This award supports research to characterize the nonlinear viscoelastic behavior of glassy polymers. Understanding the long-term mechanical behavior of glassy polymers is critical for assurance of the durability of glassy polymers in their use as stand-alone materials, or as matrix materials for advanced composites, in applications that require low-density and high specific strength materials, such as in automotive, sports, and aerospace industries. This project will take steps to address the challenge from a fundamental perspective using a novel nonlinear fingerprinting methodology that promises to bring important advances for development of premier products, hence benefiting US industry and society. In the past, the fingerprinting methodology has been used to characterize polymeric fluids. This award extends the method to polymer glasses, both through novel experiments and through appropriate nonlinear constitutive modeling and validation. The graduate students on the project will be trained in this cutting edge research, which involves experimental and computational mechanics of time-dependent materials, glassy polymer physics, and structure-property relations for engineering polymers. The project will also leverage STEM outreach programs at both Texas Tech University and University of Texas Dallas to excite K-12 students about science and engineering.
Fundamental knowledge related to the nonlinear viscoelastic behavior of solid polymers will be pursued in this project. This requires a general scheme to classify the variety of observed behaviors and use these observations to challenge the relevant constitutive equations. The research will use the combined Large Amplitude Oscillatory Shear and Mechanical Spectral Hole Burning fingerprinting methodology to provide fingerprints of the nonlinear viscoelastic behavior of a series of glassy polymers and to challenge them against a set of nonlinear constitutive law predictions from several leading models from the literature. Not only will the work show that the method can be applied to glassy polymers, but by choosing to examine polymers with different degrees of heterogeneity, as estimated by the strength of the beta-relaxation, the method can differentiate among different types of nonlinear behavior and, in principle, determine how the details of the dynamic heterogeneity impacts the fingerprints and the constitutive model parameters. This provides an important advance in ideas of structure-property relations in the nonlinear mechanics of polymers. Furthermore, the work provides a critical evaluation of the fingerprinting paradigm, currently developed for shearing deformations in nonlinear fluids, by making measurements in non-volume preserving conditions, viz., tension and compression to develop material fingerprints as a function of said deformation geometries and comparing them to the shearing results.
