Collaborative Research: New Approaches to Predicting Long-time Behavior of Polymer Glasses

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Glasses are non-equilibrium materials and their properties vary with time, thermal history, and mechanical deformation. A major impediment to the insertion of polymeric glasses in applications ranging from dielectric thin films in electronics to structural composites for aerospace, automotive, and wind turbine applications is the inability to predict long-term performance. This lack of quantitative understanding of the mechanical and thermal behavior also adversely impacts the application and engineering of amorphous glassy polymeric materials. This award supports research to facilitate the prediction of long-term behavior of polymer glasses, and thus facilitate the design and engineering of this important class of materials in advanced composites and other applications of importance to national interests. In addition, graduate and undergraduate students will be trained in polymer physics and mechanics, modeling, and experimental methods related to polymer glasses. At North Carolina State University, outreach will be conducted with K-12 summer camps to promote interest in STEM through a curriculum that includes teamwork, creative problem solving, and design challenges. Outreach activites at University of Nebraska Lincoln will focus on lifelong learner adults interested in expanding their understanding of math, physics, and programing.The Back Stress Expansion model will be investigated in order to develop a robust, yet computationally simplified, methodology that is able to describe and predict the long-term performance of glassy polymers as a function of thermal and mechanical history, including nonlinear or large deformations. In order to obtain a quantitatively correct description of material behavior, the model will be extended by incorporating i) a distribution of relaxation times, ii) the transition from the super-Arrhenius temperature dependence of the relaxation times above the glass transition to a weaker temperature dependence in the glassy state, and iii) the full multi-dimensional framework to model non-isotropic deformations. The model will be tested and validated using new experimental data, including pressure dependent structural recovery, isochoric measurements in the glassy state, and non-linear viscoelastic measurements. These experiments, which will be performed on three important engineering thermoplastics - polycarbonate, polysulfone, poly(methyl methacrylate) - will not only provide new data to validate and refine the model but will also further our understanding of the physics and mechanics of glassy polymers.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.