COMPREHENSIVE ANALYTICAL CHEMISTRY VOLUME
53
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ADVISORY BOARD Joseph A. Caruso University of Cincinnati, Cincinnati, OH, USA Hendrik Emons Joint Research Centre, Geel, Belgium Gary Hieftje Indiana University, Bloomington, IN, USA Kiyokatsu Jinno Toyohashi University of Technology, Toyohashi, Japan Uwe Karst University of Mu¨nster, Mu¨nster, Germany Gyo¨rgy Marko-Varga AstraZeneca, Lund, Sweden Janusz Pawliszyn University of Waterloo, Waterloo, Ont., Canada Susan Richardson US Environmental Protection Agency, Athens, GA, USA
Wilson & Wilson’s
COMPREHENSIVE ANALYTICAL CHEMISTRY
Edited by ´ D. BARCELO Research Professor Department of Environmental Chemistry IIQAB-CSIC Jordi Girona 18-26 08034 Barcelona Spain
Wilson & Wilson’s
COMPREHENSIVE ANALYTICAL CHEMISTRY MOLECULAR CHARACTERIZATION AND ANALYSIS OF POLYMERS
VOLUME
53 Edited by JOHN M. CHALMERS VS Consulting, 14 Croft Hills, Tame Bridge, Stokesley TS9 5NW, UK PROF. Dr ROBERT J. MEIER Institut fu¨r Agrosphere, ICG-4, Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany
Amsterdam Boston Heidelberg London New York Oxford Paris San Diego San Francisco Singapore Sydney Tokyo
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CONTENT S
Contributors to Volume 53 Volumes in the Series Preface Series Editor’s Preface
xiii xv xix xxi SECTION I: INTRODUCTION
1. Introduction
3
John M. Chalmers and Robert J. Meier 1. The Analytical Competence 2. Contents and Structure 3. Practical Considerations in the Multi-Technique Approach References
2. Polymer Chemistry and Microstructure
3 6 11 12
13
Jacques Devaux and Sophie Demoustier-Champagne 1. Introduction 2. Polymer: Definitions 3. From Molecules to Macromolecules 4. Fundamentals of Polymer Chemistry 5. Polymer Degradation 6. Summary References
3. Polymeric Materials: Composition, Uses and Applications
14 15 25 34 58 62 62
65
J.P. Candlin 1. Introduction 2. Mode of Polymerisation 3. Polymer Materials: Types and Forms 4. Polymer Additives 5. Summary Bibliography
65 66 68 87 117 118
vii
viii
Contents
SECTION II: POLYMER CHAIN ANALYSIS
4. Chain Structure Characterization
123
Gregory Beaucage and Amit S. Kulkarni 1. Introduction to Structure in Synthetic Macromolecules 2. Local Structure and Its Ramifications 3. Summary References
5. Chain End Characterisation
123 134 165 166
171
Anthony T. Jackson and Duncan F. Robertson 1. 2. 3. 4.
Introduction End Groups in Free Radical Polymers End Groups in Condensation Polymers End Groups in Polymers from Ionic Polymerisation 5. Closing Remarks Acknowledgements References
6. Determination of Molecular Weights and Their Distributions
171 175 182 194 201 201 201
205
Simone Wiegand and Werner Ko¨hler 1. Introduction 2. Distribution Functions and Averages 3. Methods Based on Colligative Properties 4. Viscometry 5. Scattering Techniques 6. Liquid Chromatography 7. Analytical Ultracentrifuge 8. Other Methods 9. Practical Examples 10. Conclusion Acknowledgements References
205 208 212 218 220 228 235 237 241 247 248 248
SECTION III: POLYMER MORPHOLOGY AND STRUCTURE 7. Phase Structure and Morphology
255
Rufina G. Alamo 1. Phase Structure and Morphology 2. Supermolecular Morphology
255 274
Contents
3. Concluding Remarks Acknowledgements References
8. Characterization of Molecular Orientation
ix
286 287 287
295
Thierry Lefe`vre, Christian Pellerin and Michel Pe´zolet 1. Introduction 2. Theory 3. Birefringence 4. Infrared Linear Dichroism (IRLD) 5. Raman Spectroscopy and Microspectroscopy 6. Fluorescence Spectroscopy 7. NMR Spectroscopy 8. X-Ray Diffraction and Absorption 9. Conclusion References
9. Polymer Networks: Elastomers
295 297 301 305 313 322 325 328 333 333
337
B. Erman and J.E. Mark 1. 2. 3. 4. 5. 6. 7.
Introduction Structure of a Typical Network Elementary and More Advanced Molecular Theories Phenomenological Theories Some Relevant Simulations Swelling of Networks and Responsive Gels Enthalpic and Entropic Contributions to Rubber Elasticity: The Force–Temperature Relations 8. Multimodal Elastomers 9. Liquid–Crystalline Elastomers 10. Novel Reinforced Elastomers 11. Further Comments on Some Characterization Techniques Acknowledgement References
338 339 341 351 352 356 358 359 365 370 374 376 377
SECTION IV: POLYMER DEGRADATION 10. Polymer Degradation and Oxidation: An Introduction
387
John M. Chalmers and Robert J. Meier 1. Introduction 2. Standard Tests and Comparative Methods 3. Spectroscopic Techniques 4. Thermal Methods 5. Chromatographic Analysis 6. Closing Remarks Acknowledgement References
388 390 402 436 442 447 448 448
x
Contents
11. The Role of Oxidation in Degradation of Polymers: The Relation of Oxidation to the Light Emission from Oxidized Polymers
451
Jozef Rychly´ and Lyda Matisova´-Rychla´ 1. Introduction, Historical Overview and Terminology Associated with the Degradation and Ageing of Polymers 2. Methodology and Analytical Techniques in Polymer Stability Studies 3. Mechanisms Leading to the Light Emission from Thermally Oxidized Polymers (Chemiluminescence), Similarities and Differences within Respective Groups of Polymers 4. General Classification of Chemiluminescence from Polymers 5. Comparison of Chemiluminescence for Isothermal and Non-Isothermal Conditions with those Obtained by DSC 6. The Effect of Initial Molar Mass on Chemiluminescence Runs 7. The Effect of Temperature on Chemiluminescence Intensity; The Mutual Relation of Isothermal and Non-Isothermal Studies 8. Relaxation Experiments (Jump Changes of Concentration of Oxygen, Temperature and Humidity) and Rate Constants of Respective Elementary Reaction Steps 9. The Effect of Antioxidants and Polymer Stabilizers 10. Isothermal Kinetics of Polymers Oxidation and Its Relation to the Concentration of Oxygen in Surrounding Atmosphere 11. The Effect of Alkaline Additives on Chemiluminescence Runs: A Possible Impact 12. Conclusions and Outline of the Further Perspectives Acknowledgements References
12. ESR and ESR Imaging Methods for the Study of Oxidative Polymer Degradation
452 461
463 471 476 478 480
482 483 487 490 493 496 496
499
Shulamith Schlick and Krzysztof Krucza"a 1. 2. 3. 4.
Introduction: Polymer Degradation in the Presence of Oxygen Fundamentals of ESR and ESR Imaging ESR as a Tool for the Study of Polymer Degradation ESR Imaging: A Nondestructive Method for Spatially Resolved Polymer Degradation 5. Concluding Remarks Acknowledgements References
500 504 513 517 521 521 522
SECTION V: POLYMER PRODUCT ANALYSIS 13. Spatial Imaging/Heterogeneity
527
Peter Wilhelm and Boril Chernev 1. Introduction 2. Raman and FTIR Microscopy
527 529
Contents
3. Examples of Heterogeneous Polymers Investigated by IR and Raman Microscopy 4. Other Methods and Comparison of Methods 5. Conclusions References
14. Additive Analysis
xi
538 550 557 558
561
John Sidwell 1. Introduction 2. Analytical Techniques for Additive Analysis 3. Analysis of Specific Additives 4. Future Developments in Analytical Methodology References
15. Failure, Defect, and Contaminant Analysis
562 563 573 603 605
607
James D. Rancourt, Jennifer Brooks, Sue Mecham, Alan Sentman, Brian Starr and Jason Todd 1. Introduction 2. Case Studies 3. Summary References
607 608 673 674
16. Surface Analysis
675
John M. Chalmers and Robert J. Meier 1. Introduction 2. A Surface and Spatial Resolution 3. Adhesion 4. Polymer Degradation 5. Polymer Fracture 6. Characterizing Polymer Surfaces 7. Delamination 8. Cure 9. Closing Remark References
675 676 677 679 679 679 679 681 682 682
SECTION VI: POLYMER AND POLYMER PRODUCT DEVELOPMENT: SUPPORT TECHNIQUES 17. The Supporting Role of Molecular Modelling and Computational Chemistry in Polymer Analysis
685
John Kendrick 1. 2. 3. 4.
Introduction Methodologies Infrared and Raman Spectroscopy Photoelectron Spectroscopy and Auger Electron Spectroscopy (AES)
686 687 693 703
xii
Contents
5. Mass Spectrometry 6. X-Ray Diffraction 7. Nuclear Magnetic Resonance 8. Electron Spin Resonance 9. Summary References
18. High-Throughput Analysis
712 718 722 727 729 730
735
Robert J. Meier 1. Introduction 2. Some General Considerations on MTE and the Corresponding Analytical Needs 3. Analytical Techniques for Medium-Throughput Polymer Analysis 4. Outlook References
Subject Index See Color Plate Section at the End of This Book
735 737 740 742 743
745
CONTRIBUTORS TO VOLUME 53 Rufina G. Alamo FAMU/FSU College of Engineering, Department of Chemical and Biomedical Engineering, 2525 Pottsdamer St., Tallahassee FL 32310, USA Gregory Beaucage Department of Chemical and Materials Engineering, University of Cincinnati, Mail location 12, Cincinnati, OH, 45221-0112, USA Jennifer Brooks Polymer Solutions Incorporated, 2903C Commerce Street, Blacksburg, Virginia 24060, USA J.P. Candlin 4 Rudby Lea, Hutton Rudby, Yarm, TS15 0JZ, UK John M. Chalmers VS Consulting, 14 Croft Hills, Tame Bridge, Stokesley, TS9 5NW, UK Boril Chernev Centre for Electron Microscopy and Nanoanalysis, Steyrergasse 17, A-8010 Graz, Austria Sophie Demoustier-Champagne Universite´ catholique de Louvain, Unite´ de Chimie et de Physique des Hauts polyme`res (POLY), Croix du Sud, 1. B-1348 Louvain-la-Neuve, Belgium Jacques Devaux Universite´ catholique de Louvain, Unite´ de Chimie et de Physique des Hauts polyme`res (POLY), Croix du Sud, 1. B-1348 Louvain-la-Neuve, Belgium B. Erman Department of Chemical and Biological Engineering, Koc University, Rumeli Feneri Yolu 34450 Istanbul, Turkey Anthony T. Jackson AkzoNobel CARG, 137 Jiangtian East Road, Songjiang Industrial Estate, Shanghai 201600, P.R. China John Kendrick Institute of Pharmaceutical Innovation, Bradford University, BD7 1DP, UK Werner Ko¨hler Physikalisches Institut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany Krzysztof Krucza"a Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland Amit S. Kulkarni Department of Chemical and Materials Engineering, University of Cincinnati, Mail location 12, Cincinnati, OH 45221-0112, USA
xiii
xiv
Contributors to Volume 53
Thierry Lefe`vre De´partement de chimie, Universite´ Laval, Que´bec, QC, G1V 0A6, Canada J.E. Mark Department of Chemistry, Crosley Tower, Martin Luther King Drive, University of Cincinnati, Cincinnati, OH 45221-0172, USA Lyda Matisova´-Rychla´ Polymer Institute, Centre of Excellence for Degradation of Biopolymers, the Slovak Academy of Sciences, 842 36 Bratislava, Slovakia Sue Mecham Polymer Solutions Incorporated, 2903C Commerce Street, Blacksburg, Virginia 24060, USA Robert J. Meier DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands; Institut fu¨r Agrosphere, ICG-4, Forschungszentrum Ju¨lich GmbH, D-52425 Ju¨lich, Germany Christian Pellerin De´partement de chimie, Universite´ de Montre´al, Montre´al, QC, H3C 3J7, Canada Michel Pe´zolet De´partement de chimie, Pavillon Alexandre-Vachon, local 1084D, Univeriste´ Laval, Que´bec, G1V 0A6, Canada James D. Rancourt Polymer Solutions Incorporated, 2903C Commerce Street, Blacksburg, Virginia 24060, USA Duncan F. Robertson Intertek-MSG, D131, Wilton Centre, Wilton, Redcar, Cleveland, TS10 4RF, UK Jozef Rychly´ Polymer Institute, Slovak Academy of Sciences, Du´bravska´ cesta 9, 842 36 Bratislava, Slovakia. Shulamith Schlick Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols Road, Detroit, Michigan 48221-3038, USA Alan Sentman Polymer Solutions Incorporated, 2903C Commerce Street, Blacksburg, Virginia 24060, USA John Sidwell Smithers Rapra Technology Ltd., Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Brian Starr Polymer Solutions Incorporated, 2903C Commerce Street, Blacksburg, Virginia 24060, USA Jason Todd Polymer Solutions Incorporated, 2903C Commerce Street, Blacksburg, Virginia 24060, USA Simone Wiegand Forschungszentrum Ju¨lich, IFF – Weiche Materie, Postfach 1913, 52425 Ju¨lich, Germany Peter Wilhelm Research Institute for Electron Microscopy, Graz University of Technology, Steyrergasse 17, A-8010 Graz, Austria
VOLUMES IN THE SERIES
Vol. 1A
Vol. 1B Vol. 1C Vol. 2A
Vol. 2B
Vol. 2C
Vol. 2D Vol. 3
Vol. 4
Vol. 5
Vol. 6 Vol. 7 Vol. 8
Vol. 9
Analytical Processes Gas Analysis Inorganic Qualitative Analysis Organic Qualitative Analysis Inorganic Gravimetric Analysis Inorganic Titrimetric Analysis Organic Quantitative Analysis Analytical Chemistry of the Elements Electrochemical Analysis Electrodeposition Potentiometric Titrations Conductometric Titrations High-Frequency Titrations Liquid Chromatography in Columns Gas Chromatography Ion Exchangers Distillation Paper and Thin Layer Chromatography Radiochemical Methods Nuclear Magnetic Resonance and Electron Spin Resonance Methods X-ray Spectrometry Coulometric Analysis Elemental Analysis with Minute Sample Standards and Standardization Separation by Liquid Amalgams Vacuum Fusion Analysis of Gases in Metals Electroanalysis in Molten Salts Instrumentation for Spectroscopy Atomic Absorption and Fluorescence Spectroscopy Diffuse Reflectance Spectroscopy Emission Spectroscopy Analytical Microwave Spectroscopy Analytical Applications of Electron Microscopy Analytical Infrared Spectroscopy Thermal Methods in Analytical Chemistry Substoichiometric Analytical Methods Enzyme Electrodes in Analytical Chemistry Molecular Fluorescence Spectroscopy Photometric Titrations Analytical Applications of Interferometry Ultraviolet Photoelectron and Photoion Spectroscopy Auger Electron Spectroscopy Plasma Excitation in Spectrochemical Analysis
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Volumes in the Series
Vol. 10 Vol. 11 Vol. 12
Vol. 13
Vol. 14 Vol. 15 Vol. 16 Vol. 17 Vol. 18 Vol. Vol. Vol. Vol. Vol.
19 20 21 22 23
Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol.
24 25 26 27 28 29 30 31 32 33 34
Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol.
35 36 37 38 39 40 41 42 43
Vol. 44 Vol. 45 Vol. 46
Organic Spot Tests Analysis The History of Analytical Chemistry The Application of Mathematical Statistics in Analytical Chemistry Mass Spectrometry Ion Selective Electrodes Thermal Analysis Part A. Simultaneous Thermoanalytical Examination by Means of the Derivatograph Part B. Biochemical and Clinical Application of Thermometric and Thermal Analysis Part C. Emanation Thermal Analysis and other Radiometric Emanation Methods Part D. Thermophysical Properties of Solids Part E. Pulse Method of Measuring Thermophysical Parameters Analysis of Complex Hydrocarbons Part A. Separation Methods Part B. Group Analysis and Detailed Analysis Ion-Exchangers in Analytical Chemistry Methods of Organic Analysis Chemical Microscopy Thermomicroscopy of Organic Compounds Gas and Liquid Analysers Kinetic Methods in Chemical Analysis Application of Computers in Analytical Chemistry Analytical Visible and Ultra-violet Spectrometry Photometric Methods in Inorganic Trace Analysis New Developments in Conductometric and Oscillometric Analysis Titrimetric Analysis in Organic Solvents Analytical and Biomedical Applications of Ion-Selective Field-Effect Transistors Energy Dispersive X-ray Fluorescence Analysis Preconcentration of Trace Elements Radionuclide X-ray Fluorecence Analysis Voltammetry Analysis of Substances in the Gaseous Phase Chemiluminescence Immunoassay Spectrochemical Trace Analysis for Metals and Metalloids Surfactants in Analytical Chemistry Environmental Analytical Chemistry Elemental Speciation – New Approaches for Trace Element Analysis Discrete Sample Introduction Techniques for Inductively Coupled Plasma Mass Spectrometry Modern Fourier Transform Infrared Spectroscopy Chemical Test Methods of Analysis Sampling and Sample Preparation for Field and Laboratory Countercurrent Chromatography: The Support-Free Liquid Stationary Phase Integrated Analytical Systems Analysis and Fate of Surfactants in the Aquatic Environment Sample Preparation for Trace Element Analysis Non-destructive Microanalysis of Cultural Heritage Materials Chromatographic-Mass Spectrometric Food Analysis for Trace Determination of Pesticide Residues Biosensors and Modern Biospecific Analytical Techniques Analysis and Detection by Capillary Electrophoresis Proteomics and Peptidomics: New Technology Platforms Elucidating Biology
Volumes in the Series
Vol. Vol. Vol. Vol. Vol. Vol.
47 48 49 50 51 52
Modern Instrumental Analysis Passive Sampling Techniques in Environmental Monitoring Electrochemical (Bio) Sensor Analysis Analysis, Fate and Removal of Pharmaceuticals in the Water Cycle Food Contaminants and Residue Analysis Protein Mass Spectrometry
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PREFACE
When, over three years ago, we were asked to commission a book on polymer analysis, our initial reaction was to decline, since we both believed that there was enough excellent books already published that dealt with the subject matter. However, we then contemplated that most existing books cover the subject from a technique point of view, that is, they tend to have a format in which individual chapters focus specifically on a single technique, such as mass spectrometry, nuclear magnetic resonance, vibrational spectroscopy, thermal analysis methods, chromatographic methods, etc. Occasionally, the scope is wider, if we can put it that way, but in these cases the focus is on a particular segment of the world of polymers. It was during our discussions on this viewpoint when our industrial experience kicked in, for which we recognised that the paramount desire of the ‘‘customer for polymer molecular characterization and analysis’’ is that of problem solving, not of championing a single or set of particular techniques. What these customers ideally require is a solution or information that enables them to solve or overcome a problem on a timescale that is relevant to the customers’ business needs, whether they be research-related, production-related, property-related, service-related, marketing-related, competitive product deformulation,y It was with this mindset that we agreed to try to commission a book, the result of which you have in front of you, in which each chapter addresses a particular problem or property, for example, chain structure or chain end characterization, polymer degradation, molecular orientation, product failure,y, and in which the techniques were secondary, that is ‘‘a means to an end’’ rather than ‘‘how can I solve this problem/measure this property by the technique for which I am a recognised specialist?’’ The task we set for many of our authors was therefore often a very difficult one: that of providing as balanced a viewpoint as possible of the various techniques and methods that may be used to tackle a particular problem or property characterization, and how they may optimally complement each other, and their comparative limitations and strengths. This was our ultimate aim; that is, a text from which the essential information to tackle a problem, for which polymer analysis is required, could be readily distilled from its pages, and from which the supportive, confirmatory and complementary positions of the various measurement techniques could be readily grasped.
xix
xx
Preface
Our industrial experience should have forewarned us: while it is relatively easy to find well-recognised technique specialists, it did turn out, however, to be not trivial to find authors fully comfortable with covering the issues and all techniques from a problem-solving/property-related point of view. Some chapters might, therefore, from some specialist readers and reviewers perceptions, still have some missing issues albeit relevant. In other cases, for example, polymer degradation, optimum coverage has been achieved by including multiple chapters. Nevertheless, considering all the possible topics to be covered and the complexity of the issues involved, we believe most topics are well covered by the contributing authors, and, notwithstanding, we believe the coverage is comprehensive, and provides a unique text on the molecular characterization and analysis of polymers, and approaches our ideal closely. As intimated above, reaching this point, that is publication of this book, has not been a straightforward task; the road has been sometimes rocky, with various professional and personal problems causing both the need to refine our original outline and the need along the way to commission late in the project new authors. To all our chapter authors, we are therefore truly indebted. For those whose chapters we have ‘‘sat on’’ for nearly two years, we especially thank you for your understanding and patience! For those who ‘‘stepped into the breach’’ near the end, we especially thank you for coming to our aid! We are also very thankful to the publishers, Elsevier Science B.V., for their patience too! This book is divided up into sections. The first three chapters provide a background; sections that follow contain chapters dealing with polymer chain analysis, polymer morphology and structure, polymer degradation, polymer product analysis and support techniques. These are listed in more detail in Chapter 1, which also expands more fully on our industrial perception of the requirements for competence and appreciation in all techniques and methods for polymer molecular characterization and analysis. We hope you find this book of value and its approach both unique and technically informative and useful. John Chalmers Robert J. Meier November 2008
S ER I E S E D I TOR ’ S P R E F A CE
This volume on ‘‘Molecular Characterization and Analysis of Polymers’’ has been edited by John M. Chalmers and Robert J. Meier, both of whom have considerable experience in the industrial sector. They have compiled a broad compilation of chapters on the various aspects of polymer analysis and placed a clear and useful emphasis on problem solving, rather than on the techniques used. The first three chapters layout the background; the following chapters deal with polymer chemistry and microstructure and polymeric materials, polymer chain analysis, polymer morphology and structure, polymer degradation, polymer product analysis, and polymer product development support techniques. The book contains a considerable amount of information on the techniques and methods used for polymer molecular characterization and analysis. Various techniques covered include analytical options for polymer product analysis, like size exclusion chromatography, thermal methods, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, mass spectrometry, infrared spectroscopy, gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry. The book is a useful addition to the Comprehensive Analytical Chemistry series and it is the first on the topic of polymer analysis in the series. A tremendous effort was made by the editors to achieve this compilation and as they say in their Preface ‘‘the road has been sometimes rocky’’. Thanks to the editors and all contributing authors for their time and efforts in preparing this comprehensive compilation of research papers that will make this book on molecular characterization and analysis of polymers a unique reference in this field. D. Barcelo´ Barcelona, April 14, 2008
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SECTION I: Introduction
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CHAPT ER
1 Introduction John M. Chalmers and Robert J. Meier
Contents
1. The Analytical Competence 2. Contents and Structure 3. Practical Considerations in the Multi-Technique Approach References
3 6 11 12
1. THE ANALYTICAL COMPETENCE Polymers are around us everywhere, and their industrial production is a major activity of the chemical industry throughout the world. As a consequence, there is a significant support related R&D (research and development) activity within both academia and industry. This applies to the search for new catalysts, synthesis of new polymers, as well as attempts to produce new polymer chain architectures, formulations, and finally the processing and shaping of polymers. Few polymers are used in their pure form: additives and fillers, including fibres, are commonly added to arrive at a material with the desired lifetime use properties. Polymers can be functionalised to improve certain properties such as adhesion or to make them suitable for use in a particular application, such as an electronic component or aerospace application. To verify that the polymer chain synthesized is what was expected, or in order to relate polymer properties to the structure of the material, one needs polymer analysis and characterisation tools. Analysis can be thought of as the breaking up of anything complex; it is the contrary of synthesis, and can sometimes be referred to as such as ‘reverse engineering’. Characterisation is the determination of properties, such as form, chemical structure, colour, etc. Morphology is the science of form. The build-up of a polymeric sample in terms of amorphous and crystalline domains, threedimensional networks, spherulites or other structural units, or the presence and distribution of glass-fibres or other filler material, constitutes the morphology of the sample. Morphological characterisation thus is a key means to establish structure–property relations. Comprehensive Analytical Chemistry, Volume 53 ISSN: 0166-526X, DOI 10.1016/S0166-526X(08)00401-7
r 2008 Elsevier B.V. All rights reserved.
3
4
John M. Chalmers and Robert J. Meier
Traditionally, analysis and, to a lesser extent, characterisation, have been viewed from the viewpoint of expertise in an individual technique, e.g., spectroscopy of polymers (infrared (IR) and Raman spectroscopies, mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy), thermal properties of polymers (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)), surface characterisation (e.g., secondary ion mass spectrometry (SIMS) and atomic force microscopy (AFM)), etc. However, particularly in an industrial context, problem solving is primarily what product group customers of analytical groups are interested in and the technique expertises should be considered as supporting activities. Moreover, in academic groups those that synthesize polymers based on newly developed catalysts often want to undertake their own analyses. Such groups are essentially interested in appropriate answers from polymer analysis and characterisation, not in building up expertise in individual analytical techniques. What is needed primarily for problem solving and general analytical support is an analytical/characterisation science competence. The competence in polymer analysis is the capability to analyse polymers for constituents and properties, each in relation to the questions or interests from the customer. Whereas in academic synthetic groups the customer and the analyst may be one and the same person, in industry the customer is usually a different person, often with conflicting priorities of timescale. Defined as a competence, the traditional individual expertises, e.g., microscopy, spectroscopy, thermal analysis, etc., are considered as supporting the competence. Foremost is the capability to investigate, analyse and characterise, the polymer, both effectively and efficiently. ‘‘What do we want to know about the polymer?’’ is the question that comes first, whereas the technique and therewith the required expertise comes second. This, obviously, is not meant to suggest that an expertise in a specific technique is not important; in many cases the involvement of a technique expert is vital. However, in both the industrial environment as well as in academic synthetic and materials research groups, the problem solving capability is the key issue. With which technique the problem is going to be solved is less relevant. In most real cases all that matters is whether a problem is addressed appropriately, both in terms of the solution information and its derived timescale; this might, for example, in a failure analysis, simply be identifying cause and not a detailed reason. Important components of the solution may be any one or a combination of accuracy or precision really required, overall time to obtain the results, price per analysis, etc. Accuracy is mentioned at this point because the traditional expert has the tendency to strive for results that are scientifically the best. This, however, is often not required. This requires experts who are well trained in analytical competence thinking. Precision is sometimes much more important than accuracy in a quality assurance measurement. The key appreciation here is the difference between technique expertise and analytical competence. While retaining the necessary expertise on individual techniques, understanding of the customer questions and aims (the problem setting) is paramount. Understanding of the strengths, weaknesses and
Introduction
5
limitations of different techniques is an integral part of optimal performance in a competence. The level of technique expertise required will likely vary from one situation to the other. The requirement on technique expertise does not imply that in each laboratory where an analytical competence is required, a high-level analytical expertise level is required. What is required is the set of skills to provide analyses up to the level required and appropriate to the problem, and perhaps its value of solution. Whether equipment is in-house or elsewhere does not have to matter, as long as it is available when required for problem solving. Having said this, it was felt therefore that there is a need for a book addressing analysis and characterisation of polymers from the point of view of what we wish to call the primary analytical question. Many excellent textbooks and reference works exist which address one or more individual analytical techniques, see, for example, references [1–10]. These books form the basis of the knowledge of the technique expert. They also contain many excellent and varied examples on successful applications of analytical techniques to polymer analysis and characterisation. There are also books which address the multitude of analytical techniques applied in polymer analysis, see, for example, references [11–24]. However, a synthetic chemist may wish to know the constitution of his/her polymer chain, a material scientist may want to find out the reasons why a fabricated sample had failed. What technique is best or optimal to study chain constitution will depend on the situation. Polymer failure may result from morphological features, which needs to be avoided, a contaminant, a surface property degradation, etc. When a sample has been processed, e.g., a film blown, molecular orientation may be the key parameter to be studied. A formulation scientist may wish to know why an additive from a different supplier performs differently. It is from such points of view that polymer analysis and characterisation is addressed in this book. The aim of this book is therefore to present and discuss modern approaches and strategies to polymer characterisation and analysis. As appropriate, however, space will be given to theory, practice and instrumentation. While state-of-the-art characterisation methods will feature highly, conventional use of analytical techniques will also be well covered. The emphasis however, whenever possible, will be pragmatic and problem solving and property determination led, illustrated by ‘real-world’ application examples. As suggested above, the real accuracy or precision required, the time allowed to arrive at a satisfactory result, and the monetary/manpower budget available to obtain meaningful results are all integral elements when the intention is to run an analytical competence in optima forma. Therefore, the emphasis must be on the synergistic, complementary, effective and efficient usage of characterisation techniques and tools and analytical methodology, expertise and skill; it cannot be technique led. Foremost must be the property characterisation, the problem solution and effective polymer analysis; techniques provide the means, and per se should not be the driver! The materials covered in this book will be organic polymers; biopolymer and inorganic polymer characterisations are not considered. The characterisations will focus on polymer and polymer product microstructure and
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John M. Chalmers and Robert J. Meier
composition; the discussions will not feature bulk property measurements, e.g., mechanical, electrical. The emphasis is on commercial materials and formulations. The reason is that commercial materials are rarely pure materials. A pure homopolymer is a rare species in the ‘real-world’ materials. To arrive at the desired material’s properties, either a copolymer is used, sometimes a blend or a dispersion, or additives or filler materials including rubber particles, carbon black or fibres of various type and make may be added, and are thus commonplace in commercial products. This implies a more complex constitution and morphology than expected for pure polymers. However, obviously, the methods described herein can be applied to pure, unmodified, polymers as well.
