Molecular Characterization and Analysis of Polymers

John M. Chalmers, Robert J. Meier

• Publisher: Elsevier Science
• Year: 2008
• eBook ISBN: 9780080932040
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Written by expert contributors from the academic and industrial sectors, this book presents traditional and modern approaches to polymer characterization and analysis. The emphasis is on pragmatics, problem solving and property determination; real-world applications provide a context for key concepts. The characterizations focus on organic polymer and polymer product microstructure and composition.- Approaches molecular characterization and analysis of polymers from the viewpoint of problem-solving and polymer property characterization, rather than from a technique championing approach- Focuses on providing a means to ascertaining the optimum approach or technique(s) to solve a problem/measure a property, and thereby develop an analytical competence in the molecular characterization and analysis of real-world polymer products- Provides background on polymer chemistry and microstructure, discussions of polymer chain, morphology, degradation, and product failure and additive analysis, and considers the supporting roles of modeling and high-throughput analysis

Chapter 7 Phase Structure and Morphology

Rufina G. Alamo

Abstract
Publisher Summary

The major components of the phase structure of semi-crystalline polymers and the most common techniques of characterization of the crystalline, amorphous, and intermediate phases have been described in this chapter. Most properties of crystalline polymers depend, to a large extent, on the relative content of these phases, which can be varied over very wide limits and in a very systematic manner by changing molar mass, molar mass distribution, structural regularity of the chain, and crystallization conditions. The morphology of organized crystalline entities at a level above the crystallographic unit cell is also described in the chapter. These entities are polymeric lamellae that integrate the long-chain connectivity between crystalline and non-crystalline regions. The thickness and extension of the lamellae are a function of molecular variables (length and structural regularity) and crystallization conditions. Lamellae may aggregate in a large variety of super molecular structures, or branch, following homoepitaxial growth.

1 Phase Structure and Morphology

The properties of semi-crystalline polymers are very dependent on the chain microstructure and the crystalline morphology adopted upon crystallization or on processing. The well-known fact that crystallinity controls the properties of these materials raised interest in investigating the details of the liquid–solid transformation, and the degree and type of chain organization, from early stages and in parallel with the development of polymer science. In this chapter we will describe in some detail the fundamental elements of the semi-crystalline structure, the large-scale macromolecular organization and most customary methods of characterization of crystalline and non-crystalline regions.

For polymer molecules to adopt long-range three-dimensional order, they must have both a high degree of chemical regularity and a high degree of relative motion in the melt.¹ A major difference between the crystalline state developed by long-chain molecules and that of small organic molecules, metals or ceramics is inherent to the covalent connectivity between repeating units in a polymer molecule. While individual atomic or low molecular weight species occupy equivalent points in each unit cell, in long-chain polymers the covalent connectivity of repeating units sets a unique correspondence to perpetuate crystallographic symmetry. The continuity of long-chain molecules impacts in a very unique way the structure and morphology that evolves from the initial complex melt.

The pioneering works of Storks [1] and Keller [2] established that lamellar crystallites are the fundamental three-dimensional entities that develop upon crystallization of chain molecules from solution or from the melt. The widespread observation of lamellar crystallites, with the chain axis preferentially oriented normal to the basal plane with thicknesses much lower than the contour length of the molecule, represented a major advance in the understanding of the morphological features of crystalline polymers [3–7]. The molecules fold back and forth and, except for the very low molecular weight polymers, may traverse a crystallite many times and connect two or more lamellae. This molecular connectivity is a unique feature of polymer crystallization. It defines the amorphous interlamellar or liquid-like region as well as the interfacial region in which the order of the chains emanating from the crystalline phase is dissipated [8].

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb's phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semi-crystalline polymers are crystalline systems well removed from equilibrium.

The crystalline state of polymers can be described by a set of hierarchical structures. At the most fundamental level there is the unit cell or primary crystallographic repeated unit. Increasing complexity and, at a larger-scale level, the polymeric lamellae integrate the long-chain connectivity between crystalline and non-crystalline regions. It is in the crystalline regions where the repeated symmetry of continuous crystallographic units is found. The next level of organization corresponds to lamellar aggregates or supermolecular structures. At this level spherical aggregates (spherulites) of different degrees of order are most often found [9–12]. The crystalline lamellae are an important feature of the semi-crystalline structure. A schematic representation of the lamellae for a melt-crystallized system is given in Figure 1. The figure displays the coexistence of the three major phases, cr...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Advisory Board
• Contributors to Volume 53
• Volumes in the series
• Preface
• Series editor's preface
• Section I: Introduction
• Section II: Polymer Chain Analysis
• Section III: Polymer Morphology and Structure
• Section IV: Polymer Degradation
• Section V: Polymer Product Analysis
• Section VI: Polymer and Polymer Product Development: Support Techniques
• Subject Index