Molecular Mobility in Polycarbonate Glasses During Active Deformation

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TECHNICAL SUMMARY:When a polymer glass is deformed, molecular mobility in the glass can be enhanced by many orders of magnitude even when the sample temperature does not change appreciably. Researchers from the University of Wisconsin-Madison will use a recently developed optical photobleaching technique to quantify changes in molecular mobility during the deformation of polycarbonate glasses. Within the framework of current models, changes in mobility are responsible for the non-linear mechanical response of polymer glasses. During deformation, the position of the system on the potential energy landscape can be altered. Understanding these changes is the central objective of this work because it is critical for an improved understanding of polymer glass deformation. Both constant strain-rate and constant stress deformations will be utilized. Constant strain-rate experiments provide controlled access to yield and thus allow changes in molecular mobility to be observed as the system is pulled up the energy landscape. Stress-aging experiments have been interpreted as the system being pulled down the energy landscape; mobility measurements during stress-aging will test this interpretation. The comparison of experimental results with simulations and model predictions should result in an increased fundamental understanding of glass deformation and better prediction capabilities. NON-TECHNICAL SUMMARY:Polymer glasses are widely used in our society. These materials can be seen everyday as bulletproof glass, compact disks, safety glasses, and automobile headlamp covers. In next generation commercial aircraft such as the Boeing 787, the wings and fuselage will be made of polymer glass (in a composite with other materials). For such applications, polymer glasses are used because of their ability to deform under stress without breaking. By increasing our fundamental understanding of polymer glass deformation, this research project at the University of Wisconsin-Madison will likely lead to an improved ability to predict the mechanical properties of polymer glasses. Better predictions, in turn, will lead to further applications for these materials in areas like the transportation sector, where lightweight materials save energy and thus contribute to a decrease in greenhouse gas emissions. Each year, these researchers will conduct a three-week summer enrichment course for 15 high school students, most of whom will be from groups underrepresented in science and engineering. For two hours each day, these researchers will provide activities designed to increase interest in science/engineering and to develop critical skills required for success in college.