Mesomorphic Order i n Polymers and Polymerization i n Liquid Crystalline Media Alexandre Blumstein, E D I T O R
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Mesomorphic Order i n Polymers and Polymerization i n Liquid Crystalline Media Alexandre Blumstein, E D I T O R
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.fw001
University
of
Lowell
A symposium sponsored by the Division of Polymer Chemistry, Inc. at the 174th Meeting of the American Chemical Society Chicago, I l l . , August 2 9 - 3 0 , 1977
74
ACS SYMPOSIUM SERIES
AMERICAN
CHEMICAL
WASHINGTON, D. C. 1978
SOCIETY
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.fw001
Library of CongressCIPData Mesomorphic order in polymers and polymerization in liquid crystalline media. (ACS symposium series; 74) Includes bibliographical references and index. 1. Polymers and polymerization—Congresses. 2. Liquid crystals—Congresses. I. Blumstein, Alexandre. I. American Chemical Society. Division of Polymer Chemistry. III. Series: American Chemical Society. ACS symposium series; 74. QD380.M47 ISBN 0-8412-0419-5
547'.84 ACSMC8
78-9470 74 1-264 1978
Copyright © 1978 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of thefirstpage of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective works, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED I N T H E UNITED STATES O F AMERICA
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.fw001
A C S Symposium Series Robert F. G o u l d , Editor
Advisory Board K e n n e t h B . Bischoff
N i n a I. M c C l e l l a n d
D o n a l d G. Crosby
J o h n B . Pfeiffer
Jeremiah P . Freeman
Joseph V. Rodricks
E. Desmond Goddard
F. Sherwood R o w l a n d
Jack H a l p e r n
Alan
Robert A. Hofstader
Raymond B . Seymour
James P . Lodge
Roy L . W h i s t l e r
J o h n L . Margrave
Aaron W o l d
C . Sartorelli
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.fw001
FOREWORD The A C S S Y M P O S I U M S E R I E S was founded i n 1 1 9 7 4 t o provide a medium for publishing symposia quickly i n book form. The format of the S E R I E S parallels that of the continuing A D V A N C E S I N C H E M I S T R Y S E R I E S except that i n order to save time the papers are not typeset but are reproduced as they are submitted by the authors i n camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book. Papers published in the A C S S Y M P O S I U M S E R I E S are original contributions not published elsewhere i n whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.
PREFACE A4Tesomorphic order in polymers is important to the field of high strength materials such as the ultra-high modulus fibers recently developed by industry from stiff chain polymers and carbon pitches. However, other fundamental aspects of polymer chemistry and physics are involved also. Mesomorphic order in polymers is tied to the problem of local segmental
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.pr001
order within macromolecular coils. It has a direct bearing on the understanding of the complex morphology of crystalline polymers in the regions of intermediate order.
T h e relationship between crystallinity, mo-
lecular order, and stereoregularity
and problems relevant to biophysics
of macromolecules, such as intramolecular crystallization, formation of tertiary structures, and formation of liquid crystalline morphoses i n living tissues and cell organelles, are some of the topics which are discussed i n this volume. Whereas the subject of stiff chain polymers has received much attention in two recent A C S Meetings ( Witco Symposium honoring D r . Paul Morgan held in New York in A p r i l 1 9 6 6 and symposium on Rigid Chain Polymers held i n N e w Orleans in March 1 9 7 7 ) , the wider aspects of mesomorphic order i n polymers have been discussed only occasionally at meetings.
T h e chapters assembled in this volume constitute the pro-
ceedings of the first symposium devoted to broader aspects of this rapidly developing field and, as such, provide a valuable source of information to the student and scientist. University of Lowell
ALEXANDRE
Lowell, M A 0 1 8 5 4
April 1 1 , 1 9 7 8
vii
BLUMSTEIN
1 X-Ray Diffraction from Polymers with Mesomorphic Order S. B. CLOUGH and A. BLUMSTEIN Polymer Program, Department of Chemistry, University of Lowell, Lowell, MA 01854
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
A. deVRIES Liquid Crystal Institute, Kent State University, Kent, OH 44242
Polymers with side chain s t r u c t u r e s i m i l a r to that of low molecular weight liquid crystalline com pounds can achieve various l e v e l s of o r g a n i z a t i o n i n the b u l k . These polymers are sometimes formed by p o l y merization of v i n y l monomers that themselves e x h i b i t mesomorphic behavior. I n other cases, they can be obtained from monomers that do not form liquid crys talline s t a t e s . At one extreme the s t r u c t u r e of the polymer i s h i g h l y organized, approaching that of crys talline polymers and g i v i n g r i s e to a number of x-ray diffraction peaks. At the other extreme the polymer chains are
diso r g a n i z e d ,
with
x-ray
diffraction
pat
terns that resemble those from amorphous polymers. The relation between the molecular s t r u c t u r e and the mesomorphic order has been p r e v i o u s l y discussed (1). In t h i s paper, the x-ray diffraction results for a number of polymers with stiff and/or bulky side groups are discussed. X-ray diffraction photographs from unoriented liquid crystalline states of low molecular weight compounds are c h a r a c t e r i z e d by an i n n e r r i n g (at 2θ = 2-5 degrees) r e l a t e d to the length of the mole c u l e , and an outer r i n g ( 2 0 - 2 0 deg). The former i s sharp f o r smectic s t a t e s where the molecules are mutually parallel and arranged in planes. The spacing c a l c u l a t e d (Bragg equation) i s the distance between these planes. The i n n e r r i n g is diffuse in the case of nematic s t a t e s , where the molecules in a volume element lie approximately parallel to each other but are not organized into a l a m e l l a r s t r u c t u r e . The sharpness of the outer r i n g ( s ) depends principally on the extent of lateral packing between molecules. In many cases a diffuse halo near 2θ = 20 deg shows the absence of long range lateral order between the molecules. 0-8412-0419-5/78/47-074-001$05.00/0 © 1978 American Chemical Society
M E S O M O R P H I C ORDER I N
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
2
POLYMERS
A n u m b e r o f d i f f e r e n t s m e c t i c s t a t e s e x i s t among t h e low m o l e c u l a r w e i g h t m e s o m o r p h i c compounds. The m o l e c u l e s , f o r e x a m p l e , m i g h t be n o r m a l t o t h e p l a n e s or t i l t e d r e l a t i v e t o the normal. The different s t a t e s have been g i v e n v a r i o u s l e t t e r d e s i g n a t i o n s ( S m e c t i c A, B, C e t c . ) a n d d e V r i e s ( 2 ) h a s classified the x-ray p a t t e r n s obtained from the v a r i o u s types of s m e c t i c (and a l s o n e m a t i c ) s t a t e s . The p a t t e r n s o b t a i n e d f r o m t h e p o l y m e r s d i s c u s s e d h e r e show f e a t u r e s s i m i l a r t o t h o s e o f l o w m o l e c u l a r w e i g h t compounds. The i n n e r r i n g i s now r e l a t e d t o the o r g a n i z a t i o n and l e n g t h o f the p o l y m e r s i d e g r o u p s , w h i l e t h e o u t e r r i n g s d e p e n d on t h e l a t e r a l p a c k i n g between the s i d e groups. X-ray patterns and p o l y m e r o r g a n i z a t i o n are d i s c u s s e d h e r e i n t e r m s of the c l a s s i f i c a t i o n of deVries. T h i s does n o t nec e s s a r i l y i m p l y m i s c i b i l i t y w i t h a low molecular w e i g h t l i q u i d c r y s t a l o f t h e same l e t t e r d e s i g n a t i o n . Indeed, the polymers d i s cussed here are not f l u i d m a t e r i a l s b u t a r e h a r d a n d b r i t t l e , and a r e n o t l i q u i d c r y s t a l s i n the u s u a l sense of t h i s term. I n most c a s e s h e a t i n g does n o t change t h e l e v e l o f o r g a n i z a t i o n up t o t e m p e r a t u r e s w h e r e t h e p o l y m e r s t a r t s t o decompose. O t h e r c o m p a r i s o n s a n d d i s t i n c t i o n s w i l l be drawn b e t w e e n t h e s e p o l y m e r s and low m o l e c u l a r w e i g h t mesomorphic s t a t e s below. Some comments w i l l a l s o be made on t h e o r g a n i z a t i o n o f t h e p o l y m e r s as c o m p a r e d t o s e m i - c r y s t a l l i n e polymers with s m a l l extent of crystallinity. Experimental The m e t h o d s o f p r e p a r a t i o n o f t h e monomers a r e i n the references i n Tables I-III. X - r a y d i f f r a c t i o n d a t a w e r e o b t a i n e d w i t h a Warh u s v a c u u m c a m e r a , and R i g a k u w i d e a n g l e ( m o d e l SG-7B) and s m a l l a n g l e d i f f r a c t o m e t e r s , a l l w i t h Cu radiation. A L e i t z Ortholux p o l a r i z i n g microscope with a M e t t i e r FP5 2 h o t s t a g e was u s e d f o r e x a m i n a t i o n of biréfringent f i l m s o f t h e p o l y m e r s and f o r d e t e r m i n i n g monomer t r a n s i t i o n t e m p e r a t u r e s . A P e r k i n Elmer DSC1B d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r was a l s o u s e d t o o b t a i n t h e t r a n s i t i o n t e m p e r a t u r e o f monomers. given
Results
and
Discussion
The p o l y m e r s a r e p r e s e n t e d i n o r d e r o f d e c r e a s i n g l e v e l s of o r g a n i z a t i o n , s t a r t i n g with h i g h l y ordered s m e c t i c and p r o c e e d i n g t o p o o r l y o r g a n i z e d n e m a t i c states.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
1.
CLOUGH E T A L .
X-Ray Diffraction from Polymers
3
Smectic. T a b l e I l i s t s t h e monomers o f some p o l y m e r s t h a t show a s h a r p i n n e r r i n g a n d t h u s h a v e smectic organization. P o l y ( a e r y l o y l o x y b e n z o i c a c i d ) , (ΡABA). When t h e c o r r e s p o n d i n g monomer i s p o l y m e r i z e d i n t h e m e l t t o h i g h degrees o f c o n v e r s i o n , * x-ray d i f f r a c t i o n from t h e p o l y m e r shows a s h a r p r i n g c o r r e s p o n d i n g t o 1 7 . 7 ^ , and t h r e e s h a r p o u t e r r i n g s i n a d d i t i o n t o t h e o u t e r h a l o (JO . The s p a c i n g o f t h e i n n e r r i n g i s l e s s t h a n twice the length o f t h e side groups, suggesting t h a t the l a t t e r a r e s t r o n g l y t i l t e d w i t h r e s p e c t t o t h e smectic planes. We w e r e u n a b l e t o o r i e n t t h e s a m p l e to confirm t h i s t i l t i n g . In the smectic state o f a low m o l e c u l a r w e i g h t a n a l o g , n o n y l o x y b e n z o i c acid, t h e m o l e c u l e s a r e t i l t e d a t a p p r o x i m a t e l y 45 d e g r e e s t o t h e p l a n e n o r m a l (J4). The x - r a y p a t t e r n m o s t c l o s e l y r e s e m b l e s t h a t o f t h e S m e c t i c E-f- ( 2 ) . The t h r e e o u t e r r i n g s show t h a t t h e r e e x i s t s some r e g u l a r p a c k i n g b e t w e e n t h e s i d e g r o u p s . I t m i g h t b e n o t e d t h a t f o r p o l y m e r s we w o u l d e x p e c t t o observe t h e h a l o , even i f t h e r e a r e sharp o u t e r r i n g s present. T h i s w o u l d b e due t o p o o r l y o r g a n i z e d r e gions . The o r d e r e d d o m a i n s a r e n o t e x p e c t e d t o e x t e n d u n i f o r m l y throughout t h e specimen. The p r e s e n c e o f the h a l o i s s i m i l a r t o t h e case o f s e m i - c r y s t a l l i n e polymers. P o l y ( m e t h a c r y l o y l o x y b e n z o i c a c i d ) , (PMBA). Films of t h i s polymer cast from dimethyIformamide give x-ray p a t t e r n s w i t h a sharp i n n e r and a sharp outer r i n g , (_5 ) i n a d d i t i o n t o t h e h a l o . This p a t t e r n resembles the p a t t e r n o f Smectic B (JO. I t i s i n t e r e s t i n g t o remark t h a t i n both abovem e n t i o n e d c a s e s , t h e monomers a r e n o t m e s o m o r p h i c . Poly(Ν-ρ-methacryloyloxybenzylidene-p-aminobenz o i c a c i d ) , (PMBABA). P o l y m e r i z a t i o n a t 213°C f r o m the nematic s t a t e g i v e s a polymer w i t h a s m e c t i c structure. The s p a c i n g c a l c u l a t e d f r o m t h e s h a r p inner r i n g i s approximately twice the length o f the s i d e c h a i n s (6_) . This suggests Smectic A o r g a n i z a t i o n w i t h t h e s i d e groups normal t o t h e planes and t h e p o l y mer m a i n c h a i n c o n t a i n e d w i t h i n t h e p l a n e s . Polyme r i z a t i o n o f t h e monomer f r o m t h e n e m a t i c s t a t e i n a l O k O e m a g n e t i c f i e l d g a v e an o r i e n t e d s p e c i m e n . X-ray t
^ P o l y m e r i z a t i o n t o low degrees o f c o n v e r s i o n and c a s t i n g o f f i l m s from a dimethyIformamide s o l u t i o n g i v e s a s e m i - c r y s t a l l i n e p o l y m e r w i t h up t o e i g h t x-ray d i f f r a c t i o n l i n e s .
3
/—ν
3
<
3
2
(CH =
/—ν
C
C H C 0 0 Y O ) "
PdiABAB
2
PChAB r~\ CH =C H C 0 0 ^ O / -
2
C H : =
0
C
0
H
[
=
C
N
H
)
O
2
y—ν
K
K
with
Smectic
1
l
1
8
8
2
2
K
2
1
2
0
1
s
2
0
6
N
1
2
8
0
1
Poly η
Κ 114.8 I (111.8C)
K
Κ 201 I
22.2
45.2
35.3
32 . 0
19-0
17.7
5. 0 8
Data n. 9
5.0
6.08
6.35
Polymer X-ray ή. Q
Organization
Monomer T r a n s i t i o n s , n<=>crm&& r
Polymers
" \ O / -
L
]
C O O H
n- /Q\^-cooh
PChMA CH =C(CH )C00[Chol]
2
PMBABA /—ν CH = C(CH ) C 0 0 ^ O / -
2
CH = C ( C H ) C O O - / O X
PMBA
2
CH = C H C 0 0 Y O V
C 0 0 H
//—ν —y
1.
PABA
A b b r e v i a t i o n o f Polymer S t r u c t u r e o f Monôme^
I.
No.
Table
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
1.
CLOUGH E T A L .
5
X-Ray Diffraction from Polymers
d i f f r a c t i o n shows t h e o u t e r h a l o t o b e p o s i t i o n e d a t 9QO t o t h e a r c s d i f f r a c t e d f r o m t h e p l a n e s . Figure 1 shows t h e x - r a y p a t t e r n f r o m an o r i e n t e d s p e c i m e n . This confirms the s i m i l a r i t y o f t h i s o r g a n i z a t i o n with t h e S m e c t i c A. No r e g u l a r p a c k i n g e x i s t s b e t w e e n t h e side chains. The a v e r a g e d i s t a n c e b e t w e e n t h e s i d e c h a i n s i s D=5.0 8^ c a l c u l a t e d f r o m 1. 117^= 2 D s i n 6 (_7). A s c h e m a t i c d r a w i n g o f t h e s t r u c t u r e i s shown i n F i g u r e 2. An u n u s u a l f e a t u r e o f F i g u r e 1 i s t h e m u l t i p l i c i t y o f higher orders o f d i f f r a c t i o n from the smectic planes. The f o l l o w i n g o r d e r s h a v e b e e n o b s e r v e d : 1, 2, 3, 5, 6, 7, 9, 1 0 , 11 a n d 1 3 . A f e w h i g h e r o r d e r peaks have been r e p o r t e d f o r polymers and low m o l e c u l a r w e i g h t compounds w i t h s m e c t i c o r g a n i z a t i o n . The p o l y m e r PMBABA m u s t h a v e a h i g h d e g r e e o f p o s i t i o n a l r e g u l a r i t y a l o n g t h e s i d e group, y e t no o r dered l a t e r a l packing o f t h e side groups. P o l y ( c h o i e s t e r y l m e t h a c r y l a t e ) , (PChMA) a n d Poly(choiesteryl p-acryloyloxybenzoate), (PChAB). The s t r u c t u r e o f t h e s e p o l y m e r s i s c l a s s i f i e d a s S m e c t i c A (J3_) . The i n t e r p l a n a r s p a c i n g i s a p p r o x i mately twice t h e length o f t h e s i d e group. The s t r u c t u r e i s d i s c u s s e d i n some d e t a i l i n a n o t h e r p a p e r i n t h i s volume. F i g u r e 3 shows t h e e f f e c t o f d i f f e r e n t m e t h o d s o f s p e c i m e n p r e p a r a t i o n on t h e s h a r p n e s s o f x - r a y l i n e s from smectic planes. The i n t e n s e , s h a r p p e a k i n F i g u r e 3 a i s f r o m t h e p o l y m e r PChMA p r e p a r e d b y b u l k p o l y m e r i z a t i o n o f t h e i s o t r o p i c monomer. D i s s o l v i n g the sample i n benzene and subsequent f i l m c a s t i n g gives a poorly organized s t r u c t u r e , (Figure 3b). S u b s e q u e n t a n n e a l i n g o f t h i s s p e c i m e n a t 180°C (Tg^-160°C) i n a v a c u u m o v e n f o r t w o h o u r s a l l o w e d r e o r g a n i z a t i o n o f the chains r e s u l t i n g i n the d i f f r a c t i o n shown i n F i g u r e 3c. I n o t h e r c a s e s i n - w h i c h smectic organization i s achieved during polymerization o f t h e monomer d i r e c t l y , r a p i d p r e c i p i t a t i o n o f t h e p o l y m e r f r o m s o l u t i o n g i v e s a p o w d e r w h i c h shows o n l y a very d i f f u s e h a l o a t t h e s c a t t e r i n g angle o f t h e inner ring. I n d i s c u s s i n g t h e mesomorphic s t r u c t u r e o f p o l y m e r s , i t i s i m p o r t a n t t h a t t h e method o f sample p r e p a r a t i o n be known. Poly(di(N-p-aeryloyloxybenzylidene)-p-diaminobenzene), (PdiABAB). The p o l y m e r i z a t i o n o f t h i s d i f u n c t i o n a l monomer a n d t h e c o p o l y m e r i z a t i o n w i t h a m o n o f u n c t i o n a l monomer h a v e b e e n p r e v i o u s l y d e s c r i b e d , P o l y m e r i z a t i o n o f t h i s monomer i n i t s n e m a t i c s t a t e i n a m a g n e t i c f i e l d o f 4, 000 Oe l e a d s t o a n o r i e n t e d polymer. The d i f f r a c t i o n p a t t e r n o f t h i s p o l y m
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
6
MESOMORPHIC
Figure 1.
ORDER
IN
POLYMERS
X-ray diffraction from PMBABA showing orientation of the smectic planes. The 32À line is partially hidden by the beam stop.
Figure 2. Schematic diagram of the smectic A structure in PMBABA
1.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
mer
CLOUGH E T A L .
X-Ray Diffraction from Polymers
i s analogous t o a p a t t e r n o f a Smectic
7
A mesophase.
Intermediate Organization. The t w o monomers l i s t e d m Table I I give r a t h e r i n t e n s e i n n e r r i n g s that are broader than those from the l a m e l l a r s t a t e s d e s c r i b e d above. F i g u r e 4 shows t h e d i f f r a c t i o n f r o m poly(N-p-butoxybenzylidene-p-aminostyrene),(PBBAS). The B r a g g s p a c i n g c a l c u l a t e d f r o m t h e i n n e r r i n g i s less than twice the length o f the side chain (with extended n - a l k y l groups). F u r t h e r , c a r e f u l measure ment o f t h e a n g l e s o f t h e f i r s t a n d s e c o n d o r d e r p e a k s , shows ( s i n θ ) s e c o n d o r d e r s 2 ( s i n &) f i r s t o r d e r . These f e a t u r e s i n d i c a t e a s t r u c t u r e w h i c h d i f f e r s from a w e l l organized smectic. Further, the high intensity o f t h e i n n e r r i n g i m p l i e s a s t a t e more o r g a n i z e d t h a n nematic. We h a v e b e e n u n a b l e t o f i t t h e s e d a t a i n t o t h e c l a s s i f i c a t i o n ( f o r low m o l e c u l a r w e i g h t compounds) of deVries. The b r o a d e n i n g o f t h e l i n e s may come f r o m d i s t o r t i o n s o f a s m e c t i c o r g a n i z a t i o n w i t h interpénét r a t i o n o f t h e n-alkoxy chains from d i f f e r e n t molecules . The o u t e r d i f f u s e h a l o shows t h a t n o l a t e r a l r e g u l a r p a c k i n g e x i s t s between t h e s i d e c h a i n s . Nematic. T a b l e I I I g i v e s t h r e e monomers t h a t p o l y m e r i z e t o s t a t e s t h a t we h a v e c l a s s i f i e d a s n e matic. S i n c e some o f t h e p r o p e r t i e s o f t h e s e p o l y m e r s d i f f e r w i d e l y from those o f low m o l e c u l a r weight n e m a t i c s t a t e s , t h e e x p e r i m e n t a l r e s u l t s s h o u l d be c l o s e l y examined. Poly(N-(p-cyanobenzylidene)-p-aminostyrene), (PCBAS). P o l y m e r i z a t i o n o f t h e monomer f r o m t h e n e m a t i c and i s o t r o p i c s t a t e s p r o c e e d s over a few h o u r period (10). S a m p l e s p o l y m e r i z e d a t 127°C f o r 24 h o u r s show a v e r y weak d i f f u s e i n n e r r i n g a t 2 9 ^ 4 . 2 degrees. T h i s i s much l e s s i n t e n s e t h a n e x p e c t e d from comparisons w i t h low m o l e c u l a r weight nematics. W i t h i n t h e outer h a l o there i s a sharp l i n e correspondi n g t o 4.43^. Films o f the polymer are transparent, b u t a r e w e a k l y biréfringent. L i g h t o f low i n t e n s i t y i s t r a n s m i t t e d between c r o s s e d p o l a r i z e r s i n t h e optical microscope. B a s e d on c o m p a r i s o n s w i t h l o w m o l e c u l a r w e i g h t c o m p o u n d s , t h e o p t i c a l c l a r i t y a n d t h e weak i n n e r r i n g suggest t h a t t h e o r g a n i z a t i o n i s i s o t r o p i c . H o w e v e r , n e m a t i c o r d e r was a s s i g n e d t o t h e p o l y m e r b a s e d o n o b s e r v a t i o n o f o p t i c a l a n i s o t r o p y ( 1 0 ) . The s h a r p l i n e o f medium i n t e n s i t y w i t h i n t h e o u t e r h a l o s u g g e s t s some p a r a l l e l p a c k i n g o f t h e s i d e c h a i n s . The s p a c i n g i s t h e same a s t h a t f o u n d f o r o t h e r s i d e
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
8
Figure 3. X-ray diffraction from PChMA prepared under different conditions, (a) Polymerized from the isotropic melt; (b) films cast from benzene solution; (c) speci men from (b) annealed at 180°C for 2 hr.
Figure 4. X-ray diffrac tion from PBBAS
MESOMORPHIC
ORDER
2 Θ, dec
IN POLYMERS
v
, — , >
6
=c h
c: H = C H
/
2
•0CCH=CH 2
PdiABH •—ν C H = C H C O O ^ Q ) C H = N-N=CH
2
,—ν . -
C H =C r f O > N = C H / 0 ) c PPMAS MAS / — ν — CH 2 \ 0 ^ = C H / 0 \ : H =N
H
H
9
Κ
K
94
S
88. 3
Ν
120.6
97.5
Ν
2
I
I
Κ 113 . 8 Ν 1 4 0 . 5 I
Monomer T r a n s i t i o n s , degrees C
Κ 140 N P o l y ' n
Κ 1 8 0 N Poly'n
29=3. 3
29=3.l
U
5.00
4.95
v e r y weak
4.2° ^ 2°
5
~
5,1
4.9
Polymer X-ray Data Inner Ring D,g
u
Polymer X-ray Data Inner Ring D,g
Organization
Organization
116
Polymers w i t h Nematic
1 3
J 4
2.
III.
C
C
PC3AS r LdAb / β — νν
Table
0
//— ν ν
=C H / Q V N = C H / O /
0
1.
2
PHBAS r nBAS
CH
r^\
/
> =C H - ^ Q ^ - N ^ C i ^ O )
A b b r e v i a t i o n o f Polymer S t r u c t u r e o f Monomer
.
CH
PBBAS
Intermediate
Monomer T r a n s i t i o n s , degrees C
Polymers w i t h
A b b r e v i a t i o n o f Polymer S t r u c t u r e o f Monomer
II.
No.
2
No.
Table
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
10
Ref
11
*
5 , 11
Réf.
ο
ο
ο
ώ
><
>.
g
ο
n r
t-
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
10
MESOMORPHIC
ORDER I N
POLYMERS
c h a i n s c o n t a i n i n g t h e S c h i f f base s t r u c t u r e and p h e n y l r i n g s when r e g u l a r p a c k i n g i s a c h i e v e d ( 9 ) . Further, t h i s sharp peak i s absent from t h e x-ray p h o t o g r a p h s from samples p r e c i p i t a t e d from s o l u t i o n . This l a t t e r method o f sample p r e p a r a t i o n l e a d s t o a l e s s o r d e r e d structure than bulk polymerization. Thus, t h e sharp l i n e m u s t b e due t o p a r a l l e l p a c k i n g b e t w e e n s i d e groups, not other i n t r a m o l e c u l a r e f f e c t s . The l o w i n t e n s i t y o f t h e x - r a y i n n e r r i n g c o u l d be due t o t w o f a c t o r s . 1) The n e m a t i c o r g a n i z a t i o n m i g h t n o t be e x t e n s i v e . As m e n t i o n e d a b o v e , t h e o r d e r e d d o m a i n s may n o t e x t e n d c o n t i n u o u s l y t h r o u g h o u t t h e s ample i n p o l y m e r s , b u t w o u l d be i n t e r r u p t e d by disordered regions. 2) The d i f f u s e i n n e r r i n g f o r n e m a t i c s t a t e s i n s m a l l m o l e c u l e s i s caused by t h e low e l e c t r o n d e n s i t y a t t h e ends o f t h e m o l e c u l e s . In the case o f polymers w i t h nematic packing o f the s i d e c h a i n s , one e n d o f t h e n e m a t o g e n i c s t r u c t u r e i s b o n d e d t o t h e p o l y m e r main c h a i n w i t h no a b r u p t d e c r e a s e i n density. Thus, there a r e fewer e l e c t r o n d e n s i t y f l u c t u a t i o n s p e r u n i t volume. To o u r k n o w l e d g e , t h e r e a r e no r e p o r t e d c a s e s w h e r e p o l y m e r s w i t h n e m a t i c o r g a n i z a t i o n o f s i d e g r o u p s g i v e an I n n e r r i n g o f m o d e r a t e intensity. The t u r b i d i t y o f t h e n e m a t i c s t a t e s o f l o w m o l e c u l a r w e i g h t compounds i s due p r i m a r i l y t o f l u c t u a tions i n the o r i e n t a t i o n of the d i r e c t o r over distances on t h e o r d e r o f t h e w a v e l e n g t h o f v i s i b l e l i g h t ( 1 2 ) . The o p t i c a l c l a r i t y o f PCBAS i n d i c a t e s t h a t t h e r e g i o n s o f n e m a t i c o r g a n i z a t i o n a r e much s m a l l e r i n s i z e . Light i s not appreciably scattered, nor are the t y p i c a l nematic textures resolvable i n the p o l a r i z i n g mic r o s cope. Poly(p-phenylene-bis(N-methylene-p-aminostyrene)), (PPMAS), P o l y ( d i ( N - p - a c r y l o y l o x y b e n z y l i d e n e ) h y d r a z i n e ) , (PdiABH). T h e s e d i f u n c t i o n a l monomers y i e l d p o l y m e r n e t w o r k s upon b u l k p o l y m e r i z a t i o n . No i n n e r r i n g s have been d e t e c t e d from t h e nematic o r g a n i z a t i o n . This h a s b e e n e x p l a i n e d (9) b y t h e f a c t t h a t b o t h c h a i n e n d s o f t h e monomer a r e i n c o r p o r a t e d i n t o t h e p o l y m e r m a i n chains. I n t h e case o f complete c o n v e r s i o n t o polymer t h e r e a r e no s i d e group ends g i v i n g l a r g e e l e c t r o n density fluctuations. Comparison w i t h C r y s t a l l i n e Polymers. The s t r u c t u r e o f t h e p o l y m e r s d i s c u s s e d above d i f f e r s f r o m t h a t i n c r y s t a l l i z a b l e polymers w i t h low degrees o f c r y s t a l l i n i t y ( p o o r l y d e v e l o p e d s t r u c t u r e ) . Here t h e t e n d e n c y t o o r g a n i z e comes f r o m t h e p a r a l l e l p a c k i n g of the side groups. There i s no e v i d e n c e t h a t p o l y m e r
I.
C L O U G H
E T
A L .
X-Rciij Diffraction from Polymers
11
main c h a i n s a r e p a r a l l e l t o each o t h e r o v e r l o n g d i s tances. I n t h e s m e c t i c A a n d C s t a t e s , we e n v i s i o n the p o l y m e r c h a i n as c o i l e d t h o u g h c o n f i n e d t o a p l a n e . In t h e u s u a l case o f polymer c r y s t a l s t h e main c h a i n s l i e p a r a l l e l i n the c r y s t a l l i n e regions. Acknowledgement T h a n k s a r e e x p r e s s e d t o t h e NSF f o r p a r t i a l p o r t u n d e r t h e G r a n t DMR-75-17397.
sup
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch001
Literature Cited 1. 2. "Liquid 3. 4. quid 5. 6. fillment 7. 8. 9. 10. 11.
12.
Blumstein, A., M a c r o m o l , ( 1 9 7 7 ) 1 0 , 872. deVries, Α., Ρramana, ( 1 9 7 5 ) 9, 93 Suppl. 1, a n d Crystals," C h a n d r a s e k a r , S. e d . , Indian Acad. Sci., Bangalore, 1975. Blumstein, A., Clough, S.B., Patel, L . , Blumstein, R.B., a n d H s u , E.C., M a c r o m o l . , ( 1 9 7 6 ) 9, 2 4 3 . Chystiakov, I., "Ordering a n d Structure of Li Crystals" in Adv. in Liquid Crystals, Vol. 1, B r o w n , G. ed., A c a d e m i c Press, (1975). Blumstein, A., Blumstein, R.B., Clough, S.B., a n d H s u , E.G., M a c r o m o l , ( 1 9 7 5 ) 8, 7 3 . L i m , L., Thesis in preparation for partial ful of requirements for Ph.D. d e g r e e . Poly mer Preprints, ( 1 9 7 5 ) 1 6 , No. 2, 2 4 1 . deVries, A., Mol. Cryst. Liq. Cryst., ( 1 9 7 0 ) 10, 219. H s u , E.C., Clough, S.B., a n d Blumstein, A., Polym. Lett., (1977) 15, 545. C l o u g h , S.B., Blumstein, A., a n d Hsu, E.C., M a c r o m o l . , ( 1 9 7 6 ) 9, 1 2 3 . H s u , E.C., a n d Blumstein, A., P o l y m . Lett., (1977) 15, 129. H s u , E.C., L i m , L . K . , Blumstein, R.B., a n d Blumstein, A., Mol. Cryst. Liq. Cryst. (1976) 33, 35. d e G e n n e s , P., The Physics of Liquid Crystals, Oxford University Press, London (1974).
Received December 8, 1977.
2 X-Ray Diffraction Studies on Mesomorphic Order in Polymers J. H. WENDORFF, H. FINKELMANN, and H. RINGSDORF
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
Deutsches Kunststoff Institut, Darmstadt, and Institut für Organische Chemie, Universität Mainz, West Germany
In the past few years a considerable number of papers were published which were concerned with liquid crystalline structures in polymeric systems. Different routes were employed to obtain polymers with liquid crystalline structures or even thermodynamica1ly stable l i q u i d crystalline phases 1). In general monomers containing mesogenic groups - groups which are known to have a tendency towards the formation of liquid crystalline s t r u c t u r e s , or rigid groups were used. Cases are known where the monomers e x h i b i t liquid crystalline phases 2). In that case the polymerization can be performed i n a n i s o t r o p i c melts; frozen-in liquid crystalline structures and textures can be obtained i n many instances 1,2). In other cases the monomers do not display liquid crystalline phases. The formation of liquid crystalline polymer structures may nevertheless be possible due to the restriction of the motions of the i n d i v i d u a l repeat units 3). B a s i c a l l y two d i f f e r e n t approaches have been used to obtain liquid crystalline structures i n polymers: E i t h e r the mesogenic or rigid group was b u i l t i n t o the polymer backbone or it was attached to the main chain as a side c h a i n . Main chain polymers. Due to the r i g i d i t y of the monomer units s t i f f polymer chains are obtained which according to the theory should have a tendency towards the formation of a n i s o t r o p i c melts 4). The properties of these melts have been widely used to obtain f i b e r s with very good mechanical properties Disadvantages of r i g i d chains are t h e i r high melting point and strong r e s t r i c t i o n s of r e o r i e n t a t i o n a l motions which are necessary f o r inducing t e x t u r a l changes. One way of overcoming these disadvantages consists i n p u t t i n g β
0-8412-0419-5/78/47-074-012$05.00/0 © 1978 American Chemical Society
2.
WENDORFF ET A L .
X-Ray Diffraction Studies
13
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
f l e x i b l e s p a c e r groups between mesogenic groups i n o r d e r t o d e c o u p l e t h e orientâtional m o t i o n s and t h e p o s i t i o n a l o r d e r o f t h e i n d i v i d u a l r e p e a t u n i t s 3J · I t c a n be e x p e c t e d t h a t i n t h i s c a s e t h e m e s o g e n i c groups w i l l form l i q u i d c r y s t a l l i n e s t r u c t u r e s i n t h e same way a s i n t h e c a s e o f l o w m o l e c u l a r w e i g h t s u b stances . Side chain polymers. In t h e case of s i d e chain p o l y mers i n w h i c h t h e m e s o g e n i c g r o u p s a r e a t t a c h e d t o t h e b a c k b o n e t w o ways o f o b t a i n i n g l i q u i d c r y s t a l l i n e structures are possible. I f the side chain i s r i g i d l y a t t a c h e d t o t h e backbone l i q u i d c r y s t a l l i n e s i d e g r o u p s t r u c t u r e s c a n o n l y be o b t a i n e d i f t h e c h a i n conformation i s d i s t o r t e d with respect t o the chain c o n f o r m a t i o n i n t h e i s o t r o p i c f l u i d s t a t e . An e x a m p l e w i l l be d i s c u s s e d w h e r e t h e b a c k b o n e i s c o n f i n e d t o a plane w i t h i n s m e c t i c l a y e r s . I f t h e s i d e groups a r e decoupled from t h e backbone w i t h r e s p e c t t o t h e p o s i t i o n a l and o r i e n t a t i o n a l o r d e r as w e l l as w i t h r e s p e c t to t h e r e o r i e n t a t i o n a l motions i t i s p o s s i b l e f o r t h e s i d e groups t o form l i q u i d c r y s t a l l i n e s t r u c t u r e s without d i s t o r t i n g the conformation of the polymer m a i n c h a i n . One t h e n e x p e c t s t h a t s t r u c t u r e f o r m a t i o n o c c u r s i n a way s i m i l a r t o t h a t known i n l o w m o l e c u l a r w e i g h t l i q u i d c r y s t a 1s. S h o r t m e s o g e n i c groups are e x p e c t e d t o f o r m n e m a t i c s t r u c t u r e s w h e r e a s l o n g groups a r e e x p e c t e d t o form s m e c t i c s t r u c t u r e s . These model c o n s i d e r a t i o n s a r e d i s c u s s e d i n d e t a i l i n conn e c t i o n w i t h t h e s y n t h e s i s o f t h e monomers a n d p o l y mers i n p a p e r -Z ) , In t h i s p a p e r t h r e e k i n d s o f p o l y m e r s w i l l be c o n s i d e r e d ( p o l y m e r s 1, 2 a n d 3) i n w h i c h m e s o g e n i c groups a r e e i t h e r a t t a c h e d t o t h e f l e x i b l e backbone v i a f l e x i b l e s p a c e r g r o u p s ( p o l y m e r s 1 a n d 2) o r d i r e c t l y w i t h o u t s p a c e r g r o u p s . The p o l y m e r s s t u d i e d a r e shown i n t a b l e 1. Experimental The s y n t h e s e s o f t h e monomers a n d p o l y m e r s b a s e d on model c o n s i d e r a t i o n s a r e d e s c r i b e d i n a s e p a r a t e p a p e r A]. The s a m p l e s u s e d i n t h e e x p e r i m e n t s w e r e o b t a i n e d e i t h e r by p r e c i p i t a t i n g t h e p o l y m e r s f r o m s o l u t i o n s o r by c a s t i n g f i 1 m s f r o m s o l u t i o n s . The f i 1 m s were s t u d i e d "as r e c e i v e d " and a l s o a f t e r h e a t i n g t h e m up t o t e m p e r a t u r e s a b o v e t h e m e l t i n g p o i n t a n d c o o l i n g them down t o room t e m p e r a t u r e . The s t r u c t u r e o f t h e p o l y m e r s was s t u d i e d by means of X - r a y d i f f r a c t i o n . The e x p e r i m e n t s w e r e p e r f o r m e d
14
MESOMORPHIC
ORDER I N
POLYMERS
Table 1 Polymers were
studied,
i n most c a s e s m e t h a c r y l i c
backbones
used-
Polymer 1 Backbone-COQ- (CH ) -0-C H -COO-CgH -R 2
(CH )^
flexible
2
n
6
4
5
s p a c e r group,
R variable
Polymer 2 Backbone-COO-C H -(CH~) - C O O - C h o l e s t e r y 1 6 4 2 η ( C H ) f l e x i b l e s p a c e r g r o u p , η = 2, 6 o r 12 C
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
2
n
Polymer 3 Backbone-COO-CgH -CH=N-C H -0-C H 4
Figure 1.