2. CONTENTS AND STRUCTURE In contrast to a book describing contributions of individual techniques, there is undoubtedly not a single way to structure the approach to be dealt with in this book. The approach taken is as follows. A polymer material consists of individual polymer chains, and all together these make the material and its morphology, which in turn relate to the material’s properties. A cluster of chapters addressing the individual chain characteristics was therefore thought appropriate, followed by another cluster on morphology. As mentioned before, as most polymeric materials for practical use are not pure polymers, a third cluster was formed dealing with analysis of the product, which includes issues such as additives and surface characteristics. Consequently, this book comprises six sections. The first section serves as an Introduction, it includes this chapter and the next two chapters, which deal with the basics of polymer chemistry and microstructure (Chapter 2) and polymeric materials and their composition, uses and application (Chapter 3). The next section (Polymer Chain Analysis: Chapters 4–6) relates to polymer chain analysis (chain structure characterisation, chain end characterisation and molecular weight and distribution). The third section (Polymer Morphology and Structure) that deals with polymer morphology and structure is subdivided into phase structure and morphology (Chapter 7), molecular orientation (Chapter 8) and polymer networks (Chapter 9). Chapters 10–12, the fourth section (Polymer Degradation), cover approaches to study and investigate polymer degradation. The next section, Polymer Product Analysis, covers spatial imaging and heterogeneity (Chapter 13), analysis of additives (Chapter 14), failure, defect and contaminant analysis (Chapter 15) and concludes with a short chapter on surface analysis (Chapter 16). The final section, which we have called Polymer and Polymer Product Development: Support Techniques, considers the role of computational chemistry and molecular modelling in supporting polymer analysis (Chapter 17), and high-throughput analysis (Chapter 18). The polymer characteristics addressed within these clusters are presented below in Table 1. Table 1 presents an overview of the many analytical/ characterisation techniques and methods used for the molecular characterisation
Introduction
7
Table 1 (a) Summary of the techniques used for molecular analysis and characterisation of polymers. The techniques and methods listed have been mostly picked from those mentioned in the chapters within this book; the order in which they appear is random, and does not reflect their relative importance or their order of most common use, particularly since they are most frequently used in synergistic, confirmatory and/or complementary combinations. (b) Abbreviation and acronym definitions for techniques cited in Table 1(a). (a): Polymer
Property to be analysed/ characterised
Suitable/primary techniques/methods
monomer, comonomer type and composition degree of polymerisation (dp) conformation (e.g., transgauche), isomerism (e.g., cis-trans) tacticity, stereoregularity chain lengths (persistence, Kuhn unit) branching (e.g., shortchain, long-chain, hyper-, star, ladder, brushes) unsaturation copolymer sequencing end-group analysis
NMR, IR, Raman
Polymer chain structure:
cross-linking
molecular weight, molecular weight distribution
NMR NMR, IR, Raman
NMR, IR SANS SEC, GPC, NMR, rheology, SANS, SAXS NMR, Raman, IR NMR, IR NMR, MS, MALDI-MS, IR, Raman, chemical titration indirect techniques, including s-NMR, gelpoint determination, rheology, LC-MS techniques can in part be applied GPC/SEC, MALDI-MS, membrane osmometry, vapour pressure osmometry, viscometry, light scattering, TDFRS, SAXS, SANS, SECHPLC, SEC-MS, SEC-IR, FFF, ultracentrifugation, MALDI-TOF-MS, NMR, capillary electrophoresis
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John M. Chalmers and Robert J. Meier
Table 1 (Continued ) (a): Polymer
Property to be analysed/ characterised
Suitable/primary techniques/methods
crystallinity, amorphous content, phase analysis
WAXS, WAXD, SAXS, SALLS, density, DSC, IR, Raman, s-NMR, AFM, optical microscopy, SEM, TEM WAXD, s-NMR, IR, Raman, fluorescence spectroscopy, birefringence CL, ESR, ESRI, IR/IR microscopy, Raman/ Raman-microscopy, GC-IR, EGA-IR, Py-IR, Py-GC-IR, UV, visible spectroscopy, fluorescence/ phosphorescence, MS, MALDI-TOF-MS, GC-MS, Py-MS, Py-GCMS, GPC, SEC, GPC-MS, GPC-IR, s-NMR, NMR, TA, DSC, TGA, oxidative-induction time (DSC), DPC, TGA-IR, TGA-MS, GC, IGC, LC, LC-CC, HPLC, LC-IR, LC-MS, XPS, SIMS, optical microscopy, oxygen uptake, carbonyl index (IR), iodometric test, titration, staining, MFI
Polymer material/product characterisation:
molecular orientation
degradation, oxidation
Product analysis: polymer network
s-NMR, gelation, swelling, birefringence, IR, NMR, STM, SANS, Brillouin scattering, pulsepropagation
Introduction
Table 1 (Continued ) (a): Polymer
Property to be analysed/ characterised
Suitable/primary techniques/methods
surface analysis
optical microscopy, XPS (ESCA), IR-ATR, Raman, AFM, SIMS, LEIS, IR optical microscopy, IR-microscopy, Ramanmicroscopy, TOF-SIMS, NIR-imaging, SNOM, MRI, ESRI, mXRF, PES, LEIS, RIS AAS, GC, GC-MS, IR, HPLC, LC-MS, ICP-AES, SEM, TGA, XRF, NMR optical microscopy, IR, Raman, XRF, DSC, TGA, NMR, SEM, SEM-EDS, XPS, SIMS, GPC/SEC, ICP-MS, GC-MS, LC-MS, AFM
spatial heterogeneity
additive analysis
failure, defect, contaminant analysis
(b)
AAS AFM ATR CL DPC DSC EDS EGA ESCA ESR ESRI FFF GC GPC HPLC IGC ICP-AES ICP-MS IR LC LC-CC
Atomic absorption spectrometry Atomic force microscopy Attenuated total reflection Chemiluminescence Differential photocalorimetry Differential scanning calorimetry Energy dispersive spectroscopy Evolved gas analysis Electron spectroscopy for chemical analysis Electron spin resonance spectroscopy Electron spin resonance imaging Field-flow fractionation Gas chromatography Gel permeation chromatography High-performance liquid chromtography Inverse gas chromatography Inductively coupled plasma atomic emission spectroscopy Inductively coupled plasma mass spectrometry Infrared Liquid chromatography Liquid chromatography under critical conditions
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John M. Chalmers and Robert J. Meier
Table 1 (Continued ) (b)
LEIS MALDI-TOF-MS MFI MRI MS NIR NMR PES Py RIS SALLS SANS SAXS SEC SEM SIMS s-NMR SNOM STM TA TDFRS TEM TGA ToF WAXD WAXS XPS XRF mXRF
Laser enhanced ionisation spectroscopy Matrix-assisted laser desorption time-of-flight mass spectrometry Melt flow index (Nuclear) magnetic resonance imaging Mass spectrometry Near-infrared spectroscopy Nuclear magnetic resonance spectroscopy Photoelectron spectroscopy Pyrolysis Resonance ionisation spectroscopy Small-angle laser light scattering Small-angle neutron scattering Small-angle X-ray scattering Size exclusion chromatography Scanning electron microscopy Secondary ion mass spectroscopy Solid state nuclear magnetic resonance spectroscopy Scanning near-field optical microscopy Scanning tunnelling microscopy Thermal analysis Thermal diffusion forced Rayleigh scattering Transmission electron microscopy Thermogravimetric analysis Time-of-flight Wide-angle X-ray diffraction Wide-angle X-ray scattering X-ray photoelectron spectroscopy X-ray fluorescence spectroscopy micro-X-ray fluorescence spectroscopy
and analysis of polymers and polymer formulations and products. While one or two techniques/methods may be dominant in a particular area, it can be readily seen that several to many can be applied depending on the information required and the depth of problem solving and understanding that is needed, along with the suitable or most common analytical techniques. The requirement and need for a multi-technique attack on a problem or study is probably most exemplified in the chapter on polymer failure (Chapter 15) and polymer degradation (see Chapter 10); in the latter many of the references cited used five or more analytical/characterisation tools in the investigation. The need for a multitechnique approach is an even more prominent need in the study of polymer morphology, although perhaps less often practised in single studies. A key
Introduction
11
feature here obviously is the various length scales involved, from single chain conformation and configuration (as they co-determine the possibility of certain larger structures to be formed) to crystalline lamellae, shperulites, etc. Key issues when polymer chain analysis or polymeric materials characterisation is applied is the choice of property to analyse, and when selected, which is the most suitable technique given the question to be addressed and circumstances, e.g., business pressure (availability, price, accuracy, speed).
3. PRACTICAL CONSIDERATIONS IN THE MULTI-TECHNIQUE APPROACH Whereas several techniques may thus be used to study a certain characteristic of a polymer sample, for instance IR and Raman spectroscopy and X-ray diffraction as well as NMR may be used to determine or infer the crystallinity level of a sample, different techniques work differently and therefore usually do not measure the same. What this means is that crystallinity levels obtained from the same sample may differ when a different technique is applied, see, for example, ref. [23] and chapter 7 and references therein. However, these differences do not necessarily imply one technique being better than another. In fact these differences may contain useful information on the sample (see, for example, ref. [25]). This brings us to the point of considering the suitability of a technique with respect to a certain analysis, particularly from an industrialist’s or contract laboratory viewpoint. This is a very important consideration, as this is not a priori straightforward. There are a number of factors influencing the choice. We will mention just a few of these in arbitrary order: (i)
(ii) (iii) (iv) (v) (vi) (vii) (viii)
When, for whatever reason, one does not wish to be involved in further method development but just apply a technique, this means that the technique must have been applied to the problem of interest before, or it must be essentially a trivial translation from a previously demonstrated application. Example: IR spectrum of an unknown, NMR can do the job; X-ray crystallinity. Does the technique need to be applied on a lab-scale or in-process? Does the technique have to be applied off-, at-, in- or on-line? Is cost of analysis relevant? What is the accuracy/precision required? Is concentration/composition to be measured? Is speed important? Do you have the equipment available (in-house or otherwise readily available)? If such as a failure analysis, is it just satisfactory to identify the cause, or does reason matter too?
The chapters following in this book should, we hope, enable one to obtain an objective assessment and comparison of the most appropriate polymer molecular
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John M. Chalmers and Robert J. Meier
characterisation or analysis tool according to the problem solving circumstances. The reader will be able to obtain experts’ overviews of the range of tools, and their strengths, convenience of use and limitations, necessary to characterise or analyse the molecular structure, composition and properties of polymers and their formulated products.
REFERENCES 1 I. Ando and T. Asakura, Solid State NMR of Polymers, Elsevier, Amsterdam, 1998. 2 F. Bovey and P. Mirau, NMR of Polymers, Elsevier, Amsterdam, 1996. 3 J.M. Chalmers and P.R. Griffiths (Eds.), Handbook of Vibrational Spectroscopy, Vol. 4, Wiley, Chichester, 2002. 4 N.J. Everall, J.M. Chalmers and P.R. Griffiths (Eds.), Vibrational Spectroscopy of Polymers: Principles and Practice, Wiley, Chichester, 2007. 5 P. Worsfold, A. Townshend and C. Poole (Eds.), Encyclopedia of Analytical Science, 2nd ed., Academic Press, London, 2004. 6 D. Briggs, Surface Analysis of Polymers by XPS and Static SIMS, Cambridge University Press, Cambridge, 1998. 7 S. Schlick (Ed.), Advanced ESR Methods in Polymer Research, Wiley, Chichester, 2006. 8 W.M. Groenewoud (Ed.), Characterisation of Polymers by Thermal Analysis, Elsevier, Amsterdam, 2001. 9 I. Wilson, C. Poole and M. Cooke (Eds.), Encyclopedia of Separation Science, Academic Press, London, 2000. 10 G. Montaudo and R.P. Lattimer, Mass Spectrometry of Polymers, CRC Press, Boca Raton, 2001. 11 B.J. Hunt and M.I. James (Eds.), Polymer Characterisation, Blackie Academic & Professional, Glasgow, 1993. 12 J.L. Koenig, Spectroscopy of Polymers, 2nd ed., Elsevier Science, New York, 1999. 13 J.I. Kroschwitz (Ed.), Polymers: Polymer Characterization and Analysis, Wiley, New York, 1990. 14 J.I. Kroschwitz, Concise Encyclopedia of Polymer Science and Engineering, Wiley, New York, 1998. 15 Polymers and Rubbers. In: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2002. 16 H.F. Mark (Ed.), Encyclopedia of Polymer Science and Technology, 3rd ed., Wiley-VCH, Weinheim, 2004. 17 J.C.J. Bart, Additives in Polymers: Industrial Analysis and Applications, Wiley, Chichester, 2005. 18 J. Scheirs, Compositional and Failure Analysis of Polymers: A Practical Approach, Wiley, Chichester, 2000. 19 J. Haslam, H.A. Willis and D.C.M. Squirrell, Identification and Analysis of Plastics, 2nd ed., Buterworth, London, 1972. 20 J.I. Kroschwitz (Ed.), Concise Encyclopedia of Polymer Science and Engineering, Wiley, Chichester, 1998. 21 R.A. Pethrick and J.V. Dawkins (Eds.), Modern Techniques for Polymer Characterisation, Wiley, Chichester, 1999. 22 M. Bolgar, J. Hubbal, J. Groger and S. Meronek, Handbook for the Chemical Analysis of Plastic and Polymer Additives, CRC Press, Boca Raton, 2007. 23 D. Campbell, R.A. Pethrick and J.R. White, Polymer Characterization: Physical Techniques, 2nd ed., CRC Press, Boca Raton, 2000. 24 R.A. Prethrick, Polymer Structure Characterization from Nano to Macro Organization, RSC Publishing, Cambridge, 2007. 25 R.J. Lehnert, P.J. Hendra, N. Everall and N.J. Clayden, Comparative quantitative study on the crystallinity of poly(tetrafluoroethylene) including Raman, infra-red and F nuclear magnetic resonance spectroscopy, Polymer, 38(7) (1997) 1521–1535.
CHAPT ER
2 Polymer Chemistry and Microstructure Jacques Devaux and Sophie Demoustier-Champagne
Contents
1. Introduction 2. Polymer: Definitions 2.1 Monomers, chain length, length distribution 2.2 Thermoplastics and thermosets 2.3 Thermal transitions and properties 2.4 Microstructure 3. From Molecules to Macromolecules 3.1 Chain length and entanglements 3.2 End groups 3.3 Chain substituents 3.4 Spatial arrangements 4. Fundamentals of Polymer Chemistry 4.1 Chain polymerisation vs. step polymerisation 4.2 Types of chain polymerisations and polymers synthesized — keypoints of mechanisms, kinetics 4.3 Radical chain polymerisations 4.4 Ionic polymerisations — active species 4.5 Olefin polymerisation processes 4.6 Processes for polyolefin syntheses 4.7 Copolymers 4.8 Types of step polymerisations and polymers synthesized — keypoints of mechanisms, kinetics 5. Polymer Degradation 6. Summary References
Comprehensive Analytical Chemistry, Volume 53 ISSN: 0166-526X, DOI 10.1016/S0166-526X(08)00402-9
14 15 15 17 17 21 25 27 27 28 28 34 34 35 35 42 45 48 50 53 58 62 62
r 2008 Elsevier B.V. All rights reserved.
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Jacques Devaux and Sophie Demoustier-Champagne
1. INTRODUCTION ‘‘A material is a solid used by man to produce items that constitute the support for his living environment’’ [1]. Such a definition, although restrictive as it excludes liquids, is simple enough. It might imply however, on the one hand, that materials as such only existed from the beginning of mankind, moreover, on the other hand, that the usual three classes of ‘‘materials’’ — ceramics, metals and polymers — have always coexisted. Indeed, most of the earth’s terrestrial crust is made of minerals, the majority of which are considered to be ceramics, that is, ‘‘resulting from the combination of a limited number of metallic elements (Si, Ca, Mg, Al, Ti, etc.) with non-metallic elements, most commonly oxygen’’ [1]. Ceramic materials marked the so-called ‘‘Stone Ages’’, the earliest stages of civilisation. A few pure (native) metals have existed as materials from the first ages of mankind, namely those that have a high oxido-reduction potential, which protects against oxidation by atmospheric oxygen (e.g., gold, platinum, silver,y). Their bright colour and inalterability led humans to use them in jewellery. However, geological science tells us that huge amounts of natural metals exist in the earth’s nucleus (e.g., Si, Al, Fe, Ni, etc.), which constitute the main part of the earth’s mass. Being oxidised through reaction with oxygen, landmarks in civilisation’s progress (bronze age, iron age) are labelled by mankind’s mastering of more and more complex metal productions (reduction). Natural polymers are far more complex materials, being exclusively ‘‘organic’’, that is, products of life (cellulose, proteins, DNA,y). Nevertheless, as nature seems to have formed (size, weight, hardness, etc.) several natural polymers (wood, bones, ivory, etc.) in such a way as to be almost immediately usable by our ancestors, they were most probably the bases of the first human tools. The commonly widespread conception that polymers are the youngest ‘‘materials’’ in the historical world is not true, but, strictly speaking, only applies to synthetic polymers, which were discovered about 100 years ago, early in the XXth century. Polymers are undoubtedly ‘‘materials’’. Knowledge and understanding of their properties belongs to the so-called field of ‘‘Materials Science’’. Study and understanding of their formation, properties and performance characteristics involves chemistry, physico-chemistry and physics, the latter including mechanics, optics, electricity, etc. Material’s knowledge is not limited to the aforementioned fields of science, but includes also technological properties, which are most often used for their classification: strength, hardness, transparency, conductivity, etc. Materials science is thus built on complex interrelationships between such fields, see Figure 1. Few properties, if any, depend directly and only on the chemistry. Indeed, as shown in Figure 1, the ‘‘route’’ implies that it is chemistry that influences (micro-) structure, which, in turn, governs properties. Processing, which is not often classified as a ‘‘science’’ but as ‘‘engineering’’, has to find its place in the picture, since it is the normal way to go from a macromolecule to a product with the desired properties.
Polymer Chemistry and Microstructure
15
Processing Synthesis
Characterisation
Microstructure
Properties
Figure 1 Polymers in materials science: Interrelationships between synthesis, processing, microstructure, properties and characterisation.
Characterisation covers a wide field of science and technology and interacts from the synthesis stage through the analysis of structure to the determination of properties. Characterisation allows one to observe, control and model (including ‘‘closed-loop’’ backward interaction) all the complex interrelationships of materials science.
2. POLYMER: DEFINITIONS 2.1 Monomers, chain length, length distribution The word ‘‘polymer’’ (first proposed by Berzelius in 1833) is made of ‘‘poly’’ from the ancient Greek word ‘‘poluB’’ meaning ‘‘many’’ and ‘‘meroB’’ meaning ‘‘part’’. Polymers are molecules built up from numerous identical chemical ‘‘units’’ spatially repeated to form a chain. From the early times and still nowadays, a distinction is often made between ‘‘natural’’ and ‘‘synthetic’’ polymers, but it is somewhat artificial as natural polymers can now sometimes be synthesized (e.g., synthetic ‘‘natural rubber’’) and some synthetic polymers, which are never found in nature, can be synthesized by natural ways (enzymatic syntheses). Nevertheless, synthetic polymers are most often made from monomers (from ancient Greek ‘‘monoB’’ meaning single), which are precursor molecules of a polymer chain. When only a few monomers are joined to form a chain, they are called oligomers (from ‘‘oligoB’’ a few, in ancient Greek). Despite this distinction between oligomers and polymers is rather ill defined, one generally conceives that polymer chains exhibit high molecular masses ranging from 104 to about 107, the masses of oligomers being conventionally limited to below about 104. Polymer is also a word the use of which is restricted to chemical compounds based on covalent links, chemically stable at ambient conditions. Most often polymers are made from carbon and hydrogen or hydrocarbon derivatives (with oxygen, nitrogen, sulfur, halogens,y) (Table 1). But, they are sometimes based on inorganic backbone atoms (silicon, nitrogen and phosphorous,y). From the above considerations, it appears that chain length is a main characteristic of polymers. Natural macromolecules, particularly in living bodies, can exhibit exact lengths, thus exact molecular masses. For instance haemoglobin molecular mass is known to within one dalton. However, a synthetic polymer
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Jacques Devaux and Sophie Demoustier-Champagne
Table 1
Examples of some ‘‘natural’’ and ‘‘synthetic’’ polymers
Name of polymer
Formula
Polyethylene
Approximate date of invention
1935
C C H2 H2 n
Polypropylene (isotactic)
1954
C CH H2 n CH3
Poly(methyl methacrylate) (PMMA)
O
C
O
1935
CH3
C C H2 n CH3
Poly(vinyl chloride) (PVC)
1872
C CH H2 n Cl
Polystyrene
1925 C CH H2 n
Poly(ethylene terephthalate) (PET)
Poly(hexamethylene adipamide) (Nylon 66)
1941 H
O
C C O H2 H2
C
C
O
O
H
N H
Cellulose
O
C N C H2 6 H
H2 C OH O H HO H H H
OH
1938
O
O
OH n
C OH C H2 4 n
H O
HO H
Natural polymer
OH H H
O C OH H2
n
Polymer Chemistry and Microstructure
17
sample contains chains having a wide distribution of chain length. The breadth of the molecular mass1 distribution depends on the synthesis process. Therefore, it is necessary to define average molecular masses to characterize a polymer sample. The simplest method — the number average molecular mass or Mn — is an arithmetic average based on the total mass of a sample divided by the total number of molecules in this sample. For correlation with most physical properties (mechanical strength, optical scattering), mass average molecular mass of a polymer ‘‘Mw ’’ appears more satisfactory. Higher-power averages like z-average molecular mass ‘‘Mz ’’ seem to better correlate with rheological properties. Expressions of Mn ; Mw and Mz are P P 2 P 3 ni mi
i Mn ¼ P n
i
ni mi
i Mw ¼ P nm
i
i
i
i
ni mi
i Mz ¼ P n m2 i
i
i
where ni and mi are the number and the molecular mass of molecules containing i units, respectively. A measure of the breadth of the molecular mass distribution is given by the ratios of molecular mass averages. The most commonly used ratio Mw =Mn ¼ H, is called the polydispersity index. Wiegand and Ko¨hler discuss the determination of molecular masses (weights) and their distributions in Chapter 6.
2.2 Thermoplastics and thermosets Thousands of polymers have been synthesized and more are likely to be produced in the future. Fortunately, all polymers can be divided into two major groups based on their thermal processing behaviour. Some polymers behave as (viscous) liquids upon heating and can be re-processed, almost indefinitely. These are classified as ‘‘thermoplastics’’ (from the ancient Greek prefix ‘‘yermo’’ for ‘‘heat’’). Some major commercial examples are polystyrene (PS), polyethylene, polypropylene and poly(vinyl chloride) (PVC). Other polymers keep their (definitive) shape after a first heating step during synthesis or subsequent treatment. These are called ‘‘thermosets’’. Once formed the crosslinked networks resist heat and solvent attack. Such properties make thermosets suitable materials for composites, coatings and many adhesive applications. Major examples of thermosets include epoxy resins, phenol-formaldehyde resins, urea/melamine formaldehyde resins and unsaturated polyesters.
2.3 Thermal transitions and properties 2.3.1 Glass transition Matter is made from atoms or molecules, for which the spatial arrangements depend on cohesion (energy from interaction forces) and thermal energy, which is proportional to temperature. Weak cohesion and/or high temperatures lead to 1
The notation M is used here for molecular masses, while X is used for chain lengths (or ‘‘degrees of polymerization’’). M ¼ XMO with MO, the molecular mass of one unit in the polymer chain.
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Jacques Devaux and Sophie Demoustier-Champagne
gases, while, in contrast, high cohesion and/or low temperatures yields solids, with liquids as an intermediate state. There are rather limited differences between liquid and solid states as specific masses of solids are usually only about 15% higher than those of corresponding liquids. Inside liquids, due to the rather low level of empty volume (the so-called ‘‘free volume’’), molecules can move due to ‘‘concerted’’ movements of the neighbouring molecules. Upon cooling down a liquid, the ratio between the thermal and the cohesion energy decreases, molecules become closer to each other and the free volume decreases. Thermal energy, which leads to molecular mobility, becomes lower, and attraction and repulsion forces between molecules begin to play a more significant role. When these latter forces predominate over thermal energy, an equilibrium is usually reached between them leading to ordered (crystalline) structures. However, this is only the case when the time needed for molecular movement (the so-called ‘‘characteristic time’’) is lower than that of cooling. When no extended ordering can occur and/or if molecular movements are extremely slow (very big and/or branched molecules) and/or if the cooling is very fast, ordering remains limited to rather small ‘‘domains’’ of molecules. The solid is said to be ‘‘amorphous’’ and retains in the solid state, the rather disordered microstructure (including the free volume) of a liquid. Figure 2 shows how atomic organisation changes with temperature. The temperature at which a liquid (highly viscous, or even rubbery for very high molecular masses) becomes an amorphous solid (rigid, stiff, often brittle) on cooling (or vice versa an amorphous solid becomes liquid on heating) is called the glass transition and noted Tg (Figure 3). In the glassy state, thus below Tg, very few molecular movements (group rotations, vibrations) are allowed. Above Tg, large-scale motions involving the whole macromolecule (translations, repetations) become possible. It has to be noticed that no isothermal volume contraction on cooling or volume expansion on heating is associated with Tg, contrary to crystallisation or melting. The latter is a true first-order transition exhibiting a discontinuity in
Figure 2 Evolution of atomic organisation with increasing temperature. Reproduced from Mercier, Zambelli and Kurz [1], Elsevier (2002) with permission of Elsevier. Copyright Elsevier 2002.
Polymer Chemistry and Microstructure
19
Figure 3 Curves of specific volumes vs. temperature for poly(vinyl acetate) measured on cooling. Equilibrium values measured 0.02 h and 100 h after cooling, as indicated. Tgu and T g are glass transitions respectively at fast and slow cooling rate. Reproduced from Ref. [2] with permission of John Wiley & Sons, Inc.
thermodynamic variables. Despite the discontinuity occurring only in the first derivative of thermodynamic variables, Tg is not a true ‘‘second-order’’ transition as it also depends on kinetics. Figure 3 also shows how Tg moves with cooling rate for a sample of poly(vinyl acetate). Tg is said to be a ‘‘kinetic transition’’.
2.3.2 Crystallisation-melting temperature The commonest state of the solid matter is the crystal. A crystalline solid is made of a periodic arrangement of atoms (or small molecules) in the three dimensions of space. A (single) crystal is a very ordered state formed by the juxtaposition of numerous identical ‘‘unit cells’’ (parallelepipedic volume containing one atom or one small molecule). A crystal structure is characterised by its symmetry [1]. Due to the length of the macromolecules, the existence of single crystals of polymers has remained a controversy for a long time. Soon after World War II, microscopic polymer ‘‘single crystals’’ were obtained by very slow cooling from
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Jacques Devaux and Sophie Demoustier-Champagne
Figure 4 Scheme of chain folding in polymer crystals.
dilute solutions. Upon cooling from the liquid state at a suitable rate, polymers whose chain segments (including a few repeat units) exhibit symmetry elements of high enough order, can thus crystallise. Such a crystallisation goes along with long-range ordering. The determination of the ‘‘unit cell’’ led to the conclusion that, within such a single crystal, one macromolecule encompasses many ‘‘unit cells’’, which, itself, includes only one or a few ‘‘repeat units’’. Moreover, upon examination by electron diffraction, it appeared also that in these ‘‘single crystals’’, the chain axis was perpendicular to the largest faces, thus, that the chains are oriented in the ‘‘thickness’’ direction. However, as such thicknesses are much smaller than extended-chain lengths, a model of ‘‘chain folding’’ was established for forming such polymer crystals (Figure 4). As a consequence of chain folding, besides the inclusion of chain ends, these ‘‘single crystals’’ exhibit at a minimum a certain amount of defects at their surface. This is a reason why polymers are never 100% crystalline. In polymers crystallised from the melt, lamellae were observed by electron microscopy, whose thickness are of the same order of magnitude as ‘‘single crystals’’ obtained from solution (Figure 5). Crystal ‘‘unit cells’’ within these lamellae are comparable to those of single crystals and the chains are also oriented in the thickness direction. Therefore, the same model of chain folding was applied to melt-crystallised polymers. However, lamellae are, by far, less perfect structures than ‘‘single crystals’’ and each individual polymer chain gives rise to only a limited number of ‘‘folds’’, its remaining part (the so-called ‘‘tie’’) protrudes from the lamella and remains in an amorphous phase. By this way, the crystallinity of melt-crystallised polymers lie within a range extending from 85% to 90% for very regular and flexible polyethylene chains down to about 30–40% maximum for most other crystallisable polymers. Polymers are often said to be ‘‘semi-crystalline’’, whereas ‘‘partly crystalline’’ should be more correct. By optical microscopy (OM), birefringent structures are observed in semicrystalline polymers, characterized by ‘‘Maltese-crosses’’ under crossed polars as seen in Figure 6. As these structures grow symmetrically in three dimensions
Polymer Chemistry and Microstructure
21
Figure 5 Electron micrograph of a portion of melt crystallised polyethylene spherulite by transmission electron microscopy (TEM) showing lamellae. Reproduced from Ref. [3] with permission of John Wiley & Sons, Inc.
from a nucleus, they are called ‘‘spherulites’’. Upon examination, they appear to be formed of ‘‘fibrils’’ growing from the nucleus. These fibrils include several lamellae most often linked by tie-molecules going from one lamella to its neighbour. In semi-crystalline polymers, ordered structures appear at different dimensional levels 1. 2. 3. 4.
the the the the
crystal cell lamella fibril spherulite.
Such a complex microstructure is illustrated in Figure 7.
2.4 Microstructure 2.4.1 Linear and branched homopolymers Polymers that are built from the repetition of identical ‘‘repeat units’’ are called ‘‘homopolymers’’ (from ancient Greek ‘‘o`moB’’ ¼ same).2 Linear homopolymer chains are obtained upon linking chemically identical units exclusively at both ends. However, repeat units are not always symmetrical in the chain direction. Depending on the orientation of the repeat unit, different microstructural 2
Copolymers that are made from the repetition of two (or a very few) different units will be the subject of a later paragraph.
