LIQUID CRYSTAL POLYMERS: FROM STRUCTURES TO APPLICATIONS

LIQUID CRYSTAL POLYMERS: FROM STRUCTURES TO APPLICATIONS

POLYMER LIQUID CRYSTAL SERIES Edited by D. ACIERNO University of Salerno. Italy W. BROSTOW University of North Texas. USA A.A. COLLYER Sheffield Hal/am University, UK

Springer-sbm Archive Dordrecht LIQUID CRYSTAL POLYMERS: FROM STRUCTURES TO APPLICATIONS Edited by A.A. COLLYER Division of Applied Physics, School of Science, Sheffield Hallam University, UK ELSEVIER APPLIED SCIENCE LONDON and NEW YORK

ELSEVIER SCIENCE PUBLISHERS LTD Crown House, Linton Road, Barking, Essex IGll 8JU, England WITH 37 TABLES AND 203 ILLUSTRATIONS 1992 ELSEVIER SCIENCE PUBLISHERS LTD and THE SOCIETY OF MATERIALS SCIENCE, JAPAN British Library Cataloguing in Publication Data Liquid Crystal Polymers: From Structures to Applications I. Collyer, A.A. 547.7 ISBN 1-85166-797-0 Library of Congress Cataloging-in-Publication Data Liquid crystal polymers: from structures to applications/edited by A.A. Collyer p. cm. - (Polymer liquid crystal series) Includes bibliographical references and index. ISBN 1-85166-797-0 1. Polymer liquid crystals. I. Collyer, A.A. Witold, 1934- III. Series. QD923.L568 1992 530.4'29--dc20 92-8744 CIP No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Special regulations for readers in the USA This publication has been registered with the Copyright Clearance Centre Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside the USA, should be referred to the publisher. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Photoset in Malta by Interprint Limited

Preface The subject of liquid crystals and their use in electronic displays and in non-linear optical systems has become of tremendous importance during the last decade; and the incorporation of liquid crystal units into polymeric materials has led to a group of new materials with diverse properties. Some of these properties have been utilized in new products and some have yet to be used. Much published work has appeared that deals with specific materials or particular applications, and it was felt that a book was needed to examine and explain the underlying principles governing the diverse properties of these liquid crystal polymers, LCPs. The current work describes the diverse nature of LCPs, their synthesis, characterization, properties and finally their applications. It describes the manner in which liquid crystallinity or mesomorphism occurs in small molecules, monomer liquid crystals and polymer liquid crystals. Chapter 1 gives a classification of the various ways in which the mesogens may be connected to the polymer chains. Currently, the bulk of LCP material is based on main chain or longitudinal LCPs for use in engineering applications. The side chain or comb polymers are intended for use in electronics and opto-electronic systems and as surfactants. Many other variants and possibilities exist but their properties have not yet been fully studied or used. In this respect it is hoped that the current work will indicate future possibilities as well as discussing current opinion. v

vi Preface Chapters 2 and 3 describe methods of characterizing the mesophases. In the former a comprehensive guide to textures is given. Chapter 4 describes the dielectric properties of LCPs. Chapters 5 to 8 inclusive deal with comb and longitudinal LCPs, lyotropic and thermotropic. In these chapters are discussed the syntheses, characterization, structures and properties of these materials. Chapter 9 deals with some applications of LCPs. One of the important topics not covered in this text is that concerning the effects of electric and magnetic fields on LCPs, and associated optical effects. To cover this vast subject would be difficult in one chapter, and the merest of introductions is given at the end of Chapter 7; readers are urged to read the recent texts texts given at the end of this preface for a thorough coverage of this subject. However, it is intended to cover this area in a future volume. 'Polymer liquid crystals or liquid crystal polymers?', is a question that baffled the editor. Several reasons for using the one or the other were put forward by several authors. Professor Brostow, who has given a classification of different architectures, prefers to go in a natural progression from monomer liquid crystals to polymer liquid crystals. On the other hand, to call the book 'Polymer Liquid Crystals' may imply that the subject is about liquid crystals when it is really about polymers. A majority of the contributing authors preferred the term 'liquid crystal polymer', and as such the editor has taken refuge in the majority view. This brief discussion then settles the aim of the book. It is intended for workers with a knowledge of polymeric materials but scant awareness of liquid crystallinity. It is hoped that this text will assist the reader in understanding the latter so that an appreciation of the way in which LCPs behave will be obtained; and, moreover, readers will be able to appraise for themselves the possibilities unfolded by the different structures and capabilities provided by these fascinating materials, with a view to extending their use in novel products and applications. A.A. COLLYER REFERENCES 1. Ciferri, A. (Ed.) LiqUid Crystallinity in Polymers: Principles and Fundamental Properties. VCH Verlagsgesellschaft, Germany, 1991.

