependence of nTx on the reciprocal of temperature for CH and C H 2 of bulk poly(vinyl methyl ether) [31]. (Data are collected by the inversion recovery method under proton noise decoupling, not by CPMAS.) Measurements are carried out at two resonance frequencies, 25.15 and 62.5 MHz for a3C nuclei. As seen from the figure, the nT1 minimum is T(=C) nTltS)
40
20 I
0.4. 0.3.
60
&
o
0
-
80
100
=-
120
I
9
\62.5 MHz
\
o\
\ -\
0.2.
/
.7
0.1 0.08
Y
25.15 MHz
0" f
0.06 0.05 0.04
. . . . . . . . . . .
'~ 3
.... 2:s
T
Fig. 7.23. Observed (( 9 and (O) for CH and CH2 carbons, respectively) and simulated (dashed lines) nT1 values for bulk PVME [31].
296
TAKESHI YAMANOBE
observed at 90~ at 62.5 MHz and 70~ at 25.15 MHz. Since T1,cH/T1,cH2 = 1.95 _ 0.12, the relaxation mechanisms, i.e., the modes of molecular motion, for CH and CH2 are the same and a simultaneous fitting of experimental data is possible. Although simulations based on the isotropic, Hall-Helfand [32] and Viovy-Monnerie-Brochon [33] models are carried out, the agreement is unsatisfactory between the calculated and observed data. The HallHelfand model assumes two motional modes" one is the correlated jumps responsible for orientation diffusion along the chain like a crankshaft motion and the other is the nonpropagative specific motions or distortions of the chain with respect to its most stable local conformation like the trans-gauche transition. The Viovy-Monnerie-Brochon model involves cross-correlation functions of a pair of distant bonds in addition to neighbouring bonds. From a detailed analysis of calculated and observed data, Dejean et al. [31] pointed out the existence of an additional motion which is not included in the three models such as a partial reorientation of the CH vectors. They assigned the additional motion to the molecular librations of the limited extent of the CH vectors about their equilibrium conformation and corresponds to oscillations inside a potential well (Dejean-Laupretre-Monnerie model). In Fig. 7.23, best fit curves are shown together with observed data. The agreement is very satisfactory. This model also can explain the TI behaviour of the polymer measured in solution, which means the motional mode of polymers in solution is the same as that in the bulk. 7.7.3
Chemical shift anisotropy and exchange
The MAS technique is used to eliminate chemical shift anisotropy. However, much information is contained in the powder pattern spectrum. Chemical shift anisotropy means an orientation dependent chemical shift. This anisotropy originates from the nonspheric distribution of electrons around the nucleus of interest. The powder pattern spectrum contains information about the orientation of the principal axis of the chemical shift tensor, which is rigourously defined with respect to the molecular axis. Therefore, an analysis of the powder pattern produces concrete motional modes at the atomic level. Figure 7.24(a) shows the powder pattern spectrum of polyethylene [34]. As is well known, polyethylene is composed of the crystalline and amorphous phases. The CPMAS technique distinguishes these two phases as separate peaks. As the powder pattern spectrum is very broad because of chemical shift anisotropy, Fig. 7.24(a) is thought to be an overlap of powder patterns for the crystalline and amorphous phases. Hughes et al. [34] have carried out fitting of powder pattern spectra by assuming the existence of both the crystalline and amorphous phases. Figure 7.24(b) is the best fit spectrum, the
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
(d)
297
~ - ~ - ~
(e)
i 90
'
i 70
'
i 50
'
I 30
P P M fron~ TMS
'
i 10
'
!
'
- 10
Fig. 7.24. (a) Powder pattern of polyethylene; (b) calculated spectrum, the sum of the crystalline and the amorphous components; (c) the crystalline tensor components;. (d) the amorphous tensor components; and (e) residual spectrum (a-b) [34].
sum of the crystalline (Fig. 7.24(c)) and the amorphous (Fig. 7.24(d)) tensor components. Figure 7.24(e) is the residual spectrum (Fig. 7.24(a-b)). As seen from these spectra, the best fit spectrum reproduces the observed spectrum very well. The obtained chemical shift tensor for each phase are summarized in Table 7.7. As seen from Fig. 7.24 and Table 7.7, the chemical shift tensor for the crystalline phase is a typical tent-like one, asymmetric with three principal values representing the chemical shift along three orthogonal directions. On the contrary, the amorphous shielding tensor is axially symmetric and quite different from that for the crystalline phase. As there is only one kind of chemical group, CH2 in polyethylene, the difference in the powder pattern can be attributed to the difference of molecular mobility in the crystalline and amorphous phases. Table 7.8 summarizes the chemical shift tensors and their directions with
298
TAKESHI YAMANOBE
Table 7. 7. Principal values of chemical shift tensor for polyethylene
2.35 T field
O'xx 0"22
o'33 O'is o
4.70 T field
Crystalline
Amorphous
Crystalline
Amorphous
51.1 33.9 15.5 33.5
40.2 36.4 13.7 30.1
50.1 34.0 15.2 33.1
39.6 36.3 12.6 29.5
Table 7.8. Chemical shift tensor and the principal axis of
Principal axis
n-C2oH42
Chemical shift tensor/ppm 17.2
/
H
38.2 H
H
H
50.2
respect to the molecular axes determined for n-C2oH42. Comparing the chemical shift tensor for n-CzoH42 with that for the amorphous phase, 633 is almost the same. ~11 and (~22 are almost equal for the amorphous phase. As there is no symmetry axis to equalize 611 and (~22 for C H 2 , and as seen from the chemical shift tensor for the crystalline phase, the three values for the chemical shift tensor should be different. Therefore, rapid anisotropic molecular reorientation about the polymer chain axis is taking place in the amorphous phase. Two-dimensional exchange NMR is a powerful technique to study exchange processes. The 2D exchange NMR experiment consists of a preparation period, an evolution period, a mixing time and a detection period. If any exchange processes couple different resonance frequencies during the mixing time, off-diagonal peaks appear. Edzes and Bernards [35] applied 2D exchange NMR with chemical shielding anisotropy (2DECSA) to polyethylene (Fig. 7.25). If there are no exchange processes for a static sample, each spin has a fixed resonance frequency depending on the orientation of its
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
299
(a) I_
II0
9
.
.
|
I
r,,O
9
0oom
----..~
r~o z
(d)
Fig. 7.25. 2DECSA spectra of the crystalline phase for polyethylene: (a) normal spectrum; (b and c) stacked plots of the 2DECSA spectra after exchange times of 10 ms and 10 s; and (d) simulated 2DECAS spectrum [35].
300
TAKESHI YAMANOBE
chemical shift tensor. The 2D spectrum has only diagonal peaks. If any motion which exchanges the orientation of the chemical shift tensor exists, the 2D spectrum has off-diagonal peaks. In this spectrum, signals from the amorphous phase are suppressed by the relatively short CP time used. The spectra obtained after exchange times of 10 ms and 10 s are very different. There is a ridge on both sides of the diagonal for longer exchange times. The ridge is evidence for the existence of exchange. The ridges begin at the intersection of 61~ and ~22 and converge toward 633. This indicates that the exchange process does not change the direction of ~33, whereas ~11 and 622 are exchanged. There are two possibilities for this exchange process: (1) a rotation by 90 ~ around the ~33 axis, and (2) a rotation by 180~ around the axis bisecting the ~11 and 622 directions. Both exchange processes (1) and (2) give the same spectrum as shown in Fig. 7.25(d). Since the upside-down jump is physically impossible, actually, there are no possibilities for (2). The exchange process of (1) is taking place. For the exchange process (1), two processes can be assumed: (la) a molecular jump by 90 ~ of the polymer around the chain axis, and (lb) interchain spin exchange between 13C nuclei located on neighbouring chains with mutually perpendicular orientations. According to the detailed investigation for variable temperature experiments and measurements with different decoupling strengths, it can be concluded that the interchain exchange process (lb) is most plausible. Thus, in the crystalline phase, almost no molecular motions are taking place. Although a measurement of the static sample yields information about chemical shift tensors, there are the disadvantages of insensitivity and overlap of peaks. On the contrary, a measurement with fast MAS cannot give any information about molecular reorientation. A slow MAS experiment circumvents these problems. Two-dimensional exchange NMR measurements under slow MAS conditions are applied to polyoxymethylene (Fig. 7.26) [36]. In these measurements, the mixing time is synchronized with the rotor since the spinner orientation is exactly the same at the end of the evolution period as at the beginning of the detection period. Figure 7.26(a) shows the 2D exchange spectrum of polyoxymethylene with the mixing time of 1 s. Polyoxymethylene is composed of the crystalline phase with T 1 = 18 S and the amorphous phase with T 1 - - 9 0 m s . The peaks in Fig. 7.26(a) correspond to the crystalline phase, because of the short T1 and the small chemical shift anisotropy of the amorphous phase. From this spectrum, it is clear that no molecular motion occurs under this condition, 316 K, and a mixing time of 1 s. Figure 7.26(b) shows the spectrum at 336 K with a mixing time of 4.5 s. There are many off-diagonal peaks, which means the change in the orientation of the chemical shift tensor. By comparing the observed spectrum with the simulated 2D exchange spectrum with the assumption of molecular motion, it was found
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
301
(a~
-2
.-
,,,
,.
i
f-
(b)
-2
-;,| -
~ .........
HI
J
-
-1
-
J
0
'
'J
1
'
,r"'
2
Fig. 7.26. 2D exchange spectra of polyoxymethylene with mixing times of (a) 1 and (b) 4.5 s [36].
302
TAKESHI YAMANOBE
that there is a rotation of the polyoxymethylene helical chains over 200 ~ From the temperature dependence of the 2D exchange spectra with slow MAS, the activation energy for the motion is deduced to be 20 kcal/mol which is in good agreement with the values obtained for c~-relaxation.
7.7.4
Chain diffusion
Chain diffusion between the crystalline and amorphous phases in polyethylene can be investigated by 2D exchange N M R with fast magic-angle spinning. As mentioned abeve, MAS experiments provide separate peaks for the crystalline and amorphous phases by about 2 ppm because of the T-gauche effect. If a polymer chain translates from the crystalline phase to the amorphous phase or vice versa during the mixing time, it is possible to detect the chain diffusion in the 2D spectrum in the form of off-diagonal peaks (Fig. 7.27(a)) [37]. Figure 7.27(b) shows the 2D exchange spectrum of U H M W P E measured with D D / M A S . In this spectrum, the peak for the crystalline phase is saturated by a short repetition time. As seen from Fig. 7.27(b), off-diagonal peaks are clearly observed, which indicates exchange of magnetization, and chain diffusion from the crystalline to the amorphous phases. This motion has been assigned to the a-relaxation process which involves a rotation and a translation by one repeat unit. From quantitative analysis and Monte Carlo simulations, the activation energy for this diffusion process is 105 kJ/mol which is in
(a)
(b) C
CO
C
=~
f 28
,,-
i 30
~'-
f 32
~,-
s3/.
e-~
r
ppm
W1
Fig. 7.27. (a) Schematic visualization of chain diffusion in polyethylene, (b) 2D exchange spectrum of UHMWPE. "a" and "c" indicate peaks for the amorphous and the crystalline phases. Peaks of "ac" or "ca" arise from the chain diffusion between the crystalline and amorphous phases [37].
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
(b)
303
cn
CH 3
ah
~ CH
(a)
CH 3
al 1
ah Fig. 7.28. 2D CPMAS NMR spectrum of atactic polypropylene acquired with mixing times of (a) 5 and (b) 500 ms [38].
304
TAKESHI YAMANOBE
good agreement with reported values of 104-116 kJ/mol for the a-relaxation process. The conformational transition between the trans- and gauche-conformations is detected by 2D exchange NMR with fast MAS for atactic polypropylene [38]. The conformation-dependent chemical shift are reported for polypropylene which show that trans-trans (tt), trans-gauche (tg) and gauche-gauche (gg) resonate at 48.6, 44.5 and 40ppm, respectively. At 250 K, the peak of the methylene carbon splits into two peaks (48.6 and 44.6 ppm), which indicates the existence of both the tt- and tg-conformations. The peak splittings coalesce at temperatures above 259 K. This means that conformational transitions exchanging the tt- and tg-conformations lead to motional averaging. Figure 7.28 shows 2D exchange CPMAS NMR spectra measured with different mixing times, tm = 5 and 500 ms at the same temperature. From Fig. 7.28(a), during a mixing time of 5 ms, the off-diagonal peak is weak. Few exchanges between the tt- and tg-conformations occur during this mixing time. The off-diagonal peaks can be easily detected in Fig. 7.28(b), where the mixing time is 500 ms. This indicates that the conformations of certain carbon segments are changing during 500 ms. Analysis of the temperature dependence of the 2D exchange CPMAS NMR spectra provides a rough estimation of the average correlation times for the conformational exchange in atactic polypropylene of 10 -3 ~< ~'c ~< 1 S. The temperature dependence of the spectra exhibit a similarity to the diffusive rotational a-relaxation process.
References
,
7. 8. 9. 10. 11. 12. 13. 14.
C.A. Fyfe, J.R. Lyerla, W. Volksen and C.S. Yannoni, Macromolecules 12 (1979) 758. W.L. Earl and D.L. VanderHart, Macromolecules 12 (1979) 762. D.L. VanderHart, Macromolecules 12 (1979) 1232. N. Bleoembergen, E.M. Purcell and R.V. Pound, Phys. Rev. 73 (1948) 679. D.M. Grant and E.G. Paul, J. Am. Chem. Soc. 86 (1964) 2984; W.R. Woolfenden and D.M. Grant, J. Am. Chem. Soc. 89 (1966) 1496; A.E. Tonelli and F.C. Schilling, Acc. Chem. Res. 14 (1981) 233. D.L. VanderHart and F. Khoury, Polymer 25 (1984) 1589. D.L. VanderHart, J. Magn. Res. 24 (1976) 467. S.J. Opella and J.S. Waugh, J. Chem. Phys. 66 (1977) 4919. D.L. VanderHart, J. Chem. Phys. 64 (1976) 830. T. Nakai, J. Ashida and T. Terao, J. Chem. Phys. 88 (1988) 6049. K. Schmidt-Rohr, M. Wilhelm, A. Johansson and H.W. Spiess, Magn. Reson. Chem. 31 (1993) 352. W.E.J.R. Maas, A.P.M. Kentgens and W.S. Veeman, J. Chem. Phys. 87 (1987) 6854. T. Nakai, J. Ashida and T. Terao, Magn. Reson. Chem. 27 (1989) 666. C.S. Yannoni and T.C. Clarke, Phys. Rev. Lett. 51 (1983) 1191.
CRYSTALLINE AND NONCRYSTALLINE PHASES IN POLYMERS
305
15. M.J. Duijvestizn, A. Manenschijn, J. Smidt and R.A. Wind, J. Magn. Reson. 64 (1985) 461. 16. T. Nakai, T. Terao and H. Shirakawa, Chem. Phys. Lett. 145 (1988) 90. 17. D.L. VanderHart, J. Magn. Reson. 44 (1981) 117. 18. T. Sorita, T. Yamanobe, T. Komoto, I. Ando, H. Sato, K. Deguchi and M. Imanari, Makromol. Chem. Rapid Commun. 5 (1984) 657; M. Takenaka, T. Yamanobe, T. Komoto, I. Ando and H. Sato, Solid State Commun. 61 (1987) 563. 19. T. Yamanobe, T. Sorita, T. Komoto, I. Ando and H. Sato, J. Mol. Struct. 131 (1985) 267. 20. I. Ando, T. Yamanobe, S. Akiyama, T. Komoto, H. Sato, T. Fujito, K. Deguchi and M. Imanari, Solid State Commun. 62 (1987) 785. 21. H. Tadokoro, M. Kobayashi, Y. Kawaguchi, A. Kobayashi and S. Murahashi, J. Chem. Phys. 38 (1963) 703. 22. H. Kurosu, T. Yamanobe, T. Komoto and I. Ando, Chem. Phys. 116 (1987) 391. 23. W.S. Veeman, E.M. Menger, W. Ritchey and E. de Boor, Macromolecules 12 (1979) 924. 24. L.A. Belfiore, F.C. Schilling, A.E. Tonelli, A.J. Lovinger and F.A. Bovey, Macromolecules 17 (1984) 2561. 25. D.L. VanderHart and E. Perez, Macromolecules 19 (1986) 1902; E. Perez and D.L. VanderHart, J. Polym. Sci., Polym. Phys. 25 (1987) 1637. 26. T. Yamanobe, H. Tsukamoto, Y. Uematsu, I. Ando and I. Uematsu, J. Mol. Struct. 295 (1993) 25. 27. B. Mohanty, T. Komoto, J. Watanabe, I. Ando and T. Shiibashi, Macromolecules 22 (1989) 4451. 28. H. Tanaka and T. Nishi, J. Chem. Phys. 85 (1986) 6197. 29. K. Takegoshi and K. Hikichi, J. Chem. Phys. 94 (1991) 3200. 30. J.R. Lyerla, C.S. Yannoni and C.A. Fyfe, Acc. Chem. Res. 15 (1982) 208. 31. R. Dejean de la Batie, F. Laupretre and L. Monnerie, Macromolecules 21 (1988) 2045. 32. C.K. Hall and E. Helfand, J. Chem. Phys. 77 (1982) 3275. 33. J.L. Viovy, L. Monnerie and J.C. Brochon, Macromolecules 16 (1983) 1845. 34. C.D. Hughes, N.K. Sethi, J.H. Baltisberger and D.M. Grant, Macromolecules 22 (1989) 2551. 35. H.T. Edzes and J.P.C. Bernard, J. Am. Chem. Soc. 106 (1984) 1515. 36. A.P.M. Kentgens, E. de Boer and W.S. Veeman, J. Chem. Phys. 87 (1987) 6859. 37. K. Schmidt-Rohr and H.W. Spiess, Macromolecules 24 (1991) 5288. 38. K. Zemke, B.F. Chmelka, K. Schmidt-Rohr and H.W. Spiess, Macromolecules 24 (1991) 6874.
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Chapter 8
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Oriented Fibers and Polymers Tetsuo Asakura and Makoto Demura Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan
8.1
Introduction
Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. Many different experimental approaches have been used to characterize orientation in polymers, such as, birefringence, ultraviolet-visible light spectroscopy, infrared dichroism, sonic modulus measurements, various X-ray techniques and nuclear magnetic resonances (NMR) [3]. Among these approaches, the rapid methodological development of solid-state NMR has opened up new possibilities for characterization [3-9]. Orientation-dependent NMR interactions, such as dipole-dipole coupling, quadrupole coupling and chemical shielding, yield structural information on polymers in the solid state. If these parameters are observed for each site in oriented polymers, solidstate NMR methods can give even the atomic coordinates of the polymers. This is especially effective in the determination of the atomic coordinates of peptides or proteins in membrane systems, and fibrous proteins in which the X-ray technique gives only little structural information. The application of solid-state NMR to proteins is reviewed in Chapter 23. Fibrous proteins are particularly difficult to study using standard structure determination techniques. X-ray diffraction from fibers, in which proteins are aligned along the long axis of the fiber, typically yields general features
308
TETSUO ASAKURA AND MAKOTO DEMURA
of molecular organization and packing, but lacks atomic resolution details. High-resolution solution NMR techniques are not applicable to fibrous proteins, since the solution state structure is generally unrepresentative of the structure in the fibrous state. On the other hand, the natural alignment of the protein within the fiber provides an important advantage that can be utilized by solid-state NMR. Selective isotope labeling of the sample is usually required in these NMR experiments for getting site-specific structural information. This is possible for the silk fibroin fiber by oral administration of isotope labeled amino acids to silkworms, or by cultivation of the silkglands which produce silk fibroin protein in medium containing the isotope labeled amino acids as described in Chapter 20. Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. As mentioned above, the determination of atomic level structure, i.e., the backbone torsion angles for an oriented protein fiber, is possible by using both solid-state NMR method described here and specifically isotope labeling. This is basically to obtain the "angle" information. Another structural parameter is "distance" between the nuclei for atomic coordinate determination. The observation of Nuclear Overhauser Enhancements (NOEs) between hydrogen atoms is a well known technique to determine the atomic coordinates of proteins in solution [14]. In the field of solid-state NMR, R E D O R (rotational echo double resonance) for detection of weak heteronuclear dipole interactions, such as those due to 13C and ~SN nuclei [15, 16] o r R 2 (rotational resonance) for detection of the distance between homonuclei, are typical methods for internuclear distance determination [17,18]. The R E D O R technique has been applied to structure determination of a silk fibroin model compound [19]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required which frequently becomes a problem. A review of these approaches has appeared elsewhere [16].
309
ORIENTED FIBERS AND POLYMERS
To extract information on molecular orientation distribution from experimental data, the most widely known technique, the Legendre moment expansion approach can be taken. In this section, this approach will be discussed first, followed by methods to elucidate atomic resolution details of the structures of ordered polymers with orientation-dependent NMR interactions, such as those from chemical shielding, dipole-dipole and quadrupolar coupling. Then, solid-state NMR studies of the torsion angles of the peptide backbone of highly ordered silk fibroin fiber, a protein that has been studied extensively as a model for fibrous proteins, will be described.
8.2
Molecular orientational distribution
The position of any polymer chain (or a segment of that chain) in an oriented (drawn) sample can be described by a rotational transformation of tensors from a molecular coordinate frame in a typical structural unit (xyz) to a sample coordinate frame (XoYoZo) with the Euler angles (a/3 y) and (al/3171) (Fig. 8.1). This step introduces the statistical distribution of units into the analysis. In the most general case, there is a distribution of orientations, P(a, /3, 7), describing the sample anisotropy. When the orientation is uniaxial, the distribution is cylindrically symmetrical about the draw axis, and P(a,/3, y) reduces to a function of one variable, P(/3) [3, 5] Zo
Z go
OtI
.J Xo
// (a)
X
24,, 9"
~y
.(b)
Fig. 8.1. (a) Orientation of a typical structural unit in the sample coordinate frame XoYoZo where Zo is the draw direction of a uniaxially stretched polymer; (b) orientation of the sample in the laboratory frame X Y Z where Z denotes the direction of Bo and Zo is the sample draw direction.
310
TETSUO
ASAKURA
AND MAKOTO
DEMURA
o0
P(13) =
E
P,oo" Pz(cos/3)
(8.1)
l=0 l even
where 2/+ 1 P,oo = ~ (P/(cos/3)) 41r
(8.2)
and (Pz(cos/3)) represents the average value of the lth Legendre polynomial Pt of cos/3 over the distribution (denoted hereafter simply by (Pz)). The sum in Equation (8.1) includes only even-ordered terms because of symmetry considerations [20]. Because the (Pz) are given by (p,) -- E
bi(cos i [3)
(8.3)
i=o l even
the expansion of P(13) entails calculation of the moments of the distribution ((cos 2 13), (cos 4/3) . . . . ) where P(/3) cos" 13sin 13 d/3 (cos"
(8.4)
="
~o
P(13) sin/3 d]3
These quantities can be used to make certain conclusions about the shape of P(fl). Of course, one cannot evaluate the sum in Equation (8.1) over an infinite number of terms. Fortunately, the truncation error can be made small for low to moderate values of (cosi 13) by including only a few terms [21]. For relatively high degrees of orientation, it may be simpler to make certain assumptions about the shape of P(13), testing this assumed shape against the experimental results. Several models have been proposed that attempt to describe the development of orientation in drawn polymers. The pseudo-affine deformation model [5] is used on occasion to describe statistical distributions in partially ordered polymers. Upon drawing, this model envisages the unique axes of structural units undergoing the same change in direction as lines connecting pairs of material points in the polymer without change in volume. The moments of the distribution are related to the draw ratio l according to
O R I E N T E D FIBERS A N D P O L Y M E R S
(COS 2/3) -- (/~ 3 __ 1) 1,2
{
1-
311
tan- 1(/~ 3 __ 1) 1/2l (/~ 3 __ 1)1/2
J
/~3 (COS ~ ] 3 ) =
( k - 2)(a 3 - 1)
{(k-
1)(cos/~-2 ] 3 ) -
1}
(8.5)
An interesting comparison has been drawn between the prediction of pseudoaffine deformation and affine deformation of a rubbery network [3] which showed that the orientation distribution functions (cos m/3) grow at a rate which increases with increasing draw ratio in contrast to the behavior of the pseudo-affine scheme. The NMR spectrum of an oriented system, such as a deformed polymer, is subject to interactions that arise from the anisotropic distribution of molecular orientations. Homonuclear dipolar coupling for 1H or 19F [3], heteronuclear dipolar coupling for 1 3 C - - 1 5 N [22] and 2H~lSN [23], quadrupolar coupling for 2H [24], and chemical shift anisotropy for ~3C, 15N, 19F etc. [25-27] in the solid state exhibit a dependence on the state of orientation of the molecular framework relative to the magnetic field direction. This dependency can be analyzed to characterize the sample anisotropy. Several investigations have used the Gaussian distribution expressed as follows: P(13)
=
Ce -sin2t3/sin2~
(8.6)
where C is a normalization constant. The dependence of the predicted moments (Pz(cos/3)) on the width of the Gaussian distribution, /3, determined from Equations (8.1) and (8.5), has been worked out [28, 29]. 2H NMR spectra of uniaxially oriented da-polyethylene (PE) (draw ratio A = 9) and d4-PE single crystal mats were analyzed using a Gaussian orientational distribution function as a function of 13 [28]. The molecular order of poly[1,4phenylene 2,5-bis (hexyloxy) terephthalate-d4] aligned in the magnetic field has been analyzed from solid-state angular-dependent aH NMR (Fig. 8.2). The macroscopic alignment of the rigid backbones can be quantitatively frozen in the solid state. Spectral simulation using two values of the angle, a, between the local axis of the terephthalate ring and the order axis, 0 ~ and 8~ weighted 1 "3, respectively, and assuming a spherical Gaussian orientational distribution function with a halfwidth of 18~ corresponding to an order parameter (Pe)= 0.9, was performed. Based on this result, the presence of two major conformers, with a = 0 for cis and 7~ < a < 9~ for trans was found [29]. Orientation in the crystalline and amorphous regions of polytetrafluoroethylene (PTFE) as a function of 13 was discussed from 19F
312
TETSUO ASAKURA AND MAKOTO DEMURA
100k"-~Hz 13o=0~
30 ~
60 ~
90 ~
Obse
Simulated ~_
-'3_
'-.-
.JL
Fig. 8.2. Experimental and calculated angular-dependent 2H NMR spectra of poly[1,4-phenylene 2,5-bis (hexyloxy) terephtalate-d4] aligned in the magnetic field; /3o denotes the angle between the direction of the magnetic field Bo and the order axis h. a, angle between the local axis of the terephthalate ring and the chain axis.
chemical shift spectra [27]. Crystalline and amorphous spectra for a number of sample orientations in the magnetic field reveal a weak angular dependence in the amorphous signal. The order parameter S and (cos //3) for the crystalline and amorphous regions have been obtained from the analysis of angular dependence of the 19F chemical shift. More recently, multidimensional NMR techniques for analyzing ordered structure in polymers have been described [1].
8.3
Determination of molecular orientation
The determination of polymer structure at the atomic level is possible by analyzing orientation-dependent NMR interactions such as dipole-dipole, quadrupole and chemical shielding anisotropy as mentioned above. The outline of the atomic coordinate determination for oriented protein fibers used here is described more fully in Ref. [30]. The chemical shielding anisotropy (CSA) interaction for 15N nucleus in an amide (peptide) plane can be interpreted with the chemical shielding tensor transformation as shown in Fig. 8.3. [31, 32]. The 15N CSA principal axis system (PAS) is a frame in which the 15N CSA is diagonal, with principal components O'11 < 0"22 ( 0"33. When the molecular symmetry axis (MSA) system is used as a reference frame, the Euler angles
313
ORIENTED FIBERS AND POLYMERS
(A)
PAS
Zrxs
(.'/3.~
13
-Xr~s
MSA ~~//
(B)
I'A~
( 0.., ONc) ZLAB i,
ZFAS
,.~lr ,11K.
Y Fig. 8.3. (A) Transformation from the 15N PAS to the MSA and to the FAS. The PAS is related to the MSA by the Euler angles aD and /3D. The PAS is related to the F A B by the Euler angles aF and /3F. The bond orientations 0NH and 0Nc are obtained according to the transformation between PAS and MSA, and PAS and FAS. (B) Transformation from the FAS to the LAB frame of reference. The FAS is related to the L A B by the Euler angles aL and /3,..
that express the relative orientations of the 15N CSA PAS and the MSA frames of reference are known as aD and /3D. Inherent in this orientation relation is the assumption that ~r33 lies in the peptide plane (i.e., the x z plane), yielding the result that yi~ = 0 ~ [33-37]. Here the fiber axis system (FAS) is a reference frame fixed in the aligned sample, and is defined such that the macroscopic fiber axis lies in the z direction. The 15N CSA PAS is also at some orientation relative to the FAS, expressed by the Euler angles a v and/3v. In this case, the location of x and y within the plane perpendicular
TETSUO ASAKURA AND MAKOTO DEMURA
314
to the fiber axis is defined such that 033 lies in the xz plane, as illustrated in Fig. 8.3(A). This position also yields the convenient result that YF = 0 ~ The N M R spectra are observed in the laboratory frame of reference (LAB), in which the applied magnetic field (Bo) lies in the Z direction. The angles aL and /3L are the Euler angles that transform the FAS into the LAB frame of reference (see Fig. 8.3(B)). Only two angles are required for this transformation because the NMR experiment is sensitive only to the component of the tensor parallel to Bo. Therefore, the position of the FAS within the XY plane is arbitrary, and the third Euler angle, YL, can be conveniently set to zero. The angles aL and /3L are set in the 15N experiment performed here by placing the fiber axis parallel ( a L = 0 ~ /3L=0 ~ or perpendicular (0 ~ < C~L< 360 ~ /3L = 90 ~ to Bo. The specific orientations of the peptide plane, N ~ H , N ~ C ' , C ' = O , and C ' ~ N bonds with respect to the fiber axis are illustrated in Fig. 8.4, and are calculated with these Euler angles aV, /3F, aD and/3D from the 15N and 13C solid-state NMR spectra. The torsion angles th and 0 of the oriented protein fibers are determined by a combination of the bond orientations as described below. 8.3.1 Orientation of chemical shielding tensor elements with respect to the oriented axis The 15N CSA principal components (O'11 < 0"22 < 0"33) of the chemical shielding tensor can be experimentally determined by observing the powder pattern arising from a randomly dispersed sample. Then the 15N CSA tensor is rotated from the PAS through the FAS to the LAB frame of reference as follows:
(YPAS
RF(a F, /3F) RL(a L, /3L) ~ dFAS )~' dLAB
where Ri(oli, ~i) represents the 3 x 3 unitary transformation matrix that rotates the original tensor to the i reference frame, where i is a single-letter abbreviation denoting the frame to which the tensor is being rotated, (c~i, [~i) are the Euler angles as previously defined, and d is the representation of the chemical shielding tensor in the reference frame denoted by its subscript. The explicit form of the above net transformation is expressed as the following series of matrix multiplications: ~LAB = RL(aL, /3L)" RF(C~F, /3F)" (TpAs" RF(ggF, /~F) T" RL(CgL, /~L) T
(8.7)
ORIENTED FIBERS AND POLYMERS
315
Fig. 8.4. Definition of the bond orientations, 0NH , 0NC , 0CO and 0CN, Each bond orientation is related to the angle between N--H, N--C', C'--O, and C'--N bond vector and magnetic field Bo.
where (~LABdenotes the final representation in the laboratory frame of the CSA, the superscript T denotes the transpose of matrix, and
dPAS =
0"11 0
0 O"22
0 t 0
0
0
o-33)
t
(s.8)
TETSUOASAKURAAND MAKOTODEMURA
316
R/(a,
fli)= (CO~[3i0--Sio[3it t COS~i 1
\ sin ~i
0
•
COS ~i /
sin c~i Ot
-sin a i
COS a i
0
0
(8.9)
The observed chemical shielding corresponds to the component of the transformed chemical shielding tensor that is parallel to Bo, which is the (3, 3) component of the resulting dLAB matrix. In the case where the fiber axis is parallel to Bo, aL = 0 and /3L = 0, and the observed chemical shielding obtained from Equation (8.7) is given by
O'par =
O'11 sin2 ~F cOS2 aF nt- 0"22 sin2 ~F sin2 aV + 0"33 COS2 ~F
(8.10)
For the case where the fiber axis is perpendicular to Bo, /3L = 90 ~ and 0 ~ < aL < 360 ~ (all values of aL will be equally represented in the spectrum). The observed chemical shielding for this situation can be obtained from Equation (8.7) and is given by
O'per(aF, /~F,
a t ) = Fll cOS2 a t -~- 2F12 cos ~ t sin aL + F22 sin 2 at.
(8.11)
where F l l = 0"11 cOS2 ]~F cOS2 a F -'}- O"22cOS2 /~F sin2 av + 0"33sin 2 / 3 F
(8.12A)
F12 = (O"22 -- O"11) COS/~F COS O~F sin g~F
(8.12B)
F22 = o11 sin 2 a v + 0"22 COS2 aF
(8.12C)
The F 0 terms are the i, j components of the CSA tensor expressed in the FAS reference frame. Equation (8.11) represents a family of curves that are manifested in the spectral line shape. The spectra obtained from oriented protein fiber samples aligned parallel and perpendicular to the magnetic field yield eight possible orientations of the peptide plane relative to the long axis of the fiber [31]. Note that it is necessary to consider only one set of aF, /3F angles per site in the spectral simulations since the remaining seven possible pairs for each site will result in identical lineshapes. A trial and error process can be utilized to simulate the lineshape by
O R I E N T E D FIBERS AND P O L Y M E R S
9
I
I
200
I
I
317
I1
i
0
100 ppm
80
60
0
20
4o
60
80
CtF (deg)
Fig. 8.5. Solid-state ~SN N M R spectra obtained from the oriented [15N]Gly silk fibroin fiber sample. Spectra were observed with the fiber axis both (A) parallel and (B) perpendicular to the magnetic field. The best-fit simulated line shapes are superimposed on the experimental spectra. Parameters for spectral simulation were found from error analysis as a function of ~F and /3F(C).
varying av and /3v. Full spectral simulations are performed in order to improve the accuracy of these two spectral angles as shown in Fig. 8.5A. A Gaussian probability distribution of fiber axis orientations was employed to account for the spectral broadening observed in both the parallel and
TETSUO ASAKURA AND MAKOTO DEMURA
318
perpendicular cases. An error analysis is performed by summing the difference between the experimental and calculated points in the spectral lineshape,
N X2= E (icalc Iexp)2 i=1
(8.13)
The minimum of this sum indicates the best fit of the av and /3F pair (Fig. 8.5B). 8.3.2 Orientation of chemical shielding tensor elements with respect to the molecular frame The powder pattern spectrum of a dipolar coupled nuclear site can be expressed in the following form [33]:
f180 (360 S(u) = ~ S(ui) = ~ i
gi[u, u~(O, oh, aD, /3D)] sin 0 00 0~b
(8.14)
i" dO=Odcfl=O
where 0 and ~b are polar angles that describe the orientation of the chemical shielding PAS with respect to the magnetic field fixed in the laboratory frame, u is the observed frequency, ui is the transition frequency for a particular orientation of the chemical shielding (Ucs) and dipolar (up) tensors, and g~ is the lineshape function for ui. The Euler angles aD and /3D that express the relative orientations of the 15N CSA PAS and the MSA frames of reference (Fig. 8.3) are determined from this spectral simulation. For example, the 13C or ~SN powder patterns of [1-13C]~[15N] double labeled peptides are modulated by the dipolar interaction between 15N and directly bonded 13C nuclei [30], and thus the Euler angles aD and /3D for [1-13C]~[15N] doubly labeled peptides are experimentally determined. 8.3.3
Orientation of the molecular frame with respect to the oriented axis
The orientation of the molecular flame with respect to the oriented axis defined as 0yI-i, 0yc, 0co and 0cN in Fig. 8.4 can be calculated with the Euler angles described above. Simple equations can be derived for 0yi-i and 0yc, the N ~ H and N ~ C ' bond orientations to the fiber axis, respectively. The resulting expression for the N ~ X bond angle relative to the fiber axis is as follows [31]:
319
O R I E N T E D FIBERS AND POLYMERS COS 0NX -- COS j~F COS/~DNX -+- sin j~F COS ~F COS CgDNx
sin j~DNX
+ sin j~F sin aF sin IXDNx sin j~DNX
(8.15)
where IXDNx and j~DNX (X is H or C'), and av, /3F pairs are the Euler angles described above. The determination of the molecular frame with respect to the oriented axis is possible from the observation of the dipolar coupling constant of uniaxially oriented [1-13C]~[15N] double labeled silk fiber. Namely, when the 13C~15N dipolar splitting is observed for the uniaxially oriented protein fiber placed parallel to Bo, the angle, 0Nc, between the 1 3 C ~ 1 5 N bond and the oriented fiber axis can be directly obtained according to the following equation [22, 38]. APobs-- vii(3 cos 2 0NC- 1)
(8.16)
where vii is the dipolar coupling constant ( - h y y y c / 4 z r 2 r 2 _ c , h" Planck's constant, ~/y and Yc: the gyromagnetic ratios of 15N and 13C nuclei, respectively, rN--C: bond length of the N ~ C peptide bond). The quadrupole splitting A v0 observed in the solid state 2H NMR spectrum of oriented 2H labeled protein fibers depends on the angle, 0CD of the C~2H bond vector relative to Bo when the oriented fiber axis is set parallel to the magnetic field [39, 40]. At,o =
3 e2qQ 4
h
(3 cos 2 0CD -- 1 + 7/sin 2 0CD COS2 a )
(8.17)
where e2qQ/h and ~ are the rigid lattice quadrupole coupling constant and asymmetry parameter (=0-0.05 in a peptide), respectively. Thus, the angle 0CD is determined directly from observed quadrupole splitting of uniaxially oriented samples. 8.3.4
Determination of torsion angles of the peptide backbone
The relative orientations of the peptide planes in a protein are conveniently described with the torsion angles ~ and 0. As shown in Fig. 8.4, the orientation of a peptide plane with respect to the fiber axis, which is set parallel to Bo, is determined by the bond orientations, i.e., 0NH and 0NC for the Ca(i- 1)~C'--N~Ca (i) plane, or Oco and OCN for the C a ( i ) ~ C ' ~ N ~ C a ( i + 1) plane. In addition, OCD is also used for this purpose. The conformational space described with 4~ and ~ angles are reduced
320
TETSUO A S A K U R A AND M A K O T O D E M U R A
from the relative orientation of two peptide plane linkages and, therefore, the bond orientations, 0NH, 0NC, 0CO, 0CN, and 0CD can be used for the determination. In a uniaxially oriented protein such as silk fibroin fiber, these 4~, qJ angles are calculated under the following assumptions [30]: 1. Bond lengths and bond angles are fixed, and the standard values are used. 2. The o~ angle of the peptide plane is fixed to be 180 ~ 3. The C a ( i - 1 ) ~ C a ( i + 1) direction with respect to the FAS is known. At first, bond orientations, 0, with respect to the C a ( i - 1 ) ~ C a ( i + 1) direction of the protein fiber are calculated as a function of (4~, qJ). Fig. 8.6 shows the bond orientations in degrees calculated with standard geometrical parameters as an example. Possible (~b, qJ) pairs are compared with the observed bond orientations. In addition, the calculated bond orientations, 0 directly correspond to the observed orientations when the C a ( i - 1 ) ~ C a ( i + 1) direction is parallel with respect to the FAS such as in the oriented /3-sheet structure which appears in the silk fiber. The most accurate (4~, qJ) values can be found from the combination of the calculated orientations which satisfy the observed bond orientations. Fig. 8.7 shows a combination of the bond orientations, 0NH, 0NC, 0CO, 0CN for the case of the Ala site of B. mori silk fiber. Here, the experimental error for each 0 value is introduced and shown as the width of the lines. The black area indicates the most accurate ~b-qJ region which satisfies all of the observed bond orientations within experimental error. By addition of the angular constraint 0CD to those obtained from 15N and 13C solid-state NMR, 0NH, 0NC, 0CO, 0CN, a further narrow (more reduced conformational space) 4~-qJ region is determined. Examples for fibrous proteins are shown in Chapter 20.
Quantitative use of chemical shielding anisotropy to determination of bond orientation 8.3.5
The solid-state N M R method described in this article is still useful in the determination of the local structure in oriented synthetic polymers as well as protein fibers. In this section, bond orientation will be treated first, as a more simple stage in the structural determination on the basis of chemical shielding. Namely, the NH bond orientation relative to the oriented axis can be calculated from the 15N chemical shielding when the oriented axis is set parallel to the magnetic field. Then the solid-state ~SN NMR method applied to the determination of silk fiber structure is shown to be also useful in the analysis of the local structure of polyamide fibers. In this case, the samples are not isotope labeled. On the other hand, a carbonyl carbon labeled PET sample
321
ORIENTED FIBERS AND POLYMERS 1 80 1 601 401 20Q~
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is used for elucidating the local structure of oriented films with solid-state N M R (Chapter 14). The 15N chemical shieldings of oriented polymers such as polyamides are sensitive to the angle between the NH bond vector and the fiber axis, 0NI-X when the oriented axis of the sample is set parallel to the magnetic field. Therefore, the relationship between the ~SN chemical shielding and the angle 0NH can be used for obtaining structural information. Figure 8.8 shows the solid-state ~SN N M R spectra (solid line) obtained from blocks of oriented natural abundance silk fibroin fiber, Poly(p-pheny-
322
TETSUO ASAKURA AND MAKOTO DEMURA
180
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,
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(deg)
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Fig. 8. 7. Combination of four bond orientations (0NH, ONe, 0CO and 0oN) of Ala site of silk fiber within - 1 8 0 ~ th < 0 ~ and 0 ~ qJ < 180 ~ The width of each line means experimental error (_+5~ There are two lines indicated by arrows for each bond orientation. The black area (A-D) indicate the most accurate 4~-~0 angular constraints satisfied with all four bond orientations within experimental error.
lene terephthalamide) (PPTA) fiber and poly (y-benzyl c-glutamate) (PBLG) film placed parallel to the magnetic field. Here the oriented film of PBLG was prepared under shearing stress on a glass plate [41, 42]. The IR spectrum of the PBLG sample indicates an ahelical conformation [43] where the angle between the NH bond and the helix axis is approximately 13 ~ [44, 45]. The 0NH angle of the oriented PPTA fiber is calculated from the coordinates reported from an X-ray diffraction analysis as 66 ~ [46]. Thus, it is noted that the 15N chemical shielding changes significantly when the angle 0NI-I is changed from 13~ to 90 ~ The observed chemical shielding values of O'pa~a~l~ -- (rico for natural abundance silk fibroin, ~SN Gly-labeled silk fibroin, PPTA, and PBLG are plotted against 0yi-i for 0 ~ < 0NH < 90 ~ in Fig. 8.9. When the values, aDNH and /3I~NI-I determined for appropriate model compounds are used [31, 17], the angle, 0yH can be obtained as a function of av and /3F as described in Section 8.3.1. The O'pa~an~- O'i~o values are calculated by searching all of the av and /3F space (0~ ~ for aF and /3F pairs that yield simultaneous solutions to both Equations (8.10) and (8.11) in Section 8.3.1. The orientational restriction for silk fibroins and PBLG are shown by the region shown by the horizontal lines in Fig. 8.8. Similarly, the region shown by the vertical lines is allowed region for PPTA. The latter
ORIENTED FIBERS AND POLYMERS
Silk Fibroin
323
A
(.
P
PBLG
I ........
1
1
250 200 i50
I
100
_
t
50
_
i
0
_j
-50
ppm from15NH4NO3 Fig. 8.8. Solid-state 15N NMR spectra of the oriented block samples of natural abundance B. mori silk fibroin, poly(p-phenylene terephthal amide) (PPTA), and poly(y-benzyl L-glutamate)
(PBLG). region is considerably more narrow than the former one, due to the smaller /3DNIa value [17]. Thus, the relationship between the 15N chemical shielding and the angle 0yH can be used for the determination of the local structure of polymers containing N H bonds such as polyamides. This method is used in Chapter 12.
8.4
Conclusions
The solid-state N M R approach on the basis of orientation-dependent N M R interactions such as chemical shielding anisotropy, dipole-dipole and quadru-
324
TETSUO A S A K U R A AND M A K O T O D E M U R A 120 8O o 40 ! I,,,
m
0
-40 -8O
0
30
60
90
0NH Fig. 8.9. Ranges of O'par - -
O'is o VS. 0NH (0-90 ~ searched over all av and/3v space. The regions shown by the horizontal lines are for natural abundance silk fibroin (O), ~SN Gly-labeled silk fibroin (O), and PBLG (A), and the region shown by the vertical lines is for PPTA ([--1). The 0NH values and the observed O'par -- r values for natural abundance silk fibroin (1), [15N]Glylabeled silk fibroin (2), PPTA (3) and PBLG (4) are included. The three ranges for O'par - - O ' i s o determined from the spectrum of the block PMIA sample are represented by pairs of arrows.
pole coupling has been developed to determine the atomic coordinates, that is, the torsion angles of the backbone chain of a protein fiber. This approach is also applied to determine the local structure of an oriented synthetic polymer containing a noncrystalline domain. In this chapter, we concentrated on obtaining "static" detailed structural information of oriented polymers, but the spectra used for the analysis also include another important information, namely "dynamics", which have been currently obtained from solidstate NMR [1]. Deformation such as drawing, compression, annealing, strain, creep and stress relaxation of polymers including fibers may produce quite different orientational behavior, the results of which can be examined with solid-state NMR from both the static and dynamic viewpoints. The accurate model produced on the basis of atomic resolution of the local structure and the local dynamics can be built up in order to interpret the mechanical properties of polymers and the deformation mechanisms. Most recently, a double-quantum solid-state NMR technique has been reported [47]. This gives detailed structural information such as torsion angles for unoriented amorphous polymers including unoriented polypeptides with NMR tensor correlation in the ~3C-13C labeled segments. This new solidstate NMR technique can be used to cover the atomic level structural analyses of polymers from amorphous to crystalline, together with the analytical methods used for oriented polymers and described in this article as well as the R E D O R o r R 2 methods for "distance" determination.
ORIENTED FIBERS AND POLYMERS
325
References
.
10. 11. 12. 13. 14. 15. 16. 17.
18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid-State NMR and Polymers. Academic Press, London, 1994. D. Kaplan, W.W. Adams, B. Farmer and C. Viney (eds.), Silk Polymers. American Chemical Society, Washington, DC, 1994. R.A. Komoroski (ed.), High Resolution NMR of Synthetic Polymers in Bulk. VCH Publishers, Deerfield Beach, FL, 1986. 3.L. Koenig, Spectroscopy of Polymers, ACS Professional Reference Book. American Chemical Society, Washington, DC, 1992. V.J. McBrierty and K.J. Packer, Nuclear Magnetic Resonance in Solid Polymers. Cambridge Solid State Science Series, Cambridge, 1993. R.N. Ibbett, NMR Spectroscopy of Polymers. Blackie Academic & Professional, Glasgow, 1993. R.Y. Dong, Nuclear Magnetic Resonance of Liquid Crystals. Springer Verlag, New York, 1994. R. Tycko (ed.), Nuclear Magnetic Resonance Probes of Molecular Dynamics. Kluwer, Dordrecht, 1994. A.H. Fawcett (ed.), Polymer Spectroscopy. John Wiley, Chichester, 1996. T. Asakura, J-H Yeo, M. Demura, T. Itoh, T. Fujito, M. Imanari, L.K. Nicholson and T.A. Cross, Macromolecules 26 (1993) 6660. J.-H. Yeo, M. Demura, T. Asakura, T. Fujito, M. Imanari, L.K. Nicholson and T.A. Cross, Solid-state NMR 3 (1994) 209. T. Asakura, J.-H. Yeo and I. Ando, Polym. J. 26 (1994) 229. T. Asakura, T. Konakazawa, M. Demura, T. Ito and Y. Maruhashi, Polymer 37 (1996) 1965. K. Wiithrich, NMR of Proteins and Nucleic Acids. John Wiley, New York, 1986. T. Gullion and J. Schaefer, J. Magn. Reson. 81 (1989) 196. T. Gullion and J. Schaefer, Adv. Magn. Reson. 13 (1989) 57. P.T. Lansbury, Jr., P.R. Costa, J.M. Griffiths, E.J. Simon, M. Auger, K.J. Halverson, D.A. Cocisko, Z.S. Hendsch, T.T. Ashburn, R.G.S. Spencer, B. Tidor and R.G. Griffin, Nature Structural Biology 2 (1995) 990. J.M. Griffiths, T.T. Ashburn, M. Auger, P.R. Costa, R.G. Griffin, and P.T. Lansbury, Jr., J. Am. Chem. Soc. 117 (1995) 3539. T. Asakura, A. Aoki, M. Demura, J.M. Joers, R.C. Rosanske and T. Gullion, Polym. J. 26 (1994) 1405. D.I. Bower, J. Polym. Sci., Polym. Phys. Ed. 19 (1983) 93. S. Nomura, H. Kawai and I. Kimura, J. Polym. Sci., A2 8 (1980) 383. T. Asakura, M. Demura, Y. Hiraishi, K. Ogawa and A. Uyama, Chem. Lett. (1994) 2249. F. Tian, K.-C. Lee, W. Hu and T.A. Cross, Biochem. 35 (1996) 11959. T. Asakura, M. Minami, R. Shimada, M. Demura, M. Osanai, T. Fujito, M. Imanari, A.S. Ulrich, Macromolecules 30 (1997) 2429. T. Asakura, M. Demura and N. Nishikawa, in Ann. Rep. NMR Spectrosc. 34, G.A. Webb (ed.) Academic Press, London (1997) 301. M. Demura, Y. Yamazaki, T. Asakura and K. Ogawa, J. Mol. Struc. (1998), in press. A.J. Brandolini, M.D. Alvey, C. Dybowski, J. Polym. Sci., Polym. Phys. Ed. 21 (1983) 2511. R. Hentschel, H. Sillescu, H.W. Spiess, Polymer 22 (1981) 1516.
326 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.
TETSUO ASAKURA AND MAKOTO D E M U R A U. Falk and H.W. Spiess, Makromol. Chem., Parid Commun. 10 (1989) 149. M. Demura, M. Minami, T. Asakura and T.A. Cross, J. Am. Chem. Soc., in press. L.K. Nicholson, T. Asakura, M. Demura and T.A. Cross, Biopolymers 33 (1993) 847. M. Mehring, Principles of High Resolution NMR in Solids, 2nd revised Edition. Springer Verlag, Berlin, 1983. Q. Teng and T.A. Cross, J. Magn. Reson. 85 (1989) 439. G.S. Harbison, L.W. Jelinski, R.E. Stark, D.A. Torchia, J. Herzfeld and R.G. Griffin, J. Magn. Reson. 60 (1984) 79. T.G. Oas, C.J. Hartzell, F.W. Dahlquist, G.P. Drobny, J. Am. Chem. Soc. 109 (1987) 5962. C.J. Hartzell, M. Whitfield, T.G. Oas and G.P. Drobny, J. Am. Chem. Soc. 109 (1987) 5966. C.J. Hartzell, T.K. Pratum and G.P. Drobny, J. Chem. Phys. 87 (1987) 4324. Q. Teng., L.K. Nicholson and T.A. Cross, J. Mol. Biol. 218 (1991) 607. R.A. Kinsey, A. Kimtanar, M-D. Isai, R.J. Smith, N. Janes and E. Oldfield, J. Biol. Chem. 256 (1981) 4146. A.S. Ulrich, A. Watts, A.I. Wallat and M.P. Heyn, Biochemistry 33 (1994) 5370. R.D. Orwoll and R.L. Vold, J. Am. Chem. Soc. 93 (1971) 5335. S. Sasaki and I. Uematsu, J. Polym. Sci. Polym. Phys. Ed. 23 (1985) 263. Y. Masuda, K. Fukushima, T. Fujii and T. Miyazawa, Biopolymers 8 (1969) 91. R.D.B. Fraser and T.P. MacRae, Conformation in Fibrous Proteins, Academic Press, New York and London, 1973. A. Elliott, R.D.B. Fraser and T.P. MacRae, J. Mol. Biol. 11 (1965) 821. M.G. Notholt, Eur. Polym. J. 10 (1974) 799. W. Gabrielse, G.H. Angard, F.C. Feyen and W.S. Veeman, Macromolecules 27 (1994) 5811.
Chapter 9
Polyethylene and Paraffins Takeshi Yamanobe I and Hiromichi Kurosu 2 1Department of Chemistry, Gunma University. Kiryu, Gunma, Japan; 2Department of Textile and Apparel Science, Nara Women's University, Kitauoya-Nishimachi, Nara, Japan
9.1
Introduction
Paraffins and polyethylene have been the target of many high resolution solid-state N M R studies on the chemical shift values, motions etc., because only one kind of methylene exists except for the terminal carbons in the molecule. In this chapter, structure and dynamics of polyethylene and paraffins are described.
9.2
Conformation and crystal structure of polyethylene and paraffins
The chemical shift value is a good index of structure for molecules of interest. For paraffins and polyethylene trans-zigzag conformation is stable independent of chain length in the solid state. In Fig. 9.1 is summarized observed chemical shift data for paraffins and polyethylene in solid, liquid states and solution [1]. Chemical shift values of paraffins and polyethylene in the solid state are in the range 33-35 ppm, which are typical chemical shift for methylene carbons taking the trans-zigzag conformation. As seen from the figure, the chemical shift of n-paraffins in the liquid state and solution appears to be of lower frequency than those in the solid state. In the liquid state and solution, paraffins and polyethylene adopt some rotational isomers containing gauche conformation. As mentioned in Chapter 7, the gauche conformation causes a low frequency shift, so-called y-gauche effect [2]. The low frequency shifts in the liquid state and solution are caused by y-gauche effect like an amorphous phase. The conformational state (how much gauche conformer exist) in the liquid state and solution can be estimated by using chemical shift in the solid state as a reference for trans-zigzag conformation and y-gauche effect. Chemical shifts in the solid state varies within about 2 ppm (33-35 ppm) which is much larger than the variation in the liquid state and in solution (less than 0.1 ppm). This large scattering is closely associated with crystal
328
TAKESHI YAMANOBE AND HIROMICHI KUROSU !
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' O
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-
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o
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-
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I
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I
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l
I
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i
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l
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T h e n u m b e r o f carbon a t o m s Fig. 9.1. 13C chemical shift of polyethylene and n-paraffins. (O) 13C chemical shift in solution; ( 9 13C chemical shift in the solid state [3, 5, 16].
structure [3, 4]. As described in Chapter 7, the chemical shifts of C H 2 for paraffins and polyethylene in orthorhombic form are shifted by about 1 ppm when compared with those in the monoclinic and triclinic forms. Quantum chemical calculations reveal that the chemical shift difference is caused by a local difference of mutual orientation for the trans-zigzag plane in intermolecular interactions in the orthorhombic form and the triclinic and monoclinic forms [5].
9.3
Fold structure of single crystal polyethylene
The determination of chain conformation in polyethylene single crystals is one of the most important problems in polymer science, and its knowledge becomes fundamental for obtaining information about the conformation of a single polymer chain in the crystalline state. Several models have been proposed to depict the conformation in the folded structure region of polyethylene single crystal: (a) sharp fold model [6]; (b) switchboard model [7]; and (c) loose loop model [8]. Since cyclic paraffins of long chain lengths assume a compact conformation in the solid state with the shape of two parallel straight chains bridged at both ends and are closely related to the
POLYETHYLENE AND PARAFFINS
329
chain-folded structure [9, 10], the chemical shifts of cyclic paraffins can be used to clarify which model is the most reasonable for the folded structure region in polyethylene single crystals [11]. Cyclic paraffins provide information about the effect of chemical shifts which arise from conformational changes, since some bonds are constrained to the gauche conformation. Anet et al. [12] reported the spectra of cyclododecane, cyclotetradecane and cyclohexadecane in solution. By cooling down the solutions to -130 and -150~ they have been able to obtain slow exchange spectra concerning the conformational interconversions. M611er et al. [13] measured c-C12H24, c-C24H48 and c-C36H72 at temperatures below their melting points and have assigned the resolved peaks on the basis of the Vicinal gauche effect in addition to the y-effect. Ando et al. [11, 14] studied a series of cyclic paraffins with a carbon number from 24 to 80 in addition to polyethylene to clarify the folded structure of the single crystal polyethylene (Fig. 9.2). c-C24H48 produces a sharp single peak, while c-C28H56and cC32H64 both present broad single peaks. The linewidth of the 13C signal of cyclic paraffins increases with an increase in the number of carbon atoms. If cyclic paraffins take a folded structure, it should be possible to obtain different chemical shifts for the methylene carbons arising from the folded and transzigzag structures. The observed single peak for c-C24H48 suggests that they have a given molecular structure without the freezing of molecular motion. This is justified by ~H NMR study [15]. As the number of carbon atoms increases, the molecular motion is frozen and packed into specified cyclic structure. This is verified by low frequency shoulders or small peaks on the main peak. The main peak (peak I) and shoulder peaks (peaks II and III from high frequency) and their differences are listed in Table 9.1. The fraction ratio of the intensity of peaks I, II and III is as follows: 5.8:1.7:1 for cC36H72, 7.2 : 1.8:1 for c-C4oH8o, 8.5:2.3 : 1 for c-C48H96 and 14:2.6 : 1 for cC64H~28. The fraction ratio of (II + III) to I decreases with an increase in the number of carbon atoms. These peaks can be assigned to carbons of folded region by means of the y-gauche effect concept. As shown in Fig. 9.3, the cyclic paraffins crystallize in a conformation characterized by two parallel all-trans, planar zigzag strands connected by two GGTGG loops. If this conformation is dynamically rigid in the solid state at room temperature, the loop carbons C3, C3,, C4 and C4, resonate at l y-gauche effect and loop carbons C~ and C~, at 2 ),-gauche effects to low frequency of the all-trans and loop carbons C2, C2, at 0 y-gauche. For c-C36H72, we would expect 24 carbons with 0 y's, 8 carbons with 1 y's and 4 carbons with 2 y's, leading to a 6:2:1 ratio of peaks, each separated by a y-gauche effect. Similarly, for c-C40H8o the ratio is 7:2: 1, for c-C48H96 9:2:1 and for c-C64H128 13:2: 1.
330
TAKESHI YAMANOBE AND HIROMICHI KUROSU
',d)
Ib)
(c)
a)
!
t
(g)
(h)
(f)
9
20ppm (e)
9
I
10 ppm
(J)
(i)
_J ,5
20 ppm
Fig. 9.2. CPMAS NMR spectra of cyclic paraffins, n-paraffin and polyethylene in the solid state [11]" (a) c-C24H4s; (b) c-C2sH56, (C) c-C32H64; (d) c-C36H72" (e) c-C40H80; (f) c-C48H96, (g) c-C64H128, (h) c-C80H160; (i) n-f32H66; and (j) polyethylene. I, II and III correspond to peaks I, II and III in the text, respectively.
Assuming - 4 to - 6 ppm as y-gauche effect, chemical shift positions of peaks I, II and III are predicted to be 34, 28-30 and 22-26 ppm, respectively. These predictions agree with the observed results quantitatively. If the peaks II and III are also observed in a spectrum of polyethylene
331
POLYETHYLENE AND PARAFFINS Table 9.1.
13C chemical shifts of cyclic paraffins, n-paraffins and polyethylenes in the solid
state.
13C chemical shift/ppm Sample
Peak I ~I
Peak II ~II
Peak III ~III
~ I - ~II
~ I - ~III
Cyclic paraffin
33.0 29.7 30.7 34.0 33.5 33.7 33.8 34.0
30.0 27.0 27.0 28.7, 26.9 -27.0
26.0 24.5 22.0 22.0 -22.0
4.0 6.5 6.7 5.1, 6.9 "-7.0
8.0 9.0 11.7 11.8 -12.0
26.5, 24.5 26.5, 24.3
20.9 21.1
6.5, 8.5 6.5, 8.6
12.1 11.9
C28H56 C32H64
C36H72 C4oH8o C48H96 C64H128 C8oH16o
n-paraffin n-C19H4o n-C22H46 n-C32H66
Polyethylene Single crystal Melt quench
33.4 33.0 33.0 33.0 33.0
single crystals, the methylene carbons in fold region take similar conformation to cyclic paraffins. The spectrum of single crystal polyethylenes can be effectively interpreted following Ando et al. [11]. As seen in the amplified spectrum of polyethylene single crystals, the small low frequency peaks II and III on the main peak can be identified. The peak positions agree approximately with those of c-C64H128. This means that the existence of a sharpfolded structure in the polyethylene single crystal as well as c-C64H128 can be identified by 13C NMR spectroscopy. Based on the intensities of the peaks I, II and III, the stem length is estimated to be about 125 A which is consistent with the crystal thickness (120-150 A) measured directly by electron microscopy.
9.4
Phase transition and molecular motion in the solid state
Solid-solid transitions are observed especially for short alkanes. Odd numbered n-paraffins with more than 9 carbon atoms, and even numbered ones with more than 22 and up to about 45 carbons, undergo the transition to the rotator phase. Solid-state NMR spectra give evidence for this transition at the molecular level. M611er et al. [16] studied the temperature dependences of several n-paraffins and polyethylene systematically. Figure 9.4 shows the
332
T A K E S H I Y A M A N O B E AND H I R O M I C H I K U R O S U
C! C 2" C 3"
o.-....
o-~ o--__. o-'"
"--o "---o
Fig. 9.3. Schematic drawing of the conformation of a cyclic paraffin [9].
temperature dependence of the ~3C CPMAS NMR spectra for n-nonadecane. n-Nonadecane shows a phase transition from the orthorhombic phase, which are taken at low temperature, to the rotator phase transition at 295 K and melts at 305 K. Because of MAS and uncertainty of the temperature measurement, the phase transition temperature observed in the NMR spectra does not strictly correspond to the DSC data. The top, middle two and bottom two spectra are spectra in the melt, in the rotator phase and in the orthorhombic phases, respectively. Chemical shift changes are observed between orthorhombic and rotator phases. Chemical shifts for terminal CH3 (15.6 ppm) and c~-CH2 (25.5 ppm) move to low frequency at this transition. These shifts are attributed to the y-gauche effect, which means that a conformational change, trans-gauche transition, takes place at chain ends. On the contrary, the chemical shift of the internal CH2 moves to high frequency (from 33.4 to 33.9ppm). Since chemical shifts in this range are
333
POLYETHYLENE AND PARAFFINS
31.11 23.9
15.1
33'91
__~jU
L ..........
331
6inppm
. . . . . ~s. . .
A.
~'-
~o,~
298K
15.6
3o'' . . . . . . ~s . . .
A
296K
9 A
290K
2'o ........
,
Fig. 9.4. Temperature dependence of CPMAS NMR spectra of n-C19H4o [16].
usually attributed to the trans-conformation, a conformational change does not occur by this transition for internal methylene. Similar low frequency shifts are reported for crystal structure differences. As stated in Chapter 7, the chemical shift of CH2 in the trans-conformation is affected by the orientation of a trans-zigzag plane of the neighboring chain. For polyethylene, a chemical shift difference of 1.3 ppm is reported for the crystal structure difference between monoclinic and orthorhombic phases. In the rotator phase, trans-zigzag chain undergoes rotation about chain axis, which means CH2 carbon feels an averaged orientation (parallel and perpendicular) to neighboring chains. As a result of both contributions from the parallel and perpendicular orientations, a smaller deshielding (0.5 ppm) is observed than for the parallel orientation (monoclinic phase, 1.3 ppm) [17]. Similar chemical shift changes caused by solid-solid phase transitions are clearly observed in a case of C36H74 . AS the number of carbon atoms increases, such a stepwise phase transition becomes ambiguous. The polyethylene sample melts continuously over a broad range of temperatures.
334
TAKESHI YAMANOBE AND HIROMICHI KUROSU ,
o
,
'|
|
9
|
9
20
|
30
"
9
'
. . . .
9
|
so
9
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_
6'0 r
Fig. 9.5. DSC diagram of cyclic paraffin, c-C24H48 [15].
For cyclic paraffins, phase transitions in addition to the melting process are reported. Figure 9.5 shows a DSC trace of c-C24H48 [15]. There are two transitions at 29 and 51~ Both the heat and entropy of the transition at 29~ are greater than those at the melting temperature, 51~ As the rotor phase transition shows a much smaller heat change, the transition at 29~ is not the same as the rotor phase transition observed in n-paraffins. Figure 9.6 shows the 1H NMR spectra for c-C24H48 measured over a temperature range of 25-50~ From 30 to 45~ the 1H signal consists of two components, which means there are two different components of molecular motion. The half-height linewidth A Ul/e for the broad component obtained from curve fitting decreases slowly from 64.5 to 52.8 kHz as the temperature increases from 0 to 25~ A ul/2 is reduced drastically from 52 to 8.1 kHz at between 25 and 30~ From 30 to 45~ A~1/2 decrease slowly from 8.1 to 7.3 kHz and then, at between 45 and 50~ it falls a few hundred Hz. For A Vl/2, similar to the DSC's result, two transitions are observed and the first transition is much larger than the second. Below the first transition temperature, 29~ T1 and Tip decrease as the temperature is increased. In this temperature range, the molecular motion is in the slow motion region (~o~ >> i). Above 30~ T1 and Tlo increase as the temperature is increased. The molecular motion is nearly in the extreme narrowing region (o~, ~ 1) above the first transition temperature. No appreciable change is observed at the melting temperature. This means that, at the first transition, a large jump of correlation time for molecular motion takes place and motional narrowing already occurs and spin diffusion occurs effectively between the mobile and immobile regions. Figure 9.7 shows the powder pattern spectrum of c-C24H48 at ambient temperature [11]. As the temperature in the CPMAS probe is thought to be more than 30~ this is the spectrum above the first transition. The powder
c-C2taH~ 8
9 t"
.._.._.___
~
J
~
~_30
Z: t~ Z
. . . .
I
,
J
> Z
10 KHz 50 ~
> >
s
~ ra,l
Ii
9 ,
oc
10 KHz 2 KHz
Fig. 9.6. Temperature-dependence of 1H NMR spectra for c-C24H48 [15].
+
_
336
TAKESHI YAMANOBE AND HIROMICHI KUROSU
Fig. 9. 7. CP powder pattern spectra of c-C24H48at ambient temperature [11].
pattern spectrum for c-C24H48 shows an axially symmetric pattern instead of the usual tent-like one. An axially symmetric pattern means that the molecular motion takes place around a certain axis and is not perfectly random. On the basis of these results the structure of c-C24H48 can be described. Below the first transition temperature, 29~ the molecular motion of cC24H48 is frozen, i.e., it assumes a compact conformation with the shape of two parallel trans-zigzag straight chains bridged at two G G T G G loops. Further in the range from 29~ to the melting temperature, c-C24H48 has two components: these are the mobile and immobile regions that comprise the folded chain structure region and the trans-zigzag structure region, respectively. The molecular motion in the mobile region comes from the fast transition between the trans- and gauche-conformers. On the other hand, the molecLdar motion in the immobile region comes from the gauche migration in the trans-zigzag chains as follows:
.... TTTTT G . . . . . . . . TTTT G T . . . . . . . TTT GTT . . . . . . TT G TTT... T GTTTT..
The motional modes allow the transition between the trans- and gaucheconformers to occur in a cyclic paraffin without a large scale change in the overall shape of the cyclic paraffins.
337
POLYETHYLENE AND PARAFFINS
9.5
Structural and dynamic studies of polyethylene
The amorphous region of 13C-labeled polyethylenes crystallized under different conditions were studied by VT 13C CP/MAS NMR [18]. The dynamics of the amorphous region were discussed by measuring the 13C spin-lattice relaxation times (T1) and dipolar-dephasing relaxation times (TDD) over a wide temperature range, from -120 to 44~ Two types of 13C-labeled polyethylene samples were prepared. One is single 13C-labeled (polymerized using 90% single 13C-enriched ethylene) polyethylene and the other is double labeled (polymerized using 90% double 13C-enriched ethylene) polyethylene. Typical 13C CP/MAS NMR spectra of the single labeled solution-crystallized polyethylene (PESL; the crystallinity is 95%) and single labeled meltquenched polyethylene (MQPESL; the crystallinity is 66%) are shown in Fig. 9.8. Each of these spectra consists of three peaks corresponding to an orthorhombic crystalline peak, O, at 33.0 ppm, a monoclinic crystalline peak, M, at 34.4 ppm (a small shoulder on the left side of the orthorhombic peak), and an amorphous peak, N, which appears at 30.8-31.3 ppm. The 13C T1 values of PESL and MQPESL over a wide range of temperatures are obtained using the inversion-recovery method with the PST (pulse saturation transfer)
PES, I
45
'
'
' '
"1
4O
'
'
'
'
I
35
'
'"'
'
I"
3O
'
'
"
'
I'"
25
'
'
'7
2O
Fig. 9.8. Typical 13C CP/MAS NMR spectra of samples of PESL and MQPESL at room temperature [18].
338
TAKESHI YAMANOBE AND HIROMICHI KUROSU 0.6
0.5
0.4 A
I---
0.3
0.2
o.1
.
,.
,i
0.0O4
.
1/T (K"1)
,.
.
,
0.005
Fig. 9.9. Plots of amorphous 13C spin-lattice relaxation times: TIS for PESL (0) and MQPESL (O) versus the reciprocal absolute temperature [18].
pulse sequence. The PST pulse sequence enhances the intensity of the signals of mobile methylene carbons. The resulting T1 values for PESL and MQPESL are plotted against the inverse of the absolute temperature (l/T) in Fig. 9.9. It had been suggested previously [19, 20] that local molecular motion in the amorphous region of polyethylene is independent of the degree of crystallinity, high-order structures or morphologies. However, these suggestions are not supported by the experimental results because: (1) the TlS of the amorphous region of PESL and MQPESL at room temperature are 570 and 440 ms, respectively, and the difference of 130ms between them is beyond experimental error; and (2) the T1 minimum values for the two samples (Fig. 9.9) are at different temperatures, the T1 minimum of PESL appearing at -10.5~ and that of MQPESL at -32~ These facts show that the local molecular motions in the amorphous regions of PESL are more constrained than those of MQPESL. In order to study whether the dynamic behavior of the two kinds of amorphous region is also different on the T2 timescale, TDD values of PESL and MQPESL were measured over a wide range of temperatures. The relative intensity of the amorphous peak obtained from the computer simulation was plotted against the delay time ~" in Fig. 9.10. It can be clearly seen that the amorphous peak of PESL relaxes more quickly than that of MQPESL. The dipolar-dephasing time, TDD, usually depends on molecular motion, carbon-proton dipolar interactions, MAS rate and spin diffusion [21]. Fundamentally, it can be said that the dipolar-dephasing time in the
339
POLYETHYLENE AND PARAFFINS 0
"
'
II
9
'
II
9
'
II
"
II
"" '
>., omm
u)
r
0
0
r
..,=,.,.
e
.01
' " 100
"
' 200
"
" 300
"
' 400
" 500
(ps) Fig. 9.10. Intensity of amorphous peaks of samples of PESL and MQPESL versus delay time r [18]. The peak intensity was obtained from computer simulation of the 13C partially relaxed dipolar-dephasing NMR spectra.
amorphous region becomes a measure of molecular motion. Therefore, the longer TDD value of MQPESL, compared with that of PESL, obviously suggests that the carbon-proton dipolar interaction is partially averaged by molecular motion on the Y2 timescale. Furthermore, Chen et al. [22] have measured the ~3C CP/MAS spectra of a ~3C-labeled solution of crystallized polyethylene at temperatures from -120 to 144~ The measurements (Fig. 9.11) were taken in order to study changes of structure and molecular motion of the polymer with temperature variation for the crystalline and amorphous regions. As the crystalline and amorphous signals are incompletely resolved in the ~3C CP/MAS spectrum of the polyethylene sample, computer-fitting of the spectra is performed, and the determined ~3C chemical shifts of the crystalline and amorphous signals are shown in Fig. 9.12. It is shown that the ~3C chemical shift of the amorphous signal decreases with an increase in temperature, whereas the ~3C chemical shift of the crystalline signal does not change greatly with temperature before the melting point.
340
TAKESHI YAMANOBE AND HIROMICHI KUROSU (b) -120"c -100"c -80"c
144"c
-60"c
132.8"c
-50'C 121.6"c
40"c -30"c
110.4~
-20"c
99.2"c
O'c
88"c
11.5"c RT I''''1''''1''''1"""~i'"''1 37 36 ~15 34
33
32
....
I ....
31
I .... 30
I''''l
29
28
65.6"c 3~ '~ "2~i' ' '314' ' '3~?.' ' '3~' ' 'Z~8' ' ' ~
Fig. 9.11. -120~
1 3 C CP/MAS NMR spectra of the PESL sample as a function of temperature: (a) to room temperature; (b) 65.6 to 144~ [22].
34
A
E (3. cL v
33
9 9e e
..ww.,C r,/)
rj E to 0
32
0
0
0
eee
9
ee
9
9 eeeee
0000 0 0 0
31
0
0000
,,,,,.
30 .140
9 ' ,.100
9 | .60
9
!
-20
9
I
20
=
I
60
=
I
0
9
100
C I
140
T(~
Fig. 9.12. Plots of the
1 3 C chemical shifts of the crystalline ( e ) and amorphous ( 9 signals of the PESL sample against temperature [22].
341
POLYETHYLENE AND PARAFFINS
O
A
E
(:3. Q.
O O
v
J~
0
0
0
O
O
"13
O
0 r !
o~ ,o
'r"
1
9
9
9 Q0000
0 0
9
D
(3 0
-140
9
I
-100
I
l
-60
9
I
-20
9
!
20
9
I
60
9
1
100
,,
I
140
T(~ Fig. 9.13. Plots of the width at half the maximum peak height of the crystalline (O) and amorphous ( 9 signals of the PESL sample against temperature [22].
This shows that an increase of the fractional population of the t r a n s conformer leads to the observed high frequency shift of the amorphous signal with a decrease of temperature. At temperatures below -80~ the 13C chemical shift of the amorphous signal does not change with temperature. Such a result indicates that the molecular motion in the amorphous region of PESL is completely frozen below -80~ on the NMR timescale. The halfheight width (halfwidth) of the crystalline and amorphous 13C signals are plotted against temperature in Fig. 9.13. The halfwidth of the amorphous region becomes a maximum at -30~ This means that molecular motion of the crystalline region occurs at a frequency corresponding to the amplitude of the proton decoupling field (about 55 kHz in this case). Contrary to the results of the amorphous peak, the halfwidth of the crystalline peak is almost constant at temperatures from 0 to -120~ The halfwidth decreases from 0.9 to 0.7 ppm as the temperature is increased from 0~ to room temperature. Further elevation of temperature from 65.6~ causes line broadening and the halfwidth becomes a maximum at 110.4~ This result may suggest that there is an a-transition, T~, in the crystalline region of polyethylene and thus the temperature (110.4~ at which the maximum of the halfwidth is observed may be correlated with T~. The structure and dynamics of polymer chains adsorbed on the surface of solids have been studied using spectroscopic methods such as IR, ESR and
342
TAKESHI YAMANOBE AND HIROMICHI KUROSU
II
II
i""
40
9
9
I
38
'
9
'
I'"
36
9
'
I'"
34
"~"
I
32
9
9
9
I
30
9
9
9
I
28
9
'
' I
" ~
26 ppm
Fig. 9.14. 13C CP/MAS spectra of 13C-labeled crystallized polyethylene [23].
so on. The NMR sensitivity is very low compared with IR and ESR. This is a problem for NMR experiments on polymers adsorbed on the surface of solids. Therefore, to obtain a high-resolution solid-state 13C NMR spectrum with a reasonable signal-to-noise (S/N) ratio, isotope-labeled polymers must be prepared [18]. Inoue et al. [23] have studied the structure and dynamics of the 13C-labeled polyethylene adsorbed on the surface of silica gel as a function of temperature by high-resolution solid-state ~3C NMR spectroscopy. A 13C CP/MAS spectrum of 13C-labeled polyethylene in bulk at room temperature is shown in Fig. 9.14. As reported previously [4, 24-35], this spectrum consists of three peaks corresponding to an orthorhombic crystalline transzigzag methylene peak II at 33.0 ppm, a monoclinic crystalline trans-zigzag methylene peak I at 34.3 ppm and a noncrystalline methylene peak III at 30.8-31.3 ppm. The methylene carbons are undergoing a fast transition between the trans- and gauche-conformers in the amorphous region, and so the observed amorphous chemical shift is an average value for the trans- and gauche-conformers. Therefore, the 13C chemical shift of the amorphous peak reflects the fraction of the gauche-conformer. 13C CP/MAS and PST/MAS NMR spectra of 13C-labeled polyethylene adsorbed on the surface of silica gel are shown in Fig. 9.15. The ~3C chemical shift values and peak intensities of the 13C CP/MAS spectrum obtained by computer-fitting are listed in Table 9.2. In the 13C CP/MAS spectrum (Fig. 9.15(a)), the three peaks appear at
343
POLYETHYLENE AND PARAFFINS
1
a)
2
3
b)
__
40
38
J
36
34
32
30
28
26
ppm Fig. 9.15. (a) 13CCP/MAS and (b) 13C PST/MAS spectra of 13C-labeled polyethylene adsorbed on the surface of silica gel [23]. Table 9.2. 13C chemical shift values and peak intensities of the 13C CP/MAS spectrum of 13Clabeled polyethylene adsorbed on the surface of silica gel. Peak number
13C Chemical shift/ppm
Peak intensity
1 2 3
32.9 30.6 27.5
16 8 1
32.9, 30.6 and 27.5 ppm, and are designated by 1, 2 and 3 from high frequency, respectively, in the ~3C CP/MAS spectrum of an NMR rotor without polyethylene and with silica gel there are no peaks in the region of 20-40 ppm. In the ~3C CP/MAS experiments, it is known that the CP efficiency strongly depends on molecular motion, and when the reorientation rate is close to the 1H decoupling frequency (~60 kHz), the CP efficiency is greatly reduced and so the signal sometimes disappears. On the other hand, in the
344
TAKESHI YAMANOBE AND HIROMICHI KUROSU
PST/MAS experiments the intensity of the peak is governed by the NOE enhancement. In the 13C CP/MAS spectrum, the intensity of peak 1 is more intense than that of peak 2, but in the 13C PST/MAS spectrum (Fig. 9.15(b)) the intensity of peak 2 is relatively increased compared with the a3C CP/MAS spectrum. This means that the molecular motion of the methylene carbons for peak 2 is faster than that for peaks 1 and 3. The 13C CP/MAS spectral pattern of adsorbed polyethylene is very different from that of bulk polyethylene. The fractions of the mobile components for these polyethylene samples are different from each other. The fraction of the mobile component in adsorbed polyethylene is larger than that of bulk polyethylene. The chemical shift value for peak 1 is close to that for trans-zigzag methylene carbons and the difference in X3C chemical shift between peaks 1 and 3 is 4.4 ppm which corresponds to the 1 y-gauche effect value. From these results, it can be said that peaks 1 and 3 come from the trans and gauche parts in which the molecular motion is frozen on the NMR timescale. This is caused by adsorption on the surface of silica gel (this part is designated as the region A). Peak 2 comes from the methylene carbons which are undergoing fast transitions between the trans- and gauche-conformations (this part is designated as the region B). The ratio of intensities for peaks 1, 2 and 3 obtained by computer-fitting is 16:8:1 in the CP/MAS spectrum of PEDA (doubly 13C-labeled polyethylene adsorbed on the surface of silca gel). As described above, because the CP efficiency depends on molecular motion, the peak intensities do not correspond exactly to the ratio of each component. However, it serves as a rough standard for the ratio of each component, especially the trans- and gauche-conformers. In order to study the dynamic behavior in region B, 13C spin-lattice relaxation times T1 are measured by varying the temperature. As an example, the partially-relaxed 13C spectra of polyethylene at room temperature obtained by the inversion recovery method are shown in Fig. 9.16. Using these results, 13C T1 values were determined as listed in Table 9.3. The 13C T1 values were plotted against inverse absolute-temperature in Fig. 9.17 together with those of the amorphous region for MQPESL (meltquenched polyethylene (single 13C-labeled)) which was melted at 150~ and quenched to -70~ and PESL (polyethylene single 13C-labeled) which was dissolved at 130~ in xylene at a concentration of 0.03% and crystallized. The T1 curves for these three samples have the minimum at different temperatures. The T1 minimum for polyethylene adsorbed on the surface of silica gel appears at the highest temperature compared with other samples. According to the BPP theory, the correlation time for molecular motion at the T1 minimum corresponds to the resonance frequency. All of the T1 values for the three samples are measured using the same
POLYETHYLENE AND PARAFFINS
345
Delay Time 1; ( x l 0 m s )
400
200
150
80
~
f
40 20
10
I
34
'
'
'
I
32
'
'
'
I
30
'
"
'
I
28 ppm
Fig. 9.16. Partially relaxed 13C NMR spectra of 13C-labeled polyethylene adsorbed on the surface of silica gel at room temperature obtained by the inversion recovery method [23].
Table 9.3. 13C T1 values of methylene carbons in the region B of 13C-labeled polyethylene adsorbed on the surface of silica gel. Temperature/K
TI/S
313 284 273
0.65 0.49 0.50
346
TAKESHI YAMANOBE AND HIROMICHI KUROSU
0.7
0.6
m
,..,
-.e-
0.5
PEDA PESL MQPESL
_
' ~ 0.4 0.3
m
% 0.2
J
-
0.1 0.003
0.004
0.005
1 / T ( K "1) Fig. 9.17. Plots of 13C spin-lattice relaxation times T1 against inverse absolute-temperature for laC-labeled polyethylene adsorbed on the surface of silica gel [23].
spectrometer operating at 67.8 MHz. For this, it is apparent that the molecular motion of the methylene carbons in the region B is more restrained, compared with that of bulk polyethylene samples. From the single correlation-time model based on the BPP theory, the correlation time, rc, for molecular motion was calculated using the determined 13C T1 values [23]. The plot of rc against inverse absolute temperature 1/T for P E D A is shown in Fig. 9.18. The temperature dependence of the correlation time usually obeys the Arrhenius form [38] as rc = roexp(AE/T), where AE is the activation energy which is corresponding to the potential barrier for the transgauche transition and ro is the pre-exponential factor. The AE value can be estimated from the slope of ln(rc) against 1/T. By this procedure, ro is found to be 1.37 x 10 -14 S and the AE value is found to be 6.60 kcal/mol for the molecular motion of the methylene carbons in the region B of PEDA. As reported previously, the AE values are 5.19 and 3.72 kcal/mol for the molecular motion of the methylene carbons in amorphous region for PESL and MQPESL samples, respectively. This means that the potential barrier for the trans-gauche transition for the methylene carbons in the region B for P E D A is larger than those in the amorphous region of PESL and MQPESL samples. It is reported that the rotational barrier for the central C-C bond
347
POLYETHYLENE AND PARAFFINS 1 0 .8
r.~
1 0 "9
L---'
10 -10 0.003
i
0.0035
0.004
I / T ( K -1) Fig. 9.18. Plots of correlation times zc in molecular motion for the methylene carbons in the region B for 13C-labeled polyethylene adsorbed on the surface of silica gel against the inverse absolute temperature [23]. in n-butane is about 3.3 kcal/mol as determined from thermodynamic data [36, 37]. Since this value is very close to that for M Q P E S L , it has been suggested that the trans-gauche transition rate in the noncrystalline region of melt-quenched polyethylene is very fast. On the other hand, in the amorphous region of PESL, the AE value is somewhat larger than that for n-butane, and so the trans-gauche transition rate is slower than that for M Q P E S L . In the region B of P E D A , the AE value is larger than those for M Q P E S L and PESL. This means that the trans-gauche transition rate is much slower than those for M Q P E S L and PESL. It is shown that the contributions of 13C--13C dipolar interaction to the 13C T1 value can be neglected, compared with the 1 3 C - - 1 H dipolar interaction as discussed below. The ratio of the contributions of the 13Cm13C and 1 3 C - - 1 H dipolar interactions to relaxation rate R = l/T1 can be expressed by R-
(1/T1)cc = (1/T1)cH
4
6
yc/rcc
2 6 ' T~TH/rCH
(9.1)
where 7c and ")/H are the magnetogyric ratios of the carbon and hydrogen nuclei, respectively ( 7 c = 0.6726 x 104 and YH = 2.675 X 104 rad s -1 gauss--i), and rcc and rcH are C ~ C and C ~ H bond lengths, respectively (rcc = 1.54 A and rCH = 1.1 A). Therefore, R can be estimated as R = (0.6726/2.675)2 x (1.1/1.54) 6 - 0.0043. It is apparent that the contribution of the 13C~13C dipolar interaction is negligibly small compared to that of
348
TAKESHI YAMANOBE AND HIROMICHI KUROSU
the 13C--1H dipolar interaction. According to the above discussion, it is considered that the morphology of polyethylene adsorbed on the surface of silica gel is as follows. Methylene chains roughly contain two components with different molecular motions. The first is directly the adsorbed part on the surface of silica gel and its molecular motion is strongly restricted. In this part, methylene carbons are frozen in t r a n s - or gauche-conformations in an NMR timescale. As described above, the t r a n s conformation is the major one and the ratio of t r a n s - and gauche-conformers is approximately 16: 1. The other part is assigned to a polyethylene which is not directly adsorbed on the surface of silica gel, i.e., away from the surface and there is a fast transition between the t r a n s - and gauche-conformations which is similar to the amorphous region of polyethylene in bulk. The mobility of this part is more restricted compared with the noncrystalline region of polyethylene in bulk. The chain length of this part may be shorter than that of the amorphous region of bulk polyethylene.
References
.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
T. Yamanobe, R. Chujo and I. Ando, Mol. Phys. 50 (1983) 1231. D.M. Grant and E.G. Paul, J. Am. Chem. Soc. 86 (1964) 2984; W.R. Woolfenden and D.M. Grant, J. Am. Chem. Soc. 89 (1966) 1496; A.E. Tonelli and F.C. Schilling, Acc. Chem. Res. 14 (1981) 233. D.L. VanderHart, J. Magn. Reson. 44 (1981) 117. D.L. VanderHart and F. Khoury, Polymer 25 (1984) 1589. T. Yamanobe, T. Sorita, T. Komoto, I. Ando and H. Sato, J. Mol. Struct. 131 (1985) 267. A. Keller, Phil. Mag. 2 (1957) 1171. P.J. Flory, J. Am. Chem. Soc. 84 (1962) 2857. E.W. Fischer and R. Lorenz, Kolloid. Z. 189 (1963) 97. B.A. Newman and H.K. Kay, J. Appl. Phys. 38 (1967) 4105. P. Groth, Acta Chem. Scand. A 33 (1979) 199. I. Ando, T. Sorita, T. Yamanobe, T. Komoto, H. Sato, K. Deguchi and M. Imanari, Polymer 26 (1985) 1864. F.A.L. Anet, A.K. Cheng and J.J. Wagner, J. Am. Chem. Soc. 94 (1972) 9250; F.A. Anet and A.K. Cheng, J. Am. Chem. Soc. 97 (1975) 2420. M. M611er, W. Gronski, H.-J. Cantow and H. H6cker, J. Am. Chem. Soc. 106 (1984) 5093. I. Ando, T. Yamanobe, T. Sorita, T. Komoto, H. Sato, K. Deguchi and M. Imanari, Macromolecules 17 (1984) 1955. M. Takenaka, T. Yamanobe, T. Komoto, I. Ando and H. Sato, Solid State Commun. 61 (1987) 563. M. M611er, H.-J. Cantow, H. Dortloff, D. Emeis, K.-S. Lee and G. Wegner, Makromol. Chem. 187 (1986) 1237. S. Ishikawa, H. Kurosu and I. Ando, J. Mol. Struct. 248 (1991) 361.
POLYETHYLENE AND PARAFFINS
349
18. Q. Chen, T. Yamada, H. Kurosu, I. Ando, T. Shiono and Y. Doi, J. Polym. Sci.: Part B: Polym. Phys. 30 (1992) 591. 19. J.J. Dechter, R.A. Komoroski, D.E. Axelson and L. Mandelkern, J. Polym. Sci. Polym. Phys. 19 (1981) 631. 20. B. Schroter and A. Posern, Makromol. Chem. 182 (1981) 675. 21. D.E. Axelson, in: R.A. Komoroski (ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, Chapter 5, p. 197, VCH Publishers, Florida, 1986. 22. Q. Chen, T. Yamada, H. Kurosu, I. Ando, T. Shiono and Y. Doi, J. Mol. Struct. 263 (1991) 319. 23. D. Inoue, H. Kurosu, Q. Chen and I. Ando, Acta Polymer. 46 (1995) 420. 24. C.A. Fyfe, J.R. Lyerla, W. Volksen and C.S. Yannoni, Macromolecules 12 (1979) 757. 25. J.J. Dechter, R.A. Komoroski, D.E. Axelson and L. Mandelkern, J. Polym. Phys. Ed. 19 (1981) 631. 26. B. Schroter and A. Posern, Makromol. Chem. 182 (1981) 675. 27. D.E. Axelson, J. Polym. Phys. Ed. 20 (1982) 1427. 28. R. Kitamaru, F. Horii and K. Murayama, Polym. Bull. 7 (1982) 583. 29. D.E. Axelson, L. Mandelkern, R. Popli and P. Mathieu, J. Polym. Sci. Polym. Phys. Ed. 21 (1983) 2319. 30. R. Kitamaru, F. Horri and K. Murayama, Macromolecules 19 (1986) 636. 31. I. Ando, T. Yamanobe, S. Akiyama, T. Komoto, H. Sato, T. Fujito, K. Deguchi and M. Imanari, Solid State Commun. 62 (1987) 785. 32. A.L. Cholli, W.M. Ritchey and J.L. Koenig, Spect. Lett. 21 (1988) 519. 33. S. Akiyama, T. Komoto, I. Ando and H. Sato, J. Polym. Sci. Polym. Phys. Ed. 28 (1990) 587. 34. D. Doddrell, V. Glushko and A. Allerhand, J. Chem. Phys. 56 (1972) 3683. 35. K.J. Packer, I.J.F. Poplett, M.J. Taylor, M.E. Vickers, A.K. Whittaker and K.P. Williams, Makromol. Chem. Macromol. Symp. 34 (1990) 161. 36. K.S. Pitzer, Ind. Eng. Chem. 36 (1944) 829. 37. W.B. Person and G.C. Pimentel, J. Am. Chem. Soc. 75 (1953) 539.
This Page Intentionally Left Blank
Chapter 10
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Polymer Blends and Miscibility Atsushi Asano I and K. Takegoshi 2 ~Department of Chemistry, National Defense Academy, Hashirimizu, Yokosuka, Japan; 2Department of Chemistry, Graduate School of Science, Kyoto University, Kitasirakawa, Sakyo-ku, Kyoto, Japan
10.1
Introduction
Here, the main discussion concerns solid-state NMR techniques as applied to a mixture of two different polymers (a polymer blend). Due to its inherent heterogeneity, even one polymer is a difficult target to characterize. Then, why be concerned by the mixture? It is because the blending of polymers is a simple and economic method to produce new materials. At least, the blending is much easier than finding new monomers and polymerization techniques. Various pairs of polymers have been examined [1] with the hope of realizing a unique set of and/or some designed properties. Macroscopic properties of a blend depend on its microscopic phase (domain) structure. One may naively expect that a homogeneous one-phase blend shows the average properties of component polymers, while a heterogeneous two-phase blend keeps the original properties of component polymers. Therefore, microscopic characterization of a blend is important and we will see that solid-state NMR is particularly useful for that. In fact, examination of heterogeneity in a polymer blend is, in a sense, easier than that in a homopolymer, because each component polymer can be discriminated by its distinct 13C peaks. The resolved 13C peaks can be used as clues to unravel the heterogeneity in a blend. Numerous NMR works have been published and some of them were conveniently reviewed recently [2, 3]. In this review, the references are not intended to provide a complete compilation of the literature, because it would be outdated in a month. We would like to take this opportunity to apologize to all authors and readers affected by this.
10.2
Miscibility
Macroscopic properties of polymer blends are influenced by the degree of mixing between component polymers [4, 5]. Miscibility is a term based on thermodynamics, and a miscible state means a homogeneous single phase on
352
A T S U S H I A S A N O A N D K. T A K E G O S H I
a molecular level. In practice, the miscibility depends on how closely we look at the blend; if the domain size is smaller than the characteristic space scale of a particular observation, the blend appears to be miscible/homogeneous. A blend which is homogeneous for a certain observation is often found to be heterogeneous by another observation with a smaller scale of observation. For instance, poly(2-methylstyrene)/poly(2,6-dimethylphenylene oxide) (P2MS/PPO) shows a single glass transition by differential scanning calorimetry (DSC). This shows that P2MS/PPO is miscible from the DSC point of view. However, this blend is observed to be heterogeneous by using solidstate NMR [6]. This discrepancy comes from the smallest domain size that can be detected by Tg (DSC) is 10-20 nm [7], while it is 2-5 nm for NMR (see below). It is true that the Tg measurement is a very useful and good method to judge heterogeneity of a blend on a 10-20 nm scale, but care must be taken if one's problem originates from the heterogeneity of much smaller scales. In this section 10.2, we review the various solid-state NMR methods used to investigate interpolymer interactions, molecular motion and the spatial structure of a polymer blend. An interaction between component polymers affects the chemical shifts and lineshapes (see Section 10.2.2.1) and the molecular motions of the component polymers (see Section 10.2.2.2). In Section 10.2.3.1, microheterogeneity from 2 to 50 nm is studied by measuring XH spin diffusion indirectly from its effects on XH spin-lattice relaxation. The ~H spin-diffusion processes can also be monitored by several methods based on the Goldman-Shen experiment [8] (see Section 10.2.3.2). Homonuclear and heteronuclear two-dimensional correlation experiments reveal how and to what extent component polymers interact with each other (see Section 10.2.3.3). Section 10.2.3.4 deals with cross-relaxation experiments. Before discussing NMR experiments, the thermodynamics of a blend is briefly outlined. 10.2.1
Thermodynamicbackground
In general, the miscibility of a pair of polymers depends on temperature and composition. Figure 10.1 schematically shows three typical phase diagrams. The ordinate and the abscissa axes represent temperature and composition, respectively. The solid line in Fig. 10.1(a), below which the blend becomes immiscible (two-phase), is referred to as an upper critical solution temperature (UCST). However, Fig. 10.1(b) shows a lower critical solution temperature (LCST) behavior. Some polymer pairs display both UCST and LCST as shown in Fig. 10.1(c). As will be shown in the following, UCST is rarely observed for a polymer blend.
353
POLYMER BLENDS AND MISCIBILITY
a)
b)
c) 9
,
,t
.
,
-
,,
Single Phase
Single Phase Single Phase
Fig. 10.1. Schematic illustration of a phase diagram for a polymer blend, showing: (a) upper critical solution temperature (UCST)" (b) lower critical solution temperature (LCST)" and (c) UCST + LCST. Ordinate and abscissa show temperature and composition, respectively.
According to the second law of thermodynamics, polymer pairs will be miscible if the Gibbs free energy of mixing AGm is negative. The AGm may be described by the following three contributions [9-11]" (1) combinatorial entropy of mixing; (2) free volume difference between the component polymers; (3) exchange interaction energy, which can be represented as
AGm ~A (liB = ~ In (/)A -~ ~ In 4~ + XAB~AIJ)B, RTV mAVrA mBVrB
(10.1)
where R is the ideal gas constant, T, q~g, Vri and m~ represent the absolute temperature, volume fraction, molar volume and polymerization degree of component-i, respectively. Thus, the first two terms represent the combinatorial entropy of mixing. /I{'AB includes both the entropic free volume contribution and the exchange interaction energy between polymers A and B. Generally, the value of XAB is positive because polymers interact with the dispersion forces, which do not favor the mixing. For polymer pairs with small m~, the first two terms are negative and larger than the positive third
354
A T S U S H I A S A N O A N D K. T A K E G O S H I
term. Therefore, AGm becomes negative. Hence, a pair of short polymers would be miscible and show the UCST phase diagram (Fig. 10.1(a)). For larger mi values, the combinatorial entropy of mixing becomes too small to overcome the disadvantage of free volume differences between the component polymers. The A Gm value becomes positive if there are no special exchange interactions between A and B. Therefore, most of the polymer pairs are immiscible. More advanced treatments have been published elsewhere [12]. For two polymers with higher molecular weight to be miscible, a certain exothermic exchange interaction is required for the third term (3) to be negative. If such exothermic interactions exist, a polymer blend shows the LCST phase diagram. Because the free volume contribution in XAB becomes large with increasing temperature, and at a certain temperature (LCST), it overcomes a negative exchange interaction. Several specific interpolymer interactions, such as charge transfer [13, 14], ion-amide [15], ion-dipole [16] and hydrogen bonding [3, 17, 18], have been found to work in miscible blends. There are a few polymer pairs which require no specific interpolymer interactions [19-21]. For example, poly(methyl acrylate) and poly(vinyl acetate) (PMA/PVAc) mix by entropic advantage [20, 21].
10.2.2
Polymer-polymer interactions
In the above, we have seen that a certain interpolymer interaction is required for different polymers to be miscible. Here, we will see that high resolution NMR enables us to locate interacting regions in component polymers. One of the most useful methods is the nuclear Overhauser effect (NOE) between ~ H ~ H and ~H~13C. NOE can be observed between spins whose distances are less than about 0.5 nm. The one- (1D) and two-dimensional (2D) NOE experiments have been used to reveal the spatial structure of biomolecules in solutions. Of course, these can be applied to locate interacting regions in a blend in solution and in solids [3]. For example, Crowther et al. [22] and Mirau et al. [23] applied NOE experiments to polystyrene/poly(vinyl methyl ether) (PS/PVME) in a toluene solution, and show that the interpolymer NOE signals between the aromatic protons of PS and the methoxy protons of PVME can be observed at polymer concentrations higher than 25 wt%. In the solid state, Heffner and Mirau [24] measured 2D ~ H ~ H NOESY (NOESY: nuclear Overhauser effect spectroscopy) spectra of 1,2-polybutadiene and polyisoprene (1,2-PB/PI) and observed NOE cross-peaks between these component polymers. White and Mirau observed interpolymer NOE interactions between the ~H spins of PVME and the ~3C spins of deuterated
POLYMER BLENDS AND MISCIBILITY
355
PS in solids [25, 26]. These homo- and heteronuclear correlations in solids can also be used to study miscibility (see Sections 10.2.3.3 and 10.2.3.4). In a miscible blend, a specific interaction would influence chemical shifts and/or lineshapes of component polymers (see Section 10.2.2.1). Molecular motion is also affected by the interpolymer interaction, and is investigated by spin-lattice relaxation, 13C and 2H lineshapes and 2D exchange 2H NMR (see Section 10.2.2.2). 10.2.2.1 Chemical shifts and lineshapes By comparing the NMR spectrum of each component polymer with that of a blend, we can often see that some changes occur in a chemical shift and/or a lineshape. Such apparent changes can be attributed to modifications of both chemical structure and polymer conformation upon blending, reflecting a specific interpolymer interaction. In solution, even a small chemical shift change upon blending can be detected due to its high resolution [27]. Unfortunately in most solid polymer blends, changes in a chemical shift and/or a lineshape are obscured by broadening of resonances. For instance, Natansohn and Simmons [13] studied poly([N-ethylcarbazol-3-yl]methyl methacrylate) and poly(2-[(3,5-dinitrobenzoyl)oxy] ethyl acetate) (PECMMA/PNBOEAc), which are mixed via a charge-transfer (CT) interaction. The aromatic 13C resonances of the blend of their monomeric units (ECMMA/NBOEAc) show low frequency shifts of 1-5 ppm due to the CT interaction. However, such shifts are not observed for the polymeric PECMMA/PNBOEAc blend. Grobelny et al. [28] observed changes in the lineshapes of imide carbonyl carbons of polyimide (PIm) in the blends with poly(ether sulphone) (PES). They attributed the lineshape changes to a polar interaction between the imide carbonyl group of PIm and ether oxygen or sulphone of PES. Among several interactions, the hydrogen-bonding interaction causes appreciable changes in a 13C spectrum. Several blends show a high frequency shift and/or a lineshape change due to hydrogen bonding between component polymers. For example, Yang et al. [29] found a broadening and high frequency shift at the carbonyl carbon resonance of poly([1-hydroxy-2,6-phenylene]methylene) in the blend with poly(N,N-dimethyl acryl amide). Several groups investigated blends of poly(4-vinylphenol) (PVPh) [30-36]. They found that the resonance of the aromatic region of PVPh is sensitive to the interactions occurring in its surroundings, particularly the hydrogen-bonding interaction. In the blends of PVPh, the hydroxy group of PVPh acts as a proton donor, and the oxygen of the carbonyl group of another component polymer acts as a proton acceptor. Belfiore et al. [30] examined the chemical shifts and lineshapes of carbonyl carbon resonances of several polymers in
356
ATSUSHI ASANO AND K. TAKEGOSHI
PEA, wt.% 10 20
30 40 50 70 80
90 100 ]I~5
18~0
17S
170
PPM
'~C Solid State Chemical Shift Fig. 10.2. 13C CP/MAS NMR spectra of the carbonyl chemical shift region of PEA in PVPh/PEA. The weight percent of PEA in the blend is indicated on the left of each spectrum. (Reprinted with permission from Ref. [30]. 9 1993, John Wiley, New York).
the blends with PVPh. For instance, Fig. 10.2 shows 13C CP/MAS spectra of PVPh/poly(ethylene adipate) (PEA) blends with several weight percent of PEA. Although a single Tg was observed for whole compositions, the coexistence of two morphologically inequivalent microenvironments are appreciable as shown by the two peaks at 174 and 177 ppm. These two peaks are attributed to carbonyl carbons in a rigid amorphous phase and those hydrogen-bonded with the hydroxyl proton of PVPh.
POLYMER BLENDS AND MISCIBILITY
357
Zhang et al. [37, 38] and Feng et al. [39] adopted poly(vinyl alcohol) (PVA) as a proton donor. The signal of the methine carbon (CH) of pure PVA shows the characteristic triplet lineshape due to hydrogen bonding [40]. On blending with a proton acceptor such as poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMAA), rearrangement of hydrogen bonding occurs and the characteristic lineshape disappears. Contrary to the widely accepted statements that hydrogen bonding produces a shielding decrease, Zhang et al. [37] observed an increase of ---3 ppm for the carbonyl carbon of PVA/ PMAA. The intrapolymer hydrogen bonding among PVA is destroyed on blending and interpolymer hydrogen bonding is formed. The former brings a low and the latter a high frequency shift. They concluded that the intrapolymer hydrogen bonding in PVA has a larger deshielding effect than the interpolymer one. Other interesting studies are found in Refs. [17, 18, 41431. Recently, it was demonstrated that the 129Xe nucleus is very sensitive to the free volume changes, and its NMR resonances provide a quick examination of miscibility. For miscible PB/PI [44] and PS/PVME [45] blends, a single 129Xe NMR signal is observed at an intermediate position of the two resonances for two component polymers. From the chemical shift differences and the 129Xe diffusion constant D (typically 10-11"-'10 -13 m 2 S-1) [46, 47], the domain size, below which only one 129Xe signal is expected, is estimated to be 90 nm [45] to 600 nm [48]. Mansfeld et al. [49] observed 129XeNMR in a rubbery copolymer of polyethylene (PE) and polypropylene (PP) embedded in a rigid PP. They find an approximate linear relation between the shift and the PE/PP ratio of the copolymer. For a blend, Miyoshi et al. [45] observed a nonlinear relation between the 129XeNMR chemical shift and the composition ratio in PS/PVME (Fig. 10.3). They showed that the observed 129Xe chemical shifts in the blend is explained by the total volume of the blend. Therefore, when an interpolymer interaction exists, the linear relation does not hold, because the total volume in a blend is smaller than that weighted average sum of the volumes of component polymers. Furthermore, they observed that the 129Xe NMR linewidth of PS/PVME = 50/50 (250 Hz) is narrower than that of PS (450 Hz), and broader than that of PVME (100 Hz). They suggested that PS in the blend becomes mobile by blending with mobile PVME. In Section 10.3.2.2, it is demonstrated that the 1D/2D 129Xe NMR is a powerful tool to determine the domain size of phase-separated blends. Line broadening of 13C CP/MAS signals is often observed at the temperature range of Te, + 30 to Tg + 60 K. For PS/PVME = 50/50, line broadening of the 13C resonance for the CH carbon of PVME on blending with PS was
358
ATSUSHI ASANO AND K. T A K E G O S H I
230
225
I
220
(_1
o,~
215
33 u 210
205 0
20
40
60
80
100
PVME c o n t e n t / w t % Fig. 10.3. Component-ratio dependence of the 129XeNMR chemical shifts in PS/PVME blends. Circles express the measured shifts. A solid line represents the calculated chemical shifts using a simple weighted sum of the volumes of pure PS and PVME. Crosses are the calculated chemical shift using the observed total volumes of the blends measured by Shiomi et al. [50] (Reprinted with permission from Ref. [45]. 9 1997 Elsevier, Amsterdam.)
observed at 311 K (Fig. 10.4) [51, 52]. This broadening is also due to the interpolymer interaction in the blend. Since the broadening effect is a motional effect, it is discussed in the next section. 10.2.2.2 Effects of blending on motion The effects of blending on motion may be caused by the different free volume of the blend compared with that of homopolymers or specific interpolymer interactions between component polymers. For example, it is well known for a miscible blend that a composition-dependent glass transition occurs at a temperature between two Tg values of the respective component polymers. The change of the original Tg value indicates that large-scale main-chain motions of a few 10 kHz are affected by blending. Macroscopic properties of a polymer, such as impact strength and ductility, are largely influenced by such molecular motions [53]. Thus, it is important to study the effects of blending on motion. In solid-state NMR, the lineshapes and relaxation rates
POLYMER BLENDS AND MISCIBILITY
Ps
a)
@/
-cH2cH-
OCH3
PVME ,,-.-..,. ",~
CH
240 220 200 180 160 140 120 100 80
b) PS
PVME
-CH2CH-
CH OCH3
CH2 ~.
5/5
•••i1••`•j•••l••••••••t••••••••1••••••.••`•••••
359
5/5
60
40
20
0
-20 -40
220 2s
t80 160 140 120 100 80
60
40
20
0
-20
Fig. 10.4. 13C CP/MAS spectra of pure PS, pure PVME and PS/PVME = 50/50 at (a) 311 K and (b) 228 K. The peak marked with asterisk (*) in (a) is from a silicon-rubber cap to prevent leaking of the blend at higher temperatures. The peaks marked with SSB denote spinning side bands. (Reprinted with permission from Ref. [52]. 9 1994 Butterworth-Heinemann, UK.) would be influenced by such large-scale motion of a few 10 kHz. For example, Kwei et al. [54] observed a transition of the 1H 7'2 values of P S / P V M E = 50/50 from 15 to 3 ms at ---323 K. This is ascribed to the onset of large-scale motion at the glass transition. The free induction decay (FID) is composed of two T2 components which are different from those observed for pure PS and PVME. Each T2 component shows a different transition temperature, indicating different "glass transition" temperatures for the component polymers. For poly(ethylene oxide)/poly(methyl methacrylate) ( P E O / P M M A ) , Brosseau et al. [55] found that the temperature dependence of T2 of P E O obeys a Williams-Landel-Ferry (WLF) equation [56] with the reference temperature shifted 50 K higher than Tg. The motional narrowing of a ~H resonance occurs when the frequency of motion exceeds the 1H static linewidth and an averaging of the 1 H ~ H dipole interaction becomes appreciable. Similar motional narrowing occurs for a dilute spin, such as t3C and 29Si. The linewidth of a dilute spin is governed
360
ATSUSHI ASANO AND K. TAKEGOSHI
by an anisotropic chemical shift interaction and a heteronuclear dipolar interaction between 1H. The linewidth in general is a few 10 kHz for a rigid solid, therefore, the linewidth of a dilute spin is also sensitive to motion of a few 10 kHz. Newmark and Copley [57] measured e9si NMR spectra of poly(dimethylsiloxane) (PDMS)/silicone at various temperatures. The linewidth decreases gradually with increasing temperature beyond Tg from --~1200 Hz at 173 K to --~100Hz at 295 K. For high-resolution solid-state 13CNMR using 1H dipolar decoupling (DD) and magic-angle spinning (MAS), the effects of motion on linewidth are rather complicated. This is because the anisotropic chemical shift interaction and the 1H--13C heteronuclear dipolar interaction are already averaged to be zero by MAS and DD. Since random molecular motion interferes with the artificial coherent averaging of DD and MAS, motional broadening is observed, instead of motional narrowing [58]. When the motional frequency is close to the MAS frequency, the chemical shift anisotropy interaction is reintroduced. Practically, however, this broadening is negligibly small when the magnetic field is less than 10 T, so that the observed broadening of the 13C resonances upon changing temperature has been attributed to the interference between motion and DD. When the motional frequency is close to the strength of DD in Hz (---50 kHz), the heteronuclear dipolar interaction is reintroduced. For most glassy polymers below Tg, the 13C linewidth is ---4 ppm and temperature independent. With increasing temperature beyond Tg, the line broadens and reaches a maximum width at --~T~ + 50 K. Further increases of temperature narrow the line to a few 10 Hz. Miller et al. [59] examined the temperature dependence of the 13C linewidths in miscible PI/poly(vinylethylene) (PVE). They found that the 13C linewidths of the two component polymers show different temperature dependencies. This shows that even though a blend is thermodynamically homogeneous, carbons on the respective components exhibit a distinct "glass transition". Takegoshi and Hikichi [51] gave an analytical equation for a temperature-dependent 13C linewidth in a glassy polymer. They explained the linewidth of 13C under DD and MAS as follows: Below Tg, the linewidth reflects a distribution of the isotropic chemical shift arising from a variety of local conformations of polymer in the glassy state. Above T~, molecular motion averages out the distribution, hence this term decreases gradually to zero (motional narrowing). With increasing temperature above Tg, the interference between the random (incoherent) modulation due to molecular motion and the coherent modulation by DD occurs. As a result, motional broadening instead of narrowing occurs. At temperatures far above T~, motional averagings of the anisotropic chemical shift and the 13C~IH dipole interaction become effective, and the line becomes
POLYMER BLENDS AND MISCIBILITY
361
narrower even without DD and MAS. The remaining linewidth is ascribed to the intrinsic one accounting for various static line broadening mechanisms such as an inhomogeneous static field. Thus, the 13C linewidth is represented as [51] 2 AM2 8 = 8111 + - arctan{a(To- T)}] + ~ 7"1"
7rO) 1
7"O)1
1+
r2 w 2
+ 60.
(10.2)
Here, the first term empirically describes the motional averaging of the distribution of local conformations, where ~1 is half the linewidth due to the distribution, a describes steepness of the transition, and To is a characteristic transition temperature. The second term describes the interference between motion and DD [58], where A is the reduction factor of the second moment M2, r is the correlation time of motion and (.O1 is the strength of DD. This shows that the interference is effective when the motional frequency comes close to the frequency of DD (ro)l "~ 1). The third term 8o is the intrinsic linewidth. Figure 10.5 shows the observed temperature dependence of the CH carbon of PVME in PS/PVME [51]. 13C CP/MAS spectra of PS/PVME = 50/50 have been already shown in Fig. 10.4. The motional broadening with a linewidth of 1000 Hz occurs at 311 K for PS/PVME = 50/50, which is about Tg + 52 K. It is clearly shown that the dispersion of the isotropic chemical shift in the glassy state is very similar for each blend at below Tg, which is about 270 Hz (---4 ppm). Figures 10.4 and 10.5 show the molecular motion of PVME is affected by the blending with PS and the curve shifts to the higher temperature side with increasing the content of PS. The solid lines in Fig. 10.5 are the "best-fit" ones to Equation (10.2). It is concluded that the onset of anisotropic short-range motion of the PVME chain is not related to the macroscopic glass transition. Menestrel et al. [60] also investigated the 13C r e s o n a n c e s of PS in PS/PVME. Both groups conclude that the PS and PVME components do not share the same local chain dynamics, even though it is thermodynamically homogeneous. This conclusion is also supported by the aforementioned result of 1H T2 [54] and the 2H NMR study [61] for PS/PVME. Similar 13C linewidth behavior showing dynamic heterogeneity has been found for PIP/PVE [59], PVPh/PEO [34, 35], PVPh/PMA [36] and PMA/PVAc [62]. Landry and Henrichs [63] applied dynamic mechanical spectroscopy and 2H NMR to investigate sub-Tg motion in polycarbonate(PC)/PMMA and PC/poly(cyclohexylene dimethylene terephthalate)(PCHDMT). Examination of 2H NMR spectra and relaxation times led them to conclude that local
362
ATSUSHI ASANO AND K. TAKEGOSHI
7.0
6.0 ~-
^ Q
4
3.o
A v
l,j
0
2.5
.!
I 3.0
I
I 3.5
I
!. 4.0
I
l 4.5
IO00K/T Fig. 10.5. Temperature dependence of the linewidth of the CH carbon of pure PVME (O), P5/PVME - 20/80 (x), PS/PVME - 50/50 (A) and PS/PVME = 80/20 (n). The solid curves through the data points are "best fits" calculated using Equation (10.2). (Reprinted with permission from Ref. [51]. 9 1991 American Institute of Physics, New York.)
motions in the PC backbone are slower in the miscible blends than that in pure PC, while local motions of PMMA are relatively unaffected by the blending with PC. It is true that blending affects the frequency of the main-chain motion of component polymers, but does it affect the mode (e.g., jump angles) of the motion? Among many NMR experiments, the 2D exchange 2 H NMR experiment is particularly useful for investigating the motional mode of component polymers [64]. The pulse sequence for the 2D 2 H exchange NMR experiment is shown in Fig. 10.6(a) [64]. Figure 10.6(a) shows two pulse sequences for the cosine and sine magnetization components in the t~ period. The initial pulse produces transverse magnetization. After the evolution time t~, the second pulse induces either longitudinal magnetization (cosine) or quadrupole order (sine). Segmental reorientation occurring during a mixing time tm alters the quadrupole coupling for a spin. After the mixing time, the stored Zeeman magnetization or quadrupole order are again transformed
363
P O L Y M E R B L E N D S A N D MISCIBILITY
a) 90, 54 . 7.,
I I .
54.7,
90,
(u.,t2)>
Zeemanorder .
.
.
.
.
_AlfX
.
_
~VV v ) < I
II
tl 90~ 54.7.,
) <
t~
') (
A 54.7,
)
A 90~
spin ali.gnmen
' 9
>
t2 <s i n ( t o ~ t ~ ) s i n ( u 2 t . 2) >
~VV ~
b)
~.t /"x
_
_A
~ttttttttt~
-150-1OO-50
O 50 1OO 150 kHz
-150-1OO-50
O 50 IOO 150 kHz
Fig. 10.6. (a) Pulse sequence for the 2D 2H exchange N M R experiment. (b) The 2D 2H exchange NMR spectrum of pure d3-PS at 373 K [65]. The contour plot is shown in the righthand side. The 7r/2 pulse length was 2.2 ms and the mixing time (tm) was 10 ms. The pulse interval for the quadrupole echo (A) is 20 ms.
364
ATSUSHI ASANO AND K. TAKEGOSHI
into transverse magnetization and detected by using the quadrupole echo with the echo period of A. Figure 10.6(b) shows the 2D 2H exchange NMR spectrum of main-chain deuterated PS (d3-PS) at 353 K, which is below the Tg of PS (---375 K), with the mixing time of 10 ms [65]. The 2D spectrum shows a typical rigid powder pattern along the diagonal axis without appreciable off-diagonal signals. This indicates that the motion of d3-PS is much slower than the order of Hz below Tg. However, the spectrum at 393 K shows appreciable cross-peaks (Fig. 10.7(a)), and the off-diagonal pattern shows no particular ridges [65]. Similar spectra have been also observed by Wefing et al. [66] and they conclude that the main-chain motion of PS can be ascribed to an isotropic rotational diffusion. For d3-PS/PVME, a similar pattern was also observed at 333 K (Fig. 10.7(b)). This shows that the mode of molecular motion of PS is not affected by the blending of PVME. Note, however, the cross-peak pattern in the blend observed at 333 K, which is much below the corresponding temperature (393 K) for homopolymer. This indicates that the main-chain motion of PS in PS/PVME = 50/50 of 100-1000 Hz commenced at temperature much below the Tg of the PS homopolymer [61]. Similar conclusions by Chin et al. [67] show that component polymers retain their distinct motional characteristics in a miscible blend, even though their dynamics in blends are very different from those in homopolymers. They studied glass transition dynamics of miscible PPO/PS = 25/75 by 2D exchange NMR experiments of ~3C and 2H. From those exchange spectra, they found that the chain motion of PPO and PS of a few kilohertz in the blends commenced at temperature of 10-15 K below the Tg (401 K). In contrast, such motion of homopolymers appeared at a temperature above their Tg's. The motion exhibited the characteristics of rotational Brownian diffusion with an associated broad distribution of correlation times. Both distributions of PPO and PS are a bimodal distribution and considerably broader than those typical for pure PPO and pure PS, respectively. They employed a statistical lattice model to evaluate local concentration fluctuations and explained the observed relative ratio of the mode. The dynamic heterogeneity in nominally miscible polymer blends has been studied also by Chung et. al. [68] by observing 2D exchange 2H NMR. They observed that the two components of PI/PVE displayed distinctly different segmental mobilities, even though this blend is characterized by a single glass transition. They explained the dynamic heterogeneity depending on PVE content using the effective glass transition temperature obtained by fitting the correlation time and WLF equation. Kumar et al. [69] explained the dynamic heterogeneity in several miscible blends by using a model based on concentration fluctuations. They assumed
365
P O L Y M E R BLENDS A N D MISCIBILITY
a)
i Y
i I
I'""!'"'"
-150-100-50
I
"'" I ' " ' I'""' I ' " ' I 0 kHz
50
100 150
'"'i'l"l""l"l'l""l
-150-100-50
0 kHz
50
''l~
100 150
b) /,
I f'''
-150--100-50
'"" I"'"I"" 0 kHz
50
I"" I
100 150
''''lll''llli'l'"'l''''
-150--100-50
0 kHz
50
l'''V'
100
150
Fig. 10.7. 2D 2H exchange N M R spectra and contour plots of (a) pure d3-PS at 393 K and (b) d3-PS in d3-PS/PVME = 50/50 at 333 K [65].
366
A T S U S H I A S A N O A N D K. T A K E G O S H I
that, although the probability of occurrence of concentration fluctuations is symmetric about the mean value in a given volume, the cooperative volume over which a fluctuation must occur for it to be detected by a dynamic probe is not a constant. They conclude that the dynamic heterogeneity is a consequence of the coupling of concentration fluctuations, which occur symmetrically about the mean composition in any fixed volume, with a cooperative volume that changes monotonically with composition for systems with significant Tg contrast. The blending effects on motion have also been studied by measuring the ~3C spin-lattice relaxation time T~. Feng et al. [70] measured ~3C T~ of PPO and PS in PPO/PS. They showed that the respective ~3C T~ values for the aromatic carbons of PPO (---10 s) and PS (---39 s) homopolymers become the same value of 17-18 s when PPO and PS are blended at PPO/PS = 60/40. They concluded that the aromatic rings of PPO drive those of PS to move cooperatively, which indicates a strong rr-rr electron conjugation interaction between the aromatic rings. This approach was also applied to investigate poly(styrene-co-acrylonitrile)/poly(ethyl methacrylate) (SAN/PEMA) and SAN/PMMA [71]. Interpolymer interactions between the phenyl groups of SAN and the carbonyl groups in PEMA or PMMA were examined. Schantz and Ljungqvist [72] measured ~3C Tx of poly(3-octylthiophene) (POT) and PPO in partially miscible POT/PPO. They found that the ~3C T~ values of alkyl side chain carbons of POT increase considerably when POT is blended with PPO at POT concentrations less than 30%. This reflects an increasing of flexibility of POT on the blending with PPO. Recently, self-diffusion constants of polymer blends and copolymers have been observed by using pulsed-field gradient NMR (PFG-NMR) techniques in solution and solids. Meier et al. [73] examined interpolymer diffusion in PDMS/poly(ethylmethylsiloxane) (PEMS) at temperatures far above its Tg by the ~H correlation spectroscopy and PFG-NMR. Miyashita and Nose [74] examined the dynamic critical behavior of PS/PVME in deuterated benzene solution at the concentration where polymer chains are weakly entangled. They applied PFG-NMR, quasielastic light scattering and shear viscosity measurements to conclude that the self-diffusion of constitutional polymers is not affected by critical fluctuations. Uemura and Macdonald [75] investigated the binding of a hydrophobic ethoxylated urethane (HEUR) associating polymer (AP) to PS latex (diameter 168 nm). They observed the enhancement of the self-diffusion of HEUR-AP on addition of PS latex due to the breakup of the associated network. Pinder [76] calculated the X parameter from selfdiffusion and slow mode diffusion constants obtained by PFG-NMR and dynamic light scattering for PS/PMMA in deuterated solvents.
POLYMER BLENDS AND MISCIBILITY
10.2.3
367
Spin diffusion and domain size
Since miscibility (degree of mixing) influences macroscopic properties of a blend significantly, it is important to know the size and morphological information of domains in a blend. In Section 10.2.3.1, the effects of spin diffusion on IH Tx and Tip are discussed, which can be used to deduce the domain size on a scale of 2-50 nm. Sections 10.2.3.2, 10.2.3.3 and 10.2.3.4 discuss several experiments to monitor spin diffusion. To monitor spin diffusion, the following three periods, which are formally analogous to cross-relaxation and chemical exchange NMR experiments in liquids, may be required: (1) the preparation of nonequilibrium magnetization (polarization gradient) among the spins of component polymers or between different domains; (2) the variable spin-diffusion time, where spin diffusion takes place; (3) the observation of the resulting magnetization. Various methods have been proposed for (1) and (3). In Section 10.2.3.2, methods for (1) based on the Goldman-Shen experiment [8] are reviewed. In Section 10.2.3.3, 2D NMR experiments, which enable us to detect the region of specific interpolymer interaction and the domain size on a scale of 2 nm, are discussed. In Section 10.2.3.4, heteronuclear cross-relaxation experiments between 1H~13C,1H~ZH,19F~13C,electron to 1H, etc., which can be applied to study the intimacy between component polymers, are reviewed. 10.2.3.1 Spin-lattice relaxation experiments In solids, different 1H relaxation rates of respective spins tend to be averaged by a mechanism called spin diffusion. Spin diffusion is the equilibration process of nonequilibrium polarizations of spins at different local sites through mutual exchange of magnetization. Since the efficiency of spin diffusion is governed by a strength of a dipole-dipole interaction, which is a function of the internuclear distance, we can obtain information about the domain size of a blend by analyzing the spin-diffusion rate among component polymers. In this section, effects of 1H spin diffusion on a 1H spin-lattice relaxation rate are discussed. Figure 10.8 shows a schematic illustration of how the T1 relaxation process for ~H spins in a blend of polymers A and B proceeds with spin diffusion (SD). Here, we assume that (1) 1H spins are divided into two species: species A for polymer A and species B for polymer B, and (2) both A and B are characterized by their common relaxation times TIA and T1B, respectively.
368
ATSUSHI ASANO AND K. TAKEGOSHI
SD
Tt
\ /
A
B
ltttt iiii Ill |
m
i
i
i
4,
Ill T1A <
~. -.-)
III;
ittt
-.) 4,
tt Fig. 10.8.
Schematic illustration of the spin-lattice relaxation process together with spin diffusion (SD). A and B denote two kinds of spin in the component polymers A and B, respectively. It is assumed that T1 of A is much shorter than that of B.
Suppose TIA is much shorter than T1B, and the whole spins are inverted by a 7r pulse. If spin diffusion between component polymers is slow, the spin system may reach a situation where all of the XH spins of polymer A are fully relaxed to be "up", while those of polymer B are still "down" (the left-top in Fig. 10.8). Spin diffusion tries to average this polarization gradient created by different T1 values, that is, to flip down the half of 1H spins in polymer A concomitant with flipping up of the same number of 1H spins ofpolymer B (the right-top of Fig. 10.8). Due to the short T1 of polymer A, the "down" spins of polymei A quickly flip up to create a polarization gradient again (the lefthand side of the second column in Fig. 10.8), and again spin diffusion tries to average it, and so on. After all, both spin species reach to thermal equilibrium. When spin diffusion is much faster than the relaxation rates, any polarization gradient created is quickly averaged, and both spins relax at a common rate. However, when spin diffusion is very slow, the polarization gradient is not averaged, and both spins relax with their respective relaxation rates. Then, the question arises how fast should the spin diffusion be to achieve a common rate, or how slow to keep different rates?
POLYMER BLENDS AND MISCIBILITY
369
The equations describing the dynamics of magnetization in the two-spin species may be given as follows [77]:
d(MA~=(fB~A -fftkMB ]
kc
fAkc)(MA) -- ~B MB '
(10.3)
with
~A = KA + fBkc, '~B = KB + fAkc,
(10.4)
where Mi, fi and Ki (i = A and B) denote the magnetization, proton molar fraction and the intrinsic relaxation rate, respectively, kc is the spin-diffusion (cross-relaxation) rate between A and B. From numerical calculations using these equations, it can be shown that when the spin-diffusion rate is at least 10 times faster than the K values, the apparent relaxation decays for both MA and MB become practically identical and can be expressed by a single exponential with a common relaxation rate. In this fast spin-diffusion case, the apparent relaxation rate Kavemay be given as a 1H mole weighted average of the intrinsic rates of component polymers as [78, 79] Kave = fAKA + fBKB .
(10.5)
In evaluating Equation (10.5), one may use the relaxation rate of the pure polymer A for KA (--1/TA) and that of B for KB ( = 1/TB), provided that blending does not alter molecular motion. When the spin-diffusion rate is comparable to the K values, the relaxation decay curves become nonsingle exponential (see below), and when the spindiffusion rate is less than 10% of the K values, the relaxation decay curve can be described by a single exponential with an intrinsic relaxation rate. From these semiquantitative estimations, one can regard that, if the observed T1 values for the component polymers are the same, the size of the domains is small enough for spin diffusion to average polarization gradient among spins A and B created by different intrinsic relaxation rates. Then, the question arises how small the size of the domains should be to realize fast spin diffusion? The maximum diffusive path length r by spin diffusion in three-dimensions for a time T1 may be given as [78-81] r 2 = 6DT1,
(10.6)
370
ATSUSHI ASANO AND K. TAKEGOSHI
where D is the spin-diffusion constant. Note that the factor 6 in Equation (10.6) may be different if one starts from a different model such as diffusion from a 1D lamellar morphology (in this case, the factor becomes 4/3) [82, 83], 1D and 2D point sources (in these cases, the factors become 2 and 4, respectively). Fortunately (?), however, such differences depending on the model chosen are not serious when one deduces domain size from the magnetic relaxation experiments. Assink [84] gave a simple relation between D and the spin-spin relaxation time T2 as D = 2re~T2,
(10.7)
where ro is the proton van der Waals radius (0.117 nm). From Equation (10.7), the D value for T2 of 50 txs is calculated to be 5.5 • 1 0 - 1 6 m e s -1. This value is in good agreement with the typical D values, which are in the range of 10-15-10 -16 m e s -1 [79, 83-86]. By putting D = 5 x 10 -16 m e s -~, we obtain r = 20---50 nm for T1 values of 0.1---1 s, and r = 2---5 nm for Tip values of 1---10 ms. In other words, if the domain size is smaller than 20 nm, spin diffusion can effectively average any polarization gradient created due to different T1 values of a few seconds. To phrase it differently, when one observes the same T1 values for both component polymers, one can regard the blend as homogeneous on a scale of 20-50 nm. Similar criteria hold for Tip experiments on a scale of 2-5 nm. By using high resolution 13C solidstate NMR, it is possible to observe the 1H relaxation curve of each component polymer individually via well-resolved 13C peaks. Due to the ease of T1 and Tip experiments, these criteria are frequently used to establish the lengthscale over which the blend is homogeneously mixed. Various factors affecting miscibility, such as molecular weight [55, 87], side-chain difference [6, 8890], number of monomer units in the copolymer [91] and tacticity [42], have been investigated. Since the 7"1 and Tip experiments have different scales of observation, there is a case that a blend is regarded as homogeneous by T1 measurements, but is observed to be heterogeneous by Tip measurements. Table 10.1 shows one such example observed for P C / P M M A = 3/7, 5/5 and 7/3 [92]. The observed T1 and Tip values together with the calculated (averaged) values obtained using Equation (10.5) are collated. The observed T1 value of PC is in good agreement with that of P M M A within an experimental error for all blends. Furthermore, good agreement between the observed and calculated 7"1 values indicates that averaging of T1 rates by spin diffusion occurs and molecular motion effective for T1 is not altered by blending. The Tip values of PC are, however, different from those of PMMA. These show that the
371
POLYMER BLENDS AND MISCIBILITY
Table 10.1. Observed and calculated values of 1H T1 (lefthand) and Txp (righthand) for P C / P M M A = 3/7,515 and 7/3. The calculated (averaged) values are obtained by the assumption that T~ or T~o values of PC and PMMA in the blend are equal to those of PC and P M M A homopolymers (Equation (10.5)). T1/ms
Pure PC
268 "+ 13
PC
362 "+ 15
Calculated/ms
Tlo/ms 5.4 "+0.3 9.1 "+0.8
357 _+ 23
3/7 PMMA
384 -+ 7
PC
339 "+ 8
5/5
10.9 _+ 0.5 13.3 + 0.5 6.3 -+ 0.3
331 -+ 20 PMMA
351 -+ 11
PC
304 _+ 13
7/3
Calculated/ms
8.8 - 0.4 9.2 _+ 0.3 5.7 + 0.1
306 _+ 17
7.2 _+ 0.3
PMMA
313 + 20
7.5 -+ 0.5
Pure P M M A
395 _+ 29
15.7 + 0.6
Error is ~r. Reprinted with permission from Ref. [92]. 9 1992 The Society of Polymer Science, Japan.
blends are homogeneous on a scale of a few 10 nm, but heterogeneous on a scale of a few nm. Similar observations have been also found for many other blends, for example, see Refs. [43, 72, 90, 93-96]. When the spin-diffusion rate is comparable to the spin-lattice relaxation rates, the relaxation decay curves deviate from a single-exponential. Figure 10.9 shows the 1H T1o decays for PPO and PS in the two kinds of PPO/PS blends [77]. Observed nonexponential decays were successfully explained by Equation (10.3) by using the fitting parameters shown in Fig. 10.9. Stejskal et al. [77] concluded that PPO/isotactic-PS (i-PS) is mixed intimately enough for PPO relaxation being aided by efficiently relaxing i-PS, and at the same time, PS relaxation is reduced by its dispersal into PPO. The spin-diffusion rate kc can be used to calculate the domain size by adopting a proper morphology [86, 97, 98]. For example, the domain size of PPO/i-PS is calculated to be 5 nm by putting kc = 135 s -1 and D = 5 x 10 -16 m 2 s - 1 into Equation (10.6). From the 1H relaxation experiments, the value of kc can be obtained only when the characteristic nonsingle exponential decay curves are observed, that is, only when kc is comparable to the KA and KB values. Unfortunately, in most miscible blends, kc is so fast that the decay curves are identical or too slow to average the decays. It is worth pointing out here that motional effects on spin diffusion should be carefully taken into account. Miyoshi et al. [99] observed nonsingle expo-
372
ATSUSHI ASANO AND K. TAKEGOSHI I
1
"
KA : 3 2 , - ' ,
-
I%"
~
148.~-'
- ' ~
PPO
kc-250~.-'
t
r" ~'S'h--A t MW)
~r[l-I)from. Tt~(SL) for PPO and, P5 Lr~ / P,5 ( 7.5/2.5 ) b zrt~
e II _ -
-
i
x,
-4oj-'
K~ - / / O l -j
1
~
1
10
.....
1
PFO
1
20
Fig. 10.9. Observed 1H Tlo
decay curves of PPO and PS in PPO/PS = 75/25. The pair of calculated curves by using Equation (10.3) with the fitting parameters indicated in the figure are shown as solid lines. (Reprinted with permission from Ref. [77]. 9 1981 American Chemical Society, Washington, DC.)
nential 1H Tlo decay curves for PEO/PMAA = 1/1 at 307 K (Fig. 10.10). These curves were analyzed by fitting to Equation (10.3) to give the spindiffusion rate kc = 1.4 x 10 3 s -1. At first glance, this blend is partially miscible and its domain size may be deduced from this kc. However, the Tao experiments at 237 K gave a single exponential decay for both PEO and PMMA, and their Txo values are almost identical. Apparently, the molecular motion of PEO is fast at 307 K and averages the interpolymer 1H dipolar interaction to lead to slow spin diffusion. In fact, the observed Tip value of PEO in the blend at 307 K is close to that of the amorphous phase of PEO homopolymer (0.5 ms), suggesting that the motional frequency of PEO in the blend is comparable to the strength of the spin-locking field (55.6 kHz). Asano et al. [52] also observed nonsingle exponential 1H Tip decay curves for PS/PVME =
373
POLYMER BLENDS AND MISCIBILITY 1 . 000.g0 0.80 0.70 N
0.60 0.50-
N
0.400.30-
0.20
I
I
O. 50
1 . O0
I I
! 59 0
2. O0
l 2.50
i 3. O0
t/ms
Fig. 10.10. 1H Ttp decay curves for P E O / P M A A = 1/1 at 307 K: (O) PEO; ( x ) PMAA. The solid curves were calculated using Equation (10.3) with parameters KA = 1.6 x 103 s -1, KB = 0.2 • 103 s -1 and kc = 1.4 x 103 s -1. (Reprinted with permission from Ref. [99]. 9 1996 Elsevier Science, Amsterdam.)
50/50 at 311 K. The observed 1H Tip decay curves were successfully explained by assuming a three-spin system of the PS, methoxy group ( O C H 3 ) of PVME, and the methine group (CH) of PVME. At 228 K, the observed T~p decay curves for PS and PVME become almost identical. Again spin diffusion is suppressed by fast motion of PVME at 311 K. In contrast to the 1H spin-lattice relaxation experiments, the examination of the effects of 13C--13C spin diffusion on 13C spin-lattice relaxation is not popular yet. This may be due to lower sensitivity of such dilute spins. Its diluteness, however, enables us to obtain high resolution spectra by using DD and MAS, and the high resolution can be used to specify a particular position of polymers and to examine the morphology of several polymeric and biological materials. The 13C--13C spin diffusion between methyl 13Clabeled P P O (13C--PPO) and normal PS in 1 3 C - - P P O / P S = 10/90 was investigated by Pavlovskaya et al. [100]. They used a rotational resonance technique [101] with DANTE [102] to observe 13C--13C spin diffusion between the methyl carbon of 1 3 C - - P P O and the aromatic carbons of PS. The rotational resonance enhancement of 13C--13C spin diffusion was observed and the distance between 1 3 C - - P P O and PS was estimated to be ---0.5 nm. A 2D ~3C spin-diffusion experiment was also applied to examine the miscibility and annealing effects on poly(ethylene terephthalate)/poly(p-hydroxybenzoic
374
ATSUSHI ASANO AND K. TAKEGOSHI
acid-co-p-hydroxynaphthoic acid) (PET/Vectra-A) [103]. The annealing effect will be discussed in Section 10.3.2.1. 10.2.3.2 Goldman-Shen experiments As shown above, even a very homogeneous miscible blend shows dynamic heterogeneity. Therefore, one simple way to achieve a polarization gradient for detecting the spin-diffusion process is to utilize mobility differences. Goldman and Shen [8] developed an experiment to monitor 1H spin diffusion by selecting 1H magnetization by T2. In their original experiment, they observed the 1H spins directly. The original Goldman-Shen experiment was further modified to incorporate high resolution 13C detection (Fig. 10.11(a)) [104, 105]. Or, a 1H Tlo difference may be used to create a polarization gradient, where a 1H spin-locking pulse is applied during the delay time tl [106, 107]. After a certain delay time tl, a shorter T2 or Tlo component vanishes and a longer component remains. The remaining 1H magnetization is flipped back by the second ~r/2 pulse to the z-direction. For a spin-diffusion (mixing) time z, the redistribution of magnetization occurs by spin diffusion. The amount of the resulting 1H magnetization at the spin-diffusion time z is measured indirectly by transforming it to 13C by CP. From now on, these experiments are referred as the T2-, Tlo-selective Goldman-Shen experiments with 13C detection, respectively. A single 1H dipolar echo sequence [82] or multiplepulse dipolar filter [86] can also be used to create gradient. Furthermore, 1H chemical-shift differences can also be used to pick up 1H spins [82, 86, 108, 109]. In these experiments, 1H direct observation may be applied if sufficient resolution is achieved. The 1H chemical-shift selection requires sufficient separation of the 1H peaks of the component polymers. Therefore, the application of CRAMPS (combination of rotation and multiple-pulse spectroscopy [110, 111]) is a pre-requisite. Figure 10.11(b) shows normalized deviations of 13C magnetizations (M(z)meq)/Meq for PVPh and PEO in PVPh/PEO = 58/42 versus the square of the spin-diffusion time z [34]. Here, M(z) is the magnetization after the time z and m e q is the magnetization at internal equilibrium. For short z, the deviations are proportional to z ~/2. This indicates that the deviation is directly proportional to the size of the initial polarization gradient. Havens and VanderHart [83] defined the effective diffusion time te as the intercept of the initial slope with the abscissa, and the mean squared distance as (r 2) - 4/3Dte. The observed te from Fig. 10.11(b) is 4.4 ms. Thus, the domain size @2)1/2 is estimated to be 0.9 nm using the D value of 1 . 5 • -16 m 2 s -1. Zhang and Wang [106] applied the Tlo-selective Goldman-Shen experiment with 13C detection to PES/poly(ethersulphone) (PPS). There are many other studies
375
POLYMER BLENDS AND MISCIBILITY 90.~
9Ox ,,
90_~
1H
,
CP
13 C
b)
Dec.
CP
M(~)-Meq
O CH2
Meq
A C1, 2 PVPh V C5
"o 0.4 ~o
PVPh
te
"
02
PEO
O " O -,
l
1
O~
0 ''~
~,-------~
3
VT ms v~
-0.2 -0.4 -0.6 -0.8
PVPh/PEO - 58/42
-1.0 Fig. 10.11. (a) Pulse sequence for the modified Goldman-Shen experiment with 1 3 C detection and (b) plots of 1H magnetization change versus square root of the spin-diffusion time r 1/2 for PVPh/PEO = 58/42 at 310 K. Straight lines are drawn through the initial linear portion of the data to determine the intercept time te. (Reprinted with permission from Ref. [34]. 9 1992 American Chemical Society, Washington, DC.)
using the modified Goldman-Shen techniques, e.g., PS/PPO [70], PEO/ PMAA [99] and PMA/PVPh [112]. Figure 10.12 shows the 1H and 13C NMR spectra of poly(ether-imide)/ poly(aryl ether ketone) (PEI/PAEK) obtained from the aH chemical-shift selective Goldman-Shen experiment with (the righthand side) and without (the lefthand side) 13C detection [108]. The signals from the aliphatic protons of PEI at ---2 ppm are resolved from the aromatic protons of PEI and
376
ATSUSHI ASANO AND K. TAKEGOSHI
tH CRAMPS
t3C CP MAS TOSS
U,H
H U,H
U,H a
UH. ,,
1 . . . .
10
U, 1,
5 ppm
t,
,
I
0
i,_
,,
,,
I,
200
,
i ,!
. . . .
150
ppm
1 . . . .
100
I
50
. . . .
!
0
Fig. 10.12. 1H CRAMPS and 13C CP/MAS spectra of PEI/PAEK; (a) spectra without any selection; (b) spectra after selection of the methyl proton magnetization of PEI (depicted by U) by ~H chemical shift filter; and (c) spectra taken after redistribution of 1Hmagnetization from spectrum (b) by spin diffusion. (Reprinted with permission from Ref. [108]. 9 1990 John Wiley, New York.)
PAEK at ---8 ppm. With the 1H chemical-shift filter using the MREV-8 pulse sequence [110, 113, 114], the peaks in the aromatic region are selectively removed. The resulting aliphatic 1H magnetization is transferred to 13C by CP, and 13C CP/MAS spectrum shows only the 13C resonances from PEI (the righthand spectrum of Fig. 10.12(b)). During the mixing time for spin diffusion, the magnetization of the aliphatic 1H spins of PEI diffuses to PEI and PAEK. The diffusion may be appreciable in the 1H spectrum in Fig. 10.12(c), however, it is difficult to discriminate the aromatic protons of PAEK. By adopting 13C high resolution detection, one can easily observe the spin-diffusion effect via the resolved 13C signals of PAEK. From the examination of the normalized deviation of magnetization, Schmidt-Rohr et al. [108] conclude that PEI/PAEK is miscible on a scale of 0.5-1 nm. Even without the 13C detectlon, the chemical-shift filtered 1H CRAMPS technique can be a powerful tool to investigate the miscibility of polymer blends in favorable cases. VanderHart et al. applied the chemical-shift filtered Goldman-Shen experiment to PEI/poly(benzimidazole) (PEI/PBI) [82, 109, 115]
POLYMER BLENDS AND MISCIBILITY
377
and poly(benzo[a,d]dithiazol-2,6-diyl-l,4-phenylene) (PBZT)/nylon [82, 109, 115, 116]. Schmidt-Rohr et al. [117] applied a 2D version of the modified GoldmanShen experiment with a3C detection to P S / P V M E - 50/50 (the 2D-WISE experiment). Figure 10.13 shows the 2D-WISE spectra for PS/PVME = 50/50. The first dimension gives the IH wideline spectrum and the second dimension the 13C high resolution spectrum. At a mixing time for spin diffusion, tm, of 0.2 ms (Fig. 10.13(d)), narrower IH lines are observed for PVME, reflecting its mobility. After tm of 5 ms, the IH magnetization has essentially equilibrated, as can be suggested from the equivalence of the silhouette of the 2D spectrum with the 1D 13C CP/MAS spectrum in Fig. 10.13(c). Due to spin diffusion, the IH narrower silhouette of PVME becomes rather broader, and the broad silhouette of PS becomes narrower. Note that the 2D-WISE spectra also demonstrate dynamic heterogeneity (see Section 10.2.2.2) that both the hard and soft regions exist although the blend is more than 50 K above its Tg. 10.2.3.3 Two-dimensional NMR experiments In this section, 2D homonuclear and heteronuclear correlation experiments are reviewed. The longer experimental time for a 2D experiment is rewarded by enhancement of resolution achieved by adopting the second dimension. This gives us a chance to get information on both miscibility and polymerpolymer interactions. For mobile polymers, MAS may be enough to narrow the 1H resonances. In such cases, ~H spin diffusion becomes inefficient while the NOE is used to correlate 1H spins. Heffner and Mirau [24] observed 2D ~H NOESY spectra of 1,2-PB/PI = 1/1 at 333 K which is above Tg of the blend. They observed a strong interpolymer cross-peak between the CH3 protons of PI and the CH2 protons of 1,2-PB. Furthermore, weak interpolymer cross-peaks between the CH3 protons of PI and the double-bonded CH protons of 1,2PB were observed, showing these 1H spins exist within 0.5 nm. For immobile polymers, resolution of 1H spins in solids achieved by CRAMPS is still not good enough to be applicable to any pair of polymers: it is barely good enough to resolve aromatic and aliphatic protons in a blend. Caravatti et al. [118] studied PS/PVME by using a 2D 1H exchange NMR experiment. Figure 10.14 shows the 2D 1H exchange spectra of PS/PVME (Fig. 10.14(B)) and its pulse sequence (Fig. 10.14(A)). For an immiscible state cast from chloroform (Fig. 10.14(B-a)), only intrapolymer cross-peaks within PS or PVME are observed. However, for a miscible PS/PVME blend cast from toluene, interpolymer cross-peaks indicated by arrows appear (Fig. 10.14(B-b)), showing direct evidence of close contact on a microscopic scale. They also observed
378
ATSUSHI ASANO AND K. TAKEGOSHI
c) 13 c
ppm
1--' ; ! I I ; : : ' 1 -
50
50 ld-lz
0
-'I
I
I
I
I
150
d)
l--I
i .... I
I
103
I
~"1
50
~
i
~
1--
0 ppm
e)
PVME
Ps 1H
1
ps
t
#"
I = I -!
150
1 ..... l
I
I
100
'1'
I
!
50
~ '!
~ 1
0 ppm
I I
tm-5ms
13 C
t m = 1 ITLg
Fig. 10.13. (a) 1D 1H wideline; (b) 1D 1H dipolar filtered wideline; (c) 13C CP/MAS; and (df) 2D-WISE NMR spectra with MAS of 4 kHz of PS/PVME = 50/50 at 320 K. The mixing time for spin diffusion is (d) 0.2 ms; (e) 5 ms; and (f) 1 ms. (Reprinted with permission from Ref. [117]. 9 1992 American Chemical Society, Washington, DC.)
the mixing time d e p e n d e n c e of the intensity of the cross-peak and c o n c l u d e d that the p e r c e n t a g e of the total p o l y m e r s c o n t a i n e d in the mixed phase is 6 0 80% assuming the t h r e e - p h a s e m o d e l of pure PS, pure P V M E and the mixed phase (see Section 10.3).
379
POLYMER BLENDS AND MISCIBILITY
A) 9 0 " aS"
4~-~-
9Q" 4~"
MREV-8
B)
MREV-8
t,
PS
_CH-CH 2I
t2
~m OCH 3
PVME
OCH
-CH-CH zI o
oom. A -F
t
' '1
8
]
I
6
@
o)
@
i
I
4
" 'i
I
CH~
I
~
2
l
0 ppm
@
o
t
b)
i
@i~0 (aJ 2
Fig. 10.14. (A) Pulse sequence for 2D 1H--1H exchange NMR and (B) 2D exchange spectra of PS/PVME with a mixing time of 100 ms. Interpolymer cross-peaks between aromatic protons on PS and the methine and methoxy protons on PVME are indicated by arrows. (Reprinted with permission from Ref. [118]. (O 1985 American Chemical Society, Washington, DC.)
380
ATSUSHI ASANO AND K. TAKEGOSHI
The time-consuming 2D experiment can be replaced by 1D selective inversion-recovery/saturation-transfer experiments [119]. Its application to study the heterogeneity of PS/PVME is mentioned afterwards (see Section 10.3.1.1). Furthermore, Campbell and VanderHart [109] showed that the selective techniques under multiple-pulse homonuclear decoupling are not necessary. They realized that at certain ~- value in a 2D exchange NMR experiment, polarization gradient necessary for spin diffusion can be achieved by the chemical shift difference. The optimum preparation period for the 1D analogue of the 2D exchange NMR experiment was discussed. Instead of using ~H spins, 13C spins may be useful, provided that inefficient 13C--13C spin diffusion is somehow enhanced. To facilitate spin diffusion, Henrichs et al. used 13C enriched polymers [120, 121]. They applied the 1D/2D 13C--13C exchange NMR to study PC/PET = 25/75. Figure 10.15 shows the pulse sequence and the 2D 13C exchange NMR spectrum of PC/ (ethylene-13C enriched) PET = 25/75 (13C-PET). The cross-peak between PC and 13C-PET is clearly observed, showing 13C~13C spin diffusion. The intensity of the cross-peak at PC is about 25% of that at 13C-PET. Due to its low gyromagnetic ratio, 13C~13C spin diffusion is less efficient as compared to 1H~IH spin diffusion. Therefore, 13C pairs must be much closer together than would be necessary for 1H pairs for spin diffusion. The observable lower limit of the domain size thus becomes much smaller than that observed by 1H spin-diffusion experiments; cross-peak is observed between the domains on a scale of 3-6 nm. Since ~3C enrichment is time consuming and expensive, the rotational resonance technique [101] may be used to recover the 13C~13C dipole interaction averaged by MAS [100, 102, 103]. This enables us to realize 2D 13C~13C exchange spectra without 13C enrichment. So far, homonuclear dipolar interactions were used to correlate spins. Alternatively, a 2D 1H~13C heteronuclear correlation (HETCOR) experiment [122] can be used to study miscibility and interpolymer interactions in a polymer blend. The original HETCOR experiment consists of three periods, i.e., (1) the evolution period, where a high resolution 1H spectrum is recorded; (2) the mixing period, where the 1H magnetization is transferred to the 13C spins nearby; (3) the detection period, where a high resolution 13C spectrum is observed. Several pulse sequences have been proposed to suppress 1H~IH homonuclear and 1H~13C heteronuclear dipolar interactions simultaneously during the evolution period [123], and one typical combination is shown in Fig. 10.16(a).
381
POLYMER BLENDS AND MISCIBILITY
a)
'm JvJw J w.......J ~
[
9
w
1
.
~c i,1
im
i
n ~ PET -CH z -
PET CO
b)
I
Ar
I
I
-0
I
.....
I
0 .... "E)
I
I ,
I
200
1
1
]
I00
I
,
'
J
,
-
I00
-
200
"Q
B
J
09
ppm
Fig. 10.15. (a) Pulse sequence for 2D
13C--13C exchange NMR and (b) 2D exchange spectrum of pC/13C-PET = 25/75 with the mixing time of 400 ms. The interpolymer cross-peak between the carbonyl carbon of PC and the 13C-labeled methylene carbons of PET is indicated by the arrow. The associated 1D spectrum is a cross-section taken from the 2D spectrum at the dashed line. (Reprinted with permission from Ref. [120, 121]. 9 1985 American Institute of Physics and 1988 American Chemical Society, Washington, DC.)
382
A T S U S H I A S A N O A N D K. T A K E G O S H I M2
=/2
_
a)
'H
I BLEW-i2
w,M: i cw,ec L
13C
'L
'~VV vv
---
b) .t "
O, /
a
b
H.....%.c/O.cH, .4
CH.CH,
" c . . cH ..
OCH,
0 Aromalics
)~ 6 ~ g E.
,z,
i , lo
15
c,
c1 , I 50
c2
c3 1O0 ppm
50
0
Carbon Chemical Shill
Fig. 10.16. (A) 2D 1H--13C H E T C O R pulse sequence and (B) 2D H E T C O R spectrum for PMA/PVPh = 1/1 cast from methyl ethyl ketone. The interpolymer cross-peak between the carbonyl carbon of PMA (b) and the hydroxyl protons of PVPh (a) is appreciable. (Reprinted with permission from Ref. [26]. 9 1994 John Wiley, New York.)
Mirau and White applied the HETCOR pulse sequence to PMA/PVPh, PMMA/PVPh and PC/polycaprolactone (PCL) blends [26, 124]. Figure 10.16(b) shows the HETCOR spectrum for PMA/PVPh = 1/1 cast from methyl ethyl ketone. The combined use of WIM-24 and MAS allows CP for aH and 13C spins in close proximity, therefore, most of the cross-peaks appear between directly bonded 1 3 C ~ H pairs. Interestingly, a cross-peak was observed for the nonprotonated carbonyl carbon of PMA. The spectrum shows
P O L Y M E R BLENDS AND MISCIBILITY
383
that this carbon is close to a proton appearing at 5 ppm. The peak intensity is considerably weaker than the other cross-peaks between the carbons and the directly bonded protons. Furthermore, this peak was not observed in PMA/PVPh whose hydroxyl protons are deuterated. From these results, Mirau and White assigned the proton signal at 5 ppm to the resonance of hydroxyl protons of PVPh, and the cross-peak represents existence of interpolymer hydrogen bonding between the hydroxyl group of PVPh and the carbonyl group of PMA. No intrapolymer correlation peaks were observed between the hydroxyl proton and the aromatic carbons of PVPh, showing that the interpolymer distance between the hydroxy proton and the carbonyl carbon is shorter than the intrapolymer distance between the hydroxy proton and the aromatic carbons. Such a situation should be rare, therefore, to observe interpolymer cross-peaks, a spin-diffusion period may be incorporated. The original H E T C O R experiment has been modified to incorporate the spin-diffusion period after the evolution period to examine the miscibility of PC and an aromatic dianine [125]. Li et al. [126] used the modified H E T C O R pulse sequence to study d5-PS/PPO, d3-PS/PPO and d3-PS/d5-PS blends. The cross-peak intensities increase with increasing the 1H spin-diffusion period, and the distance between interacting components was determined. Instead of inserting the spin-diffusion period, Takegoshi and Hikichi [127] used the conventional CP sequence for a mixing sequence to study an interpolymer interaction of PMA/PVPh. They found that with a CP contact time of 0.2 ms, no appreciable interpolymer cross-peaks appear, but with a CP time of 1 ms, the interpolymer cross-peak between the aromatic protons of PVPh and the carbonyl carbon of PMA appears. 10.2.3.4 Cross-relaxation experiments As shown above, 1H--13C cross-polarization (CP) can also be used to study miscibility. A technique frequently used is to examine interpolymer CP from protons in one component polymer to carbons of the other fully deuterated component polymer [19, 89, 128-131]. Since effective 1 H ~ I 3 c CP transfer occurs for 1 H ~ I 3 c spins within --~2 nm [132], a CP enhancement for the deuterated component polymer shows at least some protonated component polymers are nearby. This is a very straightforward approach but does require deuteration. In a favorable case, deuteration of one of the component polymers may not be required. Zhang et al. [38] observed the interpolymer CP effects for PVA/poly(vinylpyrrolidone) (PVP) blends. The CP rate of the nonprotonated carbonyl carbon of PVP increases on blending with PVA, showing the closeness of the OH proton of PVA and the carbonyl carbon of PVP.
