Nonlinear creep in a polymer glass

Results of molecular dynamics simulations are presented for the nonlinear creep response of a model polymer glass under both tension and compression. The structure of the material was examined as a function of time; it is found that the overall structure, as measured by the structure factor, changes appreciably during the deformation. The intensity of the first peak is decreased under stress and the polymer chains are deformed. The dynamics are studied using both the bond autocorrelation function and the incoherent dynamic structure factor. It is found that the dynamics are strongly correlated with the strain rate in the material. Additionally, each measure of the dynamics is uniformly enhanced by the same amount. Previous simulations have shown that the enhanced dynamics do not correlate with changes in the volume in any clear manner; here, evidence from simulations indicates that the enhanced dynamics correlate with the energy of the inherent structures of the material, suggesting that changes in the materials' position on the potential energy landscape lead to the observed enhanced dynamics. Finally, the results of simulations are discussed in terms of two theories of stress-induced dynamics.