6
4
2
5
DSC traces of a polymer disphying a smectic and a nematic phase
2. W E N D O R F F E T A L .
X-Ray Diffraction Studies
15
w i t h a K r a t k y sma11 a n g l e d i f f r a c t i o n u n i t a n d w i t h a w i d e a n g l e d i f f r a c t o m e t e r . The s c a t t e r e d i n t e n s i t y was régistered s t e p w i s e u s i n g a s c i n t i l l a t i o n c o u n t e r c o m b i n e d w i t h an i m p u I s h e i g h t d i s c r i m i n a t o r . D i a grams o f o r i e n t e d s a m p l e s - due t o i n d u c e d t e x t u r e s , w e r e o b t a i n e d by means o f a f l a t f i l m c a m e r a . The p o s i t i o n and t h e w i d t h o f t h e r e f l e c t i o n s were c a l i b r a t e d by c o m p a r i s o n w i t h t h e d i a g r a m s o f q u a r t z . A d d i t i o n a l i n f o r m a t i o n about t h e s t r u c t u r e s and t h e s t a b i l i t y r e g i o n o f t h e s t r u c t u r e s w e r e o b t a i n e d by t h e r m a l a n a l y s i s (DSC) a n d by means o f a p o l a r i z i n g microscope.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
Thermal
Properties
P o l y m e r s w i t h f l e x i b l e s p a c e r g r o u p s . P o l y m e r s obt a i n e d by p r e c i p i t a t i o n o r by c a s t i n g f i l m s f r o m s ο 1 u t i ons w e r e f o u n d t o be amorpho us as shown by X - r a y and m i c r o s c o p i c a l o b s e r v a t i o n s . The DSC d a t a r e v e a l e d t h a t on h e a t i n g up t h e s a m p l e s f o r t h e f i r s t t i m e a s t e p w i s e i n c r e a s e o f t h e s p e c i f i c heat o c c u r r e d which c o u l d be a t t r i b u t e d t o a g l a s s t r a n s i t i o n . Thus a t room t e m p e r a t u r e t h e " a s r e c e i v e d " s a m p l e s w e r e i n the i s o t r o p i c g l a s s y s t a t e , a p p a r e n t l y no l i q u i d crystalline solutions exist. J u s t a b o v e t h e g l a s s t r a n s i t i o n t e m p e r a t u r e an e x o t h e r m i c p e a k was o b s e r v e d , i n d i c a t i n g t h a t an o r d e r e d s t r u c t u r e was f o r m e d . S u b s e q u e n t X - r a y s t u d i e s s h o w e d t h a t e i t h e r nemat i c o r s m e c t i c s t r u c t u r e s we r e f o r m e d , d e p e n d i n g on t h e n a t u r e o f t h e s u b s t i t u e n t R. On f u r t h e r h e a t i n g one o r t w o s h a r p m e l t i n g p e a k s w e r e o b s e r v e d ( F i g u r e 1 ) , t h e p o l y m e r s were t r a n s f o r m e d i n t o t h e i s o t r o p i c f l u i d s t a t e , a s shown by X - r a y d a t a . On c o o l i n g t h e samp l e s t h e t r a n s f o r m a t i o n i n t o l i q u i d c r y s t a l l i n e s t a t e s o c c u r r e d a s shown by e x o t h e r m i c peaks ( F i g u r e 1 ) . At l o w e r t e m p e r a t u r e s a s t e p w i s e d e c r e a s e o f t h e s p e c i f i c h e a t was f o u n d , t h u s a t room t e m p e r a t u r e s t h e s a m p l e s a r e n e m a t i c o r smec t i c g l a s s e s . The m e l t i n g a n d c r y s t a l l i z a t i o n r a n g e s w e r e v e r y n a r r o w o f t h e o r d e r o f 1 o r 2 d e g . C. The d e g r e e o f s u p e r - c o o l i n g was v e r y l o w , a t a h e a t i n g and c o o l i n g r a t e o f 4 d e g , p e r m i n u t e t h e d i f f e r e n c e i n m e l t i n g and c r y s t a l l i z a t i o n t e m p e r a t u r e s amounted t o a b o u t 2 d e g . C. I n t h e c a s e o f t h e nemat i c t r a n s i t i o n t h e h e a t s o f f u s i o n were o f t h e o r d e r o f 2 J o u l e s / g whereas i n t h e case o f t h e s m e c t i c t r a n s i t i o n s t h e h e a t s o f f u s i o n w e r e o f t h e o r d e r o f 10 J o u l e s / g . The p r o p e r t i e s d i s c u s s e d a b o v e a r e v e r y unusual f o r p o l y m e r i c systems b u t q u i t e u s u a l f o r low m o l e c u l a r weight l i q u i d c r y s t a l l i n e systems.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
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The m e l t i n g p o i n t s o f t h e p o l y m e r s s t u d i e d w e r e i n t h e r a n g e o f 100 t o 130 d e g . C f o r p o l y m e r s 1 and a r o u n d 200 d e g . C f o r p o l y m e r s 2. At the g l a s s t r a n s i t i o n a d i s t i n c t s t e p w i s e change of t h e s p e c i f i c h e a t was o n l y o b s e r v e d f o r n e m a t i c p h a s e s whereas f o r the case of the s m e c t i c phase o n l y a change i n the s l o p e o f t h e s p e c i f i c heat t e m p e r a t u r e c u r v e was f o u n d . A d d i t i o n a l t e c h n i q u e s s u c h as d i l a t o m e t r y h a v e t o be u s e d i n o r d e r t o p r o v e t h a t a c t u a l l y a glass t r a n s i t i o n occurs. This result indicates that s i n c e the s m e c t i c s t r u c t u r e i s c l o s e r to t h a t of a c r y s t a l , the m o t i o n s which t a k e p l a c e above t h e g l a s s t r a n s i t i o n w i l l be r e s t r i c t e d . Thus t h e i n c r e a s e i n t h e s p e c i f i c h e a t w i l l be s m a l l . The t y p e s o f m o t i o n s w h i c h f r e e z e i n a t t h e g l a s s t r a n s i t i o n w i l l be d i s cussed below. I s o t r o p i c g l a s s e s a r e c h a r a c t e r i z e d by r e l a x a t i o n p r o c e s s e s , w h i c h a r e d i r e c t l y r e l a t e d t o t h e none q u i l i b r i u m nature of the glassy s t a t e . S i m i l a r processes should also occur i n l i q u i d c r y s t a l l i n e glasses. E n t h a l p y r e l a x a t i o n was o b s e r v e d f o r s a m p l e s , w h i c h had b e e n a n n e a l e d b e l o w t h e g l a s s t r a n s i t i o n t e m p e r a ture f o r s e v e r a l hours, proving the n o n - e q u i l i b r i u m n a t u r e o f s m e c t i c and n e m a t i c g l a s s e s . P o l y m e r s w i t h o u t f l e x i b l e s p a c e r g r o u p s . The DSC curves of the polymer 3 i n d i c a t e d the e x i s t e n c e of a l i q u i d c r y s t a l l i n e g l a s s y s t a t e a t room t e m p e r a t u r e . The p o l y m e r was f o u n d t o be s m e c t i c . Two m e l t i n g p e a k s w e r e o b s e r v e d i n t h e t e m p e r a t u r e r a n g e b e t w e e n 300 and 310 d e g . C. T h e s e p e a k s a r e n o t as e a s i l y o b s e r v e d as i n t h e c a s e o f t h e p o l y m e r s d i s c u s s e d a b o v e , s i n c e t h e d e c o m p o s i t i o n t a k e s p l a c e i n t h e same t e m p e r a t u r e r a n g e . The o c c u r r e n c e o f e x o t h e r m i c p e a k s on c o o l i n g n e v e r t h e l e s s i n d i c a t e s t h a t r e v e r s i b l e m e l t i n g and c r y s t a l l i z a t i o n processes take place. X-Ray
Investigations
Polymers with f l e x i b l e spacer groups. Smectic o r n e m a t i c p o l y m e r s w e r e o b t a i n e d d e p e n d i n g on t h e nature of the s u b s t i t u e n t R i n the case of polymers 1 ( T a b l e 2 ) . I t has t o be p o i n t e d o u t t h a t t h e s t r u c t u r e s a t t e m p e r a t u r e s above t h e g l a s s t r a n s i t i o n t e m p e r a t u r e s a r e e q u i 1 i b r i urn s t r u c t u r e s , s i nee t h e r m o d y n a m i c a l l y s t a b l e l i q u i d c r y s t a l l i n e phases e x i s t . T h i s i s i n c o n t r a s t t o most systems s t u d i e d u n t i l now w h e r e o n l y l i q u i d c r y s t a l l i n e s t r u c t u r e s w e r e obtained · The n e m a t i c p h a s e s w e r e c h a r a c t e r i z e d by one b r o a d halo i n the wide angle s c a t t e r i n g r e g i o n , which c o u l d
2.
WENDORFF
X-Ray Diffraction Studies
ET AL.
17
Table I I Thermal p r o p e r t i e s o f s e l e c t e d examples o f polymers 1 (T smectic t r a n s i t i o n , T nematic t r a n s i t i o n , H heat of fusion) M
b
N
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
Nematic
n=6
polymers
R=0CrL
T =121 C
H= 2.3
Joules/g
R=OCH
T
H= 2.1
Joules/g
T =129 C
H = 9.2
J o u l e s /g
T =14D°C
H=11.3 J o u l e s / g
T =ii5"c
H=1>.5 J o u l e s / g
T =17l4-°G
H= 3.1
J o u l e s /g
T =1
H= 1.0
J o u l e s /g
Smectic
N
3
= I O 5
N
c
C
polymers
h=2
R=OC^H
n=2
R=OC H13
n=6
R = o c
7
s
S
6
Diphenyl
6 i3 H
s
derivatives
6\"
n=2
R = C
n=2
R=C H 6
5
O C H
3
N
s
2k°0
T =187°C
H=
.3
Joules /g
T =160°C
H=
.8
Joules /g
T =181 °c
H= 2.3
N
n=6
R = C
6 5 H
S
N
J o u l e s /g
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
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be a t t r i b u t e d t o t h e s h o r t r a n g e p o s i t i o n a l o r d e r o f t h e s i d e g r o u p s c h a r a c t e r i s t i c f o r t h e nemat i c o r d e r . The m o l e c u l a r d i s t a n c e s d e r i v e d f r o m t h e s e d a t a c o r r e s p o n d e d t o t h o s e o f p a r a l l e l s i d e g r o u p s . No r e f l e c t i o n s w e r e o b s e r v e d w h i c h c o u l d be a t t r i b u t e d t o a p o s i t i o n a l o r d e r i n the d i r e c t i o n of the long axes o f t h e s i d e g r o u p s , t h u s no c y b o t a c t i c c l u s t e r i n g t o o k ρ l a c e J±> . The s c a t t e r i n g d i a g r a m i s v e r y s i m i l a r t o t h a t o f a m o r p h o u s p o l y m e r s . One c o n s e q u e n t l y has t o use a d d i t i o n a l t e c h n i q u e s s u c h as p o l a r i z i n g m i c r o scope techniques i n order to v e r i fy a nematic s t r u c t u r e . The n e m a t i c s t r u c t u r e i s c h a r a c t e r i z e d by a l o c a l o r i e n t a t i o n o f t h e s i d e g r o u p s . In t h e a b s e n c e o f a s p e c i f i c t e x t u r e no m a c r o s c o p i c o r i e n t a t i o n w i l l r e s u I t s s i n c e the p r e f e r r e d d i r e c t i o n changes w i t h i n the sample. S l i g h t l y o r i e n t a t e d samples were o b t a i n e d by d r a w i n g , h i g h l y o r i e n t e d samp l e s w e r e o b t a i n e d i f s p e c i f i c t e x t u r e s w e r e i n d u c e d . T h i s w i l l be discussed l a t e r . No i n f o r m a t i o n was g a i n e d a b o u t t h e confor m a t i o n o f t h e c h a i n . I t can be a s s u m e d h o w e v e r , t h a t t h e conformâtion i s v e r y c l o s e t o t h a t o f t h e i s o t r o p i c f l u i d s t a t e due t o t h e f l e x i b l e s p a c e r g r o u p s . The s m e c t i c s t r u c t u r e was c h a r a c t e r i z e d by one or more ( h i g h e r o r d e r ) r e f l e c t i o n s i n t h e sma11 angle s c a t t e r i n g r e g i o n , w h i c h w e r e s h a r p , and by a b r o a d h a l o i n t h e w i d e a n g l e r e g i o n . The s m a l l a n g l e r e f l e c t i o n s c o u l d be a t t r i b u t e d t o s m e c t i c l a y e r s , t h e wide angle halo to a two-dimensional f l u i d s t a t e w i t h i n t h e s m e c t i c l a y e r s . Thus t h e s t r u c t u r e i s c h a r a c t e r i s t i c of a smect i c A o r C m o d i f i c a t i o n . In o r d e r t o o b t a i n more i n f o r m a t i o n a b o u t t h e s m e c t i c s t r u c tures i n polymers extended s t u d i e s i n c l u d i n g s t u d i e s on o r i e n t e d s y s t e m s w e r e p e r f o r m e d on a s p e c i f i c p o l y mer d i s p l a y i n g a s m e c t i c s t r u c t u r e $ a p o l y m e r w i t h o u t a f l e x i b l e s p a c e r g r o u p was c h o s e n . T h i s w i l l be d i s c u s s e d be low. In the case of the p o l y m e r s 2 s m e c t i c s t r u c t u r e s were observed f o r the homopolymers, t h a t i s f o r polymers i n w h i c h t h e f l e x i b l e s p a c e r g r o u p was constant. In a d d i t i o n t o the homopolymers a l s o c o p o l y m e r s were s t u d i e d , i n w h i c h t h e comonomers w e r e c h a r a c t e r i z e d by s p a c e r g r o u p s o f d i f f e r e n t l e n g t h . The combinations s t u d i e d w e r e η = 2 / 6 j η « 6/12 and η = 2/12. Smectic p o l y m e r s w e r e o b s e r v e d by c o p o l y m e r i z a t i o n o f mono m e r s w i t h η = 2/6 and η = 6/12, whereas c h o l e s t e r i c p o l y m e r s w e r e o b t a i n e d by c o p o l y m e r i z a t i o n o f mono m e r s w i t h η = 2/12, i f t h e compos i t i o n was approxim a t e l y 1 : 1. A p p a r e n t l y large differences in spacer l e n g t h , which lead to s h i f t s of the mesogenic cho1e s t e r y l group r e l a t i v e t o each o t h e r are r e q u i r e d f o r
2. W E N D O R F F E T A L .
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Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
t h e d e s t r u c t i o n o f t h e s m e c t i c o r d e r . The X - r a y p a t t e r n o f t h e h o m o p o l y m e r s a n d t h e c o p o l y m e r s 2/6 a n d 6/12 a r e c h a r a c t e r i z e d by s h a r p r e f l e c t i o n s i n t h e s m a l l angle r e g i o n , whereas t h e p a t t e r n o f t h e c o p o l y m e r 2/12 i s c h a r a c t e r i z e d by i n c r e a s i n g l y b r o a d e r r e f l e c t ions i f t h e c o m p o s i t i o n 1 : 1 i s approached. At t h i s c o m p o s i t i o n o n l y a b r o a d s m a 1 1 a n g l e h a l o i s obs e r v e d . The e x i s t e n c e o f a c h o l e s t e r i c s t r u c t u r e i s e s t a b 1 i s h e d by means o f p o l a r i z i n g m i c r o s c o p e techn i q u e s a n d by t h e observât i o n o f s e l e c t i v e r e f l e c t i o n . The s i d e g r o u p s a r e p a r a l l e l t o a p r e f e r e d d i r e c t i o n i n t h e s e p o l y m e r s on a l o c a l s c a l e , t h e p r e f e r e d d i r e c t i o n c h a n g e s i n t h e s a m p l e i n s u c h a way t h a t a helical structure results. P o l y m e r s w i t h o u t f l e x i b l e s p a c e r g r o u p s . The t e n d e n c y t o f o r m s m e c t i c s t r u c t u r e s i s i ne r e a s e d i f t h e s p a c e r g r o u p i s m i s s i n g . T h i s i s p r o b a b l y c a u s e d by t h e s t r o n g coup l i n g b e t w e e n t h e p o s i t i o n o f n e i g h b o u r i n g m e s o g e n i c s i d e g r o u p s due t o t h e c h a i n b a c k b o n e . O r i e n t e d samp l e s - u s e d f o r s t r u c t u r a l a n a l y s i s , w e r e o b t a i n e d by p e r f o r m i n g t h e p o l y m e r i z a t i o n i n t h e nemat i c p h a s e o f t h e monomer u n d e r t h e i n f l u e n c e o f a m a g n e t i c f i e l d . I t s h o u l d be p o i n t e d o u t t h a t t h e f i n a l s t r u c t u r e o f t h e p o l y m e r d i d n o t d e p e n d on t h e m e t h o d by w h i c h t h e p o l y m e r was s y n t h e s i z e d . I n a l l c a s e s t h e d i f f r a c t i o n p a t t e r n was c h a r a c t e r i z e d by f o u r s h a r p r e f l e c t i o n s i n t h e s m a 1 1 a n g l e r e g i o n and one b r o a d r e f l e c t i o n i n t h e w i d e a n g l e r e g i o n . The small angle r e f l e c t i o n s corresponded t o a l a t t i c e d i m e n s i o n o f 2 0 . 3 Κ, w h i c h was r e l a t e d t o t h e d i mension o f t h e smect i c l a y e r s . The s m a 1 1 a n g l e r e f l e c t i o n s w e r e a n a l y z e d w i t h r e s p e c t t o t h e d e p e n d e n c e o f t h e i r w i d t h on t h e o r d e r o f t h e r e f l e c t i o n s . From t h i s a n a l y s i s i n f o r m a t i o n on s t r u c t u r a 1 d e f e c t s o f t h e l a y e r a r r a n g e m e n t c a n be obtained. According t o the theory of p a r a c r y s t a l l i n e d i s t o r t i o n s _§.) t h e i n t e g r a l w i d t h ί Β o f t h e r e f l e c t i o n s c a n be r e l a t e d t o t h e s i z e L o f s m e c t i c a g g r e gats ( c r y s t a l s i z e i n t h e d i r e c t i o n o f t h e l a y e r n o r m a l ) a n d t o t h e r e l a t i v e f l u c t u a t i o n s fl/l = g of t h e l a t t i c e d i m e n s i o n ( s m e c t i c l a y e r d i m e n s i o n ) 1 by the e x p r e s s i o n : / B
2
- 1 / L
2
4
* (If g h ) / I
2
w h e r e h i s t h e o r d e r o f t h e r e f l e c t i o n . I t was f o u n d t h a t t h e dependence o f t h e w i d t h o f t h e r e f l e c t i o n s on t h e o r d e r c o u I d be r e p r e s e n t e d by t h i s e q u a t i o n .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
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ORDER I N
POLYMERS
From t h e i n t e r c e p t and t h e s l o p e o f t h e l i n e a v a l u e o f 1 5 8 0 A was o b t a i n e d f o r L and a v a l u e o f 1.4 % f o r g. Thus up t o 80 l a y e r s a r e w i t h i n one s m e c t i c a g g r e gate t h e r e l a t i v e f l u c t u a t i o n o f t h e l a y e r dimension i s very s m a l l . A d d i t i o n a l i n f o r m a t i o n c a n be o b t a i n e d f r o m o r i e n t e d s a m p l e s , t h e y w e r e c h a r a c t e r i z e d by s m a l l a n g l e r e f l e c t i o n s on t h e m e r i d i o n a n d a b r o a d r e f l e c t i o n i n t h e w i d e a n g l e r e g i o n on t h e e q u a t o r . The r e f l e c t i o n s w e r e e l o n g a t e d a l o n g Debye r i n g s , due t o m i s o r i e n t a t i o n s . The f i b e r d i a g r a m r e v e a l e d t h a t t h e d i r e c t i o n o f t h e one d i m e n s i o n a l l o n g r a n g e o r d e r , w h i c h was i d e n t i c a l w i t h t h e d i r e c t i o n o f t h e f i b e r a x i s , was p e r p e n d i c u l a r t o the d i r e c t i o n , i n which only a short r a n g e o r d e r e x i s t e d . Thus t h e s m e c t i c s t r u c t u r e o f t h e p o l y m e r i s c h a r a c t e r i z e d by a s s e m b l i e s o f s m e c t i c l a y e r s i n t h e d i r e c t i o n o f t h e l a y e r norma 1 and a s h o r t range o r d e r w i t h i n t h e s m e c t i c l a y e r s . In these l a y e r s t h e s i d e groups, c o n t a i n i n g t h e mesogenic group, a r e p a r a l l e l t o each o t h e r and p a r a l l e l t o t h e l a y e r norma 1 a s i n t h e c a s e o f t h e s m e c t i c A m o d i f i c a t i o n . I f one c o m p a r e s t h e m o l e c u l a r d i m e n s i o n s o f t h e c h a i n w i t h t h e v a l u e s o b t a i n e d from t h e s c a t t e r i n g e x p e r i ment o n e h a s t o c o n c l u d e t h a t t h e p o l y m e r c h a i n s a r e a r r a n g e d i n a s t a g g e r e d way. N e i g h b o u r i n g p o l y m e r c h a i n s must be s h i f t e d i n a r e g u l a r way i n t h e d i r e c t i o n o f t h e long axes o f t h e s i d e groups. I ti s p o s s i b l e t h a t t h e CH~ g r o u p s o f t h e m e t h a c r y l i e b a c k bone d i s r u p t t h e c l o s e p a c k i n g o f t h e s i d e g r o u p s s i n c e i n t h e c a s e o f an a c r y 1 i c b a c k b o n e no s h i f t o f t h e n e i g h b o u r i n g s i d e c h a i n s was o b s e r v e d . I n t h e s m e c t i c p o l y m e r s t h e m a i n c h a i n m u s t be c o n fined to p a r a l l e l planes w i t h i n the smectic layers since the observed p e r i o d i c s t r u c t u r e i n the d i r e c t i o n o f t h e l a y e r norma 1 c a n o n l y be o b t a i n e d by t h i s a r r a n g e m e n t . The X - r a y d a t a do n o t y i e l d i n f o r m a t i o n on t h e a c t u a l t w o d i m e n s i o n a 1 conformâtion o f t h e c h a i n on t h e s e s m e c t i c p l a n e s . On h e a t i n g up t h e samp l e t h e X - r a y d i a g r a m r e m a i n e d u n c h a n g e d up t o t h e g l a s s t r a n s i t i o n temperature. Above t h i s t e m p e r a t u r e t h e s m a l l a n g l e r e f l e c t i o n s remaiη u n c h a n g e d w h e r e a s t h e w i d t h a n d t h e p o s i t i o n o f t h e w i d e a n g l e h a l o c h a n g e i n a way c h a r a c t e r i s t i c o f i n c r e a s e d m o l e c u l a r d i s t a n c e s and a b r o a d e r d i s t r i b u t i o n o f t h e m o l e c u l a r d i s t a n c e s . Thus t h e m o t i o n s which s e t i n at the glass t r a n s i t i o n occur only w i t h in the smectic layers i n a d i r e c t i o n p a r a l l e l to the l a y e r s u r f a c e . The s m e c t i c l a y e r a c t s a s a t w o - d i mensional fluid.
2.
WENDORFF E T A L .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch002
Liquid
21
X-Ray Diffraction Studies
Crystalline
Textures
L i q u i d c r y s t a l l i n e t e x t u r e s i n p o l y m e r i c systems can be o b t a i n e d i n t w o d i f f e r e n t w a y s . One way c o n s i s t s in inducing a s p e c i f i c texture i n a l i q u i d c r y s t a l l i n e monomer p h a s e . O f t e n t h e f o r c e s a c t i n g on t h e s a m p l e w i l l be s u f f i c i e n t l y l a r g e t o s t a b i l i z e t h e t e x t u r e d u r i n g t h e p o l y m e r i z a t i o n . A t e x t u r e may be o b t a i n e d i n t h i s way e v e n i n p o l y m e r s w h i c h do n o t d i s p l a y l i q u i d c r y s t a l l i n e phases. In t h e case of p o l y m e r 3 t h e p o l y m e r i z a t i o n i n t h e n e m a t i c monomer phase under t h e i n f l u e n c e o f a magnetic f i e l d l e d t o a p o l y m e r s a m p l e w i t h h i g h o r i e n t a t i o n a 1 o r d e r on a macroscopic scale. A s e c o n d way t o o b t a i n l i q u i d c r y s t a l l i n e t e x t u r e s c o n s i s t s i n h e a t i n g up p o l y m e r s a m p l e s , w h i c h d i s p l a y l i q u i d c r y s t a l l i n e p h a s e s , between g l a s s s l i d e s o r u n d e r t h e i n f l u e n c e o f e x t e r n a l f o r c e s s u c h as e l e c t r i c o r magneti c f i e Ids. Text ures w i l l then form i n a way s i m i l a r t o t h e c a s e o f l o w m o l e c u l a r w e i g h t l i q u i d c r y s t a l s . T h i s was o b s e r v e d f o r t h e c a s e o f t h e p o l y m e r s 1. On c o o l i n g down t h e t e x t u r e s c a n be f r o z e n i n s i n c e a g l a s s t r a n s i t i o n o c c u r s . Thus i t becomes p o s s i b l e t o keep a s p e c i f i c t e x t ure w i t h i n t e r e s t i n g o p t i c a l p r o p e r t i e s permanently. Literature Cited 1) 2) 3) 4) 5) 6)
Blumstein, A., "Liquid Crystalline Order in Poly mers", Academic Press in press Shibaev, V.P., V y s o k o m o l . S o e d i n ( 1 9 7 7 ) A 9 , 923 Blumstein, A., M a c r o m o l e c u l e s ( 1 9 7 7 ) 1 0 , 872 Flory, P.J., Proc.Roy.Soc.Lond. ( 1 9 5 6 ) A 2 3 4 , 73 Jackson, W . J . and K u h f u s s , H.F., J. Pol. Sci.P o l y m e r Chem. E d . ( 1 9 7 6 ) 1 4 , 2 0 4 3 Roviello, A . and Sirigu, Α., Polymer Letters Ed. ( 1 9 7 5 ) 1 3 , 455
7)
Finkelmann, Η., this issue 8) De Vries, A., 219 9) H o s e m a n n , R. and Diffraction by Matter", A m s t e r d a m 1962 RECEIVED December 8, 1977.
Ringsdorf,
Η.
Mol.Cryst.Liquid Bagchi, North
and
Wendorff,
Cryst.
(1970)
J.Η. 10,
31,
S.N., "Direct Analysis of Holland Publ. Comp.,
3 Enantiotropic (Liquid Crystalline) Polymers: Synthesis and Models H. FINKELMANN, H. RINGSDORF, W. SIOL, and J. H. WENDORFF
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
Institut für Organische Chemie, Universität Mainz and Deutsches Kunststoff Institut, Darmstadt, West Germany
In the past few years studies on liquid crystalline polymers have attracted Increasing interest because of their theoretical and technological aspects (1). In order for the liquid crystalline phase to form, the molecules must have an extended rigid molecular shape and a large anisotropic polarizability (2). If a polymer has to behave like a conventional liquid crystal, it is best to use a mesogenic molecule as a monomer component. By substituting a vinyl group, the polymerization leads to an alkyl polymer chain, where the mesogenic moieties are fixed as side chains. In the liquid state, the polymer main chain is flexible, and only the rigid mesogenic side chain moieties are required to form the mesophase. They have to build the liquid crystalline order independently from the conformation of the connecting main chain. Consequently the theoretical considerations used in the case of conventional liquid crystals have to be valid for this type of polymer. In the past few years attempts to synthesize liquid crystalline polymers have started with a monomer in a liquid crystalline state. Three different types of polymer systems have been obtained: (1)
Amorphous polymers — which exhibit properties.
no liquid
(2)
Polymers, with fixed liquid crystalline structure — if the polymerization temperature was below the glass transition temperature (T ) of the polymer. These polymers exhibit a liquid crystalline structure in the solid glassy phase because the structure of the ordered monomer phase was "frozen in" by polymerization. By heating the polymer, these frozen structures were irreversibly lost above the Tq (4,5). An additional effect was observed; starting from a nematic or cholesteric monomer in most cases, the resulting polymer structure was smectic (6,7). The decrease of mobility of the mesogenic side chains because of their G
0-8412-0419-5/78/47-074-022$05.00/0 © 1978 American Chemical Society
crystalline
3.
FINKELMANN ETAL.
Enantiotropic (Liquid Crystalline) Polymers
23
f i x a t i o n t o the polymer main c h a i n i m p l i e s an a d d i t i o n a l l o n g range order o f the centers o f g r a v i t y o f the mesogenic groups.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
(3)
L i q u i d c r y s t a l l i n e ( L C ) polymers w i t h a thermodynamically s t a b l e l i q u i d c r y s t a l l i n e phase above the T Q . These were obtained only i n a few cases ( 8 , 9 ) ·
U n t i l now there was no obvious c o r r e l a t i o n found between the monomer s t r u c t u r e and the r e s u l t i n g polymer phase. No - theor e t i c a l s t r u c t u r a l c o n d i t i o n s were d e s c r i b e d which would r e s u l t i n a l i q u i d c r y s t a l l i n e polymer w i t h a d e f i n i t e ordered phase, e.g., w i t h a nematic, a smectic, o r a c h o l e s t e r i c phase as i n c o n v e n t i o n a l l i q u i d c r y s t a l s . Although previous examples have e s t a b l i s h e d (8,9) the e x i s t e n c e o f e n a n t i o t r o p i c l i q u i d c r y s t a l l i n e s i d e c h a i n polymers, a d d i t i o n a l c o n s i d e r a t i o n s are i n order f o r a systematic s y n t h e s i s o f such polymers. Model Considerations I n c o n v e n t i o n a l L C phases the motion o f the molecule i s r e s t r i c t e d only by the a n i s o t r o p i c i n t e r a c t i o n s w i t h i t s neighbors. This l e a d s t o the formation o f the o r i e n t a t i o n a l l o n g range order and, i n the case o f smectic phases, t o an a d d i t i o n a l l a m e l l a r s t r u c t u r e . However, completely d i f f e r e n t c o n d i t i o n s normally e x i s t i n a l i q u i d c r y s t a l l i n e polymer. F i g u r e 1 s c h e m a t i c a l l y shows the t y p i c a l s t r u c t u r e f o r a L C polymer as d e s c r i b e d so f a r . The mesogenic groups are d i r e c t l y f i x e d t o the main chain. Thus f o r the l i q u i d c r y s t a l l i n e polymer s t a t e above the T Q two c o n d i t i o n s have t o be considered. (1)
The polymer main chain has t o be i n the l i q u i d s t a t e , causing a tendency toward a s t a t i s t i c a l chain conformation.
(2)
The mesogenic s i d e chains have t o be i n the l i q u i d c r y s t a l l i n e order. The a n i s o t r o p i c i n t e r a c t i o n s induce an anisotropic orientation.
These two tendencies c o n f l i c t , and s t e r i c hindrance i n the system determines which tendency w i l l dominate. The s t e r i c hindrance r e s u l t s from the d i r e c t l i n k a g e o f the main chain t o the s i d e chain. A d i r e c t c o u p l i n g o f motions o f the main c h a i n to the r i g i d mesogenic s i d e chains can be assumed. Therefore, an amorphous l i q u i d polymer i s obtained i f the a n i s o t r o p i c o r i e n t a t i o n o f the s i d e c h a i n i s hindered o r d i s t u r b e d by the main c h a i n . On the other hand, a s o l i d polymer w i t h a L C s t r u c t u r e i s obtained i f the a n i s o t r o p i c - o r d e r e d r i g i d s i d e chains hinder the normal motions o f the main chain and thus tend t o r e s t r i c t the main chain motions.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
24
MESOMORPHIC
ORDER IN
POLYMERS
The c r i t i c a l p o i n t seems to he the d i r e c t l i n k a g e between the main and s i d e chains. Thus one can assume t h a t to o b t a i n a polymer i n i t s l i q u i d c r y s t a l l i n e s t a t e , the d i r e c t c o u p l i n g has t o be avoided. I f t h i s l i n k a g e were f l e x i b l e , s t e r i c hindrance would be a l l e v i a t e d , and the motions of the s i d e and main chains would be decoupled ( F i g u r e 2). Under these c o n d i t i o n s a s t a t i s t i c a l main chain conformation as w e l l as an a n i s o t r o p i c o r i e n t a t i o n of the side chain would occur, r e s u l t i n g i n an e n a n t i o t r o p i c LC polymer. The f l e x i b l e l i n k a g e ( s c h e m a t i c a l l y represented i n F i g u r e 2 b y ^ ) can be r e a l i z e d by an a l k y l - or a l k y l o x y chain and w i l l be c a l l e d " f l e x i b l e spacer group." I f the model w i t h the decoupled main chain and s i d e chain w i t h the f l e x i b l e spacer group i s v a l i d , the p r o p e r t i e s of the r e s u l t i n g l i q u i d c r y s t a l l i n e polymer should be determined only by the meso genic moieties s i m i l a r to c o n v e n t i o n a l LC. The model can be a p p l i e d by homo- and copolymer!zation (Figure 3). Nematic as w e l l as smectic polymers may be expected w i t h homopolymerization. With regard to a homologous s e r i e s of a mesogenic components w i t h v a r y i n g para s u b s t i t u e n t s , nematic phases should be formed p r e f e r e n t i a l l y by u s i n g s h o r t para s u b s t i t u e n t s . Smectic phases should be found p r e f e r e n t i a l l y i f the para s u b s t i t u e n t i s s u f f i c i e n t l y l o n g . This analogy to low molecular l i q u i d c r y s t a l s Is expected i f the main and s i d e chains are decoupled. The knowledge of c o n v e n t i o n a l l i q u i d c r y s t a l s would then be a p p l i c a b l e to these types of polymers. With c o p o l y m e r i z a t i o n b a s i c a l l y the same p r i n c i p l e s can be expected as f o r the homopolymers. U s i n g d i f f e r e n t monomers, a wide v a r i a t i o n of the temperature r e g i o n of the mesogenic phase may occur. An a d d i t i o n a l e f f e c t has to be expected w i t h c ο polymer i ζat iο η of monomers w i t h spacers of d i f f e r e n t l e n g t h s . The r e s u l t i n g co polymers should e x h i b i t even l e s s order than the corresponding spacer-group-containing homopolymers. I t should be p o s s i b l e to f a v o r an arrangement of mesogenic s i d e groups i n which the centers of g r a v i t y are d i s t r i b u t e d s t a t i s t i c a l l y and thus a v o i d the smectic order. These models might be of i n t e r e s t , e s p e c i a l l y f o r the formation of nematic polymers. R e s u l t s and D i s c u s s i o n Homopolymers« Phenylesters of benzoic a c i d were chosen as mesogenic groups f o r the s y n t h e s i s of s u i t a b l e monomers. The a c r y l o y l - and the m e t h a c r y l o y l moieties were used as polymerizab l e groups. The f l e x i b l e spacer was an a l k y l chain or an a l k y l o x y chain of v a r y i n g l e n g t h . Thus a homologous s e r i e s of £-.0. -(2-methylpropenoyloxyalkyoxy) benzoic a c i d -p s u b s t i t u t e d phenyl e s t e r s 1 was prepared. The monomers were synthesized by the standard methods as f o l l o w s : 1
FINKELMANN ET AL.
Enantiotropic (Liquid Crystalline) Polymers
M A I N C H A I N IN
MESOGENIC S I D E C H A I N IN LC O R D E R
LIQUID STATE
TENDENCY
TENDENCY
TOWARD!
STATISTICAL
ORIENTATION
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
CONFORMATION
STERIC
TOWARD
ANISOTROPIC
CHAIN
HINDRANCE
AMORPHOUS
SOLID
POLYMER
LC
POLYMER
STRUCTURE
Figure 1. Schematic of formation of liquid crystalline order in polymers with mesogenic side groups attached directly to the main chain
MAIN CHAIN LIQUID
IN
STATE
MESOGENIC SIDE C H A I N LC
TENDENCY
TOWARD
STATISTICAL
CHAIN
CONFORMATION
ENANTIOTROPIC
IN
ORDER
TENDENCY
TOWARD
ANISOTROPIC ORIENTATION
LC
POLYMER
Figure 2. Schematic of liquid crystalline order formation in polymers with mesogenic side groups decoupled from the main chain by a flexible spacer
MESOMORPHIC
1.
ORDER
IN POLYMERS
HOMOPOLYMER IΖ AT ION
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
NEMATIC POLYMER
SMECTIC POLYMER
2.
COPOLYMERIZATION NEMATIC POLYMER
Figure 3. Schematic of nematic and smectic order in (a) copolymers and (b) homopolymers
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
3.
FINKELMANN ETAL.
Flexible spacer
Enantiotropic (Liquid Crystalline) Polymers
27
Mesogenic group
The p r o p e r t i e s o f some monomers a r e l i s t e d i n Table I . Only the d i p h e n y l d e r i v a t i v e s e x h i b i t e n a n t i o t r o p i c l i q u i d c r y s t a l l i n e p r o p e r t i e s whereas the phenylesters melt t o i s o t r o p i c f l u i d s . A monotropic l i q u i d c r y s t a l l i n e phase i s obtainable i f long spacers and para s u b s t i t u e n t s are used. Because o f the high tendency t o c r y s t a l l i z e , the monotropic phase t r a n s i t i o n s o f the monomers were not determined. A l l monomers are r a d i c a l l y polymer!zable. I n accordance w i t h the p r e d i c t i o n s d e r i v e d from the model, these polymers e x h i b i t e n a n t i o t r o p i c l i q u i d c r y s t a l l i n e p r o p e r t i e s . Even the polymers prepared from i s o t r o p i c monomers are l i q u i d c r y s t a l l i n e . Table I I l i s t s the phase t r a n s i t i o n and LC phases which were determined by DSC, x-ray i n v e s t i g a t i o n s ( 1 0 ) , and p o l a r i z a t i o n microscopy. I f one compares the polymers where R i s an a l k y l o x y group, nematic as w e l l as smectic phases are observed. I f R i s a s h o r t s u b s t i t u e n t (-0CH ), the polymer i s nematic, whereas i f R i s a longer s u b s t i t u e n t , the polymer i s smectic. The diphenyl d e r i v a t i v e s e x h i b i t same c h a r a c t e r i s t i c . I n a d d i t i o n the d i phenyl d e r i v a t i v e s w i t h o u t a para s u b s t i t u e n t have a low temperature smectic phase as w e l l as a nematic phase. These r e s u l t s show t h a t the side chain polymers f i t the model w e l l . Furthermore they confirm the expected analogy o f these spacer-group-containing polymers w i t h conventional l i q u i d c r y s t a l s t h a t nematic and smectic phases can be r e a l i z e d , dependi n g on the para s u b s t i t u e n t s o f the mesogenic m o i e t i e s . Copolymers. The second p a r t o f our model i s concerned w i t h the c o p o l y m e r i z a t i o n o f monomers w i t h d i f f e r e n t spacers. I n t h i s case a s t a t i s t i c a l d i s t r i b u t i o n o f the centers o f g r a v i t y of the mesogenic chain might be p r e f e r r e d even more. This p r e d i c t i o n was t e s t e d w i t h c h o l e s t e r y l d e r i v a t i v e s :
MESOMORPHIC
ORDER I N P O L Y M E R S
Table I . Phase T r a n s i t i o n s o f Monomers 1
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
r'°-
1.
( C H
) 2
n-
0 _ <
>
0 "
c o o
<
>
" 0 "
R
Alkylethers η
R
2
OCH
2
O C
Phase T r a n s i t i o n (°c) k 69 i
3
k 67 i
H
3
7
2
OC H
6
OCH
6
O C
6
k 59 i
13
k li-T 1
3
k I4-T η 53 i
H
6 i
3
Diphenyl d e r i v a t i v e s η
R
2
C
3
C H^OC H
3
C
6
Phase T r a n s i t i o n
(°c)
6H!+
6
C
OCH 2
6
H
6
H
k 108 η 211 i
3
5
k 123 η 202 i k 105 i
5
k 6k sm 68 η 92 5
FINKELMANN ET AL.
Enantiotropic (Liquid Crystalline) Polymers
Table I I . L i q u i d C r y s t a l l i n e Phase T r a n s i t i o n of Synthesized Polymers CH
0=C-0-(CH ) -0-<(Q)-C00^^)-R
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
2
1.
n
Alkylethers a) Nematic polymers η
R
Phase T r a n s i t i o n
2
OCH
g 101 η 121 i
6
OCH
g 95
η 105 i
b) Smectic polymers Phase T r a n s i t i o n 2
OC H 3
g 120 sm 129 i
7
2
g - ) sm ihO i a
C
H
° 6 i3
6
2.
O C
g
H
6 13
60 sm 115 i
Diphenyl d e r i v a t i v e s n
R
Phase T r a n s i t i o n
2
-0-OCH
3
-0-O
C £
g - ) a
3
B
5
g 120 sm 300 i sm 1T0 η 19T i
3
6
UT i
-o>-
Not y e t determined.
g 132 sm l6k n I8I4- i
MESOMORPHIC
30
ORDER I N
POLYMERS
2 n
= 2, 6,
12
i n which the spacer l e n g t h η i s v a r i e d . A l l synthesized monomers e x h i b i t c h o l e s t e r i c mesophases i n s t e a d of nematic phases because of the c h i r a l i t y o f c h o l e s t e r o l (Table I I I ) .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
Table I I I . Phase T r a n s i t i o n s o f C h o i e s t e r y l D e r i v a t i v e s
/ γ ""Ο η 2 6 12
(CH
^-<_
chol
2
-
2
Phase T r a n s i t i o n ( Cj ch 159 I ch 112 i ch 8l I
k 139 k 90 5 k 55
The synthesized monomers are r a d i c a l l y polymerizable, and t h e i r polymers and copolymers were p u r i f i e d by r e p r e c i p i t a t i o n . Table IV summarizes the r e s u l t s o f p o l y m e r i z a t i o n experiments. I n a l l cases e n a n t i o t r o p i c l i q u i d c r y s t a l l i n e polymers were obtained. The homopolymerε e x h i b i t smectic phases. Contrary to our expectations smectic polymer phases are obtained a l s o by copolymer!zation o f the monomers i n which the spacer l e n g t h i s n-j^ = 2, Ώ-2 = 6 and nj_ = 6, n = 12. A s i m i l a r r e s u l t was p u b l i s h e d r e c e n t l y by Shibaev e t a l . (7), who, based on i n v e s t i g a t i o n s of comb-like polymers, synthesized polymer c h o l e s t e r y l d e r i v a t i v e s w i t h e n a n t i o t r o p i c smectic phases. The expected preference o f a s t a t i s t i c a l d i s t r i b u t i o n o f the mesogenic side chain has so f a r been only r e a l i z e d by the copolymer!zation i n which n^ = 2, rip = 12 and only i f the monomer r a t i o i s 1:1. Other compositions a l s o l e d t o smectic phases. The b e s t proof f o r i t s c h o l e s t e r i c s t r u c t u r e Is i t s s e l e c t i v e r e f l e c t i o n o f l e f t - c i r c u l a r p o l a r i z e d l i g h t i n the v i s i b l e , which i n d i c a t e s a left-handed c h o l e s t e r i c h e l i x o f the polymer. The s t r u c t u r e has been e s t a b l i s h e d a l s o by x-ray i n v e s t i g a t i o n s (8). This copolymer i s the f i r s t e n a n t i o t r o p i c c h o l e s t e r i c polymer. This r e s u l t confirms the model and shows t h a t a s t a t i s t i c a l d i s t r i b u t i o n of centers of g r a v i t y o f the mesogenic side chains i s p o s s i b l e i f spacer groups of s u f f i c i e n t l y d i f f e r e n t lengths are used. 2
3.
