Molecular mobility during polymer glass deformation

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TECHNICAL SUMMARY:The deformation properties of polymer glasses are critical for applications but are poorly understood at a fundamental level. Polymer glass deformation is almost always non-linear and is complicated by the non-equilibrium nature of the initial state of the material. Changes in molecular mobility during the deformation of a polymer glass are intimately connected with the nonlinearity of the mechanical response; to a first approximation, the flow of a polymer glass is made possible by the increased mobility of polymer segments. Researchers from the University of Wisconsin-Madison will use an optical technique to directly measure segmental mobility during the deformation of poly(methyl methacrylate) and polystyrene glasses. The role of temperature in deformation-induced mobility will be investigated. In addition, using reversing strain deformations, the competition between deformation and aging in adjusting the position of the polymer glass on the potential energy landscape will be investigated. The validity of purely mechanical estimates of segmental mobility during deformation will be evaluated. The overall objectives of the proposed research are to enhance the fundamental understanding of polymer glass deformation and to test current theoretical approaches and computer simulations. A better understanding of polymer glass deformation will also influence research on the deformation of other glassy materials including metallic glasses.NON-TECHNICAL SUMMARY:Polymer glasses play an important role in society due to their low density, their useful mechanical properties, and the low energy input required for molding. They are encountered in everyday life as safety glasses, bulletproof glass, automobile headlamp covers, and as major structural components in modern aircraft (e.g., the Boeing 787). In all these applications, the polymer glass must be stiff but also should not break in response to large forces. This project at the University of Wisconsin-Madison seeks a fundamental understanding of this behavior and will likely lead to an improved ability to predict the response of polymer glasses to various forces. Better predictions are expected to lead to new applications for these materials particularly where lightweight materials are important (e.g., transportation). In addition, the funding of this proposal will advance the training of graduate students and high school students, through the integration of materials research and education activities. Each summer a polymer materials curriculum, developed for educational outreach, will be presented as a thirty-hour course for 15 high school juniors, most of whom will be from groups that have been underrepresented in engineering and science. The goal of this outreach program is to develop interest in science/engineering and to develop skills required for success in college.