384
ATSUSHI ASANO AND K. TAKEGOSHI
Zumbulyadis et al. [133-135] showed that proton to deuterium CP transfer is also useful to investigate miscibility. For a completely deuterated PMMA (d8-PMMA) homopolymer, a very weak deuterium FID signal, which was created by CP between a small amount of residual protons (<~2%), was detected (Fig. 10.17(a)) [134]. For miscible d8-PMMA/PVPh = 18.8/81.2, an appreciable enhancement was observed (Fig. 10.17(c)). However, for the phase-separated blend, the signal enhancement was small as compared to that for the miscible one (Fig. 10.17(b)). They concluded that the signal enhancement in the phase-separated blend comes from interracial regions (see Section 10.3). Several other interpolymer polarization transfer experiments have been examined. For example, Veeman et al. applied 19F~13C, 1H~19F and 19F~1H~13C CP experiments to PMMA/poly(vinylidene fluoride) PVF2 [136-140]. Electron polarization was transferred to 1H spins of the other component polymer via an interracial region of a polymer blend [141, 142]. The expected maximum signal enhancement of the 1H spins for electron-ill polarization is 660 and that of ~3C spins for electron~1H~13C polarization is 2640. These experiments are described in Section 10.3.1.3. Douglass and McBrierty [78] observed the transient NOE between 1H and 19F, and applied it to study miscibility of PMMA/PVF2 at various temperatures. They estimated the cross-relaxation rate from T1, Tip and T2 measurements and concluded that a substantial fraction of the two component polymers is intimately mixed in the amorphous regions. White and Mirau observed the steady-state equilibrium NOE enhancements of 13C PS signals for the completely deuterated PS(d8-PS)/PVME cast from toluene but not chloroform [25, 26]. Even below Tg of the blend, they observed interpolymer NOE between the methoxy group of PVME and the aromatic group of PS. The interpolymer NOE is generated by fluctuating fields produced by the rapid rotation of the methoxy group. They concluded that the phenyl ring of PS is much closer to the methoxy group of PVME than to the main-chain of PS. For this technique to be applicable, at least one of the component polymers should be mobile. Ruhnau and Veeman [143] showed that the abundant 14Nspin can be used to examine miscibility. They applied the rotational echo 14N~13C~IH triple resonance technique [144] to observed 13C signals with and without 14N irradiation for PMAA/poly(N-vinylimidazole) (PVIm). They observed a significant loss of 13C spin-echo intensity when 14N is irradiated. The interpolymer distance was estimated to be less than 0.5 nm by a numerical model, which was also supported by 1H Tip results. Some studies appreciate the effects of the additional 1H dipolar field on 13C spectra on blending. Delayed or not decoupled 13C NMR can be utilized
385
POL'~/MER BLENDS AND MISCIBILITY
!;1i111i1111',"11'1'i"1'1-111' . . . . . . . . 0..~
t.
i'il'i"il' . . . . ~ . . . . ' l l l ' i i l l ' l l i ' 2.5
.... '}l'lill'lil"i'li'iil| 3 3.5
. . . . . "~ . . . . .
"ilii|'1 ..... 4..~
mS 'iiY I .~
b)
I;'"'
.... o.~'.........
,' ,","" ;
' I '''~ . . . . . . . 0.5
I ......... ,.~'"" ...... , ........
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'"~' '~ r r r r r m l 1.5
rnl 2
,,,.~...... ,,,,,-,,,,~ .... ~.~'"'" ....
a ~ ' r n v r r ~ 2.5
1 n n rvvrrTrrr 3 5.5
,"'""~'=~'~'l,.~ ~,
TI 1 ~ ' q r n 4
rrrn
ms
,I-,~ r r , T r r , I .z 5
51
Fig. 10.17. Time-domain 2H CP/MAS NMR spectra of pure deuterated-PMMA and deuterated-PMMA/PVPh = 18.8/81.2: (a) Only a very weak 2H signal due to CP from residual 1H is observed for deuterated-PMMA; (b) 2H time-domain signal of a partially phase-separated blend cast from tetrahydrofuran; and (c) 2H time-domain signal of a homogeneous blend cast from methylethylketone. (Reprinted with permission from Ref. [134]. 9 1993 American Chemical Society, Washington, DC.)
386
A T S U S H I A S A N O A N D K. T A K E G O S H I
to show whether or not ~H dipolar interactions increase/decrease on blending [128, 131]. For deuterated PS, VanderHart et al. [145] observed broadening of residual ~H spins in 1H MAS spectra when protonated PS is introduced. The increased 1H dipolar interactions were attributed to indicate that the protons of one component polymer come close to the other component upon blending.
10.3
Heterogeneity
In practice, blends may be found between a homogeneous single-phase blend and a heterogeneous two-phase blend. So far, NMR studies on the former have been mainly reviewed. From an engineering point of view, however, a homogeneous single-phase blend is less important because, in most cases, an average property of component polymers is found. Hence, for a homogeneous polymer blend, it is difficult to keep two contradicting properties, such as shock resistance and thermostability, and heterogeneous blends with various degree of miscibility have been used. From an NMR point of view, a completely phase-separated blend is a difficult target, because most of the NMR observable, such as relaxation times, are not sensitive to heterogeneity larger than a few 10 nm. That is, we cannot discriminate whether we have two pure polymers in our sample tube or one heterogeneous two-phase blend. In Section 10.3.1, the NMR techniques, whose application for examination of miscibility of a blend is reviewed in Section 10.2, are applied to examine domain structures of partially miscible blends. For a partially miscible blend, a 1H relaxation decay curve is expressed by a sum of several exponential decays indicating that there exist regions having different relaxation times, which are not averaged by spin diffusion. Deviation of the observed T2, 7"1 and T~p values from the superposition of those of pure component polymers implies s o m e blending effects. For an immiscible blend, whose domain size is much larger than a few 10 nm, techniques with a larger observation scale, such as NMR imaging may be applied. To achieve desirable macroscopic properties, one would like to control the degree of miscibility of a blend. Heat treatment/annealing is a simple technique to modify a phase structure of a blend. Morphological changes induced by heat treatment can also affect NMR observable (see Section 10.3.2.1). Furthermore, as shown in Section 10.2.1, several blends exhibit a lower critical solution temperature (LCST) phase diagram. Such a blend phase-separates at temperatures above its LCST temperature. The compositional fluctuation during the phase-separation process is examined in Section 10.3.2.2.
P O L Y M E R BLENDS AND MISCIBILITY
10.3.1
387
Partially miscible and immiscible blends
As shown in Section 10.2, a domain/phase is defined as a group of spins having one relaxation time, T~ or T~p. In other words, we regard a group of ~H spins as a domain/phase if spin diffusion is fast within the group but is negligibly slow between the different groups. For nonnegligible interdomain spin-diffusion cases, refer to Section 10.2.3.1. For partially miscible and immiscible blends, various domain/phase structures can be invoked. Unfortunately, the resonance position of a particular spin in each domain is not appreciably affected by its respective domain structure (see Section 10.2.2.1). Therefore, we cannot expect to observe highly resolved NMR resonances for different domains. Since relaxation times are different for each domain, a relaxation curve is observed to be a featureless multiexponential one and, in most of the cases, is too monotonous to include interdomain spin diffusion. Therefore, most of the experimental results have been explained by using the simplified picture of no interdomain spin diffusion and the observed multiexponential decay is fitted to a sum of exponential functions. Practically three exponentials are enough to realize the observed decay. Each relaxation time represents one domain, thus, only a few domains can be distinguished by one resonance line. Inevitably, the heterogeneous structures deduced from NMR relaxation experiments become simple. Roughly speaking, the following four models (Models A - D ) have been proposed to explain the observed ~H relaxation curves and the spin-diffusion data (Fig. 10.18): (Model A) a component polymer forms domains dispersed in a matrix of the other polymer with a sharp boundary (an interface); (Model B) similar to Model A, but there is an interphase, where polymer chains of component polymers are intermingling; (Model C) one of the component polymers is mixed homogeneously with a part of the other polymer; and (Model D) two homogeneous domains coexist with different composition. Suppose one observes three relaxation times: two of them are identical to those of pure polymers and the third one is observed for both component polymers. Then, Model B is invoked. If the interphase is dominant in Model B, the blend becomes a homogeneous one. If the interphase region is negligibly small, Model B becomes Model A. The Model C structure is often found when one of the component polymers is amorphous and the other is semicrystalline. The Model D structure is found as a result of phase separation induced by thermal treatments. It is true that these models, and especially the assumption of no spin diffusion between domains, are too simplified. One may lose an opportunity of a detailed description of microstructure by adopting the simplified model
388
ATSUSHI ASANO AND K. TAKEGOSHI
interface
A
B
interphase
A
A
B
+
B Mode i-A
Mode I-B
Phase-X Phase-Y
A+B
B
Mode l-C
xA +
yA +
1-x)B
(1-y)B
Mode I-D
Fig. 10.18. Schematic drawing of phase structures postulated for immiscible blends.
to analyze one's data. More elaborate discussions and treatments of the spindiffusion data to elucidate polymer morphologies are found elsewhere [86, 97, 98]. 10.3.1.1 1H NMR experiments From a so-called wideline ~H spectrum, one may naively think that it is difficult to obtain the relaxation curve of a particular polymer in a blend. This is not true, because in many blends a short T2 is caused by the mobility of the one of the component polymers or that of side-chains. Thus, there is a chance to discriminate between polymers by their different T2 (mobility). For example, Segre et al. [146] observed two T2 decays for PS/PB. The fastdecaying component was attributed to rigid PS. The slow-decaying component shows the presence of two 7"1 relaxations. These were attributed to the interphase and pure rubbery PB (Model B). Parizel et al. [147] observed that the 1H FID of polyurethane (PU) in a cross-linked PMMA consists of three
POLYMER BLENDS AND MISCIBILITY
389
decays, which were attributed to rigid PMMA, the intermediate region and mobile PU. Even though the spectral resolution is low, there is a case that different relaxation times for component polymers make it possible to examine structures. The observed T~p decay for PVAc/PMMA was not a single exponential and was analyzed by fitting to three-exponential decay function [148]. Two of the observed T~p times are in good agreement with the relaxation times of pure component polymers, and the third one with an intermediate value was obtained. Model B was proposed and the intermediate T~p component was attributed to the interphase. For PCL/poly(vinyl chloride) (PVC), Albert et al. [80] observed a single exponential decay for T1 and a double exponential decay for T~p. This indicates a small-scale heterogeneity, and Model C, which PCL forms an intimate blend with part of PVC through hydrogen bonding was proposed. By analyzing the temperature dependence of T~, T~p and T2, they concluded that in the miscible domain, PCL acts as a plasticizer for PVC, while excess PVC remains relatively immobile. For PE/PP, McBrierty et al. [149] observed the increase of the 7'1 minimum for the methyl group of PP and the concomitant slight decrease of the T1 minimum for PE motion. A weak PE-PP coupling via spin diffusion was suggested and Model B was proposed. The resolution of 1H spins in solids achieved by CRAMPS is good enough to resolve aromatic and aliphatic protons in a blend. Caravatti et al. studied PS/PVME by using a 2D ~H exchange NMR experiment (Fig. 10.14) [118] and 1D selective inversion-recovery/saturation-transfer experiments [119]. In the latter inversion-recovery 1D experiment, the aromatic peak of PS was selectively inverted and the aliphatic magnetization of PVME was plotted as a function of the mixing time (Fig. 10.19). For the blend cast from chloroform (Be), the magnetization decays simply by the spin-lattice relaxation, while for the blend cast from toluene (BT), the magnetization shows a biexponential dependence. The initial increase is due to interpolymer spin diffusion, and the latter decay is the spin-lattice relaxation. The clear biexponential behavior was explained by Model B, and the concentrations within the three phases were obtained. Campbell et al. [116] used the 1H chemical-shift selective Goldman-Shen experiment with 13C detection to study PS decorated by a hydroxylated monomer (1.5mo1%) and poly(butyl methacrylate) (PS(OH)/PBMA). The aliphatic peak was selectively inverted and after a certain spin-diffusion time, the spectrum was taken (Fig. 10.20). It took 40 ms to achieve internal spin equilibration indicating nonideal mixing on a molecular level. Furthermore, the spin-diffusion time dependence of the peak intensities was quantitatively analyzed. Apparently the above-mentioned
390
ATSUSHI ASANO AND K. TAKEGOSHI
I
8
.65
Bc i i t i i i i i i i i i i i i I t i f i i i i l-t
o
50
ioo
i i i i i-
ms
Fig. 10.19. Measurement of the interpolymer spin-diffusion time constant for PS/PVME cast from toluene (BT) and chloroform (Bc), respectively. The deviation of the methine/methoxy line intensity of PVME from the equilibrium value obtained for 'I'm >~> T1 is plotted in arbitrary units vs. ~'m after selective inversion of the aromatic PS line. (Reprinted with permission from Ref. [119]. 9 1986 American Chemical Society, Washington, DC.)
models (Models A - D ) showing discrete phases with sharp boundaries are too idealized and a more realistic description of heterogeneity was attempted. 10.3.1.2 High resolution ~SC NMR experiments High resolution lSC NMR enabled us to study relaxation/spin-diffusion behaviors for the component polymers separately. This makes it possible to pick up one of the models for a blend less ambiguously as compared to the low resolution direct ~H observation. Nowadays, most NMR researchers employ high resolution lSC NMR to study heterogeneity in blends. For example, Fig. 10.21 shows 1H T~p decays for the blend of acrylonitrile/methyl acrylate/butadiene terpolymer (B210) with PC [150]. Two decays were separately observed via well-resolved lSC peaks for the individual component polymers by using high resolution ~SC NMR techniques. In the figure, the long Tip component for B210 and the short one for PC shown by solid lines represent T~p of pure B210 (15.7 ms) and PC (3.8 ms), respectively.
391
POLYMER BLENDS AND MISCIBILITY
xl
_//At
/1
x12
.-/ /
/
xlO
.,I" /
/
x5
/
xl.3
.J
t .....
1 .....
10
A! /
"..~
Mo
/ 'x.
40 ms
k._,
7 ms
/\ \ / \
5oov
~
50 vs
I
1
1
5
0
-5
__1
PPM
Fig. 10.20. Spin-diffusion spectra taken after the indicated spin-diffusion times for PS(OH)/PBMA -60/40; the Mo spectrum is given as a reference lineshape corresponding to full internal spin equilibrium. (Reprinted with permission from Ref. [116]. 9 1992 American Chemical Society, Washington, DC.)
Furthermore, the Tlo decays for component polymers show o n e Tip (---8 ms) component in common. These clearly show that a miscible phase is present, along with pure phase of both polymers (Model B). This experiment corresponds to the observation of the three Tlo components by the direct 1H experiments [118, 119]. Instead of three exponential functions, high resolution 13C NMR has enabled them to use two exponential functions for two separately observed decays from the component polymers. A two-step decay
392
ATSUSHI ASANO AND K. TAKEGOSHI
7.0[
6.0
Tip = 15.7 ms .
5.0 o I---I -
I--4 f... m
4.0
3.0m
2.6
0.0
!
I
4.0
I
I
8.0
,1
1
12.0
,!
ixt
16.0
1
I
20.0
22.0
T (ms)
Fig. 10.21. 1H Tip intensity data for B210 ( 9 ([-3)= 50/50. (Reprinted with permission from Ref. [150]. 9 1991 Butterworth-Heinemann, UK.)
for each component polymer was also observed as a result of the modified Goldman-Shen spin-diffusion experiment with 13C detection on PS/PPO [70]. In this experiment, the XH magnetization of PPO was stored and that of PS was created by spin diffusion. The first step was attributed to spin diffusion between PS and PPO in the intimately mixed range, and the second attributed to that between the PS-rich and PPO-rich phases according to Model B. Model C is mostly found for a blend of crystalline and amorphous polymers. In general, the miscibility for the crystalline/amorphous blends would be better in an amorphous component-rich system than that of a crystallinerich system. For example, when the crystalline PEO composition is more than 60 wt% in PEO/amorphous PVPh, PEO in the blend showed two T~p relaxation times (Table 10.2) [34]. One of the two Txf, agrees well with T~p
393
POLYMER BLENDS AND MISCIBILITY Table 10.2.
1H
Tip values (ms) of PVPh/PEO at 310 K PVPh C-4
PVPh C-5
PVPh C-1,2
PVPh PVPh/PEO = 73/27 PVPh/PEO = 58/42 PVPh/PEO = 40/60
7.1 3.1 1.3 1.4
7.2 3.2 1.2 1.4
7.2 3.2 1.5 1.2
PVPh/PEO = 31/69
1.4
1.3
1.9
PEO C-1 3.0 1.1 1.1 0.18 1.0 0.19 5.4 0.16
PEO
[34]. 9
Reprinted with permission from Ref. DC.
2.0
\
I
I
(%)
(75%) (25%)
(57%) (43%) (43%) (57%)
1992 American Chemical Society, Washington,
I
I
I
I
,,
I
~h
1.5 'h\
2-
-40/60
1.0
,, '8
>
0.5
i PBZT (MP) o PBZT (PLS)
\
O 0.
O o
,.~
""
S
-0.5
0
NYLON (MP)
x NYLON (PLS)
~,
,'Y" X
-1.0
+
\~ % ~O
I r<
NYLON/PBZT
ch
+
x
u
x
:~
o
A
x
+~
O
A
~x--
-4-
+ i
.. 3
,
! 6
,
tl/2 (ms 1/2 )
./
9
,
Fig. 10.22. Plots of normalized deviations from the sample-averaged proton polarizations (per spin) for nylon and PBZT protons in nylon/PBZT-40/60. MP refers to the multiple-pulse version of the spin-diffusion experiment (the modified chemical-shift selective Goldman-Shen experiment); PLS indicates the proton lineshape version (the T2-selective Goldman-Shen experiments with 13C detection). (Reprinted with permission from Ref. [82]. 9 1990 Huthig & Wepf Verlag.)
394
ATSUSHI ASANO AND K. TAKEGOSHI
of PVPh, while the other shorter one agrees with Tip of the crystalline phase of pure PEO. This clearly shows that the former Tip is from the miscible phase and the other one is from the crystalline phase of PEO, which is not disturbed by blending. The modified chemical-shift selective and T2-selective Goldman-Shen experiments with 13C detection were applied to a blend of nylon/PBZT = 40/60 [82]. In both experiments, the spins do not come to internal equilibrium quickly following their initial surge towards equilibrium (Fig. 10.22). It took 140 ms for the whole 1H spin system to recover internal equilibrium. A minimum domain dimension of --~4 nm with some scattered larger crystals of nylon was suggested. For a crystalline/crystalline blend, Yoshie et al. [151] studied blends of PVA and poly(3-hydroxybutyrate) (PHB). They found that PVA/PHB is compatible only when the blend contains a larger amount of PVA, and Model C was found with amorphous and crystalline PHB. Kwak et al. [94] studied poly(ether-ester)/PVC to find a c o m m o n T1, but double-exponential Tip decays. Model B was proposed with a mixed amorphous phase and two microcrystalline phases for component polymers. Note that Guo [95] reexamined this blend and pointed out that these assignments have to be reconsidered. The difference of relaxation times in different domains makes it possible to observe the 13C spectrum of one of the domains. Figure 10.23(a), shows the 13C Tl-selected spectrum of PVPh/PEO = 40/60 [34]. Since the 13C T1 of crystalline PEO (---15 s) is much longer than that of the amorphous phase (--~0.1 s), it is possible to observe the 13C spectrum of crystalline PEO selectively (indicated by arrow in Fig. 10.23(a)). On the other hand, for the miscible PVPh-rich blend (PVPh/PEO = 58/42), the crystalline-PEO peak is not appreciable. This is in agreement with the above-mentioned Tip results (Table 10.2). The 13C signals of mobile domains/component polymers can be observed selectively by utilizing the weaker dipolar interaction between 1H. To name a few examples, the dipolar dephasing [128,131,152], the crosspolarization-depolarization [152] and the pulse saturation transfer [151] techniques have been applied. 10.3.1.3 Triple-resonanceexperiments Characterization of the interfacial regions is important to understand the mechanical properties of incompatible polymer blends. As shown, in many heterogeneous blends, the simplifying assumption of the neglect of spin diffusion between domains is reconcilable with NMR observations. In other words, most of the NMR observables are not sensitive enough to appreciate the influences of the other domains. However, it is also true that the spins are interacting with each other via the interface. To study such interactions,
POLYMER BLENDS AND MISCIBILITY
o)
395
90Ox
13 C
CT
PVPh/PEO
",1 1.5s
90~
ACQ ,
,,
90~
b)
c) 4
PPH
200
150
100
50
0
Fig. 10.23. (a) Pulse sequence for selective measurement of the crystalline domain of PEO,
and the resulting 13C CP/MAS spectra for (b) PVPh/PEO = 58/42 and (c) 40/60. The signal from the crystalline PEO is indicated by an arrow. (Reprinted with permission from Ref. [34]. 9 1992 American Chemical Society, Washington, DC.) selective observation of the spins at the interface is required. Due to the low sensitivity of NMR, one should enhance the signals from the interfacial regions. To achieve such enhancement, electron spins were introduced to one of the component polymers either by doping a radical containing molecule to one component polymer [142], or the component polymer itself carries the unpaired electrons [141]. An enhanced signal-to-noise ratio was realized by transferring the electron polarization to the ~H spins of the other component polymer at the interfacial region, and was observed directly [141], or further transferred to the ~3C spins for better resolution [142]. Even though the signal enhancement achieved was much less than that expected from a simple theory [141], Afeworki et al. [142] could observe the aromatic carbon signals of 13Cenriched PC at the interface of PC/PS (Fig. 10.24). It was shown that the interfacial PC chains have less motion than PC chains in the bulk. To avoid leakage of polarization from interface by the fast 1H spin diffusion, they
396
ATSUSHI ASANO AND K. TAKEGOSHI DNP of pC(13C)I p S ( 1 2 C / , ) d/[/'e r e n c e
/--second )./j
or,,~oocod PS
mtc-rowove
200
100
0 PPM
c~ Fig. 10.24. 15.1 MHz CPIMAS 13C NMR spectra of PC(~3C)IPS(12CI*) with (bottom) and without (middle) 1.0 s microwave irradiation. The difference spectrum (top) has a po]ycarbonate contribution (6r 120) arising from PC chains at the interface. (Reprinted with permission from Ref. [142]. 9 1992 American Chemical Society, Washington, DC.)
attempted to transfer the electron polarization in the PS domain directly to the 13C of PC. If one of the component polymers has a third spin other than 13C and 1H, several cross-relaxation experiments become possible. Since spin diffusion occurs a few 10 nm before the polarization decays, only a small amount of the spins near the interface must be detected for immiscible blends. Section 10.2.3.4 describes the application of interpolymer cross-polarization (CP) from 1H in one component to :H of another deuterated component [133,134] to study miscibility. This method was also applied to investigate interphases. Zumbulyadis et al. [135] examined the 1H to 2H CP transfer in symmetric poly(styrene-block-methyl methacrylate) diblock copolymers P(S-b-MMA) to investigate the interphase region. The width of the interphase was estimated by comparing the 2H intensity obtained by applying a single ~r/2 pulse and that by CP. The former intensity is proportional to the total 2H
POLYMER BLENDS AND MISCIBILITY
397
Fig. 10.25. Cross-polarization pulse sequences with IH and 19F dipolar decoupling during
data acquisition: (a) 19F--13C cross-polarization; (b) 19FmlH, followed by IHmI3c crosspolarization; and (c) 1H~I9F cross-depolarization, followed by 1H~13C cross-polarization. (Reprinted with permission from Ref. [137]. 9 1991 American Institute of Physics, New York.) concentration in the sample, while the latter is a measure of the number of deuterons in contact with protons. The interpolymer CP from IH of a protonated polymer to 13C of a deuterated polymer was applied to investigate the structure of the interfacial regions [128]. A similar interpolymer CP experiment involving 19F was performed for PMMA/PVF2 [137, 138]. Figure 10.25 shows three cross-polarization schemes applied to examine its microstructure: (a) direct cross-polarization from 19F to 13C; (b) 13C magnetizations are created by contacting XH, whose magnetizations are created by contacting 19F; and (c) a part of the 1H polarization is transferred to 19F and the rest of the IH magnetization is used to
398
ATSUSHI ASANO AND K. TAKEGOSHI
polarize I3C. The first experiment is to observe PMMA in close proximity to PVF2. The second and third experiments are to determine the amount of well-mixed domain. By analyzing the contact-time dependence, the authors conclude that the blend consists of four phases: (I) one isolated phase of PVF2; (II) one mixed phase; and (III, IV) two phases for PMMA. Basically the blend is Model B, but two-spin dynamically different areas are present for the PMMA phase. One PMMA phase (III) is close to a mixed PMMA/PVF2 phase whose local magnetization density is influenced by 1H in the mixed phase, and the other PMMA phase (IV) is an isolated PMMA phase, and its magnetization is not affected by 19F. The size of the phase II was estimated to be --~0.6 nm and that of phase III to be --~0.6 nm. Tong et al. [153] observed interracial PC in PC/poly(p-fluorostyrene-costyrene) (PC/PFS-S) by applying the rotational-echo double-resonance (REDOR) technique. They labeled 13C of the carbonyl carbon in PC and a a3c-observe, a9F-dephase R E D O R was performed as following: two spectra taken with/without 19F ~r-pulses were subtracted to give the 13C difference spectrum raised only from I3C n e a r 19F. The unwanted PFS-S signals were removed by using 13C-depleted PFS-S. Interference from natural-abundance background signals was eliminated by a second subtraction using the R E D O R difference signal from an unlabeled PC blended with the same PFS-S (Fig. 10.26). 10.3.1.4 NMR imaging For a completely phase-separated blend, the domain size becomes too large as compared to the characteristic scale of observation of the spin-diffusion experiments (1-50 nm). Therefore, one needs an experiment with a much larger scale to investigate the morphology of a phase-separated blend. For example, TEM can visualize the shape of the domain ranging from 10-1000 nm. In NMR, it is possible to obtain a spatial image of the 1H distribution by NMR imaging techniques, which, in fact, have been used in clinical medicine to obtain the morphology of humans by locating water. In solids, however, the line-broadening due to the dipole interaction and the chemical shift interaction blurs the NMR image of solid materials. Therefore, application of the techniques used in medical imaging has been limited for highly mobile species in solids. Even for such materials, certain line-narrowing techniques such as multiple pulse and MAS or a large field gradient should be applied. Cory et al. [154] employed MAS for line narrowing of PB in PB/PS. The ~H linewidth was reduced from 1500 Hz for a static sample to 100 Hz under MAS of 5 kHz. By applying a field gradient of 8 kHz/mm (---18.8 G/cm), the NMR image of PB was obtained with a spatial resolution of 50 ~m. Sarkar and Komoroski [155] used a strong field gradient of 20 G/cm. To apply conventional NMR imaging techniques, actively shielded
399
POLYMER BLENDS AND MISCIBILITY
PC (~3C)/PFS-PS /ntef [oc/o/ po/,vc orbonote
l
REDOR
lobe/
~~.~.~/~~,~~,,~,fl,~
difference
AS (x5)v~.
~
1
~'~Y~~ ~
oturo/, obundonce
~
---/
spectro
sub/rooted
rolor cycles
full
echo SO
I00
!
i
i
i
3oo
200
~oo
o
'"
ppm
Fig. 10.26. Carbon-observe, fluorine-dephase R E D O R NMR spectra of a heterogeneous blend of [carbonyl-13C]polycarbonate and poly(p-fluorostyrene-co-styrene) as a function of the number of rotor cycles of 5 kHz magic-angle spinning. The natural-abundance background has been subtracted from both the top and bottom sets of spectra. The REDOR difference (top) arises exclusively from the polycarbonate carbons at the interface. (Reprinted with permission from Ref. [153]. 9 1995 American Chemical Society, Washington, DC.)
gradients capable of fast switching (50-100 I~S) have to be used. They could obtain images of 1H spins with T2 values of a few I~S, and images of elastomeric components of several tire sections with a resolution of 100-200 I~m were obtained. Recently, a much stronger field gradient of 137 G/cm was applied to obtain image of ~H with T2 of 12 I~S in polyethylene-ethylenepropylene/rubber [156]. Several multiple-pulse sequences have also been proposed to remove aH homonuclear dipolar interactions to obtain an image of rigid materials. Recent developments are summarized in Ref. [157]. 10.3.2
Thermally induced processes
As briefly reviewed in Section 10.2.1, a polymer blend is thermodynamically unstable. Thus, its thermal history is an important factor in the control of miscibility. To phrase this differently, heat treatment is a simple technique
400
ATSUSHI ASANO AND K. TAKEGOSHI
to modify the microstructure of a blend. Several processes take place upon heat treatment, that is, phase-separation, recrystallization, homogenization, degradation and reaction. In the following, NMR studies of these phenomena are reviewed. 10.3.2.1 Recrystallization, homogenization, reaction and degradation In a homogeneous crystalline/amorphous blend, the crystalline phase is destroyed to be amorphous by blending. By annealing it, one would expect recrystallization of a crystalline component polymer. Annealing/ageing of PMMA/PVF2 has been studied extensively. Grinsted and Koenig [158] observed an increase of Tip and the cross-relaxation time between ~H and 13C with ageing of PMMA/PVF2, indicating that a subtle separation in the amorphous phase occurs. By analyzing the double exponential T~p decays of PMMA/ PVF2, T6k61y et al. [159] determined the degree of crystallinity as a function of the annealing time. From the Tip values, they suggest that the crystalline phase is mainly built-up of nuclei and lamellae of small dimensions. Papavoine et al. [139] applied a triple-resonance 1 H ~ 1 3 C ~ 1 9 F CP technique to PMMA/PVF2. They compared the crystalline fraction obtained by DSC and the isolated fraction of PVF2 measured by NMR as a function of the annealing time. They observed different crystallization behaviors depending on annealing temperatures, and suggest an upper critical solution temperature (UCST) for PMMA/PVF2. PET is another well-known crystalline polymer. By examining the T1 values for the annealed bisphenol A polycarbonate (BPAPC)/PET, Henrichs et al. [121] showed that BPAPC and PET are not inherently miscible, at least at 260~ and both polymers are amorphous. When heating above the melting point of PET, followed by slow cooling, crystalline PET with a slowly relaxing T~p appears. Further heating and cooling cycles resulted in degradation of the size of the PET crystals leading to molecular mixing with BPAPC. Similar homogenization upon annealing was also found for PET/Vectra-A [103]. Both works used 13C--13C two-dimensional exchange NMR techniques to show miscibility. In some polymers, the crystalline phase shows a different ~3C lineshape as compared to that of amorphous. For these, there is a chance to observe linesbape changes due to recrystallization. In PET, the 13C resonance of the methylene carbon in the trans-conformation is narrower than that in the gauche-conformation, and appears at lower frequency. Thus, it is possible to determine the relative amount of these conformers. Upon annealing PET/ Vectra-A, Tang et al. [103] found that the relative amount of the gaucheconformer is reduced greatly, and its linewidth becomes narrower. Another example can be found for Nylon6. The recrystallization of the a-crystalline
401
POLYMER BLENDS AND MISCIBILITY
(a)
2
5
m
s
~
10ms~ . . _ ~ . . , _ , ~ , , ~
2
250 200 150 100 50 0 ppm from TMS
m
s
~
-50 -100 250 200 150 100 50 0 ppm from TMS
-50 -100
Fig. 10.27. 13C CP/MAS spectra taken at several contact times of deuterated-PS/PVME = 50/50 at 240 K. Spectra (a-d) are a mechanical mixture before heating and (e-h) are after heating to 403 K for 30 min. The additional resonances in the aromatic region are due to carbons of d8-PS which have been polarized from the protons of PVME. (Reprinted with permission from Ref. [130]. 9 1987 John Wiley, New York.)
phase of Nylon6 in Nylon6/PS-PSSA was studied by monitoring the characteristic peak of the crystalline phase [90]. As exemplified in BPAPC/PET, annealing sometimes helps mixing of immiscible blends (homogenization). Figure 10.27 shows the ~3C CP/MAS NMR spectra of mechanically mixed d8-PS/PVME with (e-h) and without (a-d) heating at 403 K for 30 min [130]. At the shortest CP time of 0.5 ms (d), the 13C signal intensity of the aromatic region (128 ppm) of d8-PS is not appreciable for the blend without heating. At longer CP contact times (ac), the aromatic resonance appears from domains where interracial mixing has occurred between PVME. However, for the blend with heating the aromatic signal appears at even the shortest CP contact time of 0.5 ms (h). This appreciable signal enhancement for d8-PS in the heat-treatment blend is evidence of miscibility of less than 2 nm. Gobbi et al. [130] concluded that the mechanically mixed blend becomes homogeneous on a segmental level when the blend is heated above Tg but below LCST. Xie et al. [160] found that poly{4'-[[2-(acryloyloxy)ethyl]-ethylamiro]-4-nitroazobenzene}/PMMA shows two different Tip values, but after heating the blend 5~ above its Tg for 1 h, a single Tap value was achieved.
402
ATSUSHI A S A N O AND K. T A K E G O S H I
Other effects of heat treatment are reaction and degradation. In fact, the above-mentioned homogenization of BPAPC/PET occurs as a result of chemical reactions that include the partial loss of the BPAPC carbonyl and a loss of symmetry of the ethylene glycol moiety [121]. For PC/PET, the chemical structures of copolymers formed in the transesterification during melt processing were studied [161]. Similarly, by examining 13C spectra, Velden et al. [162] concluded that transesterification occurred in PC/poly(butylene terephthalate) (PC/PBT) at 270~ These works indicate a suitable arrangement for interpolymer reaction of dissimilar polymers in a blend. With the hope of observing such interpolymer reactions between PVA and PAA, Zhang et al. [162] undertook heat treatment of PVA/PAA. The dehydration products of PAA are similar to those from homopolymers, but the reaction temperatures for the blends are --~100 K lower than those for homopolymers. This was ascribed to the loss of the crystalline phase of PVA and a lowering of Tg of the component polymers in the miscible PVA/PAA. 10.3.2.2 Phase separation As shown above, and naively expected, heat treatment tends to cause heterogeneity in a miscible blend. For example, in miscible PVA/PAA, at higher temperatures, the free-volume difference becomes so large to overcome the hydrogen-bonding interaction, and a phase separation occurs (a LCST phase diagram). Since the amount of the high frequency shift is roughly proportional to the strength of hydrogen bonding, it is possible to monitor phase separation of theblend by observing the spectra. Zhang et al. [163] showed that when heating PVA/PAA above its Tg, the chemical shift of the carboxyl carbon of PAA showed a low frequency shift. This shift is attributed to dissociation of hydrogen bonding between PVA and PAA. An irreversible phase separation in PMMA/PVAc after heating at temperatures higher than Tg of PVAc was observed by Tip spin-diffusion experiments [148]. Grinsted and Koenig [158] examined the ~H~13C CP rate of PMMA/PVF2 on blending and ageing. The observed decrease of the CP rate with ageing was explained by phase separation. Two mechanisms were proposed to phase separation: spinodal decomposition and nucleation growth. Figure 10.28 shows a schematic drawing of a phase diagram showing LCST. In this phase diagram, a blend with composition 4~o at temperature To is miscible. Suppose we heat it at temperature T above the spinodal curve. At T, the blend is unstable, and phase separation (shown as a broken horizontal line in Fig. 10.28) referred to as spinodal decomposition, takes place until the thermal equilibrium is achieved at coexistent compositions ~ba and ~bb. On the other hand, nucleation and growth occurs when the system is in the metastable region (i.e., the region comprised
403
P O L Y M E R BLENDS AND MISCIBILITY spinodal
coexistence
curve
curve /
2 Phases l .
g~ +~
J
' _,,/__
/ / .....
*,
r
I I i I I I
I
'
,
/
i
g~ G) C~
/
'
i, , ,
,, /
], /
/
,
/
/
I:
'/i /
/
/
I
:I
'i
it
,
rs I , I , i
1 Phase
i
-
i i ! ! i
~a
~c
~0
~b
B1 end Compos i t i on Fig. 10.28. Schematic drawing of a LCST phase diagram.
between the spinodal and the coexistence (binodal) curves). In nucleation growth, a new phase-separated phase starts from nuclei, whose composition is one of the coexistent compositions and does not change during the process. The growth of nuclei leads to the gradual change of the composition of the rest of the blend. In both cases, a phase-separated blend consists of two phases with coexistent compositions 4)~ and 4)b, which is schematically shown as Model D in Fig. 10.18. The theory of spinodal decomposition was first examined by Cahn [164]. It predicts the exponential growth of sinusoidal composition modulations at a fixed wavelength A. The size of each phase may be given by A written as [165], -1/2
(10.8) S
404
ATSUSHI ASANO AND K. TAKEGOSHI
where T and Ts denote the heat-treatment temperature and the spinodal temperature, respectively (Fig. 10.28), and l represents the Debye range of the interpolymer interaction [166]. Nishi et al. obtained l = 58 nm and Ts = 118~ for PS/PVME = 50/50 using microscopy [167]. If T = 140~ then A = 900 nm is obtained, which is much larger than the minimum domain size discriminated by Ta and Tip relaxation experiment. Therefore, spin diffusion between the two phase-separated phases can be neglected. Note that we assume fast spin diffusion within each phase in the phaseseparated blend (Model D in Fig. 10.18), i.e., each phase is homogeneously mixed from the 1H spin-diffusion point of view. In other words, one Tip or T1 value is associated with one phase. As shown in Section 10.3.1, such heterogeneity of a blend manifests itself as multiexponential relaxation decay curves. For Model D, we expect a double-exponential decay for the respective 1H spins of polymers A and B. For spin-locking Tip experiment, the two double-exponential decay curves are given for the two component polymers as
MA(r) = SeAexp(-r/T x)
+ (1 -
~A)exp(-~'/ToV),
MB(r) = sOBexp(-r/T x) + (1 - s%)exp(-~'/ToV),
(10.9)
where T x and Tov denote the Tip values of phases X and Y, respectively, and ~i denotes the 1H molar fraction of polymer-/(i = A or B) in the phase X. It is straightforward to deduce the stoichiometry of the two separated phases from the fraction of the two exponential functions. Chu et al. [96] heated PS/PVME above 139~ for 30 min and quenched it at 0~ ~H Tlo relaxation values of component polymers were observed through well-resolved 13C peaks. By comparing these values to T~p of nonheated blends at various composition determined experimentally, they estimated the coexistent compositions at various heat-treatment temperatures and a LCST phase diagram was obtained. Asano et al. analyzed relaxation curves of PC/PMMA [92] and PS/PVME [52] by using Equation (10.9). 1H T1 values of PC and PMMA in PC/PMMA = 50/50 agree well to show its miscibility on a scale of ---20 nm. They observed relaxation curves after heat treatment for 30 min at various temperatures. When heated above 150~ the relaxation curve becomes nonsingle exponential, and they attributed this temperature of 150~ to the phase-separation temperature of PC/PMMA = 50/50. The relaxation curves were fitted to Equation (10.9) to deduce the coexistent compositions at various temperatures to give a LCST phase diagram. They further studied kinetics of phase separation, which will be discussed in the following.
POLYMER BLENDS AND MISCIBILITY
405
It takes a finite time for the process to be completed: the composition ~bo slowly changes to the coexistent compositions 4~a and ~bb. The process is illustrated by the horizontal arrow in Fig. 10.28. Kinetics of phase separation in a blend has recently gained considerable interest from both a theoretical and experimental point of view. Several experimental examinations were carried out to determine the concentration fluctuation during the phaseseparation process taking PS/PVME as a model, which exhibits a LCST phase diagram. For example, Larbi et al. [168] observed fluorescence emission of anthracene-labeled PS in PS/PVME to investigate kinetics of both spinodal decomposition and nucleation growth. The time required to complete the process depends mainly on the molecular weight and a heat-treatment temperature. In general, it is difficult to estimate the exact time, but in many cases, we find conditions at which the process takes more than 10 min. Therefore, we have a good chance to monitor the process stroboscopically by quenching the blend at various heattreatment times. Analysis of the relaxation curves at various heat-treatment times would give us the composition during the process. In fact, from the beginning of the application of NMR to blends, Nishi et al. demonstrated such an approach to give direct proof of spinodal decomposition [54, 167]. They observed that a single exponential T1 decay of PS/PVME becomes double exponential when heated at 130~ The fraction of the two relaxations does change gradually with a phase-separation period, leading them to conclude a spinodal decomposition. Since they observed XH directly, the two double exponential decay curves for PS and PVME overlap, and are difficult to analyze fully by using the two-phase model. VanderHart et al. applied a high resolution 1H observation using the multiple-pulse methods and MAS to monitor spin diffusion in annealed PEI/PBI [109, 115]. This blend is miscible on a scale of 25 nm, but upon heating it for 1 h at 310~ phase separation was detected. The minimum domain dimensions and stoichiometric information were obtained during annealing. The overlap of 1H relaxations can be overcome by extending the approach using high resolution 13C techniques. Asano et al. [52, 92] showed that stoichiometries of both major and minor components in a phase-separated domain can be obtained by analyzing two 1H T1 decay curves, selectively observed for component polymers via ~3C. They determined the compositional change during a spinodal decomposition of PC/PMMA [92] and PS/PVME [52]. Figure 10.29 shows the observed 7"1o decay curves of PS/PVME = 50/50 after heat treatment at 140~ for (a) 1 min and (b) 2 min. Before heat treatment, the Tip decay curves were singly exponential, and with increasing the heat-treatment time, they deviated more from single exponential to show double exponential behavior. These curves were
406
ATSUSHI ASANO AND K. TAKEGOSHI 1.0 0.9 0.8 0.7 0.6 0.5 0.4 N Z0.3 "-'0.2 N Z
I_ o' l 0.0
1
5.0
I
10.0
,
,, I
15.0
I
20.0
,
~/ms 1.0 0.g 0.8 0.7 0.6
et_
0.5 0.4 N ~" 0 . 3 "0.2 N Z
0.1
0.0
5.0
10.0
15.0
20.0
•:/ms
Fig. 10.29. Observed 1H Tlo decay curves for PS (O) and PVME (x) in PS/PVME = 50/50 after (a) 1 min and (b) 2 min of heating at 140~ These decay curves were obtained at -10~ The solid lines are the "best-fit" curves from Equation (10.9). (Reprinted with permission from
Ref. [52]. 9 1994 Butterworth-Heinemann, UK.)
analyzed by fitting to Equation (10.9) and compositions at various heattreatment times were obtained (Fig. 10.30). Figure 10.30 shows that after 30min of heat treatment, further treatment does not cause appreciable changes in the Tip c u r v e s . This shows that after a completion of the initial fluctuation in concentration, the morphological change occurs on a larger scale that can be reflected in Tlo.
407
POLYMER BLENDS AND MISCIBILITY O- 0
IIII
m
.
i
m
0 f---q
-
C/3
--
o.5
0.4
~ 9 ~0.5 "
i,
i
--
0.6
k
1.0 0
I
1
,
,
I,, ss
2
60
Heat-treatment time / min Fig. 10.30. Compositional changes occurring during the phase separation of PS/PVME = 50/50 at 140~ (O) PS in the PVME-rich domain; (1) PS in the PS-rich domain; ([]) fraction of the PVME-rich domain r. The solid lines are for guidance only. (Reprinted with permission from Ref. [52]. 9 1994 Butterworth-Heinemann, UK.)
129Xe NMR was also applied to monitor the phase-separation process of PB/PIP [44] and PS/PVME [45]. For a phase-separated, two-component blend, the 129Xe NMR spectrum exhibits two resonances, whereas the homogeneous morphology of a miscible blend leads to a single peak. Figure 10.31 shows the spectra of 129Xe dissolved in PS/PVME = 50/50 heat-treated at 140~ for 0-10min. Phase separation produces two peaks, because the characteristic wavelength of the compositional fluctuation a --~ 900 nm is much larger than the upper limit of domain size that gives a single peak (90 nm [45] to 600 nm [49]). The high- and low-frequency peaks were assigned to the PS-rich and PVME-rich phases, respectively. The compositional change in each phase at various heat-treatment times was obtained from the composition-dependent chemical shifts (Fig. 10.3). So far, the initial stage of phase separation has been considered. After the coexistent compositions are achieved, the domains with the characteristic wavelength begin to merge to form larger domains with increasing the heat-
408
ATSUSHI ASANO AND K. TAKEGOSHI
(f)
'i
280
'
I
240
'
I
200
'
I
16f
Chemical shift/ppm Fig. 10.31. NMR spectra of 129Xe dissolved in PS/PVME = 50/50 heat-treated at 140~ for various periods: (a) no heating; (b) 1 min; (c) 2 min; (d) 3 min; (e) 5 min; and (f) 10min. (Reprinted with permission from Ref. [45]. 9 1997 Butterworth-Heinemann, UK.)
treatment time [169]. This later process does not affect either the lineshape o f 129Xe NMR or the 1H relaxation curve. The increase of the domain size at the later stage for PS/PVME was examined by the 2D aZ9Xe exchange NMR [45]. For the blend heated for 30 min at 140~ apparent cross-peaks between the two peaks of 129Xe in the PS-rich and PVME-rich phases were observed with a mixing time of 1 s. However, after heat treatment for 1200 min, no apparent cross-peaks were observed. This clearly shows that the phase-separated domains get larger with increasing the heat-treatment time. In addition to the increase of domain size, homogenization of microheterogeneity in the phase-separated domains was detected by using 2H NMR
POLYMER BLENDS AND MISCIBILITY
409
[170]. The characteristic time constant for the homogenization was estimated to be 300 min.
10.4
Summary
We have seen that miscibility, morphology and molecular motion in a blend can be appreciated using various NMR observables, such as chemical shifts and relaxation times; if any of these change on blending, there should be a microscopically mixed region in the blend. The resolution of all NMR spectrum and the shape of a relaxation decay are not, however, good enough to give a very detailed description of the microstructure of the blend. Thus, the picture deduced from NMR is inevitably blurred (Fig. 10.18). Fortunately, many macroscopic properties, which we want to appreciate, are coarse grained ones. Hence, the information given by NMR is valuable in understanding the properties of the blend. We are witnessing its success and ever-growing number of publications. This is so because most of the other spectroscopic methods cannot give even a coarse grained picture of the microstructure of a blend. These comments may also be applied to NMR studies on the other solid polymer systems described in this book.
References
.
3. 4. 5.
,
8. 9. 10. 11. 12.
13. 14.
D.R. Paul and S. Newmann (Eds.), Polymer Blends, vols. 1 and 2. Academic Press, New York, 1979. W.S. Veeman and W.E.J.R. Maas, NMR Basic Principles and Progress 32 (1994) 127. K. Takegoshi, Annu. Rep. NMR Spectrosc. 30 (1995) 97. R.A. Dickie, Ref. [1], vol. 1, pp. 353. O. Olabisi, L.M. Robeson and M.T. Shaw, Polymer-Polymer Miscibility. Academic Press, New York, 1979. L.C. Dickinson, H. Yang, C.-W. Chu, R.S. Stein and J.C.W. Chien, Macromolecules 20 (1987) 1757. D.S. Kaplan, J. Appl. Polym. Sci. 20 (1976) 2615. M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. M.L. Huggins, J. Chem. Phys. 9 (1941), 440. P.J. Flory, J. Chem. Phys. 9 (1941), 660; J. Am. Chem. Soc. 87 (1965) 87, 1833. D. Patterson, Macromolecules 2 (1969) 672. For example: (a) Ref. [1]; (b) Ref. [5]; and (c) L.A. Utracki, Polymer Alloys and Blends, Thermodynamics and Rheology. Carl Hanser Verlag, Mtinich. 1989; (d) R. Koningsveld and L.A. Kleintjens, J. Polym. Sci., Polym. Symp. 61 (1977) 221; (e) D. Patterson, Polym. Eng. Sci. 22 (1982) 64. A. Natansohn and A. Simmons, Macromolecules 22 (1989) 4426. M.C. Piton and A. Natansohn, Macromolecules 28 (1995) 1598.
410
ATSUSHI ASANO AND K. TAKEGOSHI
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Chapter I1
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Polyolefins Akira Aoki I and Tetsuo Asakura 2 1Plastic Laboratory, Tokuyama Co., Harumicho, Tokuyama, Japan; 2Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan
11.1
Introduction
Since the discovery of olefin polymerization using the Ziegler-Natta catalyst, polyolefin has become one of the most important polymers produced industrially. In particular, polyethylene, polypropylene and ethylene-propylene copolymers have been widely used as commercial products. High resolution solution NMR has become the most powerful analytical method used to investigate the microstructures of these polymers. It is well known that the tacticity and comonomer sequence distribution are important factors for determining the mechanical properties of these copolymers. Furthermore, information on polymer microstructures from the analysis of solution NMR has added to an understanding of the mechanism of polymerization. On the other hand, high resolution solid-state NMR has been available to analyze solid structures for approximately 20 years. This is a very powerful method to analyze the structures of solids, and can reveal conformation, crystallographic forms and the morphological character of solid without special sample preparations. It is expected that the information obtained from solid-state NMR will show how the mechanical properties of the solid arise.
11.2
Isotactic polypropylene
Solid-state NMR spectroscopy has been available for the analysis of polymorphs for isotactic polypropylene (i-PP) for approximately 15 years [1-3]. By different sample preparation (e.g,, the method of crystallization), i-PP forms c~, /3 and smectic forms. Figure 11.1 shows the 13C CP/MAS NMR spectra of these three forms of i-PP at 20~ In Fig. l l . l ( a ) , which shows the a-form, both methylene and methyl carbon resonances are splitted by approximately 1 ppm [2, 3]. The ratio of intensities of the high to the low frequency component is 2:1 for both carbon
416
A K I R A AOKI AND TETSUO A S A K U R A
a
CH
CH 3
CH 2
b
_
.
.
-
-
_
c Smectic
I ....
30
I "--" '-'--I -~-m
25
20
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15
1 """'"'"'1-'-'-'
10
5
' i-'
0
'
""
l-"-"
-5
"
a (opm)
Fig. 11.1. 13C CP/MAS spectra of i-PP: (a) a-form; (b) /3-form ; and (c) smectic form.
types. Bunn et al. [2] interpreted this splitting as due to the inequivalent site, A and B, produced by pairing of helixes of opposite handedness [4] (see Fig. ll.2(a)), which are also present in the ratio A : B = 2:1. The A sites correspond to a separation of 5.28 A between helical axes, while for the B sites the helixes are 6.14 ~ . Thus, these splittings depend on the mode of interchain packing in crystal. In the spectrum of the/3-form (Fig. l l . l ( b ) ) , each carbon resonance is a single peak. The chemical shifts of the methylene and methyl carbons in the /3-form are close to the low frequency part of each pair of the same resonances in the c~-form, which were attributed to the B sites [3]. Figure ll.2(b) shows the interchain packing of the/3-form of i-PP. Unlike a-form packing, all 31helixes are separated with equal intervals (6.36/~) form the other chains.
POLYOLEFINS
417
1
7
5
- -
,o..
8 i 2
"%
~--
<~
+.~I
;
+..,o
li~
6_
s
%
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a
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..6
6
,,)..,__o
,v'"
Z
o/~
"/'~
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,d
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6
4
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%'
o"0
o/e S_1oI~,
'
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j
I
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26~-, Io
o_s
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:/\
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~" 9,. 6 L ......
9"~o
o
ig.
,,
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~
,;
,
", ~...6o. o." 30
Fig. 11.2. (a) Crystal structures of a-form and (b) /3-form of i-PP. Full and open triangles indicate 31-helix i-PP chains of right- and lefthandedness, respectively. A and B label the inequivalent sites. The circles at the triangle vertices in (a) correspond to methyl carbons.
Figure 11.1(c) shows the smectic crystalline of i-PP. This spectrum is similar to that of the a-form. This suggests interchain packing in smectic crystalline is similar to that of/3-form. Recently, the peak splitting observed in the ~3C CP/MAS NMR spectrum (Fig. 11.1(a)) for the methyl and methylene carbons of a-form i-PP were interpreted tentatively in terms of the steric shielding effect proposed by Grant and Cheney [5]. They developed a steric-hindrance model that predicts the effect on the chemical shifts of the additional crowding of the C - - H bonds in the solid state. In this model, a C - - H bond is compressed, or
418
AKIRA AOKI AND TETSUO ASAKURA
C3
4
J
Fig. 11.3. The steric relationship between hydrogen atoms on 1,4-carbon atoms of n-butane.
expanded, by mutual repulsions of the bonded hydrogens and nearby nonbonded hydrogens. The chemical shift differences depend on the hydrogenhydrogen distance, r(nm), and the angle 0 between the C ~ H bond and the interhydrogen separation vector in Fig. 11.3. A13C
=
-
1680 cos 0 exp (-26.71 r).
Figure 11.4. shows the conformational dependencies of the shielding for 3 &
....
~ ~ m \
e
t // //
2
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,7--cl
-I
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o ....
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~\
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Dihedral angle ~ (degree)
Fig. 11.4. The conformational dependencies of the shielding for C1 carbon of n-butane. The solid and broken lines indicate the results of the calculation by using Grant rule and ab initio
IGLO method, respectively.
POLYOLEFINS
419
carbon of 1-butane. The solid line indicates the result of the calculation using Grants rule and the broken line indicates the result of calculations using the ab initio IGLO method [6]. The conformational dependencies calculated by using Grants method agree well with those calculated by the IGLO method. When the dihedral angle 0 is 180~ (trans-arrangement), the total shielding effect is low. On the other hand, when 0 is 60 ~ (gauche and another gauche arrangements), the total shielding is high. The difference of shielding between the trans and gauche or another gauche form is approximately 2 ppm. This shielding corresponds to the y-effect. In spite of underestimation of shielding value, this result demonstrated that chemical shift dependence on chain conformation can be evaluated by using Grants method. On the assumption that it is possible to estimate the shielding from nonbonded hydrogens in neighboring chains by this rule, interchain shielding effects were calculated by summing the shielding effect of each carbon that is marked by black ball in Fig. 11.5, from hydrogen atoms in neighboring i-PP chains. For example, the shielding effect of methyl or methylene carbons in i-PP chain 1 was calculated by summing the shielding effect from the hydrogen atoms in chains 2-5. In Fig. 11.6, peak simulations based on the shielding are shown. These
~176
O
i
"'"
~
:o
! o o, e
"""-:
9
3
~
O
4
i
~..O..._O.-. O
"'= 0
,
5
,;-
o o
! e o e
9
!
3
Fig. 11.5. The model of Crystal a-form structure to calculate shielding effect. The black balls indicate methyl and methylene carbons calculated shielding effect.
420
AKIRA AOKI AND TETSUO A S A K U R A
__j t CH2
I
'"
'
'
'
'
50
'-''
CH I'
40
'
'
'
'
'
"'
"'1"
'
'
'"'
CH3 '
"'
30
'"1
'
'
20 ppm from TMS
b)
.=__j " CH2
CH3
13C CP/MAS NMR spectra of i-PP with a-form and (b) peak simulations based on the calculated shielding for 13C NMR chemical shift in methylene and methyl carbons Fig. 11.6. (a)
regions. simulated spectra agree with experimental results. Thus, it is possible to interpret quantitatively the splitting due to the inequivalent placement of iPP molecules in the c~-form. The phase structure of i-PP isothermally crystallized from the melt was examined at various temperatures by Saito et al. [7]. Figure 11.7 shows 13C CP/MAS NMR spectra of i-PP obtained at various temperature. It is clear that the splitted peaks in the methylene and methyl carbon regions change to single peaks with increasing temperature, as a result of a high frequency shift of the low frequency lines. If the doublets truly stem from the presence of the different packing sites in the unit cell as proposed by Bunn et al. [2], such a difference may disappear at higher temperatures. In the X-ray diffraction profiles at various temperature, there is no significant change except for slight shifts of diffraction angles due to thermal expansion of the crystals. A
POLYOLEFINS
421
CI'I
CH
CFI 2
147"C
,_
:
3
J
135~
111"C
870C . .
64~
.
.
.
.
.
J
.
.
:~176176176 I
. . . .
, ....
I .....
50
I
.....
I .....
40
.
.
.
, ....
.
JkJL
.
i
I. . . .
30
ppm
, .....
1
I ....
20
t.,.~
from TMS
Fig. 11.7. 13C CP/MAS spectra of i-PP at various temperature. This sample is crystallized isothermally at 140~ spectrum.
The stick spectrum indicates the chemical shifts in solution NMR
possible explanation of this phenomenon is made by the packing effect [2] and the molecular motions in the crystalline region, which become more active at high temperature and average each carbon resonance. This molecular motion is most likely a three-fold jump rotation about the 31helical chain axis. Probably this molecular motion is related to the crystal relaxation, the so-called c~-relaxation. In addition, with increasing temperature, broad peaks appear at high frequency in the methylene and methine
422
AKIRA AOKI AND TETSUO ASAKURA
carbon regions (Fig. 11.7). This can be seen more clearly in 13C DD/MAS spectra in the same temperature range, shown in Fig. 4 in Ref. [7]. The chemical shifts of the new broad peaks are similar to those of peaks in the solution NMR spectrum, which is shown as a stick spectrum in Fig. 11.7. Consequently, these peaks are assigned to the methylene and methine carbons in the amorphous component. On the basis of 13Cspin-lattice relaxation and computer lineshape analysis, phase structures were discussed in detail. The methylene and methine resonances contain three components with different Tics, reflecting different molecular mobilities in the crystalline and amorphous regions as shown in Table I in Ref. [7]. The longest and shortest T~c components can be assigned to crystalline and amorphous components, respectively. It is expected that the intermediate Tic is due to an interfacial component. On the other hand, only two Txc values are observed for the methyl peak, probably due to the fact that two of the three values are indistinguishable. The longer T i c of the methyl peak, which is assigned to the crystalline component by the doublet lineshape, is much shorter than those of the crystalline component of the methylene and methine peaks. This is due to methyl rotation present even in the crystals as reported by Lyerla and Yannoni [8]. By the lineshape analysis for each component using the different T I c values, it was found that i-PP is composed of not only crystalline and amorphous components but also another noncrystalline component with a 31-helical conformation. Furthermore, the dependencies of the mass fractions of the three components on temperature were estimated quantitatively, as shown in Fig. 11.8. In this temperature range, the mass fraction of the crystalline components stays nearly constant. This is reasonable from the lack of melting recognized in the DSC thermograph. The noncrystalline component, which consists of a chain in 31-helical conformation, is located in the transition region between the crystalline and amorphous regions. However, such a helical chain may be transformed to a random chain at higher temperatures as seen from the change in mass fraction between noncrystalline and amorphous components. This structural change was found to be reversible, because no difference was detected in the mass fraction of each component at room temperature before and after the measurements at higher temperatures. The y-crystalline form of i-PP has been studied through best fitting of Xray diffraction powder profile [9, 10]. A novel crystal architecture was found where layers of parallel helixes two chain wide (bilayers) present a chain axis orientation with a tilt of 80 ~ with respect to the one of the adjacent bilayers. This packing can explain several experimental results and is consistent with the model proposed for the interface between parent and daughter lamellae in branching ce-form of i-PP [11, 12].
POLYOLEFINS 1.0
L
,
!
I
0c u RI M
. . . . . .
0
i"
noncrystalline
.~~(rand~ noncrystalline
~
( 31 -helix )
~4 4J
423
[. -
~---~
r,, 0.5
0,S
I/I
X crystalline
"I. "RT
6'4
1
87
I
,
111 I
1.0 135
T/"C
Fig. 11.8. Temperature dependence of the phase structure of polypropylene crystallized isothermally at 140~
Bruckner et al. [13] discussed the y-form of i-PP by using high resolution solid-state NMR, and compared it to the other crystalline forms, a and/3. In Fig. 11.9, the ~3C CP/MAS N M R spectra of the a-form (Fig. 11.9(a)) and the ,/-form (Fig. 11.9(b)) are presented. In both spectra, a clear splitting of about 1 ppm of the methyl resonances is observed. It is apparent that in the y-form spectrum the low frequency signal has roughly twice the intensity of CH3 CH (a)
+ CH=
v
SOHz ,--,
(b)
,
6 in ppm
I
~0
........
I
3O
,
I
2O
,
Fig. 11.9. 13C CP/MAS NMR spectra of i-PP recorded at room temperature (a) a-form and
(b) y-form.
424
AKIRA AOKI AND TETSUO ASAKURA
A
A
Fig. 11.10. Packing diagram of the crystal structure a -form of i-PP viewed along chain axis.
the high frequency one, thus reversing the intensity ratio observed for the same signals in the a-form. Since intramolecular effects are comparable, the differences in the CP/MAS spectra between a- and y-forms must be interpreted in terms of different packing in the two crystal structures. Figures 11.10 and 11.11 show the packing diagram of the crystal structure of a- and y-form, respectively. The similar methyls at the interface of adjacent layers belonging to the same bilayers were labeled A and the third methyl located at the periphery of the bilayers is labeled B. The presence of a 2:1 ratio of
A
A
A.... i
Q
1__ B Fig. 11.11. Packing diagram of the crystal structure y-form of i-PP viewed along one chain axis.
POLYOLEFINS
425
A and B sites can account for the 2:1 relative intensity of the two methyl resonances in the ~3C CP/MAS N M R spectra of the two phases. However, the chain packing and the interlayer spacing in the y-form is different from that in the a-form. In the a-form, an average value of 5.13 A can be attributed to distance a, and a value of 5.34 A to distance b. In the y-form an inversion occurs, with distance a being longer than distance b. This inversion is likely to be related to the corresponding inversion of the chemical shifts A and B in the two crystal forms. For atactic polypropylene (a-PP), studies still continue, because it is difficult to interpret the 13C CP/MAS N M R spectrum of a-PP, which is more broad that of isotactic or syndiotactic polypropylene. In Fig. 11.12, the 13C
CH ......
l
CH 3
70
60
I
50 L.
,,
40 1
~,,,,,-,,-",,',-"~
48
I
I
47\
30 t
.
20 I
~
[\
9
I0 l
.
0 ]
,,
46
I
ppm
Fig. 11.12. 13C CP/MAS spectrum of a-PP (top) shows broad structured resonances, especially in the methylene and methyl regions. The spread in the solution spectrum of the methylene region (bottom) is much smaller; the numerous sharp lines belong to different configurations.
426
AKIRA AOKI AND TETSUO ASAKURA
CP/MAS N M R spectra of the polymer glass and the solution are shown. Inhomogeneous broadening, which is due to conformational broadening and configurational splitting, are clearly visible. In the 13C CP/MAS N M R spectrum, the methylene resonance is richly structured, displaying two main contributions and a low frequency shoulder. The methyl peak is structured too, whereas, the methine resonance is comparatively narrow. In the solution spectrum of methylene region numerous peaks occur which reflect configurational splitting due to different hexad compositions. Recently, this broadening was simulated by Born and Spiess [14] using a combination of conformational statistics and ab initio IGLO calculations. Figure 11.13 shows the simulation in comparison with an experimental 13C CP/MAS spectrum of
Experiment
60 . . . . . . . so
,o
2o
6 [ppm]
,o
Theory
4O
5O
30
20
,~ [ppm]
10
Theory .
40
.
.
30
.
.
20
6 [ppr..l
10
.
.
.
.
0
Theory 60
50
40
6 [pp.,]
30
20
Fig. 11.13. Experimental CP/MAS spectrum of solid glassy a-PP (top) compared with simula-
tions for the various groups (rows).
POLYOLEFINS
X~
X2
X3
Ct
C~
I
I
0102
427
0304
Fig. 11.14. The model molecule for polypropylene which is used for IGLO calculation. The y-neighbors of the central methylene (A) and methyl (C1, C2) units are fixed by defining the central diads X1, X2 and X3 and the dihedral angles 01, 02, 03 and 04.
a-PP. The fine lines display the contributions of the various geometries (which are calculated by the IGLO method for the hexamer model as shown in Fig. 11.14), which have been slightly broadening for typographical reasons. The bold line shows the overall simulation where all contributions have been convoluted with a Gaussian function. The absolute heights of the resonances were adjusted arbitrarily, as the different groups have different CP efficiencies. These simulations agree well with experimental patterns. For example, in the methylene carbon region, both the two almost equally populated main contributions and the small low frequency shoulder are reproduced. By investigating the geometries contributing to these subresonances, three resonances in experimental pattern can be assigned (from left to right) to conformations (01, 02, 03, 04)= (t, *, *, t), (t, *, *, g ) + (g, *, *, t) and (g, *, *, g), respectively. The conformational state of a position denoted by an asterisk (*) is not specified. This assignment confirms the empirical y-gauche effect. Thus, by this combination of conformational statistics and ab initio IGLO calculation, the observed spread in the laC chemical shift is shown to be of predominantly conformational origin. Furthermore, it was demonstrated that configurational splitting in the solid NMR spectrum of a-PP can be estimated by theoretical methods as mentioned above.
11.3
Syndiotactic polypropylene
Contrary to i-PP, syndiotactic polypropylene (s-PP) has received little attention. This is due to the poor syndiospecificity of the Ziegler-Natta catalyst system. However, in the last few years, new metallocene catalysts have been developed, which allow very high syndiospecificity [15, 16]. Previous X-ray diffraction studies [17-20] have shown that there are two types of molecular conformations of s-PP depending on the sample preparation. One type, Form I, consists of molecular chains with helical confor-
428
AKIRA AOKI AND TETSUO ASAKURA
mation, (-TTGG-)2, and is the most thermodynamically stable crystalline form. Here T and G means trans and gauche, respectively. The other type, Form II, consists of chains with an all-trans-planar zigzag conformation. These structures have already been studied by using solid-state 13C N M R [21-23]. These N M R spectra were interpreted in terms of preferred conformations and the y-effect on the 13C N M R chemical shifts [24]. Figure 11.15(a, b) shows the ~3C CP/MAS N M R spectra of Forms I and II samples at 20~ respectively. In Fig. 11.15(a), there are two peaks at 48.29 and 39.64 ppm in the methylene region. The difference in the chemical shifts is explained using the y-effect. In the (-TTGG-)2 conformation, the methylene carbons are placed at two different positions. In one site (39.64 ppm), they receive two y-effects from two different methine carbons. In the other site (48.29 ppm), no y-effect is received.
(b)
(c)
-
ppm from T M S
Fig. 11.15. 13C CP/MASNMR spectra of s-PP at 20~ (a) Form I" (b) Form II" and (c) Form III.
POLYOLEFINS
429
On the other hand, only one peak at 48.29 ppm is observed in the methylene region of Form II as shown in Fig. 11.15(b). This indicated that the sample consists of a molecular chain with an all-trans-conformation where the methylene carbons receive no y-effect. Differences in chemical sifts between Forms I and II are also observed for methyl and methine peaks although the difference is small. The methyl carbon receives one y-effect from a methine carbon in Form I, but it receives two y-effects in Form II. The methine carbon receives two y-effects from two carbons which are methylene and methyl in Form I but two methyls in Form II. Recently, Form III with a predominantly new crystalline form was obtained by special sample preparation [23, 25-27]. Chatani et al. [25] reported this new structure of s-PP by X-ray diffraction analysis. An s-PP sample with planar zigzag form was exposed to benzene, toluene or p-xylene vapor below 50~ for several days. They concluded that the new structure of s-PP has a (-Y662TzG2-) conformation. Figure 11.15(c) shows the 13C CP/MAS NMR spectrum of a sample prepared by soaking Form II sample in toluene at room temperature for two days. It is apparent that this spectrum is different from those of Forms I and II, and not a mixture of the two spectra. For example, a methylene peak appears at 43.72 ppm between the two methylene peaks of Form I. Generally, chemical shift calculations based on the y-effect on the 13C NMR chemical shift and a rotational isomeric state (RIS) model have successfully provided reasonable assignments of stereosequence-dependent peaks in 13C NMR solution spectra of PPs [28-31] and olefin copolymers [24, 32]. Through the calculation of the conformation probabilities for dyad s-PP units, the fraction of the preferred conformations, TTTT, TTGG and G ' G ' T T are 0.33, 0.21 and 0.21, respectively, and other conformations were negligible. Apparently, the TTTT conformation is formed in Form II. On the other hand, TTGG and G ' G ' T T are formed in Form I. By combining these three preferred dyad syndiotactic sequences, longer propylene units than dyad were considered. From the calculations of conformation probability, it was reasonable to propose T662 in the case of tetrad monomer units, and TloG2, T6TzGzT2 and T4GzT4G~ in hexad monomer units, respectively. Figure 11.16 shows the stick spectra which are calculated by an additive contribution of y-effect for each gauche bond by assuming the preferred conformations predicted. The stick spectra (a) and (b) represent the -TTGG(Form I) and-TTTT- (Form II) conformations, respectively, which agree with the observed spectra. In the other spectra, the character of the spectrum (e) agrees with the observed spectrum Fig. 11.15(c). In the methylene region, six methylene carbons are placed at three different positions in the (-T6GzTzG2-) conformation. One methylene carbon receives two y-effects from two different methine carbons, and such a peak should appear on the
430
A K I R A AOKI AND TETSUO A S A K U R A (a) TTGG (form i)
[
I
(b) 1"1"1-1"(lorm II)
(c) T6 G 2
(d) TloG 2
I
I,
,
!,1
I
I
I
(e) T6 G 2T2G2
I
='tCH
~
~[
.
.
.
.
.
(f) T4G2 T4 G'2
CH 2
I
CH
I
CH3
Fig. 11.16. The
13C NMR stick spectra of s-PP predicted for the chains with preferred conformations. The chemical shift difference on the basis of the y-effect is noted in the stick spectrum (e): yCH in the CH2 region means the y-effect between CH and CH2 carbons. TCH2 and y e n 3 in the CH region mean y-effect between CH2 and CH carbons, and that between CH3 and CH carbons, respectively, y e u 2 m y C H 3 is the difference between these two y-effect. yCH in the CH3 region means y-effect between CH and CH3 carbons.
low frequency similar to the methylene peak at 39.64 ppm in Form I. On the other hand, the peak assigned to three methylene carbons which receive no y-effect should appear on the high frequency similar to the methylene peak at 48.29 ppm in Forms I and II. The other two methylene carbons receives only one y-effect from a methine carbon and the peak should appear between these two peaks. A shoulder on the high frequency side of the main peak of methine carbons, and two peaks in the methyl region at 22.25 and 19.05 ppm with an intensity ratio of 2" 1, are observed. Thus, it is clear that the spectrum of this sample (Fig. 11.15(c)) indicates a new crystalline structure Form III. The chemical shift difference among each peak in the methylene region is represented as y C H ~ C H 2 , the difference between two peaks in the methine region is (yCHz--CH)-(yCH3--CH), and the difference between two peaks in the methyl region is yCH--CH3. The observed values in the spectrum (Fig. 11.15(c)) were 4.3, 0.5 and 3.2 pp.rn, respectively. In addition, through
POLYOLEFINS
(a)
CH2 la w
CH CH3 a I ta
431
CH2
(b)
,oo~
,oo
u
i
90~
I1_
_l
I~]~
90~
8o~
/I
I
~
/1~
8o~
7o~
II
I
IA~ ]l~[
70~
o~ ~
J.
Ylt }lU,
6o~
b'L 7~
so~
JJ~ ]~
40~
so ~ !l 40~ it-';
60
. . . . . . .
II ! . . . . . . . . .
50
/ ! . . . . . . . . .
! ....
40 30 I ~ m from TMS
9. . . . . .
20
'. . . . . .
i
10
f . . . . . . . . .
60
CH CH 3
Ilal
I'"
50
" ' I ....
|'"
40
. . . . . . .
I . . . . . . . . .
30
ppm from TMS
I . . . . . . . . .
20
i
10
Fig. 11.17. 13C CP/MAS spectra of s-PP of (a) Form I and (b) Form II as a function of temperature: I, Form I; II, Form II; a, amorphous component.
a more detail analysis, it was revealed that this sample consists of a mixture of Forms I, III and amorphous structures. Figure 11.17(a, b) show the 13C CP/MAS spectra of Forms I and II with increasing temperature along with the stick spectrum of the random coil shift of s-PP in solution, respectively. As shown in Fig. 11.17(a), above 60~ a new small peak becomes visible to high frequency of the methine peak. With increasing temperature, this peak becomes sharper and the intensity increases. In addition, the methylene peak at 48.29 ppm at 40~ appears to shift slightly to low frequency, which is due to the appearance of a new peak on the low frequency side of the methylene peak. The intensity of the latter peak increases with increasing temperature. Since the chemical shifts of these new peaks are close to those of s-PP observed in solution, they can be assigned to an amorphous component. Spectral simulation of Fig. 11.17(a) shows that Form I decreases gradually from 80% (40~ to 70% (100~ The remainder is an amorphous conformation. After the NMR measurement at 100~ the spectrum of Form I was observed at 40~ No significant change
432
AKIRA AOKI AND TETSUO ASAKURA
was observed between two spectra at 40~ indicating that the transition from the crystalline Form I to an amorphous structure is reversible. By contrast, temperature dependencies of the 1 3 C CP/MAS NMR spectra of Form II are more complicated. As shown in Fig. ll.17(b), the spectrum at 100~ is quite different from that at 40~ Judging from the change in the spectral pattern, it is clear that a structural transition from Form II to coexistence of Form I and amorphous component occurs with increasing temperature. Detailed analysis of the spectral change gives the information on the structural transition. A new sharp peak at 20.5 ppm, which is assigned to the amorphous component, is clearly visible to high frequency of the methyl peak between 40 and 60~ The methine peak shifts gradually to high frequency from 40 to 60~ and a shoulder appears on the low frequency side of the main peak in the methine region. Above 70~ the peak is clearly split into a doublet. The high frequency peak is assigned to the amorphous component and the low frequency one to Form I. In methylene region, a small broad peak becomes visible at about 40 ppm above 60~ These spectral changes clearly show the transition from Forms II to I with increasing temperature. Figure 11.18 shows a plot of the fraction of Form II, I and amorphous component determined by spectral simulation of Form II spectra with temperature. Form II decreases rapidly between 40 and 80~ and the amorphous component increases gradually with increasing temperature. Form I increases between 40 and 80~ but slightly decreases between 80 and 100~ After the 1 3 C CP/MAS measurement at 100~ this Form II sample was observed again at 40~ The spectrum is quite similar to that of Form I at 40~ and, therefore, with decreasing temperature, a transition from an amorphous structure to Form I also occurs. No more Form II appears. Figure 11.19 shows the 1 3 C CP/MAS NMR spectra of a sample, which 100 Form II --'O--Form I
80
II I
A
r
._o 13 LL
60 40 20
o9
L so
,1 so
,
1, 70
, i , eo
Temperature (~
I 90
100
Fig. 11.18. The plot of the fraction of Forms II, I and amorphous component determined by the spectral simulation of Form II against temperature.
POLYOLEFINS
ell2 100~
I
433
CH CH3 Ii
aiiii
j
90~
80~
70"c
60"C
~l
I
40~
o,u oo
I
8o....... a i '
I
I
-,b ....' ....sb ....... ~ i ' .....10 ppm from TMS
Fig. 11.19. 13C CP/MAS spectra of s-PP of Form III as a function of temperature. III, Form III" I, Form I; a, amorphous component.
is a mixture of Forms I, III and amorphous component, with increasing temperature. The peaks characteristic to the Form III, i.e., one peak at 43.72 ppm in the methylene region and two peaks at 21.86 and 19.05 ppm in the methyl region, can be still observed at 90~ Above 60~ the high frequency peak at 28.5 ppm which is assigned to the amorphous component, can be observed clearly in the methine region. At 100~ the spectrum becomes similar to that of Form II although the peaks from Form I tend to be sharper in Form III. The fractions of Forms I, III and amorphous component are plotted against temperature in Fig. 11.20. Form III decreases gradually and the amorphous component increases gradually with increasing temperature. From I increases above 80~ in contrast to the case of Form II. Figure 11.21 shows the 13C CP/MAS spectra of an annealed s-PP sample [33]. This sample was hot washed with toluene, precipitated with methanol,
434
AKIRA
AOKI
AND
TETSUO
100
,
,
,,
,
I --o-- r-~rm
eo
r-
ASAKURA
t.--<>-~,.om~..
60
)
.g 40
1
LL 20
0
,
4c
'
J
so
60
,
L
,
,
70
so
90
Temperature
,
)
loo
(~
Fig. 11.20. The plot of the fraction of Forms III, I and amorphous component determined by the spectral simulation against temperature.
! .
.
.
.
I
Fig. 11.21. The 13C CP/MAS NMR spectrum of annealed s-PP sample.
dried under vacuum, melted and annealed at 140~ for 12 h. The polymorph of this annealed sample is Form I, because the spectrum of this sample is similar to that of Form I (Fig. ll.15(a)). However, in the spectrum of the annealed sample, splitting is observed in the methyl carbon region. It is considered that this splitting is due to contribution of chain packing. Sozanni et al. [33] explained this by the introduction of statistical disorder in the distribution of chirality of the chains within the lattice. In addition, they discussed the amorphous phase in detail. As mentioned above, s-PP samples contain at least 40% of the amorphous component in any case. The chemical shift of the amorphous phase in semicrystalline s-PP is an average of specific conformational sequences, and provides a precise measurement of the conformational equilibria in the amorphous bulk. The distribution of interconverting conformers is shifted toward a major content trans-sequences.
On the other hand, the molecular conformation and the phase structure
POLYOLEFINS
6O
9 20
ppm from TMS
435
I0
0
Fig. 11.22. The 13C DD/MAS spectra of s-PP/o-dichlorobenzene gel (13.6 wt%). In the lower part, the spectrum of Form I is shown as a reference.
of s-PP gel were studied by Nakaoki et al. [34]. The gel sample was dissolved completely in o-dichlorobenzene (ODCB) at 150~ and prepared by quenching in iced water. The concentration of the solution was 13.6 wt%. Figure 11.22 shows the 13C DD/MAS spectrum for the s-PP/ODCB gel in the equilibrium state. In this case, CP was not used to avoid the complication that may arise from different CP efficiencies for different phases in the gel system. For comparison, the spectrum of Form I is also shown in the lower part of the figure. These spectra are thought to reflect faithfully the contributions from all structural components in the sample. The spectrum for the gel is somewhat different from that of Form I. Several additional peaks are recognized in the gel spectrum. To provide detailed assignment of these peaks, the longitudinal and transverse relaxation rates were measured. As mentioned above, the peaks at 19.9, 27.4 and 46.4ppm of the spectrum for the gel sample are assigned to the methyl, methylene and methylene carbons of the chains in amorphous phase, respectively. However, in the methylene carbon region, a new peak at 49.9 ppm was observed. From analysis of the longitudinal and transverse relaxation, this peak was assigned to the methylene carbon in the crystalline phase. For example, the values of the longitudinal and transverse relaxation times of this peak (Tic = 53 s, T z c - 0.01 5 ms) are nearly equal to those of the peak at 47.7 ppm in Form I (Tic = 63 s, T z c 0.014 ms). For Form I, two splitted peaks are recognized for methylene carbon at 39.0 and 47.7 ppm. This splitting could be the result of the ?,-effect as mentioned above. On the other hand, three distinguishable peaks at 39.0, 47.7 and 49.0 ppm are assigned to the crystalline methylene carbon in the gel. This intensity ratio of three peaks is 1/2/3. The chemical shifts of the former two, 39.0 and 47.7, are the same as those of Form I, and that of the
436
A K I R A A O K I AND T E T S U O A S A K U R A
last peak at 49.0 ppm is identical to that of Form II. It is clear that the triple splitting of the methylene resonance of the gel cannot be attributed to the existence of a Form III crystal because of the lack of a 44 ppm peak from the GT-TT sequence with one y-effect. To confirm whether the gel structure consists of the mixture of TT and TTGG crystals or not, the infrared (IR) spectra, which are very sensitive to the molecular conformation, were measured. It was recognized that all vibrational modes of the gel were completely the same as Form I crystal. This indicates that the conformation formed in the gel takes only a TTGG sequence and all split peaks should be assigned to the TTGG sequence. A possible explanation for the results of 13CNMR and IR spectra was given by taking a molecular packing effect into account that yields a triplet resonance to the methylene carbon in the 13C DD/MAS spectrum. It was assumed that the resonance due to the TG-GT sequence is further splitted into 47.7 and 49.0ppm peaks because of the molecular packing effect, and because the GT-TG sequence stays as a singlet at 39.0 ppm. Sozzani et al. [22] observed the 49.0 ppm peak in an aspolymerized sample. They did not regard it as the TT-TT conformation. Since the TG-GT methylene locates at the outer position of the helical structure of s-PP, an enhanced interaction with neighboring helical molecular chains is expected. Therefore, there are two distinct magnetic environments possible for the TG-GT methylene to yield the splitting. The splitting width of 1.3 ppm observed for the gel is comparable to that in the case of c~-form of i-PP (1.0 ppm) as mentioned in Section 11.2. This splitting is understood to be due to the existence of inequivalent sites for methylene carbons because of the molecular alignment in the crystal lattice. In the case of s-PP gel, the same effect is predicted. Depending on the degree of stereoregurality, its mechanical and thermal history, s-PP shows different amounts of statistical disorder in its crystalline packing [18,19,37-40]. In general, the helixes of s-PP, which form the (-TTGG-) conformation (Form I), are packed in one of the two orthorhombic models shown in Fig. 11.23(a, b). The position of the chain axes, and the relative height of the methyl groups of neighboring macromolecules, characterize the two basic models of the mode of packing of the methyl groups. Figure 11.23(a) shows the packing model in the case of a pseudocentered structures on the B face (B-centered axes in (0, 0, z) and (1/2, 0, z)) [35]. Main X-ray peaks appear in the powder spectrum at d = 7.25, 5.60 and 4.31 A (20= 12.2 ~ 15.8 ~ 20.6 ~ Cu K~). On the other hand, Fig. 11.23(b) shows the packing model in the case of a pseudocentered structure on the C face (C-centered axes in (0, 0, z) and (1/2, 1/2, z)). Main X-ray peaks appear in the powder spectrum at d = 7.25, 5.22 and 4.31 ~ (2 0 = 12.2 ~ 17.0 ~ 20.6 ~ Cu K~) [36]. The ideal limit ordered structures correspond to the
POLYOLEFINS
(a)
3
-
-
3
3
3
bt (b) 3 1
3-
437
13[ -3
Q
a
~
13 -3 1 3-
3 1 -3
Fig. 11.23. Packing models in the a-b plane of macromolecular chains of s-PP, represented by rectangles in the case of a pseudocentered structure on (a) the B face and (b) the C face; the methyl carbons indicates black balls placed at the vertices of the rectangles: R = righthanded helix; L = lefthanded helix.
orthorhombic space groups Ibca [40] (Fig. 11.23(a'), helixes of opposite chirality alternate along a and b) and C2221 [36] (Fig. 11.23(b')), all helixes are isochiral), with identical a = 14.5 A, c = 7.40 A axes and b = 11.2 A (= 5.6 • 2 A) in Fig. 11.23(a'), and b = 5.60 A in Fig. 11.23(b'). On the other hand, as shown Fig. 11.24 [20], the s-PP chains which form an all trans-planar zigzag conformation (Form II), are packed in an orthorhombic lattice, centered on the C face, with a - 5.22 A, b - 11.17 A and c = 5.06 A (chain axis). The main peaks in a hypothetical X-ray diffraction powder pattern would occur at d = 5.58, 4.73 and 3.75 A (20 = 15.9 ~ 18.8 ~ 23.7 ~ Cu K~). Furthermore, for Form III with (T6G2T2G2) conformation, a triclinic cell has been proposed [25] for this polymorph with a = 5.72 A, b = 7.64 A and c = 11.6 A (chain axis), a = 73.1 ~ a = 88.8 ~ and 3' = 112.0 ~. The hypothetical X-ray diffraction powder pattern of this form would present main peaks at d = 6.84, 5.26 and 4.46 A (20 = 12.9 ~ 16.8 ~ 19.9 ~ Cu K~). Recently, Form III was interpreted in terms of disorder in the crystalline by Auriemma et al. [41]. They prepared an s-PP sample (quenched-precipitated sample) crystallized by precipitation with methanol from a solution in npentane and vacuum treated for 12 h at 60~ Figure 11.25 shows the ~3C CP/MAS spectrum of this quenched-precipitated sample compared to that of Form I. The spectrum of the quenched-precipitated sample is similar to Fig. 11.15(c). Through detail analysis of the X-ray diffraction profiles and 13C CP/MAS N M R spectra at various temperature, they concluded that the con-
438
AKIRA AOKI AND TETSUO ASAKURA Lb
88
:-----'-
-----t
"
1'
T Fig. 11.24.
Crystal structure of all trans-planar zigzag conformation of s-PP.
formational disorder in the quenched-precipitated sample was somehow connected with the presence of regions in the t r a n s - planar zigzag conformation with (-TTGG-) conformation. In Fig. 11.26(b, c) the a - c projection of possible models of locally disordered structures are shown, and in Fig. 11.26(a), for comparison, the a - c projection of an ideal ordered structure is drawn. In all three structural models, the chains are packed in an orthorhombic lattice centered on the C face. In the disordered models of Fig. 11.26(b, c) the chains are characterized by portions of chains in the helical conformation in an ideal symmetry s(2/1)2 (a-portions). All of them are isochiral and connected by portions of chain in a (-G2T6G2T6-) conformation (Fig. 11.26(b), /3-portions) or (-GET18-) (Fig. 11.26(c), /3'-portions) in such a way that a
POLYOLEFINS
439
,o ,J ii] i 60
50
40
~/
30 ppm
20
10
Fig. 11.25.13C CP/MAS NMR spectra at room temperature of (a) fully helical s-PP and (b) of the quenched- precipitated sample.
substantial parallelism among the chain axes and mean periodicity along the chain axis of 7.40 A is maintained. In the regular portions of the chain, the position of the methyl carbons is the same as in an ideal crystal with C2221 symmetry. The defective in the/3 region (Fig. 11.26(b)) form a (-TaG2T6-) conformation. They are similar to the conformational sequences in the Form III structure mentioned above (Chatani et al. [25]) and are packed in a similar manner. In addition, the formation of plane defects fl or/3' corresponds to a very low conformational energy [25, 42]. Each chain in a/3 defect comprises four methylene groups in a conformational environment GT-TT, giving a resonance at 44.9 ppm, two methylene groups in sequences TT-TT and two in sequences TG-GT corresponding to resonances at 50.2 and 49.0ppm, respectively. As far as the methyl groups are concerned, each/3-chain defect implies the presence of four methyl groups in a GG-TT chain environment, which could explain the resonance at 22.4 ppm and four in a TT-TT chain environment, which could explain the resonance at 18.9 ppm, i.e., approximately 3 ppm upfield with respect to the resonance at 22.4 ppm. In the same way, each/3'-chain defect implies two methylene groups in a conformational environment TTTG, one methylene group in the sequence TG-GT and seven in sequences TT-TT. As far as the methyl groups are concerned, eight methyl groups are found in sequences of the kind TT-TT, whereas, two are in conformational environments TT-GG. The presence of defects /3 or /3', one every 30-40 monomeric units, could quantitatively explain the features of the ~3C CP/MAS spectrum of Fig. 11.25(b). Finally, the chemical sifts of the carbon atoms in the region surrounding
9
el n el
20
9
v)
k
C
?
Fig. 11.26. a-c projection of C-centered structures: (a) ordered case; (b) and (c) two examples of disordered cases. In (b) and (c), the portions of the chains in the regions indicated with a are in (-TTGG-) conformation and have methyl groups in the crystallographic register. The conformation; The defective portions p’ in (c) to (-GZTI8-)conformation. defective portions p of the chains in (b) correspond to (-G2T6GZT6-)
POLYOLEFINS
441
the defect shown in Fig. 11.26(b, c), were estimated quantitatively by Auriemma et al. [41] at the ab initio level using the IGLO method [43, 44].
11.4
Poly(1-butene)
For isotactic poly(1-butene) (i-P1B) [45], it is well known that the solid structures are three kinds of (-TG-) helical conformations as shown in Fig. 11.27. In general, i-P1B chain form 3~-helix. In this case, the dihedral angles of trans- and gauche-conformations are 180 and 60 ~ respectively (Fig. 11.27(a)). This crystalline form is known as Form I. Above 90~ conformation is slightly changed and this chain forms an ll3-helix called Form II. In this case, the dihedral angles 9 of trans- and gauche-conformations are 163 and 77 ~ respectively (Fig. 11.27(b)). However, the sample which is crystallized by casting from chloroform or by freeze-drying from benzene, forms 41-helix called Form III. In this case, the dihedral angles 9 of transand gauche-conformations are 159 and 83 ~ respectively (Fig. 11.27(b)). Figure 11.28 shows 13C CP/MAS spectra of i-P1B. It is apparent that the
I
c Fig. 11.27. Stereoscopic drawings of three crystalline polymorphs of isotactic poly(1-butene): (a) Form I, 31-helix; (b) Form II, ll3-helix; and (c) Form III, 41-helix.
442
AKIRA AOKI AND TETSUO ASAKURA
ppm
Fig. 11.28.
I3C: CP/MAS NMR spectra of isotactic poly(1-butene). (a) Form I at 20°C; (b) Form I1 at -60°C; (c) Form I11 at -10°C; and (d) amorphous at 43°C. The vertical dashed lines represent the peak positions of Form I .
POLYOLEFINS
443
chemical shifts are different between different crystalline forms in spite of only slight differences of dihedral angles. Recently, it was shown that Form II of i-P1B exhibits significant slow conformational exchange above the grass transition by using a3C 2D exchanged NMR under conditions of MAS [46]. Comparison with spectra of Form I proved that this motion occurs within the crystalline region.
References
.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
A.E. Tonelli, Macromolecules 12 (1979) 255. A. Bunn, M.E.A. Cudby, R.K. Harris, K.J. Packer and B.J. Say, Polymer 23 (1982) 694. M.A. Gomez, H. Tanaka and A.E. Tonelli, Polymer 28 (1987) 2227. G. Natta and P. Corradini, Nouvo Cim. Suppl. 15 (1960) 40. D.M. Grant and B.V. Cheney, J. Am. Chem. Soc. 89 (1967) 5315. M. Barfield and S.H. Yamamura, J. Am. Chem. Soc. 112 (1990) 4747. S. Saito, Y. Moteki, M. Nakagawa, F. Horii and R. Kitamaru, Macromolecules 23 (1990) 3256.. J.R. Lyerla and C.S. Yannoni, IBM J. Res. Dev. 27 (1983) 302. A. Turner-Jones, J.M. Aizlewood and D.R. Beckett, Makromol. Chem. 17 (1964) 134. S. Bruckner and S.V. Meille, Nature 340 (1989) 455. B. Lotz and J.C. Wittmann, J. Polym.Sci., Part B: Polym. Phys. 24 (1986) 1541. B. Lotz, S. Graft and J.C. Wittmann, J. Polym. Sci., Part B: Polym. Phys. 24 (1986) 2017. S. Bruckner, S.V. Meille, P. Sozzani and G. Torri, Makromol.Chem. Rapid. Commun. 11 (1990) 55. R. Born and H.W. Spiess, Macromolecules 28 (1995) 7785. J.A. Ewen, R.J. Jones, A. Razavi and J.D. Ferrara, J.Am. Chem. Soc. 110 (1988) 6225. G. Balbontin, D. Dainelli, M. Galimberti and M.G. Paganetto, Makromol. Chem. 193 (1992) 693. G. Natta, M. Peralso and G. Allegra, Makromol. Chem. 75 (1965) 215. A.J. Lovinger, B. Lotz, D.D. Davis and F.J. Padden, Jr., Macromolecules 26 (1993) 3494. W. Stocker, M. Schumacher, S. Graft, J. Lang, J. Wittmann, A.J. Lovinger and B. Lotz, Macromolecules 27 (1994) 6948. Y. Chatani, H. Maruyama, K. Noguchi, T. Asanuma and T. Shiomura, J. Polym. Sci., Part C. 28 (1990) 393. A. Bunn, M.E.A. Cudby, R.K. Harris, K.J. Packer and B.J. Say, J. Chem.Soc., Chem.Commun. 15 (1981). P. Sozzani, M. Galimberti and G. Balbontin, Makromol. Chem., Rapid. Commun. 13 (1992) 305. P. Sozzani, R. Simonutti and M. Galimberti, Macromolecules 26 (1993) 5781. A.E. Tonelli, NMR Spectroscopy and Polymer Microstructure. VCH Publishers, New York, 1989. Y. Chatani, H. Maruyama, T. Asanuma and T. Shiomura, J. Polym. Sci. Part B, Polym. Phys. 29 (1991)1649. P. Sozzani, R. Simonutti and A. Comotti, Macromol. Symp. 89 (1995) 513. T. Asakura, A. Aoki, T. Date, M. Demura and T. Asanuma, Polymer J. 28 (1996) 24.
444 28. 29. 30. 31. 32.
33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.
AKIRA AOKI AND TETSUO ASAKURA F.C. Schilling and A.E. Tonelli, Macromolecules 13 (1980) 270. A. Zambelli, P. Locatelli, A. Provasoli and D.R. Ferro, Macromolecules 13 (1980) 270. T. Asakura, K. Omaki, S.-N. Shu and R. Chujo, Polymer J. 16 (1984) 717. T. Hayashi, Y. Inoue, R. Chujo and T. Asakura, Polymer 29 (1988) 138. T. Asakura, M. Demura and T. Hayashi, 13C NMR Assignments of Polyolefins and Olefine Copolymer Based on the 13C NMR Chemical Shift Calculation and 2D-INADEQUATE NMR. Annual Reports on NMR Spectroscopy. Academic Press, London, 1993. P. Sozzani, R. Simonutti and M. Galimberti, Macromolecules 26 (1993) 5782. T. Nakaoki, H. Hayashi and R. Kitamaru, Polymer 37 (1996) 4833. B. Lotz, A.J. Lovinger and R.E. Cais, Macromolecules 21 (1988) 2375. P. Corradini, O. Natta, P. Ganis and P.A. Temussi, J. Polym. Sci., Part C 16 (1967) 2477. F. Auriemma, C.D. Rosa and P. Corradini, Macromolecules 26 (1993) 5719. D.C. Rosa and P. Corradini, Macromolecules 26 (1993) 5711. A.J. Lovinger, B. Lotz and D.D. Davis, Polymer 31 (1990) 2553. A.J. Lovinger, D.D. Davis and B. Lotz, Macromolecules 24 (1991) 552. F. Auriemma, R. Born, H.W. Spiess, C.D. Rosa and P. Corradini, Macromolecules 28 (1995) 6902. B. Pirozzi and R. Napolitano, Eur. Polym. J. 28 (1992) 703. M. Schindler and W. Kutzelnigg, J. Chem. Phys. 76 (1982)1919. W. Kutzelnigg, U. Fleischer and M. Schindler, NMR Basic Princ. Prog. 23 (1991) 165. L.A. Belfiore, F.C. Schilling, A.E. Tonelli, A.J. Lovinger and F.A. Bovey, Macromolecules 17 (1984) 2561. H.W. Beckham, K. Schmidt-Rohr and H.W. Spiess, ACS Symp. Ser. 598 (1995) 243.
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved"
Chapter 12
Polyamides Isao Ando I and Tetsuo Asakura 2 1Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan; 2Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan
12.1
Introduction
Since Holmes' observation of the X-ray diffraction of nylon [1], many fruitful studies have been presented using X-ray diffraction, infrared absorption and other techniques. It can be expected that solid-state NMR provides useful information about the structure and dynamics of the crystalline and noncrystalline components of polyamides [2, 3]. Actually, solid-state 2H, 13C and 15N NMR have been successfully used to clarify various crystalline and amorphous components. In this chapter, therefore, some examples of the structural and dynamic analyses performed for polyamides such as nylon 4, 6, 7, 11 and 66, will be described including the oriented fibers of aromatic polyamides, poly(mphenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4-MPTA).
12.2
Solid-state
13C NMR
and structural characterization
The structure of polyamides has been characterized by means of 13CCP/MAS NMR, 13C LD(low decoupling)/MAS NMR and 13C PST(pulse saturation transfer)/MAS NMR methods [4, 5]. The CP/MAS method enhances the 13C signal of the immobile component and, on the other hand, the LD/MAS and PST/MAS methods enhance the 13C signal of the mobile component. These methods have been used to characterize the mobile and immobile components of nylon.
446
ISAO A N D O A N D T E T S U O A S A K U R A
§247 (~ ~
[3
3
a
tJ
C=0
]!!
[
2oo
,
,
,,
,,
,
v,
1-
,~o
" ' - "
J "
9
'"
I"
,oo
9
9
'"
9
~
'
'
9
""
'"
ppm
Fig. 12.1. Typical 13C CP/MAS NMR spectrum of nylon 4 single crystals in the solid state at room temperature. SSB" spinning sidebands.
Figure 12.1 shows a typical 13C CP/MAS spectrum of a nylon 4 single crystal sample at room temperature, which assumes the c~-crystal form [5]. Nylon 4 has a carbonyl (C----O) carbon and three types of methylene (CH2) carbons in the repeat units. The assignment for these carbon signals is straightforward. The C----O signal appears at highest frequency (--~174 ppm) compared with the CH2 carbons (20-43 ppm). The co-CH2, a-CH2 and /3CH2 signals can be assigned by 13Cchemical shift values on nylon 4 in solution [6]. These signals appear increasingly lowest frequency in the order co-CH2, a-CH2 and/3-CH2. 13C CP/MAS, PST/MAS and LD/MAS spectra of nylon 4 single crystal samples are shown as a function of temperature in Fig. 12.2. The 13Cchemical shifts for the observed peaks are listed in Table 12.1. In Fig. 12.2(a) three peaks in the CP/MAS spectrum come from the CH2 carbons in the c~-crystal form. The lineshapes and a3C chemical shifts do not change significantly between 20 and 100~ This shows that the structure for the component does not change within this temperature. In the PST/MAS spectrum (Fig. 12.2(b)), two new peaks, in addition to the above-mentioned three peaks observed in the CP/MAS spectrum, appear at about 26 and 40 ppm on the/3-CH2 and w-CH2 peaks, respectively, and are indicated by n. These peaks come from the /3-CH2 and co-CH2 carbons in a mobile state, or in the noncrystalline component, because their peak intensities are enhanced by the PST/MAS method compared with the CP/MAS method. It is noted that the noncrystalline peaks for the/3-CH2 and o~-CH2 carbons, respectively, appear at higher frequency by about 3 ppm and at lower frequency by about 2.5 ppm at the
447
POLYAMIDES
A w
100~
60t
(a) 2or pore 60
50
40
30
20
10
Fig. 12.2. (a) 13C CP/MAS NMR spectra of nylon 4 single crystals sample as a function of temperature. The CH2 region is expanded.
crystalline peaks. However, the noncrystalline peak for the ce-CH2 carbon does not appear. The carbonyl carbon appears as a single peak within the above temperature range, and is resolved into the crystalline and noncrystalline components. However, the carbonyl peak at 100~ moves significantly downfield compared with that at 20 and 60~ On the other hand, 13C LD/MAS spectra of the nylon 4 single crystal sample shown in Fig. 12.2(c), show an obvious change from 20 to 100~ In the spectrum at 20~ no peak is observed owing to the broadening caused by dipolar interactions between carbons and protons. Such broadening at 20~ cannot be overcome by the low power decoupling for protons used in
448
ISAO ANDO AND TETSUO ASAKURA
(b pOm EO
5O
~
3O
20
tO
Fig. 12.2. (b) PST/MAS NMR spectra of nylon 4 single crystals sample as a function of temperature. The CH2 region is expanded.
the LD/MAS method because of the extremely slow motion of the carbons considered here. However, at 100~ three CH2 peaks and a carbonyl peak are clearly evident. This is because the decoupling power is adequate to obtain sharp lines by fast molecular motion at the glass transition temperature Tg < 100~ The 13C chemical shifts for the/3-CH2 and to-CH2 carbons agree with those for the noncrystalline components observed by the PST/MAS method. This shows that the assignment for the noncrystalline peaks observed by the PST/MAS method is correct. The 13C chemical shift for the ce-CH2 peak agrees with that determined by the CP/MAS and PST/MAS methods. This means that the 13C chemical shifts for the noncrystalline and crystalline ce-CH2 carbons are in close agreement. The carbonyl peaks moves to high
POLYAMIDES
449
CO
,00
20':t::
(c) I
....
A
IBO"
ppm
Fig. 12.2. (c) LD/MAS NMR spectra of nylon 4 single crystals sample as a function of
temperature. In (a) and (b) the CH2 region is expanded. frequency when compared with that observed by the PST/MAS method, but is very close to that observed by the PST/MAS method. It is probable that in PST and LD/MAS spectra at 100~ the carbonyl carbon in the crystalline state is observable. Figure 12.3(a, b) shows expanded 13C CP/MAS and PST/MAS NMR spectra as functions of the CH2 carbons in the nylon 4 melt-quenched sample, which contains a larger noncrystalline fraction compared with the single crystal samples. In the 13C CP/MAS spectra, the noncrystalline/3-CH2 and o-CH2 peaks are observed with a weak intensity, but in the 13C PST/MAS spectra their peak intensities increased drastically. This leads to the correct
4~
Table 12.1.
> O
13C N M R chemical shifts (ppm) of nylon 4 in the solid state and in solution a-CH2
r
Temperature
]3-CH2
(~
Method
CO
Crystalline a
Noncrystalline b
Crystalline
Noncrystalline
Crystalline
Noncrystalline
20 60 100 100 20
CP, PST CP, PST CP, PST LD/MAS SolutionC
173.3 173.4 173.6 174.6 176.03
42.4 42.4 42.5
39.7 39.8 40.1 39.9 38.88
34.0 34.1 34.3
34.0 34.1 34.3 34.4 32.90
23.4 23.5 23.5
26.4 26.5 26.6 26.4 24.6
a Values for the crystalline state determined by CP/MAS. b Values for the noncrystalline state determined by PST/MAS. c 10% H F I P solution.
9
O
POLYAMIDES
451 a
[3
3
Wn
n
<'__
rl
(
(b 60
50
~
30
20
IO
6
.......
~o . . . .
,b . . . . . . . .
~
.....
- 2b . . . .
,b
Fig. 12.3. (a) 13C CP/MAS and (b) PST NMR spectra of nylon 4 melt-quenched sample as a
function of temperature. The CH2 region is expanded. Peaks marked with an asterisk are due to materials in the rotor cap. assignment for the noncrystalline peaks and to the determination of their exact chemical shift values. The 13C chemical shifts observed in hexafluoroisopropanol (HFIP) solution are compared with the solid-state 13C chemical shifts (Table 12.1). The /3CH2 and o)-CH2 peaks appear at higher and lower frequencies, respectively, compared with those in the crystalline state. This shows that the /3-CH2 and o)-CH2 carbons in solution are similar in conformation to those in the noncrystalline state. The carbonyl peak for the solution appears at higher frequency by about 1.4 ppm compared with that in the noncrystalline state. Such a shielding may come from solvent-solute interactions. Generally, the amide carbonyl carbon chemical shifts in peptides and polypeptides are affected by strong acidic solvents and move largely at low frequency. Similar reasoning may apply to the a-CH2 high frequency shift, because the a-CH2 carbon is placed near the amide group. Figure 12.4 shows the ~3C CP/MAS NMR spectrum for a nylon 6 single crystal sample [4]. This polymer has six magnetically nonequivalent carbons, i.e., the carbonyl carbon and five CH2 carbons, in the repeat unit. The o)CH2, a-CH2 and 6-CH2 peaks can be assigned by using 13C chemical shift values on small amide molecules. The assignment of the 13C signals for the
452
ISAO A N D O AND TETSUO A S A K U R A
y.6 NYLON 6
i i m! i CI
C..-O ,
20O |'
*
9
'
W
' 9
*
"
] 50 |
'
9
i
9
9
100 |
9
9
9
,
i
S0
9
9
9
'.
"
ppm
Fig. 12.4. Typical ~3C CP/MAS N M R spectrum of nylon 6 single crystals at room temperature.
remaining CH2 carbons is performed on the basis of the chemical shift position and peak intensity of nylon 6 in solution. For the CH2 carbons, therefore, it is found that their peaks are increasingly shifted to lower frequency in the order ~o-CH2, a-CH2, 6-CH2 or y-CH2 and fl-CH2 carbons (6-CH2 and yCH2 peaks overlap). 13C CP/MAS and PST/MAS NMR spectra of nylon 6 single crystals sample, melt quenched sample, and drawn sample at room temperature (the carbonyl peak is not shown because a single sharp line appears without significant change for all samples). The lineshapes of the CH2 peaks depend on the crystallization conditions, and, furthermore, those for the same sample obtained by CP/MAS and PST/MAS are markedly different. It is noted that the PST/MAS method, in contrast to the CP/MAS method, enhances the peak intensity for CH2 carbons, such as those in the noncrystalline state, which undergo relatively rapid reorientation. Therefore, a comparison of 13C CP/MAS and PST/MAS spectra leads to a discussion of the structure and dynamics of the crystalline and noncrystalline states in nylon 6 sample crystallized under various conditions. The 13C chemical shifts are listed in Table 12.2.
POLYAMIDES
453
Table 12.2. 13C chemical shifts of nylon 6 in the solid state 13C
Single crystal sample Crystalline state Noncrystalline state Melt-quenched sample Crystalline state Noncrystalline state Drawn sample Crystalline state Noncrystalline state a b
chemical shift (ppm)
C---O
~o-CH2
ce-CH2
(6 + "y)-CH2a /3-CH2
173.5 _b
43.0 40.8
36.4 _b
29.8 27.7
26.2 _b
173.6 _b
42.2 40.1
36.8 _b
29.8 27.5
26.2 _b
173.5 _b
42.7 40.6
36.8 _b
30.0 27.7
26.1 _b
Since 6-CH2 and "y-CH2peaks overlap, their chemical shifts cannot be read separately. Not identified.
In these connections, it is useful to consider the behavior of the 13C chemical shift of the C H : carbons of paraffins and polyethylene in the crystalline and noncrystalline components. It is known that the CH2 carbons in paraffinic chains appear at lower frequency by 4 - 6 ppm if a carbon atom three bonds away is in a gauche-conformation rather than in a trans-conformation (7-effect) [7]. In fact, cyclic paraffins which crystallize in a conformations are characterized by two parallel all-trans-planar zigzag strands connected by two G G T G G loops, it is found that the CHa carbons with a 3'effect resonates at a lower frequency by about 6.5 ppm as compared with those with no 7-effect [8, 9]. Furthermore, it is found that the CH2 carbons of cyclic paraffins, and n-paraffins in the noncrystalline state, appear at lower frequency by 2-3 ppm more than those in the crystalline state. In the crystalline state, the CHa carbons assume the all-trans-zigzag conformation, which is fixed because motion is frozen, but in the noncrystalline state a rapid transition between the trans- and gauche-conformations occurs [11]. Weeding et al. [12] obtained the same results on the noncrystalline state. Figure 12.5(a) shows 13C C P / M A S N M R spectra of nylon 66 meltquenched sample as a function of temperature [5]. The assignment of each peak was carried out using the solid-state 13C chemical shift data on nylon 4 and 6, and from the solution-state 13C chemical shift data. The carbonyl signal is a single peak. The CHa signal consists of four major peaks and a small low frequency shoulder, CeN(n)CH2, on the CeNCH2 peak. The intensity of this small peak increases as the temperature is increased. The peak can be assigned to the noncrystalline aN peak as described below. The 13C chemical shifts are listed in Table 12.3.
454
ISAO ANDO
AND TETSUO
ASAKURA
IqYI.0N6
Y.6 :'~,,,~ s AI3
b,
w a
1 .........
50 Fig.
12.5.
f ..........
40
r
ulc)
I,.,,
30
13C C P / M A S
.....
I .....
20
I .........
50
!
.........
40
IA, ....... I. .... ! . . . . . . . . .
30
ppm
20
50
I .......... I .......... I .....
40
30
20
a n d P S T / M A S N M R s p e c t r a of n y l o n 6 s a m p l e at r o o m t e m p e r a t u r e :
(a) single crystal; (b) m e l t - q u e n c h e d s a m p l e ; a n d (c) d r a w n s a m p l e .
Figure 12.5(b) shows expanded 13C PST/MAS NMR spectra of the CH2 carbons in nylon 66 melt-quenched sample as a function of temperature. The CeN(n)CH2 peak is more intense than the C~NCH2 peak, which appears as a small shoulder on the C~N(n)CH2 in the CP/MAS spectrum. At 20~ a peak appears between the (/3N, "yN)CH2and /3cCH2 peaks. This peak moves to low frequency as the temperature is increased and overlaps with the/3cCH2 peak. This peak is assigned to the noncrystalline TNCH2 carbon. Figure 12.5(c) shows the expanded 13C LD/MAS NMR spectra of the nylon 66 sample at 20, 60 and 100~ In the spectrum no peak is observed at 20~ and very broad peaks appear at 60~ At 100~ five CH2 peaks and a carbonyl peak appear clearly due to fast molecular motion (Tg =---50~ The 13C chemical shifts are listed in Table 12.3. The carbonyl chemical shift value agrees with that in the noncrystalline state. On the other hand, the 13C chemical shifts for the CeNCH2, cecCH2, ]3NCH2 and j3cCH2 carbons agree with those observed by the PST method. The 13C chemical shift value for the 7NCH2 appears at lower frequency than that observed by the PST/MAS method. Furthermore, the temperature dependence of the 13C PST/MAS and
Table 12.3. 13C N M R c h e m i c a l shifts ( p p m ) of n y l o n 66 in t h e solid s t a t e a n d in s o l u t i o n Temperature (~
CeNCH2
(~N, TNCH2)
cecCH2
Method
CO
Crystalline a
Noncrystalline b
Crystalline
Noncrystalline
Crystalline
20
CP, PST
173.4 (176.4) c
42.9
40.0
36.1
36.1
60
CP, PST
173.7 (176.4) c
42.8
40.5
36.4
100
CP, PST
173.8 (176.7) c
42.6
40.5
36.5
100
LD/MAS
174.1
~cCH2 Noncrystalline
Crystalline
Noncrystalline
31.2
25.8
25.8
36.4
31.2
25.7
25.7
36.5
31.1 d
26.0
26.2
30.0 e 40.3
36.6
30.1 d
26.2
27.3 e 20
Solution e
176.2
39.47
35.53
27.68 d
24.22
25.24 e a V a l u e s f o r t h e c r y s t a l l i n e s t a t e d e t e r m i n e d by C P / M A S . b V a l u e s f o r t h e n o n c r y s t a l l i n e s t a t e d e t e r m i n e d by P S T / M A S . c T h e C O s i g n a l splits i n t o t w o p e a k s . d F o r /3N, T N C H 2 p e a k . e F o r TNCH2 p e a k . f 10% H F I P solution. g
Not identified.
4~ L~
456
ISAO ANDO AND TETSUO ASAKURA
C--'O
60"(:
(a) ~o
"
~
. . . .
~
. . . .
;o
Fig. 12.6.(a) 13CCP/MAS NMR spectra of melt-quenched nylon 66 sample at various temperatures.
LD/MAS spectra of the nylon 66 single crystals sample was observed (Fig. 12.6). From these results, it was shown that the noncrystalline component exists even in single crystals. It is known that nylon 6 takes two types of c~- and y-forms as polymorphs. Weeding et al. [12] and Okada et al. [13] observed the 13C CP/MAS NMR spectra of nylon 6 samples with the c~- and y-forms as shown in Fig. 12.7. Table 12.4 shows that there is a significant chemical shift difference between these forms. It is demonstrated that solid-state 13C NMR provides useful information about the crystal structure of polyamides. This leads to solidstate 13C NMR studies on nylon 7 [14] and 11 [15] with polymorphs. The effect of sorbed water, fast cooling of nylon 6 melt and addition of copolymer of ethylene and vinyl alcohol to nylon 6 on 13C CP/MAS NMR spectrum and 13C T1 was studied [16]. It was found that sorbed water does not affect the spectra and the T1 values. However, differences could be observed between the spectra and T1 values of nylon 6 powder, nylon 6 film and nylon 6 in the blends.
POLYAMIDES
457
r
(~:
lOO'C
/ 60~C
(b
20"C
Fig. 12.6.(b) PST/MAS NMR spectra of melt-quenched nylon 66 sample at various temperatures.
12.3 Solid-state
lgN N M R and structural characterization
12.3.1 15N C P / M A S N M R Mathias et al. have studied the structure of polyamides, such as nylon 6, 11, 12, etc., in the solid state by solid-state 15N NMR at the natural abundance level [17-19]. Figures 12.8-12.10 show typical 15N CP/MAS NMR spectra of nylon samples in the solid state. In Fig. 12.8, the spectrum A was observed for a nylon 6, 10 sample that had been melt-pressed into a clear thin film and annealed to promote formation of the stable a-crystal form. The peak at 83.8 ppm is relatively sharp with a peak width at half-height of 3.2 ppm. The spectrum B is for a nylon 12 sample with the T-form. The peak is located at 88.7 ppm with a peak width of 4.1 ppm. These indicate that there is a chemical shift difference between the crystal forms. The spectrum C is for a nylon 6, 10 sample which had been melt-
458
ISAO ANDO AND TETSUO ASAKURA
yN
6o*c
201~
(c)
,,60' ~ - , ~ o ,;,o",~0" ,6o ~o ~
ppm
~" ~
6
Fig. 12.6.(c) LD/MAS NMR spectra of melt-quenched nylon 66 sample at various temperatures.
pressed and rapidly quenched to room temperature. It shows a broad peak from 83 to 90 ppm which encompasses the residual a-form, which was seen at 84 ppm and possibly a small amount of ,/-form which should be at 89 ppm. The broad peak contribution between these extremes is assigned to the amorphous regions as observed for nylon 6 and 11. The spectrum D of quenched nylon 11 shows a similar broad peak which encompasses the stable a-form at ---84 ppm along with shoulder for the noncrystalline component at 84-89 ppm. Hatfield et al. [20] prepared 15N-labeled nylon 6 to get the spectrum with a reasonable signal-to-noise ratio. This leads to a high quality discussion on the structure of nylon 6 compared with a natural abundance sample. The 15N CP/MAS NMR spectra of nylon 6 samples, which are predominantly amorphous (A), high in a-form crystallinity (B) and high in y-form crystal-
POLYAMIDES
i
ii
:do
i
459
1
~s ppm
i
1;o 80 ,o
,
go'
,
Fig. 12.7. ~3C CP/MAS NMR spectra of nylon with a- and y-crystal forms.
Table 12.4.
Solid-state 13C-NMR chemical shifts of nylon 6
Specimen
C--O
a-CH2
fl-CH2
T-CH2
8-CH2
o~-CH2
a-Form y-Form
171.1 171.0 -0.1 176.4
34.4 35.4 + 1.0 34.6
24.3 28.2 +3.9 24.4
28.1 28.2 +0.1 25.0
28.1 32.0 +3.9 27.2
41.1 37.9 -3.2 39.2
A(~C a
Solutionu
a A(~C : ( ~ C ( y ) - (~C(a),
b Cited from "Carbon-13 NMR of Monomers and Polymers", Sadler Res. Lab., 1981, D388C. linity (C), are shown in Fig. 12.11. The samples high in a- and y-form crystallinity each contain a sharp peak and a broad shoulder, presumably due to crystalline and amorphous nitrogens. This interpretation was confirmed by carrying out the CP T1 experiments and observing 15N (Fig. 12.12). 15 Figure 12.13 shows a 15N C P / M A S N M R spectra for N-labeled and unlabeled nylon 12 samples [18]. The spectrum F is for the labeled sample annealed >50 h and containing ,/-crystal form and noncrystalline regions.
460
ISAO ANDO AND TETSUO ASAKURA
l.
110
,
1
100
,
,!
90
,,
!
ppm
80
,
i
I
70
I
I
60
Fig. 12.8: 15N CP/MAS NMR spectra of commercial nylons: (A) nylon 6, 10 annealed a-form; (B) nylon 12 with y-crystal form; (C) nylon 6, 10 sample melt-quenched" and (D) nylon 11 containing predominantly a-crystal form.
B
10s i~0
9's
9'0
8's
do
7~
7a
6s
60
ppm
Fig. 12.9. 15N CP/MAS NMR spectra of annealed samples of: (A) nylon 7; (B) nylon 6, 8; and (C) nylon 6, 9.
POLYAMIDES
120
100
80
461
60
ppm
40
20
Fig. 12.10. (a) 15N CP/MAS NMR spectra of nylon 13 13 as obtained and (B) after annealing for several hours.
C. h i g h in - t - c r y s t a l l ~ n i l y
J
~..
f B. h i g h in ~ . - c r y s t a l l i n i t y
A. h i g h l y [""
180
_
.
amor
" "--'~! " 168
.......
..... ~_
_ -~ . . . . .
-. . . . . . . . . . . . . . . . . . 9 '"
"-I
"-"
156
" .~.~i
~ .~-v~'i.,
144
9. .
132
I '
120
"
9 "~"
i ..-i
108
,./
,-,
96
, .
~1-.
84
_ , ,
,
,
i ~
72
, ,
9 9
60
PPM
Fig. 12.11. 15N CP/MAS NMR spectra of nylon 6 samples which were: (A) predominantly amorphous; (B) high in a-crystal form; and (C) high in 7-crystal form.
The main peak at 89.3 ppm is assigned to the y-crystal form, while the amorphous peak is a broad shoulder at 86.6 ppm. The spectrum E for the annealed commercial sample shows identical chemical shifts but somewhat different peak shape and a poor signal-to-noise ratio. A shoulder appearing 90.3 ppm in the spectrum of the labeled ),-rich material has been seen. This
"(I) tuaoj [e~s,r (~I) pue 'tuaoj Ie~S,~a~-,,c (O) '.tu.~oj IelS,~.~-x. (:3) '.stuaoj IelS,~a~-x. snld-;o (fl) 'stu.~oj [elsz~a3-X. snld-x~ (V) :s~ldtues ~:I UOlz~U~IqeI!eae (~) ,qle!~a~tutuo~ pue (I) P~PqeI-Ns~ jo e:[l~ds "tlIAIN SVIN/d::) N~ "ft"g[ "g!d ~dd
099
OL
, , l.
.
.
.
Og !
061
OOL , 9
,
Oft i
S .....
O~L :l-
Y
0
9tu:~ol [~,s,~:~a-x, (fl) pu~ tu:~oj [~,s,~:~-w (V) :u! q~!q ~:~,~ ,~q, s~idtues IAIdd 09
i~/.
~8
~
96
got
0~1.
~[:t
1~|
9!;t
S
g91.
~U.lllelS/O~* n
eU:llelrJoo-s,
081.
-y
"8
g9f,
V~IFI>IVSV OFIS.LELL CINV OCINV OVSI
POLYAMIDES
463
peak may belong to material epitaxially crystallized on the surface of ycrystallites or to an ordered interphase region. The spectrum D is for the quenched-labeled sample. The corresponding spectrum of commercially available material is spectrum C. The main peaks of spectra C and D are shifted slightly to low frequency when compared to those of spectra E and F. This may be due to a larger size distribution of smaller crystallites in the former, resulting in a slight shielding effect. The spectra A and B are for mixtures of a- and y-crystal forms in commercial and synthesized materials, respectively. The high frequency resonance occurs at exactly the same chemical shift (89.3 ppm) as that of the sample prepared with mainly the y-crystal form. The peak at 84.1 ppm for the a-crystal form is in agreement with that observed for nylon 6 and 11 with the a-crystal form. 12.3.2
15N CP NMR of oriented polyamide fibers
Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined as mentioned in Chapter 8. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly(p-phenylene terephthalamide) (PPTA) [21], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [22]. Poly(p-phenylene terephthalamide) (PPTA) is a well known rigid polymer with a high Young's modulus and tensile strength [23]. Two research groups have determined independently the crystal structures of PPTA fibers using X-ray diffraction [24-26]. All reflections of the diffraction pattern are indexed satisfactorily by assuming a monoclinic unit cell with a = 7.80-7.87/k, b = 5.18-5.19 ~ and c (fiber axis) = 12.9 A. The molecular conformation in the crystalline region is found to be an all-trans fully extended structure. This conformation is primarily dictated by the extensive conjugation in the chain. The resonance effect has the tendency to stabilize the coplanarity of the amide groups and the benzene rings. Counteracting this effect is the steric hindrance between the oxygen and hydrogen atoms of the amide group, and the ortho-positioned hydrogen atoms of the p-phenylene diamine and terephthalic segments, respectively. Because of these restrictions, the polymer chain adopts an extended conformation with alternating amide group orientations [23]. The solid-state 15N NMR method developed here is used to elucidate the atomic resolution structure of PPTA fibers [27]. For this purpose, the NH and NC' bond directions of PPTA have been determined with respect to the fiber axis, as well as the orientational distribution of the fiber axis, p. These
464
ISAO ANDO AND TETSUO ASAKURA
structural parameters of the polymer chain are compared with those previously obtained by X-ray diffraction [25, 26]. From spectral simulation of the powder pattern, the chemical shielding tensor elements are determined as O'11 - - 4 8 ---+ 3 ppm, ~r22 = 68 --- 3 ppm, 033 = 195 _+ 5 ppm and Oiso = 104 __+4 ppm. The O'iso value is consistent with the chemical shielding from the 15N CP/MAS spectrum (107 ___0.5 ppm). The Euler angles, ai~Nc and /3DNC, which transform the 15N chemical shielding tensor relative to the N - - C ' bond direction (Chapter 8) are determined from the 15N powder pattern of [1-13C]m15N double-labeled benzanilide as 30 and 108 ~ respectively. Then, the cq)Ni-i and/3I~NI-I values are calculated as 0 and - 7 ~ respectively, using 115 ~ for the HNC' bond angle [24]. Figure 12.14 shows the 15N C P NMR spectra of the oriented PPTA block sample, whose fiber axis is set parallel (A) and perpendicular (B) to B0. Although the narrow linewidth of the peak in Fig. 12.14(A) indicates a highly oriented PPTA sample, a symmetric spectrum would be observed for both
iO
300
200
100
0
- 100
ppm from
15N.tI4NO 3
(B) I ' ' " ' ' l ' " ' r ' ~ " ' ' ' ' ' l ' " " ' ' ' l ' " ' ' l
300
200
I00
. . . .
I
. . . .
0 ppm
I
. . . .
I ' '
-I00
from I'~N.H,,NO.-,
Fig. 12.14. Experimental (solid line) and calculated (broken line) 15N solid-state NMR spectra of the oriented PPTA block sample set parallel (A) and perpendicular (B) to Bo.
POLYAMIDES
465
orientations if the fibers were perfectly aligned with respect to the magnetic field. However, the observed lineshape when the fiber axis is perpendicular to Bo is clearly asymmetric and, therefore, a fiber axis distribution is introduced. The best-fit parameters determined from simulations (broken lines) of the observed spectra in Fig. 12.14 are: av = 45 ~ /~F = 61 ~ and p = 19~ The angle between the NH and the fiber axis, 0NI-I, was calculated as well as the angle, 0Nc,. These coincide with data from an X-ray diffraction analysis; 0NH = 68~ and 0NC' = 49 ~ from NMR; and 0NIJ = 66 ~ and 0NC' = 49 ~ from X-ray diffraction analysis. A similar structural analysis with solid-state 15N NMR was performed for poly(m-xylene-a,a'-diyladipamide) and nylon 66 [281. Most recently, the influence of the application of macroscopic tensile stress to the PPTA fiber on the microscopic dynamic structure has been studied with deuterium NMR [29]. The aromatic polyamides, PMIA and P4M-MPTA, have excellent thermal stability and high tensile modulus. The structures of these polyamides in the crystalline domain have been reported by using X-ray diffraction methods [30, 31]. However, the fraction of the noncrystalline domain has been reported as -~70% for PMIA [30] and 75% for P4M-MPTA [31]. Thus, the atomic level analysis of these polymer structures in the noncrystalline domain is particularly required. 15N CP/MAS NMR spectra of PMIA and P4M-MPTA fibers indicated single peaks. The 15N chemical shielding values are 108 ppm (PMIA) and 107 ppm (P4M-MPTA). However, these peaks are asymmetric and broad (especially for P4M-MPTA), indicating the presence of a noncrystalline domain in addition to the crystalline domain. Actually, oriented PPTA powder with more than 80% crystalline domain gives a sharp and narrow single peak. Figure 12.15 shows the solid-state 15N NMR spectra of oriented blocks of PMIA and P4M-MPTA fibers placed parallel to the applied magnetic field (solid line). There is a sharp peak at approximately 70 ppm with broad peaks to higher frequency for each spectrum. This spectral tendency is the same between both spectra although the intensity of the broad peak increases in the P4M-MPTA sample. As mentioned in Chapter 8, the 15N chemical shieldings of oriented polymers placed parallel to the magnetic field are sensitive to the angle 0NIJ, and a plot of the ~SN chemical shielding vs. 0NIJ can be used to determine this angle. In order to resolve the spectra, the peak simulations are performed by assuming them to be Gaussian. The presence of at least three peaks in each spectrum is suggested from the peak simulation (broken line) as shown in Fig. 12.15, where the three peaks are noted as A, B and C. The fraction of each peak was determined by the simulation along
466
ISAO ANDO AND TETSUO ASAKURA
300
200
100
0
ppm
(b)
I-~" 300
" - I'"
-100 from
t:lN|14NO3
^
- -""! 200
- " - ~'i ....
! .... 100
"1" ~ ' ' ' ~
" i "'+ + " T " ' ~ 0
ppm from
- 100 I$~H4NO.I
Fig. 12.15. (a) Solid-state 15N NMR spectral simulation of PMIA and (b) P4M-MPTA oriented samples set parallel to Bo. The solid and broken lines represent observed and simulated spectra, respectively. The simulated peaks are composed of three components A, B and C.
with the chemical shielding and the half-height width. As indicated by arrows in Chapter 8, it is possible to determine the range of 0NI-I values for each peak and these are also listed in Table 12.5. The angles, 0yi-i, 75 or 80~ for PMIA, and 75 or 94 ~ for P4M-MPTA are determined for peak C in both spectra. These are in agreement with the Xray diffraction data and, therefore, peak C can be assigned to the ~SN nuclei of these samples in the crystalline domain. This assignment seems reasonable by judging from the fact that peak C is the narrowest one of the three peaks. However, the fraction of peak C is slightly different from those of the crystalline domain determined by the X-ray diffraction, 30% for PMIA and 25% for P4M-MPTA [30, 31]. This difference between X-ray diffraction and the solid-state NMR results might come from ambiguity in the determination by the X-ray diffraction method, i.e., in the determination of the base line coming from the noncrystalline domain. However, if the sequence of the chains with similar 0yi-i values is too short to form the domain, which is
POLYAMIDES
467
Table 12.5. The 15NCP parallel spectral parameters for oriented PMIA and P4M-MPTA block samples placed parallel to the applied magnetic field. The fraction of each peak and the range of the angle, 0N~ between the NH bond and the fiber axis are also listed
Oriented block samples Parameters
Fraction (%) 6 (ppm) half-height width (Hz) ~ H range (degree)
PMIA
P4M-MPTA
A
B
C
A
B
C
37 160 1000 25-35
20 105 880 46-58
43 68 400 67-90
61 149 1100 31-42
18 106 520 44-56
21 72 520 66-90
recognized as a crystalline domain from the X-ray diffraction viewpoint, the disagreement between NMR and X-ray diffraction can be interpreted. That is, the solid-state NMR gives more local structural information than X-ray diffraction. The peaks A and B in both spectra clearly originate from the noncrystalline domain in the samples, indicating that the noncrystalline domain reported from X-ray diffraction has a relatively ordered structure rather than a randomly distributed one. This conclusion derived from NMR is also supported by the conclusion that the noncrystalline domain is highly oriented in aromatic polyamides on the basis of X-ray diffraction studies [30, 32]. The relatively broad peaks show a wider distribution for each 0NH value compared with peak C, which is also reasonable. The chemical shieldings of the peaks A and B are almost the same between PMIA and P4M-MPTA. This indicates that the local structure in the noncrystalline domain is similar for these polyamide fibers. It has been reported that the fraction of noncrystalline domain in the P4M-MPTA sample is higher than in the PMIA sample. The increase in the fraction of the noncrystalline domain comes predominantly from the contribution of peak A, i.e., the structure with 0NH = 31--42 ~ which is derived from the solid-state NMR experiment. If the noncrystalline domain represents a random distribution of 0NH values in the oriented PMIA and P4M-MPTA samples, the 15N CP NMR spectrum would be a powder pattern. Clearly, the noncrystalline domains are not composed of randomly dispersed polymers, but exhibit a significant degree of order with remarkably well-defined 0NH values. In addition, the ~SN CP/MAS NMR spectra shown in Fig. 12.15 give limited structural information because, basically, only a single peak is observed in each spectrum, but the solid-state 15N NMR spectra of oriented blocks of these fibers give more fruitful structural information. The latter comes from the high sensitivity of the 15N chemical shielding to the bond orientation, 0yH.
468
ISAO ANDO AND TETSUO ASAKURA
References
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
28. 29. 30. 31. 32.
D.R. Holmes, C.W. Bunn and D.T. Smith, J. Polym. Sci. 19 (1956) 401. H. Miura, J. Hirschinger and A.D. English, Macromolecules 23 (1990) 2169. I.-S. Chuang and G.E. Maciel, in G.A. Webb (Ed), Annual Reports on NMR Spectroscopy, vol. 29. Academic Press, London, 1994, pp.169-274. K. Kubo, T. Yamanobe, T. Komoto, I. Ando and T. Shiibashi, J. Polym. Sci. B27 (1989) 929. K. Kubo, I. Ando, T. Shiibashi, T. Komoto and T. Yamanobe, J. Polym. Sci. B29 (1991) 57. H.R. Kircheldorf, J. Macromol. Sci. Chem. All (1977) 2281. W.L. Earl and D.E. VanderHart, Macromolecules 12 (1979) 762. I. Ando, T. Yamanobe, T. Sorita, T. Komoto, H. Sato, K. Deguchi and M. Imanari, Macromolecules 19 (1984) 1955. I. Ando, T. Sorita, T. Yamanobe, T. Komoto, H. Sato, K. Deguchi and M. Imanari, Polymer 26 (1985) 1864. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M.J. Albright, The 20th Meeting of NMR, Tokyo, 1981, p. 68. M. Takenaka, T. Yamanobe, T. Komoto and I. Ando, J. Polym. Sci. Polym. Phys. 25 (1987) 2165. T.L. Weeding, W.S. Veeman, H.A. Gaur and W.G.B. Huysmans, Macromolecules 21 (1988 ) 2028. A. Okada, M. Kawasumi, I. Tajima, T. Kurauchi and O. Kamigaito, J. Appl. Polym. Soc. 37 (1989) 1363. C.O. Johnson and L.J. Mathias, Polymer 35 (1994) 66. P. Holstein, J. Spevacek, D. Geschke and V. Thiele, Prog. Colloid & Polym. Sci. 85 (1991) 60. H. Ketels, L. van de Ven, A. Aerdts, G. van der Velden, Polymer Commun. 30 (1989) 80. D.G. Powell, A.M. Sikes and L.J. Mathias, Polymer 32 (1991) 2523. L.J. Mathias and C.G. Johnson, Macromolecules 24 (1991) 6114. C.G. Johnson and L.J. Mathias, Polymer 34 (1993) 4978. G.R. Hatfield, J.H. Olans and W.B. Hammond, Macromolecules 23 (1990) 1654. J.-H. Yeo, M. Demura, T. Asakura, T. Fujito, M. Imanari, L.K. Nicholson and T.A. Cross, Solid State NMR 3 (1994) 209. T. Asakura, J.-H. Yeo and I. Ando, Polym. J. 26 (1994) 229. E.G. Chatzi and J.L. Koenig, Polym. Plast. Technol. Eng. 26 (1987) 229. S. Sasaki and I. Uematsu, J. Polym. Sci. Polym. Phys. Ed. 23 (1985) 263. M.G. Northolt and J.J. van Aartsen, J. Polym. Sci. B l l (1973) 333. K. Tashiro, M. Kobayashi, and H. Tadokoro, Macromolecules 10 (1977) 413. P.T. Lansbury, Jr., P.R. Costa, J.M. Oriffiths, E.J. Simon, M. Auger, K.J. Halverson, D.A. Cocisko, Z.S. Hendsch, T.T. Ashburn, R.O.S. Spencer, B. Tidor and R.G. Griffin, Nature Structural Biology 2 (1995) 990. J.-H. Yeo, T. Asakura and H. Shimazaki, Makromol. Chem. Phys. 195 (1994) 1423. D.J. Schaefer, R.J. Schadt, K.H. Oardner, V. Gabara, S.R. Allen and A.D. English, Macromolecules 28 (1995) 1152. H. Kakida, Y. Chatani and H. Tadokoro, J. Polym. Sci. Polym. Phys. Ed. 14 (1976) 427. K. Okuyama, H. Hidaka and H. Ichige, Macromolecules 22 (1989) 3776. H. Tadokoro, Sen-i Gakkaishi 31 (1975) P-278.
Chapter 13
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Poly(imide)s Andrew K. Whittaker Centre for Magnetic Resonance, University of Queensland, Queensland 4072, Australia
13.1
Introduction
Poly(imide)s as a class of polymer exhibit a range of properties, such as high Tg, excellent thermal stability, high chemical resistance, low dielectric constant and ease of fabrication, which have lead to important uses in the semiconductor and advance composite industries. In addition, the high aromatic content of many of these polymers and consequent high stability to ionizing radiation, leads to usage of poly(imide) films and composites in the nuclear and aerospace industries. Many poly(imide)s are insoluble in their processed form, either because of interchain charge-transfer interactions, or because of the presence of crosslinks in cured poly(imide) resins. The range of analytical techniques available to characterize processed poly(imide)s is therefore limited. NMR spectroscopy, and in particular solid-state NMR [1-3], has an important role to play in the determination of structure, conformation, morphology and molecular motion in poly(imide) materials. The aim of this chapter is first, to briefly summarize the various classes of poly(imide)s, second, to review the current literature on NMR of these materials and finally, to hopefully indicate where NMR spectroscopy will make further additions to the knowledge of the properties of poly(imide)s. 13.1.1
Condensation poly(imide)s
Poly(imide)s first became commercially important with the development of the condensation poly(imide) Kapton [4, 5] in 1965. The two-step reaction of a dianhydride (pyromellitic dianhydride) with a diamine (p-phenylene diamine) to initially form a poly(amic acid), and subsequent thermal cyclization to form the poly(imide), is a common route to the formation of poly(imide)s, as well as being exploited for the synthesis of oligomeric precursors for addition poly(imide)s. Usually, such condensation polymers are insoluble
470
ANDREW K. WHITTAKER
and intractable after imidization, and usually processed as films of poly(amic acid)s, and then heated to form the final poly(imide) product. Earlier experimental effort was concentrated on the study of the complex reactions involved in the formation of the poly(amic acid) precursor, and the subsequent conversion to poly(imide) [6]. Since much of the early work on characterization of the imidization occurred before the advent of commercial CPMAS equipment in the late 1970s, the technique of choice was infrared (IR) spectroscopy [7-10], except for the few examples in which the final poly(imide) remained soluble. An alternative route to the formation of poly(imide)s is the nitro-displacement reaction to form the Ultem series of polymers, first exploited by White et al. [11] at General Electric. These, and similar materials, have application in composite materials and as specialty thermoplastics. Compared to the amic acid route described above, the nitro-displacement reaction is highly controlled, and materials of high chemical regularity produced, as demonstrated by White et al. [11] in their solution-state NMR study of Ultem poly(imide)s.
13.1.2
Addition poly(imide)s
In the early 1960s, a new class of addition polymer and addition poly(imide)s was developed by the Rhone Poulenc company. The most important of these were bis-maleimides (BMI) [12, 13], which could be crosslinked or copolymerized to form thermosets with outstanding thermal and chemical resistance. These materials cure without volatile by-products, thereby, minimizing the formation of voids, and have high glass transition temperatures and low moisture absorption. The major uses of this class of resin is in advanced composites and printed circuit boards. One of the major drawbacks of BMI resins is their extreme brittleness, which is a result of high crosslink density and absence of flexible spacers. Several different approaches have been adopted to alleviate this problem, including copolymerization with a range of monomers, or blending with thermoplastic or elastomeric polymers [14-19]. The maleimide group in BMI can undergo a wide range of possible reactions, either in the neat resin or copolymerized, with other monomers. The predominant reaction is the free radical chain reaction of the double bond ([20, 21] and references therein), which, due to the difunctionality of BMI monomers, results in a crosslinked three-dimensional network. Maleimides have been shown to undergo copolymerization with a number of monomers including methyl methacrylate [22, 23], styrene [22-24], acrylonitrile [22] and
POLY(IMIDE)S
471
2-ethylhexyl vinyl ether [22]. Other copolymerizations can be effected by the Michael addition reaction with primary amines [25-27], thiols [28-30]; the ene-reaction and subsequent Diels-Alder addition reaction with allyl phenol compounds [31], and direct Diels-Alder addition to diene monomers such as propenylphenoxy compounds [32]. A comprehensive review of aspects of the chemistry of BMI resins has been given by Stenzenberger [33]. Acetylene-terminated prepolymers [34] have the attraction that crosslinking is likely to proceed by cyclotrimerization of the acetylene endgroups, thus resulting in a three-dimensional network without evolution of volatile products. An additional advantage of these materials is their hightemperature thermal stability. Hergenrother [35] has described a large number of structures of potential acetylene-terminated resins. Greater control over the cure of this class of materials is obtained by using the phenylacetylene end cap, which undergoes polymerization at --~120K above the temperature of cure of the acetylene end caps. More recently, processing has been facilitated by Diels-Alder copolymerization with maleimide monomers [36]. The PMR range of resin materials utilize a two-step reaction, in which a norbornene-terminated prepolymer is formed in situ by the condensation reaction of low molecule weight diamines with norbornene-capped imides or acid esters [37]. The PMR-15 designation refers to polymerization of monomeric reactants, with a mixture of diaminodiphenylmethane and a dimethyl ester and a norbornene-containing monomethyl ester, to form a condensation prepolymer having molecular weight approximately equal to 1500. PMR resins are used as matrix materials in high performance composites in the aerospace industry.
13.2
NMR studies of polyimides
13.2.1 Characterization of poly(imide)s and studies of curing of poly(imide)s by 13C and 1H NMR 13.2.1.1 Studies of conversion of poly(amic acid)s to poly(imide)s As mentioned above, IR spectroscopy has often been used to study the conversion of amic acids to imides. Despite the popularity of this technique, important questions have been raised concerning the quantitative aspects of the results, especially at high conversion [38]. NMR spectroscopy, in particular solution-state NMR, has the advantage of greater chemical information, and when care is taken, provides quantitative measurements of the extent of conversion of amic acid to imide. The situation is less clear with insoluble poly(imide)s, since increased linewidth [39] in solid-state ~3C NMR spectro-
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ANDREW K. WHITTAKER
scopy reduces the information content in the spectrum. However, it is clear that the results of NMR investigations have cast doubt on the quantitative aspects of the IR analysis of poly(imide)s. In addition, NMR has the advantage of much greater flexibility in the sample type, including, primarily, the ability to analyze composite materials. In the following two sections the results of 13C NMR studies of poly(imide)s are reviewed. Seshardi et al. [40] studied the preparation of acetylene-terminated imide oligomers, using 13C solution-state NMR. The chemical shifts for carbons in a large number of model compounds was determined. The conversion of amic acid precursor to imide macromer was studied after heating in DMSOD6 solution to 423 K for up to 90min. New peaks in the carbonyl and acetylenic regions were observed and assigned to imide formed by cyclodehydration, as well as the amine monomer formed by hydrolysis. Under the conditions of this experiment, water was not removed from the reaction during heating and was available for hydrolysis. The extent of the cyclodehydration reaction at the two distinct sites of his particular amic acid oligomer was followed by comparing the extent of imidization evident in the acetylenic and carbonyl regions of the spectra. The complexity of the formation of poly(amic acid) was highlighted by Denisov et al. [41], who studied the structures formed by condensation of four different aromatic dianhydrides with both p-diphenylenediamine and benzidine, using 13C solution-state NMR. The authors assigned the complex spectra of the mixtures of poly(amic acid) isomers by comparison with the spectra of model compounds, and the consideration of the well-known effects of substituents on the 13C chemical shifts of aromatic carbons. For example, it was found that the poly(amic acid) formed by reaction of pyromellitic dianhydride with both diamines was 60% in the cis-isomer and 40% in the trans-isomer. More complex spectra were obtained for the poly(amic acid)s formed from the asymmetric dianhydrides. In all cases, the proportion of isomers was independent of the type of diamine and depended only on the dianhydride. A correlation was made between the effect of electron density on the relative rates of formation of the respective isomers, and the difference in chemical shifts of the quaternary aromatic carbons attached to the anhydride. Schneider et al. [42] reported the 13C CPMAS spectra of the poly(amic acid), poly(isoimide) and poly(imide) of pyromellitic dianhydride and diaminodiphenylmethane. All the spectra were poorly resolved, as is typical for these polymers and in particular, the peaks due to imide and amic acid carbonyl carbons were not resolvable. It is concluded that solid-state ~3C NMR is not a suitable method for measuring the extent of imidization in this
POLY(IMIDE)S
473
system. On the other hand, these authors present measurements of the extent of imidization based on IR spectroscopy. The spectra reported by Grenier-Loustalot et al. [43, 44] highlight the difficulties in resolving peaks due to the acid and amide in many poly(amic acid)s. They reported a difference in chemical shift of <1 ppm between the peaks due to amic acids (167.3 and 167.0ppm) and amide (166.3 and 166.4 ppm) [43]. The average linewidth in solid-state 13C NMR spectra of solid amorphous poly(amic acid)s is usually much larger than this. The authors studied the reaction of pyromellitic dianhydride with p-toluidene, as a model of the reaction of the anhydride with the diamine. A slight excess of the cis-isomer was observed for the room temperature condensation reaction. The solid-state spectra of these model compounds allowed resolution of the two peaks due to the carbonyls in the acid and amide residues, however, the peak due to the carbonyl in the insoluble imide model was coincident with these two peaks. FTIR spectroscopy proved a more reliable method for following the extent of conversion from both isomers to imide. The authors also reported the 1H NMR spectra of these materials. In a later study, Grenier-Loustalot et al. [40] examined the reaction of monoacids and monoamines as models for the condensation reaction. Peaks due the acid and amide were resolved in the solid-state NMR spectra, however, the linewidths in the spectra of these model compounds were appreciably smaller than those expected for the conformationally-complex poly(amic acid). In this paper, the authors used HPLC and FTIR to determine the effect of size and basicity of the amine, as well as the removal of water on the kinetics of reaction. Full imidization was achieved only on the addition of catalysts to the reaction mixture. Matsuura and coworkers [45] used 1H NMR spectroscopy to follow the imidization of poly(imide)s of a dianhydride containing a hexafluoropropane linkage with diamine having fluorinated methyl substituents. These materials were soluble in a range of common organic solvents after complete imidization. The 1H NMR spectra were very high quality, and allowed the extent of imidization to be evaluated quantitatively. The analysis of the spectra, which consisted of integration of the peak due to the carboxylic acid proton with respect to the rest of the spectrum, is reliant on the absence of water in the solvent, which may distort the intensity of the peak due to acid protons. In a later study, Ando et al. [46] exploited the higher resolution of 13C solution-state spectroscopy to examine in more detail the mechanism of reaction of these poly(imide)s. The spectra of the three amic acid isomers were assigned using model compounds. On curing at 393 K, peaks appear in the 13C NMR spectrum due to the imide, while there is evidence of amine
474
A N D R E W K. WHITTAKER
chain ends caused by depolymerization of the poly(amic acid). Curing at 473 K results in complete imidization of the poly(amic acid), while the measurement of solution viscosity indicates an increase in molecular weight at higher curing temperature. 13.2.1.2 Characterization of structure of cured materials Havens and coworkers [47] studied the 13C CPMAS spectra of Kapton, PMR poly(imide)s and several model compounds, as part of a larger study of the spectroscopy of poly(imide)s materials [48, 49]. The spectra of the amorphous polymers were considerably broader than those for the crystalline model compounds, which was ascribed to the distribution of conformations frozen in the disordered solid state. Assignments to the spectra were made partly by comparison with independent calculations of electron densities, which were related to the 13C chemical shift by the empirical equation of Alger et al. [50]. As discussed below, there is some doubt over these assignments [51, 52]. Variations in the 13C chemical shifts of the carbonyl carbons, throughout the series of models and polymers, were interpreted as indicating an increase in the degree of conjugation at the carbonyl carbon in Kapton compared with the model compounds. Similar conclusions were made for the PMR material. O'Donnell and Whittaker [51] reassigned the 13C CPMAS spectrum of Kapton, in part after consideration of the results of the dipolar dephasing experiment, and of the relative peak intensities in the NMR spectra. In addition, spectra were also obtained of solutions of Kapton in concentrated sulfuric acid. Several of the peaks in the high-quality spectra were split into doublets, which was ascribed to the presence of two different rotational conformers having equal energy. Schneider and coworkers [52] also reported the CPMAS spectra of Kapton poly(imide). The dipolar dephasing experiment was also used to aid assignment of the spectra, and it was also concluded that the assignment by Havens et al. [50] of the peak at 119-122 ppm to the nonprotonated aromatic carbons is incorrect. Nonetheless the assignment of this peak to the protonated benzimide carbon does not agree with the assignments of O'Donnell and Whittaker [51]. The disagreement between these three groups reflects the very poor resolution in the solid-state spectra of this particular poly(imide). Yang et al. [53] prepared a novel series of metal-containing poly(imide)s. Polymers of pyromellitic dianhydride with the zinc, strontium, lead, calcium and nickel salts of p-aniline sulfonic acid, were prepared and examined by 13C CPMAS NMR. There was little difference in the chemical shifts of the dianhydride carbons, compared with the chemical shifts of the poly(imide) with diaminodiphenyl methane. Finally, Marek and coworkers [54] characterized a series of poly(imide)s
POLY(IMIDE)S
475
of 3,3',4,4'-benzophenonetetracarboxylic dianhydride with 4,4'-(alkane-l,ndiylidoxy)dianilines by IR and 13C solid-state NMR. The length of the main chain alkane spacer was varied from n = 4-10. The poly(amic acid)s were confirmed by XRD to be completely amorphous, however, imidization by heating at up to 588 K resulted in semicrystalline materials. 13.2.1.3 Charge-transfer complexes in poly(imide)s Many poly(imide)s are highly coloured and insoluble, evidence in both cases for strong intermolecular association [55, 56]. The high insolubility of many poly(imide)s has precluded the determination of the equilibrium constants for formation of the charge-transfer complex. Dinan and coworkers [57] measured the 13C CPMAS NMR spectra of mixtures of two structurallysimilar model compounds for a poly(imide). Four new peaks were seen in the spectrum of the mixture prepared by solution mixing of the two models, compared with the spectrum of the mixture formed by mechanical mixing (see Fig. 13.1). The spectra of the intimate mixture, which is suggested to result in the formation of charge-transfer complexes, shows that the methoxy chain end, as well as the internal aromatic carbon adjacent to the bridging ether group in the dianhydride, exists in two isomers, whereas a single conformation is present in the pure material. An energy minimization program was used to generate low energy conformations of the molecules in question, and the molecules were then superposed to allow association of phenyl rings necessary for the formation of a charge-transfer complex. An arrangement of molecules was found in which each of the methoxy and phenyl carbons are in close proximity to one another. The authors proceeded to investigate the importance of planarity of the dianhydride molecules on charge-transfer complexation. In each case the appearance on new peaks in the solid-state 13C NMR spectra (Fig. 13.1), compared with the solution-state spectra, was taken as evidence for complexation. 13.2.1.4 Poly(imide) blends There is much interest in the formation of blends of poly(imide)s with other polymers, so as to improve properties such as toughness and processability [14-19, 58, 59]. The subject of measurement of interactions and miscibility of blends by NMR spectroscopy has been discussed by Takagoshi and Asano in Chapter 10 of this book, and will not be referred to in detail here. The use of 13C NMR to study miscibility in blends containing poly(imide)s is somewhat restricted because most poly(imide)s contain a high proportion of aromatic groups, and consequently form blends with other highly aromatic polymers. The 13C CPMAS spectra, which as discussed above are broad and
476
A N D R E W K. W H I T T A K E R
(b)
i
I
200
I
150
i
100
I
50
|
0 ppm
(a)
I
200
I
150
I
I
1
100
50
0
ppm
Fig. 13.1. Solid-state
13C CPMAS spectra of 1:1 molar ratio mixtures of two nitro- and methoxy-terminated diimides. Spectrum (a) was obtained from a physical mixture, while spectrum (b) is of a mixture recrystallized from DMF. The arrows on spectrum (b) indicate the new peaks observed in the intimate mixture. (Reproduced from Ref. [57] with permission. 9 1992 John Wiley, New York.)
poorly resolved, become further complicated by the presence of overlapping peaks from each component of the blend. Nonetheless, several detailed studies have been reported in the literature [60-62]. MacKnight and coworkers found that several poly(imide)s, such as Ultem poly(ether imide) and XU218 poly(imide), formed miscible blends with poly(benzimidazole) (PBI) [58, 59]. The miscibility of these blends is believed to result from specific interactions between the benzimide ring of the poly(imide) and benzimidazole ring of PBI. In a later study, Grobelny et al. [60] reported the ~3C CPMAS spectra of several of these blends. Slight changes in the lineshape were observed in the intimate blends, and seen as evidence of the specific interactions between the polymers. In particular, the peak in the spectra, due to the imide carbonyl carbon, was seen to broaden towards
POLY(IMIDE)S
477
higher chemical shift on blending with PBI. Two poorly-resolved peaks, assigned to H-bonded carbonyl groups, were seen to increase in intensity with increasing proportion of PBI in the blends. In addition, the authors also measured 1H T1 from the changes in peak intensity with increasing crosspolarization contact time. For the blend of XU218 and PBI, a single aH Tip, having a value intermediate to that of the two pure homopolymers was observed, and it was concluded, therefore, that the two polymers were mixed on a molecular scale. The experiment was inconclusive for the Ultem/PBI blend, since the values of ~H T1 for the homopolymers were very similar. Kostereva and coworkers [61] reported solution viscosity, IR and 13C CPMAS measurements of blends of PBI and the poly(amic acid) and poly(imide) of Kapton. The presence of PBI in the solution with the poly(amic acid) of Kapton slowed dramatically the decrease in viscosity ascribed to breakage of the amide linkage of the amic-acid. This was seen as evidence of close association of the two polymer chains. 13C CPMAS spectra, reported for samples after heating to high temperature (up to 823 K), showed an increase in the rate of opening of the imide group to form a three-dimensional crosslinked network compared with pure poly(imide). Feng and coworkers [62] studied miscible blends of several different poly(imide)s, including ether- and thioimides. However, changes in the 13C CPMAS spectra were observed and the authors incorrectly interpreted changes in the relative amplitudes of the peaks in the spectra. The results demonstrate the difficulty in analyzing spectra of mixtures of chemicallysimilar polymers. In the second part of the paper, the authors reported a study of 1H Tip for blends of the poly(thioimide) and a poly(imide) based on biphenyl dianhydride. A bi-exponential 1H T1 decay curve was observed for the latter polymer, and for blends containing this polymer, which is semicrystalline. The proportion of the slower Tip decay decreased with decreasing proportion of the biphenyl poly(imide). The calculations of the average sizes of the domains in the homopolymer and blends are in reasonable agreement with results of SAXS measurements.