FINKELMANN
Enantiotropic (Liquid Crystalline) Polymers
ETA L .
Table IV.
31
Phase T r a n s i t i o n s o f Homo- and Copolymers of C h o l e s t e r y l D e r i v a t i v e s 2 a
Homopolymers
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
η
T ^ (°C)
b
ΔΗ
(j/g)
b
Kl
Polymer Phase
ο 2
d -
d -
, . smeetic
6
l82
6.7
smectic
12
168
k.2
smectic
c
Copolymers M
U2
l
2
6
50
6
12
50
2
12
51
2
12
2 2
M
2
50
m
-Kl
b
AHm
(j/g) Kl d
_
d
Polymer Phase smectic
185
2.76
smectic
1+9
209
1.17
cholesteric
56
1+1+
212
1.25
smectic
12
3^
66
-
-
smectic
12
Us
58
-
-
smectic
a
- spacer leng5th o f components M^, M .
£l>
S
DSC
measurements.
2
2
b c X-ray i n v e s t i g a t i o n s ^
c
b
Decomposition
before c l e a r i n g .
32
MESOMORPHIC
ORDER I N P O L Y M E R S
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch003
Bulk P o l y m e r i z a t i o n . The monomers were a l s o polymerized i n b u l k . To f o l l o w the process, a p o l a r i z a t i o n microscope was used, which made i t p o s s i b l e t o observe phase separations or phase transitions during polymerization. (1)
A phase s e p a r a t i o n d u r i n g p o l y m e r i z a t i o n occurred when a nematic monomer was polymerized t o a smectic polymer, be cause the polymer was I n s o l u b l e i n the monomer.
(2)
No phase s e p a r a t i o n was observed when an i s o t r o p i c monomer was polymerized t o a nematic o r smectic polymer. The nematic or smectic polymers are s o l u b l e i n the i s o t r o p i c monomer. When a c e r t a i n conversion i s obtained, a phase t r a n s i t i o n from i s o t r o p i c t o nematic o r smectic phase takes p l a c e . This phase t r a n s i t i o n , caused by v a r i a t i o n o f the mole f r a c t i o n o f the monomer and the polymer, i s comparable w i t h a phase t r a n s i t i o n caused by v a r y i n g the temperature a t constant composition.
(3)
A homogenous ροlyme r i ζati on was achieved by the polymeriza t i o n o f a nematic monomer t o a nematic polymer where the temperature r e g i o n o f the l i q u i d c r y s t a l l i n e monomer phase overlapped the temperature r e g i o n of the l i q u i d c r y s t a l l i n e polymer phase (see Table I I ) . This p o l y m e r i z a t i o n process i s i n f l u e n c e d e i t h e r by a p r e c i p i t a t i o n or by a phase transition.
A d e t a i l e d p h y s i c a l i nve s t i gat io η o f the polymers d e s c r i b e d here i s d i s c u s s e d i n Chapter 2. Literature 1. 2. 3. 4. 5. 6. 7· 8. 9.
Cited
Shibaev, V.P., Vysokomol. Soedin (1977) A9, 923. Gray, G.W., Winsor, P.A., "Liquid Crystals and Plastic Crystals," Ellis Horwood, Chichester, 1974. Strzelecki, L., Liebert, L., Bull. Soc. Chim. (1973) N2, 605. Clough, S.B., Blumstein, A., Hsu, E.C., Macromolecules (1976) 9, 123. Bouligand, Y., Cladis, P.E., Liebert, L., Strzelecki, L., Mol. Cryst., Liq. Cryst. (1974) 25, 233. Lorkorwski, H.J., Reuther, F., Plaste Kautsch. (1976) 2, 81. Hsu, E.C., Blumstein, Α., Clough, S.B., J. Polym. Sci., Polym. Lett. Ed. (1977) 15, 545. Perplies, Ε., Ringsdorf, H., Wendorff, J.H., Ber. Bunsenges. Phys. Chem. (1974) 9, 921. Shibaev, V.P., Freidzon, J.S., Platé, N.A., Dokl. Akad. Nauk, SSSR (1976) 227, 1412.
R E C E I V E D March 2, 1 9 7 8 .
4 Thermotropic Cholesterol-Containing Liquid Crystalline Polymers V. P. SHIBAEV, N. A. PLATÉ, and Y. S. FREIDZON
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
Polymer Chemistry Department, Faculty of Chemistry, Moscow State University, Moscow, USSR
In recent years, i n v e s t i g a t o r s working in the field of physics and chemistry of high-molecular compounds have been paying a l o t of a t t e n t i o n t o the problem of c r e a t i n g l i q u i d c r y s t a l l i n e systems (1-14). The great i n t e r e s t displayed in the study of p r o p e r t i es of such systems can most probably be accounted for by two main f a c t o r s : firstly, by the advances in s t u dies i n t o the s t r u c t u r e , p r o p e r t i e s and p r a c t i c a l use of low-molecular l i q u i d c r y s t a l s in physics, technology and medicine, and, secondly, by the s t u d i e s of the nature and s a l i e n t f e a t u r e s of the l i q u i d c r y s t a l l i n e state in polymers as a s p e c i f i c state of macromolecul a r substances. Advances in t h i s field of research are associated with the development of methods f o r producing polymeric l i q u i d - c r y s t a l l i n e systems and c o n t r o l l i n g the processes of s t r u c t u r e formation in polymers. Unfortunately, at present, the relevant literature not only l a c k s classification of experimental data on polymeric l i q u i d c r y s t a l s , but a l s o criteria determining the existence of the l i q u i d c r y s t a l l i n e state i n polymers are nowhere to be found. More often than not, c e r t a i n authors r e f e r t o polymer systems under study or s p e c i f i c s t a t e s of these systems as liquid - c r y s t a l ones without s u f f i c i e n t grounds. The r e c e n t l y published reviews by Papkov on l y o t r o p i c l i q u i d c r y s t a l l i n e polymer systems (12) and by Shibaev and Plate on the l i q u i d c r y s t a l l i n e states i n polymers (13) should be regarded as the first attempts to systematize the great body of a v a i l a b l e experimental data with a view to e l a b o r a t i n g adequate techniques of producing l i q u i d c r y s t a l l i n e polymer systems. In t h i s paper, which deals e x c l u s i v e l y with t h e r motropic l i q u i d c r y s t a l l i n e polymers, we s h a l l apply the term "liquid-crystalline" to the thermodynamical0-8412-0419-5/78/47-074-033$06.00/0 © 1978 American Chemical Society
MESOMORPHIC
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
34
ORDER I N P O L Y M E R S
l y s t a b l e phase state of polymers or polymer systems characterized by a spontaneously o c c u r r i n g (independently from t h e i r state of aggregation) anisotropy of p r o p e r t i e s ( i n p a r t i c u l a r , o p t i c a l anisotropy) i n the absence of a three dimensional c r y s t a l l a t t i c e . It should be pointed out that along with t h i s d e f i n i t i o n which describes the l i q u i d c r y s t a l l i n e state as a phase s t a t e , use i s often made of the term " l i q u i d c r y s t a l l i n e s t r u c t u r e " which i s i n d i c a t i v e only of a c e r t a i n o r i e n t a t i o n ordering i n a system. Despite the narrower meaning of the l a t t e r term, the notion of a l i q u i d c r y s t a l l i n e s t r u c t u r e (or ordering) i s widely used i n the l i t e r a t u r e on s t r u c t u r a l polymer s t u d i e s , therefore, i n some instances, we s h a l l use t h i s term as w e l l . At present, f o u r types of polymer systems can be d i s t i n g u i s h e d to which, i n our opinion, the term of l i q u i d c r y s t a l l i n e (mesomorphous) state i s a p p l i c a b l e : (1) Melts of c r y s t a l l i n e polymers and amorphous polymers c h a r a c t e r i z e d by an o r i e n t a t i o n r e l a t e d t o liquid
crystalline
ordering;
(2) L y o t r o p i c l i q u i d c r y s t a l l i n e systems; (3) Mesomorphous s t r u c t u r e s of block polymers i n gels; (4) Polymers with side anisodiametric groups mod e l i n g the molecular s t r u c t u r e of low-molecular l i q u i d crystals. In t h i s paper, we s h a l l dwell on approaches t o c r e a t i n g l i q u i d c r y s t a l l i n e polymers of the l a t t e r t y pe. As f a r as the f i r s t three systems are concerned, t h e i r d e s c r i p t i o n can be found i n Refs. (12-13) and the c i t e d references. Polymers with side anisodiametric groups modeling the molecular s t r u c t u r e of low-molecular l i q u i d c r y s t a l s can be obtained e i t h e r through synthesis of monomers with l i q u i d c r y s t a l l i n e (mesogenic) groups with subsequent polymerization or through chemical a t t a c h ment of molecules of low-molecular l i q u i d c r y s t a l l i n e compounds to a polymer chain by way of polymer-analogous transformations. As can be i n f e r r e d from the bulk of works d e a l i n g with t h i s problem, at present, the f i r s t of these two methods i s used i n most cases. Ref. (13) reviews the b a s i c types of the so f a r synt h e s i z e d polymers with mesogenic groups along with a d e t a i l e d d e s c r i p t i o n of the s t r u c t u r e of these compounds. A n a l y s i s of these data suggests that the presence of mesogenic groups d i r e c t l y linked with the main chain (Figure 1), as a r u l e , does not r e s u l t i n such polymers manifesting l i q u i d c r y s t a l l i n e p r o p e r t i e s . I f some monomers do i n f a c t e x h i b i t l i q u i d c r y s t a l p r o p e r t i e s , the polymers obtained on t h e i r basis are r i g i d substances f e a t u r i n g high s o f t e n i n g points w i -
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
35
thout d i s p l a y i n g truly l i q u i d c r y s t a l l i n e properties i n accordance with our d e f i n i t i o n . Only some works O t 1 0 t 1 5 - 1 B ) provide evidence that the synthesized polymers possess the property of b i r e f r i n g e n c e , however, no d e t a i l e d s t r u c t u r a l and thermodynamic c h a r a c t e r i s t i c s of these polymers are given. In our opinion, such a macromolecular s t r u c t u r e i n which the mesogenic groups are d i r e c t l y l i n k e d with the main polymer chain renders d i f f i c u l t the packing t y p i c a l for low-molecular l i q u i d c r y s t a l s . To obtain an l i q u i d c r y s t a l l i n e s t r u c t u r e , a c e r t a i n l a b i l i t y of branches i s required, which would, despite the presence of the polymer chain, ensure a p a r t i c u l a r order i n g i n the arrangement of mesogenic groups. To overcome the s t e r i c hindrances o c c u r r i n g i n the packing of branches, one should space the mesogenic groups a c e r t a i n distance apart from the main chain, or somehow enhance i t s f l e x i b i l i t y . In c o n s i d e r i n g various approaches t o c r e a t i n g thermotropic c h o l e s t e r o l - c o n t a i n i n g polymers ( 1 1 , 1 9 2 0 ) , we proceeded from the derived notions on fïïe r e l a t i o n s h i p between the s t r u c t u r e and p r o p e r t i e s of comb-like polymers with long a l i p h a t i c branches i n each monomer u n i t ( 2 1 ) » The independent behaviour of the side chains of comb-like polymers, manifesting i t s e l f i n t h e i r a b i l i t y to form layered s t r u c t u r e s and even t o c r y s t a l l i z e , r e g a r d l e s s of the main c h a i n s c o n f i g u r a t i o n ( 2 1 ) , opens up p o s s i b i l i t i e s to obtain l i q u i d c r y s t a l l i n e comb-like polymers. Indeed, since the chemical bonding of asymmetric side pendants i n such polymers takes place only through the end groups of branches, the l a t t e r may be regarded as a s t r u c t u r a l l y organized arrangement of long-chain molecules, b u i l t around the main chain, which seems to be one of the p r e r e q u i s i t e s f o r a l i q u i d c r y s t a l l i n e s t r u c t u r e . I t could, therefore, be assumed that i f one would add, to the end groups of comb-type polymer branches, g r o ups capable of forming the l i q u i d c r y s t a l l i n e phase, i . e . space them a c e r t a i n distance apart from the main chain, the s t e r i c hindrances imposed by the main chain on the packing of branches would be much l e s s s i g n i ficant. As mesogenic groups we have s e l e c t e d c h o l e s t e r o l d e r i v a t i v e s having, i n the case of low-molecular l i quid c r y s t a l s , most extensive a p p l i c a t i o n . To t h i s end, we have synthesized c h o l e s t e r o l e s t e r s of N-met h a c r y l o y l - GO -aminocarbonic acids (ChMAA-n) having d i f f e r e n t lengths of the a l i p h a t i c r a d i c a l (index η corresponds to the number of methylene groups i n the a l k y l r a d i c a l l i n k i n g c h o l e s t e r o l to the main c h a i n ) , as follows? 1
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
36
MESOMORPHIC
ORDER I N
POLYMERS
where η « 2,5,6,8,10,11. The f i r s t stage y i e l d e d W-methacryloyl-^-aminocarbonic acids (MAA-n) whose subsequent r e a c t i o n with c h o l e s t e r o l y i e l d e d monomers of ChMAA-n. Prom these monomers, homopolymers (PChMAA-n) and copolymers with n - a l k y l a c r y l a t e s (A-m) and n-alkylmethacrylates (MA-m) were obtained by r a d i c a l polymerization (index m c o r responds t o the number of carbons i n the n - a l k y l r a d i c a l ) . In a d d i t i o n , i n order to e l u c i d a t e some of the problems r e l a t i n g to the s t r u c t u r e of l i q u i d c r y s t a l l i ne polymer compounds, monomers were s p e c i a l l y synthe s i z e d and polymers were obtained containing methyl (PMMAA), benzyl (PBMAA) and hexadecyf(PHMMA) groups i n the side chain instead of c h o l e s t e r o l : \\2Î-C{CH )-C0NH-(CH )f^C0O-Jfl, where R « -CH^ f o r η « 2,5,6,8,10,11; 5
-CH
z
(PMMAA-n) f o r n m I I (PBMAA-II);
2
-(CH ) -CH 2
15
3
f o r η « I I (PHMAA-II)
I t should be noted that Ref. (22)describes syn t h e s i s of c h o l e s t e r o l esters of poly^SF-acryloyl- <^> aminocarbonic acids through a d d i t i o n of c h o l e s t e r o l to the macromolecules of p o l y - N - a c r y l o y l - co -aminocar-
SHIBAEV E T A L .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
Thermotropic Liquid Crystalline Polymers
37
bonic a c i d s . However, the r e s u l t i n g polymers contained about 10 mol. % of carboxyls and d i d not e x h i b i t any l i q u i d c r y s t a l l i n e p r o p e r t i e s . An i n t e r e s t i n g example of synthesis of comb-like polycholesteryl-II-methacryloyloxyundecanoate has been described by Imoto et a l . (23), however, apart from the study of the phase state olf"the monomer, no data are provided on the l i q u i d c r y s t a l l i n e s t r u c t u r e of the polymer. This work i s , therefore, aimed a t : (1) developing methods f o r obtaining c h o l e s t e r o l containing monomers, as w e l l as polymers with d i f f e rent length of the side chain and frequency of occur rence of c h o l e s t e r o l groups; (2) studying the physicochemical behaviour of mo nomers of ChMAA-n and c h o l e s t e r o l - c o n t a i n i n g polymers i n the s o l i d phase, as w e l l as d e f i n i n g the conditions under which the polymers and copolymers under conside r a t i o n acquire the l i q u i d c r y s t a l l i n e s t r u c t u r e . Experimental Synthesis of Monomers N-me thacry l o y 1ω-amino-* carbonic acids (MAA-n; were obtained as f o l l o w s Added to a s o l u t i o n of 0.08 M of <^-aminocarbon i c a c i d i n a 0.6 η s o l u t i o n of HaOH (600 ml), with s t i r r i n g , were 0.25 M of c h l o r i d e of methacrylic a c i d . A f t e r 3 hours of s t i r r i n g , the r e a c t i o n mixture was a c i d i f i e d with HC1 up to pH e 3. The p r e c i p i t a t e was f i l t e r e d , d i s s o l v e d i n chloroform and washed, succes s i v e l y , with a s o l u t i o n of Ha C0^, a weak s o l u t i o n of HC1 (pH = 2-3), and water. A f t e r the s o l u t i o n had been d r i e d over MgSO,, i t was evaporated, and the mo nomer was r e c r y s t a l l l z e d from the acetone-petroleum e i t h e r mixture. The y i e l d was 70%. The melting points of MAA-n are given i n Table I. p
Table I. M e l t i n g points of H-methacryloy1- cû aminocarbonic a c i d s (MAA-n) :
η T, m
°C
• •
(-1°)
2
:
5
• 67
53
; 4
:
6
:
β
i io
•
43
: 11 •
48
68
72
C h o l e s t e r o l e s t e r s of N-me thacry loy 1-^-aminocar bonic acids (ChMAA-n) were obtained as f o l l o w s (20): 0.02 M of MAA-n, 0.02 M of c h o l e s t e r o l and ÏÏT05 g of p-toluene s u l f o n i c a c i d were d i s s o l v e d i n 200 ml of absolute benzene. The mixture was heated to the b o i l i n g point and the azeotropic mixture of benzene with the r e s u l t i n g water was d i s t i l l e d o f f . 5 hours l a t e r , the r e a c t i o n mixture was cooled down to room
MESOMORPHIC
38
ORDER I N P O L Y M E R S
temperature„ and the monomer was p r e c i p i t a t e d with methanol* The monomer was freed of c h o l e s t e r o l on a column with AlpO~. The y i e l d was 40%. The r e s u l t s of the elemental chemical a n a l y s i s of the monomers of ChMAA-n are l i s t e d i n Table I I . Table I I . Elemental analyses of ChMAA-n monomers C, %
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
11
2 5 6 8 10 11
jfound c a l c u l a t e d 77.46 78.48 78.42 78.85 79.40 79.52
77.66 78.25 78.43 78.76 79.06 79.20
:
H
• found 10.61 10.49 11.10 10. 58 10.95 11.51
>
%
%
calcula-feui found calcula' ted • 10.54 10.82 10.91 11.07 11.21 11.28
•
2.52 2.55 2.36 2.41 2.12 2.12
2.66 2.47 2.41 2.30 2.19 2.15
Cholesterylmethacrylate (ChMA) were obtained by way of i n t e r a c t i o n of c h l o r i d e of methacrylic acid with c h o l e s t e r o l i n absolute ether. The melting point of the end product was 109°C. which c o i n c i d e s with that of ChMA obtained i n (24)* Hexadecyl and benzyl ethers of MAA-II (HMAA-II and BMAA-II, r e s p e c t i v e l y ) were obtained i n a manner s i m i l a r to the synthesis of ChMAA-n. The melting point of HMAA-II was 58°C and that of BMAA-II, 5 4 ° C Synthesis of Polymers PMAA-n was obtained by radical polymerization of MAA-n i n a s o l u t i o n of d i methyl formamide at 60°C, using d i n i t r i l e of azoisobut y r i c acid (BAA) or UV i r r a d i a t i o n at room temperature i n an argon atmosphere. The obtained polymers were p r e c i p i t a t e d with acetone. Methyl ethers of PMAA-n (PMMAA-n) were obtained by t r e a t i n g PMAA-n with diazomethane i n benzene. ChMAA-n, ChMA, HMAA-II and BMAA-II were polymerized i n a benzene s o l u t i o n at 60°C i n the presence of DAA. The polymers were p r e c i p i t a t e d with acetone.Melt polymerization was conducted on the hot stage of a p o l a r i z i n g microscope, i n a s p e c i a l c e l l between cover glasses. The copolymers of ChMAA-n with A-m and MA-m were obtained by polymerization i n benzene i n the presence of DAA. The composition of the copolymers was determined by the r a t i o of o p t i c a l d e n s i t i e s of the 1.740 cm-1 (C=sO modes i n an e s t e r group) and 1.650cm"^ (C»0 modes i n an amide group) absorption bands. The r e s u l t s of t u r b i d i m e t r i c t i t r a t i o n of polymers i n benzene i n d i c a t e that the copolymers are homogeneous i n
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
39
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
composition. I n v e s t i g a t i o n Techniques O p t i c a l studies were conducted i n the crossed p o l a r i z e r s of a MIN-8 p o l a r i z i n g microscope with a hot stage. P i c t u r e s were taken by means of a " Z e n i t ^ M " camera mounted on the microscope tube v i a a micro attachment. The X-ray patterns were obtained on a URS-55 X-ray apparatus with a f l a t cassette ( i r r a d i a t i o n with CuK^ ). Small-angle X-ray patterns were obtained on a s p e c i a l l y designed camera with a temperature attachment. The sample f i l m d i s t a n ce was adjusted w i t h i n 90 to 130 mm. Thermographic studies were conducted on a "Dérivâtograph" instrument (Hungary). The glass and f u s i o n temperatures were determined from thermomechanical curves derived on a Kargin balance. The IR absorption spectra were obtained on a UR-10 spectrophotometer from samples made i n the form of f i l m s or p e l l e t s with KBr. The t u r b i d i m e t r i c t i t r a t i o n of copolymer s o l u t i o n s was conducted with methan o l . The o p t i c a l density was measured on a FEK-I phot o e l e c t r i c colorimeter. Results and Discussion Monomers Consider f i r s t the behaviour of synthesized monomers of the ChMAA-n s e r i e s at varying temperatures, as w e l l as the e f f e c t of c r y s t a l l i z a t i on c o n d i t i o n s on t h e i r s t r u c t u r e . The most a c t i v e monomers are ChMA and ChMAA-2 which polymerize r a p i d l y at elevated temperatures. For example, melting of ChMAA-2 at 125°C causes the monomer to polymerize immediately, which i s i n d i c a t e d by a sharp exothermal peak, on the thermogram of ChMAA-2, f o l l o w i n g the endothermal melting peak ( F i gure 2). In the case of melting of ChMAA-5,6,8,10 monomers, an i s o t r o p i c l i q u i d i s formed at temperatures of 108, 98, 85 and 84°C, r e s p e c t i v e l y . Rapid c o o l i n g of these melts r e s u l t s i n the formation of l i q u i d c r y s t a l l i n e phase whose temperature existence i n t e r v a l depends on the rate of c o d l i n g . At slow c o o l i n g , e i t h e r growth of s o l i d c r y s t a l s (ChMAA-10) or polymerization of monomers (ChMAA-5,6,8) i s observed. ChMAA-II forms two c r y s t a l l i n e m o d i f i c a t i o n s d i f f e r i n g i n t h e i r melting points and s t r u c t u r a l paramet e r s (Table I I I ) .
40
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
MESOMORPHIC
ORDER
IN POLYMERS
Figure 1. Structure of macromolecules with side meso genic groups, (a) Mesogenic groups are directly linked to the main chain; (b) mesogenic groups are linked to the branches of comb-like poly mers.
125 X
Figure 2.
Thermogram of ChMAA-2
80
1 {00
1 120
ι #0
1— 160
4.
SHIBAEV E T A L .
41
Thermotropic Liquid Crystalline Polymers
Table I I I . Interplanar spacings and melting points of ChMAA-II samples i n various c r y s t a l l i n e m o d i f i c a t i o n s Modification*
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
I II
I n t e r p l a n a r spacings, A 4.67 3.72
5.10 4.90
5.95 5.30
7.70 7.70
36.0 36.0
*T
°C 84 102
M o d i f i c a t i o n I i s formed i n the case of c r y s t a l l i z a t i o n from a s o l u t i o n and, at 84°C, changes to mod i f i c a t i o n I I which melts i n t o an i s o t r o p i c l i q u i d at 102°C. Cooling of the ChMAA-II melt leads to the f o r mation of an l i q u i d c r y s t a l l i n e phase which, a f t e r f u r t h e r c o o l i n g , converts to c r y s t a l l i n e m o d i f i c a t i o n I I . Sequential r e c o r d i n g of t h i s process by a camera shows how c r y s t a l l i n e s p h e r u l i t e s grow i n the flowing l i q u i d c r y s t a l l i n e phase (Figure 3a) and gradually f i l l the e n t i r e f i e l d of v i s i o n of the microscope ( F i gure 3b-d). Thus, a l l monomers of the ChMAA-n s e r i e s form a monotropic l i q u i d c r y s t a l l i n e phase of the c h o l e s t e r i c type, whose temperature i n t e r v a l of existence depends on the r a t e of c o o l i n g . The l i q u i d c r y s t a l l i n e phase i s unstable and i s transformed to c r y s t a l phase so soon that X-ray examination of the mesophase s t r u c t u r e becomes d i f f i c u l t . Nevertheless, p o l a r i z a t i o n - o p t i c a l studies have made i t p o s s i b l e to draw c e r t a i n conclusions as to the nature of the l i q u i d c r y s t a l l i n e phase of monomers. Cooling of i s o t r o p i c melts of monomers r e s u l t s i n a c o n f o c a l texture which turns to a planar one when a mechanical f i e l d i s superimposed on the sample, f o r example, by s h i f t i n g a cover g l a s s i n the c e l l of the p o l a r i z i n g microscope (Figure 4 ) . The observed planar texture e x h i b i t s the property of s e l e c t i v e l i g h t r e f l e c t i o n , which i s t y p i c a l of low-molecular cholesteric liquid crystals. Polymers and Copolymers Polymerization of a l l ChMAA-ns and ChMA i n a melt y i e l d s polymers f e a t u r i n g spontaneous o p t i c a l anisotropy. The o p t i c a l pattern observed i n crossed p o l a r i z e r s i s s i m i l a r to the conf o c a l texture of low-molecular l i q u i d c r y s t a l s and r e presents a combination of biréfringent regions 2 to 10 microns i s s i z e (Figure 5). At the same time, the presence of a d i f f u s e halo at wide angles of X-ray s c a t t e r i n g (Figure 5 ) i n the case of polymers of the PChMAA-n s e r i e s , does not give reason enough to a s c r i be c r y s t a l l i n e s t r u c t u r e to them. Consider now the manner i n which the p h y s i c a l state of a polymer and the o p t i c a l pattern vary with f
Figure 3. Optical microphoto graphs showing sequential growth of solid spherulites (b,c,d) from liquid crystalline phase of ChMAA-II (a) (crossed polarizers)
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
Ο
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Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
43
Figure 4. Optical microphoto graph of the planar texture ChMAA-II (crossed polarizers)
Figure 5. Optical microphoto graph of a PChMAA-II film at 20°C. The x-ray pattern of the PChMAA-II film, taken at 20° C, can be seen in the lower right corner.
MESOMORPHIC
44
temperature
ORDER I N P O L Y M E R S
(Table I V ) .
Table IV. Glass temperatures (T ), flow temperatures (T~) and temperatures of t r a n s i t i o n of from a n i s o t r o p i c t o i s o t r o p i c state (Α ) f o r PChMAA-n g
±
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
Polymer
V
PChMA PChMAA-2 PChMAA-5 PChMAA-6 PChMAA-8 PChMAA-10 PChMAA-11
°
>200 185 130 130 130 125 120
c
T
°C
f f
_
_
-
—
200 190 180 150 135
220 215 200 185 180
As can be seen from t h i s t a b l e , PChMAA-2 has the highest g l a s s temperature (Tg) at which the polymer s t a r t s to thermally decompose. The observed o p t i c a l pattern remain i n v a r i a b l e up t o t h i s temperature. As the methylene bridge l i n k i n g c h o l e s t e r o l t o the main chain becomes longer, the values of T~ become lower, and, i n the case of polymers of the PçhMAA-η s e r i e s with η > 5, an e l a s t i c and viscous state may p r e v a i l . I t should be emphasized that, i n the viscous state r e gion, polymers of the PChMAA-n s e r i e s are viscous f l u i d s whose flow i s accompanied by a displacement of the biréfringent regions i n a way s i m i l a r to the behav i o r of low-molecular l i q u i d c r y s t a l s . As can be i n f e r r e d from Table IV, the o p t i c a l anisotropy remains i n the e l a s t i c and v i s c o u s s t a t e s and disappears a t temperature T ^ T r a n s i t i o n from the o p t i c a l l y a n i s o t r o p i c t o an o p t i c a l l y i s o t r o p i c s t a t e at T _^ i s reverse and occurs i n a narrow temperature range (2 to 3 ° ) · Table IV shows that the values of t r a n s i t i o n temperatures T _^ s l i g h t l y decrease with i n c r e a s i n g n, which i s probably due to the e f f e c t of i n t e r n a l p l a s t i f i c a t i o n and was f r e q u e n t l y observed i n the case o f comb-like polymers ( 2 1 ) . The e v a l u a t i o n of the t h e r mal e f f e c t inherent Tii t h i s t r a n s i t i o n f o r PChMAA-II gave the value of 0.76Î0.08 c a l / g , which agrees w e l l with the values of heats of melting, corresponding t o the l i q u i d c r y s t a l l i n e phase-isotropic melt t r a n s i t i on f o r low-molecular l i q u i d c r y s t a l s (2J5)# However, i n contrast to the l a t t e r , which, as a r u l e , c r y s t a l l i z e during c o o l i n g (see, f o r example, Figure 3 / the l i q u i d c r y s t a l l i n e phase of polymers v i t r i f i e s while cooled (Figure 5)· I n other words, i n the case of polymers , the s t r u c t u r e of the l i q u i d c r y s t a l l i n e phase a
0
a
a
f
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
45
remains i n the glass state as w e l l , whereas polymers of the PChMAA-n s e r i e s e x h i b i t l i q u i d c r y s t a l l i n e pro p e r t i e s i n a l l three p h y s i c a l s t a t e s : v i t r i f i e d , e l a s t i c and viscous. The upper l i m i t temperature range of the l i q u i d c r y s t a l l i n e state i s ¥ „Α. What i s i t then that makes the l i q u i d c r y s t a l l i n e state p o s s i b l e i n the polymers under i n v e s t i g a t i o n ? To answer t h i s ques t i o n , l e t us analyze the X-ray study r e s u l t s . In the wide angles of X-ray s c a t t e r i n g there i s a d i f f u s e maximum dj whose magnitude, i n the case of ηs*5, does not depend on the branch length (Figure 6 ) . In the region of small angles, three maxima are obser ved whose p o s i t i o n changes with the number of the me thylene groups l i n k i n g c h o l e s t e r o l to the main chain (Figure b ) . To interprète these X-ray spacings, l e t us f i r s t examine the s t r u c t u r e of model compounds: PMAA-n and PMMAA-n. The macromolecules of these polymers have a chemical structure s i m i l a r to that of PChMAA-n with the d i f f e r e n c e that they do not contain c h o l e s t e r o l groups i n the side chains. The X-ray patterns of PMAA-n and PMMAA-n i n the region of large s c a t t e r i n g angles a l s o d i s p l a y a d i f fuse maximum corresponding to i n t e r p l a n a r spacings: d j = 4·6 to 4#7 A , while i n the small-angle region a s i n g l e X-ray spacings i s observed, whose p o s i t i o n depends on the branch length (Figure 6). The X-ray spacings i n the wide angle region i s s i m i l a r t o that observed on the X-ray patterns of amorphous p o l y - n - a l k y l a c r y l a t e s and poly-n-alkylmethacrylates, where i t i s r e l a t e d to the side groups i n t e r a c t i o n (21). E v i d e n t l y , both i n the case of PMAA-n,"TiiMAA-n and PChMAA-n, the X-ray spacings i n the wide angle region may be a t t r i b u t e d to the distance between the branches arranged i n p a r a l l e l . This assumption i s substantiated by the f a c t that, i n the case of u n i a x i a l o r i e n t a t i o n of polymers, the i n t e n s i t y of t h i s X-ray i n t e r f e r e n c e i n a meridional d i r e c t i o n sharply increases (Figure 7 ) . The value of d j i s 6.3 A f o r PChMA and PChMAA-2, while f o r the other polymers of the PChMAA-n s e r i e s i t i s 5.9 A These values are s l i g h t l y greater than the respective i n t e r p l a n a r spacings f o r PMAA-n and PMMAA-n (which, i n our opinion, i s due to the presence of bulky c h o l e s t e r o l groups) and are close t o the distances between the molecules of low-molecular c h o l e s t e r o l esters i n the c h o l e s t e r i c mesophase (26). The reason why the values of d d i f f e r i n PChMA and PChMAA-2, on the one hand, and i n the other PChMAA-ns, on the other, w i l l be considered below when we s h a l l discuss a polymer s t r u c t u r e model.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
Β
#
T x
46
MESOMORPHIC
ORDER
IN POLYMERS
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
0
Figure 6. Interplanar spacings vs. the number of carbons in the side chain for PChMAA-n (dj-dj, PMAA-n (d/, d,') and PMMAA-n (d/', d "). The experimental points on the ordinate correspond to the interplanar spacings for polycholesterylmethacrylate. 9
Figure 7. X-ray pattern of an oriented PChMAA-II sample. The primary beam extends at a right angle to thefiberaxis.
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
As to small-angle X-ray spacings, the dependence of spacings d (PMAA-n, PMMAA-n, PChMAA-n) and (PChMAA-n) on the branch length at n > 5 suggests that they are due to the layered ordering of the side gro ups. This i s corroborated by the X-ray texture pat terns o f oriented polymer samples. In the case of u n i a x i a l o r i e n t a t i o n , these X-ray i n t e r f e r e n c e s look l i k e s a g i t t a l arcs (Figure 7), which i s i n d i c a t i v e of the branches being arranged s u b s t a n t i a l l y at a r i g h t ang l e to the main chain a x i s . R e f r a i n i n g , at t h i s point, from f i n a l and unambiguous i n t e r p r e t a t i o n of X-ray spacings d^, we s h a l l only note that i t s magnitude i s l i t t l e dependent (and at η » 8,10 and 11, not depen dent at a l l ) on the branch length, a t t r i b u t i n g t h i s i n t e r f e r e n c e to the s c a t t e r i n g from the main chain and assuming that i t i s responsible f o r a c e r t a i n pe r i o d i c i t y associated with i t s folded s t r u c t u r e , i f such occurs. On the basis of the obtained X-ray data, the s t r u c t u r e of c h o l e s t e r o l - c o n t a i n i n g polymers of the PChMAA-n s e r i e s may be presented as f o l l o w s . F i r s t of a l l , i t i s evident that the s t r u c t u r e of PChMA and PChMAA-2 d i f f e r s from that of PChMAA-n where n>5. This d i f f e r e n c e manifests i t s e l f both i n the values of d j and the dependence of small-angle X-ray spacings ά , do, and àA on n. In the case of polymers of tKe PChMAA-n s e r i e s , where η > 5 , d j 5 9 L This value approaches that f o r the c h o l e s t e r i c mesophase o f cholesterylnonanoate (6.05 A) and c h o l e s t e r y l m y r i s t a t e (5.73 1) (26). The r e f o r e , the packing of branches i n PChMAA-n seems t o be s i m i l a r to that of molecules o f the above-mentio ned low-molecular c h o l e s t e r o l e s t e r s i n the c h o l e s t e r i c mesophase. Wendorff and P r i c e (26) who had s t u d i ed the mesophase s t r u c t u r e of these compounds sugges ted that the packing of molecules i n the c h o l e s t e r i c mesophase must be a n t i p a r a l l e l so that a c h o l e s t e r o l group i s surrounded by methylene " t a i l s * , while the l a t t e r are surrounded by c h o l e s t e r o l groups. This i s confirmed by the f a c t that the l a t e r a l s i z e of a cho l e s t e r o l group exceeds 6 1, therefore, i n the case of p a r a l l e l arrangement of molecules, d j would be greater than 6 X From the same considerations, i t can be as sumed that the packing of side chains i n polymers o f the PChMAA-n s e r i e s , where η > 5 , must be a n t i p a r a l l e l so that the c h o l e s t e r o l groups of one macromolecule are surrounded by the methylene chains of adjacent raacromolecules (Figure 8a-c). I n t h i s case, the bran ches are arranged at a r i g h t angle to the main chain and almost i n p a r a l l e l to one another, however, the long axes of the c h o l e s t e r o l - c o n t a i n i n g branches may 2
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
47
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American
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1155 18th St., N.W. Washington, D £ 20036
f
Figure 8. Modeh of PChMAA-II (a) and PChMA(a') macromolecules and packing of PChMAA-n macromolecules in oriented samples (b,b',c,c') ûi η ^ 5 (b,c) and η < 5 (b',c'); b, b —solid lines indicate macromolecules lying in the drawing plane, broken lines indicate mac romolecules lying in a plane parallel to that of the drawing; c,c'—projection along the main chain of macromolecules.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
49
extend at a c e r t a i n angle t o one another. As can be seen from Figure 8a-c, the p r o p o s e d tural
m o d e l may o c c u r o n l y
length on the sequence of methylene u n i t s permits the " r i g i d " c h o l e s t e r o l skeleton to extend along the methylene chain. C a l c u l a t i o n s and studies u s i n g molecul a r models suggest that the length of a methylene "bridge" must be at l e a s t 10-11 A . In the case of polymers of the PChMAA-n s e r i e s , t h i s i s true at n ^ 5 . At lower values of n, the c h o l e s t e r o l group cannot extend along the methylene chain, therefore, p a r a l l e l packing of branches takes place (Figure 8a -c )# This r e s u l t s i n d j i n c r e a s i n g to 6.3 A, which i s close t o a respective value f o r c h o l e s t e r y l a c e t a t e (6.48 A) and f o r which no a n t i p a r a l l e l packing i s p o s s i b l e e i t h e r . Another reason why i t i s d i f f i c u l t t o discuss the s t r u c t u r a l model of c h o l e s t e r o l - c o n t a i n i n g polymers i s the f a c t that so f a r no adequately substantiated s t r u c t u r a l model of low-molecular c h o l e s t e r i c l i q u i d c r y s t a l s e x i s t s , apart from the already mentioned packing pattern proposed i n Ref. (26). I t seems that f o r a f u l l understanding of the mesophase structure of c h o l e s t e r o l e s t e r s , one should, f i r s t of a l l , caref u l l y study t h e i r c r y s t a l l i n e s t r u c t u r e . Such a study, described i n Refs. (27. 28), has not yet lead to a complete d e s c r i p t i o n of molecular packing i n c h o l e s t e r o l esters. Although the structure of c h o l e s t e r o l - c o n t a i n i n g polymers i s not yet completely understood, there i s no doubt that the l i q u i d c r y s t a l l i n e p r o p e r t i e s e x h i bited by these polymers are due t o a p a r t i c u l a r order i n the arrangement of c h o l e s t e r o l groups. This i s c o r roborated by the r e s u l t s of studying the structure and o p t i c a l p r o p e r t i e s of model compounds not containing c h o l e s t e r o l . The above-mentioned PMAA-n and PMMAA-n are o p t i c a l l y i s o t r o p i c i n a l l three p h y s i c a l s t a t e s : v i t r i f i e d , e l a s t i c and viscous. To e l u c i d a t e the r o l e of c h o l e s t e r o l i n the f o r mation of a l i q u i d c r y s t a l l i n e s t r u c t u r e , consider now the structure and properties of model PBMAA-II and PHMAA-II polymers whose macromolecules lack c h o l e s t e r o l groups. At room temperature, PBMAA i s i n e l a s t i c state and e x h i b i t s no o p t i c a l anisotropy. On the X-ray pattern of t h i s polymer i n the small-angle region, one can see a r e f l e x corresponding to an i n t e r p l a n a r spac i n g of 28 A S u b s t i t u t i o n of the hexadecyl r a d i c a l f o r c h o l e s t e r o l r e s u l t s i n a c r y s t a l l i n e PHMAA-II polymer with a melting point of 40°C. The polymer i s c r y s t a l l i z e d i n a hexagonal c e l l , which i s i n d i c a t e d by an i n t e n s i v e X-ray i n t e r f e r e n c e at wide angles, corresponding t o an i n t e r p l a n a r spacing of 4*19 A. 1
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
struc-
i n such cases i nwhich t h e
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M E S O M O R P H I C ORDER I N P O L Y M E R S
50
In the region of small angles, three X-ray i n t e r f e r e n ces are observed, corresponding t o i n t e r p l a n a r spa cings d = 14.5 A , do « 21.6 A, and d* = 38 A Above the melting p o i n t , the i n t e r f e r e n c e i n the wide angle region becomes an amophous halo corresponding t o an i n t e r p l a n a r spacing of 4 · 6 A while i n the small-ang l e region there remains a s i n g l e i n t e r f e r e n c e (dg « 36 1) corresponding i n magnitude to the branch length. At temperatures above the melting p o i n t , the polymer i s optically isotropic. The foregoing data i n d i c a t e that i n PMAA-n, PMMAA-n, PBMAA and ΡΗΜΑA-11, the branches are a r r a n ged i n a layered manner. However, the layered orde r i n g alone i s not s u f f i c i e n t f o r manifestation of op t i c a l anisotropy. A l i q u i d c r y s t a l l i n e state may oc cur only i n the presence of groups capable of produ c i n g mesomorphous s t r u c t u r e s . I n c h o l e s t e r o l - c o n t a i ning polymers, i t i s p r e c i s e l y the c h o l e s t e r o l groups that perform t h i s f u n c t i o n , t h e i r mutual packing being responsible f o r o p t i c a l anisotropy. We have already considered the s t r u c t u r e and pro p e r t i e s of polymers c o n t a i n i n g c h o l e s t e r o l i n such monomer u n i t . I t i s a l s o of i n t e r e s t to examine the s t r u c t u r e and p r o p e r t i e s of polymers i n which the con tent of c h o l e s t e r o l groups can be v a r i e d , i . e . , copo lymers. Given i n Table V are t r a n s i t i o n temperatures f o r some copolymers that we have synthesized. Table Τ. Τ , T and T . of copolymers of ChMAA-n with a l k y l a c r y l a t e s (A-m) and alkylmethac r y l a t e s (MA-m) 2
#
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
f
f
&
• Copolymer (mol.%, ChMAA--n) i ChMAA-II 42 37 17 ChMAA-II 90 67 40 ChMAA-II 75 58 25 ChMAA-II 45 ChMAA-II 75
T , °C f
i V i > °
C
with A-4 65 60 < 20
115 100 60
160 140 100
115 105 85
140 140 135
180 170 160
90 70 < 20
140 120 90
45
70
70
110
with MA-4
with MA-10 180 170 no anisotropy
with A-16 100
with MA-22 no anisotropy
4.