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Jacques Devaux and Sophie Demoustier-Champagne
Figure 6 Spherulites of isotactic poly-1-butene (a, during growth) and of polyethylene (b, after completion) by optical microscopy (OM) under crossed polars. Reproduced from Ref. [3] with permission of John Wiley & Sons, Inc.
Growth
Intercrystalline tie chain (amorphous)
Crystalline lamellae
Amorphous phase
Nucleating seed
Figure 7 Scheme of the internal structure of a spherulite. Reproduced from Mercier, Zambelli and Kurz [1], Elsevier (2002) with permission of Elsevier. Copyright Elsevier 2002.
Polymer Chemistry and Microstructure
CH3 CH3
CH3 C H2
C H
C H2
C H
A
C H B
CH3 C H2
C H2 C
C H
23
CH3 C H2
C H
A
Figure 8 Schemes of (A) ‘‘head–tail’’, (B) ‘‘head–head’’ and (C) ‘‘tail–tail’’ microstructural arrangements in polypropylene.
SCB
LCB
Dendrimer
Hyperbranched
Figure 9 Examples of branching in polymers LCB (long-chain branched, e.g., LDPE) and SCB (short-chain branched, e.g., LLDPE), dendrimer and hyperbranched.
arrangements — ‘‘head–head’’, ‘‘head–tail’’ or ‘‘tail–tail’’ — can occur (1D microstructure); these are called positional isomers (Figure 8). Due to either accidental or intentional events during the synthesis, branching points can occur along the chain (2D microstructure). Although this needs the presence at the branching point of, stricto sensu, a ‘‘non-identical’’ unit, these macromolecules remain classified as homopolymers. Branching can also give rise to either the so-called ‘‘long-chain branching’’ (LCB) or ‘‘short-chain branching’’ (SCB), see Figure 9. Such terms are of importance particularly for polyethylene as they describe the only3 small ‘‘chemical’’ difference between technologically highly different low-density (LDPE) and linear lowdensity (LLDPE) polyethylenes. Except for polyethylene, the distinction between LCB and SCB is somewhat controversial as it can depend on several characteristics or even on the capacity of the characterisation method itself. From the branching point three or more chains can arise. This depends on the so-called ‘‘functionality’’ of the ‘‘monomer’’ at the branching point (the ‘‘branching unit’’). This functionality can be defined as the number of molecules (monomers, branches) actually linked to one branching unit. This definition can be extended to monomers purposely inserted into a chain to create branching. In this case the definition of functionality is generalized to the number of monomers that can link to the considered unit. ‘‘Normal’’ monomers have thus a functionality of 2. Branching units can exhibit functionalities of 3, 4 or more.
3
End groups are not taken into account here.
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Jacques Devaux and Sophie Demoustier-Champagne
2.4.2 Dendrimers and hyperbranched polymers Highly non-linear homopolymers can even be synthesized exclusively from monomers of functionality higher than 2. Dendrimers (from ancient Greek ‘‘dendron’’ ¼ tree) are obtained when all the reaction sites of each branch links to another one and when all branches exhibit the same length. Whereas, hyperbranched (HB) polymers result from incomplete reaction of each multifunctional monomer, see Figure 9.
2.4.3 Crosslinked homopolymers As the number of branching points per chain increases, (when it becomes W2) an almost infinite network of covalent bonds can be created. Strictly all the system belongs to only one giant macromolecule. The polymer becomes insoluble and infusible; it is said to be ‘‘crosslinked’’. Even if, formally, an infinite network of covalent bonds can give rise to a 2D infinite structure (when grown onto a surface, for instance), crosslinked polymers are generally considered as 3D structures. Such crosslinked structures can be made either by using monomers exhibiting functionalities higher than 2 (thermosets like polyepoxides, phenolics), or by inducing a reaction of preformed linear chains with ‘‘bridging’’ molecules (e.g., vulcanisation of natural rubber) (Figure 10). Crosslinked 3D networks are normally amorphous,4 infusible and insoluble. Solvents can only swell networks. However, polymers resulting from polymerisation of molecules of functionality higher than 2 can remain soluble during the course of synthesis, below a certain ‘‘critical conversion value’’. Above this state, ‘‘gelation’’ occurs. Even in this state, the system remains partially soluble. It can be separated by solvent extraction into two phases: the soluble phase is called the ‘‘sol’’ and the insoluble phase the ‘‘gel’’. As the monomer conversion proceeds, the amount of ‘‘sol’’ decreases, while the amount of ‘‘gel’’ increases. The behaviour of crosslinkable amorphous systems during their synthesis can be described in the so-called time–temperature transformation or ‘‘TTT diagram’’ (Figure 11). It has to be noticed that such a TTT diagram can only be used in the direction of increasing time (transformation). A particular characterisation of a crosslinked system is the determination of the crosslink density. Access to the measurement of such crosslink densities is possible through models requiring determining the swelling of these systems.
2.4.4 Linear and branched copolymers Copolymers are built from the repetition of two (or more) ‘‘repeat units’’. Depending on the spatial arrangement of those units (e.g., A and B) along the chain (sequencing), various types of copolymers can be made: alternating, block, random (or statistical), branched, crosslinked (see Figure 12). More sophisticated microstructures are sometimes synthesized, but the highly complex chain sequencing encountered in natural copolymers, like proteins or 4
Depending on the mobility of chain segments, some crystallisation can occur in crosslinked systems.
25
Polymer Chemistry and Microstructure
C CH C C H2 H2 H
C CH H2
Cross-link C C H2 H
A
C CH H2
CH C H2
C CH C C H2 H2 H
C H2
CH3 CH
C
C H
C H2
S
B
S C C H2 H
C
C H
CH3
Figure 10 Examples of crosslinked polymers (A, polyfunctional; B, vulcanised).
enzymes, are still not accessible to chemical synthesis at the beginning of the third millennium. Several physical properties of copolymers are directly or indirectly dependent upon the distribution of sequences. The phase behaviour (microphase separation) in block copolymers is linked to the mutual insolubility/ solubility of the sequences. This will be discussed further.
3. FROM MOLECULES TO MACROMOLECULES As mentioned previously, chemistry and physico-chemistry play major roles in polymer microstructure, and, as a consequence, on polymer properties. The characterisation of several chemical parameters of polymer chains is thus of paramount importance for controlling the final properties of polymer materials Chain length or molecular masses J Entanglement J Concentration and type of end groups Nature of chain substituents
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Jacques Devaux and Sophie Demoustier-Champagne
UNGELLED GLASS GELLED GLASS GELATION
TIME TO GELATION AND VITRIFICATION
GLASS
VITRIFICATION
RUBBER LIQUID
GELATION
Tgg
Tg∞
CURE TEMPERATURE, TCURE
Figure 11 Example of time–temperature transformation (TTT) diagram. (Increasing time in vertical direction). Reprinted from Roller and Gillham [4], published by the Federation of Societies for Coatings Technology, with permission of publisher.
A A B A A B B B A B B A
Random
B
A B
A B
A B
A A A A A B
B
A B
A B
A
Alternate
B
B
B
Block or sequenced
B
B
A A A A A A A A A A A A B
B
B
B
B
Figure 12 Examples of microstructures of copolymers.
Branched
Polymer Chemistry and Microstructure
27
Spatial arrangement J Cis–trans isomerism J Chain configuration J Chain conformation The chain structure (length, entanglement, branching, tacticity,y) of polymers is discussed further in the next chapter (Chapter 4) by Beaucage and Kulkarni.
3.1 Chain length and entanglements An important chemical characteristic of a polymer is the chain length. Chain length strongly influences the thermal properties as illustrated in Table 2 for several aliphatic compounds of different lengths. Detailed studies have been published on the influence of chain length on melting temperatures of various polymers [5] or on Tg and Tm (melting temperature) of aromatic polymers [6]. Above a characteristic chain length polymer chains may (depending on the rigidity of the polymer backbone and/or on processing (stretching)) undergo the formation of entanglements. These entanglements mainly govern rheological properties and several mechanical (viscoelastic) properties. A less known, and indirect, effect of such entanglements is a lowering of ‘‘pure’’ thermal stability of polymers as compared with corresponding lower mass compounds (non-entangled). As an example, in absence of oxygen, linear polyethylene starts degrading at about 3001C, while hexadecane can resist up to 5001C [7].
3.2 End groups A useful relationship can be written between molecular mass and end group concentration for linear polymers. Indeed, as there are exactly two end groups Table 2
Influence of chain length on melting point, T, of aliphatic compounds H–(CH2)n–H
n
Name
Melting T (1C)
1 2 3 4 5 6 8 10 12 18 30 EN
Methane Ethane Propane Butane Pentane Hexane Octane Decane Dodecane Octadecane Triacontane Polyethylene
182.4 182.8 189,9 138.3 129.7 95.0 56.5 29.7 9.6 28.0 65.5 110–135
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Jacques Devaux and Sophie Demoustier-Champagne
per chain (of n units), their concentration [EG] can be written as: ½EG ¼
2 Mn
End group quantification is thus a useful method for number average molecular mass determination. Also, if the end group concentration is higher than the value calculated from an independent Mn determination, chain branching can be deduced and quantified. Jackson and Robertson consider the molecular structure characterisation and analysis of polymer end groups in Chapter 5.
3.3 Chain substituents The nature of the chain substituents strongly influence the properties of the resulting polymer. For instance, the influence of the nature of chain substituents on Tg is illustrated in Table 3 for a series of vinyl polymers.
3.4 Spatial arrangements In addition to the type, number and sequential arrangement of monomers along the chain, the spatial arrangement of substituent groups plays also an important role in determining the polymer properties. Table 3
Influence of chemical nature of substituents on Tg
Structure
C C H2 H2
Glass transition (Tg)1C
Polyethylene
B100
Polypropylene
15
Polyvinyl chloride
+80
Polystyrene
+100
n
C CH H2 n CH3
C CH H2 n Cl
C CH H2
Name
n
Polymer Chemistry and Microstructure
29
CH3 C H2
C
C H2
CH3 CH3 H2 H2 H2 C C C C C C C C C C H H H2 H2 H
CH3 H C C
CH3 H2 H2 H C C C C C C C C C H2 H H2 H2 CH3
Gutta percha
Natural rubber
Figure 13 Chemical structures of trans and cis isomers of 1,4 polyisoprene (gutta-percha and natural rubber, respectively).
3.4.1 Cis–trans isomerism For polymers with double bonds in the backbone, a first arrangement to consider is the cis–trans isomerism. A well-known example of the influence of this isomerism is found in the natural resins from the saps of hevea (‘‘natural rubber’’) and of sapotacea (‘‘gutta-percha’’). The chemical structures of these materials are, respectively, cis-1,4 polyisoprene for natural rubber and trans-1,4 polyisoprene for gutta-percha (Figure 13). From their properties, mainly that of the chain symmetry allowing the trans isomer to crystallise more easily than the cis isomer, natural rubber achieved its huge commercial exploitation, while uses of gutta-percha remained limited mainly until the end of XIXth century to textile impregnation, and nowadays to golf balls and to dental filling.
3.4.2 Configuration — tacticity Chirality (from ‘‘weir‘‘, hand in ancient Greek) is a property of organic molecules containing at least one carbon atom (or a silicon) with four different substituents. When there is only one asymmetric centre in the molecule, two stereoisomers can be drawn. Along a long chain (considering both sides of the chain as different) carbons (or silicons) bearing two different substituents are considered as asymmetric and two structures (R or S) that would not superimpose can be drawn (Figure 14A). Upon laying out such units along a polymer chain, successive units can be either identical (R,R or S,S) or different (R,S or S,R), giving rise to different stereochemical arrangements of atoms, called configurations. Unlike the conformation, the configuration of a polymer chain cannot be altered without breaking chemical bonds. The general term for the stereoregular configurations is the tacticity of the chain. When all successive asymmetric units exhibit the same configuration, the polymer is said to be ‘‘isotactic’’, when the configurations alternate, the polymer is said to be ‘‘syndiotactic’’; it is ‘‘atactic’’5 when the configurations are randomly distributed along the chain. As illustrated in Figure 14A, three different placements 5
‘‘Heterotactic’’ is sometimes used.
30
Jacques Devaux and Sophie Demoustier-Champagne
H3C
Polypropylene
Poly(methyl-phenyl)silane
A
B
H3C C
C
C C
C
φ
H3C
H3C
H3C
Si
C C
Si
Isotactic
CH3 H3C C
C C
CH3 C
C
C C
φ
H3C
Si
C
CH3 H3C C
C
CH3 C
C
φ
C C
φ H3C
φ
φ
H3C
φ
Si
Si φ CH H3C 3
φ Si
Si
CH3 φ
CH3 H3C
Si
φ
H3C
Si
CH3 φ
CH3
Si Si
φ H3C
Si
Si Atactic
SI
Si
Syndiotactic
φ
H3C
Si
H3C
φ CH3
φ
H3C
Si H3C
H3C
φ
H3C
φ
CH3 φ
Si Si φ H3C
CH3 Si
Si φ
Figure 14 Tacticity of (A) polypropylene and (B) poly(methyl-phenyl) silane.
of the substituents (in the example, H, CH3) are possible. As the vinyl polymers contain only one stereogenic centre every two carbon atoms, the asymmetric carbons are separated by one methylene group. Therefore in an isotactic vinyl polymer, all (identical) substituents are situated on the same side of the plane formed by the extended-chain backbone, whereas they alternate in the case of a syndiotactic vinyl polymer. When each backbone atom is a stereogenic centre, the situation is exactly the opposite, substituents alternating on both sides of the chain plane form an isotactic polymer and non-alternating substituents give rise to a syndiotactic polymer (e.g., Figure 14B is the tacticity of an asymmetric polysilane [8]).
3.4.3 Conformations 3.4.3.1 Definitions. From basic knowledge of chemical bonding, a rather satisfactory representation of the backbone of a hydrocarbon polymer chain is given in Figure 15A. Carbon–carbon bond angles are about 1091 while the ˚. average bond length is 1.52 A However, this representation, while giving a good idea of the linearity of the chain, and of bond lengths and angles, remains limited to the particular situation where all the backbone atoms are located in the plane of the paper (the so-called ‘‘extended chain’’ or ‘‘planar zig-zag’’ situation). Indeed, while maintaining lengths and angles internal rotations around single bonds allows one to
Polymer Chemistry and Microstructure
H2 C
A
C H2
H2 C C H2
H2 C C H2
Energy
H3C
H2 C C H2
H2 C C H2
H2 C C H2
H2 C C H2
H2 C C H2
31
H2 C C H2
H2 C C H2 CH3
Gauche
Gauche
B Eclipsed
Eclipsed Trans = "zig-zag"
0°
180°
360° Angle
Figure 15 (A) Polyethylene chain in planar zig-zag conformation. (B) Energy diagram of n-butane as a function of ‘‘chain’’ rotation.
realise a large number of different geometrical arrangements, which are called ‘‘conformations’’. Conformations are defined as ‘‘non-identical arrangements produced by the rotation of backbone atoms around one or more single bonds’’. The different conformations are easily understood by reference to simple molecules like ethane or butane for a rotation around the central carbon–carbon single bond. ‘‘Eclipsed’’ and ‘‘staggered’’ conformations refer to the situations where two identical substituents on both central carbon atoms are aligned, or non-aligned (rotation of 601), respectively. Of course, depending on the size and shape of the substituents, all conformations are characterised by a slight energy difference. The lowest energy conformation of a linear molecule with more than four backbone atoms is always the planar or ‘‘trans’’ conformation (like the example in Figure 15A). Two other low-energy conformations are obtained by a clockwise or counter-clockwise rotation of the central bond of 1201 (Figure 15B). The two staggered conformations are said to be ‘‘gauche’’ and are skewed and non-planar. Energy differences between conformations remain, however low and, in most cases, less than the thermal energy, allowing almost free rotation around carbon–carbon single bonds at room temperature, hence, the average conformation of a polymer chain in a liquid (melt or solution) or amorphous state is a ‘‘random coil’’ (or ‘‘statistical coil’’).
3.4.3.2 Ordered conformations. As explained above, the mean conformation of a polymer chain in the amorphous state is near to being a random coil. Situations where an isolated molecule displays a highly ordered arrangement are rather uncommon. It can however result from intramolecular bonding and strong interactions. An example can be found in p-phenylene polyamides (see, e.g., Figure 16) where the phenyl groups are planar and therefore rotation around the C–N bond is restricted due to electron delocalisation. Such molecules are
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Jacques Devaux and Sophie Demoustier-Champagne
O
O
C
C
N
N
N
H
H
H
O
O
C
C
O
O
C
C
N
N
N
H
H
H
O
O
C
C
Figure 16 Chain structure of poly(p-phenylene terephthalamide) (Kevlars).
consequently asymmetric and are best visualised as long thin rods, explaining why they often behave as liquid crystals in solution. An increase in rod-like arrangement of the macromolecules can also arise by stretching a polymer either in its solid state, either in the melt or even in solution (for polymers leading to lyotropic liquid crystals such as aromatic polyamides). This is the basis of the development of synthetic fibres including high modulus polyethylene Dyneemas, polyamide Nylonss and Kevlars, polyester Tergals or Dacrons fibres. Stretching a polymer in two perpendicular directions, either successively or by blowing a bubble of molten material, leads to its biaxial orientation, which strongly improves mechanical properties in the stretching directions and/or gas permeability (e.g., biaxial orientation of polypropylene leads to BOPP (for biaxially oriented polypropylene) or biaxial orientation of poly(ethylene terephthalate) gives CO2-impermeable bottles for carbonated beverages.) (Characterisation methods for determining molecular orientation are considered in Chapter 8.) The most important way of ordering linear molecules remains however crystallisation (Section 2.3.2). In crystals, the most common conformations are those that minimize energy, for instance planar zig-zag in polyethylene. Usual intermolecular forces (van der Waals interactions, hydrogen bridges, dipole–dipole interactions) can be responsible for polymers maintaining preferred chain conformations more or less ordered as quasi-linear extended chains in crystal lattices. Hydrogen bridges occur in both the crystal phase and the amorphous (or liquid) phase of polyamides like Nylonss and are responsible for their high-melting enthalpy, hence high-melting point. They are responsible for the higher thermal properties of polyamides compared to the corresponding polyesters. Hydrogen bridges also play a prominent role in celluloses and in proteins by maintaining preferred arrangements able, for instance, to catalyse biological reactions (enzymes). In hydrocarbons like polyethylene, interactions in the crystal phase are weaker and of the van der Waals type. Dipole interactions are important in PVC as a consequence of the C–Cl bonds. In some cases, planar chain structures become impossible when replacing a hydrogen atom on the backbone of a macromolecule by a bulky group. Packing problems arise due to steric hindrance if the trans orientation of each bond is
Polymer Chemistry and Microstructure
33
: CH3 :C :H
Figure 17 Helical structure of isotactic polypropylene. (See Color Plate Section at the end of this book.)
maintained, which becomes eventually less energetically favourable than the gauche conformation. Hence, other rotational angles or a succession of rotational angles must be adapted as, for instance, in a helix (x/y)6 conformation, which, in the general case, is realised by the repetition of a trans-gauche sequence. The same gauche angle must always occur. As there are two possibilities, righthanded and/or left-handed helices can form. Examples are isotactic polypropylene (a 3/1 helix) (Figure 17), polytetrafluoroethylene (e.g., Teflons) where a 13/1 helix is found, or polyisobutylene arranged in a 8/5 helix. Helices can be included in crystal lattices.
6
An x/y helix is a helix with x repeat units in y turns.
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Jacques Devaux and Sophie Demoustier-Champagne
4. FUNDAMENTALS OF POLYMER CHEMISTRY 4.1 Chain polymerisation vs. step polymerisation It is not the purpose of this chapter to describe in detail the synthetic chemistry of polymers. From the early ‘‘Principles of polymer chemistry’’ book of Flory, the first edition of which dates from 1953 [9], numerous other books have been written on the subject, among which a few examples are referenced [3,10,11]. In addition to biological reactions, there are two main mechanisms of building synthetic polymers from their monomers: addition (or chain) polymerisation and step polymerisation. Schematically, synthesizing a polymer can be compared to playing dominos. On the one hand, it can be done by aligning a long series of them on their smallest face, then, by a small push on the first one (initiation), inducing a concerted falling down (propagation). On the other hand, dominos can be arranged in long series by associating corresponding ends step by step, like functional molecules by reacting corresponding functions. A major difference between both kind of syntheses lies in polymerisation kinetics: in the former case there is a chain mechanism (initiation, propagation, termination), in the latter case there is no chain reaction and each step is equiprobable. Main comparison points are highlighted in Table 4 [9]. For both types of polymerisation mechanisms, different polymerisation processes can be used ranging from simple bulk and solution polymerisation processes to more sophisticated ones such as suspension, emulsion, interfacial, plasma,y polymerisation processes.
Table 4
Comparison between addition and step polymerization
Chain polymerisation (addition polymerisation)
Step polymerisation (polycondensation)
Only growing species add units one at a time Monomer concentration decreases steadily throughout reaction Long chains formed at once Molecular mass changes little throughout reaction The longer the reaction time, the higher the yield Reaction mixture contains: monomer high polymer E104 part of active species
Any two molecules can react Monomer almost disappears early during reaction Molecular mass increases all along reaction The longer the reaction time, the higher the molecular mass Distribution of all species can be calculated all along the reaction
Polymer Chemistry and Microstructure
35
4.2 Types of chain polymerisations and polymers synthesized — keypoints of mechanisms, kinetics Addition polymerisation is mainly characterised by the existence of a chain mechanism. This means that the process of building the chain is always based on the sequence: initiation - propagation - termination. A long chain (high) polymer is only obtained if the propagation step is much faster than the initiation and/or termination7 reactions. Therefore, during the synthesis, a very limited amount (or number) of active species can actually react at a given instant. Moreover, the lifetime of these active species is often shorter than the total duration time of the reaction. Long terminated (non-reactive) chains thus coexist with monomer molecules throughout the reaction. Disregarding the actual chemical mechanism of each monomer reaction, a good example of polymers made by chain reactions are the vinyl polymers (by definition, polymer made from monomers having the vinyl group: CH2QCH–, as the reactive group). Formulae of some typical vinyl polymers were reported in Table 1. Chain polymerisations are often classified on the basis of their elemental reaction type as: Radical chain polymerisation Ionic chain polymerisation Transition metal insertion polymerisation. Each of these leads to particular characteristics of the polymer chain. Each will be briefly discussed in the following sections, with a special focus on the viewpoint of characterisation.
4.3 Radical chain polymerisations 4.3.1 Free radical polymerisation A schematic representation of the principal steps (initiation, propagation and termination) of a radical chain polymerisation is presented in Figure 18.
4.3.1.1 Initiation. Initiation in radical polymerisation consists of two steps: the dissociation of the initiator to form two radical species, followed by addition of a single molecule to the initiating radical (Figure 18). Initiators include any organic compound with a labile group, such as an azo (–NQN–), disulfide (–S–S–) or peroxide (–O–O–) compound. The labile bond can be broken by various ways like heat, UV light, g-irradiation or by a redox reaction. 4.3.1.2 Propagation: head-tail. As usually considered, a propagation reaction leads to the addition of a monomer molecule, with the same linear orientation as
7
When there is a termination reaction.
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Jacques Devaux and Sophie Demoustier-Champagne
I R
2R +
H H2C C
H R C CH C C H2 H2 n
R
+
H C C H2
Initiation
H H2C C
H R C CH C C H2 H2 n+1
Propagation
H R C CH C C H2 H2 n
+
C C C C R H H2 H H2 n
R C CH C CH C C C C R H2 H2 H H2 H H2 n n
by addition
by disproportionation
Termination
R
H C CH C C H2 H n
+
C C C C R H2 H2 H H2 n
Figure 18 Mechanism of radical chain polymerisation of styrene.
the previous unit to the growing radical (head-to-tail addition). The main reasons invoked to justify this behaviour are: head-to-head addition would lead to less stable radicals (case of PS), head-to-head addition is unfavourable due to steric hindrance (case of poly(vinyl acetate)), the polarity of the substituent of the vinyl moiety orients the addition to headto-tail (case of PVC).
Polymer Chemistry and Microstructure
37
Monomer addition under radical propagation conditions leads to mainly an atactic configuration. As a consequence, radical polymerisations of asymmetric vinyl polymers usually lead to amorphous materials. However, if the substituent is small enough to enter into the crystal cell, atactic vinyl polymers can crystallise (an example is poly(vinyl fluoride)).
4.3.1.3 Kinetics of radical chain polymerisation. Kinetics calculations on radical chain polymerisation are based on the three steps mechanism with notations as shown in Figure 19. Two main hypotheses are made on radicals to simplify the calculations. 1st hypothesis: Equi-reactivity. The radical reactivity remains independent on the nature or size of the radical. 2nd hypothesis: Steady state. Radical concentration instantaneously reaches and keeps its steady-state value throughout the reaction. The rate of radical formation (rate of initiation, Ri) equals the rate of radical disappearance (rate of termination Rt). d½M ¼ 2Ri 2Rt ¼ 2kd ½I 2kt ½M 2 ¼ 0 (1) dt where [I] is the concentration of initiator, [M ] the concentration of radicals, kd the rate constant of dissociation and kt the rate constant of termination. Using these two hypotheses, the overall rate of polymerisation is simply the rate of chain propagation (Rp). d½M kd 1=2 1=2 ¼ kp ½M ½M ¼ kp f Rp ¼ ½I ½M (2) dt kt where [M] is the concentration of monomer, f the fraction of primary radicals initiating a polymer chain and kp the rate constant of propagation. Equation (2) shows that the rate of radical chain polymerisation is proportional to the monomer concentration and to the square root of initiator concentration.
I M
R
+
Mn
+ M
Mm + Mn
kd ka kp
kt
2R M
Mn+1
Mn+m Mn + Mm
Figure 19 Mechanisms of radical chain polymerisation with kinetic notations.
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Jacques Devaux and Sophie Demoustier-Champagne
S + Mn S
+ M
I + Mn R
ktrS
M ktrI
+ M
M + Mn M + M
Mn + S
Mn + R M
ktrM
Mn + M M
Figure 20 Transfer reactions in radical chain polymerisation.
4.3.1.4 Chain transfer reactions 4.3.1.4.1 Kinetic vs. material chain. Kinetically, a chain reaction exists throughout the ‘‘life’’ of the radical, that is, from the initiation of a radical up to its termination by recombination or by disproportionation. The lifetime of a radical determines the so-called kinetic chain length Lk defined as the number of monomers consumed per initiating radical. Lk, by definition, can be calculated from the ratio between the propagation rate Rp to the initiation rate Ri, or, using steady-state hypothesis (Equation (1)), from the ratio between propagation rate to the termination rate Rt (Equation (3)). 2
2
kp ½M Rp kp ½M ½M kp ½M Lk ¼ ¼ ¼ ¼ 2 2k Rt ½M 2kt Rp 2kt ½M t
(3)
During the course of radical chain propagation, several types of transfer reaction can occur, leading to the interruption of the ‘‘material’’ chain, while allowing the ‘‘kinetic’’ chain to continue. ‘‘Different’’ end groups can appear as a result of such transfer reactions (Figure 20). In the absence of any transfer, only the type of termination reaction has to be taken into account to obtain the number average degree of polymerisation (or chain length) Xn . Indeed, if the reaction terminates by addition, two radicals give one chain, while if the reaction terminates by disproportionation, one radical generates one chain. Using 0.5oxo1, one obtains Xn ¼
2 2 Lk kp ½M ¼ x 2xkt Rp
(4)
Equation (4) clearly shows that the number average degree of polymerisation Xn is inversely proportional to the reaction rate Rp, meaning that, in radical chain polymerisation high reaction rates are linked to low molecular masses and vice versa. One way to avoid this dilemma is to use emulsion polymerisation where the lifetime of a radical (i.e., the ‘‘kinetic’’ chain length) is independent of
Polymer Chemistry and Microstructure
39
‘‘overall’’ monomer or radical concentrations, but only depends on the number of micelles and on the percentage of time they are ‘‘active’’ (statistically 50%).8 If transfer reactions occur, they terminate ‘‘material chains’’, thus decreasing Xn , without affecting the ‘‘kinetic’’ chain length Lk. Thus Rp P Xn ¼ (5) xRt þ Rtr P with Rtr the sum of all transfer reaction rates. Equation (5) denotes that a control of Xn can be obtained by controlling transfer reactions, that is, by adding calculated amounts of molecules able to transfer a radical thus creating new end groups. 4.3.1.4.2 The low-density polyethylene and polypropylene cases. In the course of the radical chain polymerisation of ethylene two kinds of transfer on the polymer play a major role, giving rise to LCB or SCB. These are illustrated in Figure 21. Due to both kinds of branching leading to chain irregularities, the crystallisation of radical chain-polymerised polyethylene is strongly hindered. Its maximum degree of crystallinity is limited to about 50%, its melting temperature ranges from 801C to 1151C and its density remains low (B0.92). From this latter property, it received the name of low-density polyethylene (LDPE). In radical chain polymerisation of allylic monomers (containing the chemical allyl group CH2QCR–CH3, like propylene, isobutylene,y), a transfer on the monomer can occur leading to a very stable allyl radical unable to propagate the chain. Thus radical chain polymerisation of such monomers, especially propylene is not feasible, as a competition exists between the propagation rate and the high rate of transfer on the monomer (see, Figure 22). 4.3.1.4.3 Termination reactions 4.3.1.4.3.1 LOW VISCOSITY (LOW CONCENTRATION SYSTEMS). Radicals are species that normally have short lifetimes (102 s to 10 s). In the course of a chain growth mechanism (see Figure 18) they terminate by finding a second non-paired electron to build a chemical link made of an electron pair. This requires that two radicals meet, either to pair their electrons and combine (addition) or to exchange a hydrogen, leading to the formation of one saturated bond and one double bond (disproportionation). 4.3.1.4.3.2 GEL OR TROMMSDORF EFFECT, VITRIFICATION AND LIVING RADICALS. When the initial monomer concentration is high (X40%), the viscosity of a system increases upon increase of the conversion. This increase is linked to progressive interpenetration of coiled macromolecules, leading to entanglement. Due to this viscosity increase, the mobility of the molecules, mainly the longer ones (growing radicals) decreases and the kinetics may be affected by slower diffusion. This effect is first noticeable for reactions involving two large reactive molecules, that are the termination reactions. Therefore, as the initiation reaction is less affected, the steady-state hypothesis is no more valid and the radical lifetime increases, 8
Due to the very small (nanometric) size of a micelle, only one radical at a time can exist inside.
