Mechanical rejuvenation and overaging in the soft glassy rheology model

Abstract

Mechanical rejuvenation and over-aging of glasses is investigated through stochastic simulations of the soft glassy rheology (SGR) model. Strain-and stress-controlled deformation cycles for a wide range of loading conditions are analyzed and compared to molecular dynamics simulations of a model polymer glass. Results indicate that deformation causes predominantly rejuvenation, whereas overaging occurs only at very low temperature, small strains, and for high initial energy states. Although the creep compliance in the SGR model exhibits full aging independent of applied load, large stresses in the nonlinear creep regime cause configurational changes leading to rejuvenation of the relaxation time spectrum probed after a stress cycle. During recovery, however, the rejuvenated state rapidly returns to the original aging trajectory due to collective relaxations of the internal strain.

Deformation-induced accelerated dynamics in polymer glasses

“…The measured form of the acceleration factor is, however, quite different. It would be interesting to see if using the measured form of the acceleration factor could account for some of the limitations identified in this model [30].…”

“…Below yield, the claim of mechanical rejuvenation is even more controversial [27]. In this regime, deformation increases molecular mobility at the same time as aging decreases it [28], and experiments show that both the volume [29] and the relaxation times [28] of the glass quickly return to their previous states once the load is removed [30]. In this report, we reserve the term "rejuvenation" for the erasure of aging due to deformation, without comment on whether the yielded state truly resembles a younger glass or is something else altogether.…”

Ageing and rejuvenation in glassy amorphous polymers

“…Though not conclusive at this stage, these experiments seem to point to the fact that a sample that is mechanically cycled after quenching attains a structurally different state from one that is not. An attempt to reconcile these apparently paradoxical findings with the picture that rejuvenation is the 'erasure of ageing' has been attempted by Warren and Rottler (2008).…”

“…Their simulations seem to show that the increase in yield stress in slowly cooled samples is due to the stronger intrachain bonds that are formed in a macromolecular ensemble as a result of slower cooling. Warren and Rottler (2009) have simulated degrees of ageing in coarse-grained polymeric and atomistic Lennard-Jones samples by quenching them at a fixed rate and then holding them for different lengths of waiting times t w under constant pressure. Their MD simulations show that individual atoms do undergo infrequent, sudden large 'hops' but interestingly, the frequency of only the first hop seems to depend on the waiting time t w .…”

Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model

“…The system is deemed to explore a distribution of P(σ t ) and hopping rates, which is in fact the ReeEyring extension to EM but with the average flow a sum of stressed units, rather than the other way around. 65,66 Previous shear stress relaxation models [67][68][69] have assumed such processes, which are also consistent with stretched exponential stress relaxation. 70 There is then a distribution of shear rates at any one time, whose average value over time and space is…”