Defects in Polymers
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MRS BULLETIN/SEPTEMBER 1995
areas of expertise. In some sense, the study of defects in polymers is simply an expedient means to focus attention on the relationship between microstructural and macroscopic properties that are the underlying theme of essentially all materials science. By examining the relationship between symmetry, structure, and properties in more detail, it should be possible to better motivate efforts in the molecular design of new systems, as well as to correctly interpret the microstructural hierarchy of polymer materials found in nature. This issue of the MRS Bulletin presents articles from several researchers who are all actively studying defects and defectmediated phenomena in ordered polymer systems. Their contributions to this issue provide a snapshot of our current understanding of this evolving field. Because of the interest in polymer materials for a variety of applications, including optoelectronic devices such as waveguides, thin-film transistors, gas sensors, and electroluminescent displays, we are confident that interest in this subject will continue to intensify. Wunderlich and Kreitmeier provide an overview of classification schemes for discussing defects in polymer crystals. They report on simulations of defect structure and mobility using molecular dynamics. The topology, elastic properties, and mobility of defects in liquidcrystalline polymers (LCPs) are discussed in the article by Kle'man. This discussion points out the complications associated with the experimental characterization of polymers because of their high viscosity, high defect density, and thermal degradation. Nevertheless, LCPs have considerable technological interest and in certain cases make it possible to isolate defect structure for detailed analyses. Morphological studies of defect structure, interaction, and dynamics in LCPs are discussed by O'Rourke and Thomas. Studies of defect structure make it possible to extract information about the local elasticity of the liquid-crystalline
phase. The role of hairpin defects in LCPs is discussed by Williams and Halperin. Hairpin defects in the chain conformation of an LCP have only recently received significant theoretical attention, and this article provides an interesting overview of an area for which there is very little experimental data available at present. Hudson, Amundson, Jeon, and Smith discuss the nature of defect-limited rheology of microphase-separated block copolymers. Block copolymers are ordered phases with symmetries that depend upon the relative size and composition of the components in the polymer. Studies of block copolymers have provided a wealth of new information about the stability and structure of polymers as models of condensed matter. Hudson et al. discuss the rheology of these ordered materials with particular emphasis on dislocation energetics and mobility. Martin, Wilson, Liao, and Jones discuss the properties of a class of point defects peculiar to polymer systems: chain ends. The focus of this article is the influence of chain ends on t
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