Bulk Semicrystalline Polymers: A Status Report
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H.D. KEITH
Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA ABSTRACT Discovery several decades ago that long-chain molecules crystallize by folding prompted vigorous basic research on structure-property relations in bulk semicrystalline polymers with expectation of insights that, paralleling experience in metal physics, would lead to better materials for technology. However, complexity of the crystallization process and difficulty in exploring morphology in adequate detail were both underestimated. Important aspects of behavior were recognized but improved polymeric materials have resulted more from exploratory engineering with composites and blends than from basic research on homopolymers. A summary of this research is presented with emphasis upon developments key to present status; also upon the truly formidable challenges that remain, most readily accessible questions for research being of limited technological significance. INTRODUCTION
As an opening to this symposium on crystalline microstructure in polymers Professor Cebe has asked me to present something largely "tutorial" in nature. My involvement in this subject, in which I have been interested for almost four decades, has centered upon crystallization from quiescent (unsheared) melt when many basic principles and molecular processes are manifested in their simpler, though far from simple, forms. I have chosen to attempt a reflective overview of basic research in this area where we still have problems to solve, new phenomena to ponder and new avenues to explore. It is instructive for us to reevaluate from time to time our limited understanding and the focus of our aims in ways that best serve ongoing technological development; that, after all, is what materials science is all about. In that regard, incidentally, I have a few concerns about some recent trends in research and interpretation which, put in historical perspective, seem rather unhelpful. EARLY ASPIRATIONS
In present context we may date the beginning of morphological research on synthetic semicrystalline polymers to the mid-1950's, and especially to the impetus then given it by Keller. He drew attention to general occurrence of spherulitic aggregates when crystallization takes place in melts and also to their radiating fibrillar structures [1]. Soon thereafter, he pioneered the growth of polymer single crystals from solution [2] and confirmation that unexpectedly regular habits which these crystals often exhibit (lamellae of polyhedral shape about 10 nrm thick) are attributable to chain folding. It was soon established in many laboratories that lamellar habits, also involving some measure of chain folding, are commonly produced during crystallization within viscous melts. It was recognized that fibrillar structures as, for example, seen earlier by electron microscopy in 511 Mat. Res. Soc. Symp. Proc. Vol. 321. @1994 Materials Research Society
cast films of polyesters and polyamides, are in fact clusters of highly elongated but very narrow lamellae and tha
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