(173a) A Molecular View of Structure and Dynamics in Nanocomposite Glasses At Rest and Under Active Deformation

There is considerable interest in understanding the behavior of polymeric glasses and polymer nanocomposites at multiple length scales, particularly under active deformation. Over the past two years, we have used a combination of Monte Carlo and molecular dynamics simulations to examine the local and global thermophysical properties of several coarse grain models of polymeric and nanocomposite glasses both at rest and under constant stress or constant strain rate deformation. Our simulations and recent experimental studies have demonstrated that an applied stress can lead to a decrease in the segmental relaxation times by several orders of magnitude.  Many questions regarding the origins of these behaviors remain unanswered. Our simulations of an entangled polymer glass and a nanocomposite glass suggest that nanoparticles impart mechanical reinforcement onto the polymer, requiring larger stresses to achieve the same deformation.  In both materials, the dynamics correlate well with the instantaneous strain rate, although there are situations, having to do with the amount of strain, where this correlation breaks down.  Overall, simulated trends for the dynamics during creep agree very well with recent experiments pure polymer glasses.  For large deformations, we also examine how nanoparticles of different size and shape influence the cavitation and fracture of polymeric glasses. We find that a distinct correlation exists between the local mechanical properties of the material at rest and the positions were voids are nucleated and grown into crazes, thereby providing a connection between structure and failure. Finally, results will also be presented for the structure, mechanical properties, and deformation of ultra-stable glasses prepared by vapor deposition. Such glasses exhibit unique thermophysical properties that differ considerably from those of ordinary glasses, and the origins and implications in the context of stability and aging will be discussed.