13.2.2
NMR studies of motion and chain conformation in poly(imides)
A large number of linear aromatic poly(imide)s are highly intractable and cannot be processed due to the inherent stiffness of the chains and, more importantly, due to strong interactions between aromatic rings on adjacent polymer chains. An approach to improve the processability of such "stiff macromolecules" is to attach flexible alkoxy chains to the aromatic rings (see, for example, Refs. [63-65]).
478
ANDREW K. WHITTAKER
The group at the MPI-P in Mainz reported a large body of work concerned with these polymers, including NMR studies of aromatic poly(imide)s with flexible alkoxy side chains. Generally, this type of material can exist in two distinct polymorphs, labelled either A or B, which differ in the average spacing between layers of the stiff aromatic backbones. Polymers in the Aform have an average interlayer spacing which is consistent with the alkoxy chain being close to perpendicular to the polymer main chain, while in the B-form, the spacing indicates that the alkoxy chains lie at an angle close to 50 ~ to the main chains. By considering the WAXD measurements and the 13C CPMAS spectra, Adam and Spiess [66] were able to demonstrate that the spacing depends to a large extent on the arrangement of the rigid main chains. The ~3C CPMAS spectra of the poly(imide)s show splittings of the lines because of the aliphatic carbons due to the presence of two populations of chain--those having the all-trans-arrangement and those having a proportion of gauche defects present in the side chains. The latter population of chains exists in regions of relatively high disorder. An analysis of the relative intensities of the peaks due to side chains in the disordered and ordered regions showed that the methyl end-groups on the side chains were onaverage in the regions of higher order than the methylene groups in the centre of the side chains [67]. It was concluded that a large proportion of the methyl end-groups reside within the ordered crystalline regions of the polymer. Spectra published later [68] show splittings of the peaks due to the imide carbonyl and the unprotonated phthalimide aromatic carbon, which can be interpreted as being due to different conformations of the polymer main chain. In the final paper concerned with the aromatic poly(imide)s with alkoxy side chains, Clauss et al. [68] used the WISE experiment [69] to study the molecular motion of the carbons giving rise to the various peaks in the ~3C CPMAS spectra as shown in Fig. 13.2. The results indicate that the side chains in the poly(imide) can exist either in a very rigid crystalline phase of high conformational purity, or in a disordered phase with a degree of heterogeneity of motions. Spin-diffusion measurements were then used to estimate the sizes of the crystalline domains as being approximately twice the layer spacing of the rigid polymer main chains. A pictorial representation of the structure of the poly(imide) is shown in Fig. 13.3. These papers present a very detailed study of the details of the chain packing in the solid state of aromatic poly(ester)s, poly(amide)s and poly(imide)s, and demonstrate the potential of solid-state NMR to provide information of conformation, dynamics and spatial arrangements in complex materials. The sensitivity of the ~3C chemical shift to chain conformations has been exploited also by Kricheldorf and coworkers in their comprehensive studies
POLY(IMIDE)S
479
PPPI
IH
13 c
C2-C~3
A
t tons l/gauche
L~O ....
3'0
|
2'0 pprr
,
,
,j
.
.
.
.
.
.
J,
,
150 kHz
!
,
I
i C
20 PF,,, IU
100kHz
Fig. 13.2. Expansion of the aliphatic region of the 2D WISE-NMR spectrum of poly(pphenylene pyromellitimide) having two C-16 alkoxy side chains. Also shown are the projections in the 13C and 1H dimensions. (Reproduced from Ref. [68] with permission. 9 1992 American Chemical Society, Washington, DC.)
of the properties of main chain aliphatic-aromatic liquid crystalline poly(ester imide)s ([70, 71] and references therein). The 13C CPMAS spectra [70] revealed the presence of stable all-trans-conformations in the alkyl spacers at 373 K, which contrasts with the results of aromatic polymers containing aliphatic side-chains discussed above [66-68]. This stability of the alkyl spacers is consistent with the chains being attached at both ends to relatively immobile mesogenic groups. In a later publication [71], the authors report that the conformation of alkyl chains depend on small changes in the structure of the mesogen. The spectra of the poly(ester imide)s prepared in this study indicate a high proportion of all-trans-conformation, however, there is a degree of nonaveraged conformational disorder up to higher temperatures revealed by the diffuse nature of the peak due to the methylene carbons. It appears that these polymers possess a disordered phase having nonetheless a high proportion of trans-conformations. Copolymers having alkyl spacers of different lengths display two-phase behaviour.
480
ANDREW K. WHITTAKER
PPPl-16 Fig. 13.3. Schematic representation of the morphology of poly(p-phenylene pyromellitimide) having two C-16 alkoxy side chains, derived from NMR experiments as described in the text, and indicating the domains extending over more than one layer spacing. (Reproduced from Ref. [68] with permission. O 1992 American Chemical Society, Washington, DC.)
13.2.3
Characterization of poly(imide)s by
15N
NMR
15N NMR promises to be a powerful method for determining the structure of polymers [72]. Although the low natural abundance of ~SN necessitates expensive isotopic enrichment, the wealth of information obtained from the NMR spectra more than compensates for it. The power of 15N NMR is the relatively large range of chemical shifts compared with the 15N linewidth and, thus, spectra are often relatively simple. Cross-polarization can be used to enhance the signal-to-noise ratio of this low gyromagnetic ratio nucleus. Murphy et al. [73] studied the cure and degradation of an acetyleneterminated 15N-labelled poly(imide) using 15N CPMAS NMR. Initially, the conversion of the amic acid to the imide precursor was followed. Four resolved peaks are observed due to amide and imide either attached to a phenyl ring or at the terminal position. Measurements of the rate of crosspolarization, and the dipolar dephasing experiment, assisted in the assignment to the spectra. Very different rates of cross-polarization (1/TNH), and values of 1H T1, were measured for the various structures. Imidization was incomplete after heating to 670 K for 1 h, a result at variance with the results of
POLY(IMIDE)S
481
IR measurements. A cured sample of poly(isoimide) also revealed incomplete imidization. The sensitivity of the structures to temperature and humidity were also studied. Recently, Jarrett et al. [74] characterized the magnitude and orientation of the chemical shift tensor of the 15N nuclei in model compounds for poly(imide)s. The principal values of the chemical shift tensors were evaluated from the nonspinning spectra of four model compounds. The shift tensors span --~120 ppm with the 622 element showing the greatest sensitivity to changes in structure. The orientation of the shift tensor was determined by using 13C~15N doubly-labelled materials. Experiments of this type make possible the sophisticated multidimensional NMR experiments for determining order and dynamics in polymers, as described by Schmidt-Rohr and Spiess [3].
13.2.4
NMR studies of water and other species in polyimides
The ability of poly(imide)s to absorb significant amounts of water has important implications for the use of these materials as low dielectric materials. For example, Kapton film, which is used in the semiconductor industry, absorbs --~3% of its weight in water [75]. In a series of papers, Li and coworkers [75-77] studied the state of the water in Kapton films. In their early work [75], two distinct peaks were observed in the dielectric relaxation spectra at 140 and 210 K at a 1 kHz measurement frequency. The relative strengths of the two relaxations were dependent on sample thickness and, hence, morphology and moisture content. In later work [76, 77], the authors identified a composite, angular-dependent lineshape in the 2H NMR spectra of films treated with deuterium oxide. By using saturated salt solutions the authors were able to vary the equilibrium moisture content of the films. The NMR linewidth was found to decrease with increasing moisture content--a result attributed to the presence of two superimposed lines contributing to the NMR spectrum. Similar conclusions were drawn from measurements of ~70 NMR spectra of films loaded with 170-enriched water [77]. On the basis of comparison with the results of dielectric measurements, the low temperature relaxation was associated with motion of the water molecules giving rise to the narrow NMR spectrum. The two types of water were assigned to water strongly associated with the polymer chain, and small clusters of water molecules most likely residing in defects in the polymer matrix. The latter class of water molecules are likely to be more mobile and possibly contribute to ionic conductivity. In a related study, Turoscy and coworkers [78] identified two types of ions in Kapton poly(imide) films using 23Na, 7Li and 133CsNMR. For each system,
482
ANDREW K. WHITTAKER
the NMR spectrum became narrower and moved closer to a chemical shift on the addition of small amounts of water. The ~33Cs spectrum showed two poorly-resolved peaks of differing linewidths. The T2 decays were biexponential in all cases, which was taken as evidence for the presence of two distinct types of ions. The authors reach identical conclusions to those of Li et al. [75-77], i.e., the ions in Kapton films exist in two environments, either in free volume cavities associated with the polymer chains, or in water-field defects, which are likely to contribute to macroscopic diffusion of the ions. While the above studies resulted in the characterization of the relative mobilities of water and ions in Kapton films, no evidence is presented of the precise details of the interaction of the water molecules with the polymer chains. Waters et al. [79] used ~3C CPMAS NMR to address this problem. A series of measurements of the relaxation times, 13C T1, ~H T~, 13C Tlo and TcH were made on dry and hydrated films. On hydration, 13C T1 was unaltered for all carbons except the carbonyl and unprotonated aromatic phthalimide carbons. After consideration of the various possible mechanisms for relaxation, it was concluded that relaxation at these sites in dry films is facilitated by the presence of dissolved oxygen gas, and that on hydration water molecules prevent the access of oxygen by specifically binding to the polar carbonyl groups. Quite different behaviour was observed with ~3C T~ measurements, in that the relaxation time of all the carbons, except that of the carbonyl carbons, was reduced on the addition of water. The increase in 13C T~o for the carbonyl carbons was also ascribed to blocking of paramagnetic relaxation due to oxygen, while the decrease in relaxation times of the other carbons could be due either to plasticization, or more efficient relaxation via the spin-spin relaxation mechanism. Finally, the inversion-recovery cross-polarization experiment was used to measure TcH for each carbon type. The introduction of water resulted in the appearance of a longer TcH component, which was ascribed to plasticization of the polymer chains by the water molecules.
13.3 ~3C NMR studies of acetylene- and norbornene-terminated poly(imide)s Sefcik and coworkers [80] were the first to use solid-state 13C NMR to study the curing of poly(imide) oligomers terminated with reactive groups. The authors observed a clearly-resolved peak at 84 ppm in the spectrum of the unreacted poly(imide) resin, Thermid 600, due to terminal acetylene groups. On curing at 450-640 K, the decrease in intensity of this peak allowed facile
POLY(IMIDE)S
483
analysis of the extent of cure. Additional structures formed during curing were identified by subtraction of the spectrum of the uncured resin from the spectrum of the cured sample after artificial broadening of the peaks. New peaks in the region of 120-150 ppm were assigned to the products of trimerization of the acetylene groups, and addition of acetylene groups onto the imide backbone. The conclusion that, at a maximum, only 30% of the acetylene groups undergo cyclotrimerization, is dependent on the assumption of quantitative peak intensities in the ~3C CPMAS spectra. In a later study, Swanson and coworkers [81] studied the cure of acetyleneterminated poly(imide)s selectively labelled at various positions with 13C nuclei. Curing of the sample, labelled at the imide carbonyl group, confirmed the completion of the imidization reaction on heating. The product of addition onto the carboxyl group was not observed. Four new peaks were identified in the spectrum of the cured sample labelled at the Cl-acetylene group, while a similar result was obtained for the sample labelled at the C2-acetylene position. Analysis of these results rules out the participation of coupling reactions and the biradical mechanism, which would produce triple-bond structures, but confirms the presence of the product of cyclotrimerization and Friedel-Crafts reactions. The latter mechanism is confirmed from the presence of small peaks due to aliphatic carbons in the spectra of the materials labelled at the acetylene groups. Wong and coworkers. [82, 83] used ~3C solution and solid-state NMR to study the cure of the norbornene end-capped poly(imide)s 2NE/DDM and PMR-15. At lower cure temperatures, exo-endo isomerization was observed to be a major product [84]. Loss of cyclopentadiene was suggested to result in initiation of the partner maleimide, as well as Diels-Alder addition of cyclopentadiene with norbornene. There was no evidence of internal double bonds formed by incorporation of the cyclopentadiene into the polymer backbone. The solid-state spectra were very poorly resolved, but did allow confirmation of the mechanism of reaction [83]. Spectra obtained at higher fields did not show improved resolution, indicating that the dominant mechanism for line-broadening in these materials is the dispersion of isotropic chemical shifts resulting from frozen conformations. In later work, MilhouratHammadi and coworkers [84, 85] reported solution-state ~3C and 1H NMR studies of PMR-15 prepolymers.
13.4
NMR studies of bis-maleimides
Lind and Fry reported a comprehensive study of the curing reactions of BMI resins [86, 87]. The system they studied was 1,1'-(methylenedi-4,1-
484
ANDREW K. WHITTAKER
(b)
I
I
I,II
I
200
II
I,II
I I I
i l l f l
150
.i
I l l |
l i l l l l l
1O0
I I |
II
50
el
i
i
i
I I |
0
PPM
(a)
<_.__ 200
150
1O0
50
0
PPM Fig. 13.4. 13C CPMAS spectra, obtained using the TOSS sequence of (a) pure BMI cured at 493 K and (b) 1.5:1 BMI/DDM cured for 1 h at 418 K. (Reproduced from Ref. [86] with permission from the author.)
phenylene)-bis-maleimide (BMI),
polymerized with the chain extender diaminodiphenyl methane (DDM). Excellent high-resolution ~3C CPMAS spectra were obtained, as a function of conversion and comonomer ratio. Typical spectra are shown in Fig. 13.4. The spectrum of BMI cured at 493 K for 4 h
POLY(IMIDE)S
485
(Fig. 13.4(a)) shows three sets of peaks due to, in order of increasing field, carbonyl carbons (169 and 175 ppm), aromatic carbons (110-150 ppm) and aliphatic carbons (32-55 ppm). Of much interest are the peaks due to carbonyl carbons in the reacted BMI units (175 ppm) and unreacted BMI monomer (169 ppm). In addition, the shoulder at high chemical shift on the peak at 40-50 ppm in Fig. 13.4(a) is due to methine carbons in BMI units reacted by free radical addition. The spectrum of the resin containing the chain-extender diamine (Fig. 13.4(b)) shows an additional peak at 52 ppm, which is assigned to methine carbons of BMI units having an attached amine group. The resolution of these peaks makes possible the quantitative measurement of the extent of free radical addition reaction of BMI, and the extent of reaction by chain extension. Of crucial concern in the analysis of spectra, such as those shown in Fig. 13.4, is that the peak intensities reflect accurately the number of carbons contributing to each peak. In their second paper, Fry and Lind [87] discuss the quantitative aspects of the cross-polarization experiment and demonstrated that, for their crosslinked resins, spin diffusion tends to equalize 1H T~ for each carbon type, so that, for example, the ratio of the peaks due to carbonyl carbons at 175 to 169 ppm is an accurate measure of the extent of reaction of the BMI double bonds. The authors were also able to use a curvefitting program to determine the extent of reaction of the chain extender with BMI [87]. Curing at lower temperatures (418 K) was shown to result in greater reaction of the chain extender, while high temperature curing (493 K) results in homopolymerization of the BMI component. A comparison with DSC measurement of the extent of cure with that determined from the ratio of the intensities of the peaks due to the carbonyl carbons, demonstrates that DSC cannot detect the presence of considerable proportions of trapped, unreacted BMI monomer. Thus, 13C CPMAS is suggested to be an accurate technique for determining extent of cure at high conversions in these resins. As mentioned above, bis-maleimides are often copolymerized with vinyl monomers to improve processability and impact resistance. Winter and van der Velden [24] reported a study of the mechanism of reaction of a complex BMI mixture with styrene monomer. The copolymerization of the model system, N-methyl maleimide with styrene, was found to result in polymers having approximately equimolar styrene-maleimide compositions across- the whole range of monomer feed ratios, with alternation of the two monomers. The solid-state ~3C CPMAS spectra of the cured resins were less informative. However, the absence of peaks at 147 ppm due to styrene-rich triads strongly suggest that the copolymerization with styrene results in an alternating copolymer. Liao and coworkers [88] copolymerized poly(urethane)s with BMI to im-
486
A N D R E W K. W H I T T A K E R
prove toughness of the resins. In a comprehensive study of the cure reaction and polymer properties, the authors reported the solid-state 13C NMR spectra of the cured materials using techniques such as FTIR, DMA and TEM. The spectrum of the cured pure BMI shows some evidence of unreacted monomer, while that of the poly(urethane)-BMI copolymer indicates complete cure. Shibahara et al. [89] used 13C CPMAS and solution-state NMR to study the cure of B MI resins catalyzed with triphenylphosphine. The chain extender in this case was diallylbisphenol-A. The presence of the allyl was shown to result in the formation of trimers of N-phenyl maleimide by solution-state NMR. The solid-state spectra of the resin cured at 393 K also showed evidence of the formation of trimers in the bis-maleimides. The diallyl compound did not appear to participate in the polymerization, and could be extracted with acetone after completion of reaction of the BMI. Nonetheless, the rate of reaction determined from the ratio of the peak in the 13C CPMAS spectra at 175 ppm, due to reacted BMI with that at 170 ppm because of unreacted BMI, was increased in the presence of the diallyldiol. The catalytic effect of the hydroxy group on the dially compound was confirmed by studying the reaction of p-hydroxy-N-phenyl maleimide. The use of bis-maleimides in composite materials often necessitates the use of coupling agents [90]. The addition of a small amount of coupling agent, usually a silane, can result in improved properties such as bonding to fibres and hydrolytic stability. Gambogi and Blum used 2H NMR to study the molecular motion of the interface between a bis-maleimide attached to a silica surface via a deuterated (aminoalkyl)silane coupling agent [91-93]. Spectra were recorded for chemisorbed coupling agent before and after reaction with BMI in two molar ratios. Changes in the 2H NMR shape, as a function of temperature and BMI content, were explained in terms of either a model of two-site jumping motion coupled with slow anisotropic brownian diffusion, or at lower temperatures, by jumping motion on a tetrahedral lattice. A single correlation time of motion was used, and a small activation energy of motion deduced from variable temperature measurements. Polymerization with an excess of BMI did not appreciably affect the NMR spectra. A later study [92] considered the effect of water on the rates of motion of the coupling agent. The presence of water, both for the pure silane coupling agent on silica and the system with excess BMI, lead to an increase in the rates of motion by a factor of approximately four. Finally, Blum [93] reported the behaviour of two different coupling agents, i.e., aminopropylsilane and aminobutylsilane. The latter coupling agent was reported to have motional frequencies approximately an order of magnitude higher than the shorter chain, as may be expected.
POLY(IMIDE)S 13.5
487
Summary and outlook
NMR spectroscopy and in particular solid-state NMR spectroscopy proved to be a powerful method for studying the mechanism and extent of reaction in complex poly(imide) materials. In particular, during the cure of BMI resins, careful use of 13C CPMAS NMR indicated that measurements of the extent of cure by DSC were significantly overestimated [86, 87]. This article demonstrates that NMR spectroscopy has been able to characterize the structure of condensation poly(imide)s and, more successfully, the cure of BMI, PMR and acetylene-terminated resins. For many complex poly(imide) mixtures, the NMR spectra are severely overlapped, due to the structural similarity of many of the monomer repeat units, and also the presence of a range of conformations frozen-in in the solid state. The use of isotopic enrichment of selected sites, with either ~SN [73] or 13C [81] nuclei, can of course alleviate this problem, however, such a course of action is both tedious and expensive. An alternative method for improving resolution in the CPMAS spectra is to measure the spectra above the glass-transition temperature of the materials, thus allowing averaging of conformations frozen in the glassy state. Several probe manufacturers now market CPMAS probes with high temperature limits well in excess of those required to study this class of polymers. Application of high temperature CPMAS to crosslinked epoxy resins has been reported by Sterna and Smith [94] and Harris and coworkers [95]. Routine use of such equipment promises a revolution in the study of high temperature poly(imide)s. Solid-state NMR has been applied successfully in a few cases for the study of poly(imide) blends. It can be said, however, that the techniques arising from recent advances in the analysis of polymer blends, such as selection techniques based on ~H multipulse methods, and improvements in the modelling methods of spin diffusion, have yet to be applied to the study of blends containing poly(imide)s. It is suggested that these techniques will have an important role to play in future studies of poly(imide) blends, particularly for blends such as impact-modified BMI resins. The great power of NMR compared with other spectroscopic techniques is the ability to study both the frequency and geometry of molecular rearrangements in the solid state. It is true to say that there have been few studies of molecular motion in poly(imide)s using NMR spectroscopy. The main reason for this has been the previously limited temperature range of commercial CPMAS probes. Therefore, future work in poly(imide)s is expected to exploit the ground-breaking efforts of other workers in the field of multidimensional NMR spectroscopy in the solid state [3].
488
ANDREW K. WHITTAKER
References 1. R.A. Komoroski (Ed), High-Resolution NMR Spectroscopy of Synthetic Polymers in Bulk. VCH, Deerfield Beach, FL, 1986. 2. L.J. Mathias (Ed), Solid-State NMR of Polymers. Plenum Press, New York, 1991. 3. K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid-State NMR and Polymers. Academic Press, London, 1994. 4. C.E. Scroog, A.L. Endrey, S.V. Abramo, C.E. Berr, W.M. Edwards and K.L. Oliver, J. Polym. Sci. A3 (1965) 1373. 5. R.A. Dine-Hart and W.W. Wright, Makromol. Chem. 143 (1971) 189. 6. F.W. Harris, in D. Wilson, H.D. Stenzenberger and P.M. Hergenrother (Eds), Polyimides, Chapter 1. Blackie, Chapman and Hall, New York, 1990. 7. J.A. Kreus, A.L. Endrey, F.P. Gay and C.E. Scroog, J. Polym. Sci. A14 (1966) 2607. 8. C.A. Pryde, J. Polym. Sci. A27 (1989) 711. 9. F.P. Gay and C.E. Berr, J. Polym. Sci. 6(1968) 1935. 10. M.M. Koton, T.K. Meleshko, V.V. Kudryavtsev, P.P. Nechayev, Ye.V. Kamzolinka and N.N. Bogorad, Polym. Sci. USSR A24 (1982) 791. 11. D.M. White, T. Takekoshi, F.J. Williams, H.M. Relles, P.E. Donahue, H.J. Klopfer, G.R. Loucks, J.S. Manello, R.O. Matthews and R.W. Schluenz, J. Polym. Sci., Polym. Chem. Ed. 19 (1981) 1635. 12. H.D. Stenzenber, in High Performance Polymers, Advances in Polym. Sci. 117. Springer Verlag, Berlin, 1994. 13. T. Takekoshi, in New Polymeric Materials, Advances in Polym. Sci. 94. Springer Verlag, Berlin, 1990. 14. T. Iijima, M. Hirano, W. Fukuda and M. Tomoi, Eur. Polym. J. 29 (1993) 1399. 15. R.J. Morgan, R.J. Jurek, A. Yen and T. Donnellan, Polymer 34 (1993) 835. 16. S.P. Wilkinson, T.C. Ward and J.E. McGrath, Polymer 34 (1993) 870. 17. R. Greco, P. Laurienzo, M. Malinconico, E. Martuscelli, N. Perenze and A. Sorrentino, Adv. Polym. Tech. 13 (1994) 141. 18. S. Takeda and H. Kakiuchi, J. Appl. Polm. Sci. 35 (1988) 1351. 19. H.D. Stenzenberger and P. K6nig, High Perform. Polym. 5 (1993) 123. 20. A. Matsumoto, T. Kubota and T. Otsu, Macromolecules 23 (1990) 4508. 21. A. Matsumoto, Y. Oki and T. Otsu, Polym. J. 25 (1993) 237. 22. L.E. Coleman and J.A. Conrady, J. Polym. Sci. 38 (1959) 241. 23. J.M. Barrales-Rienda and J.I. Gonzalez, J. Makromol. Chem. 11 (1977) 267. 24. H. Winter and G.P.M. van der Velden, Macromolecules 25 (1992) 4285. 25. J.V. Crivello, J. Polym. Sci., Polym. Chem. Ed. 11 (1973) 1185. 26. M.G. Gherasim and I. Zugravescu, Eur. Polym. J. 14 (1978) 1201. 27. J.E. White, M.D. Scaia and A. Snider, J. Appl. Polym. Sci. 29 (1984) 891. 28. J.V. Crivello and P.C. Juliano, J. Polym. Sci., Polym. Chem. Ed. 13 (1975) 1819. 29. J.V. Crivello, J. Polym. Sci., Polym. Chem. Ed. 14 (1976) 159. 30. J.E. White and M.D. Scaia, Polymer 25 (1984) 850. 31. J.M. Barton, I. Hamerton, J.R. Jones and J.C. Steadman, Poly. Bull. 27 (1991) 163. 32. H.D. Stenzenberger, Brit. Polym. J. 20 (1988) 383. 33. H. Stenzenberger in D. Wilson, H.D. Stenzenberger and P.M. Hergenrother (Eds), Polyimides, Chapter 4. Blackie, Chapman and Hall, New York, 1990. 34. A.L. Landis, N. Bilow, R.H. Boshan, R.E. Lawrence and T. Aponyi, Polym. Prep. 15 (1974) 537.
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P.M. Hergenrother, Polym. Prep. 21 (1980) 81. M.D. Soucek, R.H. Pater and S.L. Ritenour, Polym. Prep. 34 (1993) 530. T.T. Serafini, P. Delvigs and G.R. Lightsey, J. Appl. Polym. Sci. 16 (1972) 905. F.W. Harris, in D. Wilson, H.D. Stenzenberger and P.M. Hergenrother (Eds.), Polyimides, Chapter 1, p. 17. Blackie, Chapman and Hall, New York 1990. D.L. VanderHart, W.L. Earl and A.N. Garroway, J. Magn. Reson 44 (1981) 361. K.S. Seshardi, P.A. Antonoplos and W.J.Heilman, J. Polym. Sci., Polym. Chem. Ed. 18 (1980) 2469. V.M. Denisov, V.M. Svetlichnyi, V.A. Gindin, V.A. Zubkov, A.I. Koltzov, M.M. Koton and V.V. Kudryavtsev, Polym. Sci. USSR 21 (1980) 1644. B. Schneider, P. Schmidt, M. Marek, J. Straka, B. Bedn.r and J. Kr.lcek, Eur. Polym. J. 26 (1990) 941. M.-F. Grenier-Loustalot, F. Joubert and P. Grenier, J. Polym. Sci., Polym. Chem. Ed. 29 (1991) 1649. M.-F. Grenier-Loustalot, M. Gillard, F. Joubert and P. Grenier, J. Polym. Sci., Polym. Chem. Ed. 31 (1993) 2839. T. Matsuura Y. Hasuda, S. Nishi and N. Tamada, Macromolecules 24 (1991) 5001. S. Ando, T. Matsuura and S. Nishi, Polymer 33 (1992) 2934. J.R. Havens, H. Ishioa and J.L. Koenig,~Macromolecules 14 (1981) 1327. H. Ishida, S.T. Wellinghoff, E. Baer and J.L. Koenig, Macromolecules 13 (1980) 826. S.T. Wellinghoff, H. Ishida, J.L. Koenig and E. Baer, Macromolecules 13 (1980) 834. T.D. Alger, D.M. Grant and E.G. Paul, J. Am. Chem. Soc. 88 (1966) 5397. J.H. O'Donnell and A.K. Whittaker, Polym. Bull. 12 (1984) 319. B. Schneider, P. Schmidt, M. Marek, J. Straka, B. Bedn.r and J. Kr.lcek, Eur. Polym. J. 26 (1990) 941. X. Yang, W. Oui, C. Li, W. Zeng, J. Liu, L. Lu, X. Wang and B. Sanctuary, Polymer 35 (1994) 4300. M. Marek, B. Schneider, D. Hlavata, J. Labsky and M. Bleha, J. Macromol. Sci., Pure Appl. Chem. A33 (1996) 477. S.C. Feilich, Macromolecules 20 (1987) 973. T.L. St. Clair, in D. Wilson, H.D. Stenzenberger and P.M. Hergenrother (Eds), Polyimides, Chapter 3. Blackie, Chapman and Hall, New York, 1990. F.J. Dinan, W.T. Schwartz, R.A. Wolfe, D.S. Hojnicki, T. St. Clair and J.R. Pratt, J. Polym. Sci., Polym. Chem. Ed. 30 (1992) 111. L. Leung, D.J.Williams, F.E. Karasz and W.J. MacKnight, Poly. Bull. 16 (1986) 457. P. Musto, F.E. Karasz and W.J. MacKnight, Polymer 30 (1989) 1020. J. Grobelny, D.M. Rice, F.E. Karasz and W.J. McKnight, Macromolecules 23 (1990) 2139. T.A. Kostereva, N.G. Stepanov, A.V. Gribanov, L.A. Shibaev, G.N. Fedorova and Yu. N. Sazanov, Polym. Sci. Ser. B 36 (1994) 1304. H. Feng, C. Ye, P. Zhang, Z. Sun and Z. Feng, Macromol. Chem. Phys. 196 (1995) 2587. M. Ballauf, Angew. Chem. 101 (1989) 261. M. Ballauf, Makromol. Chem., Rapid Commun. 7 (1986) 407. M. Ballauf and G.F. Schmidt, Rapid Commun. 8 (1987) 93. A. Adam and H.W. Spiess, Makromol. Chem., Rapid Commun. 11 (1990) 249. C. Baldwin-Frech, A. Adam, U. Falk, C. Boeffel and H.W. Spiess, New Polym. Mater. 2 (1990) 267. J. Clauss, K. Schmidt-Rohr, A. Adam, C. Boeffel and H.W. Spiess, Macromolecules, 25 (1992) 5208.
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ANDREW K. WHITTAKER K. Schmidt-Rohr, J. Clauss and H.W. Spiess, Macromolecules 25 (1992) 3237. H.R. Kricheldorf, N. Probst and C. Wutz, Macromolecules 28 (1995) 7990. H.R. Kricheldorf, N. Probst, G. Schwarz and C. Wutz, Macromolecules 29 (1996) 4234. M. Andreis and J.L. Koenig, Adv. Polym. Sci. 124 (1995) 1991. P.D. Murphy, R.A. Di Pietro, C.J. Lund and W.D. Weber, Macromolecules 27 (1994) 279. W.L. Jarrett, C.G. Johnson and L.J. Mathias, J. Magn. Reson. Ser. A 116 (1.995) 156. G. Xu, C.C. Gryte, A.S. Nowick, S.Z. Li, Y.S. Pak and S.G. Greenbaum, J. Appl. Phys. 66 (1989) 5290. S.Z. Li, Y.S. Pak K.J. Adamic, S.G. Greenbaum, B.S. Lim, G. Xu and A.S. Nowick, J. Electrochem. Soc. 139 (1992) 662. S.Z. Li, R.S. Chen and S.G. Greenbaum, J. Polym. Sci., Polym. Phys. Ed. 33 (1995) 403. R. Turuscy, H. Leidheiser and J.E. Roberts, J. Electrochem. Soc. 140 (1993) 149. J.F. Waters, W.R. Likavec and W.M. Ritchey, J. Appl. Polym. Sci. 53 (1994) 59. M.D. Sefcik, E.O. Stejskal, R.E. McKay and J. Schaefer, Macromolecules 12 (1979) 423. S.A. Swanson, W.W. Fleming and D.C. Hofer, Macromolecules 25 (1992) 582. A.C. Wong and W.M. Ritchey, Macromolecules 14 (1981) 825. A.C. Wong, A.N. Garroway and W.M. Ritchey, Macromolecules 14 (1981) 832. A. Milhourat-Hammadi, H. Chayrigues, R. Levoy, C. Merienne and A. Gaudemer, J. Polym. Sci., Polym. Chem. Ed. 29 (1991) 1347. A. Milhourat-Hammadi, H. Chayrigues, C. Merienne and A. Gaudemer, J. Polym. Sci., Polym. Chem. Ed. 32 (1994) 203. A.C. Lind and C.G. Fry, Polym. Mater. Sci. Eng. 59 (1988) 466. C.G. Fry and A.C. Lind, New Polym. Mater. 2 (1990) 235. D.C. Liao, K.H. Hsieh and S.C. Kao, J. Polym. Sci., Polym. Chem. Ed. 33 (1995) 481. S. Shibahara, T. Enoki, T. Yamamoto, J. Motoyoshiya and S. Hayashi, Polym. J. 28 (1996) 752. E.D. Pleiddermann, Silane Coupling Agents. Plenum Press, New York, 1991. J.E. Gambogi and F.D. Blum, Macromolecules 25 (1992) 4526. J.E. Gambogi and F.D. Blum, Mater. Sci. Eng. A162 (1993) 249. F.D. Blum, Macromol. Symp. 86 (1994) 161. L.L. Sterna and H.C. Smith, J. Magn. Reson. 79 (1988) 528. R.K. Harris, R.R. Yeung, P. Johncock and D.A. Jones, Polymer 37 (1996) 721.
Chapter 14
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Poly(ethylene terephthalate) Tetsuo Asakura I and Takuro Ito 2 1Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan; 2Co. R&D TOyOSeikan Group, Yokohama, Kanagawa, Japan
14.1
Introduction
Poly(ethylene terephthalate) (PET) which is manufactured by a stagewise melt polymerization process consisting of transesterification, prepolymerization and finishing polymerization steps, is one of the fastest growing thermoplastic polyesters used extensively for fibers, films, bottles, injection molded parts and other products [1]. Considerable scientific effort has been made to elucidate its properties. Important aspects can be studied using solid-state NMR spectroscopy to determine morphology, orientation and mobility in the bulk material.
14.2
13C CP/MAS spectra
Figures 14.1(a,b) show typical 1 3 C CP/MAS spectra of two types of PET yarns, an amorphous yarn wound at relatively low speed and a 36% crystalline yarn wound at relatively high speed, respectively [2]. The ethylene and carbonyl carbon peaks of the amorphous yarn are shifted about 1 ppm downfield with respect to the semicrystalline yarn, as opposed to the aromatic carbons which are shifted slightly upfield. Besides differences in chemical shift, the spectrum of the 36% crystalline yarn shows narrower lines with a better S/N ratio than the spectrum of the amorphous yarn. The broader lines in Fig. 14.1(a) are attributed to a broader orientation distribution of polymer molecules, which results in a larger distribution of isotropic chemical shifts. Additional differences between both spectra are observed in the lineshape: the ethylene and carbonyl carbon peaks in Fig. 14.1(a) have a symmetric lineshape, whereas, these lines in Fig. 14.1(b) are asymmetric. The asymmetric lineshape is resolvable into two partially overlapping resonances: a relatively broad low-field component and a relatively narrow high-field
492
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5O
Fig. 14.1. 13C CP/MAS spectra of different PET yarns. Spinning sidebands are marked with an asterisk: (a) Amorphous yarn" (b) 36% crystalline yarn" and (c) deconvolution spectra of carbonyl and ethylene resonances of Fig. 14.1(b).
component (Fig. 14.1(c)). The narrow component is shifted about 1 ppm upfield with respect to the broad component. From peak deconvolution, the broad resonance line in Fig. 14.1(c) corresponds both in linewidth and in chemical shift with the resonance lines of the amorphous yarn in Fig. 14.1(a).
POLY(ETHYLENE TEREPHTHALATE)
493
Therefore, the broad resonance is assigned to the disordered NMR amorphous phase and the narrow peak to ordered NMR crystalline regions.
14.3
Motion
Sefcik et al. [3] studied quenched and annealed PET films with different crystallinities varying from 3-50%. They measured rotating-frame relaxation times of protons and carbons, Tip 1H and Tip 13C. These parameters give information about molecular motions in the tens or hundreds of kilohertz range, which is a characteristic frequency range for many important motional processes in solid polymers. Sefcik et al. were particularly interested in relaxation times, Tip 13C which, in principle, can provide information about molecular motions at specific sites within the polymer. A multiexponential behavior of the 13C magnetization decay was observed. English [4] has studied the dynamics of PET by 1H and 13C solid-state NMR measurements in detail. The PET samples used included semicrystalline powders (both fully protonated and partially deuterated), amorphous and semicrystalline fibers, and amorphous and semicrystalline films. The NMR relaxation results indicated that four separate motional processes can be distinguished. A very rapid reorientation of polymer segments in which the amplitude of the motion grows as a function of temperature. A slower specific motion of the benzene rings (perhaps flipping) that correlates with the /3 relaxation which occurs at -50~ at 100 Hz, and is thought to correspond to limited motions of all portions of the polymer in both crystalline and amorphous regions, an even slower motion that is unique to the methylene groups (perhaps trans-gauche isomerization) that correlates with the a relaxation, and an almost effectively isotropic reorientation of some amorphous polymer segments whose population is temperature dependent. Gabrielse et al. [2] observed several relaxation times such as relaxation times in the rotating frame (Tip 1H, Tip 13C) and 1H~13C cross-polarization transfer times (TcH) for PET fibers with different structures, varying from amorphous to 36% crystalline. Figure 14.2 shows the decay of the ethylene carbons plotted logarithmically against ~" in order to determine Tip 13C for 36% crystalline yarn measured at 78 kHz, where the 13C CP/MAS spectrum is shown in Fig. 14.1(b). The observation of three distinct TI o 13C values is considered to correspond to three different mobility regions, NMR crystalline, rigid NMR amorphous and mobile NMR amorphous, as summarized in Fig. 14.3. In this model, well-ordered crystalline regions alternate with less-ordered amorphous domains. Since single chains pass from several crystalline regions
494
TETSUO ASAKURA AND TAKURO ITO
Tlp(13C) 10 2
}'~1_.3 (20~)" [
O ~ ~ " ~
~
4g.7
(53~
total ''~
_
'NMR-
S
25.5
,~,
100
. . . .
0
i
. . . .
10
(277.)
. . . .
,
20
i
30
'NMRamorphous
. . . .
I
40
. . . .
.50
T(ms)
Fig. 14.2. Tip 13C relaxation behavior of the total ethylene resonance and of the NMR crystalline and NMR amorphous components. Tip 13C parameters (in ms) with fractions (in brackets) are given for each slope.
!
-- IOOA I
'NMR-Crystalline '
rigid mobile 'NMR'NMRamorphous' amorphous'
!
I
X-Ray Crystalline
X-Ray Amorphous
Fibril
i
' r
Fig. 14.3. Schematic representation of the physical structure of semicrystalline PET yarns.
POLY(ETHYLENE T E R E P H T H A L A T E ) c)
1
4
2
495
C
3 5
t ~/t.
- - - +
.
.
.
.
250 200 150 100 50
ppm
b)
ca2 2
2
cut
2
Fig. 14.4. (a) 13C 2D solid-state NMR spectra of a bundle of oriented, industrial PET fibers. The diagonal peaks are labeled according to Fig. 14.5. The 180~ phenylene motion about the
para-axis is reflected in the sharp exchange peak between positions 2 and 3. The high intensity along the diagonal is due to carbon atoms which have not changed their position and, therefore, their frequency during the mixing time tm, 1 S. (b) Contour plot of the spectrum shown in Fig. 14.4(a). The 20 linearly spaced lines between 1.5 and 17% of the maximum height of the spectrum indicate clearly the 180~ flip motion. In addition, a cut through the spectrum at peak 2 is shown. into amorphous regions, coherent structural units, called fibrils, are formed. From Tlo 1H experiments it can be concluded that both mobile and rigid regions in the amorphous yarn have dimensions smaller than 50/k. TCH measurements reveal that ethylene and aromatic groups in the mobile amorphous domains of all yarns undergo large amplitude motions. The principal elements of the chemical shift tensor of aromatic carbons remain independent of temperature up to at least 327 K, indicating that, in crystalline regions, semicrystalline yarns and rigid parts of the amorphous yarn, phenyl reorientation has a very small amplitude (not exceeding ---5~ Slow 180 ~ phenylene flips in P E T fibers were detected by Wilhelm and Spiess [5] using two-dimensional (2D) solid-state exchange N M R . Figure 14.4
496
TETSUO ASAKURA AND T A K U R O ITO
fiber axis//Bo-field
para- axis -,,
IC..' './ ,~H ' !
C=O
3 e\ 9 O
H~, / H...C
Fig. 14.5. Sketch of repeat unit of PET. A 180~ flip motion of the phenylene unit changes the
orientation between the C m H bond of a protonated aromatic carbon atom and the applied magnetic field, Bo (labeled as sites 2 and 3). Note the difference in orientation of approximately 18~ between the fiber axis and the para-axis of the aromatic ring.
shows 2D solid-state 13C NMR spectra of a bundle of oriented, industrial PET fiber, where the diagonal peaks are labeled according to Fig. 14.5. The high intensity along the diagonal is due to carbon atoms which have not changed their position and, therefore, their frequency during the mixing time tm. The sharp exchange peak between positions 2 and 3 reflects a 180~ phenylene flip motion about the para-axis. This is clearly detected in the contour plot Fig. 14.4(b) of the spectrum shown in Fig. 14.4(a) as well as the cut through the spectrum at peak 2. The exchange intensity plot with variation of the mixing time t m was found to be highly nonexponential. If the motion occurs inside a crystallite, a single exponential would be expected. In addition, only highly oriented molecules can give rise to sharp exchange peaks. Thus, the slow flip motion detected here is concluded to occur in the immobilized, highly oriented noncrystalline parts of the semicrystalline polymer fiber. A quantitative analysis of the motion was performed with a Kohlrausch-Williams-Watts function, I ( t m ) = 1 - exp{-(ktm)~} from the increase of the exchange intensity with tm [5]. For T = 293 K, the fit yields a mean rate k = 1.52 s -1 and/3 = 0.34, indicating a wide distribution of jump rates.
14.4
Orientation
Rober and Zahamann [6] studied the molecular orientation in PET drawn by necking before and after crystallization, by eH NMR. The NMR investiga-
POLY(ETHYLENE TEREPHTHALATE)
497
tions were performed on samples in which only the benzene rings were deuterated as well as those in which only ethylene groups were deuterated in order to determine separately the orientation of the benzene ring and ethylene groups. In addition, the measurements are performed by applying different "waiting times" in the NMR measurements to determine separately the orientation of chains of different mobilities. The orientation was determined by measuring the 2H NMR spectra at different angles/30 between the draw direction and the magnetic field and different angles Ao between the normal to the film plane and the magnetic field [6]. The difference of two NMR spectra is obtained by subtracting a spectrum measured at a waiting time rwl from another measured after a longer waiting time rw2. They designated the result of the subtraction as a "difference spectrum" [Zw2; rwl]. To determine the orientation of the benzene ring, difference spectrum of PET, in which the benzene ring was deuterated is observed by changing the angles/30 and Ao. Figure 14.6 shows difference spectra [4.0 s; 1.2 s] of PET in which the benzene ring was deuterated, drawn at 45~ crystallized for 2 h at 240~ and measured at 20~ On the righthand side are shown the calculated spectra that gave the best fit to the experimental curves. Excellent agreement between the calculated and the measured curves could be obtained. Forty percent of these spectra arise from benzene rings in the crystals
measured ~0 = O
calculated
9
po=90"
Xo= O"
13o=90" Xo= 75 ~ 13o= 90" Xo= 90 ~
& ~.,,#%.~
.-~.,~
Fig. 14.6. Difference 2H solid-state NMR spectra ([4.0 s; 1.2 s]) of PET in which the benzene ring was deuterated. The calculated spectra assuming a crystalline component of 40% and an amorphous component of 60% are also shown.
498
TETSUO ASAKURA AND TAKURO ITO
and 60% from benzene rings in the less mobile amorphous chains. Thus, the orientation of the benzene rings is quite high not only in the crystals but also in the amorphous regions. The chains are preferentially oriented parallel to the machine drawn (MD) direction. In addition, a parallelization of the benzene ring planes to the film plane, although to a lesser extent than in the crystals, is also observed in the amorphous regions. In contrast to these results, good agreement can be obtained between the measured and calculated 2H solid-state spectra in the samples with deuterated benzene rings that were drawn and not annealed, by assuming the benzene ring axis, rather than the chain axis, is preferentially oriented parallel to MD and the orientation distribution of the normal to the benzene ring plane shows rotational symmetry with respect to MD. In addition, by means of 2H NMR, it becomes possible to measure separately the orientation distributions of (i) the chains in regions of different chain mobilities (crystals, amorphous regions with restricted chain mobility, amorphous regions with more mobile chains) and (ii) the parts of the molecules containing different atomic groups, i.e., benzene rings and ethylene groups. The various 1D 13C CP NMR spectra of a biaxially drawn industrial PET film are shown in Fig. 14.7. The pronounced sensitivity to the orientation of the sample with respect to the applied magnetic field is also shown [7]. However, the orientation distribution is relatively complicated and, therefore, difficult to quantify from these data. For PET, overlapping resonances from carboxyl and phenylene group carbons are especially troublesome and, consequently, restrict the angular information achievable in 1D NMR experiments. Spiess et al. [7] developed a multidimensional D E C O D E R (direction exchange with correlation for orientation-distribution evaluation and reconstruction) method to measure and correlate NMR frequencies at two different sample orientations. Through this correlation, the spectra contain the equivalent of information on two Euler angles that describe the orientation of a given molecular segment. Many features of the orientation distribution are directly reflected in the intensity distribution of the 2D spectrum, from which the width of the orientation distribution of certain axes can immediately be read. The multidimensional D E C O D E R NMR experiments are applied to drawn PET fibers and thin PET films prepared under different processing conditions. Figure 14.8(a) shows a 2D 13C D E C O D E R spectrum for a bundle of uniaxial PET fibers flipped through an angle/3 = 90 ~ during the mixing time. The fibers are oriented along the axis of the receiver coil. For the simulation (Fig. 14.8(b)), a value of 18 _+ 4~ for the angle between the phenylene paraaxis and the fiber axis was used, which was found to best reproduce the features of the experimental spectrum. The full-width-at-half-maximum
POLY(ETHYLENE TEREPHTHALATE)
' ""
(:I)
(t))
"
_
OCH z
B~
2 ..
! . . . , 1
2so
9 .!
. . . .
250
499
~
W ______.'J j
. . . .
200
I
. . . .
200
1 . . . .
~so
I
. . . .
150
I
. . . .
~oo
I
. . . .
100
1
. . . .
so
I
0
[ppm]
1 . . . .
1.
50 0 [pp~]
9. t . . . ~ l
250
. . 1
. . . .
250
. . . .
200
t
. . . .
200
!
. . . .
150
I l j l .
i50
!
. . . .
!
. . . .
i00
i00
l
. . . .
1_
I
. . . .
L
50
0 [ppm]
50 0 [ppm]
Fig. 14.7. 1D static 13C NMR spectra for a biaxially drawn PET film with its machine direction
(MD) parallel to the receiver coil axis. Spectra (a) and (b) were obtained after cross-polarization and a Hahn spin echo. Spectra (c) and (d) were obtained with 13C single-pulse excitation using a 1-s recycle delay, which selects for the most highly mobile segments. Little orientation dependence is observed for the mobile components.
(fwhm) of the distribution of orientations was determined to be 20 +_ 5 ~ with a 40% contribution from a much more disordered component (fwhm 135 ___45~ For biaxially drawn films, the direct reconstruction of slices through the orientation distribution shows that in a highly oriented fraction (ca. 80%) of the sample, the chain axes are confined to the film plane (fwhm of 15~ while the in-plane distribution of chain axes is much broader (fwhm of ca. 90~ the phenylene rings and carboxyl group planes are oriented preferentially parallel to the plane of the film (fwhm of 55~ Film samples collected at different points across an industrial sheet of P E T are shown to exhibit sizable variations in ordering. For the center of the sheet, a slight preferential orientation along the transverse direction is clearly detected. Increased alignment of the PET chains (fwhm of 55~ with the preferred orientation rotated toward the machine direction, is observed near the sheet
500
TETSUO ASAKURA AND TAKURO ITO u!
(a)
,
(b) -
O-
..
OCH 2
L (,( 150
"
200-
a o = 18 ~
250 '
250
-
I
2oo
'
150
I
1o0
'
50
I
-"-~
0 [ppm]
(c)
Fig. 14.8. (a) 2D 13C DECODER spectrum for a bundle of uniaxial PET fibers flipped through an angle/3 = 90~ during mixing time. The fibers are oriented along the axis of the receiver coil. (b) Five-site simulation for/3 = 90~ which reflects the better alignment of the fibers in the coil for this sample configuration. A value of ao = 18~ for the angle between the chain axis and phenylene ring para-axis was used for the simulation, which was found to best reproduce the features of the experimental spectrum. (c) Two simulated spectra that employ ao = 12 and 24~ showing the sensitivity of the phenylene carbon features to the angle So.
edge. In uniaxial films, the fwhm of the orientation distribution is found to be 85 ~. 13C solid-state N M R spectra of uniaxially oriented carbonyl carbon 13Clabeled P E T films with different draw ratios have been r e p o r t e d as a function of the angle b e t w e e n the machine draw direction and the laboratory magnetic field, to allow analysis of the P E T samples in terms of local structural ordering [8]. The basis of the studies is the o r i e n t a t i o n - d e p e n d e n c e of nuclear spin interaction tensors which serves as a probe of the relative orientations of specific b o n d vectors. U n d e r these conditions, the contribution of the aromatic carbon peaks, which overlapped the carbonyl carbon p e a k in the natural a b u n d a n c e 13C CP spectrum, could be neglected in the analysis of 13C N M R results. The 13C N M R analysis used here is essentially similar to the 15N N M R analyses already applied to uniaxially aligned silk. The possible orientations can be readily expressed in terms of b o n d angle orientations, provided the orientation of the 13C PAS is k n o w n relative to
POLY(ETHYLENE TEREPHTHALATE)
/ .......
,'.... 29 ~
tO',,
//
\
.9
........
Ch'ainaxis
! L
..........
'
O,
I~
~"
501
r~
.........lip-./ ....
/
~(I.2 /
Fig. 14.9. The principal axis system (PAS) orientation of the carbonyl carbon relative to the bonds of interest in PET. The O-ll axis corresponds to the normal of the carbonyl ( O - - C - - O ) plane. The O-22 axis is set parallel to the carbonyl direction ( C u O ) in the carboxyl plane. The O-33 axis is perpendicular to both 0-11 and o'33 directions. The angle 0 between the phenylene para-C--C and chain axes is also shown.
the bonds of interest. The angles between the 13C PAS and molecular symmetry axis (MSA) frames of reference for the PET molecule used here are shown in Fig. 14.9. The O"11 axis is approximately perpendicular to the O ~ C ~ O plane. The 0"33 makes an angle of 29 ~ with the C ~ O bond direction and 0"22 is very close to the C ~ O bond direction in the O ~ C - - O plane. These angles are the calculated values using the FPT-INDO method with the coordinates of PET, which are very close to the values reported for protonated carboxyl groups. The 13C CP NMR spectra of uniaxially oriented PET films (draw ratio 3, and drawn at 45~ are shown in Fig. 14.10 as a function of the angle, /3L, between the MD direction and the magnetic field. The glass transition temperature Tg of PET has been reported to be about 68~ [1]. Thus, the oriented PET film used here was drawn at a temperature lower than Tg. It is clear from the angle-dependent spectra that the spectra cannot be explained by assuming only a single angular-dependent component in the material examined. For example, the small peak at about 130 ppm between/3L = 0 ~ and 30 ~ strongly indicates the presence of an angle-independent powder pattern component in this sample. By using spectral simulation (dotted lines), the fraction of the powder pattern and the oriented components in the obtained spectra were determined as 35 and 65%, respectively. For the latter component, the structural parameters were determined as /3F = 18 --+ 6~ with the distribution of the fiber axis p - 22 ~ Walls [9] applied ATR-IR to the examination of conformational change by the sample stretching in uniaxially drawn PET films. A remarkable increase in the absorbance of the PET vibration at 1340 cm -1 (CH2 wagging vibration from the glycol segments of the polymer chain in the trans conformation) and decrease in the absorbance
502
TETSUO A S A K U R A AND T A K U R O ITO
iJ Ik
90"
75*
!
60*
45"
30"
15 ~
0 9 J
9 ....
300
~ ....... ,+ . . . .
+, . . . .
,i,-
200
100
0
ppm
Fig. 14.10.
1 3 C CP NMR spectra of carbonyl carbon-labeled uniaxially draw (x3 at 45~ PET film as a function of ilL, the angle between the draw direction and the magnetic field. Solid and dotted lines show observed and calculated spectra, respectively. The fractions of the two components, amorphous (35%) and oriented (65%) were determined computer simulations of the observed NMR lineshapes. The structural parameters ~ f - 90 ~ e l f - 18 +--6~ with the distribution of the fiber axis p = 22 ~ were found to be characteristic of the oriented component.
POLY(ETHYLENE TEREPHTHALATE)
503
at 1370 cm -1 (gauche) are observed for uniaxial draw ratios between 2 and 3. The most significant changes in extended trans-content occur with samples in this uniaxial draw ratio range. In addition, the conjugated system
%/
\
/
is substantially planar. These considerations suggest that the angle a F should be approximately 90 ~ In this case, 22 - 6 ~ is obtained as the angle 0 between the phenylene para- and chain axes in the oriented chain of uniaxially aligned PET films (draw ratio of 3, drawn at 45~ Within experimental error, this is in agreement with the reported values, 20 ~ by Arnott and Wonacott [10], 24 ~ by Rober and Zachmann [6] and Harbison et al. [11] and 18~ by Chmelka et al. [7]. The assumption of two components can no longer be retained when considering spectra obtained with a PET sample with a draw ratio of 5 (Fig. 14.11). There are additional broad peaks in these spectra (marked by arrows) for/3L = 15-60 ~ indicating the presence of a highly oriented additional component beyond the oriented component observed in the spectra of the samples with draw ratios of 3 and 4. By assuming three components (one angleindependent, the remainder with spectra that are orientation angle-dependent), the spectra of the PET sample with a draw ratio of 5 could be simulated. The contribution of the powder pattern to the spectrum was thus determined to be 55%. The oriented component, which corresponds to the oriented portion of samples with draw ratios of 3 and 4, was found to be 33% with aV = 90 ~ /3F = 2 4 - 10~ and p = 24 ~ The fraction of highly oriented component of the sample was 12% with the structural parameters aF = 90 ~ /3F = 16___6~ a n d p = 5 ~ Figure 14.12 shows 13C CP N M R spectra of a sample of PET with a draw ratio of 5 after heat treatment at 170~ for 30 min at orientation angles of /3L = 0 ~ and/3L = 90 ~ along with the corresponding spectral simulations (dotted lines). These spectra clearly indicate that after heat treatment the sample is well oriented. For example, the doublet of sharp peaks in the spectrum at /3L = 90 ~ is highly consistent with this conclusion. By spectral simulation, the fraction of powder pattern Contributing to these spectra was determined to be 30%. The low oriented component (av = 90 ~ / ~ F - - 2 0 +_ 10~ and p = 8 ~ was 35%, while the highly oriented one (av = 90 ~ /3F = 11 --+ 5 ~ and p = 2 ~ was also found to be 35% of the sample. The small p values for the latter two components coincide with the well-oriented character of the sample. The angle 0 between the phenylene p a r a - C ~ C and chain axes of the component
Oriented corn ponent
Oriented
component (powder p a [ t e r n )
corn ponent (low)
(high)
L~ cD 4~
Unoriented
PL
90"
~
l
t
!
60.
0 > >
j
. . . .
P~
C ~o > > Z
i
> C ~o 0 0 ---"
300
200
100
200
100
ppm
.....
' _ - - - A , - - - . A , ,
200
100
=---"
. . . .
J ....
200
'
. . . . . .
100
Fig. 14.11. 13C CP N M R spectra of carbonyl carbon-labeled uniaxially draw ( x 5 at 80~ P E T film as a function of ilL, the angle between the draw direction and the magnetic field. Full and dotted curves show observed and calculated spectra, respectively. The fractions of the three components, amorphous (55%) and two oriented ones (33 and 12%), was determined by simulation. The structural parameters were fiE = 90o, ~ F = 24 -+ 10 ~ and p = 24 ~ (low oriented component) and GF = 9 0 ~ ~ F = 16 -+ 6 ~ and p = 5 ~ (high oriented component), respectively.
i
!
!
(high)
._t
300
200
t
.
Unoriented component (powder pattern)
Oriented component (low)
Oriented component
4
.
200
I00
I00
200
2O0
I00
I00
9 trn
ppm
rn
o-
i 7~
rn
> t>
, 300
,
'~
v-~ ~ ~.,_~, 200
100
l
.
.
.
.
.
.
.
.
.
. 9 ....
200
_
l
.
.
.
.
|
.
.
.
200
IO0
.
i
.
.
.
.
t
100
__
o,o
200
100
ppm
Fig. 14.12. 13C CP N M R spectra of carbonyl carbon-labeled uniaxially draw ( x 5 at 80~
P E T film after heat t r e a t m e n t at 170~ /3L is set as 0 and 90 ~ Full and dotted curves show observed and calculated spectra, respectively. The fractions of the three components, amorphous (30%) and two oriented ones (35 and 35%), was determined by simulation. The structural parameters were aF -- 90 ~ ~F --" 20 + 10 ~ and p = 8 ~ (low oriented c o m p o n e n t ) and O~ F = 90 ~ ~ F = 11 - 5 ~ and p = 2 ~ (high oriented c o m p o n e n t ) , respectively.
t.n ~t~
506
TETSUO A S A K U R A AND T A K U R O ITO
Table 14.1. Fraction (%) of disordered and ordered components for uniaxially oriented PET films determined from 13C CP NMR
Drawn at 45~
Drawn at 80~
Heat treatment
Draw ratio
x3
x2
x3
x4
x5
x5.66
x5
Disordered Ordered
35 65
100 0
80 20
70 30
55 33 (1) 12 (h)
47 38 (1) 15(h)
30 35 (1) 35 (h)
(1) = low ordered component. (h) = high ordered component. Table 14.2. Structural parameters (degrees) of the ordered components for uniaxially ordered PET samples determined from 13C CP NMR (az was assumed to be 90 ~
Draw ratio
p /3F 0
Drawn at 45~
Drawn at 80~
Heat treatment
x3
x3 & x4
x5 & x5.66 low high
x5 low
high
24 24 --+ 10 16 -----10
8 20 --+ 10 20 -----10
2 11 --+ 5 29 -----5
22 18 -----6 22 -----6
24 20 -----10 20 -----10
5 16 -- 6 24 -----6
p: = the orientational distribution around the fiber axis (chain axis or MD). 0 = the angle between the phenylene para-C--C and chain axes (or MD) of PET film.
is determined as 29 m 5o. The structural parameters obtained for uniaxially oriented PET films are summarized in Tables 14.1 and 14.2. The three components observed in the spectra of well-oriented PET samples might correspond to the three-region model composed of an NMR crystalline, highly oriented component, a rigid, NMR amorphous (low oriented) component, and a mobile NMR amorphous region (unoriented component) proposed by Havens and VanderHart [12], and Gabrielse et al. [2] on the basis of 13C CP/MAS NMR relaxation experiments.
References 1. J.-M. Besnoin and K.Y. Choi, J. Macromol. Sci., Rev. Macromol. Chem. Phys. C29 (1989) 55. 2. W. Gabrielse, H. Angad Gaur, F.C. Feyen and W.S. Veeman, Macromolecules 27 (1994) 5811. 3. M.D. Sefcik, J. Schaefer, E.O. Stejskal and R.A. McKay, Macromolecules 13 (1980) 1132. 4. A.D. English, Macromolecules 17 (1985) 2182. 5. M. Wilhelm and H.W. Spiess, Macromolecules 29 (1996) 1088. 6. S. Rober and H.G. Zachmann, Polymer 33 (1992) 2061.
POLY(ETHYLENE TEREPHTHALATE)
507
7. B.F Chmelka, K. Schmidt-Rohr and H.W. Spiess, Macromolecules 26 (1993) 2282. 8. T. Asakura,T. Konakazawa, M. Demura, T. Ito and Y. Maruhashi, Polymer 37 (1996) 1965. 9. D. Walls, J. Appl. Spectrosc. 45 (1991) 1193. 10. S. Arnott and A.J. Wonacott, Polymer 7 (1966) 157. 11. G.S. Harbison, V.-D. Vogt and H.W. Spiess, J. Chem. Phys. 86 (1987) 1206. 12. J.R. Havens and D.L. VanderHart, Macromolecules 18 (1985) 1663.
Chapter 15
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Crosslinked Polymers R.V. Law ~ and D.C. Sherrington 2 1Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW 7 2AZ, UK; 2D.C. Sherrington, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
15.1
Introduction
Crosslinked polymers are key materials in a wide range of technological applications ranging from the "high-tech" aerospace area to the "low-tech" use of wood-derived products. Inevitably, the bulk properties and materials performance are controlled in the final analysis by the polymer molecular structure, although to relate the backbone structure to performance in applications is a complex exercise. This often involves the requirement for numerous intermediate correlations e.g., molecular structure with micromorphology, micromorphology with phase separation, etc. Evaluating the molecular structure of linear polymers has become relatively straightforward in recent years and solution phase ~H and ~3C NMR techniques have played an important role in this context. When linear polymers are crosslinked to form an infinite network, either during the polymerisation process, or as a post-polymerisation chemical treatment, then analysis by routine solution NMR spectroscopy rapidly becomes impossible as a result of signal broadening. When the level of crosslinking remains low, solution phase methodology applied to highly solvent swollen polymer can still be useful (see Sections 4.3 and 4.4). In general though, most crosslinked polymers are relatively highly crosslinked, and become amenable to analysis by NMR spectroscopy only by using solid phase techniques.
15.2 Solid-state ~3C and ~SN cross-polarisation/magic-angle spinning (CP/MAS) NMR 15.2.1
Background
Despite the obvious benefits of quantitative solid-state NMR via single pulse excitation methodology (SPE) (see Section 3), cross-polarisation combined with magic-angle spinning (CP/MAS) [1] has many intrinsic advantages which
510
R.V. LAW AND D.C. SHERRINGTON
have proved very valuable in evaluating the structure of crosslinked polymers. The principle benefit of CP is that it enables a spectrum to be acquired very quickly, as more scans can be obtained in any given time, giving a better signal-to-noise (S/N) ratio. The increase in S/N ratio is due to the fact that in many organic materials the proton spin-lattice relaxation mechanism, by which the system can relax back to equilibrium, is approximately an order of magnitude shorter that for 13-carbon. Though it is desirable to use SPE it is often impractical to carry out a fully quantitative analysis because experimental conditions e.g., MAS at high temperatures, means that there is limited time available and therefore a compromise, using only CP, has often to be made. If time permits an ideal approach is to use a combination of both SPE and CP as these techniques complement each other well (see Section 3). The other advantage that CP brings is important information about the molecular dynamics within the polymer from CP variable contact studies. The parameters typically obtained include the cross relaxation time between 1H and 13C, TcH, and the relaxation time in the rotating frame, ~H Tip. Furthermore, there is also a large (--~ factor 4 for 13C) sensitivity enhancement for ~3C when carrying out CP as polarisation is transferred from the isotopically abundant 1H (100%) to the rare 13C (1.1%). This section will deal principally with crosslinked polymers which have been characterised with CP combined with MAS. Earlier monographs and reviews have dealt, in part, with the application of CP/MAS to crosslinked polymer systems. These include reviews by Yu and Guo [2], Andreis and Koenig [3], books by Komorski [4], Mathias [5], McBrierty and Packer [6], Ibbett [7], Bovey and Mirau [8] and the annual reports by Webb [9]. This section will focus on more recent papers dealing with the application of solid-state CP/MAS NMR spectroscopy to the analysis crosslinked polymer systems.
15.2.2
Phenol-formaldehyde resins
Phenol-formaldehyde (PF) resins have been used as model compounds for the study of pyrolysis and combustion reactions that occur in solid fuels [10]. Utilising these resins it is possible to incorporate a wide range of heteroatomic and hydrocarbon moieties to simulate compounds that arise naturally in the solid fuels. A series of phenol resins crosslinked with thiophene, dibenzothiophene, diphenylsulfide, benzyl phenyl sulfide, thioanisole, 8-hydroxyquinoline and 2-hydroxycarbazole were synthesised. These samples were then cured at 200~ (Fig. 15.2.1) and the resulting resins examined by solid-state NMR spectroscopy. The ~3C CP/MAS spectra of a standard PF resin is shown
CROSSLINKED POLYMERS
511
OH '---'- S ~
OH
Excess CH20 "OH
/
CH20H
S
CH2
Curing
/
CH2
S
CH, -
Fig. 15.2.1. Incorporation of diphenyl sulfide into the structure of the co-resite and the resultant
resite after curing.
in Fig. 15.2.2. This was compared to the partially and fully cured resins. (Figs. 15.2.3 and 15.2.4). By curing the standard resins at increasing temperatures (up to 200~ it was possible to show that peaks at 35 and 70 ppm, attributable to methylene and alkyl ether bridges respectively, were converted from the ether to methylene bridges at the final cure temperature. The aryl ether peak at 160 ppm possibly arose from the condensation of two phenolic moieties. These resins were studied in order to obtain the optimum cure conditions. The sulphurcontaining resins show a wide range of ether, ethylene and methylol constituents. The peaks at 38, 55-80 and 90 ppm arise from methylene and methylol carbons attached to the ortho and para positions of the phenol ring and hemiacetal carbons, respectively. Fully cured resins contain only aliphatic peaks at 18 and 38 ppm with no remaining alkyl ether linkages. The aryl ether peak at 160 ppm also increases slightly in intensity. A major side reaction that was
512
R.V. L A W A N D D.C. S H E R R I N G T O N
't
j .......
I
200
. . . .
90~ (48 hours) ~._
I. . . . .
100 PPM
-
--I
0
. . . . . .
Fig. 15.2.2. Solid state 13C C P / M A S N M R spectra of a normal PF resin after various cure periods. SB = spinning side band.
identified for both the normal and sulfur containing resins was the formation of an arylmethyl moiety which gave a peak at 18 ppm. Understanding the curing process in PF resins and the analogues employed in this work gave a useful insight into the mechanisms that occur in solid fuels. Though useful as model compounds, PF resins have many commercial uses in their own rights, these include thermal insulation, mouldings, and use as wood resins. Therefore the systematic understanding of their molecular structure gives an insight into their physical properties. One important physical property is the resistance of the resin towards acid and bases; understanding this enables a better approach to the correct formulation of the resins.
513
CROSSLINKED POLYMERS
(a)
(c)
200
1SO
leo PPH
50
0
Fig. 15.2.3. Solid state 13C CP/MAS NMR spectra of the partially cured (130~ containing (a) dibenzothiophene (b) thioanisole (c) phenyl benzyl sulfide.
resites
This has been studied by 13C CP/MAS NMR where the degradation of PF resins in the presence of acid (oxidising and nonoxidising), base and formalin [11] has been monitored (Fig. 15.2.5). The main structural components believed to be present in the cured resin are shown in Fig. 15.2.6. The proportions of each species depends upon the initial P : F ratio, p H value, catalyst and temperature. At relatively low concentrations (---1 M) exposure to acid and base simply leads to neutralisation or formation of the phenoxide salt of the resin. This is shown by the change in intensity of the peak at ca. 152 ppm. Also in the presence of alkali, peaks at 73 ppm disappeared giving a proportional rise in intensity at 65 ppm. This was explained by the cleavage of dimethylene ether linkages to produce the corresponding methylol groups. Treatment with formalin also produced methylol groups at positions ortho or para with respect to the phenolic linkage, producing peaks
514
R.V. LAW A N D D.C. S H E R R I N G T O N
(b)
(c)
(d)
200
150
I00 PPM
50
0
Fig. 15.2.4. Solid state 13C CP/MAS NMR spectra of the fully cured (200~ resites containing
(a) dibenzothiophene (b) phenyl benzyl sulfide (c) diphenyl sulfide (d) thioanisole.
at 65 ppm. There was also evidence, from the presence of peaks at 40 ppm, of the formylation of p,p'-methylene linkages formed. Under stronger nonoxidising acidic conditions (sulphuric acid 36 N) (Fig. 15.2.7) all of the methylol and dimethylene ether linkages are cleaved. Some of this cleavage gives rise to CHO (194ppm) and CH3 (18ppm) moieties. There is also evidence that there is sulphonation of the aromatic ring in the ortho and para position to the phenolic hydroxide group (ca. 152 ppm). The use of strong oxidising acid (15 N nitric acid) brought about more major structural changes
CROSSLINKED P O L Y M E R S
515
Phenol i c ring carbons
herl•rbons /
~
~C_H2OAr'
ArCHO
ArCH2Ar'
b
C 9
i"~'~'T
200
,"'i
1
'
150
'
'
i 1 "" 100
'
'
I
50
""'~ ' I 0
'
PPM
Fig. 15.2.5. 15.1 MHz 13C CP/MAS N M R spectra of (a) resole type resin (b) cured resin after treatment with 1.0 N sodium hydroxide under N2(g) for 65~ for 3 days, and (c) cured PF resin after treatment with 36.8% formal under N2(g) at 25~ for 1 day.
(Fig. 15.2.8). These included the conversion of phenolic rings into cyclic ketones and nitration of the rings. Methylol and methyl groups are also similarly nitrated. 15.2.3
Melamine-formaldehyde resins
These are common materials that have found many applications e.g., as hard, durable and chemically resistant surfaces. A series of uncured and cured
516
R.V. LAW AND D.C. SHERRINGTON OH
OH
OH
0
~
OH
Fig. 15.2.6. Main moieties present in a PF cured resin.
melamine-formaldehyde (M-F) resins have been examined by solution state (1H, 13C) and solid-state (13C, 15N) NMR respectively [12]. The resins were either laboratory synthesised or obtained from an industrial manufacturing process. The solid-state NMR studies involved four resins, two laboratory synthesised and two obtained from industry, all of which had different M : F ratios. The 13C solid-state NMR spectra of the synthetic resins are shown in Fig. 15.2.9. The peak at 166 ppm in readily assigned to azine ring carbons. The more interesting region, however, is the methylene region (40-60 ppm) in which there are five components at 48, 52, 58, 66, and 72 ppm. These have been assigned to the following structures,
--NHCHzNH m,
=NCH2NH--,
--NCH2N=,
mNHCH2OCH2~,
This indicated that resins synthesised with M ' F ratios of 1.0" 1.5 contain principally methylene linkages where as higher ratios of M ' F (1.0" 3.0) contain equal amounts of methylene and methylene ether linkages. The spectra, however, showed no evidence for methylol groups which should give peaks at 64 and 69 ppm. The was also confirmed by examining the 15N solid-state NMR spectra (Fig. 15.2.10). The 15N solid-state NMR spectra of model compounds containing methylolmelamines showed peaks at 87 and 107 ppm (indicative of ~ N H C H 2 O H and ~N(CH2OH)2 which were absent in the spectra of the resins. What the 15N spectra did indicate, however, was a peak at 77 ppm and a shoulder (for the M ' F 1.0" 3.0 resin) at 93 ppm indicative of ~ N H C H 2 ~ and - - N C H 2 ~ linkages. The industrially synthesised resins showed very similar spectra indicating the presence of high concentrations of branched and linear methylene and methylene ether linkages.
517
CROSSLINKED P O L Y M E R S
Phenolic ri ng carbons ot--~hercarbons
I
COCH2Ar'
J!
ArCH2OAr' ~ AI.CH2OCH2Ar'
C_OH ArCHO
ArCH2OH
\
- ArCH2Ar
a
b
C 200
!50
100
50
0
PPI4
Fig. 15.2.7. 15.1 MHz 13C CP/MAS NMR spectra of three residues of cured PF resin after treatment with sulphuric acid under N2(g) and three different conditions (a) 1.0 N sulphuric acid solution at 65~ for 3 days (b) 36 N sulphuric acid at 25~ for 1 day (c) 36 N acid at 65~ for 1 day.
15.2.4
Urea-formaldehyde resins
Currently, one of the most important commercially available materials today are the urea-formaldehyde (U-F) resins. Their applications include coatings, adhesives, castings, moulding compounds and textiles. Maciel et al. have produced a series of extensive papers in this area concentrating on both 13C and 15N CP/MAS [13-16].
518
R.V. LAW A N D D.C. S H E R R I N G T O N
a
b 0
d
-
'"
1.-
' 1"
2S0
'
' ' I '''
200
' I '''''''~
150
'
'1"
100
'""
I '
50
''
~1
~
0
'
1 '
-50
PP~I
Fig. 15.2.8. 13C CP/MAS NMR spectra of the residue of cured PF 50 resin after treatment with 15 N nitric acid in air at 25~ for 1 day (a) 50.3 MHz (b) 22.6 MHz (c) 15.1 Mhz and (d) 15.1 MHz 50-1~s dipolar dephasing. Spinning sidebands are marked with an asterisk.
A systematic 13C CP/MAS study was undertaken of a large array of different U-F resins synthesised under a wide range of conditions including pH, concentration, and U : F ratio [13]. Catalyzed by both acid and base a great variety of reactions can occur leading to a large and complex range of moieties depending upon the synthetic conditions. At high pH the principal linkages present are methylol urea (65, 72 ppm) and dimethylene ether (69, 70 ppm), also present under the basic conditions is a small amount of methylene methyl ethers (55 ppm). Under acidic conditions other reactions predominate. In addition to the formation of linear methylene linkages (47, 54,
519
CROSSLINKED POLYMERS
Ca)
L
1
300
J- .
200
.
.
.
100
.
1
. . . . . . .
0
ppm
I
-100
(b)
L
300
~
:
200
..........
l
100
.
.
.
.
ppm
__1
0
.j
-100
Fig,. 15.2.9. Solid state 13C CP/MAS NMR spectrum of M-F resin.
60ppm) resins contained a substantial amount of crosslinking methylene linkages (69, 76ppm) which increase as the F : U ratio increases. At very high F : U ratios methylene dimethylene ethers, methylol and hemi-formals occurred (69-72 ppm). Also present in large quantities were disubstituted urons (75, 79 ppm) which increased as pH increased. In an attempt to formulate alternative [14] resins N,N'-dimethylolurea or paraformaldehyde was used as a different source of formaldehyde to crosslink the urea molecules. The resins produced by these methods generally exhibited similarities to those previous synthesised using formaldehyde [13]. Small
520
R.V. LAW A N D D.C. S H E R R I N G T O N
(a)
r
(b)
i
I
I
400
300
,
J_
_
200
1
t
100 ppm 0
,
.1
-100
Fig. 15.2.10. Solid state 15N CP/MAS N M R spectrum of M-F resin.
differences between the resins produced by the different synthetic route appeared to be due principally to the problems of solubility of the N,N'dimethylolurea or paraformaldehyde. A large volume rotor MAS system was used to examine the natural abundance 15N present in urea-formaldehyde resins [15]. Increasing the amount of material which is examined has enabled the investigation of the isotopically low 15N present (0.37%) in the resins without having to resort to synthesising 15N enriched materials. There are four possible interaction sites between urea and formaldehyde (Fig. 15.2.11).
CROSSLINKED POLYMERS O II
0 II
~--N--C--NCH2OH + H20
~-~N--C--NH + HOCH2OH
I
I
O II ,~,N--C--NCH2(OCH2)nOH
I
I
0 II ,,,,,,~N--C--NCH20H 9
I
I
I
OH
,,~N--C--NCH2(OCH2)n+IOH + H20
0 II + ,,---N--C--NH
I
~
I
O O II II ,~-N--C--NCH2N--C--NH.'~ + H20
I
I
I
I
O O II II ,~,N--C--NCH2OCH2N--C--NH,w~ + H20
~-~N--C--NCH2OH
--N--C--N--N [ ~H2 CH2
0 Ii
+HOCH2OH
0 I!
o II
521
I
~
",,
O
N
~ jN
I
I
I
/ + H20
O
OH
Fig. 15.2.11. Structural units present in UF resins.
For resins synthesised under acidic conditions tertiary amides initially seen by 13C CP/MAS were confirmed by 15N CP/MAS. Under neutral or basic conditions the main constituents of the resin are N,N'-dimethylolurea (102 ppm), monomethylolurea (102 and 78 ppm) and dimethylene ether linkages (90 ppm). Using dipolar dephasing and cross-polarisation times it was possible to distinguish primary, secondary and tertiary substituted nitrogens. These results confirmed the existence of many moieties postulated by 13C CP/MAS. The widespread application of U-F resins has meant understanding the mechanism of the degradation process is important if an improvement in resin stability is to be obtained [16]. Therefore the way in which U-F resins change when they undergo hydrolysis was examined. The resins have been described previously [13]. There are a number of possible mechanisms which involved the hydrolysis of the moieties in the U-F resins. A typical degraded resin is shown in Fig. 15.2.12.
522
R.V. LAW AND D.C. S H E R R I N G T O N
A
b)---J -I-
i ....
I"
i
"!
I
i ....
200 180 160 140 120 100 80
I'
60
I
40
I"
20
I --
0 PPM
Fig. 15.2.12. (a) 50.3 MHz 13C CP/MAS NMR spectrum of a UF resin sample prepared from formalin (F) and urea (U) with an equivalent F/U/water ratio of 2.00/1.00/1.07 at pH 3 and (b) its solid residue after hydrolytic treatment at pH 4 and 86~ for 20 h. Spinning side bands are marked with asterisks.
It was demonstrated that resins prepared with an equivalent molar ratio of F : U gave the highest stability towards hydrolytic treatment. Resins which contained higher F : U ratio (2.0: 1.0) contained a wide range of moieties which were more readily susceptible to hydrolysis, the products formed include dimethylene ether linkages, poly(oxymethylene glycols) and methylols attached to tertiary amine groups. These moieties are the sources of formaldehyde when the resin degrades. Resins of different composition showed similar degradation patterns. 15.2.5
Isocyanurate based resins
In a series of papers cured resins based upon ~SN enriched 4,4'-methylenebis(phenyl isocyanate) (MDI) have been examined by utilising ~3C and 15N
CROSSLINKED POLYMERS RNCO + H20 R'NCO + RNH2 ~ ~ - ' P "
RNCO + R'NHCONHR"
523
~-" RNH2 + CO 2
RNHCONHR' ~" R"NHCON(R")CONHR
Fig. 15.2.13. Reactions of isocyanate units that occur in MDI-polyisocyanurate resins.
CP/MAS NMR. In the first of these papers the structures within the resins were examined as a function of cure temperature [17]. The chemistry of the resins is very complex but one of the principle reactions is the formation of stable isocyanurate structures from three isocyanate units. Other species are also present e.g., amine, urea and biuret. These are formed by the reactions shown in Fig. 15.2.13. 13C CP/MAS NMR spectra of the resins indicated that the optimum cure temperature was 120~ at which most of the isocyanate groups were converted to isocyanurate (Fig. 15.2.14). The peak at 150ppm is due to the isocyanurate carbonyl carbon, the benzylic substituted aromatic carbons para- to an isocyanurate moiety are shown by a peak at 145 ppm, the 130ppm peak is due to the protonated aromatic carbons, the shoulder at 125 ppm is due to both isocyanate and ortho aromatic carbons. These signals were confirmed by using dipolar dephasing spectra. However, because of the complexity of the spectra ~SN CP/MAS NMR was used to clarify the structures present at the different cure temperatures. A summary of the moieties found in the resin is given in Fig. 15.2.15. In addition Duff et al. also undertook a quantitative analysis by utilising the large difference in cross-polarisation time for protonated and nonprotonated nitrogen. In a related study a series of resins were examined in which biuret linkages predominated [18]. These were formed by the reaction of formic acid and 4,4'-methylenebis(phenyl isocyanate) (MDI). The principal reactions that occur in the resins are shown in Fig. 15.2.16. The 13C and 15N CP/MAS spectra showed that when the formic acid" MDI ratio increased the biuret linkage predominates. The pathway for this was initially the formation of MDI-based urea and formic anhydride moieties which further reacted with isocyanate groups to form the biuret linkages and possibly diformyl imide groups. In a further study the same resins were analysed straight after curing and again after a 7 months exposure to air [19]. Three different cure temperatures were used, 100~ 120~ and 160~ A typical example of the 15N CP/MAS spectra is given in Fig. 15.2.17. The predominant structure in these resins is the isocyanurate linkage. These are relatively stable and it is the chemistry of the residual isocyanate groups that dominate the formation of new bonds in the system during the
524
R.V. LAW AND D.C. SHERRINGTON
160 *C
140 ~
120 ~
100 ~
80 ~ 160
140
120
100 PPM
Fig. 15.2.14. 50.3 MHz 13C CP/MAS spectra of MDI-polyisocyanurate resins prepared at different temperatures.
exposure to the air. Using ~SN CP/MAS it was possible to identify clearly the products of isocyanate hydrolysis which involved principally the formation of amines and the urea-linkage condensation products, there was no significant formation of biuret linkages. Structural assignment for these resins was further substantiated by 13C CP/MAS. These results aided the identification of the decrease and increase in concentrations of isocyanate groups and urea linkages, respectively. Duff et al. also attempted a quantification experiment using the ~SN CP/MAS results to determine the relative concentrations of the
CROSSLINKED POLYMERS
525 15N Chemical Shift
Isocyanate
ArNCO
46
Amine
ARNH 2
53
Urea
ArNHC(O)NHAr
104
114 ArNHC(O)N(Ar)C(O)NHAr
Biuret
141
.0
L Uretidione
Ar--N
\]l/
(NH) (N)
145
N--At
O
Isocyanurate
O
'~N
N/Ar 149
I
Ar Fig. 15.2.15. Structures and 15N chemical shift data pertinent to MDI-based resins.
RNCO + R'COOH
~"
RNCO + 2R'COOH RNCO + R'NHC(O)R" RNCO + R'C(O)OC(O)R'
RNHC(O)R'
+ C02
RNHC(O)NHR + R'C(O)OC(O)R' ~--
RNHC(O)NR'C(O)R"
~
R'C(O)NRC(O)R' + C02
Fig. 15.2.16. Reactions that occur between formic acid and 4,4'-methylenebis(phenyl isocyanurate) (MDI).
526
R.V. LAW AND D.C. S H E R R I N G T O N
B
A l
200.00
J_
1
| ~ , ~0
J .....
!
i013.=
I
_
l
50,00
,,.J
!
E, O0 PPM
Fig. 15.2.17. (a) 20.3 MHz 15N CP/MAS spectra of MDI-polyisocyanurate cured at 100~ (b) Same resins after 7-month exposure to air.
different nitrogen containing moieties before and after prolonged exposure to air [20]. The differing extents to which hydrolysis occurred in the samples was interpreted in terms of the structural effect that the cure conditions had. The possibility that the different morphologies present could be responsible for the amount of hydrolysis was further investigated by study of the 1HT~p of the samples. Finally the thermal degradation of the samples was examined. In this study it was possible to show that the degradation of all biuret and uretidione linkages occurred at 230~ a decrease of residual isocyanate took place on heating to temperatures up to 240~ an increase in urea linkages occurred in samples heated up to 240~ followed by a decease in these from 250-260~ A steady increase in amine groups and a decrease in isocyanurate groups was also observed. The peaks in both the ~3C and ~SN CP/MAS NMR
CROSSLINKED POLYMERS
527
spectra broadened with increasing temperature. This was attributed to the formation of free radicals and substantiated by ESR spectroscopy. The thermal stabilities of the relevant groups were in the order biuret, uretidione < urea < isocyanurate < urea', where urea' is a urea-type more stable than isocyanurate. 15.2.6
Synthetically crosslinked natural polymers
Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state N M R spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by ~3C CP/MAS solid-state N M R spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-4~, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). For the paraformaldehyde cured species three new peaks were observed that were representative of the C4 unsubstituted flavonoid site (38 ppm) and (OH) OH H
OH
H
(OH)
Fig. 15.2.18. A typical flavonoid structure. The parentheses indicate variations in flavonoid structure with some hydroxyl groups absent.
528
R.V. LAW A N D D.C. S H E R R I N G T O N
..[
......
, ....
~00
Fig. 15.2.19. 13C C P / M A S N M R
t. . . . . . . . . .
150
l . . . . . . . . .
I00
ppm
I
. . . . . . . . .
50
! . . . . . . . . . .
I
0
spectrum of pine tannin.
the formation of methylene bridges between two phenolic rings (36.8-37 and 33 ppm). Unsurprisingly methylene bridges were formed exclusively. For the hexamethylenetetramine cured species peaks assigned to the formation of methylene bridges (identical to the first reaction) together with peaks assigned to the formation of tribenzyl- (57.5 ppm), dibenzyl- (51.0 ppm) and monobenzylamines (45 ppm) linkages were observed. In the latter case it appeared that 40-50% of the crosslinks were the benzylamine type (with tri- and monobenzylamine predominating) the remaining being methylene bridges between phenolic type structures. Further evidence for this was shown from the peak at 98 and 105-110 ppm representative of the free and reacted C6/C8 sites respectively, the former decreasing and the latter increasing in both cases with reaction with the formaldehyde and the hexamethylenetetramine. Pecan nut tannin with another predominant flavonoid form of differing reactivity was also reacted with hexamethylenetetramine. In this case the dibenzylamine and tribenzylamine units were the dominate moieties present in the
CROSSLINKED POLYMERS
.!
.........
200
f .........
150
! .........
I00
ppm
f .....
50
....
529
1 .........
f
0
Fig. 15.2.20. 13C CP/MAS NMR spectrum of pine tannin extract hardened with paraformal-
dehyde.
system, also the level of methylene bridging was much lower representing only 20% of the total crosslinks. In two closely related studies Wendler and Frazier examined, by ~SN NMR, the interaction between both model cellulose compounds [22] and wood with 15N enriched polymeric diphenylmethane diisocyanate (pMDI) [23]. The resin formed is used commercially as a wood adhesive. Previous work [17] had shown that this reaction is sensitive to moisture, the formation of different products depending upon the degree of moisture present (Fig. 15.2.22). Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence
530
R.V. LAW AND D.C. SHERRINGTON
! .....
200
, ....
! ....
150
~ ....
1 .....
, ....
100
I .....
50 ppm
, ....
t ..........
1
0
Fig. 15.2.21. 13C CP/MAS NMR spectrum of pine nut tannin extract hardened with hexamethylenetetramine.
of the large amounts of hydroxyl groups available from the cellulose to react further with the urea linkages. 15.2.7
Polyacenicpolymers
The potential applications for conducting polymers are enormous and this has stimulated a large amount of research into this area. Not surprisingly, solid-state NMR spectroscopy has been applied to study these amorphous, insoluble and in many cases crosslinked materials [24]. Looking at the 13C CP/MAS spectra of a series conducing polyacenic polymers, some of which were doped with iodine, it was possible to see the effect of the halogen upon conductivity. These resins were prepared by a conventional procedure for the preparation a Novolak-type phenol-formaldehyde resin. After synthesis, the phenol-formaldehyde resin were dissolved and solutions were cast as a film and heat treated to between 590-670~ in a Ne atmosphere to form the polyacenic film. The electrical conductivity of the films was shown to increase
531
CROSSLINKED POLYMERS
1
t
250
1
200
! ....
150
, ....
! . . . . . . . . .
100
;0 . . . . . . . . .
I
PPM
Fig. 15.2.22. ~SN CP/MAS spectrum of wood/15N-pMDI composite as a function of precure moisture content.
with higher temperature. The addition of iodine to one of the films gave rise to substantial increase in electrical conductivity. The postulated structures are shown in Fig. 15.2.23. Typical 13C CP/MAS spectra are shown in Fig. 15.2.24. By using the reference spectrum of a phenol-formaldehyde resin the peaks for the polyacenic films were assigned. The main peak at 127-130 ppm moves upfield and broadens with increasing temperature indicating an increased amount of polyacenic-type structures. The peak at 150 ppm assigned to quaternary aromatic carbon substituted by hydroxy groups and the peak at 40 ppm assigned to methylene carbons decrease in intensity with increasing temperature but are never completely removed. Using dipolar dephasing spectra the aromatic region also revealed further signals and it was possible
532
R.V. LAW AND D.C. SHERRINGTON
a
a
Fig. 15.2.23. Structures of polyacenic films. Initial phenol-formaldehyde film (top), partially cured (middle). fully cured (bottom).
CROSSLINKED
533
POLYMERS
2,3 5,6
i
~-
i
~)o
~
~
i
9
15o
, .
100
.
.
.
.
50
Fig. 15.2.24. 13C C P / M A S spectra of a polyacenic film.
to assign them using chemical shifts from model compounds. The peaks at 129.5 and 151ppm were due to protonated/nonprotonated and hydroxy substituted carbons respectively in the residual phenol-formaldehyde resin structure. The peaks at ca. 125 and 138 ppm are the methine and quaternary carbon in the polyacene. The ratios of two of the aromatic peaks, obtained by deconvolution, were related to the degree of electrical conductivity. For the iodine doped sample it was shown that the iodine interacts significantly with the polyacene part and not with the phenol formaldehyde part. This was shown by peak broadening of the polyacene type peaks. 15.2.8
Polyethers
A semi-crystalline poly(1,3-dioxolane) was examined by solid-state NMR spectroscopy looking at both linear and crosslinked polymers [25]. The
534
R.V. LAW AND D.C. SHERRINGTON
OCH2CH20
OCH20
(b)
,'l",~']"'"'i
le5
100
95
....
I ....
$0
! .... "T"'"I
85
80
....
75
I' '"" i '''~:i . . . .
7e
65
6a ~p~
i
Fig. 15.2.25. (a) 13C single pulse excitation and (b) 13CCP/MAS spectra of poiy(1,3-dioxolane).
crosslinks were formed by introduction and reaction of acrylate groups allowing the formation of a network and control of the molecular weight. The systems were examined by 13C MAS and typical spectra appear in Fig. 15.2.25 where two characteristics peaks at 67.5 ppm ( O C H 2 0 ) and 96.0 ppm ( O C H 2 0 ) are diagnostic. For the crosslinked polymer the CP spectrum, being more sensitive to static molecular motion, revealed a further peak at 93.4 ppm (the O C H 2 0 region) which was assigned to a less mobile phase. To clarify these results variable contact CP and cross-depolarisation experiments were carried out and by deconvolution of the peaks in the NMR spectrum three regions, a crystalline, an interfacial and elastomeric one, were indicated. Further study carried out using 1HTlp which is indicative of kHz motion in
CROSSLINKED POLYMERS
535
the polymer, also suggested that there were three phases present in the polymers. 15.2.9
Epoxide based resins
In a high temperature study [26] of two epoxide resins, the samples were heated to above Tg whilst still carrying out magic-angle spinning to remove residual line broadening interactions. At these temperatures (ca. 260-290~ the molecular motion of the system had increased to such an extent that it was possible to use conventional one (~H, DEPT) and two dimensional (HECTOR) solution state NMR experiments on the samples. The networks looked at were the oligomer of diglycidyl ether of bisphenol A (DGEBA) cured with 100 and 66% of diaminodiphenyl sulphone (DDS) (Fig. 15.2.26). The spectra of the 100% cured resin at ambient and high temperatures are shown in Fig. 15.2.27. The principal reactions are between the epoxy and the primary and secondary amines, further reactions between hydroxyl groups are also possible. The principal reaction moiety for the 100% cured polymer is indicated in Fig. 15.2.28. For the 66% cured polymer the situation was more complex the spectrum is shown in Fig. 15.2.29 and the major structural moiety present is indicated in Fig. 15.2.30. The increase in resolution at higher temperature is clearly evident and the possibility of using conventional solution state editing techniques is advantageous as they greatly aid peak assignment. Unfortunately this technique may be applicable only where there is a substantial increase in motion above Tg. Many highly crosslinked polymers do not show a discrete Tg and therefore would not be expected to show any substantial decrease in line broadening if they were heated to high temperatures. There is also the question as to whether further post-curing reactions may occur when polymers are heated to such temperatures. Epoxide resins made from 2,2-[4-(2,3-epoxypropyl)phenyl]propane (DGEBA) polycondensed with 4,4'-sulphonyl-dianiline (DSS) produce a three-dimensional insoluble network which was examined by CP/MAS NMR spectroscopy [27]. In this study the chemical structures and the cure kinetics were determined. A cured epoxy synthesised from a mixture of the diglycidyl ether of bisphenol A (DGEBA) and 1,3-phenylenediamine was studied by ~H NMR spectroscopy including multiple pulse techniques and spin-lattice relaxation in the rotating frame, T~o. The study [28] focused on the water distribution based upon possible variation in the cross-link density measured by spin diffusion. From the analysis involving a combination of T~p and multiple
t~ ta~
/o~
c.,---c.c.,o
Y" /~
I
o.
~---~xk ..../f--oc~c.c.,o
]/----x
I /--x.
/o~
X~ <
>, Z
DGEBA
h :z:
./
N
sch -
o u
It
N
Z
\ H
DDS Fig. 15.2.26. Structure of diglycidyl ether of bisphenol A and 4,4'-diaminodiphenylsulphone.
0 Z
II
537
CROSSLINKED POLYMERS
_ ~L_L_=_
I
180
I
I
lz~O
i
I
100
~
i
I
60
I
I
20
~c/ppm
Fig. 15.2.27. 13C MAS spectra of DGEBA-DDS with 100% stiochiometry (a) 23~ with crosspolarisation and (b) 290~ without CP (c) expanded.
HO
0
\
I
'
I
0
\
/OH
I
o
Fig. 15.2.28. Major structure present in the 100% cured DGEBA-DDS resin.
pulse it was possible to postulate that the water was molecularly dispersed in the epoxy rather than aggregated in the voids. Also there seemed to be an absence of two distinct sites for water affinity. The presence of accelerators, either magnesium perchloroate or N,Ndimethylbenzylamine (DMBA) [29], on the curing of bisphenol A diglycidyl ether with butane-l,4-diol (BADGE-BD) were studied by CP/MAS (Fig. 15.2.31). Magnesium perchlorate was shown to induce the consumption of all the diol whereas the DMBA showed only approximately 50% consumption
538
R.V. LAW AND D.C. SHERRINGTON
q I
--
I
180
I
,
,
,
'I
1/.,0
,
I
L
.....
I
100
,
I
9
9
9
I
60
LJ ,
I
c/ppm
20
Fig. 15.2.29. 13C MAS spectra of DGEBA-DDS with 66% stoichiometry (a) 23~ with crosspolarisation; (b) 260~ without CP and (c) expanded.
as indicated by the residual primary alcohols from the butanediol (Fig. 15.2.32). 15.2.10
Methacrylate-based resins
Polymer composites are increasingly used for dental applications [30], the durability and aesthetic appeal has made them ideal substitutes for the more traditional amalgam fillings. The dental polymer composites are principally composed of an organic matrix and a powdered ceramic phase. The organic matrix is composed of an aromatic or urethane dimethacrylate such as 2,2bis[4-(2-hydroxy-3-methacryloyl propoxy) phenyl]propane (bis-GMA) with
539
CROSSLINKED POLYMERS
O
o
Fig. 15.2.30. Major structure present in the 66% cured DGEBA-DDS resin.
I _
s
150
,,,
!
.....
!
110
I
,
r
70
1,
!
....
30 ppm
Fig. 15.2.31. 13C CP/MAS spectrum of BADGE-BD system with accelerator Mg(C104)2.
another monomer such as triethylene glycol dimethacrylate (TEGDMA) to alter viscosity. This system has the further advantage that it can be photopolymerised. ~3C solid-state NMR spectroscopy has been used to study the extent of the crosslinking reaction. A series of commercial and laboratory synthesised resins were examined by CP/MAS and SPE to determine more accurately the relative amounts of unreacted resin present (Fig. 15.2.33). In the synthesis of a methacrylate-based metal chelating resin, ~3C CP/MAS spectroscopy has been used to confirmed that the target resin had been made [31]. Here the imidazole ligand bis(imidazo-2-yl)methylaminomethane (bimam) was attached to a glycidyl methacrylate-co-trimethylolpropane trimethylacrylate (pGMT) resin. The peaks in the ~3C NMR spectrum were
540
R.V. LAW AND D.C. SHERRINGTON
_
t
150
!. . . . .
!
110
....
s,
t. . . .
70
! ....
t
30 ppm
Fig. 15.2.32. 13C MAS spectrum of BADGE-BD system with accelerator DMBA.
assigned as follows: 7.3 ppm, hindered methyl in trimethylolpropane residue; 24.3 ppm, backbone methyl; 41.4 ppm, ~ C H N in ligand; 46.0 and 56.1 ppm, polymer backbone; 67.5 ppm, ~ O C H 2 ~ in epoxy linkage; 127.5 and 145.6, imidazole carbons; and 176.3, carbonyl carbon. In an attempt to provide alternative supports to styrene-divinylbenzene resins [32] for use in reactive chemistry, poly(hydroxyethylmethylacrylate) (poly(HEMA)) has been employed. In this study poly(HEMA) was crosslinked with ethyleneglycol dimethacrylate and the CP/MAS recorded. The peaks were assigned as follows: 18.6 ppm is due to the methyl attached to the aliphatic backbone; 56.0 and 45.4 ppm are due to the methylene and quaternary of the backbone; the ~ O C H 2 ~ forming the crosslink are at 63.0ppm; the conjugated and unconjugated carbonyls are at 167.2 and 177.6 ppm and finally the methylene and quaternary carbons of the unreacted vinyl group are at 126.3 and 137.0 ppm. Another alternative is the use of crosslinked ethylene dimethacrylate [33]. Here, the unreacted double bonds in resins were used as a graft point for further reaction. The level of unreacted double bonds was determined by the relative areas of the carbonyl peaks at 176.3 and 166.3 ppm (Fig. 15.2.34) determined by CP/MAS before and after reaction with glycidyl methacrylate. These results were in good agreement with data from Raman spectroscopy. Spin diffusion is a valuable method by which it is possible to examine the heterogeneity of a polymer [34]. Spin-lattice relaxation times in the rotating
541
CROSSLINKED POLYMERS
260
P-20
180
140
100
60
20
0
ppm
Fig. 15.2.33. 13C CP/MAS spectra of commericially available dental acrylate resins. (a) Tetric; (b) Zl00; (c) Duo Bond; (d) Coltene bonding agent; (e) TEGMA the labels r and u indicate carbonyl peaks from reacted and unreacted methacrylate groups.
flame have been used to determine the rate of spin diffusion. Tip data from three solid polyacrylate networks made by photopolymerisation of poly(ethylene glycol) diacrylate (PEGA), trimethylolpropane triacrylate (TMPTA), and dipentaerythritol pentaacrylate (DPHPA) have been used in this way. The photopolymerisation was carried out by a laser and this was related to the degree of crosslinking that occurred which was quantified in terms of the signals from the residual double bonds in the CP/MAS NMR spectra. The level of heterogeneity in these resins was measured by the 1HTlp and was related to the degree of crosslinking.
542
R.V. LAW AND D.C. SHERRINGTON
1
190
I
180
I
170
I
160
PPM
Fig. 15.2.34. Part of the 13C CP/MAS n.m.r, spectrum of poly(ethylene dimethacrylate) showing the peaks used for the determination of the double bond content.
In an attempt to obtain high surface area glycidyl methacrylate-co-trimethylolpropane trimethacrylate resins were synthesised [35] with a variety of porogens. The degree of unreacted double bonds was determined by CP/MAS NMR spectroscopy. Oligomers containing ether-ester groups were synthesised [36] in order to obtain a crosslinking agent that gavegood cure kinetics and was uniformly distributed in the network structure. The crosslinking agents were modified so that vinylidene groups were incorporated to enable them to be polymerised free radically with styrene or methyl acrylate. The oligomer was incorporated (5-50%) in the polymer to give clear hard resins and these were characterised by CP/MAS NMR (Fig. 15.2.35). The peak at 30 ppm is due to the tert-butyl group, the broad peaks for
CROSSLINKED POLYMERS
543
/
V
200
160
ASO
t40
t20
100 ~X
60
40
2O
0
Fig. 15.2.35. Solid state 13C CP/MAS spectra of three samples of poly(methyl methacrylate)
crosslinked with 5 (lower), 20 (mid) and 50 (top) % of t-butyl acrylate end-capped oligomer containing ca. 2-3 HDDA repeat units.
the ethyl and ester carbons bonded to the oxygen are at 65-80 ppm, and the carbonyl carbon on the upfield side of the P M M A carbonyl at 178 ppm. Polyacrylamides can be synthesised by two methods, either polymerisation of an acrylamido monomer or chemical modification of another polymer e.g., poly(methylmethacrylate). In the latter case 13C CP/MAS N M R spectra show clearly the loss o f - - O C H 3 groups as these are replaced b y - - N H C H z - groups (Fig. 15.2.36). The latter approach has the advantage that polymethacrylates like P M M A can be obtained easily in a bead form of uniform size and are physically convenient for further exploitation. CP/MAS was used in a study [37] of the kinetics of reaction between P M M A and a series of amines. This was carried out by taking aliquots of the reaction mixture at certain times and recording the solid-state spectrum after the reaction had been quenched. The degree
544
R.V. LAW AND D.C. SHERRINGTON
_JA
~"-'~
720 rain.
360 min.
240 rain. 120 rain.
,
J 200.00
~
I 150.00
,
1 100.00
PpM
, 50.00
PMA
-0.00
Fig. 15.2.36. 13C CP/MAS spectra of PMMA reacted after various times with 1,6-diaminohexane.
of reaction was determined by 13C CP/MAS and further structural evidence was provided by 15N~ CP/MAS. 15.2.11
Styrene-basedpolymers
The degree of unsaturation in styrene cured polyesters was investigated [38] by using dipolar dephased 13C CP/MAS NMR data. The commercial resins contained fumarate, isophthalate and propylene glycol structural units and were cured with styrene. The use of dipolar dephasing (see Fig. 15.2.37) suppressed the strong phenyl peak at 129 ppm and therefore allowed the determination of the degree of unsaturation in the resin. The signal at 131 ppm was attributed to the isophthalate units, and the peak at 144 ppm to the quaternary substituted carbon from the styrene. This peak also showed
545
CROSSLINKED POLYMERS
180
160
lt, O
Chemico[ shift
(ppm}
12O
Fig. 15.2.37. Part of a 13C CP/MAS spectrum of a solid polyester obtained (a) without dipolar dephasing and (b) with dipolar dephasing. Chemical shift are shown by the numbers.
partial resolution into two peaks at ---142 and --~146ppm which may have been due to styrene units in sequences of differing lengths. The two carbonyl peaks at 165 and 172 ppm were ascribed to unreacted fumarate/isophthalate and reacted fumarate carbonyls, respectively. The degree of residual unsaturation was calculated by determination of the relative ratios to these peaks. From this it was possible to determine the optimum level of styrene (47%) needed to give the lowest degree of unsaturation in the resin. Polyesters derived from maleic anhydride and 2,2-di(4-hydroxyphenyl)propane were copolymerised with styrene and then studied by CP/MAS NMR [39] spectroscopy. The three dimensional-crosslinked network formed by the polymerisation was examined using spin-lattice relaxation times in the rotating frame. A correlation between reaction conditions and the structure of the resulting material was found. The degree of residual unsaturation was determined by subtraction of two relaxation times from a linear additivity model used for crosslinked polymer systems. In two closely related papers [40, 41] CP/MAS was used to examine a series of styrene-divinylbenzene (St-DVB) and chloromethylated resins. In the first part of this study the authors were concerned with trying to determine the residual amount of unreacted vinyl groups present in St-DVB resins (see Section 3). In order to increase the sensitivity of the method the authors used 13C-labelled divinylbenzene (labelled in the methine position) and combined this with unlabelled styrene (1-20% by weight). The final resins were examined by CP/MAS and it was found that even for a very lightly crosslinked
546
R.V. LAW A N D D.C. S H E R R I N G T O N
species (1% DVB-St) at 70~ residual unreacted vinyl groups were present. Post-curing reactions, carried out by swelling the resin in a solvent and then heating the resin to 155~ in the presence of initiator, showed it was possible to remove the residual unreacted vinyl groups. For resins with higher amounts of DVB (10 and 20%) it was impossible to react all the residual vinyl groups by using this method. This showed that the unreacted vinyl groups are trapped in inaccessible locations within the polymer network. The second part of the study was concerned with studying resins in the unswollen glassy state and the solvent swollen state using CDC13. St-DVB and a chloromethylated resin were examined. For samples with low levels of crosslinking (<6%) swelling in a good solvent whilst spinning at the magic angle (54.74 ~ gave the best resolution (see Section 4). 4-Acryloxy benzophenone (APB) was copolymerised with 4% divinylbenzene (DVB) and ~3C CP/MAS and other techniques were used to characterise the resin [42]. The purpose was to examine the nature of polymer-supported initiators. Data from solution state NMR analysis of the linear uncrosslinked poly(APB) was used as a model for assigning the ~3C CP/MAS NMR spectrum. The solution state spectrum of poly(ABP) gave 12 well resolved lines. The CP/MAS of the crosslinked poly(APB-DVB) copolymer gave six broad lines, and the keto carbonyl at 192.5 ppm and the ester carbonyl at 173.2 ppm. The peak at 153.8 ppm arose from the quaternary aromatic carbon attached to the oxygen atom of the ester group. The remaining quaternary and protonated aromatic carbons were visible as two broad overlapping signals at 128.8 ppm and 122.0 ppm. The signal from the DVB probably also contributes to this region and to the methylene-methine (ca. 40.9 ppm) signal but the authors did not comment upon this. The signal at 40.9 ppm was similar to the broad feature seen in the solution state NMR and is representative of the methylene and methine peaks of the backbone unit. The line broadening was attributed to the distribution of chemical shifts commonly seen in crosslinked resins. A series of copolymer hydroquinone diacrylate (HyDA) resins were characterised by using 13C CP/MAS NMR spectroscopy [43]. The series include styrene-HyDA, glycidyl methacrylate-HyDA, phenylmethacrylate (PhMA)-HyDA, 2,4,6-tribromophenyl acrylate-HyDA, and 4-acetylphenylmethacrylate-HyDA. The 13C CP/MAS spectrum of the PhMA-co-HyDA copolymer is shown in Fig. 15.2.38 with the 13C-{~H} solution NMR spectrum of poly(PhMA). Comparison of the these makes the peak assignment unambiguous. The a-methyl group of phenyl methacrylate appears as a broad peak at 16.4 ppm, the backbone methylene group shows a sharp intense line at 44.0 ppm and the small broad quaternary carbon peak is at 54.0 ppm. The O ~ C aromatic
547
CROSSLINKED POLYMERS
! 3C&SC 2C&6C /
4C
1C
(Ca)
_
I
200
t
!
160
!
,
I
120
1
!
80
1
I,
t,O
l
{~
PPM
Fig. 15.2.38. (a) 13C-{1H} NMR solution spectrum of poly(PhMA) and (b) 13C CP-MAS NMR solid-state spectrum of the PhMA-co-HyDA polymer.
peak (C1) is at 149.2 ppm, the protonated aromatic peaks are at 127.6 (C2 and C6) and 119.6 ppm (C3 and C5) and the para carbon (C4) is at 124.9 ppm. The carbonyl peak is at 172.2 ppm. Hypercrosslinked resins have also been the subject of an N M R study [44] (see Section 3.4). 13C CP/MAS NMR has been used to estimate the degree of crosslinking in a series of these species. The resins were examined in a solvent swollen and a nonswollen state to determine if with swelling in a deuterated solvent it was possible to narrow the line lines further. Determination of the level of crosslinking was carried out by deconvolution of the quaternary aromatic peak at ca. 146 ppm (Figure 15.2.39). This was broken down into two peaks, one centred at 146 and one at 140 ppm. This is a difficult undertaking as there appears to be no substantial asymmetry present in the peak. Use of SPE methodology is probably more quantitative in the context (see Section 3.4). The levels of crosslinking were shown to be high as there was no substantial narrowing of the static proton line widths even at temperatures of 200~
548
R.V. LAW AND D.C. SHERRINGTON
8
b _
-''''
J
'26o ' ' '
~
6o' :'''
Fpr~
Fig. 15.2.39. 13C CP/MAS spectra of (a) crosslinked polystyrene (0.3% DVB) and (b) hypercrosslinked sample. Side bands are indicated by the presence of an asterisk.
One of the aspects of polymer-supported reactions (see Section 4) is the ability to separate reactive centres from each other. The extent to which a benzoin condensation reaction occurs on a crosslinked polymer was examined [45]. The starting material, a polymeric benzaldehyde, was prepared by incorporation of vinyl benzaldehyde into a resin using either divinylbenzene or tetraethyleneglycol diacrylate as a crosslinker. The product was examined using ~3C CP/MAS. The spectra showed two important peaks at 86.2, 126.0 and 166.0 ppm. These were attributed to the c~-hydroxy carbon, the protonated aromatic carbons and the carbonyl carbon of the a-hydroxy ketone. This demonstrated that in the polymer the benzoin condensation reaction had occurred to a significant extent. Another metal chelating resin was looked at using 13C CP/MAS [46] (Fig. 15.2.40). This resin was based on an salicylaldehyde acrylate monomer
CROSSLINKED POLYMERS
549
OHC
Fig. 15.2.40. Structure of salicylaldehyde-containing polymer resin.
polymerised with a small amount of crosslinker, divinylbenzene. This was then further modified to form the oxime. The final product, the aldehyde and oxime, were also examined by 13C CP/MAS (Fig. 15.2.41). Only some of the peaks were assigned: the peak at 40.2ppm was the backbone from the methylene in the styrene and the peak at 126.0 ppm the protonated aromatic carbons. The phenyl ester carbonyl occurred at 160.6 ppm and the aldehydic carbonyl of the salicylaldehyde which appeard at 186.3 ppm in the initial polymer was not present in the oxime. The authors did not comment on the ratio of the peaks found between 55-105 ppm though this may be due to the poor signal-to-noise of the spectrum in this region. 15.2.12
Miscellaneous
Though it now possible to make diamond film by chemical vapour deposition, this technique has limitations and it would still be extremely advantageous to be able to make diamond or a diamond-like material synthetically. Towards this end crosslinked adamantane have been synthesised [47] by reacting 1,3,5,7-tetrakis(4-iodophenyl) adamantane with 2-methyl-3-butyn-2-ol and phenylacetylene to give two corresponding tetracetylene derivatives. These materials were then cured and the final products were examined with solidstate NMR spectroscopy. Typical spectra are shown in Fig. 15.2.42. In these the phenylacetylene crosslinked adamantane shows peaks at 79 and 84 ppm. These arise due to the free acetylene. Upon partial and full curing these lines broaden to form a single peak at 79 ppm. The adamantane carbons are at 39.5 and 50.8 ppm and the phenyl carbons appear as three peaks at 120-132 and one at 148 ppm. On curing the adamantane peaks
550
R.V. LAW AND D.C. SHERRINGTON
II
A
290
I
I
I
I
I
I
2b0
240
220
200
I@
I60
I
140
120
100
1
I
I
I
1
80
60
40
x1
0
PPM Fig. 15.241. Solid state I37c: CP/MAS spectra of ( I ) salicylaldehyde-acrylate-divinjbenzene copolymer and ( 2 ) salicylaldchydc-acrylatc-divinylbcnzcnc oxirne dcrivative.
broaden only slightly compared to thc phenyl pcaks which merge to form just two peaks. This line broadening upon curing has also been seen in other adamantane-containing polymers. From the CP/MAS spectra it was possible to deduce that the polymer cures to give essentially an all-aromatic structure
CROSSLINKED POLYMERS
551
ssb
7/~---k / 3,8 .8.9,10
ssb
180
I
160
'
I
140
'"' ...... I 120
'
'i 100
' '
1 80
'
! 60
'
1 40
'
I
20
ppm
Fig. 15.2.42. '3C CP/MAS spectra of adamantane-alkyne condensate before (bottom) and after curing (top).
containing adamantane cores to yield a material which shows excellent thermal stability. The crosslinked insoluble polymers obtained by vapour deposition techniques have been studied [48] by NMR spectroscopy. In poly(furyleneethylene) the ~3C CP/MAS spectra showed additional peaks at 106.4 and 107.5 ppm which indicated that the chains were crosslinked with furyleneethylene units (Figs. 15.2.43 and 15.2.44). A series of polymers (including polysulphones, polyethers) containing the monomer trans-l,2-diphenylcyclopropane have been synthesised [49] by solution polycondensation. 13C CP/MAS spectroscopy was used, among other techniques, to characterise the cyclopropane ring opening reaction which had occurred in the final thermally crosslinked product. One such polymer (Fig. 15.2.45) showed characteristic peaks in its CP/MAS spectrum (Fig. 15.2.46).
552
R.V. LAW AND D.C. S H E R R I N G T O N
.../~
3
2~o.
2
3
ff-----a // \\
!
1
I
Fig. 15.2.43. Structure of poly(furyleneethylene) film. Numbers indicate carbon atoms corresponding to resonances shown in the spectrum in Fig. 15.2.44.
Those at 162.7, 152.9, 136.9ppm are from the nonprotonated aromatic carbon bound to the sulfonyl group, oxygen and cyclopropane ring, respectively. The protonated aromatic signals arise at 130.3, 121.7 and 117.0 ppm. The peak at 28.7 ppm is attributed to the aliphatic cyclopropane ring carbons. Upon crosslinking the cyclopropane carbons disappear, and three new aliphatic peaks appear. This change also corresponds with a change in chemical shift of the peak at 136.9 ppm which shifts to higher field upon crosslinking. A crosslinked polymer synthesised [50] by crosslinking ethylene-vinyl acetate copolymer with dicumyl peroxide gave two large signal at 31.8 and 33.8 ppm in its 13C CP/MAS NMR spectrum These corresponded to crystalline and amorphous methylene units in the ethylene polymer.
15.2.13
Summary comments
For the characterisation of crosslinked polymer systems it has been shown that solid-state 13C and 15N CP/MAS NMR spectroscopy offers a unique insight into the structure of these insoluble polymeric materials. Often it is the only way possible to develop a fundamental understanding of these generally structurally very complex systems. The power and utility of these NMR techniques are demonstrated by their increasing application. Some authors have adopted the approach of using 13C and 15N labelled
CROSSLINKED POLYMERS
553
c~
c3',c3
C1'
CZ
side b
t side band
180
160
140
IZ0
100
80
side band l
60
40
ZO
0
Fig. 15.2.44. Solid state 13C CP/MAS NMR spectrum of a poly(furyleneethylene) film.
O
, o-
xO///-V
% //
I!_
jo
Fig. 15.2.45. Structure of polysulphone containing the 1,2-diphenylcyclopropane moiety.
compounds for the starting monomeric materials, and this can greatly aid the analysis of the final spectra. This is of course costly, time consuming, and requires the synthesis of the labelled model networks; nevertheless the benefits achieved can be very worthwhile. Conventional CP/MAS methodology applied to unlabelled polymer networks is itself very powerful, however, and further advances and developments in both software and hardware will provide greater understanding and widen the applicability of this technique.
554
R.V. LAW AND D.C. SHERRINGTON
I '
-
-
=
--
200
'
;
150
. . . .
100
50
0
PPM Fig. 15.2.46. 13C CP/MAS NMR spectra of noncrosslinked and cross-linked polymer poly(ether sulfone).
15.3 Solid-state ~3C CP and single pulse excitation (SPE) MAS NMR analysis of crosslinked particulate polystyrene-based resins
15.3.1
Background
Spherical particulate crosslinked resins based on styrene and (meth)acrylate monomers prepared by suspension polymerisation methodologies [51] have become extremely important in a number of technological areas. Perhaps the most important are ion exchange resins [52] for purification of surface, ground and waste water, for steam turbine condensation polishing, for hydrometallurgy, for nuclear process and reprocessing, for de-ionised water production, for ultra-pure water production and for use in various purification (e.g., sugar) and separation processes. The closely related sulphonic acid resins have also become important industrial acid catalysts and are now used in a growing number of large scale acid-catalysed reactions [53]. High surface area nonfunctional styrene-divinylbenzene resins are also finding increasing
CROSSLINKED POLYMERS
555
use as sorbents for both liquid and gas phase components and contaminants [54], and in this context hypercrosslinked resins [55, 56] are now becoming available from commercial sources [57] for industrial exploitation. Most resins are based on crosslinked polystyrene and generally the level of crosslinking is higher (5% upwards) than the very lightly crosslinked gel-type species used in solid phase combinatorial synthesis (see Section 4). Analysis of such resins by NMR spectroscopic techniques was and remains problematical, but solidstate CP and SPE MAS methodologies have moved the area forward significantly in recent years. The high level of crosslinking generally means that the associated increased line widths of signals cannot be reduced simply by solvent swelling although this aspect is currently being re-visited [58]. Solid-state ~3C MAS NMR spectroscopy has had much success in examining amorphous insoluble polymers [59]. In recent years, however, there has been some debate on the reliability of quantitative data derived from CP experiments [60] and work on fossil fuels in particular has highlighted the problem [61, 62]. Undoubtedly, the issues arise in the analysis of polymers as well [63-66]. While CP results in signal-to-noise enhancement and hence reduced accumulation times, carbon atoms present with no proximal protons tend to have their peak intensities reduced relative to other signals. Quaternary aromatic carbons are likely to suffer badly in this respect. The modulation of the dipolar interactions by the motion of some moieties can also introduce quantitative errors [67]. The rotation of the methyl group about its 3-fold axis of symmetry is a good example of this. Single pulse excitation (SPE) [60] however overcomes the problems that are associated with CP, i.e., that the dynamics may alter the CP rates and may therefore discriminate for or against some types of carbon [61, 68] but the long delay time needed between each acquisition (typically 5 times the ~3C spin-lattice relaxation time (30-100s)) means that the technique is time-consuming. In addition, the signal enhancement brought about by the CP sequence is lost.
15.3.2 13C MAS NMR studies of anion exchange resins and their precursors Typically anion exchange resins are synthesised by the sequence shown in Fig. 15.3.47. The chloromethylation reaction is a potentially hazardous one [69] and an important side-reaction is methylene bridging. The latter effectively increases the level of crosslinking and manufacturers have to take account of this (by experience) in tailoring the final physical properties of resins. Important issues in manufacture, in addition to the level of methylene bridging, are the degree of chloromethylation achieved and the efficiency of
R.V. LAW AND D.C. SHERRINGTON
556
//'(
CMME Lewis acid
CH2CI
~~N(CH3) 3
c~
MethyleneBridging
[~CH2~(CH3)3CIChloromethylation and amination of poly(styrene-divinylbenzene) resins to form anion exchange resins.
Fig. 15.3.47.
amination of these groups in generating quaternary ammonium ion anion exchange sites. Ford and his co-workers have used swollen gel-phase [70, 71] and solidstate CP MAS 13C NMR spectroscopy [72] to probe the products of the chloromethylation reaction, while a very detailed study and analysis of all steps in the synthesis of the final anion exchangers have been reported by Sherrington and his collaborators [73] using both CP and SPE MAS 13C NMR methods. In the latter work four precursor resins were employed, two gel-type with 3.5 and 0.5 wt% divinylbenzene crosslinkers, and two macroporous with 7.5 and 3.5 wt% divinylbenzene crosslinker. Each was chloromethylated using chloromethyl methyl ether and in the case of resin 1 ZnCla as the Lewis acid catalyst, and for resins 2-4, FeC13. 13C CP MAS NMR spectra were recorded at 25.2 MHz and the ~CHaC1 resonance was seen clearly at 46.8 ppm. The enhanced intensity at ---40 ppm confirmed the occurrence of methylene bridging but overlap with backbone CH2 and CH signals precluded even tentative quantification. CP spectra of the anion exchange resins formed by reaction of trimethylamine with the chloromethylated species showed signals at 69.0 and 53.2ppm due to and ~ N + C H 3 carbons and corresponding loss of intensity around 47 ppm. However, a significant feature remained at 46 ppm and exhaustive study of this suggested its assignment to a weak base function ~N(CH3)2 probably generated from impurity dimethylamine in the trimethylamine. Interestingly such functionality had been detected from the ion exchange behaviour of these species much earlier by resin manufacturers. Using SPE techniques with the anion exchange resin samples it did prove possible to quantify for the first
557
CROSSLINKED POLYMERS
1
.
.
.
.
.
.
.
!
Iso
.
.
.
.
!
.
1o0 PPn
.
.
.
1
so
.
.
.
.
,.,
I
o
.
Fig. 15.3.48. 13C SPE NMR spectra of: top, anion exchange resin derived from, bottom, styrene-divinylbenzene resin 1(3.5% DVB, gel-type) (see Ref. 73).
time the level of methylene bridging arising on chloromethylation. Figure 15.3.48 shows the 13C SPE MAS NMR spectra of precursor polystyrene resin 1 and its derived anion exchange form, while Fig. 15.3.49 shows the corresponding spectra for resin 2.
558
R.V. LAW AND D.C. SHERRINGTON
1
|
9
9
9
~n
1
1S0
~.
~.
.
.
1
loo PPn
I
J
a
-
|
J
$o
-
9
_a
|
, j
o
Fig. 15.3.49. 13C SPE NMR spectra of: top, anion exchange resin derived from, bottom, styrene-divinylbenzene resin 2 (0.5% DVB, gel-type). (see Ref. 73).
The signal from aromatic quaternary carbon attached to the ~CH2N+(CH3)3 group (originally the--CH2C1) moves upfield to overlap with the protonated aromatic carbon signals. The residual downfield aromatic quaternary carbon signal therefore contains only two components, that due to the carbon atom connecting to the polymer backbone (quantitatively identical to the corresponding signal in the original nonfunctional resin) and that from carbon atoms attached to methylene bridges. Careful assessment of peak areas showed that on average nearly all aromatic groups are substituted with mCH2N+(CH3)3 groups (confirming elemental analytical data) but rather surprisingly ---53% of aromatic groups are methylene bridged in the anion exchange resin derived from resin 1, and --~63% in the case of resin
CROSSLINKED POLYMERS
559
CH~(CH3}~ Fig. 15.3.50. Structural units most likely present in anion exchange resins.
2. While resin manufacturers had suspected significant levels of methylene bridging, the quantitative data is nevertheless surprising. This means of course that the predominant anion exchange site is structure (2) rather than structure (1) (Fig. 15.3.50).
15.3.3
~3C MAS NMR studies of highly crosslinked poly(divinylbenzene)
resins High surface area polystyrene and polymethacrylate-based resins without additional functionality are useful sorbents [74]. Typically they are prepared by suspension polymerisation using relatively high levels of crosslinker (usually divinylbenzene, >50 vol%) in the presence of a solvating porogen such as toluene [74, 75]. It has been known for many years that polymerisation of all the vinyl groups in these systems is not complete, and indeed residual groups can be used for chemical modification. In addition, achieving adequate detailed molecular structural characterisation of these materials has proved very difficult primarily because of their highly crosslinked nature, but also because of the complex composition of the comonomer mixtures used in polymerisations. Commercially sourced "divinylbenzene" is prepared from diethylbenzene and the grade employed contains both meta- and para-isomers as well as smaller levels of other components. Consequently dehydrogenation to yield "divinylbenzene" yields a mixture of at least four components: metaand para-divinylbenzene and meta- and para-ethylstyrene, the latter arising from incomplete dehydrogenation. Restricting the conversion to divinylbenzene isomers seems deliberate since 100% divinylbenzene is rather unstable and gels readily on storage or in transport. Two grades of commercial divinylbenzene are widely available, one containing ---50-55% divinylbenzene isomers and the other ---80% of these. Widely known resins prepared with high levels of crosslinkers are the XAD series from Rohm and Haas, XAD2 and XAD-4 being particularly much used styrene types. Recently Sherrington and his collaborators [76] have prepared model high surface area resins by polymerisation of both grades of commercially available
560
R.V. LAW AND D.C. SHERRINGTON
0 Q
ClinCH
c
/
H
!
H3
200
150
100 PPM
50
0
Fig. 15.3.51. 13C CP/MAS NMR spectra of poly(divinylbenzene) resins: top, 100% p-DVB" middle,---50% commercial DVB" bottom---80% commercial DVB. (see Ref. 76).
divinylbenzene using toluene as the porogen, and a third model species from ---100% para-divinylbenzene prepared in-house. Fig. 15.3.51 shows the ~3C CP MAS N M R spectra of the three resins obtained at 25.2 MHz. The presence of ethyl groups from ethylstyrene in the commercial divi-
CROSSLINKED POLYMERS
561
nylbenzene is seen clearly, along with signals from unreacted vinyl groups. In the case of the resin from ---100% para-divinylbenzene, ethyl groups are absent and small signals from a carbonyl containing contaminant are also apparent. The corresponding 13C SPE MAS NMR spectra for the three resins are shown in Fig. 15.3.52. Unlike the CP spectra, the latter allow quantification of the level of residual unreacted vinyl groups by integration of appropriate peak areas. For the resin prepared from ---100% para-divinylbenzene ---44% of vinyl groups remain unreacted i.e., the effective crosslink ratio is ---56%. For the resin prepared with the ---80% grade of commercial divinylbenzene --~45% of vinyl groups remain i.e., the effective crosslink ratio is ---45%, while for the resin prepared with the ---50% grade of commercial divinylbenzene ---32% of vinyl groups remain i.e., the resin is ---35% crosslinked. The levels of residual double bonds are therefore remarkably high. 13C CP MAS NMR spectra for XAD-2 and XAD-4 are shown in Fig. 15.3.53, and corresponding SPE spectra in Fig. 15.3.54. Even before attempting any quantitative analysis the clear visual similarity of these spectra with the spectra of the two model resins (from ---50% and ---80% grade divinylbenzenes) is apparent. Since the manufacturers have not disclosed the comonomer composition used to produce XAD-2 and XAD-4 the quantitative analysis of these spectra was more difficult. However, by examining the ratio of aromatic carbons to aliphatic ones it seems that no styrene comonomer is used in the polymerisation feeds (confirmed from FTIR spectra). For XAD-2 the ethylstyrene and divinylbenzene contents seem to be ---49 and ---51% respectively. Likewise the figures for XAD-4 are ---21 and 79% respectively. With this data in hand the SPE spectra were then reanalysed as for the model resins to determine the levels of unreacted double bonds. For XAD-2 this turns out to be ---47% i.e., an effective crosslink ratio of ---27%, and for XAD-4, 49% unreacted vinyl groups i.e., an effective crosslink ratio of ---40%. Thus it seems that XAD-2 and XAD-4 are manufactured directly from the two widely available commercial grades of divinylbenzene, and the high levels of unreacted vinyl groups correspond very closely to the levels found in the two model resins prepared in-house. 15.3.4
13C MAS NMR studies of hypercrosslinked polystyrene resins
Davankov and his co-workers [55, 56] first discovered these remarkable resins. They are prepared either by chemically crosslinking linear polystyrene in a post-polymerisation treatment, or similarly post-treating lightly crosslinked (with ---0.3-2% divinylbenzene) polystyrene resins. Reagent quantities are chosen to allow exhaustive Friedal-Crafts alkylation of aromatic
562
R.V. LAW AND D.C. SHERRINGTON
,
1
200
.
a
.a
.
I
IS0
a
I
9
9
1
100
PPN
~
J
~
~
1
SO
.
~
i
.
1
0
.
Fig. 15.3.52. 13C SPE NMR spectra of poly(divinylbenzene) resins" top, 100% p-DVB; middle, ---50% commercial DVB; bottom, ---80% commercial DVB (see Ref. 76).
563
CROSSLINKED POLYMERS
I
9
Fig. 15.3.53.
I
a
13C
I
I
150
,
9
.
9
|
I00 PPH
I
,a
I
J
!
SO
J
9
,
J
l
0
,
CP/MAS NMR spectra of: top, XAD-4; bottom XAD-2 (see Ref. 76).
groups and the final product resins display some remarkable properties. Firstly, surface areas measured by N2 sorption can be as high as 8001000 mZg -1, some 200-300 mZg -1 higher than conventional high surface area styrene-divinylbenzene resins, and secondly, the resins, though "totally hydrophobic" visibly swell in nonsolvents such as alcohols and even water. Two solid-state 13C NMR spectral studies of these resins have now been published [58, 77] in an attempt to evaluate their molecular structures. The results of the two studies may not be directly comparable since the source of the resins differs in each case, and hence the structures might differ. In addition, one of the studies [58] uses CP spectra and a peak deconvolution procedure and it seems likely that this approach is less quantitative than the other which utilises SPE spectra [77]. Figure 15.3.55 shows the basic chemical steps in
564
R.V. LAW AND D.C. SHERRINGTON
I
'
9
I
9
|
IsO
9
9
a
!
|
"
Ioo PPH
'
'
'
~
50
, a
a
a
'
|
0
J
Fig. 15.3.54. 13C SPE NMR spectra of: top, XAD-4; bottom XAD-2 (see Ref. 76).
synthesising the resins, where the methylene bridging side-reaction described earlier in the context of anion exchange resins is deliberately exploited. The 13C NQS and CP MAS NMR spectra of a hypercrosslinked resin are shown in Fig. 15.3.56 together with the spectral assignments. Residual ~CHzC1, and possibly ~ C H z O H groups are apparent, the former being confirmed by amination with trimethylamine. Figure 15.3.57 shows the corresponding ~3C SPE MAS NMR spectrum. The peak area ratios can be used here to quantify the various types of carbon atom present. The theoretical ratio of total aromatic carbons to total
565
CROSSLINKED POLYMERS CHsOCHtCI SnCI 4
SnCl,/
,/
\
t
CH,CI
,( rl
Fig. 15.3.55. Synthesis of hypercrosslinked resins via deliberate exploitation of methylene bridging reaction.
aliphatic carbons based on the model methylene-bridged structure in Fig. 15.3.55 is 2.40:1.00. The experimental value is 2.05:1.00. This confirms the very high level of methylene bridging, but since the ratio is even less than predicted from the simple structure, it suggests that additional aliphatic carbon functionality arises, yielding trialkyl-substituted aromatic rings. This is only possible if a very high level of double methylene bridging of aromatic groups occurs, and there appears to be a statistical distribution of such bridges between all aromatic groups rather a regular structure. In addition, elemental microanalysis confirms the presence of residual mCHzC1 groups which are located on --~10% of the aromatic groups. These must, of course, be highly hindered species. Two additional hypercrosslinked resins from a commercial source reportedly prepared from conventionally heavily crosslinked divinylbenzene resins [78] appear in one instance to exploit unreacted double bonds (and possibly additional divinylbenzene) to generate the secondary crosslinks, and in another to use SOCl2/Lewis acid to introduce very high levels of sulfoxide secondary crosslinks. Interestingly dynamic NMR experiments suggest that the latter two hypercrosslinked species retain more flexibility and mobility than that prepared via the more conventional methylene bridging technique.
R.V. LAWAND D.C. SHERRINGTON
566
_CH,O(?)
CICH,Ar I
/
HOCHIAr ~ .
.
.
CH,CH,Ar
.
H
ClC.~
~CH,CHAr
~]/~ CHsCH2Ar
l_
Fig. 15.3.56.
Ref. 77). 15.4 15.4.1
13C
~1
9
i
6,
J IS0
100 PPN
SO
0
NQS and CP/MASNMR spectra of hypercrosslinkedpolystyreneresin (see
Use of NMR spectroscopic analysis in solid phase synthesis
Background
Professor Bruce Merrifield first disclosed the use of a crosslinked polystyrene support as a macroscopic protecting group in oligopeptide synthesis in 1963 [79], and heralded the arrival of "solid phase synthesis". Some twenty years later he received the Nobel Prize for Chemistry for developing and exploiting this concept. Since his original work the use of polymer-, and indeed inorganic oxide-based supports, has expanded enormously to include polymeric reagents, polymeric protecting groups and auxiliaries and a wide range of polymer-supported catalysts [80]. The vast majority of applications employ crosslinked insoluble polymers as the support, usually in the form of spherical
567
CROSSLINKED POLYMERS
1
1
ISO
IO0 PPn
St
0
Fig. 15.3.57. 13C SPE NMR spectrum of hypercrosslinked polystyrene resin (see Ref. 77).
particulates or beads, --~20-500 Ixm in diameter, and this format provides the convenience of handling and manipulation which has led to the popularisation of the methodology. Right from the start a major weakness in solid phase synthesis, especially in applications where quite complex molecules were to be synthesised attached to a resin support, was the lack of molecular structural characterisation techniques comparable in scope and sensitivity to the methodologies available for soluble molecules. In particular, the organic synthetic chemists most powerful analytical tool, high resolution 1H and 13C NMR spectroscopy was initially not applicable. Over the last five years or so there has been an explosion in the use of solid phase synthesis when it was realised that the method could be adopted in combinatorial synthesis to allow rapid, even automated, synthesis of large libraries or mixtures of organic molecules [81-86]. This possibility has been seized upon by drug discovery groups in pharmaceutical companies as a rapid method of preparing and screening large numbers of compounds in the search
568
R.V. LAW AND D.C. SHERRINGTON
for new lead compounds. This in turn has led to a stimulation of this work in small entrepreneurial companies and in academic research laboratories worldwide [86]. The involvement of a broad group of synthetic chemists has made the need for better structural analysis more urgent, and this has brought effort and resources to bear on improving the analysis of supported reactions by NMR spectroscopy. In practice the type of support used most widely is a lightly crosslinked poly(styrene-divinylbenzene) system usually in the form of spherical beads ---50-500 l~m in diameter produced by suspension polymerisation [51]. Typically the level of the crosslinking comonomer divinylbenzene used is only ---0.5-2.0 vol%. This is crucial in terms of structural analysis by NMR, since such lightly crosslinked systems can swell considerably in suitable solvents (up to ---15 fold), such that the local environment around a functional group attached to the polymer network can approach closely to that in isotropic solution (see later).
15.4.2
Solution phase NMR studies
Structural analysis of linear polymers molecularly dissolved in a suitable solvent using 1H and 13C solution phase NMR spectroscopy is long established [87-89]. Not surprisingly therefore when a linear soluble polymer is used as a support in solid phase synthesis 1H and X3C solution phase NMR spectroscopy can be a powerful tool in following the chemical synthesis on the support [90]. Figure 15.3.58, for example, shows a series of 1H NMR spectra of dissolved linear polymer samples taken at various stages in the solid phase synthesis of oligoethers on soluble polystyrene [91]. The various chemical steps Fig. 15.3.59 are clearly demonstrated. Even in the case of linear polymers, however, if the solution concentration is increased to allow the local and macroscopic viscosity to rise significantly the mutual dipole-dipole interactions of magnetic 1H nucleii give rise to broadening of their NMR resonances, though the effect with the very much lower natural abundance of 13C nucleii is considerably less. In the case of crosslinked polymers therefore even when the level of crosslinking is very low attempts to record "solution" 1H NMR spectra of solvent swollen samples generally result only in broad signals. However, ~3C NMR spectra from swollen lightly crosslinked polymer gels can show remarkably fine line-width resonances.
"([6 "J~ oos) dols uo!looloJdop puooos 'd '.dols ~u!Idnoo puooos JO uo!legUOlO ' 3 '.dols UOgeA!lOe 'O '.dols uo!loo~mdop 'D '.do~s $u!ldnoo :to luotuqoelle '~l '.ouoJg~Sglod POl~IgqlotuoJolqo olqnlos 'V :laoddns oIqnIos Jeou!I e uo (6~'17"~I "g!d) sJoqloo$!IO jo s!soqluds oseqd p!los jo oSels qoeo le potuaoj slonpoJd jo ealoods ~IIAIN H~ uo!lnIos "8_c'P'~I "8!d
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R.V. LAW A N D D.C. S H E R R I N G T O N
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i
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CROSSLINKED POLYMERS
571
(••=•CH2CI coupling
Nail H(OCH2CH2),OCPh3
(~~CH2(OCH2CH2),OCPh3 deprotection
CF,C 0 H H20
(~~-~CH2(OCH2CH2),OH activation ( ~ - ~
CH3SO2CI DMAP/TEA
C H ~ ( O C H ~ C H ~ ) , O S O~CH3
elongation
NaH H(OCH2CH2),OCPh 3
deprotection
CF3C O2H H,O
@--~CH2(OCH2CH~).OH Fig. 15.4.59. Scheme for solid phase synthesis of oligoethers on a soluble linear polystyrene support.
15.4.3 Swollen gel-phase 13C NMR studies Early work on the application of 13C NMR conventional techniques to the study of solvent swollen crosslinked polymers was reported by Stenlicht and co-workers [92] and Schaefer [93]. The first detailed work on the chloromethylated polystyrene resins used in solid phase synthesis appears to be by Manatt and co-workers [94]. They employed a 0.095% crosslinked species supplied by the Dow Chemical Co. and chloromethylated this to varying d e g r e e s . 1H noise decoupled 13C NMR spectra were recorded in the usual
572
R.V. LAW AND D.C. SHERRINGTON
200
160
120
ppm
80
40
6%DVB
2%
!
o~
Fig. 15.4.60. 13C NMR spectra of chloromethylated polystyrene resins of 0-6% crosslink ratio swollen in CDC13 (see Ref. 97).
way at 15.1, 25.1 and 45.2 MHz using resin slurries in CHC13 or CDC13 in 10 mm o.d. sample tubes. The chemically modified Dow resin yielded spectra at various--CHzC1 loadings essentially as sharp as those of a model soluble linear polymer while the spectrum of a chloromethylated ---1% crosslinked polystyrene already started to show broadening. Interestingly the analysis showed that chloromethylation occurred almost exclusively in the 4-position of styryl residues, and significant levels o f - - C H z O H groups were also detected as a result of hydrolysis of the - - C H z C 1 groups, confirming an earlier report by Merrifield and co-workers [95]. Manatt and co-workers also described the use of a 19F NMR approach for studying peptide synthesis on swollen gels [96]. In many respects, however, Ford and his collaborators brought to a wider audience the potential scope for using conventional X3c NMR techniques for analysing solvent swollen lightly crosslinked functional polymers [70, 71, 97]. Figure 15.4.60 shows the gated decoupled 13C NMR
CROSSLINKED POLYMERS
573
spectra of polystyrenes containing 25 wt% chloromethylstyrene and 0-6% divinylbenzene crosslinker. The spectra were recorded at 25.2 MHz with the samples fully swollen in CDC13 in 12 mm o.d. tubes. Extensive relaxation experiments and attempts to quantify relative peak areas demonstrated a number of artefacts to arise in the generation of these spectra (see later). Nevertheless the work set the scene for the expansion in the use of so-called "gel-phase" 13C NMR spectroscopy applied to solid phase reactions on crosslinked polymer supports. Major contributions to gel phase 13C NMR spectral analysis in the solid phase synthesis of oligopeptides have been made by the groups of Epton [98-101] and Girault [102-104]. Epton and his co-workers [105] pioneered the use of "ultra-high load" methods in peptide synthesis using a phenolic resin, such that in the final support-peptide assemblies the anchored peptide chains comprise the major mass component. In these reactions the group have monitored N-terminal Boc and Fmoc removal by gel phase 13C NMR spectroscopy [99, 100], investigated the lability and stability of some common side-chain protecting groups [101] and characterised a number of polymerpeptide assemblies [99, 100]. Remarkably detailed 13C NMR spectra (e.g., Fig. 15.4.61) are reported in these works. Girault's group have examined the mobility of pendant groups on polymer resins and shown the importance of using the correct solvent, not only for swelling the resin, but also for minimising any intrapolymeric interactions of immobilised groups that can give rise to major line broadening in the 13C NMR spectrum [106]. The association of peptide chains during solid phase synthesis has also been studied by quadruple echo deuterium NMR spectroscopy [107]. As well as utilising lightly crosslinked gel-type resin supports Giraults group has also examined macroporous polystyrene resins, Kel-F-gstyrene, polyacrylamide and controlled pore glass supports, and have applied both 19F and 13C NMR gel-phase spectroscopy to correlate the microenvironmental mobility of protected amino acids with the differences in reactivity observed in peptide synthesis [103]. Other state-of-the-art work from the group includes the stage-by-stage gel-phase 13C NMR spectroscopic characterisation of a growing peptide chain during stepwise synthesis [102]; 31p_ NMR spectroscopic studies of oligonucleotide synthesis on lightly crosslinked polystyrene; and 2D-1H-13C correlated NMR spectra of oligopeptides bound to similar supports. In each case the crosslinked gel was swollen in CDCI3 [104]. During the above development of gel-phase NMR analysis of supported reactions, advances were also in progress with regard to the solid support itself. The most widely used lightly crosslinked polystyrene systems had con-
c.,--- co--..--
o
c.--co--..--c.--co-N.--c.--co-!
I ~,~., ~.?..
"0c", ,~..
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NH I zoco I
..J
/6
/~ C=NHI NH SO,
o I
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;= <
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180
lt0
rs
r~4
148
r~!'
]
1211
104
ppm
l0
tl
Z C~ -] 0 Z
1J 40
20
0
Fig. 15.4.61. Gel phase 13C NMR spectrum of peptide synthesised on a high load resin with the matrix swollen in (CD3)2SO (see Ref. 101).
CROSSLINKED P O L Y M E R S
575
siderable restrictions with regard to the solvents which could be employed in reactions, and this led to a number of strategies to try to overcome the problem. Perhaps the one which has had the greatest impact is the development of lightly crosslinked polystyrene gels with long polyethylene glycol (PEG) derived sidechains, where the solid phase synthesis is carried out on the free terminus of the PEG chains [108, 109]. These materials have been termed "tentacle" polymers [110, 111] and form the basis of the TentaGel | range of supports available from Rapp Polymere, Tfibingen, Germany (also known as the "Rapp resin"). These materials are unusual because of the broad solvent compatibility range they offer, but also because of the flexibility and accessibility of the functional endgroups. This is reflected in the gelphase 13C NMR relaxation times, T1, of peptides bound to the PEG terminus, and to the sharp NMR signals which are recorded [112]. When a poor solvent for the PEG tentacles is employed however, e.g., diethyl ether, broad NMR signals are obtained typical of measurements on solids in suspension. Tentagel | resins have rather low capacities (per gramme) as a result of the long PEG spacers used and this also, no doubt, contributes to the efficiency of solid phase synthesis and to the quality of 13C NMR spectra which can be obtained. However, recently Brown and Ramsden [113] have used gel-phase ~3C NMR spectroscopic analysis to demonstrate high fluidity and homogeneity in the case of 2% crosslinked polystyrene resin functionalised to a relatively high level (--~60% pendent group) with an eleven carbon atom spacer. Again the solvent employed is crucial and for a tethered tetrapeptide [2H6]DMSO proved very effective, presumably as a result of minimising interchain aggregation. Typically spectra were obtained at 126 MHz in --~1 hour at 50~ A further development in gel-phase ~3C NMR spectroscopic analysis has been the use of ~3C enriched building blocks in the synthesis [114]. This allows the use of much smaller quantities of resin (--~20 mg) containing less than 1 mg of the moiety to be analysed. In the case of synthesis on a Tentagel | resin, rapid analysis is possible because of the enhanced mobility of the system and the reduced number of transients that need to be accumulated. 15.4.4
Swollen gel-phase MAS 1H NMR studies
Although analysis of solid phase reactions by ~3C NMR spectroscopy is a very powerful methodology the move towards synthesising a diverse range of organic molecules on polymer supports had led to an increased demand for ~H NMR spectra comparable to those achievable for small molecules dissolved in a solvent. As early as 1974 Doskocilov~ and co-workers [115] reported that high resolution ~H NMR spectra of crosslinked poly(methylme-
576
R.V. LAW AND D.C. SHERRINGTON
thacrylate) gels swollen in chloroform could be achieved by spinning the sample at the "magic angle" relative to the applied stationary field, in a similar way to the acquisition of solid-state spectra. The method was effective for crosslinking levels in the range 0-1%. Despite a further report in 1985 indicating 1H line narrowing in polystyrene-divinylbenzene resins [116] by magic-angle spinning of swollen gels, this development was not picked up by those involved in solid phase synthesis. The effect was confirmed and exploited by Mashelkar and his co-workers studying super-absorbing polymer gels [117], before the much broader relevance was realised by Fr6chet and his group [118, 119]. These researchers showed that in the case of a CDC13 swollen functionalised 1% crosslinked polystyrene support with 12% of the aromatic groups with ephedrine moieties, direct polarisation 13C and 1H NMR spectra with high resolution could be obtained by spinning the sample in a normal MAS probe. Figures 15.4.62 and 15.4.63 show the 13C and 1H spectra respectively obtained using a standard 7 mm MAS probe in a Bruker AF-300 spectrometer operating at 75.47 MHz for ~3C. Spinning rates were 2000 Hz and 2350 Hz respectively for 13C and 1H spectra. In this instance the ephedrine moiety was attached directly to the polymer backbone and in retrospect it is clear that structural units attached by flexible spacers were likely to yield even better spectra. Since this approach did not require the auxiliary power amplifiers typically needed for cross-polarisation (CP) MAS in the solid-state the authors argued [118] that the method could become rather routine from both an experimental and instrumental point of view. More detailed work [119] by the group on 1-2% crosslinked resins highly swollen with solvent, as examples of polymeric systems that undergo rapid but anisotropic reorientation on the molecular scale, confirmed that the spectral line broadening due to the residual dipolar coupling (1H NMR) or chemical shift anisotropy (13C NMR) can be removed by moderate-rate magic-angle spinning, to yield highly resolved ~H and 13C NMR spectra. Furthermore, well-resolved signals attributable to the crosslink junctions themselves (in the case of a 5% dimethylsilane crosslinker) were recorded. A further advance was reported by Keifer and co-workers [120] in terms of reduced sample size. They utilised a Varian Nano-NMR probe with a 500 MHz instrument to record 1H NMR spectra of Tentagel | bound substrates. Typically 10 mg samples swollen in 30 ~L of DMSO-d6 and spun at 2000 Hz at the magic angle yielded better spectra than 100 mg of sample in a more routine gel-phase experiment not involving magic-angle spinning. Shapiros group have used a more conventional 7 mm o.d. probe to obtain 1 3 C ~ 1 H correlated NMR spectra of substrates attached to both Tentagel | [121] and Wang [122] resins. Again spinning at the magic angle yielded much better resolved signals than those from simple gel-phase experiments, and 2D corre-
577
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lations approaching those of solution phase spectra. Reduction in sample size to the ultimate analysis of a single resin bead has been reported by both Keifer and co-workers [123], and Rapp and his collaborators [124]. The former group employed the Varian Nano-NMR probe with a single bead of Wang resin carrying a 3,5-dimethoxy-benzoic residue. The bead was swollen in 30 ~l of CHzClz-d2 and the 1H NMR spectrum recorded at 500 MHz with the sample spun at the magic angle at 2000 Hz. In practice the resulting
578
R.V. LAW AND D.C. SHERRINGTON
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Fig. 15.4.63. Gel phase IH NMR spectra of ephedrine immobilised on a 1% crosslinked polystyrene resin: A, "solution state" spectrum" B, sample spun at the magic angle (see reference 118).
spectrum was complicated by large signals arising from solvent backgrounds, impurities and fingerprints on the outside of the sample cell, as well as smaller peaks due to the polystyrene backbone itself. To achieve usable spectra it was necessary to synthesise the corresponding species 13C enriched on the methyl groups of the methoxy substituents. To some extent therefore this requirement negates the value of the single bead analysis. In contrast the Rapp team [124] took a single macro bead of Tentagel | with a diameter in the range 400-750 ~m swollen in DMSO-d6 or CDC13 fixed into an insert
CROSSLINKED POLYMERS
579
and placed in a standard 4 mm o.d. solid-state NMR rotor. The detection volume was 200-400 I~L. The XH NMR spectra were recorded at 300 MHz with magic-angle spinning at 3000 Hz. Very detailed high resolution spectra (Fig. 15.4.64) of the hydantoin reaction sequence on the resin were obtained in this way. Since the above reports appeared, the use of magic-angle spinning of solvent swollen lightly crosslinked gel-type polymer supports has expanded, particularly for monitoring the progress of solid phase syntheses [125, 126]. ~H NMR spectra at 600 MHz show remarkably detailed features [126] and magic-angle spinning in acquisition of 19F NMR spectra has also proved very valuable [127]. The in-house use of the Varian Nano-NMR probe has been expanded [128] to investigate the influence of the resin structure, tether length and solvent on the quality of the high resolution 1H NMR spectra of bound peptides. Perhaps, not surprisingly, all of these factors have been shown to contribute to spectral quality. Despite the improvement in the narrowing of XH NMR signals by spinning solvent swollen gels at the magic angle, this approach still treats the signals from the polystyrene support in exactly the same way as the signals from any anchored species. Wekler and Westman [126] have used the Varian Nanoprobe with spinning at the magic angle in a 600 MHz instrument to record ~H NMR spectra of a diphenylsulphone attached to CDC13 swollen Tentagel | By combining presaturation of an upfield linker signal at 3.5 ppm, with a 180~ ~ double pulse experiment, the broad signals from the aromatic protons were almost totally suppressed (Fig. 16.4.65). Sarkar and co-workers have argued that the complication of the presence of the large peaks from the matrix could be reduced by judicious choice of the r value in the spin echo pulse sequence (90~176 [129]. This should be possible because the matrix signals are much broader presumably because of a short T2. The use of the spin echo sequence to distinguish between narrow and broad lines has been reported before [130, 131]. Figure 15.4.66 shows the 500 MHz ~H MAS NMR spectra of species (3) bound to a Wang resin swollen in CD2C12. The spectra were recorded from ---6 mg resin in a Nano-NMR probe. It is clear that by appropriate choice of the r value the effect of the polystyrene resonances can indeed be reduced significantly. At the time of writing Shapiro and his group have shown further that the loss of coupling information which accompanies the spin-echo experiment, can be restored by utilising a 2D J-resolved procedure [132]. Similarly S0rensen and his co-workers have reported the results of 2D ~H homonuclear correlations to reduce the inhomogeneous line broadening [133]. Clearly the various specialised instrumental techniques described above are yielding increasingly better quality 1H NMR spectra of resin bound
580
R.V. LAW AND D.C. SHERRINGTON I
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Fig. 15.4.64. Gel phase 1H NMR spectra of products at each stage of a solid phase synthesis of an hydantoin on a single lightly crosslinked Rapp macro-bead swollen in CDC13 and spun at the magic angle (see Ref. 124).
581
CROSSLINKED POLYMERS
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Fig. 15.4.65. Gel phase 1H NMR spectra of aromatic sulfone species attached to Tentagel | swollen in CDC13 and spun at the magic angle, showing manipulation of the pulse sequence to suppress the signals from the support hydrogen atoms (see Ref. 126).
substrates, and it is by no means clear yet how far these developments can go and where the levelling in technology will occur. Nor is it yet clear what can be regarded as routine. Much of the instrumentation and some of the techniques described are costly in terms of both hardware and man-hours, and this will restrict the number of groups capable of applying these methods, and indeed potentially limit the throughput of samples within these groups. There is little doubt however that those wishing to participate at the leading edge of solid phase synthesis methodology will have to assimilate some of these procedures. In all of these developments it is also important to bear in mind two other factors. Firstly, it is not clear yet how quantitative the various specialised methods are likely to be since application of complex chemical-physics methodologies often involve a compromise in terms of the generation of artefacts, especially with regard to the level of quantification that is possible (see Solid State SPE MAS 13C NMR analysis). The second factor to be borne
582
R.V. LAW AND D.C. SHERRINGTON
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CROSSLINKED POLYMERS
583
in mind is the dramatic limitation all these developments involve with regard to the level of crosslinking and local mobility of the support. Essentially all the work reported has involved very lightly crosslinked gel-type supports typically nominally --~1% crosslinked. While this is fine in terms of combinatorial solid phase synthesis no advantage has been reported to date in those areas (e.g., polymer-supported catalysts) using highly crosslinked macroporous resins. No doubt some forward movement will be seen here in due course, but for now it remains an area which offers a considerable challenge to NMR specialists!