SHIBAEV E T A L .
50 ChMAA-6 with A-4 45 30
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
51
Thermotropic Liquid Crystalline Polymers
40
80
no anisotropy
100 70
170 105
180 115
As can be seen from t h i s t a b l e , the i n t r o d u c t i o n of a second component, namely, a "solvent" of c h o l e s t e r o l - c o n t a i n i n g u n i t s , whereby the glass temperature i s lowered, r e s u l t e d , i n some cases, i n a longer i n t e r v a l of existence of the e l a s t i c and viscous l i q u i d c r y s t a l l i n e s t a t e s , while i n other cases, i n the f o r mation of o p t i c a l l y i s o t r o p i c polymers* Copolymers of ChMAA-II with b u t y l a c r y l a t e (A-4) and butylmethacrylate (MA-4) form a l i q u i d c r y s t a l l i ne phase i n a wide range of component r a t i o s (Table V and Figure 9a,b). For example, a copolymer containing more than 80% of A-4 can s t i l l e x i s t i n the l i q u i d c r y s t a l l i n e s t a t e . At the same time, a copolymer with MA-22 whose content i s only 25%, i s amorphous and opt i c a l l y i s o t r o p i c . As had been mentioned above, the o p t i c a l anisotropy i n c h o l e s t e r o l - c o n t a i n i n g polymers i s due to a p a r t i c u l a r ordering i n the arrangement of the c h o l e s t e r o l groups. I f , i n copolymers, the second component does not hinder packing of the c h o l e s t e r o l groups, a mesophase may be formed. In copolymers with MA-22, the long branches screen the c h o l e s t e r o l groups, and no o p t i c a l anisotropy i s manifest i n any of the three p h y s i c a l states of polymers. It should be noted that a copolymer with an e q u i molar r a t i o of ChMAA-II and MA-22 u n i t s i s amorphous, too (Table V), although the PMA-22 homopolymer e a s i l y c r y s t a l l i z e s , forming a hexagonal l a t t i c e t y p i c a l of comb-like polymers (29)* As was shown e a r l i e r (21), the i n t r o d u c t i o n of such " d i s t u r b e r s " as i s o p r o p y l a c r y l a t e i n an amount of 80% i n t o c r y s t a l l i n e comb-like polymers does not destroy the c r y s t a l l a t t i c e . In our case, the i n t r o d u c t i o n of only 50% of ChMAA-II i n t o PMA-22 prevents the l a t t e r from c r y s t a l l i z i n g . This copolymer provides a s t r i k i n g example of the impossib i l i t y of a structure c h a r a c t e r i s t i c of each of the homopolymers i n the case of copolymerization of monomers with bulky groups. Thus, copolymers of c h o l e s t e r o l - c o n t a i n i n g monomers with n - a l k y l a c r y l a t e s and n-alkyImethacrylates are capable of y i e l d i n g a l i q u i d c r y s t a l l i n e phase i n cases where the length of the a l k y l chain i n a l k y l a c r y l a t e or alkylmethacrylate does not exceed that of the methylene bridge l i n k i n g c h o l e s t e r o l with the main chain. An X-ray study of copolymers has shown that the i n t r o d u c t i o n of a second component i n t o c h o l e s t e r o l -
52
M E S O M O R P H I C ORDER I N P O L Y M E R S
c o n t a i n i n g polymers r e s u l t s i n a c e r t a i n change i n the l e t t e r ' s s t r u c t u r e . In the small-angle region, only one d i f f r a c t i o n maximum i s retained f o r a l l copolymers ChMAA-II, corresponding to i n t e r p l a n a r spac i n g d « 19*8 A which c o i n c i d e s with d f o r the PChMAA-II homopolymer. In the case of u n i a x i a l o r i e n t a t i o n , t h i s X-ray i n t e r f e r e n c e takes the form of s a g i t t a l arcs. Since copolymers of ChMAA-II are o p t i c a l l y a n i s o t r o p i c , one n a t u r a l l y assumes that i t i s prec i s e l y the high degree of ordering i n the arrangement of the side methylene chains, manifesting i t s e l f i n the occurrence of i n t e r f e r e n c e d , that permits such packing of c h o l e s t e r o l groups, which ensures the opt i c a l anisotropy i n the above polymers. This i s supported by the r e s u l t s of X-ray studies of copolymers at various temperatures. Figure 10 shows that the i n t e n s i t y of the small-angle X-ray spacings remains high i n the viscous state as w e l l when there i s an o p t i c a l anisotropy (Figures 9a,b), and sharply decreases dur i n g t r a n s i t i o n to an o p t i c a l l y i s o t r o p i c state ( F i gure 10, curve 3 ) . The r e s u l t s of studying the s t r u c t u r e and propert i e s of copolymers demonstrate that proper s e l e c t i o n of components f o r copolymerization permits obtaining a wide v a r i e t y of polymers forming a mesophase i n d i f f e r e n t temperature ranges. As f a r as the type of the l i q u i d c r y s t a l l i n e phase formed i n c h o l e s t e r o l - c o n t a i n i n g polymers i s concerned, the morphological s i m i l a r i t y of the t e x t u re appearing i n polymer f i l m s with the confocal texture of c h o l e s t e r i c l i q u i d c r y s t a l l i n e compounds seems t o suggest that we deal with a c h o l e s t e r i c type of l i q u i d c r y s t a l s . However, the planar texture t y p i c a l of c h o l e s t e r i c l i q u i d c r y s t a l s , which i s e s s e n t i a l l y a s i n g l e c r y s t a l of the c h o l e s t e r i c type, does not occur i n f i l m s of these polymers, and when a cover g l a s s i s s h i f t e d , the texture formed i n the p o l a r i z i n g microscope c e l l i s deformed (Figure 9b). T h i s i s probably due to the defects introduced i n t o the r e s u l t i n g l i q u i d c r y s t a l l i n e s t r u c t u r e by the polymer main chain. On the other hand, the layered ordering i n the arrangement of branches i s i n d i c a t i v e of a smectic s t r u c t u r e , hence, the seeming s i m i l a r i t y of the texture of low-molecular l i q u i d c r y s t a l s of the c h o l e s t e r i c type to polymers of the PChMAA-n s e r i e s and t h e i r copolymers has, i n f a c t , nothing to do with the manner i n which s t r u c t u r a l elements of comb-like macromolecules are packed i n the c h o l e s t e r i c phase. Besides, the presence of the main polymer chain h i n ders the formation of a s p i r a l s t r u c t u r e t y p i c a l of low-molecular c h o l e s t e r i c l i q u i d c r y s t a l s . A l l t h i s 2
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
2
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
53
Figure 9. Optical microphoto graphs of a copolymer of ChMAA-II with A-4 (37:63), at 130°C without superposition of a mechanicalfield(a) and after shifting a cover glass in the microscope cell (b) (crossed polarizers)
Figure 10. X-ray scattering intensity vs. scattering angle for a copolymer of ChMAA-II with A-4 (37:63) at 25(1), 130 (2) and 150°C (3)
54
MESOMORPHIC
ORDER I N P O L Y M E R S
renders i t d i f f i c u l t to c l a s s the l i q u i d c r y s t a l l i n e phase of polymers with a d e f i n i t e type of l i q u i d c r y s t a l s . E v i d e n t l y , the c l a s s i f i c a t i o n of l i q u i d c r y s t a l s proposed f o r low -molecular compounds cannot be applied to polymers whose s t r u c t u r a l arrangement i s d i f f e r e n t from that of low-molecular l i q u i d c r y s t a l l i n e systems. We wish to acknowledge the valuable c o l l a b o r a t i on of I.V.Sochava whose group at the P h y s i c a l I n s t i tute of the Leningrad State U n i v e r s i t y has determined the melting heat of the l i q u i d c r y s t a l l i n e phase of PChMAA-II Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
9
Literature cited 1. De Viseer, A.C., De Groot, K., Feyen, J . , and Bant j e s , Α., J.Polymer Science, (1971), A-1, 9, 1893. 2. Bouligand, J . , C l a d i s , P., L i e b e r t , L., and S t r z e l e c k i , L., Mol.Cryst.Liq.Cryst., (1974) 25, 233. 3. Amerik, Yu.B., and K r e n t s e l ' , B.A., i n coll. "Uspek h i k h i m i i i fiziki polimerov" (Advances i n Che mistry and physics of Polymers), p. 97, "Khimiya" Publishers, Moscow, 1973. 4. Frenkel, S.Ja., J.Polymer S c i . (1974), C-44, 49. 5. Blumstein, Α., Blumstein, R., Murphy, G., Wilson, C., Billard, J . , "Liquid C r y s t a l s and Ordered F l u i d s " , p. 277, vol.2, Plenum Press, 1974. 6. Freidzon, Ya., Shibaev, V.P., and P l a t é , N.A., Abstracts of papers at the 3rd All-Union Conference on Liquid C r y s t a l s , p. 214, Ivanovo, 1974. 7. Shibaev, V.P., T a l r o s e , R. V., Karahanova, F.J., Haritonov, A.V., and P l a t é , N.A., Dokl.AN SSSR (1975), 225, 632. 8. R o v i e l l o , Α., and S i r i g u , Α., Polymer.Lett.Ed. (1975), 13, 455. 9. P e r p l i e s , Ε., Ringsdorf, Η., and Wendorff, J . , Makromolek.Chemie (1974), 175, 553. 10. Lorkowski, Η., and Reuther, F., Plaste und Kautschuk, (1976), 23, 81. 11. Shibaev, V.P., F r e i d z o n , Ya.S., and P l a t é , N.A., Dokl.AN SSSR (1976), 227, 1412. 12. Papkov, S.P., Vysokomolek.soed. (1977), A-19, 3. 13. Shibaev, V.P., and P l a t é, N.A., Vysokomolek.soed. (1977), A-19. 923. 14. Shibaev, V.P., Freidzon, Ya.S., P l a t é , N.A., Vysokomolek. soed., (1978), A-20, i n press. 15. K r e n t s e l ' , B.A., and Amerik, Yu.B., Vysokomolek. soed. (1971) A-13, 1358. 16. Blumstein, Α., Blumstein. R., Clough, S., and Hsu, Η., Macromolecules (1975), 8, 73.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
55
17. Clough, S., Blumstein, A., and Hsu, Ε., Macromole cules (1976), 9, 123. 18. Blumstein, Α., Clough, S., P a t e l , L., Blumstein, R., and Hsu, Η., Macromolecules (1976), 9, 243. 19. Shibaev, V.P., Freidzon, Ya.S., and P l a t é , N.A., Abstracts of papers at the 11th Mendeleev Congress on General and Applied Chemistry, p. 164, vol. 2, "Nauka" Publishers, (Moscow), 1975. 20. Shibaev, V.P., Freidzon, Ya.S., and P l a t é , N.A., USSR Inventor's C e r t i f i c a t e No. 525, 709, B y u l l . i z o b r e t e n i y (1976), No. 31. 21. P l a t é , N.A., and Shibaev, V.P., J.Polymer S c i . , Macromolec.Rev., (1974), 8, 117. 22. Kamogawa, H., J.Polymer S c i . (1972), B-10, 7. 23. Minezaki, S., Nakaya, T., and Imoto, Μ., Makromolek.Chemie, (1974), 175, 3017. 24. Saeki, H., Iimura, Κ., and Takeda, Μ., Polymer.J. (1972), 3, 414. 25. Kunihisa, K., and Hagiwara, S., Bull.Chem.Soc. Japan, (1976), 49, 2658. 26. Wendorff, J . , and P r i c e , F., Mol.Cryst.Liq.Cryst. (1973), 24, 129. 27. Wendorff, J . , and P r i c e , P., Mol.Cryst.Liq.Cryst. (1973), 22, 85. 28. Barnard, J., and lydon, J . , Mol.Cryst.Liq.Cryst., (1974), 26, 285. 29. Shibaev, V.P., and Freidzon, Ya.S., Vysokomolek. soed., (1975), B-17, 151. R E C E I V E D December 8, 1 9 7 7 .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch004
4.
SHIBAEV E T A L .
Thermotropic Liquid Crystalline Polymers
55
17. Clough, S., Blumstein, A., and Hsu, Ε., Macromole cules (1976), 9, 123. 18. Blumstein, Α., Clough, S., P a t e l , L., Blumstein, R., and Hsu, Η., Macromolecules (1976), 9, 243. 19. Shibaev, V.P., Freidzon, Ya.S., and P l a t é , N.A., Abstracts of papers at the 11th Mendeleev Congress on General and Applied Chemistry, p. 164, vol. 2, "Nauka" Publishers, (Moscow), 1975. 20. Shibaev, V.P., Freidzon, Ya.S., and P l a t é , N.A., USSR Inventor's C e r t i f i c a t e No. 525, 709, B y u l l . i z o b r e t e n i y (1976), No. 31. 21. P l a t é , N.A., and Shibaev, V.P., J.Polymer S c i . , Macromolec.Rev., (1974), 8, 117. 22. Kamogawa, H., J.Polymer S c i . (1972), B-10, 7. 23. Minezaki, S., Nakaya, T., and Imoto, Μ., Makromolek.Chemie, (1974), 175, 3017. 24. Saeki, H., Iimura, Κ., and Takeda, Μ., Polymer.J. (1972), 3, 414. 25. Kunihisa, K., and Hagiwara, S., Bull.Chem.Soc. Japan, (1976), 49, 2658. 26. Wendorff, J . , and P r i c e , F., Mol.Cryst.Liq.Cryst. (1973), 24, 129. 27. Wendorff, J . , and P r i c e , P., Mol.Cryst.Liq.Cryst. (1973), 22, 85. 28. Barnard, J., and Lydon, J . , Mol.Cryst.Liq.Cryst., (1974), 26, 285. 29. Shibaev, V.P., and Freidzon, Ya.S., Vysokomolek. soed., (1975), B-17, 151. R E C E I V E D December 8, 1 9 7 7 .
5 Liquid Crystalline Order in Polymers and Copolymers with Cholesteric Side Groups 1
A. BLUMSTEIN, Y. OSADA, S. B. CLOUGH, E . C. HSU, and R. B. BLUMSTEIN
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
Department of Chemistry, Polymer Program, University of Lowell, Lowell, MA 01854
We have recently shown (1, 2) that atactic polymers with side groups characterized by chemical constitution related to mesomorphic behavior tend to organize spontaneously and independently of the conditions of their preparation if given enough segmental mobility. The organization displayed can be lamellar (smectic) or directional (nematic). In the former case the macromolecular backbone is confined to a plane, while the side groups are either perpendicular or tilted with respect to the planes containing the backbone. The side groups are arranged either in single or in double arrays in which the individual side groups are mutually parallel (3). In the latter (nematic) type of organization, the backbones are not restricted to layers but the side groups keep a mutually parallel orientation throughout the domain which can reach submicroscopic dimensions. The development of liquid crystalline order in the polymer may or may not be accompanied by crystallinity. Polymers with benzoic acid side groups were studied and found to display ordering analogous to the smectic C structures of alkoxy benzoic acids (1, 2). Polymers with Schiff base side groups were found to display ordering of the smectic A (1) and nematic type (4). Literature X-ray data (5,6) can be interpreted as being generated by lamellar (smectic) structures and seem to point to the fact that in all cases the intrinsic tendency of side groups to organize is reinforced by the connectivity imposed by the polymeric backbone. Mesomorphic order in polymers with side groups containing the cholesterol moiety has scarcely been studied in spite of the importance of cholesterol derivatives in the field of liquid crystals and biological materials. The main focus of attention has been 'Present address: College of Liberal Arts and Science, Ibaraki University, Mito 310, Japan.
0-8412-0419-5/78/47-074-056$05.00/0 © 1978 American Chemical Society
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
5.
BLUMSTEIN E T AL.
57
Liquid Crystalline Order
c e n t e r e d around t h e study o f phase t r a n s i t i o n s o f a c r y l i c ( 7 , 8, 9 ) a n d m e t h a c r y l i c ( 1 0 , 1 1 , 1 2 ) e s t e r s o f cholesterol. I n s p i t e o f c o n s i d e r a b l e c o n t r o v e r s y as to t h e l o c a t i o n o f t h e c h o l e s t e r i c phase t r a n s i t i o n i n t h e s e m o n o m e r s , t h e r e was a c o n s e n s u s t h a t p o l y m e r s o f c h o l e s t e r y l a c r y l a t e and c h o i e s t e r y l m e t h a c r y l a t e a r e amorphous. The amorphous s t r u c t u r e i s r e p o r t e d t o occur i n d e p e n d e n t l y o f t h e phase i n which t h e p o l y m e r i z a t i o n has been c a r r i e d o u t , though Hardy (7) has shown t h a t polymer-monomer m i x t u r e s ( p o l y m e r i z a t i o n a t 0 C) h a v e a s m e c t i c o r g a n i z a t i o n . From o u r p o i n t o f v i e w , t h e s e s t a t e m e n t s w e r e s u r p r i s i n g as we e x p e c t e d the c h o l e s t e r i c m o i e t i e s t o i n t e r a c t s t r o n g l y and o r ganize t h e polymer i n a fashion s i m i l a r t o other p o l y mers w i t h m e s o g e n i c s i d e g r o u p s . They w a r r a n t e d an i n q u i r y i n t o t h e s t r u c t u r e o f polymers with c h o l e s t e r i c side groups. A r e c e n t p u b l i c a t i o n (13) w h i c h a p p e a r e d w h i l e t h i s w o r k was i n p r o g r e s s d e s c r i b e d t h e s p o n t a n e o u s development o f l a y e r e d o r d e r i n polymers w i t h t h e cho l e s t e r i c moiety a t t a c h e d t o t h e backbone v i a a l o n g , f l e x i b l e m o l e c u l a r s p a c e r c o n s t i t u t e d by anCo-aminocarboxylie acid chain. L a y e r e d s t r u c t u r e s a r e deve loped w i t h t h e long s i d e groups roughly p e r p e n d i c u l a r to t h e main c h a i n . The a u t h o r s a t t r i b u t e d t h e d e v e l o p ment o f t h e s e s t r u c t u r e s t o t h e f l e x i b i l i t y o f t h e bridge connecting t h e c h o l e s t e r o l moiety. One o f t h e f a c t o r s l e a d i n g t o c o n f u s i o n i n i n t e r p r e t i n g d a t a on t h e development o f o r d e r i n p o l y m e r s w i t h mesogenic s i d e groups i s t h e f r e q u e n t phase sepa r a t i o n taking place during the polymerization process. The i n t e r m o l e c u l a r d i s o r d e r c h a r a c t e r i z i n g t h e p r e c i p i t a t e d polymer i s o f t e n "locked i n " i f the tempera t u r e o f p r e c i p i t a t i o n i s below t h e g l a s s t r a n s i t i o n temperature o f t h e polymer Tg. C a s t i n g o f f i l m s from good, s l o w l y e v a p o r a t i n g s o l v e n t s o r s k i l l f u l a n n e a l i n g i n v a r i a b l y leads t o t h e development o f mesomorphic order ( 1 , 3 ) . S i m i l a r r e s u l t s have been a c h i e v e d w i t h p l a s t i f i c a t i o n o f p o l y ( ^ f - L - b e n z y l g l u t a m a t e ) (_14). I n t h i s p a p e r we d e s c r i b e t h e s t u d y o f d i f f e r e n t polymers c o n t a i n i n g t h e c h o l e s t e r o l moiety i n t h e s i d e g r o u p a s w e l l a s c o p o l y m e r s o f ch οle s t e r y l m e t h a c r y 1 a t e (PChMA) w i t h n - a l k y l m e t h a c r y l a t e s o f v a r i o u s c h a i n lengths. The d e v e l o p m e n t o f o r d e r i n Ρ C h M A s o f d i f f e r e n t t a c t i c i t i e s i s a l s o d e s c r i b e d , p r o v i d i n g some i n s i g h t i n t o t h e f a c t o r s governing t h e development o f mesomorphic o r d e r i n polymers w i t h c h o l e s t e r i c s i d e groups. T
58
M E S O M O R P H I C ORDER I N
POLYMERS
Experimental All
s o l v e n t s and
reagents
were d i s t i l l e d
prior
to
use. Monomers. Choiesteryl p-acryloyloxybenzoate ( C h A B ) was p r e p a r e d i n t h r e e s t e p s by t h e f o l l o w i n g synthetic route : 0 Ho/0VcOOH+CH
2
0
=CH~C-Cl
Q H ^ C H = C H - C O Y Q V C 00H
Ρ
2
2) Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
0 S
Q
C
1
2
0
^ C H =CH-(:0-VQVCC1 9
H+
cholesterol
<δ)-Ν(αϊ ) 3
^ 2
0 CH
2
= CH-CO-^^Q^CQCCh]
The p r o d u c t was r e c r y s t a l i i z e d f r o m chloroform/acetone and c h l o r o f o r m / e t h a n o l t o c o n s t a n t t r a n s i t i o n t e m p e r a ture. Elemental analysis: C a l c d . f o r C'3 Hr 0 : C, 7 9 . 2 9 % ; H , 9.29%. Found: C, 7 9 . 5 1 % ; H, 9.31%7 The NMR s p e c t r a w e r e c o n s i s t e n t w i t h t h e e x p e c t e d s t r u c t u re . C h o i e s t e r y l m e t h a c r y l a t e (ChMA) was o b t a i n e d b y a p r o c e d u r e s i m i l a r t o t h e one d e s c r i b e d i n t h e l i t e r a t u r e ( 15 ) . T h i r t y f i v e grams (0.34M) o f m e t h a c r y l o y l c h l o r i d e was a d d e d s l o w l y t o a m i x t u r e o f 77 g (0.2M) o f c h o l e s t e r o l a n d 15 0 m l o f t r i e t h y l a m i n e d i s s o l v e d i n 500 ml of t o l u e n e . A f t e r 3 h a n o t h e r 15 0 m l o f t r i e t h y l a m i n e was a d d e d t o t h e m i x t u r e a n d t h e r e a c t i o n was c o n t i n u e d f o r 24 h a t 60-70°C. The c h o l e s t e r y l m e t h a c r y l a t e was p r e c i p i t a t e d t h r e e t i m e s i n t o 1.6N HC1 s o l u t i o n i n methanol from toluene. The y i e l d was 6 0 - 7 0 % The p u r e compound d i s p l a y e d a m e l t i n g p o i n t a t 114.8°C a n d a m o n o t r o p i c c h o l e s t e r i c p h a s e a t 111.8°C: Κ 114.8 I ( 1 1 1 . 8 ) i n a g r e e m e n t w i t h l i t e r a t u r e d a t a . (Κ, I a n d C d e s i g n a t e t h e c r y s t a l l i n e , i s o t r o p i c a n d c h o l e s t e r i c phase, r e s p e c t i v e l y . The e l e m e n t a l a n a l y s i s , NMR a n d I R s p e c t r a w e r e c o n s i s t e n t w i t h t h e s t r u c t u r e of cholesterylmethacrylate. M e t h y l m e t h a c r y l a t e (MMA), p r o p y l m e t h a c r y l a t e (PMA) 7
2
4
5.
BLUMSTEIN ET A L .
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Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
n - b u t y l m e t h a c r y l a t e ( B M A ) , n - o c t y l m e t h a c r y l a t e (OMA), and n - d o d e e y l m e t h a c r y l a t e (DMA), were p u r i f i e d b y d i s t i l l a t i o n u n d e r vacuum. The n - h e x a d e c y l m e t h a c r y l a t e (HMA) was r e c r y s t a l i i z e d f r o m a b e n z e n e - e t h e r m i x t u r e . Polymerization and Copolymerization. The i o n i c p o l y m e r i z a t i o n o f ChMA was c a r r i e d o u t m toluene ( d r i e d o v e r Na w i r e and d i s t i l l e d i n p r e s e n c e o f L i A l H L j . ) , a t -3QOC u s i n g B u L i a s i n i t i a t o r . The p o l y mer was p r e c i p i t a t e d t h r e e t i m e s f r o m b e n z e n e i n t o methanol. T h e f r e e r a d i c a l p o l y m e r i z a t i o n o f ChA a n d ChMA was c a r r i e d o u t t h e r m a l l y o r i n s o l u t i o n i n f r e s h l y d i s t i l l e d b e n z e n e a t 60OC u s i n g A I B N a s i n i t i a t o r . The t o t a l c o n c e n t r a t i o n o f monomer was 1.0 m o l / 1 . The c o n c e n t r a t i o n o f i n i t i a t o r was 10"" m o l / 1 . G l a s s am p u l e s were f i l l e d t o h a l f c a p a c i t y w i t h t h e s o l u t i o n , evacuated, purged w i t h N and s e a l e d . They were p l a c e d i n a t h e r m o s t a t a t 60OC. T h e p o l y m e r i z a t i o n was i n t e r r u p t e d a t c o n v e r s i o n s v a r y i n g f r o m 5 t o 1 5 % (by w e i g h t ) . T h e p o l y m e r was p r e c i p i t a t e d t w i c e f r o m b e n z e n e i n t o m e t h a n o l a n d d r i e d a t 4QoC i n v a c u o . The c o p o l y m e r c o m p o s i t i o n s were d e t e r m i n e d f r o m e l e m e n t a l a n a l y s i s o f t h e copolymer. 2
2
C h a r a c t e r i z a t i o n o f Monomers a n d P o l y m e r s . Phase t r a n s i t i o n s o f monomers w e r e d e t e r m i n e d b y means o f D i f f e r e n t i a l S c a n n i n g C a l o i m e t r y (DSC) u s i n g t h e P e r k i n E l m e r D^C I B a n d t h e r m a l p o l a r i z i n g microscopy. X - r a y s t u d i e s o n t h e s ample s w e r e p e r f o r m e d w i t h a R i g a k u w i d e a n g l e d i f f r a c t o m e t e r (SG 7B) a s w e l l a s w i t h a l o w a n g l e Warn us c a m e r a . Cu-K^. r a d i a t i o n was used. S ample s o f c o p o l y m e r s were e x a m i n e d as p e l l e t s made f r o m c a s t f i l m s . ( F i l m s w e r e c a s t f r o m a 1-2% s o l u t i o n i n b e n z e n e f o l l o w e d b y d r y i n g a t 50°C i n vacuo). S e v e r a l f i l m samples were p r e s s e d t o g e t h e r i n a d i e ( 1 2 , 0 0 0 ρ.s.i., 3 m i n . u n d e r v a c u u m o f a p p r o x . 0.1mm) t o f o r m a p e l l e t 1.5mm t h i c k a n d 10mm i n d i a meter. Samples o f homopolymers were e x a m i n e d e i t h e r as s i n g l e unpressed f i l m s o r as p e l l e t s . Spacings cha r a c t e r i s t i c o f s m e c t i c o r g a n i z a t i o n (d) were c a l c u l a t e d by t h e B r a g g e q u a t i o n . The a v e r a g e d i s t a n c e between s i d e g r o u p s (D) was c a l c u l a t e d f r o m t h e e q u a t i o n D = 1 . 1 1 7 A / 2 S i n § ) ( 1 6 ) f r o m t h e h a l o n e a r 29 = 15°. The s h a r p n e s s s o f t h e l o w a n g l e x- r a y d i f f r a c t i o n p e a k s i s d e f i n e d as t h e r a t i o o f t h e peak h e i g h t h t o t h e w i d t h w a t 1/2 h . The p o s i t i o n o f t h e l o w a n g l e p e a k f o r t h e c h o l e s t e r y l m e t h a c r y l a t e was f o u n d a t 29=2.5° a v a l u e s o m e w h a t h i g h e r t h a n t h e v a l u e 29=2.4g f o u n d f o r t h e same p o l y m e r i n a n e a r l i e r s t u d y ( 1 7 ) .
60
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The g l a s s t r a n s i t i o n t e m p e r a t u r e s w e r e e s t i m a t e d f r o m d e f o r m a t i o n t e s t s p e r f o r m e d on t h e v e r y same p e l l e t s which were used f o r x-ray s t u d i e s . The t e s t s w e r e made b y means o f a P e r k i n - E l m e r T h e r m o m e c h a n i c a l a n a l y z e r TMS-1, c a l i b r a t e d w i t h p o l y s t y r e n e a n d p o l y m e t h y l m e t h a c r y l a t e . C-13 NMR S p e c t r a w e r e r u n i n d e u t e r a t e d c h l o r o f o r m a t 1 0 w t % c o n c e n t r a t i o n a n d a t 4-5°C. A J E O L PFT-10 0 s p e c t r o m e t e r was u s e d ( r e s o l u t i o n : 1.22 I I ; p u l s e w i d t h 15 ^ t s e c ; r e p e t i t i o n : 1. O s e c ) . T a c t i c i t y a s s i g n m e n t s w e r e made f r o m t h e a n a l y s i s o f the c a r b o n y l p o r t i o n o f the spectrum. T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y was p e r f o r m e d o n s a m p l e s o f t h i n f i l m s o f c o p o l y m e r s a n d b l e n d s o f homop o l y m e r s b y means o f J E O L JEM10 0 e l e c t r o n m i c r o s c o p e . The f i l m s w e r e s t a i n e d w i t h OgOi^ p r i o r t o e x a m i n a t i o n .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
z
Results
a n d Discussiοη
Monomers. Table I gives the t r a n s i t i o n tempera t u r e s a s f o u n d b y means o f DSC a n d p o l a r i z i n g m i c r o s c o p y f o r t h e t h r e e monomers. The c h o l e s t e r i c t r a n s i t i o n s o f b o t h c h o i e s t e r y l a c r y l a t e and c h o i e s t e r y l m e t h a c r y l a t e w e r e f o u n d t o be m o n o t r o p i c i n a g r e e m e n t w i t h T o t h a n d T o b o l s k y (JO a n d d e V i s s e r (JO . While t h e e x a c t v a l u e o f t h e t r a n s i t i o n f o r ChMA i s i n a g r e e ment w i t h d e V i s s e r s (9_), t h e v a l u e f o r ChA i s a t v a r i a n c e w i t h t h e v a l u e f r o m (_8_) a n d a d d s s t i l l a n o t h e r t r a n s i t i o n temperature t o the l i t e r a t u r e o f reported v a l u e s f o r t h i s monomer. The c h o l e s t e r y l e s t e r o f t h e a c r y l o y l o x y b e n z o a t e (ChAB ) g i v e s a n e n a t i o t r o p i c t r a n s i t i o n a t 12 8°C (K—^C ). A l l t h r e e monomers d i s p l a y iridescent colors i n their cholesteric state. In the s o l i d s t a t e ChA a n d ChAB e x h i b i t e d an i n t e n s e x - r a y d i f f r a c t i o n p e a k n e a r 20=2° a s w e l l as a n u m b e r o f wide angle l i n e s . I n t h e c h o l e s t e r i c s t a t e t h e low a n g l e peak d i s a p p e a r e d . The x - r a y d i f f r a c t i o n p a t t e r n o f ChMA i n t h e s o l i d s t a t e c o n t a i n e d a l o w a n g l e p e a k a t 20=5.9° a n d a n u m b e r o f w i d e a n g l e l i n e s . This monomer p o l y m e r i z e d r a p i d l y a f t e r m e l t i n g a n d we d i d not succeed i n r e c o r d i n g t h e x-ray d i f f r a c t i o n p a t t e r n in the cholesteric state. Thus, t h e l i q u i d c r y s t a l l i n e s t a t e o f ChA a n d ChAB h a s a t w i s t e d n e m a t i c o r g a n i z ation. I t i s very probable t h a t l i q u i d c r y s t a l l i n e ChMA i s a l s o a t w i s t e d n e m a t i c . f
5.
BLUMSTEIN E T A L .
61
Liquid Crystalline Order
Table I Monomers Name ChA
Transition T e m p e r a t u r e °C
Structure CH 1
9
= CHC-OCCh)
Κ 125.8 I (124.8 C 91K)
II ο
ChMA
CH =C(CH )C-0[Ch] 2
3
Κ 114.8 I (11.8C)
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
0 ChAB
CH ο = CH C 0 Y O n0 xz/
!
) - CO [ Ch ] Κ 128 C - ^ p o l y n 0 ti
Polymers. PChMA. F o r t h e polymer p r e p a r e d by free r a d i c a l i n i t i a t i o n the very intense x-ray l i n e w h i c h c o r r e s p o n d s t o 35. 3°^ (29=2.5°) shows a s m e c t i c t y p e s t r u c t u r e f o r m ( s e e Fi.g. l b ) . T h i s s t r u c t u r e i s c o n s i d e r a b l y weakened by a b r u p t p r e c i p i t a t i o n o f t h e polymer . The d i s t a n c e f r o m t h e c e n t e r o f t h e m a i n c h a i n t o t h e e n d o f t h e s i d e group i s e s t i m a t e d a t 18-19^ from m o l e c u l a r models. The ρ r e d o m i η a n t l y i s o t a c t i c f o r m o f PChMA p r e p a r e d b y i o n i c ρ οlymeri ζ a t i o n a l s o d i s p l a y s an i n t e n s e x-ray l i n e w h i c h , a f t e r an n e a l i n g , i s l o c a t e d a t (29=2.5°) i n d i c a t i n g a l a m e l l a r b i - l a y e r e d a r r a n g e m e n t o f mac r o m o l e c u l e s s i m i l a r t o t h a t o f t h e ρ r e domi η a n t l y s y n d i o t a c t i c s p e c i m e n . F o r b o t h f o r m s , a h a l o w i t h a p e a k a t 15.5°=29 shows t h a t no l o n g r a n g e o r d e r e x i s t s b e t w e e n s i d e g r o u p s ^ The average d i s t a n c e D between s i d e groups i s 6.35^ f o r b o t h t y p e s o f PChMA. Table I I
PChMA ( A I B N ) (after annealing) PChMA ( B u L i ) (after annealing)
jO
TSO
rr
mm
d^
. 6
.1
35 . 3
6.3
.6
35 . 3
6. 3
ΡChAB. An i n t e n s e and sharp low angle r e f l e x i o n a t 4 5 . 2 g a n d a h a l o a t 6.0 8^ i n d i c a t e a s m e c t i c t y p e of o r g a n i z a t i o n ( s e e F i g . l a ) . Here a g a i n t h e d i s t a n c e from t h e c e n t e r o f t h e main c h a i n t o t h e end o f t h e
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
62
Figure 1. (a) (above) X-ray low and wide angle diffraction patterns obtained from poly(cholesterylester of the acryloyloxybenzoicacid)(PChAB). (b)(right) X-ray low angle diffraction pattern obtained from annealed sample of poly( cholesterylmethacryhte) PChMA.
M E S O M O R P H I C ORDER I N
POLYMERS
5.
BLUMSTEIN ET A L .
Liquid Crystalline Order
63
group ( 2 3 g e s t i m a t e d from m o l e c u l a r models) i s o f the d spacing. ΡChA. A b r o a d e r x - r a y p e a k t h a n f o r t h e t w o p r e c e d i n g p o l y m e r s i s o b s e r v e d n e a r 2.6°=29 ( 3 3 . 4 g ) . A b r u p t p r e c i p i t a t i o n o f t h i s p o l y m e r l e a d s t o an amor phous p o l y m e r . We a r e i n c l i n e d t o t h i n k t h a t i n a l l c a s e s p r e sented above, t h e main c h a i n o f t h e polymer i s con f i n e d t o p l a n e s , w i t h s i d e groups e s s e n t i a l l y per p e n d i c u l a r t o t h e main c h a i n on b o t h s i d e s o f t h e plane. T h i s m o d e l i s s c h e m a t i c a l l y g i v e n i n f i g u r e 2b.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
side half
Copolymers. F i g u r e 3 shows a s e t o f t y p i c a l l o w a n g l e d i f f r a c t i o n p a t t e r n s o b t a i n e d f r o m PChMA a n d c o p o l y m e r s o f v a r i o u s c o m p o s i t i o n s o f ChMA a n d BMA. One c a n s e e t h a t t h e h o m o p o l y m e r a s w e l l a s some c o p o l y mers o f h i g h ChMA c o n t e n t a r e c h a r a c t e r i z e d b y a s h a r p l o w a n g l e x - r a y d i f f r a c t i o n p e a k a t 29=2.6° c o r r e s p o n d ing t o 33.9g, w i t h h i g h e r orders (18). The p o s i t i o n o f t h i s peak does n o t v a r y w i t h t h e c o m p o s i t i o n and n a t u r e o f t h e s e c o n d comonomer, a l t h o u g h i t s i n t e n s i t y is strongly affected. The l o w a n g l e peak i s accom p a n i e d b y h i g h e r o r d e r s and a h a l o a r o u n d 29=15.5g which corresponds t o t h e average d i s t a n c e between chol e s t e r y l s i d e g r o u p s o f D= 6 . 3 5 g The v a l u e o f 33. 9 g f o r t h e l o w a n g l e x - r a y d i f f r a c t i o n i s s o m e w h a t l o w e r t h a n 3 5 . 3 g f o u n d f o r PChMA i n a former study (17). I t may w e l l b e t h a t , d u r i n g t h e s a m p l e p r e p a r a t i o n , t h e 12,0 O O p s i a p p l i e d o n t h e p e l l e t r e s u l t s i n a s l i g h t c h o l e s t e r i c s i d e group i n terpénétration ( s e e F i g . 2 a ) . We h a v e a t t r i b u t e d t h e 3 3 . 9 g peak t o t h e r e g u l a r i n t e r l a m e l i a r s p a c i n g s o f the ordered polymer. The m o d e l p r o p o s e d f o r t h e p o l y mer o f ChMA i s s t i l l a p p l i c a b l e t o t h e c o p o l y m e r s w i t h h i g h ChMA c o n t e n t , w i t h t h e c h o l e s t e r i c s i d e g r o u p s arranged i n a double array and e s s e n t i a l l y perpendicul a r t o t h e lamellar plane c o n t a i n i n g the convoluted backbone. One m u s t as s urne t h a t s i d e g r o u p s i n t e r p e n e t r a t e s l i g h t l y as t h e l e n g t h o f t h e s i d e group i s 18-19A ( e s t i m a t e d f r o m F i s h e r models from c e n t e r o f the chain t o t h e end o f t h e c h o l e s t e r i c moiety). F i g u r e 4 g i v e s t h e s h a r p n e s s s o f t h e 2.6° p e a k as a f u n c t i o n o f t h e c o m p o s i t i o n f o r d i f f e r e n t c o p o l y mers o f ChMA w i t h n - a l k y l m e t h a c r y l a t e s . T h e s e r e s u l t s a r e compared t o t h e d a t a o b t a i n e d f o r m e c h a n i c a l blends of t h e c o r r e s p o n d i n g homopolymers. The s a m p l e s o f h o m o p o l y m e r b l e n d s w e r e p r e p a r e d u n d e r t h e same c o n d i t i o n s as t h e copolymers. I t i s i m m e d i a t e l y apparent from F i g u r e 4, t h a t d e p e n d i n g on t h e s t r u c t u r e o f t h e non-mesogenic c o -
MESOMORPHIC
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I N POLYMERS
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
Figure 2. (a) (above) Cholesteric side groups (Fisher space filling molecular models), (b) (right) Proposed smectic organization of Ρ ChAB, PChMA,and PChA.
