Physical Aging and Thermal and Mechanical Rejuvenation in Glassy Amorphous Polymers: A Review

The term "physical aging" was first coined by Struik to separate the relaxation of glassy polymers from those of other time-dependent phenomena such as chemical aging. Glassy amorphous polymers are in non-equilibrium states below their glass transition temperature (Tg). A glassy polymer gradually moves toward its equilibrium state with time when it is kept below Tg. The amorphous polymer can be heated again, known as thermal rejuvenation, to erase its memory and return the polymer to its non-equilibrium state. Many experimental and mathematical simulations have been conducted so far to investigate the rejuvenation of glassy polymers by applying mechanical stresses, i.e., mechanical rejuvenation. However, to turn a glassy polymer to its non-equilibrium state by imposing mechanical stress on the polymer is still under debate. Some scientists agree and others disagree with the idea. It was found that small strains can over-age an amorphous polymer, however, large strains can cause mechanical rejuvenation. In pre-yield regime, straining a polymer will not change its state and mechanical rejuvenation does not occur. In post-yield regime, straining will change the state of the system, but, it is totally different from that of an unaged sample. In this article, our aim is to present some information on physical aging and also discuss a controversial topic of thermal and mechanical rejuvenation of glassy amorphous polymers by reviewing the recently published papers.