Mechanical rejuvenation in polymer glasses: fact or fallacy?

Abstract

There has been considerable interest in the impact of mechanical stresses on thethermodynamic state of glassy materials over the past quarter century—beginningwith Struik’s hypothesized erasure of ageing by large stresses. In particular, therehas been a recent application of the ideas of erasure or rejuvenation in the generalliterature on glasses and glass-like substances. For this reason we here examine theevidence for and against rejuvenation and show several sets of data that arguestrongly against rejuvenation or erasure of ageing as viable explanations for theinteractions 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 glasstransition allow the direct measurement of the evolution of the glassyresponse both with and without applying any stress. For example, intorsional dilatometry experiments where the volume recovery is measuredsimultaneously and in the same apparatus as the mechanical response, it is foundthat the baseline volume recovery is unaffected by the applied stress. Thesecond class of experiments has been post-yield experiments, where therejuvenation hypothesis is more difficult to reject. However, observations thatpost-yield ageing leads to evolution into a different ‘equilibrium’ state asmeasured by the recovered yield stress suggest that yielding does notrejuvenate the glass, but it may lead to a sort of polyamorphism or newdeformation induced phase. We also present results of mechanically induced‘implosion’ in polymers far below the glass temperature that suggest thatmechanical stimuli may actually accelerate ageing rather than rejuvenating theglass. Finally, the origins of the power-law dependence on ageing time(also termed elapsed or waiting time) of the mechanical relaxation timeare discussed, and the use of the elapsed time as a natural variable todescribe ageing is shown not to be appropriate for structural glasses.