Journal of Polymer Science
Figure 3. Low angle x-ray diffraction peak for PChMA and copolymers of CHMA with BMA of various composi tions are expressed in mole %. (1) ChMA (100%), (2) ChMA (91.4%)—BMA (8.6%), (3) ChMA (84.8%)—BMA (15.2%), (4) ChMA (78.1%)—BMA (21.9%) (17).
§ 4
ι. ι , 3 2 1 0
2Θ
i
BLUMSTEIN ET AL.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
m m «C
Liquid Crystalline Order
25
15
10
5
0
0.2
0.4
0.6
0.8
1.0
ChMA i n Polymer or i n Blend (mole f r a c t i o n )
Journal of Polymer Science
Figure 4. (mole % ). S as a function of composition in copolymers of ChMA with various long chain methacrylic esters and also, blends of various homopolymers (before annealing). Copolymer: (O) MAA, (X) FM Α, Ο BMA, (A) OMA. Blend: (·) MM A, (X) FM A, (U) BMA (17).
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
66
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ORDER IN
POLYMERS
monomer, t h e s m e c t i c o r g a n i z a t i o n o f t h e c o p o l y m e r i s a t f i r s t r e i n f o r c e d and t h e n v e r y r a p i d l y d e c l i n e s f o r h i g h c o m p o s i t i o n s o f t h e n o n - m e s o g e n i c comonomer. This e f f e c t i s t h e s t r o n g e s t w i t h BMA b u t p e r s i s t s f o r t h e o c t y l m e t h a c r y l a t e (OMA). In the case of s h o r t e r c h a i n e s t e r s MMA a n d ΡMA t h e o r g a n i z a t i o n i s n o t signifi c a n t l y r e i n f o r c e d and a l e s s p r e c i p i t o u s b u t s t e a d y d e c l i n e o f the peak i n t e n s i t y s e t s i n . Q u i t e s u r p r i s i n g l y , t h e c o p o l y m e r o f MMA c a n s t i l l d i s p l a y a s i g n i f i c a n t l o w a n g l e p e a k down t o l e s s t h a n 40 m o l e p e r cent of c h o l e s t e r y 1 methacrylate i n the copolymer. The v a r i a t i o n o f s w i t h t h e c o m p o s i t i o n f o r b l e n d s o f homopolymers stands i n c o n t r a s t w i t h the r e s u l t s f o r the copolymers. As one c a n s e e f r o m F i g u r e 4, t h i s v a r i a t i o n i s l i n e a r and f o r a g i v e n c o m p o s i t i o n inde pendent of the polymer admixed. The s h a r p n e s s s o f t h e l o w a n g l e x - r a y d i f f r a c t i o n peak i s a complex e x p e r i m e n t a l parameter r e l a t e d not o n l y t o the p e r f e c t i o n o f the l a m e l l a r ( s m e c t i c ) d o m a i n s i n t h e c o p o l y m e r , b u t a l s o t o t h e i r s i z e and concentration. In t h e case of blends o f homopolymers t h e s c a t t e r i n g i s due t o s e g r e g a t e d d o m a i n s o f ChMA u n i t s a n d t h e r e f o r e s w o u l d d e p e n d on t h e i r c o n c e n t r a t i o n i n the blend. T h i s seems t o be c o n f i r m e d by t h e l i n e a r r e l a t i o n s h i p b e t w e e n s h a r p n e s s s a n d b l e n d com p o s i t i o n i n F i g u r e 4. The s t u d y o f t h i n f i l m s o f c o p o l y m e r s and blends o f homopolymers by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y gives d i r e c t evidence f o r segregation i n t o separate domains o f b o t h components i n the case o f b l e n d s . This c a n be c l e a r l y s e e n i n F i g u r e 5 . The 0 0 μ s t a i n e d c h o i e s t e r y l m o i e t i e s show as d a r k r e g i o n s a n d I n d i c a t e t h a t PChMA s e p a r a t e s i n t o c l u s t e r s o f approximately 1 M . No s e p a r a t i o n was s e e n i n f i l m s o f t h e c o p o l y m e r . ' The n o n - l i n e a r a n d s t r o n g v a r i a t i o n o f s w i t h t h e composition f o r a l l copolymers i n d i c a t e s t h a t the per f e c t i o n o f t h e l a m e l l a r domains i s s t r o n g l y a f f e c t e d by t h e p r e s e n c e i n t h e c o p o l y m e r c h a i n o f non-mesoge nic alkylmethacrylates. The i n t e r f e r e n c e t a k e s p l a c e a l r e a d y a t v e r y low c o n c e n t r a t i o n s o f t h e non-mesoge n i c comonomer. T h e r e a r e g o o d r e a s o n s t o b e l i e v e t h a t t h e d r a s t i c i n c r e a s e o f s f o r c o p o l y m e r s o f ChMA-BMA a n d ChMA-OMA i s due t o an i n t e r n a l p l a s t i f i c a t i o n e f fect . P l a s t i f i c a t i o n w o u l d i n c r e a s e t h e s e g m e n t a l mo b i l i t y a n d g i v e t o c h a i n s more o p p o r t u n i t y t o " p a c k " into a lamellar array. The g l a s s t r a n s i t i o n t e m p e r a t u r e s f o r v a r i o u s c o p o l y m e r s o f ChMA and p-alkylmethac r y l a t e s a r e p l o t t e d i n F i g u r e 6 as a f u n c t i o n o f com position. One c a n s e e t h a t t h e c o p o l y m e r compositions corresponding to the strongest i n c r e a s e of s i n F i g . 4 S
BLUMSTEIN ET AL.
Liquid Crystalline Order
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
Figure 5. Electron micrograph of a castfilmof a blend of PChMA and Ρ BMA. The black regions indicate phase separated domains of PChMA.
0
0.2
0.4
0.6
0.8
1.0
ChMA i n Copolymer (mole f r a c t i o n ) Journal of Polymer Science
Figure 6. Ghss transition temperature as a function of composition of the copolymer. (O) MMA, (Q) BMA, (A) OMA (17).
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
M E S O M O R P H I C ORDER I N P O L Y M E R S
ISO
100 Slope so
Number o{ C atoms Figure 7. (a) (top) Height of the low angle x-ray diffraction peak in various copolymers of ChMA as a function of composition after annealing, (b) (bottom) Slope of lines from Figure 7a vs. number of carbon atoms in the aliphatic chain of the non-mesogenic methacrylic comonomer.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch005
5.
BLUMSTEIN E T A L .
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Liquid Crystalline Order
g i v e l a r g e d e c r e m e n t s o f Tg. The argument o f i n t e r n a l p l a s t i f i c a t i o n i s independently supported by annealing experiments. F i g u r e 7 a shows t h e e v o l u t i o n o f p e a k h e i g h t a f t e r a n n e a l i n g o f a l l samples u n t i l a c o n s t a n t v a l u e o f p e a k h e i g h t was o b t a i n e d . As one c a n s e e from t h i s f i g u r e , t h e o r g a n i z a t i o n o f a l l copolymers i s c o n s i d e r a b l y i n c r e a s e d by annealing andt h e sharp o r d e r e n h a n c e m e n t i n BMA a n d OMA c o p o l y m e r s g i v e n i n F i g u r e 4 d i s a p p e a r s as a r e s u l t o f a n n e a l i n g . This i n d i c a t e s t h a t t h e o r d e r enhancement e f f e c t observed i n u n - a n n e a l e d c o p o l y m e r s o f ChMA a n d BMA o r ChMA a n d OMA i s d u e t o a n i n c r e a s e o f s e g m e n t a l m o b i l i t y . T h e increase o f segmental m o b i l i t y through annealing en hances t h e o r d e r i n g o f copolymers w i t h h i g h Tg and d e c r e a s e s t h e amount o f d i s o r d e r l o c k e d i n b y r a p i d evaporation o fthe solvent. I t i s i n t e r e s t i n g t o note t h a t t h e r a t e o f t h e c o l l a p s e o f o r d e r as a f u n c t i o n o f t h e mole p e r c e n t o f t h e non-mesogenic component i n t h e c o p o l y m e r F i g u r e 7b i s much s l o w e r f o r MMA t h a n f o r monomers w i t h l o n g e r a l k y l c h a i n s . A drastic i n c r e a s e i n t h i s r a t e t a k e s p l a c e a t η "7 3 p r e s u m a b l y i n d i c a t i n g severe s t e r i c i n t e r f e r e n c e with t h e ordering o f a d j a c e n t c h o l e s t e r i c s i d e g r o u p s f o r comonomers s u c h a s BMA o r OMA. I t i s thus becoming i n c r e a s i n g l y apparent t h a t mesomorphic order i n macromolecules i s s t r o n g l y c o n d i t i o n e d by t h e l e v e l o f segmental motion i n the polymer. H i g h s e g m e n t a l m o b i l i t i e s " u n l o c k " t h e amorphous s t r u c t u r e o f f r e s h l y p r e c i p i t a t e d polymers w i t h mesogenic s i d e groups and f a v o r t h e f o r m a t i o n o f l i q u i d c r y s t a l l i n e order i n such polymers. T h i s can be accomplished e i t h e r b y i n t e r n a l p l a s t i f i c a t i o n ( l o w e r i n g Tg) o r annealing. Acknowledgement A c k n o w l e d g e m e n t i s made t o t h e d o n o r s o f P e t r o l e u m R e s e a r c h F u n d a d m i n i s t e r e d b y t h e A m e r i c a n Che m i c a l Society f o r p a r t i a l support o f t h i s research u n d e r t h e P R F R e s e a r c h G r a n t No. 82 85AC6C. Thanks a r e e x p r e s s e d t o t h e NSF f o r p a r t i a l s u p p o r t u n d e r t h e G r a n t DMR-75-17397. Literature
Cited
1.
Blumstein, Hsu, E.C.,
Α., Blumstein, Macromolecules,
R.B., Clough, S.B., (1975) 8, 73.
2.
Blumstein, Α., Clough, S.B., Patel, R.B., and Hsu, E.C., Macromolecules,
L., Blumstein, (1976) 9, 243
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3. 4. 5. 6. 7.
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8. 9. 10.
11. 12. 13. 14. 15. 16. 17. 18.
Clough, S.B., Blumstein, A., and Hsu, E.C., Macromolecules (1976) 8, 123. Hsu, E.C., and Blumstein, A., J. Polymer Sci. (letters), (1977) 15, 129. Newman, B.A., Frosini, V., and Maganini, P.L., J. Polymer Sci., Polymer Phys. Ed., (1975) 13, 87. Perplies, Ε., Ringsdorf, Η., and Wendorff, J.M., Makromol. Chem., (1974) 175, 553. Hardy, Gy., Nyitrai, Κ., and Cser, F., IUPAC In ternat. Symp. Macromol. Chem., Budapest, (1969) 121. Toth, W.J., and Tobolsky, Α.V., J. Polymer Sci., (1970) B8, 289. DeVisser, A. C., Feyen, J., deGroot, Κ., and Banties, A., J. Polymer Sci., (1972) B, 10, 851. Tanaka, Υ., Kabaya, S., Shimura, Υ., Okada, A., Kurihata, Υ., and Sakakibara, Y., J. Polymer Sci., (1972) B10, 261. Saeki, Η., Iimura, Κ., and Takeda, Μ., Polymer J., (1972) 3, 414. DeVisser, A.C., deGroot, Κ., Teyen, J., and Bantjes, A., Polymer Letters, (1972) 10, 851. Shibaev, V.P., Freidzon, J.S., and Plate, N.A., Dokl. Acad. Nauk. USSR, (1976) 227, 1412. Samulski, E.T., and Tobolsky, Α.V., Macromolecules (1968) 1, 555. Tanaka, T., Shioraki, Η., and Shimura, J., CA78 136543 (1973). deVries, A., Molecular Cryst. Liquid Cryst., (1970) (1970) 10, 219. Hsu, E.C., Clough, S.B., Blumstein, A., J. Poly mer Sci. (letters) (1977) 15, 545. Osada, Υ., and Blumstein, A., J. Polymer Sci., (letters), (in print).
R E C E I V E D December 8, 1977.
6 Influence of Mesomorphic Order on the Physical Properties of Poly(p-Biphenyl Aerylate) and Related Polymers B. A. NEWMAN Department of Mechanics and Materials Science, Rutgers, The State University, New Brunswick, NJ 08903
V. FROSINI and P. L. MAGAGNINI
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
Università di Pisa, Istituto di Chimica Industriale ed Applicata, 56100 Pisa, Italy
In previous investigations (1,2) the influence of the chemi cal structure on the physical properties of aromatic polyacrylates and polymethacrylates, obtained by r a d i c a l polymerization, has been extensively studied. The most interesting and stimulating result (3-6) was the discovery that some of these polymers, namely, poly(p-biphenyl acrylate) (PPBA), poly(p-cyclohexyl phenyl acrylate (PPCPA) and poly(p-acryloyloxyazobenzene) (PPAAB), display thermo dynamic properties t y p i c a l of c r y s t a l l i n e polymers. Recently the c r y s t a l l i n e character of atactic PPBA and PPCPA has been pointed out by the study of their dynamic mechanical behavior (7). How ever x-ray studies (8,9) have shown that the structure of these polymers is not c r y s t a l l i n e in the conventional sense. A onedimensional ordering showing in some cases as many as four sharp reflections and a p e r i o d i c i t y of 23.2 Å was observed i n a direc tion perpendicular to the main chain, together with a diffuse halo i n the wide angle region. In order to account for these results a smectic structure was proposed (8), the long rigid side groups being randomly directed at right angles on both sides of the main chain to form a layered structure. On the basis of this model it was anticipated (8) that a syndiotactic configuration of the macromolecules would have favor ed the attainment of a higher degree of order and perhaps also the formation of conventional c r y s t a l l i n e structures. Studies have therefore been undertaken i n order to synthesize samples of PPBA having different types of stereoregularity and to determine their morphology and physical properties. Experimental Materials. Atactic PPBA was prepared by radical polymer ization i n bulk or i n benzene solution at 60-75°C, using B z 0 as i n i t i a t o r . I t contained ^55% syndiotactic diads. Isotactic PPBA was prepared by polymerizing the monomer i n toluene at -78°C with Bu L i as i n i t i a t o r . The procedure was very similar to that reported i n the l i t e r a t u r e for the preparation of other i s o t a c t i c 2
0-8412-0419-5/78/47-074-071$05.00/0 © 1978 American Chemical Society
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Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
polyacrylates. The d e g r e e o f i s o t a c t i c i t y was ^90%. Fractiona t i o n w i t h a p p r o p r i a t e s o l v e n t s l e d t o a sample w i t h a d e g r e e o f i s o t a c t i c i t y h i g h e r than 97%. Monomers w i t h b u l k y s i d e g r o u p s do n o t f o r m s y n d i o t a c t i c r i c h polymers r e a d i l y . Several attempts t o prepare s y n d i o t a c t i c PPBA were made. The b e s t r e s u l t s were a c h i e v e d b y p o l y m e r i z i n g t h e monomer i n t o l u e n e s o l u t i o n a t -78° u n d e r U.V. i r r a d i a t i o n , when a sample was o b t a i n e d w i t h ^ 6 5 % s y n d i o t a c t i c d i a d s . A t a c t i c p o l y ( p - b i p h e n y l m e t h a c r y l a t e ) PPBMA was p r e p a r e d b y r a d i c a l p o l y m e r i z a t i o n i n b u l k a t 75°C. I s o t a c t i c a n d s y n d i o t a c t i c PPBMA were o b t a i n e d b y p o l y m e r i z a t i o n w i t h B u L i a s i n i t i a t o r a t -78°C i n t o l u e n e a n d THF r e s p e c t i v e l y . T e c h n i q u e s . N.M.R. s p e c t r a o f a t a c t i c , i s o t a c t i c a n d s y n d i o t a c t i c s a m p l e s were r e c o r d e d u s i n g a J e o l PS-100 i n s t r u m e n t a n d a V a r i a n 220 s p e c t r o m e t e r , e q u i p p e d w i t h v a r i a b l e t e m p e r a t u r e c o n t r o l l e r a n d f r e q u e n c y m e t e r ( 1 0 ) . S p e c t r a were o b t a i n e d u s i n g 5-10% (W/V) s o l u t i o n s i n C C l ^ o r 0 - c h l o r o b r o m o - b e n z e n e w i t h HMDS as i n t e r n a l s t a n d a r d . S p e c i f i c h e a t measurements were made u s i n g a P e r k i n - E l m e r DSC-IB d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r . The sample s i z e was g e n e r a l l y ^ 1 5 mg. a n d t h e d a t a was o b t a i n e d f o r a sample s u r r o u n d e d b y a d r y n i t r o g e n a t m o s p h e r e . S y n t h e t i c s a p p h i r e was used as a r e f e r e n c e . I n each run, c a r r i e d out a t a r a t e o f 8°/min, a t e m p e r a t u r e r a n g e o f 20°Κ was e x p l o r e d . D.S.C. s c a n s were o b t a i n e d a t s c a n n i n g r a t e s o f 32°C/min. The t h e r m a l o p t i c a l d a t a were o b t a i n e d u s i n g a p h o t o c e l l i n c o n j u n c t i o n w i t h a Bausch-Lomb 500yV V0M7 r e c o r d e r , programmed b y a M e t t l e r hot stage. X - r a y d i f f r a c t i o n d a t a were o b t a i n e d u s i n g a N o r e l c o w i d e a n g l e d i f f r a c t o m e t e r w i t h Cu Κα r a d i a t i o n . X-ray d i f f r a c t i o n p a t t e r n s were a l s o r e c o r d e d o n f i l m u s i n g a f l a t p l a t e , w i t h a s p e c i m e n t o f i l m d i s t a n c e o f 7.0 cm. Cu Κα r a d i a t i o n was u s e d s o
ο t h a t t h e 23.2 A s p a c i n g a p p e a r e d o n t h e same f i l m w i t h t h e w i d e angle d i f f r a c t i o n . A t h e r m o g r a v i m e t r i c a n a l y s i s was c a r r i e d o u t f o r a l l p o l y m e r s using a M e t t l e r thermogravimetric a n a l y s i s device. Both t h e a b s o l u t e w e i g h t a n d t h e r a t e o f w e i g h t l o s s were r e c o r d e d a s a f u n c t i o n o f temperature. R e s u l t s and D i s c u s s i o n a. N u c l e a r M a g n e t i c R e s o n a n c e . The t a c t i c i t i e s o f a l l t h e p o l y m e r s s t u d i e d were d e t e r m i n e d f r o m N.M.R. measurements u s i n g t h e method d e s c r i b e d i n a p r e v i o u s (10) p u b l i c a t i o n . The method i s b a s e d o n t h e c o n s i d e r a t i o n t h a t t h e two b a c k b o n e m e t h y l e n e protons o f i s o t a c t i c sequencies, because o f t h e i r d i f f e r e n t s h i e l d i n g , are magnetic non-equivalent, w h i l e those o f syndio t a c t i c s e q u e n c e s , e x p e r i e n c i n g t h e same m a g n e t i c e n v i r o n m e n t , a r e e q u i v a l e n t . F o r i s o t a c t i c PPBA t h r e e bands w i t h 1:1:1 relative
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i n t e n s i t i e s a r e e x p e c t e d , w h e r e a s s y n d i o t a c t i c PPBA s h o u l d g i v e two g r o u p s o f l i n e s w i t h 1:2 r e l a t i v e i n t e n s i t i e s . The p e r c e n t t a c t i c i t y c a n be d e r i v e d f r o m t h e s p e c t r a u s i n g t h e r e l a t i v e a r e a s u n d e r c o r r e s p o n d i n g p e a k s . These r e s u l t s f o r a l l p o l y m e r s i n v e s t i g a t e d i n t h i s s t u d y a r e shown i n T a b l e I . TABLE I
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
Polymer A t a c t i c PPBA S y n d i o t a c t i c PPBA I s o t a c t i c PPBA (a) (b) (c) A t a c t i c PPBMA S y n d i o t a c t i c PPBMA I s o t a c t i c PPBMA
Tacticity 55% 65% 97% 90% 78% 80% 90% 90%
syndiotactic syndiotactic isotactic isotactic isotactic syndiotactic syndiotactic isotactic
b. D i f f e r e n t i a l Scanning Calorimetry. D.S.C. s c a n s f r o m i s o t a c t i c , a t a c t i c a n d s y n d i o t a c t i c PPBA were made a n d t h e s p e c i f i c heats a t various temperatures c a l c u l a t e d . This data i s shown i n F i g u r e 1 f o r b o t h a t a c t i c a n d i s o t a c t i c s a m p l e s . The t a c t i c i t y o f t h e i s o t a c t i c sample u s e d f o r t h i s f i g u r e was 9 7 % . I t c a n be s e e n t h a t a l t h o u g h t h e g l a s s t r a n s i t i o n o c c u r s a t t h e same t e m p e r a t u r e f o r b o t h s a m p l e s , t h e i s o t a c t i c s a m p l e m e l t s a t a t e m p e r a t u r e a p p r o x i m a t e l y 40°K l o w e r t h a n t h e a t a c t i c s a m p l e . I t was f o u n d t h a t t h e m e l t i n g p o i n t o f a t a c t i c , i s o t a c t i c and s y n d i o t a c t i c p o l y m e r v a r i e d w i t h t a c t i c i t y a n d t h e r m a l h i s tory. I n order t o i n v e s t i g a t e f u r t h e r these e f f e c t s , polymer samples were c r y s t a l l i z e d i s o t h e r m a l l y a t v a r i o u s t e m p e r a t u r e s and t h e m e l t i n g p o i n t o f e a c h sample e s t a b l i s h e d u s i n g t h e D.S.C. scans. The r e s u l t s a r e s u m m a r i z e d i n F i g u r e 2. The d a t a shown here f o r t h e i s o t a c t i c sample were o b t a i n e d from a sample w i t h a m e a s u r e d t a c t i c i t y o f 90%. I t was f o u n d t h a t t h e m e l t i n g t e m p e r ature increased l i n e a r l y w i t h c r y s t a l l i z a t i o n temperature b u t t h a t the r a t e o f i n c r e a s e depended on t h e polymer c o n f i g u r a t i o n as shown i n F i g u r e 2. The r a t e o f i n c r e a s e o f m e l t i n g t e m p e r a t u r e w i t h c r y s t a l l i z a t i o n t e m p e r a t u r e was c o n s i d e r a b l y s m a l l e r f o r t h e a t a c t i c polymer than f o r the i s o t a c t i c and s y n d i o t a c t i c polymers. An e q u i l i b r i u m m e l t i n g t e m p e r a t u r e c a n be d e t e r m i n e d b y e x t r a p o l a t i o n o f t h e m e a s u r e d m e l t i n g p o i n t s a s shown i n F i g u r e 2. The v a l u e s o b t a i n e d were 511°K f o r t h e i s o t a c t i c p o l y m e r (90% i s o t a c t i c ) , 550°K f o r t h e a t a c t i c p o l y m e r a n d 578°K f o r t h e s y n d i o t a c t i c sample. As we have a l r e a d y d i s c u s s e d , t h e e x t e n t o f s y n d i o t a c t i c i t y i n t h e p o l y m e r h e r e d e s c r i b e d a s s y n d i o t a c t i c was a c t u a l l y q u i t e low ((^65%) T a b l e I ) , a n d i n f a c t o n l y s l i g h t l y h i g h e r t h a n t h e polymer obtained by r a d i c a l p o l y m e r i z a t i o n and here d e s c r i b e d as a t a c t i c ( 5 5 % s y n d i o t a c t i c d i a d s ) . From F i g u r e 2 i t c a n be s e e n
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
M E S O M O R P H I C ORDER I N P O L Y M E R S
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
6.
NEWMAN ETAL.
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75
t h a t t h e m e l t i n g p o i n t o f t h e a t a c t i c p o l y m e r may be g r e a t e r t h a n the m e l t i n g p o i n t o f t h e s y n d i o t a c t i c polymer a t l o w c r y s t a l l i z a t i o n t e m p e r a t u r e s . However t h e e x t r a p o l a t e d m e l t i n g t e m p e r a t u r e f o r t h e s y n d i o t a c t i c p o l y m e r i s 28°K h i g h e r t h a n f o r t h e a t a c t i c p o l y m e r . These r e s u l t s i n d i c a t e t h a t t h e e q u i l i b r i u m m e l t i n g p o i n t o f PPBA i n c r e a s e s w i t h i n c r e a s i n g s y n d i o t a c t i c i t y . This supports the conclusion that a s y n d i o t a c t i c c o n f i g u r a t i o n o f the m a c r o m o l e c u l e s o f PPBA f a v o r s t h e r e g u l a r p a c k i n g o f t h e a r o m a t i c s i d e groups, a c c o r d i n g t o t h e suggested model. The t h e r m a l p r o p e r t i e s o f i s o t a c t i c , a t a c t i c a n d s y n d i o t a c t i c PPBMA a r e i l l u s t r a t e d by t h e D.S.C. t r a c e s i n F i g u r e 3. None o f t h e s e s a m p l e s showed a m e l t i n g e n d o t h e r m . Thus e v e n a s y n d i o t a c t i c c o n f i g u r a t i o n ( e x t e n t o f t a c t i c i t y 90%) i s n o t a b l e t o promote e f f i c i e n t p a c k i n g o f t h e s i d e groups f o r these p o l y m e r s . I n t e r e s t i n g p e c u l i a r i t i e s were n o t e d i n t h e m e l t i n g b e h a v i o r o f i s o t a c t i c PPBA w i t h a l o w e r d e g r e e o f i s o t a c t i c i t y . Figure 4 shows t h e D.S.C. t r a c e s f o r a sample 78% i s o t a c t i c . The most i n t e r e s t i n g o b s e r v a t i o n i s t h a t f o r t h i s p o l y m e r two w i d e l y s e p a r a t e d e n d o t h e r m i c p e a k s c a n be o b s e r v e d , a t 490°K a n d 540°K. On t h e c o r r e s p o n d i n g c o o l i n g c u r v e two e x o t h e r m i c p e a k s a t 500°K a n d 460°Κ were o b s e r v e d . Upon r e h e a t i n g t h e e n d o t h e r m i c p e a k s were a g a i n o b s e r v e d a t somewhat l o w e r t e m p e r a t u r e s b u t t h e i n t e n s i t y o f t h e h i g h t e m p e r a t u r e peak was f o u n d t o be g r e a t l y r e d u c e d . The h i g h e r e n d o t h e r m i c p e a k , f o r t h e i s o t a c t i c s a m p l e , occurs a t a temperature c l o s e t o the m e l t i n g p o i n t o f the a t a c t i c p o l y m e r , a n d w o u l d a p p e a r , a t f i r s t s i g h t , t o be a s s o c i a t e d w i t h the f u s i o n o f a t a c t i c polymer p r e s e n t i n t h e i s o t a c t i c sample. T h i s e x p l a n a t i o n was t e s t e d b y t h e s t u d y o f b l e n d s o f i s o t a c t i c p o l y m e r ( 9 7 % i s o t a c t i c ) a n d a t a c t i c p o l y m e r . The D.S.C. t r a c e s d i d show two e n d o t h e r m i c p e a k s a t ^490°K a n d ^540°K. However, upon c o o l i n g a n d r e h e a t i n g , b o t h h i g h a n d l o w t e m p e r a t u r e p e a k s a p p e a r e d a g a i n w i t h o u t a n y a p p r e c i a b l e change. The e x p l a n a t i o n t h a t t h e i s o t a c t i c sample w i t h a s m a l l e r e x t e n t o f t a c t i c i t y c a n be s i m p l y r e g a r d e d a s a b l e n d o f i s o t a c t i c a n d a t a c t i c p o l y m e r i s t h e r e f o r e n o t s u f f i c i e n t f o r a complete e x p l a n a t i o n . c. Thermal O p t i c a l A n a l y s i s . A t h i n f i l m o f t h e polymer, cast from s o l u t i o n and o b s e r v e d between c r o s s e d p o l a r s i n t h e p o l a r i z i n g m i c r o s c o p e , showed u n i f o r m e x t i n c t i o n a n d i n d i c a t e d t h a t p o l y m e r p r e c i p i t a t e d f r o m s o l u t i o n was e s s e n t i a l l y amor phous . However f o r t h e c a s e o f a t a c t i c PPBA, upon h e a t i n g , some b i r e f r i n g e n c e d e v e l o p e d a n d a t 200°C e x t e n s i v e r e g i o n s o f s t r u c t u r e were o b s e r v e d . F u r t h e r h e a t i n g o f t h e f i l m l e d t o a m e l t i n g w i t h a consequent decrease i n b i r e f r i n g e n c e t o uniform e x t i n c t i o n . Upon c o o l i n g f r o m t h e m e l t , s t r u c t u r e w i t h b i r e f r i n g e n c e a g a i n appeared. A q u a n t i t a t i v e measure o f t h i s b e h a v i o r was p r o v i d e d b y r e p l a c i n g one t h e e y e p i e c e s w i t h a p h o t o c e l l a n d a t t a c h i n g t h e p h o t o c e l l t o a r e c o r d e r t o p r o v i d e a measure o f t h e l i g h t r e a c h ing the eyepiece. The h e a t i n g a n d c o o l i n g was programmed u s i n g
76
M E S O M O R P H I C ORDER I N P O L Y M E R S
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
Ο
U
L J
ι
300
t 400
Γ
JLJ
500
Figure 3. D.S.C. traces for isotactic (1), atactic (2), and syndiotactic (3) PPBMA
450
500
T/°K
Figure 4. D.S.C. traces for isotactic PPBA sample (c); onfirstcooling (1 ) and second cooling (2)
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
6.
N E W M A N ET AL.
Influence of Mesomorphic Order
77
a M e t t l e r hot stage. Films were c a s t from chloroform s o l u t i o n d i r e c t l y on the microscope s l i d e . The thickness of the f i l m could be v a r i e d by changing the amount of polymer used. U s u a l l y a t h i c k ness of ^0.5 m i l was s u f f i c i e n t to give changes i n b i r e f r i n g e n c e of s u f f i c i e n t magnitude. The samples were heated and cooled a t 3°C/min. The r e s u l t s f o r a t a c t i c and i s o t a c t i c PPBA are shown i n Figure 5. The i s o t a c t i c sample shown here was 78% i s o t a c t i c . For both a t a c t i c and i s o t a c t i c sample the i n i t i a l c a s t f i l m shows e s s e n t i a l l y zero b i r e f r i n g e n c e . When the temperature exceeds ^388°K (the g l a s s t r a n s i t i o n temperature 385°K) the b i r e f r i n g e n c e s t a r t s to increase, and reaches a maximum a t ^532°K f o r the atact i c sample. Premelting and melting then reduce the b i r e f r i n g e n c e u n t i l a zero value i s reached at ^543°K. The i s o t a c t i c polymer however, with 78% t a c t i c i t y , showed an increase i n b i r e f r i n g e n c e up to 493°K. A f t e r a small d e c l i n e , a subsequent increase to a second maximum at 518°K was observed. Further increase i n temperature l e d to a melting which was complete by ^543°K. The highest b i r e f r i n g e n c e observed was i n f a c t only f i r s t order grey, so that the occurrence o f the second maximum was q u i t e unexpected. I s o t a c t i c and a t a c t i c polymers both d i s p l a y e d the same behavi o r during c o o l i n g and r e h e a t i n g . Upon c o o l i n g from the melt a biréfringent s t r u c t u r e was obtained which upon subsequent r e heating melted. For the a t a c t i c polymer zero b i r e f r i n g e n c e was obtained f o r temperature ^543°K. The i s o t a c t i c sample d i d not show a second maximum i n t h i s case; the melting appeared to occur a t the l o c a t i o n of the f i r s t maximum, zero b i r e f r i n g e n c e being observed at ^523°K. I t i s i n t e r e s t i n g to compare the T.O.A. data f o r a t a c t i c PPBA i n Figure 4 with the data obtained from i s o t a c t i c polystyrene f i l m s c a s t i n the same manner. T h i s data i s shown i n F i g u r e 6. Despite the d i f f e r e n t molecular conf i g u r a t i o n the o p t i c a l behavior d e s c r i b e d here i s e s s e n t i a l l y the same. We assume that b i r e f r i n g e n c e can a r i s e from two sources; a c r y s t a l l i z a t i o n (or o r d e r i n g phenomenon) from the amorphous phase to give biréfringent s t r u c t u r e s ; and an o r i e n t i n g of macromolecu l a r chains e i t h e r i n the c r y s t a l l i n e or the amorphous phases. The c l a r i f i c a t i o n of these e f f e c t s i s best achieved using x-ray d i f f r a c t i o n methods, and these r e s u l t s w i l l p r e s e n t l y be d e s c r i b e d . However a c o r r e l a t i o n of the T.O.A. data with the D.S.C. data can be made. I t appears t h a t the o r i g i n a l f i l m c a s t from s o l u t i o n i s amorphous and without extensive molecular o r i e n t a t i o n . At temperatures above Tg the molecular m o b i l i t y i s s u f f i c i e n t l y great to induce " c o l d " c r y s t a l l i z a t i o n . The T.O.A. data f o r i s o t a c t i c PPBA p a r a l l e l the D.S.C. data. I f changes of b i r e f r i n g e n c e were to be a s s o c i a t e d only with the melting process then a p l a t e a u i n the T.O.A. data should have been observed r a t h e r than a maximum. T h i s i n d i c a t e s that important t e x t u r a l changes must a l s o accompany the complex melting behavior d i s p l a y e d by t h i s i s o t a c t i c polymer.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
MESOMORPHIC
ORDER
IN
Figure 5. T.O.A. data for atactic and isotactic PPBA, on heating cast film (1), on solidification (2), and on reheating (3)
Figure 6.
T.O.A. data for isotactic polystyrene
POLYMERS
6.
NEWMAN
ET A L .
79
Influence of Mesomorphic Order
X-Ray D i f f r a c t i o n . X - r a y d i f f r a c t o m e t e r s c a n s o f v a r i o u s a t a c t i c PPBA s a m p l e s a r e shown i n F i g u r e 7. Sample A was o b t a i n ed b y p r e c i p i t a t i o n f r o m C H C 1 s o l u t i o n d i r e c t l y a f t e r p o l y m e r i zation. Samples Β a n d C w e r e o b t a i n e d b y c a s t i n g t h i n f i l m s f r o m CHC1 s o l u t i o n onto c l e a n f l a t s u b s t r a t e s ( l e a d and g l a s s ) . Sample D was o b t a i n e d b y s l o w c o o l i n g a n d s o l i d i f i c a t i o n o f m o l t e n p o l y m e r o n a g l a s s s u b s t r a t e t o g i v e a t h i n f i l m . The p r e p a r a t i o n o f s a m p l e C was e s s e n t i a l l y t h e same a s t h a t u s e d t o p r e p a r e f i l m s f o r t h e T.O.A. s t u d i e s . Sample D gave t h e d i f f r a c t i o n p a t t e r n d i s c u s s e d i n p r e s i o u s p u b l i c a t i o n s (9_) . I t i s c h a r a c t e r i z e d b y a s h a r p r e f l e c t i o n c o r r e s p o n d i n g t o a s p a c i n g o f 23.2 A and a b r o a d h a l o a t h i g h e r B r a g g a n g l e s w i t h a maximum i n t e n s i t y a t 19.5°. V e r y weak 2nd and 3 r d o r d e r s a r e p r e s e n t . Samples A a n d D show t h e same g e n eral characteristics. The l o w a n g l e p e a k s a r e n o t a s i n t e n s e a n d c a r e f u l measurement shows t h a t t h e s e c o r r e s p o n d t o a s p a c i n g o f 21.0 A. Sample C showed v e r y l i t t l e d i f f r a c t i o n a t a l l e x c e p t f o r a v e r y s m a l l peak a t t h e B r a g g a n g l e 19.5°. Some a n n e a l i n g t r e a t m e n t s were t h e n c a r r i e d o u t o n s a m p l e s A, Β a n d C a t v a r i o u s t e m p e r a t u r e s a n d t i m e s . T h e s e r e s u l t s a r e t a b u l a t e d i n Table I I and the d i f f r a c t i o n p a t t e r n s from samples Β a n d C a r e shown i n F i g u r e s 8 a n d 9. 3
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
3
TABLE I I Sample
Annealing T°C
A
Treatment
Ratio I / I s D
Time(hrs)
170 170 210 240
none 30 3 3 3
mins hrs hrs hrs
1:4 1:1 2:1 3:1 4:1
170 170 210 240
none 30 3 3 3
mins hrs hrs hrs
1:3 1:1 2:1 3:1 4:1
170 170 210 240
30 3 3 3
mins hrs hrs hrs
30:1 50:1 120:1 140:1
Β
A f t e r a n n e a l i n g , t h e l o w a n g l e p e a k s h i f t e d t o 3.8° ( c o r r e s p o n d i n g t o a n i n t e r p l a n a r s p a c i n g o f 23.2 A ) , became more s h a r p , a n d i n c r e a s e d i n i n t e n s i t y r e l a t i v e t o t h e w i d e - a n g l e h a l o . A mea s u r e o f t h i s i n c r e a s e i n i n t e n s i t y i s g i v e n i n T a b l e I I where t h e
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
M E S O M O R P H I C ORDER I N P O L Y M E R S
Figure 7.
X-ray diffraction from atactic PPBA
6.
N E W M A N ET AL.
81
Influence of Mesomorphic Order
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
a-PPBA cast film on lead
26
25 24
23 22
21
20
18
If
« Figure 8.
17
16
15
14
13
— Bragg
12
11
10
9
έ
?
6
5
4—3
a ng ι <· —
X-ray diffraction scans from annealed films cast on lead
M E S O M O R P H I C ORDER I N P O L Y M E R S
•90 80
a-PPBA MO'c
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
170*c 210*c 240*c
for for for for
70 60
30 mi η 3 hrs. 3 hrs. 3 hrs,
70
^40 Î30 -10
-50 -40 •50
J 26 25 24 23 22 21 20 19 18 17 !6
« Figure 9.
15 14 13. 12
IΓ 10 9
8
Bragg angle
7
6
^
«20
5~4
—
X-ray diffraction scans from annealed films cast on ghss
6. N E W M A N E T A L .
intensity ratio wide-angle
Influence of Mesomorphic Order
1
(ïg/ ^
peak h e i g h t
o
f
83
the low angle peaks h e i g h t
(1^) i s l i s t e d .
(I ) t o the g
The m o s t n o t i c e a b l e f e a -
t u r e was t h a t f o r sample C t h i s r a t i o i n c r e a s e d t o o n l y 4:1.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
M o r e o v e r , f o r sample Β t h e a n n e a l i n g t r e a t m e n t
l e d t o t h e appear
ance o f a s m a l l peak a t 20 = 17.5° s u p e r i m p o s e d on t h e b r o a d h a l o . I n a l l r e s p e c t s s a m p l e s A and Β b e h a v e d t h e same. I t i s l i k e l y t h a t t h e f i l m c a s t l e a d i s v e r y s i m i l a r t o t h e random powder a g g r e g a t e o b t a i n e d b y p r e c i p i t a t i o n f r o m s o l u t i o n . How e v e r t h e f i l m c a s t o n t h e g l a s s s u r f a c e shows v e r y d i f f e r e n t x - r a y d i f f r a c t i o n d a t a . A f t e r a n n e a l i n g , t h e l o w a n g l e r e f l e c t i o n shows a much g r e a t e r i n t e n s i t y r e l a t i v e t o t h e w i d e a n g l e h a l o . This c a n be u n d e r s t o o d i f t h e r e i s some p r e f e r r e d o r i e n t a t i o n o f t h e l a y e r e d s t r u c t u r e i n these c a s t f i l m s , both the extent o f o r d e r ing and the degree o f p r e f e r r e d o r i e n t a t i o n i n c r e a s i n g w i t h a n n e a l i n g temperatures and t i m e s . I f thesmectic layered struc t u r e i s o r i e n t e d p a r a l l e l t o the g l a s s s u b s t r a t e s u r f a c e , such an enhancement o f t h e l o w a n g l e r e f l e c t i o n w o u l d be a n t i c i p a t e d f o r the r e f l e c t i o n geometry u t i l i z e d by the d i f f r a c t o m e t e r . S t u d i e s o f t h e i s o t a c t i c PPBA were a l s o c a r r i e d o u t . The c a s t f i l m s showed a n i n i t i a l s m a l l l o w a n g l e peak c o r r e s p o n d i n g t o a n i n t e r p l a n a r s p a c i n g o f 23.2 A r a t h e r t h a n 21.0 A o b s e r v e d for the case o f the a t a c t i c polymer. I n a l l o t h e r r e s p e c t s , i n c l u d i n g t h e p r e f e r r e d o r i e n t a t i o n , t h e x - r a y d i f f r a c t i o n d a t a was v e r y s i m i l a r t o the d a t a o b t a i n e d from the a t a c t i c polymer. o
Thermogravimetric
A n a l y s i s (T.G.A.)