15.5
Swollen-state N M R studies of crosslinked elastomers
Elastomeric polymers i.e., polymers whose glass-transition temperatures, Tg, lie well below ambient temperature generally have very low tensile and tear strength. This is simply because the intermolecular forces are very weak, allowing individual polymer chains to be physically pulled apart. For practical application therefore it is necessary to vulcanize or crosslink elastomers to provide toughness and strength. Although in recent years a number of thermoplastic block copolymer elastomers have been developed in which the interbonding of elastomer chains is provided by discrete microphases of glassy polymer, in practice the large majority of rubbers use crosslinking as the method of forming a tough infinite network of elastomeric chains. The level of crosslinking is not surprisingly therefore a key parameter in defining and controlling the bulk properties of an elastomer. Despite this pivotal position, it has proved very difficult to develop routine and simple methods for quantifying the level of crosslinking in a polymer. An important methodology established early on is an equilibrium volume swelling approach and application of the Flory-Rehner equation [134, 135]. While this is satisfactory for simple one component elastomer networks it is not particularly convenient experimentally and is of dubious value for multicomponent elastomers. Paradoxically the "problematical" line broadening, arising in the NMR spectra of polymers which are crosslinked (see Section 4), has been exploited to provide a simple instrumental technique for quantifying crosslinking. Although ~H NMR spectral line broadening on crosslinking had been observed earlier, Marchenkov and Khitrin [136] first published a mathematical explanation of the observation that line widths in the spectra of polymer gels are quantitatively related to the concentration of crosslinks under defined conditions. Loadman and Tinker [137] took vulcanizates of a blend of natural rubber and acrylonitrile-butadiene rubber swollen in CDC13 and recorded
584
R.V. LAW AND D.C. SHERRINGTON
CW ~H NMR spectra at 90 MHz. While the peak broadening of the methyl protons in natural rubber alone could be monitored, this proved impossible in blends because of spectral overlap. As a result the proton signals from the olefinic group were found to be more generally useful. Peak broadening (H%) was assessed as the ratio of the signal strength at a reference point on the high field side of the peak to the signal strength at the peak. Good correlation was found between H% and the level of crosslinking in each component rubber as measured by a separate solvent swelling experiments. Blends of natural rubber and cis-polybutadiene have been similarly investigated [138]. The analytical procedure has also been developed for 1H NMR spectra of swollen elastomers recorded on a 300 MHz FT instrument [139]. In this case a progressive downfield shift of the olefin proton peaks with increasing crosslink ratio is observed and this complication has to be accounted for in the peak broadening analysis. Similar studies have been reported using a 200 MHz instrument [140]. Although most studies have employed ~H NMR spectra, analogous phenomena are seen with ~3C NMR spectra obtained from solvent swollen elastomers, and similar correlations with crosslink ratio are possible [139]. This approach broadens the applicability of the technique to include elastomers whose ~H NMR spectra do not lend themselves to this analysis. Overall the use of advanced NMR spectral techniques is becoming more widespread in the study of intractable elastomers [141, 142].
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588
R.V. LAW AND D.C. SHERRINGTON
113. J.M. Brown and J.A. Ramsden, J. Chem. Soc., Chem. Comm. (1996) 2117. 114. G.C. Look, C.P. Holmes, J.P. Chinn and M.A. Gallop, J. Org. Chem. 59 (1994) 7588. 115. D. Doskocilovfi, B. Schneider and J. Trekoval, Coll. Czech. Chem. Comm. 39 (1974) 2943. 116. D. Bahneider, D. Doskocilov~i and J. Dybal, Polymer 26 (1985) 253. 117. S. Ganapathy, M.V. Badiger, P.R. Rajamohanan and R.A. Mashelkar, Macromolecules 22 (1989) 2025. 118. H.D.H. Stover and J.M.J. Frdchet, Macromolecules 22 (1989) 1574. 119. H.D.H. Stover and J.M.J. Fr6chet, Macromolecules 24 (1991) 883. 120. W.L. Fitch, G. Detre, C.P. Holmes, J.N. Shoolery and P.A. Keifer, J. Org. Chem. 59 (1994) 7955. 121. R.C. Anderson, M.A. Jarema, M.J. Shapiro, J.P. Stokes and M. Ziliox, J. Org. Chem. 60 (1995) 2650. 122. R.C. Anderson, J.P. Stokes and M.J. Shapiro, Tetrahedron Lett. 36 (1995) 5311. 123. S.K. Sarkar, R.S. Garigipati, J.L. Adams and P.a. Keifer, J. Am. Chem. Soc. 118 (1996) 2305. 124. M. Pursch, G. Schlotterbeck, L.H. Tseng, K. Albert and W. Rapp, Angew. Chem. Int. Edn., Engl. 35 (1996) 2867. 125. I.E. Pop, C.F. Dhalluin, B.P. D6prez, P.C. Melnyk, G.M. Lippens and A.L. Tartar, Tetrahedron 52 (1996) 12209. 126. T. Wehler and J. Westman, Tetrahedron Lett. 37 (1996) 4771. 127. M.J. Shapiro, G. Kumaravel, R.C. Petter and R. Beveridge, Tetrahedron Lett. 37 (1996) 4671. 128. P.A. Keifer, J. Org. Chem. 61 (1996) 1558. 129. R.S: Garigipati, B. Adams, J.L. Adams and S.K. Sarkar, J. Am. Chem. Soc. 61 (1996) 2911. 130. D.L. Rabenstein, K.K. Mills and E.J. Strauss, Anal. Chem. 60 (1988) 1380A. 131. P.F. Agris and I.D. Campbell, Science 216 (1982) 1325. 132. M.J. Shapiro, J. Chin, R.E. Marti and M.A. Jarosinski, Tetrahedron Lett. 38 (1997) 1333. 133. A. Meissner, P. Bloch, E. Humpfer, M. Spraul and O.W. SCrensen, J. Am. Chem. Soc. 119 (1997) 1787. 134. P.J. Flory and J. Rehner, J. Chem. Phys. 11 (1943) 521. 135. P.J. Flory, J. Chem. Phys. 18 (1950) 108. 136. V.V. Marchenkov and A.K. Khitrin, J. Chem. Phys. 3 (1986) 2214. 137. M.J.R. Loadman and A.J. Tinker, Rubber Chem. Technol. 62 (1989) 234. 138. P.S. Brown and A.J. Tinker, J. Nat. Rubber Res. 8 (1993) 1. 139. P.S. Brown, M.J.R. Loadman and A.J. Tinker, Rubber Chem. Technol. 65 (1992) 744. 140. V.A. Shershnev, I.K. Shundrina, V.D. Yulovskaya and I.A. Vasilenko, Polymer Science 35 (1993) 1428. 141. C.D. Hull, K.D.O. Jackson and M.J.R. Loadman, J. Nat. Rubber Res. 9 (1994) 23. 142. I. Fontao, L. Gonzalez, M.L. Jimeno and A. Marcos, Kautsch. Gummi Kunstst. 46 (1993) 431.
Chapter 16
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Electrically-Conducting Polymers Hiromichi Kurosu Department of Textile and Apparel Science, Nara Women's University, Kitauoya-Nishimachi, Nara, Japan
16.1
Introduction
High-resolution solid-state NMR spectroscopy provides useful information about the structures and dynamics of electrically-conducting polymers in the solid state, which sometimes cannot be determined by X-ray diffraction. NMR chemical shifts are very sensitive to the structure in the solid state and the separable resonance lines lead to an exact structural analysis.
16.2
Polyacetylene
Polyacetylene (PA), the simplest linear conjugated polymer, has been actively studied for two main reasons. First, the discovery of the direct synthesis method of PA films on the surface of a Ziegler-Natta catalyst solution [1]. Second, the discovery of a large increase in electronic conductivity, due to a synthetic metal by doping with small quantities of electron-attracting species such as iodine, AsFs, etc., or with an electron donor such as sodium. However, because of its high reactivity and poor solubility, it is difficult to obtain the experimental structural data of PA. Typically, PA is synthesized as the cis-isomer and the trans-isomer can be obtained from the cis-isomer by heating. The first NMR measurement on polyacetylene was carried out by Maricq et al. [2] by the 13C CP/MAS technique. The chemical shift positions differ by 10 ppm; the peak position of the trans-isomer is 139 ppm and the cis-isomer is 129 ppm. Gibson et al. studied the isomerization of the cis- to trans-polyacetylene by heating. As shown in Fig. 16.1, the sample without heating showed only one peak (128.5 ppm) which is characteristic of cis-polyacetylene. By isomerization at high temperature, a trans-peak appears at 137.3 ppm. It is weak at first but increases in intensity as the heating continues. Finally, the cis-peak disappears completely. Terao et al. [3] found that at the middle stage of isomerization, the trans-peak position shifts to low frequency with respect to that of a
590
HIROMICHI KUROSU ! t i i
4
c
j
1
i 15o
1
1
i ,..
1.
13o
r 1
PPM FROMM~4SI
1
1~o
Fig. 16.1. 13C CP/MAS NMR spectra of partially isomerized polyacetylenes: (a) pure cispolyacetylene; (b) partially isomerized polyacetylenes of initially high cis content; and (c) pure trans-polyacetylene.
pure trans-polyacetylene and the linewidth is substantially broader. This low frequency shift demonstrates that the structure of the trans part in partially isomerized polyacetylene is different from that of pure trans-polyacetylene. On the other hand, the presence of the cis-peak indicates that there is no disorder in the cis part. This implies the existence of isolated cis regions in partially isomerized polyacetylene, and that the isomerization proceeds inhomogeneously. In order to understand the chemical shift behavior, Yamanobe et al. carried out quantum-chemical consideration [4, 5]. In these papers, it is shown that the calculated results of the single chain model for undoped
E L E C T R I C A L L Y - C O N D U C T I N G POLYMERS
591
polyacetylene agree favorably with the experimental data. The electronic structures of polymers with a periodic structure have been studied by the tight-binding (TB) MO theory which is well known in solid-state physics. The chemical shift tensor component, which predominantly contributes to the difference in 6iso between the cis- and trans-isomers, is 633 of the principal values (the most shielded), and the direction of 633 is perpendicular to the chain plane. From this, it is assumed that there is a large difference in the 7r -electron interchain interactions between cis- and trans-polyacetylenes and, hence, these affect the chemical shift [6]. In order to investigate the effect of interchain interactions on the 13C NMR chemical shifts and the electronic structure of undoped polyacetylene chains in the solid state, Kurosu and coworkers carried out shielding (chemical shift) calculations by using a multichain model [7]. The geometrical parameters used for the calculations are given in Fig. 16.2. The arrangement of the interchains is based on the lattice parameters (the a and b axes and "setting angle" 4~ which indicates the angle between the plane of the chain and the (010) plane of the crystal) determined by the X-ray diffraction data of Perego et al. [8]. A small decrease from the original crystal-lattice values, determined by the X-ray method, leads to energetically stable results in the calculation for both of the polyacetylene isomers (Fig. 16.3). With respect to the bandgap, the smaller the b values are, the smaller the bandgap values become continuously. The decrease of the lattice parameters in the calculation corresponds to the PA system under high pressure where the shrinkage of the interchain distance occurs. Therefore, it is suggested that the electronic conductivity of PA samples will increase with an increase in pressure. Comparing both isomers, the bandgap value of the trans-polyacetylene is smaller than that of the cis-polyacetylene. This result explains the experimental fact that transpolyacetylene has a higher conductivity than cis-polyacetylene. The calculated values are very large for the arrangement with the original lattice parameters. The arrangement with the small value of b is near to that of the real crystal, as mentioned above. It was shown experimentally that the lattice parameters a and b for cis-polyacetylene are larger than those for trans-polyacetylene, i.e., the interchain distance of cis-polyacetylene is longer than that of transpolyacetylene. This may be due to the different shapes of the two polyacetylene chains. In the comb-like cis-polyacetylene chains, some of the hydrogen and carbon atoms tend to be very close in interatomic distance and so they become repulsive (in fact, the nearest-neighbor C ~ H distance between the interchains in cis-polyacetylene is shorter than that in trans-polyacetylene in the case using the original geometries). Therefore, the cis-polyacetylene crystal becomes more stable for the arrangement with the larger lattice para-
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a) ~
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,
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H
e
r"
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o
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.
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TRANS
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y
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Fig. 16.2. Geometrical parameters of cis- and trans-polyacetylenes: (a) valence geometries and (b) lattice parameters.
meter than for the trans-polyacetylene crystal, and these long interchain distances lead to the large bandgap and, hence, to the small conductivity of cis-polyacetylene. In the arrangement with the original lattice parameters, the calculated values of Oiso do not reproduce the observed large difference between both the isomers. However, in the arrangement with a short interchain distance, the calculations agree well with the experiments. As to the shielding tensors, not only 0"33 but also the other components O"11 and O'22 move to high frequency as the interchain distance is decreased (Fig. 16.4). The most changeable component for a change of the interchain distance
ELECTRICALLY-CONDUCTING POLYMERS
593
A) -198.5 5 d ~" -198.7 t.9 n... w z w -198.9 J
o -199.1 -1.5
-1
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0
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Fig. 16.:3. The dependence of the calculated (A) total energy; (B) bandgap; and (C) isotropic 13C N M R chemical shift of P A chains in the seven-chains model on b at A ~ - 0 ~ ( 9 cis; @: trans).
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022 033
{~II ~ b = - 1.5-
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9
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I -260
9I
-z6o
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1
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-,60 pp~
-,60
-s0
I
~p~
Fig. 16.4. The dependence of the calculated chemical shift tensors of the central PA chain in the seven-chains model on b at A4~ = 0 ~ The observed values are indicated at the bottom, and the arrows lefthand side indicate the directions of the calculated principal axes. (a) cis and (b) trans.
is 0"22 , the direction of which is along the molecular chain. The effect of the interchain interactions might appear strongly in this direction. At Ab = - 1 A, the difference in 0"22 between both isomers quantitatively agrees with the experimental data. From these results, it can be said that interchain interac-
ELECTRICALLY-CONDUCTING POLYMERS
595
tions are important in discussions of the 13C NMR shielding and electric structure of conducting polymers, such as polyacetylene in the solid state.
16.3
Polypyrrole
Polypyrrole is one of a series of heterocyclic polymers which has attracted much attention due to its characteristic electric and electronic properties. However, there are some problems relating to the physical and material properties associated with its structure. The fundamental structural formulae shown in Fig. 16.5 have been generally proposed for the structures of dedoped and doped polypyrroles, where the aromatic form corresponds to the dedoped state and the quinoid form corresponds to the doped state [9-11]. However, the actual structure appears to be more complicated. At present the exact structure is not known because the polymer is amorphous and insoluble. Consequently, various structures have been proposed for polypyrrole [10]. The structure of polypyrrole in the solid state has been studied by means of high resolution solid-state ~3C NMR spectroscopy [12]. However, the structure is insufficiently analyzed because of the complexity of the unresolvable broad aromatic 13C signal. This is because there are several magnetically non-equivalent aromatic carbons as shown in Fig. 16.5.
N\
(a)
IN (b) Fig. 16.5. (a) Aromatic and (b) quinoid structures for polypyrrole.
596
HIROMICHI KUROSU
The structure of polypyrrole, prepared electrochemically, has been analyzed by using high resolution solid-state 15N NMR spectroscopy. The sample used is 15N-labeled in order to obtain 15N spectra with a high signalto-noise ratio, as attempted by Wehrle et al. [13]. However, they could not carry out a successful analysis of the 15N CP/MAS spectra of the polymers in the solid state because of insufficient resolution. As expected from Fig. 16.5, 15N NMR spectroscopy will provide a simpler spectral pattern, when compared with I3C NMR spectroscopy, because a given ~SN resonance line may correspond to a given structure. Therefore, the structure of doped and dedoped 15N-labeled polypyrrole films can be successfully studied by high resolution solid-state NMR [14, 15]. Doped and dedoped samples were prepared by electrochemical polymerization [16] using 20-30% 15N-labeled pyrrole. To obtain a dedoped sample, the electrodes were inverted after the doping experiment and the same voltage applied. The observed 15N CP/MAS NMR spectrum of polypyrrole (sample c; electrical conductivity = 4 • 10 -6 s/cm) is shown in Fig. 16.6(a). It is shown that the 15N signals of the polypyrroles consist of two major and two minor peaks, which are designated a, /3, 3/ and 6 with decreasing shielding. The 15N chemical shift values of the four peaks are approximately 90, 113, 129 and 145 ppm. This chemical shift range is very large and the results indicate that the polymer has at least four types of structure corresponding to the four peaks. The determined 15N chemical shifts, halfwidths and relative peak intensities of samples (a) (electrical conductivity- 7 • 10 -4 s/cm), (b) (electrical conductivity = 2 • 10 -3 s/cm) and (c) are summarized in Table 16.1. As shown in this table, the relative intensities of peaks a and/3 increase from 5.5 to 8.0% and 28.7 to 30.2%, respectively, on going from sample (b) doped to sample (c) dedoped. However, the relative intensity of peak ~, decreases from 53.7 to 49.4% by dedoping. Hence, the relative intensities of peaks a and/3 increase with a reduction in conductivity, but peak 3, decreases. In addition, the relative intensity of peak 6 does not change with the increase in conductivity. When the 15N CP/MAS experiment is performed using a contact time of 100 ~s, the intensities of the peaks a and /3 are relatively enhanced as shown in Fig. 16.6(b), and the chemical shifts and halfwidths of the observed shoulder peaks are determined accurately. Furthermore, the difference of the intensity enhancement between peaks a,/3 and peaks y, shows the difference of the magnetic environments, i.e, a difference in TNH (contact relaxation time between ~SN and 1H) values between 15N and 1H and in Tip, between peaks a , / 3 and 7, 3. To get more detailed information about the 15N NMR chemical shift behavior and physical properties of polypyrrole in the solid state, quantum
597
ELECTRICALLY-CONDUCTING POLYMERS
-
(a)
OBSERVED SPECTRUM
---- THEORETICAL SPECTRUM -- DECOMPOSED SPECTRUM
IbO
(b)
-
is0
StO
$0.
. bp.1
OBSERVED SPECTRUM
Fig. 16.6. A 50.55 MHz "N CP/MAS NMR and simulated spectra of polypyrrole (sample c) in the solid state. The four peaks were decomposed by computer fitting: (a) contact time (CT) = 800 ps and (b) CT = 100 ps.
598
HIROMICHI KUROSU
Table 16.1. Observed 15N chemical shifts, halfwidths and relative peak intensities of doped and dedoped polypyrrole sample a
Sample
Peak
13C chemical shift (ppm)
Halfwidth (ppm)
Relative peak intensity (%)
(a)
a /3 7 6
91.0 113.6 129.1 145.5
30.0 23.0 20.6 19.0
5.1 28.8 53.9 12.2
(b)
a /3 7 6
91.0 113.6 129.1 145.5
30.0 23.0 20.6 19.0
5.5 28.7 53.7 12.2
(c)
a /3 7 6
87.5 112.8 128.2 142.5
30.0 23.0 18.6 19.0
8.0 30.2 49.4 12.4
a Determined by computer fitting.
chemical calculations have been performed. As suggested above, it is thought that polypyrrole predominantly takes the aromatic and quinoid forms (Fig. 16.7). The isotropic 15N NMR chemical shifts calculated by the finite perturbation theory (FPT)-INDO method for the aromatic forms are listed in Table 16.2, and those for the quinoid forms are listed in Table 16.3. The calculated values are shielding constants, and so the negative sign indicates deshielding. Since the observed values are the relative chemical shifts and the positive sign corresponds to deshielding, only the relative difference in the calculated 15N NMR shielding constants (o-) should be compared with the observed data (6). As seen from Tables 16.2 and 16.3, the calculated 15N NMR chemical shift for the quinoid form appears towards a high frequency with respect to that for the aromatic one. Polypyrrole in the solid state is in an amorphous state [10]. Therefore, some local structures are assumed. The calculations for these structures show that the 15N NMR chemical shift moves considerably to high frequency when a hydrogen atom bonded to a nitrogen atom (N2 in Fig. 16.7(b) and N4 in Fig. 16.7(d)) is very close to a hydrogen atom bonded to a carbon atom of the other ring. From these calculated results, and the experimental findings that the observed 15N NMR chemical shift for peak y appears towards a high frequency with respect to that for peak/3, and the fact that the intensity of peak y for doped polypyrrole is larger than that for dedoped polypyrrole, it can be concluded that the major peak y, at ---129 ppm, is assigned to the nitrogen
599
ELECTRICALLY-CONDUCTING POLYMERS
(b)
~4 ! c..
.C;.Ha
(Cl)
~Ns
3
Fig. 16. 7. The structure of models used in the FPT-INDO calculation.
atoms in the quinoid form. The other major peak/3, at --~113 ppm, is assigned to the aromatic form, and the minor peak 6 at ---145 ppm probably comes from the nitrogen atoms which are bonded to hydrogen atoms approaching other hydrogen atoms bonded to different atoms. From the calculations performed, the other minor peak a, at ---90 ppm, cannot be assigned.
600
HIROMICHI KUROSU
Table 16.2. Calculated
15N shielding constants for the aromatic models by the FPT-INDO
method Model
Nitrogen species
Calculated 15N shielding constant Criso(ppm)
(a)
N1 N2 N3 N4 N5
-318.81 -317.33 -317.34 -317.23 -319.49
(b)
N1 N2 N3 N4 N5
-319.18 -379.48 -319.54 -312.89 -320.56
Table 16.3. Calculated 15N shielding constants for the quinoid models by the FPT-INDO
method Model
Nitrogen species
Calculated 15N shielding constant O'iso (ppm)
(c)
N1 N2 N3 N4 N5
-322.67 -325.73 -325.51 -325.72 -322.67
(d)
N1 N2 N3 N4 N5
-324.41 -329.29 -315.82 -367.05 -324.00
Table 16.4. Calculated 15N shielding constants and bandgaps for the aromatic and quinoid polypyrrole methods by the tight-binding INDO/S-SOS method
Structure
Calculated 15N shielding constant Oiso (ppm
Bandgap (eV)
Aromatic form Quinoid form
-223.50 - 232.51
5.12 2.86
In o r d e r to o b t a i n i n f o r m a t i o n a b o u t the 15N c h e m i c a l shifts a n d e l e c t r o n i c e n e r g y b a n d s t r u c t u r e s of an infinite p o l y p y r r o l e chain with a r o m a t i c or q u i n o i d f o r m s , calculations w e r e c a r r i e d o u t by the tight b i n d i n g ( T B ) I N D O / S m e t h o d . A s listed in T a b l e 16.4, the c a l c u l a t e d 15N N M R c h e m i c a l
(a)
Electronic Energy Band Stnucture Oensity of States
EnergyleVI
(b)
Electronic Energy Band Structure
Density of States
Energy[eVi t0
t"
0
~q m
f
-10 L- .,
>
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.628
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Fig. 16.8. Electronic energy band structure and density of states of an infinite polypyrrole chain calculated by the TB-INDO/S methods: (a) aromatic form and (b) quinoid form.
602
HIROMICHI KUROSU
shift for the quinoid form appears at high frequency compared with that for the aromatic form. This agrees with the results calculated by the FPT-INDO method. The calculated band structures for both the aromatic and quinoid forms are shown in Fig. 16.8. The bandgap is an important factor in determining electrical properties such as electric conductivity, where the bandgap is the energy difference between the highest occupied band and the lowest unoccupied band. Therefore, if this value becomes smaller, the electric conductivity increases. The bandgaps for the aromatic and quinoid forms are 5.1 and 2.9 eV, respectively. This result shows that the electric conductivity for the quinoid form is larger than that for the aromatic form. Therefore, it can be expected that polypyrrole with a high electric conductivity can be obtained if the amount of the quinoid form is increased.
16.4 Polyacenic polymers The structure of polyacenic polymers, one of the typical conducting polymers, has been studied by infrared (IR), X-ray diffraction and ESR [17, 18]. From these studies, it was found that polyacenic polymers obtained by heating resins are amorphous and their structures are very complicated. Therefore, their exact structures have not been clarified because o f their amorphous nature and insolubility. Kurosu et al. [19] studied the relationship between the structure and electrical conductivity of polyacenic polymer samples by high resolution solid-state NMR spectroscopy. The polyacenic polymer films are obtained by heating phenol-formaldehyde resin (PFR) at 590, 640 and 670~ abbreviated as PAc590, PAc640 and PAc670, respectively. Table 16.5 shows the results of the elemental analysis. The hydrogen/carbon atomic ratio varied from 0.26 to 0.42. Table 16.5. Electrical conductivity and elemental analysis data of polyacenic polymers and phenol-formaldehyde resin Sample
PFR a PAc590 PAc640 PAc670 I-PAc590 a
Heat-treating temperature/~
Electrical conductivity/S-cm- 1
590 640 670 590
1.8 X 10 - 1 ~ 1.4 x 10 -7 6.6 x 10 -6 3.2 X 10 - 4
Phenol-formaldehyde resin.
C
H
O
I
1.00 1.00 1.00 1.00 1.00
0.857 0.418 0.334 0.263 0.418
0.143 0.044 0.035 0.027 0.044
0.029
ELECTRICALLY-CONDUCTING POLYMERS
r
A) .
o. .
603
, !.
1
CH
B)
11
,r--~ b f
(1)
Y YhYY f
b e g
e
(2)
a
r
b
/I"
c
(3)
Fig. 16.9. Structures of (A) phenol-formaldehyde resin and (B) polyacenic polymers.
The structures of PFR and polyacenic polymers are shown in Fig. 16.9. Figure 16.10 shows the relationship between electrical conductivity and heattreatment temperature. This shows that the electrical conductivity of polyacenic polymer is increased as the heat-treatment temperature is increased and there is a linear relationship between electrical conductivity and heattreatment temperature. Therefore, an increase in electrical conductivity is caused by the growth of the polyacenic structure. Furthermore, the electrical
604
HIROMICHI KUROSU
0.5
9
I
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9
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9
9
9
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,
,
9
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9
9
9
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9
600 620 640 660 Hear-Treatment Temperature PC
680
Fig. 16.10. Plots of electrical conductivity against heat-treatment temperature.
conductivity of the polyacenic polymer (PAc590) is increased by iodinedoping (I-PAc590) from 1.8 x 10 - 1 ~ (s.cm -1) to 3.2 x 10 - 4 ( s . c m - 1 ) . Figure 16.11 shows 13C CP/MAS spectra of PFR and PAc samples. The peak assignment of PFR was performed straightforwardly from reference data by Yamabe et al. [20] as shown in Fig. 16.11, where the numbers on the peaks correspond to the carbons of PFR as shown in Fig. 16.9. As seen from these spectra, the main peak (at 127-130 ppm) moves to low frequency and the peaks are broadened as the heat-treatment temperature is increased. Therefore, it can be considered that this spectral change originates from the structural change in going from PFR to polyacenic polymers by heat-treatment, as shown in Fig. 16.9(B). The intensities of the peaks at about 150 ppm, which are assigned to PFR carbons bonded to the hydroxyl group (carbon 1), and those at ---40ppm which are assigned to methylene carbon of PFR (carbon 7), decrease as the heat-treatment temperature is increased. This shows that PFR is converted to polyacenic polymer by heat treatment. However, these peaks still remain even in the spectrum of PAc670. This shows that a small amount of PFR exists in the PAc670. The 13C signals of the methine and methylene carbons, obtained by the dipolar-dephasing (DDph) method, are very broad and their peak intensities
ELECTRICALLY-CONDUCTING POLYMERS
#
i
,
I
200
i
I^
i
,
'
!
150
i~
'
'
4
'
605
7
'
i
~100
~'
'
c
'"'1
50
I
l
!
PPH I
Fig. 16.11. Observed 13C CP/MAS NMR spectra of: (a) phenol-formaldehyde resin (PFR);
(b) polyacenic polymer heat-treated at 590~ (PAc590); (c) polyacenic polymer heat-treated at 640~ (PAc640); and (d) polyacenic polymer heat-treated at 670~ (PAc670). Asterisks indicate spinning sidebands.
606
HIROMICHI K U R O S U Ca) OBSERVED
SPECTRUM
....
THEORETICAL
-- --
DECOHPOS
SPECTRUM
150
/ / ~
140
130
120
ppm
120
ppm
(b)
8
1/11~\ ~-;--~-.-G . . . .
150
/I / 111"\~:~\
I - - "~ ::~','r
140
I
---O "~
130
(c)
150
140
i 30
120
ppm
Fig. 16.12. Observed and simulated 13C NMR spectra of polyacenic polymer heat-treated at 590~ (PAc590) by: (a) CP/MAS; (b) DDph/MAS methods; and (c) iodine-doped polyacenic polymer heat-treated at 590~ (I-PAc590).
ELECTRICALLY-CONDUCTING POLYMERS
607
Table 16.6. Observed 13C chemical shifts, halfwidths and relative intensities of polyacenic polymer samples
Sample
Peak
13C chemical shift (ppm)
Halfwidth (ppm)
Relative peak intensity (%)
PAc590
a /3 y 6
124.8 129.5 138.0 150.9
10.0 7.8 9.6 8.5
46.1 23.7 23.0 7.2
PAc640
c~ /3 y 6
125.2 129.5 138.0 151.1
11.2 7.4 10.8 7.8
49.4 17.0 28.1 5.5
PAc670
c~ /3 3/ 6
125.0 129.5 138.0 151.4
12.7 7.5 11.5 8.0
57.1 10.8 28.5 3.6
decrease because of proton-carbon dipolar interactions. Figure 16.12(a, b) shows 13C CP/MAS and D D p h / M A S spectra and computer-fitted spectra of PAc590. These figures show that the main peak at 127 ppm in the 13C CP/MAS spectrum is shifted to high frequency to 131 ppm, compared with that in the 13C D D p h / M A S spectrum. On the basis of these experiments, the decomposed four peaks are designated c~, /3, y and 6 with decreasing shielding by computer fitting as shown in Fig. 16.12. The 13C chemical shift, halfwidth and relative intensities for the individual peaks obtained by computer fitting are listed in Table 16.6. Peak/3 can be assigned to PFR carbons 2, 3, 5 and 6 and peak 6 to PFR carbon 1 from the assignment of PFR (Fig. 16.9). In the 13C D D p h / M A S spectrum, the intensities of peaks y and increase and those of c~ and /3 decrease compared with the 13C CP/MAS spectrum. From this result, it can be assigned that the major component of peak c~ originates from the methine carbon of PAc and that of peak y originates from the quarternary carbon of PAc. The peaks of the polyacenic polymer can be assigned with the aid of reference data as shown in Fig. 16.13. From these data, peak c~ is assigned to carbons a, b, d and f and peak 3, is assigned to carbons c, e, g and h. Figure 16.14 shows the relationship between electrical conductivity and relative intensity ratios I~/I~ and I~/I~. This figure shows that I~/I~ and I~/I~ are increased as the electrical conductivity is increased. Hence, the peak intensity ratios, I~/I~ and I~/I~, are a
608
HIROMICHI KUROSU
b)
a) 5
6
/5/ppm 1 135 2 132 3 128 4 127 5 126 6 120
3
4C
30
3
4
4
1
!
~i/ppm 1 131 2 127 3 125 4 125 5 125
5/ppm i 132 2 128 3 126 4 125
8/ppm 1 137 2 127 3 126
Fig. 16.13. Structures and 13C chemical shifts of some aromatic compounds: (a) perylene" (b) pyrene; (c) anthracene" and (d) 9,10-dihydroanthracene.
measure of the change of electrical conductivity and, therefore, it can be concluded that the conductivity is due to the structures of peaks cr and 3'. PAc590 was doped by iodine vapor and its conductivity became 3.2 x 10 - 4 (s/cm). Figure 16.12(c) shows 13C CP/MAS NMR spectrum and computerfitted spectrum of iodine doped polyacenic polymer (I-PAc590). The chemical shifts, halfwidths and relative peak intensities are shown in Table 16.7. All of the 13C chemical shifts of four peaks, and the halfwidths of peaks/3 and 8 which are assigned to the carbons of PFR, are unchanged by doping. However, the halfwidths of peaks a and 3' which are assigned to the carbons of the polyacenic part are increased by doping. This shows that the doped iodine interacts significantly with the polyacenic part but not with the PFR part. It seems that the distribution of conformations caused by heat-treatment leads to a chemical shift distribution.
609
ELECTRICALLY-CONDUCTING POLYMERS
18.0
- ~ 9, - 4
16.0 9"
peak -- peak ~'
14.0
o,,,~
e~
,~
12.0
=
10.0
8.o
f
6.0
>
e~
4.0 .
2.0 -10
.
.
.
|
.
.
.
.
-9
|
-8
.
.
.
.
.
.
.
.
.
-7
1
~
9
,
-6
9
-5
Electrical Conductivity lc)g ~ / S cm 1 Fig. 16.14. Plots of relative peak intensities against electrical conductivity of polyacenic polymers. Table 16.7. Observed 13C chemical shifts, halfwidths and relative intensities of polyacenic polymers and iodine doped polyacenic polymer samples
Sample
Peak
13C chemical shift (ppm)
Halfwidth (ppm)
Relative peak intensity (%)
PAc590
a /3 3' 6
124.8 129.5 138.0 150.9
10.0 7.8 9.6 8.5
46.1 23.7 23.0 7.2
I-PAc590
a /3 3' 6
125.2 129.5 138.0 151.1
12.2 8.2 11.3 8.3
44.9 19.0 30.3 5.8
From the above results, it can be concluded that peak a is assigned to the carbons a, b, d and f of the polyacenic structure and y is assigned to the carbons c, e, g and h. P e a k / 3 is assigned to the carbons 2, 3 and 5 of the PFR and 6 is assigned to the PFR carbon 1. The halfwidths of peaks a and 3' depend on electrical conductivity and, therefore, are a measure of electrical conductivity of polyacenic polymer.
610 16.5
HIROMICHI KUROSU
Other conducting polymers
The structural analysis using high resolution solid-state NMR has been successfully performed on other conducting polymers such as polyphenylene vinylene [21], polyaniline [22-24], polyphenylene sulphide [25, 26] and polyp-phenylene [27-29].
References
.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci. Poly. Chem. Ed. 12 (1974) 11. M.M. Maricq, J.S. Waugh, A.G. MacDiarmid, H. Shirakawa and A.J. Heeher, J. Am. Chem. Soc.100 (1978) 7729. T. Terao, S. Maeda, T. Yamabe, K. Akagi and H. Shirakawa, Chem. Phys. Lett. 103 (1984) 347. T. Yamanobe, R. Chujo and I. Ando, Mol. Phys. 50 (1983) 1231. T. Yamanobe and I. Ando, J. Chem. Phys. 83 (1985) 3154. T. Yamanobe, I. Ando and G.A. Webb, J. Mol. Struct. 151 (1987) 191. T. Ishii, H. Kurosu, T. Yamanobe and I. Ando, J. Chem. Phys. 89 (1988) 7315. G. Perego, G. Lugli, U. Oedretti and E. Cernia, J. Phys. Colloq. C3 (1983) 93. J.L. Bredas and G.B. Street, Acc. Chem. Res. 18 (1985) 309. M. G. Kanatzldis, Chem. Eng. News 3 (1990) 36. J.L. Bredas, B. Themans, J.G. Fripiant and J.M. Andre, Phys. Rev. B 29 (1984) 6761. F. Devreux, G. Bidan, A.A. Syed and C. Tsintavis, J. Phys. 46 (1985) 1595. B. Wehrle, H.H. Limbach, J. Mortensen and J. Heinze, Synth. Meth. 38 (1990) 293. M. Kikuchi, H. Kurosu and I. Ando, J. Mol. Struct. 269 (1992) 183. H. Kurosu, M. Kikuchi and I. Ando, J. Polym. Sci.:Part B: Polym Phys. 33 (1995) 769. N. Ogata, Doudenseikoubunshi (Conducting Polymers). Koudansha, Tokyo, 1990. K. Tanaka, K. Ohzeki, T. Yamabe and S. Yata, Synth. Met. 9 (1984) 41. K. Tanaka, T. Koike, T. Yamabe, J. Yamaguchi, Y. Deguchi and S. Yata, Phys. Rev. B 35 (1987) 8368. H. Kurosu, Y. Nakajima and I. Ando, J. Mol. Struct. 355 (1995) 27. T. Yamabe, S. Yamanaka, K. Tanaka, T. Terao, S. Maeda and S. Yata, Phys. Rev. B 37 (1988) 5808. J. Nouwen, P. Adriaensens, D. Franco, D. Vanderzande, H. Martens, J. Gelan, Z. Yang and H. J. Geise, Synthetic Metals, 48 (1992) 143. S. Kaplan, E.M. Conwell, A.F. Richter and C. Tsintanis, Synthetic Metals 29 (1989) E235. C. Mnardo, M. Nochtschein, A. Roussean, J.P. Travers and P. Hany, Synthetic Metals 25 (1988) 311. P. C. Stein, C. J. Hartzell, B. S. Torgensen and W. L. Earl, Synthetic Metals 29 (1989) E297. J. Tsukamoto and K. Matsumura, Jpn. J. Appl. Phys. 24 (1985) 974. P. Hrusyka, J. Jurga, B. Brycki and N. Pislewski, Polymer 32 (1991) 2921. C.E. Brown, M.B. Jones and P. Kovacic, J. Polymer Sci. Polym. Lett. 18 (1980) 659.
ELECTRICALLY-CONDUCTING POLYMERS
611
28. C.E. Brown, I. Koury, M.D. Bezoari and P. Kovacic, J. Polymer Sci. Chem. Ed. 20 (1982) 1697. 29. K. Kume, K. Mizuno, M. Mizoguchi, K. Nomura, Y. Maniwa, J. Tanaka, M. Tanaka and A. Watanabe, Mol. Crys. Liq. Crys. 83 (1982) 285.
This Page Intentionally Left Blank
Chapter 17
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
Inorganic Polymers Toshio Takayama Department of Applied Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama, Japan
17.1
Introduction
In recent years, the literature has furnished a wealth of information concerning chemical structures from multinuclear (e.g., ~3C, 29Si, 31p...) solidstate NMR studies of crystalline and noncrystalline polysilanes, polysiloxanes, polyhosphazenes, composite polymers and other inorganic polymers. It has been demonstrated that cross-polarization/magic-angle spinning (CP/MAS) NMR spectroscopy is the most powerful means available to characterize the structure and dynamics of inorganic polymers in the solid state [1]. The most recent development of a variable-temperature (VT) spinner provided an indispensable means to study slow chemical exchange, molecular motion and phase transitions of inorganic polymers. This chapter will review the applications of the solid-state multinuclear NMR methods to several inorganic polymers.
17.2
17.2.1
Polysilanes
Poly(methylphenylsilane)
Polysilanes are a unique class of polymers in which the o--electrons are delocalized entirely along the sp3-bonded silicon backbone, causing their electronic absorption properties to be strongly dependent on the conformation of silicon backbones [2]. This property has created much interest in the structure of these polymers in the solid state [3]. In spite of the usefulness of solid-state NMR, there are few systematic studies on the 29Si CP/MAS NMR of polysilanes [4-6]. Most recently, it has been demonstrated that the VT 29Si CP/MAS NMR experiment is very useful to study the conformational features of polysilanes in the solid state [7]. Measurements of the 29Si CP/MAS NMR spectra of poly (methylphenylsilane) (PMPS), in the solid state over a wide range of temperatures, are performed and the conformation
614
TOSHIO TAKAYAMA
and dynamic behavior through the temperature change of the main chain are shown. To discuss the above experimental results in detail, the 298i chemical shifts of the main chain Si atoms are calculated by means of the finite perturbation theory (FPT) within the CNDO/2 MO framework. 17.2.1.1 Experimental Material: PMPS was prepared by a method similar to the Trujillo method [8]. The polymer is a clear, colorless and brittle solid with an elemental analysis for C y H s S i of C = 65.8% (69.9%), H = 6.73% (6.71%) (figures in parentheses are calculated values). The X-ray diffraction powder patterns show that this polymer is amorphous. The IR spectrum shows vibrational peaks for Si-phenyl at 1433 and 1100cm -1 (s), Si-methyl at 1250cm -1 (vs) and Si~Si at 462 cm -1 (s). The IR spectrum is similar to that reported by Trujillo [8]. The polymer has a monomodai molecular weight, Mw = 7.2 x 103, as determined from the gel permeation chromatography (GPC) elution profile (GPC was carried out using a Waters GPC150-C Module using spectrograde THF as the eluent and is relative to polystyrene standards). Differential thermal gravimetric analysis shows that the polysilane melted at --~250~ and decomposed at --~360~ under a nitrogen atmosphere.
NMR
m e a s u r e m e n t : g T - 2 9 8 i CP/MAS NMR spectra were obtained at temperatures from -120~ (153 K) to 120~ (393 K) using a JEOL GX270 spectrometer equipped with a variable-temperature CP/MAS accessory operating at 53.54 MHz.The sample was contained in a cylindrical-type rotor and spun at 4-5 kHz. Contact time was 5 ms and repetition time 15 s. Spectral width and data points were 10kHz and 8k, respectively. Spectra were usually accumulated ca. 200-300 times to achieve a reasonable signal-to-noise ratio. 298i chemical shifts were calibrated indirectly through the 298i peak of polydimethylsilane and were converted to the value from tetramethylsilane (TMS). The variable temperature controller was used for all of the probe temperatures at which measurements were taken.
Calculation of 29Si NMR chemical shift: The orbital energies of oligosilanes and polysilanes are very sensitive to molecular conformation. Empirical force field, semiempirical CNDO, MINDO/3 and MNDO and ab initio singledeterminant molecular orbital geometry optimizations have been performed for several parent oligosilanes. Among the semiempirical methods, MINDO/3 appears to be superior in that it produces nearly correct Si~Si bond lengths, while those predicted by MNDO and CNDO are too short and too long, respectively. However, these empirical calculations on chains up to 30 Si atoms in length led to the conclusion that the interactions are additive
615
INORGANIC POLYMERS
(a) H
H +
Ii
II II
Si-----Si
H li
II
Si
It
It
II
Si ~
II
II
II
(b) v lld"
q---si'." %H
2~Si.,
3
/ \
v . V = - C H = C I I 2, M=-CH 3
~
"
i
5
vt
v
M
\ ,,,,.+ .
s i - - - - Si % M
2
si - - - s i
v/".... "M 8 ~II
Fig. 17.1. (a) A schematic drawing of the n-SivHx6 chain and (b) a schematic illustration of a pentad sequence in n-2,3,4,5,6,7-hexavinyloctasilane.
in this approximation and that it is sufficient to evaluate the conformational properties of pentasilane [9]. The calculations were performed using the FPT CNDO/2MO method [1012] on model compounds: compound (a) are oligosilanes (n-SinHzn+2, n = ca. 7-15) and compound (b) is n-2,3,4,5,6,7-hexamethyl-2,3,4,5,6,7-hexavinyloctasilane (Fig. 17.1). In this calculation, the SimSi, SimC, Si--H, C~---C (vinyl) and C ~ H bond lengths used were 2.40, 1.87, 1.48, 1.40 and 1.09/k, respectively, and the bond angles for the vinyl groups were 120 and 109.47 ~ respectively [13]. All the calculations were carried out by means of a FACOM VP-30 computer at the Computer Center of Kamagawa University and a HITAC-280H computer at the Tokyo Institute of Technology. 17.2.1.2 Results and discussion CP/MAS NMR spectra of PMPS at various temperatures are shown in Fig. 17.2. At room temperature, the 29Si signal is a broad peak having some high frequency shoulder peaks. As the temperature is increased, the spectral patterns change considerably because the individual peaks move to high frequency and their linewidths also change with a change of temperature. The linewidth largely decreases as the temperature is increased. The broad line at 153 K may come predominantly from dispersion of the chemical shifts 29Si
616
TOSHIO TAKAYAMA Me
I ~i-) n I 'h
T(K 393
333
298
203
IS3 il~l ~1~ ~ ~1~ I l l l l i i I l l ll~l
-25
-35
-45
29Si ~ / ' p p m
Fig. 17.2. 29Ni VT-CP/MAS NMR spectra of poly(methylphenylsilane) in the solid state from 153 to 393 K.
of various conformers which are exchanging slowly on the NMR timescale. As the PMPS prepared here is amorphous, it is assumed to take any equilibrium state with a mixture of gauche- and trans-conformations: rotational isomeric state models are assumed for the polysilane by considering all backbone bonds, each restricted to a few rotational states by their inherent rotational barriers. For SimSi with two single bonds usually in three states, one trans (T) and two gauche (G and G') conformations are assumed. According to such a rotational isomeric state model, gauche-gauche (GG or G ' G ' ) , gauche-trans (GT or G'T) [or trans-gauche (TG or TG')] and trans-trans (TT) conformations, where GG' can be ignored due to the large steric hindrance, were assumed. To analyze the 29Si signal behavior over a wide range of temperatures, the 29Si signal was separated into some components which are assumed to arise from GG, GT (TG) and TT conformations undergoing very slow exchange on the NMR timescale. The lineshape analysis was completed as shown in Fig. 17.3, where the 29Si signal is deconvoluted with Gaussian broadening functions. The chemical shifts, linewidths and relative peak inten-
INORGANIC POLYMERS
617
GG
331
153
/~ ~/ppm
29Si
Fig. 17.3. The lineshape analysis of the VT C P / M A S N M R spectra of PMPS from 153 to 393 K. Solid line with dots = the experimental spectrum; solid line -- theoretical line' and broken line = theoretical line convoluted by the Gaussian function.
sities determined from the lineshape analysis are given in Table 17.1. As shown in Table 17.1, the magnitude of relative peak intensity for the conformations in PMPS is in the order of GG (14) < GT(TG) (21) < TT (65) at 153 K, while it is in the order of TT (7) < GT(TG) (18) < GG (75) at 393K. Halfwidths of these peaks decrease as the temperature is increased from 153 to 393 K, however, their chemical shift values are almost independent of temperature. Next, the large spectral change associated with the structural alteration of PMPS in the solid state will be discussed. The thermochromic behavior exhibited by polysilanes generally arises from a change in the temperature of conformation in the main chain. It is known that as the temperature is increased, the population of the TT conformation decreases and those of the GG and GT (TG) conformations increase [14]. Therefore, TT, GT (TG) and GG are considered for the main chain conformation. From the VT 29Si CP/MAS experiments, it is suggested that PMPS in the solid state undergoes slow exchange among the TT, GT (TG) and GG conformations on the NMR
618
TOSHIO TAKAYAMA
Table 17.1. The observed 298i chemical shifts, halfwidths and relative intensities for poly(methylphenylsilane) in the solid state at various temperatures Temp.
Conformation"
~Si chemical shift b
Half-width
Relative intensity
393
TT
- 4 2 . 3 (0.0)= - 39.5 (2.8) - 36.3 (6.0)
2.0 2.0 3.0
7 18 75
GT (3G
- 43.0 (0.0) - 39.3 (3.7) - 35.5 (7.5)
4.0 4.0 4.0
11 71 18
TT (37' (3(3
- 4 2 . 1 (0.0) - 39.6 (2.5) - 36.2 (5.9)
4.0 4.0 5.0
45 32 23
TT
- 4 2 . 2 (0.0) - 38.7 (3.5) - 3 5 . 5 (6.7)
5.0 5.0 5.0
65 21 14
GG
- 4 2 . 8 (0.0) - 39.5 (3.3) - 36.2 (6.6)
5.5 5.5 5.5
68 20 12
(3T GG
- 43.2 (0.0) - 3 9 . 1 (4.1) - 36.4 (6.8)
5.5 5.8 5.8
65 21 14
(K)
CaT GG 333
298
243
TT
GT GG 203
T'T (3T
153
(ppm)
(ppm)
(%)
aSee text. bRelative to TMS. CThe chemical shifts are converted with respect to the conformation TT. The positive sign in parenthesis means downfield shift.
timescale at temperatures from 153 to 393 K. As shown in Fig. 17.2, the 298i signal separates into three peaks corresponding to the three conformations. However, at this stage the peaks cannot be straightforwardly assigned. To do this, the FPT CNDO/2 calculation of the 298i chemical shifts is used to obtain information about the 29peak assignment. Figure 17.4 shows the 298i nuclear shielding of the model compounds with various conformations for the main chain of PMPS calculated by the FPT CNDO/2 MO theory (a), together with the observed 298i chemical shifts of PMPS (b) at 393, 333 and 153 K. For convenience, they are schematically represented by the stick spectrum. The relative peak intensity is indicated by the stick height. As shown in Fig. 17.4, the calculated 298i nuclear shielding is converted with respect to that of the trans-TT conformation. The chemical shifts for the conformers of TT, GT (TG) and GG were ca. 0-0.7, 4.0-4.9 and 9.1-10.1 ppm, respectively. The calculated 298i chemical shifts for all the
619
INORGANIC POLYMERS
(a)
CALCULATED
TT
TTTGTTTG'
GG
~Oel
confor~tton
G T (TG)
~k~
n
TTGTTGTTGTTG TGTGTG
TGTG' < ~
n
TGGTG'G' TGGGTG'G'G' GGGG
I
[~ I I0
(b)
n
CI ~
N_
n n
V
TTGGTTGG
TT
I
I
|
I
j
.5
I
v
i
I
!
i
llppm
i
!
I
9
0
OBSERVED 153 k
m
333 k
n
o
n
393 k
H I0
i
i
n I
5
!
i
~Ippm
0
Fig. 17.4. (a) The calculated stick spectra of n-Si.H2. + 2 (n = ca. 7-15) with various conformations by the FPT CNDO/2 method and (b) the observed stick spectra of poly(methylphenylsilane) in the solid state. The chemical shift is converted with respect to that of the TT conformation. The positive sign in the 6-scale means a high frequency shift. The stick height indicates the relative peak intensity.
TT conformers in the model compounds were almost identical, and this situation is the same for all the TT, GT (TG) and GG conformers. Their peaks appear at high frequency in the order TT, GT (TG) and GG. The trend of these calculated chemical shifts was used to assign the three peaks which come from the TT, GT (TG) and GG conformations for PMPS of interest. From a comparison of the calculated and observed results, the most intense and most shielded peak in the spectrum at 153 K can be assigned to that which comes from the TT conformation, and the most intense signals at 333 and 393 K can be assigned to peaks from the GT (TG) and GG conformations, respectively. Therefore, on the basis of the calculated results, it can
620
TOSHIO T A K A Y A M A
Table 17.2. Calculated 298i NMR chemical shifts for pentad model compounds of poly(methylphenylsilane) by the FPT CNDO/2 method
chemical shift b (ppm)
Pentad stereosequence"
29Si
mmmm mrmr rnrrm rrrr
- 345.6 -345.2 - 345.2 -344.9
(0.0)" (-0.4) (-0.4) (-0.7)
aAll-trans-zigzag conformation was used in the calculation, bThe nuclear shielding value averaged for the Si atoms 3, 4, 5 and 6 in model compound (b). CThese values are relative to mmmm. Negative values indicate upfield chemical shifts.
be said that at low temperature a TT sequence mainly exists, at high temperature a GG sequence mainly exists, and at intermediate temperature a GT (TG) sequence exists. It appears that the overall trend for the conformation dependence of the observed 298i chemical shifts in PMPS can be explained qualitatively by the present calculations. The stereosequence dependence of 298i chemical shifts for PMPS will be discussed next. A silicon atom in the main chain possesses a methyl and phenyl ~substituent and is a pseudoasymmetric center. Schilling et al. [15] reported that the solution state 298i NMR spectrum of poly(methyl-n-hexylsilane) has a broad peak with a halfwidth of about 0.5 ppm, containing at least seven subpeaks corresponding to pentad configurational sequences. Therefore, in order to investigate the configurational-sequence dependence of 298i NMR chemical shifts of PMPS, the 298i chemical shift of pentad model compounds (Fig. 17.4(a)) of PMPS was calculated. 298i NMR chemical shifts calculated for some pentad stereosequences mmmm, mrmr, mrrm and rrrr by the FPT CNDO/2 MO method are presented in Table 17.2, where m and r indicate meso and racemic dyads, respectively. As shown in Table 17.2, the calculated 298i chemical shifts for pentad stereosequences appear at high frequency in the order mmmm, mrmr, mrrm and rrrr and their spread is ---0.7 ppm. Therefore, it can be said that the calculated 298i chemical shift range, which arises from the stereosequence effects, is much smaller compared with that which arises from the conformation effect. Next, the effect of ring currents from phenyl groups on the 298i chemical shifts of the main chain Si atoms in PMPS will be discussed. For convenience, a syndiotactic structure with the all-trans-zigzag is considered, where phenyl rings are set either perpendicular (type A) or coplanar (type B) to the Si--Si bonds (Fig. 17.5). The effect of ring currents produced by neighboring phenyl
INORGANIC POLYMERS
621
3
Type A
Y Type B
ii Fig. 17.5. A portion of a poly(methylphenylsilane) chain with the all-trans-zigzag conformation.
The phenyl rings are set perpendicular (type A) and coplanar (type B) to the Si--Si bond.
by the Johnson-Bovey table [16]. The ring currents for any specified Si atom for types A and B lead to high frequency shifts of 0.7 and 1.7 ppm, respectively. However, the maximum difference in chemical shifts between any specified Si atom is <1 ppm and so the ring-current effects on each tend to be cancelled out. Therefore, it can be said that the effect of ring-currents make only a relatively minor contribution compared with the conformation effects. Finally, it can be concluded that variable-temperature 298i CP/MAS N M R confirms that PMPS is mainly in TT conformation form at low temperatures and mainly in GT (TG) or GG forms at high temperatures. The FPT C N D O / 2 M O calculation is very useful to assign the observed 298i N M R signal to some conformations. 17.2.2
Copoly(dimethylsilyldiphenylsilane)
In Section 17.2.1, it was demonstrated that VT 29Si CP/MAS N M R spectroscopy is a powerful means available for characterizing the structure of polysilanes in the solid state [7, 17], as well as VT 13C CP/MAS N M R experiments in solid polymers [1]. As a continuation of this study, 29Si CP/MAS N M R spectra of copoly(dimethylsilyldiphenylsilane) (CPMPS) in the solid state over a wide range of temperatures were measured and the configurational and conformational aspects through temperature change of the main chain 29Si chemical shifts
622
TOSHIO TAKAYAMA
were also measured. To provide detailed discussion of the experimental results, the calculation of the 298i chemical shifts of the main chain Si atoms by means of FPT CNDO/2 MO were attempted. 17.2.2.1 Experimental Materials: CPMPS was prepared by a method similar to that described by Zhang and West [18]. Here, 1.84 g (0.08 mol) of sodium and 30 ml of dried toluene were added to the reaction flask and the stirred mixture heated under gentle reflux. Then, 0.04 mol of each mixture of dichlorodimethylsilane [(CH3)2SIC12] and dichlorodiphenylsilane [(C6H5)2SIC12] (molar ratios 3:1, 2: 1, 1:1 and 1:2) in 20ml of dried toluene was added dropwise quickly enough to maintain a gentle reflux under an Ar atmosphere. After the addition was completed, the mixture was heated to reflux for 3 h. The reaction mixture was then cooled to room temperature and quenched with 5 ml of ethanol and 30 ml of water. The organic layer was separated by filtration. The solvent was then stripped off on a rotary evaporator and the greasy residue dissolved in 10ml of tetrahydrofuran(THF). A mixture solution (160 ml) of methanol and 2-propanol (volume ration 1:1) was then added to the THF solution while stirring to precipitate the polymers. The precipitate was dried in a vacuum oven at 100~ for 3 h. The obtained polymers are random copolymers, clear, colorless and brittle solids. The molar ratios of [ ~ S i ( M e ) 2 ~ ] and [ ~ S i ( P h ) z ~ ] l ( M e = methyl, Ph = phenyl) (m) for these copolymers are identified to be ~--~ 3 by elemental analysis. The IR spectra for the copolymers with various values of m are shown in Fig. 17.6. The characteristic absorption bands in these spectra are almost the same and in agreement with the expected copolymeric structures (In = 1) [18]. However, the relative intensities of bands at 2880 and 2940 cm -1 (aliphatic C ~ H bond) and 1250 cm -1 ( S i ~ M e bond) increase and those of 3040 and 3060 cm-1 (aromatic C ~ H bond) decrease as in is increased. Obviously, this shows that the amount of [ ~ S i ( M e ) 2 ~ ] in the copolymer increases as m is increased. The X-ray diffraction powder patterns show that these copolymers are amorphous. The copolymers began to soften gradually from about 100~ Differential thermal gravimetric analyses show that the copolysilanes began to decompose slowly at 280-300~ under nitrogen atmosphere.
NMR measurement: VT 29Si CP/MAS NMR spectra for the copolymer with m = 1 and for the remaining copolymers were obtained at temperatures from - 1 2 0 to 120~ and 25 to 120~ respectively, using a JEOL GX270 spectrometer equipped with VT CP/MAS accessory operating at 53.54 MHz. Usually, spectra were accumulated 300-500 times to achieve a reasonable
623
INORGANIC POLYMERS In =
l-st {He) 2-l/l-sl (Ph) 2-1
3
2
1/2
Cll
Si-ph ---
{arosattc)
I
4000
I
3000
' "i
2000
'1
1500
I
1000
HAVENUHBER,
cm
I
TOO
-~.
'
I
400
Fig. 17.6. Infrared spectra of C P M P S with various values of m. In this m e a s u r e m e n t , the K B r pellet m e t h o d is used. signal-to-noise ratio. The other conditions of measurement were similar to that described above.
Calculation of 29Si NMR chemical shift: The calculations were performed, employing the FPT C N D O / 2 MO method, for H3SimSiXz--SiYzmSiZ2 Sill3 (X, Y, Z = Me or Ph) triads and the same pentads as the model compounds for the Si backbone of CPMPS, where the conformation around
624
TOSHIO TAKAYAMA
the Si~Si bond takes trans or gauche. In this calculation, the Si~Si, Si~C, S i g H , C ~ C (phenyl ring) and C ~ H bond lengths used were 2.40, 1.87, 1.48, 1.40 and 1.09 A, respectively, and the used bond angles for phenyl group and the other groups were 120 and 109.47 ~, respectively [13]. All the calculations were carried out as described above. 17.2.2.2 Results and discussion 1 298i CP/MAS NMR spectra of CPMPSs with in = ~, 1, 2 and 3 at 120~ are shown in Fig. 17.7. Many peaks appear in the spectra. Each peak in the spectrum is numbered from low frequency. The peak intensities for the 298i spectra change by a change of m. The intensities for peaks 1, 2, 5, 7 and 8 increase, and those for peaks 3, 4 and 6 decrease, as m is decreased. Peak 9 1 appears only when m is ~. Random addition of monomer moieties [~Si(Me)2~] and [~Si(Ph)2~] produces several magnetically-nonequivalent environments for silicon atoms in its moieties and so should result in 298i resonances with different chemical shift positions. As in is decreased, the intensity of peaks coming from some [~Si(Ph)2~] moieties increases. If the symbols M and P are used for the moieties [~Si(Me)2~] and [~Si(Ph)2~], respectively, the triad configurations of CPMPS may be represented as MMM, MMP, PMP, MPM, MPP and PPP. The observed 298i peaks by six triad configurations will be assigned. Peak 9 appears when an amount of moiety [~Si(Ph)2~] is large (In = 2). For this reason, peak 9 can be assigned straightforwardly to PPP triad as a result of the experimental finding that the 298i chemical shift for peak 9 (-23.5 ppm) is the same as that of polydiphenylsilane consisting of only P units. Peak 4 can be assigned to MMM triad because the 298i chemical shift of peak 4 (-34.1 ppm) is the same as that of polydimethylsilane consisting of only M units, and it has the largest peak intensity when an amount of moiety [~Si(Me)2~] is greater. At this stage, the other peaks cannot be assigned conclusively. For this reason, the FPT CNDO/2 calculation of the 298i chemical shift for the triad model compounds will be used to obtain useful information about the peak assignment. As triad model compounds of CPMPS, H3Si~ Si(Me)z--Si(Me)z--Si(Me)z--SiH3 (MMM), H3Si--Si(Me)z--Si(Me)z-Si(Ph)z--SiH3 (MMP), H3Si--Si(Ph)z--Si(Me)z--Si(Ph)z--SiH3 (PMP), H3Si--Si(Me)z--Si(Ph)z--Si(Me)z--SiH3 (MPM), H3Si--Si(Me)z--Si(Ph)2 --Si(Ph)z--SiH3 (MPP) and H3Si--Si(Ph)z--Si(Ph)z--Si(Ph)z--SiH3 (PPP) are used. The 298i nuclear shieldings of these triads were calculated by the FPT CNDO/2 method. On the basis of these results, the observed peaks will be assigned. Figure 17.8 shows the stick spectra for the calculated 298i nuclear shieldings of the model compounds with all-trans- and all-gauche-confor-
625
INORGANIC POLYMERS I
I 5
I
I
m 1 /2
I -20
I
I
I
I
-25
I
-30
-35
-40
-45
Fig. 17.7. VT "Si CPlMAS NMR spectra of CPMPS in the solid state of m. Fur cunvcnicncc, the pcaks are numbered from low frequency.
I -50
at 120°C as a function
mations. The conformation dependence of 29Si nuclear shieldings is very small compared with the configuration dependence. These conformations were assumed due to the experimental finding that at low temperature polysilanes mainly take a trans-rich conformation and at high temperature a guucherich conformation [14]. Note that the calculated u is a nuclear shielding constant and so the
626
TOSHIO TAKAYAMA r~
gouche 1
,b
9
~
9
trons
I
-420
....
I"
-4 O0
I
-380
29S|
....
i
-360
....
I
-340
Ocalc d/PPm
Fig. 17.8. The calculated 298i nuclear shieldings for the model compounds, H3Si--SiX2-SiYz--SiZz--SiH3 (X, Y, Z = Me, Ph) with all-trans or all-gauche forms.
negative sign means deshielding. On the other hand, the negative sign of the observed chemical shift 6 means shielding. Therefore, the relative difference in the calculated shielding should be compared with the observed chemical shift [10]. As shown in Fig. 17.8, the calculated 298i chemical shifts for the compound with trans-conformation appear from high frequency in order of PPP, PPM, MPM, MMM, MMP and PMP. The resonance of the Si atom in moiety [--Si(Me)2--] shifts to low frequency compared to that in the moiety [--Si(Ph)2--]. This trend can be used to assign the six peaks which come from six types of triads for CPMPS of interest. As mentioned above, from a comparison of the calculated results with the observed ones, peaks 4 and 9 can be assigned to MMM and PPP, respectively, and peaks 1 or 2, 3, 5 or 6 and 7 or 8 are assumed to be assigned to peaks which come from PMP, MMP, MPM and PPM, respectively. Peak 3 can be tentatively assigned to triad MMP, but the peaks in the observed spectra number more than six. Hence, in order to clarify such a situation, the pentad configuration was introduced, for which the 298i nuclear shielding effect is induced by more remote moieties as compared with triads. In connection with PMP, MPM and PPM triads, 298i chemical shift of any specified pentads, MPMPP, PPMPP, MMPMP, PMPMP, MPPMP and PPPMP pentads particularly is calculated. MPMPM, MMPMM, and MPPMM pentads were excluded because their r fractions are very small at m = 5. The calculated 298i shielding o- for pentads appear from high frequency in the order of PPPMP (-387.2 ppm), MPPMP ( - 3 8 4 . 8 p p m ) , PMPMP (-364.3ppm), MMPMP (-362.7ppm), PPMPP (-350.1 ppm) and MPMPP (-349.9 ppm). From this calculation, when two
I N O R G A N I C POLYMERS
627
consecutive moieties of [--Si(Ph)2 m] in pentads are placed, the P's resonance of its pentad tends to shift to high frequency as compared with the other cases. It can be said that the calculated 298i chemical shift range arising from pentads is very large. Consequently, according to the chemical shift calculation, Peaks 1, 2, 5, 6, 7 and 8 can be assigned to peaks which come from MPMPP, PPMPP, MMPMP, PMPMP, MPPMP and PPPMP, respectively. Next, the spectral behavior over a wide range of temperatures are discussed. 298i CP/MAS NMR spectra of CPMPS with in = 1 at temperatures from - 1 2 0 to 120~ are shown in Fig. 17.9. The assignment of the observed 298i peaks was made as shown above. The linewidth largely increases as the temperature is decreased. The broad linewidth at low temperature may come predominantly from the dispersion in the chemical shifts for various conformers, which are slowly exchanging on the NMR timescale. At 25~ the 298i signals a broad peak with some shoulder peaks which appear to low frequency. As the temperature is increased further, the spectral patterns change considerably because high frequency peaks clearly appear. As the CPMPS used here is amorphous, rapid transition between trans- and gauche-conformers occurs on the NMR timescale and the observed chemical shift is the averaged value over possible conformers. It is known that the thermochromic behavior exhibited by polysilanes generally arises from a temperature change of the main chain conformation, and as the temperature is increased, the population of the TT conformation decreases and the populations of GG and GT conformations increase [14]. As shown in Fig. 17.8, the calculated 298i chemical shift in the allgauche-conformation shifts to high frequency compared with that of the alltrans-conformation. This means that the 298i chemical shifts of the peaks move to high frequency as the temperature is increased because the population of the gauche-conformation increases with temperature. This agrees with the observed results. The results of the lineshape analysis for the observed 298i spectra at -120, 25, 60, 90 and 120~ are shown in Fig. 17.10, where the 298i signal is deconvoluted with Lorentzian broadening functions. The chemical shifts, linewidths and peak intensities at temperatures from - 1 2 0 to 120~ determined from the lineshape analysis, are given in Table 17.3. As shown in Table 17.3, the halfwidths of these peaks decrease as the temperature is increased from - 1 2 0 to 120~ This means that the decrease in linewidths for the 298i resonances comes mainly from the increase of mobility as the temperature is increased. All the 298i peaks shift to high frequency as the temperature is increased from - 1 2 0 to 120~ According to the calculations, this means that the populations of the GG and GT conformations increase. Next, the effect of ring currents [16] from phenyl groups on the 298i
628
TOSHIO TAKAYAMA
T(~
120
1;0
-120
i -20
'I
' 'I' -30
I ....
29 Si
I -40
'I
~I -50
6/ ppm
Fig. 17.9. 29Si C P / M A S N M R spectra for copoly(dimethylsilyldiphenylsilane) (m = 1) at temperatures from - 120 to 120~
chemical shifts of the main chain Si atoms in CPMPS will be discussed. In polymethylphenylsilane, the ring-current effects on each Si atom tend to be cancelled out [17]. From this, it may be said that the ring-current effects serve only as relatively minor contribution compared with the configurational and conformational effects.
INORGANIC POLYMERS
4
A
T(~
629
II
120
60
S~2 1
2.5
S~432
..12
I
I
~
I
i'
-25
-30
-35
-40
-45
29Sl 6/
Pi3~
Fig. 17.10. The lineshape analysis of the VT 298i CP/MAS NMR spectra of CPMPS at temperatures from -120 to 120~ Circle line, experimental spectrum" solid line, theoretical line; and broken line, theoretical line convoluted by the Lorentzian function.
Finally, it may be concluded that variable temperature 298i CP/MAS NMR confirms that the configurations of CPMPS in the solid state can be assigned and the high frequency shift, exhibited by the copolysilanes, arises from the increase of the populations of the GG and GT conformations in the main chain as the temperature is increased. The FPT CNDO/2 MO calculation is very useful in assigning the observed 298i NMR signal to configuration and conformation of copolysilanes.
630
TOSHIO TAKAYAMA
Table 17.3. Observed 295i chemical shifts (6 ppm) for copoly(dimethylsilyldiphenylsilane) in the solid state at various temperatures Temp.
Peak b
('c)
1
9
- -
.
2
.
.
.
3
.
.
.
.
4
.
.
5
6
7
,
8
120
zJSi6 HW Fraction
-39.0 1.1 7
-37.7 1.1 7
-35.9 1.1 7
-34.5 1.2 43
-33.0 1.3 8
-31.5 1.2 9
-29.4 1.0 8
-27.9 1.6 12
90
~Si 6 HW Fraction
-39.3 1.2 7
-38.1 1.2 7
-36.1 1.8 19
-35.0 1.5 24
-33.9 2.0 23
-31.9 1.2 4
-30.4 1.2 6
-29.1 1.8 II
60
~Si6 HW Fraction
-39.7 1.1 12
-38.3 1.1 12
-36.6 1.1 9
-35.5 1.5 20
-34.1 1.3 33
-32.5 1.0 3
-31.5 1.1 5
-31.0 1.1 6
25
nSiJ HW Fraction
-40.0 I.I 10
-38.4 I.I 12
-36.8 1.8 17
-35.6 1.8 21
-34.3 I.I 25
-32.7 1.0 3
-32.0 1.3 6
-31.2 1.3 6
-30
~Si6 HW Fraction
-40.2 4.0 8
-38.7 8.0 8
-36.7 4.3 29
-35.8 4.0 17
-34.4 3.0 16
--33.0 4.0 5
-32.1 4.0 4
-31.3 5.0 14
--80
~Si6 HW Fraction
-42.3 5.0 8
-40.3 4.5 9
-38.5 4.8 17
-37.0 5.0 30
-35.2 4.5 24
-33.1 4.0 3
-32.2 5.0 2
-31.5 4.0 6
120
zDSi6 HW Fraction
-42.5 3.5 7
--40.5 3.5 5
--38.7 3.5 17
--37.1 4.0 29
--35.5 4.0 21
-33.4 3.4 8
--32.3 3.5 5
--31.6 3.5 8
aThe negative sign means upfield shift from TMS. bSee text.
17.2.3
Short-alkylgroup substituted polysilanes
An investigation of poly(di-n-alkylsilane)s with short side chains assumes a critical importance for a generalized understanding o f the structures, energetics and phase transition of this novel class of polymers. The structures of the poly(di-n-alkylsilane) family are far more complicated than presumed to date, requiring a re-examination of our current understanding of the structure-property relationships of these important polymers. For this reason, this section reports an investigation of polysilanes with shorter side chains. These are: (1) the symmetrically substituted polysilane derivatives, dimethyl (PDMS) and diethyl (PDES) derivatives; (2) the copolymer formed from two symmetrically substituted monomers, PM-co-ES with
INORGANIC POLYMERS
631
the structure and properties of the PDMS and PDES homopolymers; and (3) the unsymmetric polysilane that forms an interesting class of materials exhibiting various degrees of intra- and intermolecular disorders due to the mismatch of side chain lengths, poly(ethylmethylsilane) (PEMS). 17.2.3.1 Experimental Materials: Preparations of substituted polysilanes were performed by the following method [19]. Some pieces of metal sodium were added to dry toluene contained in a flask equipped with,~ mechanical stirrer, a condenser, an additional funnel and an Ar inlet. The mixture was stirred under reflux until the sodium melted and dispersed into small beads. Then the substituted dialkyl dichloro-silane in toluene was added over 30 min. The reaction mixture was refluxed for 2 h and then stirred at room temperature overnight. The residual sodium was quenched by very slow dropwise addition of water. Finally, more water was added. The solution was filtered to collect the insoluble material, which was the desired high molecular polysilane. For PDMS and PDES polysilanes, highly insoluble fractions are obtained in reasonable yields. Unfortunately, molecular weights of the polysilanes could not be determined because they are insoluble in all solvents. For PM-co-ES and PEMS polysilanes, insoluble materials are not always obtained. So, the toluene layer was separated, washed with water and dried over MgSO4. The toluene was evaporated and the residue dissolved in THF, and reprecipitated with methanol to give, after drying, a pure, white and flocculent polymer. PM-co-ES and PEMS have monomodal molecular weights, Mw = 6.3 • 103 and 7.2 • 103, respectively, as determined from the gel permiation chromatography (GPC) elution profile (GPC was carried out using a Waters GPC, 50C Module using spectrograde THF as the eluent and is relative to polystyrene standards). The X-ray diffraction powder patterns for the polysilanes, except for PDMS (ordered), show the presence of an ordered and disordered phases at room temperature. Differential scanning calorimetry (DSC) analyses do not show the endothermic peaks upon heating at 10~ up to 300~ except for PEMS.
NMR experiments" 29Si and ~3C NMR spectra were obtained with a JEOL EX270WB spectrometer. Samples were contained in a cylindrical rotor made of zirconia. Line narrowing was achieved by high-power 1H decoupling and magic-angle spinning (MAS). The spinning rate was set to ~ 6 k H z . GHD/MAS means a simple 90 ~ pulse sequence with high-power proton decoupling (GHD) without CP. T1 CP is the pulse sequence to measure spinlattice relaxation times using the cross-polarization method. T1 MAS is the pulse sequence to measure spin-lattice relaxation times using the inversion
632
TOSHIO TAKAYAMA
recovery pulse sequence modified for solid-state measurements. 13CMASDL, which is the CP/MAS method with dipolar diphasing, is used to assign between the CH3 carbons, nonprotonated carbons and protonated carbons. The other conditions of measurements were similar to those described above. Calculation: In this section the calculations were performed employing the
FPTCNDO/2 MO method on the model compound Si11H24. The present theoretical understanding of the electronic nature of oligosilanes and idealized polysilanes in terms of molecular orbital theory has been fairly satisfactory [7, 17, 20]. In this calculation, the Si~Si and S i g H bond lengths used were 2.40 and 1.48 A, respectively, and the bond angles for S i ~ S i ~ S i and S i ~ S i ~ H were 109.47 ~ All the calculations were carried out by means of a FACOM VP-30 computer at the Computer Centre of Kanagawa University. 17.2.3.2 Results and discussion Polydimethylsilane (PDMS) CH3
I I CH3 Lovinger and coworkers [21] reported that PDMS takes the trans-conformation form at room temperature and at higher temperatures, the main chain could be realized, in principle, through changes in molecular conformation. However, the exact structural change of the PDMS molecule has not been determined by solid-state 29Si NMR. So, to obtain a better understanding of the structural change, the 29Si CP/MAS NMR spectra of PDMS at various temperatures are measured (Fig. 17.11). At 50~ a sharp, intense signal occurs at -34.11 ppm. As temperature increases, one broad signal appears at the low frequency side and gradually increases. And at 168~ the peak intensity of the signal is mostly identical with the high frequency peak. During cooling from 168 to 52~ the peak intensity of the signal gradually decreases and the signal almost disappears at 52~ The peak-intensity and chemical-shift change was reversible. To obtain the ratios between the two peak, the spectrum is divided into two areas. Figure 17.12 shows 29Si NMR spectra of PDMS at various temperatures and these deconvolute into two Lorentzian constituents. For example, at
633
INORGANIC POLYMERS
86% 168"C
~ ~
52"C
~
168"C
44%
68% I 1 J
32%
81
50"C
-30
29SI
-35 ~/ppm
-40
Fig. 17.11. Variable temperature 29Si CP/MAS NMR spectra of PDMS.
142~ the ratio between the high and low frequency peaks is 57 and 43%, respectively. These deconvolution curves fit very well with the experimental spectra. So, these spectra are shown to consist of two components. The results of the ratios the two peaks at various temperatures are shown in Fig. 17.12.
634
TOSHIO TAKAYAMA
'i
57%
i
!42~ 3i
:1 t,..-;:.t
..:f .it '~\..
._-~ . . . .
~_~~/,
'~2.~.'~.
68
.....
~
/ .-30
....
116"C
.-~.!_.
86
._~ ._
k
-35 29Si 6/ppm
50~
-40
Fig. 17.12. 29Si N M R spectra of PDMS and their deconvolution into two Lorentzian constituents.
To understand why the 29Si peaks shift to low or high frequency with increasing temperature, FPT CNDO/2 MO calculations of the 29Si chemical shifts were carried out. The calculated 29Si nuclear shielding of the model compound using the FPT CNDO/2 MO theory is plotted against the dihedral angle 0 around the Si--Si bond in Fig. 17.13, together with the structure of the model compound n-Si11H24. This structure just shows the trans-zigzagconformation of the model compound. Newman projections of a hydrogen-
INORGANIC
635
POLYMERS
mlm
d
I
I
2"--"k
$i
*"
N
2--"t
II
II
$;
-315
Ii
I
--
gauche
trans
-320
-325 -330
-335 tt
"D o
-340
__
%1 I
u
uo
% I
-3q5
I t
l ! I I
-350
-3ss
' !
1
30
,
,, I
60 (g0uche)
, I I
90
,,
i
.
.
.
.
.
.
.
.
120
I
I
150
180
(he I Ix )
,
(trons)
dlhedrol ongle e ( d e g r e e )
Fig. 17.13. The calculated 298i nuclear shieldings for the central Si atom in the model compound n-Si11Hz4 as a function of the dihedral angle 0 by the FPT CNDO/2 MO method.
substituted silicon backbone, viewed along a Si--Si bond in the trans- and gauche-conformations. The computation for the dihedral angles 0, less than 30 and between 60 to 120~ diverges as a result of greater steric hindrance. Note that the calculated chemical shielding o" as such that a negative sign indicates deshielding. However, the negative sign of the observed chemical shift ~; indicates shielding.
636
TOSHIO TAKAYAMA
Therefore, only the relative difference in the calculated shieldings should be compared with the observed chemical shifts. As shown in Fig. 17.13, when 0 deviates a little from 180 ~ (this is the trans form), the chemical shift moves to low frequency. At near 150 ~ (this is the helix form), the calculated 29Si shielding was near the maximum value. It can be said that as 0 deviates from 180 (trans form) to 140 ~ the 29Si chemical shift moves to low frequency. On the other hand, as 0 deviates largely from 180 ~ (trans form) to 60 ~ (gauche form), the calculated 29Si shielding decreases. So, according to these calculations, it may be expected that as the 29Si shielding increases with an increase of temperature, the conformation of the polysilane changes from the trans to the helix form. On the other hand, as the 29Si chemical shielding decreases, the conformation changes greatly from the trans to the gauche form. As shown in Fig. 17.13, as the temperature increases, the ratio of the small deviation form increases and at 168~ the ratio of the small deviation form becomes about one-half. According to the FPT CNDO/2 MO calculations, the Si~Si main chain conformation changes with a small deviation (herical form) from the trans form. This would imply that with an increase of temperature, the trans-conformation of PDMS takes a small statistical deviation (helical form) from the trans-zigzag-planarity.
Polydiethylsilane (PDES) CH2CH3
CH2CH3
For PDES, the ethyl groups interfere with each other during the rotation. The ethyl groups are shown in eclipsed conformation, which minimizes the steric interactions among consecutive ethyl groups along the Si backbone. For the slightly more stable staggered ethyl conformation, the terminal methyl interactions will be much more severe. For all of these reasons, bond distortions (primarily torsional) occur along the Si backbone. Under these situations, the analyses of the structure and molecular motion for PDES are of general interest. In order to consider the relationship between the silicon chemical shift and silicon bond conformation, the 29Si NMR spectra with CP/MAS and G H D / M A S of PDES at various temperatures are measured (Figs. 17.14 and
INORGANIC POLYMERS
637
29S i CPMAS
1oo
5O
j
-30
____J 0
29
-20-
-40
Si 6/ppm
Fig. 17.14. Temperature dependence of 29Si CP/MAS NMR spectra of PDES.
17.15, respectively). From a 29Si CP/MAS spectrum, a single peak at -60~ is observed at -25.2 ppm (tentatively ordered phase (I)). And as the temperature increases, one broad signal appears at high frequency (disordered phase (I): -20.5 ppm at 10~ Peak intensity of the signal increases gradually up to 10~ compared with the low frequency peak. Moreover, the two peaks gradually shift to high frequency with increasing temperature. This means that ordered phase (I) transforms slowly with disordered phase (I) within the
638
TOSHIO TAKAYAMA
29
Si GHDMAS "c
25
I
0
........
~"
-I0
'1'
''I
-20 ''Si
-~0
"'
-40
i
Fig. 17.15. Temperature dependence of 29Si GHD/MAS NMR spectra of PDES.
NMR timescale, and the extent of the disordered phase (I) increases with an increase of temperature. At 25~ the ordered phase (I) disappears dramatically (although not confirmed by the DSC thermogram) and the disordered phase (I) only remains. As the temperature is increased more from 25~ the signal of the disordered phase (I) gradually shifts to high frequency. From the 29Si GHD/MAS spectra (Fig. 17.15), a single, broad resonance is observed at temperatures from - 3 0 to 125~ This resonance also shows a high frequency shift as the temperature is increased. The chemical shift of this peak is identical with the one of the high frequency peak in the 29Si CP/MAS spectra. Afterall, the deshielding means that the disordered phase (I) transforms
639
INORGANIC POLYMERS
gradually to a perfectly-disordered phase (II) (all-gauche-conformation) within the NMR timescale. The well-ordered phase implies that the backbone has an all-trans-conformation, although the complete crystal structure of polysilanes has not been reported yet. The side chains are arranged in a direction nearly perpendicular to that of the backbone, with a nearly alltrans-conformation, although for steric reasons. The structure at temperatures above the transition temperature shows that both the side chains and the backbone are conformationally disordered, but the side chains remain organized preferentially on planes perpendicular to the polymer chain axis. The resulting increase in lattice dimensions renders the density of the disordered phase less than that of the ordered phase. The disordering transition in which the ordered phase (I) is converted into the disordered phase (I) is reversible, and by varying the crystallization conditions, room-temperature samples, rich in either ordered (I) or disordered phase (I), can be produced. The dramatic shift in the 29Si NMR chemical shift observed at the transition temperature is the result of this conversion of the conformationally ordered phase into the conformationally disordered phase. To gain additional insight into the nature of the chain motions of PDES, the spin-lattice relaxation times, T1, of the silicon nucleus was measured. The data are reported in Table 17.4. Measurements were made using both T1MAS and TICP methods. The T1CP method is used to probe motions in the ordered phase. The 29Si--T1 values obtained by the TICP method are fairly large at low temperature. T1 value at -30~ is 161 s, which represents a high value implying high structural rigidity.
Table 17.4.. Temperature dependence of 29Si and
13C spin-lattice relaxation times (Tl/s) of
PDES T amp./'(3
Nuoleu=
='Si (TICP) (Tt~LAS)
"C
(T, CP) CH, (1) (2) CH, (1) (Z)
-60
-30
-10
25
"i5
125
237
181 15. 1
144 I0. 8
10. 1 10. 2
18. 8 16. 5
34. 9 34. 1
I. 0 i. 0
I. 4 I. 4
2. 0 I. 5
I. 3
I. 7
a. 2
13. 3 12. 2
10. 6 I0. g
g. 4 8. 7
0. g
1.2
2.0
640
TOSHIO TAKAYAMA
This is attributed to the crystallographic packing of the side chains, which locks the Si backbone into an exceptionally rigid structure. The values decrease gradually with an increasing temperature, at 25~ these decrease dramatically, and increase again with a further increase in temperature. On the other hand, the 29Si~T1 values obtained by the T1 MAS method are almost identical to those from the 29Si~T1CP method between 25 and 125~ The values of 29Si~T1 at - 3 0 and -10~ are largely different depending on the method used for the T1 measurements. The small value observed for the silicon nucleus of PDES probably reflects the proximity of the freely rotating ethyl group. The disordered phase (I) of PDES shows gauche-conformational deviations from the trans planarity has a silicon Tx value of only 15.1 s at -30~ These data may reflect that PDES retains both ordered and disordered phases in the range of - 6 0 to -10~ Above 25~ PDES takes only a disordered phase and the molecular motion is in the fast-motion region for the single correlation-time model based on BPP theory [22], because the 29Si T1 values increase as the temperature is increased from 25 to 125~ That is to say, the disordered phase (I) is conformationally disordered but shows rudimentary intermolecular packing and reflect a single motional state. The 13C NMR spectra were measured to investigate the side-chain structure in PDES. The 13C spectra of PDES with CP/MAS and GHD/MAS are shown in Figs. 17.16 and 17.17, respectively. All of these spectra taken with CP/MAS and GHD/MAS methods show identical resonances. The resonances are themselves split into nearly equal components at lower temperatures as shown in Fig. 17.16. Normally, the observation of such splittings would be attributed to the presence of two highly ordered forms. This doubling of the carbon resonances suggests the existence of two different, but approximately equal, populations of the side chains in slow exchange. These may correspond to two alternative modes of side-chain packing within the same polymer molecule. At 25~ one resonance (6 12.2) disappears dramatically and the other resonance (6 11.6) remains. Above 25~ only one resonance shows. The T1 values for CH2 groups also were shown in Table 17.4, where two Tl-values are reported for individual T1 results for the two peaks of the carbon nuclei, but found that they were almost identical (10.6 and 10.9 s at -30~ At 25~ the CH2 peak is no longer split and the T1 value for the CH2 peak becomes considerably reduced. This means that the side chains of PDES become mobile at 25~ and above 25~ the side chains move with a higher velocity.
INORGANIC POLYMERS
1 3C CP
~"'
641
~ llS
100.
_
SO
2S
_
_
J
o
[h~
% 30
, , 20
-10 J_ _
;, ,3C
,
-~,
;
Fig. 17.16. Temperature dependence of 13C CP/MAS NMR spectra of PDES.
Copolydimethylsilyldiethylsilane
CH3
CH2CH3
I ~(--Si--)n--(--Si~)m--
I CH3
CHzCH3
(PM-co-ES)
TOSHIO TAKAYAMA
642
13
C GHDMAS
(oC)
ell3
]L2S
~ 2
100 _
_-
75
25
:_
0__J
-lo 'u
30
..U
....
]
20
.
.
.
.
9 .
lo
.
.
.
9
o
:
m
-1o
z'C 6 Fig. 17.17. Temperature dependence of
13C
GHD/MAS NMR spectra of PDES.
The copolymer formed from two symmetrically substituted monomers has been examined. The structure of PM-co-ES with the structure of the PDMS and PDES homopolymers is described. This copolymer is of particular interest, since both of the comonomers adopt an ordered phase in their
INORGANIC POLYMERS
643
respective homopolymers at low temperature. Additionally, PDMS is the lowest homologue of this family and its methyl side chains have no conformational degrees of freedom to influence the structure of the Si backbone (this is in contrast to PDES). The 29Si spectra recorded with CP/MAS and G H D / M A S for the PM-coES from - 8 0 to 120~ are shown in Figs. 17.18 and 17.19, respectively. These 29Si spectra obtained with CP/MAS and G H D / M A S show identical chemical shifts and shapes. It implies that PM-co-ES retains an identical phase over a wide range of temperatures. From the 29Si G H D / M A S spectra (Fig. 17.19), the peak intensity of the two 298i signals indicates a diethyl/dimethyl comonomer ratio of about 50:50. For the 29Si CP/MAS spectrum (Fig. 17.18) of PM-co-ES at -80~ two broad signals are observed at -24.3 ppm for the PDES moiety and at - 3 5 . 4 ppm for the PDMS moiety. The signal of the PDMS moiety is almost independent of temperature during - 8 0 to 120~ while that of the PDES moiety gradually shifts to high frequency. This shift seems to be due to changes in the main chain conformation. This may mean that a disordered phase occurs which shows a mean chemical shift on the NMR timescale and transform slowly to a perfectly-disordered phase (all gauche-conformation). The PDES moiety is more sensitive to temperature than the PDMS moiety within this temperature range. To gain additional insight into the nature of the phase of PM-co-ES, the spin-lattice relaxation times, T1, of the silicon and carbon nuclei were measured. These data are reported in Table 17.5. The 29Si T1 values of PM-co-ES at -30~ were 15.7 and 43.4 s for the PDES and PDMS moieties, respectively. At -30~ the T1 of PDES moiety in PM-co-ES is tenfold less than the extremely long T~ (161 s) of the ordered phase in the PDES comonomer. The 29Si Ta value of the PDMS comonomer is 900s at room temperature, which represents a high value implying extremely high structural rigidity [21]. This means that the PDMS moiety of PM-co-ES is more flexible compared with the PDMS comonomer and the PDES moiety of the copolymer has almost the same mobility as the disordered PDES comonomer (15.1 s at -30~ Above all, PM-co-ES retains a disordered phase between - 3 0 and 120~ The assignment of the 13C resonances is based on the CP method with the dipolar diphasing (MASDL) method (Fig. 17.20) and a comparison with the spectrum for a homopolymer PDES. The splittings observed in most of the resonances reflect the distribution of comonomers along the copolymer chain. This chemical shift dispersion results from variations in the silicon bond conformation and changes in the valence angles. The latter reflects the large difference in the steric size of the methyl and ethyl substituents. The resonances are clearly resolved at the triad level. The MMM triad assignment is
644
TOSHIO TAKAYAMA
29
Si GHDMAS Et
~ Me oC
120
gO
9 -lO
.
-
.
i
.
-
.
-20
9 -30
29
.
-
.
, -40
.
-
9 -50
Si 6/ppm
Fig. 17.18. Temperature dependence of 295i CP/MAS NMR spectra of PM-co-ES.
645
INORGANIC POLYMERS
29
Si CPMAS Me
Et
9
-1o
. . . .
(oC)
.
-2o
-
..
..
-~o
29
_,
-~o
._
'~,o
Sid
Fig. 17.19. Temperature dependence of 298i GHD/MAS NMR spectra of PM-co-ES.
646
TOSHIO TAKAYAMA
Table 17.5. Temperature dependence of 29Si and PM-co-ES T,,p.
Nucleus -30 .
.
.
.
.
.
.
.
15.7
Xe
/ 'C
25 (T, MAS)
.
''Si(T, CP) Et ' 3C
0
spin-lattice relaxation times (T~/s) of
13C
80
.
.
.
90
.
.
.
.
120 .
.
14.9
22.1(16.9)
22.4
18.8
16.8
43.4
40.4
47.4(24.7)
27.2
20.3
23.0
I.I 0.7 Z.3
1.5
1.8
(1.7
3.2
0.5
0.7
(0.7)
2.5
2.5
(2.3
3.2
2.6 "(2.5
(T, CP)
KC(CH,) (CH,) Me(1) (2)
2.8
)
2.3
2.9
0.7
0.5
0.8
)
2.1
2.'3
3.1
)
2.3
2.8
3.1
PDES
PM-co-ES
Ma(1)
Me
.
.
.
.
l
2o
"
-~
~"
-
i
,o
.
.
.
.
.
;
o
.
.
.
.
9 .
.,o
.
.
.
.
.
.
.
.
.
.
i ' ' ~
~
.
.
.
.
.
i
,o
*
. . . . . . .
i
o
..10
Fig. 17.20. 13C MASDL spectra of PDES and PM-co-ES.
made by comparison to the chemical shift of the methyl carbon of PDMS. The EMM and MME triads are assigned to dimethyl units with one diethyl neighbor and the EME triad is assigned to the dimethyl unit with diethyl units on either side. These assignments assume that the presence of neighbor-
I N O R G A N I C POLYMERS
647
13C CPMAS
CH3 .C~2(2)iMe(ll
(~ 9O
6_00___.
<..._
- 1 0_q__._~
-60 j
,__
-a0~
2.0.
.___
le
" 9 -to 1:3C 6 / P t : ~
Fig. 17.21. Temperature dependence of 13C CP/MAS NMR spectra of PM-co-ES.
ing diethyl units produces an additive low frequency shift in the resonance position for the carbon in the dimethyl unit [3(e)]. The ~3C spectra recorded with CP/MAS and GHD/MAS for the PM-coES from -110 to 120~ are shown in Figs. 17.21 and 17.22, respectively. Both spectra show identical shapes and chemical shifts. This demonstrates that the side chains of PM-co-ES retain identical phases over a wide range of temperatures. At -110~ broad signals are observed at 12.3 and 6.4 ppm (CH3 and CH2 groups for the PDES moiety, respectively) and at -0.1, - 0 . 7 and - 2 . 9 ppm (Me(3): MMM, Me(2): MME or EMM and Me(l): EME from the triad for the PDMS moiety, respectively). The I3C chemical shifts of the PDMS and PDES moieties are almost independent of temperature between -110~ and 120~ All of the peaks from the PDMS and PDES moieties are sharp and intense at higher temperatures. This implies that the mobility of
648
TOSHIO TAKAYAMA
13C GHDMAS ~3cH2 (~
.- c2~.i1~
i,~
3.20
90
~.
~
~176I -10 -30 -60 -80
20
Fig. 17.22. Temperature dependence of
13e .6/pl:~
13C
tO
"
9
-LO
GHD/MAS NMR spectra of PM-co-ES.
the side chains of PM-co-ES are gradually increased as the temperature is increased. The 13C-T1 values of CH2 group of the PDES moiety, in the range of - 3 0 to 120~ are similarly small compared to those of the disordered phases for the PDES and PEMS (see below) comonomers. The 13C-T1 values of the two Me(l) and Me(2) groups for the PDMS moiety are almost similar although a little larger when compared with that for the CH3 group of the PDES moiety. The data may reflect some increased local motion in the backbones of PMco-ES, but more likely the shorter TI'S are the result of the substantially increased motion of the side chains. Side-chain motions are particularly important in this polymer since it is primarily the dipolar interaction with the side-chain protons that causes the relaxation of the silicon nuclei.
INORGANIC POLYMERS
649
Polyethylmethylsilane (PEMS)
CH3
I --(--Si--).--
CH2CH3 The unsymmetric polysilanes form an interesting class of materials which exhibit various degrees of intra- and intermolecular disorder due to the mismatch of side-chain length. Because the role of side-chain packing in determining the backbone conformation is so important in the planar zigzag structures, this mismatch in side-chain length also causes various amounts of conformational disorder in the backbone. Polyethylmethylsilane (PEMS) has been examined. The 29Si and 13C NMR spectra with CP/MAS of PEMS at various temperatures are shown in Figs. 17.23 and 17.24, respectively. From the 29Si CP/MAS spectra of PEMS, a single peak is observed at -27.7 ppm at -70~ while the chemical shifts of the 29Si peak are almost independent of temperature during - 7 0 to 120~ This means that the conformation of the Si backbone does not change in this temperature range, so reflecting the substantial local chain motion involved in conformational averaging. However, the 29Si~T1 values gradually decrease with an increase in temperature from 24~ and at 70 to 90~ the 29Si~T1 value decreases significantly (33.2 to 10.6 s) (Table 17.6). This means that the Tl-data may reflect sidechain motion increases and the dipolar interaction with the side-chain protons that causes the relaxation of the silicon nuclei. Above the solid-state phase transition of the polysilane nearly identical silicon T1 data are found, indicating similar backbone and side-chain motions in the high-temperature disordered phase of polysilanses. From the ~3C CP/MAS spectrum of PEMS at -70~ (Fig. 17.24), three broad signals were assigned to 12.7, 10.3 and - 1 . 9 ppm for CH3 (2) (ethyl group) and CH2 (1) (ethyl group) and CH3 (Me) group, respectively. The chemical shifts of these spectra do not significantly change over the range of - 7 0 to 75~ However at 80~ the intensity of CH2 (1) increases dramatically and all of the signals move to low frequencies (11.5, 7.7 and -3.5, respectively). It means that the disordered phase of the ethyl group is converted to the all-disordered phase [23]. This change was confirmed from a DSC thermogram of PEMS. Upon heating from 25~ the endothermic peak is observed in the DSC datum at 80~ with a AH of 2.3 cal/g. This change can be understood from the 13C-T1 values in PEMS. These data are also reported in Table 17.6. The ~3C-T~ values for CH2 group of
650
TOSHIO T A K A Y A M A
29
Si CPMAS
(~
120
9o
7o
5o
24
lo
_J t___
lo
.j ~ -3o ~
~_-so
J'i -10
-30
-5o
29 S i. 6/1:~1~
Fig. 17.23. Solid-state 29Si CPMAS NMR spectra of PEMS at -70 to 120~
PEMS in the range of 70 to 75~ strongly decrease. This demonstrates that the ethyl group of PEMS becomes more motional over this temperature range. Above all, from the high resolution 29Si and ~3C N M R spectra and the relaxation times (Tx) of the poly(di-n-alkylsilane)s with shorter side chains in the solid state, this paper concludes that the conformation of PDMS takes
INORGANIC POLYMERS
13
C CPMAS
651
Me
I ~,o"~' Ioo
_L,,
B0
!
1
2
H~-s|{ - c H 2CH 3
2
I
CP
lO |
-i
"
i
30 lo
i
9 "
-1o-.1o
9
___J]~ :~0 _
13C MASDL NMR spectra at 24~
~-so
.
_.___.a ~ J ~ L-7 ~ 9
'
_
,,
,
|
;o ;o-Io-,o 11r ~/l:,P~
Fig. 17.24. Solid-state 29Si GHDMAS NMR spectra of PEMS at -70 to 120~
652
TOSHIO TAKAYAMA
Table 17.6. Temperature dependence of 29Si and 13C spin-lattice relaxation times (T1/s) of
PEMS Nuoleus
Temp. / ' C 24
.
.
.
50 .
'70
.
.
80
9.0
120
.
(T, CP)
41. 0
"C
(T, CP) (CHs) (CH,)
3. 6 2. 0
3. 0 I. 8
2. 2 0. 8
i. 8 0. 5
4.9
3.9
2.3
1.2
Me
6
75 .
"Si
Et
38.
.
33. 2
I0. 6
14. 5
small, statistical deviations from the exact trans-planarity with an increase of temperature. PDES takes on an ordered phase at low temperatures, with an increase of temperature it takes on a disordered and ordered phase and when the temperature further increases, a transformation to a perfectly-disordered phase (all-gauche-conformation) occurs slowly. For PM-co-ES with an increase of temperature, the signal of the PDMS moiety is almost independent of temperature, but that of the PDES moiety gradually shifts to high frequencies. This is due to a change in the main chain conformation. The mobility of the PDES moiety is more sensitive to temperature compared with the PDMS moiety. For PEMS, the conformation of the Si backbone changes little over the range from - 7 0 to 120~ but the ethyl and methyl groups convert to the perfect-disordered phase at 80~
17.3 17.3.1
Polysiloxane
Poly(diethylsiloxane)
Solid-state NMR experiments show the dynamic nature of the disordering. It was possible to monitor the type and the changes of segmental motions within the mesomorphic phase and at the corresponding phase transitions for typical examples. Moeller and coworkers [24] clearly elucidated the thermal behavior of poly(diethylsiloxane) (PDESO) by high resolution solid-state 29Si NMR. PDESO gave one sharp signal at - 1 7 . 9 p p m for the fully-ordered crystalline state at 190 K, which does not change on raising the temperature until the/31-/32 transition is reached. At the/31-/32 transition, the resonance is shifted to low frequency by Au = 3.3 to -21.2 ppm. Increasing the tempera-
INORGANIC POLYMERS
653
ture further results in a gradual shielding increase from -21.2 ppm at 220 K to -22.2 ppm at 280 K, which is directly below the /32-1x transition. On passing the/32-1x transition, the isotropic chemical shift of the silicon atoms shifts to -23.6 ppm. Further heating, even above the melt transition (isotropization), does not result in further variation of the chemical shift. The observed low frequency shifts indicate changes in the molecular packing and the bond conformation as the sample is converted from a highly-ordered crystal to the isotropic melt. In the melt, the 29Si resonance gives the fast exchange-averaged chemical shift for a dynamic equilibrium between different rotional isomeric states of the S i l O and S i ~ C bonds. The fact that the 29Si chemical shift is identical for the melt and the/z-phase, demonstrates a dynamically disordered conformational state also below the isotropization transition. This is confirmed by 29Si spin-lattice relaxation experiments. The value of Ta time is 23 s for the melt at 330 K and 25 s for the /z-phase at 300 K. Thus, the motional state and the conformational equilibrium of the molecular segments remains very much the same. 17.3.2
Polysiloxane comprising rigid segment
It is well known that organosiloxane polymers have an exceptional ability to exhibit and retain superior mechanical properties over an extremely wide temperature range because of their unique combination of high thermal stability and low temperature flexibility. This has been attributed to specific rotation about the S i l O bond [25, 26]. The barrier height for the potential hindering rotation about the S i l O bond is 0.6--~ 1.0 kcal mol -~ [27]. This barrier is small compared with the value of 2.9 kcal mo1-1 obtained for the C ~ C bond in ethane [27]. For this reason, organosiloxane polymers possess good low-temperature flexibility, which is uncommon in hydrocarbon polymers. Nevertheless, they are susceptible to degradation by ionic reactions when exposed to temperatures above 200~ for an extended period of time. This property can significantly reduce their long-term thermal stability and, consequently, narrow the field of possible applications. Thus, in order to improve the thermal stability, while still retaining the desirable low glasstransition temperature of the polymers, previous investigations have been directed toward the preparation of polymers with more thermally stable structure units within a siloxane-based backbone [28]. Recently, the copoly(tetramethyl-p-silphenylenesiloxane/dimethylsiloxane) (CPTMPS/DMS) containing exactly alternating arylenedisilane and siloxane units has been prepared (as discussed in Section 17.3.2.1). As expected, this polymer has a considerably higher glass-transition temperature [29]. Copolymers comprising of rigid and soft segments are an alternative approach to obtaining polymers
654
TOSHIO TAKAYAMA
with specific uses. However, until now, there has been no systematic study of the molecular structure and segmental motion of these novel copolymers in the solid state by using variable-temperature (VT) high resolution solidstate 298i NMR spectroscopy. In previous papers [7, 17, 20], it has been demonstrated that VT 298i CP/MAS NMR spectroscopy is a powerful means for characterizing the structures of solid-state polysilanes and other solid polymers [1, 30]. In this work, high-resolution 298i NMR spectra and spin-lattice relaxation times, T~, of CPTMPS/DMS in the solid state over a wide range of temperatures were measured and the conformational behavior and molecular motion of the main chain discussed. To discuss the experimental results in more detail, calculation of the 298i shielding constants of the main Si atoms by means of FPT CNDO/2 MO framework was attempted. 17.3.2.1 Experimental Materials: Exactly alternating arylenedisilane-siloxane polymer CPTMPS/ DMS was prepared by a method similar to that of Dvornic and Lenz [31]. The basic reaction for the preparation of the alternating arylenedisilanesiloxane polymer is a step-growth polymerization reaction in which a phenylenedisilanol reacts with bis(1,1-tetramethylene-3-phenylureido)dimethylsilane (bisureidosilane) as shown below. High molecular weight polymer fractions were separated from oligomers by repeated precipitation in methanol.The polysiloxane obtained was a white powder melted at 132~ The structural characterization of the polymer was carried out by 1H NMR, 13C NMR and infrared spectroscopy. The X-ray diffraction powder pattern shows that this polysiloxane is amorphous. The molecular weight of the polymer was 2.6 x 104, as determined from gel permeation chromatography (GPC) elution profile (GPC was carried out with a Waters GPC 150-C Module using spectrograde THF as the eluent and polystyrene standards).
NMR measurement: VT 298i CP/MAS NMR spectra were recorded over the temperature range -100 to 80~ using a JEOL GSX270 spectrometer equipped with a VT CP/MAS accessory operating at 53.54 MHz. The other conditions of measurements were similar to that described above. Calculation of 29Si NMR chemical shifts: The calculations were performed, employing the FPT CNDO/2MO method, on two model compounds for the S i l O backbone of CPTMPS/DMS. Compound (a) is 1,1,2,2,3,3-hexamethyl1,3,-diphenyl-trisiloxane ( P h S i M e z ~ O ~ S i M e z ~ O ~ S i M e z P h ) , and compound (b) is 1,4-bis(dimethyl-(trimethylsiloxy)silyl)benzene ( M e 3 S i ~ O ~ SiMez~C6H4~SiMez~O~SiMe3). In these calculations, the values used for
655
I N O R G A N I C POLYMERS
'
Me
9 ~
'
I~
"11" 0
I
Me
"Y
0
~s
TMPS
Me ,
Scheme 1.
, ,
"~
I
~
I
Me
i
- ( - S l - O - S l ~ ~ - o - ) ~ I i
Copoly(tetramethyl-p-silphenylenesiloxane/dimethylsiloxane)(CPTMPS/DMS).
the Si--O, SimC (Me and Ph), C = C (Ph) and C m H (Me and Ph) bondlengths were 1.66, 1.87, 1.40 and 1.09 A, respectively, and the values used for the Si--O--Si, C = C - - C (phenyl ring) and ( H - - C - - H , C ~ S i ~ C ) bond angles were 144, 120 and 109.47 ~ respectively [13]. 17.3.2.2 Results and discussion The 298i CP/MAS NMR spectra of CPTMPS/DMS at various temperatures are shown in Fig. 17.25. At -100~ a sharp intense signal and a broad weak signal, appear at - 0 . 8 ppm (for the TMPS moiety) and at -21.1 ppm (for the DMS moiety), respectively. The peak intensity of the TMPS moiety is almost independent of temperature in the -100 to 80~ range, but that of the DMS moiety gradually decreases with an increase of temperature and, at temperatures above -30~ the signal disappears. This means that the mobility of the DMS moiety is more sensitive to temperature compared with the TMPS moiety within this temperature range. (The CP efficiency largely depends on the mobility.) The 298i GHD/MAS NMR spectra of CPTMPS/DMS at various temperatures are shown in Fig. 17.26. The 298i signal of the TMPS moiety shifts to low frequency (A6 = 0.4 ppm) as the temperature changes from -100 to 80~ However, the 298i signal of the DMS moiety shifts further to low frequency (A6 = 1.1 ppm) during the same temperature change. The observed low frequency shifts seem to be due to changes in the main chain conformation. Besides the TMPS and DMS peaks, some peak broadening occurs (marked by an asterisk). These peaks come from materials used in the NMR probe. In order to understand why the 298i peaks shift to low frequency as the temperature is increased, FPT CNDO/2MO calculations of the 298i chemical shift were carried out. The calculated 298i nuclear shielding of the model
656
TOSHIO T A K A Y A M A
TMPS
DMS
(oc) 80 L _ _
_
27
-30
-4O
-6o
-100
I'1 20
t'Sl
..... I .... 0
I .....
chenJcal
I.... -20
.hlft
I
I .... -40
5 /ppl
Fig. 17.25. VT 298i CP/MAS NMR spectra of CPTMPS/DMS in the solid state at temperatures from -100 to 80~
compound for the DMS moiety using the FPT CNDO/2MO theory are plotted against the dihedral angle 0 in Fig. 17.27, together with the structure of the model compound (with a dihedral angle 0 of 180 ~ relative to the cisconformation). The computations for dihedral angles 0 less than 60 ~ diverge as a result of greater steric hindrance. As shown in Fig. 17.27, the calculated shielding of the central Si atom in the model compound changes passing through the maximum at 0 = 90 and 150 ~ on going from 60 to 180 ~ It can
657
I N O R G A N I C POLYMERS TMPS
(oc)
DMS
80
.@
27
t
|
I
-30
I
-40
-60
-I00
20
2~
0 chGmlcal
-20
-40
shill
5 /ppm
Fig. 17.26. VT 29Si GHD/MAS NMR spectra of CPTMPS/DMS in the solid state at temperatures from -100 to 80~
Background noise from the NMR probe is labeled by (*).
be said that when 0 deviates from 180 ~ (trans form), the signal for the central Si atom moves to low frequency. For the TMPS moiety, the 29Si nuclear shieldings are calculated as a function of 0 using the FPT C N D O / 2 MO method as shown in Fig. 17.28. The calculated 29Si shielding of the arylenedisilane group moves through a maximum as 0 changes from 0 to 180 ~ It can be said that as 0 deviates from 180 ~ (trans form), the 29Si signal moves to low frequency. This is similar
658
TOSHIO T A K A Y A M A
M•M!I.__ 0
-246
.,-I
",-I
0
Mg'd-
-248 BI la,
.el o M al
r
O
-250
b
911
-252 O O
I 60
(gauche)
I
90
I
I
120
dihedral anglo
150
180 (trana)
0 (degree)
Fig. 17.27. The calculated 29Si nuclear shieldings for the central Si atom in the model compound as a function of the dihedral angle 0 by the FPT CNDO/2 method.
to the case of the DMS moiety, but the magnitudes of moving are smaller compared with those of the DMS moiety. From the NMR experimental finding by Moiler [32] that the 29Si signal for the Si atom of poly(dimethylsiloxane) moves to low frequency due to a conformational change from the trans to the gauche form with an increase of temperature, it may be expected that at low temperatures the DMS moiety predominantly takes the trans form and at high temperature it predominantly takes the gauche form. However, the 29Si signal for the TMPS moiety is almost independent of temperature. This experimental result means that the conformational change of the TMPS moiety is constrained. For the above reason, the DMS moiety is called the "soft" moiety and the TMPS moiety the "rigid" moiety. The CPTMPS/DMS copolymer retains both rigid and soft properties over a wide range of temperatures. This experiment is also concerned with the dynamics of the CPTMPS/DMS copolymer through the 29Si T1 experiment [33-36]. It is assumed that 29Si
659
INORGANIC POLYMERS Mex M_d'/ eMe
N ,i.I o U
-
"1= G j.) N
Me Me
..
" ~ I :~-(~--,51_ 1/ Mg.l~v" ~ \O--SINM e
-271
In ,.,.4 It~ o
,~
-27:]
u t~
-275
U nl U
I 30
I 60 (gauohe)
I
I.
I
90
120
150
dlhodral anElo
I (
180 trans
)
0 (do~reo)
Fig. 17.28. The calculated 29Si nuclear shieldings for the silphenylene Si atom in the model compound as a function of the dihedral angle 0 by the FPT CNDO/2 method.
T1 occur values solely by intramolecular dipole-dipole interactions with the attached protons, CH3--Si--CH3. This is undoubtedly true for motions encountered in the copolymer. As determined from Fig. 17.26, the ratio of the peak intensities of the TMPS and DMS moieties is approximately 2:1 at -100~ However, the intensity of the DMS peak is increased as the temperature is increased from - 1 0 0 to -60~ and gradually decreases as the temperature is further increased from - 6 0 to 80~ This means that the 29Si Tx values of CPTMPS/DMS vary greatly with the temperature change. The 29Si T~ values of CPTMPS/DMS in the solid state using the inversion recovery method ((180 ~ ~" - 90 ~ - 150 s) pulse sequence) at - 9 0 , - 6 0 , - 3 0 , 27 and 80~ were measured. Figure 17.29 shows the 29Si G H D / M A S spectra of CPTMPS/DMS at 80~ The magnitude of the two signals recovers from a negative value to the equilibrium positive value through the zero value. As seen from this figure, the TMPS signal recovers faster, exponentially, than that of DMS. The determined values of 29Si T~ for the TMPS and DMS moieties from - 9 0 to 80~ are shown in Table 17.7 and plots of T~ against I/T (K -~) are shown in Fig. 17.30. The T~ value for the TMPS moiety decreases as the temperature is increased from - 9 0 to 80~ This indicates that the molecular motion is in the slow-motion region [22, 37] over this temperature range. However, the T~
660
TOSHIO
TAKAYAMA
THPS
9
'~
DHS
0.3
"
.5
a
n
150 a
o
t'$1
Fig. 17.29.
29Si G H D / M A S
-lo chsllcal
NMR
-20
shift
-30
5
/ppa
spectra of C P T M P S / D M S
in t h e solid s t a t e at 80~
using
t h e i n v e r s i o n r e c o v e r y m e t h o d (180 ~ - r - 90 ~ - 150 s) as a f u n c t i o n of r .
Table 17.7. 298i Ta value(s) for the TMPS and DMS moieties in CPTMPS/DMS in the solid state at various temperatures (~
Temperature
TMPS
DMS
- 90 -60
62 49
15 10
- 30
43
18
27
27
33
80
23
44
value for the DMS moiety is increased as the temperature is increased from - 6 7 to 80~ becomes a minimum at -67~ and is increased again as the temperature is decreased from - 6 7 to -90~ This indicates that the molecular motion is in the extreme narrowing region above -67~ From the experimental T~ values and the single correlation-time model based on BPP theory [22], the correlation time % for molecular motion was calculated [38]. From the Tx minimum, the correlation time ~c for molecular motion at megahertz frequencies is obtained. The plot of log (~'c) vs. 1/T for the DMS moiety in Fig. 17.31 shows a good linear relationship. The temperature dependence of the correlation time ~'c usually obeys the Arrhenius form [39] as ~'c = ~o e x p ( - A E / R T ) , where ~'o is the prefactor, R is the gas constant and T is the absolute temperature. The activation energy AE, which is considered to correspond to the barrier height for the potential hindering rotation, can
661
INORGANIC POLYMERS
L
I
I
70
('C) -30
27
8O
-90
-60 -67
I
I
II THPS
60
5O 0
~0
u 4.)
30 20 I~S
o[,
I
2.5
3.0
I
3.5
I
I
q.O
I
q.5
5.0
I
5.5
103 1 T (K- l )
Fig. 17.30. Temperature dependence of the observed 29Si T1 value in the solid state.
-8.5
-9.0 u w, c~ o
-9.5
-i0.0
2,5
I 3.0
I 3.5
! 11.0
1, q,5
I 5.0
1 5,5
i0 ~ / T (K-l)
Fig. 17.31. Plot of 29Si correlation time for the DMS moiety vs. reciprocal absolute temperature.
662
TOSHIO T A K A Y A M A
022 (711
033 oat
all
O1| -tO
ott
['
'1 ' 40
u
I
30
u"
'1
0
'
i
-20
'
i
-40
'*Ill o / Pl:"*
29SiCP NMR powder pattern spectra of CPTMPS/DMS at -90, 25 The principal values of the shielding tensors ~11, 1~22, ~33 and the anisotropy widths a r e shown.
Fig. 17.32. The observed and ll0~ 0~11-
~331
be estimated from the slope of log (re) vs. 1/T. By this procedure, a AE value of 1.8 kcalmo1-1 is obtained for the DMS moiety. The potential barrier for the hindering rotation in the DMS moiety is only slightly larger than that in poly(dimethylsiloxane) [27], which is itself very small. So, it can be said that the soft properties of the DMS moiety come from these low potential barriers for the hindering rotation. In order to obtain further information about the molecular motion of the siloxane chain, the 29Si powder pattern spectra were recorded [40]. Figure 17.32 shows the 29Si CP NMR powder pattern spectra for the CPTMPS/DMS copolymer obtained without MAS at -90, 25 and ll0~ These powder pattern spectra predominantly come from the TMPS moiety, because the CP efficiency for the DMS moiety is negligibly small when compared with that for the TMPS moiety as seen from the 29Si CP/MAS NMR experiments. The anisotropy widths 10-11- O'331 obtained from the tent-like powder pattern spectrum are 57.8, 55.0 and 52.0ppm at -90, 25 and ll0~ respectively. The anisotropy width decreases as the temperature is increased. This indicates
INORGANIC POLYMERS
663
that the mobility for the TMPS moiety is slightly increased as the temperature is increased. Above all, the DMS moiety predominantly takes the trans form at low temperatures and the gauche form at high temperatures, whereas the conformation of the TMPS moiety is almost independent of temperature. The molecular motion of the DMS moiety is fast, while that of the TMPS moiety is restricted. This indicates that CPTMPS/DMS retains both rigid and soft parts in the main chain.
17.4
17.4.1
Miscellaneous
Polyphosphazene
Phase transitions in thermotropic polyphosphazenes have created considerable interest in recent years and many experimental techniques have been used to monitor them. Recently, Young and coworkers [41] showed the phase transitions in polyphosphazenes using solid-state 31p and I3C NMR. Several polyphosphazenes with phenyl and halogenated phenyl groups have been synthesized, purified and characterized. A series of MAS NMR measurements and results show that this technique is useful for the study of in situ molecular chain dynamics, chain conformation and specimen crystallinity of polyphosphazenes. Often, an ordered (crystalline) and disordered (amorphous) contribution are exhibited by the chain backbone below the thermotropic transition (T(1)), whereas, above this temperature a single highly mobile two-dimensional phase exists in an expanded state depicted by dilatometry measurements. 31p MAS NMR results for the halophenoxyphosphazenes, poly[bis(3-bromo-phenoxy)phosphazene] (PB(3-Br)PP) support the idea of two resonances below T(1), one belonging to the amorphous (disordered) regions and the other to the crystalline (ordered) regions of the polymers. A single broad peak is noted at 27~ As the polymer is heated, a peak appears at - 2 9 ppm, which corresponds to the T(1) mesophase and two distinct peaks also coexist above this transition. Upon cooling the sample in the spectrometer, the - 2 9 ppm resonance decreases and then disappears reversibly at 20~ This behavior contracts with the results of other polyphosphazenes and may be attributed to steric interactions involving the polar msubstituted bromine atom on the phenyl ring. The 13C CP/MAS spectra of PB(3-Br)PP the same polymer taken at 25 and 100~ At 25~ two broad resonances are present, but at 100~ one disappears and the other splits into a doublet. This feature is probably an artifact of CP since this technique is designed for rigid systems; PB(3-Br)PP is not a rigid system because of its
664
TOSHIO T A K A Y A M A
very low crystallinity. It seems that the bromine-substituted carbon is extremely mobile below and above T(1), so that the 13C resonances cannot be observed due to a time-averaged superposition. 17.4.2
Compositepolymer
Spiese and coworkers [42] used solid-state NMR techniques to study the microphase structure of poly(styrene-b-methylphenylsiloxane) (PS-bPMPSO) diblock copolymers with different compositions and molecular weights and are able to detect heterogeneities on a sacle of _->2nm in systems that exhibit only a single Tg in the DSC measurement. The microphase structure of block copolymers is a research subject of both scientific and practical importance. NMR signals of solid samples are broadened by dipoledipole interactions and anisotropic chemical shifts. These may be partially averaged by various kinds of molecular motions. The NMR lineshape then contains useful information about the structure and dynamics of the molecules in the solid. Of particular interest in block copolymers is the interaction between the different components, especially the interfacial region. The higher the mixing level, the stronger is the interaction between the components resulting in a greater modification of the NMR spectrum relative to that of the pure components. Therefore, the deviation of the spectrum of the copolymer from the superposition pattern of the constituent homopolymers allows a direct judgment of the phase structure. The detailed determination of structural heterogeneity and domain dimension is based mainly on observing the time dependence of nuclear magnetization transfer from one distinct part of the spin system to another by spin diffusion. This magnetization transport is achieved by the magnetic dipoledipole interaction. However, the nuclear environment can be probed down to a scale of <1 nm. On the other hand, the magnetization can migrate over a significant distance, so that domain dimensions over a range of tens of nanometers can be detected. Indeed, various spin diffusion techniques have been used to probe chemical or physical heterogeneities in polymer systems.
17.S
Conclusion
Solid-state multinuclear NMR experiments have successfully provided very useful information about inorganic-polymer conformations. The relationship between the solid-state conformations and NMR chemical shifts is of widespread interest. In this work, the structural behaviors of several substituted polysilanes in inorganic polymers were studied by means of 29Si CP/MAS
INORGANIC POLYMERS
665
N M R spectroscopy over a wide range of t e m p e r a t u r e s . T h e dynamics experim e n t s of these polysilanes were also u n d e r t a k e n t h r o u g h 29Si~T1 m e a s u r e ments. In o r d e r to consider the relationship b e t w e e n the silicon chemical shift and silicon b o n d c o n f o r m a t i o n , 29Si-chemical shifts were calculated by the F P T C N D O / 2 M O m e t h o d . It was shown that V T 29Si C P / M A S N M R chemical shifts (6), 29Si-relaxation times (T1) and calculated 29Si-shielding constants (tr) are very useful in studying the solid-state o r d e r - d i s o r d e r transitions in the polysilanes.
Acknowledgments T h e a u t h o r is deeply i n d e b t e d to Professor I. A n d o , T o k y o Institute of T e c h n o l o g y , for his helpful discussions on this work. T h e a u t h o r also gratefully acknowledges two financial supports from G r a n t - i n Aid for Scientific R e s e a r c h (C) of the Ministry of E d u c a t i o n , Science and Culture, Japan.
References 1. (a) R.A. Komoroski (Ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk. VCH, Deerfield Beach, FL, 1986; (b) H. Saito and I.Ando, Annu. Rep. NMR Spectrosc. 21 (1989) 209. 2. F.C. Schilling, A.J. Lovinger, F.A. Bovey and J.M. Zeigler, Adv. Chem.Ser. 224 (1990) 342. 3. (a) J.F. Rabolt, D. Hofer, R.D. Miller and G.N. Fickes, Macromolecules 19 (1986) 611; (b) F.C. Schilling, F.A. Bovey, A.J. Lovinger and J.M. Ziegler, Macromolecules 19 (1986) 2660; (c) R.D. Miller, B.L. Farmer, W. Fleming, R. Sooriyakumaran and J. Rabolt, J. Am. Chem. Soc. 109 (1987) 250; (d) B.L. Farmer, R.D. Miller, J.F. Rabolt, W.W. Fleming and G.N. Fickes, Bull. Am. Phys. Soc. 33 (1988) 657; (e)F.C. Schilling, A.J. Lovinger, D.D. Davis, F.A. Bovey and J.M. Zeigler, Macromolecules 25 (1992) 2854. 4. R.K. Harris, K. Metcalfe and E. Hengge, Polyhedron 4 (1985) 1319. 5. F.C. Schilling, F.A. Bovey, A.J. Lovinger and J.M. Zeigler, Macromolecules 19 (1986) 2663. 6. G.C. Gobbi, W.W. Fleming, R. Sooriyakumaram and R.D. Miller, J. Am. Chem. Soc. 108 (1986) 5624. T. Takayama, S. Ando and I. Ando, J. Mol. Struct. 220 (1990) 243. 8. R.E. Trujillo, J. Organomet. Chem., 198 (1980) C27. 9. J.R. Damewood, Jr., R. West, Macromolecules 18 (1985) 159. 10. I. Ando A. Nishioka and M. Kondo, J. Magn. Reson. 21 (1976) 429. 11. I. Ando and G.A. Webb, Org. Magn. Reson. 15 (1981) 111. 12. T. Takayama and I. Ando, Bull. Chem. Soc. Jpn. 60 (1987) 3125; 62 (1989) 1233. 13. L.E. Sutton (Ed), Tables of Interatomic Distances and Configuration in Molecules and Ions, Chem. Soc. Spec. Publ. No.11. The Chemical Society, London, 1958. ~
666 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.
41. 42.
TOSHIO TAKAYAMA
J.R. Damewood, Jr., Macromolecules 18 (1985) 1793. F.C. Schilling, F.A. Bovey and J.M. Zeigler, Macromolecules 19 (1986) 2309. C.E. Johnson, Jr. and F.A. Bovey, J. Chem. Phys. 29 (1958) 1012. T. Takayama and I. Ando, J. Mol. Struct. 222 (1990) 275. X.H. Zhang and R. West, J. Polym. Sci. Polym. Chem. Ed. 22 (1984) 159. P. Trefonas III, P.I. Djurovich, X.H. Zhang, R. West, R.D. Miller and D. Hofer, J. Polym. Sci. Polym. Lett. Ed. 21 (1983) 819. T. Takayama and I. Ando, J. Mole. Struct. 243 (1991) 101. A.J. Lovinger, D.D. Davis, F.C. Schilling, F.J. Padden, Jr. and F.A. Bovey, Macromolecules 24 (1991) 132. N. Bloembergen, E.M. Purcell and R.U. Pound, Phys. Rev. 73 (1948) 679. I. Ando, T. Yamanobe, S. Akiyama, T. Komoto, H. Sato, T. Fujito, K. Deguchi and M. Imanari, Solid State Commun. 62 (1987) 785. G. Koegler, A. Hasenhindl and M. Moeller, Macromolecules 22 (1989) 4190. M.G. Voronkov (Ed), The Siloxane Bond. Consultants Bureau, New York, 1978. F.O. Stark (Ed), Comprehensive Organometallic Chemistry, Vol.2. p. 305. Pergamon, London, 1982. J.E. Mark and P.J. Flory, J. Am. Chem. Soc. 86 (1964) 138. (a) J.E. Mark and J.H. Ko, J. Polym. Sci., Polym. Phys. Ed. 13 (1975) 2221; (b) N. Okui, H.M. Liand, J.H. Magill, Polymer 19 (1978) 411; (c) J.H. Magill and H.M. Li, Polymer 19 (1978) 416. L.W. Breed, R.L. Blliott and M.E. Whitehead, J. Polym. Sci. Part A 5 (1967) 2745. I. Ando, T. Yamanobe and T. Asakura, Prog. NMR Spectrosc. 22 (1990) 349. P.R. Dvornic and R.W. Lenz, J. Polym. Sci., Polym. Chem. Ed. 20 (1982) 593,951, 2277. M. Moller, Adv. Polym. Sci. 66 (1985) 59. R.K. Harris and B.J. Kimber, J. Magn. Reson. 17 (1975) 174. F. Heatley and A. Begun, Polymer 17 (1976) 399. B. Mohanty, J. Watanabe, I. Ando and K. Sato, Macromolecules 23 (1990) 4908. Q. Chen, T. Yamada, H. Kurosu, I. Ando, T. Shiono and Y. Doi, J. Polym. Sci., Polym. Phys. Ed. 91 (1992) 591. D. Doddrell, V. Glushko and A. Allerhand, J. Chem. Phys. 56 (1972) 3683. F.A. Bovey and L.W. Jelinski, J. Phys. Chem. 89 (1985) 571. A.A. Jones, in R.A. Komoroski (Ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, Chap. 7, p. 260. VCH, Deerfield Beach, FL, 1986. (a) G. Kogler, A. Hasenhindl and M. Moller, Macromolecules 22 (1989) 4190; (b) V.M. Litvinov, A.K. Whittaker, A. Hagemeyer and H.W. Spiess, Colloid Polym. Sci. 267 (1989) 681. S.G. Young, M. Kojima, J.H. Magill and F.T. Lin, Polymer 33 (1992) 3215. W.Z. Cai, K. Schmidt-Rohr, N. Egger, B. Gerharz and H.W. Spiese, Polymer 34 (1993) 267.
Chapter 18
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Fluoropolymers Robin K. Harris x, Gustavo A. Monti ~ and Peter Holstein 2 ~Department of Chemistry, University of Durham, UK; and 21nstitutfar Experimentelle Physik I, Universitil't Leipzig, Germany
18.1
Introduction
There are a number of commercially-important polymers containing fluorine, notably: (a) Poly(tetrafluoroethylene), (CF2CF2)n, PTFE -- sometimes known by the tradename Teflon. (b) Poly(trifluoroethylene), (CFzCFH)n, PTrFE. (c) Poly(vinylidene fluoride), (CFzCHz)n, PVDF (also referred to as PVF2). (d) Poly(chlorotrifluoroethylene), (CFzCFC1)n, PCTFE--known by its tradename kel-F. (e) Copolymers of vinylidene fluoride and perfluoropropene, (CHzCF2)n (CF3CFCFz)m--known by the tradename Viton. Another version of Viton is a terpolymer with tetrafluoroethylene. After a short section mentioning NMR techniques that are particularly appropriate for solid fluoropolymers, there will be separate sections below on different classes of these materials, commencing with homopolymers.
18.2
Techniques
It is obvious that the special characteristic of fluoropolymers for NMR is the incorporation of the 19F nucleus, which is in many ways particularly suitable for study and can act as a unique probe to examine the chemical microstructure, domain structure and mobility at the molecular level for these materials. However, some special techniques are frequently required for the best results from 19F NMR of solid polymers. These are fully described in Section 6.6. The net result of the difficulties encountered in obtaining high-resolution solid-state 19F spectra is that relatively few research papers to date have reported spectra with optimised resolution. Therefore, most reported 19F
668
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
work on fluoropolymers has either consisted of broadline/relaxation studies or has involved spectra of systems with sufficient mobility that resonances are naturally significantly narrowed (e.g., obtained well above glass transition temperatures) -- thus obviating any requirement for CRAMPS or for highpower proton decoupling. This situation is now changing rapidly and a wider variety of systems is being examined as the realisation dawns that the "special techniques" are feasible. Most homopolymers containing fluorine have a very limited range of other elements present, thus restricting the NMR nuclei that can be accessed. Indeed, PTFE has only carbon and kel-F has only carbon plus chlorine (the latter usually not being useful for NMR). Accessing 13C spectra for these systems would require either direct polarisation or cross polarisation from fluorine. Neither method has proved popular in this area. PVDF and copolymers such as Viton also contain protons, which constitute both a bane and a blessing--the latter because additional experiments become possible, and the former because to take optimum advantage of NMR then requires tripleresonance (1H, 19F, 13C) probes and triple-channel spectrometers. Although 13C, 19F, 1H triple-channel experiments have been performed on special probes since the early 1980s (see Ref. 1, and references therein), it is only relatively recently that reliable commercial probes (and spectrometers!) capable of such studies have become available. However, Veeman and coworkers [2] achieved significant success in the early 1990s with a probe modified inhouse and have published a series of papers on copolymers and blends involving PVDF (see Section 18.5). Given the requisite equipment, experiments can be devised to distinguish various dipolar interactions. Some suitable pulse sequences are illustrated in Fig. 18.1. These produce two-dimensional spectra to give DCF, Dciq or Dcv + Dcia depending on the decoupling regime during tl. Discriminating experiments for 13C spectra of the homopolymers usually take the form of distinguishing subspectra of crystalline and amorphous regions via the relaxation properties of the proton or fluorine spin baths. In block copolymers, such experiments can be valuable in understanding domain structures. However, spin diffusion among protons or among fluorine nuclei becomes of substantial importance. In fact, all the sophisticated techniques commonly used for 13C studies of polymers are feasible for fluoropolymers, but they usually require a third channel for decoupling the "unwanted" nucleus during appropriate time periods. For a discussion of double CP (19F ~ 1H ~ 13C) and cross-polarisation experiments, see Section 18.5. The fluoropolymers which contain protons also lend themselves to study by deuterium NMR, following isotopic enrichment. Use of other nuclei, such
FLUOROPOLYMERS
669
tH DD
DD
rd2 19F
13C
CT
-~
n/Vr
J--b"[--] -~m
n/Vr
r
VVV~'--
refocus
tH DD
rd2 t9F DD
DD
~3C
CT
I~
n/Vr
n/Vr v
~ rl--,
"
r
AA^^,, ACQ A. . . . . . .
VVv"-
refocus
1H DD
~/2 19F
13c ! ]~cT ,,,, n~, ~M .-9 <
./Vr
r-
IAAA^ Aco . . . . . . . .
refocus
Fig. 18.1. Pulse sequences for obtaining two-dimensional spectra to give information on dipolar coupling strengths for fluoro-polymers from 13CNMR. Top" for DCF. Middle" for DCH. Bottom: for DCF + Dci-i. Rotor synchronisation is essential (Vr = rotation rate)
670
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
as 170, 15N or (e.g., in Nation membrane systems) 23Na or 7Li, is relatively rare. The high magnetogyric ratio of 19F gives rise to large dipolar coupling constants. This fact can be put to good use to measure interatomic distances if molecules containing single fluorine atoms are isotopically-enriched in a site-specific manner by 13C. Schaefer and coworkers [3] have used this principle to obtain (C,F) distances of ca. 12 A in dendrimer systems.
18.3
Perfluorinated homopolymers
Most N M R work on fluoropolymers not containing hydrogen has concentrated on poly(tetrafluoroethylene) (PTFE) because of its high commercial importance. PTFE is a very unusual solid material in that its bonds are among the strongest known (the C - - F bond dissociation energy in PTFE is 480 kJ mo1-1) while its weak ones are among the weakest of such interactions [4,5] ( - - C F 2 - - interchain van der Waals' forces are only 3.2 kJ mol-1). PTFE is also unusual from the point of view of the arrangement of atoms in the polymer chain. The molecules of PTFE are unbranched and generally very long. The CF2 groups are equally spaced along the polymer chain; the conformation in the crystalline domains is a zigzag of carbon atoms with the whole chain twisted into a helix. The helical structure arises because fluorine atoms are too large to allow a planar zigzag conformation, so the carbon chain twists to accommodate the steric interaction [6,7]. The crystallinites of PTFE estimated by X-ray methods range from 90% for a powder as produced in the polymerisation reaction, to ca. 75% for melted and slowly cooled material, to ca. 50% for melted and rapidly cooled material. It has been established [8] that rapidly cooled PTFE, although lower in crystallinity, possesses the same molecular conformation and basic crystalline structure as does slowly cooled PTFE. PTFE has outstanding thermal stability so its upper temperature limit for practical use is determined by mechanical properties. The usually quoted upper use-temperature is 260~ At the lower end of the temperature scale, PTFE is equally remarkable. Elongation capability decreases, as expected, but not to a state of glassy brittleness so typical of most other polymers. The complex morphology of PTFE is one of the topics of interest. There are at least two crystalline phase transitions (in addition to fusion), plus amorphous changes. Since 1953, 19F NMR results have been used [9] to discuss crystallinity. Much of the early work was summarised and supplemented by McCall et al. [10], who reported the measurement of T1, T2 and Tio for 19F of static samples as a function of temperature, over the maximum range - 1 5 0 to +200~ The F I D data were fitted to sums of
FLUOROPOLYMERS
671
Lorentzian and Gaussian contributions, assumed to represent amorphous and crystalline fractions, respectively. A sharp change in T2 for the latter at ca. 293 K was associated with a crystalline phase transition (triclinic to hexagonal). Minima in T~ and Txp were discussed in relation to this and other transitions. McBrierty et al. [11] extended such work using highly oriented PTFE fibres, measuring spin-spin relaxation characteristics as a function of both temperature and orientation of the fibres in the magnetic field. The results confirm the onset of chain rotation at the 20 ~ crystalline transition, and support the view that longitudinal chain translation sets in for the crystalline region at, or near, the 35 ~ transition. Questions of crystallinity and motion at the molecular level have been further investigated by English and Vega [12-14] using simplified 19F spectra obtained from the multiple-pulse technique (REV8) at various temperatures. They did not use MAS but deconvoluted the bands (e.g., at 259 K--see Fig. 18.2) into an isotropic line (amorphous) and an axially symmetric powder pattern (crystalline). Changes in the shapes with temperature (Fig. 18.3) and processing allowed the authors to observe both the /3- and ~/-polymer
f Fig. 18.2. Fluorine-19 REV8 spectra of three samples of PTFE of differing crystallinity at
259~ Top: low crystallinity (--~50%). Middle: medium crystallinity (---70%). Bottom: high crystallinity (-95%). The bandshapes may be viewed as a convolution of a crystalline contribution (axially symmetric) and an amorphous contribution (single line). [Spectra taken with permission from Ref. 12.]
672
R . K . HARRIS, G. A. MONTI AND P. HOLSTEIN
23*
100 Hz
14"
50Hz
Fig. 18.3. Left: fluorine-19 REV8 bandshapes for the crystalline component of PTFE between 14 and 28~ Right: calculated bandshapes assuming rotational diffusion about the chain axis, with the indicated diffusion coefficients. [Spectra taken with permission from Ref. 14.]
relaxations, the latter shown to be due to reorientation about a local chain axis in the amorphous regions (not previously reported by NMR). The bandshape TREV results were supplemented by TI, Tlo and -lXZ measurements. Solid-echo 19F NMR FID measurements have also been used [15] (in conjunction with other techniques) to determine the crystallinity of melt-quenched PTFE samples of different molecular weights. Recently, factor analysis has been applied [16] to 19F time-domain data, obtained using MAS, for a substantial number of PTFE samples, and the relationship of the factors to crystallinity was discussed. Several reports [12-14,17-19] provide data on 19F shielding tensor components for PTFE, obtained from multiple-pulse methods. These have been summarised by McBrierty and Packer [20]. The values reported lie in the chemical shift ranges (in order from highest to lowest) ~11 = - - 2 7 to - 5 3 ppm, 622 = -141 to -156 ppm, ~33 = -156 to -205 ppm, with 622 = 633 at
FLUOROPOLYMERS
!
0
i
i
|
673
i
-I00 -200 (~F/PPm
Fig. 18.4. Fluorine-19 CRAMP spectra of short- and long-chain PTFE: (a) C2oF42 (MAS rate 5.1 kHz); (b) highly crystalline PTFE (2.5 kHz). Arrows indicate resonances from CF3CF2 end-groups in (a). [Spectra taken with permission from Ref. 22.]
high temperature [13]. Using bundles of drawn PTFE fibres, Garroway et al. [19] showed that, at 77 K, the most shielded tensor component lies along the C - - F bond, while the least shielded component is aligned ca. 20 ~ to the molecular chain axis. Fluorine-19 CRAMP spectra have been briefly reported [21,22] for shortand long-chain PTFE, with resolved peaks obtained for CF3 and CF2 endgroups of the former (a sample of perfluoroeicosane, C2oF42) (see Fig. 18.4). Katoh et al. [23] showed that MAS rates of over 12 kHz were required to obtain sufficiently narrowed lines for C2oF42, whereas, 5.1 kHz was found earlier [21,22] to suffice when CRAMPS is used. "Oligomers" (Mw ~ 700-11,500) of PTFE, obtained by reaction of longchain PTFE with fluorine gas, were studied [23] by 19F MAS NMR (but without multiple-pulse operation) in some detail, and the various signals assigned to a range of fluorine environments. It was shown that CF3 endgroups affect the surface properties of PTFE oligomers. The authors used n-C2oF42 as a standard for comparison of the MAS spectra. More sophisticated ~9F NMR techniques are beginning to be applied to fluoropolymers. For instance, the multiple-quantum (MQ) method has been used [24] for 19F in a study of polymer dynamics in static samples of PTFE
674
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN
10 20 30
40 50
60
70 80 90
Fig. 18.5. Fluorine-19 MREV8 spectra of static PTFE samples: (left) under tension (draw ratio 1.40); and (right) under compression (draw ratio 0.82), with (top) the draw axis parallel to the magnetic field, and (bottom) the draw axis perpendicular to the magnetic field. [Spectra taken with permission from Ref. 28.]
as a function of temperature. The correlated behaviour of multiple nuclei interacting through dipolar coupling is monitored. Although the results were not fully understood, the experiments were able to differentiate between samples with various thermal histories. A series of articles by Brandolini, Dybowski and co-workers [25-29] reported the use of 19F NMR for static samples of PTFE to obtain information on orientation distribution of crystallites following deformation by tensile stress. The spectra were obtained using the MREV8 pulse sequence to remove homonuclear dipolar coupling so as to observe chemical shift dispersion as a function both of the draw ratio and of the angle between the draw direction and applied magnetic field (Fig. 18.5). The moments of the spectra were used [25-28] to determine the order parameters of the samples up to (Ps(cos 0)). The orientation distribution functions were then calculated
675
FLUOROPOLYMERS cO
o.. f 0 e
g
.~
d"
',. "~
",,\ \ \
.~.
\
\,'~
._
....
...-"o,..,"
oo..
C
.o~176 o.~176176176176176176 ~
..,. . . . . . . . . .
0
~'--.~. ~,\ \~ ~
"-... "~
~,,~
~,.
- - , . ~ . # . . . ~ . , ~--=~"~-.~ . . . . . .
3'0
!
8
60
!
90
Fig. 18.6. Orientation distribution functions N(0)) for PTFE samples of draw ratios: (a) 0.82; (b) 1.00; (c) 1.40; (d) 1.75; (e) 2.10; and (f) 2.40. [Taken with permission from Ref. 28.]
(Fig.18.6) and interpreted [28] in terms of the mechanical properties of the polymer. The disorientation during annealing at 300~ was followed [29]. Even after prolonged annealing, totally random orientation was not achieved. The effects of 7-rays on PTFE have been monitored [30] using broadline 19F NMR (in association with ESR). As expected, second moments increase with irradiation. Fluorine-19 imaging has been applied [31] to PTFE. Large oscillating magnetic-field gradients were necessary to provide adequate spatial resolution because of the short values of T2. Carbon-13 spectra, obtained with MAS, CP from 19F and high-power 19F decoupling, have been reported [32,33] for PTFE as a function of decoupler power and temperature. A single peak was observed, as expected. The authors stressed the requirement for even higher decoupling powers for 19F than for 1H in corresponding systems because of the high chemical-shift dispersion in the former. A broadening of the signal above ca. 20~ was attributed to a first-order transition associated with the onset of molecular motion about the long axis of the helical chain in crystalline domains of the polymer (see above). Much less NMR work has been carried out on solid perfluorinated poly-
676
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
mers other than PTFE. Poly(chlorotrifluoroethylene), (CFC1CF2)n,has been studied [34,35] by 19F transverse relaxation. The results have been discussed in terms of a model involving flexible polymer chain ends (cilia) characterised by bend and twist between segments. However, the model is not very successful for PCTFE (in contrast to PE and PVDF). The authors suggest consideration of polymer folds is necessary for PCTFE. As mentioned in Section 6.6, Kel-F, (CFC1CF2)n, was chosen as an example of the first use of CRAMPS [36] and high-speed MAS [37] for 19F spectra of solids. Kel-F was also studied using 13C-{19F} CPMAS techniques by Fleming et al. [32,33]. Two peaks were seen at room temperature, the resolution being insufficient to resolve tacticity effects. A broadening of the CFC1 signal at low temperatures was attributed to second-order interactions with the quadrupolar chlorine nuclei. Magic-angle spinning was shown to be a viable technique, without multiple-pulse homonuclear decoupling, for several perfluoropolymers with highly mobile chains. Therefore, for a system with long chains of detailed fine structure enables polymer-sequencing information to be obtained [38]. Fast MAS has allowed polymeric material produced from a hexafluoropropene plasma to be studied [39], though the spectral detail is not highly developed. Fluorine-19 CRAMPS has been reported [21,22] for some novel polymer samples of composition --(C(CF3)=e(eF3))n--.
18.4
Homopolymers containing both fluorine and hydrogen
Among polymers, poly(vinylidene fluoride), PVDF, (see Ref. 40 for a simple review) exhibits some peculiarities arising mainly from the extremely strong piezoelectricity and pyroelectricity as first described by Kawai [41]. These outstanding electric properties make PVDF suitable for a variety of applications. Parallel to the large number of papers in the scientific and engineering literature presenting the complexity of the properties and their potential for application, several attempts have been made to obtain relevant information from wideline NMR spectra (using both continuous-wave and pulsed radiation). PVDF is semicrystalline. The glass transition temperature is ca. -35~ The crystalline lamellae generally represent about half of the material. Several crystalline phases have been reported. The structures of the a (nonpolar) and/3 (polar) modifications are shown in Fig. 18.7. The most common technique for obtaining macroscopically polar films combines stretching and poling. Drawing induces a change of the originally spherulitic structure to an arrangement of crystallites whose molecules are
677
FLUOROPOLYMERS H
H
a -phase .....
.,
.< H
H
b=964
A
H
H
B-phase
1
(3) ,,r II ..el
a=8.58 A Fig. 18. 7. Unit cell contents of the c~-phase (top) and/3-phase (bottom) of crystalline PVDF. The chain conformations are tg § and all-trans, respectively. Note the complete cancellation of C--F dipoles between the two chains for the a form but the reinforcement of dipoles for the/3 form.
oriented in the direction of the mechanic treatment. The orientation process can be accompanied by a change of the chain conformation, depending on the temperature. Stretching at high temperatures (ca. 150~ does not change the original tg*tg - conformation. That means the crystal structure is still nonpolar (a-modification). Stretching forces the molecules into the most extended chain conformation (tttt) at ca. 80 to 100~ (see Fig. 18.8). Thus, stretching at lower temperatures produces two effects: (i) a highly-oriented chain structure; and (ii) a change from the nonpolar a-phase to the polar/3phase. An electric field can be applied to achieve a net dipole moment (Fig. 18.8(c)). The molecular dipoles tend to orient into the direction of such a field. Alpha crystallites can also be oriented in electric fields. Poling in intermediate fields (1-3 MV/cm) does not change the chain conformation but alters the unit cell arrangement (the so-called polar a or 6 form). The combination with semicrystallinity leads to complexity for many properties of PVDF. PVDF has been investigated since the early days of solid-state NMR.
678
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN Melt solidified PVF 2
~~~
,~
Spherulites
of the nonpolar a-phase
n
Mechanical extension (~ 300%)
g
Uniaxially oriented PVF 2
' " ~ - ~ ~i7 ~ R -.
t
--~''":~~ Oriented /~ ~/...-.~--~. molecules ' of the polar ....~ " B-phase andom dipole directions
Electrical
poling
Polarized PVF2 film /(~Electric field dir. A .... O) Stretch dir. .ur~en ! eo,, / _,Z---'-_ ,,,o,o~ Z ~ Transverse dir. ~ / " - o f the polar ~--T ~ ~, ~ ~:-~ ~ " J " 8-phase ~" Dipoles normal to film Fig. 18.8. Production of piezoelectrically active (polar) films of PVDF by stretching and poling. [Figure reproduced with permission from Ref. 40.]
Proton and fluorine NMR wideline techniques (linewidths, second moments and relaxation times) played an important role in revealing dynamic properties of the semicrystalline polymer. The temperature dependence of the second moment of PVDF was first investigated by Slichter [42] as part of a study of polyethylene and its fluoroderivatives (PE, PVF, PVDF, PtrFE and PTFE). Investigations of PVDF were often directed to understanding the anisotropic molecular properties of the material in film form. For example, Lando et al. [43] measured the dependence of the second moment as a function of the angle between the magnetic field Bo and the draw direction for film material at liquid nitrogen temperature, which is far below the glass transition temperature. By comparison with the theoretically predicted angular dependency of M2, the orientational behaviour could be described using a two-component model. The orientation, due to stretching, occurred only within the crystalline regions (at least up to modest stretching ratios). The crystallinity was estimated to be about 50%, which is in accordance with X-ray results.
FLUOROPOLYMERS
679
Sasabe et al. [44] used a combination of dielectric spectroscopy and wideline NMR to identify molecular motions and relaxational processes in the crystalline regions of PVDF. A similar study was carried out by Kakutani [45], who compared stretched and unstretched material. Douglass et al. [35] made model calculations of the effect of molecular motions of defects on the surfaces of crystallites on relaxation times. The same group [46] used pulsed NMR (at 30 MHz) to measure different 1H and 19F NMR relaxation times (T1, Tlo, T2). Four relaxational processes were found (3/, /3, /3', a) which describe the complex semicrystalline nature of the polymer. Problems which arise from the strong dipolar coupling between protons and fluorine nuclei have been discussed. In a review on polymer NMR, McBrierty and Douglass [47] used PVDF as a typical example for the discussion of properties of semicrystalline polymers. The relaxation processes 3/,/3,/3' and a (from lower to higher temperatures) have been interpreted. The relaxations denoted as 3/,/3 and/3' are referred to motions in the amorphous phases whereas the relaxation is assigned to chain motion in the crystallites. The 3/relaxation is caused by the onset of chain rotations in the amorphous regions. It leads to a small step in the temperature dependence of T 2 which is also weakly reflected in Tlo. The/3 process is the micro-Brownian motion in the amorphous phase (glass transition). This process is the dominating feature in the T2 and Tlo curves. The so-called /3' transition which has been found in some dielectric investigations [45] is only weakly indicated in the wideline investigations. It could be caused by chains on the surfaces of crystalline lamellae (ca. 20% of the material) which are often discussed as forming an intermediate phase. Obviously, head-to-head or tail-to-tail polymerisations strongly influence the c~-relaxation, as proved by the addition of a small amount of HFP (hexafluoropropylene), which artificially increases the defect concentration. McBrierty and Douglass [48] have used transient Overhauser measurements to obtain information about cross relaxation between the 1H and 19F spin systems in static PVDF powder over the temperature range -196 to + 160~ The authors comment that the experiment provides good resolution of T1 components, and they were able to observe two minima, at 25 and 110~ where other measurements were unable to detect adequately the latter (which probably relates to/3 relaxation). McGarvey and Schlick [49] observed (by both 1H and 19F NMR) a twocomponent behaviour of the longitudinal relaxation in PVDF material which contains the modification a in its crystalline phase. The results have been interpreted by incorporating cross-relaxation effects. The investigation of orientational effects arising from drawing is one of the main applications for NMR wideline measurements of PVDF. Thus, Clements et al. [50] used a two-component model for the deconvolution of
680
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
the 1H wideline spectrum for drawn material containing crystalline (form/3) and amorphous domains. This model also described the decrease of the degree of crystallinity at increased temperatures. The effects of the application of tensile stress on the rigid fraction (degree of crystallinity) have been investigated "in situ" by Clements et al. [51]. The application of stress parallel to the draw direction provides a reversible increase of the rigid fraction, but this fraction is decreased if the stress is applied perpendicular to the draw direction. The influence of 7-radiation on structure and molecular mobility has been discussed by Temnikov et al. [52] from pulsed 1H NMR results, using a threecomponent model for T2. Attempts have been made to correlate the outstanding electric properties (piezoelectricity, pyroelectricity) of PVDF with parameters obtained from wideline NMR experiments. Samples processed by different electric methods have been included in NMR experiments. Wideline NMR studies of irreversible effects induced by relatively high static electric fields have been reported by several authors. The results of wideline N M R (1H) have been used [53] in a combined investigation with a piezoelectric resonance method to find any effects of electric poling (0.78 MV/cm at 120~ on structure and/or orientation. However, no indications of structural or orientational changes have been found. In a series of publications by Ishii et al. [54-57], effects of the electric field on structural changes in the amorphous regions, accompanied by an additional relaxation process, were discussed. These effects are reflected in the angular dependence of the second moment at different temperatures. The separation of any orientational effects due to poling from stretching effects were made by the preparation of different sample types. The complications for such a separation arose from the facts that (i) mechanically induced effects on chain orientations are much larger than that of the (electric) dipole reorientation and (ii) after poling only a small irreversible electric polarisation remains. Investigations of poling effects in the crystalline regions by 60 MHz pulsed 1H wideline NMR have been presented by McBrierty and co-workers [58,59]. The angular dependence of the second moment was discussed in terms of a series of Legendre polynomials for the mathematical description of the orientation. The experiments were carried out using a spin-locking sequence to suppress the signal of the amorphous component, as proposed by Bergmann [60] for semicrystalline polyethylene. In contrast to the results of Ishii et al. [54], the poling effect occurred only in the crystalline regions of the film. The orientational parameter obtained by static 1H NMR could be correlated with the piezoelectric coefficient d31.
FLUOROPOLYMERS
681
Similar results have been obtained by Holstein, Geschke and coworkers [61-64] by the application of wideline and multiple-pulse methods (T2, T2eff) on poled PVDF samples. The incorporation of a direct observation (in situ) of electric field effects revealed that the reversible part of the polarisation is much larger than the irreversible one which is used in applications. There is also a reversible increase in the degree of crystallinity during the field application. The high voltage (several kV) was directly applied via electrodes on the sample in the NMR probe. Fluorine-19 multiple-pulse investigations of poled PVDF films have been carried out by Frigge et al. [65], leading to similar results, which were complementary to the proton work [61]. Of course, 19F spectroscopy is a more direct approach than ~H NMR for the observation of electrically influenced CF2-dipoles. Doverspike et al. [66] reported a deuteron NMR orientational lineshape study for perdeuterated (99%) PVDF. The experiments examined only the crystalline part of the drawn and poled samples. The maximum remanent polarisation attained with deuterated PVDF homopolymer film was 1.0/.~C/cm 2. A Gaussian distribution about the draw direction characterises the chain axis reorientation in the stretched samples. The PVDF (draw ratio 3) has a Gaussian distribution of width 220 (half-width at e -~ of maximum). Angular dependence of the magnetisation for the poled PVDF homopolymer films was not expected due to the low remanent polarisation of the samples. Hirschinger et al. [67] have applied two-dimensional (2D) exchange deuteron NMR to study the ultraslow chain motion in the crystalline c~-phase of PVDF and specifically the c~-relaxation that occurs in the crystalline regions at around 97~ The reorientational angle distribution (RAD) P(0), obtained from the 2D experiment, indicates a jump motion of the C ~ 2 H bond by an angle of 67 ~ or 180-67 = 113 ~ in the crystal lattice. P(0) also indicates an uncertainty of the reorientational angle of ---3~ Since the angular resolution in 2D-exchange 2H NMR is as high as ___0.5~ the broader RAD in PVDF clearly reflects the nonideal packing in the polymer crystal. From the intensities of the diagonal and off-diagonal parts of the spectrum, it was concluded that the jump motion included only two sites. By changing the mixing time (tm) in the 2D experiment, the timescale of the motion was evaluated. The final-state RAD was reached for tm I> 200 ms. From the RAD at tm = 2 and 20 ms, the calculated correlation time of the two-site jump motion ~-~ was 20 +_ 5 ms at 97~ The timescale of these motions agreed well with the so-called a-relaxation detected by dielectric and mechanical spectroscopy. Comparision with X-ray data showed that the NMR results were consistent only with the conformation of Takahashi et al. [68]. They proposed the existence of disorder between four possible chain orientations in the unit cell. Transitions between these polarisation states require molecular reorientation.
682
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
Four different models of molecular motion were in agreement with the jump angle determined by NMR. However, of these possible motions only one was in agreement with the dielectric relaxation results of Miyamoto et al. [69]. This motion is defined by a dipole-moment transition and a conformational change (tg+tg-~--~ g-tg*t) yielding an effective dipole-moment reversal only along the chain axis and a reorientation angle of 113 ~ for the C ~ 2 H bond directions. As discussed in Section 6.6, proton high-power decoupling during 19F observation is not an easy technique to implement, because of the proximity of the relevant frequencies. An alternative to continuous wave 1H decoupling, involving proton 180 ~ pulses during windows of MREV8 19F pulse sequences, has been shown [70] to be effective for PVDF, giving an improved shielding powder-pattern bandshape. Until recently, surprisingly little attention has been paid to high-resolution (i.e., using MAS) 19F spectra of PVDF or other fluoropolymers which also contain protons. However, Harris and coworkers have now conducted a number of studies [71-75], utilising high-power proton decoupling. Without such decoupling, PVDF powder obtained from the melt shows, at ambient probe temperature, a major signal at ~ ~ -91 ppm, together with hints of shoulders (on both high and low frequency sides) and a weak doublet at lower frequency ( ~ - -116 and -120 ppm). Proton decoupling has a significant, but not dramatic, effect by improving the resolution of the shoulders (see Section 6.6, Fig. 6.6.2). Spectral quality also relies on MAS rates being high (> 10 kHz). Solution-state 19F-{1H} NMR [76-78] indicates that the main peak arises from the regular alternating CF2CH2 structure expected for the polymer, whereas the weak doublet can be assigned to chain imperfections of the type H T H T T H H T (where H and T refer to "head" CH2 and "tail" CF2 groups, respectively). It is noteworthy that a TT link is followed by a HH link to resume the original monomer sequence (thus giving t w o 19F signals for the imperfections). The solid-state shifts for the most intense signal and for the tail-to-tail doublet in the solid are very similar to those for solutions (6F = --91.6, --113.6 and -116.0 ppm for N,N-dimethylacetamide as the solvent [76]), indicating that chain conformations are similar in the two phases. The shoulders in the solid-state spectra can be shown to arise from crystalline domains in the sample. For material produced from the melt, these domains are of the a form, and NMR techniques of discrimination proved to be very effective [73,74]. Cross polarisation is more efficient for crystalline regions than for amorphous parts, since the chains in the latter are more mobile. Discrimination in favour of the crystalline domains can be enhanced
FLUOROPOLYMERS
683
a.