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Jacques Devaux and Sophie Demoustier-Champagne
Long Chain branching
CH
+
CH2
C H2
n H 2C
+
H2 C
CH2
CH
+
CH3
H 2C
H 2C
H2 C
CH 2
LCB
CH C H2 H 2C
H 2C
Short Chain branching
H2 C
CH
H H2C
H2 C
H2 C C H
CH2 CH2
H 3C
H2 C
H2 C
H2 C
C H
CH2
CH 2 CH 2
CH2 H 3C
+
n H 2C
CH2
H2 C
CH
H2 C
H2 C C H2
C H2
(CH2)3 CH3
SCB
Figure 21 Transfer reactions in radical chain polymerisation of ethylene.
together with their concentration (Equation (1)), leading to an auto-acceleration of the overall polymerisation reaction. As this phenomenon is due to entanglement of coiled chains and a large increase of viscosity, it is called, somewhat improperly, the ‘‘gel effect’’ (even if there is no true ‘‘gel’’ because there is no crosslinking). It is also sometimes named the ‘‘Trommsdorf effect’’ [3]. Such effects are especially observable when polymerisation occurs in the bulk (mass polymerisation). In this case, another effect can be observed at high
Polymer Chemistry and Microstructure
R
+
R
C C H2 H
H2 C
C H
CH3
CH3
+
H 2C
R
C H
C C H2 H
CH3
R
C C CH3 H 2 H2
41
CH3
R
C CH C C H2 H2 H CH3
CH3
H 2C
C H
CH2
H 2C
C H
CH2
+
stable allyl radical unable to propagate
Figure 22
Transfer reactions inhibiting propylene radical polymerisation.
conversion. If the final synthesized polymer is characterized by a Tg higher than the actual temperature at which the polymerisation reaction is carried out, it happens that the Tg of the system, which increases with the conversion degree, reaches the reaction temperature. At this point, the whole reaction medium vitrifies and the molecular mobility vanishes, stopping the reaction without termination, at a monomer conversion below 100%.
4.3.2 Controlled/Living radical polymerisation Recently, controlled/living systems based on radical mechanism have been reported [12,13]. The new concept for controlling chain growth processes relies on forming an equilibrium between active species (radicals) and various types of dormant species. Very small amounts of the active species equilibrate with the dormant species, slowing down the rate of polymerisation and allowing tolerance to impurities. There are three approaches to the exchange process. Two of them are based on the persistent radical effect and are spontaneous (nitroxide-mediated polymerisation, NMP) or catalysed (atom transfer radical polymerisation, ATRP). The third approach uses a degenerative transfer mechanism with either alkyl iodides or dithioesters (reversible addition fragmentation transfer, RAFT) as transfer agents [12]. These new synthetic polymerisation processes provide efficient routes to precisely control molar mass and its distribution, macromolecular architecture as well as microstructure (tacticity and sequencing). Controlled/living radical polymerisation (CRP) is currently a fast developing area in polymer synthesis and it allows preparation of many advanced polymeric materials, including thermoplastic elastomers, surfactants, gels, coatings, biomaterials, materials for electronics and many others.
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Jacques Devaux and Sophie Demoustier-Champagne
4.4 Ionic polymerisations — active species Ionic chain polymerisations refer to chain mechanisms in the course of which the propagation step consists of the insertion of a monomer into an ionic bond. The strength of this ionic bond can vary, depending on the nature of the species, the temperature and the polarity of the solvent, between a closed ionic pair in contact up to free ions (see Figure 23). Final polymer microstructure (configuration,y) and molecular mass distribution depend on the actual nature of the active ionic species. Ionic chain polymerisations follow the same basic steps as radical chain polymerisation and are said to be either cationic or anionic depending on the nature of the ion formed in the initiation step. Schemes of the insertion during the propagation step in cationic and anionic chain polymerisations are shown in Figure 24. Polymerisation of vinyl monomers with electron-donating substituents can proceed by a cationic mechanism, while monomers with electron-withdrawing group can polymerise by an anionic pathway.
4.4.1 Cationic polymerisation A cationic polymerisation mechanism is illustrated in Figure 25 for the most industrially important example of polyisobutylene. Unlike radical chain polymerisation, initiation in cationic polymerisation uses a true catalyst that is recovered at the end of the polymerisation and is not incorporated at one end of the growing chain. Catalysts for cationic chain polymerisation are molecules able to withdraw electrons, mainly Bro¨nsted (H2SO4, H3PO4) and Lewis acids (BF3, AlCl3, SnCl4). The choice of solvent for cationic polymerisation is also important because it plays a major role in the association between cation and counter ion. A too tight association will prevent monomer insertion during the propagation step. However, the use of ‘‘stabilized’’ A
B
n
n A
Aδ(+)
B
Bδ(−)
aggregate
A+B−
A+//B−
ion pair solvated ion pair
A+ + B− free ions
Figure 23 Active species in ionic polymerisations. cationic A
H2 + C CH C CH B− H2 n R R
+
H2C
CHR
A
+
H2C
CHR
B
H2 + C CH C CH H2 n+1 R R
B−
anionic B
H2 − C CH A+ C CH H2 R n R
Figure 24 Schemes of propagation steps in ionic polymerisations.
H2 − C CH C CH A+ H2 n+1 R R
43
Polymer Chemistry and Microstructure
CH3 F3BOH OH +
Initiation
+ H2C
C
CH3
ki
H 3C
CH3
CH3 CH3 H2 C C + F3BOH C C H2 n CH3 CH3 CH3
CH3
Propagation
H3C
C
CH3
- + HC 2
H 3C
kp
CH3 CH3 H2 C C + F3BOH C C H2 n CH3 CH3 CH3
C
-
CH3 H 3C
CH3 CH3 H2 C C + F3BOH C C H2 n CH3 CH3 CH3
kt
C
Transfer to monomer
CH3 CH3 H2 C C + F3BOH C C H2 n+1 CH3 CH3 CH3
C
CH3 CH3 H2 C C OH C C H2 n CH3 CH3 CH3
C
C
+
BF3
ktr
CH3
CH2 CH3 H2 C C C C H2 n CH3 CH3 CH3
C
-
CH3
H 3C
CH3
CH3
H 3C
-
CH3
H 3C
CH3
- + HC 2
F3BOH
CH3
CH3
CH3
Termination
C
C+
+
H 3C
C+
F3BOH
-
CH3
Figure 25 Overall cationic chain polymerisation mechanism of isobutylene.
carbocation sites allows the cationic chain polymerisation to be undertaken under ‘‘living’’ conditions at low temperatures, where termination and transfer reactions are almost suppressed [13]. Typical termination can occur by deactivation of the cationic species from the growing chain, possibly leading to a functional end group (Figure 25). Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically 1001C. Commercial polyisobutylene is often copolymerised with a few percentage of isoprene in order to allow for crosslinking (butyl rubber).
4.4.2 Anionic polymerisation Anionic polymerisation mechanism is illustrated in Figure 26 with the example of styrene polymerisation. The active species in anionic chain polymerisations are anionic growing chain ends. The main characteristic of such a process is the almost total absence of termination and transfer reactions. For this reason, anionic polymerisation is often called ‘‘living polymerisation’’.
44
Jacques Devaux and Sophie Demoustier-Champagne
Initiation C4H9-
+ Li
+
H 2C
H C
C4 H9
- + C CH Li H2
Propagation
H2 C CH C H2
-+
CH n-1
Li
+
H2 C
H C
H2C CH C H2
-+
CH Li n
NO TERMINATION
Figure 26 Overall anionic chain polymerisation mechanism of styrene initiated by n-butyllithium.
Initiators are species able to transfer either an electron or an electron pair to the monomer: Bro¨nsted or Lewis bases, or, most often, organometallic derivatives like alkyllithium (very strong bases) are used. Note that in this case the initiating species (e.g., the n-butyl group) is incorporated as an end group in the polymer chain. When the initiation step is very fast (almost instantaneous), the absence of termination during living polymerisation can lead to a very narrow molecular mass distribution. Therefore, anionic chain polymerisation is often used to synthesize monodispersed samples used as standards for size exclusion chromatography (SEC) (see Chapter 6 for discussion of SEC). In the absence of termination reactions each monomer has an equal probability of fixing to an anion site. Therefore, the number averaged degree of polymerisation is simply equal to the ratio of initial monomer concentration to initial initiator concentration. In anionic polymerisation the molecular mass increases linearly with the conversion of the monomer [14]. During the propagation step, depending on the nature of the active ionic species, a limited control on the tacticity of the final polymer is possible. Ion pairs can, indeed, require the insertion of the monomer under a defined orientation, while free ions are unable to orient the insertion. If the starting reagents are pure and if oxygen and traces of water are excluded from the reaction, propagation can proceed indefinitely or until all the monomer is consumed. However, introducing selected additives to react with active anionic end groups allows one to either functionalize the chain end, or to start a new polymerisation of the same or of another monomer, leading to building of block copolymers. Typical functionalisation of end groups are listed in Table 5.
45
Polymer Chemistry and Microstructure
Table 5
Typical end-functionalisation reactions in anionic chain polymerisation
End-functionalisation reaction
Functional end group
C
+ O2
C
O
C
+ CO2
C
C
O
O O
C
+
H2C
CH2
C
O
C H2
C
+ H2O
C
H
C
+
C
H
RH
H2 C
O
+ OH
+ R
4.5 Olefin polymerisation processes With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler–Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link.
4.5.1 Phillips process The Phillips process can be used for the synthesis of polyethylene only. It makes use of a Cr-based heterogeneous catalyst on the surface of which the chain grows on metallic atoms of low oxidation level (CrII). Termination and transfer reactions in the Philips process both lead to unsaturated end groups, which results in chains having one saturated and one unsaturated end group (Figure 27).
46
Jacques Devaux and Sophie Demoustier-Champagne
Figure 27 Propagation, termination and transfer reactions in ethylene polymerisation by the Phillips process.
A major characteristic of the Phillips process chain polymerisation of ethylene is that it leads to very limited branching. The resulting polymer is thus highly linear and can reach high levels of crystallinity, hence high densities approaching 0.96–0.97. Such a polyethylene is known as HDPE for ‘‘High-density polyethylene’’.
4.5.2 Ziegler–Natta process The naming ‘‘Ziegler–Natta’’ given to processes for polyolefin syntheses originates from the use of Ziegler-type catalysts and in their use by Natta for isotactic propylene synthesis. They are based on heterogeneous catalysts made from complexes of group I–III organometallic compounds (or hydrides) and compounds of a group IV–VIII transition metal. Typical initiators of Ziegler– Natta polymerisation process are listed in Table 6. A common catalyst is the complex a-TiCl3–AlR3. The chain grows by insertion of the olefin molecule into the Ti–C link. The main characteristics of most of these heterogeneous catalysts is that, due to the size and shape of the complex, the insertion is only possible for one particular spatial orientation of the monomer, which, in the case of an asymmetric monomer like propylene, leads to a good control of tacticity. While use of Ti-based catalyst can lead to isotactic polypropylene, syndiotactic polypropylene is obtained using V-based catalysts. Ziegler–Natta catalysts were also designed to synthesize polyethylene (HDPE) and copolymers of ethylene with longer chain a-olefins (n-butene,
Polymer Chemistry and Microstructure
Table 6
47
Examples of typical components of Ziegler–Natta initiators
Group I–III metal
Group IV–VIII transition metal
(C2H5)3Al (C2H5)2AlCl (C2H5)AlCl2 (C2H5)2Be C4H9Li C5H11Na
TiCl3 TiCl4 TiBr3 VCl4 Ti(OC4H9)4 ZrCl4
Figure 28 Hydrogen transfer reaction in propylene polymerisation by the Ziegler–Natta process.
n-hexene, n-octene,y), to introduce controlled amounts of SCB and a limited crystallinity. These polymers are known as linear-low density polyethylene or LLDPE. The importance of Ziegler–Natta processes result from their broad synthesis range possibilities.
Isotactic polypropylene9 HDPE and LLDPE Ethylene — propylene copolymer (ethylene–propylene rubber or EPR) Polybutadiene (PB) and polyisoprene with cis–trans controlled microstructure (synthesis of ‘‘natural rubber’’).
The termination of Ziegler–Natta synthesis is obtained by neutralisation of the catalytic site using, for instance, alcohol. The reaction can also simply be stopped (with no clear termination) by embedding of the catalyst into the polymer. Industrially, the control of final molecular mass is often accomplished by a hydrogen transfer reaction (see Figure 28).
4.5.3 Metallocenes and others Based on an early process discovered by Natta in the 1950s, soluble transition metal catalysts like metallocenes were developed mainly in the 1950s as initiators for polyolefin syntheses. Others are still now under investigation, like the so-called LTM (for ‘‘late transition metal’’) catalysts. Metallocenes seem 9
This result led to Ziegler and Natta jointly receiving the Nobel Prize for Chemistry in 1963.
48
Jacques Devaux and Sophie Demoustier-Champagne
Typical metallocenes:
Proposed MAO structures:
CH3
H3C Al
O
Al
CH3 O
CH3
H3C Al
Al
O
Al
CH3 O
Al
CH3
H3C
m
n O
Figure 29 Structures of typical metallocenes and proposed structure of methylaluminoxane (MAO).
potentially even more versatile than Ziegler–Natta catalysts as they retain their internal chiral site due to their ‘‘sandwich’’ structure. There are numerous available molecular structures of metallocenes on the basis of Ti, Zr, Hf, Yb and other transition metals. To act as a polyolefins’ synthesis catalyst, they need to be activated by a co-catalyst, for example, by methylaluminoxane (MAO), which is used in high excess by comparison with the metallocene. Typical structures of a few metallocenes and of MAO are reported in Figure 29. Polyolefins obtainable through the use of metallocene catalysis include isotactic, syndiotactic, and atactic polypropylenes, polyethylenes and copolymers including polar comonomers. A characteristic of polyolefins synthesized with metallocene catalysts is their significantly lower polydispersity compared to one obtained by using heterogeneous Ziegler–Natta catalysts. Such narrower molecular mass distributions can lead to different mechanical properties of the resulting material.
4.6 Processes for polyolefin syntheses Table 7 shows synthesis processes and microstructures of available polyethylenes together with several of their properties.
Table 7 Synthesis processes and microstructures of polyethylenes with characteristic properties Structure data
HDPE Phillips (Ziegler–Natta) 0.96–0.94(0.95) 94 (80–87) 128–135 1.3 (4) 1.2 (0.6)
LLDPE Ziegler–Natta 0.915–0.93 46–60 120
LDPE Radical chain polymerisation 0.915 – 0.93 46–60 110 25 0.6
Polymer Chemistry and Microstructure
Abbreviation Synthesis Typical density Percentage crystallinity range Melting range (1C) –CH3/1,000 carbons –vinyl/1,000 carbons
49
50
Jacques Devaux and Sophie Demoustier-Champagne
4.7 Copolymers 4.7.1 Linear copolymers On the basis of the kinetic characteristics of chain polymerisation reactions, it is possible to predict the final microstructures available by a so-called random process from a simple mixture of two comonomers. Indeed, the global mechanism of copolymerisation can be illustrated as presented in Figure 30. From this kinetic scheme, two characteristic parameters can be defined, r1 and r2 r1 is a measure of the ratio between the rate of addition of monomer 1 to the rate of addition of monomer 2 on a growing radical terminated by monomer 1, while r2 is the ratio between the rate of addition of monomer 2 to the rate of addition of monomer 1 on a growing radical terminated by monomer 2. r1 ¼
k11 k12
r2 ¼
k22 k21
The composition of the copolymers as a function of the composition of the mixture of monomers can be best understood from the practical example of styrene and methyl methacrylate (MMA) as shown in Figure 31. For radical chain copolymerisation of styrene and MMA, r1Er2o1. The copolymer content in styrene is not very different from the monomer kd
I
2R
ka1
R + M1
ka2
R + M2
M2
k11
M1 + M1
k12
M1 + M2
k21
M2 + M1
k22
M2 + M2
M1 +
M1
M1 +
M2
M2 +
M2
Initiation
M1
kt11 kt12 kt22
M1
+ M1
M1
+ M2
M2
+ M1
M2
+ M2
Propagation
copolymer
copolymer
Termination
copolymer
Figure 30 Mechanism of ‘‘random’’ chain copolymerisation of two comonomers (M1 and M2).
Polymer Chemistry and Microstructure
51
Copolymer: fraction of styrene
1
Cationic
0.8
0.6 Radical
0.4
0.2 Anionic 0 0
0.2
0.4
0.6
0.8
1
Monomer mixture : fraction of styrene
Figure 31 Copolymer composition as a function of monomer mixture composition in the case of styrene methyl methacrylate mixtures. Reproduced from Mercier and Mare´chal [15], Reproduit avec l’autorisation de l’e´diteur. Tous droits re´serve´s.
composition and does not change a lot during the course of the copolymerisation, nevertheless the slight difference between r1 and r2 prevents the composition following a straight line (diagonal if r1 ¼ r2 ¼ 1). In the case of cationic copolymerisation of styrene and MMA, r1 r26¼1 and r1W1 and r2o1. As it adds faster, the styrene content in the copolymer is always higher than in the monomer mixture, where it decreases throughout the reaction. The styrene content in the copolymer decreases thus also throughout the reaction. However, in the anionic copolymerisation of styrene and MMA, r1 r26¼1 and r1o1 and r2W1. As it adds slower, the styrene content in the copolymer is always lower than in the monomer mixture. In summary, the copolymer composition depends on the monomer ratio in the monomer mixture and on r1 and r2 reactivity ratios.
4.7.2 Graft copolymers An important class of copolymers copolymers, synthesized in order to of a rubber phase. Examples are the for High-Impact PolyStyrene and Both are synthesized in two steps.
made by chain copolymerisation is graft toughen brittle materials through inclusion cases of styrenic copolymers called ‘‘HIPS’’ ABS for Acrylonitrile–Butadiene–Styrene.
First step: radical polymerisation of butadiene in the presence of a chain transfer agent to control its molecular mass. Second step: polymerisation of styrene (to get HIPS) or styrene and acrylonitrile (to get ABS) with partial grafting of PS or SAN (styrene/ acrylonitrile) sequences onto the PB chains.
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Jacques Devaux and Sophie Demoustier-Champagne
Addition ("grafting to")
Mn
M n• or
+
C C H2 H
C H
C C H2 H or
C H2
• C H
C H2
• C CH C H2 H R
R•
C H2
Transfer & grafting ("grafting from")
Mn • or
MnΗ +
C C H2 H
R•
C H
C H2
C C H2 H or
or
C H
RΗ
• C H
C H
C H
• C H
• C H
C H2
•
Mn + nM
C H
Figure 32 Grafting mechanisms in styrenic graft copolymers.
Figure 32 shows the mechanism of grafting involved, including the grafting (addition) of growing chains onto the butadiene double bonds (sometimes referred to as ‘‘grafting to’’) and transfer of the radical to the butadiene followed by the growing of a new chain from this transferred radical (sometimes referred to as ‘‘grafting from’’). The succession of both mechanisms may lead to crosslinking.
4.7.3 Block copolymers Another important class of copolymers synthesized by chain polymerisation are block (or sequenced) copolymers; diblock and triblock copolymers being the most important ones. They are very useful as compatibilisers (emulsifiers) in immiscible polymer blends. Another major use is as thermoplastic elastomers. Both uses are best explained through the example of butadiene–styrene block copolymers. PS and PB homopolymers are immiscible. Any added PB–PS block copolymer in a PS–PB blend will have one sequence miscible in PS and one sequence miscible in PB, hence they will localise at the interface. As a consequence, the interfacial energy will decrease, greatly helping dispersion and providing phase adhesion, thus a transfer of mechanical properties. In a pure PB–PS block copolymer, both sequences are immiscible and the microstructure will be diphasic (at a supramolecular or nanoscopic level). If the ratio is such that PS particles are dispersed into a matrix of PB, below the Tg of PS, the system behaves like crosslinked PB, hence as an elastomer. However, above the Tg of PS, the system becomes viscous and can be processed like a thermoplastic.
Polymer Chemistry and Microstructure
A Spheres
A Cylinders
AB Lamellae
B Cylinders
53
B Spheres
Figure 33 Available morphologies in biphasic sequenced copolymers.
Synthesis of vinyl block copolymers is accomplished by living polymerisation, mostly by anionic polymerisation. Several strategies can be used, illustrated here by the example of the Styrene–Butadiene–Styrene (or SBS) triblock copolymer. Synthesis of a PB sequence with two living ends, using a difunctional initiator, followed by the addition of styrene to build two sequences of PS (two steps). Synthesis of a PS sequence with one living end, adding and polymerising butadiene, while keeping its living end, then adding a new amount of styrene to build the third sequence (three steps). Sequential synthesis of a diblock styrene–butadiene copolymer, while keeping the living end onto the butadiene end. Then coupling the living end groups by a suitable bifunctional agent (e.g., COCl2) (three steps). Making use of the higher reactivity of butadiene in anionic polymerisation (r1o1, r2 W1) to get the triblock SBS copolymer in two steps. The first step is the synthesis of a PS sequence with a living end, then, upon addition of a mixture of styrene and butadiene, butadiene will add first, building a ‘‘pure’’ PB sequence, and styrene will finally build the third sequence (two steps).
4.7.4 Links with microstructure (phase diagrams) Copolymers (graft or block) made of immiscible sequences give rise to biphasic morphologies depending on the ratio of immiscible sequences (or of their lengths). Such possible microstructures are reported in Figure 33. A minor phase can be dispersed as nodules (spheres) or filaments (cylinders) while, when concentrations of both phases get similar, lamellar (interpenetrated) structures can appear. It should be noted that rather similar morphologies could also be found in (compatibilised) polymer blends.
4.8 Types of step polymerisations and polymers synthesized — keypoints of mechanisms, kinetics 4.8.1 Step polymerisation vs. polycondensations The most important difference between chain and step polymerisations is the absence of a kinetic chain in the latter. Among the different chemical reactions usable to synthesize polymeric materials by step polymerisation are esterification, amidation, nucleophilic aromatic substitution and urethane (carbamate) formation. Polymerisation
54
Jacques Devaux and Sophie Demoustier-Champagne
usually proceeds by the reactions between two different functional groups, for instance, hydroxyl and carboxyl (to get esters), amino and carboxyl (to get amides), hydroxyl and halogen (to get ethers) or isocyanate and hydroxyl groups (to get carbamates or urethanes). The type of reactions involved also leads to use (sometimes improperly) of the terms polycondensation and polycondensates for step polymerisation and polymers made by them, despite there not always being a condensable small molecule among the by-products.
4.8.2 Monomers — ‘‘repeat’’ vs. ‘‘structural’’ units Step polymerizations of linear chains can involve either two different bifunctional monomers in which each monomer possesses only one type of functional group (commonly represented by X–X or Y–Y), or a single monomer containing both types of functional groups (common representation X–Y). However, whatever the monomer type, a linear polymer molecule contains, on average, one functional group of each species per chain (molecule). In order to unify most calculations in step polymerisation a notation, which will be used throughout this section, was introduced by Odian [10], the first step of which is the clear distinction between two definitions. A repeat unit is the sequence of atoms that is repeated to build a polymer chain. This definition, although very simple, distinguishes between X–Y and X–XY–Y units. For instance, the repeat units of polyamide 6 and polyamide 6610 are: O N
C C C C C C H2 H2 H2 H2 H2
H
POLYAMIDE 6 (repeat unit) n
structural unit N H
C C C C C C N H2 H2 H2 H2 H2 H2 H
structural unit
O
O
C
C C C C C H2 H2 H2 H2
POLYAMIDE 66 (repeat unit) n
structural unit
A structural unit or monomer can be defined as the residue from each monomer molecule, whatever its chemical species. In the case of X–Y monomers, a structural unit is identical to a repeat unit (e.g., in polyamide 6), while in the case of X–X, Y–Y monomers, there are two structural units per repeat unit (e.g., in polyamide 66). In the latter case, the mass of a structural unit is obtained by averaging the masses of the two different chemical species. For polyamide 6 and polyamide 66 the masses of structural units are thus 113 in both cases. 10
Polyamides are conventionally named from their number of carbon atoms in structural units with one figure only for X–Y type and two figures (first one for the amine residue, second one for the acid residue) for X–YX–Y type.
Polymer Chemistry and Microstructure
55
A last definition is needed for N (N0 at time 0), which is the total number of molecules present in the system at any time. With the above definitions, N0 initial molecules correspond to N0 monomers and N0 structural units whatever be the chemical species. Each molecule contains on the average one functional group of each kind. Calculations are thus easier and will preferably be performed using structural units.
4.8.3 Carothers equation Assuming p is the conversion ratio equal to the ratio between the actual number of one type of functional group to its initial number N0 N p¼ N0 As there is one functional group of each type per chain, one can write for Xn (number average chain length or degree of polymerisation) Xn ¼
N0 N
Xn ¼
N0 1 ¼ N 0 ð1 pÞ 1 p
or
p¼1
1 Xn
The last equation commonly named the Carothers equation is of general use in step polymerisation. From this equation it can first be deduced that the successful synthesis of high polymers by step polymerisation (i.e., polymers exhibiting molecular mass sufficiently high to be useful for technological application) requires very high conversion levels. A common characteristic to all the chemical reactions involved in step polymerisation that should be emphasised is that they are most often equilibrated reactions. For instance, the polyesterification reaction is based on the esterification/hydrolysis equilibrium K COOH
+
OH
COO
+
H2O
4.8.4 Control of molecular mass The molecular mass in step polymerisation can be controlled by Controlling the duration of the reaction (kinetic control). Controlling the water concentration (thermodynamic equilibrium control). Controlling the stoichiometry by adding either difunctional monomers or a monofunctional reactant. Performing what is known as post-condensation. Most step polymerisations are exothermic and, consequently, the equilibrium constant K decreases with increasing temperature. Hence, one way to increase the molecular mass would be to decrease the polymerisation temperature, but kinetics prohibits using a too low temperature as it will lead to an excessively long residence time in the reactor and/or too high viscosities. Thus, in order to reach very high molecular
56
Jacques Devaux and Sophie Demoustier-Champagne
masses, a second step of reaction is sometimes performed under high vacuum for long periods of time in the solid state at a temperature where K is more favourable (post-condensation). Such a process finds industrial use for bottlegrade PET. It can be demonstrated that in step polymerisation, the polydispersity index H ¼ Mw/Mn ¼ 1+p. Consequently, the equilibrium polydispersity of step-grown polymers is often close to 2.
4.8.5 Livingness of the step polymers A consequence of the nature of step reactions is that macromolecules exhibit a ‘‘living’’ character, as their functional end groups are always able to react. As most of the reactions involved in step polymerisation are equilibrated, interchange reactions can occur. Interchange mechanisms can involve reactions between the terminal functional group of one polymer molecule with the internal repeating linkage of another macromolecule. Alcoholysis, acidolysis and aminolysis are other examples of reactions involved in the process. Interchange is also said to occur between internal links of two different macromolecules (through ester or amide interchange for instance). Most interchange reactions involve breaking and building of links of exactly the same nature. These reactions are thus most often athermal being only entropy driven. An example of such interchange equilibrium can be given for a polyester: O CHAIN#1
CHAIN#1
C O
+ CHAIN#2
O
CHAIN#2
C O O
CHAIN#1
CHAIN#2
C O
+ CHAIN#2
O
CHAIN#1
C O
The consequence of such interchanges are important and numerous Polymerisation — depolymerisation (hydrolysis for instance) equilibrium. Equilibrium molecular mass distribution in step polymers. Decrease of molecular masses upon end capping, due to re-equilibration.
Polymer Chemistry and Microstructure
57
Randomisation in miscible polymer blends (formation of random copolymers). Instability of block copolymers due to randomisation. Equilibrium formation-opening of cyclic oligomers.
4.8.6 Functionality — gelation — crosslinked macromolecules — networks The previous sub-sections of Section 4 were devoted to linear polymers made from monomers which were assumed to be bifunctional, or able to link to two other ones (Section 2.4). Vinyl (or similar) monomers used in chain polymerisation are bifunctional when they contain one vinyl (or similar) group per molecule. Monomers used in step polymerisation were so far assumed to contain two functional groups, being thus bifunctional. Crosslinked macromolecules can be formed from bifunctional monomers by first polymerising them, then by performing coupling reactions onto existing chains. The latter mechanism is often called vulcanisation.11 Several chemical routes can be followed to accomplish vulcanisation, depending on the functions available on polymer chains. In the section following monomers with more than two functional groups (fZ2) and polymers made thereof, defined as thermosets, will be discussed from a chemical viewpoint.
4.8.7 Crosslinking in chain polymerisation – polydienes Structures of monomers able to crosslink by chain mechanism can be rather different, but the majority of them are of the diene type, with two equivalent vinyl groups per monomer.12 A crosslinked system, including units of the diene type, can be considered as formed by linear chains (the initial chains) linked by randomly distributed bridging links. The bridging density r is defined as: n r¼ N0 where N0 is the total number of structural units and n the number of structural units belonging to a bridging link. It can be calculated [16] that the critical bridging density rc, at which a 3D network will appear is given by 1 1 rc ¼ yw yw 1 where yw is the mass average degree of polymerisation of initial chains.
