Atomic mobility in a polymer glass after shear and thermal cycles

Molecular dynamics simulations and energy landscape analyses are carried out to study the atomic mobility of a polymer glass during the physical aging process that follows shear and thermal cycles. The mobility is characterized by the fraction of atoms moving more than their diameter in a given time interval. The mobility is enhanced after a shear or thermal cycle, and this enhancement decays with time. These mobility results are related to the position of the system on the energy landscape, as characterized by the average energy of the energy minima visited by the system; the mobility over longer time scales increases with the average energy of the energy minima visited, but the mobility over shorter time scales does not show a correlation with this average energy. From these results, we conclude that barriers separating metabasins composed of proximate energy minima, rather than barriers between individual energy minima, control the physical aging process. We also show that, after some finite time, the mobility following shear and thermal cycle appears to behave similarly to the mobility without perturbations; however, the system is at different regions of the energy landscape in these two cases, which implies that mobility alone does not characterize the state of the system.