Characterization of the Ductile to Brittle Transition in Model Polymer Glasses

While recent studies have developed a deep understanding of the origins of localized plastic rearrangements in disordered solids, the organization of these rearrangements into failure modes remains poorly understood. At low temperatures, glassy systems are known to exhibit brittle failure after these localized rearrangements, referred to as shear transformation zones, organize into shear bands and fracture. One reason for the poor understanding of failure is the lack of simple molecular models that exhibit a ductile-to-brittle transition. Molecular Dynamics (MD) simulations using common model glass-formers, which are generally based on the conventional Lennard Jones (LJ) pair potential, have been shown to have significant ductility even at temperatures far below the glass transition temperature. In this work, I will describe our efforts to design a model system that exhibits a ductile-to-brittle transition that is accessible to MD simulations. Following ideas proposed by Falk and co-workers, we modify the LJ potential to a form that increases the elastic modulus of the material and leads to brittle failure at temperatures far below the glass transition. I will describe the characteristics that lead to brittle failure and how they depend on temperature and sample history.