F o r f u r t h e r c h a r a c t e r i z a t i o n , d e g r a d a t i o n o f t h e p o l y m e r was s t u d i e d u s i n g the M e t t l e r thermogravimetric apparatus. F i g u r e 10 shows t h e w e i g h t l o s s c u r v e s f o r a t a c t i c PPBA when h e a t e d a t 6°C/min. No a p p r e c i a b l e w e i g h t l o s s o c c u r r e d a t t e m p e r a t u r e s up t o 613°K. I s o t a c t i c p o l y m e r s showed s i m i l a r t r a c e s a n d no a p p r e c i a b l e d e g r a d a t i o n o c c u r r e d a t t e m p e r a t u r e s l e s s t h a n 573°K. T h e s e d a t a c o n f i r m t h a t d e g r a d a t i o n p r o c e s s e s do n o t a p p e a r t o p l a y a r o l e i n the thermal s t u d i e s d e s c r i b e d here. Conclusions The r e s u l t s d e s c r i b e d h e r e i n d i c a t e t h a t d i f f e r e n c e s i n t h e d e g r e e a n d t y p e o f t a c t i c i t y o f PPBA s a m p l e s f a i l t o i n f l u e n c e t h e s t r u c t u r e o f these polymers as r e v e a l e d by x-ray d i f f r a c t i o n . This i s a very important f i n d i n g since i t i m p l i e s t h a t the s t r u c t u r e o f the polymer i s , i n t h i s case, independent o f the p o l y m e r i z a t i o n procedure. S e v e r a l a u t h o r s have i m p l i c i t l y g r a n t e d t h e p o s s i b i l i t y t o o b t a i n p o l y m e r s w i t h mesomorphic s t r u c t u r e l i e s i n t h e a v a i l a b i l i t y o f m e s o g e n i c monomers, a n d o n t h e p o s s i b i l i t y t o p o l y m e r i z e them i n t h e m e s o m o r p h i c p h a s e . Our r e s u l t s c o n t r a d i c t the suggestions. The o n l y d i f f e r e n c e one c a n r e a s o n a b l y e x p e c t between p o l y m e r s p r e p a r e d i n the mesomorphic and i s o t r o p i c phase
M E S O M O R P H I C ORDER I N P O L Y M E R S
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
a-PPBA S. W.^9.02 mg H. c/min.
Figure 10.
T.O.A. data for atactic PPBA
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch006
6.
N E W M A N ET A L .
Influence of Mesomorphic Order
85
( b e s i d e s d e g r e e o f p o l y m e r i z a t i o n ) may b e i n r e l a t i o n t o a d i f f e r e n t s t e r e o r e g u l a r i t y . Our r e s u l t s show t h a t e v e n marked d i f f e r e n c e s i n s t e r e o r e g u l a r i t y do n o t a l t e r t h e s m e c t i c s t r u c t u r e o f PPBA, t h u s d e m o n s t r a t i n g t h a t t h e l a t e r a l o r d e r i n g depends c r i t i c a l l y on the very s t r u c t u r e o f the aromatic s i d e groups r a t h e r than the main c h a i n c o n f i g u r a t i o n . Our r e s u l t s a l s o i n d i c a t e t h a t t h e r m a l h i s t o r y and s a m p l e p r e p a r a t i o n c o n d i t i o n s c a n a l t e r t h e m o r p h o l o g y and t e x t u r e o f t h e p o l y m e r and a l s o i n f l u e n c e t h e m e l t i n g p o i n t . The p r e f e r r e d o r i e n t a t i o n of the smectic l a y e r e d s t r u c t u r e o f the c a s t f i l m s on t h e g l a s s s u b s t r a t e i s n o t a l t o g e t h e r u n e x p e c t e d s i n c e s u c h o b s e r v a t i o n s a r e commonly f o u n d among m e s o g e n i c compounds. The d i f f e r e n t t h e r m a l b e h a v i o r o f PPBA s a m p l e s w i t h d i f f e r e n t t a c t i c i t i e s i s i n t r i g u i n g and p e r h a p s a s s o c i a t e d w i t h s m a l l d i f f e r e n c e s i n t h e mode o f p a c k i n g o f a r o m a t i c s i d e g r o u p s w i t h i n the l a y e r s , not observable by x - r a y d i f f r a c t i o n . Our r e s u l t s a l s o i n d i c a t e t h a t t h e m e l t i n g p o i n t o f PPBA i n creases w i t h i n c r e a s i n g extent of s y n d i o t a c t i c i t y . This supports the c o n c l u s i o n t h a t a s y n d i o t a c t i c c o n f i g u r a t i o n f a v o r s the model p r e v i o u s l y proposed f o r the l a y e r e d smectic s t r u c t u r e .
Literature Cited (1) Pizzivani, G., Magagnini, P. L., and Giusti, P., J. Polym. Sci. (1971) 9, A2, 1133. (2) Pasquini, M and Frosini, V., IUPAC International Symposium on Macromolecules, Madrid 1974, Reprint III 5-13. (3) Baccaredda, M, Magagnini, P. L., Pizzivani G., and Giusti, P., Polym. Letters (1971) 9, 303. (4) Magagnini P. L., Marchetti, Α., Matera, F., Pizzivani, G., and Turchi, G., European Polym. J., (1974) 10, 585. (5) Frosini, V., Magagnini, P. L., and Newman, Β. A., J. Polym. Sci. (1974) 12, 23. (6) Balloni, P., Thesis, University of Pisa (1972). (7) Frosini, V., Magagnini, P. L., and Newman, Β. A., J. Polym. Sci. in press. (8) Newman, Β. A., Magagnini, P. L., and Frosini, V., Advances in Polymer Science and Engineering, Pae, K. D., Morrow, D. R., and Chen, Yu, Plenum Press, New York (1972) 21. (9) Newman, Β. A., Frosini, V., and Magagnini, P. L., J. Polym. Sci. Phys. Ed. (1975) 13, 87. (10) Ceccarelli, A., Frosini, V., Magagnini, P. L., and Newman, Β. A., Polym. Letters (1975) 13, 101. RECEIVED
December 8,
1977.
7 A New Series of Mesomorphic Monomers: Bulk Polymerization of 4-Cyano-4'-biphenyl (n + 2)-alkenoates J. C. DUBOIS, J. C. LAVENU, and A. ZANN
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
Thomson-CSF, Domaine de Corbeville, 91401 Orsay, France
Polymers which exhibit mesomorphic order may have interesting anisotropic properties such as high o p t i c a l , e l e c t r i c a l , or mechanical anisotropies. Such polymers may be obtained either by polymerization of a non-mesomorphic monomer i n a l i q u i d c r y s t a l l i n e solvent (1,2) or by direct polymerization of a mesomorphic monomer i n its l i q u i d c r y s t a l l i n e state ( 3 - 9 ) . The organization i n the resulting polymers may be improved if the polymerization is done under orienting conditions i n e l e c t r i c or magnetic f i e l d s or within suitable boundary conditions (10,11,12,13). In this chapter a new series of mesomorphic monomers i s presented -- the 4-cyano-4'-biphenyl (n+2)-alkenoates. Their bulk polymerization i n the nematic state has been investigated, and t h i n layers obtained by thermal polymerization with or with out an orienting e l e c t r i c field have been studied. D i e l e c t r i c properties are discussed also. Monomers The monomers which have been synthesized have the general formula:
with n equal to 0, 1, or 2. Synthesis. The preparation of these compounds involved three steps. The first step was the synthesis of 4-cyano-4'hydroxybiphenyl (14):
0-8412-0419-5/78/47-074-086$05.00/0 © 1978 American Chemical Society
7. D U B O I S E T A L .
New Series of Mesomorphic Monomers
87
The complex was broken down by u s i n g concentrated h y d r o c h l o r i c a c i d and f e r r i c c h l o r i d e i n aqueous s o l u t i o n a t 60°C. The second step was the p r e p a r a t i o n of the a c i d c h l o r i d e from the s e l e c t e d v i n y l i c a c i d (15): C H = C H - ( C H ) - C 0 0 H +/ 0 YcOCl * CH =CH-(CH ) -C0Cl ~" \—/ heating 2
2
2
n
2
n
The a c i d c h l o r i d e was s imultane ou s l y d i s t i l l e d and c o l l e c t e d on c o o l hydroquinone. The l a s t step, e s t e r i f i c a t i o n , was conducted a t room temperature i n Ν,Ν -dimethylaniline medium (l6): 1
C H = C H - ( C H ) - C 0 C l + H0-( Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
2
2
kS h r
0 )-( 0 ) - C N
n
CH =CH( 2
CH
2
) coo n
-^T^X^^)-
CN
The r e s u l t i n g compound was p u r i f i e d by chromatography on a s i l i c a g e l column w i t h a benzene/hexane mixture as e l u a n t and then was r e c r y s t a l l i z e d two or three times i n e t h a n o l . Mesomorphic P r o p e r t i e s . The s y n t h e s i z e d compounds have been i n v e s t i g a t e d by means of a p o l a r i z i n g microscope equipped w i t h a h e a t i n g and c o o l i n g stage ( M e t t l e r FP5). The temperature and e n t h a l p i e s of t r a n s i t i o n were determined by d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC 1 B-Perkin Elmer). The r e s u l t s are l i s t e d i n Table I where Κ, Ν, I a r e , r e s p e c t i v e l y , the c r y s t a l l i n e , nematic, and i s o t r o p i c phases, and AE^ i s the m e l t i n g enthalpy. The meso morphic p r o p e r t i e s d i f f e r g r e a t l y a c c o r d i n g t o the l e n g t h of the c h a i n . For comparison, the compounds o f the homologous s a t u r a t e d s e r i e s (17) are mesomorphic a t lower temperatures. The presence of the v i n y l l i n k a g e i n the unsaturated compounds s t a b i l i z e s the mesophase. Table I .
T r a n s i t i o n Temperatures and E n t h a l p i e s f o r the U-Cyano-^ -biphenyl(n+2)-alkenoates 1
η
Κ
0
.
100
.
I30
1
. .
136 73
. .
166 [73]
2
Ν
I
ΔΗ^
(Kcal/mol)
k,6
·
3,7
I n order t o widen the nematic range of the monomers and t o decrease t h e i r m e l t i n g temperatures, we have made b i n a r y mixtures of compounds η = 0 and η = 1 (Table i ) . We observed t h a t t h e experimental diagram d i d not e x h i b i t an e u t e c t i c , as i s g e n e r a l l y observed w i t h other l i q u i d c r y s t a l s . The two compounds are probably m i s c i b l e i n a l l p r o p o r t i o n s i n t h e i r c r y s t a l l i n e s t a t e ( s o l i d s o l u t i o n ) . The phase diagrams, experimental and c a l c u l a t e d , are p l o t t e d on F i g u r e 1.
M E S O M O R P H I C ORDER I N P O L Y M E R S
88
The d i e l e c t r i c a n i s o t r o p y € o f the f i r s t monomer, the i f - c y a n o - V - a c r y l o y l o x y b i p h e n y l , was measured. The £ o f a 0.10/0.90 m o l a r - f r a c t i o n mixture o f t h i s compound, w i t h a nematic compound having an £ equal t o 0.1, i s 2.1. Since the a n i s o t r o p i c s a r e a d d i t i v e , the a n i s o t r o p y of the considered monomer i s approximately equal t o 2.0. fl
a
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
Bulk P o l y m e r i z a t i o n Bulk p o l y m e r i z a t i o n of each monomer has been s t u d i e d by means o f d i f f e r e n t i a l scanning c a l o r i m e t r y . The procedure f o r f o l l o w i n g t h e k i n e t i c s was s i m i l a r t o t h a t d e s c r i b e d by Blumstein and Hsu ( l 8 ) . A s m a l l q u a n t i t y o f t h e compound was sealed i n a DSC sample pan. The temperature was i n c r e a s e d l i n e a r l y t o the m e l t i n g p o i n t and was then s e t q u i c k l y t o the d e s i r e d v a l u e . The i s o t h e r m a l p o l y m e r i z a t i o n was performed d u r i n g a given time t . A t t h e end o f t h i s time, the sample pan was quenched w i t h a dry ice-methanol mixture t o stop the p o l y m e r i z a t i o n and t o a l l o w the r e c r y s t a l l i z a t i o n t o take p l a c e . A thermogram was then taken again. F o r each p o l y m e r i z a t i o n time, a new sample was used. The r a t e o f conversion i n fo i s equal t o ( l -ΔΗ/ΔΗ ) Χ 100 where H i s the m e l t i n g enthalpy o f the pure monomer, and Δ Η i s the t r a n s i t i o n enthalpy o f the remaining monomer a f t e r p a r t i a l p o l y m e r i z a t i o n . Thermal p o l y m e r i z a t i o n o f the f i r s t monomer o f the s e r i e s i n i t s nematic s t a t e i s very f a s t . The e v o l u t i o n o f i t s m e l t i n g peak i s shown i n F i g u r e 2. This f i g u r e a l s o shows t h a t the presence o f oxygen reduces the r a t e of p o l y m e r i z a t i o n . Since the two other monomers need a d d i t i o n o f i n i t i a t o r t o polymerize, only the k i n e t i c s t u d i e s o f thermal p o l y m e r i z a t i o n o f the i f - a c r y l o y l o x y - V -cyanobiphenyl a r e presented. The r a t e s of conversion i n wt. °jo vs. time f o r three temperatures are p l o t t e d on F i g u r e 3· The curves are p e r f e c t l y l i n e a r up t o 8θ/ο conversion, monomer and polymer are m i s c i b l e , and no phase segregation occurs. From these curves, the l o g a r i t h m o f the p o l y m e r i z a t i o n r a t e vs. ΐ/τ has been p l o t t e d (Figure k) and the energy o f activation c a l c u l a t e d . E i n t h e nematic phase i s equal t o 119 k J/mol. The b u l k polymer obtained a f t e r 80 min a t 1 1 0 ° C i n Np atmosphere i s white, very hard,and b r i t t l e . The t e x t u r e o f a t h i n f i l m o f t h i s polymer looks l i k e a d i s o r d e r e d mesophase t e x t u r e . The polymer i s i n s o l u b l e i n a l l the c l a s s i c a l s o l v e n t s except i n the dimethyIformamide. I t s w e l l s i n chloroform. The thermogram o f t h i s polymer does not e x h i b i t any t r a n s i t i o n from room temperature u n t i l 260 C. 0
0
}
a
1
Thin Layer P o l y m e r i z a t i o n of ^ - A c r y l o y l o x y - ^ - c y a n o b i p h e n y l I n v e s t i g a t i o n o f o p t i c a l and e l e c t r i c a l p r o p e r t i e s o f t h i s polymer may be f a c i l i t a t e d by making t h i n l a y e r s . Thus t h e p o l y m e r i z a t i o n has been done i n a ^Ο-μια t h i c k c e l l . Such a c e l l
New Series of Mesomorphic Monomers
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
DUBOIS E T A L .
1. t-=o 2. aïbtLr to ι** ait 1 Ι ο ( 1 Z. « f t ^ r ^ O H i v i ait H o ° C 6
Figure 2. Polymerization of 4-cyano4 -acryïoyloxyhiphenyl f
MESOMORPHIC
ORDER I N
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
!2û°d
Figure 3. Conversion of 4-cyano-4'-acryloyloxybiphenyl to polymer as a function of time at various temperatures
E
A
= iw
kJ/Mol.
o.i
o.oi
Figure 4. Arrhenius plot for the polymerization of 4-cyano-4 -acryloyloxyhiphenyl in the nematic phase f
POLYMERS
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
DUBOIS E T
AL.
New Series of Mesomorphic Monomers
Figure 5. Cell for thermal polymerization of a 50-pm film of a liquid crystal monomer orientated by an electric field
Vacuum, o r
\w4st<^a<
<:1*ΑΚΑ.4>«.Γ
M
Figure 6. Apparatus for thermal polymerization of mesomorphic monomers under electric field
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
M E S O M O R P H I C
ORDER
I N
P O L Y M E R S
18c
Figure 7.
Evolution of the static dielectric constant e of 50-μτη film of p
CH = 0Η-€ΟΟ-ζθ^-ζθ^-€Ν 2
during polymerization
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
7.
DUBOIS E T A L .
New Series of Mesomorphic Monomers
93
(Figure 5) i s made up o f two conductive g l a s s p l a t e s separated by T e f l o n spacers 50yum t h i c k . The monomer i n the s o l i d s t a t e was placed a t the i n l e t o f the c e l l , degassed under 20 mm Hg a t 70°C, and then i n t r o d u c e d by c a p i l l a r i t y on m e l t i n g a t 100°- 102°C. P o l y m e r i z a t i o n was achieved a t 110°C i n 90 min under N2 atmosphere. W e l l polymerized l a y e r s , d i s p l a y i n g f l a t thermograms up t o 260°C, were obtained. Observation w i t h p o l a r i z i n g microscope shows biréfringent and d i s o r d e r e d t e x t u r e . A l o n g - d i s t a n c e order i s induced i n a nematic l i q u i d c r y s t a l by a p p l y i n g an a p p r o p r i a t e magnetic o r e l e c t r i c f i e l d . I f such a f i e l d i s a p p l i e d d u r i n g p o l y m e r i z a t i o n , a mesomorphic order may be a t l e a s t p a r t i a l l y maintained. Therefore we have performed a s e r i e s o f t h i n l a y e r p o l y m e r i z a t i o n s under d.c. o r a.c. e l e c t r i c f i e l d w i t h values from 6 t o 20 kV/cm. The c a p a c i t y o f the c e l l was measured simultaneously d u r i n g the p o l y m e r i z a t i o n by an automatic capacitance b r i d g e (H.P. 4270A). The scheme o f the apparatus i s drawn i n F i g u r e 6. An example o f t h e e v o l u t i o n o f the d i e l e c t r i c constante.ρ o f the polymer being formed i s given i n F i g u r e 7 · The l a s t 6 ρ value i s independent o f the value o f the a p p l i e d f i e l d s and always reaches 3-2. We can conclude t h a t the p o l a r i z a t i o n o f o r i e n t a t i o n , which i s very important i n the monomer when é,^>/ 20, disappears i n the polymer. The d i p o l e s are blocked. I n order t o know i f the mesogenic s i d e groups have kept the mesomorphic, smectic, o r nematic order, x-ray i n v e s t i g a t i o n s are now b e i n g performed w i t h a small-angle d i f f r a c t o m e t e r . Comparison w i l l be made w i t h the r e s u l t s obtained on the p o l y (ρ-biphenyl a e r y l a t e ) , which e x h i b i t s a smectic A order w i t h spacing o f about 20 Â between t h e smectic plane ( l g , 2 0 ) . Conclusion Three mesomorphic monomers b e l o n g i n g t o a new s e r i e s , the ^—cyano-V - b i p h e n y l (ri+2)-alkenoates, were prepared. One o f them, the h-cyano-k -acrylatebiphenyl, i s nematic from 100 t o I3O C; i t s thermal p o l y m e r i z a t i o n i n t h e nematic phase i s easy. A 50->im polymerized f i l m has been p l a c e d between two conductive g l a s s p l a t e s . When t h e p o l y m e r i z a t i o n o f such a l a y e r i s performed under o r i e n t i n g e l e c t r i c f i e l d s , the d i e l e c t r i c constant decreases from t h e high έ ^ value o f the l i q u i d c r y s t a l t o the c l a s s i c a l value f o r the polymer. Mesomorphic order i n the r e s u l t i n g polymer w i l l be i n v e s t i g a t e d by x-ray s t u d i e s . 1
0
Literature Cited 1.
K r e n t s e l , B.A., Amerik, Y.B., Vysokomol Soedin. (1971) A 13
n°6 2. 3.
1358.
Blumstein, Α., Billard, J . , Blumstein, R., M o l . C r y s t . Liq. C r y s t . (1974) 25, 83. Paleos, C.M., Labes, M.M., M o l . C r y s t . Liq. C r y s t . (1970) 11, 385.
94
4. 5. 6. 7. 8. 9. 10. 11. 12. Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch007
13. 14. 15. 16. 17. 18. 19. 20.
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Liebert, L., Strzelecki, L . , et al. Bull. Soc. Chim. F r . (1973) nº2, 597. I b i d . , (1973) n°2, 603. I b i d . , (1973) n°2, 605. I b i d . , (1975) n°9, 10, 2073. I b i d . , (1975) n°11-12, 2750. Perplies, V . E . , Ringsdorf, H . , Wendorff, J . H . , Ber. Bunsenges. Phys. Chem. (1974) 78, 921. Liebert, L., S t r z e l e c k i , L., C.R. Acad. Sci. Paris (1973) 276, n° 8, 648. Perplies, Ε., Ringsdorf, H . , Wendorff, J . H . , Polym. Lett. Ed. (1975) 13, 243. Lorkowski, H.J., Reuther, F., Plaste Kautsch. (1976) 23 Jahrgang, 81. Clough, S.B., Blumstein, Α., Hsu, E . C . , Macromolecules (1976) 9, 123. Gray, G.W., Harrison, K.J., Nash, J . Α . , Constant, J., Hulme, D.S., Kirton, J., Raynes, E . P . , L i q . Cryst., Ordered Fluids (1974) 2, 637. Stempel, G . H . , et al., J. Am. Chem. Soc. (1950) 72, 229S t r z e l e c k i , L., Liebert, L., Bull. Soc. Chim. F r . (1973) 2, 597. Dubois, J . C . , Zann, Α., J. Phys. Colloq. C3 (1976) 37, C3-35Hsu, E . C . , Blumstein, Α., Polym. Lett. Ed. (1977) 15, 129Newman, B . A . , F r o s i n i , V . , Magagnini, P.L., ACS Symp. Ser. (1978) 74, 71. Clough, S . B . , Blumstein, Α., de Vries, Α., ACS Symp. Ser. (1978) 74, 1.
Received March 2, 1978.
8 Polymerization in the Liquid Crystalline State: Monomer-Polymer Interactions F. CSER, K. NYITRAI, and G. HARDY
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
Research Institute for Plastics, H-1950 Budapest, Hungary
Sadron e t al. (1) prepared l y o t r o p i c liquid crystalline systems u s i n g polymers and a polymerizable s o l v e n t and attempted t o fix t h e mesomorphic s t r u c t u r e by p o l y m e r i z i n g t h e monomeric s o l v e n t . Bouligand e t al. (2) attempted t o prepare liquid crystalline substances w i t h f i x e d s t r u c t u r e by copolymerizing mono- and bifunctional mesomorphic monomers. However, n e i t h e r group i n v e s t i g a t e d the phase c o n d i t i o n s o f the monomers o r o f the monomer and polymer. I n both cases identical homogeneous phases were assumed before and after p o l y m e r i z a t i o n . I n s o l i d - s t a t e p o l y m e r i z a t i o n , however, the polymer formed in the r e a c t i o n i n t e r a c t e d w i t h the initial monomer phase, forming a new thermodynamic system (3,4,5,6,7). The r e a c t i o n s were e i t h e r heterogeneous o r homogeneous phase topochemical r e a c t i o n s . The polymer remained isomorphous w i t h its monomer forming a one-phase system in homogeneous r e a c t i o n s only ( 3 , 6 ) . This type o f r e a c t i o n is, however, very r a r e (7,8,9,10). Heterogeneous r e a c t i o n s , in which the polymer and the monomer a r e not isomorphous, a r e much more frequent. I n many cases, polymerizat i o n s s t a r t i n g as homogeneous change i n t o a r e a c t i o n which is predominantly heterogeneous. The determining c o n d i t i o n f o r a homogeneous r e a c t i o n is the isomorphism o f the tactic polymer w i t h the monomer crystals. This isomorphism can o n l y be realized under s p e c i a l c o n d i t i o n s r e q u i r e d by the thermodynamics of the systern. These c o n d i t i o n s a r e as f o l l o w s : the chain p e r i o d o f the polymer must c o i n c i d e w i t h a translational period of the monomer crystal lattice, the o v e r l a p p i n g volumes o f monomeric u n i t s in the polymer chain should not differ g r e a t l y from t h a t o f t h e monomer molecule (11), and the volume c o n t r a c t i o n d u r i n g the c h a i n formation should not beint h e direction of the chain growth. These c o n d i t i o n s a r e fulfilled in c r y s t a l s o f monomers w i t h a long paraffinic chain s u b s t i t u e n t (12). F i g u r e 1 shows an example o f the isomorphous solid s o l u t i o n of p o l y ( c e t y l v i n y l e t h e r ) in its s i n g l e l a y e r o f pgg l a y e r symmetry. The isomorphism o f monomer c r y s t a l s and o f the
0-8412-0419-5/78/47-074-095$05.00/0 © 1978 American Chemical Society
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
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polymer formed has been reported by s e v e r a l authors (8,2,10,13). The topochemical aspect of the r e a c t i o n v a r i e s depending on the type o f mesomorphic s t a t e . Topochemical p o l y m e r i z a t i o n can be expected i n smectic mesophases (l2) where the f u n c t i o n a l groups o f the l a y e r formed by the p a r a l l e l chains are l o c a t e d i n one plane (lA,15). Homogeneous topochemical r e a c t i o n s may proceed i n a l a y e r w i t h e i t h e r u n i - or b i m o l e c u l a r s t r u c t u r e . I n the p o l y m e r i z a t i o n of c e t y l v i n y l e t h e r (l6) the polymer formed remained isomorphous w i t h the monomer, and the m e l t i n g p o i n t o f the system i n c r e a s e d . This isomorphism was destroyed, however, when the system reached i t s m e l t i n g p o i n t . A f t e r c o o l i n g i t formed a two-phase system. I n the p o l y m e r i z a t i o n o f c h o i e s t e r y l a c r y l a t e the i s o morphism o f the polymer w i t h the monomer could be preserved up t o the conversion l i m i t o f TOfo a t 0° C (lî), b u t a t 30° C the system became r a p i d l y heterogeneous because o f the increased m o b i l i t y of the molecules. A t both temperatures the monomer was i n the smectic G s t a t e . The experimental evidence i s shown i n F i g u r e 2. The decrease i n the m e l t i n g and/or c l e a r i n g p o i n t s o f the monomers caused by the presence o f the polymer was detected i n both the smectic s t a t e p o l y m e r i z a t i o n of v i n y l o l e a t e (l8,19) and i n the p o l y m e r i z a t i o n o f c h o i e s t e r y l a c r y l a t e i n the c h o i e s t e r i c s t a t e (lî)· The p o l y v i n y l o l e a t e formed i n t h i s r e a c t i o n was c r y s t a l l i n e w h i l e the p o l y c h o l e s t e r y l a c r y l a t e was amorphous. The temperature o f the phase t r a n s i t i o n was reduced i n both cases. The s i t u a t i o n i s completely d i f f e r e n t i n the c h o l e s t e r i c and the nematic s t a t e s . -The r a t e s o f p o l y m e r i z a t i o n s a r e u s u a l l y lower, and the a c t i v a t i o n energies o f r e a c t i o n s a r e higher than those i n the l i q u i d i s o t r o p i c s t a t e measured o r e x t r a p o l a t e d t o the same temperature (20,21,22). The c h o l e s t e r i c and nematic s t a t e s do not seem t o favor topochemic a l processes, and, t h e r e f o r e , a s t r u c t u r a l isomorphism cannot be expected e i t h e r . Because i n t h i s s t a t e the molecules are not packed densely, the f o r m a t i o n o f a homogeneous s o l u t i o n a l s o can be expected w i t h g r e a t e r d i f f e r e n c e s i n o v e r l a p p i n g volumes. The polymer/monomer s t a t e diagram o f ρ-methyl, JD - a c ryloyloxyazoxybenzene (23) i n F i g u r e 3 shows t h a t near the m e l t i n g p o i n t of the monomer a heterogeneous two-phase system i s formed i n which both phases have a nematic s t r u c t u r e . Above the m e l t i n g p o i n t o f the monomer the s o l u t i o n i s a homogeneous nematic one; the polymer i s d i s s o l v e d i n the i s o t r o p i c l i q u i d phase monomer. Therefore, i n the nematic phase of the monomer a heterogeneous phase p o l y m e r i z a t i o n can be expected. When the r e a c t i o n i s s t a r t e d i n the i s o t r o p i c l i q u i d phase, i t i s homo geneous up t o a g i v e n conversion, where a phase s e p a r a t i o n takes p l a c e . T h e r e a f t e r , as the r e a c t i o n proceeds, the nematic polymer/monomer s o l u t i o n d i s s o l v e s the remaining i s o t r o p i c 1
E T A L .
Polymerization in the Liquid Crystalline State
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
CSER
5
10
15
20
25
30
35
*0
Figure 2. X-ray powder pattern of chotesterylacryhte monomer (a), of its precipitated polymer (d), of polym erizing systerns polymerized at 0°C up to 7Οψο conversion (b), and at 30°C up to 28% conversion (c).
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
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monomer, and the r e a c t i o n becomes homogeneous. The Tg o f the p l a s t i c i z e d polymer system i s reached, and the r e a c t i o n terminates. The polymer/monomer s t a t e diagram of c h o l e s t e r y l v i n y l s u c c i n a t e shows another type of i n t e r a c t i o n (2^4-, 25 ) as can be seen on F i g u r e k. I n one of the three p o s s i b l e c r y s t a l l i n e phases a homogeneous t o p o t a c t i c s o l i d s t a t e p o l y m e r i z a t i o n takes p l a c e . The polymorph i s formed i n the presence o f even a t r a c e of polymer, and there i s a r e a l isomorphism o f the polymer and o f the monomer. The h i g h e s t polymer content i n t h i s s o l i d s o l u t i o n i s c a . 25$. A t h i g h e r tempera t u r e s the isomorphous polymer/monomer system has a smectic s t r u c t u r e . T h e r e a f t e r , i n the c h o l e s t e r i c s t a t e the system becomes heterogeneous. I n samples w i t h a h i g h polymer content two smectic s t a t e s were detected. A low v i s c o s i t y smectic s t r u c t u r e could be detected a t higher temperatures. The p o l y m e r i z a t i o n i n the c h o l e s t e r i c s t a t e o f t h i s monomer i s a heterogeneous phase r e a c t i o n . The polymer formed i s pre c i p i t a t e d as a smectic phase, and as the conversion i n c r e a s e s , the c h o l e s t e r i c monomer i s d i s s o l v e d i n the i s o t r o p i c system, and a homogeneous phase i s formed. As the r e a c t i o n proceeds f u r t h e r , the system becomes smectic (25). For a l o n g time we searched without success f o r a homo geneous r e a c t i o n i n the c h o l e s t e r i c s t a t e u n t i l we began t o study the c h o l e s t e r y l v i n y l f u m a r a t e monomer. I n the f o l l o w i n g we present the polymer/monomer s t a t e diagram of t h i s substance. F i g u r e 5 shows the phase t r a n s i t i o n p o i n t s found i n t h i s system w i t h a 10$ polymer content. F i g u r e 6 d i s p l a y s the DSC t r a n s i t i o n heats as a f u n c t i o n o f the composition. The next two f i g u r e s show the polymer/monomer s t a t e diagrams determined by DSC ( F i g u r e 7), by p o l a r i z i n g microscopy, and by thermomechanical methods ( F i g u r e 8). The f i g u r e s show a good agree ment between d i f f e r e n t methods. F i g u r e 9 d i s p l a y s wide-angle x-ray d i f f r a c t o g r a m s o f some c h a r a c t e r i s t i c compositions o f the polymer/monomer systems. The monomer i s i n the smectic G s t a t e (26). Systems w i t h polymer content o f l e s s than kO°jo c o n s i s t o f two phases - - a n amorphous phase o f the p l a s t i c i z e d monomer and a smectic Β phase o f the monomer c o n t a i n i n g the polymer. When the polymer content i s g r e a t e r than kO°jo, o n l y the amorphous phase i s detected. The upper p a r t o f F i g u r e 9 shows the x-ray d i f f r a c t o g r a m s o f systems p o l y m e r i z i n g a t 1^5 C i n the presence of 0.2$ benzoylperoxide as a f u n c t i o n o f r e a c t i o n time. I n these p o l y m e r i z i n g systems s i m i l a r i n t e r a c t i o n s occur as i n the melted and cooled systems. Samples w i t h polymer contents l e s s than 50$ d i s p l a y the green-blue r e f l e c t i o n c h a r a c t e r i s t i c o f the c h o l e s t e r i c s t a t e . The c h a r a c t e r i s t i c s o f the s t a t e s are as f o l l o w s : When the polymer content i s h i g h e r than 70$, the systems c o n s i s t o f a homogeneous phase which contains the p l a s t i c i z e d 0
Polymerization in the Liquid Crystalline State
CSER E T A L .
p
t°c 300260A
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220-
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140-
J
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/
S\ 1
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S e
100-
-ΛΓ—Λ—*—Α,^ΙΕ ;D
6020 C>
Figure 3. Polymer/monomer state diagram of p-methyl, p'acryloyloxyazoxybenzene ob tained by polarizing microscopy M composition (weight fraction of polymer). Characteristic points: (Αλ decreasing birefrin gence; (yf), mesomorphic transi tion; mesomorphic melting; (Φ), solidus; (O), liquidas. The meaning of the areas: (A), iso tropic liquid; (B), isotropic liquid + mesomorphic plasticized poly mer; (C), liquid; (D), glassy states of plasticized polymer in mesomorphic phase; (E), crystal line monomer -f- mesomorphic plasticized polymer in the glassy state; (F), crystalline monomer -f mesomorphic plasticized poly mer in the liquid state; (G), mesomorphic monomer -f- meso morphic plasticized polymer in the liquid state.
1
.2
.4
.6
.8
1.
_
Figure 4. Polymer/monomer state diagram of cholesterylvinyhuccinate. M , composition (weight fraction of monomer). ([J), T ; (O), change in the thermal exponent of deformation; (S7), T/, all measured by thermomechanics. (V), shift in the base line, if M < 0.5 or second peak if M > 0.5; (Φ), first peak on the DTA trace. (-{-), clearing point; (V), melting begins; (X), phase transformation, all measured by polarizing microscopy. The mean ing of the areas: (A), isotropic liquid; (B, C, D, and E), homo geneous mesomorphic plasticized polymer in liquid (B), in high elas tic (C and D), and in glassy state (E), respectively. Solid solutions are in solid (G) and in plasticized states (11). Mesomorphic solutions are in smectic (I) and in cholesteric (J) states. G, II, I and J are biphasic in the concentration range of0.5<M <0.8. m
g
m
m
m
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Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
•OX
.04 -
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ass
120
140
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t
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Figure 5. DSC traces ( ), depohrized light intensity (· · ·), and deformation ( - X - ) vs. temperature for cholesterylvinylfumarate polymer/monomer systems with 0.1 p/p polymer. The symbols of the curves relate to data on which the state diagrams of Figures 7 and 8 are based. The shadowed areas were converted to heal/mol and are represented in Figure 6.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
8.
CSER E T
0
Polymerization in the Liquid Crystalline State
AL.
2
M
.6
.8
1. Mp
380
101
systems
Figure 7. Polymer/monomer state dia gram of cholesterylvinylfumarate ob tained by DSC. (V), ΔΗ initial stage of transition, (ψ), second peak; ( A , X and +), peaks or shoulders on the main transition of Δ Η . (Φ), Δ Η ; initial stage of transition, (O), second peak on ΔΗ transition. Χ
2
Λ
4
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
MESOMORPHIC
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Figure 8. Polymer/monomer state dia gram of cholesterylvinylfumarate ob tained by thermomechantes and polar izing microscopy. (Q), T ; (-{-), initial stage of phstic deformation; (X), change in the thermal coefficient of deformation; (±), T . (ψ), decrease in the birefringence; (V), cholesteric tran sition, with a gap in the birefringence; (€)), decrease in the birefringence; (Φ), peak or shoulder in the depolarized light intensity; (O), clearing point. g
f
POLYMERS
AL.
Polymerization in the Liquid Crystalline State
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
CSER E T
Figure 9. Cu-Ka x-ray diffractograms of cholesterylvinylfumarate polymer/monomer systems prepared by different ways
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polymer. When the polymer content i s below 70$, the systems c o n t a i n two phases. Based on thermomechanical o b s e r v a t i o n s , the enthalpy change, Δ H i , i s c o n s i s t e n t w i t h t h e Tg o f the whole system. The enthalpy change Δ H2 i s a s s o c i a t e d w i t h the monomer phases c o n t a i n i n g the polymer, and i t c o n s i s t s o f s e v e r a l t r a n s i t i o n s l o c a t e d above the m e l t i n g temperature, Tf. The g r e a t e s t changes i n enthalpy a r e observed a t Tf, b u t they a r e lower than would have been expected (on the b a s i s o f molecular dimensions) from crystalline-to-mesomorphic t r a n s i t i o n s . The c h o l e s t e r i c s t a t e was observed a t the f l o w temperature under a p o l a r i z i n g microscope. The c h o l e s t e r i c r e f l e c t i o n s a r e observed a t lower tempera-tares than Tf i n systems c o n t a i n i n g polymers o r a t Tf of the monomer i t s e l f . This means t h a t there i s a phase t r a n s f o r m a t i o n i n the c h o l e s t e r i c s t a t e too, which corresponds t o the l a s t peak of Δ H^. The dependence o f the t r a n s i t i o n s on c o n c e n t r a t i o n i s s i m i l a r t o t h a t o f the s o l i d s o l u t i o n . The c h o l e s t e r i c - t o - i s o t r o p i c l i q u i d t r a n s i t i o n i s of an e u t e c t i c type. The enthalpy changes,Δ 5ΐμ a r e r e l a t e d t o the c h o l e s t e r i c phases. The composition of the system w i t h the lowest m e l t i n g p o i n t does not c o i n c i d e w i t h the composition o f the homogeneous polymer (^8$ and 72$, r e s p e c t i v e l y ) . The t r a n s i t i o n from c h o l e s t e r i c t o i s o t r o p i c s t a t e takes p l a c e simultaneously w i t h an exothermic t r a n s i t i o n ( Δ H3) a s s o c i a t e d w i t h p o l y m e r i z a t i o n . This exothermic t r a n s i t i o n depends on the polymer/monomer r a t i o of the samples w i t h a m i n i mum and a maximum value a t polymer contents o f 20$ and 60$, r e s p e c t i v e l y . Since t h i s t r a n s i t i o n can be found a l s o i n the p r e c i p i t a t e d polymer, i t may r e s u l t from the t r a n s i t i o n o f c y c l o p o l y m e r i c chains c h a r a c t e r i s t i c o f vinylfumarates (27). The r e f l e c t e d c o l o r o f the cooled samples proves t h a t the polymer d i s s o l v e d i n the monomer phase f i x e s t h e c h o l e s t e r i c s t r u c t u r e o f the monomer, which i s p o s s i b l e o n l y f o r a c h o l e s t e r i c homogeneous mixture of the monomer w i t h the polymer. From t h i s s o l u t i o n a smectic Β phase monomer i s p r e c i p i t a t e d by c o o l i n g , and thus a two-phase system i s obtained c o n s i s t i n g o f a smectic monomer and a c h o l e s t e r i c polymer p l a s t i c i z e d by i t s monomer. A t room temperature, t h e r e f o r e , we have a three-phase system: a p l a s t i c i z e d polymer w i t h a polymer content o f 70$, a polymer phase w i t h a c h o l e s t e r i c s t r u c t u r e e n r i c h e d i n monomer, and a monomer phase w i t h a smectic Β s t r u c t u r e . Consequently, the p o l y m e r i z a t i o n which s t a r t e d i n the c h o l e s t e r i c s t a t e i s a homogeneous r e a c t i o n w i t h a thermodynamically s t a b l e homogeneous phase which i s s o l i d i f i e d by c o o l i n g before a phase s e p a r a t i o n begins. The r e a c t i o n r a t e i s decreased t o a great extent i n the h i g h l y e l a s t i c s t a t e of the p l a s t i c i z e d polymer, and a l i m i t i n g conversion i s a t t a i n e d . F i g u r e 10 shows the polymer/monomer s t a t e diagram of a co polymer prepared from jo-methyl, JD -acryloyloxyazoxybenzene and c h o l e s t e r y l v i n y l s u c c i n a t e and i t s o r i g i n a l monomer mixture. The 1
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
8.
CSER E T A L .