11
Historically, the first crosslinking of macromolecules (natural rubber) was performed with sulfur thus given the name of Vulcanus, Roman god of volcanoes from where sulfur was extracted.
12
Conjugation phenomena, which can modify the reactivity of the second vinyl group, will not be considered here.
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Jacques Devaux and Sophie Demoustier-Champagne
A bridging index can be defined as: X ny ry P d¼ ¼ ryn ny where ny is the number of initial chains of degree of polymerisation y and yn the number average degree of polymerisation of initial chains. The critical bridging index is y 1 d c ¼ rc y n ¼ n ¼ yw H The critical bridging index is thus the reciprocal of the heterogeneity factor H of the initial chains.
4.8.8 Crosslinking in step polymerisation-modified Carothers equation At the critical conversion value, the conversion p, the functionality f and the degree of polymerisation are linked. The simplest calculation model is due to Carothers. It is based on the hypothesis that the degree of polymerisation reaches infinity at the critical conversion, meaning that all the system is made of only one macromolecule. If N0 and N are the numbers of molecules at the beginning and during the reaction, fN0 is the total number of functional groups. Providing there is no intramolecular ring formation, the amount of functional groups disappearing during the reaction is 2(N0N). Therefore, p¼ which transforms as: p¼
2ð N 0 N Þ N0 f
2 N 2 1 1 1 ¼ f N0 f Xn
The latter equation is a modification of the Carothers equation and can be used to approximate the critical conversion value at the gelation. Indeed, when Xn goes to infinity the above equation leads to 2 p¼ f This critical value is, however, only an approximation leading often to an overestimation of the critical conversion value. The main reason to this failure is that gelation actually occurs, at least for some molecules, at a finite degree of polymerisation. Equations based on a statistical approach can provide better results in the estimation of gel point.
5. POLYMER DEGRADATION Similarly to polymer synthesis, there are essentially two main mechanisms of degradation of synthetic polymers involving main chain links and leading to
59
Polymer Chemistry and Microstructure
decrease of molecular mass: chain depolymerisation or unzipping and random (or statistical) degradation. Comparable distinctions can be made between the two mechanisms of chain and step polymerisations and degradations (see Table 8). Representative examples of such degradation mechanisms can be respectively found in radical depolymerisation of poly(methyl methacrylate) (PMMA), Figure 34, and in random thermolysis of poly(butylene terephthalate) (PBT), Figure 35. In random degradation molecular mass decreases early, while in chain degradation the molecular mass of the polymer remains almost constant. Characterisation methods for molecular mass are thus very sensitive methods to follow random degradation. In contrast, as monomer is produced in chain depolymerisation, weight loss measurement techniques are the best methods to follow this kind of degradation. (Chapters 10–12, in Section IV, of this book focus on the methods used in the molecular characterisation and analysis of polymer degradation and polymer degradation mechanisms.)
Table 8 Comparison between chain degardation (depolymerisation — unzipping) and step degradation (random degradation) Chain degradation
Step degradation
Needs initiation Monomer concentration increases steadily throughout the reaction High molecular masses remain all the along reaction The longer the reaction time, the higher the yield in monomer Reaction mixture contains: monomer high polymer E104 part of active species
Any bond can break at any time Almost no monomer produced except at the end of the reaction Molecular mass decreases rapidly
Depolymerisation (‘‘unzipping’’)
Random degradation
The longer the reaction time, the lower the molecular mass Distribution of all species can be calculated all along the reaction
CH3
CH3
CH3
CH3
CH3
O
O
O
O
O
C
O
C
C C C C H2 H2 CH3 CH3
C
O C H2
C CH3
O
C
O
C
C C C C H2 H2 CH3 CH3
CH3 C
O + H2 C
O
C CH3
Figure 34 Example of propagation step in chain depolymerisation mechanism (unzipping) of polymethylmethacrylate (PMMA).
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Jacques Devaux and Sophie Demoustier-Champagne
O
O C
C O
T°
H
H2 CH C
O C H2
O
C
C
C O H2
C
H C
+ OH
O O
O C
O
O
H2 C
H2 C
O C C O H2
C
O O
Figure 35 Example of random degradation mechanism: thermolysis mechanism of poly(butylene terephthalate) (PBT).
It has also to be emphasised that polymers synthesized by chain polymerisation can undergo random degradation (as polypropylene for instance), while certain polymers synthesized by step polymerisation can depolymerise (e.g., polyamide 6). In the case of random degradation, the molecular mass drop due to chemical reaction with a contaminant (e.g., H2O, alcohol, acid, for a polyester) can easily be calculated. Consider the example of hydrolysis of a step polymer. There are two end groups on a linear polymer chain. Thus, their molar fraction is 2 X n0 where the subscript (0) indicates the beginning of the reaction. The molar fraction of new end groups due to hydrolysis is just the difference between the molar fractions before and after hydrolysis. As there are two end groups created by each water molecule, the molar fraction of new end groups is also twice the molar fraction of water. This can be written as: 2 2 2nH2 O ¼ N0 X n X n0 where nH2 O is the number of water moles and N0 the total number of structural units (moles) in the polymer. Multiplying both denominators by M0, which is the (mean) molecular mass of a structural unit, and dividing each member by 2 gives 1 1 nH2 O ¼ M n M n0 N 0 M 0 The denominator on the right side is the total mass of the polymer. Thus, multiplying the numerator and the denominator of the right-hand term by the molecular mass of water (18) leads to ½H2 Ow=w 1 1 ¼ 18 M n M n0 where ½H2 Ow=w holds for weight concentration of water.
Table 9 Summary of structure — property relationships of polymers Molecular structure
Double-bond (vinyl) monomers or bifunctional monomers
Plurifunctional monomers or bifunctional monomers+ crosslinking
Network ¼ giant macromolecule
Linear chains Branched or grafted chains
Homopolymers Copolymers
Glass transition
Order-crystallinity
WRoom temperature
Solubility
Effect of T1
Good
WoroRoom temperature
Disordered (amorphous) Amorphous thermoplastic (Partly) ordered ¼ (partly) Semi-crystalline crystalline thermoplastic
Softening to (viscous) liquid Softening+ melting
WRoom temperature
Usually disordered (amorphous)
Thermoset (crosslinked)
Insoluble (swells)
Non-meltable
Elastomer
Swellable but insoluble
Non-meltable
oRoom temperature
Type of material
Difficult
Notes: Within amorphous thermoplastics some are, due to asymmetry, non crystallisable while others are usually amorphous because they are too slow to crystallise during normal processing.
Polymer Chemistry and Microstructure
Monomers
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Jacques Devaux and Sophie Demoustier-Champagne
This expression is particularly simple and useful for predicting the molecular weight after hydrolysis. It can be easily extrapolated for random degradations by reaction with any little molecule (‘‘contaminant’’) other than water.
6. SUMMARY As a summary to this chapter focusing on polymer chemistry and microstructure, major characteristics of polymers are reported in Table 9, classified under The type of monomers The (chemical) microstructure of the macromolecule The physico-chemical microstructural features leading to common classification J Glass transition J Order (crystallinity) Basic properties J Solubility J Thermal behaviour. As stated in the beginning of this chapter, few properties, depend directly and only on the chemistry. Table 9 summarises how chemistry influences (micro-) structure, which, in turn, governs properties. In this chapter, the ways synthesis’ processes influence the (chemical) microstructure were discussed. Processing methods, by providing the material its (physical) microstructure, also influence its properties. It is therefore the role of characterisation to gain insight into and control all the stages from chemical synthesis to the building of microstructure as well as the final polymer properties. The composition, uses and applications of commercial polymers are discussed in the next chapter (Chapter 3).
REFERENCES 1 2 3 4 5
6
7 8 9 10 11
J.P. Mercier, G. Zambelli and W. Kurz, Introduction to Materials Science, Elsevier, Paris, 2002. A.J. Kovacs, J. Polym. Sci., 30 (1958) 131. F.W. Billmeyer, Textbook of Polymer Science, Wiley, New York, 1962. M.B. Roller and J.D. Gillham, J. Coat. Technol., 50(636) (1978) 57. J. Carlier, R.L. Devaux and P.T. McGrail, Percentage of Rigid Chain Length, a new concept for predicting glass transition temperatures and melting points of poly(aryl ether ketones) and poly(aryl ether sulphones), Macromolecules, 25 (1992) 6646. V. Carlier, J. Devaux, R. Legras and D.J. Blundell, Extrapolation of short chain oligomers melting temperatures at infinite molecular weight, J. Poly. Sci., Part B: Poly. Phys., 36 (1998) 2563–2571. N. Grassie and G. Scott, Polymer Degradation and Stability, Cambridge University press, New York, 1985. B. Jambe, Ph.D. Thesis: Thermal Behavior of Poly(methyl n-alkyl silane)s (Prom. J. Devaux) Louvain-laNeuve (1997). P.J. Flory, Principles of Polymer Chemistry, Cornell, New York, 1953. G. Odian, Principles of Polymerization, Wiley, New York, 1981. J.R. Fried, Polymer Science and Technology, Prentice Hall PTR, USA, 1995.
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63
K. Matyjaszewski, Prog. Polym. Sci., 30 (2005) 858. K. Matyjaszewski and P. Sigwalt, Polym. Int., 35 (1994) 1. G. L’Abbe and G. Smets, J. Polym. Sci., A-1, (5) (1967) 1359. J.P. Mercier and E. Mare´chal, Chimie des polyme`res in ‘‘Traite´ des mate´riaux vol 13’’, Presses polytechniques Universitaires Romandes, 1993, pp. 107–108. 16 J.P. Mercier and E. Mare´chal, Chimie des polyme`res in ‘‘Traite´ des mate´riaux vol 13’’, Presses polytechniques Universitaires Romandes, 1993, pp. 137–142.
12 13 14 15
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CHAPT ER
3 Polymeric Materials: Composition, Uses and Applications J.P. Candlin
Contents
1. Introduction 2. Mode of Polymerisation 2.1 Oligomers, macromers and telechelic macromers 3. Polymer materials: Types and forms 3.1 Thermoplastics and thermosets 3.2 Amorphous and crystalline polymers 3.3 Elastomers (rubbers) and thermoplastic elastomers 3.4 Fibres 3.5 Flat films and sheets 3.6 Surface coatings and paints 3.7 Advanced materials 3.8 High performance polymers 4. Polymer Additives 4.1 Polymer-processing modifiers 4.2 Chemical property modifiers 4.3 Mechanical property modifiers 5. Summary Bibliography
65 66 68 68 68 70 74 77 79 81 84 84 87 88 98 112 117 118
1. INTRODUCTION Polymers play an integral role in modern society. Over 150 million tonnes are made annually. The scale of production varies enormously from a 200,000 tonnes per annum continuous operational plant for commodity polymers to a batch process producing a few kilograms of a specialised polymer (advanced material).
Comprehensive Analytical Chemistry, Volume 53 ISSN: 0166-526X, DOI 10.1016/S0166-526X(08)00403-0
r 2008 Elsevier B.V. All rights reserved.
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J.P. Candlin
Commodity polymers include: low density PE (LDPE) Polyethylene (PE)
linear low density PE (LLDPE) high density PE (HDPE)
Polypropylene (PP) Polystyrene (PS) Poly (vinyl chloride) (PVC)
Although some applications use these commodity polymers as structural components, their principal use is in packaging, e.g., films, bottles and containers. They are each made on a scale of over 10 million tonnes annually. Engineering polymers are often used as a replacement for wood and metals. Examples include polyamides (PA), often called nylons, polyesters (saturated and unsaturated), aromatic polycarbonates (PCs), polyoxymethylenes (POMs), polyacrylates, polyphenylene oxide (PPO), styrene copolymers, e.g., styrene/ acrylonitrile (SAN) and acrylonitrile/butadiene/styrene (ABS). Many of these polymers are produced as copolymers or used as blends and are each manufactured worldwide on the 1 million tonne scale. Advanced materials can be used in extreme conditions, e.g., high temperatures (W2001C), severe chemical environments (e.g., polytetrafluoroethylene (PTFE) with concentrated H2SO4). They are often used as a critical component in a workpiece and are frequently reinforced with glass, carbon or aramid (e.g., Kevlars) fibres. Organic polymers are sometimes referred to as plastics (although, this should be confined to thermoplastic polymers), macromolecules or resins, though the latter is often used to describe raw polymeric material awaiting fabrication. Many polymers are used in various forms that are not associated with normal plastic materials. These include paints and coatings, elastomers (rubbers), adhesives, sealants (caulks), surfactants and also their use in various industrial applications, e.g., ion-exchange resins, membranes.
2. MODE OF POLYMERISATION Polymers are formed via two general mechanisms, namely chain or step polymerisation, originally called addition and condensation, respectively, although some polymerisations can yield polymers by both routes (see Chapter 2). For example, ring opening of cyclic compounds (e.g., cyclic lactides and lactams, cyclic siloxanes) yield polymers either with added catalyst (chain) or by hydrolysis followed by condensation (step). Many polymers are made via vinyl polymerisation, e.g., PE, PP, PVC, poly(methyl methacrylate) (PMMA). It could be argued that the ethylenic double bond is a strained cyclic system. Another aspect is that the two reactants should yield a condensation product with the elimination of a small molecule, e.g., H2O. This is not essential. Thus in
Polymeric Materials: Composition, Uses and Applications
67
the formation of polyurethanes, the key step is the reaction: RNCO þ HOR0 ! RNHCOOR0 The general mechanism for chain polymerisation is (where M = monomer):
M
catalyst
M
M*
activated
rapid
M2*
nM
Mn*
termination rapid M n
whereas, for step polymerisation, the mechanism is: M þ M ! M2 M2 þ M ! M3 M2 þ M2 ! M4 ; etc: In general, Mn þ Mm ! MðnþmÞ The differences between chain growth and step growth polymerisations are shown in Table 1.
Table 1
Chain growth vs. step growth polymerisations
Chain mechanism
Catalyst or initiator required
Step mechanism
Polymerisation can proceed without catalyst Viscosity increases steadily throughout Viscosity increases rapidly at end of reaction polymerisation Monomer concentration decreases Monomer concentration decreases slowly during reaction rapidly before any high polymer formed Intermediates not usually isolatable Oligomers can be isolated at any stage (low concentrations of growing chains) MW independent of time MW increases with time MW generally decreases with MW independent of temperature increasing temperature (because termination reactions are enhanced) Rate of polymerisation is zero initially, Rate of polymerisation is a maximum at rises to a maximum as active centres the start and decreases continuously are formed from the initiator, and as the concentration of the functional then remains constant, before falling groups decreases off when the monomer is consumed Note: MW, molecular weight.
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J.P. Candlin
2.1 Oligomers, macromers and telechelic macromers If the polymerisation is halted deliberately by adding a terminating or chain transfer agent, e.g., a monofunctional agent, or accidentally, e.g., adventitious introduction of poisons (e.g., air, water, solvents), then a polymer with a low molecular weight (MW) may result with a degree of polymerisation (DPn) of perhaps n = 4–20. These materials are called oligomers. With chain polymerisation this is carried out at the start of polymerisation and the chain transfer agent statistically terminates the polymerisation. The MW of the polymer depends on the ratio of chain transfer agent to monomer, for example, the MW of PE and PP prepared by the catalytic polymerisation of ethylene or propylene, respectively, can be controlled by the continuous addition of hydrogen to the feed. With step polymerisation, the progress of polymerisation is measured by viscosity. It can also be estimated from the amount of condensate removed. It can be arranged that these oligomers contain a reactive end group, e.g., hydroxyl, carboxyl, allowing subsequently further polymerisation with a different reactant monomer to take place. These reactive oligomers are called macromers. Finally, some polymerisations can be directed such that the final oligomer or polymer contains two or more reactive end groups capable of extended polymerisation with different monomers. These materials are named telechelic macromers or telechelic polymers.
3. POLYMER MATERIALS: TYPES AND FORMS 3.1 Thermoplastics and thermosets The terms thermoplastic and thermoset refer to the processability of a particular polymer and the properties of the finished article. Thermoplastic polymers are mostly a linear or branched linkage of monomers containing many thousands of repeat units. All the commodity polymers and most of the engineering polymers are thermoplastic. Thermoplastic polymers can be homopolymers or copolymers where two or more different monomers are used. With copolymers, different arrangements of the monomers are possible (see also Chapter 2). Random copolymer Alternating (or regular) copolymer Block copolymer
Graft copolymer
Arbitrary arrangement of monomers in the polymer chain Reciprocal array of repeat units (this occurs when two or more monomers react together to form a complex, which is then polymerised) Contains long sequences of one linkage joined onto another sequence with a different linkage. Di- and tri-blocks are possible Obtained by chemically reacting a second monomer/ polymer chain onto a pre-formed polymer
Polymeric Materials: Composition, Uses and Applications
69
The chemical nature of the repeat units determines the physical and mechanical properties of the polymer. Thermoplastics soften when heated and form a viscous melt; on cooling they solidify and become stiff. This process can be repeated. In the molten condition they can be shaped by injection moulding or extruded into tubes, films or fibres. The viscosity of the molten polymer varies with chemical linkage and MW of the polymer. The higher the MW, the higher the viscosity at a given temperature. As a general rule, the acceptable melt viscosity for extrusion, etc., is 1501C above Tg and 501C above Tm, where Tg and Tm are the glass transition and crystalline melt temperatures, respectively. Inter-chain attraction between the polymer chains determines the DPn necessary to give the polymer the required toughness, strength, etc. For polymers containing heteroatoms, e.g., polyesters, polyamides, DPn of 100–200 is adequate, whereas for polymers with little or no inter-chain attraction, e.g., PE and PP, then DPnW1,000 is recommended. Thermoset polymers (sometimes called network polymers) can be formed from either monomers or low MW macromers that have a functionality of three or more (only one of the reagents requires this), or a pre-formed polymer by extensive crosslinking (also called curing or vulcanisation; this latter term is only applied when sulfur is the vulcanising or crosslinking agent.) The crosslinks involve the formation of chemical bonds — covalent (e.g., carbon–carbon bonds) or ionic bonds. The act of crosslinking can either be an addition reaction (e.g., radical/radical) or a condensation reaction (e.g., alcohol/carboxyl). In the latter, the formation of volatile condensates is often not desirable because they may weaken the structure. Crosslinking reactions involve creating radicals along the polymer chain, which can couple to form a crosslink. A problem is that chain scission may occur. These radicals can be generated from added peroxide or, with more control, by high-energy radiation, e.g., X-rays (synchrotron), electron beams or g-rays (Co-60). For example, crosslinked PE is much tougher, more environmental stress cracking resistant and is often used for wire coating. When cold stretched and then heated it returns to its original shape, hence its use for shrink-wrap films or tubes. Depending on the strength of the crosslink, under most conditions, thermoset polymers do not melt, although with weak crosslinks, e.g., ionic, dipole-dipole, H-bonding, they are capable of being moulded. The normal procedure for the fabrication of thermosets is to add the reagents to a mould and leave to complete polymerisation. An alternative method is to stop the reaction before completion. Then, depending on the extent of polymerisation, the material can be injection or compression moulded, or, if the product at this stage is solid, it can be ground into a powder and compressed (moulding powders). The finished article is then cured by raising the temperature. As the reactants are often liquid, solvents are not used because they may get entrapped within the growing polymer matrix giving a poor surface finish. Conversely, the addition of volatile solvents at this stage is used to generate rigid foams. Polymer linkages that do not release condensates on being formed include polyurethanes (diisocyanate/polyalcohol) and epoxy (ring opening of the epoxy
70
Table 2
J.P. Candlin
Comparison of thermoplastic and thermoset polymers
Thermoplastic
Thermoset
Long-chain molecules, often without branching Soluble in some solvents
Cross-linked 3D structures
Crystalline and/or amorphous Variable MW; determined by solution techniques Can be flexible, soft or brittle Will flow under pressure Can be moulded or fabricated Many highly flammable Sometimes fail catastrophically at high temperatures Can be converted to thermoset by post-treatments
Can be swollen in solvents, but always insoluble Amorphous Very high MW; cannot be determined Rigid, brittle structure Structurally stable Only moulded articles (injection moulding possible with RIM) Many self-extinguishing Have good dimensional stability or fail slowly under load at elevated temperatures Impossible to remove crosslinks
group with polyamines). An advantage of liquid monomers is the ease by which they can be handled. Thus polyurethane articles can be made using reaction injection moulding (RIM). The two reactants are pumped into a mixing chamber then into a mould. Large articles, e.g., car bumpers, garage doors, can be made using this technique. Table 2 contrasts some of the properties of thermoplastic and thermoset polymers.
3.2 Amorphous and crystalline polymers No polymer is 100% crystalline, such as benzoic acid or NaCl, although PTFE can be over 90% crystalline. Conversely, polymers may be 100% amorphous; this occurs when the polymer chains cannot align themselves into a close, organised regular structure. About 50–100 uniform repeat units are necessary for crystallisation to take place. Amorphous regions in the bulk polymer are created when the polymer chain is capable of cis–trans configuration, rotational movement around a single bond or a crankshaft displacement involving several units in the polymer chain. Bulky substituents on the main polymer chain also lead to amorphous regions. Tacticity (see Chapter 2) of the polymer chain (i.e., extent of stereoregularity) also determines the possibility of crystallisation. Stereoregular, isotactic or syndiotactic polymers are often crystalline, whereas the corresponding atactic polymers are amorphous. Thus amorphous atactic PS has a softening temperature of 1001C and atactic PP is a viscous gum at room temperature, whereas syndiotactic PS and isotactic PP have crystalline melting
Polymeric Materials: Composition, Uses and Applications
71
temperatures (Tm) of 2701C and 1601C, respectively. These chain rotations, bulky substituents, etc., require space in which to move. This space is called free volume; it can be estimated by plotting volume (i.e., volume per unit mass) against temperature. Thermoset polymers are not crystalline because the multiple crosslinks do not allow the main chain polymer molecules any movement to form crystals. For all polymers (except those prepared by ‘‘living’’ growth mechanisms, see Chapter 2) because of molecular weight distribution (MWD), amorphous polymers do not have a sharp melting or softening point. Instead they have a broad temperature range of approximately 501C over which, on heating, the polymer changes from a brittle glassy solid to a pliable rubbery material. This change occurs at the glass transition temperature, Tg. The value of Tg depends on the chemical linkage of the polymer chain. Aromatic polymers with stiff, rigid para-linkages have high Tgs, e.g., poly(aryl ether sulfone) (PES) with a Tg of 2251C, whereas aliphatic carbon–carbon linkages, e.g., natural rubber, may have a Tg as low as –731C. In the rubbery, semi-molten state the polymer chains rotate, but on cooling the polymer behaves like a frozen liquid with a completely random structure. The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. Some of the properties and differences that amorphous and crystalline polymers exhibit are: (i)
(ii)
(iii)
(iv) (v)
Amorphous copolymers are brittle below Tg; elastomeric above Tg, although amorphous advanced materials with aromatic chains with high Tgs (e.g., PES) are tough at low temperatures. Amorphous polymers display small volume changes (compared with crystalline polymers) on cooling from the polymer melt to a solid, because in the melt the polymer chains are randomly oriented. Thus amorphous and crystalline polymers exhibit about 0.5% and 2.0% shrinkage, respectively. This shrinkage must be allowed for in precision or large articles. It can be compensated for by adding a small amount of foaming agent or filler during fabrication. Films made from amorphous polymers are more permeable to gases than crystalline polymers. More free volume is available for transport of the gases through the film. The amorphous phase is lower in density than the crystalline phase. Amorphous polymers are often transparent. Usually the crystalline phase has a different refractive index than the amorphous phase. This causes scattering and refraction giving the crystalline polymer a milky appearance. Unusually, the refractive index of crystalline isotactic poly(4-methyl pentene-1) is the same as the amorphous phase rendering
72
(vi)
(vii)
J.P. Candlin
the polymer transparent. This makes it particularly suitable for transparent sheets and laboratory ware (replacing glassware). Amorphous polymers will swell or dissolve in solvents. Crystalline polymers are more resistant to solvents; solvation has to overcome the attractive forces that exist within the covalent crystal lattice. If the affinity between the solvent and the polymer chain is low the crystal regions remain intact. Solvents can induce crystallisation. If the solvent is in contact with an amorphous polymer (but potentially crystallisable) diffusion of the solvent into the amorphous regions (plasticisation) lowers the Tg, allowing movement and reorganisation into crystalline lattices; the solvent itself is excluded from the lattice.
Although the chemical structure of the polymer chain may allow crystallisation, this may not be possible for the following reasons: (i)
(ii)
(iii) (iv)
Crystallisation from the polymer melt takes time and may require nucleation. If the cooling is rapid the organisation of the polymer chains into crystals may be too slow, resulting in an amorphous phase. To promote crystallinity the polymer must be re-heated to temperatures between Tg and Tm. This technique is called annealing. Another approach is stretching the polymer to promote crystallisation. This occurs in blow moulding of bottles and the biaxial stretching (drawing) of films. This increases the gas-barrier properties and because the induced crystallites are small (compared to the wavelength of visible light), the polymers are transparent. Amorphous cellulose tri-acetate sheets are used for overhead projector sheets for use in photocopying machines; they do not ‘‘curl up’’ or distort in the heated atmosphere. If poly(ethylene terephthalate) (PET) sheets are used, the biaxial stretching must be equal or it will curl up. Polymeric fibres increase in strength (increased tenacity) on being drawn. This orientation is performed between the Tg and Tm temperatures. (Methods for determining the extent of molecular orientation in polymers are discussed in Chapter 8.) Polymers vary in their rate of crystallisation depending on side chains, etc. Thus with PE crystallisation is almost instantaneous (less than 1 s), whereas nylon 6,6 and PVC (over 1 year to complete crystallisation) are sluggish. PET is slow to crystallise, but poly(butylene terephthalate) (PBT) is more rapid. Thus consideration of final application is critical, such as transparency and dimensional stability. Physical entanglement of the polymer chains restricts the translational movement necessary for alignment and crystallisation. Imperfections in the polymer chain. Insertions for chain polymer growth require ‘‘head-to-tail’’ for the correct stereoregularity (tacticity). A false ‘‘head-to-head’’ inclusion will disrupt the regular pattern. In a similar manner, atactic (non-stereoregular) insertions will produce amorphous polymers.
Polymeric Materials: Composition, Uses and Applications
73
As described above, crystallinity in polymers occurs when the polymer chain is regular and uniform and contains no defects (which can include external degradation, e.g., oxidation), and is critical for fibre applications. This arrangement is aided by inter-chain attraction. If a polymer does crystallise then the role of the amorphous phase becomes less important especially when considering mechanical strength and stiffness and the maximum service temperature. This is because the crystalline regions behave like temperaturesensitive crosslinks. They act like a thermoset when cold, but on heating (above Tm) are mouldable. This arises because emanating from each crystalline region are polymer chains (called tie molecules) that may be incorporated into separate different crystalline regions. The higher the MW of the polymer, the more this occurs and the polymer is much tougher. A significant aspect is the manner in which glass–fibre reinforcement increases the service temperature of semi-crystalline polymers; this is especially important in advanced materials. A value of the maximum service temperature can be obtained from the heat distortion temperature (HDT). This is measured by observing the temperature at which the polymer distorts under controlled stress. Normally with crystalline polymers this is between Tg and Tm of the polymer. With amorphous polymers this is below Tg. This can be seen by plotting modulus vs. temperature. An amorphous polymer has only one thermal transition (Tg), whereas a semi-crystalline polymer has two (Tg and Tm). When reinforced with glass fibres, the HDT of the crystalline polymer is raised to temperatures approaching Tm, whereas with the amorphous polymer the increase in HDT is small and remains below Tg. An illustration is shown in Figure 1 of the increase in HDT at a fixed modulus by the addition of glass fibres to amorphous and semicrystalline polymers. Amorphous
Semi-crystalline neat
glass-filled
glass-filled
modulus
modulus
neat
a fixed modulus
a fixed modulus
HDT neat increase temperature
HDT (glass-filled)
HDT neat
increase
HDT (glass-filled)
temperature
Figure 1 Illustration showing the increase in HDT at a fixed modulus by the addition of glass fibres to amorphous and semi-crystalline polymers.
74
J.P. Candlin
Table 3
Comparison of HDTs for amorphous and semi-crystalline polymers
Amorphous
Semi-crystalline
Heat distortion temperature (1C) Neat
PES
20% glass-fibre reinforced
B215 315 142 250 223 260
203 165 130 100 200 B100
PEEK PC Nylon 6,6 PEI PPS
Notes: PES, poly(aryl ether sulphone); PEEK, poly(aryl ether ether ketone); PC, polycarbonate; PEI, polyetherimide; PPS, poly(phenylene sulphide).
This improvement can be seen from the comparisons in Table 3. Some polymers, especially those with a rod-like, linear aromatic backbone, e.g., X X
X
X
X
X
O where X =
O
,
NH
,
C
O O
or
S
,
O
have a tendency to form crystals even whilst in the molten state or in solution. They are called liquid crystal polymers. They have very high melting points making them difficult to process, although careful introduction of flexible groups (e.g., –CH2–, meta-phenylene) does lower this to some extent. However, in the molten state or in solution with applied shear (e.g., extrusion processes) the polymer chains align together in the direction of flow and the viscosity decreases drastically allowing fabrication. In the solid state they are extremely anisotropic (uni-directional) and when combined with reinforcing fibres, e.g., carbon–carbon fibres, yield some of the highest service temperatures (approaching 4001C) known for polymers. Kevlars (produced from highly oriented poly(paraphenylene terephthalamide) is an example of a liquid crystal polymer.
