Aging dynamics in a model polymeric glass-former far from equilibrium

Glasses age towards equilibrium in a manner characterized by signatures identified by Kovacs over 50 years ago. Here we probe the range of validity of these signatures via simulations of aging in polymer glasses, driven much farther from equilibrium and at higher temperatures than typically possible in experimental glasses. Simulated glasses exhibit Kovacs signatures of glassy aging only when both the temperature at which they depart equilibrium and their aging temperature are below the onset temperature T_A of non-Arrhenius equilibrium dynamics. For higher temperatures, Kovacs signatures are lost, signaling that the substance is not genuinely ‘glassy’ despite being removed from equilibrium. Below T_A, the we find that the timescale for aging in overdense glasses can be predicted with zero parameters from equilibrium behavior via an Arrhenius interpolation between T_A and the temperature of departure from equilibrium, again identifying T_A as key to understanding the nature of glasses. We compare these findings to expectations of aging behavior from the Tool-Narayanaswamy-Moynihan model.