Preface VII 2. McArdle, C.B. (Ed.), Side Chain Liquid Crystal Polymers. Chapman and Hall, New York, 1989. 3. Takeda, M. (Ed.), Applied Liquid Crystal Polymers. A special issue of J. Mol. Cryst. Liq. Cryst., 1989, vi. 4. Weiss, R.A. & Ober, c.k. (Eds), Liquid-crystalline Polymers. American Chemical Society, 1990.

Contents Preface... v List of Contributors.................................................... xv 1. An Introduction to Liquid Crystallinity.... W. BROSTOW 1.1 Introduction.......................................... 1 1.2 A brief history of MLCs and PLCs... 2 1.3 Mesophases and their types... 3 1.4 Heterogeneous composites, molecular composites and PLCs.. 4 1.5 The nature of liquid crystallinity... 8 1.6 Phases of liquid crystals... 11 1.7 Classification of PLCs... 14 1.8 Molecular structures, properties and packing... 21 1.9 From structures to applications... 23 References... 25 2. Characterization of Mesophases... 31 C. NofOL 2.1 Introduction... 31 2.2 Typical molecular structures... 32 2.2.1 Low-molar-mass liquid crystals (LMMLCs)... 34 2.2.2 Polymers... 41 IX

x Contents 2.3 Mesophases of rod-like molecules... 45 2.3.l Nematics (N)... 46 2.3.2 Cholesterics (Ch)... 48 2.3.3 Smectics (S)... 48 2.3.4 Compounds with highly polar end groups... 53 2.4 Textures and polymorphism of LC polymers... 55 2.4.l Nematic textures... 55 2.4.2 Cholesteric textures... 60 2.4.3 Smectic textures... 62 2.5 Miscibility tests... 70 2.6 X-ray diffraction patterns... 76 2.6.1 X-ray diffraction patterns for powder samples... 77 2.6.2 X-ray diffraction patterns for oriented samples... 82 2.7 Conformation of LCPs as revealed by small-angle scattering methods... 87 References... 91 3. NMR Studies of Thermotropic Polymers... 103 F. LAUPR~TRE 3.l Introduction... 103 3.2 NMR investigation of orientational conformational phenomena in mesomorphic polymers... 104 3.2.l Orientational order parameters... 104 3.2.2 Principles of the NMR measurements... 105 3.2.3 NMR studies of orientational and conformational order in longitudinal thermotropic polymers... 107 3.2.4 NMR studies of orientational and conformational order in side chain thermotropic polymers... 115 3.2.5 NMR studies of orientational and conformational order in disco tic thermotropic polymers... 122 3.3 NMR investigation of local dynamics in mesomorphic polymers 123 3.3.l Glass-liquid transition and secondary transitions... 123 3.3.2 Principles of the NMR experiments... 125 3.3.3 NMR studies of local dynamics in longitudinal liquid crystal polymers... 127 3.3.4 NMR studies of local dynamics in side chain thermotropic polymers... 133 3.4 NMR investigation of slow motions in mesomorphic polymers 137 3.5 Conclusions... 138 References... 139 4. Dielectric Relaxation in Macromolecular Liquid Crystals... 143 J.K. MOSCICKI 4.l Introduction... 143 4.2 Principles of dielectric relaxation spectroscopy... 144 4.3 Dielectric spectroscopy of liquid crystals... 156

Contents Xl 4.3.1 Dielectric relaxation in the uniaxial phase... 163 4.3.2 Ferroelectric modes in chiral smectic C* phase... 171 4.4 Dielectric relaxation in polymers... 174 4.4.1 Polymers in dilute solutions... 177 4.4.2 Flexible polymers in bulk... 179 4.4.3 Rod-like polymers in concentrated solutions... 184 4.5 Dielectric spectroscopy of liquid crystal polymers... 187 4.5.1 Lyotropic polymers...... 191 4.5.2 Thermotropic polymers... 195 References... 231 5. Lyotropic Side Chain Polymer Liquid Crystals... 237 P.J. HALL and G.J.T. TIDDY 5.1 Introduction... 237 5.2 Physical properties of surfactants... 239 5.2.1 Dilute micellar solutions... 239 5.2.2 Micelle size and shape... 243 5.2.3 Liquid crystal formation of small-molecule surfactants... 245 5.2.4 Techniques of characterization... 250 5.3 Synthesis of lyotropic side chain polymer liquid crystals... 253 5.4 Phase behaviour of lyotropic side chain polymer liquid crystals 256 5.5 Polymerization in oriented monolayers and vesicles... 267 References... 270 6. Lyotropic main chain liquid crystal polymers... 273 M.G. NORTHOLT and D.l. SIKKEMA 6.1 Introduction and synthetic aspects... 273 6.1.1 Aromatic polyamides... 274 6.1.2 Rigid-rod heterocyclic (ladder) polymers... 276 6.2 Order in lyotropic polymer solutions... 278 6.2.1 The modified Maier-Saupe mean field theory... 281 6.2.2 Flow behavior of lyotropic solutions... 286 6.2.3 Spinning of lyotropic solutions... 290 6.3 Morphology of fibers and films... 296 6.3.1 Chain conformation... 296 6.3.2 Results of X-ray and electron diffraction studies... 298 6.3.3 Characterization by optical and electron microscopy... 305 6.3.4 Coagulation and structure formation... 310 6.4 Mechanical and thermal properties... 315 6.4.1 Elastic behavior... 318 6.4.2 Creep and stress relaxation... 324 6.4.3 Strength of fibers and films... 330 6.4.4 Compressive strength of fibers... 334 6.4.5 Thermal properties... 337 References... 340

