As plastic materials displace in increasing amount many of the well-established traditional materials the study of mechanical performance of polymeric glasses has become more important. Improving mechanical properties of this important class of materials requires a deeper molecular-level understanding of what factors and processes affect the ultimate strength. This project aims to establish the polymer physics on a molecular level that can be applied to provide predictive design principles on how to make stronger bulk polymeric glasses. Specifically, theoretically motivated experiments and computer simulations will be carried out to elucidate the unique mechanical characteristics due to chain connectivity and address the questions of why polymer glasses can be ductile and how they can be made even more resistant to brittle failure. The success of the research program will allow applications to be further broadened to increase the economical values of this class of modern engineering materials. Since new models and concepts are expected to emerge from the research activities, the work should offer substantial advances to the general knowledge of polymer physics concerning plastics in the glassy state and enhance the curriculum of graduate education in polymer science and engineering.
The project integrates various experiments with pertinent molecular-dynamics simulations to develop a conceptual framework for molecular mechanics of polymeric glasses and to search for physical principles concerning the origin of stress, the nature of stress relaxation, yielding, as well as brittle-to-ductile transition during large deformation. The research has four objectives: A) characterize brittle-ductile transition (BDT) in compression using both experiment and molecular-dynamics (MD) simulation; B) elucidate the origin of stress with both experiment and MD simulation including intrachain contributions; C) depict how the chain network is able or unable to drive a polymer glass into a plastic state through activation and mobilization of vitreous segments, where experiment and simulation will explore the effect of diluting the chain network by incorporating a low molecular-weight component to make mixtures of different compositions; D) examine how deformation at different rates and temperatures enhances molecular mobility -- can deformation always lead to enhanced segmental mobility as implied by the Eyring idea of activation? (e.g., does stress always emerge to lower the activation barrier as depicted in the Eyring formula for the molecular mobility?) Unlike most constitutive modeling studies in continuum mechanics, this project takes a phenomenological and molecular approach to determine the prerequisites for the application of the Eyring idea (for a single particle) to such a complex many-body system as polymer glasses. Explicitly, the research will examine the causality for yielding and ductility in polymer glasses in terms of the displacement and deformation of the chain network as causes for segmental activation and eventual macroscopic yielding.