John Wiley - Handbook of Polymer Crystallization

Table of Contents

Copyright 2013 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:

Handbook of polymer crystallization / edited by Ewa Piorkowska, Polish Academy of Sciences, Centre of Molecular and Macromolecular Studies, Lodz, Poland and Gregory C. Rutledge, Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA, USA.

pages cm

Includes index.

ISBN 978-0-470-38023-9 (cloth)

1. Crystalline polymers. I. Piorkowska, Ewa, editor of compilation. II. Rutledge, Gregory Charles, editor of compilation.

QD382.C78H36 2013

547.7dc23

2012037881

Preface

Synthetic thermoplastic polymers form an important class of materials that has expanded dramatically over the past half century, finding utility in a variety of end-use applications. Thermoplastics comprise amorphous polymers that are unable to crystallize and polymers that are crystallizable. Since the melting temperatures of crystallizable polymers tend to be approximately 50% higher than their glass transition temperatures, the polymers that crystallize generally find use over a broader temperature range. Crystallization in polymers is a complex phenomenon that differs significantly from the crystallization of low molecular weight substances.

To crystallize, long polymer chains must partially disentangle from other chains and forego conformational entropy to fit into a crystal phase. As a consequence, polymers crystallize from the molten state at temperatures that can be up to several tens of degrees below their thermodynamic melting temperatures. This large supercooling is one of the easiest observed differences that distinguish polymers from other substances. The slow kinetics of crystallization allow for many polymers to be cooled into the glassy state, only to crystallize later when heated back above their glass transition temperatures. During crystallization, flexible polymer chains fold upon themselves to form a crystal. In melt-crystallized structures, thin lamellar polymer crystals form that are interspersed with noncrystalline layers in which fragments of polymer chains are still entangled, giving rise to the semicrystalline state of polymers. The noncrystalline layers include also chain ends, loops, and tie molecules that connect adjacent crystals. The prevalence of the semicrystalline state is another important feature that distinguishes crystallizable polymers from other solids. Frequently, lamellae form polycrystalline aggregates that grow outwards from common nucleation sites. Thus, the overall crystallization kinetics is determined by both the nucleation rate and the growth rate of crystals. In addition, polymers are able to solidify in the form of mesophases that exhibit various degrees of order, although less perfect than that of the crystalline phase.

Usually, the thicknesses of the lamellar polymer crystals and intercrystalline amorphous zones do not exceed a few tens of nanometers. Semicrystalline polymers are in fact Nature's nanocomposites: self-assembled nanocomposite materials in which a combination of crystals and rubbery amorphous phase may coexist, resulting in the remarkable ductility and toughness of these materials. Another consequence of the relatively small thickness of polymer crystals and the high surface energy of their basal surfaces is a strong dependence of melting temperature on the crystal thickness.

The relatively slow crystallization kinetics of polymers at small supercooling make it possible to control, to some extent, the temperature of crystallization. This enables researchers to study solidification at predetermined isothermal conditions and to link the crystallization and emerging structure with temperature. The temperature at which crystallization occurs influences not only the nucleation and growth of crystals but also the sizes and shapes of the crystals and the overall degree of crystallinity.

In industrial processing, polymeric materials usually crystallize during cooling. Their low thermal conductivity and diffusivity can result in temperature gradients across the product thickness, especially when the release of latent heat of fusion contributes to development of the temperature gradient. Moreover, during processing steps such as extrusion, injection molding, film blowing, or fiber spinning, the flow of material can lead to orientation of the polymer chains, which in turn affects both the crystallization kinetics and the emerging morphology. Depending on the processing conditions and molecular characteristics of a polymer, different structures are observed, from spherulites to shishkebabs. Crystallization and resulting morphology are strongly related to the temperature, applied shear rate (or strain rate), and total strain achieved during flow. Complex thermomechanical conditions determine the supermolecular structure of polymeric materials and, as a consequence, their properties. The understanding of how the polymer morphology develops during processing is a key issue to linking the processing conditions with final properties of the product.

Different monomers may be copolymerized to modify the properties of thermoplastics. For the same purpose, homo- and copolymers are frequently mixed with other substances, including other polymers, various fillers, and nanofillers. The presence of comonomers in macromolecules, as well as interactions between macromolecules in miscible blends, can affect both crystallization and morphology of the polymeric material. Interfaces and the confinement of polymer chains within a finite volume influence the solidification and morphology of immiscible polymer blends and polymer-based composites. They are also of special importance in ultrathin polymer layers where the thickness is comparable to or smaller than the lamellar crystal thickness itself.