Polymerization in the Liquid Crystalline State
Figure 10. Polymer/monomer state diagrams of a copolymer prepared from a inixture of 0.4 mol of choleste rylv iny hu ce i η a t e and 0.6 mol of pmethyl,p acryloyloxyazoxybenzene and of the original mixture of monomers. (•), T,,; (O), T ; both obtained by thermomechanics. (H), in crease; (X), shoulder; (^f), peak; (A), constant value of depolarized light intensity; (·), clearing point. The meaning of the areas: (A), isotropic liquid; (B), different smectic phases of the plasti cized copolymer in the liquid, (C) in the high elastic, and (D) in the glassy states, respectively. Other areas correspond to biphasic systems, where the plasticized copolymers are in the glassy (J), in highly elastic (I), and in liquid states. (EH).In addition the mixture of monomers in crys talline (I, J), molten eutectic + crystalline (II), and in the cholesteric (E, F, G) states is given. B, and B are biphasic; where one phase is B , the other is either isotropic-liquid or B, . f
f
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f
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c o n d i t i o n s of mesophase formation are much more complicated here than i n the case of homopolymers and - w i l l not be d i s c u s s e d i n d e t a i l . However, the main c o n c l u s i o n t h a t can be made from t h i s and from the other s t a t e diagrams o f the same monomers w i t h d i f f e r e n t compositions (28) i s t h a t there are a t l e a s t one or more ranges o f temperature and composition where two phases e x i s t w i t h d i f f e r e n t compositions and d i f f e r e n t mesophases. Homogeneous phases can be observed a t h i g h e r polymer contents where t h e s t r u c t u r e i s determined by the behavior of the polymer i t s e l f and does not depend on the phase of the pure monomer.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch008
Conclusion In our present work the mesomorphic s t a t e p o l y m e r i z a t i o n was i n v e s t i g a t e d from the p o i n t o f view of s o l i d - s t a t e p o l y m e r i z a t i o n . The thermodynamic i n t e r a c t i o n s of monomers and polymers can be represented by s t a t e diagrams. E f f e c t s r e p o r t e d by Sadron e t a l . may only be expected i n the homogeneous areas o f the s t a t e diagrams. The thermodynamic c o n d i t i o n s l e a d i n g t o such areas a r e f u l f i l l e d f r e q u e n t l y i n smectic s t a t e s b u t r a r e l y i n c h o l e s t e r i c o r nematic ones. Acknowledgement The authors are indebted t o J . M e i s e l ( T e c h n i c a l U n i v e r s i t y of Budapest) f o r the DSC measurements. Literature 1. 2. 3. 4. 5. 6. 7. 8. 9· 10.
Cited
Sadron, C . , Pure A p p l . Chem. (1962) 4, 347. B o u l i g a n d , Y., C l a d i s , P.E., L i e b e r t , L., Strzeleeki, L., Mol. C r y s t . Liq. C r y s t . (1974) 25, 233. K a i s e r , J., Wegner, G . , F i s c h e r , E.W.,Isr. J . Chem. (1972) 10, 157. Coulson, J.P., Renecker, D . H . , J. A p p l . Phys. (1970) 41, 4296. Hardy, G., Cser, F., Takács, E., J. Polym. Sci. (1973) C42, 662. Hardy, G . , " K i n e t i c s and Mechanism of P o l y r e a c t i o n s , " Main and P l e n a r y L e c t u r e s , Akadémiai Kiadó, Budapest, p. 571, 1970. Hardy, G., Cser, F., Kovács, G., Fedorova, N., Samay, G., A c t a Chim. Acad. Sci. Hung. (1973) 79, 143. Wegner, G . , Shermann, W., Colloid Polym. Sci. (1974) 252, 655. Hardy, G., Kovács, G., Koszterszitz, G . , Fedorova, Ν., Cser, F., P r o c . Tihany Symp. R a d i a t . Chem. 3 r d . 1971 (1972) 601. Cser, F., Hardy, G . , A c t a Chim. Acad. Sci. Hung. (1975) 84, 297.
8.
CSER
11. 12. 13.
E T
A L .
Polymerization in the Liquid Crystalline State
107
K i t a i g o r o d s k i i , A . I . , Organic Chemical C r y s t a l l o g r a p h y , Consultant Bureau, N.Y., 1961. Hardy, G., Cser, F., Nyitrai, K., Fedorova, N., Proc. Tihany Symp. R a d i a t . Chem. 4 t h . (1977) in p r e s s . Baughman, R.H., Yee, K.C.,J.Polym. Sci. Chem. (1974) 12, 2467.
14.
Rosilio,
C., Ruaddel-Texier, Α., J. Polym.Sci.Chem. Ed.
(1975) 13, 2459.
15.
Putermann, M., F o r t , T., Lando, J.B.,
16.
Hardy, G., Nyitrai, K., Cser, F., C s e l i k , G., Nagy, I., Eur. Polym. J. (1969) 5, 133. Hardy, G., Cser, F., Kalló, Α., Nyitrai, K., Bodor, G., Lengyel, M., A c t a Chim. Acad.Sci.Hung. (1970) 65, 287. Amerik, Y.B., K r e n t s e l , B.A., Dokl. Akad. Nauk. S.S.S.R.
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17. 18.
Colloid
Interface
Sci.(1974)47,705.
(1965) 165, 1097.
19. 20. 21. 22. 23.
Hardy, G., Cser, F., Fedorova, N., Bátky, M., A c t a Chim. Acad.Sci.Hung. (1977) 94, 275. P a l e o s , C.M., Labes, M.M.,Liq.C r y s t s . I n t .Liq.C r y s t . Conf. Pap. 3rd. (1972) 1065. P e r p l i e s , E., Ringsdorf, H., Wensdorf, J.H., Makromol. Chem. (1974) 175, 553. N y i t r a i , K., Cser, F., Lengyel, M., S e y f r i e d , É., Hardy, G., Eur. Polym. J. (1977) 13, 673. Cser, F., Nyitrai, Κ., S e y f r i e d , Ε., Hardy, G., Eur. Polym. J.
24. 25. 26. 27. 28.
(1977) 13, 679.
Nyitrai, Κ., Cser, F., B u i Due Ngoc, Hardy, G., Magy. Kém. F o l y . (1976) 82, 210. N y i t r a i , K., Cser, F., Csermely, G., B u i Duc Ngoc, Hardy, G., Eur. Polym.J.,accepted f o r p u b l i c a t i o n . de V r i e s , Α., Mol. C r y s t .Liq.C r y s t . (1973) 24, 337. Yamada, M., Takase, I., Y u k i Gosei Kagaku (1963) 20, 180. Cser, F., Nyitrai, K., K o c s i s , J., Hardy, G., unpublished data.
RECEIVED April 1 1 , 1978.
9 Applications of Thermotropic Mesophase Reactions RAJ N. GOUNDER
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
Corporate Research and Development, Lord Corporation, Erie, PA 16512
Recent r e s e a r c h in t h e areas o f m o l e c u l a r / m i c r o structural e n g i n e e r i n g o f polymer solids has resulted in a number o f t e c h n o l o g i c a l b r e a k t h r o u g h s . As a result, we a r e now c l o s e t o realizing t h e theoretically p o s s i b l e maximum performance from these microstructurally engineered polymeric m a t e r i a l s . Table 1 lists a number o f t o o l s t h a t are now a v a i l a b l e f o r t h e microstructural d e s i g n i n g o f u l t i m a t e polymer solids. The potentials and limitations o f t h e s e v a r i o u s tools f o r m o l e c u l a r / m i c r o s t r u e t u r a l e n g i n e e r i n g o f polymer s o l i d s are t h e s u b j e c t s o f a monograph c u r r e n t l y under preparation. Some o f t h e s e t e c h n o l o g i e s offer g r e a t p o t e n t i a l s for producing polymeric s o l i d s with ultimate behaviors in all t h r e e crystallographic directions. Thus, t h e t e c h n i q u e o f solid s t a t e p o l y m e r i z a t i o n has shown t h e practicality of p r o d u c i n g d e f e c t - f r e e s i n g l e c r y s t a l s o f c h a i n extended p o l y m e r i c systems, p r o v i d e d c e r t a i n stringent criteria may be satisfied . U l t i m a t e properties along specific d i r e c t i o n s o n l y , however, a r e o b t a i n e d more easily by a variety o f t e c h n i q u e s . Thus, a number o f t e c h n i q u e s utilizing certain extrinsic v a r i a b l e s have been-very w i d e l y i n v e s t i g a t e d . These i n c l u d e t h e c o n t r o l o f m e c h a n i c a l and t h e r m a l p a r a meters on polymer solids , c o n t r o l of mechanical ( h y d r o s t a t i c ) and t h e r m a l parameters on polymer melts and solutions , as w e l l as t e c h n i q u e s utilizing the effects o f electric and magnetic fields i npolym e r i c systems. A number o f t e c h n o l o g i e s have a l s o been d e v e l o p e d utilizing t h e effects o f intrinsic parameters f o r controlling the m i c r o s t r u c t u r a l a r c h i t e c tures o f polymeric solids. Examples o f these t e c h n i q u e s utilize polymer-solvent interactions , polymerpolymer interactions , and interchain f e a t u r e s — 7 . Two t e c h n i q u e s w i t h maximum s u c c e s s utilize thermotropic mesophase systems and lyotropic liquid crystalline 1
2-5
6-7
8-10
11-12
13
14-15
1-6
0-8412-0419-5/78/47-074-108$05.00/0 © 1978 American Chemical Society
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systems r e s p e c t i v e l y . The Kevlar® f i b e r s were d e v e l oped from l y o t r o p i c l i q u i d c r y s t a l l i n e systemsi-L; t h e e q u a l l y i n t e r e s t i n g u l t r a - h i g h modulus g r a p h i t e f i b e r s are made from t h e r m o t r o p i c mesophase p i t c h e s . The a p p l i c a t i o n o f t h e r m o t r o p i c l i q u i d c r y s t a l l i n e o r the s o c a l l e d mesophase r e a c t i o n s f o r the c o n t r o l of m o l e c u l a r / m i c r o s t r u c t u r a l a r c h i t e c t u r e s i n macro-molecular systems w i l l be the s u b j e c t o f t h i s r e v i e w .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
A p p l i c a t i o n s o f Carbonaceous Mesophase P i t c h The f o r m a t i o n o f mesophase s t r u c t u r e s from the l i q u i d p y r o l y s a t e d u r i n g the heat t r e a t m e n t o f p o t e n t i a l l y g r a p h i t i z a b l e m a t e r i a l s such as a r o m a t i c h y d r o carbons, c o a l - t a r , and p e t r o l e u m p i t c h e s was f i r s t r e p o r t e d by Brooks and T a y l o r L i z i J . The t o t a l l y i s o t r o p i c t a r o r p i t c h t r a n s f o r m s t o an almost comp l e t e l y a n i s o t r o p i c mesophase p i t c h a t temperatures r a n g i n g from 300-50Q°C by the p a r a l l e l a l i g n m e n t o f l a r g e p l a n a r m o l e c u l e s d e v e l o p e d by a r o m a t i c p o l y m e r i z a t i o n a t such températures» Hot s t a g e m i c r o s c o p i c s t u d i e s — ~JLï. have r e v e a l e d t h a t on h e a t i n g a f u l l y i s o t r o p i c t a r o r p i t c h under q u i e s c e n t c o n d i t i o n s a t a temperature of about 350-450°C, s m a l l i n s o l u b l e l i q u i d spheres b e g i n t o appear ( F i g u r e 1 ) . These spheres g r a d u a l l y i n c r e a s e i n s i z e as h e a t i n g i s c o n t i n u e d . These spheres a r e found t o be h i g h l y a n i s o t r o p i c i n c h a r a c t e r . P o l a r i z e d l i g h t m i c r o s c o p y as w e l l as e l e c t r o n d i f f r a c t i o n s t u d i e s show t h a t t h e s e spheres c o n s i s t o f l a y e r s of o r i e n t e d s h e e t - l i k e molecules a l i g n e d i n n e a r l y the same d i r e c t i o n ( F i g u r e 2 ) . These s h e e t l i k e o r p l a n a r m o l e c u l e s were d e v e l o p e d by a r o m a t i c p o l y m e r i z a t i o n upon h e a t i n g a t temperatures over 300°C ( F i g u r e 3 ) . Based upon t h e i r e l e c t r o n d i f f r a c t i o n and p o l a r i z e d l i g h t a b s o r p t i o n s t u d i e s , Brooks and T a y l o r proposed a model f o r the a n i s o t r o p i c spheres as shown i n F i g u r e 4. A c c o r d i n g to t h i s model, the l a y e r a t the e q u a t o r i a l p l a n e i s p e r f e c t l y f l a t whereas the l a y e r s away from the e q u a t o r i a l p l a n e shows i n c r e a s i n g amounts of curvature. As h e a t i n g i n the range o f 300-500°C i s c o n t i n u e d , t h e s e spheres c o n t i n u e t o grow u n t i l they come i n t o c o n t a c t w i t h one a n o t h e r , whereupon they g r a d u a l l y c o a l e s c e w i t h each o t h e r t o produce l a r g e masses o f a l i g n e d l a y e r s ( F i g u r e 5 ) . Thus domains o f a l i g n e d m o l e c u l e s much l a r g e r than t h o s e o f the o r i g i n a l spheres are formed. These domains come t o g e t h e r t o form a b u l k mesophase wherein the t r a n s i t i o n from one o r i e n t e d domain t o another sometimes o c c u r s smoothly ©Dupont t r a d e mark f o r p o l y a r a m i d f i b e r s
M E S O M O R P H I C
ORDER
I N
P O L Y M E R S
TABLE 1
MOLECULAR/MICROSTRUCTURAL ENGINEERING OF POLYMER SOLIDS Reactions in Pre-Oriented Media Solid State Reactions Mesophase Reactions Thin Layers
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
Effect of Intrinsic Parameters Solvent-Polymer Interactions Polymer-Polymer Interactions Interchain Features Effect of Extrinsic Parameters Mechanical and Thermal Parameters on Polymer Solids Mechanical and Thermal Parameters on Polymer Melts Mechanical Parameters on Polymer Solutions Electric and Magnetic Fields
Metallography
Figure 1.
Photomicrograph of mesophase spherules (41 )
Thermotropic Mesophase Reactions
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
GOUNDER
(I)
(II)
(HI)
Figure 3. Formation of planar molecules by aromatic polymerization
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
112
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ORDER I N P O L Y M E R S
Figure 4. (left) Mesophase sphere with section including polar diameter; (right) cross section of mesophase sphere
Molecular Crystals and Liquid Crystals
Figure 5.
Coalescence of mesophase spheres (1977) 38,177
9.
113
Thermotropic Mesophase Reactions
GOUNDER
and c o n t i n u o u s l y through g r a d u a l l y c u r v i n g l a m e l l a e and sometimes through more s h a r p l y c u r v i n g l a m e l l a e . The d i f f e r e n c e i n o r i e n t a t i o n between the domains c r e a t e s a complex a r r a y o f p o l a r i z e d l i g h t e x t i n c t i o n c o n t o u r s i n the b u l k mesophase c o r r e s p o n d i n g t o v a r i o u s t y p e s of l i n e a r d i s c o n t i n u i t y i n m o l e c u l a r a l i g n m e n t . The m o l e c u l a r arrangements w i t h i n the mesophase p i t c h e s thus o b t a i n e d resemble t h a t of c o n v e n t i o n a l nematic l i q u i d c r y s t a l s . I t has been shown t h a t l i k e c o n v e n t i o n a l nematic l i q u i d c r y s t a l s ~ the mesophase p i t c h e s p o s s e s s p l a s t i c p r o p e r t i e s - ! , h i g h degrees of a n i s o t r o p y L i , magnetic f i e l d e f f e c t s - ! , conf i n i n g s u r f a c e s - ! , and e u t e c t i c e f f e c t s ! - ! . However, the mesophase p i t c h e s p o s s e s s o t h e r p r o p e r t i e s which are d i f f e r e n t from the b e h a v i o r s o f c o n v e n t i o n a l nematic liquid crystals. F o r example, the mesophase i s formed i r r e v e r s i b l y ; i t i s r e l a t i v e l y i n s o l u b l e i n benzene and p y r i d i n e — . Mesophase p i t c h e s e x h i b i t h i g h a c t i v a t i o n e n e r g i e s (38-85 K C a l m o l " ) which c o r r e s p o n d t o c h e m i c a l r a t h e r than p h y s i c a l processes- -!"" — . Further, increases i n molecular w e i g h t r and C/H r a t i o s r and the k i n e t i c s i - i of mesophase format i o n i n p i t c h e s a l l i n d i c a t e chemical processes of mesophase f o r m a t i o n . These are n o t p r o p e r t i e s o f conv e n t i o n a l nematic l i q u i d c r y s t a l s . Thus, the mesophase p i t c h e s w h i l e b e i n g l i k e nematic l i q u i d c r y s t a l s i n the s t r u c t u r e s they p o s s e s s , are u n l i k e nematic phases i n t h e i r b e i n g formed through c h e m i c a l p r o c e s s e s . S i n c e the d i s c o v e r y o f mesophase p i t c h e s by Brooks and T a y l o r i n 1965, numerous s t u d i e s have been c a r r i e d out to u n d e r s t a n d and o p t i m i z e the e f f e c t s o f v a r i o u s parameters on the mesophase n u c l e a t i o n , growth, and coalescence. T a b l e 2 l i s t s a number o f parameters t h a t e f f e c t the f o r m a t i o n o f mesophase spheres - t h e i r n u c l e a t i o n , growth and c o a l e s c e n c e as w e l l as the p r o p e r t i e s of the r e s u l t i n g mesophase p i t c h . For example, use o f h i g h p r e s s u r e s d u r i n g heat t r e a t m e n t t o produce mesophases has been shown -! t o i n h i b i t mesophase c o a l escence and produce a l i q u i d c r y s t a l system composed o f uncoalesced spheres. Mesophase sphere growth and c o a l escence are a l s o h i n d e r e d by the p r e s e n c e o f i n s o l u b l e carbon p a r t i c l e s ! ! and m e t a l o x i d e p a r t i c l e s ! * . Of c o u r s e , the s t r u c t u r e and s i z e of mesophases o b t a i n e d by the heat treatment of carbonaceous p i t c h e s v e r y much depend upon the c h e m i s t r y o f p a r e n t s u b s t a n c e s ! ! and the n a t u r e o f r a d i c a l i n t e r m e d i a t e s o f the p y r o l y t i c process!- . The amount and the c o m p o s i t i o n o f the mesophase as w e l l as any c o - e x i s t i n g i s o t r o p i c phase i n a p y r o l y s a t e p i t c h depend upon temperature -**, . The 2 2
2 5
2
2
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2
1
3
1 9
2 1
30
3
8
39
1
3
5
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v i s c o s i t y o f the p y r o l y s a t e v e r y much c o n t r o l s the c o a l e s c e n c e o f mesophase spheres and hence the forma t i o n o f a n i s o t r o p i c domains. V a r i o u s s t u d i e s ~ * have been performed t o s t u d y the e f f e c t s o f temperature, h e a t i n g time and v i s c o s i t y on the mesophase sphere f o r m a t i o n , growth and c o a l e s c e n c e . Yamada, e t a l — have shown t h a t removal o f low m o l e c u l a r weight mole c u l e s from p a r t i a l l y t r a n s f o r m e d p i t c h e s by s o l v e n t e x t r a c t i o n a f f e c t s the v i s c o s i t y and hence p r e v e n t s sphere c o a l e s c e n c e d u r i n g a d d i t i o n a l h e a t i n g . Thus, a body o f knowledge has been d e v e l o p e d e x p l o r i n g the e f f e c t s o f v a r i o u s parameters on the n a t u r e of meso phases formed i n carbonaceous p i t c h e s . D u r i n g t h i s same p e r i o d , the t e c h n o l o g y o f p r o d u c i n g carbon f i b e r s from v a r i o u s p i t c h e s was a l s o b e i n g established. ~ The a l r e a d y e x i s t i n g body o f know l e d g e c o n c e r n i n g mesophase f o r m a t i o n i n p i t c h e s has g r e a t l y impacted on the development o f p i t c h p r e c u r s o r c a r b o n f i b e r t e c h n o l o g y . Thus, about the same time Brooks and T a y l o r o f A u s t r a l i a p u b l i s h e d t h e i r r e s u l t s on the mesophase f o r m a t i o n i n carbonaceous p i t c h e s , O t a n i U o f Japan p u b l i s h e d h i s work on the m e l t s p i n n i n g o f carbon f i b e r s from c o n v e n t i o n a l i s o t r o p i c carbonaceous p i t c h e s . O t a n i s process involved several s t e p s as shown i n F i g u r e 6. F i r s t , carbon f i b e r s were spun from p i t c h e s s p e c i a l l y p r e p a r e d by p y r o l y z i n g p o l y v i n y l c h l o r i d e a t a temperature of 400-415°C f o r 30 minutes i n a n i t r o g e n atmosphere. The melt-spun f i b e r was made i n f u s i b l e by o x i d i z i n g i n ozone o r a i r and was then c a r b o n i z e d a t a temperature i n the range o f 500-1350°C. The f i b e r s p r e p a r e d i n t h i s manner e x h i b i t e d a t e n s i l e modulus and s t r e n g t h o f 8 χ 10^ p s i and 256 χ 1 0 p s i r e s p e c t i v e l y . L a t e r l , carbon f i b e r s w i t h s i m i l a r p r o p e r t i e s were a l s o p r e p a r e d from n a t u r a l l y o c c u r i n g p i t c h e s such as p e t r o l e u m a s p h a l t and c o a l - t a r p i t c h e s . A few y e a r s l a t e r , Hawthorne and c o w o r k e r s L i m Canada showed t h a t the m e c h a n i c a l p r o p e r t i e s o f p i t c h f i b e r s c o u l d be g r e a t l y enhanced by s t r e t c h i n g the f i b e r d u r i n g the c a r b o n i z a t i o n s t e p and c a r r y i n g out the c a r b o n i z a t i o n a t a much h i g h e r temperature o f 2000-2800°C (see F i g u r e 7 ) . The f i b e r s thus o b t a i n e d by s t r a i n i n g d u r i n g c a r b o n i z a t i o n e x h i b i t e d an e l a s t i c modulus as h i g h as 70 χ 10^ p s i and a t e n s i l e s t r e n g t h of almost 375 χ 10 p s i . Such f i b e r s were shown to e x h i b i t a h i g h degree o f c r y s t a l l i t e o r i e n t a t i o n p a r a l l e l t o the l o n g i t u d i n a l f i b e r a x i s . However, the i n d i v i d u a l c r y s t a l l i t e s were found t o be t u r b o s t r a t i c and e s s e n t i a l l y d e v o i d o f t h r e e - d i m e n s i o n a l o r d e r c h a r a c 39
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
POLYMERS
h k
5 0
1
3
2
I
2
9.
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115
TABLE 2
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
EFFECT OF PARAMETERS ON MESOPHASE FORMATION
Isotropic Pitch
1.
Pressure
2.
Insoluble Particles
3.
Temperature
4.
Parent Material
5.
Nature of Radical Intermediate
6.
Viscosity
7.
Heating Rate and Time
Melt Spinning
Spun Fiber
Oxidize in Ozone
Infusible Fiber
w
Εj
= 8 x 1 0 psi
Oj
= 256 χ 1 0 psi
Carbonize 500-1350° C
Carbon Fiber
6
3
Carbon
Figure 6.
Isotropic Melt ^ — Pitch Spinning
w
Fibers from isotropic pitches (51)
Spun Oxidize Infusible ^ >» Fiber in Ozone Fiber Εj
= 70 χ 1 0 psi
σ
= 375 χ 1 0 psi
τ
Strain & Carbonize Oriented _ >. Carbon 2000 - 2800 C p i b e r
6
3
Oriented Fibers Turbostratic Devoid of 3D Order Nature
Figure 7.
Fibers from strained isotropic pitches (53)
MESOMORPHIC
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116
ORDER IN
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t e r i s t i c of p o l y c r y s t a l l i n e graphite. Only r e c e n t l y , the u s e f u l n e s s o f m e l t - s p i n n i n g carbon f i b e r s from a carbonaceous p i t c h i n i t s meso phase s t a t e has been r e a l i z e d and patented!!!!!! . According to these p a t e n t s , petroleum p i t c h , c o a l t a r p i t c h o r a c e n a p h t h y l e n e p i t c h i s f i r s t c o n v e r t e d t o the mesophase s t a t e by h e a t i n g under q u i e s c e n t c o n d i t i o n s and under vacuum o r n i t r o g e n atmosphere a t a tempera t u r e o f about 3 5 0 - 4 5 0 ° C (see F i g u r e 8 ) . The c a r b o n aceous p i t c h h a v i n g a mesophase c o n t e n t o f 5 5 t o 6 5 p e r c e n t thus produced i s spun i n t o f i b e r s by c o n v e n t i o n a l t e c h n i q u e s such as m e l t s p i n n i n g , c e n t r i f u g a l s p i n n i n g o r blow s p i n n i n g . F o r 5 5 t o 65 p e r c e n t mesophase c o n t e n t , s u i t a b l e s p i n n i n g temperatures range from 3 4 0 ° C to 3 8 0 ° C . The t h e r m o p l a s t i c carbonaceous f i b e r s thus o b t a i n e d are g i v e n a t h e r m o s e t t i n g treatment by h e a t i n g i n an o x y g e n - c o n t a i n i n g atmosphere a t temperatures below the s o f t e n i n g p o i n t o f the f i b e r s but above a t l e a s t 2 5 0 ° C f o r 5 to 60 minutes. The i n f u s i b l e f i b e r s o b t a i n e d by t h e r m o s e t t i n g are then c a r b o n i z e d by h e a t i n g i n an i n e r t atmosphere a t a temperature r a n g i n g from 1 5 0 0 ° C t o 1 7 0 0 ° C f o r about 1 to 5 m i n u t e s . The carbon i z e d f i b e r s are f u r t h e r heat t r e a t e d i n an i n e r t atmos phere a t a temperature of 2 8 0 0 ° C t o 3 0 0 0 ° C f o r about 1 minute. The f i b e r s thus o b t a i n e d p o s s e s s the t h r e e dimensional order of p o l y c r y s t a l l i n e graphite. The f i b e r s c o n s i s t o f h i g h l y o r i e n t e d g r a p h i t i c domains o f 5 0 0 0 A to 4 0 , 0 0 0 A i n s i z e . As a r e s u l t , the f i b e r s obtained e x h i b i t extremely high mechanical p r o p e r t i e s w i t h the Young's modulus i n the range of 7 5 χ 1 0 ^ p s i to 1 2 0 χ 1 0 p s i and t e n s i l e s t r e n g t h i n the range o f 250 χ 1 0 p s i , to 350 χ 1 0 p s i . B e s i d e s the a p p l i c a t i o n s i n the p r o d u c t i o n o f u l t r a h i g h modulus c a r b o n f i b e r s , the mesophase p i t c h e s are used i n making t h e r m a l - s h o c k - r e s i s t a n t aerospace g r a p h i t e s — and g r a p h i t e e l e c t r o d e s u f o r the s t e e l industry. Needle cokes are produced by d e f o r m a t i o n t h r o u g h t u r b u l e n c e and bubble p e r c o l a t i o n o f c a r b o n aceous mesophase p i t c h e s ' . Such a n i s o t r o p i c n e e d l e cokes form f i l l e r m a t e r i a l s i n the p r o d u c t i o n o f t h e r m a l - s h o c k - r e s i s t a n t a e r o s p a c e g r a p h i t e s and g r a p h i t e e l e c t r o d e s . The n e e d l e cokes are shown t o con s i s t o f a c i c u l a r - s h a p e d , c o r r u g a t e d a r o m a t i c mesophase l a y e r s t h a t are t i g h t l y f o l d e d and entwined. Increased c o n t e n t of the a c i c u l a r phase i n g r a p h i t e s i s shown to d e c r e a s e the t h e r m a l e x p a n s i o n î l z l l and i n c r e a s e the r e s i s t a n c e t o c l e a v a g e and f r a c t u r e - ! . 6
3
3
60
6 2
6
9.
GOUNDER
Thermotropic Mesophase Reactions
Development o f L i q u i d C r y s t a l l i n e
Polyesters
Another recent development i n v o l v i n g the use o f t h e r m o t r o p i c mesophases i s the d i s c o v e r y o f p o l y esters exhibiting l i q u i d c r y s t a l l i n e melts. These p o l y e s t e r s are prepared by s y n t h e s i z i n g polymers cont a i n i n g m o i e t i e s known t o l e a d t o l i q u i d c r y s t a l l i n i t y i n non-polymeric materials. The s t r u c t u r a l r e q u i r e m e n t s f o r the l i q u i d c r y s t a l f o r m a t i o n h a s been w e l l - s t u d i e d by GrayiL .. Most c o n v e n t i o n a l nematic and smectic l i q u i d c r y s t a l s approximate a general s t r u c t u r e as shown i n F i g u r e 9. F i g u r e 9 a l s o shows a f e w e x a m p l e s of t h el i n k a g e groups and t e r m i n a l groups t h a t f a c i l i tate theformation o f l i q u i d c r y s t a l s . The r e c e n t d e v e l o p m e n t o f l i q u i d c r y s t a l l i n e p o l y e s t e r s make u s e o f s u c h m o i e t i e s t h a t a r e known t o f a c i l i t a t e l i q u i d c r y s t a l formation. Thus, Kuhfuss and coworkers successfully prepared a l i s t of polyesters containing moieties l i s t e d i n F i g u r e 1 0 . Many o f t h e s e p o l y e s t e r s a n d c o p o l y e s t e r s are f o u n d t o be l i q u i d c r y s t a l l i n e i n t h e i r m o l t e n s t a t e . For example, these workers prepared a c o p o l y e s t e r o f PET w i t h a n u m b e r o f l i q u i d c r y s t a l f o r m i n g m o i e t i e s . T h e s e c o p o l y e s t e r s , i n a d d i t i o n t o t h e PET s e g m e n t s I I I , c o n t a i n e d t h e l i q u i d c r y s t a l f a v o r i n g segments I I and IV (see F i g u r e 1 1 ) . These c o p o l y e s t e r s were prepared by t h e a c i d o l y s i s o f P E T w i t h p - a c e t o x y b e n z o i c a c i d and polycondensation through t h eacetate and c a r b o x y l groups. Thus a l i s t o f c o p o l y e s t e r s c o n t a i n i n g v a r i o u s m o l e f r a c t i o n s o f p a r a o x y b e n z o y l m o i e t i e s were o b t a i n e d . I t was f o u n d t h a t m e l t s o f c o p o l y e s t e r s c o n t a i n i n g 40-90 m o l e p e r c e n t p a r a o x y b e n z o y l s e g m e n t s e x h i b i t e d behaviors s i m i l a r t o those o f nematic l i q u i d c r y s t a l s . As s h o w n i n F i g u r e 1 2 , m e a s u r e m e n t s o f t h e m e l t v i s c o s i t i e s o f c o p o l y e s t e r s c o n t a i n i n g v a r y i n g mole p e r c e n t p a r a o x y b e n z o y l r e v e a l e d v e r y i n t e r e s t i n g f e a t u r e s . Up t o 30 m o l e p e r c e n t p a r a o x y b e n z o y l c o n t e n t t h e m e l t viscosity increased. H o w e v e r , h i g h e r t h a n 30 m o l e p e r cent para oxybenzoyl r e s u l t e d i n a decrease i nthe melt v i s c o s i t y l e a d i n g t o a m i n i m u m a r o u n d 60 m o l e p e r cent para oxybenzoyl. Further, copolyesters containing i n t h e r a n g e o f 0-30 m o l e p e r c e n t p a r a o x y b e n z o y l e x h i b i t e d c l e a r m e l t s , whereas c o p o l y e s t e r s c o n t a i n i n g g r e a t e r t h a n 40 m o l e p e r c e n t p a r a o x y b e n z o y l l e d t o opaque m e l t s . T h i s was a c l e a r i n d i c a t i o n o f l i q u i d c r y s t a l f o r m a t i o n a t g r e a t e r than 4 0 mole p e r c e n t p a r a oxybenzoyl contents. The c o p o l y e s t e r s p r o c e s s e d f r o m s u c h l i q u i d c r y s t a l forming melts e x h i b i t very i n t e r e s t i n g mechanical properties. F i g u r e 13 s h o w s t h e e f f e c t o f p a r a o x y b e n z o y l content on the m e c h a n i c a l p r o p e r t i e s ( i n t h e 2
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117
66- 69
118
MESOMORPHIC
ORDER IN P O L Y M E R S
Isotropic Pitch Inert A t m . , Quiescent Cond. 400 - 450° C 55 - 6 5 % Mesophase Pitch Melt Spin 340 - 380° C Spun Fiber Thermoset in Oxygen 300 - 390° C
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
U.S. Patent 4,005,183
Infusible Fiber Carbonize 1 5 0 0 - 1700° C Graphitize 2 8 0 0 - 3 0 0 0 ° C
Figure 8. Carbonfibersfrom meso Ultrahigh Modulus Carbon Fibers phase pitches (58)
- C H = CH -
Ν =Ν — C - O -
il Ο -C = C -
— CH = Ν
Ο CH
3
-
(CH ) 2
RO
-
II C
Ο
Figure 9. (top) Structure of liquid crystals; (middle) examples of linkage groups; (bot tom) examples of terminal groups
Thermotropic Mesophase Reactions
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
GOUNDER
m Figure 10.
Ο
Polymers containing moieties known to lead to liquid crystalUnity
Ο r s
CH C—O 3
C — O H
I 0
-o—f
0
V-c—ο—f
V-c-
-OH + C H C — O H 3
(1)
II "0
0
0 -OCH CR,02
+
€—OH-y
CH C—0 3
(2)
III Ό
0
0
0 C—0CH CR,0-f2
0
7
0
0
C—OCH CH 02
2
+
CH^OH
(3)
IV Figure 11.
Synthesis of copolyesters containing liquid crystalline segments
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
120
MESOMORPHIC
ORDER IN
POLYMERS
Figure 12. Effect of p-OB content on the melt viscosity of copolyesters
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
9.
GOUNDER
Thermotropic Mesophase Reactions
121
Figure 13. Effect of p-OB content on the mechanical properties of copolyesters
MESOMORPHIC
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
122
ORDER IN
POLYMERS
i n j e c t i o n molding d i r e c t i o n ) o f i n j e c t i o n molded c o polyesters. The c o n t i n u o u s l i n e r e p r e s e n t s the f l e x u r a l modulus and the d o t t e d l i n e denotes the t e n s i l e strength. The p l o t i n d i c a t e s t h a t maximum i n t h e s e p r o p e r t i e s are a c h i e v e d around 60 mole p e r c e n t p a r a o x y b e n z o y l c o n t e n t , thus i n d i c a t i n g the e f f e c t o f l i q u i d c r y s t a l f o r m a t i o n on the m e c h a n i c a l p r o p e r t i e s o f r e s u l t i n g polymer s o l i d . T a b l e 3 summarizes the m e c h a n i c a l p r o p e r t i e s o f an i n j e c t i o n molded l i q u i d c r y s t a l l i n e polymer cont a i n i n g 60 mole p e r c e n t p a r a o x y b e n z o y l . The mechani c a l p r o p e r t i e s were measured p a r a l l e l t o the i n j e c t i o n m o l d i n g d i r e c t i o n ( l o n g i t u d i n a l ) as w e l l as t r a n s v e r s e t o the i n j e c t i o n m o l d i n g d i r e c t i o n ( t r a n s v e r s e ) . The f l e x u r a l moduli, t e n s i l e s t r e n g t h s , e l o n g a t i o n to break and impact s t r e n g t h o f c o p o l y e s t e r s e x h i b i t i n g l i q u i d c r y s t a l l i n e m e l t s are found to be s i m i l a r t o o r h i g h e r than t h o s e o f commercial g l a s s f i b e r r e i n f o r c e d polyesters. M c F a r l a n e and coworkers -! have s y n t h e s i z e d o t h e r polymers c o n t a i n i n g o t h e r moitiés t h a t are a l s o known to l e a d to l i q u i d c r y s t a l l i n i t y i n n o n - p o l y m e r i c forms. Thus, a wide v a r i e t y o f h i g h m o l e c u l a r weight copolymers e x h i b i t i n g l i q u i d c r y s t a l l i n e m e l t s may be p r e p a r e d . S t r u c t u r a l shapes c o n s i s t i n g o f a h i g h degree o f molec u l a r o r i e n t a t i o n s and c h a i n e x t e n s i o n s may be e a s i l y f a b r i c a t e d from such l i q u i d c r y s t a l l i n e m e l t s . 6
Polymers
w i t h L i q u i d C r y s t a l l i n e S i d e Chains
E x t e n s i v e work has been done i n a number o f l a b o r a t o r i e s - on the s y n t h e s i s and c h a r a c t e r i z a t i o n o f s e v e r a l polymers w i t h l i q u i d c r y s t a l l i n e s i d e c h a i n s . V a r i o u s o f the p a p e r s i n t h i s monograph d e a l w i t h v a r i o u s a s p e c t s o f such polymers c o n t a i n i n g mesomorphic s i d e c h a i n s . A r e v i e w of the p o t e n t i a l a p p l i c a t i o n s of such s i d e c h a i n l i q u i d c r y s t a l l i n e polymers t h e r e f o r e , w i l l be redundant. 70
75
Conclusion In c o n c l u s i o n , i t may be s a i d t h a t the thermotropic l i q u i d c r y s t a l l i n e reactions offer a highly e f f i c i e n t t e c h n i q u e f o r the c o n t r o l of the m o l e c u l a r / m i c r o s t r u c t u r a l a r c h i t e c t u r e s i n macromolecular s y s tems . A v a r i e t y o f n o v e l p o l y m e r i c systems e x h i b i t i n g unique m e c h a n i c a l and p h y s i c a l p r o p e r t i e s have been p o s s i b l e through s p e c i a l m o l e c u l a r / m i c r o s t r u c t u r a l d e s i g n s o b t a i n e d by u t i l i z a t i o n o f t h e r m o t r o p i c mesophase r e a c t i o n s .
9.
Thermotropic Mesophase Reactions
GOUNDER
Literature 1. 2. 3. 4.
Cited
R. N. Gounder and S. H. C a r r , "Molecular/Microstructural E n g i n e e r i n g o f Polymer S o l i d s " , in preparation. R. H. Baughman and K. C. Yee, J. Macromol. Sci. Review, in p r e s s . R. H. Baughman, "Proceedings o f t h e 8th Biennial Polymer Symposium o f Division o f Polymer C h e m i s t r y " , ACS (Plenum P r e s s , 1 9 7 7 ) . D. B l o o r , L . K o s k i and G. C. S t e v e n s , J . Materials
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S. L. Kwolek, U. S. P a t e n t 3 , 6 7 1 , 5 4 2 a s s i g n e d t o DuPont (June 2 0 , 1 9 7 2 ) . Β. K. D a n i e l s , J. P r e s t o n and D. A Z a u k e l i s , U.S. P a t e n t 3 , 6 0 0 , 2 6 9 , assigned t o Monsanto (August 17,
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R. T. L e w i s , "Extended A b s t r a c t s o f C o n f e r e n c e on Carbon", (The American S o c i e t y , 1 9 7 5 ) , pp. 2 1 5 . D. O. R e s t e r , ibid, pp. 2 2 7 . G. W. Gray, " M o l e c u l a r S t r u c t u r e and o f L i q u i d C r y s t a l s " (Academic P r e s s ,
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L. A. Joo e t al, U.S. P a t e n t 3,718,493 a s s i g n e d t o G r e a t Lakes Carbon Corp. (Feb. 27, 1973). M. Toyoguchi e t al, U. S. P a t e n t 3,767,741 a s s i g n e d t o M i t s u b i s h i Oil Co. (October 23, 1973). S. O t a n i , Carbon, 3, 31 (1965). S. O t a n i , e t al, Carbon, 4, 425 (1966). H. M. Hawthorne e t al, Nature, 227, 946 (1970). L. I . Grindstaff and M. P. W h i t t a k e r , U. S. P a t e n t 3,787,541 (January 22, 1974). R. C. S t r o u p , U. S. P a t e n t 3,814,566 a s s i g n e d t o Union C a r b i d e Corp. (June 4, 1974). D. A. S c h u l z , U. S. P a t e n t 3,919,376 a s s i g n e d t o Union C a r b i d e Corp. (November 11, 1975). I . C. Lewis, U. S. P a t e n t 3,995,014 a s s i g n e d t o Union C a r b i d e C o r p o r a t i o n (November 30, 1976). L. S. S i n g e r , U. S. P a t e n t 4,005,183 a s s i g n e d t o Union C a r b i d e Corp. (January 25, 1977). R. Didchenko e t al, "Extended A b s t r a c t s o f 12th B i e n n i a l Conference on Carbon" (The American Carbon S o c i e t y , 1975), pp. 325. J . E. Zimmer and J. L . White "Extended A b s t r a c t s o f 12th Biennial C o n f e r e n c e on Carbon" (The American Carbon S o c i e t y , 1975), pp. 223. O. Wegener e t al, U. S. P a t e n t 3,761,387 a s s i g n e d to Rutgerswerke A k t i e n g e s e l l s c h a f t (Sept. 25, 1973). Y. Suetsugu e t al, U.S. P a t e n t 3,799,865 a s s i g n e d to N i t t e t s u c h e m i c a l I n d u s t r i a l Co. L t d . (March 26, 1976). S. Mrozowski, "Proc. 1 s t and 2nd C o n f s . Carbon" (Waverly P r e s s , 1953), pp. 31. G. P i e t z k a , G. W i l h e l m i and H. P a u l s , "Interna tional Carbon C o n f e r e n c e " (Baden-Baden, West Germany, 1972), pp. 362. A. L. S u t t o n and V. C. Howard, J. N u c l e a r M a t e r . 7, 58 (1962). H. F. Kuhfuss and W. J. J a c k s o n , Jr. U. S. P a t e n t 3,778,410 a s s i g n e d t o Eastman Kodak Co. (Dec. 11, 1973). H. F. Kuhfuss and W. J. J a c k s o n , Jr., U. S. P a t e n t 3,804,805 a s s i g n e d t o Eastman Kodak Co. ( A p r i l 16, 1974). W. J. J a c k s o n , Jr., and H. F. Kuhfuss, J. Polymer Sci. Polymer Chem. Ed., 14, 2043 (1976). F. E . M c F a r l a n e , V. A. N i c e l y , and Τ. G. D a v i s , "Proceedings o f the 8th Biennial Polymer Symposium o f D i v i s i o n o f Polymer C h e m i s t r y " , ACS (Plenum P r e s s , 1977). A. B l u m s t e i n , "Proceedings o f t h e 3 r d M i d l a n d M a c r o m o l e c u l a r C o n f e r e n c e on P o l y m e r i z a t i o n in
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M E S O M O R P H I C
O r i e n t e d Systems" 71. 72.
