Chain-End Defects in Extended-Chain Polymer Solids

Abstract

Understanding the influence of local variations in symmetry (“defects”) on the macroscopic properties of polymers in the condensed state is an ongoing experimental and theoretical challenge. Studies of defects in solids require the most information-intensive description of microstructure since it is not possible to describe a “defect” without understanding the morphology of the majority phase as well.The nature of defects in polymers has been discussed elsewhere, including other articles in this issue of the MRS Bulletin. The structure, properties, and mobility of defects in polymers are all profoundly influenced by the covalently bonded chain backbone. In polymers, there are unique defects such as chain folds and twists that have no obvious analogue in materials of small molar mass. Here, we examine a particular type of defect that is present in all polymer systems with finite molecular weight: chain ends. Our interest will focus on chain ends in polymers that are essentially fully extended parallel to a certain preferred orientation axis.The extended-chain microstructure was originally envisioned by Staudinger as a “continuous crystal” in which high-molecular-weight polymers would be perfectly oriented and close-packed together laterally. Extended-chain polymer fibers such as poly(paraphenylene terephthalamide) (PPTA or Kevlar®), gelspun polyethylene (Spectra®), and the rigid-rod polymers poly(paraphenylene benzobisthiazole) (PBZT) and poly(paraphenylene benzobisoxazole (PBZO or PBO) (Structure 1) closely approach this conceptual limit. The outstanding tensile moduli (100–400 GPa) and tensile strengths (2–4 GPa or higher) of these fibers have generated considerable interest for lightweight structural applications. Extendedchain polymers can also be prepared by solid-state polymerizations of appropriate monomer precursors. Perhaps the most familiar of this latter class of materials are the polydi-acetylenes, first developed by Wegner.