Mechanical rejuvenation in polymer glasses: fact or fallacy?

There has been considerable interest in the impact of mechanical stresses on the thermodynamic state of glassy materials over the past quarter century-beginning with Struik's hypothesized erasure of ageing by large stresses. In particular, there has been a recent application of the ideas of erasure or rejuvenation in the general literature on glasses and glass-like substances. For this reason we here examine the evidence for and against rejuvenation and show several sets of data that argue strongly against rejuvenation or erasure of ageing as viable explanations for the interactions between large mechanical stimuli and the structure of the glass. Two particular experimental conditions are of significance here. First, measurements that are made in the sub-yield region and close to the glass transition allow the direct measurement of the evolution of the glassy response both with and without applying any stress. For example, in torsional dilatometry experiments where the volume recovery is measured simultaneously and in the same apparatus as the mechanical response, it is found that the baseline volume recovery is unaffected by the applied stress. The second class of experiments has been post-yield experiments, where the rejuvenation hypothesis is more difficult to reject. However, observations that post-yield ageing leads to evolution into a different 'equilibrium' state as measured by the recovered yield stress suggest that yielding does not rejuvenate the glass, but it may lead to a sort of polyamorphism or new deformation induced phase. We also present results of mechanically induced 'implosion' in polymers far below the glass temperature that suggest that mechanical stimuli may actually accelerate ageing rather than rejuvenating the glass. Finally, the origins of the power-law dependence on ageing time (also termed elapsed or waiting time) of the mechanical relaxation time are discussed, and the use of the elapsed time as a natural variable to describe ageing is shown not to be appropriate for structural glasses.