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

Abstract

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 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 relaxation bears dominant role in both phenomena in proximity of the glass transition temperature, , a broad view on a much wider temperature range indicates that vitrification and physical aging are mediated by non‐ 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‐ 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.

Advancements in Novel Mechano-Rheological Probes for Studying Glassy Dynamics in Nanoconfined Thin Polymer Films

“…As the relaxation time increases tremendously on reducing the temperature below the T g , it is evident physical aging and glass transition are related, and the interplay is complex. 26 For a given glass with a thermomechanical history (e.g., cooling rate), the transition point of it into a glass is rather described by fictive temperature (T f ), the temperature in which a glass at a certain thermodynamic state would be at metastable equilibrium corresponding to the supercooled liquid (see Figure 1). So, the evolution of T g with physical aging certainly signifies thermodynamic driving (recovery) force toward reaching the most stable state of the metastable equilibrium and entropy of the polymer leading to densification.…”

“…The polymer initially cools in state (a) (see Figure ). If it could, it would go to state (c) and crystallize, but it ends up in state (b), where it slowly traverses back toward state (a) upon physical aging. , T g exhibits a dependence on cooling rate, with faster cooling rates resulting in the formation of glasses characterized by higher T g values (e.g., T g1 > T g2 ) in Figure . The time available for thermodynamic establishment of faster fluctuations is reduced at high cooling rates, resulting in higher T g at experimental time scales.…”

“…The structural relaxation time of physical aging depends on the temperature and time associated with the aging of the polymer. As the relaxation time increases tremendously on reducing the temperature below the T g , it is evident physical aging and glass transition are related, and the interplay is complex . For a given glass with a thermomechanical history (e.g., cooling rate), the transition point of it into a glass is rather described by fictive temperature ( T f ), the temperature in which a glass at a certain thermodynamic state would be at metastable equilibrium corresponding to the supercooled liquid (see Figure ).…”

“…Most of the review articles on confined glassy polymers ,,− have been focused particularly on the glass transition phenomenon in great detail. ,,, Those works certainly considered aspects of pseudo-thermodynamic thermal transitions and viscoelastic dynamic relaxation processes. Still, descriptions of “mechanical and rheological testing” approaches were largely missing.…”