De Rosa Claudio - Crystals and crystallinity in polymers: diffraction analysis of ordered and disordered crystals

Table of Contents

Copyright 2014 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

De Rosa, Claudio.

Crystals and crystallinity in polymers : diffraction analysis of ordered and disordered crystals / by Claudio De Rosa, Dipartimento di Chimica Paolo Corradini, Universit di Napoli Federico II, Complesso Monte Sant' Angelo, Napoli, Italy, Finizia Auriemma, Dipartimento di Chimica Paolo Corradini, Universit di Napoli Federico II, Complesso Monte Sant' Angelo, Napoli, Italy.

pages cm

Includes bibliographical references and index.

ISBN 978-0-470-17576-7 (hardback)

1. Crystalline polymers. 2. Crystallization. I. Auriemma, Finizia. II. Title.

QD382.C78D43 2013

547'.7dc23

2013011106

Preface

With this book, we are indebted to our late mentor, Paolo Corradini. He introduced us into the fascinating world of the Fourier space and diffraction, and taught us the invaluable importance of crystallography for understanding the macroscopic properties of polymers at the molecular level. Several concepts illustrated in the book are the result of our long, illuminating, and tireless discussions with Paolo Corradini, which were animated in the framework of his own remarkable researches. We absorbed his unique way of looking at macromolecules and crystals due to his never-ending teaching ability.

Polymer crystallography is an essential part of materials science and is of fundamental importance in physics, chemistry, and engineering courses. This book takes an interdisciplinary approach to the subject of polymer crystallography and is, therefore, suitable for undergraduate students in all those courses. The book is also addressed to postgraduates and specialists in the field of polymer science and strikes a balance between the need for resorting to a rigorous crystallographic approach for structural analysis and the large variability of the crystalline state of polymers due to their high tendency of including a large amount of disorder inside the crystals.

The book is divided into seven chapters. covers the relationships between the crystal structure and final properties of polymers and illustrates the importance of understanding material properties at the molecular level.

Some sections of the book are concerned with more advanced topics, such as the concepts of symmetry breaking and frustration and the principle of density (entropy)-driven phase formation in polymer crystallography (. These advanced topics make this book suitable also to postgraduate students and experts in polymer science.

The concepts of crystal and crystallinity in polymeric materials are complex and very far from the ideal concept of crystal as a periodic array of identical motifs that implies a complete three-dimensional long-range order of all atoms. Although this ideal concept is valid for the solution of the crystal structure of polymers (see the classic books of polymer crystallography, Structure of Crystalline Polymers by H. Tadokoro and X-Ray Diffraction by Polymers by M. Kakudo, N. Kasai), it is useful only for establishing limit ordered models of the crystal structure to which real macromolecular systems approach when they crystallize in conditions close to the thermodynamic equilibrium.

In polymer crystals, indeed, the true three-dimensional long-range order of all atoms is never present and the structural disorder inside crystals is a rule rather than an exception. One of the main characteristics of polymeric materials is that, at variance with other crystalline materials such as metals, they are never completely crystalline. They are semicrystalline and are generally composed of crystals (lamellae) embedded into an amorphous phase, producing a highly interconnected network. The peculiar semicrystalline character of polymers, associated with the presence of structural disorder in the crystals and polymorphism phenomena, determines their outstanding physical properties.

In this book, a new view of the concepts of crystallinity and crystals in synthetic polymers is presented. The first concept is that crystallinity in polymeric materials is compatible with the absence of true three-dimensional long-range order. Second, the disorder may be described as a structural feature, using the methods of interpretation of the X-ray scattering and electron diffraction. The structures of semicrystalline polymers are discussed in terms of idealized limit models of crystals, where long-range order may be achieved only for some structural features that are not necessarily coincident with single atoms and are not necessarily point centered. Typical examples of non-point-centered structural features are straight lines, corresponding, for instance, to the chain axes of polymer molecules. In this case, the order regards only the positioning of the chain axes and no lateral correlations between atoms of neighboring chains are present.

All the possible types of structural disorder generally present in crystal of polymers are analyzed and the influence of every kind of disorder on the X-ray and electron diffraction patterns is described. Simple rules for the interpretation of the diffuse scattering present in the diffraction patterns of polymers are given. This can be very useful to recognize the disorder present in polymer crystals just from inspection of the diffraction pattern. The methods for the analysis of disordered structures in polymer crystals are illustrated from theoretical and practical perspectives. The important role of X-ray diffraction techniques in the wide angle are outlined, providing numerous examples and practical rules that allow the use of our book also as a manual and textbook.