'
I
-60
~
I
-80
J
I
-100
~
I
-120
Fig. 18.9. Fluorine-19 CPMAS spectra of PVDF (biaxially stretched film), showing the discriminatory effect of a precontact delay (with 1H spin-locking) following a 90 ~ proton pulse, combined with the use of a short (100/xs) contact time. (a) Precontact delay zero. (b) Precontact delay 40 ms. The signal for amorphous domains does not appear in (b). The sample contains both a and/3 crystallites. The signals from head-to-head units are also lost in (b) and, therefore, reside principally in amorphous regions. [Figure reproduced with permission from Ref. 72.]
by spin-locking the protons prior to CP contact (since T1H is substantially shorter for amorphous chains) (see Fig. 18.9). Such an experiment (known as a proton Tip filter) allows the investigation of the effect of processing on the polymeric material. Drawing in one or two dimensions converts a crystallites into the /3 form. Figure 18.10 shows the spectra of a and/3 forms (the sample of the latter contains ca. 20% of the former). It is clear that the /3 form gives a single signal at 6F = - - 9 8 ppm, whereas, the a crystallites give two signals at 6F = --82 and - 9 8 ppm. These results can be understood in terms of the known conformations. The/3 form has an all-trans conformation, whereas the a polymorph is tg§ Figure 18.7 shows clearly that the two CF2 fluorines are equivalent for the former, but nonequivalent for the latter. Moreover, the/3-fluorines have two 7-gauche interactions with carbon, as has one of the a-fluorines, thus explaining the near coincidence of the signals (see Fig. 18.11). The second a-fluorine has one 7-gauche and one 7-trans carbon, and its chemical shift reflects this fact. Thus, the difference in shifts of the a fluorines represents one 7-gauche (C,F) effect, which is therefore ca. - 1 6 ppm, in good agreement with the value estimated [77] from solution-state NMR. Discrimination in favour of the amorphous domains may be achieved by
684
R . K . HARRIS, G. A. MONTI AND P. HOLSTEIN
I'"'I""I""I
-60
-80
.... I'"'I""I'"'I
-100
%/pp m
-120
Fig. 18.10. The effect of crystallite phase on PVDF 19F CPMAS spectra. Top. sample crystallised from the melt (a form). Bottom: 9-1xm thick biaxially drawn film (/3 form with a little a). Spectra were obtained using the T~o filter (precontact ~H spin lock 40 ms, contact time 50 Ixs).
,,.C
A F"
] C
tg+tg -
C all-trans
- FORM
/~ - FORM
Fig. 18.11. Chain conformations for PVDF to show T-gauche interactions of carbon to fluorine. Left: a-form, where FA has two T-gauche interactions with carbon, but FB has one T-gauche and one T-trans. Right: /3 form, with two y-gauche interactions for each fluorine.
the dipolar-dephasing pulse sequence [73,74], since static spectra are relatively narrow because of the extensive chain motion. This reveals that the tail-to-tail imperfections are largely in the amorphous domains and spinning sidebands are not as prominent for amorphous signals as for those from the crystalline regions. Both effects are as expected. Proton and fluorine relax-
685
FLUOROPOLYMERS
'
9
I
,'
.
.
.
. 1 .
.
.
40
'
'
I
. . . .
40
.
.
.;
.
.
.
.
.
.i
i
0
i
. . . .
i
0
..v..
40
-40
. . . .
i
. . . .
i
,
-40
,
'
'
!
0
. . . .
40
I
-40
'
. . . .
0
" +
'
,
. . . .
i
'
'
-40
Fig. 18.12. Slices in the 1H dimension for a 19F/1H WISE spectrum of PVDF. The two lefthand spectra are for crystalline domains. The right-hand spectra are for the amorphous region (upper right) and the head-to-head defects (bottom right). The fluorine nuclei were decoupled during the time tl. The frequency scale in kHz. [Figure reproduced with permission from Ref. 75.]
ation times (T~ and Tao) have been measured by several different methods [72,73] (see Section 6.6). The influences of temperature and poling on the spectra have been studied [74]. Cross polarisation from 19F to 1H has also been carried out [71] for solid PVDF, though the potential of this experiment is probably not great. The two-dimensional (1H,19F) wideline separation (WISE) sequence has been applied [75] to PVDF, both with and without 19F decoupling during the 1H evolution time tl. The resulting spectra yield values of proton second moments, discriminated in terms of the 19F chemical shifts by the second dimension, thus quantifying the mobility differences between the amorphous and a-crystalline chains (Fig. 18.12). Moreover, a suitable preparation period in the pulse sequence, namely application of a 1H Tlo filter, allows signals of the crystalline region to be obtained selectively in the two-dimensional plot [75]. A period for spin diffusion facilitates the study of phase separation. Full restoration of amorphous magnetisation occurs in 16 ms. A 13C spectrum of solid PVDF, obtained with cross polarisation from 1H plus simultaneous 1H and 19F decoupling appears to have been first reported by Fleming et al. [33], on a static sample. "Reasonably well-defined" powder patterns for separate CF2 and C H 2 signals were obtained and shielding tensor principal components reported. On the other hand, T6k61y et al. [79] showed
686
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
j A
I
200 $C
I
I
I"
100 ppm 0 200 $C
I
100 ppm
I
0
Fig. 18.13. Carbon-13 CPMAS spectra of PVDF. Left-hand: 19F'-~13C CP. Right-hand"
1H----~13C CP. Top: 1H-decoupled. Middle: 19F decoupled. Bottom: Double-decoupled, {19F,1H}, triple-resonance spectra. [Figure reproduced from Ref. 83.]
the rather poor-quality CPMAS spectra that are obtained with only proton decoupling. Veeman and coworkers [2,80,81] have combined the triple-channel 13C-{1H,19F} and MAS experiments, using them as the basis for studying blends containing PVDF. Peaks at 6 c - - 4 3 ppm (CH2) and 120 ppm (CF2) were seen for PVDF itself (see Fig.18.13), but no additional signals such as have been observed [78,82] for triple-channel 13C-{1H,19F} experiments on PVDF solutions. However, Holstein and Harris [83], using 13C-{1H,19F} triple-channel experiments, have shown that the c~ and /3 polymorphs give rise to slightly different spectra (peak separations ca. 4 ppm), but such spectra are inferior to those of 19F for distinguishing domains. The CP dynamics have been explored [83]. Relatively little solid-state NMR work appears to have been carried out on homopolymers in this category other than PVDF. However, some work has been reported on poly(trifluoroethylene) (PTrFE). McBrierty et al. [84] measured proton T1, Tlo and T2 as a function of temperature over the range -1 - 1 0 0 to + 140~ Above ca. +60 ~ two-component T2 (measured as the e point of the FID) was observed as motion in the amorphous region became significantly faster than in the crystalline region. A crystallinity of ca. 55% was indicated. The authors suggested that there are three minima in Tip, which were assigned to c~,/3 and 3' relaxation processes. The c~and y processes were attributed to the amorphous region, whereas the /3 process was tent-
FLUOROPOLYMERS
687
atively associated with the crystalline region. Measurements of T1 provided corroboration for these conclusions. Katoh and Ando [85] have reported I3C MAS and pulse-saturation transfer experiments (the latter giving enhanced signals by the NOE) for two poly(Lglutamate) samples with n-fluoroalkyl side chains over a temperature range from 30 to 100~ It was deduced from the measured chemical shifts that the sample with short (~,-trifluoroethyl) side chains takes the helix form whereas the one with longer (~/-n-2-perfluorodecylethyl) side chains is in the/J-sheet form. Zumbulyadis et al. [86] selected a fluorinated polyphosphazene, containing CF3CF2CF2CF2CH20 side chains (attached to phosphorus atoms in the polymer backbone) to explore the potential of 19F MAS/NOESY experiments. MAS at 3.82 kHz sufficed to resolve all four 19F sites, though effects of isotropic indirect coupling were obscured by the linewidths. Cross peaks are seen in the two-dimensional NOESY spectrum. The authors argue that these arise from NOE processes and not from spin diffusion. Wooley et al. [3] have used 13C, 19F R E D O R experiments to study the solid-state shape, size and intermolecular packing of a series of benzyl ether dendrimers (generations 1-5) based on 3,5-dihydroxybenzyl alcohol. A fluorine label was placed at the core, and samples contained site-specific 13C isotopic enrichment near the chain ends. Dipolar coupling constants were measured, giving average intramolecular (C,F) distances of ca. 12 A for generations 3-5, indicating inward folding of chain ends. Intermolecular measurements were consistent with decreased interpenetration for larger dendrimers. Some polystyrene-diluted materials were also examined.
18.5
Copolymers and blends involving only one fluorinated component
In this section, copolymers of the above type are dealt with first because, in principle, discriminating experiments originating from the 19F magnetisation in one component are possible, giving a more clear-cut situation than when both components contain fluorine. Within this category, even further simplifications become possible if the fluorinated component does not contain hydrogen. This is the situation for PE/PTFE blends, which have been examined by at least three groups of authors. Nagumanova et al. [87] studied low-density PE/PTFE mixtures with a range of compositions. Proton second moments were used to obtain mobility information and it was concluded that there were conformational restrictions at the LDPE/PTFE interface. Nagarajan and Stachurski [88] expanded on the scope of this work, using broadline 1H and 19F resonances to determine second moments as a function
688
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
of composition and of sintering times in the preparation. They proposed a model for the interaction of PE and PTFE units involving interdiffusion at the interface. Sugiura et al. [89], on the other hand, studied blend films of ultra-high molecular weight polyethylene with PTFE using 13C MAS NMR. Carbon-13 relaxation times T1 and T2 were measured by the Torchia method and by direct polarisation saturation recovery for the former, and direct polarisation spin-echoes for the latter. The signals from the Torchia method for the orthorhombic crystal form of PE were monitored in this process. The T1 decay curves were decomposed into three components. The direct polarisation results were assigned to amorphous PE units. Whereas T1 values were almost independent of composition, T2 (for the amorphous region) increased with PTFE content. It would appear that no direct measurements relating to the PTFE were made in this study. None of the above three investigations have capitalised, therefore, on the presence of fluorine in the systems examined. This is also the case for a 15N NMR investigation [90] of curing, post-curing and hydrolytic degradation of a polyimide resin produced with hexafluoroisopropylidene bis-phthalic hemi-methylester as one component. Douglass and McBrierty [91] reported a detailed study of 19F relaxation in PVDF/PMMA and PVDF/PEMA blends. In addition to measuring T1F, T~ and Tl~p, they determined the parameters affecting cross relaxation between the 1H and 19F spin systems using the transient Overhauser experiment. Thus they monitored the 19F magnetisation at a variable time after a 180~ proton pulse. All measurements were carried out over the temperature range -200 to +160~ and were compared to the analogous data for the relevant homopolymers. The authors concluded that a substantial fraction of the PVDF units are at nearest-neighbour distance from PMMA or PEMA units and that there is evidence of extensive premelting of PVDF crystallites in the blends. Blends of PVDF and PMMA were studied by T6k61y et al. [79] using 13C MAS NMR, with cross polarisation from protons. They obtained data on PMMA 13C magnetisation as a function of 1 H ~ 13C contact time for the homopolymer and a series of blends (both quenched and annealed). For high contents of PVDF, the plots showed a double-stage character, which the authors interpreted in terms of partial crystallisation of PVDF. The conclusions were reinforced by the experiments on annealed samples. However, the lack of fluorine decoupling meant that no direct study of PVDF signals was feasible. A major series of articles by Veeman and coworkers [2,80,81,92,93] has remedied that situation. Using a commercial probe modified in-house, they obtained [2] 13C spectra of blends of PVDF with PMMA using {1H,19F} double decoupling and 19F ~ 1H ~ 13C double cross
FLUOROPOLYMERS
t9 F
I,
H
689
1
I
'a C Fig. 18.14. Pulse sequence used by Veeman and coworkers [80,81,92] to determine the intimacy of mixing of PVDF and PMMA (see text).
polarisation. The PMMA carbonyl and methoxy resonances, being well removed from PVDF signals, were monitored as a function of contact time in 19F----~13C{1H,19F} experiments. Fluorine and carbon Tlo values were determined separately. Note that a "physical" mixture of PMMA and PVDF yielded no PMMA signals under the procedure used, whereas the data on the blends allowed an average distance between a fluorine atom and carbons on PMMA to be calculated, the result ranging from 2.6 to 3.1 ~ . Further work [80] used, (i) 19F----~IH----~13C double cross polarisation, and (ii) 1H ~ 19F cross depolarisation followed by 1H ~ 13C cross polarisation (see Fig. 18.14). Both techniques are affected by proton spin diffusion. This renders technique (i) only valuable in a qualitative sense, showing intimate mixing (Fig. 18.15), but the ability of proton spin diffusion to transfer magnetisation from PMMA protons to PVDF protons under experiment (ii) was successfully used to quantitatively distinguish various phases in the PVDF/PMMA blends. The authors used a model with four phases: "isolated" PVDF, "isolated" PMMA, "intimately mixed" PMMA/PVDF (giving 1H ~ 19F depolarisation); and PMMA sufficiently close to the mixed phase that spin diffusion from it is important. Radial spin diffusion was assumed, with a sphere of intimately mixed phase of radius A and a shell of "close" PMMA of thickness L - A (up to a point of abutment with a neighbouring sphere). Figure 18.16 shows experimental data for the 60:40 PMMA/PVDF blend, which is fitted to A = 6 A, L = 12 ~ and 30% "isolated" PMMA. The same experiment was then employed [81], together with 19F ~ 13C{1H,X9F} CP, to study the influence of PMMA tacticity on PMMA/PVDF miscibility, yielding evidence for a specific interaction between segments of the two polymers. The mixed PMMA/PVDF phase was determined to have a mean radius of 6-8 A, with some dependence on PMMA tacticity. Large differences were found in the
690
R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN
(bl
200 ........
"'
1 0 0
....
0 ....
*c/ppm
Fig. 18.15. CPMAS spectra of a P M M A / P V D F 60:40 blend: (a) 19F---->13C direct CP (contact time 2 ms); (b) 19F ----->1H ----->13C double CP (19F ----->1H contact time 0.4 ms, 1H ~ 13C contact time 2 ms). Experiment (b) clearly shows the intimate contact between PMMA and PVDF chains. [Figure reproduced with permission from Ref. 80.]
amounts of isolated PMMA between melt-mixed and coprecipitated blends. This theme was continued by a study [92] of the crystallisation behaviour of PMMA/PVDF 60:40 blend, and an amorphous PVDF interphase was found to be present. The fraction of isolated PVDF increased rapidly as a function of annealing time (see Fig. 18.17). Polymer interdiffusion above Tg was then monitored [93] by using technique (ii) at ambient probe temperature following various times during which the polymer system was held at 190~ Initially, the sample consisted of stacks of discs cut from PMMA and PVDF sheet (average thicknesses 92.5 and 88.2 txm, respectively) alternately. Evaluation of the diffusion equation gave intrinsic diffusion coefficients (7 _ 5) • 10 -11 and (15 ___5) • 10 -11 c m 2 s -1 (at 190~ for PMMA and PVDF, respectively. Amorphous polymers or regions of polymers can be regarded as microporous materials, and can be studied, therefore, by monitoring the 129Xe. chemical shifts of adsorbed xenon gas. Mansfeld et al. [94] used this method to distinguish between incompatible blends (of polypropylene with a polypropylene/polyethylene copolymer) and compatible blends (PMMA and PVDF). In the former case, two 129Xe signals were observed, whereas only
FLUOROPOLYMERS
691
1.0 -I
_
:~o.5
0.0
-
0.0
I
0.5
cross
I
1.0
I
1.5
I
2.0
depolarization
i
2.5
time
I
3.0
I
3.5
(ms)
Fig,. 18.16. Experimental data for a PMMA/PVDF 60" 40 blend, obtained for the 13C carboxyl
resonance of PMMA. The solid line is a calculated depolarisation curve. The dashed line represents the offset from 30% of "isolated" PMMA. [Figure reproduced with permission from Ref. 80.] one was seen in the latter. However, for P M M A / P V D F blends, the 129Xe linewidth shows variations with composition that are not yet explained.
18.6
Other copolymers and blends containing fluorine
A number of synthetic, noncrystalline fluorocarbon copolymers exhibit elastomeric properties when vulcanised. Such elastomers are of commercial interest because they have unusual combinations of properties; e.g., high melting point, high thermal stability, insolubility, low coefficient of friction and flexibility at low temperatures. They are designed for demanding service applications in hostile environments characterised by broad temperature ranges and contact with chemicals, oils or fuels. Copolymers of vinylidene fluoride (VDF) (CF2=CH2) and hexafluoropropylene (HFP) (CF2=CFCF3) were reported 40 years ago by Dixon, et al. [95]. Ferguson [96] carried out a 19F NMR solution-state analysis of the samples. The gross structure was deduced from the spectral assignments on the assumption that it was a random linear copolymer, except that no adjacent
692
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN 0
.0
,
,
,.
o
I
I
I
I
I
'l
I
O 30-
0
0
O [30 O
I
V
I
==
"0 -~,,W
I
20-
EX)O 10-o ID
0-
0
I
I
200
'
O 120
~
o
140
"C
I
'
4o0
I
600
'
I
8o0
'
I
lOOO
'
I
2oo
(rain.) Fig. 18.17. Fraction of isolated PVDF in a 60"40 PMMA/PVDF blend as a function of
annealing time at 120 and 140~ [Figure reproduced with permission from Ref. 92.] hexafluoropropylene-hexafluoropropylene units were present. The assumption was justified by the lack of spectral evidence for branched and/or unsaturated structures and on chemical grounds. Randomness in the incorporation of the two monomers was expected from the method of polymerisation. Significant homopolymerisation of hexafluoropropylene under similar conditions has not been reported, and it appeared to be impossible to incorporate more than 50 mol% hexafluoropropylene into the copolymer. In the late 1950s, such copolymers were developed on a commercial scale by 3M (Fluorel) and by DuPont (Viton). Also, copolymers of vinylidene fluoride and chlorotrifluoroethylene (CTFE), CF2zCFC1, became available from Kellog in 1955 under the trademark Kel-F. In the 1960s, terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene (TFE) (CF2~CF2), were developed and commercialised by DuPont as Viton B. These kinds of elastomers from various companies typically contain a range of impurities or additives such as from initiators (organic or inorganic peroxy compounds, e.g., ammonium persulfate), emulsifying agents (usually fluorinated acid soaps) and chain-transfer reagents such as carbon tetrachloride,
FLUOROPOLYMERS
693
Table 18.1. Typical fluorocarbon elastomer polymerisation recipe
Component
Grams
Vinylidene fluoride Hexafluoropropylene Carbon tetrachloride Potassium persulfate Perfluoro-octanoic acid Potassium phosphate, dibasic Water
61 39 0.12 1.2 0.90 3.6 340
methanol, acetone, diethyl malonate and dodecylmercaptans [97]. Most fluorocarbon elastomer gums also contain a cure system, consisting of an organic onium cure accelerator, such as triphenylbenzylphosphonium chloride, and a bisphenol cross-linking agent, e.g., hexafluoroisopropylidenediphenol. For complete formulation, reinforcing fillers and metallic oxides are added, the latter as acid acceptors. Raw Viton gums contain no curatives. A typical polymerisation recipe for a vinylidene fluoride and hexafluoropropylene copolymer is shown in Table 18.1. The relationship between the molecular motion and properties of copolymers of VDF and CTFE was investigated by Katoh et al. [98] by means of wideline ~H pulse NMR over a wide range of temperatures (20-120~ For the proton spin-spin relaxation time (T2) measurements, the solid-echo pulse sequence was used. Copolymers of VDF, CTFE and unsaturated peroxide (CHzzCH--CHzmO--CO--OmOmtmBu), referred to as "base polymers", were prepared at low temperature with different number-average molecular weights: 5.6 x 10 4 and 2.6 x 10 4, designated H and L, respectively. Following side-chain cleavage at elevated temperature, further polymerisation with VDF occurred, producing "graft polymers" H and L, respectively. Two blend copolymers were obtained by mixing base polymer H with PVDF and base polymer L with PVDF at 180~ The Y2 curves of the base copolymers consist of two decays. For the H base copolymer the T2 constants are 65 and 29 ms, assigned to amorphous and "interfacial" phases, respectively. The T2 curves for the graft copolymers consist of three decays, with decayconstants of 108, 33 and 15 ms. By comparison with neat PVDF (Y2 = 16 ms) the shortest Y2 is assigned to the crystalline component of VDF units. The remaining two components correspond to those observed for the base copolymer. The nature of the "interfacial" phase in the base polymer is unclear, but its value of Y2 is close to the intermediate value in the graft system. It was shown that the molecular motion of the base copolymer is restrained with increase of molecular weight, but that the molecular motion of the graft
694
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
polymer is independent of the molecular weight of its base polymer. The base polymers showed different mechanical properties and different fractions of the immobile component. However, graft polymers H and L have the same mechanical properties and the same 1H T2 behaviour. The authors concluded that the molecular motions are clearly associated with mechanical properties. The temperature-dependence of T2 for the blends lies between those for the base and graft systems. Several wideline NMR experiments were carried out on copolymers of vinylidene fluoride and trifluoroethylene (TrFE) [84,99-112]. Such copolymers are of special interest since TrFE in proportions greater than 10% induces the VDF component to preferentially crystallise in the /3 form. In contrast with neat PVDF, the copolymers exhibit a "ferroelectric" phase transition at a temperature well below melting. McBrierty et al. [84,99] studied proton relaxation behaviour of unpoled [84] and poled [99] 52/48 mol% vinylidene fluoride and trifluoroethylene copolymer. They reported TI, Tip and T2 data as a function of temperature in the range -120 to 120~ (Fig.18.18). The unpoled sample showed a discontinuity in the relaxation parameters at 70~ more pronounced in the T2 data, which, below 70~ were represented by two components. The discontinuity in T2 occurs in both long and short components. The magnitude of the short T2 above the transition is comparable to that for pure crystalline TrFE. The absence of a T2 characteristic of the crystalline phase of neat PVDF was also noted. The behaviour of T2 around the transition temperature indicates that both amorphous and crystalline regions are affected at the 70~ transition. The transition may be interpreted in terms of either motional or order-disorder fluctuations such as the proposed ferroelectric transition. In the poled sample (Fig. 18.3), the "ferroelectric" transition is shifted to higher temperatures by 10 to 15 K. Additionally, the transition becomes sharper. The poled material has improved crystallographic packing in the ordered regions. The activation of chain rotation in the ordered region as the temperature is increased is associated with the onset of the "ferroelectric" transition. Legrand et al. [100] reported 19F wideline NMR measurements on a 70/30 mol% VDF-TrFE random copolymer, made in order to study molecular motion both below and above the ferroelectric transition temperature, Tc. The samples consisted of semicrystalline copolymer films of 0.51 mm thickness, with biaxial orientation of the crystalline axis. The samples were rolled (without poling) at 70~ with a draw ratio of 300%. The 19F resonance was chosen, rather than the proton resonance, because the abundance ratio of 19F to 1H nuclei is 1.4. In addition, the 19F free-induction decay (FID) lasts longer than that of the proton, which decreases the influence of spectrometer deadtime. FID analyses were made assuming a simple superposition of two
695
FLUOROPOLYMERS t
I
~ I-
i
I
'
I
'
I
'
I
'
I
'
TI
1o0
1p (n
uJ
-2
9
10
9
FZ o_ F<{ X
A
lo-3
_J i11 E 10_4_
o o 80.
10. 5 _
I0
-6 _ - 120
,
-80
I
l
40
I
t
I 0
l
40
I
i
80
I
i
120
TEMPERATURE ( ~
Fig. 18.18. Temperature dependence of the proton relaxation parameters for poled and drawn VDF/TrFE (52/48 mol%) copolymer, showing the ferroelectric transition. The filled triangles correspond to the case where three components for Tip can be resolved. [Reproduced with permission from Ref. 99.]
components: a short decay time attributed to the crystalline phase and a longer value assigned to the amorphous phase. This assignment was unambiguous at high temperature (T > 80~ but not at lower temperatures, and especially around room temperature as the glass transition temperature is approached (Tg is about 0~ The FID analyses showed that the relative intensity of the crystalline signal starts to increase at about the temperature of the rolling process (70~ reaching 65% just before the transition occurs. Another increase of the crystallinity up to 85% occurred on cooling throughout the transition temperature Tc. The transverse relaxation rate of the amorphous phase exhibits the same behaviour upon heating and cooling, while the linewidth for the crystalline phase narrows by a factor of two around 100~ upon heating and returns to its initial value, with a hysteresis of 40~
696
R . K . HARRIS, G. A. MONTI AND P. HOLSTEIN
upon cooling. These changes are associated with the anomalies of the specific heat at the ferroelectric transition. The results show that the disorder of the high-temperature paraelectric phase (T < Tc) is of dynamical origin. Fluorine19 spin-lattice relaxation was also investigated. For measurements at 9.14 MHz the observed T1 appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well-defined minima in T1 were observed, and associated with the ferroelectric transition. Hirschinger et al. [101] have studied the structure and morphology of VDF-TrFE copolymers containing 30 to 100 mol% VDF. They used ~H and 19F wideline NMR to obtain information on the crystalline fraction and crystalline morphology of the copolymers, and on the relative fraction of VDF and TrFE units in the rigid and mobile phases. Films of copolymers have been cast at 180~ quenched at room temperature and annealed at 120~ A variable-temperature study of the 70/30 copolymer reveals that amorphous and two crystalline phases must be considered to describe the ferroelectric transition. Proton FID analysis of the 70/30 P(VDF-TrFE) copolymer reveals that the total second moment undergoes a substantial change above 60~ with a large hysteresis loop reflecting the first-order crystalline transition, in agreement with the 19F r e s o n a n c e results of Legrand et al. [100]. At low temperature, the structural change between the crystalline forms with increasing TrFE content is detected clearly. At room temperature, the morphology of VDF-rich copolymers is readily analysed, with two components having the same 1H to 19F ratios. On the other hand, below 70 mol% VDF, the two components have different 1H to 19F ratios, which implies segregation between TrFE-rich and VDF-rich sequences. In other articles [102,103] the authors reported studies of the 70/30 P(VDF-TrFE) copolymer. They examined the dynamics and morphology of the copolymer in the ferroelectric [102] and in the paraelectric [103] phase. The spin dynamics of protons and fluorines were investigated by measurements of T1 and Tip (the spin-lattice relaxation time in the rotating frame as a function of the spinlock angle 0), the transient Overhauser effect (TOE) and the FID. The 0 results obtained from Tip in the ferroelectric phase demonstrated the presence of local motions within the crystallites, implying the existence of crystalline "defects". These defects act as sinks for relaxation via spin diffusion and probably have an important role in the initiation of the Curie transition. The authors found that a simple model treating cross relaxation and spin diffusion was appropriate to describe all the relaxation data in the laboratory frame. The model uses the Hunt and Powles [113] correlation function associated with a one-dimensional diffusion process. In particular, when compared with the results in the paraelectric phase [103], the shape of the correlation
FLUOROPOLYMERS
697
function in the amorphous phase confirms the existence of a similar highfrequency mode both below and above Tc. For the paraelectric phase, proton and fluorine relaxation times T1, from 6 to 300 MHz, and Tip from 3 to 100 kHz, were measured at different temperatures, and analyses of the FID signals on both oriented and nonoriented samples were carried out. No transient Overhauser effect was observed over the whole temperature range. Two relaxation modes were found: (i) a fast anisotropic motion showing the characteristic "one-dimensional" 0) - 1 / 2 dispersion of l/T1 and 1/Tip, and (ii) a slow motion considered to be isotropic. The two modes are present in the amorphous phase, while the fast motion alone is involved in the dynamics of the crystalline phase. The fast process was described by means of three-bond motions. Their important effect is to cause diffusion of the existing C ~ C orientations along the chain, giving 1D fluctuations. In a series of articles, Ishii et al. [104-108] reported proton spin-lattice relaxation time and linewidth measurements (obtained by field-sweep methods and reported in field units, AH), over a wide range of temperatures, in drawn film samples of copolymers of VDF and TrFE (with VDF contents of 72, 65 and 52 mol%). Three kinds of T1 relaxation processes, referred to a s j~, of t and a~, were observed, respectively, below the Curie Temperature (Tc), around Tc, and in a certain temperature region above Tc. The thermal hysteresis of T~ and AH vs. temperature for each relaxation process became smaller as the VDF content of the copolymer decreased. The motional processes in the /3 region are associated with a flip-flop of TRFE-rich groups correlated to "free" rotation of the VDF segments. In the O~t and a~ regions, the motional modes are associated with the conformational change from trans to gauche bonds. The minimum value of T1, and the discontinuous change of AH in the O~t region observed for each copolymer, are attributed to a ferroelectric phase transition and their thermal hystereses are related to their first-order nature. The behaviours of both Wl and AH in the paraelectric phase for each polymer are described in terms of a one-dimensional diffusional motion of conformational defects along the chain. The activation energy of the motion, obtained from the dependence of Tx on temperature, is about 36 kJ/mol and does not depend on the PVDF content. The anomalous behaviour of T1 and AH, observed around Tc, was investigated [107-109] by applying an effective Ising spin model to the configurational fluctuation of the trans segment. Tanaka et al. [110] studied the ferroelectric phase transition of a 52/48 random copolymer of PVDF and TrFE by pulsed proton NMR. Attention was focused on the dynamic properties of the phase transition near the Curie point. Two samples were studied; one was isothermally crystallised at 125~ and the other was rapidly quenched in liquid nitrogen from the melt. The
698
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
FID curves were well fitted by a single Weibull function for each sample. The authors analysed the FID, fitting the Kubo-Tomita [114] relation to the transverse magnetisation and, hence, obtained the correlation time (~c) of the local field and the second moment of the rigid lattice separately. The correlation time showed a cusp at the Curie point, which reflect the critical slowing down of the order parameter fluctuation in that region. The second moment decreases rapidly up to Tc and levels off at Tc. There is almost no change above Tc. The temperature behaviour of the second moment was explained by a structural change (accompanied by an intramolecular conformational, trans to gauche, process) and the abrupt lattice expansion around the Curie point. The relaxation rate, l/T1, showed a logarithmic divergence at the Curie point, explained by long-range dipolar interactions or co-operative conformational changes. The authors also concluded that the amorphous phase is not mobile enough to cause the motional narrowing, and that there is no significant difference in motion between crystalline and amorphous phases. Clements et al. [111] carried out proton broadline NMR measurements on an oriented 70:30 copolymer of VDF and TrFE. The spectra show two components, one broad and one narrow, identified with the crystalline and amorphous regions, respectively, as discussed above. A procedure was devised for modeling the rigid-lattice NMR lineshape of the copolymer and used to decompose the signal into the two components. The rigid mass fraction was determined by calculating the ratio of its integrated signal intensity to the total integrated intensity. By following the evolution of the rigid mass fraction with the temperature, the authors found an appreciable reversible change in crystallinity with temperature. Calculations showed that this change could be a significant contribution to the pyroelectricity response of the material. In addition, they observed the increasing libration of the chains prior to the Curie transition, which could also contribute to the pyroelectric response. Stock-Schweyer et al. [112] reported a high-pressure effect on molecular motions in the paraelectric phase of a (70/30) VDF and TrFE copolymer. Fluorine-19 NMR relaxation times (T1 and Tip) w e r e studied over a range of pressures from 0.1 to 200 MPa. Correlation times of the molecular motions, as functions of pressure and temperature, were obtained and the activation parameters determined. The experimental data confirmed the presence of a slow motion in the amorphous phase in addition to the fast anisotropic motion. The results indicated that the relaxation times of the copolymer are controlled by the effects of both temperature and volume. The authors concluded that ---40-50% of the mobility increase of segments with increasing temperature under constant pressure results from volume expansion.
FLUOROPOLYMERS
699
Kochervinskii and Murasheva [115] studied the microstructure of copolymers of vinylidene fluoride and tetrafluoroethylene of 71:29 composition using 19F NMR. They showed that there is 5 mol% of diads in the tetrafluoroethylene blocks and 2.5 mol% of head-to-head defects in the VDF blocks. A pioneering work on the WAHUHA multiple-pulse sequence applied to 19F NMR of a fluorinated polymer was reported by Ellett et al. [116]. They obtained resolved chemical shifts for the OCF3 and CF2 peaks (separation 73 ppm) of a copolymer of 60/40 TFE and perfluoromethylvinyl ether. Good agreement between the area of the peaks and the known composition of the copolymer was obtained. The anisotropy of the chemical shift of the CF2 groups was approximately determined. Vega and English [13] obtained 19F spectra and relaxation times for static samples of a copolymer of tetrafluoroethylene and hexafluoropropylene (TFE-co-HFP; 85 mol% HFP) by the multiple-pulse technique (MREV8) at various temperatures (see Fig. 18.19). They observed CF2 group lineshapes in crystalline and amorphous regions, and also CF3 and CF lineshapes. After subtracting the crystalline contribution to obtain the amorphous lineshape, the latter contribution was analysed as a function of temperature to obtain information about the type of molecular motion present. The/3 and 3i relaxCFz ~ 2 9 1 " 215" 174"
143" 82 ~
31" 5* -:58* -I05~
Fig. 18.19. Fluorine-19 multiple-pulse (REV8) spectra of a TFE/HFP (8.5 mol% HFP) copoly-
mer as a function of temperature. [Reproduced with permission from Ref. 13].
700
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
ations were observed as in the case of TFE homopolymer [13], but the amorphous-region local chain-axis reorientation is inhibited by the sterically larger CF3 groups below the melting point. An indication that one-third of the CF2 groups are in the crystalline environment was also obtained. As mentioned above, most commercial cured fluorocarbon elastomers are solvent resistant. Hence, it is important to achieve 19F high-resolution solidstate spectra of these polymers in order to understand their microstructure directly in the state in which they have practical applications. Dec et al. [117] studied copolymers of VDF and HFP, copolymers of VDF and CTFE, and terpolymers of VDF, TFE and HFP. They have demonstrated the feasibility of obtaining 19F high-resolution solid-state spectra, at room temperature, of these samples using direct polarisation and MAS speeds of about 18 kHz. They were able to make assignments of chemical shifts in terms of five-carbon sequences (pentads). Most of the pentad assignments for each chemical shift were made by reference to the solution-state work of Ferguson [118] and of Murasheva et al. [119]. All the major features apparent in the 19F NMR spectra of the solubilised polymers [118,119] are readily measured with highspeed MAS techniques. Some indication of a very small amount of polymerisation of HFP was found. Sufficient resolution was obtained to determine the relative concentration of each carbon pentad and also to determine monomer compositions from the 19F NMR data. Good agreement with known compositions was obtained. We have studied a commercial sample of Viton provided by Goodfellow (Cambridge, UK). The sample is a black sheet 2-3 mm thick, density 2.0 g/cm 3, lower working temperature - 10 to -50~ upper working temperature 220-300~ All 19F NMR spectra were recorded at 188.29 MHz on a CMX-200 spectrometer. The 19F NMR spectra, with and without proton decoupling, were recorded using direct polarisation with 7r/2 pulses of 3/xs duration. Spinning speeds up to 12 kHz were used. Relaxation delays of 4 s were sufficiently long to ensure quantitative peak intensities. Chemical shifts were measured with respect to CFC13 on a sample of C6F6 without any irradiation at the proton frequency. In Fig. 18.20, the 19F NMR spectrum obtained at a spinning speed of 12 kHz is shown. Following the work of Dec et al. [117] we have assigned chemical shifts to structural features. The results are given in Table 18.2. The peaks indicated with a question mark in Table 18.2 do not correspond to any known or possible pentad for this kind of copolymer. They may be attributed to resonances of unsaturated monomer units [120]. Fig. 18.21 shows a computer deconvolution of the 19F spectrum. Lorentzian lineshapes were used to simulate the peaks. Relative integrated intensities for each peak of the sample are given in Table 18.2. Corrections for the intensity of the spinning sidebands were made. An analysis of the
FLUOROPOLYMERS
701
4
3
5
7 9
1
10
6
C" !
|
100
50
,
!
,
0
-
i
|
|
!
|
-50
-100
-150
-200
-250
6(ppm)
,,
,
-300
Fig. 18.20. Fluorine high-resolution (MAS) spectrum of a commercial sample of Viton. Direct polarisation without proton decoupling was used. The asterisks indicate spinning sidebands. The spinning speed was 12 kHz. See the text for further details of the experimental conditions. The peak numbers are referred to in Table 18.2, which lists the assignments. Table 18.2. Peak deconvolution and structural assignment of a V D F - H F P copolymer Peak
Chem. shift a (ppm)
Area (%)
1 2 3 4 5 6 7 8
-56.6 -62.5 -71.0 -75.1 -89.7 -103.9 -110.0 -114.3
1.9 1.1 6.4 19.5 32.3 4.2 12.9 2.4
9 10
-118.1 -183.7
11.0 8.3
Structural assignment b
--CF2---CH2---C*F2---CH2---CF2---CF2~CH2----C*F2~CF(CF3)--CF2~ --CF2--CH2--C*F2---CF2--CH2--
---CF2mCH2--C*F2--CH2--CH2~ --CF2mCF2---C*F(CF3)mCH2---CF2~
The fluorine(s) concerned are indicated by an asterisk. From direct polarisation experiments without proton decoupling. Therefore, there is no Bloch-Siegert effect.
a
b
relative 19F signal intensities determines the composition as VDF" HFP = 72"28 for this copolymer. Resolution in the 19F spectra is not improved by application of r.f. proton.decoupling at ambient temperature. In this kind of rubbery sample a spinning speed of about 10 kHz, combined with molecularlevel motion of the polymer chains, is enough to efficiently average 1H-I9F
702
R . K . H A R R I S , G. A. MONTI AND P. HOL ST E IN
I
A
v ~
0
''''I''''I
-50
....
-100
I''''I''''I''''I
F/pp
-150
....
I'''
Fig. 18.21. Deconvolution of the 19F high-resolution spectrum of Viton shown in Fig. 18.3.
dipolar interactions. However, the Bloch-Siegert effect [121] results in the chemical shifts for decoupled spectra appearing ca. 1.7 ppm to low frequency of the corresponding shifts in coupled spectra. Fluorine-19 spin-lattice relaxation times in the laboratory frame, T1, and 19F and 1H spin-lattice relaxation times in the rotating frame, T i p , w e r e measured under high-resolution conditions (i.e., selectively) at ambient probe temperature (--23~ and are reported in Table 18.3. Relaxation times T1 w e r e obtained using the inversion-recovery technique (Tr-r-~r/2-acq), while the 19F T i p relaxation times were measured by means of the variable-time spin-lock technique, and 1H Tip was obtained via 19F resonance by a variable 1H spin-lock followed by {1H ~ 19F} cross polarisation. In all cases decoupling of the complementary nucleus was not implemented during the variable time allowed for relaxation. The 19F relaxation times all show single-exponential decay of the magnetisation as a function of Table 18.3. 19F and 1H spin-lattice relaxation times for a Viton sample (ppm) -56.6 -70.6 -75.1 -89.9 -110.4 -118.0 -183.9 a
19F T1 (ms) (-+ 10 ms)
19F Tip (ms) a
1H Tip (ms) a
375 381 379 361 388
2.16 1.61 1.64 0.98 0.86 0.83 1.60
1.1 1.2 1.2 0.9 1.3
The errors quoted are statistical.
_ 0.04 _ 0.05 ___0.01 ___0.01 ___0.01 ___0.01 --_ 0.01
__+0.1 ___0.1 +_ 0.1 __. 0.1 _ 0.1
703
FLUOROPOLYMERS
the relaxation delays. The values obtained for Tx are very similar for all the resolved lines, whether they are associated with VDF or HFP units. We conclude that spin diffusion is effective enough to produce a single T1 value over the entire sample. The relaxation times T~o for the lines associated with CF/CF3 of the HFP units (resonances at - 7 1 , - 7 5 and -184 ppm) are slightly higher than for those associated with CF 2 fluorines. However, the signals at - 9 0 , -110 and -118 ppm also have contributions from HFP units. Proton Tlo relaxation times associated with all the 19F peaks show similar values, as expected. It is known that some copolymers of VDF and HFP show different degrees of crystallinity, and the presence of HFP decreases the crystallinity of the copolymer. This effect increases when the amount of HFP in the copolymer is increased [122-125]. However, our Tlo and WISE [126,127] results (see Section 6.6 for the latter) do not discriminate between amorphous and crystalline phases of the copolymer studied. The evolution of the 19F magnetisation obtained from the standard CP experiment is shown in Fig. 18.22 as a function of the contact time. The maximum intensity of the magnetisation is reached around 900 ms. This relatively short value occurs in spite of the high mobility of the polymer and even when the 19F~lH dipolar coupling is additionally averaged by MAS. This is presumably because we are dealing with two abundant nuclei and with similar and strong dipolar-couplings, resulting in
1.00
ooOOO(X)
o oo r 1 6 2
~---~-~ 9
o
z~
*o
lo~u
~ o
:3
9
9 o
A 9
0.10
9
o
9
-75
ppm CFs
o
-90
ppm CF2 (PVDF)
9
- 110 p p m CFz
[]
- 118 p p m CFz
A
-184
p p m CF
9
o,_ In c o c o~,
0.01 -
0.0
o
I
I
2.0
4.0
I
6.0
contact t i m e ( m s )
Fig. 18.22. Evolution of the 19F magnetisation as a function of the contact time in a standard cross-polarisation experiment for a commercial Viton sample.
704
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN Table 18.4. Effective
relaxation times of a Viton sample
(ppm)
Teff (ms) a Xlp
-75.1 -89.9
2.3 ___0.1 1.6 __ 0.1 1 . 2 _ 0.2 1.2 _ 0.2 2.4 _ 0.1
- 110.8
-118.0 -183.9 a
Teff --lp
The errors quoted are statistical.
an effective magnetisation transfer process. Effective proton-fluorine Tip values are given in Table 18.4. The values were obtained by fitting the decay of the magnetisation in the cross-polarisation experiments by the simplified equation:
M(r) =
Mo (1 - THF/Tlo )
(1 - exp[--(1/THF -- 1/Tlo)'r]) exp(-'r/Tlo).
The experimental data obtained using the T O R Q U E sequence [128] (Section 6.6) reveal in more detail the evolution of the 19F magnetisation. They show that the peaks associated with CF and CF3 fluorines gain magnetisation more slowly than the peaks associated with CFe groups. To have a reasonable idea of the cross-polarisation times, TI-IF, the T O R Q U E experimental data should be fitted, using T1H and T~p values from independent measurements. We have obtained 13C high-resolution spectra by means of 1H ~ 13C and 1 9 F ~ 13C cross polarisation. In both cases 13C spectra were obtained with 1H d e c o u p l i n g , 19F decoupling, or both 1H and 19F simultaneous decoupling during acquisition. The 13C spectra of the commercial Viton sample are shown in Figs. 18.23 and 18.24. Cross polarisation works as a discriminating technique that allows us to observe only ~3C atoms bonded to protons or bonded to fluorines. The high mobility of the Viton sample makes cross polarisation only effective for 13C atoms directly bonded to protons in the case of XH ~ 13C cross polarisation or bonded to fluorines in the case of 1 9 F ~ 13C cross polarisation (in contrast to the case for crystalline PVDF). The resonance lines were assigned by comparison with the 13C spectrum of PVDF [2]. The peaks around ~ c - 120 ppm correspond to CF3, CF2 and CF groups; peaks around 8c = 40 ppm correspond to C H 2 groups. The two peaks observed in the latter region reflect conformational effects of PVDF. The position of the main peak in this region is shifted by 10 ppm with respect to the main resonance of VDF homopolymer. The position of the small peak is coincident with the resonance of pure PVDF.
FLUOROPOLYMERS
705
1
~'
............
I
200
'
'1
100
'
I
0
[i(ppm) Fig. 18.23. 1H ---) 13C cross-polarisation spectra of P(VDF-co-HFP)" (a) 1H and 19F decoupling;
(b) IH decoupling; and (c) 19F decoupling. The experimental conditions were" proton zr/2 pulse 5/zs; contact time i ms; proton and fluorine decoupling power equivalent to 50 kHz; spinning speed 4 kHz; and acquisition time 25 ms.
Doverspike et al. [66] reported a deuteron N M R orientational lineshape study of perdeuterated (99%) PVDF and the copolymer of vinylidene fluoride with tetrafluoroethylene having 80 mol% VDF. The experiments examined only the crystalline part of the drawn and poled samples (crystallinity of P(VDF-co-TFE) estimated by X R D to be 50%). The maximum remanent polarisation attained with deuterated PVDF homopolymer film was 1.0/~C/cm 2, and for the copoplymer was 3.0/zC/cm 2. A Gaussian distribution about the draw direction characterises the chain-axis reorientation distribution in the stretched samples. The PVDF (draw ratio 3) has a Gaussian distribution of width 22 ~ (half width at e -1 of maximum), whereas, the P(VDF-co-TFE) has a distribution of width 18 ~ reflecting the better alignment of the more highly stretched copolymer film (draw ratio of 4). The authors found that as the magnetic field is rotated in the plane perpendicular to the stretch direction, the deuterium spectra of the poled copolymer sample do not change. It is concluded that the occurrence of electrical polarisation in the absence of orientation dependence of the deuterium lineshape indicates molecular reorientation through 180 ~ in the copolymer. Schmidt et al. [129] carried out deuteron N M R studies on a 70/30 mol%
706
R . K . HARRIS, G. A. MONTI AND P. HOLSTEIN
c) '
I
200
'
I
100
'
I
0
'
,5(ppm) Fig. 18.24. 19F--->13C cross-polarisation spectra of P(VDF-co-HFP) with: (a) 1H and 19F decoupling; (b) 19F decoupling; and (c) 1H decoupling. The experimental conditions were: proton ~r/2 pulse 5 txs; contact time 1 ms; proton and fluorine decoupling powers equivalent to 50 kHz; spinning speed 4 kHz; and acquisition time 25 ms.
random copolymer of deuterated vinylidene fluoride and normal trifluoroethylene for temperatures above and below the ferroelectric-paraelectric phase transition of the crystalline portion of the copolymer. The observed narrowing of the deuterium lineshape in the paraelectric crystalline phase was interpreted as a result of motional narrowing caused by C n 2 H bonds making tetrahedral angles with the chain axis at all times, but rotating about that axis. A calculation was presented for the deuteron quadrupolar NMR spin-lattice relaxation caused by kink-3-bond motions along disordered helical chains in the paraelectric phase.
18.7
Composites and miscellaneous materials
In this section, papers reporting work on some materials containing both fluoropolymers and small molecules will be mentioned briefly. There are a number of NMR reports on ion-exchange membranes of the Nation | type. In these ionomers, perfluorinated sulfonates are attached as pendant groups to PTFE backbones. Several of the publications [130-136]
FLUOROPOLYMERS
707
deal with proton relaxation and bandshape measurements for water contained in the material, yielding information on mobility and domain formation, usually as a function of temperature. One paper [133] extends such a study to 2H spectra, which yielded two lines for Nation (due to unaveraged quadrupolar coupling) but only one for the related Dow system (XUS), suggesting that hydrogen bonding is weaker in the latter than in the former. Other papers [134,135,137-139] report 19F NMR measurements (T1, Tlo, T2 and bandshapes) taken under variable-temperature conditions. These give information about the mobility of the backbone and side chains. One article [139] reports a study of Nation/liquid crystal composite membranes, the presence of the liquid crystal apparently not appreciably affecting the mobility of the polymer matrix. A series of publications [134-136] have appeared reporting NMR studies on Nation using IH, 2H, 170 and 19F resonances at elevated pressures (with temperature held constant), and giving the activation volumes deduced. All the work on Nation involves static samples, and neither heteronuclear nor homonuclear decoupling was employed. Composite polymer electrolytes based on PVDF (at least 35%) containing lithium salts were studied [140] by a number of techniques including 7Li NMR. Spin-lattice relaxation measurements were used to show that localised lithium motion is significantly impeded in some samples but not in others. Fluorinated charcoal has been studied [141] by 19F MAS and 13C--->19F CPMAS spectroscopy. Dipolar dephasing and variable contact experiments yielded information on asignments and on quantification. Four types of carbon site were recognised, namely graphitic (C), CF, CF 2 CF2 and CF3. Fluorinated charcoal has been studied [141] by 19F MAS and 13C--> 19F CPMAS spectroscopy. Dipolar dephasing and variable contact experiments yielded information on assignments and on quantification. Four types of carbon site were recognised, namely graphite (C), CF, CF2 and CF3.
18.8
Postscript
While the early broadline and relaxation studies of fluoropolymers were very informative, particularly with regard to mobility at the molecular level, modern high-resolution techniques for both 19F and 13C are now being applied and show considerable potential for an expansion in the areas of applicability. The more sophisticated pulse sequences (e.g., for multi-dimensional and multiple-quantum spectra) have scarcely been used to date, with a few notable exceptions. The unusually favourable properties of the 19F nucleus offer the prospect of more detailed studies than are feasible with polymeric systems
708
R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN
containing only 1H and 13C as NMR-active nuclei. Expanded activity in this area is to be expected.
Note added in proof Recent (1997) papers report (i) 19F MAS studies [142] of Kel-F, which showed that substantially improved resolution is obtainable at elevated temperatures, (ii) an examination [143] of the ferroelectric phase transition for P(VDF/TrFE) using 13C CPMAS N M R , and (iii) a 13C CPMAS investigation [144] of perfluorooctyl acrylate/methylacrylate copolymer blends. Also, molecular motion in liquid-crystalline fluoropolymers of both main-chain and side-chain types has been extensively studied [145] by 1H ~ 13C and 19F ~ 13C CPMAS N M R , including two-dimensional isotropic/anisopropic chemical shift and WISE experiments. The reader's attention is also drawn to two reviews [146,147] summarising the work of Veeman and Maas, discussed in Section 18.5, on P M M A / P V D F copolymers and blends.
Acknowledgements One of us ( G . A . M ) thanks C O N I C E T (Argentina) for a post-doctoral fellowship, during the tenure of which this article was written. We are grateful to the Deutsche Akademischer Austauschdienst and the British Council for support to enable the collaborative work to occur. We also thank the U.K. E P S R C for research grant L02906.
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712
R . K . HARRIS, G. A. MONTI AND P. HOLSTEIN
133. G. Xu and Y.S. Pak, Solid State Ionics 50 (1992) 339. 134. J.J. Fontanella, C.A. Edmondson, M.C. Wintersgill, Y. Wu and S.G. Greenbaum, Macromolecules 29 (1996) 4944. 135. R.S. Chen, P.E. Stallworth, S.G. Greenbaum, J.J. Fontanella and M.C. Wintersgill, Electrochim. Acta 40 (1995) 309. 136. J.J. Fontanella, M.C. Wintersgill, R.S. Chen, Y. Wu and S.G. Greenbaum, Electrochim. Acta 40 (1995) 2321. 137. N.G. Boyle, V.J. McBrierty and A. Eisenberg, Macromolecules 16 (1983) 80. 138. S. Schlick, G. Gebel, M. Pineri and F. Volino, Macromolecules 24 (1991) 3517. 139. J.A. Ratto, S. Ristori, F. Volino, M. Pineri, M. Thomas, M. Escoubes and R.B. Blumstein, Chem. Mater. 5 (1993) 1570. 140. F. Croce, G.B. Appetecchi, S. Slane, M. Salomon, M. Tavarez, S. Arumugam, Y. Wang and S.G. Greenbaum, Solid State Ionics 86 (1996) 307. 141. D.K. Murray, E.W. Hagaman and G.D. Del Cul, Prepr. Amer. Chem. Soc. (Fuel Div.) 42 (1997) 232. 142. P.K. Isbester, T.A. Kestner and E.J. Munson, Macromolecules 30 (1997) 2800. 143. F. Ishii and A. Tsutsumi, Polym. Prepr. 38 (1997) 816. 144. K.L. Altmann, E.H. Merwin and R.D. George, Polym. Prepr. 38 (1997) 786. 145. M.J. Duer and E.C. Stourton, unpublished work. 146. W.S. Veeman and W.E.J.R. Maas, NMR Basic Princ. Prog. 32 (1994) 127. 147. W.E. Maas, Amer. Chem. Soc. Symp. 598 (1995) 274.
Ctzapter 19
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Hydrogen-Bonded Polymers Fumitaka Horii and Kenji Masuda Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
19.1
Introduction
There are many kinds of hydrogen-bonded polymers in which hydrogen bonds are formed between OH groups, OH groups and carbonyl groups, amido groups or isocyanate groups, etc. In this chapter, the structure and hydrogen bonding of poly(vinyl alcohol) is mainly described as analyzed by CP/MAS ~3C NMR spectroscopy, since other representative hydrogen-bonded polymers, such as polyamides, polypeptides, protein and polysaccharides, are described in detail in other chapters. Poly(vinyl alcohol) (PVA), which is highly crystalline even in almost atactic form and hydrophilic due to the existence of OH groups [1, 2], has recently received much attention as one of the potential raw materials for highperformance hydrogels and high-tenacity fibers. However, it is very important, in developing such materials, to characterize intra- and intermolecular hydrogen bonding of PVA in the dissolved and solid states in detail. Infrared (IR) spectroscopy has been used frequently to investigate the hydrogen bonds in terms of the absorption band at --~3000 cm-~, ascribed to the OH stretching or its overtone at ~6700 c m - 1 [3-6]. In particular, the latter absorption band was resolved into three lines assigned to the OH groups associated with intramolecular, intermolecular and no hydrogen bonding in the order of increasing wavenumber. However, it may be difficult to further separate the contributions from the crystalline and noncrystalline regions into these three lines. High resolution solid-state NMR spectroscopy is also a very powerful method for characterizing the solid structure and the local motion of different solid polymers. We recently characterized the crystalline-noncrystalline structure for different crystalline and liquid crystalline polymers, such as polyolefins [7-12], polyesters [13-15], polyether [16], polyurethanes [17, 18] and polysaccharides, including cellulose [19-29], amylose [30, 31] and dextran [32]. On the basis of these analytical methods, we also investigated the intra- and intermolecular hydrogen bonds of PVA in both crystalline and noncrystalline regions as well as in the frozen solution state. In this chapter,
714
FUMITAKA HORII AND KENJI MASUDA
we review mainly these e x p e r i m e n t a l results of the h y d r o g e n bonding f o r m e d in different P V A samples with different tacticities.
19.2
CP/MAS
13C NMR spectra of PVA
Figure 19.1 shows C P / M A S 13CN M R spectra m e a s u r e d at r o o m t e m p e r a t u r e for dried P V A films with different tacticities [33]. H e r e , the degree of polymerization (DP) and the triad tacticity of each sample are as follows: D P = 1590, m m = 0.19, m r = 0.48 and r r - 0.33 for S-PVA; D P = 1700, m m = 0.23, m r = 0.50 and rr = 0.27 for A - P V A ; D P = 300, m m = 0.57, m r = 0.35 and rr = 0.08 for I - P V A . These films were subjected to annealing above
CH2
CH
I[[ II
S-PVA
I~PVA
Solution ,
,
,
III ,
I
80
,
,
,
III ,
I
60
,
,
,
,
I
40
I
I
v
I
I
I
t
20
ppm from Me4Si
Fig. 19.1. CP/MAS ~3C NMR spectra of different dried PVA samples measured at room temperature.
HYDROGEN-BONDED POLYMERS
715
O-ring
~sample iliiiiiii!i!iiiiiiiiiiiii~!iii: i!Li~i~i!!i!!iiiiii~ili!i!i!!i!
1 Fig. 19.2. Schematic diagram of a MAS rotor with an O-ring seal.
150~ In addition, a cylinder-type MAS rotor shown in Fig. 19.2, (originally developed by Horii et al. [33] and now available from JEOL), was used for the dried samples to prevent the absorption of moisture during NMR measurements. The CH resonance line of each sample shown in Fig. 19.1 clearly splits into a triplet, lines I, II and III, and the relative intensity of the three lines depends greatly on the triad tacticity. The extent of the line splitting is much higher compared to the case of the splitting due to the triad tacticity in the solution-state 13C NMR spectrum, as shown in a stick-type spectrum at the bottom of Fig. 19.1. On the basis of these results, lines I, II and III were initially assigned to the CH carbons with two, one, and no intramolecular hydrogen bond(s) in the triad sequences of the PVA chain with planar zigzag structure [34], as shown in Fig. 19.3. Here, the OH group bonded to the CH carbon I may form intramolecular hydrogen bonds with two OH groups on both sides in the mm sequence. In contrast, the OH groups associated with CH carbons II and III will form one and no hydrogen bond with the neighboring OH groups in mr and rr sequences, respectively. According to the crystal structure of PVA [35, 36], the O...O distance is estimated to be 0.252 nm in the intramolecular hydrogen bonds. Therefore, it is assumed that strong deshielding that induces a large downfield shift should occur for the CH carbons I and II, in analogy with the case of hydroxybenzaldehyde crystals
716
FUMITAKA HORII AND KENJI MASUDA
mr t
H
I
H
t
H U
mm
I
rr
Fig. 19.3. Schematic diagram for intramolecular hydrogen bonding in the triad sequences of PVA.
[37]. In these crystals, marked downfield shifts of resonance lines were observed for the carbons chemically bonded OH groups with decreasing O...O distance, when this distance is less than ---0.27 nm in the intramolecular hydrogen bonding between the hydroxyl and carbonyl groups. Therefore, the largest downfield shift should appear for the CH carbon I in solid PVA, while the medium-level downfield shift will be observed for the CH carbon II. In contrast, the CH carbon III is not associated with the intramolecular hydrogen bonding, but the O H group chemically bonded to this carbon may form the intermolecular hydrogen bonding with O H groups in the neighboring chains. However, the intermolecular hydrogen bonding will induce no appreciable downfield shift for the CH carbon, because the O...O distance is more than 0.27 nm in the intermolecular hydrogen bonds of PVA crystals. Therefore, it seems plausible to assume that resonance lines of the CH carbons I, II and III appear in the order of increasing field as shown in Fig. 19.1. In contract, it was proposed from the evaluation of 13C N M R results for solid ethylene-vinyl alcohol copolymers that the large split of the CH resonance line would be possibly induced by the substituent effect due to the introduction of O H groups to polyethylene chains [38]. More recently, ab initio gauge-included atomic orbital (GIAO) calculations were carried out for solid PVA and the triplet of the CH resonance line seemed to be well interpreted in terms of the formation of the intramolecular hydrogen bonding [39]. However, an appearance of a similar triplet was suggested by the similar calculation for the planar zigzag chain of polypropyrene, which has methyl groups as substituents in place of OH groups. Therefore, it was concluded in this case that the main cause of the triplet of the CH resonance line would be the substituent effect. To confirm the assignment of the triplet of the CH
HYDROGEN-BONDED POLYMERS
717
line of solid PVA, further experiments should be conducted on the basis of the solid-state NMR theories and computer-aided analyses. In the following sections, we describe some experimental results supporting the initial assignment that lines I, II and III are ascribed to the CH carbons bonded to OH groups associated with two, one and no intramolecular hydrogen bond(s). One of the most important experiments is to separate the contributions from the crystalline and noncrystalline regions, because the degree of crystallinity is less than 0.5 in each sample. Such a separation will be also useful for elucidating the relative intensities of lines I, II and III for different PVA samples shown in Fig. 19.1. In fact, the relative intensities of lines I, II and III are not in accord with the relative fractions of mm, mr, and rr sequences, e.g. the relative intensity of A-PVA significantly deviates from 1:2:1 that can be assumed from the triad tacticity.
19.3
Spectra of the crystalline and noncrystalline components
To separate the contributions from the crystalline and noncrystalline components, ~3C spin-lattice relaxation times T~c were measured for the PVA samples by the CPT1 pulse sequence [40]. In Fig. 19.4, the logarithmic peak intensities of line II are plotted against the decay time for A-PVA films at room temperature [33]. The total decay curve, indicated by open circles, was successfully resolved into three components with different T~c values by the least-squares method, as shown by the solid and dashed lines in Fig. 19.4. Similarly, three components were also recognized for other resonance lines of A-PVA as well as all lines of S-PVA and I-PVA. Since the glass-transition temperature of PVA is about 70 or 85~ [1], the segmental motion of PVA chains must be restricted at room temperature. In this situation, the longer T~c indicates less molecular mobility of the component. Therefore, the longest Txc component was assigned to the crystalline component, whereas the medium and shorter T~c components are ascribed to the less mobile noncrystalline and mobile noncrystalline components, respectively. The detailed estimation of the degree of crystallinity supports these assignments [33]. The T~c values for the crystalline component are more than 5 times larger than the values for the noncrystalline component. Using this difference in T~c, CP/MAS ~3C NMR spectra of the two components are recorded separately by selective measurements of the crystalline component by the CPT1 pulse sequence and the following spectral subtraction method. Figure 19.5 shows the CH resonance lines of the crystalline and noncrystalline components of A-PVA films thus obtained [33]. In this figure, the results of the computer lineshape analyses for these resonance lines are also shown. Here,
718
FUMITAKA
HORII AND KENJI MASUDA
(
CH(If)
5.0 4-)
o
------4.0 I'-"E2"-~:~,
TIC=65.0 s
3.0-
r
N N k
I-.-i,
2.01.0-
A
\ TIC:I4 6s \
TIC=I .2s I
j
l
20
i
\
9
X
X
a q i
40
x
i
x
x
i
x
\
x i
\
60
i
80
i
I
100
Time/s Fig. 19.4. Semilogarithmicplot of the peak intensity of the resonance line II of dried A-PVA
films as a function of time. The solid line is the composite decay curve for three components with different T i c values, which are shown by broken lines (Ref. [33]). each line was assumed to be described as Gaussian. The composite curve of the three lines I, II and III, shown by a broken line, is in good agreement with the experimental spectrum for each component. As is clearly shown in Fig. 19.5, the integrated fractions of lines I, II and III are still in discord with the fractions 0.23:0.50:0.27 of the mm, mr and rr sequences for both components. This discordance will be interpreted by assuming that some of O H groups in the m sequences do not form intramolecular hydrogen bonds but they are associated with the intermolecular hydrogen bonding. A detailed discussion will be given in the later section.
19.4
Effects of casting solvents for film preparation
Figure 19.6 shows the CP/MAS 13C N M R spectra of unannealed A - P V A films prepared from aqueous, dimethyl sulphoxide (DMSO) and hexafluoroisopropanol (HFIP) solutions [41]. It is found that the relative intensities of the CH triplet of the sample prepared from the aqueous solution are signifi-
HYDROGEN-BONDED POLYMERS II
719
IiI
I
(b)noncrystall
90
80
70
60 ppm from Me4Si
Fig. 19.5. CH resonance lines of (a) crystalline and (b) noncrystalline components in A-PVA
films (Ref. [33]). cantly different from those for the samples prepared from DMSO and HFIP solutions. Table 19.1 files the T~c values of the respective resonance lines for A-PVA films prepared from different solvents, which were measured by the CPT1 pulse sequence. There are also three components with different T~c values, in accord with the results of annealed A-PVA described in Section 19.3. As for the longest Tic values, A-PVA films prepared from the aqueous solution have the longest Txc values, while the sample prepared with HFIP has the shortest T~c values. Such differences in Tic may be due to the difference in the size of the crystallites. Using the difference in T~c between the crystalline and noncrystalline components in these samples, the respective spectra of the crystalline and noncrystalline components are also recorded separately. According to these results, line I is increased in intensity and line III is concomitantly reduced in intensity in the order of H20, HFIP and DMSO as casting solvents in both crystalline and noncrystalline components. This fact suggests that intramolecular hydrogen bonds will be more preferably produced in this order in the crystalline and noncrystalline regions. More detailed discussion will be made in the later section.
720
FUMITAKA HORII AND KENJI MASUDA
CH2
CH Ill
III a)
(b
I
. . . .
100
!
80
'
~
'
I
60
. . . .
i
40 2O ppm from Me4Si
Fig. 19.6. CP/MAS 13C NMR spectra of unannealed A-PVA films prepared from different solutions: (a) H20; (b) DMSO; and (c) HFIP solutions.
Table 19.1. 13Cspin-lattice relaxation times of the respective resonance lines for different PVA samples, measured at room temperature Sample
T1c/S CH I
A-PVA a A-PVA b A-PVA c
78.0 50.1 40.2
7.9 8.5 3.7
II 1.9 0.1
69.0 52.9 36.1
15.0 13.0 4.6
CH2
III 3.0 2.0 0.5
aUnnannealed sample prepared from H20 solution. bUnannealed sample prepared from DMSO solution. CUnannealed sample prepared from HFIP solution. -, not measured.
79.0 67.6 38.1
29.7 12.5 5.5
5.0 1.8 0.5
68.0 52.0 38.1
11.3 8.7 5.6
0.7 1.0 1.1
HYDROGEN-BONDED POLYMERS 19.5
721
Structure in the hydrate state
CP/MAS 13C N M R measurements are also powerful in characterizing hydrated polymer samples if the water content is not reduced by high centrifugation due to MAS during N M R measurements. We originally developed the MAS rotor with an O-ring seal, shown in Fig. 19.2, for hydrated samples [30-33] and successfully obtained CP/MAS 13C N M R spectra and spin-relaxation times for different polymer samples without any loss of water. Figure 19.7 shows CP/MAS 13C N M R spectra of A - P V A films with different water contents using the MAS rotor with an O-ring seal [33]. Here, the respective samples were exposed to atmospheres of different relative humidities at 24~ in a desiccator for about 1 week to obtain the equilibrium state. The water content is expressed as (g of H 2 0 / g of dry PVA) x 100%. As clearly shown in Fig. 19.7, an additional resonance line, which is termed line IV, appears between lines II and III for the sample with a water content CH2
CH
II III water content PE
0%
3%
18%
27%
l•''.I''.•I''''l'''.i''''I''''l'''••''''I''''l''''I''''l''''I''''I''''1''''I'''' 80
60
40 20 ppm from Me4Si
Fig. 19. 7. CP/MAS 13C NMR spectra of annealed A-PVA films with different water contents
(Ref. [33]).
722
FUMITAKA HORII AND KENJI MASUDA
CH2 CH
....
! ....
, ....
I ....
80
, ....
I ....
, ....
I ....
60
, ....
I ....
, ....
! ....
v ....
! ....
v ....
!
40 20 ppm from Me4Si
Fig. 19.8. Dipolar-decoupled MAS 13C NMR spectra of the rubbery component in annealed A-PVA films with a water content of 18%, which were measured by the modified 13C spinecho method (Ref. [33]).
of 24%. Since T i c values of this component are of the order of 0.1-0.2 s, line IV can be assigned to the rubbery component where the intra- and intermolecular hydrogen bonds may be broken by water molecules. In fact, the real line shape of line IV, which was selectively observed by using the 13C spin-spin relaxation time T2c of this component as shown in Fig. 19.8 [33], was in good accord with the solution-state 13C NMR spectrum. Figure 19.9 shows the result of the lineshape analysis for the spectrum of the crystalline component of A-PVA with a water content of 18% [33], which was carried out in the same way as for dried PVA films. Although an additional Gaussian curve must be introduced upfield for line III, the composite curve of the four lines, which is described by a broken line, reproduces well the experimental spectrum of the crystalline component. The additional upfield line can be assigned to the component free from the intra- and intermolecular hydrogen bonds, which may probably appear as a result of the enhancement in molecular mobility by water. In Fig. 19.8 the integrated fractions of lines I, II and III are also shown. The fractions of lines I and II significantly increase compared with those for the dry sample shown in Fig. 19.5. This may suggest that the probability of intramolecular hydrogen bonding is increased upon addition of water.
H Y D R O G E N - B O N D E D POL YME R S
0.52 II
80
70
723
0.33 III
60 ppm from Me4Si
Fig. 19.9. Lineshape analysis for the CH resonance line of the crystalline component of annealed A-PVA films with a water content of 18%. The broken line is the composite curve of the four components shown by the solid lines and the numerical values are the integrated fractions of lines I, II and III (Ref. [33]).
19.6
Fibers: effects of drawing
It is well known that ultrahigh molecular weight linear polyethylene samples can be drawn up to 100-200 times (super drawing) when they are prepared as films, or spun as fibers, from the gels produced at an appropriate polymer concentration [42-44]. The Young's modulus of the samples thus obtained reaches 235 GPa, which corresponds to the Young's modulus along the molecular chain axis for polyethylene crystals [45]. This fact suggests that the polymer chains are highly extended by the super drawing. PVA also adopts the planar zigzag-conformation in the crystalline region as polyethylene, and the Young's modulus along the molecular chain axis for PVA crystals is significantly higher (250 GPa [46]) than for polyethylene crystals. Therefore, different methods were tried to produce high tenacity PVA fibers, but such trials are still unsuccessful possibly because of the difficulty in controlling intra- and intermolecular hydrogen bonds in the case of PVA. Here, we describe briefly the CP/MAS ~3C NMR results of PVA fibers with different draw ratios, which were spun from aqueous or DMSO solutions [47]. Figure 19.10 shows the CH resonance lines of the crystalline components of PVA fibers spun from the 10 wt% DMSO solution, which were selectively measured by the CPT1 pulse sequence. This figure also shows the results of the lineshape analysis by the computer-aided least-squares method. In these fibers, it is necessary to introduce two Gaussians, lines IIIb and IIIf, for line
724
F U M I T A K A H O R I I AND KENJI M A S U D A
!H
III b ;IIlf ~
..
x=4.1
t
_~.
_ z,,, ^
annealed
spun
t'v'X~
j L n v l , , , v l t , , , l , , , , l , , , , I v v v , l , v , , l , , , ,
80
70
60 ppm from Me4Si
Fig. 19.10. CH resonance lines of the crystalline components of PVA fibers spun from the 10% DMSO solution (Ref. [47]).
III, as in the case of hydrated PVA samples shown in Fig. 19.9. Since the chemical shift of line IIIf is in good accord with that of the upfield line for the hydrated PVA, line IIIf can be assigned to the CH carbons chemically bonded OH groups, free from the intra- and intermolecular hydrogen bonding. As clearly shown in Fig. 19.10, the fraction of OH groups, free from hydrogen bonds, is increased significantly with increasing draw ratio. Similar results were obtained for PVA fibers spun from the aqueous solution. In contrast, much different effects of drawing were observed for the noncrystalline component of PVA fibers, as shown in Fig. 19.11. Here; are the CH resonance lines of the noncrystalline components of PVA fibers spun
725
H Y D R O G E N - B O N D E D POL YME R S
II
/
P<_
6.o
_/ _
."
ed
pu 80
70
60 ppm from Me~Si
Fig. 19.11. CH resonance lines of the noncrystalline components of PVA fibers spun from the 10 wt% DMSO solution (Ref. [47]).
from the DMSO solution are shown. There is no contribution from line IIIf, but lines I, II and III change markedly in intensity with drawing. Namely, line I increases in intensity with increasing draw ratio, while the intensity of line III is concomitantly reduced. As a result, the relative intensities seem to attain 1:2:1 at higher draw ratios. This fact suggests that the intermolecular hydrogen bonds may be broken by drawing, and the intramolecular hydrogen bonds are more preferably formed in the mm and mr sequences with increasing draw ratio.
726 19.7
F U M I T A K A H O R I I AND KENJI M A S U D A
CP/MAS
13C NMR spectra of frozen PVA solutions
To obtain further information about the hydrogen bonding of PVA, CP/MAS 13CNMR measurements were conducted for frozen PVA solutions [48]. Each PVA aqueous or DMSO-d6 solution was packed into the MAS rotor with an O-ring seal at room temperature and then frozen in the rotating state in a CP/MAS probe by cooling to -50~ Figure 19.12 shows CP/MAS 13C NMR spectra of frozen A-PVA DMSO-d6 and aqueous solutions measured at -50~ For reference, the CP/MAS ~3C NMR spectrum for A-PVA films prepared from the aqueous solution is also shown in Figure 19.12. Interestingly, two lines which correspond to lines II and III for films are observed even in both frozen PVA solutions. However, the relative intensities of the two lines significantly depend on the solvents: line II appears just as a downfield shoulder of line I in the frozen aqueous solution, whereas this line is much enhanced in intensity in the frozen DMSO solution similar to the case of films. This fact may suggest that the intramolecular hydrogen bonds are produced much more in the frozen DMSO solution, although the gauche contribution due to the so-called y-gauche effect [49] should be correctly evaluated in these cases. CH
CH2,CH3(DMSO-d6)
(a) DMSO
(
100
80
60
40
20
p p m from Me4Si Fig. 19.12. CP/MAS ~3C NMR spectra of frozen A-PVA solutions measured at -50~ DMSO" (b) H20; (c) films prepared from the 10 wt% aqueous solution.
9(a)
HYDROGEN-BONDED POLYMERS
727
I I CI"I2, CH3(DMSO'd6)
CH
(a)DMSO-d6
I II (c) Films "
!
100
'
'
~
'
'
I
80
.
.
.
.
i
60
'
'
'
'
i
'
'
'"
'
I
40 20 ppm from Me4Si
Fig. 19.13. CP/MAS 13C NMR spectra of frozen HI-PVA solutions measured at -50~ DMSO; (b) H20; (c) films prepared from the 10 wt% aqueous solution.
(a)
Figure 19.13 shows the CP/MAS 13C N M R spectra of frozen DMSO-d6 and aqueous solutions of highly isotactic P V A (HI-PVA); DP = 9100, mm 0.79, mr = 0.19 and rr = 0.02 for HI-PVA [50]. The spectrum of the frozen aqueous solution, including the CH2 resonance line, differs greatly from the spectrum of the films. The splitting of two CH lines appear to correspond to lines II and III for PVA films and for the frozen A - P V A solutions, but their chemical shifts are significantly higher than those of lines II and III. Also, there is no contribution from line I as in the cases of the frozen A - P V A solutions. Moreover, only a single resonance line is observed for the frozen DMSO-d6 solution of HI-PVA, as shown in Fig. 19.13(a). Since the chemical shift of this line is somewhat higher than that of line III, this line cannot be assigned to line III. These experimental facts indicate that the probability of the formation of the intramolecular hydrogen bonds may be considerably low in the frozen HI-PVA solutions, but conformational effects due to the Ygauche effect should be taken into account in detail to interpret these spectra.
728
F U M I T A K A HORII AND KENJI M A S U D A
19.8
Statistical calculation for the formation of hydrogen bonds
As described in the preceding sections, lines I, II and III change significantly in intensity depending on the differences in morphological components, casting solvents, draw ratios and frozen solvents, even for PVA samples with the same tacticities. According to the initially proposed assignment, these experimental observations should be interpreted in terms of the changes in fractions of intra- and intermolecular hydrogen bonds, as well as in fractions the trans- and g a u c h e - c o n f o r m a t i o n s . Here, we simply describe the statistical calculation for the formation of the intra- and intermolecular hydrogen bonds for PVA chains with the planar zigzag conformation, and apply this calculation to the interpretation of the relative intensities of lines I, II and III for the crystalline component in different PVA samples. Although the crystal structure of PVA has extensively been studied by many research groups, the detailed structure has not been confirmed. In these investigations there is no conflict for a two-chain monoclinic unit cell with a - 0.783 nm, b - 0.252 nm (fiber axis), c - 0.551 nm and/3 - 92 ~ [35, 36]. However, the relative positions of the molecular chains in the b-projection and the hydrogen bonding are still controversial subjects. Figure 19.14 shows the projections of the molecular chains to the a - c plane in the crystal structure models previously proposed [35, 36]. In both models, oxygen atoms, which are drawn as larger circles, are placed randomly in either site of the two possible positions. However, there are significant differences in relative positions of the respective atoms, and in hydrogen bonds depicted by broken lines between these two models. Because of the
I
il
~ Q
(a)
= a
(b)
Fig. 19.14. Crystal structure models of PVA: (a) Bunn's model [36] and (b) Sakurada's model [35]; projection of the chains to the a - c plane.
729
HYDROGEN-BONDED POLYMERS
simplicity of the hydrogen bonding, we describe here the calculation using the model proposed by Sakurada et al. [35]. A similar statistical calculation [51] can be also carried out by using the Bunn model [36], although it is somewhat more complicated. According to the model of Sakurada et al. (Fig. 19.14b), one neighboring PVA chain associated with the intermolecular hydrogen bonding with a given PVA chain is placed in the same or neighboring unit cell. The molecular chains connected through the intermolecular hydrogen bonding are shifted with each other by half of the fiber period (0.126 nm) along the b axis. Accordingly, we focus our attention on the mm and mr sequences along the central chain shown in Fig. 19.15 [52, 53], and examine the formation of the intra- and intermolecular hydrogen bonds for the possible pairs between the triad sequences in the central chain, and the tetrad or triad sequences in the neighboring chains. For example, three pairs of mm-mrr', mm-mrm' and mr-mr, associated with the intra- and interF
--0~ --0
I I I I
,0-I I
'LO - - -
I l
t
l i ,
0--
lllrln'
F
Ill
inl]]
I I .I
___
_Of- 0-
I I
[
__0 f ~
I I I I
I
---o ,,
lo--
t_
mr
III
. . . .
I ..]
mrl [-----
' ,',
I L_
--0
I I I
0---
I I i
"'O--
--o~I I
I I
mrr'
Fig. 19.15. Triad-tetrad and triad-triad pairs which may form intra- and intermolecular hydrogen bonds along three PVA chains in the crystalline region in the case of the crystal structure proposed by Sakurada et al. [35] (Ref. [52]).
730
F U M I T A K A H O R I I A N D KENJI M A S U D A
Table 19.2. Observed and calculated fractions of the CH lines I, II and III for different P V A
samples Sample
Crystalline S-PVA a A-PVA a I-PVA a A - P V A , a hydrated A-PVA d A-PVA e A-PVA f Noncrystalline A-PVA a
Observed fraction
Calculated fraction
I
II
III
I
II
III
pa b
Fa c
0.064 0.109 0.386 0.154 0.092 0.106 0.147
0.350 0.457 0.351 0.513 0.421 0.436 0.479
0.586 0.434 0.263 0.333 0.486 0.458 0.374
0.048 0.090 0.368 0.143 0.091 0.118 0.146
0.364 0.473 0.519 0.522 0.423 0.473 0.478
0.588 0.437 0.113 0.335 0.486 0.413 0.375
0.08 0.32 0.89 0.74 0.14 0.19 0.56
0.24 0.34 0.77 0.46 0.29 0.31 0.42
0.170
0.493
0.337
0.147
0.524
0.329
0.77
0.47
aAnnealed sample prepared from the aqueous solution. bprobability for the formation of the intramolecular bond in the m e s o sequence. CFraction of O H groups associated with intramolecular hydrogen bonds. dUnannealed sample prepared from the aqueous solution. eUnannealed sample prepared from D M S O solution. fUnannealed sample prepared from H F I P solution.
molecular hydrogen bonds, are shown in Fig. 19.15. Here, closed circles indicate the CH carbons asscribed to lines I, II and Ill of the CP/MAS 13C NMR spectrum of PVA, and each sequence with a prime symbol is a mirror image of the corresponding sequence without the symbol. In addition, each arrow indicates the direction of the hydrogen bond involving the OH group. On the basis of this model, we introduce one important parameter, the probability Pa for the formation that the OH group in the m sequence forms the intramolecular hydrogen bond. Then 1 - p~ (=Pe) is the probability of the formation of the intermolecular hydrogen bond in the corresponding OH group. After the complete evaluation of the formation of two kinds of hydrogen bonds, molar fractions of the CH carbons I, II and III, which correspond to CH resonance lines I, II and III, can be described as a function of Pa for different tacticities. The fraction F~ of OH groups forming intramolecular hydrogen bonds can be also determined as a function of p~. Table 19.2 files the integrated fractions of lines I, II and III of the CH triads obtained for the crystalline components in different PVA samples by the lineshape analyses described above. Here, the observed integrated fractions were corrected by considering the difference in Tic for the respective CH lines. In this table, the calculated fractions of lines I, II and III, which were obtained to fit the observed fractions by the least-square method, are also shown together with p~ and F~ values. The calculated fractions agree
HYDROGEN-BONDED POLYMERS
731
fairly well with the observed ones, except for I-PVA. The probability Pa is much less for annealed S-PVA than for annealed A-PVA, although the difference in the meso content is only 0.05 between these two samples. This suggests that the intermolecular hydrogen bonding may be preferentially formed for samples with the m fraction less than some threshold value. In contrast, there is no large difference in probabilities for the intra- and intermolecular hydrogen bondings for annealed A-PVA; P a - - 0 . 3 2 and Pe 0.68. In annealed I-PVA, however, the accordance between the observed and calculated fractions is not so good. This may be due to the fact that Pa depends on the sort of i-j pairs, particularly for the samples with higher isotacticities. For example, Pa may be high for the mm-m'm'r' pair in the mm-rich two chains where almost all OH groups are located at the opposite sites to each other. In contrast, when these OH groups in the two chains are placed at the same side, Pa should become low, for example, for the mmmmr pair. On the other hand, the fractions of lines I and II increase by 0.05 for the crystalline component of hydrated A-PVA compared to those for the dried A-PVA. According to the statistical calculation, this change is induced by the increase in Pa from 0.32 to 0.74 upon the absorption of water. The Xray crystal structure analysis revealed that water does not change the crystal lattice of PVA. However, Tic values of the CH and CH2 carbons for the crystalline component of A-PVA are significantly decreased by the addition of water. Accordingly, a small amount of water, which is not enough to expand the crystal lattice, must diffuse into the crystalline region and break exclusively the intermolecular hydrogen bonding. As a result, new intramolecular hydrogen bonds will be produced in the mm and mr sequences, leading to the increase in the fractions of lines I and II. This change in hydrogen bonding may also induce the enhancement in molecular mobility of PVA chains in the crystalline region, which is also associated with the decrease in TIC of this component. Similar significant changes in Pa are also recognized for unannealed A-PVA films prepared using different casting solvents. The Pa value is significantly increased in the order of H20, HFIP, and DMSO. This fact suggests that the promotion, or disturbance, of the formation of the intra-or intermolecular hydrogen bonds is induced by casting solvents. In addition, it is found that, for A-PVA films prepared from aqueous solutions, annealing induces the significant increase in Pa. This means that the intermolecular hydrogen bonds are broken down, possibly owing to the enhanced molecular motion during annealing, and then more stable intramolecular hydrogen bonds are newly formed. As for the noncrystalline component in annealed A-PVA films, the calcu=
732
FUMITAKA HORII AND KENJI MASUDA
lated fractions of lines I, II and III are also in good accord with the observed fractions as shown in Table 19.2. However, such an accordance may not be very meaningful, although it will be possible to assume that the local structure in the noncrystalline region is quite similar to the structure in the crystalline region in PVA. However, it is very important to consider the effects of gauche conformations, which were neglected in the statistical calculation described above, on the formation of intra- and intermolecular hydrogen bonds in the noncrystalline region, particularly for PVA chains in the frozen solutions. For example, the same types of intramolecular hydrogen bonds are formed in the mr sequence as in the mm sequence, when the chain conformations are tttg- for mr and tttt for ram, respectively. Here, the conformations trans (t) and gauche (g) should be defined for the triad sequence C H ( O H ) ~ C H 2 ~ C H ( O H ) ~ C H 2 ~ C H ( O H ) . Moreover, because of the y-gauche effect, the chemical shift of the Co carbon moves downfield by a given ppm depending the nucleus Xv in the successive C o ~ C ~ C t 3 ~ X v sequence [49]. We have already derived equations for the integrated intensities of lines I, II and III by using two probabilities for the formation of the intramolecular hydrogen bond in the m sequence and for the gaucheconformation in each C ~ C bond. The details of the calculation and the comparison with experimental results will be published elsewhere.
19.9
1H cramps spectra of PVA films
The high resolution solid-state 1H spectra, measured for different dried PVA films by combined rotation and multiple pulse spectroscopy (CRAMPS), are shown in Fig. 19.16. Here, a fully main-chain deuterated atactic PVA (APVA-d3) was also used as a film for comparison. From the downfield side, resonance lines assignable to OH, CH and CH2 protons can be clearly observed, although the OH lines are rather broad and superposed on the neighboring CH lines. To discriminate well between the contributions of OH and CH protons, these superposed lines were resolved into their respective contributions, as shown in Fig. 19.16. In this analysis, each line was assumed as a Gaussian curve, and the validity of this assumption was confirmed by the good fitting for A-PVA-d3. The chemical shifts and linewidths thus obtained for OH resonance lines are plotted against the mm fraction in Fig. 19.17 [53]. The chemical shift increases significantly from 4.25 to 5.35 ppm with increasing mm fraction, while the linewidth becomes markedly narrower from 1.76 to 0.66 ppm with the increase in isotacticity. According to previous 13C N M R results shown in Table 19.2, the fraction of intramolecular hydrogen bonds increases with
733
HYDROGEN-BONDED POLYMERS CI-I2
CH (a) HI-PVA V
-
- -
(b) I-PVA ,.
_
_
(c) A-PVA
(d) A
-
P
V
A
-
'
'
;
d
3
~
~
(e) S-PVA ppm i
1~0
'
. . . .
6 '
Fig. 19.16. 1H CRAMPS spectra of PVA films with different tacticities measured at room temperature (Ref. [53]).
increasing mm fraction. Since the 0 . . . 0 distance is much shorter for the intramolecular hydrogen bond than for the intermolecular hydrogen bonds in PVA, the deshielding effect may induce the significant increase of the chemical shift for higher isotactic samples. On the other hand, the linewidth will possibly reflect the diversity of the structure of the intermolecular hydrogen bonding" the existence of the distribution in 0 . - . 0 distance, O ~ H . . . O angle and the number of hydrogens associated with one oxygen atom. In contrast to such complexity of the intermolecular hydrogen bonding, the structure of the intramolecular hydrogen bond seems very simple and homogeneous as suggested by the structural model described above. Accordingly, the dependence of the linewidth on the isotacticity will also stem from the change in the fractions of the intra- and intermolecular hydrogen bonds. More detailed analyses will be reported somewhere in the near future.
734
FUMITAKA HORII AND KENJI MASUDA '
I
'
i
'
i
'
I
'
3
~5 c~ .,-.t
2-~
~4
---___c_
O .,..~
x:3 L) 2
J
0
I
0.2
L
!
0.4
i
I
0.6
I
I
0.8
,
O
1
mm fraction Fig. 19.17. Plots of the chemical shifts and linewidths of OH resonance lines for different PVA samples as a function of the mm fraction (Ref. [53]).
19.10
Concluding remarks
As described above, we are now developing the precise analytical m e t h o d of b o t h h y d r o g e n bonding and conformations of P V A in the noncrystalline and frozen solution states by considering the upfield shifts of the C H resonance line due to the f o r m a t i o n of the intramolecular h y d r o g e n bonds and the downfield shifts induced by the y - g a u c h e effect. After the establishment of this m e t h o d , high resolution solid-state 13C N M R spectroscopy should become m o r e powerful in characterizing detailed local structure of P V A in different states. F u r t h e r experimental results obtained by this m e t h o d will contribute to the d e v e l o p m e n t of high-performance P V A materials. M o r e detailed analyses will be r e p o r t e d s o m e w h e r e in the near future.
References 1. 2. 3. 4. 5. 6. 7.
I. Sakurada, Polyvinyl Alcohol Fibers. Marcel Dekker, New York, 1985. C.A. Finch, Polyvinyl Alcohol. John Wiley, Chichester, UK, 1992. K. Imai, J. Ukita and S. Matsumoto, Kobunshi Kagaku 16 (1959) 597. S.N. Zhurkov and Y. Levin, Dokl. Akad. Nauk, SSSR 67 (1949) 87. L. Glatt and J.W. Ellis, J. Chem. Phys. 19 (1951) 449. T. Akahane, Y. Kazusa and H. Nakayama, Kobunshi Ronbunshu 37 (1980) 383. R. Kitamaru, F. Horii and K. Murayama, Macromolecules 19 (1986) 636; and related references of polyethylene therein. 8. R. Kitamaru, F. Horii, Q. Zhu, D.C. Bassett and R.H. Olley, Polymer 35 (1994) 1171.
HYDROGEN-BONDED POLYMERS
735
9. S. Saito, Y. Moteki, M. Nakagawa, F. Horii and R. Kitamaru, Macromolecules 23 (1990) 3257. 10. T. Kimura, K. Neki, N. Tamura, F. Horii, M. Nakagawa and H. Odani, Polymer 33 (1992) 4140. 11. R. Kitamaru, F. Horii, M. Nakagawa, K. Takamizawa, Y. Urabe, Y. Ogawa, J. Mol. Struct. 335 (1995) 95. 12. K. Kuwabara, H. Kaji and F. Horii, Macromolecules 30 (1997) 7516. 13. H. Tsujii, F. Horii, M. Nakagawa, Y. Ikada, H. Odani and R. Kitamaru, Macromolecules 25 (1992) 4114. 14. H. Kaji and F. Horii, Macromolecules 30 (1997) 5791. 15. T. Kawaguchi, A. Mamada, Y. Hosokawa and F. Horii, Polymer, in press. 16. a. Hirai, F. Horii, R. Kitamaru, J.G. Fatou and A. Bello, Macromolecules 23 (1990) 2913. 17. M. Ishida, K. Yoshinaga and F. Horii, Macromolecules 27 (1996) 8824. 18. H. Ishida, H. Kaji and F. Horii, Macromolecules 30 (1997) 5799. 19. F. Horii, A. Hirai and R. Kitamaru, Polym. Bull. 8 (1982) 163. 20. f. Horii, A. Hirai and R. Kitamaru, Macromolecules 20 (1987) 2117. 21. A. Hirai, F. Horii and H. Odani, Macromolecules 20 (1987) 1440. 22. F. Horii, H. Yamamoto, R. Kitamaru, M. Tanahashi and T. Higuchi, Macromolecules 20 (1987) 2946. 23. F. Horii, P.E. Pfeffer and W.V. Gerasimowicz (Eds). Chap. 10. Nuclear Magnetic Resonance in AgricultureCRC Press, Boca Raton, FL, 1989. 24. Y. Yamamoto, F. Horii and H. Oadni, Macromolecules 22 (1989) 4130. 25. Y. Yamamoto and F. Horii, Macromolecules 26 (1993) 1313. 26. Y. Yamamoto and F. Horii, Cellulose 1 (1994) 57. 27. Y. Yamamoto, F. Horii and A. Hirai, Cellulose 3 (1996) 229. 28. A. Hirai, M. Tsuji and F. Horii, Cellulose 4 (1997) 239. 29. F. Horii, H. Yamamoto and A. Hirai, Macromol. Symp. 120 (1997) 197. 30. F. Horii, A. Hirai and R. Kitamaru, Macromolecules 19 (1986) 930. 31. F. Horii, H. Yamamoto, A. Hirai and R. Kitamaru, Carbohyd. Res. 29(1987) 160. 32. A. Hirai, T. Ito, F. Horii, R. Kitamaru, K. Kobayashi and H. Sumitomo, Macromolecules 23 (1990) 1837. 33. F. Horii, S. Hu, T. Ito, R. Kitamaru, S. Matuzawa and K. Yamaura, Polymer 33 (1992) 2299. 34. T. Terao, S. Maeda and A. Saika, Macromolecules 16 (1983) 1535. 35. I. Sakurada, K. Futino and Okada, Bull. Inst. Chem. Res., Kyoto Univ. 23(1950) 78. 36. C.W. Bunn, Nature 4102 (1948) 929. 37. F. Imashiro, S. Maeda, K. Takegoshi, T. Terao and A. Saika, Chem. Phys. Lett. 99 (1983) 189. 38. H. Ketel, J. Haan, A. Aerdts and G. Velden, Polymer 31 (1990) 1419. 39. F. Imashiro and S. Obara, Macromolecules 28 (1995) 2840. 40. D.A.J. Torchia, Magn. Reson. 44 (1981) 117. 41. K. Masuda, S. Hu, H. Kaji and F. Horii, Polymer Prep., Japan 45 (1996) 3260. 42. P. Smith, P.J. Lemstra, B. Kalb and A. Pennings, Polym. Bull. 1 (1979) 733. 43. P. Smith and P.J. Lemstra, J. Mater. Sci. 15 (1980) 505. 44. P. Smith, P.J. Lemstra and H.C. Booij, J. Polymer. Sci., Polym. Phys. Ed. 19 (1981) 877. 45. I. Sakurada and K. Kaji, J. Polym. Sci., C 31 (1970) 57. 46. I. Sakurada, T. Ito and K. Nakamae, J. Polym. Sci., C 15 (1966) 75. 47. S. Hu, F. Horii, H. Odani, H. Narukawa, A. Akiyama and K. Kajitani, Kobunshi Ronbunshu 49 (1992) 361.
736
FUMITAKA HORII AND KENJI MASUDA
48. F. Horii, K. Masuda and H. Kaji, Macromolecules 30 (1997) 2519. 49. A.E. Tonelli, NMR Spectroscopy and Polymer Microstructure: The Conformation Connection. VCH, New York, 1989. 50. H. Ohgi and T. Sato, Macromolecules 26 (1993) 559. 51. S. Hu, PhD dissertation, Chap. 3. Kyoto Univ., 1991. 52. S. Hu, F. Horii and H. Odani, Bull. Inst. Chem. Res., Kyoto Univ. 69 (1991) 165. 53. S. Hu, M. Tsuji and F. Horii, Polymer 35 (1994) 9516. 54. F. Horii, S. Hu, K. Deguchi, H. Sugisawa, H. Ohgi and T. Sato, Macromolecules 29 (1996) 3330.
Chapter 20
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Polymer gel systems Hidekazu Yasunaga, ~ Masatoshi Kobayashi 2 and Shingo Matsukawa 3 XDepartment of Chemistry and Materials Technology, Kyoto Institute of Technology, Gosyokaido-tyo, Matugasaki, Sakyo-ku, Kyoto, Japan; 2Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan; 3Department of Food Science and Technology, Tokyo University of Fisheries, Konan, Minato-ku, Tokyo, Japan
20.1
Introduction
Crosslinked polymer gels have distinct chemical and physical properties. One of them is a drastic volume change in response to the application of an electric field [1] and to changes in solvent composition [2], pH [3] and temperature [4]. Such properties of a polymer gel depend on the chemical structure and composition of the monomer units, the primary and higherorder structure, interactions between polymer chains and solvents, molecular motion and so on. Especially, contrary to linear polymers, their unique properties arise from the three-dimensional network structure formed by crosslinkages. However, the network of gels has also made their characterization and measurements difficult. The chemical and physical properties and characteristic values of polymer gels have not been sufficiently determined. We have not so sufficient information on them as solid, liquid and gaseous states. From such a situation, NMR techniques have been applied to polymer gel systems to characterize them. These studies demonstrate that the NMR spectroscopy is one of the powerful means for investigating the structure and dynamics of polymer gel systems. NMR techniques are noninvasive methods that provide information on microscopic chemical structure through chemical shifts, signal splittings and shapes, and molecular motion through relaxation times, correlation times and self-diffusion coefficients. The advantages of NMR techniques for characterizing polymer gels are as follows: (1) noninvasive, (2) no probes are needed, (3) the NMR signal from the main chain, the side chain and the functional group of a sample are detected separately and individual information on them is obtained, (4) change in the structure and the motion of a sample with time is measurable.
738
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
As described above, polymer gel systems composed of the network polymer and solvent are a special state. The solution NMR techniques are available to characterize the solvent contained in the gels, but not to the network polymer. We cannot obtain reasonable NMR signals with high intensities, S/N ratio and resolution for gel samples by solution techniques. This is caused by the restraint of motion, the strong dipole-dipole interactions, the chemical shift anisotropy and their long T~ and short T2 values. High resolution solid-state NMR techniques are powerful means for characterizing the network polymer of polymer gel systems. They give useful information on the network polymer. The function of this chapter is to review the researches on synthesized polymer gels by high resolution solid-state NMR techniques. The research on polymer gels composed of natural materials, such as proteins and polysaccharides, are reviewed in other chapters.
20.2
High resolution solid-state NMR techniques for polymer gels
Rapid isotropic tumbling of molecules is restrained for the network polymers in the gel state. A proton dipolar broadening of many kilohertz is observed in an NMR spectrum due to the strong dipolar-dipolar interaction and chemical shift anisotropy as a result of the restraint on the molecular motion. One method used for the removal of proton dipolar broadening is to employ a high-power proton decoupling field [5]. The 13C-1H scalar couplings are removed by the high-power proton dipolar decoupling (DD) and a resolutionimproved spectrum is obtained. The 13C T1 values for network polymers are so long that lengthy repetition time is needed to get reasonable NMR spectrum with high S/N ratio and high resolution. The energy of 13C spins in the excited state (high spin temperature) should be transferred to the NMR lattice to shorten the T1. The energy is transferred from the 13C spins at high spin temperature to the 1H spins by the cross-polarization (CP) technique [6, 7]. The 1H energy levels have four times the separation of the 13C levels in the laboratory frame. That makes the energy transfer an unfavorable process. When the 13C and 1H spin systems are in frames rotating at the same rate, the energy levels of both are matched and the transfer is allowed. That is the condition where the Hartmann-Hahn condition is satisfied. The ~3C signal created by CP is four times the original magnetization in ideal conditions. In the CP method, enhancement of 13C magnetization is effective for immobile components in a polymer gel. On the other hand, in the 13C pulse saturation transfer (PST) method [8],
POLYMER GEL SYSTEMS
~12
IH
13 c
r
oo
739
I,
-
L (a) PST / MAS
~/2
,-~/2
1 oo 13 C
I 1 t-- ~ -H
..(b) PST / MAS + Inversion Recovery Fig. 20.1. Timing diagrams of the NMR pulse sequences: (a) 13C pulse saturation transfer (PST)/magic-angle spinning (MAS) and (b) 13C inversion recovery (IR) combined with PST/MAS. DD: dipolar decoupling; FID: free induction decay; rr/2 and ~r: rf pulses; ~': recovery time; and n and m: number of repeating units.
the nuclear Overhauser effect (NOE) enhances the 13C magnetization in the mobile components in the gels. The proton spins are saturated by the ~r/2 pulse train. The pulse sequence of the X3C PST method is shown in Fig. 20.1(a). Since the motion of the polymer chain in the gels is intermediate between a solid and solution sample, the PST method is suitable for characterizing them. A magic-angle spinning (MAS) method is employed to remove the chemical shift anisotropy [9]. A gel sample placed in a cylindrical rotor is rotated about an axis making an angle of 54.74 ~ (magic angle) with the magnetic field at 800-5000 Hz by air. The gel samples are placed in a cylindrical rotor made of zirconia and sealed with an O-ring. The O-ring sealing prevents loss of solvent by vaporization from gel samples. The DD and the MAS methods are usually performed with the CP or PST techniques to obtain high resolution spectra.
740 20.3
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
Relaxation times
The relaxation times are measured to get information on the tumbling motion of the main and side chains in the network polymer of gel systems. The relationship between structure and motion and the sol-gel transition are studied through the relaxation measurements. The 13C Tx is generally measured by the inversion recovery (IR) method [10]. The repeated 7r - r - ~r/2 rf pulse trains are used in the method. The Ta follows the Equation (20.1) derived from Bloch's equation: In (A~ - A,) = In 2A~ - r/T1,
(20.1)
where A~ and A , are the magnitude of the recovering vector of magnetization evolved by a 7r/2 rf pulse at time t = ~ and r, respectively. The T1 is evaluated from the plot of l n ( A ~ - A , ) against r. There is a relationship between the T, and the viscosity, r/, and temperature, T [11]: l/T1 = (1287r31x4/h2)" (a3/r6) 9(rl/kT),
(20.2)
where /x is the nuclear moment, a is the effective radius of a spherical molecule and r is the distance from the observed nucleus to its magnetic neighbors. T1 varies inversely with 7q/T and a 3 and increases with r 6. When the motion of polymer chains of a gel sample is fast, T1 decreases with rl/T. T1 for a network polymer with slow motion takes a large value. In the relationship between T1 and the correlation time, which is a kind of indication of molecular motion, there is a minimum unusual temperature ranges when the relaxation occurs according to the dipole-dipole interaction [12]. The correlation time, rc, is given to an approximation by
rc = 47r3a3rl/3kT.
(20.3)
The 13C T1 measurements of polymer gels are carried out by an IR method combined with the 13C PST/MAS or CP/MAS techniques. The pulse sequence for the IR combined with the PST/MAS is shown in Fig. 20.1(b). In the pulse sequence, a 13C 7r pulse and a recovery time r are inserted and the recovery time is changed in series to measure 13C T1. The 13C T2 values for the network polymers of gel are generally very short and the motion of main chains especially lies in the slow motion region of the BPP theory of N M R relaxation [12]. In this region, the T2 value is shown to be almost constant. Therefore, the T1 measurements are performed in general to get information on the motion of polymer gel systems.
POLYMER GEL SYSTEMS
20.4
741
Poly(vinyl alcohol) gel
Section 20.4 is concerned with the structure and dynamics of the poly(vinyl alcohol) (PVA) gel and the mechanism of the gel formation. The PVA gel prepared by repeating freeze-thaw cycles from its aqueous solution is one of the physical gels. In physical gels, crosslinking points are formed by strong intermolecular interactions such as Coulombic, dipole-dipole, van der Waals, charge transfer, hydrophobic and hydrogen bonding interactions. The crosslinkages make a three-dimensional network polymer out of the soluble linear polymers. Some of the physical gels are thermoreversible gels, which show a reversible transition from the sol to the gel state with a change in temperature [13]. At the beginning of the gelation of a PVA aqueous solution, the phase separation into polymer-rich and polymer-poor regions arises with a decrease in temperature [14]. It was suggested that the gel formation is due to the formation of intermolecular hydrogen bonds in the PVA [15]. The structure and amount of hydrogen bonds may play an important role for the nature of the PVA gel. 20.4.1
Structure as a function of polymer concentration
Kobayashi et al. [16] studied the PVA gels by making high resolution solidstate 13C NMR experiments with the CP/MAS and PST/MAS methods. The degree of polymerization and the degree of saponification of the PVA they employed were 1700 and 99.9%, respectively. The PVA gel was prepared from PVA/water solution (9% wt/wt) by four repeating freeze-thaw cycles (frozen at -20~ for 20 h, and then melted and sustained at 25~ for 4 h). The PVA gel samples with different polymer concentrations were prepared by evaporation of water from them. Fig. 20.2 shows the 13C NMR spectra for the PVA in (A) the solution; (B-D) gel; and (E) solid states as measured by solution and solid-state NMR methods [16]. The NMR spectrum for the PVA aqueous solution obtained by the solution 13C NMR method (Fig. 20.2(A)) shows each of the signals for the CH and CH 2 carbons split into multiple peaks due to its stereochemical configuration. The triply split peaks for the CH carbon are assigned to the ram, mr and rr triads from high frequency [17-19] and, furthermore, each of the triad peaks splits into pentad peaks. The split peaks for the CH2 carbon come from tetrad tacticity. In the solution 13C NMR spectrum for the PVA gel shown in Fig. 20.2(B), the 13C signal for the CH carbon splits into three peaks due to triad tacticity. The signals for the CH and CH2 carbon become broader as compared with those for the PVA solution. This is caused by the
742
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA CH2 C-'H
I!
A
Ill
mr
~
(E)
(c) m r
CH2
CH mm
. . . . . .
.~
I
'
'r'''
100
'
. . . . . . . .~_,..._.__,
I
90
'
'
'
~
I
80
'
'
.
'
~
I
70
'
'
'
'
"
60
~
I
50
.
"
.
'
'
.
["' 40
.
'
'
.
'
I ' 30
(A)
'
PPN '"''1
20
Fig. 20.2. 13C NMR spectra for the PVA in the solution, gel and solid states as measured by several NMR methods. Solution NMR method: (A) the PVA/D20 solution and (B) the PVA gel. Solid-state NMR method: (C) the PVA gel by PST/MAS; (D) by CP/MAS; and (E) the solid PVA by CP/MAS.
increase in the dipole-dipole interactions between the polymer chains in the network polymer formed in the gel. The ~3C PST/MAS NMR spectrum of the PVA gel shown in Fig. 20.2(C) is very similar to its solution NMR spectrum (B). The NOE enhancement is used to obtain the higher ~3C signal in the PST method. The PST method effectively enhances peak intensity for mobile regions in samples compared with the CP method. This means that the mobile components in the PVA gel are observed in both spectra (B) and (C).
POLYMER GEL SYSTEMS
743
On the other hand, the CP method effectively enhances peak intensity for rigid regions in samples as compared with the PST method. The 13C signal for the CH carbon of the solid PVA has three split broad peaks in the 13C CP/MAS N M R spectrum as shown in Fig. 20.2(E). The chemical shift difference between these three peaks is considerably larger than the splitting due to stereochemical configuration. Terao et al. [20] explained such a splitting by the number of intramolecular hydrogen bonds formed among neighboring hydroxyl groups. Assuming that the 13C chemical shift value for the CH carbon shifts to high frequency by 6 ppm per inter- or intramolecular hydrogen bond, the most deshielded peak I can be assigned to the mm triad with two hydrogen bonds, the central peak II to the mm and mr triads with one hydrogen bond and the most shielded peak III to the mm, mr and rr triads with no hydrogen bonds, respectively. The proposed schematic crosslinked structure of PVA gel formed by inter- and intramolecular hydrogen bonds is shown in Fig. 20.3. In the 13C CP/MAS spectrum for the PVA gel (shown in Fig. 20.2(D)), the CH signal is composed of the three split peaks corresponding to the triad tacticity and the broad peaks as found in the solid PVA. This means that both the immobile and mobile components of the PVA gel are observed by the CP/MAS method at the lower polymer concentration. Fig. 20.4 shows the 13C CP/MAS N M R spectra of the PVA gels with
[ peaklli(65ppm)l \
I/C\ H2 /C\ H2 /C\ H2
CH
CH
CH
H2
/c\ / CH
CH
I A. O\ i OH HO
i
A HO
OH
~intram~
hydrogen
NO
H, 'H\O ,H\o 1 ,H H\O OH "" "" ! V ! ! V intermolecular hydrogen bond t CH CH CH CH CH /
J
',
%
\c/l[ \c/,~\c/l\c/ \ [ H2
H2
H2
H2
[ peakI''7~P,Pm'/ [ peak'(77Ppm'I -
I
-
II
I
I
Fig. 20.3. The schematic diagram for crosslinked structure of a PVA gel formed by inter- and
intramolecular hydrogen bondings. The most deshielded peak I is assigned to the CH carbon with two hydrogen bonds, the central peak II to one hydrogen bond and the most shielded peak III to no hydrogen bonds.
744
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA CH
CH2
(D)
9 (C)
(B)
(A) PPM 100
90
80
70
60
50
40
30
20
Fig. 20.4. 13C CP/MAS NMR spectra for the PVA gels with different polymer concentrations. The polymer concentrations are (A) 9.1" (B) 11.8; (C) 13.8; and (D) 35.0% (wt/wt), respectively.
different polymer concentrations [16]. The polymer concentrations of samples A, B, C and D are 9.1, 11.8, 13.8 and 35.0% (wt/wt), respectively. The relative peak intensities for the 13C spectra of the four kinds of PVA gels and solid PVA are shown in Table 20.1. The intensities of the three split peaks, due to stereochemical configurations, decrease and those of the three peaks I, II and III increase with the decrease in the water fraction in the gel. In sample D, the three peaks due to stereochemical configurations disappear completely. Peaks I, II and III can be assigned to the CH carbons with two intra- or intermolecular hydrogen bonds, one hydrogen bond and no hydrogen bonds, respectively. The spectrum for the sample D is very similar to that of solid PVA, except that the intensity of peak III in the former is somewhat larger than that in the latter. These results show that the increase
745
P O L Y M E R G E L SYSTEMS
Table 20.1. Relative
13C peak
intensities for the C H carbons in P V A Intensity fraction of peaks
Samples
Peak I
Peak II
Peak III
mm
mr
rr
0.05 0.07 0.09 0.09
0.33 0.36 0.36 0.50
0.27 0.33 0.35 0.41
0.09 0.05 0.04 -
0.17 0.11 0.10 -
0.09 0.08 0.06 -
0.14
0.49
0.37
0.05
0.27
0.68
P V A gel polymer concentration
9.1% (w/w) 11.8 13.8 35.0 P V A (solid) P V A (solution, at - 5 0 ~
in the fraction of the immobile components, and the formation of hydrogen bonds, are promoted by the increase in the polymer concentration of the gel. Fig. 20.5 shows X3C PST/MAS NMR spectra of the PVA gels with different polymer concentrations. The signals from the mobile components in the gel are observed clearly by the PST/MAS method differing from those by the CP/MAS method. While the spectra for the samples A, B and C are similar to each other and their CH signal splits into three peaks corresponding to the triad configurations, in the spectrum for D, the CH signal shows a broad peak. The broadening is due to the dipole-dipole interactions resulting from an increase in the restraint of molecular motion. This result means that the mobility of the mobile components in the PVA gel is restrained by an increase in the polymer concentration (>35.0%) in the gel. The NMR measurement was made on PVA solutions at low temperature in order to clarify the mechanism of gel formation by freeze-thaw cycles [16]. The 13C CP/MAS NMR spectrum for the PVA solution was measured at -50~ by going from room temperature to -50~ (Fig. 20.6). The signals for the CH group shows that instead of the three peaks, which arise from the splitting by tacticity, peaks I, II and III appear. The result shows that in the PVA solution at -50~ some hydroxyl groups form hydrogen bonds as in the PVA gel and so the crosslinked structure is formed in the PVA solution. This means that the gel formation is induced in going from room temperature to -50~
20.4.2
Effect of tacticity
Kobayashi et al. [21] are concerned with the effect of the tacticity on the amount of the crosslinked region for the PVA gel. In this study, the three
746
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA CTt
CH2
~.
(D)
mm
(C)
PPM I
80
. . . .
I"-- . . . .
70
I
60
. . . .
I " ' ' '
50
r---l'--'r--'r---""'
40
1
30
Fig. 20.5. 1 3 C PST/MAS NMR spectra for the PVA gels with different polymer concentrations. The polymer concentrations are" (A) 9.1; (B) 11.8; (C) 13.8; and (D) 35.0% (wt/wt), respectively.
PVA samples with different tacticities, such as isotactic (iso-), atactic (at-) and syndiotactic (syn-) ones were used. The degree of polymerization, and the fractions of mm, mr and rr triads, are shown in Table 20.2, where m and r indicate the meso and racemic dyads, respectively. The 13C CP/MAS NMR spectra for the three kinds of PVA gels with different tacticity (9% of polymer concentration) are shown in Fig. 20.7. As described in Section 20.4.1, the CH peaks are composed of both the three sharp peaks corresponding to the triad configurations (mm, mr and rr) and the three broad peaks (I, II and III at about 77, 71 and 65 ppm, respectively). In the ~3C CP/MAS NMR spectrum for the iso-PVA gel (Fig. 20.7(A)), peak I, together with three sharp peaks for the triad configurations, is obviously observed while peaks II and III are not so obvious. However, in the spectra for the at-PVA and syn-PVA gels shown in Figs. 20.7(B) and 20.7(C), respectively, peaks II and III are observed together with peak I. The signal areas for each of the configurations may reflect the fractions of the triad tacticity of the main chain of the PVA gel.
747
P O L Y M E R G E L SYSTEMS Cl-I
CH2 III
I!
\
I'
iO0
"''
...I..' ' ' '
90
I '"'
80
"I'''
70
'I''''
60
I'''
50
r"l''
40
....". '"I
30
'"'""'
Fig. 20.6. 13C CP/MAS N M R spectrum for the PVA solution at - 5 0 ~
I"'"'
20
'"'
PPM I '"' ' i
iO
0
in the process of
cooling.
Table 20.2. The characterization of three PVA samples with different tacticities
Sample
D.P.a
mm
mr
rr
iso-PVA at-PVA syn-PVA
15000 1700 1500
80 29 15
16 44 48
4 27 37
aDegree of polymerization.
The spectra for the CH groups of the PVA gel consist of overlapping peaks from six kinds of signals corresponding to the immobile and mobile regions. For example, Fig. 20.8 shows, that the lineshape analysis of the CH carbon signal in the 13C CP/MAS NMR spectrum of syn-PVA gel. Peaks I, II and III were deconvoluted by computer fitting with a Gaussian function, and the mm, mr and rr peaks with a Lorentzian function. The cross-polarization enhancements for the triad peaks and the other three peaks are different due to different molecular motions. For this reason, the relative amount of the triad fractions with peaks I, II and II cannot be compared quantitatively. However, the fraction of the triad tacticity itself and the
748
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
IIIIII
CH
CH2
lllr 9
(A)
~
,~, . . . .
~ ....
~, . . . . . . ~, . . . .
~.-~..~~~_ ~ ....
~, . . . .
z ....
~, . . . .
~ ....
,~,,,~,
(}3) f
I
100
'~''
"'
i""
90
'
"'V
,','1
. . . .
80
!
70
. . . .
i
60
,
"'
*
i
50
'
'
'
,
i
40
,
,
,
" " 1 " , " ' "
30
'
i
20
. . . .
i
10
. . . .
1
0
(C)
~
~
Fig. 20. 7. 13C CP/MAS NMR spectra for (A) the iso-PVA gel" (B) at-PVA gel" and (C) synPVA gel.
749
P O L Y M E R G E L SYSTEMS
OBSERVED SPECTRUM THEORETICAL SPECTRUM DECOMPOSEDSPECTRUM
(~ i! / i
/// ',, ! ,-,,,\ /';' ',"',,2 I , x
,_
/-" ,-J //
BO
-/,/' i '"',i!'k !I i!
\ i \ ~/
/
.,',\!/
"../
/ ',~
,/'-~ .... ,/I
75
\ !I
70
k
~
I ii
1
\\ \ "x \
",.
\ "---_,_=_
3-------- -r '- --
65
60
{ppm)
Fig. 20.8. The lineshape analysis of the CH carbon signal in 13CCP/MAS NMR spectrum for the syn-PVA gel. hydroxyl groups forming hydrogen bonds can be discussed on the basis of the discussion in Section 20.4.1. In the iso-PVA gel, the mm fraction is high and the mm triad with two hydrogen bonds is preferentially formed. The iso-PVA gel is much softer than the at- and syn-PVA gels, although the peak-I intensity of the iso-PVA gel is higher than those of the at- and syn-PVA gels. Therefore, it can be said that the amount of intermolecular hydrogen bonds in the iso-PVA gel is lower than those in the at- and syn-PVA gels. The intramolecular hydrogen bonds are formed mainly in the iso-PVA. On the other hand, the rr fraction is high in the syn-PVA gel and it is the hardest of the three kinds of gel. In the syn-PVA gel, the peak II corresponding to the mm and mr triads with one hydrogen bond should come more from the intermolecular hydrogen bonds that from the crosslinking regions. 20.4.3
13C T1 for mobile and immobile regions o f P V A gel
Kobayashi et al. [21] measured the 13C T1 values of the at-PVA gel by using high resolution solid-state 13C N M R to clarify the molecular motion for the
750 Table 20.3.
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
13C spin-lattice relaxation time (T1) of PVA gel 13C Tl(S)
Method
CH2
CH mm
mr
rr
Torchia's pulse sequence (CP method) Inversion-recovery pulse sequence (PST method)
0.37
0.28
0.32
II
III
7.8
7.6
5.2
0.16
mobile and immobile components. The 13C T1 values for the mobile component were determined by the IR method combined with the PST/MAS technique. The 13C T1 value for the immobile component was determined by the CP/MAS technique combined with the Torchia's pulse sequence [22]. The 13C TI values are summarized in Table 20.3. The T1 values for the triads and the CH2 are smaller than those for peaks II, III and the CH2. The immobile components correspond to the CH carbons in the crosslinked regions formed by inter- and intramolecular hydrogen bonds in the gel, and the mobile ones to the regions away from the crosslinked regions which undergo fast molecular motion in the gel [21, 23]. The results show that the mobility of the network polymer in the PVA gel is restrained by hydrogen bonds and, especially, that of the crosslinked region is very low. The 13C T1 values for the immobile components in the PVA gel are very close to those for the noncrystalline components of the solid PVA [21]. This shows that the crosslinking regions in the PVA gel have amorphous rather than crystalline structures.
20.5 Poly(methacrylic acid) gel This section is concerned with the poly(methacrylic acid) (PMAA) gel as the chemical gel where crosslinkages are composed of chemical bonds. The hydro-swollen polyelectrolyte gels, such as the PMAA, deform and change their volume in response to the change in environment as described in Section 20.1. Many researches have been conducted to clarify the mechanism of the volume change of polyelectrolyte gels. The 1H NMR imaging experiments have been made and provided spatial information on the change in the distribution and motion of water in the PMAA gel induced by stress [24] and electric fields [25-27]. The solvent, water, plays an important role in the deformation of the hydro-swollen PMAA gel and Yasunaga and Ando [28-
POLYMER GEL SYSTEMS
751
30] studied the water content by the solution NMR techniques. The experimental results on 1H T1, 1H T2 and the self-diffusion coefficient for water molecules (DH2o) in the P M A A gel showed that the increase in crosslinking causes increasing restraint in the rotational and translational motion of water molecules in the gel. The molecular motion of water in the gel is sensitively affected by the size of the network. From such a situation, the importance of the study of the network polymer in the P M A A gel was recognized. However, as described in the introduction of this chapter, it is difficult to measure the network polymer of gels by normal solution NMR techniques because of their dipole-dipole interactions and chemical shift anisotropy. The solid-state NMR techniques are very powerful in the study of the network polymer gels. 20.5.1
Structure as a function of degree of swelling
Yasunaga and Ando [31] studied the hydro-swollen PMAA gel as a function of the degree of crosslinking by 13C high resolution solid-state NMR techniques. The PMAA gel was prepared by radical copolymerization of methacrylic acid (MAA) and N,N'-methylenebisacrylamide (MBAA) in aqueous solution initiated by K2820 8. The mole ratio of MBAA (t/MBAA) to MAA (nMAA) in the preparation of the PMAA gel, F, is defined as F
(20.4)
= (nMBAA[nMAA).
The degree of swelling of the PMAA gel (q) is defined by the ratio of the mass of swollen polymer gel (Mswollen) to dried polymer (Mdry) as q = Mswollen/Mdry.
(20.5)
13C PST/MAS spectra for PMAA gels, with three kinds of F and q, are shown in Figs. 20.9(a-c). The chemical shift values for the PMAA gels are listed as a function of q in Table 20.4. The peaks of the PMAA gel can be assigned to the carboxyl (C---O) (about 182 ppm), methylene (CH2) (about
I
55 ppm), quaternary ( ~ C ~ ) (46 ppm) and a-methyl (CH3) (about 20 ppm)
I
carbons from high frequency. The expanded spectra for the C--O and ceCH3 signals in the PMAA gel (q = 3.4) are shown in Fig. 20.10(a, b). Each of the C = O and ceCH3 signals is split into three peaks by triad configurations of PMAA [32]. These peaks are designated by C---O (rr) (182.5 ppm), C---O (mr) (181.5 ppm), C--O(mm) (180.8 ppm), CH3 (mm) (22.3 ppm), CH3 (mr)
752
H. YASUNAGA,
M. KOBAYASHI
AND
S. M A T S U K A W A
(a)
(b) CH3(rr)
CH 3
C=O(,rr)
(m0
/
~~/~O(mr)
(c)
PPM I I ' ' ' ' I ' ' " I ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ~1""""' i ' " ' ' ~ I '''~ ' ' I '
200 180 160 140 120 I00 80
Fig. 20.9. 13C P S T / M A S
spectra for the PMAA
60
40
20
gels: (a) F = 0 . 0 0 1 , q = 105.8; (b) F = 0 . 0 0 3 1 ,
q = 21.7; a n d (c) F = 0 . 0 2 0 , q = 3.4.
(19.8 ppm) and C H 3 (rr) (18.2 ppm) carbons from high frequency. The peaks are assigned to the isotactic [ C = O (mm) and C H 3 (mm)], heterotactic [ C = O (mr) and C H 3 (mr)] and syndiotactic [C=O(rr) and C H 3 (rr)] triads, respectively [33, 34]. The relative area ratios of the triad carbons for the C = O and ~ x C H 3 signals are shown in Table 20.5. From the c ~ - C H 3 signals, the triad fraction is evaluated as rr: mr: mm = 67.4: 27.1 : 5.5%. The main chain of the network polymer in the PMAA gel prepared by radical polymerization has mostly rr triads. The 13C chemical shift values for the PMAA gel are almost independent of q . If the formation of hydrogen bonds, or the dissociation of carboxyl groups, is caused by the change in q and they are sufficiently large, the chemical shifts for the C--O group would change. In fact, the chemical shift
POLYMER GEL SYSTEMS Table 20.4.
753
13C NMR chemical shift a of PMAA gel as a function of the degree of swelling (q)
at 300 K
13C chemical shift/ppm
I q
C~O (rr)
105.8 59.4 21.7 4.7 3.7 3.4
182.4 182.9 182.8 182.9 182.7 182.5
C---O (mr)
181.8 181.5
C~O (mm)
180.8
CH2
~C~ ]
54.7
45.3 45.9 45.9 45.7 45.8 46.0
54.6 54.1 54.2 54.6
CH3 (mm)
22.2 21.7 22.2 22.3
CH3 (mr)
CH3 (rr)
19.3 19.2 19.5 19.8
18.3 17.4 17.9 17.8 17.9 18.2
appm from tetramethylsilane.
changes are found for water in the PMAA gel caused by the change in q [29]. No change in the chemical shifts for the C = O group indicate that the C O O H groups are surrounded by water molecules and no hydrogen bond formation between the C O O H groups is caused by the decrease in q values of 3.4-105.8. The PMAA gel are so rich in the rr-triads main chain that the signals for the C = O (mr) and (mm) are found only for the samples with q values of 3.4 and 3.7 for which concentration of the network polymer is higher. 20.5.2
13C T1 as a function of the degree of swelling
The PMAA gel with a low degree of crosslinking and high q is harder than that with a high degree of crosslinking and low q. The increase in q should lead to an increase in the molecular motion of the network polymer in the PMAA gel. Yasunaga and Ando [31] measured the 13C T1 value for the network polymer in the PMAA gel as a function of q. The 13C T1 values for the C = O (rr) and C U 3 (rr) carbons increase with an increase in q, while the
I
T1 for the ~ C ~
I
carbons decrease with an increase in q. The 13C T1 values
I
for the C - - O (rr), C H 3 (rr) and ~ C - - c a r b o n s are plotted against q - 1 as
!
I
shown in Fig. 20.11. The T1 value for the C~----O (rr) and ~ C ~
carbons
decreases and increases linearly w i t h q - l , respectively. The T1 value for the CH3 (rr) carbon decreases abruptly from 0.017 to 0.046 of q-1 and is constant from 0.046 to 0.30. We have to be careful in interpreting T1 results and
754
H. Y A S U N A G A , M. KOBAYASHI AND S. MATSUKAWA
C=O(rr) ~/~
(a)
'''I'''
186
C=O(mr)
/
I'''I'''I
t84
i82.
PPN ' ''l'''li80 178 i76 CHs(rr)
CH,(mr)
A /
ells(
Co)
'''
I''
24
' I'
22
'" i"''
20
I'''
18
I''
16
'f
14
Fig. 20.10. 13C PST/MAS spectra for (a) the C - - O and (b) a-CH3 carbons of the P M A A gel (q = 3.4) expanded from the spectrum shown in Fig. 20.9(c).
Table 20.5. Relative ratio of the triad C - - O and a-CH3 carbons in PMAA gel (q = 3.4) Group
Triad
Ratio/%
mm mr rr
5.1 24.9 70.0
mm mr rr
5.5 27.1 67.4
C--O
a -CH3
755
POLYMER GEL SYSTEMS
2.5
,
o-- o
,
r
c = o (rr)
2.0
1.5 F-r..)
oo
- 0
i
0 v
I
-Ci
1.0
0.5
a CH 3
O
(rr)
!
0
0.1
!
q-1
a
.......
0.2
0.3
I
Fig. 20.11. Plots of 13C T1 for the C---O (rr), CH3 (rr) a n d - - C - - c a r b o n s of the PMAA gel against q-1 at 300 K. I
I
cannot compare the T1 absolute value for C ~ O or ~ C ~
I
with that for CH3,
because the relaxation processes are different as a consequence of the number of attached protons being different. However, the q-dependence of the Tx has significant meaning. The increase in q leads to a decrease in the density of the network polymer in the gel and an increase in the molecular motions of the network. In fact, Yasunaga and Ando [28] have reported that 1H T1, ~H T2 and DH2O of water molecules contained in the PMAA gel increase with an increase in q controlled by the degree of crosslinking. Hence, the molecular and translational motions of the water molecules in the gel increase with an increase in q. In the PMAA gel system, the spin-lattice relaxation is governed predominantly by the dipole-dipole interactions and q corresponds to the temperature in the
756
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
BPP theory of NMR relaxation [12]. Therefore, an increase in q-1 means an increase in the inverse of temperature. According to the BPP theory, Ta decreases with increasing temperature and increases after passing through a minimum as the temperature is increased further. As shown in Fig. 20.11, the decrease in ~3C T1 for the C - - O (rr) and CH3 (rr) carbons of PMAA in the gel with the increase in q-1 means that the motion of the side chain carbons is in the extreme narrowing region at the lefthand side of the BPP curve. However, the increase in T1 for ~ C ~
means that the motion is in
the slow motion region at the righthand side of the BPP curve. Therefore, it can be said that segmental motion for the side chains of the network polymer in the PMAA gel is faster than that for the main chains. The main chains seem to behave like solid molecules and the side chains like liquid molecules. This feature of the gel comes from its complex structure composed of a three-dimensional network polymer and solvent.
20.5.3
13C TI as a function of the degree of crosslinking
The water content in the PMAA gel contributes to the segmental motion of the network polymer as described above. On the other hand, the restraint of chain mobility in poly(N-vinylpyrrolidone) gel, caused by the presence of crosslinking and entanglement of polymer chains, was identified [35]. The 13C T1 for the PMAA in the gel was also measured as a function of the degree of crosslinking with constant water content. The 13C T1 values obtained for the main and side chain carbons of the network polymer in the PMAA gel (q = 3.4), as a function of the mole ratio of MBAA to MAA (F) and uncrosslinked PMAA solution (29.4 w%), are plotted in Fig. 20.12(a, b). The TI values for the C - - O (rr) and C ~ O (mr) carbons decrease in the range of F from 0 to 0.0031 and increase from 0.0031 to 0.020. The efficiency of the NMR relaxation of the carboxyl carbon becomes largest at 0.0031 for F. The results suggest that the motion of the C - - O changes from the extreme narrowing region to the slow motion region in the BPP curve with increasing the degree of crosslinking. The T1 values for the CH2, CH3 (mm), CH3 (mr) and CH3 (rr) carbons decrease with increasing F. The order of the T~ values for the triad methyl carbons is CH3 (mm) > CH3 (mr) > CH3 (rr). This shows that the molecular motion of the a CH3 carbon is fastest in the mm triad structure and slowest in the rr triad structure is the. The molecular motions for the CH3 (mm), CH3 (mr) and CH3 (rr) are in the extreme narrowing region.
POLYMER GEL SYSTEMS
2.5 . . . .
757
2.0
.~. !
1.5 ' ~ "~ Io 1.0
0 0-"" 0
"8
.,-',::'"
'- "'" "
,
0
(a)
.-'""
COOH (mr)
ii "
,
, I
t
0.005 0.010 0.015 0.020 F
0.6 A
0.5
m)
0.4 i i
F-,- 0.3 o 0.2
i i
e-)
0.1
(b)
0
i i
i,lk
~i.~ .................... CH 3 (mr)
.
~ ...................
'& ....................
7~. c.~ ........" ............c..~(4 9......
_
0
I,
!
,,
i,
::::::2::::] I 1
0.005 0.010 0.015 0.020
I I
Fig. 20.12. (a) Plots of 13C T~ for C--O (rr), C--O (mr) a n d - - C - - c a r b o n s and (b) CH3 (mm), CH3 (mr), CH3 (rr) and (CH2) carbons of the PMAA gel (q =3.4) against F at 300 K.
758
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
I
The T1 for ~ C ~
I
increases with an increase in F. The motion of ~ C ~
I
I
in the slow motion region is restrained with increasing the degree of crosslinking. On the other hand, the T1 for the main chain CH2 carbon
L
decreases with increasing F in contrast to the ~ C ~ .
This can come from the
I
difference in their relaxation processes. The two protons and two carbons are attached to the CH2 carbon, while four carbons one attached to the
I
~C~.
The results show that the segmental motion of the network polymer in the PMAA gel is strongly affected by the degree of crosslinking when the water content in the gel is constant.
20.6 Poly(N,N-dimethylacrylamide-co-acrylic acid)gel-poly(ethylene glycol) system The structure and dynamics of a polymer gel containing linear polymers are affected by various kinds of intermolecular interactions between the network polymer of a gel and the linear polymers. The interactions include hydrogen bonding, hydrophobic, Coulomb interaction and so on. It is known that a complex between poly(ethylene glycol) (PEG) and poly(acrylic acid) (PAA) is formed by hydrogen bonds between the oxygens of the PEG and the carboxyl groups of the PAA [36-38]. The network polymer of a poly(methacrylic acid) (PMAA) hydro gel also forms a complex with PEG by hydrogen bonding [23]. The justification for the formation of polymer-polymer complexes has been carried out by macroscopic methods, such as viscosity measurements for solutions and mass measurement for gels. Nevertheless, the microscopic structure and the formation mechanism of the complexes have not been fully understood. To clarify such intermolecular interactions between a probe polymer and a network polymer of the gel systems, systematic studies by microscopic methods such as NMR spectroscopy are needed. 20.6.1
Motion of PEG in poly(DMAA-co-AA) gel
Matsukawa and Ando [39, 40] studied the effect of an intermolecular hydrogen bond between a PEG and a network polymer of poly(N,N-dimethylacrylamide-co-acrylic acid) (DMAA-co-AA) gel on their dynamic behavior. In
759
POLYMER GEL SYSTEMS '
12
'
' I
'
'
'
'
'
'
8
l"" t
(D
6
I
'
FlZl
10 -7
' "
in solution w
51
J
-
-
lgi
II 51
~
Et
El
Ud
r"l
4 -
2-
,,Ik [] ,
I
i
i
!
10
,,
[] ,
,
,
,
i
50
s
,
!
i
100
Degree of Swelling q Fig. 20.13. Dependence of the diffusion coefficient of the PEG with Mw of 4250 (DPEG) in the poly(DMAA-co-AA) gels on the degree of swelling (q) at 303 K. The diffusion coefficient of the PEG with Mw of 4250 in 1 wt% aqueous solution at 303 K is indicated by the dashed line. The mole fraction of the AA in gels, fAA, are (I--l) 0, (El) 20, (gq) 50, (~l) 90, (11) 100 and (A) 100 (soaked in 0.15 mM of HC1).
order to investigate the translational motion of the PEG in the poly(DMAAco-AA) gels, the diffusion coefficient of the PEG (DpEG) in the gels was determined by the pulsed field gradient spin echo (PGSE) 1H NMR method [23]. The measurements of DpEG were made for a probe polymer with Mw = 4250 at 303 K varying q and the mole fraction of AA in the copolymer gel (fAA). q is the degree of swelling of the polymer gel defined by Equation (20.5). The DpEG plotted against q are shown in Fig. 20.13. The D1-EG values in the P D M A A gel (fAA = 0) are smaller than those in an aqueous 1 wt% PEG solution (dashed line) and increase with increasing q. This behaviour is followed by DpEG/D ~ =
e x p ( - KR),
(20.6)
where D O is the diffusion coefficient for an isolated probe polymer, K - 1 the dynamic screening length and R the hydrodynamic radius of the probe polymer [41]. It can be said that the translational motion of PEG in the P D M A A gel (fAA = 0) is restrained by hydrodynamic interaction with the polymer
760
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
-~ CH2-- CH )n C=O OH I
-(CH2- CH2- O-~m
-6 CH~-- C H ~ --
i
C=O I
.,~
,.
9 H
-( CH2-CH2- O-'~m
Scheme 20.1.
network. This means that the polymer network is working only as a spatial obstruction for the displacement of the PEG [42]. The Dpzo value in the PAA ( f A A - 1) gel is much smaller than that in the P D M A A gel. This suggests that the loose complex of the PEG and the PAA network is formed by intermolecular hydrogen bond interactions between the oxygen atoms of the PEG and the carboxyl groups of PAA [38]. In the poly(DMAA-co-AA) gels, the DpEG value is almost constant in the range of fAA < 0.5, and is drastically decreased in the range of fAA ~> 0.9. These results show that when the AA unit is isolated in the D M A A sequences, the molecular motion of the PEG is not strongly restrained by the intermolecular interaction with AA units. On the other hand, when consecutive AA units are distributed in the network, intermolecular interactions between the PEG and consecutive AA segments are enhanced and this results in the strongly restrained molecular motion of PEG [43]. By adding a small amount of HC1 into the gel, it shrinks and the DpEG decreases as shown by the arrows in Fig. 20.13. The complex formation arises from an intermolecular hydrogen bond between the oxygen atoms of the PEG and the carboxylic groups of the AA units in the undissociated state are shown in Scheme 20.1. It can be said that the addition of HC1 leads to an increase in the amount of undissociated carboxylic groups on going from the lefthand to the righthand side in Scheme 20.1. This may enhance the formation of the complex. The complex formation and the undissociation induce the shrinkage of the gel and lead to slow diffusion of PEG. The T2 value reflects the molecular motion [10, 12]. In the molecular motion of PEG in D 2 0 solution, the segmental motion is a dominant factor compared with the translational motion and rotational motion of a whole molecule. The 1H T2 of PEG ( M w - 4250) was measured by the CPMG method [10] at 303 K varying q and fAA of the poly(DMAA-co-AA) gel. Fig. 20.14 shows the plot of the 1H T2 values obtained against q. The 1H T 2 values for PEG in the gels with fAA < 0.5 decrease as q is decreased. This means that the segmental motion of the PEG molecule decreases with decreasing size of the network. The segmental motion of PEG may not be strongly
761
POLYMER GEL SYSTEMS 0.8
" ..... '
1
l"!
N r
[--,
EEl
N El
0.6
I::F! 51
0.4
,....,,
I
0.2
0
,,
v
t
10
~
t
z
J
,t
!
50
~
t
I
,
100
Degree of Swelling q Fig. 20.14. Dependence of 1H T2 of the PEG with Mw of 4250 in the poly(DMAA-co-AA) gels on the degree of swelling (q) at 303 K. The fAA are ([3) 0, ([]) 20, ([]) 50, (D) 90 and (ll) 100.
restrained by intermolecular hydrogen bonds with the AA units in the gels for fAa ( 0.5 as indicated by the experimental results on DpEo. The 1H T2 values of PEG for fAA ) 0.9 are much smaller than those for fAa ( 0.5. This result is similar to that of DpEo. The complex between PEG and the AA units in the network polymer restrains the segmental motion of PEG as well as the translational motion. From the above results, the complex should be stabilized through hydrogen bonding between the PEG chain and the consecutive AA units in the network polymer.
20.6.2
13C chemical shift
The hydrogen bonds may play an important role in the poly(DMAA-co-AA) gel-PEG system as described above. It is important to investigate hydrogen bonds formed between the AA units in the gel and PEG. It is known that the formation of hydrogen bond leads to a change in the 13C chemical shift. When the exchange rate between hydrogen bonded and unbonded form is larger than I / i ~ - 6~bl, where ~ and ~ub are the 13C chemical shifts in
762
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
hydrogen bonded form and unbonded form, respectively, the observed 13C chemical shift (6) can be expressed by -- Pb ~, + Pub
(20.7)
(~ub,
where Pb and Pub a r e the mole fractions of the carbons in the hydrogen bonded form and unbonded form, respectively [44]. Matsukawa and Ando studied the PAA gel-PEG system by a solid-state NMR technique. Fig. 20.15 shows the expanded 13C NMR spectra for (a) the PEG solution; (b) the PAA gel (q = 18); (c) the PAA gel soaked in 0.5 wt% of PEG solution (q = 7); and (d) the PAA gel soaked in 1 wt% of -C_H2C_H20-
a)
-C_OOH
b)
c)
d)
WM
1184
:182 ~
178
:176
I' I' I ~ I' i' I' I ~ I'I' I'r] 5.74 74 72 70 68 ~ F_,4
Fig. 20.15. Expanded 13C NMR spectra of (a) the PEG solution; (b) the PAA gel; (c) the PAA gel soaked in 0.5 wt% of the PEG solution (q = 7)" and (d) the PAA gel soaked in 1 wt% of the PEG solution (q < 5). Spectra (a), (b) and (c) were measured by normal solution NMR and spectrum (d) was measured by the PST/MAS NMR.
763
POLYMER GEL SYSTEMS
PEG solution (q < 5). The spectra (a), (b) and (c) were measured by a solution NMR technique, and spectrum (d) was measured by solid-state PST/MAS NMR method. The 13C chemical shift of the carboxylic carbon in PAA shows the low frequency shift from 178.62 ppm in the spectrum (b) to 178.56 ppm in the spectrum (c) and 178.45 ppm of spectrum (d). However, no signal is observed at the chemical shift position to be expected for the hydrogen bonded form. The chemical shift difference (A6= 178.62- 178.45 = 0.17 ppm) is smaller than that expected for the hydrogen bonded form. The amount of the hydrogen bonded form between the oxygen atom of the PEG and, the carboxylic carbon of the PAA in the complex, is so small that the average exchange rate between the hydrogen bonded and unbonded forms is larger than 1/16b- 6ubl. This is consistent with the result measured by potentiometric titration that the ratio of the number of binding groups to the number of potentially interacting groups in solution of PEG(Mw = 3000) and PAA(Mw = 1840000) is 0.05 [45]. From the results on the D~,EG and 1H T2 for the poly(DMAA-co-AA) gel-PEG system, it can be said that the polymer complex is formed by intermolecular interaction between the PEG and consecutive AA units in the network polymer. Therefore, the intermolecular interaction is caused by a small amount of hydrogen bonding between the oxygen atoms of PEG and the carboxylic acid groups
CHC! 3
C~H, I v ==C-- OCH=
-OCHs
2
PMLG
%
2
C-Oest~r
1,4-dio~me
C-Oax~d e
pp~ 2
0
Fig. 20.16.
150 13C
t00
50
PST/MAS NMR spectrum for the PMLG gel in CHC13.
764
H. Y A S U N A G A , M. K O B A Y A S H I A N D S. MATSUKAWA
50
30
20
,
15
/ 10
__~
jj'
!
"'
1~0
'
1
5
~,
'
~7~.
'
~
i
i;'6
~ '
"
I
'
s7~
'
'
I
~"-' '
.~72
'
'
I
rTo
' ' l ' ' ' l ' - ' ~ ' l " " ' l ' ' " i ' ' ' l ' ' ' i ' ' ' l
62
Go 5o
~
54
.~.
~
5o
4e
'~l'"~
34
32
~"
' ' u ~ l ' ' ' l ' ' ' l ' ' j l ~ ' ' ' ' l
ao
~n
~
~4
2~. ~o
T F A (%)
Fig. 20.17. 13C PST/MAS N M R spectra for the P M L G gel in the various volume fraction of T F A in a mixture of CHC13 and TFA.
POLYMER GEL SYSTEMS 177.5
|
765
!
177 O.--
176.5
.B" . . . . . . .
ID-
(a)
.E
-
176 175.5 175 174.5 174
O
173.5 E 0., I~
!
I
I
I
i
t
I
58
(b)
,....;
o e~o
57
--1
56
u., 0
55
== r/l
o
54
".r
53
~.o-
e---
e- ......
5Z
o
-O- - -.,
!
32
r,J r,.)
31
m .-.I
,,.
,,I
I o
- .. . . . . . . .
I
-.
I
I
I
(c) _
-
30 29 o,_-o . . . . .
28
-
27
Lll, ="& ,
26 0
-0 -
f
I
zo
I
I
40
60
,
-- "
"1
I
80
100
Volume Fraction of TFA (%)
Fig. 20.18. 13C chemical shifts for the PMLG gel as a function of the volume fraction of TFA in the mixture" (a) amide carbonyl carbon (O) and ester carbonyl carbon (O); (b) Ca(O) and OCH3(O)" and (c) C,/(O) and Ct3(O ).
of the PAA gel, which are formed and rapidly broken. This temporary hydrogen bonding should cooperatively form the polymer complex.
20.7
Poly(y-methyl L-glutamate) gel
The properties of the polymer gels, swollen by organic solvents, are studied in addition to water-swollen polymer gels [46]. In this section, we are concerned with poly(y-methyl L-glutamate) (PMLG) crosslinked by the esteramide exchange reaction in 1,4-dioxane using diaminododecane as a
766
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA 0
...... ,
,
,
i
,
.
,
i
','
.,
,
i
,
,
,
i
,
,
,
turbid C) transparent
40 v
.=_
:30
r.~ o
o
20
(
10
0 0
,
20
40
60
80
,
100
Volume Fraction of TFA (%)
Fig. 20.19. Dependence of the degree of swelling (q) of the PMLG gel on the volume fraction of TFA in the mixture (fTFA). Appearance of the PMLG gel is expressed by ( 9 for transparent gel and by (O) for turbid gel.
crosslinker, and swollen by chloroform (CHC13) and trifluoroacetic acid (TFA). 20.7.1
Conformationchange
It is known that PMLG takes an a-helix form in CHC13, and the random coil form in a mixture of TFA and CHC13 [47]. When the solvent composition is changed in the crosslinked PMLG gel, its volume and conformation are changed. A 13C PST/MAS NMR spectrum for the PMLG gel in CHC13 is shown in Fig. 20.16 together with peak assignments. The peak intensity for the side chain carbons is more intense than that for the main chain carbons. The side chains undergo faster molecular motion, compared with those of the main chain. The 13C PST/MAS NMR spectra for the PMLG gel are shown as a function of the volume fraction of TFA (fTFA) in a mixture of CHC13 and TFA at room temperature in Fig. 20.17. The obtained 13C chemical shifts of individual peaks plotted against fTFA are shown in Fig. 20.18. The amide carbonyl (amide C = O ) and C~ carbons resonate at 176.5 and 57.6ppm, respectively, in CHC13 (fvFa = 0). This indicates that PMLG network takes the a-helix form as determined by reference data of solid polypeptides [48-
767
P O L Y M E R G E L SYSTEMS 3.5
.~
09 v
.-
I
I
) (a) C--Oester
I
extreme narrowing region
O
2.5 2
O
()
1.5
O
O
()
O
0.5 3
I I
(b) C=Oamide
2.5
I
O (
---
I
O
I I
slow motion region
0
~7 1.s 0
0.5 0
o
I
I
5
1o
I
15
C 20
Volume Fraction of TFA (%)
Fig. 20.20.
13C T1 of (a) ester carbonyl carbon and (b) amide carbonyl carbon as a function of fTrrA measured at 20~ (O) and 40~ (O).
50]. By the addition of TFA to the PMLG gel system, the 13C chemical shifts of the amide C - - O and C~ carbon transitions move to low frequency to 174.1 and 54.4 ppm, respectively, at fTFA = 0.2. They are independent of fTFA above 0.2. Such transitional low frequency shifts show that the main chain conformation of the PMLG network changes from the a-helix to the random coil form. However, the 13C chemical shifts of the side chain carbons also show small change at fTFA = 0.2. In Fig. 20.19, the degree of swelling for the PMLG gel is plotted as a function of fTFA. The degree of swelling is transitionally decreased at 0.05-0.2 of fTFA and is gradually increased for fTFA > 0.2. This shows that the helix-coil transition leads to the shrinkage of the PMLG gel. 20.7.2
13C T1 as a function of solvent composition and temperature
Fig. 20.20 shows the TFA content dependence of the 13C T1 values for the amide C---O and ester C - - O carbons of the PMLG gel measured by PST/MAS NMR combined with the IR method at 20 and 40~ As the temperature is decreased, the T1 for the amide C---O carbon increases and
768
H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA
that for the ester C = O carbon decreases. The results shows that molecular motion of the amide C = O carbon is in the slow motion region and that for the ester C - - O carbon is in the extreme narrowing region according to the BPP theory [12]. Therefore, the increase in T1 for the amide C - - O carbon, and the decrease for the ester C - - O carbon with increasing fTFA, can be interpreted as being due to the decrease of motion of the PMLG network. In the range of fTVA = 0.05--0.2, T1 for the amide C = O carbon decreases with increasing fTFA and that for the ester C = O carbon does not change apparently. The content of the random coil form is increased gradually With increasing fTFA in this range (Fig. 20.18), and the degree of swelling is constant (Fig. 20.19). The results suggests that the molecular motion of the main chain increases when the conformation of the PMLG main chain changes from a-helix to random coil.
References
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
Y. Osada, M. Hasebe, Chem. Lett. (1985) 1285. T. Tanaka, Phys. Rev. Lett. 40 (1978) 820. T. Tanaka, Sci. Am. 244 (1981) 110. Y. Hirokawa and T. Tanaka, J. Chem. Phys. 81 (1984) 6379. R.A. Komoroski (Ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk. VCH, New York, 1986. S.R. Hartmann and E.L. Hahn, Phys. Rev. 128 (1962) 2042. A. Pines, M.G. Gibby and J.S. Waugh, J. Chem. Phys. 59 (1973) 569. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M.J. Albright, The 20th Meeting on NMR. Tokyo, 1981, p. 68. E.R. Andrew, Progress in Nuclear Magnetic Resonance Spectroscopy, J.W. Emsley, J. Feeney, L.H. Sutcliffe (Eds), Vol. 8, Part 1, p. 1, Pergamon Press, Oxford, 1971. T.C. Farrar and E.D. Becker, Pulse and Fourier Transform NMR--Introduction to Theory and Methods. Academic Press, New York, 1971. F.A. Bovey, High Resolution NMR of Macromolecules. Academic Press, New York and London, 1972. N. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev. 73 (1948) 679. J.M. Guenet, Thermoreversible Gelation of Polymers and Biopolymers. Academic Press, London, 1992. D.R. Paul, J. Appl. Polym. Sci. 11 (1976) 439. A. Takahashi and S. Hiramitsu, Polymer J. 6 (1974) 103. M. Kobayashi, I. Ando, T. Ishii and S. Amiya, Macromolecules 28 (1995) 6677. T.K. Wu and D.W. Ovenall, Macromolecules 6 (1973) 582. Y. Inoue, R. Chujo and A. Nishioka, J. Polym. Sci., Polym. Phys. Ed. 11 (1973) 393. Y. Inoue, R. Chujo, A. Nishioka, S. Nozakura and H. Iimuro, Polym. J. 4 (1973) 244. T.Terao, S. Maeda and A. Saika, Macromolecules 16 (1983) 1535. M. Kobayashi, I. Ando, T. Ishii and S. Amiya, J. Mol. Struct. 440 (1997) 155. D.A. Torchia, J. Magn. Reson. 30 (1978) 613.
POLYMER GEL SYSTEMS
769
23. H. Yasunaga, M. Kobayashi, S. Matsukawa, H. Kurosu and I. Ando, Ann. Rept. NMR Spectroscopy, 34, G.A. Webb. (Ed.), Academic Press, London, 1997, p. 39. 24. H. Yasunaga, H. Kurosu and I. Ando, Macromolecules 25 (1992) 6505. 25. T. Shibuya, H. Yasunaga, H. Kurosu and I. Ando, Macromolecules 28 (1995) 4377. 26. H. Kurosu, T. Shibuya, H. Yasunaga and I. Ando, Polym. J. 28 (1996) 80. 27. Y. Hotta, H. Kurosu, H. Yasunaga and I. Ando, Polym. Gels Networks (1998) in press. 28. H. Yasunaga and I. Ando, Polym. Gels Networks 1 (1993) 83. 29. H. Yasunaga and I. Ando, J. Mol. Struct. 301 (1993) 125. 30. H. Yasunaga and I. Ando, Polym. Gels Networks 1 (1993) 267. 31. H. Yasunaga and I. Ando, J. Mol. Struct. 301 (1993) 129. 32. C. Pichot, A. Hamoudi, Q.T. Pham and A. Guyot, Eur. Polym. J. 14 (1978) 109. 33. J. Schaefer, Macromolecules 4 (1971) 98. 34. E. Klesper, A. Johnsen, W. Gronski and F.W. Wehrli, Makromol. Chem. 176 (1975) 1071. 35. K. Yokota, A. Abe, S. Hosaka, I. Sakai and H. Saito, Macromolecules 11 (1978) 95. 36. K.L. Smith, A.E. Winslow and D.E. Petersen, Ind. Eng. Chem. 51 (1959) 1361. 37. E. Tsuchida and K. Abe, Adv. Polymer Sci. 45 (1982) 2. 38. O.E. Philippova, N.S Karibyants and S.G. Starodubtzev, Macromolecules 27 (1994) 2398. 39. S. Matsukawa and I. Ando, Macromolecules 29 (1996) 7136. 40. S. Matsukawa and I. Ando, Macromolecules 30 (1997) 8310. 41. R.E. Cameron, M.A. Jalil and A.M. Donald, Macromolecules 27 (1994) 2708. 42. P.G. de Gennes, Macromolecules 9 (1976) 594. 43. I. Iliopoulos and R. Audebert, Macromolecules 24 (1991) 2566. 44. J.A. Pople, W.G. Schneider and H.J. Bernstein, High-resolution Nuclear Magnetic Resonance. McGraw-Hill, New York, 1951, p. 218. 45. Y. Osada, J. Polymer Sci., Polym. Chem. Ed. 17 (1979) 3485. 46. C. Zhao, S. Matsukawa, M. Kobayashi and I. Ando, J. Mol. Struct. (1998), in press. 47. Y. Suzuki, Y. Inoue and R. Chujo, Makromol. Chem. 181 (1980) 165. 48. H. Saito and I. Ando, Ann. Rept. NMR Spectrosc. 21 (1989) 210. 49. I. Ando, T. Yamanobe and T. Asakura, Prog. NMR Spectrosc. 22 (1990) 349. 50. A. Shoji, S. Ando, S. Kuroki, H. Yoshimizu, I. Ando and G.A. Webb, Ann. Rept. NMR Spectrosc. 26 (1993) 55.