73.
ORDER
(Gordon and B r e a c h , New
I N
P O L Y M E R S
York,
1977).
L. Strzelecki, L. L i e b e r t and P. Keller, Bull. Soc. Chem. 11-12, 2750 (1975). Y. B. Amerik e t al, Khim. V o l o k n a , 5, 67 (1975). H. R i n g s d o r f e t al, D i e Makromol. Chemie, 176,
2029 (1975).
74.
A. C. d e V i s s e r e t al, D i e Makromol. Chemie, 176 ,
495 (1975). 75.
Y. Tanaka e t al, Polymer L e t t e r s ,
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch009
RECEIVED
December 8, 1977.
10,
261
(1972).
10 Nematic Polymers: Excluded Volume Effects Y. KIM and P. PINCUS
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
Department of Physics, University of California, Los Angeles, CA 90024
I.
Introduction
There e x i s t s a mushrooming i n t e r e s t i n the mesomorphic behavi o r of macromolecules i n s o l u t i o n . The existence of macroscopic anisotropy r e q u i r e s (at l e a s t p a r t i a l ) chain rigidity. This may a r i s e from next nearest neighbor s t e r i c r e s t r i c t i o n s f o r polymers constructed from bulky backbone monomers as i n the polyamides e.g.,
The current symposium, to a great extent, has focused on comb-like systems wherein mesogenic side groups are attached to a flexible backbone, as 2,3
where x is a rigid l i n k a g e such as - C = N - and the end group R may be of the form - 0 - C H . The present work mainly concerns helixm
coil
2m+1
systems as e x e m p l i f i e d by p o l y p e p t i d e s ,
-Glutamate
e.g.,
Poly-Benzyl-L
(PBLG)
4-5
Supported i n part by the N a t i o n a l Science Foundation and the O f f i c e of Naval Research. 0-8412-0419-5/78/47-074-127$05.00/0 © 1978 American Chemical Society
M E S O M O R P H I C ORDER IN P O L Y M E R S
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
128
i n solvents such as d i c h l o r o a c e t i c a c i d or dioxane. Under c e r t a i n c o n d i t i o n s of temperature, pH, i o n i c s t r e n g t h , e t c . , hydrogen bonding occurs along the polypeptide backbone between monomers separated by 4 or 5 u n i t s l e a d i n g to a r i g i d h e l i x . Under such c o n d i t i o n s , we are e s s e n t i a l l y d e a l i n g with a s o l u t i o n of r i g i d r o d s . If the solvent i s good, the only important intermolecular i n t e r a c t i o n s are then s t e r i c short range excluded volume r e p u l s i o n s . The mesomorphic behavior of a s o l u t i o n of r i g i d rods has been studied by s e v e r a l a u t h o r s · f, ! In p a r t i c u l a r , f o r long rods of length b and r a d i u s a, OnsagerL has shown that the e f f e c t i v e excluded volume per rod i n the i s o t r o p i c phase i s of order b a . The a c t u a l volume occupied per rod i s ïïa b. A l y o t r o p i c t r a n s i t i o n to a nematic phase occurs at a rod concentration O* given by 2
2
a*b a = k
(1.1)
2
where k i s a constant i n the range 3-10. This i m p l i e s that f o r volume f r a c t i o n s i n excess of π k ( a / b ) the system should be i n an ordered phase. T y p i c a l a x i a l ratios— f o r PBLG are b/a ^ 10 l e a d ing to c r i t i c a l volume f r a c t i o n s i n the range 10-40%. Indeed such phase t r a n s i t i o n s are o f t e n observed. Our i n t e r e s t here, however, i s to consider a somewhat more d e l i c a t e s i t u a t i o n where the i s o l a t e d p o l y p e p t i d e chains are i n e s s e n t i a l l y random c o i l conforma tions. We then suggest that as the concentration of chains i n c r e a s e s , a phase t r a n s i t i o n may occur wherein the molecules become simultaneously r i g i d and mesomorphic. T h i s induced r i g i d i t y may a r i s e when the r e d u c t i o n of the excluded volume energy that obtains i n the nematic s t a t e of r i g i d rods balances the decrease i n entropy associated w i t h the l o s s of chain f l e x i b i l i t y . This may occur f o r concentrations i n excess of that associated with O* of prestretched chains. The goal of the present study i s to i n v e s t i g a t e the phase boundary separating ordered and i s o t r o p i c phases, as a f u n c t i o n of concentration and parameters r e l a t i n g to chain r i g i d i t y . In p a r t i c u l a r , t h i s p r e l i m i n a r y report w i l l be devoted to the l i m i t where the i s o l a t e d macromolecules have extremely sharp h e l i x - c o i l t r a n s i tions . In S e c t i o n I I , we s h a l l b r i e f l y r e c a l l the s t a t i s t i c a l mechan i c s of the h e l i x - c o i l t r a n s i t i o n f o r i s o l a t e d c h a i n s . We s h a l l then present an argument showing that there i s l i t t l e tendency f o r n o n - i n t e r a c t i n g molecules to e x h i b i t s u b s t a n t i a l mesomorphic fluctuations. A mean f i e l d theory f o r i n c o r p o r a t i n g the i n t e r chain i n t e r a c t i o n s w i l l be developed i n S e c t i o n I I I i n the coopera tive l i m i t for h e l i x - c o i l t r a n s i t i o n . Some f i n a l remarks w i l l be made i n S e c t i o n IV. 2
10.
II.
κίΜ
129
Nematic Polymers
A N D PINCUS
I s o l a t e d Molecules
The h e l i x - c o i l t r a n s i t i o n of a polypeptide chain c o n s i s t i n g of Ν monomers i s u s u a l l y described i n terms of two parameters : (II.1)
s = exp[-AF /k T] l
B
where ~^F^ i s the gain i n f r e e energy per monomer when i t i s part of a r i g i d h e l i c a l segment; Σ = exp[-2AF /k T] 2
(II.2)
B
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
where AF^ i s the cost i n f r e e energy to create a boundary s e p a r a t ing
h e l i c a l and randomly c o i l e d segments.
only nearest neighbor i n t e r a c t i o n s
For an I s i n g model with
AF^ = 0
(Σ = 1);
on the other
hand, f o r polypeptides with f i n i t e range hydrogen bonding i n t e r a c -2 -4 t i o n s , Σ may t y p i c a l l y be rather small i n the range 10 - 10 Small values of the c o o p e r a t i v i t y parameter, Σ, favor sharp h e l i x c o i l t r a n s i t i o n s which n e c e s s a r i l y have very few segments. In terms of these parameters, the f r e e energy per macromolecule may be simply w r i t t e n asLlL? F
(II.3)
= -Nk Τ in λ ; Β
ο
λ = y {
(1+s)
+ [(1-s)
2
+ 4Σ ] 3
1 / 2
}
(II.4)
The r o l e of Σ i n determining the sharpness of the t r a n s i t i o n may be g r a p h i c a l l y seen by c o n s i d e r i n g the p e r s i s t e n c e l e n g t h , V , which i s the average number of monomers per h e l i c a l segment,
V
=
d in
λ/ds
d In
X/dZ
c\
fτ τ
Λ
-
1
(
Ι
Ι
·
5
)
In F i g u r e 1, we sketch the p e r s i s t e n c e length as a f u n c t i o n of s f o r s e v e r a l values of Σ. For small values of the c o o p e r a t i v i t y parameter i t i s too c o s t l y to have very many domain w a l l s and, thus, the system tends to a s i t u a t i o n of a completely random c o i l or a r i g i d h e l i x depending on whether s i s smaller or l a r g e r than unity. Let us consider an i s o l a t e d chain with p e r s i s t e n c e length V . Such a system may be viewed as a random c o i l of rods of length Va (a i s a c h a r a c t e r i s t i c monomer dimension). The question a r i s e s of whether or not there i s any s i g n i f i c a n t o r i e n t a t i o n a l ordering among the segments f o r the i s o l a t e d c h a i n . For a p o l y m e r i z a t i o n index N , the number of rods i s (N/v) leading to a c o i l radius (for a Gaussian chain) R = (N/v)
1 / 2
va ^ N
1
/
2
v
1 / 2
a
(II.6)
MESOMORPHIC
130
ORDER I N
POLYMERS
The (N/v) r i g i d segments are then confined to a volume of order R , g i v i n g a rod d e n s i t y 3
0
^
( N / V ) R~ 3 ^ ( N V ) " 5
1
/
2
a"
(II.7)
3
which i s small compared to the Onsager c r i t i c a l concentration - (Nv)~
σ/σ*
1 / 2
«
ι
σ*, (II.8)
.
Excluded volume i n t e r a c t i o n s w i l l f u r t h e r decrease the d e n s i t y . > This i s a strong argument suggesting that overlapping c o i l s (semid i l u t e s o l u t i o n s are necessary to obtain l i q u i d c r y s t a l l i n e order i n s o l u t i o n s of worm-like macromolecules. Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
1 1
III.
Mean F i e l d Theory
We now turn to the development of a crude approximation f o r the case of s e m i - d i l u t e s o l u t i o n s of h e l i o g e n i c macromolecules where the i n t e r c h a i n i n t e r a c t i o n s are important. We envisage the molecules at some f i x e d values (s,Σ) as l i n e a r arrays of a l t e r n a t ing r i g i d and c o i l e d segments. The mean number of monomers per h e l i c a l segment i s V ( I I . 5 ) and the number of such segments i s given by η = (Σ/λ) (3λ/3Σ)
.
(III.l)
We assume, f o r t h i s model, that only excluded volume type i n t e r a c t i o n s are important. We then approximate the r i g i d rod Onsager i n t e r a c t i o n — by the lowest angular dependent term i n i t s m u l t i p o l e expansion which i s of quadrupolar symmetry: U..
= -ek TV.V.Q.Q.
,
(III.2)
ε i s a dimensionless constant, v. . are the numbers of monomers th th »3 on the i and j segments; Q^ i s a quadrupole component a s s o c i ated with the o r i e n t a t i o n of each segment Q = \
(3 c o s e - 1) 2
(III.3)
where θ i s the angle between the axis of the segment and the eventual d i r e c t i o n of nematic o r d e r i n g . Each rod experiences mean i n t e r a c t i o n energy = -egk
Β
TOVQ
a
(III.4)
where g i s the volume f r a c t i o n of the sample occupied by the mac romolecules, Θ i s the average f r a c t i o n of monomers i n h e l i c a l seg ments, and < Q > i s the o r i e n t a t i o n a l order parameter ( < Q > = 0 i n the i s o t r o p i c s t a t e ; < Q > = 1 i f a l l the molecules are r i g i d and
1 2
10.
κίΜ
A N D
Nematic Polymers
PINCUS
131
completely a l i g n e d ) . The thermodynamic p r o b a b i l i t y that a given rod makes an angle θ with the nematic axis i s p r o p o r t i o n a l to h
9
e ^ >
(III.5)
and the dimensionless
mean f i e l d h i s given by .
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
h = eg0
(III.6)
In order to s i m p l i f y the algebra, l e t us s p e c i a l i z e to the case of sharp h e l i x - c o i l t r a n s i t i o n s , i . e . , Σ 0 as i s common f o r the s y n t h e t i c polypeptides. (The general s i t u a t i o n w i l l be d i s cussed i n a l a t e r p u b l i c a t i o n . ) For t h i s l i m i t , a given chain i s e i t h e r a f l e x i b l e c o i l or a r i g i d h e l i x , i . e . , V = 0 or Ν ( F i g . 1). The f r e e energy i s separable i n t o two c o n t r i b u t i o n s : (1) the i n t r a - m o l e c u l a r i n t e r a c t i o n s a s s o c i a t e d with the h e l i x - c o i l t r a n s i t i o n ; (2) the i n t e r m o l e c u l a r excluded volume i n t e r a c t i o n s which are r e s p o n s i b l e f o r any nematic order. The d i f f e r e n c e i n f r e e energy per chain between a h e l i c a l and randomly c o i l e d molecule a r i s i n g from i n t r a m o l e c u l a r i n t e r a c t i o n s i s -N £n s. The i n t e r m o l e c u l a r excluded volume i n t e r a c t i o n s i n the mean f i e l d Meier-Saupe l i k e approximation [Eqs. (III.2) - (III.6) with V = N] gives a c o n t r i bution to the f r e e energy d i f f e r e n c e , ( A F ) ^ which i s sketched i n Figure 2 as a f u n c t i o n of the nematic order parameter < Q > f o r s e v e r a l values of the monomer concentration c. For s > 1, the molecules are r i g i d and a f i r s t order l y o t r o p i c phase t r a n s i t i o n i s p r e d i c t e d to occur a t 3
c* = Νσ* = k ( N a )
- 1
.
(III.7)
For s < 1, a phase t r a n s i t i o n may only occur i f the gain i n f r e e energy of ordering exceeds the entropie cost i n l o s s of f l e x i b i l i ty of the i n d i v i d u a l chains. In the (s,c) plane, the phase boundary w i l l then be given by s = exp[(AF) /Nk T] MS
B
.
For s < 1, t h i s implies ( A F ) ^ < 0 or c > c*.
(III.8) Is there a minimum
value of s (=s ) below which the nematic s t a t e i s never stable? We m may estimate that the minimum value of (AF) by s e t t i n g V, = Ν and Q. = 1 i n ( I I I . 2 ) . Then, we f i n d 1
(AF)
(HI.9)
3
m s
> -£k TN(ca ) B
3
where the f a c t o r c a i s the volume f r a c t i o n of the sample occupied by polymer. Thus at the highest concentrations, c a 1, we obtain 3
( A F )
MS " -
£ N k
B
T
(III.10)
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
132
M E S O M O R P H I C
ORDER
I N
P O L Y M E R S
Figure 1. Persistence length ν as a function of s for several values of X. For the I sing limit % = 1, the slope of the straight line is N" . For % < < 1, the value v(s = 1) = σ~ . 1/2
1
OC*
Figure 2. Difference in free energy between nematic and isotropic phases of a rigid rod solution for differ ent values of the monomer concentration c. There is a predictedfirst-orderlyotropic transition at c = c* = Νσ* = k/Na . 3
10.
κίΜ
A N D PINCUS
Nematic
133
Polymers
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
MESOMORPHIC PHASE
ISOTROPIC PHASE
c*
Figure 3. Phase boundary separating the isotropic and nematic phases in the cooperative limit X - » 0. The area to the right of the solid line and below s = 1 represents the region of induced rigidity.
134
M E S O M O R P H I C ORDER IN P O L Y M E R S
or s
- e . For ε - 1, s ^ 1/2, which is a significant renormal in * m * ization of the helix-coil transition. This phase boundary is sketched in (Fig. 3) for Σ 0. The experimental signature of the induced rigidity part of the boundary would be, for example, jumps in both the rotary dispersion (helix-coil transition) and optical birefringence (isotropic-nematic transition) at the same polymer concentration. Indeed for PBLG in dichloroacetic acid, the early work of Robinson et al.— gives some indications of such behavior. More recent studies in the same system by Frenkel et al.-H and UematsuLi provide further evidence in support of the induced rigidity concept.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
IV.
&
Final Remarks
The principal goal of this work is to pursue the concept of induced rigidity.— Our current investigations include extensions to the cases of arbitrary values of the cooperativity parameter 1 > Σ > 0, more accurate treatments of the rod-rod excluded volume interaction,— and the inclusion of some attractive dispersion forces. While we refer to the phase transition as lyotropic, in fact we do expect that i t also occurs as a function of temperature (as, well as pH, ionic strength, etc.) because of the dependence of s on these parameters. We must further emphasize the semi quantitative nature of our results. Indeed under most actual cir cumstances, s and c will not be completely independent variables. Nevertheless we believe that the physical picture presented here has some relation to reality. Abstract 9
We have previously suggested that a cooperative helix-coilnematic phase transition may occur in solutions of heliogenic macromolecules such as the synthetic polypeptides (e.g., PBLG). In this study, we extend the model to the more explicit case of excluded volume interactions only. For the special case of fully cooperative isolated molecule helix-coil transitions, we sketch out the phase boundary between isotropic and nematic regions. Literature Cited 1. 2. 3. 4.
Such systems were discussed in detail at the ACS Symposium on Rigid Chain Polymers, New Orleans, March, 1977, Polymer Pre prints (1977) 18, 1, and J . Poly. Sci. (to be published). ACS Symposium on Mesomorphic Order in Polymers, Chicago, August, 1977, Polymer Preprints (1977) 18, 2. Blumstein, A., "Liquid Crystalline Order in Polymers," Academic Press, New York, 1977 (to be published). Robinson, Ward, and Βevers, Discuss Faraday Soc. (1958) 25, 29.
10.
5. 6. 7. 8. 9. 10.
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch010
11. 12. 13. 14. 15. 16.
κίΜ
A N D PINCUS
Nematic Polymers
135
Murthy, N . S . , I. R. Knox, and Ε. T . Samulski, J. Chem. Phys. (1976) 65, 4835. Onsager, L., Ann. Ν. Y . Acad. Sci. (1949) 51, 627. F l o r y , P. J., Proc. Roy. Soc. (1956) A234, 73. Wadati, Μ . , and A . I s i h a r a , M o l . C r y s t . Liq. C r y s t . (1976) 1 7 , 95. P i n c u s , P., and P. G . de Gennes, Polymer P r e p r i n t s (1977) 18, 131, and J. P o l y . Sci. (to be p u b l i s h e d ) . The statistical mechanics of helix-coil transitions i s well discussed in B i r s h t e i n , Τ. Μ . , and O. Β . Pititsyn, "Conforma t i o n s of Macromolecules," I n t e r s c i e n c e , New York, 1966, Chapters 9 and 10. Yamakawa, Η . , and W. H . Stockmeyer, J . Chem. Phys. (1977) 57, 2843. P i n c u s , P., "Macromolecules" (to be p u b l i s h e d ) . See, f o r example, de Gennes, P. G., "The Physics of L i q u i d Crystals," Clarence P r e s s , Oxford, 1975, Chapter 2. F r e n k e l ' , S. Ya., L . G. S h a l t y k o , and G. K. E l y a s h e v i c h , J . Polymer Sci. (1970) C 56, 47. Uematsu, Υ., P r i v a t e Communication. An e x c e l l e n t review of t h i s subject appears i n S t r a l e y , J. P., Molecular C r y s t a l s and L i q u i d C r y s t a l s (1973) 22, 333.
RECEIVED December 8, 1977.
11 Structure and Properties of Polyglutamates in Concentrated Solutions Y. UEMATSU Department of Industrial Chemistry, Tokyo Institute of Polytechnics, Atsugi, Kanagawa, 243-02, Japan
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
I. UEMATSU Tokyo Institute of Technology, Megro, Tokyo, 152, Japan
It is well known that some polypeptides form lyotropic liquid crystals at sufficiently high concentrarion of the polymers and exhibit the optical properties characteristic of the cholesteric mesophase. The solutions show the regular striation arranged in a finger - printed pattern under a polarization microscope. Robinson et at have reported that the distance between the striation,S, which is the half pitch of the twisted structure, varies with polymer concentration in an inverse manner. They also discussed the molecular arrangement in the twisted structure in the light of X-ray and other measurements. They suggested that the long-range, electric dipole-dipole interactio plays an important role in the formation of the cholesteric structure, and the helical arrangement of dipoles in the polypeptide molecules would tend to impose a unidirectional twist. From the smallness of the twist angle and the fact that it decreases on dilution, they pointed out that the twistangle arises from a dynamic equilibrium. Samulski et al suggested that an asymetric helical molecular conformation may provide an explanation f o r the c h o l e s t e r i c s t r u c t u r e , and very small oscillations of h e l i c e s r e l a t i v e to t h e i r neighbors in the liquid c r y s t a l would be biased to one side of the parallel p o s i t i o n because of the chirality of the van der Waals surface of the polypeptide molecules. Further, they found a l i n e a r r e l a t i o n s h i p between the p i t c h and the r e c i p r o c a l of temperature. This r e l a t i o n holds quite w e l l f o r a numb er of thermotropic l i q u i d crys t a l sys t ems and was explained t h e o r e t i c a l l y by Keating!) , who treated the macroscopic twis t as a r o t a t i o n a l analogue of thermal expansion with the dominant anharmonic forces coming from nearest neighI)
2)
0-8412-0419-5/78/47-074-136$05.50/0 © 1978 American Chemical Society
11.
U E M A T S U
bors.
A N D
U E M A T S U
The t h e o r y
Poly glutamate s in Concentrated Solutions
gives
the f o l l o w i n g
137
relationship:
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
^dlw AkT where k i s the Boltzmann c o n s t a n t , d , the s p a c i n g b e t w e e n n e i g h b o r i n g p l a n e s o f m o l e c u l e s , w, t h e f r e q u e n c y o f t h e e x c i t e d t w i s t m o d e s , I , t h e moment o f i n e r t i a o f t h e m o l e c u l e , and A , t h e c o n s t a n t i n t h e c u b i c a n h a r m o n i c i t y term of the anharmonic e q u a t i o n of m o t i o n d e s c r i b i n g t h e s y s t e m . However, the p i t c h for the p o l y p e p t i d e l i q u i d c r y s t a l s increases with temperature, w h i l e i t decreases f o r the thermotropic liquid crystals. Recently, there are several report discuss ins t h e t w i s t mode o f t h e b i n a r y s y s t e m o f t h e t h e r m o t r o pic liquid crystals composed of a n e m a t i c s o l v e n t and a c h o l e s t e r i c s o l u t e . Another i n t e r e s t i s i n a t t e m p t i n g to pursue the s t r u c t u r a l s i m i l a r i t y between t h e r m o t r o p i c l i q u i d c r y s t a l s and l y o t r o p i c o n e , e s p e c i a l l y f o r t h e d e p e n d e n c e o f c h o l e s t e r i c p i t c h on t h e temperature. One o f t h e b e s t m e a s u r e s o f t w i s t i s t h o u g h t t o be t h e c h o l e s t e r i c p i t c h . The t e m p e r a t u r e dependence of c h o i e s t e r i c p i t c h i s t h e n m e a s u r e d f o r p o l y - γ b e n z y l - L - , and p o l y - y - b e n z y 1 - D - g l u t a m a t e s ( PBLG, and PBDG ) a n d p o l y - y - a l k y 1 - L - g l u t a m a t e s , i n v a r i o u s s o l v e n t s y s t ems. T h e t e m p e r a t u r e d e p e n d e n c e o f some p h y s i c a l p r o p e r t i e s were a l s o m e a s u r e d . Experimental. PBLG ( mw: 2 0 1 , 0 0 0 ) a n d PBDG ( mw: 1 9 7 , 0 0 0 ) u s e d i n t h i s s t u d y were sup 1 i e d b y A j inomo t ο C o . L t d . J a p a n , a n d some s p e c i m e n s o f PBLG w e r e p r e p a r e d b y p o l y m e r i z a t i o n of N - c a r b o x y a n h y d r i d e of γ - b e n z y 1 - L glutamate. P o l y - y - a l k y 1 - L - g l u t a m a t e s ( PALG ) w e r e p r e p a r e d f r o m p o l y - y - m e t h y 1 - L - g l u t a m a t e ( mw: 2 5 0 , 0 0 0 ) by a l c o h o l y s i s by w h i ch a l l m e t h y l r a d i c a l s i n t h e s i d e c h a i n s c a n be r e p l a c e d w i t h t h e d e s i r e d a l k y 1 radicals. P o l y - y - a l k y 1 - L - g l u t amat es u s e d i n t h i s s t u d y w e r e w i t h a l k y 1 g r o u p s s u c h as e t h y l , n - p r o p y 1 , n - b u t y 1 , η - a m y 1 , n - h exy1 , n - h e p t y 1 , and n - o c t y 1 . From t h e v i s c o s i t y measurement s i n d i c h l o r o a c e t i c acid t h e m o l e c u l a r w e i g h t s o f p o l y g l u t a m a t e s were est imat e d . T h e s o l u t i o n s were p r e p a r e d b y w e i g h i n g the p o l y m e r and s o l v e n t i n a s t o p p e r e d b o t t l e . A f t e r s h a k i n g s l o w l y on a magnet i c s t i r r e r f o r long time, t h e s o l u t i o n s were p u t c a r e f u l l y i n t o t h e c e l l , w h i c h was h a v i n g p a r a l l e l s i d e s 2mm a p a r t . Then t h e top o f t h e c a p i l l a r y o f t h e c e l l was s e a l e d t o p r e v e n t t h e
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
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M E S O M O R P H I C
ORDER
I N
P O L Y M E R S
l o s s o f s o l v e n t by v a p o r i z a t i o n . The c o n c e n t r a t i o n was c a l c u l a t e d by u s i n g 0 . 7 8 7 m l / g as t h e s p e c i f i c v o l u m e o f P B G , and 0 . 8 3 m l / g as t h a t o f PALG s , and e x p r e s s e d i n the u n i t s v o l / v o l . A f t e r on s t a n d i n g , u n t i l a r e g u l a r s t r u c t u r e a p p e a r e d , we m e a s u r e d t h e half p i t c h with a p o l a r i z a t i o n microscope. The t e m p e r a t u r e o f t h e s a m p l e was r e g u l a t e d w i t h i n + 0 . 2 ° C by u s e o f a c o n s t a n t t e m p e r a t u r e c i r c u l a t o r . The measurements were c a r r i e d out a f t e r m a i n t a i n i n g the s p e c i m e n s f o r 12 h r s a t e a c h temperature. D i l a t o m e t r i c measurements were c a r r i e d out u s i n g Hg as a c o n f i n i n g l i q u i d , w h i c h was t r a n s f e r r e d i n t o the d i l a t o m e t e r a f t e r s o l i d i f y i n g the sample s o l u t i o n by l i q u i d n i t r o g e n . CD s p e c t r a , were m e a s u r e d w i t h JASCO O R D / U V - 5 0 o v e r t h e r e g i o n f r o m 220 t o 300nm. NMR s p e c t r a w e r e m e a s u r e d by u s e o f J E O L M H - 1 0 0 . The t i m e r e q u i r e d t o a r r i v e a t e q u i l i b r i u m v a r i e d f r o m one t o s e v e r a l h o u r s . The s o l v e n t s u s e d h e r e were p u r i f i e d by s t a n d a r d m e t h o d s . R e s u l t s and d i s c u s s i o n . In g e n e r a l the c h o l e s t e r i c p i t c h of t h e r m o t r o p i c l i q u i d c r y s t a l s decreases with temperature. The theore t i c a l treatments f o r t h i s b e h a v i o r have been r e ported3,4). H o w e v e r , i t was f o u n d t h a t f o r a b i n a r y s y s t e m , t h e d e p e n d e n c e o f t h e c h o l e s t e r i c p i t c h on t e m p e r a t u r e changes f r o m ρ o s i t i v e to n e g a t i v e at a c r i t i c a l compos i t i o n . F o r the b i n a r y s y s t e m of t h e r m o t r o p i c l i q u i d c r y s t a l s , c o m p o s e d o f c h o i e s t e r i c and nemat i c s u b s t a n c e s , d S / d T i s g r e a t e r t h a n z e r o i n t h e nemat i c r i c h r e g i o n , and d S / d T i s l e s s t h a n z e r o i n the c h o l e s t e r i c r i c h r e g i o n . I n t h i s p a p e r , we h a v e i n v e s t i g a t ed t h e t e m p e r a t u r e d e p e n d e n c e o f cholesteric p i t c h f o r the p o l y p e p t i d e l i q u i d crystals. R e c e n t l y DuPre et a l l } reported that , S increases l i n e a r l y with temperature r i s e . Qualitatively, their r e s u l t s are c o n s i s t e n t w i t h o u r s . However, the time r e q u i r e d to r e a c h the e q u i l i b r i u m p i t c h , v a r i e d w i t h temperature, t h e c o n c e n t r a t i o n o f p o l y m e r , and a l s o the t h e r m a l h i s t o r y . F o r PBLG s o l u t i o n i n d i c h l o r o e t h a n e (EDC) , w h i c h c o n c e n t r â t i o n i s 0 . 1 2 v o 1 / v o 1 , the v a r i a t i o n o f p i t c h w i t h t i m e was m e a s u r e d a t a c o n s t a n t t e m p e r a t u r e by T - j ump m e t h o d . F i g . 1 shows t h e t i m e d e p e n d e n c e o f c h o i e s t e r i c p i t c h by t h e T - j ump m e t h o d f r o m - 2 ° C t o + 3 0 ° C , 4 0 ° C and 50°C r e s p e c t i v e l y . I t i s c l e a r t h a t the time r e q u i r e d to a r r i v e at the equilibrium pitch i s s h o r t e r at h i g h e r temperature but i s s t i l l over s e v e r a l h o u r s . T h e r e f o r e , the e q u i l i b r i u m p i t c h mu s t be m e a s u r e d a f t e r p r o l o n g e d a g i n g at each m e a s u r i n g t e m p e r a t u r e . 11 was f o u n d t h a t the
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
11.
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A N D U E M A T S U
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139
h a l f p i t c h , S , i n c r e a s e d mono t o n i c a l l y w i t h t e m p e r a t u r e , and f i n a l l y , r e t a r d a t i o n l i n e d i s a p p e a r e d a t a c r i t i c a l t e m p e r a t u r e , and t h e b i r e f r i n g e n c e c h a r a c t e r i s t i c of t h e nemat i c s t r u c t u r e was ob s e r v e d . P l o t o f S v e r s u s t e m p e r a t u r e f o r PBDG i n c h l o r o f o r m i s shown i n f i g . 2 . T h i s b e h a v i o r c o r r e s p o n d s to the b e h a v i o r of b i n a r y m i x t u r e of t h e r m o t r o p i c l i q u i d c r y s t a l s i n t h e nemat i c r i c h r e g i o n . As shown i n f i g . 3, t h e r e c i p r o c a l o f t h e h a l f p i t c h wh i c h i s p r o p o r t i o n a l to the a n g l e of t w i s t between ( 10Ï0) ρ l a n e s , i s a l i n e a r f une t i o n o f t e m p e r a t u r e . The critical t e m p e r a t u r e , w h e r e 1/S = 0, was ob t a i n e d f r o m t h e e x t r a p o l a t i o n o f t h i s c u r v e , and was d e f i n e d as t h e n e m a t i c t e m p e r a t u r e , T^. The o p t i c a l p a t t e r n c h a r a c t e r i s t i c o f t h e nemat i c s t a t e was o b s e r v e d i n t h i s r e g i o n under the p o l a r i z a t i o n m i c r o s c o p e . DuPre et a l def i n e d t h i s t e m p e r a t u r e as t h e c l e a r i n g p o i n t ' ,s i n c e the t w i s t e l a s t i c c o n s t a n t k22> o b t a i n e d f r o m t h e o r i e n t a t i o n i n a magnet i c f i e l d t e n d s t o w a r d z e r o a t t h i s temperature. H o w e v e r , f r o m t h e ob s e r v a t i o n by a p o l a r i z a t i o n m i c r o s c o p e , i t i s o b v i o u s t h a t the newly ap p e a r e d p h a s e i s n o t an i s o t r o p i c o n e . The f a c t t h a t k22 t e n d s t o w a r d z e r ο , c a n be e x p l a i n e d w i t h t h e p r e sence of the s i z e of the c l u s t e r of c o h e r e n t l e n g t h , i n w h i c h t h e m o l e c u l e s w o u l d be a r r a n g e d p a r a l l e l . In o r d e r t o o b t a i n t h e r e l a t i o n s h i p b e t w e e n S and the c o n c e n t r a t i o n C, a d o u b l e l o g a r i t h m i c p l o t o f S a g a i n s t C a t e a c h t e m p e r a t u r e was made as shown i n f i g . 4. I t c a n be s e e n t h a t l o g S i s p r o p o r t i o n a l t o l o g C a n d , i t s s l o p e n i s an i n c r e a s i n g f u n e t i o n o f t e m p e r a t u r e , c h a r a c t e r i s t i c of the specimens used. The e x t r a p o l a t e d l i n e o f l o g S - l o g C c r o s s e d e a c h other at a c r i t i c a l c o n c e n t r a t i o n C a t wh i c h S s t a y s c o n s t a n t and i n d e p e n d e n t ο f t e m p e r a t u r e . These r e s u l t s sugges t t h a t the temperature dependence of the c h o l e s t e r i c p i t c h would i n f l e c t at the concentrât i o n higher than C . T h i s i s analogous to the b e h a v i o r of t h e r m o t r o p i c l i q u i d c r y s t a l s comp o s ed o f c h o i e s t er i c s o l u t e and nemat i c s o l v e n t , w h e r e t h e s i g n o f dS/dT r e v e r s e s a t a c r i t i c a l c o n c e n t r a t i o n . 11 i s u n d e r s t o o d t h a t t h e b e h a v i o r o f b o t h t h e r m o t r o p i c and l y o t r o p i c l i q u i d c r y s t a l s i s comparable p r o v i d e d that the nemat i c s ub s t a n c e s o f t h e f o rmer a r e s u b s t i t u t e d w i t h the s o l v e n t s of the l a t t e r . The c r i t i c a l c o n centration C i s a b o u t 0.41 v o l / v o l and t h i s v a l u e i s v e r y c l o s e to the c o n c e n t r a t i o n at which the s i d e c h a i n s on ne i g h b o r i n g m o l e c u l e s o f t h e p o l y m e r come to c o n t a c t e a c h o t h e r ( r e f e r t o f i g . 5 ) . From t h e s e r e s u l t s , i t i s e x p e c t e d t h a t t h e o r i g i n o r me c h a n i s m of t w i s t w o u l d c h a n g e a t t h i s c o n c e n t r a t i o n C . The T
Q
Q
0
Q
MESOMORPHIC
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
140
3
0
0
ORDER IN
Time (min.)
Figure 1. The change of cholesteric pitch with time for PBLG-EDC. Polymer concentration is 0.12 vol/vol.
Figure 2. The change of S with temperature for PBDG-CHCk. Numbers represent the concentration of polymer.
POLYMERS
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
U E M A T S U
A N D
U E M A T S U
Ρ olij glutamate s in Concentrated Solutions
Figure 3. 1/S vs. temperature for PBDG—EDC. Concentration (from top to bottom) 0.30, 0.23, 0.175, 0.135 (vol/vol).
141
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M E S O M O R P H I C ORDER IN P O L Y M E R S
e x t r a p o l a t i o n of the p l o t of l o g S v e r s u s l o g C , to C = 1, g i v e s the h a l f p i t c h of pure p o l y m e r , which i s a d e c r e a s i n g f u n c t i o n of t e m p e r a t u r e ; and t h i s b e h a v i o r i s a n a l o g o u s to t h a t of t h e r m o t r o p i c cholesteric liquid crystals. I t i s c o n v e n i e n t to use the r e d u c e d temperature T/T^j, where i s the nematic t e m p e r a t u r e , i n s t e a d of temperature T , i n the temperature range near T . Fig. 6 shows t h e o b s e r v e d v a l u e s o f 1/S v e r s u s Τ / T , f o r PBDG i n c h l o r o f o r m , a t v a r i o u s concentrations. The s l o p e Β d e p e n d s on t h e c o n c e n t r a t i o n , and i t i n c reases with concentration. If the c h o l e s t e r i c p i t c h was m e a s u r e d a t c o n s t a n t c o n c e n t r a t i o n , t h e f o l l o w i n g equation holds: N
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
N
1/S
= Β (
1 -
T/T
N
)
The d e p e n d e n c e o f Β on t h e c o n c e n t r a t i o n o f p o l y m e r was o b t a i n e d f r o m t h e p l o t o f l o g 1/P o r 1/S against l o g C , at each T / T . The v a l u e s o f t h e s l o p e η a r e constant and i n d e p e n d e n t o f t e m p e r a t u r e ( f i g . 7 ). A c c o r d i n g t o Rob i n s o n i ) , t h e f o l l o w i n g r e l a t i o n holds : N
θ
Hence
=
Ιττά/Έ
or
ud/S
θ = ( A/d2n-l)(
and
l -
T
C <x
l/d2
/T ) N
where d i s t h e d i s t a n c e between ( 1 0 Ϊ 0 ) p l a n e s , and 2n-l v a r i e s f r o m 2 t o 6. The f a c t t h a t t h e t w i s t i n g a n g l e θ was i n v e r s e l y p r o p o r t i o n a l t o d 2 n - 1 would s u g g e s t t h a t t h e t w i s t i n g power o r i g i n a t e s f r o m a l o n g range r e p u l s i v e f o r c e between the m o l e c u l e s , which m i g h t be an e l e c t r o s t a t i c r e p u l s i v e f o r c e o r d i p o l e dipole interaction. The v a l u e s o f A a r e calculated f r o m t h e s l o p e o f t h e p l o t o f 1/S v e r s u s T / T ^ , and C , at each c o n c e n t r a t i o n . The o b t a i n e d v a l u e s o f A f o r PBLG i n v a r i o u s s o l v e n t s a r e l i s t e d i n t a b l e 1. As shown i n t h e t a b l e , t h e s e v a l u e s a r e c l e a r l y c o n s t a n t and i n d e p e n d e n t o f c o n c e n t r a t i o n . In o r d e r to s t u d y the mechanism of the t r a n s i t i o n of c h o l e s t e r i c to n e m a t i c s t r u c t u r e , several physical p r o p e r t i e s w e r e m e a s u r e d as a f u n e t i o n o f temperature f r o m room t e m p e r a t u r e t h r o u g h t h i s t r a n s i t i o n r e g i o n . n
S p e c i f i c volume measurement. The s p e c i f i c v o l u m e s o f PBLG s o l u t i o n i n c h l o r o f o r m , d i c h l o r o m e t h a n e , d i c h l o r o e t h a n e and d i o x a n e were m e a s u r e d r e s p e c t i v e l y by d i l a t o m e t r y . I t c a n be s e e n , f r o m f i g . 8 t h a t t h e e x p a n s i o n c o e f f i c i e n t v a r i e s a b r u p t l y at T J J . These
11.
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Ρ oltj glutamate s in Concentrated Solutions
143
Figure 6. 1/S vs. T/T.v for PBLG-CHCl
to Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
s
-^0.96
•05
log y?
Table
Th e va " u e s o f c o r s t a n t A
I
* 0. 12
0. 14
0. 16
0 .18
0. 20
0. 23
0.26 0. 30
4. 08
3 10
3. 42
3. 09
3 .90
2 90
3.45
31 CHC1
1 .42
3
Figure 7. Log 1/S vs. log C for PBLG-CHCk
ave.
3 0 7 3.5 6. 3
6 .32
2 2 CH C1CH C1
1 .58
6. 71
1 .42
7 .10
7 .00
7 .50
8. 20
CHC1 CHC1
0 . 86 3. 47
4 .50
4 .43
4.46
4 .46
2 . 04 25 . 7
32 . 7
31 . 7
33. 7
32 . 7
C
H
C
2
1
2
2
2
CH C1CHC1CH C1 2
2
The
5 .96
c ο n ce n t r a t i on o f
PBLG
7. 5
r e ρ r e s en t e d i n v o l / v o l
144
M E S O M O R P H I C ORDER IN P O L Y M E R S
Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0074.ch011
r e s u l t s s u g g e s t t h a t the c h o l e s t e r i c to n e m a t i c t r a n s i t i o n may be a s e c o n d o r d e r t r a n s i t i o n a n d t h e o c c u r e n c e o f some m o l e c u l a r m o t i o n s h o u l d be e x p e c t e d . T h i s i s a l s o s u p p o r t e d by t h e f a c t t h a t a λ - l i k e e n d o t h e r m i c p e a k was o b s e r v e d n e a r t h e n e m a t i c temperature i n the d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y . Circular dichroism. CD s p e c t r u m f o r PBLG i n e t h y l e n e d i c h l o r i d e was m e a s u r e d . The i n d u c e d CD b a n d by ττ-π* t r a n s i t i o n of p h e n y l r i n g i n the s i d e c h a i n a p p e a r e d , c e n t e r e d a t 2 60nm. I t i s c l e a r t h a t the band i s i n d u c e d by c h o l e s t e r i c S t r u c t u r e since its inten s i t y d e c r e a s e s w i t h t h e o r i e