3.3 Elastomers (rubbers) and thermoplastic elastomers Elastomers are defined as polymers that can be repeatedly stretched to B200% elastic elongation and will return to their original length when released. They can
Polymeric Materials: Composition, Uses and Applications
75
be either thermoplastic or thermoset, although with the latter the extent of crosslinking is low, typically 1–5%, i.e., 1–5 crosslinks per 100 backbone chain units. Most metals and ceramics have elastic elongation of less than 2%. Stretching in these materials involves the movement of atoms. In contrast, polymers can be easily elongated because the applied stress promotes linear formation of the polymer backbone. The MW of an elastomer must be high enough to promote chain entanglement thus preventing polymer chains from sliding past each other on applying stress. Because of the linear, organised arrangement of the polymer backbone on stretching, crystallisation may occur. This also helps to increase the strength on stretching (tie molecules). The requirements for an elastomer are: (i) (ii)
(iii) (iv) (v)
High MW to promote chain entanglement. Amorphous, although crystallinity when stretched is often advantageous. This promotes tie molecules and strengthens the elastomer at maximum extensions. The Tm of the crystalline region must be near or below the working temperature such that the crystallites can melt when the stress is removed, e.g., natural rubber, Tg –731C; Tm 251C. Flexible backbone, allowing chain rotation, etc. Low Tg allowing chain movement at ambient temperatures. Thus for room temperature usage this requires a Tg of less than –501C. Low creep value (this is the flow of polymers when stress is applied over long time periods). This is achieved by low level crosslinking (less than 5%).
In some cases the modulus and strength are so low that fillers are often added for stiffness. Carbon black, as a filler, is used extensively in car tyres, and it is thought that the surface bonding with the rubber occurs, thus providing stiffness. Finely divided silica, which has been surface treated with organometallic silicon and titanium compounds, is also used. Covalent crosslinks, once formed, are non-reversible. Once made, they cannot be broken. This limits the processing options to low shear techniques, such as calendaring in which a mangle arrangement with three parallel rollers presses the polymer into sheets, or to adding the crosslinking agent at the last stage. Heat reversible crosslinks, however, allow the elastomer to be treated similarly to a thermoplastic, capable of being extruded or injection moulded at a high temperature whilst behaving like an elastomer at low temperatures. There are various ways of forming covalent crosslinking: (i)
(ii)
Olefinic polymerisation, e.g., ethylene/propylene to which is added a diolefin (diene), where the diene is 1,4-hexadiene or 4-ethylidene norborn-2ene. The resulting polymer is usually called EPDM (ethylene propylene diene monomer). Vulcanisation of unsaturated rubbers, e.g., poly(1,4-butadiene), polyisoprene, random styrene/butadiene rubber (SBR) with sulfur. The sulfur attacks the allylic C–H group to form inter-chain –S–S– linkages.
76
(iii)
(iv)
(v)
J.P. Candlin
Peroxide cure of saturated olefinic polymers, e.g., random, non-crystalline ethylene/propylene copolymers (prepared by Ziegler catalysts). The peroxide generates carbon radicals, which couple together forming inter-chain covalent bonds. Dimethyl silicone polymers have no strength. Reaction with peroxide gives random radicals, which link together and stops the silicone chains from sliding past each other on being stretched. Neoprenes is a polymer formed from chloroprene (2-chlorobutadiene). On heating with zinc oxide the neoprene polymer rearranges to give a pendant labile allylic chloride group (–CH = CH–CH2Cl). Further reaction with zinc oxide yields zinc chloride and a stable ether link between the two polymer chains. The copolymer of vinylidene difluoride and per-fluoropropylene (Vitons) on heating releases HF and gives an unsaturated fluoropolymer. Reaction with a diamine introduces crosslinks. This fluoroelastomer has a very high service temperature of over 2001C.
The linkage between two chains can also be ionic. Thus the copolymer between ethylene and methacrylic acid (MA) (up to 15% MA), made by free radical polymerisation, yields a polymer with pendant carboxyl groups. Neutralisation with zinc ions gives a crosslinked, thermo-reversible polymer (Surlyns). The resulting polymer (ionomer) has limited properties, although it is the favoured material for the outer covering of golf balls. Crosslinks prevent the flexible polymer chains from sliding past each other; crystallinity could play a similar role as long as the percentage of crystallinity is small. (Polyethylene has the potential to be an elastomer, but its crystallinity is too high — greater than 60% and therefore behaves like a thermoplastic). A big advantage, however, would be that the elastomer would be thermo-processible. Another approach would be to design a copolymer prepared by block polymerisation, which has one segment that can form crystals. If that is not achievable, one could have an amorphous segment with a high Tg (higher than the service temperature) to form a glassy domain. The other segment would consist of a flexible chain with a low Tg. These segments are often referred to as hard and soft segments, respectively. Following that approach, many thermoplastic elastomers (TPEs) have been synthesised using this principle, e.g., see Table 4. Careful control of the chain length of each block has to be made. An interesting observation arose with the thermoplastic elastomer styrene/ butadiene (S/B) tri-block copolymer (Kratons). These are made by anionic Table 4
Some thermoplastic elastomers
Hard segment
Soft segment
Poly(butylene terephthalate) Aromatic urethane Aromatic polyimides Aromatic polyesters or carbonates
Poly-1,4-butane diol (Hytrels) Poly-1,4-butane diol (spandex) Perfluoropropylene oxide Polydimethylsiloxanes
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77
polymerisation (living polymerisation). Thus either polystyrene/polybutadiene/ polystyrene (SBS) or polybutadiene/polystyrene/polybutadiene (BSB) tri-blocks can be made. The former has much better elastomeric properties than the latter. This is because the polystyrene end segments have the freedom to move and form a glassy domain (PS Tg, 1001C) together with tie molecules, whereas with polystyrene end-capped with polybutadiene the polystyrene segment is restricted in its movement. The polybutadiene segments have a low Tg of –1001C and therefore will not form a glassy rigid structure at ambient temperatures necessary for added strength on stretching. Polymers with blocks containing different tactcities can be produced, e.g., atactic PP (amorphous)/isotactic PP (semi-crystalline) can be made using metallocene catalysts. They behave in a manner similar to SBS thermoplastic elastomers.
3.4 Fibres Synthetic fibres are thermoplastic materials and are fabricated by either melt processing or from a solution of the polymer in a solvent. Both processes use a spinneret. The main structural requirement is that the polymer must have a linear backbone (with little branching). On drawing or stretching the polymer chains align promoting crystallinity giving the fibre added strength. In principle, there are many polymers suitable for fibres, but for apparel (clothes) and domestic (carpets) use only four types dominate: polyester, polyamide, polyacrylonitrile (often as a copolymer with vinyl chloride) and polypropylene. For melt spinning, it is necessary that the melting temperature of the polymer is not too high as to cause decomposition, and that the melt viscosity is low since it passes through the spinneret holes (diameter approximately 120 mm). Each spinneret plate contains hundreds of these holes. Conversely, the Tg of the fibre must be low enough to permit ironing, but the Tm must be sufficiently high to prevent melting during ironing. For melt fabrication the temperature of the melt is approximately 501C higher than Tm (although with polypropylene this is 1001C higher). Melt spinning is used to make polyester, polyamides (nylon 6 and 6,6) and polyolefin fibres. Wet or solution spinning is used for natural fibres (cellulose derivatives) and some synthetic fibres. This can be achieved by passing a concentrated solution of the dissolved polymer through the spinneret into a coagulating bath, e.g. polyacrylonitrile/dimethyl formamide (DMF) and cellulose/N-methyl morpholine N-oxide (NMMO) (Tencels) into water. With wet spinning, the solvent is evaporated by heat, e.g., aramid/concentrated H2SO4 into water and polyurethane elastomeric fibres/DMF followed by coagulation with water. The main structural/property requirements for fibre formation are: (i)
The polymer chain must be uniform with no defects in chemical composition or stereochemical arrangement. This condition is stricter than for other applications.
78
(ii) (iii) (iv)
(v)
(vi)
J.P. Candlin
Sufficient molecular weight to exhibit chain entanglement, promoting fibre strength during drawing. Linearity of backbone, with little or no branching. The inter-molecular forces between the polymer chains enhance structural cohesion giving the polymer strength during spinning and drawing. This is achieved by incorporating heteroatoms (e.g., N, O) in the polymer backbone thus encouraging H-bonding, dipole–dipole or aromatic– aromatic p-electron attraction. Crystallisation during the drawing process caused by aligning the polymer chains. These small crystallites encompass polymer tie molecules giving the fibre added strength. With amorphous copolymer fibres, e.g., acrylonitrile/vinyl chloride (acrylic fibres), crystallisation does not occur, but the act of drawing optimises the strong dipole–dipole forces and increases the strength of the fibre. Orientation of the crystallites parallel to the fibre axis gives the fibre added strength (tenacity), and because of the rapid rate of drawing, the size of the crystallites is small when compared to the diameter of the fibre.
In principle stretching or drawing of fibres should be undertaken between Tg and Tm, thus minimising brittle fracture. Thus polyester (e.g., PET) and polyacrylonitrile (PAN) fibres are heated during drawing. With wet spinning, the presence of the solvent acts as a plasticising agent and thus decreases Tg, e.g., nylon drawn in an atmosphere of high water humidity. The extent of draw (draw ratio) affects the tenacity of the fibre. Thus for fibres for apparel, i.e., soft, flexible fibres, a low draw is used (100% elongation), whereas for fibres with a high strength, e.g., ropes, tyres, requires a high draw (400%). Originally, this led to irreversibility in finished garment, e.g., kneed stockings. This has been overcome by the steam treatment of the drawn fibre (PET) before weaving. Many other properties have to be considered, especially for apparel fibres, e.g., moisture absorption, ability to dye, drape, texture, weaving characteristics, etc. Many of the properties are influenced by the cross-section profile of the fibre. Thus cotton and some rayons (an artificial synthetic fibre derived from cellulose) are a hollow round fibre; silk has a triangular shape giving it a fine lustre and drape. Using co-extrusion it is possible to make fibres with a nylon 6,6 inner core (Tm 2641C) and a nylon 6 outer shell (Tm 2151C). By applying pressure and heat between these two temperatures to bundles of these two fibres, a non-woven mat can be made. The elastomeric fibre, elastane or spandex (the anagram of ‘‘expands’’) is a hard/soft segmented block copolymer. The soft segment (with a low Tg) can be a polyglycol (e.g., polyethylene glycol or polybutylene glycol) or an aliphatic polyester glycol and has a DPn of approximately 25. This constitutes up to 80% of the polymer weight. The hard segment is a polyurethane or polymer prepared from aromatic di-isocyanates. Strong inter-molecular forces (e.g., H-bonding, crystallinity) and phase separation of the two segments renders the copolymer elastomeric, which allows 600% extensions with full recovery (similar to natural
Polymeric Materials: Composition, Uses and Applications
79
rubber, but without the inherent oxidative instability and attack by ozone). The polymer can be processed into fibres by either melt spinning or from solution (using DMF as the solvent). The latter method is used when polyurea is the hard segment, since they decompose on melting. The main application of spandex (e.g., Lycras) is for stretched apparel clothing. It is used to make yarns when combined with other natural and synthetic fibres. Although the main usage of fibres is for apparel, many applications are unique to synthetic fibres. Examples include (i)
(ii)
(iii)
(iv)
High modulus polyethylene fibres are made by dissolving ultra high MW polyethylene (W106) in decalin or paraffin oil (2% solution) at 120–1701C and drawing through a hexane bath. Extended chain polyethylene fibres (e.g., Dyneenas) are used as ballistic-resistant yarns, and have strengths comparable to aramid fibres, but they do not possess high temperature resistance. Melt-spun HDPE (fluff-like, similar to candy floss, made by flash-spinning technology) is loaded onto a moving bed to give a random mat that is consolidated by heat and pressure (Tyveks). These mats allow moisture but not liquid water to pass through. Their main application is in construction, but it also has many other uses, e.g., envelopes, protective clothing. Biodegradable polyester copolymers of lactic/glycolic acids are used as sutures in medical operations. They degrade and dissolve harmlessly in the body. Hollow amorphous aromatic polysulfone fibres, presenting a high surface area, are used as membranes for separating liquids and gases.
3.5 Flat films and sheets Films and sheets are defined as planar flat materials that are self-supporting and flexible. Generally films have a thickness of less than 250 mm, above this thickness they are referred to as sheets. They may consist of one or more layers, usually of different compositions. Films or sheets can be used alone or co-extruded with paper, foil or fabrics. The main application for films is for flexible packaging material, but because of their versatility they can be modified to fulfil many uses, such as electrical insulators, a base for printing and writing, permeable membranes, and many more. Sheet materials tend to be greater than 1 mm thick and are often used as replacements for glass, wood and metal. Examples include window glazing and outdoor signs. Sheets can be transformed into shapes by vacuum moulding (thermoforming).
3.5.1 Manufacturing of films and sheets The techniques following are used for the manufacture of films and sheets: (i)
Mould casting between planar parallel moulds. The polymer or macromer is injected into the mould as a viscous liquid together with a catalyst.
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(ii)
(iii)
J.P. Candlin
Another approach is the use of RIM using reactant monomers and a catalyst. The viscosity must be low enough to allow for flow of the reactants. This sometimes precludes polymer melts with high viscosity. Solvents are not used because volatilisation may cause bubbles. Continuous casting. In contrast to mould casting, this can use solutions of polymer. The mixture is fed onto a horizontal moving belt and the solvent is removed by evaporation. This technique has the advantages over extrusion process: a. No heat stabiliser is used, thus the sheet has greater clarity without distortion. b. Uniformity of thickness. c. Very wide sheets (over 2 m) can be processed. This is difficult with extruded sheets because of the intricate design of the clothes-hanger die used to prevent warpage caused by polymer chain orientation. Continuous casting however, using molten polymer, has been used providing the viscosity is low enough. Many polymers have been processed by casting, e.g., acrylics, polystyrene, polyamide (nylon 6), phenolics, PVC/plasticiser. Many of these are used in a pre-polymer form, which polymerise on the casting belt, or the polymerisation can be completed later by application of heat. Extrusion processes. Two techniques are used to make very thin films of less than 10 mm thickness. a. Stenter or tenter process. Extruders continuously force the molten polymer through a clothes-hanger die, which is then cooled. The sheet (approximately 500 mm thick) is re-heated (between the Tg and Tm of the polymer) and drawn by pinch or nip rollers, which stretches the sheet in the forward (machine) direction. Simultaneously, clamps (tenterhooks) stretch the sheet sideways. The finished film is then collected on a cooled rotating drum. The resulting film is between 2 and 5 times the original length and width with a corresponding reduction in thickness. This stretching, similar to the drawing of fibres, which promotes orientation and crystallite formation, is called biaxial orientation. It gives the film added strength and gas-barrier properties. In some processes, monoaxial (uniaxial) drawing is employed, e.g., polypropylene, which is then slit into thin strips and fabricated into heavy duty sacks, carpet backing, etc. The stenter process is used to make biaxial oriented poly(vinylidene dichloride) (‘‘cling’’ film), polyester, polyamide and polypropylene films. b. Blown films. The molten polymer is passed through a tubular die, the centre of which contains an air-inlet. Increasing the pressure of the air forces the tube to expand like a bubble, the top of which is clamped together by rotating nip rollers. Thus a continuous thin film, tubular roll is fabricated. The nip rollers are set to stretch the film in the forward direction, whilst the bubble expands the film in the sideways direction. Thus biaxial stretching is achieved. Bi-layers and multilayer films with different polymers are made by having two or
Polymeric Materials: Composition, Uses and Applications
(iv)
81
more concentric dies. This can be used to optimise properties, e.g., one layer has good gas-barrier properties, and the other layer has good tear strength. Biaxial polypropylene (Tm 1651C) is difficult to heat seal (large shrinkage occurs on melting), but co-extrusion with polyethylene (Tm 1151C) allows the bi-layer to be sealed below the melting temperature of polypropylene. Films of polyolefins, polyamides and poly(vinylidene dichloride) are made using this technique. As most of the films are used for flexible packaging, further down-stream surface treatments are usually applied to improve performance. For example, aqueous polymer emulsions, e.g., poly(vinylidene dichloride), or delaminated clay particles improve the barrier properties as will metallising with aluminium vapour. Corona discharge, causing slight surface oxidation, improves printability. Calendering. This method is used to process heat-sensitive polymers, such as plasticised PVC, and also to blend polymer mixtures or add additives. The polymers are fed and heated on hot rolls and squeezed between two or more parallel nip rollers into the form of a thin film or sheet. The process can be easily modified to incorporate backing material, e.g., metal foil, paper and textiles. Only polymers with a high viscosity can be processed by this method.
3.6 Surface coatings and paints Several million tonnes of polymers are incorporated annually into coatings and paints. Coatings are applied for several reasons apart from being aesthetically pleasing, for example: (i) (ii) (iii) (iv)
They form a protective layer, which inhibits corrosion (with metals) and bacterial attack (with wood). They fill and smooth imperfections. They provide a durable decorative and alternative surface finish, e.g., matt or gloss. They are resistant to household cleaning products such as bleach and solvents.
The first paints were based upon linseed oil (obtained from flax). This is an unsaturated long-chain triglyceride, which, with metal activators, crosslinks via radical pathways to form a continuous film on the substrate. Modern paints use synthetic polymers together with either a solvent or suspending medium (e.g., water), which evaporates leaving the deposited film. Exceptions are powder coatings, which require heat for completion. The main requirements for a coating are that it is easy to apply (i.e., low viscosity) and dries to a non-tacky finish within a reasonable time. Originally, this equated to the solution and evaporation of the solvent. However, because of environmental concerns, formulations have tended to shift the concentration of solvents from 80% to less than 20% (known as high solids coatings). Many industrial processes use polymer powders (of approximately 40 mm diameter),
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J.P. Candlin
which are attracted electrostatically to the substrate before being cured by melting or electron beam treatment.
3.6.1 Polymers, resins and binders used in paints These are the most important components of paint. Coatings can be either clear or opaque (containing pigments) and either solvent- or water-based. With solventbased paints, after application, most of the solvent is lost through evaporation. These solvent-based coatings are mostly alkyds (name derived from alkyl/acid) or modified alkyd resins. Normally they contain 30% polymer solids; higher solids content is limited by the increasing viscosity of the system. Alkyds are crosslinked polyesters made from 3- or 4-functional alcohols, e.g., glycerol or pentaerythritol, and a 2-functional carboxylic acid, e.g., phthalic acids and maleic acid. Also added are long-chain unsaturated oils, e.g., vegetable oils. The oligomerised product is dissolved in mineral oil or toluene. On application to a surface, the solvent evaporates and the remaining mixture undergoes catalytic (cobalt naphthenate catalyst) autooxidative crosslinking to form a continuous coating. Modified alkyds include the addition of silicones, phenol-formaldehyde, urethanes, styrene, epoxies and acrylics, which are grafted onto the alkyd backbone, e.g., urethane and epoxy groups are grafted using excess hydroxyl groups in the alkyd formed by reaction with isocyanate and oxirane (epoxy) groups, respectively. Styrene and acrylics modifications are obtained by radical polymerisation and phenol-formaldehyde by reaction of methylol groups with hydroxyl groups. Urethane alkyds are the most important. If the coating is for both outdoor and indoor use then aliphatic di-isocyanates are used. Aromatic di-isocyanates yellow and may crack when used for outside applications. Latex or emulsion polymers are prepared by emulsification of monomers in water by adding a surfactant. A water-soluble initiator is added, e.g., persulfate or hydrogen peroxide (with a metallic ion as catalyst), that polymerises the monomer yielding polymer particles, which have diameters of about 0.1 mm. The higher the concentration of surfactant added, the smaller the polymer particles. In suspension polymerisation, so-called because of the addition of watersoluble suspending agents, e.g., poly(vinyl acetate) (PVA), starch or surfactant, a monomer-soluble catalyst is added (e.g., organic peroxide) resulting in larger porous particles (approximately 500 mm) that readily absorb plasticisers. This is sometimes called bead or pearl polymerisation because of the smooth spherical shape of the polymer particles. Several different monomers may be used together to yield the desired properties. High molecular weight polymers, giving increased polymer toughness, can be prepared without the attendant increase in viscosity or without the necessity for crosslinking. With smaller polymer particle sizes, more pigment may be added to give greater opacity. However, this requires more surfactant resulting in less durability. A high polymer content (W65%) in the emulsion can be obtained. A small amount (o5%) of high boiling solvent is added, e.g., glycols, glycol ethers or alkyl-aromatic compounds depending on the polymer. These solvents plasticise the polymer and promote coalescence of the polymer particles after evaporation of the aqueous phase.
Polymeric Materials: Composition, Uses and Applications
83
The greatest use of latex paints for indoor applications is based upon vinyl acetate, which constitutes 85% of the total polymer usage, the remainder being acrylates (e.g., n-butyl acrylate). For outdoors or bathroom usage, acrylates are the major components. For various applications, e.g., wood paints, styrene copolymerised with acrylates are used. However, yellowing of the styrene units occurs. The mode of polymerisation and crosslinking for coatings is very similar to that of bulk polymers. An important requirement is that premature polymerisation should not take place before application. In the presence of activators, e.g., cobalt naphthenate, many paints on exposure to air polymerise by radical oxidation resulting in crosslinked structures. Stepwise growth polymerisation, e.g., urethanes, is promoted by heat; therefore storage at high temperatures (W501C) should be avoided. A growth area is UV or electron beam curing. This can be via radical polymerisation, e.g., a benzoin derivative, O
OR
C
CHPh
O
OR
C
CHPh
UV Ph
Ph
or cationic polymerisation (with epoxy resins) using aromatic (Ar) onium salts, e.g., sulfonium salts, Ar3S BF4
UV
Ar + Ar2S
BF4 H (from solvent)
Ar2SH BF4
Ar2S + HBF4
The strong acid, HBF4, acts as the catalyst. Cationic polymerisation is used for coatings and printing inks. Several conditions are necessary for satisfactory surface coatings, e.g., nondrip applications. There is a balance between the ease of use and sagging of the paint during and after administration. Thixotropic additives, e.g., organomontmorillonite clays are added. These form 3-dimensional networks, which are easily broken down by shear. Various finishes can be achieved — gloss, satin (or egg-shell) or matt. This is accomplished by the addition of particles of size 1–5 mm of, for example, silica, china clay or the white pigment, TiO2. The degree of mattness depends on various factors, such as particle size, surface treatment of the particles, rate of film formation, and the polymer composition, e.g., urethane/acrylate compared with epoxy/acrylate. The former requires smaller particles; larger particles cannot be used as they create a rough surface.
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One of the largest uses of paints is in the automotive industry and the coatings of metals for decorative and anti-corrosion purposes. This is achieved using electrophoresis. The first pre-treatment provides corrosion resistance by phosphating using a solution of zinc or iron phosphate. Next a water-insoluble polymer is solubilised by converting it into a soluble salt through neutralisation. Thus alkyd resins containing free carboxyl groups (polymer–COOH) are treated with KOH to give the water-soluble salt (polymer–COO~K). In a similar manner, treatment of an epoxy/amino resin with a carboxylic acid, e.g., acetic acid, converts the polymer into a water-soluble quaternary ammonium salt, polymer NR2 þ CH3 COOH ! polymer NR2 HY OOCCH3 Electrophoresis of water solutions of these salts and using steel as either the anode or the cathode, respectively, liberates the water-insoluble polymer coating through the reaction: Anode deposition; polymer2COOY K þ
H
ðfrom anodeÞ
! polymer2COOH ðinsolubleÞ
Cathode deposition; polymer2NR2 HY OOCCH3 þ
OHY
ðfrom cathodeÞ
! polymer2NR2 ðinsolubleÞ
For various reasons, cathodic deposition is the most favoured. The main advantages are that complete surface coverage is obtained and also, because the coating-insoluble polymer is an insulator (inhibiting deposition), a uniform coating is achieved.
3.7 Advanced materials These are materials that can be used under extreme conditions such as high temperatures, greater than 2001C, or severe chemical environments or permit a unique application, e.g., highly branched polyethylene oxide does not crystallise; it serves as an electrolyte for lithium ion batteries, with no crystalline regions inhibiting the transport of ions. Many also exhibit properties not normally commonly associated with polymers, e.g., conducting electricity or semiconducting or displaying piezoelectricity. Some can also show (so-called) ‘‘smart’’ behaviour, whereby they can counteract any outside influence or stress.
3.8 High performance polymers These are normally prepared by a step growth mechanism. They can be thermoplastic or thermoset and are often used in composites. Composites are materials that contain two or more phases, one of which acts as a reinforcement
Polymeric Materials: Composition, Uses and Applications
85
for the other, e.g., a binder polymer is combined with fibres made, for example, from glass, carbon, boron or aramid, to generate a tough, stiff, yet mouldable material. Thermosetting polymers are brittle because of the crosslinking that occurs during polymerisation. However, the precursors are liquid and in this state can easily wet and impregnate long fibres masses, mats and cloth. High performance thermoplastic polymers are tough but can deform under pressure and heat. They are often processed via extrusion or injection moulding with short fibres (1–5 mm). Care must be taken to promote random orientation thereby giving isotropic strength (similar to polymer crystallisation of drawn fibres and films). New techniques have been developed, e.g., pultrusion, whereby a continuous web of fibres is passed through a bath of molten polymer, which can be a thermoplastic, e.g., PEEK, or a semi-cured thermoset, e.g., epoxy or unsaturated polyester. The resulting coated fibre is called a prepreg and can be used to wrap around a mould and finally cured at a higher temperature or by radiation. High performance plastics have high temperature stability. They have a very high limiting oxygen index (LOI, sometimes called critical oxygen index (COI)), which is increased when reinforced with glass fibres. Because of the high aromatic content they tend to char on combustion and do not release flammable volatiles. Furthermore, radical decomposition is inhibited by the delocalised aromatic system, which stabilises the propagating radical intermediates. Polymers containing heterocyclic or di-substituted aromatic groups, often called ladder polymers, have outstanding thermal stability (over 5001C for short periods). The ladder structure allows individual bonds to fracture without breaking the chain. They have high strength to weight ratio (comparable with metals), high modulus and are tough and not brittle at ambient temperatures even when they are amorphous. Examples of high temperature polymers are: Poly(ether imides), O
O
C
C
CH3 C
N
N C
C
O
CH3
O
O
n
Ultem® O
O
C
C O
N
N C
C
O
O
n Kapton® Vespel®
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J.P. Candlin
Poly(aryl ether ketones),
O
O
C O
O
C O
n Victrex®
PEK
n
PEEK
Poly(aryl ether sulfones), O O
S n
O PES
O S
CH3 C
O
O
CH3
O n
®
Udel
Aromatic polyamides (aramids),
HN
NH.CO
NH.CO
CO n
HN
CO n
Kevlar®
Nomex®
Aromatic polyesters, O O O
C
C O n
Vectran®
Kevlars has the highest tensile strength and is often used as a reinforcing fibre in composites with, e.g., epoxy, PEEK. The thermotropic liquid crystal polymer Vectrans is made by melt polymerisation of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid, (the corresponding hydroxy acids decompose on melting). Because of its liquid crystal properties the polymer can be spun into fibres from the melt. Kevlars is spun from a solution in concentrated sulfuric acid, and can be melt drawn to give a high modulus (stiff) polymer. Vectrans
Polymeric Materials: Composition, Uses and Applications
87
fibres have high tenacities (strength), comparable with Kevlars and has service temperatures up to 3001C. It has many applications, e.g., cut-resistant gloves and clothing, catheters, reinforcement of ropes and yarns. Although Nomexs fibres have tensile strength properties similar to apparel fibres, they also have very high service temperatures (up to 3701C) and high flame resistance. Nomexs is self-extinguishing when removed from a flame because of the formation of char resulting in low smoke generation. It is extensively used in protective clothing and materials, e.g., by fire-fighters, for aircraft fabric, and by racing-car drivers. Nomexs is prepared from the corresponding di-acid chloride/diamine and wet spun from a solution of dimethylacetamide/5% lithium chloride.
4. POLYMER ADDITIVES This is an extremely important area. Commercial polymers are very rarely prepared or used without additives and they often contain a combination of additives. These improve processability of the polymer, its durability, service life under adverse conditions (e.g., temperature, UV light and various chemical environments), strength (e.g., using reinforcing fillers), appearance (colorants), etc. A recent use of additives is to facilitate reprocessing or recycling or, alternatively, to promote decomposition (which is often difficult because the stability is inherent in the polymer’s chemical structure). (The analysis of additives in polymers is the subject of Chapter 14.) Polymer additives may be classified into five groups (although some additives can serve more than one purpose, e.g., plasticisers, carbon black): (i) (ii)
(iii)
(iv)
(v)
Additives that facilitate or control polymer processing, e.g., lubricants, mould-release agents, blowing agents and plasticisers. Chemical property modifiers. These enhance stabilisation against degradation and aging during processing or in use. Degradation usually involves chain cleavage and is usually caused by energy (e.g., shear forces during processing, heat, UV light) or by chemical attack (e.g., oxidation, hydrolysis). Thus many polymers require anti-oxidants, light or heat stabilisers, etc. The promotion of new desirable properties for the polymer such as improved mechanical or electrical properties or dimensional stability. Fillers enhance many of these properties. Aesthetic property enhancers. These additives (e.g., colorants) will improve the polymer’s appearance and make it suitable for various applications. Degradation additives. These additives are usually added for environmental purposes in order to promote the decomposition of polymers after use.
The total world demand for additives is ca. 2 million tonnes per annum (excluding fillers and colorants — with fillers this becomes 20 million tonnes per
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annum). Their concentration can vary from a few ppm to W50% for flame retardants and fillers. The average is ca. 20 wt% of all polymers consumed.