This Page Intentionally Left Blank
Chapter 21
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Biodegradable polymers Yoshio Inoue Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Japan
21.1
Introduction
Today, a variety of and large amount of synthetic polymeric materials are produced and are supporting our comfortable daily life as well as modern high technologies. The major reasons for their success are their strength, durability, resistance to chemical and biological corrosion and low production cost. On the one hand these properties have created various fields of applications, but on the other have given rise to the problem of waste polymers. Currently, almost all plastic wastes are treated by either landfill or incineration. As great concern in conserving the global environment is growing worldwide, other methods of plastic waste treatment without raising environmental problems are eagerly sought. Recycling is one choice for reducing the polymer waste. But, recycling is only effective when waste materials are easily collectable and can be recycled to useful materials without significant deterioration of properties. Recycling also includes the chemical degradation of plastics to monomeric materials which are renewable to give polymeric materials. At present, only a few kinds of plastics, such as polystyrene, polyolefins and polyesters, are recycled on atrial scale. Another choice is using environmentally friendly biodegradable polymers. The term biodegradable polymer is used here in the sense of any polymer which is degraded to carbon dioxide, water and biomass using environmental micro-organisms. Hence, biodegradable polymers contribute to reducing environmentally released plastic waste. Some biodegradable polymers, such as polysaccharides, can be treated as compost and biologically recycled to produce natural products. Sometimes, according to a broad definition, biodegradable polymers also include biomedically useful polymeric materials which degrade and are absorbed into the animal body and again contribute to polymer waste reduction. Biodegradable polymeric materials can be classified into three categories based on their origins, i.e., chemically synthetic materials, natural products and composites of both chemical and natural products. Several aliphatic
772
YOSHIO I N O U E
polyesters, such as poly(glycolic acid), poly(lactic acid), poly(E-caprolacton) and poly(ethylene adipate), are well known examples of biodegradable polymers [1]. Poly(vinyl alcohol) [2, 3] and lower molecular weight poly(ethylene oxide) [4-6], which are typical water-soluble synthetic polymers, are also known to be biodegradable. There are many kinds of natural biodegradable polymers. They are classified into three types according to their chemical structures, i.e., polysaccharides, polypeptides/proteins and polynucleotides/nucleic acids. Among them, polysaccharides, such as cellulose, chitin/chitosan, hyaluronic acid and starch, and proteins, such as silk, wool, poly(y-glutamic acid), and poly(E-lysin), are well known and particularly important industrial polymeric materials. Recently, a series of bacterially synthesized polyesters called poly(hydroxyalkanoic acids) (PHA) have attracted much attention, because they are naturally occurring biodegradable thermoplastics (for review articles, see Refs. 1, 2, 7-10). Now, various types of micro-organisms are known to accumulate PHAs as an intracellular storage material for biological energy and carbon source [11]. Until now, more than 90 different PHAs with different chemical structures have been reported as constituents of biosynthetic PHA [10]. They are mainly produced by the feeding of designed precursor substrates, including unnatural synthetic compounds, as the substrate specificity of PHA synthases, key enzymes of PHA biosynthesis, is unexpectedly broad. Thus, it is expected that new PHA constituents will continue to be developed by using new substrates or new types of bacteria, resulting in new functional biodegradable and biocompatible polymeric materials. The productivity of these polyesters by some typical PHA-producing micro-organisms, such as Alcaligenes eutrophus and Pseudomonas oleovorans, increases by cultivating them under proper conditions, including a sufficient amount of carbon sources but lacking one of the growth factors such as nitrogen, phosphorus and oxygen. In addition to PHA-producing micro-organisms, a variety of aerobic and anaerobic PHA-degrading bacteria and fungi have been isolated from various environments, such as farm and forest soil, bottom sediments in lakes and rivers, sewage sludge and sewage sludge supernatent [12-18]. These microorganisms have as a rule, little or no ability to biosynthesize PHAs, but they can hydrolyze environmental PHAs by secreting extracellular PHBdepolymerases. They utilized the hydrolyzed products as nutrients. Thus, PHAs are expected to be fully biodegraded into carbon dioxide and water in a variety of environments. The third category of biodegradable polymeric materials is composites of both chemical and natural products. They are designed to have superior properties compared to those of the component materials, or to supply
BIODEGRADABLE POLYMERS
773
deficiencies of components to each other and, furthermore, to control biodegradability. There are many types of biodegradable polymeric composites [2]. The development of biodegradable composite materials is one of the growth areas of polymer research. The properties of solid-state biodegradable polymeric materials, such as mechanial strength and biodegradability, should be affected not only by chemical structure but also by the physical and the morphological state of materials. Almost all biodegradable polymeric materials are usually used in the solid state. Hence, high resolution solid-state NMR is expected to be a powerful method for the study of structure/morphology-properties relationships of solid-state biodegradable polymeric materials. Solid-state 13C NMR spectra should provide an independent means of discriminating between chemically equivalent carbon nuclei distributed among different environments, such as different crystalline and noncrystalline and/or amorphous regions of semicrystalline materials, and the miscible and immiscible domains of polymer blends. In this chapter, solid-state structure and properties relative to the morphologies of several chemically and bacterially synthesized biodegradable polymeric materials are described based mainly on the results obtained for bacterially synthesized polyesters by high resolution solid-state 13C NMR spectroscopy. This chapter briefly discusses polymer blends, which also includes polysaccharides and proteins, since more details are given in other chapters of this book. Several books on biodegradable polymers have been published [1, 2], and many review articles on structure and properties of bacterially synthesized polyesters have also been published elsewhere [7-10, 19-22].
21.2 21.2.1
Poly(hydroxyalkanoic acid)s Chemical structure and some physical properties of bacterial polyesters
Among a variety of bacterially synthesized poly(hydroxyalkanoic acid)s (PHAs), poly(3-hydroxybutyric acid), P(3HB), CH3
is the most popular and widely distributed homopolyester produced efficiently by various kinds of micro-organisms [10]. P(3HB) is the first example of
774
YOSHIO INOUE
PHAs discovered by Lemoigne [23] more than 70 years ago. The P(3HB) is produced and stored inside the bacterial cell walls in granules. Alcaligenes eutrophus, which was the first strain used for the semi-industrial production of PHAs, can accumulate large quantities of P(3HB) as discrete intercellular granules by careful control of the fermentation process, i.e., up to 80% of the weight of the dried cell can be in the form of P(3HB) granules [7]. P(3HB) is recovered from the cell by various procedures such as solvent extraction. As will be shown in the next section, this recovery process changes amorphous P(3HB) granules into plastic. The resulting P(3HB) material is a highly crystalline thermoplastic, whose several characteristics, such as melting point, degree of crystallinity and glass-rubber transition temperature, are comparable to those of isotactic polypropylene [24]. However, it is stiff and brittle. Furthermore, its melting temperature (ca. 170~ is near the temperature at which it thermally decomposes [25-27], limiting its processing in the molten state and, hence, its extensive industrial applications, although a number of mouldings, extrudates, films and fibres have been produced [7]. There are two possible ways to improve thermal processability and mechanical properties of P(3HB), i.e., copolymerization of 3HB with other monomers and blending P(3HB) with another polymeric materials. There are many kinds of bacterially synthesized copoly(hydroxyalkanoic acid)s [10]. A well-known typical copolyester is poly(3-hydroxybutyric acidco-3-hydroxyvaleric acid), P(3HB-co-3HV), CH3
I CH3
CH2
which is produced by fermentation with a high yield comparable to those of P(3HB), so it is an industrially important polymeric material. We can now synthesize bacterially a range of P(3HB-co-3HV)s with 3HV mole fractions ranging from 0% [P(3HB)] to 100% [P(3HV)] [28, 29]. These P(3HB-co3HV) samples are essentially random copolymers [28]. Several mechanical and physical properties of P(3HB-co-3HV)s have been found to vary widely with their comonomer composition [19, 20, 22]. Some of such composition dependences of properties should be ascribed to composition-dependent morphological changes in copolymers in the solid state. Hence, it is very interesting to investigate the composition dependence of the solid-state structures of P(3HB-co-3HV)s in detail. In the first section of this chapter, some solid-state properties of P(3HB-
BIODEGRADABLE POLYMERS
775
co-3HV)s are described in relation to their morphology elucidated by solidstate a3C NMR. Furthermore, properties and morphologies of some related bacterial copolyesters, such as poly(3-hydroxybutyric acid-co-4-hydroxybutyric acid), P(3HB-co-4HB), CH3
and poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid), P(3HB-co3He), CH3
are shown in order to discuss the molecular origins of the similarities and differences of their properties and morphologies. It will be exemplified that even small differences in the chemical structures of comonomer units induce significant changes of morphology and morphology-dependent properties. The second method to improve the properties of P(3HB) is a blending strategy, which has been widely applied [20-22]. In addition to improving the desired properties, the blending is also expected to reduce the marketing price of P(3HB) materials, which is one of the major reasons limiting its industrial applications, if the blending partners selected are less expensive biodegradable polymers. As the degree of mixing of the polymer blend systems influences the physical and chemical properties, it is important to know how morphologically heterogeneous the blend is. Solid-state NMR can provide molecular level detailed information on the mixing state of polymer blend systems. In the later sections of this chapter, the results of NMR studies of some P(3HB)-based polymer blend systems are shown.
21.2.2 Solid-state structure of poly(hydroxyalkanoic acid)s studied by NMR 21.2.2.1
Structure of poly(hydroxyalkanoic acid) granules in vivo
As mentioned above, P(3HB) isolated from bacterial cells is highly crystalline, with melting point of about 170-180~ and a glass-transition temperature of about 5-10~ while in the intact cell it is found in granules whose physical states have attracted much attention. As described below, some studies including solid-state NMR analysis of dried granules indicate the presence of
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YOSHIO INOUE
highly crystalline structures, leading to the expectation that the same highly crystalline structures are also present in native granules. High resolution solution-state 13C NMR spectra of aqueous suspensions of A. eutrophus cells containing P(3HB) intact granules show the well resolved P(3HB) resonances even near room temperature [30], indicating that the P(3HB) molecules in the bacteria are in a mobile state to a great extent. Nascent P(3HB) granules are found to be isolated as a milky suspension [31]. The mobile state was found to remain stable in an isolated granule suspension of P(3HB-co-3HV) for several months at room temperature. Both transmission and scanning electron microscopy show that their structures are made of two distinct components, a solid shell composed of overlapping lamellar crystals and a soft noncrystalline core. Glycerol triacetate, which is a minor component of the granule, is found to be a good solvent [31]. The glycerol derivatives should contribute to plasticized poly(hydroxyalkanoic acid) granules as reported [32], although the amount of the glycerol derivatives (combined amount of proteins and lipids is up to 2% by weight [33]) may not be sufficient to fully plasticize the entire P(3HB) molecules [31]. Thus, it is not yet clear what maintains the core P(3HB) as a noncrystalline mobile state. A model of a state of P(3HB) granule core has been proposed in which the core is in a metastable gel state due to interactions between high molecular weight P(3HB) molecules and a plasticizing medium [31]. The influence of various types of treatments, such as freeze drying, separation of the granule from the cell debris by centrifugation, incubation of the granule at higher temperature etc., on the mobility and crystallinity of P(3HB) in whole cells of A. eutrophus H16 and native granules has been studied using solution-state 13C NMR spectroscopy and X-ray powder diffraction. The results have been correlated with the known biological effects of these treatments [34]. In this investigation, NMR spectroscopy is used as a highly sensitive diagnostic means to monitor the physical changes induced by different treatments. That is, when the P(3HB) included in a granule sample is completely converted from the mobile elastmer state to the immobile solid state, its resonances are predicted to disappear completely in solution-state NMR, while a sample consisting of a mixture of elastmer and rigid solid should show resonances with reduced intensities, but with the same linewidth as those of the elastomer. A sample consisting of polymers with lower chain mobility should show resonances with broader linewidth without loss of integrated intensity. The strains used in this study were grown on a minimal salt medium including glucose as the sole carbon source at 30~ under phosphate limitation which stimulates P(3HB) accumulation. These experiments discriminate between four different states of the P(3HB) granule: (1) the native state found
B I O D E G R A D A B L E POLYMERS
777
in live cells is a mobile elastomer which gives well-resolved NMR signals and no X-ray diffraction pattern; (2) freeze-drying can lead to a nonnative but relatively mobile amorphous state which has been partially characterized by NMR but not less X-ray diffraction studies. The dehydrated amorphous relatively-mobile granules are found to be rehydrated to a state which is spectroscopically indistinguishable from the native state, while the crystalline material cannot rehydrate to a native state; (3) extended centrifugation of the native granules in aqueous suspension, or treatment with hydrophobic detergents under certain conditions, is found to induce crystallization; (4) heating to 90~ or refrigeration has no detectable effect on mobility but leads to inactivation of the granule. Based on these results, the authors concluded that at least water is responsible for P(3HB) plasticization in vivo, and that only native mobile P(3HB) is susceptible to depolymerases, and another, probably hydrophobic, component was suggested to be involved either as a plasticizer or a nucleation inhibitor [34]. Transmission electron microscopy, differential scanning calorimetry, wideangle X-ray powder diffractometry and Fourier transform infrared spectroscopy have been used to investigate the crystallization behavior of P(3HB) granules from A. eutrophus isolated by enzymatic purification [35]. From the results of these investigations, water is suggested to be responsible for keeping the core of nascent P(3HB) granules in a noncrystalline state, and a model was proposed for the biosynthesis where emerging P(3HB) chains in an extended conformation are simultaneously hydrogen bonded to water molecules. The crystallization kinetics of native P(3-hydroxyalkanoic acid) (PHA) granules isolated from the strains of A. eutrophus and P. oleovorans have been studied by measurements of the glass-transition temperature with a differential scanning calorimeter [36]. The comparison is made between PHA in vivo and the isolated polymer. It is demonstrated that the native granules do not contain a plasticizer and the amorphous state of in vivo PHA can be explained by straightforward crystallization kinetics. High resolution solid state 13C NMR spectroscopy is useful to characterize the composition and solid-phase morphology of dried P(3HB) granules. A series of commercial powder preparations of extracellular microbial polysaccharides, such as gellan, welan and rhamsan, have been characterized by cross-polarization (CP) magic-angle sample spinning (MAS) a3C NMR spectroscopy [37]. The spectra indicate that samples contain a noncarbohydrate component exhibiting four inequivalent carbon nuclei attributable to those of P(3HB). It is suggested that P(3HB) may be a covalent adduct of these polysaccharides, i.e., a bacterial polyester may exist as a substituent associated with certain extracellular microbial polysaccharides in addition to its
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YOSHIO INOUE
well-known role as an intracellular energy reserve [37]. I n fact, short-chain complexed P(3HB) is a ubiquitous constituent of cells and has been isolated from the plasma membranes of bacteria, from a variety of plant tissues, and from the plasma membrane, mitochondria and the microsomes of animal cells [38]. Solid-state NMR spectra of whole cells containing poly(hydroxyalkanoic acid) have been observed [39, 40]. The CP/MAS 13C NMR spectrum of P(3HB-co-3HV) containing 21 mol% 3HV in the freeze-dried cells is found to be qualitatively similar to that of a solution- or melt-cast film of the isolated copolymer [41]. The metabolic pathways of P(3HB) and polyphosphate in the micro-organism A. eutrophus H16 have been studied by several analytical techniques including ~H, 13C and 31p NMR spectroscopy [42]. Solid-state CP/MAS 13C NMR spectroscopy has been used to monitor the biosynthetic pathways of P(3HB) and other cellular biomass components from 13C-enriched acetate. The CP/MAS 13C NMR spectra of lyophilized intact cells grown on (1-13C) acetate (6% 13C)indicate that the carbonyl carbon of the acetate is selectively incorporated into both the carbonyl and methine carbons of P(3HB) and into the carbonyl carbons of proteins. From the 31p NMR analysis of A. eutrophus cells in suspension, the syntheses of both intracellular P(3HB) and polyphosphate are found to be closely related to each other. In contrast, under aerobic and anaerobic conditions, P(3HB) is degraded, but little polyphosphate is degraded. The rate of P(3HB) degradation is faster under anaerobic rather than aerobic conditions. Furthermore, under anaerobic conditions, 3hydroxybutyrate and acetate are found to be produced as the major extracellular metabolites. From these results, the pathways of P(3HB) and polyphosphate metabolism in A. eutrophus are suggested [42]. As shown here, solid-state NMR spectroscopy combined with solution NMR is a useful method to study not only metabolically important events in polyester-containing micro-organism cells but also to investigate the crystallization mechanism during polyester-isolation procedures. 21.2.2.2 Solid-state structure of P(3HB-co-3HV) Some important physical properties of P(3HB-co-3HV) have been found to show notable changes with their comonomer composition changes [19, 20, 22]. For example, Fig. 21.1 shows the plot of melting point (Tm) versus 3HV mole fraction (Fu) of random copolymer samples [28]. The melting point was measured by differential scanning calorimetry (DSC) at a heating rate of 20~ In order to eliminate the effect of thermal history, all samples for the DSC measurements were heated first to 200~ sufficiently higher than the highest melting point (namely, that of P(3HB) homopolymer), then
B I O D E G R A D A B L E POLYMERS
779
200
150
o
I00
50
0.0
'
'
0.2
t
|
o,q
0,6
Fv
t
0.8
1,0
Fig. 21.1. Plots of melting point (TM) vs. 3HV mole fraction (Fv) of bacterially synthesized random P(3HB-co-3HV)s. The circles indicate Tm values for P(3HB-co-3HV) samples which show a single DSC melting peak. The Tm values indicated by triangles are those for P(3HB-co3HV) samples which were bacterially synthesized as mixtures composed of two main copolymer components with different 3HV mole fractions, and so show well-resolved two DSC melting peaks. For these mixed samples, the 3HV contents of two components were determined by analyzing the solution XH NMR spectra based on the statistical model (for details of spectral analysis, see Ref. [28]). Hence, for these samples, the Tm values are plotted against the 3HV fractions of the component copolymers. (Reproduced from Ref. [28] with permission.)
quenched to room temperature at a rate of ca. 200~ and left at least 5 days [28, 43]. The melting point becomes a minimum at the F~ value of about 40 mol%, indicating clearly the occurrence of a crystalline phase transition in this composition range. According to the results of X-ray diffraction studies and intramolecular potential energy calculation of the crystalline structures, P(3HB) [44, 45] and P(3HV) [46] have crystalline structures similar to each other, that is, both are orthorhombic of the space group P21212x. The unit cell parameters for P(3HB) and P(3HV) are, respectively, a - 0.576 nm, b - 1.320 nm, c (fiber period) = 0.596 nm; and a = 0.932 nm, b = 1.002 nm, c (fiber period) = 0.566 nm. The fiber period of P(3HB) is very close to that of P(3HV). The crystalline structure has been refined by applying the whole-fitting method to the powder X-ray diffraction data [47]. Furthermore, it has been found by X-ray analysis that P(3HB-co-3HV)
780
YOSHIO INOUE
copolymers containing less than 40 mol% 3HV unit crystallize in the P(3HB) crystalline lattice, while those containing more than about 40 mol% of the 3HV unit crystallize in the P(3HV) crystalline lattice [48, 49]. That is, the transition from the P(3HB) to the P(3HV) crystalline lattice occurs at a 3HV unit composition of ca. 40 mol%, where the minimum of the melting point is observed. Upon a phase transition, only the a parameter of the crystalline unit cell of P(3HB) increases slightly, while the b and c parameters remain almost unchanged, irrespective of comonomer composition. X-ray data also show that the degree of crystallinity of P(3HB-co-3HV)s remains high, 5270%, with respect to the samples with comonomer compositions of 095 mol% 3HV [49]. As shown in Fig. 21.1, the melting point of P(3HB-co-3HV) is depressed rapidly as the 3HV content increases from 0 to 40mo1%. This behavior cannot be predicted by the Flory equation [48, 50], which was derived based on the assumption that copolymer crystals are composed of only one kind of comonomer component and the others exist only in the noncrystalline region. The analysis of the crystalline structure of P(3HB-co-3HV) samples with 3HV content up to 30 mol% indicate that only a small fraction of the 3HV units are included in the P(3HB) crystalline lattice as defects [51, 52]. A possible model of the crystalline structure which can explain the experimental results obtained by X-ray, solid-state 13C NMR and DSC is the one based on the assumption that both the 3HB and 3HV units are simultaneously cocrystallized into the same crystalline lattice [48]. Such a so-called isomorphism is expected to occur because the chemical structure of both of the monomeric units are not so significantly different from each other. In order to confirm the possibility of cocrystallization and to estimate how much of the minor comonomer component enters into a crystalline lattice of the major one, the solid-state structures of P(3HB-co-3HV)s with 3HV content ranging from 0%(3HB homopolymer) to 93.1 mol% were analyzed by 67.9 MHz high resolution solid-state 13C NMR spectroscopy [53, 54]. All the samples used for this NMR study are random copolymers and not mixtures of random copolymers as confirmed by the solution 13C NMR sequence analysis and by DSC melting-point measurements [43, 54]. Before taking NMR measurements, possible effects of the thermal history and recrystallization are avoided as mentioned above. Figure 21.2 shows the 13C CP/MAS NMR spectra of a series of P(3HBco-3HV) samples, where the 3HV content is indicated, for example, as P(3HB-18.3%-3HV). Each carbon resonance shows more or less a discontinuous chemical shift change between the spectra of P(3HB-31.6%-3HV) and P(3HB-55.4%-3HV). A relatively large chemical shift change of about 2 ppm,
B I O D E G R A D A B L E POLYMERS
781
P(BHB)
HB-18.3%-3HV)
t
PI3HB-316%-3HV)
P13HB-40.7%-3HV)
P (3HB-55.4%-3HV)
....L
~
P(3HB-93. 1%-3HV)
L '
'
'
i-'
180
'
'
'-!
''
160
'
'
i
'
140
'
'
'-I'
120
'
'
'
I
"'
'i00
'"'
!
80
'
'
'
'
I ' "
60
''
I
40
'
'
'"''I"'""'I'"
20
0
Fig. 21.2. 67.9 MHz 13C CP/MAS NMR spectra of a series of P(3HB-co-3HV) samples (2-ms contact time, 5-s pulse repetition time, 1000 FID accumulations. 's' indicates spinning side band). (Reproduced from Ref. [53] with permission.)
is observed for the main-chain methine carbon resonance. The chemical shifts for the resonances of the HB and HV units are listed in Table 21.1. As the CP efficiency in the crystalline region is in general larger than that in the noncrystalline region in the CP/MAS NMR spectra of semicrystalline polymers, the chemical shift changes of resonances which appear in the spectra are shown in Fig. 21.2 and Table 21.1 and should reflect predominantly the differences in the crystalline environment, that is, the crystalline
--.a b~
Table 21.1. Chemical shifts of 13C resonances of P(3HB-3HV) samples in the solid state Chem shift/ppma
X/%c
CO
0.0 18.3 31.6 40.7 55.4 93.1
170.0 169.8 169.8 169.8 169.9 169.8
CH(V) b
CH(B) b
LB
LB
72.5 72.6 72.6
LV
70.8 70.9 70.7
LV
68.6 68.5 68.6 68.7
CH:(B) b
CH2(V)
LB
LB
LV
42.9 42.9 42.9 42.9 67.5 67.2
LV
402 d 394 d 40.6 40.3
40.5 40.6 40.3
s-CH2(V) b
CH3(B) b
CH3(V) b
LB
LB
LB
LV
LV
21.4 21.1 20.9 21.0
267 d 26.9 27.2 29.0 28.7
LV
9.7 9.9 19.5 19.4
10.5 10.7 10.5
appm from TMS. bB, V, LB, and LV indicate the HB unit, the H V unit, the P(3HB) lattice, and the P(3HV) lattice, respectively, s-CH2 indicates side-chain methylene of H V units, c x indicates H V mol% content of P ( 3 H B - 3 H V ) copolymers, dBroad peak. (Reproduced from Ref. [53] with permission).
BIODEGRADABLE POLYMERS
783
lattice structure. Considering the results of X-ray analysis [49], and comparing the observed chemical shifts for the copolymer samples with the different 3HV content, the main peaks appearing in the CP/MAS spectra of P(3HB) and copolymers with 3HV contents of 18.3 and 31.6 mol% are reasonably assigned to the contributions from the nuclei in the crystalline phase of the P(3HB) lattice and those of the copolymers with 3HV contents of 55.4 and 93.1 mol% from the nuclei in the crystalline phase of the P(3HV) lattice. In the spectrum of P(3HB-co-40.7%-3HV), doublet peaks are clearly observed for the main-chain methine carbon resonances of the 3HV unit, indicating the coexistence of both types of crystal phase. This result suggests that the phase separation is induced in the crystalline region on crystallization. The composition range of the crystal transition, where both the P(3HB) and P(3HV) crystal phases coexist, was investigated by wide-angle X-ray scattering and DSC for copolymer samples compositionally fractionated with a solution of an acetone-water solvent nonsolvent system [55]. The results of the X-ray analysis obtained for the fractionated P(3HB-co-3HV) samples with the 3HV contents range from 40.9 to 85.0 mol% indicate that the samples whose 3HV content range is 40.9-55.2 mol% show the coexistence of both the P(3HB) and P(3HV) crystal phases. The samples of 3HV content lying within this relatively wide range are found to show always the same mixed X-ray patterns of the P(3HB) and P(3HV) crystal phases. At this range, the crystal lattice parameters of the P(3HB) phase are found to expand from the original ones of P(3HB), a = 0.576 nm, b - 1.320 nm and c - 0.596 nm [44, 45] up to the maximum values for the P(3HB-co-55.2%-3HV) sample; a = 0.602 nm, b - 1.343 nm and c = 0.604 nm [54] the latter parameters are not the same as those for the P(3HV) lattice [46]. To study quantitatively the crystal morphology, the comonomer compositions in the crystalline, as well as in the amorphous, phases are measured by analyzing the solid-state NMR spectra. To make quantitative measurements, first, the I3C spin-lattice relaxation time T1 was measured by Torchia's pulse sequence [56]. The T1 decaying curves for all carbon nuclei are found to be practically reproduced by assuming slow and fast two-component relaxation processes. The slow and fast processes should be ascribed to the crystalline and noncrystalline regions, respectively. The T1 values for the protonated main-chain carbons are longer than ca. 60 s, while those for the side-chain methyl carbons are shorter than 5 s. The dipolar-decoupled (DD) MAS 13C NMR spectra (without CP) are measured in order to estimate the relative peak intensities of the methyl resonances of the 3HB and 3HV units in the crystalline and the amorphous phases. For this purpose, the pulse repetition time was adjusted to be longer than at least 5 times the longest T1 value of the methyl resonances.
784
YOSHIO INOUE P ( 3~B}
____~
P {3HB-93.1~-3HV)
~CH 3 IV)
PP14
13C DD/MAS NMR spectra of P(3HB) and P(3HB-co-3HV). (Reproduced from Ref. [53] with permission.) Fig. 21.3. Methyl resonances in the 67.9 MHz
Figure 21.3 shows the methyl resonance region of the 67.9MHz 13C DD/MAS NMR spectra of P(3HB) and P(3HB-co-3HV) samples. Each of the 3HB and 3HV methyl resonances was resolved into two peaks contributed from the crystalline and amorphous phases, respectively. Hence, from these measurements, the relative contributions from the crystalline and noncrystalline regions can be quantitatively estimated. The resulting chemical shifts and the relative peak intensities are listed in Table 21.2. These data clearly indicate that both comonomer units of P(3HB-co-3HV) are distributed among the crystalline and noncrystalline regions irrespective of the crystalline lattice types. That is, cocrystallization occurs in this copolyester system. From the relative intensity data shown in Table 21.3, the degrees of crystallinity are estimated to be ca. 60-70%, which are independent of comonomer composition and again indicate the occurrence of cocrystalliza-
T a b l e 21.2. Chemical shifts and peak intensities of methyl carbon resonances in DD/MAS NMR spectra
3HV unit b X/% ~
0.0 18.3 31.6 40.7 55.4 93.1
LB
11.32 11.56
LV
10.74 11.01 10.78
0 ~D
Rel peak intensity/%
Chem shift/ppm a 3HB unit b NC
10.11 10.29 9.73 10.31 10.22
LB 21.34 21.24 21.34 21.42
LV
21.89 20.77
3HV unit b NC 20.06 20.45 20.74 20.62 19.98 19.73
LB
6.2 24.5
LV
14.2 42.5 68.1
3HB unit b NC
19.4 25.7 18.0 13.2 25.8
LB 72.3 59.4 54.0 4.1
LV
20.1 16.5
NC 27.7 21.1 14.2 19.1 27.9 6.1
appm from TMS. oNC, LB and LV indicate noncrystalline component and crystalline components in the P(3HB) and P(3HV) lattices, respectively, cX indicates 3HV mol% content of P(3HB-3HV) copolymers. (Reproduced from Ref. [53] with permission).
> >
0
786
YOSHIO INOUE
Table 21.3. Comonomer composition, crystallinity, and heat of fusion for P(3HB-3HV) samples
xa/% 0.0 18.3 31.6 40.7 55.4 93.1
xa/%
xa[%
Eb
0.0 10.2 14.2 58.6 72.1 100.0
47.8 64.4 48.6
0 0.246 0.242 0.484 0.481 0
32.1 80.8
hc/%
AHd/kj mo1-1
72.3 59.4 60.2 62.9
11.2 6.2 3.6 4.8
58.9 68.1
7.6 10.0
ax, Sc, and Xa indicate 3HV mol% content of whole P(3HB-3HV) copolymers, that in the crystalline region, and that in the non-crystalline region, respectively, bE indicates the crystallizing ability of the secondary component, cA indicates the degree of crystallinity, dAHf indicates the heat of fusion of the pure crystal. (Reproduced from Ref. [53] with permission).
tion. The more important finding is that the comonomer composition of the crystalline phase is not the same as those of the whole copolymer molecule. The mole fraction of the minor comonomer component unit in the crystalline lattice of the major one is less than that in the whole copolymer. The fraction of the minor component incorporated into the crystalline lattice of the major component increases with increasing composition of the minor component in the whole copolymer. The crystallizing ability of the minor component as the crystalline component in the lattice of the major component can be discussed quantitatively by the term E defined as follows: when a crystal has the P(3HB) lattice, i.e., X is smaller than ca. 40%, E is given by E
=
(1 -
X)Xr
X(1 - X ~ ) ' where X and Xr indicate the 3HV mol% content of the whole copolymer and that in the crystalline region, respectively. When X is larger than ca. 40%, i.e., a crystal has the P(3HV) lattice, then the value of E is given by E=X(1 (1
- Xc) -
XlXc
As can be seen from the above definition, the E value represents the fraction of the minor component that is capable of crystallization. The E value of the main component is defined to be unity. When the comonomer composition in the crystalline region is the same as that of the whole copolymer molecule,
BIODEGRADABLE POLYMERS
787
the E value become equal to unity. When the minor component is fully excluded from the crystal, the E value becomes zero. The experimental results are also shown in Table 21.3. The E values for the 3HV units in the P(3HB) lattice are about 0.24 and those for the 3HB units in the P(3HV) lattice are about 0.48. Thus, the ability of the 3HB unit to crystallize in the P(3HB) crystalline lattice is found to be twice as large as that of the 3HV unit to crystallize in the P(3HB) crystalline lattice. This is reasonable, because the side-chain of the 3HV unit is more bulky than that of the 3HB unit. Recently, VanderHart et al. [57] characterized a series of bacterially produced P(3HB-co-3HV) copolymers with a 3HB unit content of 0-27 mol% by CP/MAS ~3C NMR and determined the degree of cocrystallization of the two comonomer components. They also find that the 3HV minor component is incorporated into the P(3HB) type crystalline phase over the 3HV composition range of 0-27 mol% and that the ratio of the 3HV content in the crystalline phase to the overall 3HV composition increases with increasing 3HV content. They indicate for the copolymers with 3HV compositions of 21 and 27 mol% that the 3HV content in the crystalline phase is roughly 2/3 of the overall 3HV content. In general, most of the random copolymers form crystals composed of the major comonomer units of more crystallizable comonomer units alone, as incorporation of the minor component units into the crystalline phase need a large amount of excess free energy. So the cocrystallization of polymers is a rare phenomenon and a very few examples, such as poly(vinylidene fluoride)/vinylidene fluoride-tetrafluoro-ethylene copolymers system [58] and high-density polyethylene/linear low-density polyethylene [59], have been reported. Hence, the occurrence of cocrystallization found for the P(3HBco-3HV) copolymer is one of the rare examples. When the copolymer is composed of two types of units of different repeating-unit length, it is not expected to form an isomorphous crystal. However, for binary random copolyesters of ethylene terephthalate (ET) and 1,4-cyclohexenedimethylene terephthalate (CT), cocrystallization is found to be realized to some extent [60]. The comonomer contents in the crystalline phases are determined by high resolution solid-state X3C NMR spectroscopy. In the copolymers with an ET unit composition of 80-100 mol%, only the ET components form the crystalline phase, but in those with a CT unit composition of 66-100 mol%, a detectable amount of ET units are incorporated into poly (CT) crystalline lattice. The ET-CT copolyester system shows a minimum melting temperature at the intermediate comonomer composition range, quite similar to the case of 3HB-3HV copolyesters. According to the unit-cell dimensions, the sequence length of four ET repeating units is almost
788
YOSHIO INOUE
the same as that of three CT repeating units. This similarity should be the main reason that cocrystallization is realized for the ET-CT system.
21.2.2.3 Theoreticalstudy of cocrystallization of P(3HB-co-3HV) An involvement of different kinds of comonomer component units in the crystalline lattice of the main component unit of a copolymer should influence the bulk properties of a solid-state copolymer. As found for P(3HB-co-3HV) copolymers, the comonomer composition in the crystalline phase is not always the same as that in the noncrystalline phase. Hence, it is of great interest to investigate theoretically a comonomer composition in a crystalline phase. The cocrystallization ability of P(3HB-co-3HV) copolymers has been investigated from two different viewpoints, i.e., thermodynamically [61] and in terms of molecular mechanics [62]. Thermodynamic equations are formulated for the isomorphic behavior of A-B type random copolymer systems, in which both A and B comonomer units are allowed to cocrystallize in the common lattices analogous to, or just the same as, those of the corresponding homopolymers poly(A) or poly(B). It is assumed that, in the lattice of poly(A), the B units require free energy relative to the A units and vice versa. On the basis of the derived thermodynamic equations, phase diagrams are proposed for the A-B random copolymers with cocrystallization. The melting point versus comonomer composition curve predicted by this diagram is very consistent with that experimentally observed for the P(3HB-co-3HV) copolymers, as shown in Fig. 21.1. It is suggested that the minor comonomer unit with a less bulky structure cocrystallize thermodynamically simpler than that with a more bulky structure. The content of the minor comonomer units in the crystalline phase of the major ones is predicted to increase with respect to that in the whole copolymer. This prediction is also consistent with the results of 13C NMR observations. The theory also predicts that the content of the minor comonomer units in the crystalline phase decreases with a rise in the crystallization temperature [61]. To confirm this prediction, the contents of comonomer units in the crystalline as well as in the amorphous phases were again measured by high resolution solid-state 13C NMR spectroscopy for the P(3HB-co-3HV) copolymers crystallized at various temperatures [54]. The copolymer samples used were quenched from the melt into the crystallization temperature and left for 5 days at this temperature and then left at room temperature for more than 5 days. For the copolymer with a 3HV content as a whole of 18.3%, in which the 3HV unit is the minor component and this copolymer was crystallized in the P(3HB) lattice, the 3HV content in the crystalline phase (Xc) was found to decrease from 7.6 to 7.2% and the melting point
BIODEGRADABLE POLYMERS
789
was found to rise from 112.8 to 127.1~ with a rise in the crystallization temperature from 0 to 80~ A more striking change was observed for the copolymer with the 3HV content as a whole of 55.4% (in this case the 3HB unit is the minor component and this copolymer was crystallized in the P(3HV) lattice). This copolymer shows an increase in the Xr value from 59.2 to 68.9% and a rise in the melting point from 68.7 to 78.2~ with a rise in the crystallization temperature from 0 to 40~ All of the results are consistent with the predictions based on the thermodynamic theory. The abilities of different monomeric units of the copolymer to cocrystallize into the same lattice should depend on the degree of structural similarity between them. The minor component should at least have the chemical structure causing no significant distortion in the backbone geometry of the homopolymer chain of the major component. To investigate how the involvement of the 3HV and 3HB units as the minor components in the homopolymer chains of P(3HB) and P(3HV), respectively, disturb their backbone conformations, the conformational properties have been theoretically studied for the P(3HB-co-3HV) copolymers on the basis of MM2 molecular mechanics calculations [62]. For this purpose, the model polymer chains consisting of 15 monomer units, are considered. The optimized conformations were obtained by starting from those of the X-ray crystal structures. The MM2 calculation is confirmed to be reliable, as each rotational angle along the backbone in the region extending from the 3rd to the 12th unit of the energetically optimized geometry for the P(3HB) homopolymer model having 15 monomer units, i.e., the B15 model, is found to agree well with that for the crystalline P(3HB) determined by X-ray method [45], as shown in Fig. 21.4. Here, the rotational angles are defined in Fig. 21.5. Thus, the 3rd to 12th region of B15 is assumed to be a better analogue of the P(3HB) homopolymer chain. The similar regular structure along the backbone is again found in the same region for the P(3HV) homopolymer model chain having 15 monomer units, i.e., the V15 model. The MM2 calculations reveal that the isolated model chains of both B15 and V15 have a tendency to form stable 21-helices, as found in the crystals by X-ray analysis for both the P(3HB) and P(3HV) homopolymers [44-46]. The effects on the B15 and V15 chain conformations of the incorporation of one or two 3HV and 3HB units, respectively, have been investigated. First, the central (8th) unit of B15 model chain is replaced by an HV unit, this copolymer model is called B14Vl[8], and the conformation was optimized starting again from the X-ray structure of P(3HB). The rotational angles of the optimized conformation are shown in Fig. 21.6. Special attention was paid to the conformational changes in the region
790
YOSHIO I N O U E
+10
CD
+5
(I)
"1:3 CD cM
c o o
-5
-10 [
1
/
1
5
I0 Unit
15
no.
Fig. 21.4. Each rotational angle along the backbone in the B15 model after optimization from
the initial X-ray structure versus the monomer unit number. A zero value for the rotational angle corresponds to the X-ray structure. (O)qJ; (A)~b; (D)o~; (O)0. (Reproduced from Ref. [62] with permission.)
from the 3rd to 12th unit, which corresponds to the regular structure region of the parent homopolymer model B15. The rotational angles ~b and 0 at the 7th unit, i.e., around t h e - - ( T t h u n i t ) C H 2 - - C O ~ and I O ~ C ( C H 3 ) (8th unit) I bonds, respectively, show large changes by + 11.4 and - 1 1 . 1 from those of B15. Such large changes are not found in the other angles of the same unit as well as in those in the other units. These conformational changes are found to be independent of the position of the 3HV substitution in the 3rd to 12th region. Thus, the substitution from 3HB to 3HV influences only the angles ~b and 0 of the nearest neighboring 3HB unit preceding the incorporated 3HV unit. This conformational change is attributable to the steric interaction between the ethyl side group of 3HV and the nearest carboxyl group. The overall
BIODEGRADABLE POLYMERS
e
c=o CH2
791
~
.O-
CH3
Fig. 21.5. 3HB monomer unit and definition of rotational angles.
increase in steric energy introduced by changing one monomer unit from 3HB to 3HV can be relaxed by allowing rotation around the angles ~b and 0 of the 7th unit only. Hence, the conformational distortion induced by incorporating the 3HV unit is limited in the vicinity of the substituted unit and not propagated to more distant units. As a result, the 2~-helical structure of the parent homopolymer is almost unchanged in the B14Vl[8] copolymer model. This is also found to be valid in the di- an tri-3HV substituted copolymer models. For example, the optimized structures of the disubstituted models B13Vl[7]Vl[8] and B13Vl[6]Vl[8], in which the 3HV units are introduced at the 7/8th and 6/8th units, respectively, are shown in Fig. 21.7. In both cases, large conformational changes are found only in the nearest units preceding the incorporated 3HV units, indicating that the conformational properties of the resulting structures should not depend on the position of substitution. The copolymer models rich in 3HV units have been examined in the same way. For example, the result for the V14Bl[9] model, in which the central (9th) 3HV unit of V15 is replaced by a 3HB unit, is shown in Fig. 21.8. It is interesting to note that no apparent change in the backbone conformation occurs even around the incorporated 3HB monomer unit in contrast to the results for the B15-based models. The results indicate that substitution of one 3HB unit for one 3HV unit, i.e., the substitution of the methyl side chain for an ethyl one, causes no significant steric hindrance to the backbone of the model compound. The results of calculations for model compounds of di- and tri-substituted copolymers with various distributions of 3HB units indicate also that no significant conformational change is induced by incorporating 3HB units into the 3HV helix. Thus, it is concluded that 3HB units can
792
YOSHIO INOUE +15
+10
+5 Cf~ "[3
C
O3
0
O3
C 0 .m
O3
0
rr"
-5
-10
-15
~ 1
10
S Unit
15
no.
Fig. 21.6. Each rotational angle along the backbone in B14V1 [8] model after optimization versus the monomer unit number. A zero value for the rotational angle corresponds to the Xray data for P(3HB). Symbols are as given in Fig. 21.4. (Reproduced from Ref. [62] with permission.)
be incorporated into the 3HV-rich copolymer models without any appreciable distortion of the polymer backbone conformation when the MM2 optimization starts from a reasonable initial structure [62]. From the results of MM2 calculations, it is expected that the incorporation of the 3HB unit as the minor component into the P(3HV) type lattice may cause no apparent influence on the crystalline P(3HV) morphology. This is
793
B I O D E G R A D A B L E POLYMERS + 1.5
_b
+15
+10
+1o
+5
,.-,
cn
+$
c m
~
t~ c o
0
o
o
o
-10
-10
-15
-15 1
5
10
Unit
no.
15
1
5
lO
Unit
no.
Fig. 21.7. Each rotational angle along the backbone in (a) B13Vl[7]Vl[8] and (b)
B13V116]V118] models after optimization versus the monomer unit number. A zero value for the rotational angle corresponds to the X-ray data for P(3HB). Symbols as given in Fig. 21.4. (Reproduced from Ref. [62] with permission.)
consistent with the X-ray analysis, which shows that the lattice parameters are almost independent of the comonomer composition in the range of higher 3HV content. On the contrary, incorporation of the 3HV unit into the P(3HB) type lattice causes a substantial influence on the P(3HB) lattice. The MM2 predictions shown here are not inconsistent with the experimental conclusions based on the results of solid-state 13C N M R spectroscopy. The conformational aspects of P(3HB), as well as of P(3HB-co-3HV) in chloroform solution, have been investigated on the basis of the 1H spinspin couplings and N O E data [63]. The preferred conformation around the backbone of the methine-methylene bonds of both the 3HB and 3HV units in solution is found to be the same and coincides with that in the crystalline state [45, 46]. The different sequences and/or different monomer units do
794
YOSHIO INOUE
+10
+5 ol
o3 CO
co c o o~ r o c~
-5
x. -10 1
1
!
5
10 Unit
15
no.
Fig. 21.8. Each rotational angle along the backbone in the V14Bl[9] model after optimization versus the monomer unit number. A zero value for the rotational angle corresponds to the X-ray data for P(3HV). Symbols as given in Fig. 21.4. (Reproduced from Ref. [62] with permission.) not affect the short-range chain conformations in solution. The ~H NMR results indicate that the P(3HB-co-3HV) copolymers with different 3HV mole fractions have a higher possibility of forming isomorphic crystals.
21.2.2.4 Solid-state structure of P(3HB-co-4HB) It is very interesting to compare the crystallization behavior of P(3HB-co4HB) with that of P(3HB-co-3HV). The 4HB unit has no side chain which interacts sterically with neighboring monomeric units. The number of backbone carbon atoms in the 4HB unit is four, whereas that in the 3HB and 3HV units is three. Hence, a crystallization behavior different from that of P(3HB-co-3HV) is expected for P(3HB-co-4HV). The tendency of comonomer composition dependences of several physical properties of P(3HB-co-4HB) is quite different from that of P(3HB-co-3HV)
B I O D E G R A D A B L E POLYMERS
795
[49]. The melting temperatures of P(3HB-co-4HB) samples decreases slightly from 178 to 150~ as the 4HB content increases from 0 to 18 mol% and are almost constant in the range from 18 to 49 mol% 4HB. On the contrary, the enthalpy of fusion decreases monotonously with an increase in the 4HB unit in the composition range of 0-49 mol%. The degree of crystallinity, measured by an X-ray method, decreases from 55 to only 14% as the 4HB content increases from 0 to 49 mol%. The X-ray data indicate the existence of the P(3HB)-type of crystalline lattice at low 4HB content. A P(3HB-co-4HB) sample containing 82 mol% 4HB unit was found to show a sharp DSC melting endotherm at about 45~ [64, 65], which is close to the melting point (54~ observed for the P(4HB) homopolymer [66]. P(3HB-co-4HB) samples with 4HB contents over the range 85-100mo1% are found to have the non-P(3HB) type of crystalline lattice with a degree of crystallinity of 30-40% [65]. These experimental results suggest that the 3HB and 4HB units cannot cocrystallize in the same lattice. The factors which prevent the formation of cocrystals in the P(3HB-co4HB) copolymer, were investigated by molecular mechanics calculations for the model copolymer chains consisting of 15 monomer units [64]. Conformational analysis by the MM2 method of a P(3HB-co,4HB) model chain, including only one 4HB unit at the central position, revealed that 4HB unit cannot be included within the P(3HB) crystal lattice mainly because of the difference in the numbers of the main-chain carbon atoms of the repeating monomer units. The results of MM2 calculations also reveal that the 3HB unit, included within the P(4HB)-type lattice, takes a conformation which significantly deviates from a complete planar-zigzag conformation, which is the putative preferred one for the crystalline P(4HB) [64]. This is chosen in order to relax the steric hindrance between the methyl side chains of the 3HB units and the adjacent carbonyl oxygen atoms. Thus, the structural difference between the 3HB and the 4HB units is too large for them to cocrystallize into the same P(3HB)- or P(4HB)-type crystalline lattice.
21.2.2.5 The solid-state structure of P(3HB-co-3HP) There are several possible important factors which affect the cocrystallizability of comonomer units with shorter side chains in copoly(hydroxyalkanoic acid). These are such as the number of main-chain carbon atoms, the existence and the size of side-chains, steric configurations, crystallization temperature and the rate of crystallization. From a chemical point of view, the 3HP unit is expected to be easily incorporated into the P(3HB)-type crystalline lattice as a crystal component, since it has no side chain and its main chain consists of the same number of carbon atoms as the 3HB unit.
796
YOSHIO INOUE
To investigate whether, or not, cocrystallization occurs, in the P(3HB-co3HP) copolymer, the solid-state structure was analyzed by high resolution 13C NMR, X-ray diffraction, DSC and optical polarizing microscopy, for a series of P(3HB-co-3HP) samples with the 3HP content ranging from 0 mol% (bacterially synthesized 3HB homopolymer) to 100mol% (chemically synthesized 3HP homopolymer, i.e., polypropiolactone, PPL) [67]. The P(3HB-co-3HP) samples containing 0-37 mol% 3HP unit form the distinct spherulite when the melts of samples were cooled rapidly to the crystallization temperature on the optical polarizing microscope. The growth rate of spherulite deceases with an increase in the 3HP content. The spherulite formation could not be observed for copolymers with a higher 3HP content, while all copolymers show a DSC melting endotherm. The copolymers containing up to 37 mol% 3HP show melting peaks of about 160~ which is close to that of the P(3HB) homopolymer. The value of the enthalpy of fusion rapidly decreases with an increase of the 3HP content from 0 to 37 mol% 3HP, indicating a decrease of crystallinity with an increase of 3HP content. The crystal lattice parameters observed for these copolymers by X-ray diffraction are found to be almost identical to those of the P(3HB) homopolymer. The degree of crystallinity determined from the X-ray diffraction decreases steeply from 60 to 23% as the 3HP content in the copolymers increases from 0 to 37 mol%. The trends of composition dependence of the thermal properties and the X-ray diffraction are very similar to those for P(3HB-co4HB) but not to those for P(3HB-co-3HV), as already mentioned above. Thus, it is reasonable to assume that in the P(3HB-co-3HP) samples containing up to 37 mol% 3HP unit, the 3HB units exist in the crystalline as well as in the noncrystalline regions, while almost all of the 3HP units exist only in the latter. To further investigate this point, the 13C NMR relaxation times have been measured for the films cast from a chloroform solution. The CP/MAS 13C NMR spectrum of a P(3HB-co-37%3HP) sample is shown in Fig. 21.9, in which the signal intensities of the 3HB units, relative to the 3HP units, are much larger when compared to the corresponding relative intensities found in solution-state NMR. As described above, the CP efficiency is larger in the more rigid crystalline region than in the more mobile amorphous region. As a result, the signals arising from the crystalline region are emphasized in the CP/MAS 13C spectrum. The temperature (ca. 45~ at which the CP/MAS 13C NMR spectrum was measured, is much higher than the glass-transition temperature of this sample (ca. -5~ [68]. Hence, Fig. 21.9 indicates that fewer fractions of the 3HP units are incorporated into the crystalline region. In the CP/MAS 13C NMR spectra of the P(3HB-co13%3HP) and P(3HB-co-27%3HP) samples, the signal intensities of the
797
B I O D E G R A D A B L E POLYMERS
C=O
CH3 (B)
C H (B) I CH (B) CH 3
O ii
-(- o - cI , -
O
/3 3HB
,C
'
,
i
I
200
I
"'
,'
~
'
I
I00
'
-):--
a
3HP
H2 (P-B)
]
0
,
i
f
i
6 (ppm)
Fig. 21.9. 67.9 MHz 13C CP/MAS NMR spectra of P(3HB-co-37%-3HP). Most of the unassigned small signals are spinning sidebands and some others may arise from the rotor with polyimide end caps (2-ms contact time, 5-s pulse repetition time, 500 FID accumulations). "B" and "P" indicate that the signals arise from the 3HB and 3HP units, respectively. (Reproduced from J. Mol. Struct. 441 (1998) 119 with permission.)
carbons arising from the 3HP units are too weak to be detected, suggesting that the existence of a crystalline 3HP component is less probable in these samples. Figure 21.10 shows the partially relaxed solid-state 13C NMR spectrum of a P(3HB-co-37%3HP) sample measured by Torchia's pulse sequence [56]. Figure 21.11 shows the decaying curves of signal intensities estimated from the spectra shown in Fig. 21.10. In general, a spin-lattice relaxation process of a semicrystalline polymer, observed at temperature much higher than its glass-transition temperature, is decomposable as a rough approximation into two components, a slow and a fast relaxation processes due to the rigid crystalline and mobile amorphous region, respectively. Figure 21.11(e, f) indicate that the relaxation process of the carbon nuclei in the 3HP units is composed of only one component with a single relaxation time. However, as shown in Figs. 21.11(a-d), the T1 decaying curves for the 3HB carbons show a nearly biexponential behaviour. Hence, the T1 values of the 3HB units are obtained by the analysis assuming that the relaxation process for each carbon nucleus is composed of two components. In Fig.
798
Y O S H I O
I N O U E
3000 2500 2000 1500 1000 800
500 300 200
.~_._~
100 50
30
V < " ~ ' ~ ' " - " ~ " ~ " '""
J~ - ~ "
,
20
SSB ~ --,
II--I--I
i
I
~ I
200
" -- ,
""'"~
SSB CHz(P-~ ) ICI-h(B)lICI42(P-#)
r/ms --
"
)
]
i
150
' ....
,"
I ,
i
l
I00
~
~ --
I
I ~
50
"i I
~
~
~
I
' l
,I
I ,
I [
1--
0
6 in ppm
Fig. 21.10. 6 7 . 9 M H z lSc C P I M A S N M R spectra of P(3HB-co-37%-3HP) measured by Torchia's pulse sequence (t: pulse delay time). (Reproduced from Ref. [67] with permission.)
21.11(a-d), the semiexponential plots at the side of a longer delay time show linear decays, so that the T~ values of the slow components are determined from the slope of these parts. The T~ values of the fast components are determined from the residual intensities obtained by substracting the estimated intensities of the slow components from the original intensities. The plots of the residual intensities also become linear as shown in Fig. 21.12, supporting the validity of a two-component analysis. The T~ values of P(3HB-co-3HP)s are listed in Table 21.4, in which those for P(3HB) and the chemically synthesized PPL are also included. The methyl carbon of the 3HB unit shows a shorter Ta value, reflecting its faster internal
799
B I O D E G R A D A B L E POLYMERS
o....o
(a)
"0""0.
:5
(b)
..
~
.G
"-..
.__--
_
ttl IlJ
.~
........O........ CH
0
10
20
30 0
10
20
r~ s
3( r~ s
( 5 %o...o. d
(c) J lO
._c >,
........o ........ CI-13 (d)
........O...
slow)
IZ
2 ._c ---O~ 0 )
CH2(slow)
[
I
1
l
10
20
30 0 r/s
9
.,
(e)
5 d .c_
---o---
20
10
r/s
30
0
----o~
CH=(c~)
Cm(~3)
I1/
.r
!
1
0
10
r/s
20 0
10
r/s
20
Fig. 21.11. Semiexponential plots of the signal intensities for each carbon: (a) carbonyl; (b)
methine; (c) methylene; (d) methyl in the 3HB units; (e) c~-methylene; and (f)/3-methylene in the 3HP units of a P(3HB-co-37%-3HP) sample. (Reproduced from Ref. [67] with permission.)
rotation. All of the 3HB units in P(3HB) and P(3HB-co-3HP)s show the two fast- and slow-relaxing components. PPL also shows two components. The 13C NMR T1 values (4 and 2 s) observed for carbons of the 3HP units in P(3HB-co-37% 3HP) are close to corresponding to those of the fast-relaxing component of the 3HB methylene carbon (5 s) in the same copolymer as well as in PPL. Thus, the T1 data indicate again that almost all of the 3HP units in P(3HB-co-37%3HP) exist only in the amorphous region. The T~ value of the fast-relaxing component of each carbon in the 3HB
800
YOSHIO INOUE
(b)
(a) C=0(fast)
5 O
._c
:~
r =0.990
0J
_
o
5 d .c
lO
20
o
lO
20
r/s
L
r/s
' (c)
CI-h(Fast) )w
r :
Ct-I3(fast)
0.904
r =0.974
I/I C
.c_
l
0
I0
0
20 r/s
10
20
r/s
Fig. 21.12. Semiexponential plots of the signal intensities of the fast components for each carbon: (a) carbonyl; (b) methine; (c) methylene; and (d) methyl in the 3HB units of a P(3HBco-37%-3HP) sample (r: correlation coefficient for a linear plot). (Reproduced from Ref. [67] with permission.)
Table 21.4. 13C spin-lattice relaxation times (qS~ in s) of PHB, PPL and P(3HB-co-3HP) on the assumption of a biexponential decay obtained by 67.9 MHz 13C high-resolution solid-state N M R measurements Sample
C--O
CH(B)
CH2(B)
CH3(B)
CH2(P-ce)
CH2(P-/3)
PHB
fast slow
10 170
15 80
12 180
4 22
-
-
P(3HB-co13%3HP)
fast slow
8 150
11 70
10 170
2 17
n.d. n.d.
n.d. n.d.
P(3HB-co27%3HP)
fast slow
5 160
5 70
9 160
2 17
n.d. n.d.
n.d. n.d.
P(3HB-co37%3HP)
fast slow
4 120
5 70
5 120
2 12
4 N.D.
2 N.D.
PPL
fast slow
N.D. 130
-
-
-
7 112
4 83
n.d. indicates that the intensities of resonances were too weak to be detected. N.D. indicates that the slow component did not exist. (Reproduced from Ref. [67] with permission).
B I O D E G R A D A B L E POLYMERS
801
unit is found to decrease as the 3HP content increases, indicating that the mobility of the 3HB unit in the amorphous region increases with the content of the 3HP unit. The less bulky 3HP unit may make the copolymer segment more flexible in the amorphous region. All of experimental results shown above indicate that a cocrystallization of 3HB and 3HP units in the same crystal lattice does not occur in the P(3HBco-3HP) samples containing up to 37 mol% 3HP. A possible explanation of the fact that a cocrystallization is obstructed in this copolymer system may be the chain flexibility of the segments included the 3HP comonomer units. This allows the 3HP-containing segments to take several conformations, other than the 21-helix, found in the 3HB crystalline lattice. In fact, it has been reported for PPL that there are several crystalline conformations, including helix and planar zigzag conformations [69-71]. Among them, the paracrystalline state (planar zigzag) of PPL seemed to be very stable [71]. The imbalance of the solid-state morphology composed of 3HB and 3HP units, which has the tendency to form different crystal forms, i.e., 21-helix and planar zigzag, respectively, may be one of the possible reasons that cocrystallization does not occur.
21.2.3 Molecular dynamics in the solid state of poly(hydroxyalkanoic acid)s studies by NMR Segmental mobility of polymer chains is one of the important properties of polymer materials. It has intimate relations to several physical properties of polymer materials. NMR relaxation measurements are wellknown as powerful techniques for elucidating the segmental motions of polymer materials not only in solution but also in the solid state. Segmental motions of P(3HB) and P(3HB-co-27%3HV) in chloroform-d solution have been studied by measuring 13C NMR relaxation times and NOE factors as a function of temperature [72, 73]. Analysis of the relaxation data on the basis of the Dejean-Laupretre-Monnerie (DLM) model, which describes the dynamics of polymer chains [74], indicates that the local dynamics of a comonomer unit, e.g., 3HB, are independent of the presence of a nearby 3HV unit and vice versa that segmental motion of the P(3HBco-27% 3HV) copolymer described by cooperative conformational transitions [73] is similar to that for the P(3HB) homopolymer [72]. These motional characteristics of the P(3HB-co-3HV) copolymer chain are consistent with the conformational characteristics derived by the analysis of 1H spin coupling as shown in Section 21.2.2.3 [63] and are consistent with the occurence of cocrystallization in this copolymer system. 13C NMR relaxation is a tool for the elucidation of the rates and mechan-
802
YOSHIO INOUE
isms of backbone segmental motion and side-chain internal rotations in solution, it is also useful for the study of the amorphous bulk state of polymers, because polymer chains in the amorphous phase are expected to exhibit a liquid-like rapid molecular motion on the NMR timescale at temperatures well above the glass-transition temperature. As already mentioned in Section 21.2.2.1, the poly(hydroxyalkanoic acid)s in vivo, which exist in the mobile state, show liquid-like well-resolved a3C NMR spectra [30]. The differences in solid-state 13C spin-lattice relaxation times, reflecting differences in the segmental motions, have been used widely to discriminate between the carbon nuclei in the crystalline phase from those in the amorphous phase, as already exemplified for P(3HB) and P(3HB-co-3HV) [53] (in Section 21.2.2.2) and for P(3HB-co-3HP) [67] (in Section 21.2.2.5). The compositional dependence of the segmental motions of P(3HB-co4HV) has been found to be different from that of P(3HB-co-3HV) [75], reflecting the fact that cocrystallization occurs in the latter (see Sections 21.2.2.2 and 21.2.2.3) but not in the former (Section 21.2.2.4). The 67.8 Hz CP/MAS 13C NMR spectra of P(3HB-co-4HB) with the 4HB unit contents of 11, 33 and 49 mol% in the powder form have been observed with a contact time of 10 ms [75]. From the analysis of NMR and DSC, the crystallinity of the P(3HB-co-4HB) samples is found to decrease with an increasing fraction of the 4HB units. In the CP/MAS 13C NMR spectrum of a P(3HB-col l%4HB) samples, the resonances appear at almost identical chemical shifts with those in the P(3HB) homopolymer, and the 4HB resonances are hardly detectable, indicating that the CP is ineffective for the carbon nuclei of the 4HB units in this copolymer due to rapid molecular motion with a frequency comparable to that of CP [76, 77]. In contrast, in the spectra of the P(3HBco-33% and 49To4HB) samples, the 4HB resonances with highly sharp lines appear at chemical shifts, which are approximately consistent with those found in their solution 13C NMR spectra, suggesting that the segmental motion of the 4HB units becomes too rapid to induce motional narrowing of the linewidth [76]. These results indicate that the 4HB units of the P(3HBco-4HB) samples investigated here exist in the highly mobile amorphous phase. The segmental dynamics in the amorphous phase of bulk samples of bacterially synthesized P(4HB) homopolymer and P(3HB-co-18% and 69%4HB) random copolymers have been studied by measuring the 13C NMR spinlattice relaxation times (T1) and NOE factors at two frequencies, 75.4 and 125.7 MHz, as a function of temperature from 25 to 125~ [78, 79]. The glass transition temperature (Tg) and melting temperature (Tm) of these samples measured by DSC are P(4HB) (Tg = -48~ Tm=54~ P(3HB-co-
BIODEGRADABLE POLYMERS
803
69%4HB) (Tg = -36~ Tm = 50~ and P(3HB-co-18%4HB) (Tg = -4~ Tm = 165~ The degree of crystallinity of P(4HB) and P(3HB-co-18%4HB) measured by DSC are 25 _ 5 and 30 +_ 5%, respectively. The temperature at which the NMR spectra were measured is well above Tg. Samples for NMR relaxation measurements were prepared by placing finely ground pieces of the polymer in NMR tubes, heating above the melting point until a homogeneous melt phase was produced, and holding them at room temperature for several days prior to conducting any experiments, 13C T1 relaxation times and NOE factors were measured by the standard inversion-recovery and the gated decoupling method, respectively. The unimodal dynamic models, such as the Cole-Cole distribution model [80], the Jones-Stockmayer model [81] and the Hall-Weber-Helfand model [82], which attribute relaxation to a single motional mode and were originally derived for the study of polymer dynamics in solution, have been found to be incapable of reproducing all of the experimental 13C relaxation data of the amorphous phases of P(4HB) and P(3HB-co-4HB)s [79]. But, the 13C T1 and NOE data of these samples measured as functions of temperature and magnetic field have been successfully interpreted by the Dejean-LaupretreMonnerie (DLM) model [74, 83]. The DLM model is derived from the original Hall-Weber-Helfand model by including fast librational motions of the backbone C ~ H vectors in addition to the backbone segmental motion. The mobility of both the 3HB and 4HB units was found to increase with increasing the content of flexible 4HB units in the copolymer [79]. This result is consistent with the increasing elastomeric behavior reported for the P(3HBco-4HB) copolymers with a high 4HB unit content [8, 84, 85]. Furthermore, the segmental motion of the 4HB unit in the amorphous region of both copolymers was found to be 2-4 times faster than that of the 3HB unit [79]. These results indicate that the segmental motions which affect the 13C relaxation processes in bulk polymers have local character. The activation energies for the backbone motion of P(4HB) and the 3HB and 4HB units in the P(3HB-co-4HB)s, derived from the DLM model analysis, are found to be similar and in the range 42-47 kJ/mol [79]. This range is typical of amorphous polymers at temperatures above Tg, but they are greater than typical ones for polymers in solution, possibly due to the increased apparent viscosity exerted by the amorphous matrix on the moving backbone segment [79]. The activation energy observed for the backbone motion of P(3HB) in chloroform solution is 17 kJ/mol [72]. As the segmental dynamics probed by a3C relaxation measurements of P(4HB) and the 3HB and 4HB units in two P(3HB-co-4HB) samples followed the Williams-Landel-Ferry empirical equation [86], they are considered to be involved in the glass-transition phenomena and so the decrease in Tg values
804
YOSHIO INOUE
on going from P(4HB) to P(3HB-co-69%4HB) and P(3HB-co-18%4HB) is considered to be in agreement with the slowing down of the segmental motion as described by the DLM model. The linewidths of the high resolution scalar decoupled a3C and 1H NMR have been studied for semicrystalline P(4HB) and P(3HB-co-18%4HB) in the amorphous phase and in the melt as functions of temperature and magnetic field strength [87]. There are two classes of origins, which exert linebroadening of the NMR spectra of amorphous or semicrystalline polymers, i.e., broadening due to a distribution of chemical shifts and the second one is that related to relaxation processes. The latter includes the natural linewidth. From measurements of the 13C spin-spin relaxation times by the standard Carr-Purcell-Meiboom-Gill pulse sequence under the same experimental conditions, the natural linewidth is found to be a minor contributor to the line-broadening observed in the 13C NMR spectra of the solid P(4HB) and P(3HB-co-4HB). Possible contributions from various line-broadening mechanisms have also been examined by using a variety of coherent averaging solid-state NMR methods [87]. The crystalline phase was found to play a crucial role in modulating the ~3C linewidths of the amorphous regions of P(4HB) and P(3HB-co-18%4HB). It was shown that magnetic susceptibility and chemical shift dispersion are the major causes for the line-broadening of the proton and carbon resonances of P(4HB) in the amorphous phase and the melt, respectively. For P(3HB-co-18%4HB) in the amorphous phase, incomplete motional narrowing due to a slow motional mode restricted in amplitude by presence of crystallites and/or chain constraints was found to be the major line-broadening factor. It is interesting to investigate effects of long side chains on the molecular dynamics and related physical properties of poly(hydroxyalkanoic acid)s. The poly(hydroxyalkanoic acid)s with longer side chains, such as poly(3hydroxyoctanoic acid) [P(3HO)], have quite different mechanical properties from P(3HB) and P(3HB-co-3HV), being thermal elastmers with low glass transition temperatures ranging from - 2 5 to -40~ [88] and a much lower crystallinity of about 25-33% [89]. P(3HO) produced by P. oleovorans from sodium octanoate as the sole carbon source is a copolyester composed of 85% octanoate monomer units (with n-pentyl side chains). The remaining monomer units are roughly equally distributed between valerate, caproate and decanoate units [88]. This sample has a Tu of -36~ and Tm of 61~ [88]. The chain dynamics of the amorphous part of this P(3HO) sample have been studied by measuring variable-temperature 13C NMR spin-lattice relaxation times T~ and the nuclear Overhauser enhancements at two magnetic fields [90, 91]. Well-resolved 13C spectra of
BIODEGRADABLE POLYMERS
805
this polymer in the amorphous region were observed without CP and MAS even at room temperature, indicating that the chains in this region undergo rapid and nearly isotropic motion. The temperature dependence of the T~s of the side chain and backbone carbons reveals that these two moieties undergo different motions. The relaxation data of the backbone carbons have been analyzed by employing a number of motional models. As found for P(3HB-co-4HB), among the motional models, the DLM model was again found to be superior to the unimodal dynamic models, i.e., the relaxation data of the backbone carbons have been interpreted in terms of conformational transitions and librational motions of the backbone C ~ H vectors [91]. Temperature- and frequency-dependent TI and NOE data of the carbon nuclei in the n-pentyl side chain have been well described by internal rotations of limited amplitude superimposed on the backbone motion [91]. The temperature dependence of the linewidths of the protonated carbon resonances of P(3HO) has been discussed in terms of the semicrystalline nature of this polyester [91].
21.3 Polymer blends containing poly(3-hydroxyalkanoic acid)s studied by NMR 21.3.1 PHA containing polymer blends Blends of poly(3-hydroxyalkanoic acid)s (PHAs) with various natural and synthetic polymers have been reported as reviewed in Refs. [21,22]. By blending with synthetic polymers it is expected to control the biodegradability, to improve several properties, and to reduce the production cost of bacterially synthesized PHAs. The polymers investigated as the blending partners of PHAs include poly(ethylene oxide) [92, 93], poly(vinyl acetate) [94], poly(vinylidene fluoride) [95], ethylene propylene rubber [94, 96], poly(epichlorohydrin) [97, 98], poly(E-caprolactone) [99], aliphatic copolyesters of adipic acid/ethylene glycole/lactic acid [100] and of E-caprolactone/lactide [101], poly(vinylphenol) [102] and polymethacrylates [103]. The blends of bacterially synthesized polyesters with bacterially synthesized copolyesters, such as P(3HB) with P(3HB-co-3HV) [104-106] and P(3HV) with P(3HBco-3HV) [107], have been also studied. Some of these are biodegradable blends and the others are biodegradable/nonbiodegradable blends. PHA blends with natural biodegradable polymers such as starch [108], pullulan [109], dextran, amylose and alginate [110], have been reported. Furthermore, blends of bacterially synthesized fully-biodegradable P(3HB)
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YOSHIO INOUE
with chemically synthesized partially-biodegradable P(3HB) have also been reported [111-113].
21.3.2 NMR studies of PHA containing polymer blends The miscibility and molecular dynamics of several PHA containing blends have been studied by solid-state NMR spectroscopy. Some cellulose derivatives and P(3HB) and P(3HB-co-3HV) have been found to show good compatibility [114-116]. These are chemically modified natural and natural biodegradable polymer blend systems. Blends obtained by melts compounding P(3HB) with cellulose acetate butyrate (CAB, degrees of butyrate and acetate substitution are 2.50 and 0.18, respectively) have been found to be miscible over the whole composition range by DSC and dynamic mechanical spectroscopy [116]. Blends in the composition range 20-80 wt% of CAB (degrees of butyrate and acetate substitution are 1.6 and 1.0, respectively; Tg and T m measured by DSC are 129 and 169~ respectively) and P(3HB-co-10%3HV) (Tg and T m measured by DSC were - 2 and 166~ respectively), have been prepared by thermal compounding [117]. The blends containing 20-50% P(3HB-co10%3HV) are found to be amorphous, optically clear miscible blends, while those containing 60-80% P(3HB-co-10%3HV) are semicrystalline, partially miscible blends. High resolution 13C NMR spectra of CAB, P(3HB-co10%3HV) and 50%CAB/50% P(3HB-co-10%3HV) blend in the bulk melt have been observed at 100 MHz and at 165-185~ and 235~ (CAB only) by using a high-temperature solution-state probe. All of the P(3HB-co10%3HV) carbon resonances observed at 165~ are relatively narrow, indicating the presence of liquid-like motion, and their linewidths are largely unaffected as the temperature is increased to 185~ (temperature is limited to 185~ due to the thermal instability of this copolymer). In contrast, the CAB glucopyranosyl ring carbon resonances and butyryl and acetyl substituent carbon resonances are lost in the baseline at 185~ a characteristic for polymers with solid-like mobility. At 235~ the sugar ring carbon resonances are still spread in the baseline, but the resonances of the methyl and methylene carbons of the substituents are observed. In the spectrum of the CAB/P(3HB-co-10%3HV) 50/50 blend observed at 185~ the P(3HB-co10%3HV) carbon resonances show considerably broadened lines and the CAB butyryl methyl resonance with a more narrow line appears. The results indicate that P(3HB-co-10%3HV) has a decreased mobility, while the CAB side chain has increased mobility in the blend. Thus, even in a homogeneous melt, the blend components of CAB and P(3HB-co-3HV) can have much
B I O D E G R A D A B L E POLYMERS
807
different mobilities. This blend system has been characterized also by X-ray and mechanical analysis [117]. Poly(vinyl alcohol) (PVA) is a chemically synthesized well-known watersoluble polymer which is biodegradable [118]. In comparison with P(3HB), atactic PVA has excellent mechanical properties. Thus, by blending with PVA, it is expected to improve the mechanical properties of P(3HB). Both PVA and P(3HB) are semicrystalline polymers. So, PVA/P(3HB) blends are thought to be crystalline-crystalline polymer blend systems. It is not easy to analyze the miscibility of such blend systems, because in general miscibility is a property of the amorphous phase. Observations of glass-transition temperatures are usually employed to judge the miscibility of amorphous polymer blends. It is often difficult to judge from Tu data whether the blend system is miscible or immiscible, because the existence of the crystalline phase prevents the observation of Tu of blends with high crystallinity. The crystalline phase sometimes increases the Tg value. Measurements of NMR relaxation parameters can provide an alternative means to judge the miscibility of polymer blends on a molecular level. Crystallization and compatibility of the PVA/P(3HB) blend have been studied by high resolution solid-state 13C NMR spectroscopy, DSC, FTIR, and density measurements [119-121]. The polymer samples used were bacterial P(3HB) (Mw 360000), atactic PVA (degree of polymerization 2000; degree of saponification 99%; Tm 224.0~ triad tacticity mm - 22%, mr = 50%, rr = 28%) and syndiotactic PVA (degree of polymerization 1690; degree of saponification 99.9% ; Tm 246.1~ triad tacticity mm = 15%, mr 50%, r r - 35%). The PVA/P(3HB) blends were prepared by casting from solutions in hexafluoroisopropanol, which is a common solvent for PVA and P(3HB) [119]. The DSC thermogram of each blend sample shows three distinct endothermic peaks, representing the fusion of P(3HB) crystals, the fusion of the PVA crystal and the decomposition of P(3HB) chains in order of increasing temperature [119, 120]. Any peaks corresponding to glass transitions of both polymers are not observed, probably due to their high crystallinity [120]. Two blend systems, atactic (a)- and syndiotactic (s)PVA/P(3HB), show similar blend composition dependencies of melting temperatures, Tm, and heat of fusion. The Tm of the P(3HB) phase shows small decreases with increase of the PVA content, while the Tm of PVA phase remains constant in both blend systems [120]. Thus, it is likely that P(3HB) and PVA crystallize separately. The crystal structure of the PVA phase in the blends is considered to be the same as that before blending, while the lamellar thickness of the P(3HB) crystal is reduced by blending with PVA. The DSC peak representing the fusion of the P(3HB) crystal is not observed for the blends containing more than 70 wt% PVA.
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YOSHIO INOUE
The heat of fusion of P(3HB) phase decreases with an increase of the PVA content in both of the blends, indicating the decrease of P(3HB) crystallinity. No P(3HB) crystals were formed in the blends containing more than 75 wt% a- and s-PVA. In the same composition range, the heat of fusion of the s-PVA phase also shows about 15% decreases, indicating the decrease of s-PVA crystallinity. These results indicate the partial compatibility of these blend systems [120]. The heat of fusion of the a-PVA phase could not be estimated precisely from the DSC thermograms because of overlapping of the peaks of the fusion of a-PVA and the decomposition of P(3HB). The DSC results suggest the presence of a specific PVA-P(3HB) intermolecular interaction, which reduces the degree of crystallinity of both components and leads to compatibility. A probable interaction expected for these blend systems is a hydrogen-bonding interaction between the P(3HB) carbonyl and the PVA hydroxyl groups. This possibility has been examined by observing the solid-state 13C NMR spectra of these blends [120]. Solidstate 13C NMR spectroscopy is a powerful tool for the investigation of hydrogen-bonding. In general, 13C nuclei participating in hydrogen bonds exhibit, more or less characteristic high frequency shifts [122]. In the high resolution X3C CP/MAS NMR spectra of isotactic-PVA, a-PVA and s-PVA, the fixation of the intramolecular hydrogen bonds in solids has been found to produce a large deshielding of the methine carbon resonance [123]. The miscibility of the semicrystalline PVA with the amorphous poly(N-vinyl-2-pyrrolidone) (PVPy) has been investigated by ~3C CP/MAS NMR and DSC [124]. The a3C CP/MAS NMR spectra show that the PVA-PVPy blends are miscible on a molecular level over the whole composition range, and that the intramolecular hydrogen bonds of PVA are broken and intermolecular hydrogen bonds between PVA and PVPy are formed in the blends. 13C CP/MAS NMR spectra, which emphasize the resonances from 13C nuclei in the crystalline phase, have been observed for a series of PVA/P(3HB) blends [120]. The chemical shift of the P(3HB) carbonyl carbon, which is expected to participate in the hydrogen-bonding interaction with a PVA hydroxyl group, is found to be independent of the composition of both the a- and s-PVA/P(3HB) blends, indicating that there are no detectable PVA-P(3HB) intermolecular hydrogen-bonding interactions in the crystalline phase. The relative intensities of the P(3HB) resonances are found to be smaller than those calculated from the P(3HB) content of the blend. This result means that the crystallinity of the P(3HB) phase decreases by blending with PVA. This is consistent with the observation of heat of fusion as shown above. Intermolecular interactions in the relatively mobile amorphous phase of PVA/P(3HB) blends have been investigated by solid 13C pulse saturation transfer (PST) MAS NMR spectroscopy [125]. Figure 21.13 shows the shift
809
BIODEGRADABLE POLYMERS 172
E
O. Q.
171 r--
(/3 O O
"~
O
170
clJ tO
169
0
'
~S
.
.
PVA
80
.
.
wt
%
.
90
100
Fig. 21.13. Composition dependence of the chemical shift for the carbonyl carbon of P(3HB) in the 13C-PST MAS NMR spectra of s-PVA/P(3HB) (O) and a-PVA/P(3HB) (O) blends. (Reproduced from Ref. [120] with permission.)
variation of the P(3HB) carbonyl resonances with blend composition [120]. A significant deshielding is observed for the blends containing higher than 80wt% PVA, indicating the formation of hydrogen bonds between the P(3HB) carbonyl and the PVA hydroxyl groups. This hydrogen-bonding interaction should cause compatibility between PVA and P(3HB) in the amorphous phase. The deshielding observed for the s-PVA/P(3HB) blend is slightly larger than that for the s-PVA/P(3HB) blend. This result seems to show that the capacity to make intermolecular hydrogen bonds differs with the tacticity of PVA. More hydrogen bonds are formed between s-PVA and P(3HB) than between a-PVA and P(3HB). It was reported for the triad tactic sequences of PVA that the oxygen atom bonded to the central methine carbon atom of the isotactic, or the heterotactic triad, can make two or one intramolecular hydrogen bond(s), respectively. However, no intramolecular hydrogen bond is expected for the syndiotactic triad [123]. If these observations are also valid for PVA in the blends with P(3HB), then s-PVA has a larger capacity than a-PVA to form intermolecular hydrogen bonds. The 1H relaxation time can provide information on the domain size in the solid polymer. If two component polymers, each of which has inherent 1H relaxation times in its pure state, in the binary blends are intimately mixed and the domain sizes of the two components are small enough for effective spin diffusion during 1H relaxation, both components exhibit the same ~H relaxation time. When the domain sizes are larger, that is, the components
810
YOSHIO INOUE (a)
() .
5
0
0
0
"1"
0
00
3
1
0
100
5O wt %
a-PVA
(b)
CO
O
I-.
O
2
-
0
9
L
L
t
.
.
.
50 s-PVA
.
.
100 wt %
Fig. 21.14. Composition dependence of the proton T1 of P(3HB) (&) and PVA(O) in (a) aPVA/P(3HB) and (b) s-PVA/P(3HB) blends. (Reproduced from Ref. [120] with permission.)
exist in separated phases, each component shows a different relaxation time. A scale for effective spin diffusion during I H relaxation of a typical bulk polymer is 200-300 A (see Chapter 10). The domain size of PVA/P(3HB) blend systems was studied by observing the IH spin-lattice relaxation time, T I , by the CPMAS NMR technique [126]. Each of the pure polymers, P(3HB), a-PVA, and s-PVA, shows only one TI value, indicating that the crystalline and amorphous domains of these pure polymers are smaller than is the scale of effective spin diffusion. Figure 21.14 shows the variation of the TI values with the blend composition [120]. In the blends, the T1 values of PHB and PVA approach each other with increasing
BIODEGRADABLE POLYMERS
811
PVA content, indicating that the compatibility between PVA and P(3HB) improves with increasing PVA content. In the s-PVA/P(3HB) blends containing more than 55 wt% s-PVA, and in the a-PVA/P(3HB) blends containing more than 80 wt% a-PVA, the T~ values of P(3HB) agree very closely with those of PVA. This result implies that the domain sizes of PVA and P(3HB) in these blends are less than 200-300 A. The s-PVA/P(3HB) blend system is compatible over a wider range of composition than is the a-PVA/P(3HB) blend system. This is consistent with the results obtained from the chemical shifts of the carbonyl carbon resonances. In short, the PVA/P(3HB) blend is compatible in the amorphous phase and the compatibility between them depends on the tacticity of PVA.
21.4
21.4.1
Other biodegradable polymers studied by solid-state NMR
Natural biodegradable polymers other than PHAs
There are several kinds of natural biodegradable polymers in addition to bacterial PHAs, such as proteins, nucleic acids and polysaccharides. Among them, particulary important polymers such as industrial materials are polysaccharides, such as starch, cellulose, chitin and chitosan. The solid-state structure and properties of starch and amylose [127], cellulose [128] and chitin [129], have been extensively studied by high resolution 13C NMR spectroscopy. The details of the NMR study of solid-state polysaccharides are described in Chapter 24 of this book. The molecular structure as well as the dynamics of natural polymers are important in understanding their properties. For intact plant materials such as lignin, cutin and suberin, the principal chemical constituent moieties have been identified and quantified by solid-state 13C NMR [130-133]. The dynamics of intact lime cuticle and its two major component polyesters, cutin and wax, have been studied by the MAS 13C NMR experiment [134]. By the measurements of 13C spin-lattice relaxation times and spinlattice relaxation times in the rotating frame which characterize respectively the megahertz- and kilohertz-regime motions, it is indicated that motional restrictions are present at the crosslinks of the cutin polymer and along the alkyl chains of the wax. The values of relaxation times, which differ for analogous carbon sites of cutin and wax individually, approach common values for the two materials in the intact lime cuticle. These results are considered to provide evidence for hydrophobic association within the plant cuticle of the long aliphatic chains of cutin and wax.
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YOSHIO INOUE
21.4.2 Chemicallysynthesized biodegradable polymers There are several types of biodegradable synthetic polymer with vulnerable chemical moieties susceptible to enzymatic attack. The most typical ones are aliphatic polyesters, such as poly(glycolic acid), poly(lactic acid), and poly(ecaprolactone). Highly isotactic-, syndiotactic- and atactic-homopolyesters of 3-hydroxybutyric acid [135] and its copolymer with valeric acid [136] have also been chemically synthesized and their solid-state structure and properties have been compared with those of their natural counterparts. Recently, aliphatic polyesters based on lactic acid and lactide have attracted much attention, because they can be formed from L-lactic acid produced from natural renewable sources and they decompose rapidly and completely under a typical compost condition. Solid-state 13C NMR spectra have been reported for CH3
CH3
I poly(lactide) poly(L-lactide) (PLLA) and poly(i>lactide) [137-139]. The crystallinity and morphology of highly crystalline, partially crystalline and amorphous PLLA have been studied by solid-state CP/MAS 13C NMR spectroscopy [140]. The amorphous domains show broad resonances with Gaussian lineshapes, while the crystalline domains show narrower resonances with a high degree of Lorentzian character. The splittings of the resonances indicate that there are at least five crystallographically inequivalent sites in the crystalline domains of highly crystalline PLLA samples. For the partially crystalline PLLA sample, spectral features of both amorphous and crystalline domains are observed. The short-range (nanometer) as well as the longrange order (micrometer) in PLLA appear to affect the CP/MAS 13C NMR spectrum. A simple method which can be used to determine the crystallinity and morphology of PLLA from their NMR spectra has been investigated [1401. Molecular dynamics of/3-propiolactone (PL) homopolymer and its block copolymer with/3-butyrolactone (BL) in the solid state have been investigated by broad-line and pulse NMR [141].
BIODEGRADABLE POLYMERS
813
CH3
I --(--CH--CH2--CO--O--)x--(--CH2--CH2--CO--O--)y--
BL-PL block copolymer Proton spin-lattice relaxation times, T1 and Tip in the rotating flame have been measured in the temperature range from 113 K to the melting points (about 350 K), and have been interpreted in terms of molecular motion and phase structure of the polymers. Two regions with different mobilities in the amorphous phase of the 50/50 BL-PL block copolymer have been found. The temperature dependences of both of the relaxation times indicate the presence of molecular motions in the amorphous phase of the PL homopolymer (glass transition temperature Tg = 258.6 K) and BL-PL block copolymer (Tg = 263.8 K) far below their melting points. Rotation of the methyl groups in the block copolymer is observed, and spin-diffusion-limited relaxation is suggested. Poly(vinyl alcohol) (PVA) and poly(ethylene oxide), which are water soluble synthetic polymers, are also biodegradable. Solid-state CP/MAS 13C measurements have been performed for PVA samples with different tacticities in order to obtain information about the structure and hydrogen bonding in the crystalline and noncrystalline region [123,142-144]. The details of the NMR study of solid-state PVA are described in Chapter 19 of this book.
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BIODEGRADABLE POLYMERS
815
46. M. Yokouchi, Y. Chatani, H. Tadokoro and H. Tani, Polymer J. 6 (1974) 248. 47. S. Bruckner, S.V. Meille, L. Malpezzi, A. Cesaro, L. Navarini and R. Tombolini, Macromolecules 21 (1988) 967. 48. T.L. Bluhm, G.K. Hamer, R.H. Marchessault, C.A. Fyfe and R.P. Veregin, Macromolecules 19 (1986) 2871. 49. M. Kunioka, A. Tamaki and Y. Doi, Macromolecules 22 (1989) 694. 50. P.J. Flory, Trans. Faraday Soc. 51 (1955) 848. 51. H. Mitomo, P.J. Barham and A. Keller, Sen-i Gakkaishi 42 (1986) 589. 52. H. Mitomo, P.J. Barham and A. Keller, Polym. J. 19 (1987) 1241. 53. N. Kamiya, M. Sakurai, Y. Indue, R. Chujo and Y. Doi, Macromolecules 24 (1991) 2178. 54. N. Yoshie, M. Sakurai, Y. Indue and R. Chujo, Macromolecules 25 (1992) 2046. 55. H. Mitomo, N. Morishita and Y. Doi, Macromolecules 26 (1993) 5809. 56. D.A. Torchia, J. Magn. Reson. 30 (1978) 613. 57. D.L. VanderHart, W.J. Orts and R.H. Marchessault, Macromolecules 28 (1995) 6394. 58. J. Datta and A.K. Nandi, Polymer 35 (1994) 4804. 59. K. Tashiro, R.S. Stein and S.L. Hsu, Macromolecules 25 (1992) 1801. 60. N. Yoshie, Y. Indue, H.Y. Hod and N. Okui, Polymer 35 (1994) 1931. 61. N. Kamiya, M. Sakurai, Y. Indue and R. Chujo, Macromolecules 24 (1991) 3888. 62. N. Nakamura, N. Kamiya, M. Sakurai, Y. Indue and R. Chujo, Polymer 33 (1992) 817. 63. N. Yoshie, Y. Indue, Y. Yamamoto, R. Chujo and Y. Doi, Macromolecules 23 (1990) 1313. 64. K. Nakamura, N. Yoshie, M. Sakurai, and Y. Indue Polymer 35 (1994) 193. 65. M. Scandola, G. Ceccorulli and Y. Doi, Int. J. Biol. Macromol. 12 (1990) 112. 66. S. Nakamura, Y. Doi, and M. Scandola, Macromolecules 25 (1992) 4237. 67. M. Ichikawa, K. Nakamura, N. Yosie, N. Asakawa, Y. Indue and Y. Doi, Macromol. Chem. Phys. 197 (1996) 2467. 68. A. Cad, M. Ichikawa, T. Ikejima, N. Yoshie and Y.Inoue, Macromol. Chem. Phys. 198 (1997) 3539. 69. T. Wasai, T. Saegusa and J. Furukawa, Kogyo Kagaku Zasshi (J. Chem. Soc. Jpn. Ind. Chem. Sect.), 67 (1964) 601. 70. S. Asahara and S. Katayama, Kogyo Kagaku Zasshi (J. Chem. Soc. Jpn. Ind. Chem. Sect.), 69 (1966) 152. 71. K. Suehiro, Y. Chatani and H. Tadokoro, Polym. J. 7 (1975) 352. 72. M.E. Nadea, R.H. Marchessault and P. Dais, Polymer 33 (1992) 1831. 73. P. Dais, M.E. Nadea and R.H. Marchessault, Polymer 33 (1992) 4288. 74. R. Dejean de la Batie, F. Laupretre and L. Monnerie, Macromolecules 21 (1988) 2045. 75. Y. Doi, M. Kunioka, Y. Nakamura and K. Soga, Macromolecules 21 (1988) 2722. 76. W.P. Rothwell and J.S.Waugh, J. Chem. Phys., 74 (1981) 2721. 77. J.R. Lyerla, C.S. Yannoni, and C.A. Fyfe, Acc. Chem. Res. 15, (1982) 208. 78. A. Spyros and R.H. Marchessault, Macromolecules 28 (1995) 6108. 79. A. Spyros and R.H. Marchessault, Macromolecules 29 (1996) 2479. 80. F. Heatley, Annun. Rep. NMR Spectrosc. 17 (1986) 179. 81. A.A. Jones and W.H. Stockmayer, J. Polym. Sci. Polym. Phys. Ed. 15 (1977) 847. 82. C.K. Hall and E.J. Helfand, J. Chem. Phys. 77 (1982) 3275; T.A.Weber and E.J. Helfand, J. Phys. Chem. 87 (1983) 2881. 83. R. Dejean de la Batie, F. Laupretre, and L. Monnerie, Macromolecules 21 (1988) 2052; ibid. 22 (1989) 122. 84. Y. Doi, A. Segawa and M. Kunioka, Int. J. Biol. Macromol. 12 (1990) 106.
816
YOSHIO INOUE
85. Y. Saito and Y. Doi, Int. J. Biol. Macromol. 16 (1994) 99. 86. J.D. Ferry, Viscoelastic Properties of Polymers, 3rd edn. Wiley, New York, 1980, Chapter 11. 87. A. Spyros and R.H. Marchessault, J. Polym. Sci. Part B 34 (1996) 1777. 88. R.A. Gross, C. Demello, R.W. Lenz, H. Brandl and R.C. Fuller, Macromolecules 22 (1989) 1106. 89. R.H. Marchessault, C.J. Monasterios, F.G. Morin and P.R. Sundararajan, Int, J. Biol. Macromol. 12 (1990) 158. 90. F.G. Morin and R.H. Marchessault, Macromolecules 25 (1992) 576. 91. A. Spyros, P. Dais and R.H. Marchessault, J. Polym. Sci., Part B, Polym. Phys. 33 (1995) 367. 92. M. Avella and E. Martuscelli, Polymer 29 (1988) 1731. 93. M. Avella, E. Martuscelli and P. Greco, Polymer 32 (1991) 1647. 94. P. Greco and E. Martuscelli, Polymer 30 (1989) 1475. 95. H. Marand and M. Collins, Polym. Prep. Am. Chem. Soc. 31 (1990) 552. 96. M. Abbate, E. Martuscelli, G. Ragosta and G.Scarinzi, J. Mater. Sci. 26 (1991) 1119. 97. E. Dubini Paglia, P.L. Beltrame, M. Canetti, A.Seves, B.Marcandalli and E. Martuscelli, Polymer 34 (1993) 996. 98. P. Sadocco, C. Bulli, G. Elegir, A. Seves and E. Martuscelli, Macromol. Chem. Phys. 194 (1993) 2625. 99. F. Gassner and A.J. Owen, Polymer 35 (1994) 2233. 100 J.-S. Yoon, M.-C. Chang, M.-N. Kim, E.-J. Kang, C. Kim and I.-J. Chin, J. Polym. Sci. Part B, Polym. Phys. 34 (1996) 2543. 101. N. Koyama and Y.Doi, Macromolecules 29 (1996) 5843. 102. P. Iriondo, J.J. Iruin and M.J. Fernandez-Berridi, Macromolecules 29 (1996) 5605. 103. N. Lotti, M. Pizzoli, G. Ceccorulli and M. Scandola, Polymer 34 (1993) 4935. 104. S.J. Organ and P.J. Barham, Polymer 34 (1993) 459. 105. H. Satoh, N. Yoshie and Y. Inoue, Polymer 35 (1994) 286. 106. N. Yoshie, H. Menju, H. Satoh and Y. Inoue, Polymer J. 28 (1996) 45. 107. R.P. Pearce and R.H. Marchessault, Macromolecules 27 (1994) 3869. 108. B.A. Ramsey, V.Langlade, P.J.Carreau and J.A. Ramsey, Apple. Environ. Microbiol. 59 (1993) 1242. 109. J.M. Mayer, M. Greenberger, D.H. Ball and D.L.Kaplan, Polym. Mater. Sci. Eng. 63 (1990) 732. 110. M. Yasin, S.J. Holland, A.M. Jolly and B.J. Tighe, Biomaterials 10 (1989) 400. 111. Y. Kumagai and Y.Doi, Makromol. Chem., Rapid Commun. 13 (1992) 179. 112. R. Pearce, J. Jesudason, W. Orts, R.H. Marchessault and S. Bloembergen, Polymer 33 (1992) 4647. 113. H. Abe, Y. Doi, M.M. Satkowski and I. Noda, Macromolecules 27 (1994) 50. 114. M. Scandola, G. Ceccorulli and M. Pizzoli, Biomaterials 5 (1991) 115. 115. M. Scandola, G. Ceccorulli and M. Pizzoli, Macromolecules 25 (1992) 6441. 116. M. Scandola, M. Pizzoli and G. Ceccorulli, Macromolecules 26 (1993) 6722. 117. C.M. Buchanan, S.C. Gedon, A.W. White and M.D. Wood, Macromolecules 25 (1992) 7373. 118. R. Fukae, T. Fujii, M. Takeo, T. Yamamoto, T. Sato, Y. Maeda and O. Sangen, Polym. J. 26 (1994) 1381, and references cited therein. 119. Y. Azuma, N. Yoshie, M. Sakurai, Y. Inoue and R. Chujo, Polymer 33 (1992) 4763. 120. N. Yoshie, Y. Azuma, M. Sakurai and Y. Inoue, J. Apply. Polym. Sci. 56 (1995) 17.
B I O D E G R A D A B L E POLYMERS 121. 122. 123. 124. 125. 126. 127.
128.
129.
130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144.
817
T. Ikejima, N. Yoshie and Y. Inoue, Macromol. Chem. Phys. 197 (1996) 869. D.L. VanderHart, W.L. Earl and A.N. Garroway, J. Magn. Reson. 44 (1981) 361. T. Terao, S. Maeda and A. Saika, Macromolecules 16 (1983) 1535. H. Feng, Z. Feng and L. Shen, Polymer 34 (1993) 2516. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M. J. Albright, The 20th Meeting of NMR, Tokyo, 1981, p.68. M.J. Suilivan and G.Maciel, Anal. Chem. 54 (1982) 1615. For examples: (a) H. Saito and R. Tabeta, Chem. Lett. (1981) 713; (b) R.P. Veregin, C.A. Fyfe, R.H. Marchessault and M.G. Taylor, Macromolecules 19 (1986) 1030; (c) M.J. Gidley and S.M. Bociek, J. Am. Chem. Soc. 110 (1988) 3820. For examples: (a) R.L. Dudley, C.A. Fyfe, P.J. Stephenson, Y. Deslandes, G.K. Hamer and R.H. Marchessault, J. Am. Chem. Soc. 105 (1983) 2469; (b) D.L. VanderHart and R.H. Atalla, Macromolecules 17 (1984) 1465; (c) P.C. Belton, S.F. Tanner, N. Cartier and H. Chanzy, Macromolecules 22 (1989) 1615; (d) A. Isogai, M. Usuda, T. Kato, T. Uryu and R.H. Atalla, Macromolecules 22 (1989) 3168; (e) H. Yamamoto, F. Horii and H. Odani, Macromolecules 22 (1989) 4130. For examples: (a) H. Saito, R. Tabeta and S. Hirano, Chem. Lett. (1981) 1479; (b) H. Saito, R. Tabeta and K. Ogawa, Macromolecules 20 (1987) 2424; (c) B. Focher, A. Naggi, G. Torri, A. Cosani and M. Terbojevich, Carbohydr. Polym. 17 (1992) 97. G.E. Maciel, J.F. Haw, D.H. Smith, B.C. Gabrielson and G.R. Hatfield, J. Ag. Food Chem. 33 (1985) 185. N.G. Lewis, E. Yamamoto, J.B. Wooten, G. Just, H. Ohashi and G.H.N. Towers, Science 237 (1987) 1344. T. Zlotnik-Mazori and R.E. Stark, Macromolecules 21 (1988) 2412. J.R. Garbow, L.M. Ferrantello and R.E. Stark, Plant Physiol. 90 (1989) 783. J.R. Garbow and R.E. Stark, Macromolecules 23 (1990) 2814. P.J. Hocking and R.H. Marchessault, Macromolecules 28 (1995) 6401, and references cited therein. S. Bloembergen, D.A. Holden, T.L. Bluhm, G.K. Hamer and R.H. Marchessault, Macromolecules 22 (1989) 1663. H. Tsuji, F. Horii, M. Nakagawa, Y. Ikada, H. Odani and R. Kitamaru, Macromolecules 25 (1992) 4114. C. Howe, S. Sankar and A.E. Tonelli, Polymer 34 (1993) 2674. C. Howe, N. Vasanthan, C. MacClamrock, S. Sankar, I.D. Shin, I.K. Simonsen, and A.E. Tonelli, Macromolecules 27 (1994) 7433. K.A.M. Thakur, R.T. Kean, J.M. Zupfer, N.U. Buehler, M.A. Doscotch and E.J. Munson, Macromolecules 29 (1996) 8844. S. Glowinkowski, J. Kapturczak, Z. Pajak, P. Kurcok, M. Kowalczuk and Z. Jedlinski, Polymer 30 (1989) 519. H. Ketels, J. de Haan, A. Aerdts, and G. van der Velden, Polymer 31 (1990) 1419. F. Horii, S. Hu, T. Ito, H. Odani, R. Kitamaru, S. Matsuzawa and K. Yamamura, Polymer 33 (1992) 2299. S. Hu, M. Tsuji and F.Horii, Polymer 35 (1994) 2516.
This Page Intentionally Left Blank
Chapter 22
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Polypeptides Isao Ando ~, Tsunenori Kameda ~, and Naoki Asakawa 2 1Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan; 2Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Japan
22.1
Introduction
Synthetic polypeptides consist of a repeating sequence of certain amino acids and their primary structures are not as complicated as those in proteins. The polypeptides are very important polymers in both polymer and protein science. The characteristic properties related to the structure lead to possible expansion for research in the field of polymer science, to provide very different moplecules from conventional synthetic polymers. For example, the concept of the liquid crystal is expanded by revealing the variety of structures and properties of liquid crystals. Furthermore, the polypeptides are sometimes used as biomimic materials. On the other hand, synthetic polypeptides are sometimes used as model biomolecules for proteins because they take the c~-helix,/3-sheet, w-helix structure, and so on, under appropriate conditions. From such situations, it can be said that synthetic polypeptides are "interdisplinary" macromolecules and are very important for research work in both polymer and protein science.
22.2
Conformations and 13C NMR chemical shifts
As is well known, most of the peptides, polypeptides and proteins considered here consist of repeating sequences of peptide bonds with 20 different types of substitutents at the C~ carbon. The limited conformations such as c~-, oJ-,/3-sheet, etc., are taken by a possible set of dihedral angles (oh, g') around the N ~ C ~ and C ~ C ( - - - O ) bonds. In the solution state, the NMR chemical shift of these biomolecules, with possible rotation around the bonds, becomes the averaged value because of rapid rotation about the peptide bonds on the NMR timescale. In the solid state, however, the chemical shift is characteristic of specific conformations because internal rotation around the peptide bonds is fixed. This shows that the NMR chemical shift can be used for elucidating the conformation of polypeptides and proteins in the solid state. It has
820
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA 1801
I-~fl-sheet-~"l~
"~ 1 7 5 ~ ~ ~ - ~
a-heliXo
o ?h-o-
=O
~70 55 -O
.~ 50
a
,-
O
o---r
o
o-~ 0---o.
45 m 20 15 10
-O ~
~
) I'~'~J~l
5
J
10
,
'
i ~lj~l
I
50 100 DPn (DPn)
I
I j ~llll~}
I
500 1000 2800
Fig. 22.1. Plots of the 13C chemical shifts of (Ala)n against the number-average degree of
polymerization (DPn).
been experimentally and theoretically shown that the NMR chemical shift of polypeptides and proteins is a very important NMR parameter for determining the main-chain conformation. More recently, studies on the structural characterization of polypeptides and proteins by using such a methodology has been introduced [1]. As an example, it is shown that the 13C chemical shifts of the C~, Ct3 and amide ~ O carbons of poly(L-alanine)[(Ala)n], are related to particular conformations [2]. ~3C cross-polarization/magic-angle spinning (CP/MAS) NMR spectra of solid poly(L-alanine) shows that the C~, Ct3 and amide ~ O carbon signals are well resolved between the a-helix and/3-sheet forms. The 13C chemical shifts are plotted against the numberaverage degree of polymerization in Fig. 22.1 [2]. Clearly, 13C chemical shifts of (Ala)n (n > 16) are unchanged for the peptides of various molecular weights within experimental error and, thus, can serve to characterize the ahelix form. The chemical shifts of the C~ and carbonyl carbons of the ahelix are displaced significantly to high frequencies by 4.2 and 4.6 ppm, respectively, relative to those of the/3-sheet form, while the chemical shift of the Ct3 carbon of the a-helix is displaced to low frequencies by ---5 ppm with respective to that of the/3-sheet. For this reason, the value of the X3C chemical shift can be used to describe the local conformation. In addition, the 13C chemical shifts of randomly coiled (Ala)~ in trifluoroacetic acid solution have values between the a-helix and/3-sheet forms. The existence of such characteristic displacements of 13C chemical shifts
POLYPEPTIDES
821
Table 22.1. 13C chemical shifts of polypeptides characteristic of a-helix, /3-sheet and o~-helix
forms Sample a
Conformationb
(Gly*). (Gly*). (Gly*)5
/3
(Gly).
/3
(Gly).
31
(Ala, Gly*).
a
(Ala).
a
(Leu, Gly*),, (Leu).
a a
(Glu(OBzl), Gly*). (Glu(OBzl).
a a
(Asp(OBzl), Gly*),, (Asp(OBzl), Gly*),, (Asp(OBzl),,
a w a
31
al s
(Val, Gly*),, (Val).,
/3 a
/3
Carbonyl c
168.5 + 171.8 + 171.4 + 168.4 169.2 172.3 172.1 176.7 171.7 + 176.4 176.8 171.8 172.2 171.4 + 175.7 170.5 172.1 + 175.6 175.4 171.0 172.2 172.0 + 171.1 + 174.9 174.9 171.i 171.3 169.6 168.5 + 174.9 171.8 171.5
13C chemical shift
Ca
Ct3
43.2 43.2 41.7 43.2 44.3 43.2 42.0 52.2
14.6
52.4 52.8 48.2 49.3 55.4 55.7 50.5 56.5 56.4 56.8 51.2 51.1 53.2 50.8 53.4 53.6 50.9 50.5 49.2 58.0 65.5 58.4 58.2
14.9 15.5 19.9 20.3 39.0 39.5 43.3 25.3 25.6 25.9 29.0 29.7 33.8 33.2 33.8 34.2 33.8 32.9 35.1 32.0 28.7 32.4 32.4
Phenyl
Benzyl
127.5 --- 128.5
65.2 66.0
127.8 127.7 129.8 128.2 129.2 129.0 129.2
65.9 65.3 65.7 66.1 66.1 65.3 65.9
aSymbol * indicates carbonyul carbon enriched by 13C. ba-Helix is right-handed unless otherwise specified. CSymbol + indicates chemical shift of glycine rsidue.
is not limited to the Ala residue. Table 22.1 summarizes the 13C chemical shift values of various amino acid residues in the a-helix and/3-sheet forms [1]. It is seen here that the C~ and C - - O peaks of the a-helix form are all displaced to high frequencies with respect to those of the/3-sheet form, which is consistent with the data for (Ala),. Furthermore, it is significant to show
822
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
Table 22.2. 13C chemical shifts of various conformations of poly(/3-benzyl L-aspartate) in the solid state (ppm from TMS; ---0.5ppm) Sample
Conformation Ca
Ct3
C~O (amide) C=O (ester)
Phenyl CH20
Polymer
ag-helix aL-helix to-helix /3-sheet
53.4 50.9 50.5 49.2
33.8 33.8 32.9 38.2; 35.1
174.9 171.1 171.3 169.6
167.0 169.0 167.8 169.5
129.8 129.2 129.0 129.2
/3-sheet /3-sheet
49.1 49.5
38.0 36.8
169.8 169.6
168.2 168.0
129.0 65.5 128.4 65.9
65.7 66.1 65.3 65.9
Oligomers DP,,5a DP,,10
aDP = degree of polymerization. the 13C chemical shift values for various conformations of poly(/3-benzyl Laspartate) (PBLA) such as the aR-, aL-, ~OL-helix and /3-sheet forms in Table 22.2, which are taken by appropriate treatments [3]. The absolute 13C chemical shifts of the C~ and C~ carbons are affected by the chemical structure of the individual amino acid residues, and can be used effectively for conformational studies on particular amino acid residues in polypeptides and proteins. On the other hand, the C ~ O chemical shifts do not seem to be affected by residue structure and can be used for diagnosing the main-chain conformation. 22.2.1
Origin of the conformation-dependent 13C NMR chemical shift
Such sizeable displacements of the 13C chemical shifts can be characterized by variations in the electronic states of the local conformation as defined by the dihedral angles (4), qJ) [lg, 2, 4]. The calculated contour map for the C~ carbons of an alanine dipeptide was made using the FPT I N D O method within the semiempirical MO framework as shown in Fig. 1.2 in Chapter 1
[3]. It should be noted that the negative sign of the shielding constant oindicates deshielding, and so shielding variations can be compared with the experimental chemical shift 6 where a positive sign denotes deshielding. Such a chemical shift map successfully predicts the ~3C chemical shifts and conformations of the L-alanine (L-Ala) residues in polypeptides and proteins. For example, the experimental ~3C chemical shift for the a-helix form appears at lower frequencies by ---5.5 ppm from that for the /3-sheet form. The calculated map reasonably predicts the experimental data. Most recently, the calculated I3C chemical shift maps of the C~ and Ct3 carbons of the L-AIa residue were obtained using the G I A O - C H F method with an ab initio 4 - 3 1 G basis set as shown in Fig. 1.3 in Chapter 1 [4], of
fl-sheet
cr-helix i
i
110 '
Hm
I
I
<
H il
ISN shielding (pprn)
100
L~
90 ~ '
~
80 '
'
O !_.
PPI
PPll
OrL-, OJL-helix
PGII
PGI
Fig. 22.2. Diagram of the observed isotropic 15N shielding of some homopolypeptides (X), with various conformations (a-helix, B-sheet, aL-, OiL-helix, PGI, PGII, PPI and PPII forms).
tO
824
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
PCH:3
/
.._=, _con
// o-.e,,
p-sheet i
~
B
.
__(3 !
15
.
.
l^ll
. ~_.__~ .
.
i
10
9
9
... , ............., 9
!
5
0 ppm
Fig. 22.3. 300 MHz 1H CRAMPS NMR spectra of (Ala), in the solid state. (A) H-(Ala)8NHBu(fl-sheet form) and (B) (Ala), (a-helix +/3-sheet form) and (Ala),, (a-helix).
which the reliability may be higher when compared with the case of the semiempirical MO calculations. The chemical shift behavior as shown in these maps, is very similar to that obtained by the FPT INDO MO method. On the basis of these calculated maps, the relationship between the experimental 13C chemical shift and the main-chain conformation for a series of L-Ala containing peptides and polypeptides in the solid state was elucidated as shown in Tables 22.1 and 22.2. The calculated isotropic 13C shielding constants for the Ct3 carbon are 186.4 ppm for the /3-sheet conformation, 189.4 ppm for the righthanded a(aR)-helix, 189.6 ppm for the lefthanded a(aL)-helix, 188.7 ppm for the 31-helix, and 186.5 ppm for Silk I and (AlaGly(glycine)),, form I [3, 5]. Here the larger value means lower frequency shift. The experimental isotropic ~3C chemical shifts are 21.0 ppm for antiparallel fl-sheet, 15.5 ppm for the aR-helix, 15.9 ppm for the aL-helix, 18.4 ppm for the 31-helix and 17.6 ppm for (AlaGly)~ form II. Here the larger value means a higher frequency shift. It is found that the change of the main-chain dihedral angles dominates the isotropic chemical shift behavior of the Ala residue C~ carbon, except for Silk I and (AlaGly), form II. It is predicted that the structure of Silk I and (AlaGly)n form II might be a loose fourfold helical conformation, which was originally proposed from a X-ray diffraction study. From the calculation, the loose helix model could not be accepted either; at
POLYPEPTIDES
825
this point, however, it is still difficult to determine the main-chain dihedral angles. The calculated 13C chemical shieldings for the C~ carbon are 160.4 ppm for the /3-sheet, 159.6 ppm for the aR-helix, 159.2 ppm for the aL-helix, 161.4 ppm for the 3x-helix, and 157.9ppm for Silk I and (AlaGly)n form II. For these calculated chemical shieldings, the experimental isotropic 13C chemical shifts are 48.7 ppm for the /3-sheet, 53.0ppm for the aR-helix, 50.1 ppm for the aL-helix, 49.7 ppm for the 3x-helix and 51.5 ppm for (AlaGly)~ form II. Although it is obvious that there is a main-chain dihedralangle dependence on the chemical shift for the C~ carbon, it seems more complicated than that for the Ct~ carbon, because the orientation of the chemical shift tensor for the C~ carbon with respect to the molecular fixed frame, varies from one structure with a given set of dihedral angles to another. Also because, in the case in which the L-Ala residue carbonyl or amide group would form the hydrogen bond, the hydrogen bonding structure can also affect the behavior of the 13C chemical shift of C~, as regards the structure of (AlaGly)n form II and Silk I, they require a chemical shift calculation that takes hydrogen bonding into consideration. An investigation of the chemical shift anisotropy of the carbonyl carbon is needed [6]. The side-chain conformation of poly(L-proline) (PP; (Pro),,)) forms II and I in the crystalline state has been studied by a combination of solid-state I3C NMR and the tight-binding INDO/S sum-over-states theory [7] which is used for calculating chemical shieldings of polymer chains with an infinite regular structure [8]. In the poly(L-proline) form II, the pyrrolidine ring takes two types of conformations, and undergoes no chemical exchange within the temperature range from - 8 6 to 175~ On the other hand, in poly(k-proline) form I, the pyrrolidine rings in the crystalline state was clarified to be undergoing chemical exchange [9]. The principal values of the 13C chemical shift tensor components (611, 622 and ~33), determined from the 13C powder pattern spectrum, are valuable as parameters for obtaining structural information which can be related to the electronic state more directly than the isotropically averaged value (6iso = (~11 "[- ~22 -[" ~33)/3. Nevertheless, very few data are available for the 13C chemical shift tensor for carbon atoms in polypeptides. There have been a few studies of conformation using 13C chemical shift tensors of polypeptides in the solid state. The solid-state 13C CP/MAS NMR spectra of a variety of polypeptides containing [1-13C]glycine residues as a minor component ( < 8 % ) have also been recorded [9]. This was done to obtain the 13C isotropic averaged chemical shifts, and the principal values were incorporated into the homopolypeptides. It was found that the magnitudes of the displacements of the chemical shift on conformational changes are larger for ~22 and ~33 than for the isotropic chemical shift values,
826
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
while 611 is almost unaffected by any conformational change. A solid state a3C NMR study of some polypeptides containing [2-13C]glycine residues has been carried out in order to clarify the conformational effect on the 13C chemical shift tensors. It is found that the isotropic chemical shifts, and the principal values of the chemical shift tensor of Gly C~ incorporated in some homopolypeptides, are displaced.
22.2.2
Conformation and the 15N NMR chemical shift
It has been demonstrated that the isotropic 15N chemical shifts of systems, such as (Gly)n(polyglycine(PG)), (Ala)~, (Leu)n(poly(L-leucine)), (Ileu)n(poly(L-isoleucine)), (Val)n(poly(L-valine)), (Phe)n (poly(L-phenylalanine)), (Glu(OMe))n (poly(y-methyl L-glutamate)), (Asp(OBzl))n(poly(/3-benzyl Laspartate)) and (Pro)~ in the peptide backbone of polypeptides in the solid state, exhibit a significant conformation-dependent change [5, 6]. Both experimental observation and theoretical calculations confirm this. The 6iso for the a-helix form (97.0-99.2 ppm) appears to low frequencies by about 1.210.0ppm with respect to that for the fl-sheet form (99.5-107.0ppm) of polypeptides, which obviously depends on the structure of the individual amino acid residue, as shown in Fig. 22.2 (relative to 15NH4NO3). Some 15N chemical shift differences are rather small, but are meaningful in the context of polypeptides. The variations of 6i~o for various kinds of polypeptides are 2.5 ppm in the a-helix form and ---7.5 ppm in the/3-sheet form. In addition, the 6iso of the 13-sheet form of the Leu, Val and Ileu residues, which possess alkyl side chains, appear to high frequencies with respect to that of the Ala residue. In contrast, the 6i~o value for the 13-sheet form of the Asp (OBzl), Glu (OBzl) and Glu (OMe) residues (where Glu is glutamic acid), which possess a side-chain ester, is to low frequencies with respect to that of the Ala residue. These results indicate that the 15N chemical shift difference between the a-helix and/3-sheet forms depends on the side-chain structure of individual amino acid residues. Furthermore, 6i~o gives information about the helix sense (aR- or aL-helix) of (Asp(OBzl))~ (w-helix: 99.2ppm and aL-helix: 97.0ppm). In addition, the 15N chemical shift value of (Asp (OBzl))~ with a low molecular weight (13-sheet form) is identical to that of a high molecular weight (/3-sheet form), indicating that the 15N chemical shift solid polypeptides is independent of the chain length, if no conformational changes occur. Accordingly, the 15N chemical shift depends mainly on the conformation and side-chain structure of individual amino acid residues. To support this view, theoretical calculations of the 15N chemical shift of a dipeptide fragment forming hydrogen bonds with two formamide molecules has been calculated using the FPT
POLYPEPTIDES
827
INDO theory [5a]. The relation between the isotropic 15N chemical shift, or chemical shift tensor, and the structures of various kinds of synthetic copolypeptides in the solid state have been studied. It was found that the 15N chemical shifts indicate strong neighbor sequence effects [5, 6].
22.2.3
Conformation and 1H and 170 NMR chemical shifts
Most recently, it has been reported that there is a large difference in solidstate aH chemical shifts between the a-helix and/3-sheet for polypeptides as shown in Fig. 22.3 [7]. In (Ala)n, the 1H chemical shift of the C~H proton for the a-helix form appears at lower frequency by 1.2-1.4 ppm than for the /3-sheet form. The NH proton signal is very broad due to the quadrupolar interaction with the 14N nucleus. In (Leu),, and (Glu(OBzl))n, it was found that similar behavior exists. These experiments show that solid-state ~H NMR chemical shifts have the potential possibility for structural elucidation in polypeptides. As described in Section 1.4, the oxygen-17 nucleus is very important for obtaining information about the hydrogen-bonded structure and conformation of peptides and polypeptides. From the 170 solid-state NMR experiments on aTO-labeled (Glu)~ and (Ala)~, it has been elucidated that the chemical shift tensor components and its average value, and quadrupolar coupling constants, provide useful information about the hydrogen-bond length associated with the conformation [8]. These experiments show that solid-state 170 NMR will become important for clarifying the hydrogen-bonded structure and conformation in polypeptides and proteins.
22.2.4 Applications of the conformation-dependent 13C chemical shift to the conformational characterization of polypeptides The conformational characterizations of homopolypeptides, copolypeptides and more complicated polypeptides in the solid state have been carried out successfully using the characteristic 13C chemical shifts as mentioned above. Also, such characteristic 13C chemical shifts have become reference data for characterizing the conformation of polypeptides in the solution state [1]. Some recent works are introduced here. The 13C CP/MAS NMR spectra of PBLA in the solid state at temperatures up to 173~ are observed as shown in Fig. 22.4 [10]. The assignment of peaks is straightforward. At room temperature (23~ 13Cchemical shift values of the amide C ~ O , C~ and Ct~ carbons are 175, 55 and 35 ppm, respectively. These values are characteristic of the aR-helix form. In the temperature range from 23 to 117~ the major conformation is the aR-helix form. At 128~ the main-chain conformation
828
ISAO ANDO, TSUNENORI K A M E D A AND NAOKI ASAKAWA f C=O
~11! i l['i
C-a C-fl i1! i!l CH20! ~ ~ii
Phenyl C_i A
......... ~ i9~ " - ' ~
Amide Ester [ i i i i I 200 150
:'
i
,
i
i
I
100
i
,
~
i
TemPl(;C)
'~
I
50
i
128
i
,
i
ppm
Fig. 22.4. VT 13C CP/MAS NMR spectra of PBLA in the solid state at various temperatures" dashed line, aR-helix; light solid line, OJL-helix form; and bold line,/3-sheet form.
the aR-helix, but small peaks which come from the COL-helix form appear at 33.6 (Ct3) and 52.5 (C~) ppm, and also the small minor peaks which come from the /3-sheet form, appear at 39.6 (C~) and 51.4 (C~) ppm. At 139~ the amide C - - O peaks shift to low frequency and the ester C ~ O peak shifts to high frequency, hence, the two peaks overlap with each other. Similarly, the Ct3 and C~ peaks become broad and shift to low frequency. This indicates that the conformation is a mixture of the C~R-helix, COL-helix and /3-sheet forms. At 150~ the peaks corresponding to the aR-helix form completely disappear. At 150 and 161~ two peaks due to Ct3 at 34.5 and 39.5 ppm can be recognized. The C~ and amide C - - O and ester C ~ O signals become a broad peak. In this temperature range, the COL-helix (34.5 ppm) and/3-sheet (39.5 ppm) conformations are mixed. At 173~ the peaks corresponding to the COL-helix form completely disappear. Only the peaks corresponding to the/3-sheet form appear. The temperature change in the fraction of conformations for the C~R-, COL-helix and/3-sheet forms is obtained by peak computer-fitting as shown in Fig. 22.5. Poly(aspartates) and poly(glutamates) with n-alkyl chains as the side chains take on the liquid crystalline state under any conditions such as temperature variation, the side-chain length variation, etc., as the macroscopic structure, and also take various main-chain conformations. In poly(aspartates) and poly(glutamates) with long n-alkyl side groups such as n-octadecyl groups, between which the difference is the number of CH2 carbons between the C~ and ester carbonyl carbons, i.e., the former has one C H 2 carbon and the latter two CH2 carbons. By elevating the temperature the latter forms a
829
POLYPEPTIDES
188
/
88 o
o.,,t
.... =,i / ".. /
68
o -
:'b ",1
o,8
213 ,.,..,..,....o.........,, 23
\
117 128 139 158 161 173 Tempera ture ("(3)
Fig. 22.5. Relative peak intensity of C~ and ~ O amide and ester carbons in the aR-helix, ~oL-helix and fl-sheet forms as deconvoluted computer fitting: (closed circle) C~, aR-helix, (open square) ~ O , aR-helix, (open triangle) ~ O , ~OL-helix" (closed triangle) C~, 13-sheet; and (open circle) C=O,/3-sheet.
liquid crystal above ca. 40~ However, the former does not form a liquid crystal in a wide range of temperatures. The discrepancy of such behaviors has been clearly elucidated. From the VT 13C CP/MAS experiments on the latter's polypeptide in the temperature range from 27 to 100~ [11, 12], it was concluded that the n-octadecyl CH2 13C signal transitionally moves from ca. 33 to ca. 30 ppm above ca. 40~ by the melting of a crystalline phase composed of n-alkane-like crystallites. However the amide carbonyl and the C~ chemical shifts of the main-chain are 176 and 57.6 ppm, respectively, which comes from the aR-helix form and do not change over a wide range of temperatures. Also, the amplitude of the main-chain carbon signals in the vicinity of 40~ which is considerably decreased by a great reduction of the CP efficiency due to the reorientation of the main chain around the a-helical axis at a frequency of---60 kHz in the liquid crystalline state. On the other hand, from the VT 13C CP/MAS NMR experiments on poly(aspartate) with n-octadecyl side chains in the temperature range from 23 to 100~ [13], it was noted that the n-octadecyl CH2 13C signal transitionally moves from ca. 33 to ca. 30 ppm above ca. 40~ by melting of the crystalline phase as in the case of poly(n-octadecyl L-glutamate), and the main and side-chain carbonyl carbon and C~ carbon signals greatly change with an increase in temperature (as shown in Fig. 22.6). By comparing these experimental 13C chemical shifts and reference data for PBLA (Table 22.2), it was found that the
C = ~
-oo-~
C=O(esief)
-
> 0 > Z
c.~
0 H r.~ C 2: bl :Z 0 > ul > >, :Z
Ji
175
9
~
_J
o
6 / PPM
165
. , .
-go
,
.
.I,
6/r~
.
_ .
so
_
.
:Z > 0
>
Fig. 22.6. Expanded 13C CP/MAS N M R spectra of the main- and side-chain carbons of poly(/3-octadecyl L-aspartate) in the solid state as a 3> function of temperature.
POLYPEPTIDES
831
conformational change in the main chain from the a R - to the aL-helix occurs with the temperature elevation. Such a conformational change of the main chain leads to the preventation of the formation of thermotropic liquid crystals. The introduction of fluorine atoms into polymers induces new physical properties by changes of intra- and intermolecular interactions. From this point of view, we are interested in elucidating how the conformational behavior of the main chain of poly(glutamate) changes on the introduction of fluorine atoms in the side chains. In the ~3C CP/MAS NMR spectrum of poly(y-n-2-(perfluorodecyl) ethyl L-glutamate) (P10FLG) with long n-fluoroalkyl side chains, the chemical shifts of the amide carbonyl and Ca carbons are 172.7 and 52.3 ppm, respectively [14]. These values are very close to 172 ppm for the amide carbonyl carbon and 51 ppm for the C~ carbon for poly(~/-benzyl L-glutamate) (PBLG) and poly(y-n-octadecyl L-glutamate) with the j3-sheet forms. It can be said that the energetically stable state of the main chain of P10FLG goes from the a-helix to the/3-sheet form. In the 13C CP/MAS NMR spectrum of poly(y-trifluoroethyl L-glutamate)(PFLG) with short n-fluoroalkyl side chains, the chemical shifts of the amide carbonyl and Ca carbons are 174.5 and 53.5 ppm, respectively [14]. Although the values obtained are different from those for the typical aR-helix form, they are closer to the a-helix than the /3-sheet form. This indicates that PFLG assumes an a-helix form different from the typical aR-helix form. Poly(y-methyl L-glutamate) (PMLG) network was synthesized, and the PMLG gel with chloroform (helix solvent) and a mixture of chloroform and trifluoroacetic acid (coil solvent) was prepared. The helix-coil change of the gel, induced by changes of the mole ratio of a mixture of chloroform and trifluoroacetic acid, was elucidated using reference data of characteristic 13C chemical shifts for polypeptides [15]. The conformational characterization of poly(L-glutamic acid) ((Glu)n)and its sodium salt, poly(L-aspartic acid) ((Asp)n) and irregular poly(D,L-aspartic acid) in the solid state have been carried out by the characteristic solid state 13C chemical shifts. These results show that (Glu)n and (Asp)n in the free acid form exhibit both a- and /3sheet forms. Clarifying the structure and functions of protein materials in the solid state provides an index with respect to the design of artificial biomaterials. Solidstate NMR has been used as a powerful means for elucidating structure and dynamics in addition to the X-ray diffraction method [1el. The structure and dynamics of some fibrous proteins, such as wool, silk, collagen, tropomyosin, etc., have been characterized using characteristic solid-state NMR chemical shifts as stated above, and much more new information obtained in addition to the results provided by X-ray diffraction. And the individual advantages of these two methods are complementary with each other. Details of appli-
832
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
cations to proteins using the characteristic chemical shifts are described in other chapters.
22.2.5 Hydrogen-bonded structure and the NMR chemical shift 22.2.5.1 Hydrogen-bonding effects on the 13C chemical shift of the carbonyl-carbon in several amino acid residue [24] Figure 22.7 shows the plots of the observed isotropic chemical shifts (6iso) of Gly, L-Ala, L-valine (L-Val), D and L-leucine (L-Leu), and L-aspartic acid (LAsp) C ~ O carbons against the N...O hydrogen-bond length (RN...o) (Table 22.3). It is found that a decrease in RN...o leads to a high frequency shift and there is an approximately linear relationship between 6i~o and RN...o of peptides considered here as 6iso = a + b R N . . . o , where a and b are constants. The expressions for these relationships as determined by the least-mean squares method are given in ppm relative to tetramethylsilane (TMS). 6iso(Gly) = 2 0 6 . 0 - 12.4RN...o 6iso(L-Ala) = 237.5 - 21.7RN...o 6iso(L-Leu) = 2 0 2 . 2 - 10.0RN...o
166
(22.1a) (22.1b) (22.1c)
I
I
I
I
L/3
168o
/
a~ 170/
a. 1 7 2 _ I
O
9
9
A
,, z / A
"-4 174:U
-8 176U
A
/
/
178 J I 2.7 2.8
/
x
~G!y
-~-Ala
A
-,~"-Val ~ L e u --O--Asp
A I
I
2.9 3.0 RN...o(A)
i
.
3.1
3.2
Fig. 22. 7. Plots of 13C chemical shifts for the carbonyl-carbons in Gly, L-Ala, L-Val, D, L-Leu and L-Asp residues in peptides in the solid state, against (RN...o).
833
POLYPEPTIDES
Table 22.3. Observed isotropic 13C chemical shifts and tensor components of the Gly, L-Val, L-Leu, L-Ala residues in peptides and polypeptides, and the hydrogen-bond lengths determined by X-ray diffraction
Samplea
Gly residue Gly*-Gly C1 Ac-Gly*-Gly Als-Gly*-Gly Val-Gly*-Gly Gly*-Gly HNO3 Poly(Gly)/3-sheet Poly(Gly) 31-helix Poly(Ala, Gly*) a-helix Poly(Ala, Gly*)/3-sheet Poly(Leu, Gly*) c~-helix Poly(Val, Gly*)/3-sheet Val residue Val*-Gly-Gly Leu residue Boc-Pro-Ile*-Gy DL-Leu*-Gly-Gly Asp residue Asp*-Gly Ala residue Ac-AIa*-NHMe Poly(Ala) a-helix Ala*-Gly-Gly Ala*-Ser
Carbonyl 13C chemical shift (ppm)
Hydrogen-bond length
8iso
611
(~22
633
611 q- 633 RN...o c
168.1 170.1 170.6 169.2 168.3 169.6 173.2 173.0 168.8 172.8 169.6
242 244 240 245 248 2485 247 243 241 241 240
174 176 177 170 168 173 182 181 171 180 169
88 91 94 93 89 91 91 95 95 97 99
331 335 334 338 37 336 338 338 336 338 340
2.97 2.82 2.93 3.05 3.12 2.91 2.73
169.2
245
170
93
338
3.05
173.0 172.0
249 246
183 178
88 92
336 338
2.83 3.06
170.3
242
175
93
336
2.98
177.0 175.9 176.8 172.6 170.1
241 245 243 245 249
196 189 194 179 172
94 96 94 93 89
335 341 337 338 338
2.72 2.92 2.87 3.00 3.04
(~)b
-
aAmide carbonyl 13C chemical shifts for the asterisk-marked amino acid residues were measured. bDetermined by X-ray diffraction. CAmide-amide hydrogen-bond length (A) between nitrogen and oxygen atoms in a hydrogen bond.
6iso(L-Val) = 2 1 5 . 4 - 14.2RN...o 6iso(L-Asp) = 1 9 9 . 0 - 9.6RN...o
(22.1d) (22.1e)
where 6is o is expressed in ppm and RN...o in A,. These relationships indicate that hydrogen-bond length can be determined through the observation of the 13C chemical shift of the carbonyl carbon in the amino acid residues in peptides and polypeptides. The slope b of the variation of 6iso against the hydrogen-bond length for these amino acid residues decreases in the order
834
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
L-Ala > L-Val > Gly > L-Leu L-Asp. The magnitude of the intercept decreases in the order L-Ala > L-Val > Gly > L-Leu L-Asp. The magnitude of the slope b decreases in the same order as the intercept a. From these results, it can be said that the values of a and b are characteristic for individual amino acid residues. It is noted that the carbonyl carbon chemical shifts give a similar hydrogenbond length dependence not only in oligopeptides (dimer or trimer) but also in polypeptides (polyglycine and poly(L-alanine)). This suggests that the 13C chemical shifts of any amino acid carbonyl-carbon accepting the hydrogenbond which is formed between the amide > C - - O and amide > N ~ H , are influenced strongly by the hydrogen-bond length.
22.2.5.2 The hydrogen-bonded structure and 13C NMR chemical shift tensor of amino acid residue carbonyl carbons of peptides and polypeptides It is expected that the principal values of the 13C chemical shift tensors (611, ~22 and 633 in order of increasing shielding) are, in principle, more valuable as parameters for obtaining detailed information on hydrogen bonding, to be related to the electronic structure, than the isotropic 13C chemical shift (6iso = (~11 nt- ~22 nt- ~33)/3). Kameda et al. [25] studied hydrogen-bonding effects on the principal values of the 13C chemical shift tensor for the Gly, L-Val, LLeu and L-Asp residue carbonyl carbons of peptides and polypeptides in the crystalline state, and elucidated the relationship between the tensor components and hydrogen-bond length. The spinning-side band (SSB) analysis has been applied [26-28], i.e., the Herzfeld-Berger analysis, to determine the exact principal values of the carbonyl carbons. The tensor components of the other samples are determined in the same manner. It has been reported that ~11 is in the amide sp 2 plane and lies along the direction normal to the C ~ O bond, the ~22 component lies almost along the amide C - - O bond, and the 633 component is aligned perpendicular to the amide sp 2 plane [29]. Figure 22.8 shows the plots of the 13C chemical shift tensor components ((~11, ~22 and 633) against RN...o for Gly, L-Ala, L-Val, L-Leu and L-Asp residues. The d22 values for Gly, L-Leu and L-Ala residues move linearly to high frequency with a decrease in RN...o [30]. The slope and intercept of the variation of the plot of 622 against RN...o varies depending on the amino acid residue. The slope of the variation of (~22 against RN...o for the Ala residues in peptides is large compared with those for the other amino acid residues. This shows that the value of 622 for the Ala residue is the most sensitive to a change in RN...o compared with the cases of the other amino acid residues. The expression for the relationship as determined by the least-mean squares method for oligopeptides containing a Gly residue is expressed as follows:
hrh r162 OO
9s.teq :to.uo ,(q polea!pu! o:Ie zzg pue ~g jo sIoI.to leluotu!Iodxo Oq,L "~ oql lsu!ege sop!ldod ut sonp!so:t (ol:3I./3 uodo) ds V pue (olgue!:n uodo) no'-I '(puome!p uodo) leA '(o2enbs p!Ios) elV '(oI3:t!3 p!Ios) ' 9 0 oql u! suoq:te3 I,(uoq:te3 op!tue oql :tojZZg (~) pue ' ~ g (q) '~:g (e) :toj sluouodmo3 :tosuol 1j!qs Ie3!tuoq3 Dz: p0A:t0sqo oql JO SlOld "g':~" "~'.2at
(y) g'g
O.--N~
l.'g O'g: 6"g
s
L'g
g'g
u~
(y) o'"N l
I-'g O'g: 6"g 9":g L" 1
Ot~
-- g IO [
[u~ 9
'
1
'
_
n g61.
(y)
g'g l.'g o'g 6"g g ' g Z'g '
i
'
1
'
i
'
i
'
-09g
.i
-
0
e
tC)
[]
9 [] 9
-
[]
~
9
g6
i 9
[]
''
'
(o) '
Ogg
v
o~
A
O_
~
-
v
9
..
m
'
_ ,
!
,
(q) !
,
,,1
,
I
_
,
B
i1_O ~ g
gLL i
9
/011~ 9
g/
(e)
g9[
'
'
'
'
'
'
'
'
.......
ogg
836
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
~22 = 2 6 2 . 9 - 30.2 RN...o
(22.2)
where 622 and RN...o are in ppm and A, respectively. This relationship indicates that RN...o can be determined through the observation of 622 for the carbonyl carbon of the Gly residue in oligopeptides within an error of <0.7 ppm. It should be noted that the Gly C ~ O carbon chemical shifts for polyglycine with the /3-sheet form and the 31-helix form are located on a straight line as expressed by Equation (la). This shows that the relationship can be applied to the characterization of a hydrogen bond in polypeptides. Next, we are concerned with 611 and 633. As seen from Fig. 22.8, the experimental data for 6~1 and 633 of the Gly, Ala and Leu residues scatter significantly. The 611 and 633 values are insensitive to changes in RN...o, but it seems that 611 and 633 move slightly to low and high frequency, respectively, with a decrease in RN...o. In order to clarify the relation between 611 and 633, 6al is plotted against 633 as shown in Fig. 22.9. The slope of the linear relationship between the 611 and 633 values is about - 1 . When 611 moves to high frequency, 633 moves to low frequency by the same magnitude. Therefore, the sum of 611 and 633 is almost constant (337.5-+3.5ppm) and is independent of the amino acid residue, except for GlyGly. This leads to the experimental finding that there is a linear relationship between 6i~o and 622 as expressed by Equation (22.2). From the above results, it can be said that
86
9
,~ GlyGly
9
9
9O
00
E
IS/. CL
v
t"O t"r
94
tl
oO
98
II
611+ 633 =337.5 ppm
252
248
244 240 (ppm)
236
Fig. 22.9. Plots of the observed 611 against 633 for various peptides. The experimental errors are indicated by error bars. The slope of the line is expressed by ~11 "~- ~33"
POLYPEPTIDES
837
the large high frquency shift of the isotropic 13C chemical shifts, diso, with a decrease of RN...o comes from the behavior of the 622 component in overcoming that of the 3~x component. Therefore, it can be said that the large deshielding of 6iso, with a decrease in RN...o, is predominantly governed by a decrease in 322. Similar relationships have been reported by Oas et al. [31] for peptides containing the Gly amino acid residue. To understand the relationships between the tensor components and hydrogen-bond length, FPT-INDO calculations on 13C shielding tensors of some model peptides have been carried out. From the FPT-INDO calculations, it is found that o22 is the most sensitive to a change of RN...o and moves linearly to high frequency with a decrease in Ry...o. Correspondingly, tr11 moves increases with a decrease in RN...o, whereas, 0"33 is insensitive to changes in RN...o. The results of the theoretical calculations agree well with the experimental results. Such an agreement indicates that the ~3C chemical shift changes originate predominately from the change of the electronic state of the amino carbonyl groups caused by the hydrogen-bond length variation. Furthermore, it can be said that the amino acid residue-dependence of the calculated tensor components is similar to the experimental one.
22.2.5.3 Application of Equations (22.1) and (22.2) to the determination of the hydrogen-bond length in solid polypeptides and Gly-containing polypeptides Table 22.4 shows the values of RN...o, for some solid polypeptides, determined by using Equations (22.1a-e) through the observation of the amide carbonyl-carbon chemical shift as listed are: solid polyglycine[(Gly),,], poly(L-alanine)[(Ala)~], poly(L-valine)[(Val)~], and poly(L-leucine)[(Leu)~] with several conformations such as righthanded c~-helix (C~R-helix), /3-sheet, 31- and WE-helix. In these homopolypeptides, the RN...o values determined for the /3-sheet form are constant in the range of 3.0-3.1A, regardless of amino acid residue species. The RN...O value for the 31-helix in (Gly)~ is 2.8/~; only one result for the 31-helix form is available. The RN...O values for the aR-helix form are distributed in the range of 2.7-2.8/~, thus it can be said that the RN...o value for the/3-sheet form is much longer than that for the a R- and 31-helix forms. Furthermore, from this Table 22.4, it can be seen that the RN...o values of 3.0 ~ for the/3-sheet form, and 2.8 A for the 31-helix form in (Gly)~ are very close to the corresponding RN...O values of 2.95 and 2.73/~, respectively, which are determined by X-ray diffraction. Also, the RN...o values of 2.78 A for the C~R-helix form in (Val)~ and of 2.89 A for the aR-helix form in (Ala)~ are not far from the RN...o values of 2.89 and 2.87 ~ , respectively, as determined by X-ray diffraction. According to the X-ray diffraction results on (Ala)~, the RN...O value for the/3-sheet
838
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
Table 22.4. Hydrogen-bond lengths for some polypeptides and copolypeptides incorporated with Gly residues determined using Equations (22.1) and (22.2) through the observation of 13C chemical shift tensor components (~22and isotropic chemical shift
Samples
(Gly)n (Gly),, (Leu),, (Leu, Gly*),, (Val),, (Val, Gly*),, (Ala),, (Ala, Gly*)n (Ala),, (Ala, Gly*)n
Conformation Hydrogen bond length (A)
-helix /3-sheet a-helix a-helix /3-sheet /3-sheet a-helix a-helix /3-sheet /3-sheet 31
R~...oa
Rn...o b
2.7 3.0
2.8 3.0
2.7
2.8
3.1
3.0
2.7
2.8
3.0
3.1
*-N.-r)"I.Oc
R~V...o d
RN...vO e
2.7 3.0 2.8 3.0
aDetermined using Equation (22.2) and 13C chemical shift tensor component ~22 for the Gly residue in copolypeptides. bDetermined using Equation (22.1a) and isotropic 13C chemical shift for the Gly residue. CDetermined using Equation (22.1c) and isotropic 13C chemical shift for the Leu residue. dDetermined using Equation (22.1d) and isotropic 13C chemical shift for the Val residue. eDetermined using Equation (22.1e) and isotropic 13C chemical shift for the Ala residue.
form is smaller by 0.06 A than that for the aR-helix form. This shows that only the N M R results for (Ala)n conflict with the X-ray diffraction results. A re-investigation would be needed to determine whether or not, X-ray diffraction for the/3-sheet form underestimates the hydrogen-bond length. Now, we are concerned with the RN...o values for Gly residue incorporated into (Ala)n, (Leu)n and (Val)n as shown in Table 22.4. This table shows that the RN...o value for the aR-helix form is in the range of 2.7-2.8 A and the /3-sheet form is 3.0 A. The RN...o value for the/3-sheet form agrees with that for homopolypeptides as mentioned above. Also, the RN...o values for aR are widely distributed to some extent; these results are quite similar to those for homopolypeptides. Moreover, the average value is almost the same as that for the homopolypeptides. This indicates that the glycine residue is incorporated completely into the homopolypeptides with the aR-helix and/3sheet forms. Here, we are concerned with the application of Equations (22.1) and (22.2) to the determination of the RN...o value for the guest Gly residue incorporated into host copolypeptides. The RN...o values of the guest Gly residue in some host polypeptides, determined using Equation (22.2), from the observation of the 622 values of Gly residues, are listed in Table 22.3.
POLYPEPTIDES
839
The I3C chemical shift tensor component 622 for the Gly residue leads to the result that the hydrogen-bond lengths of the guest Gly residue (RN...o) in (Leu, Gly*)~ and (Ala, Gly*)n, of which the host Leu and Ala residues take the aR-helix form, are 2.7 A as estimated by using Equation (22.2). This value is in agreement with the values lengths of 2.7 and 2.8/k for the host Leu and Ala residues, respectively, determined using Equations (22.1c) and (22. lb) through the observation of the 6iso for Leu and Ala residues in homo(Leu)~ and (Ala)n with the aR-helix form, respectively. Moreover, the RN...o values of the guest Gly residue in (Leu, Gly*),, and (Ala, Gly*)~ are very close to the value of 2.8/k for the guest Gly residue (RN...o) determined using Equation (22.1a) through the observation of the ~iso value for the Gly residue in (Leu, Gly*)~ and (Ala, Gly*)~. This indicates that the guest Gly residue is incorporated completely into host polypeptides with the aR-helix form. In the case of (Val, Gly*)~ and (Ala, Gly*)n with the /3-sheet form, similar results are obtained. Therefore, it can be concluded that the guest Gly residue is incorporated completely into the host polypeptides with the /3-sheet form. These results indicate that the hydrogen-bond length for the host residue in copolypeptides and proteins can be determined through the observation of (~22 for the Gly residue in addition to the method using 6iso.
22.2.5.4 Hydrogen-bonded structure and the 15N NMR chemical shift for the amide nitrogen in glycine-containing peptides High-resolution 15N NMR spectroscopy has been increasingly applied to the investigation of peptides, polypeptides and proteins in the solid state [3243]. Shoji et al. [17] demonstrated that the isotropic 15N chemical shifts of a number of homopolypeptides in the solid state, as determined by the CP/MAS NMR method, are significantly displaced as much as 1.2-10.0 ppm, according to their particular conformations such as the a-helix and /3-sheet forms. Such a large chemical shift difference may come from changes in the hydrogen-bond length and angle and through a change in the dihedral angles (4~, 0). Figure 22.10 shows the plots of the observed 15N chemical shifts of Gly NH of GlyGly peptides against the N-..O hydrogen-bond length (RN...o). However, it is found that there is no clear relationship between RN...o and 15N chemical shifts. This is different from the case of 13C chemical shifts of the carbonyl carbons associated with the hydrogen bond, described above, where the 13C signals of the carbonyl carbons are linearly deshielded with a decrease in RN...o. Figure 22.11 shows the plots of the observed 15N chemical shifts of the glycine residue in X-Gly-Gly against the N ~ H bond length (RN--I-X) asso-
840
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
o ~
zo 85 o
IX, s
._O~
0
0
90
0
0
O O
O
z
O
95
O
2.7
2.8
2.9
3.0
3.1
3.2
3.3
RN...o(A) Fig. 22.10. Plots of the observed
15Nchemical
shift of oligopeptides in the solid state against
the RN...o.
ciated with a hydrogen bond. It is found that there is a clear relationship between these parameters and the decrease of RN--I~ leading to a linear increase inshielding. The expression for this relationship is
6 i s o ~---
57.73 + 39.32RN_H.
(22.3)
Such a trend is very different from that obtained from the corresponding 13C NMR study. Amide 15N chemical shifts are closely related to the length of the N ~ H bond but not related to the RN...o. This implies that the 15N chemical shift value gives useful information about the length of the N ~ H hydrogen bond. It seems that the hydrogen bond angle ( < N ~ H - . - O ) is also related to the 15N chemical shift. 15N NMR chemical shifts of the glycine residue of BocGly peptides in thesolid state" Figure 22.12 shows the plot of the observed isotropic 15Nchem-
POLYPEPTIDES I.'
85 o'
ed
o
I
....
841 i
I .....
r
o
~:~l.e 9
90
~
o ._~ ~o
z
95-
0 !
0.7
1
0.8
....
I
0.9
I
.
1.0
N-H iength(A ) Fig. 22.11. Plots of the observed 15N chemical shift of oligopeptides in the solid state against
the NmH bond length (RN--H).
ical shifts (diso) of Gly NH of BocGly peptides against the value of RN...O. It is found that there is a clear relationship between diso and Ry...o, and a decrease of RN...o leads to a decrease in shielding. This trend is similar to that found for the carbonyl 13C chemical shift and RN...o. Figures 22.13(ac) show the plot of the observed principal values (811, ~22 and t~33) of the 15N chemical shift tensor of Gly NH against RN...o, respectively. It is found that t~ll and t~33 are more sensitive than t~22 to changes in Ry...o. A change of 0.2 ~ in RN...o leads to a change of 20 ppm in ~11 and t~33 , although the change of 633 is 5ppm. However, only t~33 decreases linearly with a decrease of RN...o, although for t~ll and 622 there is no clear relationship with RN...o.
842
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
0
%
52
54
tt't
o
E~ r
56
o .w_
58
60
Z
62
3.0
3.1
3.2
RN...o(A) Fig. 22.12. Plots of the observed isotropic lSN nuclear shielding (Oiso)in the solid state against the RN...o.
These results show that such behavior is governed not only by the hydrogenbond length, but also by the hydrogen-bond angle. To obtain a deeper understanding between the experimental finding that the isotropic 15N chemical shift 6iso and the principal value of t~33 depend upon the hydrogen-bond length, theoretical calculations of 15N chemical shifts have been carried out by the FPT-INDO method. A decrease of RN...o leads to a decrease of the calculated 15N isotropic shielding in agreement with the experimental results. Therefore, such a relationship suggests that the isotropic 15N chemical shift value can be used in the estimation of RN...o. This is similar to the case of the carbonyl 13C chemical shift described above.
22.2.5.5 Hydrogen-bonded structure and the 170 NMR of glycine and L-alanine containing peptides and polypeptides The oxygen atom is one of the more important ones forming hydrogenbonded structures in peptides andpolypeptides. Nevertheless, solid-state x70
843
POLYPEPTIDES
(a)
~oo
(,b)
....
.3o
,
(c)
,
-
-2o
-10
110
--_=
0 E
.,...
,.-
O O
0
~o
0
r
Oo
~ 0 r
O
0
10 t
130
140
2.9
'
3,0
'
3.1
RN...O(A)
,
3.2
70
2.9
'
!
3.0
,
I
3.1
RN--.o(A)
,
_
3.2
20
2.9
1 "
=
3.0
'
'
3.1
'
3.2
RN...o(A)
Fig. 22.13. Plots of the observed principal values of 15N nuclear shielding tensor (a) 611" (b) 622" and (c) 633, respectively, in the solid state against the RN...o.
NMR study of peptides and polypeptides has not been carried out. This is due to the very weak sensitivity of solid-state x70 NMR measurement which arises from the two followings. One is that the ~70 nucleus has a very low natural abundance of 0.037%. The other is that the 170 nuclear spin quantum number (I) is 5/2, thus the nucleus is quadrupolar, and the 170 signal is broadened by nuclear quadrupolar effects in the solid. Solution-state 170 NMR spectroscopy has been employed successfully to elucidate a number ofstructural problems in organic chemistry [44-47], because the 170 signal becomes very sharp due to the removal of some quadrupolar interactions by isotropic fast reorientation in solution. For example, as the oxygen atom is directly associated with the formation of hydrogen bonds, hydrogen bonding of the carbonyl group in various compounds often results in large upfield shifts of the carbonyl ~70 NMR signal [48, 49]. From these results, solutionstate ~70 NMR has been established as a means for performing structural characterizations. From such situations, it can be expected that the solidstate ~70 NMR provides understanding of the hydrogen-bonding structure in solid peptides and polypeptides, as described previously [50]. In Section I.xx, solid-state 170 NMR spectra of Poly(Gly) form I (PG I), Poly(Gly) form II (PG II), glycylglycine (GlyGly) and glycylglycine nitrate (GlyGlyHNO3) are discussed with a view to understanding the relationship between the hydrogen-bonding structure and the 170 NMR parameters.
844
I S A O A N D O , T S U N E N O R I K A M E D A A N D N A O K I ASAKAWA
22.2.5.6 The relationship between the helical conformation and the 13C NMR chemical shift of the carbonyl carbons of polypeptides in the solid state It has been demonstrated that the 13C chemical shift contour map for the Ct3 carbon reasonably explains the experimental ~3C chemical shift behavior for the L-Ala residue with the c~-helix, /3-sheet, etc. However, from the experimental results reported previously [24, 30, 51, 52], it has been shown that the 13C chemical shift of the amino carbonyl carbon of various amino acid residues, such as those of Gly, L-Ala, L-Val, L-Leu and L-Asp residues, is closely related to the hydrogen-bond length. From these previous results, it can be appreciated that the 13C chemical shift of the the amide carbonyl carbon in polypeptides is closely related to not only the main-chain conformation, but also the hydrogen-bond length. In the helical region it is apparent from the structural requirement that in the helical conformation formed by intramolecular interactions there is a close relationship between the dihedral angles (4~, 0) and hydrogen-bond length for the peptides and polypeptides. Therefore, if in the righthanded helix region (the dihedral angles ( 4 ) = -110--~ - 2 0 ~ and 0 = - 1 1 0 ~ -20~ the dihedral angles (4), 0) are fixed, the hydrogen-bond length and angle are immediately determined. This means that the conformation-dependence of the 13C chemical shift of the amide carbonyl carbon comes from the hydrogen-bond length variation through a change of the dihedral angles (4), 0). With this in mind, the ~3C chemical shift contour map of the amide carbonyl carbon of some amino acid residues is produced as a function of the dihedral angles (4), qJ) in the vicinity of the helix region by using the relationship between the ~3C chemical shift and hydrogen-bond length reported previously. This contour map has been used satisfactorily used to explain the experimental 13C chemical shift behavior. Polypeptides with a helical conformation form intramolecular hydrogen bonds in order to stabilize its conformation. The hydrogen-bond length and angle are closely related to the main-chain helical conformation with specified dihedral angles (4), 0) in the vicinity of the c~-helix form. Thus, when standard geometries are used for the bond lengths and angles [53], the main-chain conformation and, also the hydrogen-bond length and angle can be immediately determined by giving the dihedral angles (~, 0) [54, 55]. In geometrical calculations on intramolecular RN...o between nitrogen and oxygen atoms in the > C = O . . . H ~ N < form and hydrogen-bond angle ( < C ~ O . . . N ) , we adopted a pentapeptide molecule as a model for polypeptides. The a-helical conformation for the pentapeptide is shown in Fig. 22.14, where the dotted line shows the intramolecular hydrogen bond. The bond lengths and angles are assumed to be the same for all of the amino acid
POLYPEPTIDES
845
Fig. 22.14. Molecular structure of pentapeptide molecule. The possible hydrogen bond obtained by forming the helix form is shown by the dashed line. The definition of the dihedral angles (05, 0) is indicated. For the bond angle (
residues except for < C ( ~ O ) ~ C ~ N . The bond lengths for the N ~ C ~ , C ~ C ( ~ O ) , C ( - - O ) ~ N and C ~ O bonds are assumed to be 1.453, 1.530, 1.325 and 1.230A, respectively, and the bond angles for the < C ~ C (---O)~N, < O~C~N, < O~C~N, < C~ C - - - O an d < C ( ~ O ) ~ N ~ C ~ are assumed to be 115.0, 124.5,120.5 and 121.0 ~ respectively [54]. As the bond angle 0, 111.0 ~ for the Gly residue and 109.3 ~ for the L-Ala, L-Leu, L-Val and L-Asp residues are used [53]. The angle o) is fixed at 180~ for all of the amino acid residues. By using these bond lengths and angles, the hydrogen-bond length and angle for any specified helix form in polypeptides can be calculated as a function of the dihedral angles (~b, 0) in the skeletal bonds. The calculations were carried out at intervals of 1~ by changing the dihedral angles (~b, 0) from -110 to - 2 0 ~ in the vicinity of the right-handed c~-helix form (05 = - 5 7 . 4 ~ and 0 = - 4 7 . 5 ~ The obtained hydrogen-bond lengths and bond angles for the Gly residue are shown as the contour maps as functions of the dihedral angles (~b, 0) in Fig. 22.15. In the previous works on the effect of hydrogen-bonding on the ~3C chemical shift of the amide carbonyl carbon in solid polypeptides and oligopeptides [24, 30, 51, 52], it has been demonstrated that the isotropic chemical shifts (6iso) of the Gly, L-Ala, L-Val, L-Leu and L-Asp residues are predominantly governed by the hydrogen-bond length between the nitrogen and oxygen atoms in the hydrogen bond, and move linearly to high frequency with a decrease in RN...o expressed in ppm by the Equations (22.1) and (22.2).
846
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
-20
--~. ~ .
-~.
~r3.o.-~ ~ . ~ \ \ \ \
-30
o
-.,~.
x'~\x,~'-(, " 2 . ~ , ~ ~ " \
-40
-"--.
-50
~" " ~ ' ~ "-x
\
\
\ \
N \ ~,,
-60
\
.
-,.,.
\
\
\
\
,x.
\
- ~ -70
I
-80 -90
\\<,i0
-I00
\NlOO, 9O
-110 -110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Fig. 22.15. The RN...o and hydrogen-bond angle contour maps as functions of the dihedral angles (4~, qJ) for the Gly residue. The RN...o values and hydrogen-bond angles contour line are represented by the solid and dotted lines, respectively. The RN...o and hydrogen-bond angle are expressed in A and degree (~ respectively.
Using these relationships, the RN...o values of polypeptides in the solid state have been determined through the observation of the carbonyl 13C chemical shift. Furthermore, it has been shown that the RN...o values for the guest Gly residue incorporated into host polypeptides and proteins is successfully determined through the observation of the carbonyl 13C chemical shift [24, 52]. Using Equation (1), and the calculated RN...O contour map, the 13C chemical shift contour map for Gly residue was produced as a function of the dihedral angles (4~, 0) as shown in Fig. 22.16. On the basis of these contour maps, the main-chain helical conformation is analyzed below. The bold solid line in Fig. 22.16 shows the RN...o contour maps as a function of the dihedral angles (05, ~) for the Gly residue in the vicinity of the righthanded C~R-helix form. RN...O is expressed in A at 0.3 A intervals in the range from 2.5 to 3.02 ~. It can be seen that each of the contour lines on the map distributes from the top left (t# = - 1 1 0 ~ ~ = - 2 0 ~ to the bottom right (th =
847
POLYPEPTIDES 20
13O
( -..,..~
-...
"~ ,~. #,,.1~ . o , . ~ _ ~
...
"~ ~
"~, \
":..-.,._ i... ,.,~7o.0~'-,, "-'
17~.,
"~\\ ....
\
,,
\
\
x
\
\
\ 0
60
\
\ \
-70
\
\
X )
-80 -90 -100
\ 99 0
-110
-110 -100
-90
-00
-70
-60
,(o)
-50
-40
-30
-20
Fig. 22.16. (a) The 6iso contour maps for Gly and (b) the 622 contour map for the Gly residue carbonyl carbon as functions of the dihedral angles (4), qs). The RN...o values and hydrogenbond angles contour line are represented by the solid and dotted lines, respectively. The RN...o and hydrogen-bond angle are expressed in A, and degree (o), respectively.
- 2 0 ~ @= - 1 1 0 ~ in Fig. 22.15, and the region of the contour line decreases with a decrease in RN...o. As shown in Fig. 22.15, as the dihedral angles (4), @) are varied, the hydrogen-bond angle changes together with change in Rn...o. The broken lines show the hydrogen-bond angle ( < C z O . . . N ) contour map as a function of the dihedral angles (4), @). The angle < ~ O . . . N is expressed in degree (~ at 20 ~ intervals in the range from 90 ~ to 180 ~ As seen from these figures, the deviation of the dihedral angles (4), @) from (4) = - 5 7 . 4 ~ @= -47.5 ~ (the a-helix form) leads to a deviation of the angle < C - - O . . . N from 180 ~ (the linear hydrogen bond). The hydrogen-bond angle is within a given specified range, in which hydrogen bond forms. Most of the < C ~ O N values for oligopeptides and polypeptides used in our previous works [24, 30, 51, 52] are within the range from 145 ~ to 165 ~ averaged value and the standard deviation are 156 and +4 ~, respectively. From this limitation, the acceptable region in the 6iso contour
848
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA -20 ~
\
~."-...
