Evidence for heterogeneous bulk melting dominating the transition of organic stable glasses

For over a century, scientists have tried to understand the mechanisms behind the glass transition and the dynamics of glassy systems. Part of the difficulty arises from the time scales involved in these processes. The low stability of conventional glasses, obtained by cooling down a melted material, have determined for years how we study glass dynamics, focused basically on the aging regime, i.e., performing isothermal treatments below the limiting fictive temperature (T_f) of the glass, allowing the glass to evolve towards a more stable configuration. But the evolution of glasses above their T_f in isothermal regime, what we could call softening of rejuvenation as the glass progressively loses stability, has been barely explored at temperatures not far above the conventional glass transition temperature of the material, T_g. The time scales in this case are extremely short for conventional glasses, and in order to study rejuvenation one would need either glasses of high stability (to increase the measuring time) or advanced instrumentation (able to work in very short times). This scenario has changed with the ability to prepare glasses of a wide range of stabilities by means of PVD and the development of fast scanning nanocalorimetry. By performing isothermal treatments at temperatures spanning more than 30 K above the conventional T_g of the material we show that the rejuvenation process in stable glasses takes place via two different mechanisms: i) a homogeneous softening of the glass and ii) the formation of liquid patches in the bulk of the glass, which grow transforming the glass straight into the liquid, dominating the pace of the transformation and resembling the nucleation and growth process characteristic of the melting of crystals. Surprisingly, these two mechanisms are present even in glasses with stabilities close to those of the conventional glass, although clear differences can be found in the number of initial liquid patches and the transformation rate.