Fillers Plasticisers Reinforcing agents Flame retardants Colorants
Consumption (%) (wt additive/wt polymer) 58 16 (almost solely with PVC) 10 6 5
The additives can be added during the polymerisation (either in the form of a reactive monomer or as an inert material — advantages include better dispersion and increased retention), but they are usually added immediately postpolymerisation of the polymer and extruded with the polymer. Often a concentrated mix with the plastic (master-batch) is prepared by the plastic manufacturer and this can be sold to the fabricators to be added during subsequent processing. When using additives the following precautions should be taken: (i)
(ii)
(iii)
The additives should not exude to the surface of the plastic article (there are some exceptions when surface modifications are required) or lost by volatilisation or decomposition during processing. Defining the end-use of the plastic. Strict controls are enforced when plastics have contact during food preparation or packaging. This is a difficult area because often the plastic manufacturer may not be aware of the final application. The fabricator must therefore ensure that the plastic article containing the additive is in compliance with the regulatory laws of the country where the article is sold. Another difficulty is the toxicity of the additive, e.g., carcinogenic or poisonous (e.g., lead or cadmium pigments) and asbestos fillers. Many additives (especially for plastics with food contact use) require approval from regulatory authorities. This costs money and time and therefore the general pattern is to use proven additives whenever possible. Interaction between the additives. Conversely some additives may serve two purposes (e.g., UV and heat stabilisation; lubricants cannot only aid processing but can also promote mould release).
It is difficult to generalise additives in that they may work with a particular polymer but not with others because of chemical compatibility, processing temperature and conditions.
4.1 Polymer-processing modifiers 4.1.1 Heat stabilisers Some polymers degrade readily when heated to the processing temperature by bond scission. In contrast, if the polymer has a completely aromatic structure (e.g., PES, PEEK) the thermal stability is high (greater than 4001C for short
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periods). Thus, when PVC is heated above 1001C an autocatalytic decomposition occurs and at 1801C it becomes dark brown within a few minutes. Thermal degradation of polymers can occur by three mechanisms (see Chapters 11 and 12): (i)
Decomposition by loss of a side group (probably by a free radical mechanism, possibly activated by radical initiators), CH2
CH
CH
+ HX
CH
X
O where X =
Cl(PVC),
O
C
CH3 (poly(vinyl acetate)).
The Lewis acid, HX, promotes further decomposition eventually giving a conjugated polyene (W6 conjugated bonds are coloured). The mechanism is possibly, H CH2 HC Cl
H C
CH2
CH
CH
CH
Cl
Cl
Cl
(-H and -Cl (=HCl)) H
H
HC
CH
Cl
Cl
C H
H C
HC Cl
CH2
+ MZCl
CH
C
CH
Z
Cl
Cl
stable
-HCl
H C
H C
CH
Cl
H C
H
CH2 C H
allylic Cl (labile)
HC
MZ2
H C
C
CH2 C H
CH Cl
Cl allylic Cl (labile) etc.
Thermal stabilisers for PVC are often heavy metal stearates (lead, barium), tin thiolates (R2Sn(SR2) = MZ2), and work by reacting the allylic
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(ii)
J.P. Candlin
chloride (which is labile) to form stable compounds. Other stabilisers for PVC are metal salts of maleic acid, which possibly work by a Diels-Alder reaction with the intermediate unsaturated conjugated system. If the PVC is used for food applications (e.g., squash (fruit juice) bottles, chocolate dividers in chocolate boxes) calcium mercaptides may be used. Depolymerisation (un-zipping). The main chain is broken and radicals are formed, which can then undergo intra-molecular rearrangement, e.g., H2 C
H2 C
H2 C
H2 C
H2 C
CH
CH
CH
CH
X
X
X
X
+
CH2
CH
CH
X
X
breakage
H2 C CH
+
X
H2C CH X (monomer)
etc. (leading to depolymerisation)
(iii)
This depolymerisation is inherent in the polymer structure and can be prevented by either making a copolymer (such that when un-zipping reaches the co-monomer moiety it is stopped from going any further (e.g., POM (polyoxymethylene) in which a few percent of ethylene oxide has been incorporated), or by using free radical traps (see anti-oxidants). When depolymerisation occurs, high percentage yields of monomers may be obtained, e.g., POM (100%), PMMA (95%), PS (40%), natural rubber (20%). This information can be utilised in recycling of defect materials by conversion back to the monomer. If the homopolymer decomposes at the fabrication temperature another approach is to make a copolymer that can be melt processed at a lower temperature. For example, polyhydroxybutyrate decomposes at the processing temperature (1901C), whereas the copolymer with valeric acid can be processed at B1601C without decomposition. These aliphatic polyesters are biodegradable and most importantly, the decomposition products are not toxic, hence their use in medical applications (e.g., sutures).
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Polymeric Materials: Composition, Uses and Applications
(iv)
Random chain scission. The initial attack is the same, but an intermolecular reaction occurs. H2 C
H2 C CH X
CH X
formation by bond breakage -see above
+
H2 H2 C C CH H C X
X
H2 C
H2 C CH X
H2 C
H2 C +
CH2 X
C
CH
X
X
+
HC
undamaged polymer chain
H2 C
CH2 C X
X (which can attack undamaged polymer)
When this decomposition occurs the degradation does not give high monomer yields (compared with un-zipping); the products tend to be low MW products. The chain scission and propagation is inhibited by free radical traps (see anti-oxidants). Sometimes heat de-stabilisation is actively sought, e.g., (i)
(ii) (iii)
Mastication. If the MW of the polymer is too high resulting in a high melt viscosity (e.g., natural rubber) or the MWD is too broad giving unstable melt processing conditions (e.g., PP) then mastication using high mechanical shear machinery, e.g., twin-screw extruders, results in a melt processible material. Free radical initiators are sometimes added to accelerate the process. This works by preferentially cleaving the longest chains, thus lowering and narrowing the MWD. Manufacture of carbon fibres, e.g., PAN fibres can be controllably decomposed in an inert atmosphere to give high carbon content fibres. Ablative materials. These polymers decompose to give off volatile materials and a surface char that acts as an insulator (e.g., paints, spacecraft re-entry vehicles). Glass-coated phenolic resins are used in the latter application.
4.1.2 Lubricants These additives reduce the friction between the polymer aggregates or crystallites and also lower the adhesion between the polymer melt and metal surfaces (which occurs in rollers, extruders). This results in: (i)
Lower melt viscosity, thus reducing energy consumption required for processing.
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(ii) (iii)
J.P. Candlin
Improves the dispersion of fillers and pigments. Reduction of wear of metal surfaces, hence improving polymer gloss, smoothness and surface regularity.
Lubricants can be classified as internal or external lubricants. Internal lubricants should be partially miscible with the polymer at processing temperatures (i.e., behave similar to a plasticiser), but phase separate at ordinary temperatures. Whereas plasticisers are completely miscible with the bulk polymer, lubricants have a limited solubility. Examples of internal lubricants are fatty acid derivatives, e.g., stearates (for PVC), waxes and polyethylene oligomers containing polar groups obtained by partial oxidation. Not all polymers require lubricants; LDPE, nylon and PET are self-lubricating. External lubricants have a lower affinity for polymers and exude to the surface (causing blooming) and act by reducing the adhesion between the polymer and the metal surface during fabrication. Excessive lubrication must be avoided because during extrusion some friction between the screw flights and the polymer is necessary for transporting the polymer through the die. Too much lubrication also causes surface haze. Examples of external lubricants are metal soaps, paraffin waxes, silicones and fluoroplastics.
4.1.3 Slip and anti-slip agents Slip and anti-slip agents are added, especially in film production, to either promote or inhibit slip or sliding of film surfaces, respectively. Their function is to control the surface properties such that there is a balance between adhesion or tackiness of film layers and, on the other hand, the slippage of films. Thus slip agents are added during fabrication of plastics that are in contact, for example, with gears and bearings, whereas, anti-slip agents (adhesion promoters), for example, are incorporated to stop the sliding of plastic rolls or stacked bags.
4.1.4 Anti-blocking agents Because of the high quality of surface smoothness of films, wind-up onto rollers causes adjacent film layers to adhere together, either by static electricity or coldflow (creep). This is overcome by adding finely divided fillers (less than 1 mm diameter) of, for example, chalk, silica, crosslinked polyacrylates, which roughen the surface allowing entrapped air to keep the successive layers apart.
4.1.5 Mould-release agents These materials prevent adhesion between chilled rollers in film production or moulds for injection moulding. They behave like external lubricants, and should serve the following requirements: (i) (ii) (iii)
Form a smooth continuous film on the surface. Good chemical resistance and oxidative stability. No detrimental effect on the polymer surface, e.g., blistering, peeling, streaking, discoloration, poor printability, stress cracking.
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93
For injection-moulded articles, although spraying the mould with a release agent is feasible (this is often done for one-off samples) it has many disadvantages, for example, being time consuming or the mould requires cleaning after use to remove residue build-up. For this reason, external lubricants are used. Permanent release metal surfaces can be achieved by treatment with a solution of fluorocarbon oligomers followed by evaporation of the solvent and curing by heat.
4.1.6 Antistatic agents Plastic surfaces can become electrostatically charged through friction, either during processing or post-fabrication handling. This generates several disadvantages: (i)
(ii)
(iii) (iv)
Attraction of dust, which leads to unsightly appearance or malfunction, in applications such as plastic furniture, recording tapes, CDs and DVDs, and packaging. Electrostatic shock from apparel and carpet fibres and plastic floor coverings. This can constitute a fire hazard when handling flammable solvents or fine powders. An uneven feed problem in the hopper (often called bridging) when processing polymer powders. Failure of sensitive electronic equipments, e.g., computers, televisions.
Polymers are inherent insulators and have a high surface and bulk electrical resistance. Reduction of the surface and bulk resistivity is achieved by: (i)
(ii)
(iii)
Surface-active antistatic agents. These are hygroscopic materials and, under humid conditions, form a thin conducting layer of water. They are applied by spraying or dipping and are classified as external antistatic agents. They are not resistant to contact with solvents and can be removed by rubbing. Examples include: cationic (quaternary ammonium, sulfonium or phosphonium salts), anionic (long-chain, sulfonic, phosphonic salts) or non-ionic (ethoxylated fatty acids, sorbitol fatty esters). Internal antistatic agents are added to the bulk polymer before fabrication. They have a low affinity for the parent polymer and migrate to the surface. They offer long-term protection against electrostatic charges. They can be added as a master-batch or concentrate (similar to many additives), which can be diluted as required. Contact food approval has been granted for many of these additives with the exception of some anionic compounds. Conductivity additives (or fillers). These compounds reduce the resistivity of the bulk and the surface of the polymer. Below 10–20% concentration, the resistivity is not affected until a threshold is reached and conduction is observed. Examples include: carbon black, metal powders or fibres (copper, aluminium and silver) and some conducting conjugated polymers. Increasing the concentration of these additives yields conducting polymers that are used for shielding against electromagnetic interference (EMI).
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Conversely, cling film (plasticised PVC/PVDC, (poly(vinyl chloride)/poly (vinylidene chloride), copolymer, which has very high gas-barrier properties) on peeling from a roll generates static electricity thus promoting adhesion to a surface, e.g., ceramics, but not metallic surfaces which conduct the static electricity away.
4.1.7 Plasticisers Additives that soften and make flexible, hard brittle polymers are called plasticisers. They also lower the processing temperature by decreasing the polymer melt viscosity. This allows conversion of polymers, which may decompose at the processing temperature. They achieve this through lowering the Tg by providing internal lubrication between the polymer chains. However, plasticisation does reduce the heat deflection or distortion temperature (HDT) of the polymer thus limiting their usage temperature. Plasticisers are compatible and miscible with their host polymer causing them to swell and allowing them to be used for coating, moulding, spraying (when used as a suspension in a liquid solvent) or calendering, extrusion and injection moulding (when used as a polymer melt). More than 90% of all plasticisers are used with PVC, the remainder being used with PVDC, cellulose diacetate, poly(vinyl acetate) (PVAC), nylons, urethanes and acrylates. There are several theories for the action of plasticisers: (i) (ii)
(iii)
Decreasing inter-polymer chain attraction in the amorphous region of the polymer thus lowering the Tg. Gels are present in polymer melts. These are formed by high molecular weight physical (chain entanglements) or chemical crosslinks. These aggregates are swollen by plasticisers within the gel allowing movement of the polymer chain. Free volume is the internal space in the bulk polymer not occupied by the polymer chain. Thus deformation and flexibility can be visualised as polymer chains hopping from one engaged volume to a free volume. Plasticisers have a large free volume and this permits more migration of the polymer chains.
The most common plasticisers for PVC are phthalate esters prepared from aliphatic C8 alcohols. For high temperature applications higher molecular weight esters are used, e.g., from mellitic acid, which are resistant to volatilisation and are used for PVC interiors in cars (to prevent fogging of windscreens). Aromatic phosphate esters impart flame retardency (although PVC has good flame retardency). When premium-grade PVC products are required, for example, for extraction and migration resistance, high wearability and low temperature flexibility, polymeric esters made from di-functional acids/alcohols are used as plasticisers, for example, adipic acid/propylene glycol, polycaprolactone and ethylene/vinyl acetate/carbon monoxide terpolymers. With suspension polymerised PVC, the polymer particles are 100–500 mm diameter. They are highly porous and the dry powder will absorb large amounts
Polymeric Materials: Composition, Uses and Applications
95
Figure 2 Two methods for applying surface coating of a plastisol onto an uneven surface. Reproduced with permission from D.H. Morton-Jones, Polymer Processing, Chapman & Hall, London, 1989.
of plasticiser thus allowing PVC to be processed by standard techniques, e.g., extrusion. In contrast, with emulsion polymerised PVC the small polymer particles (0.1 mm diameter) are essentially non-porous and when mixed with a plasticiser a mobile mixture called a plastisol (or paint) is produced. On heating to about 1601C, the plasticiser becomes absorbed into the PVC to give a flexible polymer. Plastisols are frequently used for coating materials (e.g., fabric (leather, floor covering), paper (wall-paper), car-body underseal). Two methods of plastisol coating are shown in Figure 2 depending on whether a flat surface (using a knife-on-roller method) or even-thickness coating (reverse roll coating) is required. The use of plasticisers, with other than PVC applications, is extensive. Many polar rubber sealants or caulking materials are plasticised in order to make them more pliable, e.g., polysulfides, polychloroprene (Neoprenes), nitrile rubber. Esters, similar to those employed with PVC, are used to render cellulose diacetate (‘‘Acetate’’) overhead projection sheets more flexible.
4.1.8 Blowing or foaming agents Blowing and foaming agents are solids, liquids or gases (the latter used under pressure) such that when heat and pressure are applied under processing conditions they generate a gas thus producing a foam or cellular plastic. Careful control of process variables (temperature, time and pressure in the moulds) and the nature of the blowing agent is necessary. The amount of blowing agent can vary from less than 0.1 wt% to 5–15 wt%. The former small concentration is added to counteract depressions in thick sections, which occurs in injectionmoulded articles because of the large volume shrinkages of polymers on solidification. The higher concentrations yield lightweight packaging materials, sponge rubber articles, etc. Foamed polymeric materials can have either a closed or open cellular structure. Closed cells tend to be favoured when the pressure is maintained during the expansion. Both types are used for different applications, e.g., closed for thermal and sound insulation, open cell for soaking up liquids by capillary
96
J.P. Candlin
action. Weight for weight, foamed plastics are stiffer than non-foamed plastics. Although most foamed plastics are fabricated using injection moulding, expanded polystyrene articles are compression moulded using pre-formed expanded beads. Foams can be rigid, semi-rigid or flexible. This depends on the nature of the final polymer. Thermoset polymers yield rigid foams, whereas elastomers give flexible materials. The size of cells varies from 50 to 100 mm and depends on the polymer and its end use. Cells of uniform size are a necessity. Small, uniform cells are formed by addition of nucleating agents. These are finely divided solids, e.g., silicates, and are particularly useful with physical blowing agents. There are two forms of blowing agents (physical or chemical) and these must satisfy the following requirements: (i) (ii) (iii) (iv) (v) (vi)
Long-term storage stability. Gas release in a controlled temperature, time and pressure range. Low toxicity, odour and colour of both the blowing agent and its decomposition products. Does not affect the stability and processing conditions of the polymer. Produces cells of uniform size. The gas must not diffuse out of the polymer matrix during fabrication too quickly, causing the foam to collapse.
4.1.8.1 Physical blowing agents. These are usually compressed gases (e.g., CO2, N2). These gases, which dissolve in the molten polymer under pressure, are released as bubbles when the pressure is reduced. Nucleating agents are frequently used. Low boiling liquids, e.g., pentane, perform the same function. Originally low temperature boiling chlorofluorocarbons (CFCs), for example CFC 11 and CFC 12, were used for closed cell materials. These have low thermal conductivity when compared with air, CO2 and similar gases, and hence made excellent foamed thermal insulators. CFCs have now been withdrawn because they are responsible for stratospheric ozone depletion. 4.1.8.2 Chemical blowing agents (CBAs). These are solids that decompose to give a gas at the processing temperature. In addition to the demands above, the CBA must possess the following: (i)
(ii) (iii)
The decomposition temperature must be matched with the processing temperature of the polymer (which can be in the range 150–3501C). The decomposition must be rapid (less than one minute) and within a narrow temperature range. The CBA should give a high yield of gas (B200 ml/gm). The blowing agent should have a narrow particle size range (to give cells of uniform size) and be easily dispersed in the polymer.
Very few CBAs have been developed for use with polymers for food contact applications. The most commonly used CBA, which has approval, is powdered wax-coated sodium bicarbonate/citric acid, which decomposes at 160–2101C
Polymeric Materials: Composition, Uses and Applications
97
giving off CO2. The azo compound 1,1-azobisformamide (ABFA), which decomposes giving N2 at B2201C, has also approved food contact use and is employed with PVC plastisol, LDPE, HDPE, PP, EVA and PS. For higher melting polymers, for example, PC, nylon, PET, which require temperatures greater than 3001C for fabrication, blowing agents which decompose at this higher temperature are used, e.g., substituted tetrazoles which give off nitrogen gas. The surface finish of injection-moulded foamed articles are not ideal because of external defects, burst bubbles, etc. This can be avoided by sandwich moulding. This is accomplished by initially injecting a slug of polymer melt into the mould followed immediately by a polymer containing a blowing agent, which chases the initial polymer into all the interstices. Embossed wallpapers (‘‘blown vinyl’’) are made by continuously coating a roll of paper with a liquid PVC plastisol containing a blowing agent. This is then passed through a heating oven causing the plastisol to expand. This is immediately followed by an embossing roller that develops a repeating relief pattern. Cushionflors vinyl floor coverings uses a different technique. This uses the plastisol and a CBA containing a ‘kicker’ or accelerator, e.g., ABFA and ZnO. This mixture decomposes at B1901C compared with B2201C without the kicker. When an ink pattern is printed on the plastisol surface it contains an inhibitor (e.g., fumaric acid and trimellitic anhydride), which complexes the kicker ZnO and prevents it from accelerating the decomposition of the CBA. A final layer of plastisol without the CBA is spread onto the surface, see Figure 3. On heating to B1901C the non-inked areas expand, whereas the inked areas (containing the inhibitor) do not swell. Familiar patterns (tiles, floral) with a relief are produced. A different method of making foams is to generate the blowing agent at the same time as the polymer is being formed. Thus urethane polymers are prepared
cross-section profile
Backing
Coated with plastisol + CBA + kicker (ZnO)
Print; ink, contains inhibitor (fumaric acid)
Coated with transparent wear layer (plastisol without CBA)
Heat to 190°C. Foam produced only in the absence of ink
Figure 3 Cushionflors techniques for obtaining printed/relief PVC plastisol floor covering. Adapted with permission from D.H. Morton-Jones, Polymer Processing, Chapman & Hall, London, 1989.
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J.P. Candlin
by reaction of di-isocyantes and diols in the presence of catalysts (tertiary amines, tin compounds),
OCN
R
NCO + OH
R′
OH
CONH
R
NHCOO
R′
O n
polyurethane
where the hydrocarbon groups, R and Ru, determine the nature of the polymer. If water is present in the diol, or adding water to the low molecular weight prepolymer (which contains isocyanate end groups), reaction with di-isocyanates generates CO2, R
NCO + H2O
RNH2 + CO2
This acts as a blowing agent. If this is the sole source of blowing agent then, again, dependent on the groups R and Ru, flexible foams are generally made by this route. Rigid foams are often made by the addition of a physical blowing agent, e.g., volatile liquids.
4.2 Chemical property modifiers 4.2.1 Anti-oxidants These prevent or retard the autoxidation by atmospheric oxygen of polymers, which often leads to discoloration, embrittlement, cracking, reduction of gloss, and a loss of mechanical properties. They are used in the manufacture of plastics, films, fibres, elastomers, paints, adhesives, etc. The actual mechanism of deterioration varies with the parent polymer, but they all involve radical pathways. The initiation can come from catalyst residues (metallic and nonmetallic), contamination from reactor vessel, UV light or processing equipment or from heat and mechanical processing treatment of the finished polymer. Many of the disintegration pathways involving radicals are common to both autoxidation and decomposition by UV light (see Chapters 11 and 12). The anti-oxidant itself is often consumed in performing its function, although hindered amine light stabilisers (HALS) are self-regenerating. Depending on the polymer, their concentration in the polymer may vary from a few parts per million (e.g., in PE) to several per cent (e.g., in ABS, natural rubber). Because it is impossible to determine all the polymer down-stream end uses, nearly all have food contact approval. To fulfil their role, anti-oxidants must have the following properties: (i) (ii) (iii)
They must be thermally stable and non-volatile at processing temperatures. Be miscible in the polymer. Not leach out of the polymer when in contact with solvents.
Polymeric Materials: Composition, Uses and Applications
(iv) (v)
99
Not corrode the processing equipment with acidic decomposition products. Must be odourless, tasteless and non-coloured (including its degradation products).
Oxidative degradation of polymers is initiated by radicals (Rd) generated in the polymer by heat or mechanical shear during processing or by exposure to UV light. These radicals, in turn, react with O2 to form peroxy and hydroperoxide radicals that promote radical reactions. energy RH
R +H O2 (peroxy radical)
ROO RH
ROOH (hydroperoxide)
ROO + RO + H2O
RO + OH
ROOR where RH = polymer
Thus, if formation of the initial polymer radical (Rd) can be prevented or, if formed, can be rendered harmless, then the stability of the polymer will be enhanced. Polymers that are easily capable of forming radicals are those containing tertiary C–H and allylic C–H groups, e.g., H2 C
H2 C
energy
CH
C CH3
CH3 polypropylene back-bone
H2 C
attack on tertiary group
H C C H
H2 C
H2 C
energy C H2
poly 1,4-butadiene back-bone
H C
H C C H
attack on allylic group
C H2
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J.P. Candlin
Polymers containing no secondary or tertiary C–H or are aromatic (PEEK or Kevlars) or perfluorpolymers (PTFE) are generally resistant to oxidation and require no anti-oxidant additives. However, the high temperature necessary for curing or fabrication of high performance polymers (especially for aerospace applications) can cause oxidation. For this reason, these materials are often prepared and fabricated under a nitrogen atmosphere. Anti-oxidants can be divided into two classes depending on which part of the radical chain they quench. Primary anti-oxidants are radical scavengers and will react with alkyl chain radicals (Rd) or hydroperoxides (ROOH). Secondary antioxidants work in combination with primary anti-oxidants and principally act by converting peroxide radicals (ROOd) into non-radical stable products. Synergism often works when both classes are used together.
4.2.1.1 Primary anti-oxidants. These are sterically hindered phenols and amines (HALS) and aromatic secondary amines. They intercept the radicals as they are formed: O
OH (H3C)3C
(H3C)3C
C(CH3)3
C(CH3)3
R + RH
CH3
CH3 2,6-di-t-butyl-p-cresol (butylated hydroxy toluene, BHT)
The resulting phenoxy radicals are stabilised by: (i) (ii)
Steric hindrance by the bulky t-butyl groups and therefore cannot attack new polymer chains. Resonance to give canonical forms that can dimerise and generate a new anti-oxidant dimer.
(H3C)3C
(H3C)3C
(H3C)3C
C(CH3)3
dimerise O (H3C)3C
CH3
HO (H3C)3C
CH2
HO (H3C)3C
CH2CH2
OH C(CH3)3
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101
Formation of a quinoid structure.
(H3C)3C
(H3C)3C
O
CH3
O CH3
(H3C)3C
(H3C)3C R
(H3C)3C R O CH3 (H3C)3C
which can react with further radicals. However, quinoid structures do impart a yellow colour to the polymer. A major problem with BHT is that at elevated temperatures volatilisation may occur. Higher molecular weight hindered phenols can be used. Aromatic amine radical scavengers that are derived from 1,4 di-aminobenzene or diphenylamine are also used.
RHN
NHR
R = alkyl group
CH3 Ph
C CH3
CH3 NH
C
Ph
CH3
These act similarly to the phenol anti-oxidants; the N–H group in the amine reacts with the polymer free radical Rd to give a stabilised nitrogen radical and RH. Amine anti-oxidants are more effective than hindered phenols. Thus they are used with unsaturated elastomers and rubbers, which readily form radicals.
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Similar to phenols, they can cause staining and are often used in conjunction with carbon black filled elastomers (e.g., tyres) — although carbon black itself has antioxidant capacity. For non-staining applications, e.g., polypropylene carpets, a sterically hindered amine is used, e.g., H H3C
CH3
N
H3C
CH3
2,2,6,6-tetramethyl piperidene
These compounds also inhibit photo-oxidation and are known as HALS. The mechanism is thought to be (R is the polymer group):
H
R
O
N
OR
N
N
O2
ROO
R = O + ROH
A high level of stabilisation is achieved at low concentrations. This is because of a cyclic process whereby the HALS are regenerated rather than consumed during the stabilisation process. This also promotes long-term protection. To avoid leaching of the anti-oxidant when in contact with oils (vegetable and mineral), petrol and solvents, polar amine compounds are used: O NH
NH
S
CH3
O
Alternatively, an amine monomer that can be incorporated into the polymer chain during polymerisation can be used, e.g., CH3 NH
NH
methacrylamide derivative
CO
C
CH2
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103
4.2.1.2 Secondary anti-oxidants. These compounds react with the hydroperoxide and peroxide intermediates. They are usually di-valent sulfur or tri-valent phosphorous compounds:
ROCOCH2CH2 2S
H3C
O
P 3
They function by reducing the intermediates to the corresponding alcohols: R′2S + ROOH
R′2SO + ROH
R′3P + ROOH
R′3PO + ROH
Synergy between primary and secondary anti-oxidants occurs and often a mixture is employed. Also included are metal complexing agents, e.g., EDTA (ethylenediaminetetraacetic acid), citric acid, the purpose of which is to deactivate extraneous metal ions that catalyse polymer oxidation.
4.2.2 Anti-ozonants Although only 10–50 ppb ozone is present in the atmosphere at ground level, higher concentrations occur near photocopying machines; this is sufficient to degrade polymers, especially those with unsaturation in the polymer chain, e.g., elastomers. O
O
O
O
O
O
CH
CH
CH +
CH
O3 CH
CH
polymer chain
ozonide
bond breakage
This can cause surface cracks perpendicular to the direction of stress; stretched elastomers are more susceptible to this cracking. Ozone degradation is a surface phenomenon and hence a physical antiozonant must form a protective barrier. A chemical anti-ozonant can be added during polymer fabrication, but again it must diffuse to the surface. It must also provide a defence over the lifetime of the article. Anti-ozonants can function in different ways: (i)
By diffusing to the surface and forming a physical layer between the elastomer and ozone. Paraffin waxes (CnH2n+2 where n = 20–50) behave in
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(ii)
(iii)
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this manner — there is no chemical reaction between the wax and ozone. Care must be taken when the article is exposed to solvents (e.g., petrol), which may dissolve the protection. Reaction with ozone directly. This reaction must be more rapid than the reaction of ozone with the polymer. Examples include aromatic amines, e.g., 1,4-diaminobenzene derivatives. Although not illustrated in the reaction scheme above, the down-stream reaction pathways involve radicals. Therefore, anti-oxidants that scavenge these radicals are also added.
4.2.3 Ultra-violet stabilisers UV stabilisers protect polymers by restricting UV penetration to the surface and therefore confine the damage to surface layers. Protection is important because the energy possessed by UV radiation is sufficient to break chemical bonds. The initial breakage can either be by a radical (Norrish type I) or non-radical (Norrish type II) pathway. The effects are similar to degradation of the polymer by oxidation routes; the radical intermediates can be neutralised by anti-oxidants. Many polymers contain chromophores, which may be a result of their chemical structure or generated during their preparation or fabrication, arising from monomer, solvent, trace metal and catalyst residues. These chromophores will absorb UV light and be excited to a high-energy state. Examples include olefinic, aromatic and ketonic groups. This energy can be lost by radiation, e.g., fluorescence or phosphorescence, or heat or energy transfer to neighbouring chromophores. If none of these pathways are available, photodegradation takes place with bond cleavage and the possible formation of radicals. These radicals react with oxygen to form perxoy radicals and hydroperoxy compounds. Photoexcitation can also act like a sensitiser and convert relatively non-reactive ground-state triplet oxygen into highly reactive singlet oxygen, which again will promote autoxidation of the polymer. Polymers containing keto groups are particularly susceptible to photodecomposition (indeed they are often added to stable polymers to make them photo-degradable). Norrish type I (radical): O CH2
CH2
C CH2
O
UV CH2
CH2
C
CH2
+ CH2
radical decomposition
CH2 + C O
CH2
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Norrish type II (non-radical): O CH2
C
O
H
UV
CH2
CH
H2C
CH2
non-radical decomposition
H +
C CH2
HC
H2C
O CH2
H
C C H2 O
CH2
C CH3
Several techniques can be used to stop UV degradation: (i) (ii) (iii)
Screens or coatings. These block out UV radiation, e.g., pigments (TiO2), carbon black. Radical scavengers, which intercept the generated radical pathways — similar to anti-oxidants. Often HALS are used. Energy quenchers. These absorb energy from the photo-excited state of reactive singlet oxygen and dissipate it as heat. They are often metal chelates and their action is:
UV chromophore
excited chromophore
triplet O2 quencher
singlet O2 (reactive)
quencher chromophore + excited quencher
quencher + heat
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Examples of energy quenchers are:
octyl
O S
S NH2R
Ni
M
C
RN2
S O
octyl
2
where M = Ni or Co
(iv)
These compounds are multifunctional additives. They can act as heat stabilisers, radical traps, decompose hydroperoxides, UV absorbers, etc. UV absorbers. This is the largest class of UV stabilisers. They work on the same principle as sun-screen lotions; they contain chromophores that can absorb light in the 280–400 nm region and release the excess energy as heat and not high-energy radiation. They must be stable under processing conditions and should not react with the polymer nor decompose with UV radiation.