.~
~
'~
..~~. t \ (j 7 0 . ~ , , 0 " . _
", \
\
', "~" -70
"~
-90
-
-I00
-
-110
-110
t4o
00, -100
-90
-O0
-70
-60
-50
-40
-30
-20
Fig. 22.16(b).
maps is between the hydrogen-bond angle contour lines at < C---O...N = 152 and 160 ~ and, hence, so the optimum dihedral angles (oh, qJ) are predicted to be at the crossing point of the hydrogen-bond angle contour line at
POLYPEPTIDES
849
in d e t a i l t h a t s o l i d - s t a t e 15N N M R is a v e r y u s e f u l m e a n s for e l u c i d a t i n g t h e s t r u c t u r e of p o l y p e p t i d e s in t h e solid s t a t e [ld].
References
10.
11. 12. 13. 14.
15. 16. 17. 18.
For example, (a) H. Saito, T. Tabeta, A. Shoji, I. Ando and T. Asakura, in G. Govil, C.L. Kheterapal and A. Saran (Eds), Magnetic Resonance in Biology and Medicine, Tata McGraw-Hill, New Delhi, 1985, p. 195; (b) H. Saito and I. Ando, Ann. Rept. NMR Spectrosc. 21 (1989) 210; (c) I. Ando, T. Yamanobe and T. Asakura, Prog. NMR Spectrosc. 22 (1990) 349; (d) A. Shoji, S. Ando, S. Kuroki, I. Ando and G.A. Webb, Ann. Rept. NMR Spectrosc. 26 (1993) 55; (e) H. Kurosu, S. Ando, H. Yoshimizu and I. Ando, Ann. Rept. NMR Spectrosc. 28 (1994) 189; (f) I. Ando and S. Kuroki, in D.M. Grant and R.K. Harris (Eds), Encyclopedia of NMR, John-Wiley, New York, 1996, p. 4458; (g) I. Ando, ibid., p. 176; (h) I. Ando, Macromol. Symp. 101 (1996) 371; (i) N. Asakawa, T. Kameda, H. Kurosu, S. Ando and I. Ando, Ann. Rept. NMR Spectrosc. (1997) in press. H. Saito, R. Tabeta, A. Shoji, T. Ozaki and I. Ando, Macromolecules 16, (1983) 1983. H. Saito, R. Tabeta, I. Ando, T. Ozaki and A. Shoji, Chem. lett. 1437 (1983). I. Ando and G.A. Webb, Theory of NMR Parameters, Academic Press, London, 1983. I. Ando, T. Yamanobe, H. Kurosu and G.A. Webb, Ann. Rept. NMR Spectrosc. 22 (1989) 205. I. Ando, H. Saito, R. Tabeta, A. Shoji and T. Ozaki, Macromolecules 17 (1984) 457. N. Asakawa, H. Kurosu and I. Ando, J. Mol. Structure 323 (1994) 279. H. Saito, R. Tabata, T. Asakura, Y. Iwanaga, A. Shoji, T. Ozaki and I. Ando, Macromolecules 17 (1984) 1405. N. Asakawa, H. Kurosu, I. Ando, A. Shoji and T. Ozaki, J. Mol. Structure 317 (1994) 119. For example, (a) T. Yamanobe and I. Ando, J. Chem. Phys. 83 (1985) 3154; (b) H. Kurosu, T. Yamanobe and I. Ando, J. Chem. Phys. 89 (1988) 5261; (c) T. Ishii, H. Kurosu, T. Yamanobe and I. Ando, J. Chem. Phys. 89 (1989) 7315; (d) T. Yamanobe, I. Ando and G.A. Webb, J. Mol. Structure (Theochem.) 151 (1987) 191. M. Sone, H. Yoshimizu, H. Kurosu and I. Ando, J. Mol. Structure 317 (1994) 111. M. Sone, H. Yoshimizu, H. Kurosu and I. Ando, J. Mol. Structure 301 (1993) 227. S. Ando, T. Yamanobe, I. Ando, A. Shoji, T. Ozaki, R. Tabeta and H. Saito, J. Am. Chem. Soc. 107 (1985) 7648. (a) A. Shoji, T. Ozaki, T. Fujito, K. Deguchi and I. Ando, Macromolecules 20 (1987) 2441; (b) A. Shoji, T. Ozaki, T. Fujito, K. Deguchi, S. Ando and I. Ando, Macromolecules 22 (1989) 2860; (c) A. Shoji, T. Ozaki, T. Fujito, K. Deguchi, S. Ando and I. Ando, J. Am. Chem. Soc. 112 (1990) 4693. A. Shoji, S. Ando, S. Kuroki, I. Ando and G.A. Webb, Ann. Rept. NMR Spectrosc. 26 (1993) 349. A. Shoji, H. Kimura, T. Ozaki, H. Sugisawa and K. Deguchi, J. Am. Chem. Soc. 118 (1996) 7604. S. Kuroki, A. Takahashi, I. Ando, A. Shoji and T. Ozaki, J. Mol. Structure 323 (1994) 197. T. Akieda, H. Mimura, S. Kuroki, H. Kurosu and I. Ando, Macromolecules 25 (1992) 5794.
850
ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA
19. T. Yamanobe, M. Tsukahara, T. Komoto, J. Watanabe, I. Ando, I. Uematsu, K. Deguchi and M. Imanari, Macromolecules 21 (1988) 48. 20. E. Katoh, H. Kurosu, S. Kuroki and I. Ando, J. Mol. Structure 318 (1994) 123. 21. M. Okabe, T. Yamanobe, T. Komoto, J. Watanabe and I. Ando, J. Mol. Structure 213 (1989) 213. 22. E. Katoh and I. Ando, J. Mol. Structure 378 (1996) 225. 23. S.H. Zhao, S. Matsukawa, H. Kurosu and I. Ando, J. Mol. Structure, in press. 24. K. Tsuchiya, A. Takahashi, N. Takeda, N. Asakawa, S. Kuroki, I. Ando, A. Shoji and T. Ozaki, J. Mol. Structure 350 (1995) 233. 25. T. Kameda, N. Takeda, S. Kuroki, H. Kurosu, S. Ando, A. Shoji and T. Ozaki, J. Mol. Structure 384 (1996) 17. 26. J. Herzfeld and A. Berger, J. Chem. Phys. 73 (1980) 6021. 27. J. Fenzke, B. Maess and H. Pfeifer, J. Magn. Reson. 88 (1990) 172. 28. S. Vega, E.T. Olejniczak and R.G. Griffin, J. Chem. Phys. 80 (1984) 4832. 29. R.E. Stark, L.W. Jelinski, D.J. Ruben, D.A. Torcha and R.G. Griffin, J. Magn. Reson. 55 (1983) 266. 30. N. Asakawa, S. Kuroki, H. Kurosu, I. Ando, A. Shoji and T. Ozaki, J. Am. Chem. Soc. 114 (1992) 3261. 31. T.G. Oas, C.J. Hartzell, T.J. McMahon, G.P. Drobny and F.W. Dahlquist, J. Am. Chem. Soc. 109 (1987) 5956. 32. G.A. Webb and M. Witanowski, Proc. Indian Acad. Sci. (Chem. Sci.) 94 (1985) 241. 33. G. Harbison, J. Herzfeld and R.J. Griffin, J. Am. Chem. Soc. 103 (1981) 4752. 34. T.A. Cross, J.A. Diverdi and S.T. Opella, J. Am. Chem. Soc. 104 (1982) 1759. 35. T.A. Cross, M.H. Fray and S.T. Opella, J. Am. Chem. Soc. 105 (1983) 7471. 36. H.G. Forster, D. Muller and H.R. Kricheldorf, Int. J. Biol. Macromol. 5 (1983) 101. 37. T.H. Huang, W.W. Achovchin, R.G. Griffin and C.M. Dobson, Biochemistry 23 (1984) 5933. 38. E.O. Stejeskal, J. Schaefer and R.A. MaKay, J. Magn. Reson. 57 (1984) 471. 39. C.N. Matthews, R. Ludicky, J. Schaefer, E.O. Stejeskal and R.A. MaKay, Origins Life 14 (1984) 243. 40. T.A. Cross and S.T. Opella, J. Mol. Biol. 182 (1985) 367. 41. B.A. Choi, J.E. Roberts, J.N.S. Evans and M.F. Roberts, Biochemistry 25 (1986) 557. 42. J. Schaefer, J.R. Garbow, G.E. Jacob, T.M. Forest and G.E. Willsin, Jr., Biochem. Biophys. Res. Commun. 137 (1986) 736. 43. P.L. Stewart, K.G. Valentine and S.J. Opella, J. Magn. Reson. 71 (1987) 45. 44. D.W. Boykin (Ed), ~70 NMR Spectroscopy in Organic Chemistry. CRC Press, Boca Raton, FL, 1991. 45. A.L. Baumstark and D.W. Boykin, ~70 NMR Spectroscopy: Applications to Structural Problems in Organic Chemistry, in A.L. Baumstard (Ed), Advances in Oxygenated Processes, Vol. 3, JAI Press, 1991, pp. 141-176. 46. W.G. Klemperer, in J.B. Lambert and F.G. Riddell (Eds), The Multinuclear Approach to NMR Spectroscopy, Dordrecht, Holland, 1983, pp. 245-260. 47. J.P. Kintzinger, in P. Laszlo (Ed), Newly Accessible Nuclei, Academic Press, Vol. 2, New York, 1983, pp. 79-104. 48. D.W. Boykin and A.L. Baumstark, New Journal of Chemistry 16 (1992) 357. 49. D.W. Boykin and A. Kumar, J. Heterocyclic Chem. 29 (1992) 1. 50. S. Kuroki, I. Ando, A. Shoji and T. Ozaki, J. Chem. Soc., Chem. Commun., Issue 5 (1992) 433.
POLYPEPTIDES
851
51. S. Ando, T. Yamanobe, I. Ando, A. Shoji, T. Ozaki, R. Tabeta and H. Saito, J. Am. Chem. Soc. 107 (1985) 7648. 52. T. Kameda and I. Ando, J. Mol. Structure 412 (1997) 197. 53. F.A. Momany, R.F. MacGuire, A.W. Burgess and H.A. Sheraga, J. Phys. Chem. 79 (1975) 2361. 54. S. Arnott and S.D. Dover, J. Mol. Biol. 30 (1967) 209. 55. S.N. Rao and R. Parthasarathy, Acta Cryst. B29 (1973) 2379.
This Page Intentionally Left Blank
Chapter 23
Proteins Tetsuo Asakura 1, Makoto Demura 1, Naoki Nishikawa 1 and Hiroaki Yoshimizu 2 1Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan; 2Department of Materials Engineering, Nagoya Institute of Technology, Gokiso, Shouwa-ku, Nagoya, Japan
23.1
Introduction
Proteins are a class of biopolymers that participate essentially in all aspects of a living system, and have been the focus of extensive research aimed toward elucidating structure-function relationships. Many three-dimensional structures are known today. The primary method for high resolution structure determination has been X-ray diffraction from single crystals, which provided details of the molecular architecture of numerous proteins. However, the application of X-ray crystallography has been restricted to proteins from which well-ordered crystals can be grown. This difficulty excluded a large number of biologically relevant proteins, and has been the impetus for the development of new techniques for determining atomic resolution structures. Fibrous proteins are particularly difficult to study using standard structure determination techniques. X-ray diffraction from fibers, in which proteins are aligned along the long axis of the fiber, typically yields general features of molecular organization and packing, but lacks atomic resolution details. High resolution solution NMR techniques are not applicable to fibrous proteins, since the solution state structure is generally not representative of the structure in the fibrous state. However, the natural alignment of the protein within the fiber provides an important advantage that can be utilized by solidstate NMR. Solid-state NMR has been applied to study the structure of highly oriented DNA fibers [1-4], as well as a number of highly oriented protein systems [6-14]. More recently, Bombyx mori silk [14-18] and dragline spider silk [19, 20] have been studied using solid-state NMR. In this section, solid-state NMR of fibrous proteins is reviewed. For solidstate NMR studies of oriented samples, orientation-dependent nuclear spin interaction tensors serve as probes with which the relative orientations of specific bond vectors can be determined as described in Chapter 8.
TETSUOASAKURAET AL.
854
Hibernation
iapausee g g ( ~ - -11:~41~,,~t,
. \
"
(M IJ
: arching
rearing
~
... ls,, ,ns',ar
Moth~'~~..jJNon~deaPause 3---4daysJ,1st molting ~/i~.'~.~ 1 / g 2ridin~arys~ . ~"jl:-"A"" g g ~E ~ I / 3rd instal: ~ ~ " 2nd molting Eclosion[10 --15~ays ~athz.~3" 4days ~ 3rd molting
"Uc acoonS
/~J pupation~ 5da 4-
~
5-
6days ~ 4th molting
~ s I~ t u6-8da"s r /e .....
Fig. 23.1.
23.2
5th instar
larva
Spinning
Lifecycleof B. mori.
Bombyx mori silk
The life cycle of B. mori is summarized in Fig. 23.1 [21]. In about 50 days it completes its life cycle of four different metamorphosing phases; egg or embryo, larva, pupa and adult (moth). Of the life cycle, about half is the larval stage, the only stage at which they consume food, mulberry leaves. Pupation occurs at the end of spinning (or cocoon formation); the latter takes 3-4 days. Thus, silkworm silk is produced primarily at one stage in the life cycle, during the fifth larval instar just before the molt to the pupa. The silk from each cocoon comprises a single thread ranging between 10 and 25 ~zm in diameter, and between 300 and 1200 m in length. Silks are of interest in textiles and other material applications due to their visual appearance, texture (or "feel"), environmental stability and their unique mechanical properties [21, 22]. Silk fibroin from B. mori silkworm is a fibrous protein whose primary structure largely consists of a repeating sequence of six residues, (Gly-AlaGly-Ala-Gly-Ser), [23-27]. Details of the primary structure are described in Ref. [27]. Two crystalline forms, Silks I and II, have been reported as the dimorphs of silk fibroin from B. mori in the solid state on the basis of X-ray
PROTEINS
855
fiber diffraction [28-35], electron diffraction [36], conformational energy calculations [37, 38], infrared [34, 39-41] and 13C and 15N CP/MAS NMR spectroscopies [42-48]. In 1955, Marsh et al. [30] reported a fiber diffraction study on native B. mori silk fibroin in which the antiparallel/g-sheet model was first proposed for the silk II structure although their proposed structure was based on the quantitative intensity estimation of only six equatorial reflections, and did not yield good agreement between the observed and calculated structure factors (discrepancy factor R = 37%). The structural model was revised slightly by Fraser et al. [26, 31]. Takahashi et al. [35] reported a more detailed X-ray fiber diffraction analysis of the silk II structure of B. mori silk fibroin fiber with 35 quantitative intensities observed. They considered four kinds of/3-sheet models, differing from one another in the mutual orientation of adjacent hydrogen-bonded polypeptide chains (parallel vs. antiparallel), and in the position of side chains on neighboring strands relative to the plane of the sheet (polar vs. antipolar). In this terminology, Marsh's model is of the polar-antiparallel type. Based on the R factor obtained for the four types of models, the antipolar-antiparallel /3-sheet model was proposed. Fossey et al. [38] reproduced the antiparallel/3-sheets that constitute the silk II structure from conformational energy computations on stacked sheet structures of poly(L-Ala-Gly). In this section, solid-state NMR analysis is applied to determine the torsion angles of the amino acid residues of silk fibroin fiber. The less stable form, silk I, has remained poorly understood [37]. In general, attempts to induce orientation of its polymer chain tend to cause the silk I to convert to the more stable silk II. If an oriented silk fibroin sample with silk I form can be obtained, the solid-state NMR analysis for an oriented molecular system described here may be used to determine the atomic coordinates of silk fibroin with the silk I form. 23.2.1
13C, 15N and 2H labeling
of B. mori silk fibroin
Selective isotope labeling of silk fibroin is required for these NMR experiments to get site-specific structural information. The labeling is achieved biosynthetically through the use of an artificial diet supplemented with the isotope labeled amino acids during the fifth instar larval stage [34, 49-54]. Figure 23.2 shows typical [2-~3C]Gly incorporation into B. mori silk fibroin, which was monitored using in vivo NMR of the silkworm larva [54]. Metabolic flux of [2-13C]Gly to silk fibroin was calculated with NMR peak intensities. The labeled silk fibroin samples are obtained from the cocoon and used for NMR study after removing another silk protein, silk sericin coating the silk fibroin. The method for removal of the silk sericin, i.e., the degumming process, is described elsewhere [34, 43, 46, 49]. Figure 23.3 shows an ex-
856
TETSUO ASAKURA ET AL.
(A)
(e) .-~1.0 gl = A c "~m0-5
Z
0
,
~
,
u
....
O
0 1 2 3 4 5 6 7 Time(hour)
Fig. 23.2. [2-13C]Gly incorporation into B. mori silk fibroin, which was monitored using in vivo NMR of the silkworm larva.
panded carbonyl region of the solution 13C NMR spectra of natural abundance silk fibroin, and [1-13C]Ala and [1-13C]Gly labeled silk fibroins [49, 55]. The assignment is described in detail in Ref. [49]. The labeling ratio was high for both isotope labeled silk fibroin samples and is suitable for solidstate NMR experiments. In the ISN labeling, a [15N]Gly labeled silk fibroin sample with a sufficiently high labeling ratio was also obtained [51]. However, the labeling ratio was low for [15N]Ala labeling of the sample. This is due to the high activity of transaminases in silkworms as follows [56]. Ala + a-ketoglutaric acid ~ pyruvic acid + glutamic acid Ala + oxaloacetic acid ~ aspartic acid + pyruvic acid Although glutamic acid or aspartic acid is added to the artificial diet containing [15N]Ala, in order to avoid the transamination from Ala to these amino acids, the [15N]Ala labeling of the sample was still low [18]. Thus, another isotope labeling method is required.
PROTEINS
rU
857
u
C
No. H,O
S,k
Natural abundance I
t ,
i
,
I I:,I
;: 9
I I , :
e
,
, ,,
I I
,
,
w
,(I
,
SdkFabrom I.
._l
....
177
,jo..l
176
, "',,'
I
I
I
~
i
I
I o
I
i
I I
I a
I
I I
"'' ; :!
....
;
I
I
I I
,,
I
, ....
l ....
175
I
'.
I I
,
: .
....
I ....
173
, ....
...13 L"
. l ....
,... , ....
I ~
II;II
I
I
.
I
,
I
', ', ,,
: .,
,
i
,
'
174
I~ :
:
e I
,.~..I..
I
'
,
I I -' ' ,
I I , , , ,,,, l
'
I
I I
, ,.,,,
,.
',
,
I li!,a'
,
!
.~1
.
.I'i
,l
,'
H20
,,. , { I J ~ C l G i v , ", )nHzO .
'1 _1
.
,I
i' ,o
.
I
I
', ", n
'I
_
I
I
9
CJAla
I' ~ ,:',',,: tl ', 'i,,:, ; '.,,,,:,
'.
,
I I
I ,=
'
,
illl
,
"
,
I
.
I
13
L, I ....
j ....
l _
172 171 170 p p m f r o m ext. T M S
Fig. 23.3. Expanded carbonyl region of the solution 13C NMR spectra of natural abundance silk fibroin, and [1-~3C]Ala and [1-13C]Gly-labeled silk fibroins.
Cells of the silk gland from B. mori silkworm synthesize an enormous quantity of silk proteins, fibroin and sericin, during a brief period in larval development without cell division. Silk fibroin is exclusively synthesized in the posterior silk gland (each cell produces 1015 fibroin molecules, for example, about 80 mg during a period of only 3 to 4 days fifth instar larva) [57]. Therefore, cultivation of the silkgland [21] was attempted to prepare [15N]Ala silk fibroin with a high isotope labeling ratio. The rotation culture of the silk gland [62] was performed by adding [15N] labeled amino acid to Grace's insect medium [58], which is currently used as a medium for insect cell culture [59-64]. The 15N enrichment of the Ala residue in silk fibroin was 20 times above background as shown in Fig. 23.4, which was sufficiently high for solid-state 15N NMR experiments [18]. Similarly, [15N]Tyr, [15N]Ser and [15N]Val labeled silk fibroins, as well as the [15N]Gly labeled sample with a high labeling ratio, were obtained by culture of the silk gland [62, 74]. 2H labeled silk fibroin samples were prepared through the use of an
858
TETSUO A S A K U R A ET AL.
ppm f r o m ext. NH4NO3 ....
110
I ' " "
....
100
t ' " "
V i
....
90
80
I ' ' " ' " ' " 1 " - ' '
"'
i co
<~
Fig. 23.4. 15N enrichment of the Ala residue in B. mori silk fibroin.
artificial diet supplemented with the isotope labeled amino acid during the fifth instar larval stage again. Interestingly, a highly 2H labeled sample of the methyl group of the Ala residue is obtained only by 2H20 administration. The possible metabolic pathway of the incorporation of 2H20 to silk fibroin is summarized in Fig. 23.5 where the incorporation occurs between fumarate and malate in the TCA cycle [65]. 23.2.2 13C--15Ndipole-dipole interaction If 13C--15Ndipolar splitting can be observed for oriented fiber samples placed parallel to the magnetic field, the angle between the 15C--15Nbond and the oriented fiber axis can be directly obtained as described in Chapter 4. For this purpose, it is necessary to prepare a highly isotope double labeled [113C]Gly-[15N]Ala silk fibroin fiber. Cultivation of the silk glands is tried in the presence of both [1-13C]Gly and [15N]AIa in the medium. Figure 23.6 shows the Ala region of the 15N solution NMR spectra of the natural abundance B. mori silk fibroin and the [1-13C]Gly-[15N]Ala double labeled silk fibroin in aqueous solution. The [1-13C]Gly-[~SN]Ala direct coupling of 15 Hz [66] is observed clearly along with the [1-12C]-[15N]Ala peak
PROTEINS
859 Glutamic
ATP
C02H I
acid
C=O C-OPO3 H3 II 2H CH PEP ~k
vPA Kinase
~
It
+c%
. . . .
UNA
iiI~
,ill II
2HOCHCOOH
NAD+
I
~
~jjjt,
L-Malate
IIIIIIIIii1,.
'V
~
"H OCHCOOH
jr
~1 2H
III,
~|11, 'l~t, I.
CH 2HCOOH
. IIIIIIIIIIIIIIIllllllllllilllllll illullllll ,~pu UlI
,,,,,'"'"'
II,
111111
L-Ala
2H+
~ .- NADP+
"~
H
H2NC.H I 2H CH2
~
Malic enzyme
NADH +2H+
I H-CO 2H
COOH
J.
\[~NADPH +
PEP carboxylase O=C-CO2H
acid
I -.i 2H Ct_l "-"'2 Pyruvate
GDP
I~GTP
ot-Ketoglutaric
~
'"'"',,,,,. IIII |
I
CH 2HCOOH
III 1 Mitochondria
CHCOOH II HOOCCH
~'",t,-
Fumarate
',illi
r
,lJ
,111 ,i ItI ,,0~ 1'
_., ~at~
iii IIj"
'111111111111111111111111111111111111111111111111111111111111 IIIIm'l
Fig. 23.5. Metabolic pathway of the incorporation of 2H20 to Ala-Cfl2H of silk fibroin.
in the latter spectrum. Figure 23.7 shows the 15N solid-state NMR spectra of (e) a block of [1-13C]Gly-[15N]Ala silk fibroin rods whose macroscopic fiber axes were placed parallel to Bo, along with the [15N]Ala powder (d) and a block of single labeled [15N]Ala silk fibroin rods (b), whose fiber axes were also placed parallel to Bo. In the spectrum (e), the peak at about 20 ppm is assigned to free [15N]Ala amino acid which is involved in the silk fibroin rods. It is difficult to remove the [15N]Ala by only washing the silk gland after cultivation in water. By subtracting the spectrum (d) from the spectrum (e), the spectrum (c) without [15N]Ala peak is easily obtained. The spectrum (c) consists of a peak of [15N]Ala single labeled silk fibroin (center) and two peaks due to [1-13C]Gly-[15N]Ala dipolar splitting. By subtraction of the
860
T E T S U O A S A K U R A ET AL.
110 9
I
ppm from N_H4NO3
105 "
"'"
"
"~
I
"
.
100
.
|
"
.
Natural abundance
I
Cultured with both
[1-13C]Gly and [15NlAla
t2C~
II
O
!/
15 ~
!
~
15
!!
0
Fig. 23.6. Ala region of the 15N solution N M R spectra of the natural abundance B. mori silk fibroin and the [1-13C]Gly-[15N]Ala double labeled silk fibroin in aqueous solution.
spectrum (b) from the spectrum (c) and by taking into account the 15C--15N double labeling ratio, the dipolar splitting is determined as AVobs= 1 . 0 8 - 0.08 kHz (spectrum (a)). The observed splitting directly reflects the angle, qyc, between the 15N--13C peptide bond and the macroscopic fiber axis. The four possible angles were 39, 141, 76 and 104 ~. Among these angles, 141 ~ is in agreement with the values of qyc reported from X-ray diffraction analysis (0NC = 139.5 ~ by Marsh et al. [30], 140 ~ by Takahashi et al. [35]). Although it is difficult to determine the structure of B. mori silk fibroin from only these dipolar coupling data, the number of unique orientations possible for a given site can be reduced as will be described below.
23.2.3
Angular dependent solid-state 13C and 15N NMR
In this section, the solid-state NMR method described in section 4 is applied to determine the torsion angles, 4) and $, of the Ala and Gly residues in B.
PROTEINS
FIBER AXISI
II Bo
13
-IAI-
c.
861
is
cou,,n0
,_.I
vobs=l.08
kHz
(b) (c) = (e)
(d)
(e) I
200
I
.-
!
i
I
_._....i
100 0 ppm from ext. NI-I~NOj
Fig. 23.7. 15N solid-state NMR spectra of the block of [1-13C]Gly-[15N]Ala silk fibroin rods (e) whose macroscopic fiber axes were placed parallel to Bo. The spectra of [15N]AIa powder (d) and the block of single labeled [15N]Ala silk fibroin rods (b), whose fiber axes are also placed parallel to Bo are also shown. The spectra (a) and (c) are difference spectra, (c) = (e)-(d) and (a) = ( c ) - ( b ) .
862
TETSUO ASAKURA ET AL.
mori silk fibroin. Biosynthetic labeling was employed to incorporate [15N]Ala, [15N]Gly, [1-13C]Ala and [1-13C]Gly into silk fibroin. In addition, several 15C~15N double labeled peptides synthesized as models of local structure of silk fibroin were used in order to observe the 15C~15N dipolar modulated powder pattern [14, 67]. Figure 23.8 shows the solid-state 13C NMR spectra of an oriented block of [1-13C]Gly silk fibroin fibers as a function of the angles between the oriented fiber axis and Bo [68]. Spectral simulations were performed according to the similar manner of angular dependent 15N solid-state NMR analysis described in Chapter 4. The agreement between the simulated and observed spectra is good, indicating the high reliability of the structural parameters. The Euler angles, a F and fie for the carbonyl carbon site of Gly are obtained from the simulation. The final values of the Euler angles, C~Fand fiE for the 13C carbonyl carbons and lSN nuclei of the Gly and Ala residues are summarized in Table 23.1 with the 13C and 15N chemical shielding tensors determined from the simulations of the powder pattern spectra. Next, the Euler angles, ai~ and /3D (see Equation (4.13)) are determined with two double labeled peptides: Boc-Gly-Ala- [1-13C]Gly- [15N]Ala-Gly-Ala-OPac, B oc-Ala- Gly- [1-13C]Ala-[ 15N]Gly-Ala- Gly- OPac. For example, the 13C carbonyl carbon powder pattern of [1-13C]Gly silk fibroin is modified by dipolar interaction with the 15N nucleus of the Ala residue bondeddirectly in the peptide. From a simulation which takes into account such a dipolar interaction, aDCN and /3DCN values for the carbonyl carbon are determined. The results are also listed in Table 23.1 together with the values for other sites. By the combination of aF and j~F with aDNX and/3I~NX for the 15N nucleus, the angle of the bond orientation, 0NH between the NH bond and the fiber axis, and 0NC between the NC' bond and the fiber axis, can be determined for the Gly and Ala residues according to Equation (4.15). Similarly, the angle, 0co between the C ' = O bond and the fiber axis, and 0CN between the C'N bond and the fiber axis can be determined for the 13C carbonyl carbons of the Gly and Ala residues. The values for [15N]Ala site are summarized in Table 23.2. The error in these 0 values is evaluated to be less than 5~ by taking into account the experimental error in the simulation, OF- +--5~ /3F = -+2~ /3DX = --+2~ There is a total of eight possible sets of pairs of av and /3F values. A comparison of the 0NI4 angles calculated from the combination of aF, flF, aDNH and/~DNH for the 15N site and those determined from 15N~IH dipolar coupling data can be used to select among the possible sets [14]. In
PROTEINS
863
BO
v
~
6
300
200
100
0
ppm from TMS Fig. 23.8. Solid-state 13C NMR spectra of an oriented block of [1-13C]Gly silk fibroin fibers as a function of the angles between the oriented fiber axis and Bo.
addition, on the basis of the fiber period along the c axis (fiber axis) observed from X-ray fiber diffraction [30, 35] and conformational energy calculations [38], suggesting a two residue repeat unit along the peptide chain direction, i.e., an orientation of Cc~-helixc~(i- 1)-Cc~-helixc~(i + 1) parallel to the fiber axis, unique bond orientations can be selected from the calculated bond
864
TETSUO ASAKURA ET AL.
Table 23.1. Euler angles of the 13C and lSN CSA principle axis system (PAS) relative to fiber axis system (FAS) for [1-13C]Gly, [1-13C]Ala, [lSN]Gly and [lSN]Ala sites of B. mori silk fibroin samples and that relative to molecular symmetry axis (MSA) for [1-13C]-[15N]double labeled sites of Boc-Gly-Ala- [1-13C]Gly-[XSN]Ala-Gly-Ala-OPac and Boc-Ala-Gly-[1-13C]Ala[lSN]Gly-Ala-Gly-OPac
Euler angle
Gly/deg.
Ala/deg.
15N CSA 25 72
aV
/3F
2 70
aDN C ~DNC
0
0
104
109
aDN H ~DNH
0 18
0 13
13C C S A 0~F j~F
0
6
152
165
aDC N
0
0
~DCN
35
33
O~DCO flDCO
0
0
89
91
Experimental error of each Euler angle was +5 deg.
Table 23.2. The Euler angles, aV and/3F and bond orientation data, 0NH and 0NC for [15N]Gly sites of silk fibroins with silk II form
aV
r
ONH
~c
2 2 178 178 182 182 358 358
70 110 70 110 70 110 70 110
83 123 57 97 57 97 83 123
39 2 178 141 178 141 39 2
[15N]Ala CSA
orientation (Fig. 8.6). Within the conformational range, - 180 ~ > ~b > 0 ~ and 0 ~ qJ > 180 ~ which includes/J-sheet structure, the combinations of allowed 0 values are selected from Table 23.2 The C ' ~ N bond orientation of the oriented B. mori silk fibroin fiber is also obtained from the 1 3 C ' - - 1 5 N dipolar splitting of [1-13C]Gly-[15N]Ala double labeled sample using solid-state 15N N M R [48]. The reasonable orientations are 39 or 141 ~ for the 0NC of Ala residue. These angles are in agreement with the angle, 0NC for Ala residue
PROTEINS
865
selected from Fig. 8.6, indicating a high accuracy in the selection process of the angle, 0 in Section 8. The combination of the bond orientations, 0NC, 0NH, 0CN, 0CO for Ala is shown in Fig. 8.7. The width of each line corresponds to the experimental error of -+5~ for each 0 value. There is only one overlapping area which satisfies all of the 0 constraints within experimental error as shown in Fig. 8.7 this is considered to be the/3-sheet structure. Using a least-squared fitting of 0 values, the best fitted torsion angles (4~, qJ) of the Ala residue for this region A are obtained as (-140, 142~ The Gly C a - h e l i x a ( i - 1)-Gly Cahelixa(i + 1) distance, which corresponds to a unit cell length along the c axis (fiber axis) is calculated to be 6.98 A by using the best fitted torsion angles (4), qJ) of Ala determined here, indicating a good agreement with the X-ray fiber diffraction data [30, 35]. A similar combination for the Gly site is obtained. Fujiwara et al. [69] applied solid-state NMR to a structural study on oriented [1-13C]AIa silk fibroin fiber from B. mori. They found that the Euler angles obtained from the simulated lineshapes of the Ala carbonyl group are slightly different from that of a typical anti parallel /3-sheet, [30] raising questions concerning the accuracy of the current models for silk II structure. However, our data are in agreement with the X-ray diffraction model within experimental error. As mentioned above, [15N]Ser, [15N]Tyr and [15N]Val B. mori silk fibroin were obtained by cultivation of the silk gland. The torsion angles of these residues have not been reported because these minor amino acid residues give basically no X-ray diffraction data. The solid-state NMR analysis described here is the only method for obtaining the torsion angles [64]. On the other hand, the R E D O R (Rotational Echo Double Resonance) technique for the detection of weak heteronuclear dipole interactions such as those due to the 13C and ~SN nuclei [70, 71], has been applied to the structure determination of a silk fibroin model compound [17]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required. It is possible to determine the dihedral angles using the atomic distances obtained from R E D O R experiments. 23.2.4
Angular dependent solid-state 2H NMR
In order to use solid-state 2H NMR for atomic coordinate determinations, the angle of the C2H bond vector relative to the fiber axis was determined for [2,2-2H2]Gly and [3,3,3-2H3]Ala labeled silk fibroin fibers from B. mori with 2H quadrupole echo NMR spectroscopy [65]. This structural information
866
TETSUO ASAKURA ET AL.
160
"
0
KHz
'
" -100
'
Fig. 23.9. (A) 2H quadrupole echo spectrum of (A) an oriented block and (B) unaligned silk fibroin fiber of [2,2-2Hz]Gly-labeled samples.
is used complementarily for the determination of the backbone chain conformation of the Gly and Ala residues obtained from solid-state 15N and 13C NMR studies. The 2H labeled silk fibroin samples are prepared by the oral administration of isotope labeled amino acids or 2H20 to fifth instar larvae as described above. Figure 23.9(A) shows the 2H quadrupole echo spectrum of an oriented block of [2,2-2H2]Gly labeled silk fibroin fibers when the fiber axis was set parallel to the magnetic field direction. The 2H quadrupole echo spectrum, Fig. 23.9(B), of unaligned [2,2-2H2]Gly labeled silk fibroin fiber is also observed as a powder pattern. Both spectra are split into doublets, which give the value of the quadrupole splitting, Avo as 117.8 kHz for the [2,2-2H2]Gly site; the values are the same in each case. The full rigid-lattice width of about 126 kHz should be observed when the motion is frozen [72]. Thus, it is concluded that the motion of the methylene groups of the Gly residue is almost frozen at room temperature, which is in agreement with the prediction from the intermolecular hydrogen bonding network in the silk fibroin backbone chain with an antiparallel/3-sheet conformation. The quadrupole splitting, Avo of 117.8kHz observed for the oriented block of [2,2-2H2]Gly labeled silk fibroin fiber is the same as the value in the spectrum of an unaligned sample as shown in Fig. 23.9(B). Therefore, if the latter quadrupole splitting is used to provide a value of the proportionality constant, (3/4)e2qo/h, which describes the relationship between the quadrupole splitting and the angle 0CD (the angle of the C2H bond vector of Gly relative to the fiber axis), 0CD is calculated as 90~ By taking into account an
PROTEINS
867
experimental error of +_0.2 kHz in the determination of the constant because of a slightly broader peak than the uniaxially aligned spectrum, the 0 values are calculated as 90 _+ 2 ~ 2H solid-state N M R spectra of an ordered block of [2,2-2H2]Gly labeled silk fibroin fibers were observed as a function of the angle between the fiber axis and Bo in order to check the validity of the angle, 0 - 90 ~ obtained here. A series of observed spectra is shown in Fig. 23.10 along with lineshape simulations assuming 0 = 90 ~ The details of the method of simulation are described elsewhere by Ulrich et al. [72, 73]. The agreement between the observed and simulated spectra is good. The solid-state 2H N M R spectrum of an oriented block of the [3,3,32H3]Ala labeled silk fibroin fiber was also observed. The quadrupole splitting, Avo, is 39.8 kHz and is the same as the value in the spectrum of the unaligned sample. This is the same as the case of the [2,2-2H2]Gly labeled silk fibroin fiber. The smaller value of the quadrupole splitting for the Ala site indicates the presence of three-fold fast rotation about the Cc~-C/3 axis [50, 72]. This
Experimental
Simulated
cx=90 200
100
0
KHz
-100
-200
200
100
0
-100
-200
KHz
Fig. 23.10. Experimental and calculated 2H solid-state NMR spectra of an ordered block of
[2,2-2H2]Gly-labeled silk fibroin fibers as a function of the angle between the fiber axis and Bo.
868
TETSUO ASAKURA ET AL.
Fig. 23.11. A restricted (4~, qJ) region for Gly in the Ramachandran map.
has also been supported by the observation of a spin-lattice relaxation time (T~) minimum at about - 7 0 to -80~ in the 90 MHz 1H pulsed NMR study of B. mori silk fibroin fiber [74]. For a rapidly spinning methyl group, the direction of the three individual bonds is time averaged, and the effective bond vector is that of the methyl rotor axis, that is, the C~C2H3bond axis relative to the fiber axis. The angle of the C c~-helixa~Cc~-helix/3 bond vector of Ala with respect to the fiber axis was calculated to be approximately 90 ~ if the quadrupole splitting observed for the unaligned [3,3,3-2H3]Ala labeled silk fibroin sample is used to calculate the constant (3/4)e2qo/h. A restricted (4~, 0) region for Gly in the Ramachandran map ( - 1 8 0 ~ 6 > 0~ 0 ~ 0 > 180~ was obtained as the overlap of each region obtained experimentally from the NH, NC, CN, and CO bond orientations as described above. The 2H quadrupole echo spectrum of the oriented [2,22H2]Gly labeled silk fibroin fiber yields an angle, 0 = 90 -- 2 ~ for the C2H2 bond vector of Gly residue relative to the fiber axis. A further narrow
PROTEINS
869
restricted (4~, ~) region for the Gly residue in the Ramachandran map is obtained as shown in Fig. 23.11. Here the width of each line denotes the experimental error of +5 ~ in the determination of the angle, q, for each bond vector. An overlapping region is obtained for the antiparallel/3-sheet region. Thus, the angle constraints from solid-state 2H NMR can be used effectively to narrow the allowed region obtained from previous solid-state 13C and lSN NMR studies. A similar result is obtained for the Ala residue.
23.3
Spider silk
Spider dragline silk is a remarkable biopolymer: its mechanical properties combine high tensile strength and high elasticity as well as B. mori silk [75]. The spider silk most investigated is the dragline forcibly silks from the neotropical golden silk spider Nephila clavipes. The molecular weight of dragline silk was determined to be of the order of 200-350 kDa [76]. The primary structure of the major ampullate gland protein (dragline) is still not known completely. The results of Lewis [77] suggest that N. clavipes dragline silk is composed from two different proteins designated as spidroin I and II. In an independent study Mello et al. [76] have confirmed the existence of spidroin I. The primary structure of spidroin I contains (Gly-Gly-X)m segments (where X = Gln, Ala, Tyr, Ser, or Leu and m = 3-6 if minor sequence errors are tolerated) and Alan segments (with n = 4-7). In a simplified view, silk may be looked at as a block copolymer with glycine-rich and alaninerich domains. Diffraction results [78, 79] clearly indicate that dragline silk is a heterogeneous material with "amorphous" and "crystalline" domains. It is generally accepted that the "crystalline" domains consist of protein segments that adopt an ordered /3-sheet conformation. Controversy has, however, arisen as to whether these /3-sheets are formed by the alanine-rich [77, 80] or the glycine-rich [79, 81] segments of the protein. In addition, the molecular origin of the exceptional mechanical properties of spider silk is unclear. The local structure of dragline silk from the spider Nephila madagascariensis has been investigated by two-dimensional spin-diffusion NMR [20]. In order to obtain 13C labeled samples, [1-~3C]Ala or [1-13C]Gly was used, and a resulting enrichment of 60 % was found in the silk. Two-dimensional spindiffusion NMR experiments show that the alanine-rich domains of the protein form/3-sheet structure in agreement with one-dimensional NMR results from a different species of the genus Nephila [80] but at variance with diffraction results. The microstructure of the glycine-rich domains was found to be ordered (Fig. 4.11). The simplest model that explains the experimental finding
870
TETSUO ASAKURA ET AL.
is a 31-helical structure. Random coil, planar fi-sheets, and a-helical conformations were not found in significant amounts in the glycine-rich domains. On the other hand, solid-state 2H NMR data from unoriented, oriented, and supercontracted fibers, indicate that the crystalline fraction of dragline silk consists of two types of alanine-rich regions, one that is highly oriented and one that is poorly oriented and less densely packed. A new model for the molecular-level structure of individual silk molecules and their arrangement in the fibers has been proposed [19].
23.4
Keratin
The keratins are fibrous proteins derived from human hair [82], wool, feathers, nails, etc., and chemical analyses indicate that no particular amino acid predominates but that there is a high content of polar residues, Cys, and Pro [83]. Wool keratin consists of several kinds of proteins and can be separated into three main fractions after reducing the disulfide bonds and protection of the resulting thiol groups with iodoacetic acid to form S-carboxymethyl kerateine (SCMK) [84, 85]. On the other hand, from the viewpoint of morphology, the native wool fiber consists of intermediate filaments (termed "microfibrils") composed of low-sulfur proteins which are embedded in a nonfilamentous matrix. The nonfilamentous matrix usually contains two classes of proteins; one is a high-sulfur protein and the other is a protein containing Gly and Tyr residues [83]. High resolution solid-state 13C NMR spectra of the wool keratin proteins were systematically measured to clarified their conformational features in the solid state [86-90]. In this stage, four kinds of SCMK samples extracted from wool [low-sulfur fractions (SCMKA), helix-rich fragments prepared from SCMKA by partial hydrolysis with c~chymotrypsin (SCMKA-hf), high-sulfur fractions (SCMKB), and high-GlyTyr fractions (HGT)] were used. The content of helix-forming amino acid residues such as Asp, Glu, Ala, and Leu increase but the helix-breaking amino acid residues such as Thr, Ser, Pro, Gly, and Cys decrease in the order of wool, SCMKA, and SCMKA-hf. On the other hand, SCMKB contains Thr, Ser, Pro, and Cys as its major components, and HGT contains Set, Gly, Tyr, and Phe as its major components. From these findings, it has been suggested that the conformational features of wool and the SCMKs may be different from each other. Figure 23.12 shows the ~3C CP/MAS TOSS NMR spectra of wool and four kinds of SCMKs. In Fig. 23.12, the spectral patterns of each sample are different from each other due to their amino acid compositions and higher order structures. Since the ~3C NMR chemical shift value of the main chain carbonyl carbon
PROTEINS
871
rj
tj rj ,~
<
Wool
A
o~
S
PPM i ' ' ''
200
Fig. 23.12. 13C C P / M A S
TOSS NMR
I'
150 spectra
'''
' ' ' I' 100 50 I"
of wool,
' ' ' I
SCMKA,
0 SCMKA-hf,
SCMKB,
and
HGT.
is strongly influenced by the conformation of the main chain [91, 92] it can be said that the lineshape of 13C signals in the carbonyl region for proteins varies with their conformations, therefore, it is very useful for the characterization of the main chain conformation of polypeptides and proteins in the solid state. The expanded 13C signals in the carbonyl region of wool and SCMKs are shown in Fig. 23.13. It can be seen that the carbonyl 13C signals of wool, SCMKA, and SCMKA-hf consist of two major peaks, but the relative intensity of the peak appearing at about 172 ppm decreases in the
872
TETSUO ASAKURA ET AL. o,-~
u
-~, .=
?
Wool
SCMKA i
.
i
SCMKA-h~ 0
sCMI B
i
i
i/ _.~~.
HGT i"'"i
....
I ....
I ....
I ....
PPM I ....
I
190 185 180 175 170 165 160
Fig. 23.13. The expanded 13C CP/MAS TOSS NMR spectra for the carbonyl carbon region
of wool, SCMKA, SCMKA-hf, SCMKB, and HGT. order of wool, SCMKA, and SCMKA-hf. The carbonyl 13C signals of SCMKB and H G T show that the peak appearing at about 172 ppm is an intense single peak, and in particular, that of SCMKB is asymmetric with a shoulder at about 176 ppm. From these findings, it can be said that the type and fraction of conformations present in each sample are different. In order to discuss the difference more quantitatively, the carbonyl 13C signal is decomposed to its consistent peaks by computer-fitting with Gaussian functions (Fig. 23.14) and the results for the main chain carbonyl carbons are summarized in Table 23.3 together the data of tropomyosin and carboxypeptidase A obtained by same procedure. The relative intensity of a low field peak at ca.
PROTEINS SCNKA f i l m
casted
from
873
aqu.sol.
OBSERVED SPECTRUN
....
THEORETICAL
DECOMPOSED
SPECTRUM ^ SPECTRUH /
e
-,-"
~
t__ o 9- "O
I
ffl
I
/ 190
I| /i I | / I
\/ t80
i
o
\
, t70
~
t60
t-50- (IX)")
Fig. 23.14. 13C N M R spectra for the carbonyl carbon region in SCMKA deconvoluted by computer fitting with Gaussian functions. The minor peak appeared at about 166 ppm comes from the N M R rotor.
176 ppm corresponds to the a-helix content because the peak comes from the main chain carbonyl carbons in the a-helix form. The a-helix content of carboxypeptidase A obtained here (=41%) is almost the same as that calculated from its 3D structure determined by X-ray diffraction [93] (=38%). Therefore, a quantitative evaluation of the secondary structure for polypeptides and proteins in the solid state by deconvolution of the carbonyl 13C signal is feasible. The a-helix content increases in the order of wool, SCMKA and SCMKA-hf. On the other hand, a high field peak at 172 ppm comes mainly from the main chain carbonyl carbons in the /3-sheet form. From conformational characterization on the basis of the above assignment, it can be said that the/3-sheet content is high form is rich in SCMKB and HGT. However, the halfwidth of the higher field peak (172.2ppm) of HGT is relatively broader than that of other samples. This implies that several kinds of conformations besides the a-helix and/3-sheet forms also exist in HGT. These findings were also confirmed from the peaks in other carbon regions. In the spectrum of SCMKB, the peaks assignable to the Thr and Ser Cacarbons in the/3-sheet form appear at 71.8 and 67.8 ppm, respectively. On the other hand, although the composition of Ser in H G T is almost the same as that in SCMKB, no peak appears at 65-68 ppm in the spectrum of HGT,
874
TETSUO ASAKURA ET AL.
Table 23.3. Observed 13C NMR chemical shifts, halfwidths and relative peak intensities of the main-chain carbonyl carbons in SCMKA, SCMKA-hf, SCMKA-am, HGT, a-tropomyosin and carboxypeptidase A a
13C chemical shift (ppm)
Halfwidth (ppm)
Relative intensity (%)
Upfield peaks SCMKA SCMKA-hf SCMKA-am SCMKB HGT a-tropomyosin Carboxypeptidase A
172.5 173.0 172.5 172.5 172.2 172.5 172.6
4.8 4.5 5.5 4.6 5.6 3.5 4.7
44b 35b 85b
Downfield peaks SCMKA SCMKA-hf SCMKA-am SCMKB HGT a-tropomyosin Carboxypeptidase
176.2 176.4 176.5 176.0 176.6 176.4 175.9
4.0 3.6 4.0 3.7 4.0 4.1 4.7
56 b
75 b
92b 16b 59 b
65 b 15 b 25 b
8b 84 b
41b
aDetermined by computer fitting. bFor only main-chain carbonyl carbons (see text). indicating that many of the Ser residues in H G T are not in the/3-sheet form. It has been reported that 13C peaks of the Ser Ca-helix/3 carbons of silk fibroin samples in the silk I form appear at 59.0-61.5 ppm [34, 44]. The peak appearing at 60.2 ppm in the spectrum of H G T may come from the Ser C a helix/3 carbons in a silk I form. Further, it was suggested that a coiled-coil structure exists in wool, S C M K A , and SCMKA-hf, because the 13C chemical shift values of Ala Ca-helix/3 carbons in these samples coincide within experimental error with those of the Ala residue located in the internal site of the coiled-coil structure in tropomyosin. This is consistent with the fact that typical coiled-coil heptad repeat sequence is found in S C M K A (especially in S C M K A - h f ) , as well as in tropomyosin [83]. This conformational analysis using conformation-dependent 13C C P / M A S N M R chemical shifts was applied to estimate conformational transitions of S C M K A by stretching, heating, or steam-treating [87-90]. It is confirmed that t h e / J - s h e e t form appears and the a-helix content decreases upon stretching and steam-treating, as observed for native wool fiber. For S C M K A heated at 200~ for 3 h in vacuo, the 13C C P / M A S N M R spectrum shows the peaks are broader than those of other treated samples. This indicates the existence
PROTEINS
875
of various conformations and/or different microenvironments in heat-treated SCMKA. Thus, it can be said that the random coil form appears on heating. From X-ray diffraction, the c~-helix form completely vanishes in SCMKA heated under the same conditions [94]. The result obtained by 13C CP/MAS N M R spectroscopy, however, indicates that the c~-helix form remains to an appreciable extent in the heat-treated sample, although that is an increase in random coil content. The difference between the X-ray diffraction and N M R spectroscopy results suggests that only the packing of the ordered structure (the c~-helix form) in the SCMKA is disrupted by heating, with the secondary structure being retained. Figure 23.15 shows the observed and simulated 13C CP/MAS N M R spectra for the Cc~-helixc~ methine carbon region in SCMKA films. A spectral simulation was performed by taking into account the experimental fact that the Simulated
:131% 100:0
Observed
_2 `4
.
unstretc
58:42
200% stretch
.~54:46 ! ,~..
.
.
7:53
300% stretch 70
50 PPM 0:100
70
50 PPM
Fig. 23.15. Observed and simulated 13C CP/MAS NMR spectra for the Ca methine carbons
region in SCMKA films prepared from an aqueous solution. The details are described in the text.
876
T E T S U O A S A K U R A ET AL.
13C chemical shift value of the Ca methine carbon is influenced by both the main-chain conformation and the sequence of amino acid residues in contrast to the 13C chemical shift behavior of the carbonyl carbons [91, 92]. A Gaussian lineshape for a peak with halfwidth 3.5 ppm were assumed, and the chemical shift positions of the amino acid residues constituting SCMKA determined from reference data of homopolypeptides are used. Further, the corresponding peak heights were estimated from the amino acid composition of SCMKA. Under these assumptions, the spectral simulation was carried out as follows; At first, two reference spectra were simulated, one is for the case in which all the amino acid residues have the c~-helix form, and the other is for the case in which all the amino acid residues have the/3-sheet form. Next, using the two reference spectra, spectra with any specified ratio of the c~-helix to /3-sheet content were simulated, where this ratio was determined from the carbonyl carbon 13C signal. As shown in Fig. 23.14, the increase in intensity with stretching ratio of an upfield peak appearing at about 52-3 ppm can be interpreted by the conformation-dependent 13C chemical shifts. 13C CP/MAS NMR spectra of horse hoof, horse hair, parrot feather, and human hair were reported by Kricheldorf and Muller [95]. The proportion of the/3-sheet form in parrot feather was larger than that in other samples. The dynamics and structure of mouse keratin intermediate filaments which were prepared by isotopic labeling were investigated by means of solid-state 13Cand 2H NMR spectroscopies. Recently, studies of epidermal keratin using NMR were carried out by Mack et al. [96]. The dynamic structure of mouse epidermal keratin intermediate filaments (KIF), which were labeled by both administration and tissue culture with isotopically enriched amino acids, was investigated by means of solid-state 13C and 2H NMR spectroscopies. It has been recently proved that KIF consists of a central rod domain, flanked by end domains. The structure of these two domains was predicted from the amino acid sequence, the former having a coiled-coil structure similarly to tropomyosin, and the latter having non-helical structure. In order to investigate the dynamic structure of end domains, 13C NMR experiments were performed on keratin IF labeled with [1-13C]Gly, as more than 90% of glycines in keratin IF have been located in the end domains. The dynamics were assessed using spectra which were observed by low-level broadband decoupling with WALTZ-16 and high-power coherent decoupling (Fig. 23.16). The observation of narrow, isotropic glycine carbonyl signals in spectra obtained with WALTZ-decoupling (A) suggested that there was a high degree of intrinsic dipolar decoupling of the glycine carbonyl carbons from neighboring protons. This follows because WALTZ decoupling is insufficient to
PROTEINS
i 200
I IO0
ppm
877
I 0
Fig. 2.3.16. 13C spectra at 25~ of KIF reassembled in vitro from native prekeratin labeled with [1-13C]Gly, acquired at 62.98 MHz. (A) Spectrum obtained under conditions of low-level WALTZ-16 decoupling with Overhauser enhancement. (B) Spectrum obtained with high-power coherent decoupling with NOE. About 200 Hz of line broadening was applied. Each spectrum is the average of 12600 acquisitions. Recycle times of 5 s were used.
average the short-range carbon-proton dipolar couplings of glycyl residues whose motions are highly restricted. The large static chemical shift anisotropy of the glycyl carbonyl carbon is also completely averaged by molecular motion. Furthermore, a high-power decoupled spectrum (B) revealed a slight increase compared with the WALTZ-decoupling spectrum (A) in carbonyl peak amplitude and line sharpening. This is evidence that molecular motion alone decouples the carbonyl carbon from neighboring protons. Therefore, the glycyl residues, which are almost exclusively located in the end domains, undergo a high degree of segmental motion. Measurement of relaxation times, T1 and T2, of labeled carbonyl carbons give average correlation times of about 2 ns. These results indicated that the end domains of KIF are remarkably flexible and have little or no structural order. To compare the backbone mobility of the rod domain with the end domains, keratin IF was labeled with L-[1-13C]leucine, as more than 90% of the
878
T E T S U O A S A K U R A ET AL. G
(D)
I 2,00
I 100
I 0
ppm
Fig. 23.17. 13C spectra at 25~ and 62.98 MHz of isolated KIF. (A) A WALTZ-16 decoupled spectrum of a sample incorporating [2-13C]Gly. The a-carbon resonance is at ca. 42 ppm. S and G mark the locations of the serine and glycine c~-carbon peaks. (B) A spectrum of the same sample taken under high-power coherent decoupling. (C) Spectrum of a sample incorporating L[1-13C]leucine, WALTZ decoupling. (D) A high-power decoupled spectrum of the same sample as in (C). The upfield aliphatic regions of spectra C and D serve essentially as natural abundance comparisons for the analogous regions of spectra A and B of the c~-labeled sample. Lorentzian broadening of 20 Hz is used in each spectrum. Each spectrum is the result of ca. 16,000 scans.
leucyl residues are located in the central rod d o m a i n . T h e signal intensities with W A L T Z and h i g h - p o w e r decoupling in the carbonyl c a r b o n region (Fig. 23.17(C, D ) ) are only slightly larger than natural a b u n d a n c e signals (Fig. 2 3 . 1 7 ( A , B)). In t h e o r y the leucine carbonyl c a r b o n should be ca. 15-fold e n r i c h e d , thus it is suggested that most (roughly 7 0 % ) of the labeled leucyl
PROTEINS
879
(e)
(
(g)
{d)~
~
(h|
I
!
!
-50
0 kHz
50
.
.
I
-5O
,
L
0 kHz
I
5O
Fig. 23.18. Comparison of 76.77 MHz 2H spectra of L-[2Hlo]leucine labeled KIF with lineshapes generated by a discrete two-site jump model. Spectra at temperatures (a) 26, (b) 5, (c) -23 and (d) -45~ each an average of 2500 scans. Simulations with two-site half-angle of 54.75~ between two equally populated equilibrium sites with "fast" and "slow" jump rate (kf and ks) assigned to two different classes of "molecules". (e) k f - 1 x 106, k s - - 1.65 x 104; (f) kf = 5 x 105, ks = 1.65 x 104; (g) kf = 6.5 • 104, ks = 5.73 x 103; (h) kf = ks = 3.8 • 103. A 1:1 ratio of slowly and rapidly jumping components is assumed in the simulations.
carbonyl signal is not detected. These result suggested that the majority of leucyl residues occupy regions of relatively low flexibility peptide b a c k b o n e . Subsequently, d e u t e r i u m N M R experiments were p e r f o r m e d on K I F labeled with L-[2Hlo]leucine. A series of 2H spectra r e c o r d e d at 26 to - 4 5 ~ Fig. 23.18, consists mainly of p o w d e r lineshapes rather than isotropic resonances. This result revealed a m a r k e d degree of leucyl residue rigidity within the coiled-coil, which confirmed the conclusion of the above 13C N M R experiment. M a n y of the spectra at higher t e m p e r a t u r e display a very sharp, narrow c o m p o n e n t which can be accounted for as arising from residual H D O in the
880
TETSUO ASAKURA ET AL.
interstitial fluid of the buffer and from the ca. 10% fraction of leucine side chains of KIF that are known to reside within the end domains. It provided an experimental proof of the fact that the motions of the peptide backbone in the end- and center-rod domains of the KIF are relatively high and low, respectively, consist with the conformations and higher order structures are predicted from the amino acid sequence. In a more advanced study, they investigated the dynamic structure of the macrofibril which is a the supermolecular organization of KIF within epidermal cell networks [97, 98]. It has been shown that formation of the macrofibril is mediated by interaction with KIF filaggrin, which is a relatively low molecular weight protein with apparent capability of binding numerous KIFs into tight fibrillar arrays. The homology of amino acid sequences of filaggrins were analyzed, and it was found that the consensus sequence motif comprised of a periodical tetrapeptide /3-turn block which bears unique charge and hydrophobicity profiles. Based upon these data, they proposed a model in which filaggrin interacts with KIF by the docking of the frequent polar and
Fig. 23.19. Schematic of the ionic zipper motif demonstrating the notion of the presentation of alternating positive and negative charges at apices of the frequent turns of filaggrin which dock with surface changes within the rod domains of abutting KIF.
PROTEINS
881
charged groups of its/3-turn structure with charged and polar residues within the coiled-coil rod domain of KIF. Mack et al. dubbed this an "ionic zipper" hypothesis (Fig. 23.19), and solid-state NMR experiments provided effective data to support this hypothesis. In order to study the interactions of side chains in the rod domains within the macrofibrils consisting of KIF/filaggrin, 2H NMR experiments were per-
Keratin IF
Macroflbrll$
(r)
(8)
(g)
(c)
(d)
(m)
(ti
(l)
__, -200
.
- I O0
;,
9
0
kHz
I
;, 2
.
-200
;
- I 0
+
.
,
0
I O0
200
kHZ
Fig. 23.20. A series of 2H NMR spectra taken at 76.77 MHz o f mouse KIF and mouse macrofibrils where, in both creases, the keratin chains have been labeled with L-[4,4,5,52H4]lysine. Each spectrum was acquired using the quadrupole-echo sequence and is an average of 50000 acquisitions. The temperatures at which these equilibrium spectra were taken are (a,f) 25, (b,g) -10, (c,h) -20, (d,i) -35, (e,j) -45~ Even at the lowest temperature, variations in the lineshapes indicate that the onset of the quenching of side chain motions is occurring more significantly for the lysines of the macrofibrils than for those of the KIF.
882
TETSUO ASAKURA ET AL.
formed on KIF labeled with L-[4,4,5,5-2H4]lysine, as more than 90% of lysine in KIF is located inthis domain. Macrofibril (KIF/filaggrin) was prepared in vitro by mixing KIF labeled with L-[4,4,5,5-2Ha]lysine with unlabeled filagrin in 1:1 molar ratio. Spectra of both KIF alone and the macrofibril, as a function of temperature, were shown in Fig. 23.20. The narrow component in the spectra at higher temperature was caused by residual H D O and the ca. 10% fraction of leucine residues existing in the end domains. The lineshape of both samples at low temperature was a static pattern characteristic of rigidly oriented C ~ 2 H bonds. However, the lineshapes obtained for the macrofibril sample were broader by 10-15% than those obtained for KIF alone at each temperature. The broader lineshape for the macrofibril suggests that the lysine side chain motions were damped in amplitude in comparison to those of KIF, especially at - 2 0 ~ was remarkable. The degree of restriction for the macrofibril occurred about 10~ higher. In addition, measurements of T1 and T2 were performed on both samples. The T1 values at 25~ were the same at 15 +_ 0.7 ms, and at -25~ the T1 value for KIF was essentially unchanged at 13 _ 0.4 ms, but the T1 value for the macrofibril was significantly increased to 28 +-- 1.4 ms. The T2 value was also increased for the macrofibril to 114.1 Ixs from 77.5 Ixs for KIF alone. These data indicated that the lysine side chains of the rod domain of associated KIF were more constrained than those in KIF alone. This implies that the macrofibril is formed by ionic interactions between KIFs and filaggrins, in agreement with the "ionic zipper" hypothesis.
23.5
Tropomyosin
Tropomyosin is a muscle protein, and has an unusual amino acid sequence consisting of seven residues, (a-b-c-d-e-f-g)~. There is the tendency for hydrophobic amino acid residues to be located at positions a and d in this sequence, and in many cases, the hydrophilic amino acid residues are in other positions (b, c, e, f and g). It has been suggested that this sequence forms coiled-coil a-helix ropes, which is one of the supersecondary structures of proteins. A schematic picture of the cross-section of the coiled-coil structure is shown in Fig. 23.21. In the coiled-coil structure, two different sites which are characterized as the internal hydrophobic site and the external hydrophilic site can be defined [99]. The 13C CP/MAS N M R spectra of tropomyosin in the solid state were measured, in order to elucidate the higher order structure of the protein through the observation of the 13C N M R chemical shifts of the amino acid residues, and their mobility [100-102]. A 13C CP/MAS N M R spectrum of
PROTEINS
883
External hydrophilic site e
b
Cl
f
Internal hydrophobic site Fig. 23.21. A schematic picture of the cross-section of the coiled-coil structure in tropomyosin.
tropomyosin from chicken-breast muscle is shown in Fig. 23.22. It confirms that the main conformation of tropomyosin is a-helical from the 13C NMR chemical shift of the main chain carbonyl carbons. Since Ala is one of the major components of tropomyosin, the 13Csignal which appears at 15-17 ppm
Side chain oliphatic carbons
C=0
,
,
Ca
I
'~L-Ala
J 150
100
50
0
6/PPM
Fig. 23.22. The typical ~3C CP/MAS NMR spectrum of tropomyosin at room temperature.
884
TETSUO ASAKURA ET AL.
C
|
,",',
21
L-AlaCp
9|
20
9,
9,
I"
19
"""
'
I
'
18
"
""
I""
17
'
"'"
I
. . . .
16
I
""
15
""
I"
14
"
~ "i
13
6/PPM
Fig. 23.23. The expanded 13C NMR spectra of the Ala C/3 carbon atoms of tropomyosin, measured using the inversion-recovery method at room temperature. Recovery time, (a) 10 ms;
(b) 500 ms; (c) 3500ms.
can be easily assigned to the Ala Ca-helix/3 carbons, and it can be seen that this signal consists of at least two peaks. Figure 23.23 shows the expanded Ala Cc~-helix/3 signal, measured using an inversion-recovery method. From these spectra, the carbons contributing to the peak at 15.8 ppm have a longer T1 value than those at 16.7ppm. This indicats that the former carbons are more mobile than the latter carbons, because the Ala Ca-helix/3 carbons in tropomyosin have correlation times in the extreme narrowing region at room temperature. The distance between the two a-helical axes in
PROTEINS
885
the coiled-coil structure is shorter than that for the coiled-coil helices which are packed in parallel in native muscle from X-ray studies [103]. Therefore, it can be said that the mobility of the Ala Ca-helix/3 carbons in the internal site is expected to be more restricted than that in the external site. From such a consideration, the two peaks at 15.8 and 16.7 ppm are assigned to the Ala Ca-helix/3 carbons in the external and internal sites of the coiled-coil structure, respectively. These findings suggest that the 13C NMR chemical shift values are useful for the estimation of high-order structures of proteins.
23.6 Amyloid protein Amyloid deposits are characteristic of many diseases including Alzheimer's disease (AD) [104, 105] and type II diabetest [106]. The deposits, or plaques, comprise protein fibrils which share affinity for certain dyes [107] and a regular, repeating structure [108]. The naturally derived amyloid plaque from AD brain has been analyzed by X-ray fiber diffraction and produces a distinctive pattern of reflections [109]. This diffraction was compared with a cross/3 sheet model which consists of an antiparallel peptide chain arranged perpendicular to the direction of fibril growth. However, single crystals suitable for crystallographic studies have not been produced. Recently, the solid-state NMR technique of rotational r e s o n a n c e (R 2) has been applied to the accurate measurement of carbon-to-carbon distances of amyloid formed from a synthetic fragment (HzN-Leu Met Val Gly Gly Val Val Ile Ala-COZH) of the amyloid-forming protein of AD [110]. This synthetic nine amino acid peptide (/334-42) represents the carboxy-terminal portion of the 42 amino acid/3/A4 protein. The/3/A4 protein is the major constituent of the extracelluar amyloid plaque that characterizes the brains of victims of AD and advanced Down's syndrome [111]. The 34-42 portion of the /3/A4 sequence is thought to be part of the transmembrane sequence in the amyloid precursor protein [104, 105] and is extremely insoluble and readily forms cross-/3 fibrils which can be observed by electron microscopy [112]. A soluble fragment of the protein has been studied and its structure compared to a mutant which arises in a related clinical condition [113]. The rotational r e s o n a n c e (R 2) determination of the distance between members of a homonuclear spin pair is based on rotation-enhanced transfer of Zeeman magnetization. This transfer occurs when the MAS frequency O) r is adjusted to satisfy the R 2 condition 6 = nWr, where 6 is the isotropic chemical shift difference between the two resonances of the spin pair [110], n is a small integer, and (.Or is the spinning speed. For carbon-carbon distances, appropriate samples may be engineered by selective pairwise 13C labeling.
886
TETSUO ASAKURA ET AL.
The R 2 technique has been applied to distinguish between two possible structures on the basis of a single distance measurement [111, 112]. Three kinds of 13C labeled/334-42 fragments (c~37-38, 37-c~38, 37-c~39) were synthesized in order to obtain distance constraints between two labeled sites within the same chain.
H2N-DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA-CO2H /334-42 a37-38; H2N-LMV[2-13C]G[1-13C]GVVIA-CO2H 37-a38; H2N-LMV[1-'3C]G[2-13C]GVVIA-CO2u 37-a39; H2N-LMV[1-13C]GG[2-13C]VVIA-CO2u Fig. 23.24 (top) shows a series of 13C MAS spectra of/334-42 labeled at the a carbon of Gly37 and the carbonyl carbon of Gly38 (c~37-38 distance).
ms
..,..__._,M~
I
0 ms
I
15
10
I
I
5
0
I
-5
Frequency (kHz)
1,0
A N
0,6
"~176176 ~e ~
-
.
"~
"~
"
V ~
0~
u
I
0.0
I
20.0
I
40.0
tlme (ms)
A series of 13C MAS spectra of/334-42 labeled at the a carbon of Gly37 and the carbonyl carbon of Gly38 (a37-38 distance). Fig. 23.24.
PROTEINS
887
The spectra were obtained at a 13C frequency of 100 M H z and with a MAS frequency of 12.487 kHz corresponding to the n = 1 rotational resonance; that is the spinning speed was equal to the difference in isotropic shifts between the labeled sites. The time shown denotes the interval between inversion of the labeled methylene resonance, which occurs immediately after cross polarization, and application of the observation pulse. Thus, 0 ms corresponds to the initial nonequilibrium state established by the selective inversion. The rate of this decrease may be modeled to determine the distance between the labeled sites (Fig. 23.24 (bottom)). Comparison of magnetization exchange data (filled circle) for a 3 7 - 3 8 along with the calculated curves for four distances" 4.74 A (dotted line), 4.0 A (dashed line), 3.9 A (solid line) and 3.8 A (dot-dashed line). The 4.74 A distance would be expected for an idealized antiparallel/3-strand. The best fit to the data is 3.9 A. This result is for the undiluted a 3 7 - 3 8 sample. To eliminate possible effects due to intermolecular interactions, measurements were performed on isotopically pure samples and on samples in which the doubly labeled peptide was diluted 1:5 and 1:10 in unlabeled peptide. This produced a corrected distance for a 3 7 - 3 8 of 4.0 A and in all cases, the distance could be defined to within 0.15 A or less. Experimental 37-c~38 = 2.4 ce37-38 = 4.0 3 7 - a 3 9 = 4.8
Results _+ 0.2/~ _+ 0.2/~ _+ 0.2 A~
/3-strand 2.45 A 4.74 ]k 5.77 A,
conf. C 2.49 ,/k 4.00 A 4.84 A,
conf. T 2.45 4.25 A, 4.80 A~
Possible conformations for Gly37-Gly38-Va139 were compared of an idealized /3-strand, conformations C (o937-38 = 0 ~ 4~38 = - 1 2 1 ~ q~38 = 25 ~ 4~39 = - 4 1 ~ 0937-38= 180 ~ and conformations T (o937-38= 180 ~ ~b38 = 0 ~ q~38 = - 1 3 4 ~ ~b39 = - 4 1 ~ o937-38 = 180~ respectively. The experimental distance constraints indicated that an unusual structure, probably involving a cis amide bond, is present in the aggregated peptide amyloid. This structure is incompatible with the accepted models of fibril structure.
References
1. T.M. Alam, J. Orban and G. Drobny, Biochem. 29 (1990) 9610. 2. H. Sindo, Y. Hiyama, S. Roy, J.S. Cohen and D.A. Torchia, Bull. Chem. Soc. Jpn. 60 (1987) 1631. 3. T.B. Nail, W.P. Rothwell, J.S. Waugh and A. Rupprecht, Biochem. 20 (1981) 1881. 4. H. Sindo, J.B. Wooten, B. Pheiffer and S.B. Zimmerman, Biochem. 19 (1980) 518.
888 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
TETSUO ASAKURA ET AL. Q. Teng, L.K. Nicholson and T.A. Cross, J. Mol. Biol. 218 (1991) 607. L.K. Nicholson and T.A. Cross, Biochem. 28 (1989) 9379. K.-J. Shon, Y. Kim, L.A. Colnago and S.J. Opella, Science 252 (1991) 1303. R.S. Prosser, J.H. Davis, F.W. Dahlquist and M.A. Lin-dorfer, Biochem. 30 (1991) 4687. B. Bechinger, Y. Kim, Y.L.E. Chirlian, J. Gesell, J.-M. Neumann, M. Montal, J. Tomich, M. Zasloff and S.J. Opella, J. Biomol. NMR 1 (1991) 167. A.W. Hing, S.P. Adams, D.F. Silbert and R.E. Norberg, Biochem. 29 (1990) 4144. B.A. Cornell, F. Separovic, A.J. Baldassi and R. Smith, Biophys. J. 53 (1988) 67. T.A. Cross and S.J. Opella, J. Mol. Biol. 182 (1985) 367. T.A. Cross and S.J. Opella, J. Am. Chem. Soc. 105 (1983) 306. L.K. Nicholson, T. Asakura, M. Demura and T.A. Cross, Biopolymers 33 (1993) 847. T. Asakura, J-H Yeo, M. Demura, T. Itoh, T. Fujito, M. Imanari, L.K. Nicholson and T.A. Cross, Macromolecules 26 (1993) 6660. M. Demura, Y. Yamazaki, T. Asakura and K. Ogawa, J. Mol. Struc., in press. T. Asakura, A. Aoki, M. Demura, J.M. Joers, R.C. Rosanske and T. Gullion, Polym. J. 26 (1994) 1405. T. Asakura, M. Demura, T. Date, N. Miyashita, K. Ogawa and M.P. Williamson, Biopolymers, in press. A.H. Simmons, C.A. Michal and L.W. Jelinski, Science 271 (1996) 84. J. Kummerlen, J.D. van Beek, F. Vollrath and B.H. Meier, Macromolecules 29 (1996) 2920. T. Asakura and D.L. Kaplan, in C.J. Arutzen (Ed), Encyclopedia of Agricultural Science, Vol. 4. Academic Press, London, 1994. D.L. Kaplan, S.J. Lombardi, W.S. Muller and S.A. Fossey. in D. Byrom (Ed), 1. Silk, in Biomaterials; Novel Materials from Biological Sources. Stockton, New York, 1991. D.J. Strydom, T. Haylett and R.H. Stead., Biochem. Biophys. Res. Commun. 3 (1977) 932. F. Lucas, J.T.B. Shaw and S.G. Smith, Biochem.J. 83 (1962) 164. T. Ohmachi and K. Shimura, J. Biochem. 89 (1981) 531. R.D.B. Fraser and T.P. MacRae, in Conformation in Fibrous Proteins. Academic Press, New York and London, 1973. K. Mita, S. Ichimura and T.C. James, J. Mol. Evol. 38 (1994) 583. M. Shimizu, Bull. Imp. Seric. Stn., Tokyo 10 (1941) 475. O. Kratky, Trans. Faraday Soc. 52 (1955) 58. R.E.Marsh, R.B. Corey and L. Pauling, Biochim. Biophys. Acta 16 (1955) 1. R.D.B. Fraser, T.P. MacRae and F.H. Stewart, J. Mol. Biol. 19 (1966) 580. K. Hirabayashi, M. Uchiyama, H. Ishikawa and Y. Go., Sen'i Gakkaishi 23 (1967) 538. T. Konishi and M. Kurokawa, Sen-i Gakkaishi 24 (1968) 550. T. Asakura, A. Kuzuhara, R. Tabeta and H. Saito, Macromolecules 18 (1985) 1841. Y. Takahashi, M. Gehoh and K. Yuzuriha, J. Polym. Sci., Polym. Phys. Ed. 29 (1991) 889. B. Lotz and H.D. Keith, J. Mol. Biol. 61 (1971) 201. B. Lotz and F.C. Cesari, Biochimie 61 (1971) 205. S.A. Fossey, G. Nemethy, K.D. Gibson and H.A. Scheraga, Biopolymers 31 (1991) 1529. J. Magoshi, M. Mizuide, Y. Magoshi, K. Takahashi., M. Kubo and S. Nakamura, J. Polym. Sci., Polym. Phys. Ed. 17 (1979) 515. T. Hayakawa, K. Kobo, S. Yamamoto and J. Noguchi, Kobunshi Kagaku 27 (1970) 300. T. Miyazawa, T. Shimanouchi and S. Mizushima., J. Chem. Phys. 29 (1958) 611.
PROTEINS
889
42. H. Saito, Y. Iwanaga, R. Tabeta, M. Narita and T. Asakura, Chem. Lett. (1983) 427. 43. H. Saito, R. Tabeta, T. Asakura, Y. Iwanaga, A. Shoji, T. Ozaki and I. Ando, Macromolecules 17 (1984) 1405. 44. T. Asakura and T. Yamaguchi, J. Seric. Sci., Jpn 56 (1987) 300. 45. H. Saito, M. Ishida, M. Yokoi and T. Asakura, Macromolecules 23 (1990) 83. 46. M. Ishida, T. Asakura, M. Yokoi and H. Saito, Macromolecules 23 (1990) 88. 47. M. Demura and T. Asakura, J. Membr. Sci. 59 (1991) 39. 48. M. Demura, M. Minami, T. Asakura and T.A. Cross, J. Am. Chem. Soc. 120 (1998) 1300. 49. T. Asakura, Y. Watanabe and T. Itoh, Macromolecules 17 (1984) 2421. 50. H. Saito, R. Tabeta, A. Kuzuhara and T. Asakura, Bull. Chem. Soc. Jpn. 59 (1986) 3383. 51. T. Asakura, H. Yoshimizu and F. Yoshizawa, Macromolecules 21 (1988) 2038. 52. T. Asakura, H. Suzuki and T. Tanaka, J. Seric. Sci., Jpn. 54 (1985) 504. 53. T. Asakura, Y. Kawaguchi, M. Demura and M. Osanai, Insect Biochem. 18 (1988) 531. 54. T. Asakura, M. Nagashima, R. Sakaguchi, M. Demura and M.Osanai, Insect Biochem. 21 (1991) 743. 55. T. Asakura, Y. Watanabe, A. Uchida and H. Minagawa, Macromolecules 17 (1984) 1075. 56. F. Koide, H. Nagayama and K. Shimura, Nippon Nogei Kagaku Kaishi 29 (1955) 987. 57. Y. Suzuki, L.P. Gage, D.D. Brown, J. Mol. Biol. 70 (1972) 637. 58. T.D.C. Grace, Nature (London) 261 (1967) 613. 59. Y. Chinzei, Appl. Ent. Zool. 10 (1975) 136. 60. H. Inoue and J. Mitsuhashi, J. Seric. Sci. Jpn. 53 (1984) 108. 61. T. Asakura, H. Yamada, M. Demura and M. Osanai, Insect Biochem. 20 (1990) 261. 62. T. Asakura, R. Sakaguchi, M. Demura, T. Manabe, A. Uyama, K. Ogawa and M. Osanai, Biotechnol. Bioeng. 41 (1993) 245. 63. T. Asakura, M. Demura, A. Uyama, K. Ogawa, K. Komatsu, L.K. Nicholson and T.A. Cross, in.D. Kaplan et al. (Ed), Silk Polymers, Materials Science and Biotechnology ACS Symposium Series. ACS, Washington, DC, 1994, p. 148. 64. T. Asakura, M. Demura and Y. Hiraishi, manuscript in preparation. 65. T. Asakura, M. Minami, R. Shimada, M. Demura, M. Osanai, T. Fujito, M. Imanari and A.S. Ulrich, Macromolecules 30 (1997) 2429. 66. M. Kainosho and T. Tsuji, Biochem. 21 (1982) 6273. 67. T. Asakura, Y. Yamazaki, K.W. Seng, M. Demura and I. Ando., Rep. Prog. Polym. Phys. Jpn. 36 (1993) 633. 68. T. Asakura, M. Demura, Y. Hiraishi, K. Ogawa and A. Uyama, Chem. Lett. 1994, 2249. 69. T. Fujiwara Y. Kobayashi, Y. Kyogoku and K. Kataoka, J. Mol. Biol. 187 (1986) 137. 70. T. Gullion and J. Schaefer, J. Magn. Reson. 81 (1989) 196. 71. T. Gullion and J. Schaefer, Adv. Magn. Reson. 13 (1989) 57. 72. A.S. Ulrich and A. Watts, Solid State NMR 2 (1993) 21. 73. A.S. Ulrich, A. Watts, A.I. Wallat and M.P. Heyn, Biochem. 33 (1994) 5370. 74. T. Asakura, M. Demura, Y. Watanabe and K. Sato, J. Polym. Sci., B 30 (1992) 693. 75. P.M. Cunnif, S.L. Staufer, R.V. Lewis, in D. Kaplan, W.W. Adams, B. Farmer, C. Viney (Eds), Silk Polymers-Materials Science and Biotechnology. ACS, Washington, DC, 1994, p. 222. 76. C.M. Mello, K. Senecal, B. Yeung, P. Voudros, D. Kaplan, in D. Kaplan, W.W. Adams, B. Farmer, C. Viney (Eds), Silk Polymers-Materials Science and Biotechnology. ACS, Washington, DC, 1994, p. 67.
890 77. 78. 79. 80. 81. 82. 83.
84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113.
TETSUO ASAKURA ET AL. R.V. Lewis, Acc. Chem. Res. 25 (1992) 392. J.O. Warwicker, J. Mol. Biol. 2 (1960) 350. B.L. Thiel, D.D. Kunkel, C. Viney, Biopolymers 34 (1994) 1089. A. Simons, E. Ray and L.W. Jelinski, Macromolecules 27 (1994) 5235. Z. Dong, R.V. Lewis and C.R. Middaugh, Arch. Biochem. Biophys. 284 (1991) 53. N. Nishikawa, Y. Tanizawa, S. Tanaka, Y. Horiguchi, H. Matsuno and T. Asakura, Polymer 39 (1998) 1001. For example, (a) R.D. Fraser, T.P. MacRae and G.E. Rogers, Keratins: Their Composition, Structure and Biosynthesis, Charles C. Thomas, Springfield, IL, 1972. (b) J.H. Bradbury, Adv. Protein Chem. 27 (1973) 111; (c) R.D. Fraser and T.P. MacRae in Conformation in Fibrous Proteins, Chapter 16. Academic Press, N.Y.1973. I.J. O'Donnel and E.O.P. Thompson, Aust. J. Biol. Sci. 17 (1964) 973. L.M. Dowling and W.G. Crewther, Prep. Biochem. 4 (1974) 203. H. Yoshimizu and I. Ando, Macromolecules 23 (1990) 2908. H. Yoshimizu, H. Mimura and I. Ando, Macromolecules 24 (1991) 862. H. Yoshimizu, H. Mimura and I. Ando, J. Mol. Struc. 246 (1991) 367. H. Yoshimizu, Ph.D. Thesis, Tokyo Institute of Technology, 1991. H. Mimura, H. Yoshimizu and I. Ando, 30th Meeting on NMR, Tokyo, 1991. H. Saito and I. Ando, Ann. Rep. NMR Spectrosc. 21 (1989) 210. I. Ando, T. Yamanobe and T. Asakura, Prog. NMR Spectrosc. 22 (1990) 349. F.A. Quiocho and W.M. Lipscomb, Adv. Protein Chem. 25 (1971) 1. H. Sakabe, T. Miyamoto and H. Inagaki, Sen-i Gakkaishi 37 (1981) 273. H.R. Kricheldorf and D. Muller, Colloid & Polym. Sci. 262 (1984) 856. J.W. Mack, D.A. Torchia and P.M. Steinert, Biochem. 27 (1988) 5418. J.W. Mack, A.C. Steven and P.M. Steinert, J. Mol. Biol. 232 (1993) 50. J.W. Mack, A.C. Steven and P.M. Steinert, NOBCChE'94 21 (1994) 81. P.L. Privalov, Adv. Protein Chem. 35 (1982) 1. S. Tuzi and I. Ando, J. Mol. Struct. 196 (1989) 317. S. Tuzi, S. Sakamaki and I. Ando, J. Mol. Struct. 221 (1990) 289. S. Tuzi, Ph.D. Thesis, Tokyo Institute of Technology, 1990. C. Cohn and K.C. Holes, J. Mol. Biol. 6 (1963) 423. D.J. Selkoe, Science 248 (1990) 1058. G.G. Glenner, Cell 52 (1988) 307. M. Nishi, T. Sanke, S. Nagamatsu, G.I. Bell and D.F. Steiner, J. Biol. Chem. 265 (1990) 4173. J.H. Cooper, Lab. Invest. 31 (1974) 232. R.A. Crowther, Biochim. Biophys. Acta 1096 (1991) 1. D.A. Kirschner, C. Abraham and D.J. Selkoe, Proc. Natl. Acad. Sci. U.S.A. 83 (1986) 503. R.G.S. Spencer, K.J. Halverson, M. Auger, A.E. AcDermott,, R.G. Griffin and P.T. Lansbury Jr., Biochem. 30 (1991) 10382. C.L. Master Proc. Natl. Acad. Sci. U.S.A. 82 (1985) 4245. K.J. Halverson, P.E. Fraser, D.A. Kirschner and P.T. Lansbury Jr., Biochem. 29 (1990) 2639. K. Sorimachi and D.J. Craik, Eur. J. Biochem. 219 (1994) 237.
Chapter 24
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Polysaccharides and biological systems Hazime Sait6, Satoru Tuzi and Akira Naito Department of Life Science, Himeji Institute of Technology, Kamigori, Hyogo, Japan
24.1
Introduction
Biological macromolecules such as polysaccharides, fibrous or membrane proteins characteristically adopt a unique secondary structure which may be related to a variety of physical or biological properties. Elucidation of the three-dimensional structure of these systems is not always straightforward, because, in many instances, they are insoluble in ordinary solvent systems, and crystallization for X-ray diffraction study is extremely difficult. A possible disruption of a particular secondary structure should also be anticipated, when they are solublized in a solvent or detergent. Therefore it is essential to clarify their secondary structures either in the solid, gel or membranebound state without any attempt at solubilization. For this purpose, it has been demonstrated that the high resolution solidstate NMR approach provides one with an alternative and convenient means to distinguish a variety of crystalline polymorphs and to reveal the secondary structures of biological macromolecules, because the X3C chemical shifts of backbone carbons are displaced (up to 8 ppm) [1, 2] depending on their respective conformations. In addition, it is emphasized that this type of empirical approach can be used as a very valuable constraint to construct the three-dimensional structure of biological molecules, such as peptides and proteins, based on a set of accurately determined interatomic distances measured by a partial dipolar recoupling method, such as R E D O R (rotational echo double resonance) [3-6].
24.2
Polysaccharides
A variety of monomers such as glucose, galactose, etc., and linkage positions such as 1-~ 2, 1 ~ 3, 1-~ 4, etc., can have anomeric forms such as a or/3, with a degree of branching which leads to an extraordinary variety of primary structures for polysaccharides. The secondary structure of an individual polysaccharide is defined by a set of torsion angles (4~, q~) about the glycosidic
892
H A Z I M E SAITO, SATORU TUZI AND AKIRA NAITO
Fig. 24.1. Definition of torsion angles for (1 --~ 4)-a-D-glucan (I) and (1 -~ 3)-fl-D-glucans (II).
linkages, as illustrated for the case of amylose ((I):(1 ~ 4)-Ce-D-glucan) and (1 ~ 3)-/3-D-glucan (II) (Fig. 24.1). On the basis of the conformation-dependent 13C chemical shifts, it is expected that 13C NMR spectra can be utilized to distinguish one of the crystalline polymorphs in these polysaccharides from anothers. It is also probable that the 13C chemical shifts of carbons at the glycosidic linkages are displaced in line with their particular conformations. Sait6 et al. [7] first suggested a correlation of the 13C chemical shifts of the C-1 and C-4 carbons of (1--~ 4)-a-D-glucans with their torsion angles, 4, and q~, respectively, although some variations of such relations were later proposed [8, 9]. It is also emphasized that distinction of the single and triple helices of (1--~ 3)-fl-D-glucan is made possible by a careful examination of the 13C chemical shifts of polysaccharides at carbons not always close to the glycosidic linkages [10]. In practice, it is not always straightforward to be able to determine such torsion angles at the glycosidic linkages by a fiber X-ray diffraction study, because the experimental data points may not be sufficient to arrive at the final structure. In fact, three-dimensional structures have been revised several times as in the case of B amylose [11-13]. In particular, distinguishing between multiple-stranded helices and nested single helices is often one of the most difficult problems found in interpreting fiber diffraction [14]. It is thus a major advantage to record 13C NMR spectra of these polysaccharides to reveal the secondary structure, because structural information is equally available from non-crystalline samples. In this section, we demonstrate how 13C NMR methods can be utilized to clarify the secondary structure of some gel-forming polysaccharides, and to relate the resulting secondary structures with gelation mechanism as well as biological properties.
24.2.1 24.2.1.1
Distinction of polymorphs
(1 ~ 3)-[3-D-glucans [15-19] A variety of (1 ~ 3)-/3-D-glucans have been isolated from different sources: a low molecular-weight glucan from sea weed (laminaran) is freely soluble in aqueous solution, while high molecular-weight glucans from bacteria (curd-
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
893
lan) or fungi (lentinan, schizophyllan, HA-/3-glucan, etc.) are insoluble in a neutral aqueous solution. It has been demonstrated that gel-forming and antitumor activity [20] are characteristic of these high molecular-weight glucans. Therefore, it appears that elucidation of the secondary structures of these glucans by observing 13C NMR is essential to gain insight into a relationship between the secondary structure and these biological or physical properties. A linear high molecular-weight glucan, curdlan, is not soluble in a neutral aqueous solution and is capable of forming an elastic gel by heating its aqueous suspension at a temperature above 60~ followed by cooling. Three clearly different 13C NMR spectra were available from curdlan in the solid state (Fig. 24.2) by changing the manner of sample preparation. The "anhydrous" sample (A) as-received from a commercial source (Wako Pure Chemicals, Osaka) or lyophilized from DMSO solution was converted to the "hydrated" form by hydration (B) by placing it in a desiccator at 96% relative humidity overnight. The "annealed" sample (C) was prepared by heating an aqueous suspension at a temperature above 150~ followed by slow cooling [10, 16, 21]. The annealed sample C has been identified as the triple helix form, on the basis of powder X-ray diffraction as compared with the data
i" ,"
"
C. Annealed
J e
150
100
ppm
5O
0
Fig. 24.2. 13C NMR spectra of curdlan in (A) anhydrous; (B) hydrate; and (C) annealed state
[lO].
894
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
from a fiber diffraction study [22]. The hydrated sample B is considered to be a somewhat flexible single helix form, because the 13C chemical shifts turn out to be identical to those observed in an elastic gel by high resolution NMR measurements on liquid and solid samples [21]. The anhydrous sample A is thought to be a single chain form, because this is a dehydrated form of sample B and can be readily converted to the hydrated form by exposure to high relative humidity. The C-3 13C chemical shift of the single chain form (89.8 ppm) is displaced to high frequency by 2.5-3.3 ppm from that of the single helix (87.3 ppm) or the triple helix (86.5 ppm). At first glance, it appears that the 13C chemical shifts of the single helix are very similar to those of the triple helix. Distinction of these two forms is made possible when the peak-separation between the C-5 and C-2 carbons are compared (2.0 and 3.2 ppm for the former and latter, respectively) rather than the C-3 peak-position. The reason why the C-3 13Cchemical shifts are not significantly different between the single and triple helix forms could be ascribed to the fact that the torsion angles between the two types of helices are very similar as manifested from the lengths of the c-axis (fiber axis) being 18.78 ~ (triple helix) [23] and 22.8/k (single helix)[24]. As mentioned already, distinction of single and multiple helical chains is not easy by the fiber diffraction method. This problem is very easily solved by the 13C NMR method, if these polymorphic structures can be identified with the aid of sample history and other experimental techniques and mutual conformational conversions among them can be manipulated by a series of physical treatments under a controlled manner, as illustrated in Fig. 24.3 [16-19]. The single chain form can be obtained from a sample of either the single helix by dehydration or the triple helix by lyophilization from DMSO solution. Even a multiple-stranded helix can be completely dispersed as a result of the conformational transition to a random coil form in DMSO
Triple Helix i
// ( B r a n c h e d ) ~ ~ Lyophilized from DMSO
//0.3M Na~rH
I ydration / Annealing )
solution
Ill Single Chain
Hydration _
1 Dhydratio: [,
Single.e,ix ii1
Fig. 24.3. Conversion diagram of (1 ~ 3)-/3-D-glucans by various physical treatments [17].
POLYSACCHARIDES AND B I O L O G I C A L SYSTEMS
895
solution. Surprisingly, the triple helix form, as judged from the 13C NMR peak-positions with reference to those of the annealed curdlan as illustrated in Fig. 24.2, is obtained from lyophilization of low molecular-weight glucans such as laminaran or high molecular-weight branched glucans such as schizophyllan from aqueous solution. The single helix form is readily obtained by hydration of the single chain form, as judged by their peak-positions as compared with those of gel samples as will be described later and also reversible mutual conversion from the single chain form. It appears that formation of the most energetically stable triple helix form may be hampered in the case of more hydrophobic, linear high molecular weight glucans because of an insufficient extent of hydration and the resulting single helix form is obtained. Conversion to the triple helix is made possible only when these single heilical chains can be dispersed at a temperature above 150~ followed by slow cooling. It is emphasized that this diagram was also obtained from our studies on a variety of branched (1 ~ 3)-/3-D-glucans [16-19]. This approach proved to be very useful for the distinction of the single chain/multiple-stranded chains utilizing cycles of mutual conversion among several conformations extended to various types of polysaccharides. A very similar conversion diagram has been obtained for (1 ~ 3)-/3-D-xylan, which adopts the three similar conformations, single chains, single helix or triple helix [25]. It turns out, however, that the triple helix form of (1 ~ 3)-/3-Dxylan is much more stable than that of (1 ~ 3)-/3-D-glucan, because the more hydrophobic nature of the former is achieved due to the lack of a hydroxymethyl group at the C-6 position. Surprisingly, the triple helix structure of (1 ~ 3)-/3-D-xylan is retained even if it dissolves in DMSO solution. In such case, it was found that dissolution in aqueous zinc chloride is essential to achieve the dispersed single chain structure in aqueous solution. We also used a similar approach to prepare a single stranded agarose by dissolving in N,N-dimethylacetamide [26]. As mentioned above, hydration is a very important step in the conversion to a secondary structure of polysaccharides, even if they do not dissolve in aqueous media. 24.2.1.2 Amylose and chitin/chitosan It was previously demonstrated that the crystalline conformation of a single chain for both (1 ~ 4)-c~-D- and (1 ~ 3)-/3-D-glucans may arise from large amplitude helices, although the extended form can be stabilized in the (1 ~ 3)-c~-D- and (1---~ 4)-/3-i~-glucans [27]. It has been demonstrated that amylose and starch with c~-D-(1 ~ 4)-linkages exhibit the polymorphs, V, A, B and C, as examined by a fiber X-ray diffraction study [28]. The V-form exists as complexes with small organic molecules and adopts a left-handed single helix form, because its C-1 and C-4 signals give rise to single lines and
896
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
6 O
(A)
-
1i
/ o
O .
i
o
.
:~. ::]
i
iii i i
150
1 0 0 PPM
50
Fig. 24.4. ~3C NMR spectra of amylose film from high molecular weight sample (DP 1000). (A) Anhydrous; (B) hydrated; (C) hydrated iodine complex" and (D) anhydrous iodine complex
[29]. are displaced to high frequency from those of the B form (Fig. 24.4) [29]. The C-1 peaks of the A and B forms are split into a triplet and doublet, respectively [30-32] and readily distinguished by ~3C NMR spectra. It is interesting to note that the 13C NMR spectra of both A- and B forms of amylose and starch are substantially distorted by drying to give a spectral profile of amorphous form [33-35]. In contrast, it was shown that hydration
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
897
of amorphous amylose of low molecular weight (DP 17) results in complete conversin to the B-type form [27]. We noticed that the V-form amylose of high molecular weight (DP 1000) complexed with DMSO was convertd to the B-form by humidification at 96% relative humidity for 12 h, as shown in Figs. 24.4A and 4B. A similar result was obtained for amylose of low molecular-weight (DP 17). A conversion diagram of amylose by various physical treatments similar to that of (1--~ 3)-/3-D-glucan was proposed [19, 29]. The B form was initially considered as a single helical conformation [11], since the conversion of V- to B-amylose takes place on humidification [36]. Later, the structure was refined as a right-handed double-helix in an antiparallel fashion [12]. The handedness of the double-helix form was also recently revised as a left-handed one [13]. Still, it seems to be very difficult to understand why the simple humidification of amylose in a desiccator results in a drastic conformational conversion from the single stranded helix (V form) to the double stranded helix (B form) [29], associated with the unfolding/folding process, if the secondary structure of the B-form of amylose is really doublestranded as proposed. In this connection, it is likely that complete dissolution in aqueous media is prerequisite for the conversion of the single helix to the triple helical polymer chain, as encountered for the conversion diagram of (1 -~ 3)-/3-D-glucan [16-19]. This means that unfolding of the polymer chain followed by refolding in aqueous media is a necessary step to achieving an efficient conversion process. In addition, it is mentioned that the single to triple helix conversion was associated with fragmentation of molecular chains due to thermal depolymerization of the chain from the initial molecular weight of 106 to the resultant weight of 103 Dalton [37]. It appears that this is the most important step for this coversion from the topological point of view. In this connection, the most important problem seems to be to clarify how conformational features of the native B form so far studied are different from those of the B-form converted from the V-form, in spite of the similarity of the peak-positions. Chitosan, a polymer of/3-(1--> 4)-linked 2-amino-2-deoxy-D-glucose residues, is formed on deacetylation of chitin. As pointed out already, this polysaccharide takes an extended conformation similar to that of cellulose. Deacetylation of chitin is very easily evaluated in view of the 13C NMR spectra, as illustrated in Fig. 24.5. The three polymorphs of chitosan, "tendon-chitosan" (from crab shell), "L-2" (from shrimp shell), "Annealed"(from crab shell chitosan annealed at 22~ in the presence of water) are easily distinguished, consistent with the data for the polymorphs as obtained by a powder X-ray diffraction data [38, 39]. The observed non-equivalence of two chitosan chains, as viewed from the splittings of the C-1 and C-
898
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO u I
"i
A Shrimp shell chitosan ~
,
i i .
9
B Crab shell chilosan
(annealed at 220"C) t
[
i:i
~
i:
!t, . I
Fig. 24.5. (A-C) 13C NMR spectra of three preparations of chitosans; and (D) chitin [38].
4 signals in the first two polymorphs, turned out to be removed by dehydration of water molecules loosely bound, during either annealing or complex-formation with transition metal ions. Conformational changes accompanied with metal-binding were also conveniently examined by means of 13C NMR spectroscopy [38, 39].
24.2.2 Conformation and dynamics of the gel network The network structure of gels is generally highly heterogeneous from the structural and dynamic points of view. The existence of solid-like domains from the cross-linked region is characteristic of the formation of the gel network. Such a domain in polysaccharide gels is ascribed to formation of cross-links due to the physical association of chains adopting an ordered conformation. It is now obvious that the secondary structure of such ordered polysaccharide chains is readily determined on the basis of the conformation-
899
P O L Y S A C C H A R I D E S AND B I O L O G I C A L SYSTEMS ca
?
ss
o
ca
ii
i
!
t
D. Gel (CP-MAS) 9o
~ :
:
9 . :
' A
C. Gel (MAS)
!
J~ ;1 f V
!
1~o
Fig. 24.6.
13C
~6o
i
;il
'
5'0
'
NMR spectra of curdlan gel recorded by a variety of experimental conditions
[21].
dependent displacements of the ~3C chemical shifts with reference to the corresponding peaks of crystalline polymorphs as described above. Naturally, there remain, however, considerable proportions of the polymer chains undergoing isotropic reorientational motions which are characteristic of the liquid-like domain. In fact, rather sharp 13C NMR signals were clearly visible from an elastic gel of curdlan by a conventional high resolution NMR spectrometer or by broad band decoupling (Fig. 24.6B) and ascribed to the existence of such a liquid-like domain [15, 16, 21, 40, 41], whereas these signals were completely suppressed for brittle gels of branched (1 ~ 3)-/3-Dglucans [42]. This means that the network structures of gels consisting of (1 ~ 3)-/3-Dglucans are different from each other with respect to the linear and branched forms. The C-1 and C-3 13C chemical shifts from the liquid-like domain of
900
H A Z I M E S A I T 0 , SATORU TUZI AND A K I R A NAITO
curdlan are appreciably displaced from those of oligomers taking the random coil conformation [15, 40, 41]. It is also interesting that the 13C NMR peakpositions of the solid-like domain (Fig. 24.6D) as well as those of the liquidlike domain (Fig. 24.6B) are very close to those of the hydrated curdlan (Fig. 24.6A). The amount of the triple helix form is nominal, if any, as indicated by the arrow of Fig. 24.6D, as long as the heating temperature is kept below 80~ On the other hand, we found that the 13C NMR signals characteristic of the triple helix from are dominant in the CP-MAS NMR spectra of gels consisting of branched (1--~ 3)-/3-D-glucans including HA-/3-glucan, schizophyllan and lentinan, although they take the single chain form when they are lyophilized from DMSO solution, as illustrated in Figs. 24.7(C) and 7(A), respectively. Of course, these signals are not visible by using a conventional spectrometer. This means that gelation of the branched glucan proceeds from partial association of these triple helical chains [18, 19]. It was shown that amylose gel contains two kinds of 13C NMR signals: the B-type signals from motionally restricted regions as recorded by the CP-MAS NMR technique and the signals identical to those found in aqueous solution [43]. The latter signals could be ascribed to flexible molecular chains adopting "7 O
~.d ~.
C. Hydrate(lyophillzedsample I~
9
!
p B. Hydrate(lyophllizedsample / j
150
100 f,p,.
510
"
()
Fig. 24.7. ;3C NMR spectra of HA-/3-g]ucan at (A) anhydrous; and (B) and (C) hydrated state
[211.
901
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
Table 24.1. 13C spin-lattice relaxation times of starch gel (33%) by DD- and CP-MAS NMR method (s, from Ref. [19])
Liquid-like domain Solid-like domain
C-1
C-4
C-3
C-2
C-5
C-6
0.36 9.2
0.29 11.8
0.30 11.9
0.32 11.9
0.29 11.9
0.16 2.1
a random coil conformation in the liquid-like domain. On the other hand, the former peaks are ascribed to the solid-like domain of cross-links, either double helical junction zones [43] or aggregated species of single helical chains as discussed already [21]. It is found that two such domains in the gel network are very clearly distinguished when the 13C spin-lattice relaxation times were compared as in starch gel as summarized in Table 24.1. It is interesting to note that the 13C T1 values of the liquid-like domain are in the vicinity of the T1 minimum, ~c ~ 10-8 s, whereas those of the solid-like domain are on the lower temperature side of the T1 minimum, ~'c > 10 -8 s. Here, ~c denotes the correlation time of motional reorientation. Obviously, the solid-like domain of the starch gel arises from the crystalline portion as obtained from the solid state in view of their 13C T1 values as compared with those of crystalline samples. On the contrary, the T1 values from the liquidlike domain arose from flexible molecular chains undergoing isotropic motions even if their mobility is restricted to some extent due to the presence of the cross-linked region. It is also interesting to examine whether or not the single and triple helical chains of the solid-like domain in the gel network from (1 ~ 3)-/3-D-glucans are distinguishable by this approach. It turns out that this approach is unsuccessful probably because such comparison should be made by relaxation parameters sensitive to lower frequency motions rather than the T~ values which are sensitive to high frequency motions. For this reason, we measured the ~3C resolved 1H spin-lattice realxation times in the rotating frame (Tlo) and the cross polarization time (TcH) of linear and branched (1 ~ 3)-/3-D-glucans, as summarized in Table 24.2. It is interesting to note that the TCH and Tlo values of curdlan taking the single helix form are significantly longer than those of schizophyllan and HA-/3-glucan taking the triple helix form. This means that the single helical curdlan is able to afford low frequency motions of the order of 10 -5 s in the solid-like domain, although the triple helical glucans are not. The well-documented network model of agarose gel arises from the junction zones consisting of associated double helical chains [44]. Nevertheless, it is seen that intense ~3C N M R signals of agarose gel are clearly visible from the liquid-like domain (Fig. 24.8, top trace) either by conventional N M R spectrometer or DD-MAS experiment, in addition to the intense signals from
902
H A Z I M E SAITO, S A T O R U T U Z I A N D A K I R A N A I T O
Table 24.2. The observed TCH and Tlo of linear and branched (1 ~ 3)-/3-D-glucans (from Ref. [19]) C-1
Curdlan Schizophyllan HA-/3glucan
,
' t20
C-3
C-5
C-2
C-4
C-6
TCH Tip
TCH Tip
TCH Tip
TCH Tip
TCH Tip
TCH Tip
ixs
ms
Ixs
ms
Ixs
ms
Ixs
ms
Ixs
ms
Ixs
ms
128
17.9
138
16.6
137
17.0
145
19.2
110
14.0
82.2
22.9 10.0
' ' '
67.3
3.32
62.8
4.18
53.1
4.30
56.4
4.09
32.0
5.69 22.1
56.6
5.04
35.4
5.38
64.4
6.27
47.8
5.34
50.7
5.80 53.0
l It O '
' ' '
I'
,
tO0
'"''
I'
90
,
' ' '
I ' ' ' ' 1 '
O0
70
'''
I"''''
60
I
50
'
'
"'
7.28
Pl~m
Fig. 24.8. 13C NMR spectra of agarose gel. Liquid-like domain (top) and solid-like domain (bottom) (Ref. [19]).
the solid-like domain (Fig. 24.8 bottom) recorded by CP-MAS experiment [19]. Note that the intense signal at l l 0 p p m in DD-MAS spectrum arises from the materials used for the probe assemby. A significant spectral change at 77-78.5 ppm should be ascribed to the conformation-dependent displacements of the peaks of the C-3 and C-4 carbons for (1 ~ 3)- and (1 ~ 4)linked galactosyl residues, respectively. Therefore, it is expected that the peaks which exhibit the conformation-dependent change of agarose are the
P O L Y S A C C H A R I D E S AND B I O L O G I C A L SYSTEMS
903
C-3 peak of (1 ~ 3)-linked galactosyl and the C-4 peak of (1 --~ 4)-linked 3,6anhydro-a-L-galactosyl residues, respectively. Usov [45] demonstrated that these peaks resonate at 81.9 and 77.0ppm, respectively. Therefore, the above-mentioned spectral change as manifested from the two types of spectral data, DD- and CP-MAS experiments, is ascribed to the conformational change from random coil to an ordered conformation [19]. It is now obvious that the network model by Arnott et al. is too simplified to account for the presence of the liquid-like domain as manifested by the NMR experiment. So far, several workers [14, 46, 47] have questioned the validity of the doublehelical junction zones and proposed an alternative model of gel network containing extended single helices. We also examined the 13C NMR spectra of dried agarose film prepared from N,N-dimethylacetamide solution, followed by drying at 80~ under anhydrous condition and its hydrate [26]. We found that the 13C NMR spectrum thus obtained is identical to that obtained from agarose gel. This finding is consistent with the view that the network structure of agarose gel consists mainly of single helical chains. It is now obvious that 13C NMR is a unique technique for the analysis of both the conformation and dynamics of polysaccharides in solutions, solids and gels. Especially, it is very useful to distinguish between the liquid- and solid-like domains by use of DD- and CP-MAS techniques. In addition, a systematic study of a converison diagram among polymorphs by a series of physical treatments is especially useful in order to clarify whether the polysaccharide chain under consideration takes a single or multiple chain both in the solid and in the gel.
24.3
24.3.1
Structural and membrane proteins
Conformation-dependent 13C Shifts
It is well recognized that all of the I3C chemical shifts of amino acid residues which are more than two residues away from a chain end in peptides and proteins adopting unfolded conformations in solution are effectively independent of all neighbouring residues except for proline [49]. Therefore, it is expected that the 13C chemical shifts of the backbone C~ and C = O and sidechain Ct~ signals of peptides and proteins are significantly displaced (up to 8 ppm) depending on their local secondary structure as defined by a set of torsion angles in the peptide unit (4~, q~) (Fig. 24.9), irrespective of there being a variety of neighbouring amino acid residues [1, 2]. To demonstrate this view, we have recorded the I3C NMR signals of polypeptides in the solid state whose secondary structures such as a-helix, /3-sheet, 31-helix, etc. are
904
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
C
N/
Fig. 24.9. Definition of the torsion angle in the peptide unit.
known from X-ray diffraction or other spectroscopic techniques [50-54]. In particular, the two major conformations, a-helix and /3-sheet forms, are readily distinguished from the peak-position of the ~3C NMR signals" the C~ and C ~ O 13C NMR signals of the a-helix form are displaced to high frequency by an amount of 3-8 ppm with respect to those of the/3-sheet forms, whereas the Ct3 signals of the a-helix was displaced to low frequency with respect to those of the/3-sheet form, as summarized in Table 24.3. In addition, it has been demonstrated that seven conformations including the random coil form can be distinguished by the conformation-dependent displacements of peaks as manifested from those of Ala residues [1-4]. This means that the local conformation of particular amino acid residues from any polypeptides, structural, globular and membrane proteins is readily evaluated, in an empirical manner, by means of the conformation-dependent displacements of 13C chemical shifts of respective residues with reference to the data base so far accumulated from a number of polypeptides, because the transferability of these parameters from a simple model system to more complicated proteins proved to be excellent. For globular proteins in solution, specific displacements of 13C NMR peaks with respect to those of the random coil have been utilized as a convenient probe to lead to a sequential assignment of the secondary structures of proteins [54, 55], as inspired by the success of the compilation of the conformation-dependent 13C chemical shifts as mentioned above. The existence of the conformation-dependent ~3C chemical shifts was theoretically evaluated as a contour map of 13C chemical shift (nuclear magnetic shielding constant) for the Ala residue from N-acetyl-N'-methyl-alanine amide [56, 57] as a function of the above-mentioned torsion angles.
905
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
Table 24.3.
13C chemical shifts characteristic of the c~-helix and/3-sheet forms (ppm from TMS)
(Ref. [2]) Amino-acid residues in polypeptides Ala Leu GIu(OBzl) Asp(OBzl) Val Ile Lysc Lys(Z) Argc Phe Met Gly
C-a
C-B
c~-
~-
helix
sheet
52.4 52.3 55.7 55.8 56.4 56.8 53.4 65.5
48.2 48.7 50.5 51.2 51.2 51.1 49.2 58.4 58.2 57.8 51.4
6.2
53.2 52.2 43.2
8.1 5.0
63.9 57.4 57.6 57.1 61.3 57.2
Aa
C=O
o~-
/~-
helix
sheet
Aa
4.2 3.6 5.2 4.6 5.2 5.7 4.2 7.1
14.9 14.8 39.5 43.7b 25.6 25.9 33.8 28.7
-5.0 -5.2 -3.8 (4.1) -3.4 -3.8 -4.3 -3.7
6.1
34.8 29.9 29.3 28.9 35.0 30.2
19.9 20.0 43.3 39.6 29.0 29.7 38.1 32.4 32.4 39.4 28.5
-0.8
39.3 34.8
-4.3 -4.6
-4.6
o~-
fl-
helix
sheet
176.4 176.2 175.7 175.8 175.6 175.4 174.9 174.9
171.8 171.6 170.5 171.3 171.0 172.2 169.8 171.8 171.5 174.9 172.7 176.5 175.7 170.4 176.8 175.2 169.0 175.1 170.6 168.4 171.6~ 168.5
Aa
4.6 4.6 5.2 4.5 4.6 3.2 5.1 3.1 2.2 5.3 6.2 4.5 3.1
aDifference in the 13C chemical shifts of the a-helix form relative to those of the/3-sheet form. bMistyping or erroneous assignment. This assignment should be reversed. CData taken from neutral aqueous solution, dAveraged values from the data of polypeptides containing 13Clabelled glycine residues.
24.3.2
Structural proteins
It w o u l d s e e m r e a s o n a b l e to e x p e c t t h a t the d i s p l a c e m e n t s of the 13Cc h e m i c a l shifts could be u s e d as an intrinsic p r o b e of local e n v i r o n m e n t of a given a m i n o acid r e s i d u e , if the t r a n s f e r a b i l i t y of the c o n f o r m a t i o n - d e p e n d e n t shifts of p o l y p e p t i d e s to m o r e c o m p l i c a t e d p r o t e i n systems is g u a r a n t e e d . F i b r o u s p r o t e i n s such as silk fibroin, collagen a n d collagen-like p o l y p e p t i d e s can serve as ideal s y s t e m s to justify this view, b e c a u s e several crystalline p o l y m o r p h s are available d e p e n d i n g on a variety of physical t r e a t m e n t s a n d the spectral p a t t e r n is very simple as c o m p a r e d with those of g l o b u l a r p r o t e i n s b e c a u s e of the limited n u m b e r s of a m i n o acid residues involved. Crystalline silk fibroins are k n o w n to exist in o n e of the p o l y m o r p h s , e i t h e r silk I or silk II a n d e i t h e r the c~-helix o r / 3 - s h e e t f o r m s , d e p e n d i n g on the species of s i l k w o r m , Bombix mori or Philosophia cynthia ricini respectively.
906
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
It is straightforward to distinguish the two polymorphs o f silk fibroin from B. mori by 13C NMR spectra (Fig. 24.10) [58], because the variety of amino
acid residues is limited to the following four kinds: Gly (42.9%), Ala (30.0%), Set (12.2%) and Tyr (4.8%). It is noteworthy that the C~ signals of Ala and Ser residues of silk I (designated as I) are displaced to high frequency by about 2 ppm as compared with those of silk II (designated as II), whereas the Cl3 signals of silk I are displaced to low frequency by 3.4-4 ppm with respect to those of silk II. Consistent with expectation, it was found that the C~, C~ and C - - O chemical shifts of the silk I and II samples are the same as those of (Ala-Gly), II and I, respectively, within experimental error [59]. It appears, however, that the 13C NMR signal of the Gly C~ carbon is not sensitive to the present conformational change, although the 13C chemical shift of the C - - O group is very sensitive to this change. The major advantage of the present 13C NMR approach is to be able to estimate the relative proportion of material which is not readily converted to each other (20-30%) [60]. In addition, distinction of the a-helical and/3-sheet signals in the Ala residue of P.c. ricini fibroin and the hydration-induced conformational change from the less stable ce-helix to /3-sheet region are also very conveniently examined by the 13C NMR approach [59].
L U
-..,~..-j.~-m
i ili
O i.L.
8-J~" ~ | "7
A S,,.K,
"
! Ji
i !
It
.,
:i
1
:
;
:
~
i
!
!ill ~ iAl i
9
i :
i
__J ._,1
150
._.d.
,
100
....L_
50
i._
0
ppm
Fig. 24.10. 1 3 C NMR spectra of crystalline fraction of B. mori fibroin taking (A) silk I" and (B) silk II forms [58].
907
P O L Y S A C C H A R I D E S A N D B I O L O G I C A L SYSTEMS
o
'i! i!
_
!: u ou )'i i~
I i
___jil.
9
i i
!
i
200
I
150
i
100
: : :
'1
i ,
:.
"':i
"I
i.
; i: ol
ii'.
9
i
9,.
~:
I
50
~-'-
:
:i
i
:"
:
i :
"
:
!
PPM
Fig. 24.11. 13C N M R spectra of collagen from (A) bovine achilles tendon; and ( B - D ) of model polypeptides taking collagen-like triple helical structure. (B) (Pro-Ala-Gly)." (C) (Pro-ProGly)lo; (D) (Hyp). [61].
In a similar manner, most of the 13C NMR signals of the collagen fibril, arising from the major amino acid residues, which amount to approximately 65% (Gly, 33 +- 1.3%; Pro, 11.8 +- 0.9%; Ala, 10.8 +- 0.9%; Hyp, 9.1 ___1.3%), can be readily assigned, on the basis of the peak positions from model polypeptides as indicated at the top of the individual peaks (Fig. 24.11) [53, 61]. The assignment of peaks was made by referring to the 13C chemical shifts of appropriate model polypeptides, because individual triple-helical
908
HAZIME SAIT0, SATORU TUZI AND AKIRA NAITO
chains of collagen are composed of a repeating pattern of (Gly-X-Y)~, where X and Y are frequently occupied by prolyl, 4-hydroxylprolyl or alanyl residues. The similarity of the ~3C chemical shifts of the respective amino acid residues between the collagen fibril (Fig. 24.11A) and model triple-helical polypeptides (Fig. 24.11B-D) thus confirmed previous conclusions as to the tertiary structure of collagen analyzed by X-ray diffraction studies [53]. The peaks B and C were ascribed to the C~ and Cv.~ carbons of the remaining amino acid residues (ca. 35%) such as Ser, Glu, Leu, Arg, Lys, Val, etc. by spectral simulation [61], although the Ct3 signals might be suppressed due to the presence of low-frequency molecular motions with a timescale of 104105 Hz interfered by the proton decoupling frequency [62]. It appears that distinction of the collagen-like triple helix from the assembly of the single 31 helices is not always feasible by means of the ~3C NMR data alone, because of the similarity in the torsion angles, ( - 8 0 ~ 150 ~ and ( - 7 2 ~ 153 ~ for the 31 and collagen-like triple helix, respectively. This problem was readily solved by a ~SN NMR study, because the 15N chemical shift is very sensitive to the presence or absence of ( G l y ) N ~ H . . . O - - C inter-chain hydrogen bonds which are essential for the stabilization of the triple helical conformation [63]. In fact, it was found that the Gly N ~ H ~SN chemical shift of (Pro-Pro-Gly)~o as a model of the collagen triple helix is displaced to high frequency by 4.9 ppm as a result of the formation of the inter-chain hydrogen bond stabilizing the triple helix when this peptide is fully hydrated. This is not the case for collagen in which this particular hydrogen bond is retained even if it is obviously dried. Therefore, the 15N peak-positions of the collagen fibril and (Pro-Ala-Gly)~ are found to be very close to that of this hydrogen bonded species. It is interesting to note that the ~3C spin-lattice relaxation times (T~'s) of the collagen fibrils and model polypeptides are substantially different among the carbons of a variety of amino acid residues. It is noteworthy that the T1 values of the Ct3 and Cv carbons of both the Pro and Hyp residues in collagen fibril and (Pro-Pro-Gly)~o are substantially reduced as compared with those of a variety of crystalline oligopeptides [61]. Such a significant reduction was interpreted in terms of the presence of rapid puckering motions in the pyrrolidine rings of the Pro and Hyp residues in the solid state with a timescale of 10 -8 s. Further, it is also pointed out that the dynamic feature of the side chains in the Ser and Tyr residues in silk fibroin and related polypeptides are conveniently examined by a similar reduction of the spin-lattice relaxation times [64]. We have recorded the ~3C CP-MAS and DD-MAS NMR spectra of the dry and hydrated barley storage protein, C-hordein (a fibrous protein of approximate molecular weight of 40,000) and its synthetic model peptides, (Pro)2(Gln)6 and (Pro-Gln-Gln-Pro-Phe-Pro-Gln-Gln)3 under dry and hy-
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
909
drated conditions, with the expectation to be able to relate to its viscoelastic property [65]. The spectral features of C-hordein as well as these peptides appreciably differ from each other depending on the extent of hydration, reflecting different domains that adopt different types of conformations as well as dynamics. In particular, considerable proportions of the peak intensities are lost in the CP-MAS and well-resolved 13C NMR signals emerge in the DD-MAS NMR spectra owing to the acquisition of molecular motions by swelling.
24.3.3
Membrane proteins
Membrane proteins are integral parts of a membrane and have at least one segment of peptide chain traversing the lipid bilayer. They are not soluble in ordinary solvents owing to the presence of both hydrophilic and hydrophobic regions in the same molecule. Thus, solution NMR studies are very difficult because of the high molecular weight complex. Crystallization is extremely difficult as compared with soluble proteins: whole membrane lipids are first solubilized and replaced by appropriate detergent molecules prior to crystallization.
24.3.3.1 Three-dimensional crystal: cytochrome c oxidase Tuzi et al. have recorded the 13C CP-MAS NMR spectra of a three-dimensional crystal of bovine heart cyctochrome c oxidase [66] which is a membrane protein of 400 kDa containing 70 detergent molecules per protein. The observed 13C NMR signals give rise to a spectral resolution comparable to that of crystalline lysozyme. In contrast to fibrous proteins, it is difficult to assign signals to individual carbons of these membrane- and globular proteins unless otherwise specifically 13C-labeled proteins were used, due to severe signal overlaps from many amino acid residues. It is emphasized that the 13C NMR signals are not seriously overlapped with the detergent signals, because the observed peak intensity of the polar heads in detergent BL8SY, C H 3 ( C H z ) 1 1 ~ ( O C H z C H z ) s O H , is only about 10% of the anticipated values at 1 ms contact time, owing to the presence of rapid tumbling motions in the crystal as detected by the spin-lattice relaxation times. The molecular motions of the detergent molecules attached to the proteins were found to be highly heterogeneous. It is pointed out that the whole three-dimensional structure of this protein was recently revealed by an X-ray diffraction study at 2.8 resolution [67, 68].
910
H A Z I M E SAITO, SATORU TUZI AND A K I R A NAITO
24.3.3.2 Two-dimensional crystal: bacteriorhodopsin Bacteriorhodopsin (bR) is the only protein present in the purple membrane (PM) of Halobacterium salinarium which is active as the light-driven proton pump to translocate protons from the inside to the outside of the cell. In addition to studies on the mechanism of proton pump activity by 13C NMR, this protein can also serve as an ideal model system to gain insight into the general aspects on conformation and dynamics of membrane proteins, because bR in PM is organized as two-dimensional crystals and a large scale preparation of 13C-labeled bR is exceptionaly simple as compared with other membrane proteins. ~3C NMR signals of specifically 13C-labeled bR are clearly distinguished from those of the unlabeled preparation (Fig. 24.12) [69]. It is more preferrable to utilize the 13C-labeled Ala residue for the sake of conformational probe, because the lSc chemical shifts of Ala residues have been most througly examined for a variety of local conformations (Table 24.4). The 13C NMR spectra of bR have been recorded under several conditions: lyophilized preparation, lyophilized preparation followed by hydration, and hydrated pellets of PM [69-71]. It was found that dehydration of bR by lyophilization resulted in substantial conformational distortion of the protein backbone as manifested from the obvious line broadening of 13C NMR signals [69], although such a distortion was partially recovered by a subsequent hydration experiment [69, 71]. Therefore, it is strongly recommended to use hydrated pellets to avoid ambiguity arising from such a conformational distortion. It has been demonstrated that the spectral pattern of [3-~3C]Ala-bR is significantly different when obtained by CP-MAS and DD-MAS NMR (Fig. 24.13) [70, 71], because the local flexibility of the peptide backbone is substantially different among the transmembrane a-helix, loop and N- or C-terminus regions, as inferred from the primary structure of bR (Fig. 24.14). We have found that seven Ala residues located both at the N- and C-terminus residues are missing in the 13C CP-MAS NMR but they are fully recovered in the 13C DD-MAS NMR spectra [70, 71]. This is because these Ala residues are located at these terminal regions and undergo rapid isotropic tumbling motions with correlation times of 10-8s which average out dipolar interactions essential for cross polarization. This is consistent with the fact that the XSc spectral pattern recorded by CP-MAS NMR (22 Ala residues) was unchanged even if the C-terminus moiety containing six Ala residues are cleaved by papain [70, 71]. This is of course not true for the ~3C NMR spectra recorded by DD-MAS NMR spectra (29 Ala residues). Clearly, both the 13C CP-MAS and DD-MAS NMR spectra of [3-13C]Ala bR resonating at 14-18 ppm arise from at least seven resolved 13C NMR signals. The individual peaks are ascribed to the portions of the transmem-
POLYSACCHARIDES
AND BIOLOGICAL
911
SYSTEMS
Ala C[3 13CH~
A
i
~N/~-I~c /
I
*
II
H
i
o
9
'20o . . . .
,~o
. . . .
wl,
,'oo
. . . .
~'o
Val C=O
'
'
'
'
g
~(ppm)
CHa~ /CH3 CH2
I
B
C 13
ssb
I
II
H
O
ssb
'2~
. . . .
,'5~ . . . .
,'oo'
'
'
'
"o
.
.
.
.
g
~(ppm)
Fig. 24.12. 13C CP-MAS NMR spectra of [3-13C]Ala-bR and [1-13C]Val-bR. Peaks designated by the asterisk from 13C-labeled lipids [69].
brane ai-helix, aii-helix, loop, N- or C-terminus, with reference to the conformation-dependent 13C chemical shifts as summarized in Table 24.4. The reference data for the ai-helix (so called a-helix) and an-helix were taken from the 13C N M R spectra of (Ala), in the solid and hexafluoroisopropanol (HFIP) solution. The presence of the latter was previously proposed by Krimm and Dwivedi to explain anomalously higher amide I frequency of 1665 cm -1 in the IR spectra of bR and can be extended to the assignment of 13C N M R signals by adopting the same reference sample [70, 71]. As indicated by the top trace, these types of a-helices are well distinguishable. The
912
HAZIME SAITt3, SATORU TUZI AND AKIRA NAITO
Table 24.4. Conformation-dependent 13C chemical shifts of Ala-residues (ppm from TMS) a
C,, C~ C--O
al-helix (aR-helix)
aIi-helix
aL-helix
/3-sheet
Collagenlike triple helix
Silk I
52.4 14.9 176.4
53.2 15.8 178.4
49.1 14.9 172.9
48.2 19.9 171.8
48.3 17.6 173.1
48.3 16.6 177.1
Random coil b
16.9
all. Sait6 and I. Ando, Annu. Rep. NMR Spectrosc. 21 (1989) 209. bS. Tuzi, S. Yamaguchi, A. Naito, R. Needleman, J.K. Lanyi and H. Sait6, Biochemistry 35 (1996) 520.
DDMAS
Loop i~^,,i VV/,.,
'
'
'
'
helix
i
ppll
~o
'
'
'
'~
.
.
.
.
;o
et n -helix CPMAS Loop I ~ ' t~ 'J
.
'
'
.
~
.
I txI-helix
.
.
.
ppnl
.
.
'~
.
.
.
.
.
/o
"
Fig. 24.13. (A) 13C DD-MAS; and (B) CP-MAS NMR spectrum of [3-13C]AIa-bR [71].
913
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS Ser Thr ( ~ ( ~ ( ~
Val S~ Asp G4y P,r9 Met
G~
~1~3 Lys Ser
Pro Glu PGlum(~)
Met
Arg Vail Pro Asp Glu Set Leul Asp 10r Gin Thr Phe; / ~ al Arg Leu _ Leu ThrGly Gly ~ Leu - - -~-'- - t.e~rI..... lie" . . . . . . . . . . l"he. . . . ~. ~ ....Tbr.., ;e ,- - lle-. . . . . . . 9 Phe ~Leu Phe! Phe Leu Asp Leu eA~ 11c Leu GlY220 ValLys Leu II Vd Phe Leul Leu Thr !!Val ~Gly TYrlS011e Leu AsnArg Gly(~( Leu ~u Pro Thr 10 i Asp Tyr Val V~ Gly Leu Thr Thr Vd ~ m 5Ol~ Phe lie et Vai 180 Leu Asp Leu I Met Trp lle GIY' 20 ~ Set Thr ~Asp TJ~/Thr Pa Trp J l LeuThr,...Vaj110~ ~ (~a~ Tyr Gly lie 14o~ P Met Leu Tyr Leu T~' . . . . . . . VaIL. . . . . . . . . .I'D. . . . . .V.,.-.m..yir . . . ,:w.,.. . . . 4.4u- - - ~ L..''| ,,. Ty~TM 0 "/~ly Trp Trp | |Glu l r V Leu Pra Gly Tyr Asn Lys Tyr Gly Val Set Ser Leu GlyLeu Gin 130 Tyr G|u Pro 200 Thr Glu Met Gly G~(~G ~ hV-' Val Gly Pro70Phe Asp
~ ~
Gly Asp G~y (~2Ser
LyS3o L~. ,Y~I..~.oLO.,i...L.Y'(~. TF
ThrL~ Leu
Gy
Met 20
w
Trp . . . . . . "Ire'" Trp 10 Glu Pro Arg
G~
Thr
]
."
!1 ii II
/
~
V
Fig. 24.14. Schematic representation of the secondary structure of bR after Henderson et al. [72, 73]. Residues enclosed by the boxes take transmembrane a-helices.
largest part of the intense peak at 16.9 ppm is ascribed to the Ala residues taking a random coil conformation as a result of undergoing rapid reorientational motion in the N- and C-terminal regions and is visible by the DDMAS NMR experiment alone [70]. A part of the peak at 16.9ppm (three Ala residues involved by CP-MAS experiment), however, is not ascribed to random coil form in spite of the peak-position but to highly distorted ~ . helix of Ala residues located at the membrane surface, because this signal is displaced to lower frequency on removal of retinal [74]. The remaining two peaks resonated at 17.3 and 17.9ppm are then ascribed to the loop region connecting the transmembrane helices. This sort of regio-specific assignment is useful in the beginning but further site-specific assignment is essential in relation to gaining insight into a relationship between the conformation and its biological function. The following approaches for this purpose have so far been utilized. Proteolytic digestion is a very convenient means of locating signals which are strongly affected by a cleavage of the specific site by an enzyme, as demonstrated in Fig. 24.15. For instance, two peaks emerge in the difference spectrum (Fig. 24.15C) between intact (Fig. 24.15A) and papain-cleaved bR's
914
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
B
t',.---.,..,-
C
22.0 !
20.0 I
18.0 !
16.0 !
ppm
I~I.0
12.0 !
Ib.O
Fig. 24.15. 13C NMR spectra of (A) intact; (B) papain-treated bR: and (C) its difference (C) [71]. The arrows indicate the peak-positions of random coil and a~-helix forms from the high frequency to the low frequencey, respectively.
(Fig. 24.15B), 16.9 and 15.9ppm, which are unequivocally ascribed to six Ala residues at the C-terminus: the peak at 16.9 ppm is assigned to Ala 245248 taking random coil conformation at the terminal end, whereas 15.9 ppm is assigned to Ala 228 and 233 taking the all-helix conformation [70, 76]. Especially, the signal from Ala 246 and 247 is also assigned, in view of the difference spectrum between carboxypeptidase A- and papain-cleaved bR [70]. An alternative site specific assignment is to compare the ~3C NMR spectrum of the wild type (Fig. 24.16A) with that of site-directed mutants (Fig. 24.16B and C) unless otherwise the three-dimensional structure of mutants is severely altered by such a site-directed mutagenisis. For instance, the assignment of [3-13C]AIa 53 is very easily done from the difference spectra using two mutants, A53G and A53V (Fig. 24.16D and E). This approach is very versatile and valuable for performing an unambigous assignment of peaks, although it is laborious to prepare a mutant of interest. Such an assignment has been made for further ten kinds of site-directed mutants of bR. The site-specific assignment of peaks is also possible with reference to
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
22.0
20.0
18.0
]6.0
14.0
12.0
915
10.0
ppm
Fig. 24.16. 13C NMR spectra of (A) [3-13C]Ala-bR; (B) -A53G; and (C) -A53V. Panels (D) and (E) denote the difference spectra between (A) and (B) and between (A) and (C), respectively [71].
the 13C NMR spectra of the 13C-labeledfragment of bR by chemical synthesis inserted into lipid bilayers. For this purpose, the [3-13C]Ala 14- and the Ala18-fragment (6-42) and -Ala51-fragment (36-71) of bR were chemically synthesized and incorporated into a bilayer of DMPC (dimyristoylphosphatidylcholine) (1:10 molar ratio) [77]. It is interesting to note that the 13C NMR signals of [3-13C]Ala51-fragment (36-71) in the bilayer resonate at the peak position of the c~ii-helix (16.2 ppm) in lipid bilayer, in contrast to the peakposition of the c~i-helix in the solid state (15.4 ppm) (Fig. 24.17). This finding suggests that the aix-helix form proposed by Krimm and Dwivedi is well reproduced by its fragment obtained by inserting into the bilayer. A detailed picture of this form can be examined in more detail by directly measuring the 13C.--15N interatomic distance of [1-13C]Ala 14, [15N]AlalS-doubly labeled A fragment of bR as 4.5 _+ 0.1 A within bilayers by R E D O R experiment
916
HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
3o'C
20G
]0G
OT;
ppm ',"
g6 "
"
....
16"
,
'
.
i
'2'o'-
J
'
'
I
i
.
.
'fo
Fig. 24.17. 13C NMR spectra of bR fragment, [3-~3C]Ala51-1abeled B (36-71) [77].
[78]. This type of work is also very important in order to clarify whether or not thermal fluctuation of the helix in the bilayer is responsible for the existence of the an-helix, because no such conformation has been taken into account by model building based on cryo-electron microscopy at low temperatures [72, 73]. This approach is also very useful to delineate the manner of helix-helix interactions in the bilayer. In contrast, the 13C DD-MAS NMR spectra of the [1-~3C]-amino acid (Ala, Leu, Val, etc. )-labeled bR detect signals only from the residues at the flexible surface area as illustrated for [1-~3C]Ala-bR in Fig. 24.18, whereas the CP-MAS NMR spectrum is selectively able to detect signals from the transmembrane helices and loops [76]. This distinction simply arises from the fact that the ~3C T~ valuses of the 1-13C carbons in the N- or C-terminus and
917
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
"'
'
""
'
I
180
'
'
'
'
I
175
'
'
'
'
I
170
'
'
'
'
ppm
Fig. 24.18. (A) 13C DD-MAS" and (B) CP-MAS NMR spectra of [1-a3C]Ala-bR.
transmembrane helices and loops are of the order of 1 s and 10 s, respectively [71, 76]. On the contrary, the TI values of the 3-13C carbons are of the order of 0.5 s and DD-MAS NMR measurement is able to detect signals from whole area of the sample under consideration [71]. Detailed examination of the carbonyl 13C NMR signal is also informative as far as the secondary structure of bR is concerned. This is because at least 13 signals of the carbonyl carbons are resolved for the transmembrane and loop regions and they are expected to be sensitive to the hydrogen bonding interactions [76]. It is also
918
HAZIME SAIT(~, SATORU TUZI AND AKIRA NAITO
emphasized here that the present N M R approach is very useful in the analysis of conformation and dynamics of bR associated with the interaction with retinal, lipid, cations, detergent, light, etc. Extensive works according to these lines are under way in our laboratory. Some illustrative examples have been described in our previous papers [71, 74, 76, 79].
24.3.3.3 Biologically-active peptides It is demonstrated that polymorphs of the crystalline biologically-active peptides, Leu- and Met-enkephalins, are also easily distinguished by 13C N M R on the basis of displacements of 13C chemical shifts [80, 81]. Recently, we showed that high resolution solid-state 13C N M R approach is a very useful mean to delineate local conformational change from a-helix to/3-sheet associated with fibril formation of selectively 13C-labeled human calcitonin [82]. In addition, we also demonstrated that chemical shift data were very conveniently used as an initial constraints to construct three-dimensional structures of Leu-enkephalin, based on accurately determined interatomic distances by R E D O R [6, 83]. This aspect of work was already described in Chapter 2.
24.4
Conclusion
We demonstrate here how the 13C N M R approach is a very useful means to reveal the conformation and dynamics of biological macromolecule with reference to the conformation-dependent displacements of peaks, with illustrative examples from polysaccharides, structural and membrane proteins and biologically active peptides. It is emphasized here that careful examination of the displacements of 13C or 15N chemical shifts can serve as an excellent probe when referred to an accumulated data base of reference samples of known secondary structure.
References
1. 2. 3. 4.
H. Sait6, Magn. Reson. Chem. 24 (1983) 835. H. Sait6 and I. Ando, Annu. Rep. NMR Spectrosc. 21 (1989) 209. H. Sait6, S. Tuzi and A. Naito, Annu. Rep. NMR Spectrosc. 35, in press. H. Sait6, S. Tuzi, S. Yamaguchi, S. Kimura, M. Tanio, M. Kamihira, K. Nishimura and A. Naito, J. Mol. Struct. 441 (1998) 231. 5. T. Gullion and J. Schaefer, Adv. Magn. Reson. 13, 57 (1990). 6. A. Naito, K. Nishimura, S. Kimura, S. Tuzi, M. Aida, N. Yasuoka and H. Sait6, J. Phys. Chem. 100 (1996) 14995.
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.
40. 41. 42.
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H. Sait6, G. Izumi, T, Mamizuka, S. Suzuki and R. Tabeta, J.C.S. Chem.Commun. (1982) 1386. J.A. Ripmeester, J. Inclusion Phenom. 4 (1986) 129. R.P. Veregin, C.A. Fyfe, R.H. Marchessault and M.G. Taylor, Carbohydr. Res. 160 (1987) 41. H. Sait6, M. Yokoi and Y. Yoshioka, Macromolecules 22 (1989) 3892. J. Blackwell, A. Sarko and R.H. Marchessault, J. Mol. Biol. 42 (1969) 379. H.C. Wu and A. Sarko, Carbohydr. Res. 61 (1978) 7. A. Imberty and S. Perez, Biopolymers 27 (1988) 1205. S. A. Ford and E.D.T. Atkins, Biopolymers 28 (1989) 1345. H. Sait6, ACS Symp. Ser. 150 (1981) 125. H. Sait6, ACS Symp. Ser. 489 (1990) 296. H. Sait6, Annu. Rep. NMR Spectrosc. 31 (1995) 157. H. Sait6 in D.M. Grant and R.K. Harris (Eds), Encyclopedia of Nuclear Magnetc Resonance, John Wiley and Sons, 1996, pp. 3740-3745. H. Sait6, H. Shimizu, T. Sakagami, S. Tuzi and A. Naito, in P. S. Belton, I. Delgadillo, A.M. Gil and G.A. Webb (Eds), Magnetic Resonance in Food Science, Royal Society of Chemistry, London, 1995, pp. 257-271. Y. Yoshioka, N. Uehara and H. Sait6, Chem. Pharm. Bull. 40 (1992) 1221. H. Sait6, Y. Yoshioka, M. Yokoi and J. Yamada, Biopolymers 29 (1990) 1689. H. Sait6, R. Tabeta, M. Yokoi and T. Erata, Bull. Chem. Soc. Jpn. 60 (1987) 4259. C.T. Chuah, A. Sarko, Y. Deslandes and R.H. Marchessault, Macromolecules 16 (1983) 1375. K. Okuyama, A. Otsubo, Y. Fukuzawa, M. Ozawa, T. Harada and N. Kasai, J. Carbohydr. Chem. 10 (1991) 645. H. Sait6, J. Yamada, Y. Yoshioka, Y. Shibata and T. Erata, Biopolymers 31 (1991) 933. H. Sait6, M. Yokoi and J. Yamada, Carbohydr. Res. 199 (1990) 1. R.H. Marchessault and Y. Deslandes, ACS Symp. Ser. 126 (1980) 221. A. Sarko and P. Zugenmaier, ACS Symp. Ser. 141 (1980) 459. H. Sait6, J. Yamada, T. Yukumoto and H. Yajima, R. Endo. Bull. Chem. Soc. Jpn. 64 (1991) 3528. M.J. Gidley and S.M. Bociek, J. Am. Chem. Soc. 107, 7040 (1985). R.P. Veregin, C.A. Fyfe, R.H. Marchessault and M.G. Taylor, Macromolecules 19 (1986) 1030. F. Horii, H. Yamamoto, A. Hirai and R. Kitamaru, Carbohydr. Res. 160 (1987) 29. R.P. Veregin, C.A. Fyfe and R.H. Marchessault, Macromolecules 20 (1987) 3007. M.J. Gidley and S.M. Bociek, J. Am. Chem. Soc. 110 (1988) 3820. F. Horii, A. Hirai and R. Kitamaru, Macromolecules 19 (1986) 930. F.R. Senti and L.P. Witnauer, J. Am. Chem. Soc. 70 (1948) 1438. J. Aketagawa, S. Tanaka, H. Tamura, Y. Shibata and H. Sait6, J. Biochem. 113 (1993) 683. H. Sait6, R. Tabeta and K. Ogawa, Macromolecules 20 (1987) 2424. H. Sait6, R. Tabeta and K. Ogawa, in M. Yalpani (Ed), Industrial Polysaccharides: Genetic Engineering, Structure/Property Relationship and Applications, Elsevier, New York, 1987, pp. 267-280. H. Sait6, T. Ohki and T. Sasaki, Biochemistry 16 (1977) 908. H. Sait6, E. Miyata and T. Sasaki, Macromolecules 11 (1978) 1244. H. Sait6, T. Ohki and T. Sasaki, Carbohydr. Res. 74 (1979) 227.
920 43. 44. 45. 46. 47. 48.
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HAZIME SAITO, SATORU TUZI AND AKIRA NAITO
M.J. Gidley, Macromolecules 22 (1989) 351. S. Arnott, W.E. Scott, D.A. Rees and C.G.A. McNab, J. Mol. Biol. 90 (1974) 253. A.I. Usov, Botanica Marina 27, 189 (1984). M.R. Letherby and D.A. Young, J. C. S. Faraday I 1981 (19??) 1953. I.T. Norton, D.M. Goodall, K.R.I. Austin, E.R. Morris and D.A. Rees, Biopolymers 25 (1986) 1009. H. Sait6, R. Tabeta, A. Shoji, T. Ozaki, I. Ando and T. Asakura, in G. Govil, C.L. Khetrapal, A. Saran (Eds), Magnetic Resonance in Biology and Medicine, Tata McGrawHilll, New Delhi, 1985, pp. 195-215. O.W. Howarth and D.M.J. Lilley, Prog. Nucl. Magn. Reson. Spectrosc. 12 (1978) 1. T. Taki, S. Yamashita, M. Satoh, A. Shibata, T. Yamashita, R. Tabeta and H. Sait6, Chem. Lett. (1981) 1803. H. Sait6, R. Tabeta, A. Shoji, T. Ozaki and I. Ando, Macromolecules 16 (1983) 1050. H. Sait6, R. Tabeta, T. Asakura, Y. Iwanaga, A. Shoji, T. Ozaki and I. Ando, Macromolecules 17 (1984) 1405. H. Sait6, R. Tabeta, A. Shoji, T. Ozaki, I. Ando and T. Miyata, Biopolymers 23 (1984) 2279. S. Spera and A. Bax, J. Am. Chem. Soc. 113 (1981) 5490. D.S. Wishart and B.D. Sykes, Meth. Enzymol. 239 (1994) 363. I. Ando, H. Sait6, R. Tabeta, A. Shoji and T. Ozaki, Macromolecules 17 (1984) 457. N. Asakawa, H. Kurosu and I. Ando, J. Mol. Struc. 323 (1994) 279. M. Ishida, T. Asakura, M. Yokoi and H. Sait6, Macromolecules 23 (1990) 88. H. Sait6, R. Tabeta, T. Asakura, Y. Iwanaga, A. Shoji, T. Ozaki and I. Ando, Macromolecules 17 (1984) 1405. T. Asakura, A. Kuzuhara, R. Tabeta and H. Sait6, Macromolecules 18 (1985) 1841. H. Sait6 and M. Yokoi, J. Biochem (Tokyo) 111 (1992) 376. W.P. Rothwell and J.S. Waugh, J. Chem. Phys. 74 (1981) 2721. A. Naito, S. Tuzi and H. Sait6, Eur. J. Biochem. 224 (1994) 729. H. Sait6, M. Ishida, M. Yokoi and T. Asakura, Macromolecules 23 (1990) 83. A.M. Gil, K. Masui, A. Naito, A.S. Tatham, P.S. Belton and H. Sait6, Biopolymers 41 (1997) 289. S. Tuzi, K. Shinzawa-Itoh, T. Erata, A. Naito, S. Yoshikawa and H. Sait6, Eur. J. Biochem. 208 (1992) 713. T. Tsukihara, H. Aoyama, E. Yamashita, T. Tomizaki, H. Yamaguchi, K. Shizawa-Itoh, R. Nakashima, R. Yano and S. Yoshikawa, Science 269 (1995) 1069. T. Tsukihara, H. Aoyama, E. Yamashita, T. Tomizaki, H. Yamaguchi, K. Shinzawa-Itoh, R. Nakashima, R. Yano and S. Yoshikawa, Science 272 (1996) 1136. S. Tuzi, A. Naito and H. Sait6, Eur. J. Biochem. 218 (1993) 837. S. Tuzi, A. Naito and H. Sait6, Biochemistry 33 (1994) 15046. S. Tuzi, S. Yamaguchi, A. Naito, R. Needleman, J.K. Lanyi and H. Sait6, Biochemistry 35 (1996) 520. R. Henderson, J.M. Baldwin, T.A. Ceska, F. Zemlin, E. Beckmann and K.H. Downing, J. Mol. Biol. 213 (1990) 899. N. Grigorieff, T.A. Ceska, K.H. Downing, J.M. Baldwin and R.H. Henderson, J. Mol. Biol. 259 (1996) 393. M. Tanio, S. Tuzi, S. Yamaguchi, A. Naito and H. Sait6, Eur. J. Biochem., submitted. S. Tuzi, A. Naito and H. Sait6, Eur. J. Biochem. 239 (1996) 294. S. Yamaguchi, S. Tuzi, T. Seki, M. Tanio, R. Needleman, J.K. Lanyi, A. Naito and H. Sait6, J. Biochem. (Tokyo), 123 (1998) 78.
POLYSACCHARIDES AND BIOLOGICAL SYSTEMS
921
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Chapter 25
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved
NMR characterization of functionalized polysiloxanes Gary E. Maciel Department of Chemistry, Colorado State University, Fort Collins, Colorado, U.S.A.
25.1
Introduction
Polysiloxanes are polymeric materials prepared by the condensation of suitably substituted silanes. For the most common form, polydimethylsiloxane (PDMS)--or "silicone rubber," the synthesis can be represented formally by the equation, n(Me)2Si(OR)2 ~ [(Me)2SiO-],, + nR20
(1)
where R = ethyl (Et) or methyl (Me). A variety of functionalized polysiloxanes can be prepared by analogous condensations of monomers of the type, R'Si(OR)3 (I), where R' is an organic moiety which includes a specific functional group, or by condensation of a combination of monomers of type I and Si(OR)4 (II). The products of condensation of type I and type II monomers contain structural moieties with local structures of types, Si(--Om)4 and R'Si(mO~)3. The chemical functionality of the group R' is, of course, chosen to convey some specific property to the functionalized polysiloxane. Much work has been carried out with ligands grafted onto silica surfaces or other metal oxides [1-10], and substantial research on functionalized polysiloxane ligands has been published [11-18]. These immobilized ligand systems have many potential applications, including preconcentration and separation of metal ions from aqueous solutions [5, 19-22]. A number of useful spectroscopic and chemical tools have been employed to study ligandmodified silica systems [19-27], and high-resolution solid-state NMR techniques have been used to characterize structures of organosilane-modified silica surfaces [6-10, 28-42]. In contrast, comparatively little detailed attention has been paid, until very recently, to the chemical structures of the analogous polysiloxane-immobilized ligands that have been made through the sol-gel process. Substantial research has focused on immobilized organofunctional groups,
924
GARY E. MACIEL
either anchored on silica [1-10, 43] or incorporated into a polysiloxane framework [11-14, 44] through hydrolytic condensation processes. Various organofunctional groups have been employed in such systems, including amines [11, 14, 16-18], phosphines [13, 18], and others [12, 37, 15, 16, 18]. High-resolution solid-state nuclear magnetic resonance (NMR) techniques [6-10, 15-18, 28-41], as well as other spectroscopic methods [22, 25, 35, 4650], have been used to examine the structural properties of these systems. Polysiloxane-immobilized ligands have several potential advantages over those grafted onto silica surfaces, including the fact that two ligands can be introduced simultaneously in a desired molar ratio in the polysiloxane polymerization process. Thus, new types of polysiloxane-immobilized ligand systems, in which two different types of ligand groups, e.g., amine and thiol, or amine and phosphine, or phosphine and thiol, are incorporated together into a three-dimensional structure by the sol-gel process. Despite the potential advantages of functionalized polysiloxanes, their chemical structures and properties have until recently remained uncertain. Solid-state NMR proved useful in elucidating the structures of these kinds of systems. The NMR approaches that have been employed include natural-abundance, high-resolution solid-state 13C, 15N, 29Si, and 31p NMR with cross polarization and magic-angle spinning (CP-MAS) [51] and 1H NMR with CRAMPS (combined rotation and multiple-pulse spectroscopy) [52]. 13C NMR is useful in establishing the primary structures of the pendant groups of some functionalized polysiloxanes and in confirming these structures in other cases. Establishing the primary pendant group structure is also aided in appropriate cases by 15N, 31p and/or 1H NMR data. 15N results can also play a role in establishing the presence and nature of proton transfer and/or hydrogen bonding, e.g., in amino-functionalized polysiloxanes. 29Si CP-MAS NMR experiments are valuable for assessing the nature of the polysiloxane framework and the local siloxane structures in the region of the pendant group attachments. Relaxation studies, including measurement of 1H spin-lattice relaxation times via X-detected 1H--.X cross-polarization techniques (e.g., for X = 13C, 29Si or 31p) are useful for both assessing the general degree of atomic-level mobility and establishing domain-based homogeneity/heterogeneity. This paper summarizes the extensive solid-sample NMR results obtained by this research group (initiated by I.M. E1-Nahhal) over the past few years (15-18) on functionalized polysiloxanes prepared by the sol-gel method. Additional details can be found in the Ph.D. dissertation [71] of Jane Jie Yang and in references 15-18.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
25.2
925
Experimental section
Most of the high-resolution solid-state 13C NMR experiments were carried out at 25.1 MHz on a home-built spectrometer using cross polarization (CP) and magic-angle spinning (MAS) with high-power IH decoupling [51]; some were carried out at 64.8 MHz on a modified Chemagnetics M-260S spectrometer. 15N CP-MAS experiments were carried out at 20.3 MHz on a hybrid spectrometer (severely modified Nicolet NT-200), using a 2.5 cm 3 MAS system (Chemagnetics Pencil). 298i CP-MAS experiments were carried out at 39.8 MHz on the hybrid NT-200 spectrometer and at 51.2 MHz on the modified M-260S spectrometer. High-resolution solid-state 1H NMR experiments, based on the CRAMPS [52] method, were carried out at 360 MHz on a hybrid spectrometer (severely modified NT-360). 31p CP-MAS spectra were obtained at 67.4 MHz on a modified 150 MHz spectrometer (Nicolet NT-150) or at 80.9 MHz on the modified Nicolet NT-200 spectrometer. All the 13C, 31p and 298i CP-MAS spectra, except where otherwise indicated, were obtained with a 1-ms CP contact time and 1-s recycle delay. 15N CP-MAS spectra were obtained with a 0.6-ms CP contact time and 0.6s recycle delay. The chemical shift conventions are in ppm, determined via sample substitution with respect to the 298i, 13C or 1H resonance of liquid tetramethylsilane (TMS), with respect to the 15N resonance of liquid ammonia at 25~ and with respect to the 31p resonance of 85% H3PO4. Higher numbers correspond to lower shielding. Most of the functionalized polysiloxane systems were made by hydrolytic condensation of Si(OEt)4 and (R'O)3Si(CHz)3X compounds, where X = amine, phosphine, thiol or chloro groups, and R' = Me or Et. If an amino group was present in the reaction mixture, a separate catalyst was not required. In the absence of amino groups, gelation required the presence of a catalyst, either HC1 or ( n - B u ) z S n ( O C O C H 3 ) 2 . In a few cases (vide infra) the desired functionalized polysiloxanes were synthesized from an intermediate, (S)-CHzCHzCHzC1 (where (S) represents the polysiloxane framework), by displacement reactions with suitable reagents.
25.3
25.3.1
NMR results
13C CP-MAS spectra
Functionalized polysiloxanes of the type, (S)-Y. Figures 25.1-25.7 show representative 13C CP-MAS spectra of the functionalized polysiloxanes studied in which there is only one type of pendent group per sample. Strict quantitation
926
GARY E. MACIEL 1
3 ~--_.O
1
2
~ (
,.
3
2
|
I
B 1 2 3 4 5 i--CH2CH2CH2NHCH2CH2NH2 1
D --
,,
1 2 3 4 5 6 7 i--CH2CH2CH2NHCH2CH2NH CH2CH2NH2 A4,5,6,7
1
E 200
100
6 13C (PPM)
0
Fig. 25.1. 13C CP-MAS NMR spectra of the untreated (A) and protonated (pH = 1) (B) polysiloxane-immobilized monoamine ligand, (S)-CH2CH2CH2NH2, of the untreated (C) and protonated ( p H - 1) (D) polysiloxane-immobilized diamine system, (S)CH2CHCH2NHCH2CH2NH2, and of the untreated polysiloxane-immobilized triamine sample, (S)-CH2CH2CH2NHCH2CH2NHCH2CH2NH2 (E). Ref. 17.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
~)-c~i--CH2CH2CH2CI ,,,"J "__O
1
2
3
3
927
2
A
E" t"
E) r
_.2.,
.Q
<.<_. U) C:
,..., r m
Y 2',3'
1I
2. 3. I~'--/SiCH2CHaCH2S ~ ~,,,. ,
, 9
200
,C 9
9
9
|
100
9
=
13C (PPM)
9
9
|
9
9
9
0
Fig. 25.2. 13C CP-MAS NMR spectra of 3-chloropropylpolysiloxane, (S)CH2CH2CH2CI, via HCl-catalyzed preparation (A) and via (n-Bu)2Sn(O2CCH3)2-catalyzed preparation (B) and of the polysiloxane-immobilized thiol system,(S)-CH2CH2CH2SH (C). Ref. 17.
cannot be claimed for the populations of structural moieties detected in CPMAS experiments, unless the relevant CP dynamics are characterized in detail and this was not done for most of the samples represented in this paper (see the exceptions below). Nevertheless, because a consistent set of experimental CP parameters were used for each nuclide in this investigation, the CP-MAS intensities obtained for each type of material are valid qualitatively, and in some cases semi-quantitatively. As indicated above, in general, solid-state 13C NMR is very useful in characterizing the organic
928
GARY E. MACIEL
A
/i...-m..~.~lt~-~
~
,,
-~_
~.,"
-----
B ...
