Polymorphism in Poly(L-lactide) and Physical Property Ramifications


Shaw Ling Hsu
Polymer Science & Engineering
University of Massachusetts, Amherst

Recently, elucidation of the poly(lactic acid) (PLA) polymorphic phases has led to remarkable improvements in the associated physical properties. PLA currently accounts for less than 0.2% of world’s polymer usage. In contrast, even under severe regulatory pressures, poly(vinyl chloride) (PVC) accounts for approximately 20% of world’s polymer usage. This situation occurs even when PLA is considered to be “green” because it is mostly synthesized from biomass feedstocks, an initiative that resonates with societal aspiration to reduce our dependence on petroleum. The limitation of PLA is due to its rapid crystallization resulting in a high degree of crystallinity. If it is uncontrolled, embrittlement occurs. Therefore, a clear definition of the crystallization process is necessary. This involves the clarification of the various crystalline phases that currently are being discussed in published literature. The disagreement in this area of study have led to ill-defined processing methods and a general lack of agreement even regarding the degree of crystallinity that exists. It can be said that even the enthalpic contribution at melt is strongly debated. The existence of metastable phases also is not well established. In addition, the changing properties as a function of time and temperature need to be improved. Therefore, we have carried out a number of studies utilizing oligomers and model polymers to characterize PLA polymorphic structures, including its stereocomplex. In fact, by controlling the crystallization kinetics of copolymers including the isomeric structure, we were able to develop a new class of tough PLA.

Speaker Background

Dr. Shaw Ling Hsu currently is a professor in the department of Polymer Science and Engineering at the University of Massachusetts at Amherst. His research interests encompass spectroscopic analysis of polymers; development of environmentally appropriate materials; characterization of multiphase materials including reactive blends; deformation induced structural transformation in polymers; physics of polymer surfaces and interfaces; and aging behavior of polymers.
Currently, his research group focuses on performance improvement of environmentally appropriate polymer systems via incorporation of plant-based feedstock. Nature provides the variety in chemical composition and structure that can be further altered in our laboratories for use in various applications with emphasis on coatings and adhesives. Through our research, we have the potential to alter drug delivery kinetics, increase performance of biomedical devices, better utilize agricultural and forestry residuals and develop alternative energy to achieve a better environment. Theory and experiments are being carried out to characterize the phase equilibria of these multi-component systems including polyurethanes. In collaboration with our industrial partners we have also characterized the deformation behavior of various natural biodegradable polymers such as poly(lactic acid) and their copolymers.

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