Molecular Mobility in Polymer Glasses Under Stress

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

TECHNICAL SUMMARY: Measurements of molecular mobility in polymer glasses during active deformation will be performed. Recent results show that stress can increase segmental mobility in a glass by more than a factor of 1000. This suggests that plastic flow should be understood as stress creating sufficient segmental mobility that the glass effectively is transformed into a very viscous liquid. From this perspective, understanding how stress creates mobility in a glass is the key issue that must be addressed in order to develop better predictions of polymer glass deformation. In this work, segmental mobility will be measured during the deformation of polymethylmethacrylate, polycarbonate, polystyrene, and a fully cured Epon epoxy resin. In addition to single step tensile creep measurements, multistep creep and constant strain-rate measurements will be performed. Previous mechanical experiments have shown an important influence of the thermal and mechanical history of polymer glasses on their subsequent deformation; aging a glass, for example, can transform a ductile response to brittle failure. It is anticipated that these phenomena are all controlled or significantly influenced by stress-induced mobility and this view will be tested with measurements of molecular mobility during deformation after various aging times. Collectively these experiments will provide new insights into the dynamics of polymer glasses and provide data needed to develop better predictions of polymer glass deformation. NON-TECHNICAL SUMMARY: Polymer glasses are important materials and, for some applications, their ability to deform significantly without breaking is a critical attribute. This work will measure how deformation accelerates the rate at which polymer chains can move in glass; this information is needed to make accurate predictions about how polymer glasses will deform in a wide range of applications. This work may have a significant economic and environmental impact because of the broad use of polymeric materials. As an example, measurements of mobility changes during the deformation of an epoxy resin should be directly relevant to the development of advanced composites needed to manufacture lighter, more efficient aircraft. In addition, the funding of this proposal will advance the training of graduate, undergraduate, and high school students through the integration of materials research and education activities. The scientists and students supported by this grant will work with the University of Wisconsin-Madison?s PEOPLE program to prepare high school students from under-represented groups for college. Each summer, those supported by this grant will staff a thirty-hour polymer materials course for 15 high school juniors. The PEOPLE program has a proven track record of preparing students to succeed in college; currently, the University of Wisconsin-Madison has more than 220 undergraduates who are graduates of the PEOPLE program. In collaboration with a high school science teacher, modules from the polymer materials summer course will be modified for use in Wisconsin public high schools.