.~ -~
2us ~
lr
-
lr -m
~
-
"
-"
20 us
I,..
50 us
...
(n
C
2us
20 us
/
~ =
5{:)us 200
9
"
'"
~
I
100
.
.
.
.
9
0
"
"
"
813C (PPM)
Fig. 25.3. Dipolar-dephasing 13C CP-MAS NMR spectra of the untreated (A) and Hg2+treated (B) polysiloxane-immobilized monoamine system and the untreated 3-chloropropylpolysiloxane, prepared with HC1 as catalyst (C). Dephasing period shown in IXS. Ref. 17.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
929
C =h
5
#
2#
#
, '
1
300.0
i
j I
200.0
2,3 1
'
!
!
1
100.0
0.0
-100.0
S 13C (PPM)
Fig. 25.4. 1 3 C CP-MAS NMR spectra of polysiloxane-immobilized phosphine systems. (A) diphenylpropylphosphine, (S)-CHECHECH2PPh2; (B) diphenylethyl-phosphine, (S)-CHzCHzPPh2. Ref. 18.
structures in polysiloxane-immobilized ligands and the degree of hydrolysis of the ethoxy and/or methoxy groups. Figure 25.1 shows the 13C CP-MAS spectra of (S)-CH2CH2CH2NH2 (A), its protonated form (B), the diamine (S)-CH2CH2CH2NHCH2CH2NH2 (C and D) and the triamine (S)-CH2CH2CH2NHCH2CH2NHCH2CH2NH2 (E). Figure 25.2 shows the 13C CP-MAS spectra of (S)-CH2CH2CH2C1 samples prepared with two different catalysts (A and B) and of the thiol-immobilized polysiloxane, (S)-CH2CH2CH2SH (C). Figure 25.3 shows 1H-13C dipolardephasing 13C CP-MAS spectra of samples represented in Figures 25.1 and 25.2. Dipolar-dephasing spectra are obtained by inserting a dephasing period between the CP period and detection; during the dephasing period, the carbons with directly attached hydrogens (except methyl groups, for which rapid rotation dramatically attenuates 1H-13C magnetic dipolar interactions) undergo rapid dephasing (on a time scale of tens of ~s); and hence their 13C magnetizations are dramatically depleted before (and during) 13C detection. Figure 25.4 shows the ~3C CP-MAS spectra of the phosphine-immobilized polysiloxanes, (S)-CH2CH2CH2PPh2 (A) and (S)-CH2CH2PPh2 (B). Figure 25.5 shows 13C CP-MAS spectra of the polysiloxane-immobilized diethylpro-
930
GARY E. MACIEL 1
3
2
4
C
2
D --
.
-
-
m
I
!
!
!
240.0
160.0
80.0
0.0
130
(PPM)
Fig. 25.5. 1 3 C CP-MAS NMR spectra of amine-based functionalized polysiloxanes. (A) Primary 1 2 3 amine, (S)-CH2CH2CH2NH2. (B) Primary amine system of Fig. 25.5A after silylation of accessible silanols with hexamethyldisilazane, (Me3Si)2NH. C) Diethyl-propylamine system, D)Trime1 2 3 + 4 thyl-ammonium chloride system, (S)-CH2CH2CH2N(CH3)3C1. Asterisks indicate signals from unhydrolyzed ethoxy and methoxy groups. Ref. 15.
pyl amine ligand, (S)-CH2 CH2CH2N(CH2CH3)2 (C) and the analogous trimethammonium chloride system, (S)-CHzCHzCHzN+(CH3)3C1- (D), together with the parent monoamine sample before (A) and after (B) silylation of the accessible silanols (--~SiOH groups) with hexamethyldisilazane, (Me3Si)zNH. Figures 25.6 and 25.7 present 13C CP-MAS spectra of (S)-CHzCHzCHzX systems in which X=~NHCHzCOzCH3 (6A), ~SCHzCOzCH3 (6B),
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 931 1 2
6
3
A a__ h _
_
6
i
240.0
i
160.0
i
80.0
2,3 1
i
0.0
130 (PPM)
Fig. 25.6. 13C CP-MAS NMR spectra of some polysiloxane-immobilized ligand systems. (A) 1 2 3 4 5 6 1 2 3 4 5 6 (S)-CH2CH2CH2NHCH2CO2CH3. (B) (S)-CH2CH2CH2SCH2CO2CH3.Asterisks indicate signals from unhydrolyzed ethoxy and methoxy groups. Ref. 15. --O2CCH2NH2 (7A),--O2CCH2NHCH2CO2 (7B), and--O2CCH3 (7C). Table 25.1 summarizes the 13C chemical shifts of functionalized polysiloxanes of the type (S)-Y. Functionalized polysiloxanes of the type, X-(S)-Y. Figures 25.8-25.11 show 13C CP-MAS spectra of functionalized polysiloxanes in which more than one type of pendant group is present in each sample, prepared (in most cases) by the simultaneous use of (RO)3SiCH2CH2CH2X reagents with two types of functional groups, X. Figure 25.8 shows 1 3 C CP-MAS spectra of polysiloxane samples in which both mCH2CH2CH2NH2 and mCHzCH2CH2SH moieties (A and B) or both ~CHzCHzCHzNHCHzCHzNH2 and ~CHzCH2 CHzSH moieties (C and D) are present, before (A and C) and after (B and D) HC1 treatment. Figure 25.9 shows dipolar-dephasing results corresponding to the samples not treated with HC1. Figure 25.10 presents 13C CP-MAS spectra of samples containing binary combinations of ~CHzCHzCHzPPh2 or ~CHzCHzPPh2 with --CHzCHzCHzNH2 or mCHzCHzCHzNHCHzCHzNH2 groups. Figure 25.11 shows 13C CP-MAS spectra of samples containing combinations of mCHzCHzCHzSH pendant groups and mCHzCHzCHzPPh2 (A) or
932
G A R Y E. M A C I E L 1 3
2
4
t
1,,
4,7
B _ ,,. . . . . .
~
~
-
~
_= 3
5
4
L
___
__ ~
-
-
- ~
I
240.0
-
|
160.0
-
~
_ _ - _ ~ . . . -
-
!
|
80.0
0.0
-.___~.a..._
_
130 (PPM)
Fig. 25.7. (A) 1
13C CP-MAS NMR 1 2 3 45 (S)-CH2CH2CH202CCH2NH2 2 3 45
CH2CH2CH202CCH3.
spectra of some polysiloxane-immobilized ligand systems. .
1 2 3 45 (B)(S)-CH2CH2CH202CCH2NHCH2CO
6
7
2.
(C)(S)-
Asterisks indicate signals from 3-chloropropylpolysiloxane, (S)-
~I-I2~-I2~I-I2C1. Ref. 15.
(B) pendant groups. Table 25.2 collects 13C chemical shifts of functionalized polysiloxanes of the type, X-(S)-Y.
m CH2CH2PPh2
25.3.2
15N CP-MAS results
Although 15N MAS spectra are typically difficult to obtain in natural abun-
dance (0.3%), the combination of signal-enhancement due to cross polarization and large sample volumes (e.g., about 2.5 cm 3 in the present case) makes it possible to obtain usable 15N spectra on polysiloxane-immobilized amines. One expects that 15N NMR data should be useful in assessing the acid/base and/or hydrogen-bonding status of pendant amino groups. Figure 25.12 shows 15N CP-MAS spectra of the polysiloxane monoamine system at various pHs. Figure 25.13 shows 15N CP-MAS spectra of treated (protonated
Table 25.1. a3c chemical shifts (ppm) a of the polysiloxane-immobilized systems of the type, (S)-Y. Sample
C 1
1
2
C 2
C 3
C 4
C 5
(S)-CH2CH2CH2X X = NH2 NH2 (treated with [Me3Si]2NH)
27.6 27.9
45.4 45.2
11.7
24.0
42.9
53
53
12.0
24.3
43.0
52
52
12.6
21.8
57.0
47.8
12.6
11.2
18.6
69.1
54.0
11.1
22.4
43.7
60.8
9.7
28.3
28.3
35.8
65.0 66.4 (47.8) 66.6 47.8
175.8 br 172.5 br 170.9
C1
9.7 10.9
26.5 (22.9) 27.3 (23) 27.4 27.3
SH
12.6
28.7
28.7
PPh2
14.9
27.9
27.9
6.8
27.4
5
NHCH2CHzNH2 4
5
6
7
NHCHzCHzNHCH2CHzNH2 4
5
N(CHzCH3)2 +
4
N(CH3)3C14
5
6
NHCH2CO2CH3 4
5
6
SCHzCO2CH3 45
02CCH2NH2 45
6
10.4 7
OzCCHzNHCH2CO2
10.9
45
02CCH3
1
OEt
and/or
OMe
Other ~z
11.3 11.2
4
C 6
3
2
(S)-CHzCHzPPh2
2.0(SiMe3) 52(C-7)
52
27.2(C[) b N >
60.0 168.4 171.9 172.8
ZZ >
53.2 53.2
9 Z 9
18.9, 60.7 51.9
55.5 br 10.9, 27.3 47.8 b 20.5
47.8
18.0, 60.1 51.7 19.0, 60.8 51.9 19.1 60.5 18.7, 60.2 51.6
O Z
(C;) b
N
9 131.1 139.5sh (C-Ph) 130.2 (Ph)
a 13C chemical shifts in ppm relative to tetramethylsilane (TMS)" br --- broad peak" sh = shoulder; estimated errors in the values of 13C chemical ~, 2, 3, shifts are -+0.5 ppm. b Signals from 3-chloropropylpolysiloxane (S)-CH2CH2CH2C1. '
t-" 9 X > Z
t.~ ta3
934
GARY E. MACIEL
t
i
2 3,4 A5 lull ' I I V I
.% 1 2 3 4 5 ~iCH2CH2CH2N HCH2CH2NH2 ;4,,.1' 2' 3'
2 2
J .,,%1
~
I
"1 . . . . 200
I . . . . 100
2
3
iCH2CH2CH2NH2
j , j , , l ' a' 3' :;2f--/SiCH2CH2CH2S H
I 0
. . . .
1 . . . . -100
I -200
513C (PPM) Fig. 2.5.8. 13C CP-MAS NMR spectra of untreated (A) and treated (with 0.11 M HC1) (B) polysiloxane-immobilized thiol-monoamine system and of untreated (C) and treated (with 0.11 M HC1) (D) polysiloxane-immobilized thiol-diamine system. Ref. 16.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
935
C
B
A
'
'"
'
I
. . . .
200
I 1oo
. . . .
i o
. . . .
i -'too
. . . .
l -200
130 (PPM) Fig. 25.9. 13C CP-MAS NMR spectra obtained with 50-1xs dipolar dephasing. (A) Polysiloxaneimmobilized thiol system; (B) polysiloxane-immobilized thiol-monoamine system; and (C) polysiloxane-immobilized thiol-diamine ligand system. Ref. 16.
or Cd2+-treated) polysiloxane-immobilized monoamine (A and B), diamine (C and D) and triamine (E) systems. Figure 25.14 shows 15N CP-MAS dipolar-dephasing spectra of protonated samples of the polysiloxane-immobilized monoamine (A), diamine (B) and triamine (C) systems. 25.3.3
31p CP-MAS results
Figure 25.15 presents 31p CP-MAS spectra of (S)-CH2CH2CH2PPh2 (A), (S)-CHzCHzPPh2 (C) and their corresponding analogues with monoamine pendent groups (B and D), respectively. 25.3.4
1H NMR spectra
Although often considered to be difficult or unpromising experiments, 1H NMR experiments based on the CRAMPS technique [52] are relatively straightforward with a modern solid-state spectrometer, yielding linewidths that typically have no significant contribution from 1H-1H dipolar interactions (unless molecular dynamics interfere with the line-narrowing methods) and are dominated by chemical shift dispersion. For the functionalized polysiloxanes on which this paper focuses, one expects that the 1H chemical shifts observed can confirm the primary pendent-group structures indicated by 13C (and perhaps 15N or 31p) NMR results, can perhaps provide some information on silica-like regions (e.g., displaying silanols) and perhaps shed light on the acid/base and/or hydr0gen-bonding characteristics of the pendent groups.
936
GARY E. MACIEL
i
2.2' 3'4' ~# 1,1'
D Cph ~
2,3,2' 3',4'~/~1.1'
11
Cp
~
22'
cP./
#
! !
I
300.0
200.0
1,1'
23 , 1
100.0
I 0.0
1 -100.0
[i 13C (PPM) Fig. 25.10. 13CCP-MAS NMR spectra of polysiloxane-immobilizedphosphine-amine systems. (A) Diphenylpropylphosphine-monoamine system. (B) Diphenylethylphosphine-monoamine. (C) Diphenylpropylphosphine-diamine. (D) Diphenylethylphosphine-diamine. The symbol # indicates signals from unhydrolyzed ethoxy and methoxy groups. Asterisks indicate spinning sidebands. Ref. 18.
Figure 25.16 shows 1H CRAMPS spectra of the polysiloxane-immobilized thiol system (A), thiol-monoamine system (B), the thiol-monoamine system treated with HC1 (C), the thiol-diamine system (D) and the thiol-diamine system treated with HC1 (E). Figure 25.17 shows 1H CRAMPS spectra of the untreated (A), protonated (B), CdZ+-treated (C) and HgZ+-treated (D)
NMR C H A R A C T E R I Z A T I O N OF F U N C T I O N A L I Z E D P O L Y S I L O X A N E S
Cph
937
#
2,2',3'
1,1 I
A I
300.0
k___.vJ I
200.0
1
i
100.0 5 130
L_ 0.0
.........
itaw
I
-100.0
(PPM)
Fig. 25.11. 13C CP-MAS NMR spectra of polysiloxane-immobilizedphosphine-thiol systems.
(A) Diphenylpropylphosphine-thiol. (B) Diphenylethylphosphine-thiol. Ref. 18. polysiloxane-immobilized monoamine and of the untreated chloropropyl system, (S)-CHzCHzCHzC1 (E). 25.3.5
29Si C P - M A S
results
29Si NMR data have proven to be enormously valuable on silicas and derivatized silicas [6-10, 33, 34, 36, 41], and have also been found to be very useful in characterizing the polysiloxane backbones of functionalized polysiloxanes [15-18]. Figure 25.18 shows 298i CP-MAS spectra of the polysiloxane-immobilized primary amine, (S)-CHzCHzCHzNH2, before (B) and after (C) silylation with (Me3Si)zNH; the polysiloxane-immobilized trimethylpropyl ammonium chloride system, (S)-CHzCHzCHzN+(CH3)3C1- (D); and silica gel for comparison (A). Figure 25.19 presents 29Si CP-MAS spectra of two different preparations of the polysiloxane-immobilized monoamine system (A and B), two different preparations of (S)-CHzCHzCHzC1 (C and D), the polysiloxane-immobilized diamine system (E) and the triamine system (F), and of the monoamine system after HC1 treatment (G). Figure 25.20 shows 29Si CP-MAS spectra of (S)-CHzCHzCHzNHCHzCOzCH3 (A), (S)CHzCHzCHzSCHzCOzCH3 (B), and (S)-CHzCHzCHzSH. Figure 25.21 presents 29Si CP-MAS spectra of (S}-CHzCHzCHzC1 (A), (S)CHzCHzCHzN(CHzCH3): (B), (S)-CHzCHzCHzOzCCHzNH: (C), (S)CH2CH2CH202CCH2NHCH2CO 2 (D) and (S)-CH2CH2CH202CCH3 (E).
vO r
Table 25.2 913C Chemical shifts (ppm) 1 of polysiloxane-immobilized systems of the type X-(S)-Y Sample
Cl
C2
C3
C[
C2
C3
11.9 (12.0) b
28.0 (22.8)
44.7 (43.7)
11.9 (12.0)
28.0 (28.3)
28.0 (28.3)
12.1 (12d) b
25.8sh (22.9)
52.1 (51.6)
12.1 (12.1)
28.3 (27.9)
28.3 (27.9)
/CH2CH2CH2PPh2 (S). \CH2CH2CH2NH2
10.9
26.5
26.5
10.9
26.5
45.2
/CH2CH2CH2PPh2 (S). \ CH2CH2CH2NHCH2CH2NH2
11.7
1
2
3
1
2
3
1'
2'
3'
/CH2CH2CH2NH2 (S), ,, 2, ~, \CH2CH2i~H2SH 4
5
/CH2CH2CH2NH2NHCH2CH2NH2 (S)\
C,~
C~
52.1 (51.6)
42.3 (39.3)
Cph
OEt and/or t'n OMe >
"CH2CH2CH2SH 1
1' 1
1'
2
2' 2
2'
3
3' 3
3'
4'
5'
22.3
22.3
11.7
22.3
51.9
129.6, 132.9, 140.2 51.9
41.3
128.7, 132.5, 140.2
19.3, 59.2
18.2, 58.4, 51.9
a
2
3
/CHzCHzCH2PPh2 (S).
14.9
29.1
29.1
14.9
29.1
29.1
131.6
19.3, 60.2 51.0 Z
\CH2CH2CH2SH 1'
2'
3'
2
/CH2CH2PPh2 (S),
4.4
26.8
-
10.9
26.8
45.8
6.8
22.9
-
10.9
22.9
51.4
130.0, 138.4sh
19.0, 60.9 50.8
a: >
129.8, 18.5, 59.5, 139.3sh 51.4 H 2 N
E
\CH2CH2CH2NH2 1'
2'
a
2
3'
/ CH2CH2PPh2 (S)Q C H 2 1'
2'
1
2
3'
/ CH2CHzPPh2 (S).
C 4'
H
2
C
H
2
N
H
C
H
2
C
51.4 H
2
41.5 N
~>
5'
6.8
28.4
-
14.5
28.4
28.4
131.4,
19.2 , 59 .8 51.6
O:Z 9
\CH2CH2CH2SH a' 2' 3'
a a3C chemical shifts in ppm relative to tetramethylsilane (TMS)" sh = shoulder" estimated errors in the values of 13C chemical shifts are _+0.5 ppm. 0 b Numbers in parentheses correspond to samples after treatment with HC1. :Z > N 0 .< 0 > Z rID tao
940
GARY E. MACIEL
A ~9
B
pH
3.5
C
pH
5.0
~
e-
s
[
9'
,,
I ' ' ' '
150
I ' ' ' '
100
I ' '
50
'
'
I ' ' ' '
0
,
I'
-50
'
'
'
I
-100
'
15N (PPM)
Fig. 25.12. 15N CP-MAS NMR spectra of the polysiloxane-immobilized monoamine system, (S)-CHzCH2CHzNH2, at various pH's. Ref. 17.
Figures 25.22A and 25.23A show 298i CP-MAS spectra of (S)CHzCHzCHzPPh2 and (S)-CHzCHzPPh2, respectively. Figure 25.24 shows 298i CP-MAS spectra of the polysiloxane-immobilized thiol-monoamine system (A), and of the thiol-diamJne system without treatment (B) and after treatment with HC1 (C). Figures 25.22B, 25.22C, and 25.22D show 298i CP-MAS spectra of the diphenylpropylphosphine-monoamine, diphenylpropylphosphine-diamine, and diphenylpropylphosphinethiol systems respectively. Figures 25.23B, 25.23C and 25.23D show 29Si CPMAS spectra of the diphenylethylphosphine-monoamine, diphenylethylphosphine-diamine and diphenylethylphosphine-thiol samples, respectively. For purposes of avoiding the quantitation problems associated with CPMAS spectra, for which intensities in the spectra of Figs. 25.18-25.24 have not been corrected on the basis of the relevant spin dynamics (not characterized for these samples), a few 29Si MAS spectra were obtained via direct polarization (DP-MAS), i.e., with no cross polarization involved, so that 298i magnetization is generated directly via 29Si spin-lattice relaxation.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
"
L'I
-_
941
2
1 2 A
--....
1
2
,,,.,
o~ re-
A.3
~-_o.
,
2
3
F
9 I " 150
'
''""1
'" 100
'
'
I" 50
'
'
0 I ' 0
'
'
'
t . . . . -50
"1 ' -100
"-
5 I'SN (PPM)
Fig. 25.13. ~SN CP-MAS NMR spectra of: the untreated (A) and Cd2+-treated (B) polysiloxane-immobilized monoamine samples; the nonprotonated (C) and protonated (pH = 1) (D) polysiloxane-immobilized diamine system; the untreated polysiloxane-immobilized triamine system (E); the untreated thiol-monoamine system (F); and the untreated thiol-diamine system (G). Ref. 17.
942
GARY E. MACIEL
r
I
A ._.j
A i-
D .Q L.
r(1) r-
c
1
150
100
50 0 15N (PPM)
-50
-100
Fig. 25.14. Dipolar-dephasing 15N CP-MAS spectra of the protonated (pH = 1) polysiloxaneimmobilized monoamine system (A), protonated (pH = 1) polysiloxane-immobilized diamine system (B) and protonated (pH = 1) polysiloxane-immobilized triamine system (C). Dephasing period shown in Ixs. Ref. 17.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
943
~
t'--
t_ t~ v ,,,.,. oN gO t-" (I) t'-,.i
c
o
16C).0
,It
i
80.0
i
0.0
,It
i
-80.0
~531p (PPM)
Fig. 25.15. 31p CP-MAS NMR spectra of polysiloxane-immobilized phosphine systems. (A) diphenylpropylphosphine, (S)-CH2CH2CH2PPh2; (B)diphenylpropylphosphine-monoamine; (C) diphenylethylphosphine, (S)-CH2CH2PPh2" and (D) diphenylethylphosphine-monoamine. Asterisks indicate spinning sidebands. Ref. 8.
Figure 25.25 shows 298i DP-MAS spectra of (S)-CH2CH2CH2NH2 and (S)CHzCHzCHzC1. These spectra were obtained with repetition delays of 300 s, which was shown by a rough evaluation of T si to be more than adequate for avoiding intensity distortions.
944
GARY E. MACIEL
~._~
~a'.o
81o
4:o
o'.o
1'
2'
3'
iCH2CH2CH2SH
-4'.o
-~.o
a 1H (PPM)
Fig. 25.16. 1H CRAMPS spectra. (A) Polysiloxane-immobilized thiol system" (B) polysiloxaneimmobilized thiol-monoamine system; (C) polysiloxane-immobilized thiol-monoamine system treated with 0.11 M HC1; (D) polysiloxane-immobilized thiol-diamine system; and (E) polysiloxane-immobilized thiol-diamine system treated with 0.11 M HC1. Ref. 16.
25.3.6
Relaxation measurements
~H spin-lattice relaxation times were measured on a few selected samples via 13C, 29Ni or 31p detection in 1H ~ X (X = 13C, 29Si or 31p) CP-MAS experi-
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
945
2 1
/~
/J -'_O
4
_
1
2
3
4
i-CH2CH2CH2NH2
A
1.r
A (/) r
::) ,.Q i,_
>.,
C
u~ t(9 ..,.,.
~_o
8:0
41o
o:o
-41o
-8'.0
a 1H (PPM)
Fig. 25.17. 1H CRAMPS NMR spectra of the untreated (A), protonated (pH = 1) (B), Cd 2+treated (C), and Hg2+-treated (D) polysiloxane-immobilizedmonoamine system; and untreated
3-chloropropylpolysiloxane (E). Ref. 17.
ments with 1H inversion-recovery, at various magnetic field strengths. The T~ results are summarized for various functionalized polysiloxanes of types (S)-Y and X-(S)-Y in Tables 25.4 and 5, respectively.
4~
Table 25.3. 1H Chemical shifts of some functionalized polysiloxanes a.
Sample
MeO, E t O
H2, H:~
H3, H;
/H + b
0.8 1.1
/Cd2+ b
0.9 0.9
1.7 2.0 1.9 1.9
2.7 3.1 2.9 3.0
I. 1
2.0
3.6
1.4, 3.6-4.1
>
1.1
1.9
2.7
1.4, 3.7, 3.9
,<
0.9
1.7
2.6
1
2
3
(S)-CH2CH2CH2NH2
/Hg2+ b 1
2 2
3
(S)-CHzCHzCHzSH 1
2
3
/CHzCHzCH2NH2 (S)\
1'
5.1br 7.8 7.3 7.8
3
(S)-CHzCH2CH2C1 1
H4, Hs
NH, NH2
H1, H[
2'
>
3,
\CH2CH2i~H2SH /H + b 1
2
3
4
/ CH2CH2CH2NH2NHCH2CH2NH2 (S)\
1'
2'
1.1
1.8
1.2
1.8
5
7.8
2.7 3.1br
3'
\CHzCH2i~H2SH /H + b a Chemical shifts in ppm relative to tetramethylsilane -+0.1 ppm. br = broad. After treatment with HCl(aq), Cd2+(aq) or Hg2+(aq).
b
3.7br
8.1br
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
i
50.0
i
0.0
'
I
-50.0
i
-100.0
947
!
-150.0
8 29Si (PPM)
Fig. 25.18. 298i CP-MAS NMR spectra of (A) silica gel" (B) primary amine system, (S)CH2CH2CHzNH2; (C) silylated p+rimary amine system; and (D) trimethylpropylammonium chloride system, (S)-CHzCHzCHzN(CH3)3C1-.Ref. 15. 25.4
25.4.1
Discussion
13C NMR spectra
Functionalized polysiloxanes of type (S)-Y. 13C NMR spectra of the polysiloxane-immobilized amine ligands (Fig. 25.1) show the absence of residual ethoxy or methoxy signals, whereas 13C spectra of the polysiloxane-immobilized 3-chloropropyl and 3-propylthiol systems (Fig. 25.2) show strong signals from residual ethoxy (18 and 60ppm) and methoxy (51 ppm) groups. This shows the catalytic effect that amino groups appear to have in promoting the hydrolysis/condensation reactions responsible for the formation of the gel/polymer systems. The 13C NMR spectrum of the untreated polysiloxane-immobilized monoamine ligand (Fig. 25.1A) shows three signals, at 10.7, 27.4 and 44.9ppm, whereas the spectrum of the corresponding protonated system (washed with 0.1 M aqueous HC1) of Fig. 25.1B displays three signals, at 10.5, 21.3 and 43.1 ppm. By analogy with literature reports on the corresponding modified
948
GARY E. MACIEL
o..
e-
~ r e"
E
l-
1
o.oo
I
-so.oo
1
-loo.oo
i
.lso.oo
8 ~Si (PPM)
Fig. 2.5.19. 29Si CP-MAS NMR spectra of different preparations of: the polysiloxane-immobilized monoamine system (different ratios of reactants) (A and B)" the 3-chloropropyl system, prepared via catalysis by 0.1 M HC1 (C) and by (n-Bu)2Sn(O2CCH3)2 (D); the polysiloxaneimmobilized diamine system (E)" the polysiloxane-immobilized triamine system (F)" and the polysiloxane-immobilized monoamine system after treatment with 0.10 M HC1 for 24 hours (G). Ref. 17.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
949
A
B --__L
U
1
50.0
0.0
i
1
-50.0 29Si
Fig.
25.20.
298i CP-MAS
NMR CHzCHz"CHzNHCHzCO2CH3; (B) 1 2 3 CH2CH2CH2SH" Ref. 15. 1
2
.,
4
5
6
-100 .0
--___,,.'.
[
- 150.0
(PPM)
spectra of polysiloxane systems. 1 2 3 4 5 6 (S)-CH2CH2CH2SCH2COzCH3 and
(A) (C)
(S)(S)-
silicas [37-41, 53], these signals are assigned to C(1), C(2) and C(3) carbon atoms, respectively. Carbon atom C(2) manifests a substantial increase in shielding from 27.4 ppm to 21.3 ppm upon protonation (Figs. 25.1A and 25.1B), whereas C(1) and C(3) display only minor shifts (1-2 ppm) on protonation. This behavior is in agreement with previous results reported in the literature on aminopropylsilane(APS)-modified silica [6, 30, 38]. A low-shielding chemical shift to about 27.4 ppm for C(2) was found for the polysiloxane-immobilized amine ligand upon washing the acid-treated material with aqueous 0.1 M NaOH. The 21ppm C(2) chemical shift in the spectrum of Fig~ 25_5D is consistent with what one might have expected for the the 3-(trimethylammonium) propylpolysiloxane system and the 27 ppm chemical shift is consistent with the C(2) value reported for an APS-modified silica in which most of the residual silanol groups were capped by a silylation reaction, eliminating them from participation in hydrogen bonding or proton transfer [38]. Caravajal et al. [6] have indicated, on the basis of 13C chemical shift arguments, that the amino groups of APS grafted onto uncapped silica are involved in hydrogen bonding and/or BrOnsted protonation by acidic silanols of the silica surface and the large increase in shielding of C(2) is due to protonation of the amino group. The 13C NMR spectrum of the untreated polysiloxane-immobilized di-
950
GARY E. MACIEL
D
E
I
50.0
. . . . . . . .
1.
0.0
.
.
.
I'
-
-50.0
1
-
-100.0
9
-
-150.0
--
(PPM)
29Si
Fig. 25.21. 295i CP-MAS NMR spectra of polysiloxane-immobilized systems. (A) (S)1
2
3
1
2
3
4
5
1
2
3
45
CH2CH2CH2CI" (B) (S)-CH2CH2CH2N(CH2CHs)2; (C) (S)-CH2CH2CH202CCH2NH2; (D) 1 2 3 45 6 7 1 2 3 45 (S)-CH2CHzCH202CCH2NHCH2CO~; and (E) (S)-CHzCH2CH2OzCCH3. Ref. 15.
amine system (Fig. 25.1C) shows four signals at 11.7, 24.0, 42.9 and 53.2 ppm. As shown in Fig. 25.1D, the spectrum of the protonated form of this diamine ligand displays four signals, at 10.5, 20.8, 39.2 and 51.5 ppm. These were readily assigned, as shown in Fig. 25.1 and Table 25.1, on the basis of spectral data taken from the literature [30, 38]. The ~3C spectrum of the polysiloxane-immobilized triamine system (Fig. 25.1E) shows four signals, at 12.0, 24.3, 43.0 and 51.9 ppm, a pattern similar to that of the diamine system. The signal at 51.9 ppm is very intense, because it involves the four carbon atoms, C(4), C(5), C(6), and C(7), as identified in Fig. 25.1. The signal at 43.0 ppm is weak and results from one type of carbon, C(3). This interpretation implies that the chemical form, ~S)CH2CH2CH2NHCH2CH2NHCH2CH2NH2, shown in Fig. 25.1 is in fact the
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 951
A A
o ~
v
r _=
-
__
!
0.0
J
1
~
-50.0
I
-100.0 29Si
~
v
I
i~
-150.0
(PPM)
Fig. 25.22. 29Si CP-MAS spectra of polysiloxane-immobilized systems. (A) diphenylpropylphosphine; (B) diphenylpropylphosphine-monoamine" (C) diphenylpropylphosphine_diam_ ine; and (D) diphenylpropylphosphine-thiolsample. Ref. 18.
form that was obtained from the synthesis employed. Had the other potential reaction product, (S)-CH2CH2CH2N(CH2CH2NH2)2, been produced in the synthesis, then one would have expected a ~3C spectral pattern different from that observed for the diamine ligand, i.e., a pattern in which a signal due to carbon atoms attached to the tertiary amine nitrogen should appear at 5758 ppm [54]. The ~3C CP-MAS spectra of 3-chloropropylpolysiloxane samples prepared with HC1 or (n-Bu)2Sn(OCOCH3)2 as catalyst, shown in Figs. 25.2A and 25.2B, indicate that the proportion of the residual ~ O M e (51 ppm) and ~ O E t (18 and 60ppm) moieties on the 3-chloropropylpolysiloxane prepared by HC1 catalysis are higher than those on the sample prepared by
952
GARY E. MACIEL
A ~ r
E', .D
E
c
,
!
0.0
~
!,
,!
-50.0
!
-100.0
!
!
,
!
-150.0
5 29Si (PPM)
295iCP-MAS spectra of polysiloxane-immobilized ligand systems. (A) diphenylethylphosphine; (B) diphenylethylphosphine-monoamine" (C) diphenylethylphosphine-diamine; and (D) diphenylethylphosphine-thiol sample.Ref. 18. Fig. 25.23.
(n-Bu)2Sn(OCOCH3)2 catalysis. The 13C CP-MAS spectrum of (S)CH2CH2CH2SH, shown in Fig. 25.2C, displays two functionalized-polysiloxane signals, at 12.6 and 28.7 ppm, corresponding to C(1), and C(2), C(3) sites, respectively, in agreement with data on silica systems grafted with 3mercaptopropyltriethoxysilane [41]. The signals at 19.0, 60.8, and 51.9ppm are assigned to unhydrolyzed ethoxy (CH3 and CH2) and methoxy groups, respectively. The 13C NMR chemical shifts summarized in Table 25.1 show that the polysiloxane-immobilized monoamine systems prepared in this study display chemical shifts that are similar to those of the bulk self-polymerized 3aminopropyltriethoxysilane (APS) polysiloxane system (i.e., with no Si(OEt)4 employed) [30]; but the chemical shifts measured in this study are slightly different from those of APS-modified silica [30]. The apparent low-
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
!
50.00
I
I
0.00
t.
1.
-50.00
,
!
-100.00
I
!
953
I
-150.00
29Si (PPM)
Fig. 25.24. 295iCP-MAS spectra of polysiloxane-immobilized thiol-monoamine system (A) and of polysiloxane-immobilized thiol-diamine system without treatment (B) and treated with 0.11M HC1 (C). Ref. 16.
shielding shifts of all peaks in the 13C NMR spectra of the polysiloxaneimmobilized amine systems, compared with those of APS-modified silica, may be due at least in part to the bulk magnetic susceptibilities of these materials. The same reason, as well as small structural differences, might account for the observed slight differences in the 29Si chemical shifts of
954
G A R Y E. M A C I E L
A t--
t~ .13 t__
< e-
e-' .m
0.0
I
I
30.0
I
I
60.0
L
!
90.0
I
,!
120.0
~
!
_
150.0
29Si (PPM)
Fig. 25.25. 298i DP-MAS spectra of polysiloxane-immobilized ligand systems. (A) Experimental and (B) deconvoluted spectra of polysiloxane-immobilized monoamine system. (C) Experimental and (D) deconvoluted spectra of polysiloxane-immobilized 3-chloropropyl system. Ref. 17.
pendent groups, compared to previous results on modified silica systems [37411. The relatively broad peaks in the 13C NMR spectra of the immobilized amine ligands (Fig. 25.1), compared with those of the 3-chloropropylpolysiloxane or (S)-CH2CH2CH2SH spectra (Fig. 25.2) and of the 3-(trimethylammonium)propyl-polysiloxane (Fig. 25.5D) suggest that one source of line broadening in spectra of the nonprotonated amine ligand may involve hydrogen bonding between the amine groups and surface silanols or with other ligand groups, as shown below:
7< Table 25.4. 1H spin-lattice relaxation results on functionalized polysiloxane systems of the type, (S)-Y.
13C_Detected
Sample
260 MHz TH(s) a
(S)-CH2CH2CH2NH2
(S)CH2CH2CH2NHCH2CH2NH2
29Si-Detected (298i chemical shifts) b 150 MHz
260 MHz
TY(S) a
TH(s) b
0.69 d 0.76 d
0.67 0.72
200 MHz TH(s) b
(S)-CH2CH2CH2SH
(S)-CH2CH2CH2PPh2 (S)-CHzCH2PPh2
(10.9 (27.3 (47.8 (60.0
ppm) ppm) ppm) ppm)
0.56 0.57 0.56 0.55
(12.6 (28.7 (51.9 (60.8
ppm)0.75 ppm) 0.73 ppm) 0.75 ppm) 0.71
0.59
31p-DetectedC
150 MHz TH(s) b
150 MHz T~(s) c
> XJ >
0.68 0.75
0.48 0.43
0.47 0.44
~z N >
0.67 1.0 0.50
0 :Z
0.56
0.70 1.0 0.52
/H + /Cd 2+
(S)-CH2CH2CH2C1
~Z
0 nl :Z -]
0.69, 0.72
0 2: > IN
0.71, 0.72
0.95 d 1.1 d
0.90 1.2
0.93 1.2
1.0 1.1
1.0 1.1
,-o 0
a Measured via 1H-13C cross polarization, with 1H frequency indicated. Estimated error: ---8%. b Measured via ~H-29Si cross polarization, with 1H frequency indicated. Estimated error: ---8%.
c Measured via ~H-31P cross polarization, with 1H inversion-recovery, at 150 MHz. Estimated error: +-6%. d Only one 1H relaxation behavior observed for all carbon signals measured.
V(D
Table 25.5. XH spin-lattice relaxation results on functionalized polysiloxanes of the type, X-(S)-Y
Sample
aaC-Detected"
1 2 3 /CH2CH2CHaNH2 ('').S.~ 1' 2' 3' CH2CH2CH2SH 1 2 3 4 5 / CH2CH2CH2NHCH2CH2NH2
~.S.\ ~'
"29Si-Detected (29Si chemical shifts) b
3ap-DetectedC
260 MI-Iz
150 MHz
260 MI-Iz
150 MI-Iz
150 MHz
~(s)
~(s)
~(s)
~(s)
~(s)
(-60 ppm)
(-100 ppm)
(-60 ppm)
(-100 ppm)
-PPh2
-P(O)Ph2
0.76
0.79
0.81
0.88
0.94
0.90
0.65
0.65
0.64
0.64
0.63
0.77
0.77
0.74
0.73
0.74
2' 3'
CH2CH2CH2SH /CH2CH2CH2PPh2
<s>.
\CH2CH2CHzNH2 /CH2CH2PPh2
<s>.
\CH2CH2CH2NH2
" Measured via 1H-13C cross polarization, with aH inversion-recovery, at the aH frequency indicated. Only one proton spin-lattice relaxation behavior noted for each sample. Estimated error: -+8%. b Measured via 1H-29Si cross polarization, with aH inversion-recovery, at the aH frequency indicated. Estimated error" -+8%. ~ Measured via all-alP cross polarization, with aH inversion-recovery. Estimated error: -+8%.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 957
I~./H
/H
H~ /H
../H
~N\H,,,,, o,H ,/~,/~,/////J//,
/////////~///// III
IV
V
Possible reasons why hydrogen bonding (or related proton transfers, leading to hydrogen-bonded structures like VI or VII) could lead to line broadening include the following: (1) Proton-transfer at rates comparable to the inverse of the relevant 13C chemical shift differences due to
////I/////)/t VI
/
Y"/////////SJ//
VII
hydrogen bonding or protonation of amines [6, 38, 39]. (2) Hydrogen bonding and proton transfer enhance the possible number of chemically different structures, which can result in inhomogeneous line broadening due to the dispersion of isotropic chemical shifts. (3) Hydrogen bonding networks can impart a kind of three-dimensional rigidity to an otherwise flexible system, thereby "freezing-in" a distribution of configurations (e.g., conformations) and preventing or attenuating the line-narrowing that could otherwise result from motional averaging of different isotropic chemical shifts (such as the case for 3-chloropropyl siloxane system). Another likely source of line broadening in the spectra of amine systems is the effect of the 14N quadrupole interaction in interfering with MAS averaging of the 14N~13C dipolar interaction [55-60], an effect that is especially strong when unprotonated amine groups, which have large 14N quadrupolar couplings, are present; apparently this effect is dramatically attenuated in quaternary ammonium forms, in which the quadrupole coupling constant is markedly reduced. The quadrupolar effect of 35C1 or 37C1 on 13C is largely absent due to the rapid motion of the 3-chloropropyl moiety. The 13C CP-MAS spectrum of polysiloxane-immobilized diphenylpropylphosphine, (S)-CH2CH2CH2PPh2, shown in Fig. 25.4A, displays signals at 14.9, 19.1, 27.9, 60.5,128-141 ppm. The strong signal in the 128-141 ppm region is assigned to the phenyl carbons of ~PPh2 groups. The signals at 14.9 and 27.9 ppm are assigned to C(1) and C(2), C(3), respectively, where
958
G A R Y E. M A C I E L
C(1) is attached to silicon. The peaks at 19.1 ppm and 60.5 ppm are from the
CH3 and CH2 carbons, respectively, of residual unhydrolyzed ethoxy groups. Previous studies have shown that the ~3C chemical shifts of phosphine oxide moieties are close (within about _+2 ppm) to those in corresponding phosphine moieties [61]. Hence, the 13C signals in phosphine oxide moieties are not resolved from the corresponding 13C signals of the phosphine moieties in these spectra. The strong phenyl carbon signals in the 128 to 141 ppm region of the 13C NMR spectra shown in Fig. 25.4 might have been expected to consist of four components at about 128, 131,133 and 141 ppm, due to the various aromatic carbon sites, according to a previous study on diphenylethylphosphine moieties immobilized on silica gel via surface modification [61]. The 13C chemical shifts anticipated for the various aromatic carbon sites are shown below for immobilized phosphine and phosphine oxide moieties [61]: 128 131~128 Ii II
.._
--'CH2CH2P~
131
C6H5 "
,,c0H i--CH2CH2P= 0 128 (PPM) 128 131
The 13C CP-MAS spectrum of polysiloxane-immobilized diphenylethylphosphine, as shown in Fig. 25.4B, exhibits peaks very similar to those of the propyl analog, with similar chemical shifts (Table 25.1). The 13C NMR spectra shown in Fig. 25.4 reflect the fact that the relative amplitudes of the four anticipated chemical shift components depend upon the relative amounts of the phosphine and phosphine oxide moieties in the samples. Of course, only qualitative conclusions can be drawn from comparisons of the various 13C CP-MAS spectra, because the CP spin dynamics were not studied in detail for these samples. The 13C NMR spectra of the polysiloxane-immobilized primary amine ligand system before and after silylation of accessible silanols with hexamethyldisilazane, given in Fig. 25.5, show very similar peak maxima positions for each of the three methylene carbons (Table 25.1), although there is apparently a small peak narrowing for C(2) and C(3) in the (CH3)3 Si-capped
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
959
sample, for which Fig. 25.5B shows the distinct (CH3)3 Si peak near 0 ppm. This comparison is of interest primarily for examining any chemical/structural changes in the CH2CH2CH2NH2 group, especially as reflected in the C(2) ~3C signal, as the surface silanols are "capped" by Me3Si groups to reduce the number of surface hydroxyls that might otherwise be expected to interact with amino groups. A similar strategy was used for 3-aminopropyltrimethoxysilane-derivatized silica [38]. The spectra in Figs. 25.5A and 25.5B display very similar positions of peak maxima for each of the three methylene carbons (Table 25.1), although there is apparently a small peak narrowing for C(2) and C(3) in the (CH3)3Si-capped sample. The slightly broader peaks for the uncapped material may reflect a greater structural inhomogeneity associated with hydrogen bonding between surface silanols and at least some of the pendant ligand groups. The 13C NMR spectrum of the polysiloxane-immobilized diethylpropylamine system in Fig. 25.5C shows signals at 12.6, 21.8 (shoulder at 27.2), 47.8 and 57.0 ppm. The shoulder at 27.2 ppm may come from C(2') of residual, unreacted 3-chloropropyl groups, due to incomplete reaction of 3-chloropropylpolysiloxane with diethylamine. The C(1') and C(3') peaks of the residual precursor ((S)-CH2CH2CH2C1) are overlapped with the C(1) and C(4) peaks of the product, polysiloxane-immobilized diethylpropylamine system. This interpretation of the 27.2 ppm shoulder is in agreement with the elemental analysis data, which show that unreacted polysiloxane-immobilized 3-chloropropyl material remained in the product. Peaks in the 13C NMR spectrum of the polysiloxane-immobilized diethylpropylamine system (Fig. 25.5C) are in general sharper than those of the corresponding polysiloxane-immobilized primary amine system (Fig. 25.5A). The reason for the apparently enhanced broadening in the spectrum of the immobilized primary amine system may be that the geometrical arrangements and/or mobilities of the aminopropyl moieties of these two systems may differ from each other in terms of whether the amine group is "free" or hydrogen bonded, where this kind of dual behavior would yield a range of slightly different chemical shifts. In the case of the polysiloxane-immobilized diethylpropylamine system, the absence of the possibility of amine-amine hydrogen bonding may eliminate one source of structural inhomogeneity, resulting in somewhat smaller linewidths. The absence of amine-amine hydrogen bonding may also permit increased mobility of the pendent ligand groups, rendering some degree of motional averaging of a diversity of isotropic chemical shifts associated with geometrical (e.g., conformational) variations. The 13Cspectrum of the polysiloxane-immobilized propyltrimethylammonium chloride system (shown in Fig. 25.5D), displays four peaks, at 10.8, 18.7, 54.5 and 69.1 ppm, which are assigned to C(1), C(2), C(4) and C(3),
960
GARY E. MACIEL
respectively (as identified in Fig. 25.5D, 25.5B and Table 25.1). The small shoulder at 60.5 ppm is assigned to CH2 carbons of unhydrolyzed, residual ethoxy groups. The corresponding CH3 carbon signal of the unhydrolyzed ethoxy groups, at 18 ppm, is overlapped with the C(2) peak, at 18.7 ppm, of the product. The 13C CP-MAS spectrum of this sample (Fig. 25.5D), in which the nitrogen atom cannot possibly be involved in hydrogen bonding with surface silanols or other ligand groups, also displays at least one line that is relatively sharper than the peaks in the spectrum of the corresponding primary amine system (Fig. 25.5A), due to the same reasons discussed for the (S)-CHzCHzCHzNEt2 sample. The 13C NMR spectrum of the polysiloxane-immobilized system that includes a pendent mCHzCHzCHzNHCHzCOzMe group, shown in Fig. 25.6A, displays in the sp 3 carbon region four signals, at 11.1, 22.4, 43.7 and 53.2 ppm, and a shoulder at 60.8 ppm, which were assigned [37, 39, 45] to C(1), C(2), C(3), C(6) and C(4), respectively, as shown in Fig. 25.6A and Table 25.1. This spectrum also includes a "doublet" with maxima at 168.4 and 171.9ppm, due to two carbonyl carbon resonances. These two peaks were assigned on the basis of 13C NMR data reported previously [37, 45, 55] to a "free" carbonyl and a carbonyl group that interacts via hydrogen bonding with an amine group or with a surface silanol. It had been shown previously that hydrogen bonding to the oxygen atom of a carbonyl group decreases the shielding of the carbonyl carbon [37, 45, 55]. In order to find out if 14N quadrupolar interactions have some effect [56-60] on the broadening of the carbonyl peak, 13C CP-MAS spectra of these two products were obtained at a higher field, 64.8 MHz for 13C. Since the 14N quadrupole line-broadening effect decreases at higher field, a sharper peak would be expected at 64.8 MHz if the main effect of the line-broadening of the carbonyl peak arises from the 14N quadrupolar interaction. In fact, similar broad doublets were observed for the carbonyl peak at 64.8 MHz. Therefore, in both cases, the line-broadening of the carbonyl signals mainly comes from the involvement in hydrogen bonding of the carbonyl group, and is not the result of a 14N quadrupole line-broadening effect. The 13C CP-MAS spectrum of the polysiloxane-immobilized ligand system bearing mCHzCHzCHzSCHzCOzMe groups, shown in Fig. 25.6B, has one relatively sharp carbonyl resonance at 172.8 ppm. The carbon signals at 9.7, 28.3, 35.8 and 53.2ppm were assigned to C(1), C(2) and C(3), C(4) and C(6), respectively, as shown in Fig. 25.6B and Table 25.1. The lack of a very large linewidth of carbonyl signal in the 13C spectrum of (S)CHzCHzCHzSCHzCOzCH 3 (Fig. 25.6B) may suggest the absence of a distribution of hydrogen-bonding interactions of the carbonyl group. One might have expected to see a broad carbonyl signal if some substantial (but not
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 961 dominant) faction of the carbonyl groups of this system were also involved in hydrogen bonding with surface silanols [62]. The 13C NMR spectrum of the polysiloxane-immobilized system that bears ~CHzCHzCH2OzCCH2NH2 (propylglycinate) groups, given in Fig. 25.7A, shows resonances at 10.4, 26.5, 55.0 (broad), 65.0 and 175.0 (broad) ppm, which were assigned [37, 39, 45] to C(1), C(2), C(5), C(3) and C(4), respectively (as defined in Fig. 25.7A and Table 25.1). The peak at 65.0, which was assigned to C(3), is consistent with the resonance position of methylene carbons attached to an ether oxygen atom, as reported for similar materials [39]. The carbonyl signal at 175 ppm is broad and appears to be a doublet, ostensibly with a high-shielding component due to the "flee" carbonyl and a lower-shielding component due to carbonyl groups involved in hydrogen bonding with amine or silanol groups. In the reaction of the polysiloxaneimmobilized 3-chloropropyl system with sodium glycinate, the basis of the preparation of this sample, there are in principle, two possible products: (S)CHzCHzCHzOzCCHzNH2, from the substitution of the chloride group by the carboxy group, or (S)-CHzCHzCHzNHCHzCOzNa, from substitution of the chloride group by the amine group of glycinate. The 13C spectrum of Fig. 25.7A clearly indicates that, in the reaction of the 3-chloropropylpolysiloxane with sodium glycinate, the chlorine atoms are replaced by the carboxylate group and not by the amine group. If the latter material had been produced in the synthesis, we would have expected a 13C spectral pattern similar to that of the polysiloxane-bearing ~ N H C H z C O z M e case shown in Fig. 25.6A, in which the C(3) resonance occurs at about 43.7 ppm; this chemical shift is not observed in the spectrum of the product (Fig. 25.7A). The 13C NMR spectrum of the polysiloxane-immobilized system bearing iminodiacetate ligand groups displays a pattern (Fig. 25.7B) that is similar in some respects to that of Fig. 25.7A for the glycinate polysiloxane system (i.e., it shows signals in the 66 and 172 ppm regions). The presence in the spectrum of Fig. 25.7B of strong signals of the starting 3-chloropropylpolysiloxane material (10.9, 27.3 and 47.8 ppm) and only weak or largely masked peaks for the expected product (66.4 and 172 ppm) indicate that in the iminodiacetate case the substitution reaction was incomplete under the conditions used. The incompleteness of the reaction was also indicated by elemental analysis data showing a large amount of chlorine present in the product. The 13C NMR spectrum of a polysiloxane-immobilized acetate system, bearing -CHzCHzCHzOzCCH3 groups, was obtained as a model for the C(3) peak of a system that contains the immobilized -CHzCHzCH2~O~C(O)CI-~2 group; its spectrum, shown in Fig. 25.7C, displays the 66.6ppm carbon resonance of the C(3) methylene group in this moiety. This is consistent with the 13C spectra of the glycinate and iminodiacetate polysiloxane systems,
962
GARY E. MACIEL
where signals in the range, 65-66 ppm, are seen for the methlyene groups attached to an ether oxygen of an ester moiety. The assignments for other carbons of the polysiloxane-immobilized propylacetate are as shown in Fig. 25.7C and Table 25.1. 13C NMR spectra of functionalized polysiloxanes of type, X-(S)-Y. The 13C CP-MAS spectra of X-(S)-Y samples, shown in Figs. 25.8-25.11, can largely be interpreted as linear combinations of X-(S) and (S)-Y spectra. Of course, there are distortions of intensities from what would predict from a simple spectral addition, because of variations in conformational averages (thus, chemical shifts) and in CP spin dynamics (thus, intensities). The 13C chemical shifts derived for X-(S)-Y polysiloxanes are summarized in Table 25.2. It is interesting to note that while the presence of a pendant amino group on a propylthiol-functionalized polysiloxane apparently is effective in promoting hydrolysis/condensation processes to the extent that no residual ethoxy or methoxy signals are apparent in the 13C CP-MAS spectra of Fig. 25.8, such amino groups do not lead to elimination of methoxy and ethoxy signals in the spectra of phosphine-functionalized polysiloxanes with incorporated amino groups (Fig. 25.10).
25.4.2
15N NMR spectra
The 15N CP-MAS spectra of the polysiloxane-immobilized monoamine samples treated with aqueous HC1 or NaOH solutions of various pH values (Fig. 25.12) show that, for the pH = 7 case, the spectrum has a 15N signal at 25 ppm, with a small shoulder at 34 ppm. For the pH = 1 case, the spectrum shows a peak at 45 ppm, with a weak shoulder at 33 ppm. For the pH - 13 case, the signal is a sharper, more intense peak at 25 ppm, accompanied by a small shoulder at 33 ppm. This behavior, in which the intensity of the shoulder at 34 ppm increases as the pH is changed from 13 to 3.5, is consistent with the idea that there is a simple analogy between hydrogen bonding and protonation [51]. The presence of the shoulder at 34 ppm between the free amine peak at 25 ppm and the ammonium cation peak at 45 ppm may indicate the involvement of the NH2 group in hydrogen bonding with acidic surface silanols and perhaps with other amine groups. According to 15N NMR data in the literature [53, 63-66], one can assign the signals at 45, 34 and 25 ppm in the spectra of Fig. 25.12 to the ammonium cation form (VI, VII or VIII), hydrogen bonded amine forms (III or IV), and the nonhydrogen-bonded amine form (IX and X), respectively.
NM R C H A R A C T E R I Z A T I O N OF F U N C T I O N A L I Z E D P O L Y S I L O X A N E S
H\|
N!/H VIII
..........;.,. IX
963
H /I-I X
At pH = 1 (Fig. 25.12A), the cation form (VIII) is favored, whereas at pH = 13 (Fig. 25.12E), the ionic amine form (X) would be favored; the proportion of various hydrogen bonded forms depends on the pH value. The ~SN NMR spectrum of the Cd(II)-treated sample derived from the polysiloxane-immobilized monoamine ligand, shown in Fig. 25.13B, consists of a broad signal, with most of its intensity at lower shielding than for the corresponding uncomplexed ligand system (Fig. 25.13A) between that of the ammonium cation (45 ppm) and hydrogen bonded forms (33 ppm) (vide supra). The observed slight broadening of the C(2) and C(3) signals in the 13C spectra of the Cd(II)- and Hg(II)-complexed polysiloxane-immobilized monoamine system (not shown here), compared with the spectrum of the corresponding uncomplexed polysiloxane-immobilized amine system (Fig. 25.1A), may result from an increase in chemical structural heterogeneity, indicating that not all ligand sites are accessible to metal ions, thereby remaining uncoordinated and therefore having somewhat different chemical shifts from those groups involved in coordination. This inaccessibility of metal ions to some amino sites may be due to a combination of steric hindrance and electrostatic repulsions between metal ions approaching amino groups and metal ions already complexed to the immobilized amino groups. This view, that some of the ligands remain uncoordinated, is in agreement with the suggestions of other workers for other polysiloxane systems [13]. Another possible cause of the increase in chemical structural heterogeneity might be the presence of different complexation forms of the amine groups with the metal ions, e.g., the number of ligands complexed to each metal ion. The broad signal observed in the 15N NMR spectrum of the Cd(II)-treated sample derived from the polysiloxane-immobilized monoamine ligand (Fig. 25.13B) also suggests that a substantial fraction of the immobilized monoamine that has been treated with aqueous Cd(II) solution is involved in coordination to Cd(II). The breadth of the peak in Fig. 25.13B suggests that a portion of the ligand groups remain uncoordinated to metal ion in the sample. Of course, there may be a rapid exchange of C d 2+ ions between different amino sites, which could result in another line-broadening mechanism. The ~SN NMR spectrum of the polysiloxane-immobilized diamine system
964
GARY E. MACIEL
(Fig. 25.13C) shows two signals, at 22.2 and 37.0ppm. On the basis of 15N NMR data taken from the literature [63-66], these peaks are assigned to primary and secondary amine groups, respectively. The spectrum of the protonated form of the polysiloxane-immobilized diamine ligand (washed with 0.1 M aqueous HC1) is given in Fig. 25.13D; it shows that both signals move to lower shielding, at 45.0 ppm and a shoulder at 52 ppm, upon protonation. These signals are assigned on the basis of 15N data taken +from the literature [65 ~ 66] to the ammonium cation sites of types ~ N H 3 and + ~ N H 2 ~ , respectively. The 15N NMR spectrum of the polysiloxane-immobilized triamine system presented in Fig. 25.13E shows two signals, at 18.2 and 33.6ppm, which are expected for the primary and secondary amine groups, respectively. In comparing peak intensities of the diamine system (Fig. 25.13C) with those of the triamine system (Fig. 25.13E), one sees that there is a significant increase in the secondary amine intensity relative to that of the primary amine peak in the triamine case, compared with the diamine case. This is consistent with the presence of two secondary amine groups for each primary amine group in the triamine ligand system. Furthermore, the absence of a tertiary amine signal, which would be expected to occur at about 50-55 ppm [63-66] and to survive 1H-15N dipolar dephasing very well (vide infra), confirms the absence of structure XII in the product and shows that structure XI is the product of the reaction between diethylenetriamine and the 3chloropropylpolysiloxane precursor; this result is consistent with the 13C results described above.
.~H2 H XI
"~
H2
XII
The 15N NMR spectrum of the polysiloxane-immobilized thiol-monoamine system, presented in Fig. 25.13F, shows a broad signal at about 28 ppm. The breadth and the lower-shielding intensity of the peak shown in this spectrum, in comparison with that of the corresponding polysiloxane-immobilized monoamine ligand (Fig. 25.13A), may be associated with hydrogen bonding [54] involving the thiol group. The 15N NMR spectrum of the polysiloxaneimmobilized thiol-diamine ligand system (Fig. 25.13G) shows two signals, at 22.0 and 37.2 ppm; these are assigned, according to 15N NMR data in the literature [42, 63], to primary and secondary amine groups. A similar pattern
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 965
was seen above for the polysiloxane-immobilized diamine ligand system, with no thiol component. 25.4.3
31p N M R results
Solid-state 31p NMR spectroscopy is useful for obtaining information on the phosphine ligands, e.g., the phosphorus oxidation state. The 31p CP-MAS NMR spectra of the polysiloxane-immobilized diphenylpropylphosphine (Fig. 25.15A), the diphenylpropylphosphine-monoamine (Fig. 25.15B), the diphenylethylphosphine (Fig. 25.15C), and the diphenylethylphosphine-monoamine (Fig. 25.15D) systems all reveal the presence of the following two types of structural moieties, which have been identified on the basis of previously reported 31p NMR chemical shifts [61, 67-70]" (A) phosphine moieties: -17 ppm for diphenylpropylphosphine samples and -11 ppm for diphenylethylphosphine samples; and (B) phosphine oxide moieties: 3436 ppm for diphenylpropylphosphine samples and 34 ppm for diphenylethylphosphine samples. 31p NMR spectra of related samples (not shown here) also display the presence of both the phosphine moiety and phosphine oxide moiety. The ratio of the peak amplitudes of phosphine moiety to phosphine oxide moiety varies from 0.1 to 5.5 among the samples. 31p spectra were obtained twice on the samples, with a 1-year measurement interval; no changes in the relative amounts of phosphine and phosphine oxide moieties were observed over this 1-year period, which indicates that further oxidation of phosphine moiety to phosphine oxide moiety did not occur during sample storage. Simulations [69, 70] of the spinning sideband patterns of 31p spectra obtained at 80.9 MHz with a low MAS speed (1-2 kHz) (not shown here) were carried out on all the polysiloxane-immobilized phosphine samples [71] to provide values of the principal elements of the chemical shift tensor, which often are valuable in elucidating chemical structure. Two model compounds, diphenyl-isopropylphosphine oxide and 1,6-bis(diphenylphosphino)hexane, were used to provide CSA tensor elements that would be approximately representative for the polysiloxane-immobilized phosphine oxide and phosphine moieties, respectively. The experimental 31p spectra were then simulated on the basis of the principle tensor elements obtained on the two model compounds: 117.1, 86.8 and - 9 3 . 7 p p m for diphenyl-isopropylphosphine oxide and 7.3, -29.8 and -40.9 ppm for 1,6-bis(diphenylphosphino)hexane. The simulations showed that the 31p chemical shift anisotropies of both phosphine and phosphine oxide moieties are qualitatively similar within each of these two categories for all the samples examined, implying very similar
966
GARY E. MACIEL
local phosphine structures (e.g., bond angles, bond lengths) within each category. 25.4.4
1H NMR results
1H CRAMPS spectra of the polysiloxane-immobilized monoamine system, its protonated form and two of its metal complexes, and of the polysiloxaneimmobilized monoamine and 3-chloropropyl systems, given in Fig. 25.17, show that for the protonated monoamine ligand (Fig. 25.17B) and products of treatment with aqueous metal ion solutions (Figs. 25.17C and 25.17D) there are strong, broad signals for the ~ N H ~ resonance at about 7-8 ppm. The methylene proton signals are also broad and partially overlapped, and probably obscure the weak silanols signals. In contrast to these spectra, the IH NMR spectra of the polysiloxane-immobilized 3-chloropropyl system (Fig. 25.17E) and thiol system (Fig. 25.16A) show sharp signals. Besides the methylene proton signals at about 1.1, 2.0 and 2.7 or 3.6 ppm, these spectra also show the presence of residual unhydrolyzed methoxy and ethoxy groups at about 1.4 and 3.6-4.1 ppm. The broadening of peaks in the 1H NMR spectra of the various polysiloxane-immobilized monoamine samples (Fig. 25.17) are presumably caused by some combination of effects of the types discussed above for 13C spectra of these systems. Also, the effects of metal-ion complexation (e.g., "freezing in" a variety, or range, of structures and isotropic chemical shifts) can be analogous to what were discussed above for hydrogen-bonding effects. The explanation that hydrogen bonding may be one source of line broadening in the 1H CRAMPS spectra of polysiloxane-immobilized amine ligands is supported by the 15N NMR results. In Fig. 25.12 one can see that the intensity of the shoulder at 34 ppm in the 15N CP-MAS spectrum of the polysiloxaneimmobilized monoamine system increases as the pH is changed from 13 to 3.5. This is consistent with the idea that there is a simple analogy between hydrogen bonding and protonation; the presence of the shoulder at 34 ppm between the free amine peak at 25 ppm and the ammonium cation peak at 45 ppm may indicate the involvement of the NH2 group in hydrogen bonding with acidic surface silanols and perhaps with other amine groups. The 1H CRAMPS spectrum of the polysiloxane-immobilized thiol-monoamine ligand system (Fig. 25.16B) shows no signal due to NH2 protons. This may be in part because the ~H signal of the NH2 groups is broadened by the 14N quadrupole effect on MAS averaging of the 1H-14N dipolar interaction, and in part because of effects of proton exchange of NH2 protons. Quadrupolar line broadening effects of 14N on attached protons have previously been observed in 1H CRAMPS spectra of amino acids and other systems (72-74).
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 967
The spectrum of the corresponding protonated material (Fig. 25.16C) shows a broad ~Nt-I~3 signal at 7.8 ppm; the observation of ~NH~3 signals under conditions in which m N H 2 signals are not observed may reflect the higher local electric symmetry around nitrogen of the ~Nt-I~3 group, and correspondingly smaller quadrupolar coupling constant and line broadening effect, as well as altered proton exchange behavior. The methylene proton signals of the protonated ligand system show a slight shift to lower shielding in comparison with those of the parent ligand. Separate silanol proton signals are not seen in the spectra due to the strong overlap with broad methylene signals. In the 1H NMR spectra of the polysiloxane-immobilized thiol-diamine system and its protonated form shown in Figs. 25.16D and 25.1E, the breadths of spectral lines of the thiol-amine systems, compared with those of the thiol system, may be due to a diversity of hydrogen bonding of the amine groups with surface silanols and thiol groups. 25.4.5
29Si NMR spectra. The polysiloxane framework
298i CP-MAS NMR spectra are expected to provide information on the nature
of attachments of the various silicon atoms, as well as the relative populations of the ligand-bearing silicon moieties and nonligand-bearing silicon moieties. Of course, conclusions on these populations can be only qualitative in the absence of detailed studies of the relevant spin dynamics, which would be necessary for quantitative interpretations. Nevertheless, since the same set of experimental set-up parameters was used for all 298i CP-MAS experiments, and since all of the samples in this study are physically similar (amorphous polysiloxane solids with organic ligand moieties having substantial mobilities), comparisons of relative amplitudes should be at least qualitatively valid. In general, peak linewidths in the 298i NMR spectra are larger than those in the 13C NMR spectra of corresponding samples. This observation is due to the fact that, in a polysiloxane framework, each type of silicon site, such as the single silanol (Q3), (~SiO)3SiOH, has many structural variations in terms of bond angles, chemical environments (e.g., neighbors of the silicon atom) and the absence or detailed form of hydrogen bonding (e.g., hydrogen bonded to silanols or water on the surface), etc. Such variations in one specific type of silicon site give rise to corresponding variations in chemical shifts, resulting in inhomogeneous broadening of the peaks in the 298i NMR spectra. Compared to the silicon atoms in a polysiloxane framework, the 13C atoms of a specific type of site in an organic group (e.g., in m C H z C H z C H z S H ) have much narrower local-structural variations (at least in a time-average sense) than those of silicon atoms, which results in smaller linewidths in the 13C NMR spectra.
968
GARY
E. MACIEL
The 2 9 8 i CP-MAS spectra displayed in Figs. 25.18-25.24 in general show two regions of major intensity, centered at about -100 to -105 and about - 6 0 p p m . These two peak positions correspond to to S i ( ~ O ~ ) 4 and R S i ( ~ O ~ ) 3 units, respectively, where R is an organic group containing the pendent functionality. The -105 ppm pattern is composed of at least three contributions, at about -91, -100 and -108 ppm, due to the following types of species: (~SiO)2Si(OR')2 (Q2), (~SiO)3SiOR' (Q3), and (~SiO)4Si (Q4), respectively, where R' = Et or H. The relative importance of Et and H for R' in the Q2 and Q3 sites can be inferred from the 13C NMR spectra (vide supra), which show that there are few remaining ethoxy groups in systems with amino groups present, and more residual ethoxy groups in the 3-chloropropylpolysiloxane system and the corresponding propylthiol system. The lower-shielding pattern is composed of at least one peak at about - 6 4 and a shoulder at -57 ppm, due to species containing ligand groups, RSi(OSi--~)3 and RSi(OSi~)2OR', respectively, where R ' = H, Me or Et. These assignments are in agreement with previous results on modified silica systems [3741]. For comparison purposes, the 29Si CP-MAS spectrum of silica made by the sol-gel process through the HCl-catalyzed hydrolytic condensation of Si(OEt)4 is given in Fig. 25.18A. This spectrum shows one major pattern centered at about -100ppm, composed of a shoulder at - 9 0 ppm, a clear peak at - 9 9 p p m and a shoulder at -109ppm. These components were assigned to (~SiO)2Si(OH)2, (~SiO)2Si(OEt)2 and (~SiO)2Si(OH)OEt, \ . \ . ./ - 9 0 p p m ; (~S10)3SiOH, and (~S~O)3SiOEt, - 9 9 p p m ; and Si(OSIs)4, -109 ppm. The presence of unhydrolyzed residual ethoxy groups indicated b~r the 13C spectrum (not shown here) leads to the silicon assignments: (~SiO)2Si(OEt)2, (~SiO)3Si(OH)OEt and (~SiO)3SiOEt. When a compound of type RSi(OR')3 (where R is an organic ligand group and R' = CH3 or CzHs) is added to the initial reaction mixture of Si(OEt)4, H20 (and HC1 in some cases) and MeOH, the 2 9 8 i NMR spectrum of the resulting product shows two major patterns centered at about - 6 0 and -102 ppm, as seen in Fig. 25.18B for the case R = CHzCHzCHzNH2. In this figure the - 6 0 ppm signal consists of a main peak at - 6 4 p p m , with a shoulder at - 5 7 ppm, .J .J . corresponding to RSi(OS1s)3 and RSi(OSI~)zOH structures, respectively [6, 33, 36, 41]. Since the 13C NMR spectrum of the polysiloxane-immobilized primary amine system (Fig. 25.1A) indicates that there are no, or few, unhydrolyzed, residual ethoxy groups in the product, the silicon signal at / - 5 7 p p m corresponds only to RSi(OSis)zOH structures, not including ./ RSi(OS~s)2OEt. For those samples m which the ~3C NMR spectra indicate that there are unhydrolyzed, residual ethoxy groups (see Table 25.1 and .
.
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
969
Figs. 25.1, 25.2 and 25.3), the silicon signal at - 5 7 p p m corresponds to RSi(OSi~)2OR' ( R ' = H, Me or Et). In the 29Si NMR spectrum of the 3-aminopropylpolysiloxane sample after silylation of the accessible silanols by (Me3Si)2NH (Fig. 25.18C), the peak at 12.1 ppm corresponds to Me3SlmOS1s groups. It can be seen, by companng ~pectra of Figs. 25.18B and 25.18C, that the relative intensity of the Si(OSi ~)4 peak at - 1 0 9 p p m is increased and the relative intensity of RSi(OSi~)2OH at - 5 7 ppm is decreased in the spectrum of 3-aminopropylpolysiloxane after silylation (Fig. 25.18C) compared with that of the amine ligand system before silylation (Fig. 25.18B); but there is still substantial (slightly decreased) intensity of the single silanol peak at - 9 9 p p m after silylation. It was estimated by spectral deconvolution of the spectra of Figs. 25.18B and 25.18C (not shown here) that ---76% of the silanols of the R(~SiO)2SiOH sites are silylated, while ---15% of the silanols of the (TSiO)3SiOH sites are silylated, which indicates that hydroxyl groups attached to ligand-bearing silicon atoms are more accessible and/or more reactive toward the silylating reagent, (Me3Si)2NH. The estimated percentage of . . . . . unreacted sdanols, both R(TS10)2SIOH and (TS10)3SIOH, m the sflylauon of the polysiloxane-immobilized primary amine system is 55%. In underivatized silica gel, approximately 58% of the silanols are not accessible for reaction with hexamethyldisilazane under roughly the same conditions as employed in the studies described here [33]. Figure 25.18D shows the 29Si CP-MAS NMR spectrum of the polysiloxane-immobilized trimethylpropylammonium chloride system, (S)C H 2 C H 2 C H ~ ( C H 3 ) 3 C I - , prepared via catalysis by HC1. This spectrum shows a higher intensity (relative or absolute) of the single silanol peak at - 9 9 ppm and a lower intensity of the Si(O~Si~)4 peak at -109 ppm, compared with those of the immobilized primary amine system (Fig. 25.18B), where the amino group has served as a basic catalyst in the polymerization. This implies that the polymerization was incomplete in the preparation of the polysiloxane-immobilized trimethylpropylammonium chloride system, and that this material is less cross-linked. The incomplete polymerization may be due to electrostatic repulsion among the positive charges of the polysiloxane-immobilized trimethylpropylammonium cations, which might prevent the small polysiloxane entities formed initially from getting sufficiently close together to undergo further polymerization. The same reason may account for the formation of a precipitate, instead of a clear homogeneous gel in the preparation. Of course, the absence of an "internal" amine catalyst may also be partly responsible for the incomplete polymerization. The 29Si NMR spectrum of the polysiloxane-immobilized monoamine sys9
,/Y
9
\
9
~
.
.
\
970
GARY E. MACIEL
tem made from a mixture with a high (2:1) Si(OEt)4-to-(EtO)3Si(CH2)3NH2 molar ratio (Fig. 25.19A) shows a lower intensity of the RSi(O~SI~)3 s~gnal at - 6 4 ppm, relative to the Si(O~Si--~)4 signal, in comparison to the ratio seen in the spectrum of the polysiloxane-immobilized monoamine sample prepared from a mixture with a 1:1 molar ratio (Fig. 25.19B). This suggests that the material prepared from a reaction mixture with a lower relative Si(OEt)4 content is less cross-linked, i.e., has fewer Si(OuSi~)4 sites, but more RSi(O~Si~)3 sites. In the 29Si NMR spectra of the polysiloxane-immobilized 3-chloropropyl systems made by using HC1 or organotin catalysts, shown in Figs. 25.19C and 25.19D, the sample prepared with a 0.1 M HC1 catalyst (Fig. 25.19C) shows stronger peaks at -100 ppm and -57 ppm than in the spectrum of the polysiloxane-immobilized 3-chloropropyl sample prepared with the organotin catalyst (Fig. 25.19D). This indicates that with HC1 as catalyst the crosslinking of both organosilane-containing moieties and silica-like moieties was less complete; i.e., there are more RSi(OSi--~)2OR' and (~SiO)3SiOR' sites. The 13C spectra of the 3-chloropropylpolysiloxane samples (Fig. 25.3) also show substantial amounts of residual ethoxy and methoxy groups. The 29Si spectrum of 3-chloropropylpolysiloxane prepared using the organotin catalyst also shows an extra peak at -82 ppm, perhaps due to --~Si--OmSi(OR')3 sites ( R ' = H, Et). Because there are not any residual methoxy or ethoxy groups in the polysiloxane-immobilized amine systems, as indicated by 13C NMR spectra (Figs. 25.1 and 25.2), the -57 and -100 ppm peaks of the 29Si NMR spectra in Figs. 25.19A and 25.19B represent RSi(OSi--~)2OH and HOSi(OSi~)3 moieties, respectively. 29Si NMR spectra also indicate that there are substantial amounts of crosslinking in the polysiloxane-immobilized amine systems in both the organosilane-containing moieties (as evidenced by the RSi(OSi--~)3 peak at - 6 4 ppm) and silica-like moieties (as evidenced by the Si(OSi~)4 peak at -108 ppm). The rather high relative intensities of the -57 and - 6 4 p p m peaks in Figs. 25.19C and 25.19D reveal the presence of substantial amounts of RSi(OSi~)2OR' and RSi(OSi--~)3, with R ' = H, Me or Et, in the two 3-chloropropylpolysiloxane samples; i.e., the degree of crosslinking in both the organosilane-containing and silica-like moieties is less in the 3-chloropropylpolysiloxane samples than in polysiloxane-immobilized amine systems. 29Si NMR spectra (Figs. 25.19C and 25.19D) and 13C NMR spectra (Fig. 25.3) show that (n-Bu)2Sn (OCOCH3)2 is a more effective catalyst than HC1 (aq) for condensation of the 3-chloropropylpolysiloxane system, and 13C NMR spectra (Fig. 25.2) show that (n-Bu)2Sn(OCOCH3)2 is a better catalyst than HC1 for the hydrolysis of both Si(OEt)4 and CI(CH2)3Si(OMe)3. Compared to the 29Si spectra of polysilxoane-immobil9
o
. ~
9
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES
971
ized 3-chloropropyl systems (Figs. 25.19C and 25.19D), the 29Si spectra of polysiloxane-immobilized monoamine ligand systems (Figs. 25.19A and 25.19B) show much stronger Q4 intensity relative to Q3, indicating that the condensation in the latter case is more complete, and the material has more crosslinking Of course, since the signal intensities in CP-MAS spectra are affected by CP dynamics, the results obtained from 29Si CP-MAS spectra are only qualitative. The 29Si CP-MAS NMR spectrum of the polysiloxane-immobilized diamine system, shown in Fig. 25.19E, displays features similar to those of the spectrum of the polysiloxane-immobilized monoamine ligand system (Fig. 25.19B), in the terms of relative intensities of 29Si signals in the - 6 0 p p m region and -105 ppm region, because the same molar ratios (1" 1) of Si(OEt)4 to (R'O)3SiCH2CH2CH2X (X = NH2, R' = Et; X = NHCH2CH2NH2, R' = Me) were used for both preparations. However, in both the - 6 0 ppm and -105 ppm regions, there is evidence of more crosslinking (higher relative intensities at - 6 4 ppm vs. - 5 7 ppm, and -108 ppm vs. - 1 0 0 p p m ) in the diamine ligand system than in the monoamine ligand system; therefore, the diamine ligand serves as a better catalyst for crosslinking than the monoamine ligand does. These results indicate that the hydrolysis/polymerization processes are very similar, with the amine groups acting catalytically and yielding analogous polysiloxane frameworks. The 29Si CP-MAS spectrum of the polysiloxane-immobilized triamine system is shown in Fig. 25.19F. This spectrum displays features that are similar to those of the spectrum of its precursor, 3-chloropropylpolysiloxane prepared via (n-Bu)2Sn(OCOCH3)2 catalysis (Fig. 25.19D). The 295i spectrum of the polysiloxane-immobilized triamine system (Fig. 25.19F) shows higher intensities at - 6 4 and -108 ppm, relative to those at - 5 7 and -100 ppm, respectively. These results indicate that under the reaction conditions of preparing the polysiloxane-immobilized triamine system, the polysiloxane framework is basically well-formed and has a rather high degree of crosslinking in both the organosiloxane and silica-like regions, even though the polysiloxaneimmobilized triamine system is prepared from the 3-chloropropylpolysiloxane, which we have found to contain a substantial quantity of unhydrolyzed alkoxy groups (hence, uncrosslinked silicons; see Fig. 25.2). This result is an indication that the triamine ligand acts as a good catalyst for hydrolysis of ~ S i ~ O E t or --~Si~OMe and for the crosslinking in both organosiloxane and silica-like regions. Comparison of the 295i NMR spectrum of the polysiloxane-immobilized monoamine system after treatment with 0.1 M aqueous HC1 solution (Fig. 25.19G) to the spectrum of the untreated sample (Fig. 25.19B), reveals that the spectrum of the sample treated with HC1 (Fig. 25.19G) has higher relative
972
G A R Y E. MACIEL
intensity in the -105 ppm region in comparison to the - 6 0 ppm region; in this sense the spectrum in Fig. 25.19G is a much more "silica like" spectrum, and shows increases in the relative intensities of peaks at -57 ppm, - 9 0 and -100 ppm and decreases of relative intensities at - 6 4 ppm and at -108 ppm, i.e., much less crosslinking in both organosilane-containing moieties and silica-like moieties. These results, together with elemental analysis data showing that 12.3% of the carbon and 10.9% of the silicon of the original polymer were extracted into the aqueous HC1, were explained by postulating that some leaching of small oligomeric material containing ligand species into the solution is facilitated by aqueous HC1. This presumably results from hydrolysis of some S i ~ O ~ S i linkages, especially those near the immobilized ligand. This kind of leaching of small oligomeric species by acidic aqueous solution has been discussed by others for amine ligands grafted onto silica surfaces [75]. The 29Si NMR spectra of the polysiloxane-immobilized thiol (Fig. 25.20C), thiol-monoamine (Fig. 25.24A) and thiol-diamine (Fig. 25.24B) systems, and the HCl-treated polysiloxane-immobilized thiol-diamine system (Fig. 25.24C) display two major regions of signals, centered at about -105 and - 6 0 ppm relative to TMS. These regions correspond t o S i ( ~ O ~ ) 4 and R S i ( ~ O ~ ) 3 units, respectively, where R is an organic (thiol, monoamine or diamine) ligand. As indicated above, the -105 ppm spectral region is composed of at least three peaks or shoulders, at about -109, -100 and - 9 1 p p m , which \ 9 \ . \ . correspond t o ( ~ S I ~ O ) 4 S i ; ( T S I ~ O ) 3 S i O H and/or (~SI~O)3SiOEt; and (~Si--)2Si(OH)2, (--~Si--O)2Si(OH)(OEt) and/or (--~Si--O)2Si(OEt)2 sites, respectively; these features can be readily distinguished in the 29Si CP-MAS spectrum of the polysiloxane-immobilized thiol system in Fig. 25.20C. The structures given below show the kinds of ligand-containing species that one can readily envision on the surface of the polysiloxane framework. Details of the attachment of silicon to the polysiloxane framework are intentionally left unspecified.
R R'O~/i/OR'
R\ /OR'
,
otiS\
i
I
0
XIII
0
I
XIV
\
R I
-/Sko-']
XV
R=--(CH2)3NH2,--(CH2)3NH(CH2)2NH2,--(CH2)3SH; R'= H, CH3, C2H5.
The - 6 0 ppm spectral region in Fig. 25.24 includes at least two peaks, at Y , Y - 5 6 and - 6 4 ppm, due to RSi(O~Si~)zOR (XIV) and RSi(O~Si~)3 (XV)
NMR CHARACTERIZATION
OF FUNCTIONALIZED
POLYSILOXANES
973
sites, respectively, where R ' = H, C H 3 o r C 2 H 5 . The 298i spectrum of the polysiloxane-immobilized thiol ligand system (Fig. 25.20C) shows much higher relative intensities of peaks at -56, -91 and -100ppm, compared with intensities of those peaks in the spectrum of the polysiloxane-immobilized thiol-monoamine and thiol-diamine ligand systems (Figs. 25.24A and 25.24B). The -56 ppm peak in Fig. 25.24A is an indication of incomplete hydrolysis and/or condensation of (MeO)3Si(CHz)3SH; the -91 and - 100 ppm peaks can be attributed to the incomplete hydrolysis and/or incomplete condensation of Si(OEt)4 (e.g., leavin~ ( ~ S i O ) 2 S i ( O E t ) 2 , \ . \ . . . \ . (~S10)3SiOEt and/or (7810)2Si(0H)2 moieties and (TS10)3SiOEt, and/or (~SiO)3SiOH moieties). The incomplete hydrolysis of (MeO)3Si(CHz)3SH and Si(OEt)4 is also indicated by the 13C NMR spectrum shown in Fig. 25.2C. The 298i spectrum of the thiol ligand system (Fig. 25.20C) also shows a shoulder at -47 ppm, which is assigned to RSi(OR')2OSi--~ structures (I) [41]. From the intensity patterns of the two 298i CP-MAS NMR spectra of the polysiloxane-immobilized thiol-diamine ligand system before (C) and after (D) treatment with 0.11 M hydrochloric acid, shown in Fig. 25.24, it is possible to say that the relative peak intensities of the single (-100ppm) silanol and geminal (-91 ppm) silanol peaks are increased and that of the RSi(OSi--~)3 peak ( - 6 4 ppm) is decreased somewhat by treatment with hydrochloric acid. From relative intensities of the (~SiO--)3SiOH, ( ~ S i O - - ) 2 S i ( O H ) 2 and RSi(O~Si--~)3 peaks in the 298i CP-MAS spectra of the polysiloxane-immobilized thiol-diamine system before and after 0.11 M HCI treatment (Fig. 25.24), one sees that the silanol intensities are somewhat increased and the RSi(O~Si--~)3 intensity decreased by HC1 treatment. This behavior is consistent with the small amount of leaching (1-2%, gravimetrically) caused by this treatment, i.e., some breaking of S i ~ O ~ S i bonds that are responsible for attaching the pendent ligand groups to the polysiloxane matrix. This view is consistent with the elemental analysis data, which showed a preferential leaching of pendent ligand groups. The significant hydrolytic instability of silica-bound alkylamine ligands under protonation was also suggested by Jezorek et al. [43]. The 298i NMR spectrum of ( S ) - C H z C H z C H z N H C H z C O z C H 3 (Fig. 25.20A), a derivative of the polysiloxane-immobilized primary amine, displays features that are very similar, in terms of relative signal intensities of the ligand-bearing silicon moieties ( - 6 0 ppm region) and nonligand-bearing silicon moieties (-100 ppm region), to those of the polysiloxane-immobilized primary amine system (Fig. 25.18B). Small changes in the relative intensities of individual silicon sites, such as(~SiO)3SiOH/,_ (Q3, -100 ppm) and Si(OSi~)4 (Q4, -109ppm), RSi(OSi~)3 ( - 6 4 p p m ) and RSi(OSi~)2OH
974
GARY E. MACIEL
(-57
ppm),
are
observed
in
the
spectrum
of
(S)-
CH2CH2CH2NHCH2CO2CH3, compared with that of the precursor, the pol-
ysiloxane-immobilized primary amine. The same kinds of patterns as mentioned above for the comparison between the 29Si NMR spectra of (S)CH2CH2CH2NHCH2CO2CH3 (Fig. 25.20A) and its precursor (S)CH2CH2CH2NH2 (Fig. 25.18B), are also observed among the 29Si NMR spectra of the derivative, (S)-CH2CH2CH2SCH2CO2CH3 (Fig. 25.20B), and its precursor, (S)-CH2CH2CH2C1 (Fig. 25.21A), as well as the derivatives, (S)-CH2CH2CH2N(C2Hs)2 (Fig. 25.21B), (S)-CH2CH2CH202CCH2NH2 (Fig. 25.21C), (S)-CH2CH2CH202CCH2NHCH2CO2 (Fig. 25.21D) and (S)CH2CH2CH202CCH3 (Fig. 25.21E), prepared from 3-chloropropylpolysiloxane (Fig. 25.21A). That is, the 29Si NMR spectra of these derivatives show features that are very similar to those of their precursors; only small changes in the relative intensities of individual silicon sites are observed. In general, these results indicate that the polysiloxane frameworks of the precursors are not attacked significantly during derivatization. Only small degrees of further hydrolysis or cross-linking, if any, occur under the reaction conditions used in the preparations of these derivatives. As expected, the 29Si CP-MAS NMR spectra of polysiloxane-immobilized phosphine, phosphine-amine and phosphine-thiol ligand systems (Figs. 25.22 and 25.23) show two major regions of signals, centered at about -105 and - 6 0 p p m relative to TMS. These regions correspond to S i ( ~ O ~ ) 4 and R S i ( ~ O ~ ) 3 units, respectively, where R is a pendent ligand (phosphine, monoamine, diamine or thiol) group. The -105 ppm spectral region in Figs. 25.22 and 25.23 is composed of three peaks or shoulders, at about -109, -100 and - 9 2 These correspond, respectively, to ( T S l ~ O ) 4 S i ; (~Si~O)3SiOH ppm. and (~Si--O)3SiOEt; and (~Si--O)2Si(OH)2, (~Si~O)2Si(OH)(OEt) and (~Si~O)2Si(OEt)2 sites. The - 6 0 ppm spectral region in Figs. 25.22 and 25.23 includes at least two peaks, at -57 and - 6 4 ppm, due to RSi(O~Si--~)2OR' and RSi(O~Si--~)3 sites, respectively, where R' = H, CH3 or C2H5. As indicated in the 13C spectra shown in Fig. 25.11, incomplete hydrolysis of Si(OEt)4 and RSi(OR')3 (R is a pendent organic hgand, R = Me or Et) can leave (TSI~O)3SiOEt (-100ppm), N \ (TSi--O)2Si(OH)(OEt) (-92ppm), (~SiO)2Si(OEt)2 ( - 9 2 p p m ) and )i/ R S i ( ~ O ~ S i s ) 2 O R (-57 ppm) groups in the functionalized polysiloxane. It can be seen from Fig. 25.22 that the polysiloxane-immobilized propylphosphine-monoamine system (Fig. 25.22B) and phosphine-diamine system (Fig. 25.22C) have higher relative amplitudes in the - 6 0 p p m region, in comparison to the polysiloxane-immobilized phosphine system (Fig. 25.22A), because higher molar ratios of ligand-bearing reagents, RSi(OR')3, to the nonligand-bearing reagent, Si(OEt)4, were used in the starting materials in 9
9
9
?
~
9
~
9
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 975 the former two cases. This result is consistent with the elemental analysis data, which showed that the polysiloxane-immobilized phosphine-monoamine and phosphine-diamine systems contain more ligand groups in the solid products than does the polysiloxane-immobilized phosphine system. However, the 29Si spectrum of the polysiloxane-immobilized phosphine-thiol system (Fig. 25.22D) displays a much higher relative amplitude in the - 6 0 p p m region in comparison to the amplitude in the -105 ppm region than in the spectrum of the phosphine system (Fig. 25.22A), despite the fact that almost the same relative numbers of moles of ligand-bearing starting materials and Si(OEt)4 were used in both cases. Trends similar to what were described above for the polysiloxane-immobilized diphenylpropylphosphine series were observed for the polysiloxaneimmobilized diphenylethylphosphine series, for which the 295i NMR spectra are shown in Fig. 25.23. The polysiloxane-immobilized diphenylethylphosphine-thiol system (Fig. 25.23D) shows much lower relative amplitude in the -105 ppm region, in comparison to the - 6 0 ppm region, than in the 29Si NMR spectra of the other samples. This pattern indicates that the structures of the polysiloxane networks in these polysiloxane-immobilized systems are not only affected by the molar ratios of the ligand-bearing starting materials to the nonligand-bearing starting materials, but are also strongly affected by the specific types of organoligand moieties. The small relative amplitude of the signals in the -105 ppm region in the two 295i spectra of phosphine-thiol ligand systems (Figs. 25.22D and 25.23D) indicates that, in each case of the phosphinethiol systems, more ligand-bearing silicon moieties are incorporated into the polysiloxane product, and the material is less cross-linked. This pattern may imply that thiol groups in some way retard the hydrolysis/condensation of Si(OEt)4 more than that of (R'O)3Si(CH2)nR. In order to avoid issues centering on 1H-29Si cross polarization dynamics, and their impact on analytical quantitation [76-79], we also carried out 29Si MAS NMR experiments that employed direct polarization (DP) of the 29Si nuclei via 29Si spin-lattice relaxation on (S)-CH2CH2CH2NH2 and (S)CH2CH2CH2C1 samples. The resulting DP-MAS 295i spectra, obtained with a repetition delay of 300 s to avoid intensity distortions (Fig. 25.25), have relative intensities that are qualitatively similar for the 3-chloropropylpolysiloxane sample to those of the corresponding CP-MAS spectrum shown in Fig. 25.19D. The DP-MAS spectrum of the polysiloxane-immobilized monoamine system (Fig. 25.25A) shows higher intensity in the -105 ppm region relative to that of the - 6 0 ppm region, in comparison with the corresponding CP-MAS spectrum (Fig. 25.19A). From peak areas of computer-deconvoluted CP-MAS 29Si spectra of Fig.
976
GARY E. MACIEL
Table 25.6. Relative populations of structural moieties determined from DP-MAS 29Si spectra
% RSi(-O-)3 a
Sample
(S)-CH2CH2CH2NH2
(S)-CHECH2CHEC1
Elemental Analysis
DP-MAS 295i NMR
33
28
52
Fraction RSi(OSi~)3 b DP-MAS
Fraction Si(OSi~)4 c DP-MAS
295i NMR
295i NMR
25 39
41 20
a Percentage of silicon with organoligand group attached. b Fraction of RSi(-O-)3 moieties that are RSi(OSi~)3. c Fraction of Si(-O-)4 moieties that are Si(OSi~)4.
25.25, it was possible to determine (expected error: ---5%) the relative amounts of R - S i ( n O - - ) 3 and S i ( m O - - ) 4 moieties of various types, the relative proportions of which can be estimated from the areas of deconvoluted peaks. Of course, elemental analysis data can also be used to estimate the relative amounts of R - S i ( n O ~ ) 3 and Si(mO~)4 moieties in the polysiloxane-immobilized monoamine ligand system. Results of these estimates are shown in Table 25.6. This table shows that, for the samples examined, 28% and 52% of the silicon atoms of the polysiloxane-immobilized monoamine system and 3-chloropropyl system, respectively, have organoligand groups attached. Figure 25.26 shows hypothetical representative structures that are consistent with the structural data derived for these two samples, on the basis of the DP-MAS 29Si spectra. As the 13C CP-MAS spectra show that the polysiloxane-immobilized amine samples contain no unhydrolyzed methoxy or ethoxy groups, none are shown in the structure of Fig. 25.26A. Such groups are included, in proportion to the corresponding intensities in the 13C CP-MAS spectrum, in the structure of Fig. 25.26B, representing the polysiloxane-immobililzed 3-chloropropyl system, for which hydrolysis/condensation of methoxy and ethoxy groups is not complete. 25.4.6
Time-domain (relaxation) results
Dipolar dephasing. 1H-13C dipolar-dephasing experiments are designed to identify relatively immobile CH and CH2 groups by virtue of their efficient 1H-13C dipolar dephasing. Representative dipolar-dephasing spectra of the polysiloxane-immobilized monoamine system and its Hg 2§ complex and of the 3-chloropropylpolysiloxane prepared with HC1 as catalyst are shown in Fig. 25.3. The dephasing rate of 13C magnetization of the uncomplexed
977
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES A
"YiN
~
~" / -~i~. o-'~iN
~-c)i S; " i HO ~ \O--''~,/IN''7 ~
y
9
-3
~QNS.i I L1
"-'
\/
~N
.--ii~o )Si-R I 0 r~l-I
-, ./'-"
?-~',
HO
_ j.
~
H
.,,~Lx,,-, Y t - "~ei/'-' ~0 ~ -C~-.~/.-'O~i" " ~
,-st'-
~..o-,
~'
o
"1_
o
%~/
OI-I
o~. t
,
biN.-./Si~_ _" 2. I /U / O .qi/O~6L, "~Si / I t . . r ~ "O-SiN,,.c~i~r~,,,.~;ix,.,.,~ L / ,r~ l 0 f'~n " " HO ! .v ~ '-' / u- ~%., , / v \ , . , . / "( ~- O "~i fOH rl OH R v---b{ ~ ~...I ,,S~,--
HO
O
R
OH
(:~
.(9
T
•
OH
R~
R
R
OEI
Fig. 25.26. Representative (hypothetical) structures of polysiloxane-immobilized systems based on 29Si D P - M A S spectra of Fig. 25. (A) Polysi]oxane-irnmobilized monoamine system. (B) Po]ysi]oxane-immobilized 3-ch]oropropy] system. Ref. 17.
polysiloxane-immobilized monoamine system decreases in the order, C(1) > C(2) > C(3) (C(3) attached to the NH2 group) (Fig. 25.3A). The fact that the 1H-13C dipolar-dephasing rates displayed in Fig. 25.3A follow the order, C(1)> C(2)> C(3), suggests that the amino end of the pendant group has some mobility, so that C(3) and C(2) are more mobile than C(1), which is closer to the point of attachment to the polysiloxane polymer framework. In the 13C dipolar-dephasing spectra (Fig. 25.3B) of the Hg(II) complex of the polysiloxane-immobilized monoamine, the 13C magnetization due to C(3) dephases only slightly more slowly than that of C(1) and C(2), and all three carbons dephase somewhat faster than in the uncomplexed system. From these observations, it appears that complexation of the polysiloxane-immobilized monoamine system by Hg (II) is through the
978
GARY E. MACIEL
amine group, as expected. The enhanced dephasing rate of the 13C signal intensity in the 13C dipolar-dephasing spectra of the Hg(II) complex of the polysiloxane-immobilized amine occurs because amino complexation by Hg e+ has reduced the mobility of the pendent ligand group. The 13C dipolar-dephasing spectra of the 3-chloropropylpolysiloxane prepared with HC1 as catalyst, displayed in Fig. 25.3C, shows that 13C magnetization due to C(3) (attached to C1) decays only moderately even with a 50 Ixs dipolar-dephasing time and ~3C magnetizations of all three carbons of the 3chloropropyl ligand decay more slowly than the 13C magnetization of the corresponding three carbons of the polysiloxane-immobilized monoamine (Fig. 25.3A). This result indicates that the 3-chloropropyl ligands have much greater mobilities (especially at the chain end) than the monoamine ligand. The 13C magnetizations of the residual ~ O M e and the methyl group of residual ~ O E t decay only slightly with even a 50 Ixs dipolar-dephasing time, because of the rapid rotation of CH3 groups. The 50 Ixs dipolar-dephasing 13C CP-MAS NMR spectra of the polysiloxane-immobilized thiol ligand system (Fig. 25.9A) indicates that the thiol ligands in the thiol-ligand-only system have large mobilities, so the 13CNMR signals corresponding to these ligands can survive the 50 Ixs dipolar-dephasing reasonably well. In contrast, Fig. 25.9B and 25.9C, for the thiol-amine systems, show that the thiol ligands in the thiol-amine systems are rather immobile, so most of the 13C NMR signal intensity attributed to these ligands can not survive the 50 Ixs dipolar-dephasing period; this behavior probably reflects a more rigid hydrogen-bonding network. Analogous IH-15N dipolar-dephasing 15N CP-MAS experiments carried out on the polysiloxane-immobilized amine samples with a 120 s dephasing time (Fig. 25.14A), show that for the polysiloxane-immobilized monoamine system treated at pH = 1, the 15N magnetization is only moderately dephased in 120 Ixs. In contrast to this partial dephasing behavior shown for the protonated polysiloxane-immobilized monoamine system (Fig. 25.14A), analogous experiments on the polysiloxane-immobilized diamine system treated at pH = 1 (Fig. 25.14B) show that both the primary and secondary ammonium cation signals are completely dephased at 120 Ixs. This difference in behavior may indicate that the local mobility of the polysiloxaneimmobilized monoamine system is greater than that of the corresponding diamine system, a difference that may result from some combination of steric and hydrogen bonding effects. The absence of a tertiary amine signal, which would be expected to occur at about 50-55 ppm [63-67] and to survive 1H-15N dipolar dephasing very well, confirms the absence of structure XII in the functionalized polysiloxane and shows that structure XI is the product of the reaction between diethyl-
NMR C H A R A C T E R I Z A T I O N OF F U N C T I O N A L I Z E D P O L Y S I L O X A N E S
979
enetriamine and the 3-chloropropylpolysiloxane precursor; this result is consistent with the 13C results described above. 1H spin-lattice relaxation results. The CP-derived T H results summarized in Tables 25.4 and 25.5, based on 13C, 29Si and 31p CP-MAS detection, are useful for assessing local mobility and domain homogeneity/heterogeneity in these potentially very heterogeneous systems. For some of the 13C-detected T H values given in Tables 25.4 and 25.5, only one value is given for a particular sample, reflecting some combination of apparent peak overlaps (preventing the determination of separate T H values from individual 13C peaks) or apparently uniform T H behavior reflected throughout the entire 13C CP-MAS spectrum. The proton-spin-lattice relaxation time, T H, in a diamagnetic sample is typically dominated by the dipole-dipole interactions among the protons, and is particularly sensitive to molecular motions; therefore, T11-Ivalues have been used widely to derive information about domain uniformity or phase separation in solids. In single-phase solids with an abundance of protons, the protons are usually so strongly coupled by homonuclear dipolar interactions that possible differences in proton spin-lattice relaxation are averaged out by spin diffusion, and a single T H will be observed in this case. In heterogeneous samples, different T H values can be observed for different phases. The T H approach based on 13C detection is particularly suitable for examining the distribution of the organic groups in the polysiloxane network of the polysiloxane-immobilized thiol and chloropropyl systems; in these systems there are unhydrolyzed ethoxy groups and methoxy groups, which can be seen clearly in the ~3C NMR spectra (Figs. 25.2 and 25.3). The unhydrolyzed ethoxy groups and methoxy groups come from the nonligandbearing starting material, Si(OEt)4, and ligand-bearing starting material, (MeO)3SiCHzCHzCHzNH2 or (MeO)3SiCHzCHzCHzSH, respectively, and could a priori represent spatially different regions or phases in an inhomogeneous material (i.e., if they are not largely homogeneous). The examination of T H values measured from the 13C signals of ethoxy groups, methoxy groups and pendent (ligand) groups should provide information on the distribution of the ligand groups in the sample: if the ligand groups in a particular sample are uniformly distributed in the polysiloxane network, then the same value would be expected for T H measured via the 13C signals of the ethoxy, methoxy and ligand groups. Further information can be obtained by comparing these 13C-detected T11-Ivalues with the results of analogous measurements based on 29Si signals of SiOH groups. The protons of organic ligand groups are expected to be mainly responsible for the T H values of the pendent ligand groups detected via ~3C resonances.
980
GARY E. MACIEL
They may represent different regions of the material from those regions in which are concentrated SiOH groups, which are at least substantially (if not totally) responsible for 29Si-detected T11-Ivalues. Therefore, comparison between the 29Si-detected T1-I values and 13C-detected T11-Ivalues should provide information about domain uniformity or phase separation. The T~ results obtained on the polysiloxane-immobilized thiol and chloropropyl systems (Table 25.4) show that the 13C-detected T11-Ivalues obtained for the CH2 peak of the ethoxy group at 60.8 ppm, the CH3 peak of the methoxy group at 51.9ppm, the C(2), C(3) peak of the 3-mercaptopropyl group at 28.7 ppm and the C(1) peak of the 3-mercaptopropyl group at 12.6 ppm are essentially the same within experimental error (about ---0.06 s). This indicates that the pendant organo-ligand groups and unreacted methoxy groups of the organofunctionalized region of the material and the ethoxy groups from the silica-like region are not phase separated, but in close physical proximity. 29Si NMR signals of the - 6 0 ppm region are due to silicon sites bearing the 3-mercaptopropyl ligand groups; the 29Si signals of the -100 ppm region are due to silicon sites without ligand groups attached. Comparison between the T11-Ivalues obtained for these two regions would provide information about whether or not these two regions are in the same phase or different domains. The same T~I value obtained for the two silicon resonance regions of the polysiloxane-immobilized thiol system suggests that the silicon sites in the silica-like network and the silicon sites bearing the organo-ligand functionalities are in the same phase. The fact that the same T~ value was obtained for both 29Si detection and 13C detection is further evidence that the 3-mercaptopropyl functionalities are not phase separated from SiOH groups that are at least partially responsible for the 29Si-detected T11-Ivalues. All these results indicate that the organo-ligand functionalities are evenly distributed in the polysiloxane network, not phase separated from a silicalike network. Similarly, as seen in Table 25.5, a single T~I value was obtained for the polysiloxane-immobilized thiol-monoamine system and a single value for the thiol-diamine system (Table 25.5). These results imply that the two types of ligand groups (thiol and monoamine in one sample, thiol and diamine in another) are uniformly mixed in the polysiloxane network; they are not phase separated. The fact that the 13C-detected T~ value and the 29Si-detected T11-Ivalue for the silicon sites of the - 6 0 ppm region and the - 100 ppm region are all the same for a given sample, within experimental error (Tables 25.4 and 25.5), suggests that the organofunctionalities in each of these two samples are in the same phase as the silica-like network. Analogous results are seen for the other samples in Tables 25.4 and
NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 981
25.5, and hence similar conclusions apply. For organo-phosphine containing samples, 31p-detected T~ values are also available. For some of the organophosphine-functionalized polysiloxanes (see Tables 25.4 and 25.5), CP-derived T~ values were obtained via separate CP-MAS experiments with I3C, 29Si and 31p detection. The fact that a single T~ was obtained on all the peaks within the 13C, 31p or 29Si spectrum of each sample and that the same T~'s (within experimental error) were obtained from 29Si detection, I3C detection and 31p detection for each sample examined (Tables 25.4 and 25.5) indicates that the ligand groups in the polysiloxane-immobilized ligand systems are evenly distributed in the polysiloxane network; the materials are not domain-structured. The fact that, for each of the samples represented in Table 25.4, the T~ value measured at 260 MHz is greater than T~ measured at 200 MHz indicates that the motional correlation time ~'c associated with the motions responsible for proton spin-lattice relaxation is larger than (260 x 106Hz) -1, i.e., >3.8 • 10 -9 S. The fact that T~Ip measurements (not presented here) reveal that T~Ip ~T1H for each case represented in Table 25.4 shows that Zc ~> (200 • 10 6 Uz) -1, i.e., >>5 x 10-9 S. For the polysiloxane-immobilized diamine sample represented in Table 25.4, both T~ and T~p (results not given here) are increased by treatment with HCl(aq), or Cd2+(aq). If one assumes that both treatments reduce mobility in these polysiloxane-immobilized amine systems (by introducing ionic interactions and complexation), the increases imply that Zc in treated samples is larger than about (40 x 10 3 Hz) -1, i.e., >2.5 x 10 -5 s.
25.5
Summary and conclusions
Solid-state NMR techniques can provide a high degree of structural and dynamical detail of specific sites in functionalized polysiloxanes. 29Si NMR data provide details on the siloxane network and pendent-group attachments in these systems. I3C CP-MAS techniques, and to a lesser degree IH CRAMPS experiments, are useful in elucidating structure and dynamics in the pendent organic groups that bear the desired functional groups. When applicable, 15N and 31p MAS experiments are useful. Relaxation studies not only contribute information on local dynamics (in terms of motional correlation times), but also provide a means of interrogating these samples with regard to domain homogeneity/heterogeneity-- homogeneous in the functionalized polysiloxanes studied to date.
982
GARY E. MACIEL
Acknowledgements The author gratefully acknowledges that much of the work described in this paper was supported by the National Science Foundation (NSF Grant No. CHE-9021003) and the key roles played by coworkers, especially Dr. Jane J. Yang and Dr. Issa M. E1-Nahhal.
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983
24. V.F. Dovganyuk, L.I. Lafer, V.I. Isaeva, Zh. L. Dykh, V.I. Yakerson and V.Z. Sharf, Bull. Acad. Sci. USSR 36 (1987) 2465. 25. D.J. Kelly and D.E. Leyden, J. Colloid Interface Sci. 147 (1991) 213. 26. I. Taylor and A.G. Howard, Anal. Chim. Acta. 271 (1993) 77. 27. H. Ishida, C.-H. Chiang and J.L. Koenig, Polymer 23 (1982) 251. 28. D.E. Leyden, D.S. Kendall and T.G. Waddell, Anal. Chim. Acta. 126 (1981) 207. 29. G.R. Hays, A.D.H. Clague, R. Huis and G. Van Der Velden, Appl. Surf. Sci. 10 (1982) 247. 30. C.-H. Chiang, N.-I. Liu and J.L. Koenig, J. Colloid Interface Sci. 86 (1982) 26. 31. A.M. Zaper and J.L. Koenig, Polym. Compos. 6 (1985) 156. 32. H.-J. Kang and F.D. Blum, J. Phys. Chem. 95 (1991) 9391. 33. D.W. Sindorf and G.E. Maciel, J. Phys. Chem. 86 (1982) 5208. 34. R.W. Linton, M.L. Miller, G.E. Maciel and B.L. Hawkins, Surf. and Interface Anal. 7 (1985) 196. 35. K.-P. Hoh, H. Ishida and J.L. Koenig, Polym. Compos. 11 (1990) 121. 36. D.W. Sindorf and G.E. Maciel, J. Am. Chem. Soc. 105 (1983) 3767. 37. E. Bayer, K. Albert, J. Reiners and M. Nieder, J. Chromatogr. 264 (1983) 197. 38. E.J.R. Sudholter, R. Huis, G.R. Hays and N.C.M. Alma, J. Colloid Interface Sci. 103 (1985) 554. 39. J.W. De Haan, H.M. Van Den Bogaert, J.J. Ponjee and L.J.M. Van De Ven, J. Colloid Interface Sci. 110 (1986) 591. 40. J.M.J. Vankan, J.J. Ponjee, J.W. De Haan and L.J.M. Van De Ven, J. Colloid Interface Sci. 126 (1988) 604. 41. G.E. Maciel, D.W. Sindorf and V.J. Bartuska, J. Chromatogr. 205 (1981) 438. 42. C.W. Chu, D.P. Kirby and P.D. Murphy, J. Adhesion Sci. Technol. 7 (1993) 417. 43. J.R. Jezorek, K.H. Faltynski, L.G. Blackburn, P.J. Henderson and H.D. Medina, Talanta 32 (1985) 763. 44. E. Yacoub-George, E. Bratz and H. Tiltscher, J. Non-Crystal. Solids 167 (1994) 9. 45. A.M. Zaper and J.L. Koenig, Adv. Colloid Interface Sci. 22 (1985) 113. 46. T.G. Waddell, D.E. Leyden and M.T. DeBello, J. Am. Chem. Soc. 103 (1981) 5303. 47. C. Fulcher, M.A. Crowell, R. Bayliss, K.B. Holland and J.R. Jezorek, Anal. Chim. Acta. 129 (1981) 29. 48. R.S.S. Murthy and D.E. Leyden, Anal. Chem. 58 (1986) 1228. 49. S.R. Culler, H. Ishida and J.L. Koenig, Polym. Compos. 7 (1986) 231. 50. M.W. Urban and J.L. Koenig, Appl. Spectrosc. 40 (1986) 513. 51. J. Schaefer and E.O. Stejskal, J. Am. Chem. Soc. 98 (1976) 1031. 52. G.E. Maciel, C.E. Bronnimann and B.L. Hawkins, Adv. Magn. Reson. 14 (1990) 125. 53. G.E. Maciel, C.E. Bronnimann, R.C. Zeigler, I. Chuang, D.R. Kinney and E.A. Keiter in H.E. Bergna (Ed), The Colloid Chemistry of Silica, ACS Advances in Chemistry Series, 234, American Chemical Society, Washington, D.C. (1994), p. 269. 54. J.B. Stothers, Carbon-13 NMR Spectroscopy. New York: Academic Press, (1972), p. 152. 55. S.J. Opella, M.H. Frey and T.A. Cross, J. Am. Chem. Soc. 101 (1979) 5856. 56. C.J. Groombridge, R.K. Harris, K.J. Packer, B.J. Say and S.F. Tanna, J. Chem. Soc. Chem. Common. (1980) 174. 57. A. Naito, S. Ganapathy and C.A. McDowell, J. Chem. Phys. 74 (1981) 5393. 58. N. Zumbulyadis, P.M. Henrichs and R.H. Young, J. Chem. Phys. 75 (1981) 1603. 59. A.C. Olivieri, L. Frydman and L.E. Diaz, J. Magn. Reson. 75 (1987) 50. 60. R.K. Harris and A.C. Olivieri, Prog. Nucl. Magn. Reson. Spectrosc. 24 (1992) 435.
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61. W.E. Rudzinski, T.L. Montgomergy, J.S. Frye, B.L. Hawkins and G.E. Maciel, J. Catal. 98 (1986) 444. 62. G.E. Maciel and J.J. Natterstad, J. Phys. Chem. 42 (1965) 2752. 63. M. Witznowski, L. Stefaniak and G.A. Webb, Ann. Reports on NMR Spectros. 25 (1993) 1. 64. G.C. Levy and R.L. Lichter, Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy. New York: Wiley-Interscience, 1979. 65. H.R. Kricheldoff, Polymer Bull. 3 (1980) 53. 66. W. Von Phillipsborn and R. Muller, Angew. Chem. Int. Ed. Engl. 25 (1986) 383. 67. J. BRimel, Inorg. Chem. 33 (1994) 5050. 68. L. Bemi, H.C. Clark, J.A. Davis, C.A. Fyfe and R.E. Wasylishen, J. Am. Chem. Soc. 104 (1982) 438. 69. The simulation program used in the studies described was FTNMR, Version 4.8, Hare Research, Inc., 1986. 70. J. Herzfeld and A.E. Berger, J. Chem. Phys. 73 (1980) 6021. 71. J.J. Yang, Ph.D. Dissertation. Colorado State University, 1996. 72. G. Scheler, U. Haubenreisser and H. Rosenberger, J. Magn. Reson. 44 (1981) 134. 73. R.K. Harris and P. Jackson, J. Phys. Chem. Solids 48 (1987) 813. 74. A. Naito, A. Root and C.A. McDowell, J. Phys. Chem. 95 (1991) 3578. 75. J.R. Jezorek, K.H. Faltynski, L.G. Blackburn, P.J. Henderson and H.D. Medina, Talanta 32 (1985) 763. 76. M. Mehring, Principles of High Resolution NMR in Solids, 2nd edn. Berlin: SpringerVerlag, 1983. 77. G.R. Hatfield and G.E. Maciel, Macromol. 20 (1987) 608. 78. M. Zhang and G.E. Maciel, Fuel 69 (1990) 557. 79. J.V. Muntean, L.M. Stock and R.E. Botto, Energy & Fuels 2 (1988) 108.
Conclusions
In this book it has been demonstrated that solid-state NMR spectroscopy is a very powerful means for elucidating the structure and dynamics of the mobile and immobile components of polymers in the solid state and its application is spreading rapidly in polymer science and technology. Various methodogies for elucidating the structure and dynamics in solids are increasing in the field of solid-state NMR. There are many places for developing instrumental techniques and methodologies in solid-state NMR. From such a situation it can be said that solid-state NMR is evergreen in NMR spectroscopy.
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Subject Index
Ab initio calculations, 7 Ab initio IGLO method, 419 Ab initio MO calculations, 4, 7 Absolute constraints, 230 Acetylcholine receptor, 230 Acrylonitrile/methyl acrylate/butadiene terpolymer (B210), 390 Activation energy, 288, 346 Activation energy zXE, 660 Adipic acid, solid, high resolution proton NMR, 176 Adsorbed, 341 Agarose gel, 902, 903 (AlaGly),,, 824 (Ala-Gly),, I, 906 (Ala-Gly)n II, 906 Alanine, 854, 869, 870 Alanine-rich domain, 869 n-Alkane, 275 Alpha-crystalline form, 416 Alzheimer's disease, 885 Amorphous, 614, 679, 688, 690, 694, 695, 698 Amorphous phase of polyethylene, 268 Amorphous phase, 267, 297 Amorphous yarn, 491 /3-Amyloid protein, 45 Amylose, 895 Anisotropic diamagnetic susceptibility, 228 Anisotropic parameter, 69 Anisotropy widths, 662 Annealed polymer, 433 Antiparallel/3-sheet, 46 (Asp(OBzl))., 826 Asymmetry parameter, 193, 199, 200 Atactic polypropylene, 303,304
ATR-IR, 501 Auto-correlation functions, 56 Average Hamiltonians, 30, 167, 169 Azimuthal angle, 25 Back projection, see NMR imaging chemical shift, 134 Backbone and side-chain flexibility, 191 Backbone dynamics, 191 Backbone motion, 37 Bacteriorhodopsin, 47, 230, 910 Band gap, 20, 602 Band structures, 602 Basis operator, 204 Beta-crystalline form, 416 Biaxially drawn film, 498 Binodal, 403 Biodegradable polymers, 771-773,775,805, 811,812 Biologically-active peptides, 918 Bisureidosilane, 654 Blends, 687,688 Bloch-Siegert effect, 256, 702 Bond angle, 271,276 Bond length, 271,276 Bond orientations, 314 Box-type distribution, 57, 65 BPP theory, 640, 660 BR24, 254 Broadening of peak, 291 13C, 218 C--H dipolar interaction, 293, 51 Carr-Purcell-Meiboom-Gill, 208 Carr-Purcell-Meiboom-Gill (CPMG), 202, 206, 208
988
SUBJECT INDEX
Catalysts, 970 13C chemical shift contour maps, 844, 846, 848 13C chemical shift map, 8 13C chemical shifts, 933,938 13C chemical shift tensor, 834 13C~13C exchange NMR, 380 13C CP/MAS, 829 ~3C CP/MAS NMR of: acrylate resins, 546 13C CP/MAS NMR of: anion exchange resins, 555 13C CP/MAS NMR of: epoxide based resins, 535 ~3C CP/MAS NMR of: glycidyl methacrylate-co-trimethylolpropane trimethylacrylate, 539 a3C CP/MAS NMR of: hypercrosslinked polystyrene resins, 561 13C CP/MAS NMR of: hypercrosslinked resins, 547 ~3C CP/MAS NMR of: isocyanurate based resins, 522 13C CP/MAS NMR of: melamineformaldehyde resins, 515 ~3C CP/MAS NMR of: methacrylate resins, 546 13C CP/MAS NMR of: methacrylate-based resins, 538 13C CP/MAS NMR of: phenolformaldehyde resins, 510 13C CP/MAS NMR of: polyacenic polymers, 530 13C CP/MAS NMR of: polyacrylamides, 543 13C CP/MAS NMR of: poly(divinylbenzene) resins, 559 13C CP/MAS NMR of: polyethers, 533 ~3C CP/MAS NMR of: poly(hydroxyethylmethylacrylate), 540 ~3C CP/MAS NMR of: polyphenolic tannins, 527 13C CP/MAS NMR of: styrene-based polymers, 544 ~3C CP/MAS NMR of: urea-formaldehyde resins, 517 13C CP/MAS NMR spectrum, 743 ~3C CP/MAS spectrum for the PVA gel, 743
13C--13C spin diffusion, 380 13C--13C two-dimensional exchange NMR, 4OO 13C DD/MAS NMR, 422 Cellulose, 713 Chain diffusion, 302 Chain motion, 679 Charge transfer, 354 Charge-transfer (CT), 355 CHARMM energy, 231 Chemical functionality of protons in solid polymers, and proton NMR, 166 Chemical shielding anisotropy, 312 Chemical shielding tensor, 14 Chemical shift, anisotropic, of protons, 169 Chemical shift anisotropy, 39, 51, 52, 68, 74, 296 Chemical shift distribution, 293 Chemical-shift filtered Goldman-Shen experiment, 376 Chemical shift interaction, 220 Chemical shift map, 822 Chemical shift of polyethylene, 279 Chemical-shift selective, 393 Chemical shift tensor of polyoxymethylene, 282 Chemical shift tensors, 220, 221,228, 270, 297, 298, 495,591 Chemical shifts, 27, 134, 355 Chitin, 897 Chitin/chitosan, 895 Chitosan, 897 Chloromethylation of poly(styrenedivinylbenzene), 556 C-hordein, 908 13C-labelled divinylbenzene, 545 13C-labelled polyethylenes, 337 13C NMR spectroscopic analysis: Rapp resin, 575 13C NMR spectroscopic analysis: TentaGel | 575 Cocrystallization, 780, 784, 787, 788, 796, 801,802 Coherence transfer, 28 Collagen, 905,908 Combinatorial synthesis, 567 Configuration, 5 Conformation jump models, 59, 67
SUBJECT INDEX Conformational maps, 43 Conformational space, 319 Conformational transion, 304 Conformation-dependent 13C chemical shift, 827 Conformation-dependent 13C shifts, 903 Conformations, 280, 677, 681,683,898 Conformations and 13C NMR chemical shifts, 819, 822 Contact relaxation time, 596 Continuous diffusion, 199 Contrast, change of matter, 139 Contrast, filter, see magnetization filters Contrast, functionalized, 137 Contrast, in NMR imaging, 123, 138 Contrast, material properties, 139 Contrast optimization, 139 Contrast, parameters, 138, 140, 142, 157 Contrast, state of matter, 139 Copolydimethylsilyldiethylsilane (PM-coES), 641-648 Copoly(dimethylsilyldiphenylsilane), 621630 Copoly(tetramethyl-psilphenylenesiloxane/dimethylsiloxane), 653 Correlation spectroscopy, 110 Correlation time filtering, 207 Correlation times, 57, 59, 61, 63, 64, 65, 66, 141,346 COSY, 263 CP, 256, 259 CP efficiency, 829 CP/MAS 13C NMR, 713,714, 717, 718, 720, 721,723,726, 727, 730 CP/MAS NMR, 613 CPMAS, 23,676, 684, 686, 707, 708 CPMAS of polyethylene, 268 CPMAS spectra of melt-quenched polyethylene, 292 CPMAS spectra of polyoxymethylene, 281 13C PST/MAS, 740 13C PST/MAS NMR spectra of the PVA gels, 745 13C PST/MAS NMR spectrum for the PMLG gel, 766 13C PST/MAS spectra for PMAA gels, 751 13C pulse saturation transfer (PST) method, 738
989
CRAMPS, 186, 254, 264, 673,944, 966 CRAMPS (combination of rotation and multiple-pulse spectroscopy, 374 Cross-link density, 143, 145, 147, 157 Cross polarisation, 219, 256, 257, 689, 706 Cross-polarization (CP) technique, 738 Cross polarization dynamics, 260, 975 Cross-polarization from 19F to 13C, 397 Cross polarization time (TcH), 901 Cross-relaxation experiments, 383 Crystal lattice, 436 Crystal structure, 275 Crystalline, 694 Crystalline and noncrystalline components, 717, 719 Crystalline domains, 395,670 Crystalline phase, 267, 297 Crystalline phase of polyethylene, 268 Crystalline polymorphs, 415,891 Crystalline yarn, 491 Crystallinity, 458, 686 Crystallization process, 289 CSA tensor, 965 13C SPE NMR of: anion exchange resins, 556 ~3C SPE NMR of: hypercrosslinked polystyrene resins, 564 13C SPE NMR of: poly(divinylbenzene) resins, 561 13C TI, 366 13C T1,753 CTFE, 692, 700 ~3C T1 selected spectrum, 394 Curdlan, 893 Curie temperature, 697 Cw irradiation, 117 Cyclic paraffins, 275,277, 453 Cytochrome c oxidase, 909 d8-PS/PVME, 401 DANTE pulse sequence, 79 DECODER, 498 2DECSA, 298 2DECSA spectrum, 300 Dedoped, 596 Defect, 285 Degradation, 400 Degree of crosslinking, 756
990
SUBJECT INDEX
Degree of swelling, 753 Dehedral angle, 441 2D 1H exchange NMR experiment, 377 2D 1H--13C heteronuclear correlation (HETCOR), 380 2D homonuclear and heteronuclear correlation experiments, 377 Dendrimer, 670 Density operator, 167, 203,204, 28 Deshielding effect, 733 Deuterium, 190 Deuterium exchange, 223 Deuterium labels, 190 Deuteron NMR, 681,705 2D-exchange, 681 2D exchange 2H NMR, 362, 364 2D exchange NMR, 298 2D exchange NMR experiments of 13C and 2H, 364 2D exchange spectroscopy, 198, 201,208 2D exchange spectrum, 301 Diamagnetic term, 2 Diffusion, case I, 145 Diffusion, case II, 145 Diffusion, concentration dependent constant, 145 Diffusion, Fickian, see case I Diffusion processes, 191 Diffusion tensor, 102 Diffusional rotation, 61, 63 Dihedral angles, 28, 845 Dipolar coupling, 170, 255,271,679 Dipolar decoupling, 23 Dipolar dephasing, 258, 604, 684, 928, 929, 935,942, 976, 977, 978 Dipolar-dephasing relaxation times, 337 Dipolar interactions, 23,220-223,228, 229, 232, 318 Dipolar interactions of protons in polymers, 169 Dipolar recoupling, 97 Dipolar tensors, 222 DIPSHIFT, 264 Direct polarization, 940, 975 Discriminating experiments, 668 Discrimination, 682 Disordered phase, 637-640, 643 Distributions of correlation times, 57
Domain, 387 Domain homogeneity/heterogeneity, 979 Domain sizes, 110, 367 Domain-structured, 981 Domain structures in homopolymers; and spin diffusion, 166 Domains in polymers and spin diffusion, 182 Doped, 595,596 Double cross polarisation, 689 Double decoupling, 688 Double quantum coherence, 202 Double quantum relaxation, 207 Double-quantum solid state NMR, 323 Double rotation (DOR), 218 DRAMA, 27 Draw ratios, 723,725,728 1D selective inversion-recovery/saturationtransfer experiments, 380 2D version of the modified Goldman-Shen experiment, 377 2D-WISE spectra, 377 2D 129Xeexchange NMR, 408 D values, 370 Dynamic angle spinning (DAS), 218 Dynamic averaging, 224 Dynamic screening length, 759 Dynamics, 51,663,898 Echo amplitude, 27 Effective diffusion time, 374 Effects of blending on motion, 358 Effects of drawing, 723,724 Elastomers, 144, 145, 147, 157 Elastomers, thermal aging, 147 Electric field gradient (cf. page 193), 200 Electric field gradient (EFG) cf. page 200, 192 Electrical aging, 155 Electrical conductivity, 596 Electronm1H--13C polarization, 384 Electronic spectroscopy, 219 Electronic structures, 1,278, 591 Emerimicine, 42 End group, 284 Energy minimization, 43 Ethyl branch, 285 Ethylene-propylene copolymer, 415
SUBJECT INDEX Euler angles, 314, 32 Evolution time, 128, 136 Extreme narrowing region, 660 Factor analysis, 672 Fast exchange, 198 Fd bacteriophage, 230 Ferromagnetica, 157 19Fn1Hm13C CP, 384 Fibers, 307 Fibre, 196 Fibrous proteins, 307, 891 Field gradient, 398 Field gradient tensor, 69 Film, 196 Finite perturbation theory (FPT) within the CNDO/2 MO finite perturbation theory, 598 Flip-flop term, 24 180~ Flip motion, 70, 72, 77 Fourier components, 38 FPT (finite perturbation theory), 3 FPT CNDO/2 MO, 614 FPT-INDO, 501 FPT INDO method, 3, 7, 8 FPT INDO MO method, 824 FPT-INDO calculations, 837 Framework, 614 Frequency dependencies of T1 and NOE, 65, 67 Frequency encoding, see NMR imaging Frozen solutions, 732, 734 Full-width-at-half-maximum, 498 Functionalized polysiloxanes, 924 Gamma-crystalline form, 422 Gamma-shielding effect, 419 Gauche-conformations, 269, 280, 616 y-Gauche effect, 269, 726, 727, 732, 734 Gauche form, 616, 636 Gaussian function, 427 Gaussian orientational distribution, 311 Gaussian probability distribution, 317 Gel network, 898 GG conformer, 616 4-31G-GIAO-CHF method, 17 y-Radiation, 680 y Relaxation, 288
991
GHD/MAS, 631 GIAO, 4 GIAO-CHF, 7 Glass transition temperature, 501,676, 678 Glassy polymers, 51 (1 -~ 3)-/3-D-Glucans, 892, 899 Goldman-Shen experiment, 374, 375 Goldman-Shen experiment, and spin diffusion in polymers, 182 Gradient tensor, 124, 135 Gradient vector, 125, 135 Gradient(s), 124 Gradient, oscillating, 133, 154 Gradient, rf, 136 Gradient, strong/strength, 132, 137 Gradient, time dependent, 125 Gramicidin, 230 Gramicidin A, 222, 223,225-227 GT (TG) conformer, 616 Gyromagnetic ratios, 219, 223 ~H, 218 Hall-Helfand model, 296 Hartmann-Hahn matching, 259 1HM~3C cross-polarization (CP), 383 1H--~3C cross-polarization transfer time, 493 1H chemical shifts, 946 1H chemical-shift selective Goldman-Shen experiment, 375,389 ~H CRAMPS spectra, 733 ~H DEPT NMR of: epoxide based resins, 535 1H--13C--19F CP, 400 ~H to 2H CP, 396 Heat-treatment, 404 31 Helices, 908 a-Helix, 42, 820, 903 a~-Helix, 911 c~L-Helix, 13 c~R-Helix, 13, 828 wL-Helix, 828 3~-Helix, 13,903 3~o-Helix, 42 3/1 Helix, 283 9/5 Helix, 280 c~-Helix form, 844 Herical form, 636
992
SUBJECT INDEX
Herzfeld-Berger analysis, 834 HETCOR, and high resolution NMR of protons in polymers, 177 Heterogeneity, 191,386 Heterogeneous blend, 47 Hexafluoropropylene, 679 HFP, 700, 703 High-performance hydrogels, 713 High-power decoupling, 682 High-power proton decoupling, 255,256 High resolution solid-state NMR, 23 High resolution solid-state NMR of protons in polymers, 175 High-tenacity fibers, 713 2H NMR, 190 2H NMR experiments, 206 2H NMR pulse sequences, 203 2H NMR theory, 190, 192 3H NMR, 212, 213,216 3H NMR of solids, 212 Homogeneous blend, 47 Homogenization, 400, 401 Homonuclear dipolar interaction, 27 Homonuclear two-spin system, 23 2H quadrupolar, 230 2H quadrupolar interaction, 51 1HDIH spin diffusion, 380 1H spin-lattice relaxation, 955,956 1H spin-lattice relaxation times in the rotating flame (TTp), 901 1H T1,367 1H T1 decay, 405 1H Tip, 392 Hydrated samples, 721 Hydrodynamic radius, 759 Hydrogen bonding, 354, 957, 962, 966 Hydrogen bonds, 713,715,716, 718, 719, 722-726, 728, 729, 731-734, 743 Hydrogen-bond angle, 847 Hydrogen-bond angle contour, 848 Hydrogen-bonded polymers, 713 Hydrogen-bonded structure, 832 Hydrogen-bonding structure, 236, 251 Hydrolysis, 973 Imaging, 265,675 Imaging polymers by NMR, 166 Indole, 224, 226
Infinite polymer chains, 18 Influenza A virus, 230 Infrared (IR) spectra, 436 Infrared spectroscopic data, 224 Inhomogeneous fields, 157 Interactions, bilinear, 134 Interactions, dipolar, 134 Interactions, linear, 134 Interactions, quadrupolar, 134 Interatomic distances, 23 Interchain interactions, 591,594 Interchain packing effect, 416 Interface, 387 Intermediate timescale, 198, 201 Intermolecular hydrogen bonding, 713, 731 Interphase, 387 Intramolecular hydrogen bonding, 713, 731 Inversion recovery, 206 Inversion recovery (IR) method, 740 Inversion recovery method, 659 Ion-amide, 354 Ion-dipole, 354 iPP, 273 Irregularity, 284 Isotactic poly(1-butene), 441 Isotactic polypropylene, 415 Isotopic labeling, 218 Isotropic random motion, 57, 61 Isotropization, 653 Jeener-Broekaert, 206, 207 Johnson-Bovey table, 621 Kave, 369 kc, 369 k space, see NMR imaging Kel-F, 254, 667, 676 Kinetic master equation, 87 Kohlrausch-Williams-Watts function, 496 Kuhn segments, 144
Laboratory coordinate system, 32 L-alanine Ca carbons, 14 Lamella structure, 422 LCST, 352, 401,402 LD(low decoupling)/MAS, 445 Legendre moment, 309 Lentinan, 900
SUBJECT INDEX Leu-enkephalin, 37, 918 (Leu),,, 827 Librational dynamics, 224 Librational motion, 63 Lightly crosslinked poly(styrene, divinylbenzene), 568 Line broadening mechanism, 292 Lineshape analysis, 422, 722, 723 Linewidth, 291 Linewidth of 13C, 360 Liquid crystal, 196 Liquid crystalline polymers, 713 Liquid crystalline state, 829 Liquid-like domain, 901 Log-g 2 distribution, 59, 65 Long n-alkyl side groups, 828 Long n-fluoroalkyl side chains, 831 Longitudinal magnetization, 27 Longitudinal relaxation, 202, 205 Long-range order, 102 Loop region, 913 Magainin, 42 Magainin-2, 230 Magic-angle sample spinning, 97 Magic-angle spinning, 23, 170, 218, 254 Magic-angle spinning (MAS), 223, 739 Magic-angle turning, 170 Magnetic dipole-dipole interaction, 52, 53, 55 Magnetic field gradients, see gradients Magnetic resonance imaging, see NMR imaging Magnetic resonance microscopy, see NMR imaging Magnetization filters, 139 Magnetization filters, intrinsic/extrinsic parameters, 142 Magnetization, time evolution, 134 MAS, 254, 255,256, 675,701 MASDL, 632 MAS rotor with an O-ring seal, 715,721, 726 Mechanical relaxation, 141 Mechanism of spin diffusion, 85 Medium effects on NMR chemical shifts, 3 MELODRAMA, 28 Membrane, 42
993
Membrane proteins, 891,909 Metallocene catalyst, 427 Metastable region, 402 Methyl end group, 285 Miscibility, 351 Mobility in solid polymers and high resolution NMR of protons, 177 Mode of the motion, 362 2y Model, 59, 60, 65 3y Model, 59, 61, 62, 65 2r Model, 59, 60, 65 3r Model, 59, 61, 62, 65 "Model-free" approach, 225 Model-free treatments, 57, 63, 64, 65, 66, 68, 69 Molecular axis system, 32 Molecular dynamics simulationmolecular dynamics simulation, 43 Molecular mechanics, 43 Molecular miscibility, 191 Molecular mobility, 256 Molecular motions, 51, 60, 66, 67, 68, 72, 74, 75, 80,662 Molecular symmetry axis frame, 501 Monomer unit, 429 Monte-Carlo chains, 7 Motional averaging, 226 Motional correlation time, 981 Motionally-driven spin diffusion, 91 MREV8, 254, 674, 682, 699 MRI, see NMR imaging Multidimensional NMR, 28 Multiple quantum coherence and chain dynamics, 171 Multiple quantum coherence and magicangle spinning in solid polymers, 181 Multiple quantum coherence, among coupled protons in polymers, 166, 171 Multiple quantum coherence; applications to polymers, 179 Multiple quantum detection, 218 Multiple-correlation-time models, 60, 254 Multiple-pulse sequences, 95, 117 Multiple-quantum, 673
14Nn13CnlH triple resonance technique, 384 N- or C-terminus, 916
994
SUBJECT INDEX
n-Paraffin, 277 N--H bond length, 839 Nation, 707 Natural abundance, 218 Natural abundance nuclei, 35 15N chemical shifts, 826 15N chemical shift tensor, 841 14N--13C dipolar, 957 15N CP/MAS NMR of: isocyanurate based resins, 522 15N CP/MAS NMR of: melamineformaldehyde resins, 516 ~SN CP/MAS NMR of: model cellulose, 529 ~SN CP/MAS NMR of: urea-formaldehyde resins, 517 Necking, 496 Neutron diffraction, 223,224 Newman projections, 634 NMR and X-ray diffraction, 467 NMR chemical shift, 1 NMR imaging, 5, 123,398 NMR imaging, 13C, 134, 135 NMR imaging, 19F, 154 NMR imaging, 27A1, 156 NMR ~magmg, 2D Fourier, 128 NMR imaging, 2H, 137, 151 NMR imaging, applications, 144 NMR imaging, back projection, 127, 128, 135 NMR imaging, chemical shift, 153 NMR imaging, coal, 156 NMR imaging, constant evolution time, 128, 136 NMR imaging, contrast, see contrast NMR imaging, diffusion, 138 NMR imaging, doublequantum, 137, 151 NMR imaging, dynamic shear, 150 NMR imaging, elastomers, 144, 145, 147 NMR imaging, flow/velocity, 138 NMR imaging, fluids, 144 NMR imaging, frequency encoding, 126, 132 NMR imaging, k space, 126, 128, 134 NMR imaging, line narrowing, 130, 134, 136 NMR imaging, liquids, 138 NMR imaging, magic angle spinning (MAS), 131, 135
NMR imaging, magic echo, 134, 136, 152 NMR imaging, material properties, 142 NMR imaging, mechanical load/deformation, 149 NMR imaging, molecular dynamics, 137, 139 NMR imaging, molecular orientations, 137, 139 NMR imaging, multipulse, 131, 134, 136 NMR imaging, multiquantum, 137 NMR imaging, parameter images, 142, 152 NMR imaging, phase encoding, 128, 136, 152 NMR ~magmg, polymer morphology, 154 NMR ~magmg, polymer reactions, 146 NMR ~magmg, polymers, 144 NMR imaging, principles, 123 NMR imaging, projections, 124, 132 NMR imaging, rheology, 138, 144 NMR imaging, signal-to-noise, 131, 136 NMR Imaging, single point, 128, 136 NMR ~magmg, slice selection, 129, 138 NMR imaging, solid echoes, 134 NMR imaging, solid rocket fuels, 156 NMR imaging, solids, 130, 151 NMR imaging, spatial resolution, 124 NMR ~magmg, special techniques, 130 NMR imaging, spin diffusion, 154 NMR Imaging, stray field (STRAFI), 132 NMR ~magmg, stress/strain, 151 NMR ~magmg, swelling polymers, 144, 149 NMR ~magmg, temperature distributions, 150 NMR imaging, time suspension, 134 NMR imaging, vulcanization, 148 NMR MOUSE, 157 NMR, multidimensional, in solid polymers, 166 NMR, proton, in polymers, 166 NMR relaxations, 51 NMR spectroscopic analysis in solid phase synthesis, 566 15N NMR chemical shift, 839 NOE, 384 NOESY, 377 NOESY: nuclear Overhauser effect spectroscopy, 354 Noncrystalline phase, 267
SUBJECT INDEX
14N quadrupole effect, 966 NR (natural rubber), 147 nT1 minimum, 295 Nuclear Overhauser effect (NOE), 354 Nuclear Overhauser enhancements (NOEs), 51,308 Nuclear quadrupolar effects, 843 Nucleation growth, 403 Nutation NMR, 273 Nutation NMR spectrum, 275 Nylon, 4, 6-8, 10-13,445,446, 451,452, 456, 458-461 170, 218 O...O distance, 715,716, 733 Off-diagonal intensity, 208 OH groups, 713,715-718, 730, 731 170 NMR, 236-238, 242, 246, 249-251,842 Opioid peptide, 43 170 solid-state NMR, 827 Order parameters, 64, 65,200, 311 Ordered phase, 637, 640, 643 Orientation distributions, 110, 675 Orientation-dependent NMR, 307 Orientational constraints, 196, 228-230, 232 Oriented polyamide fibers, 463 Oriented samples, 191, 195, 196, 222, 228 O-ring, 739 Orthorhombic space group, 437 Overhauser effect (NOE), 739 P2MS/PPO, 352 Pake-doublet, 273 Pake-pattern, 194 Paramagnetic term, 2 Partially miscible and immiscible blends, 387 1,2-PB/PI, 377 PB/PS, 398 PBLA, 829 PC/PET, 380 PC/PMMA, 370, 404 PC/poly(p-fluorostyrene-co-styrene) (PC/PFS-S), 398 PC/PS, 395 PCTFE, 667, 676 PE, 273, 275,678 PEI/poly(benzimidazole) (PEI/PBI), 376
995
PEMA, 688 Pentad, 620, 626 PEO, 273 PEO/PMAA, 373 Peptides, 33,236, 237,240-242, 244-247, 251 Perfluoromethylvinyl ether, 699 Permanent magnets, 157 Perturbation treatment, 89 Perturbed Hamiltonian, 54 PES/poly(ethersulphone) (PPS), 374 Pfl bacteriophage, 230 Phase encoding, see NMR imaging Phase separated, 980 Phase separation, 400, 402 Phase transition, 191,671,696 Phenylacetylene crosslinked adamantane, 549 Phosphine ligands, 965 Phosphorus oxidation state, 965 Photointermediates, 47 PI/PVE, 364 Piezoelectricity, 680 PIP/PVE, 361 PISEMA, 232 PMA/PVAc, 361 PMA/PVPh, 382 31p MAS NMR, 663 PMMA, 688, 689 PMMA/poly(vinylidene fluoride) PVF2, 384 PMMA/PVF2, 397, 400 Polar angle, 25 Polarization-echo, 87 Polarization-exchange process, 83 Poling, 676, 677, 680, 694 1,2-Polybutadiene, 354 Poly(m-phenylene isophthalamide) (PMIA), 308 Poly(p-phenylene terephthalamide) (PPTA), 308, 321 Poly(4-me th yl-m-phe nylene terephthalamide) (P4M-MPTA), 308 Poly(ethylene terephthalate) (PET), 72, 73, 308, 320, 491 Poly(gamma-benzyl L-glutamate) (PBLG), 322 Poly(e-alanine), 7, 820 Poly(k-glutamate), 687
996
SUBJECT INDEX
Poly(L-proline), 825 Poly(/3-benzyl L-aspartate), 822 Poly(y-n-alkyl L-glutamate), 293 Poly([N-ethylcarbazol-3-yl]methyl methacrylate) and poly(2-[(3,5dinitrobenzoyl)oxy] ethyl acetate) (PECMMA/PNBOEAc), 355 Poly(N,N-dimethylacrylamide-co-acrylic acid) (DMAA-co-AA) gel, 758 Poly(n-octadecyl L-glutamate), 829 Poly(N,N-dimethylacrylamide-co-acrylic acid)gel-poly(ethylene glycol) system, 758 Poly(e-caprolactone), 289 Poly(y-octadecyl L-aspartate), 830 Poly(y-n-hexyl L-glutamate), 294 Poly(y-methyl L-glutamate) (PMLG), 765 Poly(y-n-alkyl L-glutamate), 286 Poly(y-n-octadecyl L-glutamate), 287 Poly(y-oleyl L-glutamate), 287 Poly(1-butene), 283 Poly(2-methylstyrene)/poly(2,6dimethylphenylene oxide) (P2MS/PPO), 352 Poly(3-hydroxybutyrate) (PHB), 393 Poly(3-hydroxybutyric acid), 773 Poly(3-hydroxybutyric acid-co-3hydroxypropionic acid), 775 Poly(3-hydroxybutyric acid-co-3hydroxyvaleric acid), 774 Poly(3-hydroxybutyric acid-co-4hydroxybutyric acid), 775 Poly(4-vinylphenol) (PVPh), 355 Poly(benzo [a, d]dithiazol-2,6-diyl-1,4phenylene) (PBZT)/nylon, 377 Poly(butyl methacrylate) (PS(OH)/PBMA), 389 Poly(diethylsiloxane) (PDESO), 652-653 Poly(dimethylsiloxane), 662 Poly(dimethylsiloxane) (PDMS)/silicone, 360 Poly(ether-ester)/PVC, 393 Poly(ether-imide)/poly(aryl ether ketone) (PEI/PAEK), 375 Poly(ethylene glycol) (PEG), 758 Poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA), 359 Poly(ethylene terephthalate), 72, 73,491 Poly(ethylene terephthalate)/poly(p-
hydroxybenzoic acid-co-phydroxynaphthoic acid) (PET/Vectra-A), 373,374 Poly(ethylene terephthalate)/poly(phydroxybenzoic acidpoly(furyleneethylene) film, 552 Poly(hydroxyalkanoic acid), 773,775,776, 778, 801,802, 804 Poly(methacrylic acid) (PMAA) gel, 750 Poly(methyl acrylate) and poly(vinyl acetate) (PMA/PVAc), 354 Poly(methyl methacrylate), 65 Poly(methylphenylsilane), 613-621
Poly(styrene-co-acrylonitrile)/poly(ethyl methacrylate) (SAN/PEMA), 366 Poly(styrene-b-methylphenylsiloxane), 664 Poly(trifluoroethylene), 258 Poly(vinyl alcohol), 713,772, 807, 813 Poly(vinyl alcohol) (PVA), 713 Poly(vinyl alcohol) (PVA) gel, 741 Poly(vinyl methyl ether), 295 Poly[1, 4-phenylene 2, 5-bis (hexyloxy) terephthalate], 311 Polyacenic polymers, 602 Polyacetylene, 273,589 Polyamides, 445 Polycarbonate (BPAPC)/PET, 400 Polycarbonate(PC)/PMMA, 361 Polycarbonate, 47, 78 Polycarbonate, shear bands, 152 Polycarbonate, swelling with acetone, 146 Polydiethylsilane (PDES), 636 Polydimethylsilane (PDMS), 632-636 Polydimethylsiloxane (PDMS), 923 Polyethylene, 268, 277, 415 Polyethylene (LDPE), 155 Polyethylene (PE), 311 Polyethylene, crystallinity, 170 Polyethylmethylsilane (PEMS), 649-652 Polyimide, 688 Polyisoprene (I,2-PB/PI), 354 Polymer blends, 351 Polymer blends, miscibilities, 166 Polymer gel, 435,737 Polymer interactions, 354 Polymerization reaction, imaging, 146 Polymers, 47 Polymers, imaging by NMR, 166
SUBJECT INDEX Polymorphs, 43,456, 683 Polyolefins, 415 Polyoxymethylene, 280, 299 Polypeptides and proteins, 7 Polypeptides, 819, 904 Polyphosphazene, 663,664, 687 Polypropylene, 415 Polypyrrole, 595 Polysaccharides, 891 Polysiloxane-immobilized ligand, 924 Polystyrene resin: methylene bridging, 557 Polystyrene, isotactic, solid, 1H NMR, 176 Polystyrene/poly(vinyl methyl ether) (PS/PVME), 354 Polytetrafluoroethylene (PTFE), 154, 311 Polyvinylchloride (PVC), 157 POM, 273 Populations, 976 POT/PPO, 366 Powder lineshape, 195 Powder patterns, 68 Powder pattern spectra, 662 Powder pattern spectra of polyoxymethylene, 281 Powder pattern spectrum of polyethylene, 296 Powder spectrum, 194 PPO/PS, 364, 371 Preferred conformation, 429 Principal axis systems, 17, 501 Principal components, Principal values, 591 Probability of the formation of the intramolecular hydrogen bonds, 727, 730 Propane, 225 (Pro-Pro-Gly)lo, 908 Proteins, 33,904 Proton-driven spin diffusion, 91, 92 Proton to deuterium CP, 384 PS/PB, 388 PS/PVME, 359, 361,364, 377,404 Pseudocentered structure, 436 PST(pulse saturation transfer)/MAS, 445 PTFE, 263, 667, 670-672, 674, 675,678, 687 PTFE-perfluoromethoxyethylene copolymer, 255 PTrFE, 667, 678, 686
997
Pulse NMR, 286 Pulse sequences, 206 Pulsed-field gradient NMR (PFG-NMR), 366 Purple membrane, 910 PVA/PHB, 393 PVA/PMAA, 357 PVDF, 257, 263, 667, 668, 676-683, 685, 686, 688, 689, 705 PVF, 678 PVPh/PEO, 361,374, 394 PVPh/PMA, 361 Pyrrolidine rings, 825 Quadrupolar broadening, 960 Quadrupolar interaction, 190, 192, 204, 209 Quadrupolar polarization, 203 Quadrupolar relaxation, 207 Quadrupole echo, 198, 200, 201,205,206 Quadrupole splitting, 193, 195 Quality of alignment, 191,197 Quantitation, 975 Quantum mechanics, time-dependent, and NMR, 167 Quasi-equilibrium spectra, 108 Quasi-equilibrium state, 108 Quenched-precipitated sample, 437 R.f.-driven spin diffusion, 92, 93, 117 R 2 (rotational resonance), 308 Random coil conformation, 914 Random walk, 102 Reaction, 400 Real time measurement, 289 Reconstruction from projections, see NMR imaging (back projection) Recoupling schemes, 98 Recrystallization, 400 REDOR (Rotational Echo Double Resonance), 27, 232, 308, 687 Regio-specific assignment, 913 Relaxation, 976 Relaxation time measurements, 202, 205 c~ Relaxation, 302 13 Relaxation, 288 Reorientation, 493 Representative structures, 977 Residual ethoxy, 968
998
SUBJECT INDEX
Residual methoxy, 970 Resolution of proton NMR in solid polymers, 166 REV8, 671 RFDR, 28 Rigid moiety, 658 Ring-current effects, 621,627 Rotating-frame relaxation time, 493 Rotational-echo double-resonance (REDOR), 398 Rotational isomeric state model, 429 Rotational resonance, 98 Rotational resonance technique, 373 Rotor-driven spin diffusion, 98, 118 RR, 27 Salicylaldehyde-containing polymer resin, 549 SBR (styrene butadiene rubber), 147, 150 Scaling factor, 79 Schizophyllan, 900 Second moments, 687 Secondary crystallization, 291 SEDRA, 28 Selecting 1H magnetization by T2, 374 Selective excitation SASS, 74, 78, 80, 129 Selective inversion pulse, 40 Self-diffusion, 191 Semicrystalline, 676 Semicrystalline polymer, 434 Separated local field, 223, 271 /3a-Sheet, 13 /3-Sheet, 230, 820, 828, 903 Shielding anisotropy, 170 Shielding tensor, 672 Side branch, 284 Sidebands, 26 Side-chain flips, 191 Silanols, 962, 973 Silk, 230 Silk fibroin, 905 Silk fibroin fiber, 320 Silk I, 13, 824, 906 Silk II, 906 Silylation, 949 Single-correlation-time model, 63 Single helix, 894 Single quantum relaxation time, 41
SLF spectra, 272 Slow MAS, 299 Slow-MAS spin-diffusion, 92 Slow motional limit, 198 Smectic crystalline form, 415 Soft moiety, 658 Solid-like domain, 902 Solid phase synthesis, 566 Solid phase synthesis: solution phase NMR studies, 568 Solid-state 15N NMR, 457 Solid-state 170 NMR, 843 Spatial resolution, see NMR imaging Spatial spin diffusion, 83 Spatially resolved NMR, see NMR imaging Spectral densities, 56 Spectral density function, 227 Spectral spin diffusion, 83 Spin density, 124, 140 Spin diffusion (SD), 28, 83, 154, 232, 367, 368, 404, 685 Spin diffusion and domains in polymers, 182 Spin-diffusion (cross-relaxation) rate, 369 Spin-diffusion constant, 370 Spin-diffusion spectra, 391 Spin-diffusion time constant, 390 Spin I = 1, 192 Spin-l, 190, 203,204, 205,206 Spin-exchange, 24 Spin-lattice relaxation, 202, 367, 944, 979 Spin-lattice relaxation time T1, 51,337, 717, 720 Spinning axis, 79 Spinning sideband, 965 Spinodal, 403 Spinodal decomposition, 403 Spin-spin relaxation, 202, 207 Spin-spin relaxation time T2, 51,722 Statistical calculation, 728, 729, 731,732 Statistical weight factors, 5 Steady-state equilibrium NOE, 384 Stereoregurality, 436 Steric hindrance model, 417 Stimulated echo, 206, 207 Stimulated echo sequence, 207 STRAFI, 265 Structural heterogeneity, 963 Structural proteins, 905
SUBJECT INDEX Sum-over-states (SOS) method, 2 Super drawing, 723 Surface coil, 157 Susceptibility, magnetic, 134 Switching-angle sample spinning (SASS), 74 Swollen gel-phase 13C NMR studies, 571 Swollen gel-phase 19F NMR, 572 Swollen gel-phase MAS I9F NMR, 579 Swollen gel-phase MAS lh NMR studies, 575 Swollen-state NMR: crosslinked elastomers, 583 Synthesis of hypercrosslinked resins, 565 T1,202, 205,370 T1 CP, 631 Tlo, 227, 367, 370, 596 Tip decay, 405 Tlo-selective Goldman-Shen experiment, 374 T2-, Tip-selective Goldman-Shen experiments, 374 T1 MAS, 631 T1 minimum, 344 T1 relaxation, 219, 227 T2, 202, 205,208 Tz-selective Goldman-Shen experiments, 393 TNH, 596 Tacticity, 745 TB MO, 278 TB MO calculation, 19 TB MO theory, 18 TEDOR, 27 Teflon, 667 Temperature dependencies of T1 and NOE, 66 Tensor-correlation experiment, 105 Tetrafluoroethylene/perfluoromethyl vinyl ether, 254 TG conformation, 283 Thermotropic liquid crystal, 286 Three-dimensional structure, 33,891 Three-fold jump rotation, 421 Three-spin system, 32 Tight binding, 282 Tight-binding INDO/S sum-over-states theory, 825
999
Tight-binding MO, 278, 591 Tight-binding molecular orbital (TB MO) theory, 2 Tilt series, 197 Tilted rotating frame, 41 Time fluctuations, 52, 55, 60, 63 Torchia method, 688 Torchia technique, 259 TORQUE, 259, 262, 704 Torsion angles, 28, 314 Trans-conformations, 280, 616 Trans-gauche isomerization, 493 Transient Overhauser effect, 253,696 Transient Overhauser, 679 c~-Transition, 341 Transition probabilities, 53, 54, 55 Transmembrane helices, 917 Transmembrane loops, 917 Trans-planar zigzag conformation, 437 Transverse magnetization, 27 Transverse relaxation, 202, 205 Transverse relaxation time, T2, 36, 127, 143 Trans(zigzag) form, 616, 636 Trans-zigzag conformation, 269 TrFE, 694, 696, 697 Triad, 624 Triad configurations, 751 Triad tacticity, 714, 715,717 Triple helix, 894 Triple-channel, 668, 686 Triple-resonance, 394, 668 Tritiated compounds, 212, 213,216 Tritium, 212 TT conformer, 616 /3-Turn I structure, 43 /3-Turn II structure, 42 Two-dimensional spectra, 668 Two-site exchange model, 77 Two-site jump, 199, 200, 201 Two-spin system, 35, 53, 54 UCST, 352 UHMWPE, 302 Ultraoriented linear polyethylene, 270 Uniaxially oriented protein fiber, 319 Unit cell, 420 van der Waals, 230
1000
SUBJECT INDEX
Varian Nano-NMR probe, 576 VDF, 696, 697, 700, 703 Vibrational amplitude, 223,224 Vibrational spectroscopy, 219 Vinyl end group, 285 Viovy-Monnerie-Brochon model, 296 Virtual echo, 207 Viton, 260, 262, 264, 667, 692, 700, 702, 703,704 Volume-selective NMR, 124, 129 Voxel, 124 VT 13C CP/MAS NMR, 829 VT CPMAS, 291 WAHUHA, 254, 699
Weighted average of the intrinsic rates, 369 Wigner rotation matrices, 76 Williams-Landel-Ferry (WLF), 359 WISE, 263,264, 685,703, 708 WLF equation, 364
129Xe, 690 129Xe NMR, 357 129Xe NMR, 407 (1 ~ 3)-/3-D-Xylan, 895 Zeeman interaction, 192, 204, 25 Zero quantum transverse magnetization rate, 40 Zero-angle spinning, 99 Ziegler-Natta catalyst, 415