xii Contents 7. Thermotropic Side Chain Liquid Crystal Polymers... 349 DJ. SIMMONDS 7.1 Introduction... 349 7.1.1 Historical development and literature... 351 7.1.2 Scope and nomenclature... 351 7.2 General structural features... 353 7.2.1 Polymer backbone... 354 7.2.2 Spacer linkage... 355 7.23 Mesogen.......................... 356 7.2.4 Other variables: disco tic and double systems... 357 7.3 Structure-property correlations... 359 7.3.1 The backbone... 359 7.3.2 The flexible spacer... 365 7.3.3 The mesogen... 370 7.4 Copolymers... 377 7.4.1 Copolymers of mesogens with non-mesogens... 377 7.4.2 Copolymers with two different mesogens... 379 7.4.3 Cross-linked polymers-network LC elastomers... 381 7.5 Synthesis of comb and parallel systems... 383 7.5.1 Addition polymerization of mesogenic monomers... 384 7.5.2 Ionic polymerization... 387 7.5.3 Condensation polymerization... 388 7.5.4 Polymer modification reactions... 389 7.5.5 Hydrosilylation of alkenes... 390 7.6 Applications and applicable materials... 391 7.6.1 Electro-optical applications... 391 7.6.2 Liquid crystal elastomers... 399 7.6.3 Chromatographic applications... 400 7.6.4 Miscellaneous...................... 401 References... 402 8. Thermotropic Main Chain Liquid Crystal Polymers... 407 W.A. MACDONALD 8.1 Introduction.............................................. 407 8.2 Design of thermotropic main chain LCPs... 408 8.2.1 Frustrated chain packing... 410 8.2.2 Polymers with flexible spacers... 411 8.2.3 Non-linear links... 412 8.3 Synthesis of thermotropic main chain LCPs... 416 8.4 Characterization and morphology of thermotropic main chain LCPs... 419 8.4.1 Solution and melt characterization... 419 8.4.2 Morphology and structure of MCLCPs... 420 8.5 Properties of thermotropic main chain LCPs... 432 8.5.1 Rheology... 432 8.5.2 Mechanical properties... 433 8.5.3 Miscellaneous properties... 440

Contents xiii 8.6 Applications... 441 8.7 Conclusions... 442 References... 443 9. Applications of LCP Materials... 447 J.-F. JANSSON 9.1 Introduction.......................................... 447 9.2 Injection-moulded products... 450 9.3 Extruded LCP rods and profiles... 454 9.4 Oriented sheets and films... 455 9.5 Thermoforming and blow moulding... 456 9.6 Matrix materials for composites... 456 9.7 Fibers... 457 9.8 Coatings...................... 461 References... 462 Index... 465

list of Contributors W. BROSTOW Center for Materials Characterization and Department of Chemistry, University of North Texas, Denton, Texas 76203-5308, USA P.J. HALL Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK, L63 3JW J.-F. JANSSON Polymeric Materials, Royal Institute of Technology, Stockholm, Sweden F. LAUPRtTRE Laboratoire de Physico-chimie Structurale et Macromoleculaire associe au C.N.R.S., 10 rue Vauquelin, 75231 Paris Cede x 05, France W.A. MACDONALD ICI Wilton Materials Research Centre, P.O. Box 90, Wilton, Middlesbrough, Cleveland, UK, TS6 8JE J.K. MOSCICKI Institute of Physics, Jagiellonian University, Krakow, Poland; and Cornell University, Baker Laboratory, Ithaca, New York, USA C. NOtL Laboratoire de Physico-chimie Structurale et Macromoleculaire associe au C.N.R.S., 10 rue Vauquelin, 75231 Paris Cede x 05, France xv

xvi List of Contributors M.G. NORTHOLT Akzo Research Laboratories, P.O. Box 9300, 6800 SB Arnhem, The Netherlands D.J. SIKKEMA Akzo Research Laboratories, P.O. Box 9300, 6800 SB Arnhem, The Netherlands D.J. SIMMONDS Division of Chemistry, School of Science, Sheffield Hallam University, Pond Street, Sheffield, UK, Sl 1 WB G.J.T. TmDY Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK, L63 3JW