Nonlinear Viscoelastic Behavior of Glassy Polymers: Unification of the Mechanical and Enthalpic Response

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Glassy polymers are some of the most important engineering materials in use today. They are used in a wide variety of products ranging from the matrix resin in aerospace composites to high impact transparent materials used in aerospace, automotive and home applications to numerous consumer goods. Notwithstanding the commercial importance of these materials, a fundamental understanding of the glassy state remains one of the outstanding questions in science and engineering. The design of products made from glassy polymers requires a fundamental understanding of how these materials respond when deformed. This award supports research to uncover the mechanisms that control the deformation behavior of glassy polymers and then incorporate this understanding in quantitative models that describe the relationship between the deformation and the forces required to cause that deformation. This research is a major step towards developing the constitutive models that describe the material response to complex deformation ? models that are at the heart of the computer codes that are used to design products made from glassy polymers. Because of the economic importance of products made using these materials, this research will eventually benefit the U.S. economy, and thus, advance national health, prosperity, and welfare. The award will also support undergraduate research through supervision of honors thesis, and motorsports-based STEM outreach to middle and high schools in partnership with the IndyCar racing community.The current state-of-the-art in modeling the deformation behavior of glassy polymers does not incorporate one of the key molecular concepts of polymer physics. Specifically, although glassy polymers may appear to be spatially uniform, on the nanometer size scale glassy material exhibits significant dynamic heterogeneity, where neighboring spatial regions can have order-of-magnitude differences in local mobility. This award supports research that will continue development of a new class of models that explicitly incorporates dynamic heterogeneity. In addition, when materials are deformed they exhibit both deformation behavior and a thermal response; however, existing models ignore the thermal response which can contain up to half of the overall deformation energy. A major objective of this research grant will be to unify the deformation and thermal response to provide a more physical description of the deformation process in glassy polymers. Finally, researchers supported by this award will participate in a major STEM program, where high school students design, build and test electric go-karts that they then race as part of the Indy500 festivities. Material properties are a key component of the classroom aspects of this STEM program, where mechanical properties of polymers are explicitly analyzed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.