Because it is the UV-B radiation (280–320 nm) that causes the degradation, the absorption spectra of the UV-absorber must coincide with these wavelengths. UV-A (320–400 nm) does not cause damage (it is not energetic enough) and UV-C (wavelength less than 280 nm) does not reach the troposphere (it is filtered out by ozone in the stratosphere). The problem is to find an additive that absorbs UV-B but does not have an absorption ‘‘tail’’ in the UV-A and visible wavelengths, and therefore would have a yellow appearance. Many UV absorbers are based upon either 2-hydroxybenzophenone or 2hydroxytriazole derivatives, (the corresponding 3-hydroxy-compounds are not effective). These work via the following mechanisms: H
H O
O
O UV
C
energy absorption
O
C
release of energy (as heat) H
O
UV energy absorption
N
H N
N N
O
N heat release
N
To increase the longevity, UV absorbers with reactive groups which can be incorporated into the polymer chain or high molecular weight substituents are
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used. Because many of the decomposition pathways involve radicals, a combination of anti-oxidants and UV stabilisers are often employed.
4.2.4 Flame retardants Many polymers are inherently flammable. Furthermore, they often melt, volatilise and emit flammable and toxic vapours. Legislation now dictates the many polymers used in construction, fibres, furniture, advertising signs in shopping malls, must possess combustion resistance. The combustion cycle may be represented as shown in Figure 4. The initial step is the thermal degradation of the polymer giving rise to volatile products. At this stage there is probably no involvement of oxygen. These volatile products, e.g., monomers by depolymerisation, react with oxygen to give heat and flames, which does not involve radicals. Heat liberated by the flames causes further degradation of the polymer, which, together with the initial source of ignition, promotes further combustion, etc. Interference with any of these steps will stop or delay total combustion. These include: (i) (ii) (iii) (iv)
Use thermally stable polymers. Promote char formation, (many of the advanced materials have high char yields). Use radical scavengers. Use endothermic reactions to counterbalance exothermic combustion.
Many of these approaches have been employed to contribute to fire or flame retardency: (i)
Use polymers with naturally low flammability. The threshold of polymers can be qualitatively estimated by determining the LOI of the polymer. This gaseous products
flame gas phase oxygen
volatile products
heat
heat from ignition source
thermal degradation
solid polymer char residue
phase boundary
heat
thermal oxidation
Figure 4 Combustion pathways illustrating the burning of polymers.
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uses polymer samples of a standard size, and is measured by recording the volume percentage of O2 in an O2/N2 mixture that will sustain flaming combustion for 3 min after the source of the ignition has been removed. Some examples of LOI are: Polymer
LOI
Polyoxymethylene Polyethylene and polypropylene Poly(methyl methacrylate) Polystyrene Cellulose Poly(ethylene terephthalate) Nylon 6 and nylon 6,6 Polycarbonate Poly(phenylene oxide) Aramid (Kevlars) PES and PEEK PVC (unplasticised) PVDC (unplasticised) Carbon (char) PTFE
15.3 17.3–17.6 17.5 18 20 22 23–26 29 31 31 35–37 45 60 65 90
(ii)
↑ flammable ------- air -----------↓ non-flammable
Adding flame retardant additives. These reduce the efficiency of combustion by: a. Cooling of the polymer’s pyrolysis zone, e.g., aluminium trihydrate, Al2O3.3H2O (used in B50% of applications). Above 2501C, water of hydration evolves endothermically thereby lowering the temperature of the flame and diluting flammable gaseous pyrolysis products. Additionally, the residual Al2O3 provides a thermal barrier by coating the polymer surface. Al2O3 also suppresses smoke generation. b. Reducing radical pathways. The most important propagating combustion reactions are: Hd þ O2 ! HOd þ Od
ðexothermicÞ
Od þ H2 ! HOd þ Hd ðexothermicÞ Organo halogen compounds (RX, where X = Cl, Br) on heating generate HX by reaction with the polymer (poly–H): RX Ð Rd þ Xd poly H þ Xd Ð polyd þ HX H2 þ Xd Ð Hd þ HX which react with the free radical propagating intermediates to give less reactive species: Hd þ HX Ð H2 þ Xd HOd þ HX Ð H2 O þ Xd
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Either high melting point compounds (often aromatic bromine compounds) or halogen-containing monomers that have been incorporated into the polymer (e.g., p-bromostyrene in PS) can be used. If antimony compounds (e.g., Sb2O3) are also included, a synergistic interaction occurs forming SbX3, which is a very efficient radical trap: Sb2 O3 þ 6HX Ð 2SbX3 þ 3H2 O
(iii)
(iv)
Furthermore volatile SbX3 decomposes in the heat of the flame to generate fine particles of Sb2O3, which form sites for radical recombination. A problem with these gas-phase radical quenching agents is that the volatiles emitted are acidic and corrosive. Promoting char formation by adding phosphorus compounds (e.g., red phosphorous or ammonium polyphosphate, (NH4PO3)n). These additives cause the polymer to swell (intumescence) and form a carbonaceous char on the polymer surface that thus insulates the bulk of the polymer (ammonium polyphosphate is particularly useful for cellulose textile fibres (e.g., cotton)). Melamine and melamine salts (e.g., borates, phosphates) act in a similar manner promoting char formation and intumescence. Thermosets or highly crosslinked polymers give high char yields. Similarly, many high performance polymers, e.g., PEEK, PES, polyetherimide (PEI), not only have high LOI values but also tend to decompose to a char. Surface oxygen barrier formation by using boron compounds (e.g., H3BO3, borax (Na2B4O7)). They prevent combustion by forming a glass-like coating. They are not very efficient, but they are not expensive.
Allied to flammability is smoke density suppression especially in confined spaces, e.g., airliners, houses, warehouses. Many aromatic compounds burn with a smoky flame (e.g., styrene), whereas corresponding aliphatic compounds tend to burn with a clean ‘‘transparent’’ flame. This is because air-borne poly-aromatic vapours decompose to give volatile carbon (smoke) in low oxygen environments. Finely divided magnesium or aluminium hydroxides (or a 3:1 combination) are currently the best smoke suppressants. They also neutralise the acidic vapours produced from halogen-containing flame inhibiters. The more finely divide they are the more efficient they become. Unfortunately, most fatalities in fires occur by inhalation of toxic vapours. These can be carbon monoxide (which arises from incomplete combustion), cyanides (from nitrogen-containing polymers) and chlorides (from chloropolymers). These are the adverse consequences of flammable polymer combustion. They can be overcome by using breathing apparatus, face masks, etc.
4.2.5 Biocides (also called fungicides, bactericides, slimicides, anti-microbials, etc.) Most polymers of high molecular weight in their pure state are resistant to attack by microorganisms, for example, although commercial polyethylene is resistant,
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oligomers of less than 600 MW are eventually consumed. Bacterial attack on the polymer can lead to discoloration, mildew, odours, embrittlement and eventually structural degradation and product failure. Ironically, an active field of research is to promote bacterial degradation in landfill sites. Most trading polymers contain a variety of additives, e.g., plasticisers, lubricants, stabilisers, etc., and it is these additives that promote bacterial attack on the polymer. At the same time, degradation by other routes provides pathways for bacterial attack by creating hydrophilic surfaces. Because many industrial and domestic services are supplied by underground plastic pipes and cables, care must be taken when choosing additives to make sure that the polymer is not vulnerable to attack. There are two considerations to be applied when using biocides: (i)
(ii)
Reduce the amount of biocide. This is particularly true for plasticised PVC. Alternatively choose plasticisers that are less liable to attack. Thus aliphatic plasticiser esters are very prone, whereas phthalates, polymeric esters and chlorinated hydrocarbons are fairly resistant. To be effective the biocide should migrate to the surface of the polymer in sufficient concentration to perform its function. This migration should not occur too quickly (bacterial attack is a slow process, sometimes taking years to begin) or it will be leached away.
Not all additives increase biological attack. Heat stabilisers for PVC fabrication, e.g., tin and lead organometallic compounds, promote fungal resistance. Biocides are naturally toxic to lower organisms and therefore must be handled with care. Strict government rules control the sale and use of biocides, especially those used in food contact applications. They are added at the fabrication stage. The morphology of the polymer article is important, e.g., high surface area articles, such as foams, biodegrade more rapidly. The use of biocides is spread across the whole polymer range, e.g., paints, ropes, textiles, fibres, etc. Many are copper, silver or arsenic compounds and also various heterocyclic compounds, e.g., isothiazolines (which have some structural resemblance to penicillin).
4.2.6 Anti-fogging agents Fogging of plastic films used for packaging or agriculture, spectacle lenses, crash helmets, etc., is caused by water droplets. This decreases the transparency and is aesthetically not desirable. Surface-active agents may be added during the processing of films (internal addition) or by surface treatment of the film (external addition). These tend to reduce the surface energy of the film/water droplet interface promoting a continuous film of water thus enhancing transparency. Examples include hydrophilic surfactants, such as sorbitol or glycerol fatty acid mono- or di-esters. If the packaging is in contact with food it must have the necessary approval.
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4.2.7 Degradation additives This is a relatively recent area of research and development. It has come about because of various reasons: unsightly plastic articles, over-flowing landfill sites, enhancing humus in soils thus avoiding combustion methods that may generate toxic vapours, etc. These additives promote disintegration of polymers into non-toxic low molecular weight fragments through various routes. This is achieved by thermal, photo-oxidation, hydrolytic, chemical, mechanical or biological pathways. Many degradation avenues are available: random chain scission, side group elimination and un-zipping of the polymer chain. Three main routes have been followed: (i)
(ii)
(iii)
Photodegradation. This involves initiation by sunlight or UV radiation. The intermediate radicals eventually lead, via hydroperoxides and peroxides, to organic carbonyl compounds. These, in turn, are oxidised to carboxylic acid derivatives, which are biodegradable. The blending of polymeric organic carbonyl compounds, e.g., ethylene/carbon monoxide copolymer, with the parent polymer, e.g., polyethylene, gives a plastic film material that degrades within 3 months. Although dithiocarbamates of nickel(II) and cobalt(II) are multifunctional for the prevention of heat and UV degradation, the iron(III) analogue behaves differently. Initially, when added to the polymer it acts normally, but after an induction period (1–2 weeks) it functions like a photo-degradent. Furthermore, the time for the polymer film to degrade to flakes can be controlled by the amount of iron(III) dithiocarbamate added. For instance, when used in agriculture the complete breakdown of the film can be timed to coincide with the harvesting season. The degradation period can be controlled from weeks to 1–2 years. Biodegradation. This principle is to blend a completely biodegradable polymer with an inert polymer matrix. Many naturally occurring polymers are biodegradable, e.g., the polysaccharides, pullulan and chitin, in addition to synthetic polymers, e.g., polycaprolactone, polyethylene succinate and poly(vinyl alcohol). The most important additive is starch. This can be blended with polyethylene using water or glycerol as a plasticiser — up to nearly 50% starch, although normally 6–15% is used. Synthetic polymers. A different approach is to use a synthetic polymer that is biodegradable (after initial hydrolysis) and can be processed using standard methods. These include copolymers from hydroxybutyrate/ hydroxyvalerate (made from bacteria), polylactic acid and polyglycollic acid (made from the corresponding cyclic anhydrides). These are too expensive to compete with starch/PE but are used for added-value products, e.g., sutures.
Perhaps the greatest benefit of plastic degradation is in the use of plastic films (e.g., PE, PP) in agriculture, especially as mulching films. This involves the covering of growing plants with film. Gains include: less water required for irrigation, increased temperatures (killing dangerous microorganisms), avoiding
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rain run-off of fertiliser. The greatest advantage is the ease of disposal; after use, the degraded polymer flakes are just ploughed into the soil.
4.3 Mechanical property modifiers 4.3.1 Fillers and coupling agents Fillers were initially introduced to reduce costs; they have significantly lower cost than low cost commodity polymers. However, fillers are now added to polymers to enhance performance in many applications. Many of these benefits have emerged because of the greater adhesion between the polymer-filler interface. This arises from the use of coupling agents, which provide a bond, both physical and chemical, between the filler and the polymer matrix. The size and shape of the filler plays an important role. Generally, the smaller the size the greater the property improvement. Size distribution is also paramount. Packing of the filler within a polymer matrix is improved when a combination of large and small particles is used (the smaller particles occupying interstices left by the larger particles). Higher concentrations (or loadings) of the filler are possible and improved properties result. The shape also affects the properties of the filled polymer. Thus, using fibres brings about reinforcement of the polymer. The properties that can be affected arising from fillers include: (i) (ii) (iii)
(iv) (v) (vi) (vii)
Improved stiffness or rigidity (modulus). Improved protection from heat distortion. Reduced coefficient of thermal expansion. This reduces the shrinkage that occurs in the conversion melt to solid during processing. Also, compatibility between polymer/metal adjoining closures over the range –201C to 401C. Plastics expand and contract several times that of metals. Alteration of density of the polymer to a either higher or lower value by using solid or hollow fillers, respectively. Reduced creep (long-term stress). Improved abrasion resistance. Increased surface hardness.
Many other specific properties have been noted, e.g., decreased flammability and anti-blocking. Fillers can be grouped into various categories depending on shape. Spherical fillers include calcium carbonate (size B1 mm diameter and quantitatively the most important filler). This can be ground mechanically to a fine powder or, for smaller particles, by precipitation from solutions or slaked lime using carbon dioxide. Dolomite (calcium/magnesium carbonate) and barium sulfate are prepared and applied similarly. Silica can be either pulverised from quartz or flame hydrolysed from silicon tetrachloride to give ultra-fine, sub-micron diameter powder. These fillers increase the density of the base polymer. Decreasing the density can be achieved using hollow glass microspheres, flyash (from coal-burning power stations) or volcanic ash.
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Other spherical fillers include carbon black. This has several roles particularly in combination with elastomers, e.g., black pigment, anti-oxidant and UV stabiliser, reinforcing filler, and an electrical conductor when used at 60% concentration. Wood flour is particularly effective in phenol/formaldehyde and melamine or urea/formaldehyde thermoset resins because the phenolic lignin component in the wood reacts with the methylol groups (–CH2OH) in the growing polymer. An important feature of fillers is their aspect ratio (length versus diameter or thickness). When this is large, as occurs in fibres (glass, polymeric and carbon), whiskers, plate-like natural minerals (e.g., micas, talc, kaolin), graphite, etc., many properties are improved, especially tensile strength, rigidity, etc. However, none of these improvements are possible unless the interfacial force between the filler and the bulk polymer is strong enough. Since many of these fillers are inorganic with surface hydroxyl groups a bond between the polymer and filler is likely to be weak. It would also be prone to hydrolytic cleavage. Thus a coupling agent is necessary to bridge the different materials together, and enable transfer of any external stress from the polymer to the reinforcing filler. Coupling agents are chemicals with a dual function. They react with the surface groups (usually hydroxyls) on the filler and contain chemical groups compatible with the polymer matrix, either by physical (e.g., entanglement) or chemical means (e.g., hydrogen bonding, dipole/dipole interaction) (Figure 5). For silica fillers, hydrolytic stability is best when M = Si; for other fillers, e.g., calcium carbonate then titanate- or zirconate-coupling agents are preferred. In some examples it is not necessary to prepare the organometallic-coupling agent R2MX2 beforehand. Thus coating glass fibres with a heptane or alcohol solution of titanium alkoxide promotes adhesion between the fibre and polymer, e.g., acrylic, polyolefins, polyester, polyamide and other thermoplastic resins. Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). Glass fibres are fragile and break easily because of surface defects. This is overcome by treating virgin fibres with water-soluble sizing agents. These protect
+ OH
R
X M X
R
-2HX
R
O filler
filler
OH
M O
R
Figure 5 Schematic of the attachment of coupling agent to a filler particle. X = Cl, OCH3, carboxyl; M = Si, Ti, Zr; R = long-chain alkyls or oligomers that contain grafted polar groups, e.g., amides and carboxyl.
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the glass fibres before usage and can be easily washed off. Sometimes the sizing agents can act as coupling agents.
4.3.2 Impact modifiers This is a growth area — how to improve the properties, particularly impact properties of the base polymer. Many plastics can be either tough or ductile with a high impact strength and can withstand a rapid force without fracturing, or they can be brittle and shatter easily, especially at low temperatures. To overcome this, polymers are blended with a more flexible polymer, which dissipates the energy of impact through facile polymer chain movement. This implies that the impact modifiers must have a low glass transition temperature. Furthermore, they must not be miscible with the parent polymer, in contrast with plasticisers, which must have compatibility with the polymer. They must retain or increase the heat distortion temperature, not lower it as occurs when plasticisers are used. Impact modifiers are added at the blending stage and should be welldispersed discrete particles of diameter B1 mm. They must have good adhesion at the modifier/polymer interface in order to transfer the energy efficiently. Loadings of 5–15% of modifier are usually required to be effective. Transparency is often required. This is achieved by arranging that the particle size of the modifier to be below that of the wavelength of visible light (0.4–0.8 mm). This can normally be achieved by emulsion polymerisation, e.g., polybutadiene, polystyrene. Adhesion and surface compatibility between the polymer and modifier can be achieved by surface grafting of polar groups, e.g., acrylonitrile, various acrylates, onto the impact modifier surface before blending. Crystalline polymers, e.g., nylon, poly(butylene terphthalate), are not easily impact-modified. The crystalline domains can act as crack initiation sites. Amorphous polymers with high Tgs are more amenable to modification, e.g., PS, PVC, PC, although PC is tough because of H-bonding that occurs between the polymer chains. The largest use of modifiers are with PS and PVC. High-impact PS (HIPS) was made originally by mechanically blending polybutadiene (PB) and PS. The current method is to dissolve PB in styrene monomer and initiate emulsion polymerisation. This results in a grafted copolymer with PB acting as the core and PS as the surface graft. This forms a single-phase transparent alloy with PS; the homopolymers PS and PB are non-miscible and will not form a clear blend. There are various requirements for impact-modified PVC. The most demanding is for outdoor sidings and window frames, where lifetimes of 20 years are expected. Because butadiene polymers or copolymers (e.g., acrylonitrile/butadiene/styrene (ABS), methyl methacrylate/butadiene/styrene (MBS)) are susceptible to UV degradation these polymers are usually not employed; instead acrylate polymers are used for these applications. Although the main use of impact modifiers is with thermoplastics, thermosets also benefit. The agent is added at the monomer stage. Thus epoxy polymers can be made less brittle by the addition of rubbers. Care has to be taken that the high temperature properties of the thermoset are not compromised.
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4.3.3 Nucleating agents Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). All of these parameters are not under control. To deliver a more uniform distribution of crystallites, specific nucleating agents are added whilst processing. It has also been shown that many of the desirable properties are the result of small regular spherulites (or nascent crystals). Furthermore, other advantages of controlled nucleation include: (i) (ii) (iii)
Increase of the crystallisation rate. Increase of the temperature at which crystallisation commences on cooling from the melt, thereby reducing injection moulding time. Small crystallite size, less than 0.5 mm, reduces light scattering, thereby making the polymer transparent.
Nucleating agents should possess the following requirements: (i) (ii) (iii) (iv) (v) (vi)
Small particle size (preferably less than 0.5 mm diameter). High melting point. Insolubility in the polymer melt. Surface compatibility with the polymer allowing good dispersibility. Adequate thermal stability with no odour or staining during processing. Have food contact approval.
There appears to be no physical (e.g., crystal habit or morphology) or chemical (e.g., surface groups) correlation to identify which nucleating agent works best with which polymer matrix. However, it is known that dibenzylidene sorbitol works best with PP (it is sometimes called a clarifier because it enhances transparency), and inorganic benzoate salts are successful with PET.
4.3.4 Aesthetic property modifiers These enhance the appearance of the polymer, but play no role in the chemical, physical or mechanical properties of the base polymer. The main difficulty is that if the finished assembled article is made from different grades of the same polymer or from different polymers, then, particularly with different polymers, the combination must be uniform. For example, bathroom suites are often made from different materials, e.g., ceramics, baths (polyacrylates), trimmings (PVC, PP). The colorants therefore have to undergo different processing conditions and it is essential that in the final products the colour is the same.
4.3.4.1 Colorants. This is often the determining feature of a plastic article. Two types of colorants are used: pigments and dyes. Pigments are mostly inorganic
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and insoluble in the polymer matrix. They should be dispersed uniformly throughout the polymer. Dyes are organic based and tend to be soluble in the polymer. Dyes are liable to be more brilliant and vivid and more transparent than pigments, which are duller and opaque. This is because the colour with pigments is by reflection only, rather than the removal of the complementary colours by dyes. Both pigments and dyes must fulfil the following requirements: (i) (ii) (iii) (iv)
Must be thermally stable under process conditions. Have chemical compatibility with either the base polymer or accompanying additives. Have a reasonable lifetime without fading, colour change, bleeding, etc. Be non-toxic or have food contact approval. Also, on disposal (combustion or landfill) must not generate harmful materials.
Pigments can be either inorganic or organic. White pigments are the most widely used (they are often added with other colorants to give the finished article increased brightness). Titanium dioxide is by far the most important white pigment, since it imparts a high degree of reflectance and whiteness because of its high opacity (maximum light scattering with minimum light absorption). Two crystalline forms exist, rutile and anatase. Rutile (0.25 mm diameter) has the higher refractive index and is capable of greater reflectance in the visible spectrum. Anatase (0.18 mm diameter), on the contrary, scatters and therefore reflects more into the UV part of the spectrum. This confers a bluish tinge, which compensates the yellowness of some polymers. TiO2 actually promotes photoand oxidative degradation of the polymer and at the surface erodes a white powdery deposit of TiO2 results (chalking). This is prevented by initially coating the TiO2 particles with silica and alumina. Other pigments include zinc sulfide (white), iron oxides (brown), chromium oxides (green) and carbon (black). Good dispersibility is essential. To aid this the surface of the pigment is often coated with hydrophobic groups (similar to fillers). Because of toxic considerations, both lead chromate (yellow) and cadmium sulfide (red) are being phased out of usage. Synthetic ultramarines (made by fusing kaolin, sodium carbonate and sulfur) can be manufactured to give blue, pink and violet shades. Organic pigments include azo condensation products and metal phthalocyanins. These tend to agglomerate during compounding giving a spotted appearance. Dyes are miscible with the polymer and hence do not have dispersion problems. They include many of the dyes that have been developed for dying fibres, e.g., azo and anthroquinone derivatives. They are particularly useful for transparent articles, e.g., automotive taillights, decorative film. They can be added either as a dry powder or as a colour concentrate (requiring 10-fold dilution). Colorants have been developed for special effects. Pearlescent or iridescent give an attractive appearance. They are often used for coating paper (e.g., cosmetic packaging). The pigment consists of thin platelets, less than 1 mm thickness, which have a high refractive index, e.g., mica coated with TiO2.
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Interference patterns with the reflective and refractive light occurs that varies with the viewing angle. Thin metallic flakes of, for example, aluminium, copper, bronze, coated with a dye are used extensively in automobile wheel hub-caps, and ‘‘metallised’’ car-body paint finishes. Optical brighteners, optical bleachers or fluorescent pigments absorb UV light (B350 nm) and instantly emits it in the blue/violet region (B425 nm) of the spectrum. This has the effect of counterbalancing the yellow tinge of many plastics. However, they tend to perform a similar function as UV stabilisers. If they are both present, more brightener has to be added. Typical concentrations are 10–500 ppm and they are often based on stilbene structures. Their main application is in toys, packaging and road/safety signs. Phosphorescent pigments work by absorbing UV-visible light and slowly emitting it at longer wavelength light as an after-glow. Di-valent cation substitution of zinc sulfide is often the most popular choice.
5. SUMMARY The use of plastics in society is increasing at the rate of 5–10% per annum. The main application is in packaging and all the major commodity polymers are involved. Although excess packaging has been blamed for saturating landfill sites, it must be remembered that packaging does reduce the loss of goods which occurs through breakage and especially food spoilage. In the developed world, packaging results in less than 2% food wastage (an improvement from 15% which occurred before World War II) compared with 30–50%, which currently occurs in the under-developed world. Packaging also allows easy handling of goods. Improved tear strength (LLDPE compared with LDPE) has permitted a halving of the amount of polymer used in disposable shopping bags. Over the past 20 years no new commodity polymer has been developed. This is because of the advances in fabrication, blends (both miscible and nonmiscible), fibre reinforcement, etc. Thus films with up to 11 different polymer layers have been developed. High performance polymers are being used to make articles that are normally manufactured from metals. This is because of a higher strength-to-weight ratio. Modern passenger aircraft are now made with an ever-increasing usage of composites (e.g., epoxy/carbon fibres). Military aircraft development has led the way in this area. In the future, plastics and polymers will be important in developing ‘‘smart’’ materials. These are materials that can counteract any external force by reinforcing itself. An example is the repair of cracks in airplane fuselage and wings. Incorporated in the composites are hollow cenospheres containing separately the components for making a thermoset polymer, e.g., epoxy oligomer and catalyst (e.g., amine). As the crack progresses, the cenospheres are broken and the ingredients mix to form a polymer and heal the fracture.
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J.P. Candlin
BIBLIOGRAPHY There are many reference sources that expand the knowledge of polymer composition, uses and applications. These include scientific encyclopaedias (often arranged in an alphabetic manner), together with speciality books — especially on how additives improve and extend the uses of polymers. In multi-volume references and other specialist books listed below, the ISBN(13) has been given. The original sources can be traced through Internet sites (e.g., GOOGLEs) using, for example, where appropriate, the ISBN number or by searching the title given in italics. 1 2 3 4 5 6 7 8 9 10 11 12
Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed., Wiley, Chichester, 2007. Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed., Wiley-VCH, Weinheim, 2007. Encyclopedia of Polymer Science and Technology, H.F. Mark (Ed.), 3rd ed., Wiley, New York, 2004. Encyclopedia of Polymer Science and Technology, Concise, H.F. Mark (Ed.), 3rd ed., Wiley, New York, 2007. ISBN-13: 978-0470046104. M.P. Stevens, Polymer Chemistry: An Introduction, 3rd ed., Oxford University Press, Oxford, 1999, ISBN-13: 978-0195124446. Polymer Handbook, J. Bandrup, E.H. Immurgut and E.A. Grulke (Eds.), 4th ed., Wiley, Chichester, 2003. ISBN-13: 978-0471479369. A.B. Strong, Plastics: Materials and Processing, 3rd ed., Prentice Hall, New Jersey, 2006, ISBN-13: 0131145580. C.A. Harper, Handbook of Plastics, Elastomers and Composites, 4th ed., McGraw-Hill Professional, New York, 2002, ISBN-13: 978-0071384766. F. Rodriguez, C. Cohen, C.K. Ober and L. Archer, Principles of Polymer Systems, 5th ed., Taylor & Francis, London, 2003, ISBN-13: 978-1560329398. J.C. Salamone, Concise Polymeric Materials Encyclopedia, CRC Press, Boca Raton, 1998, ISBN-13: 978-0849322266. Concise Encyclopedia of Polymer Processing and Applications, P.J. Corish (Ed.), Pergamon Press, New York, 1991. ISBN-13: 978-0080370640. N.P. Cheremisinoff, Concise Encyclopedia of Polymeric Engineering Terms, Butterworth-Heinemann, Oxford, 2001, ISBN-13: 978-0750672108.
Polymer additives: 13 J. Murphy, Additives for Plastics Handbook, 2nd ed., Elsevier B.V., Amsterdam, 2001, ISBN-13: 978-1856173704. 14 Plastics Additives Handbook, H. Zweifel (Ed.), 5th ed., Hanser-Gardner, Cincinnati, 2001. ISBN-13: 978-1569902950. 15 Polymer Modifiers and Additives (Plastics Engineering), J.T. Lutz and R.F. Grossman (Eds.), Marcel Dekker, New York, 2001. ISBN-13: 978-0824799496. 16 Chemistry and Technology of Polymer Additives, S. Al-Malaika, A. Golovoy and C.A. Wilkie (Eds.), Blackwell Science Inc., Malden, 1999. ISBN-13: 978-0632053384. 17 Speciality Polymer Additives: Principles and Applications, A. Al-Malaika, A. Golovoy and C.A. Wilkie (Eds.), Blackwell Science, Oxford, 2001. ISBN-13: 978-0632058976. 18 Plastics Additives: An A-Z reference, G. Pritchard (Ed.), Kluwer Academic Publishers, Dordrecht, 1998. ISBN-13: 978-0412727207. 19 Plastics Additives Handbook, R. Gachter and H. Mu¨ller (Eds.), 4th ed., Hanser-Gardner Publications, Inc., Cincinnati, 1993. ISBN-13: 978-3446175716. 20 Plastics Additives: An Industry Guide, E.W. Flick (Ed.), Vol. 1, 3rd ed., Plastic Design Library/William Andrew Publishing, New York, 2001. ISBN-13: 978-0815514640.
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21 B. Wendel, Plastic Additives: Guide and Reference, CRC Press, Boca Raton, 2002, ISBN-13: 9781587160752.
The section on polymer additives has been modelled on three articles in the Journal of Chemical Education by M.P. Stevens; these are: J. Chem. Ed., 70(6) 444; 70(7) 535; 70(9) 713; (1993).
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SECTION II: Polymer Chain Analysis
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CHAPT ER
4 Chain Structure Characterization Gregory Beaucage and Amit S. Kulkarni
Contents
1. Introduction to Structure in Synthetic Macromolecules 1.1 Dimensionality and statistical descriptions 1.2 Chain persistence and the Kuhn unit 1.3 Coil structure and chain scaling transitions 1.4 Measures of coil size R
