Physical aging and vitrification in polymers and other glasses: Complex behavior and size effects

The paradigmatic view on the transformation of a supercooled liquid into a glass, so-called vitrification or glass transition, and the subsequent time evolution of the non-equilibrium glass, addressed as physical aging, relies on the exclusive role of the main $\alpha$ relaxation with super-Arrhenius temperature dependence. The aim of the present review is to carefully scrutinize the wealth of recent experimental results, above all in polymeric glasses, showing the relevance of other relaxational mechanisms in both vitrification and physical aging. While the $\alpha$ relaxation bears dominant role in both phenomena in proximity of the glass transition temperature, $T_g$, a broad view on a much wider temperature range indicates that vitrification and physical aging are mediated by non-$\alpha$ mechanisms of equilibration. This review also shows that a reduction of the typical time scale of equilibration can be achieved in glasses with large free interface, namely with reduced sample size. In this way, fast non-$\alpha$ mechanisms of equilibration can be exploited to convey glasses to low energy states in experimentally feasible time scales, thereby attaining information on the existence of the so-called “ideal glass” in small sized samples.