Hierarchical aging pathways and signatures of thermodynamic transition in molecular glasses

Abstract

When a liquid is supercooled, its structural equilibration time τeq increases sharply approaching the glass transition temperature Tg, below which it is kinetically arrested in the out of equilibrium glassy state. Upon annealing below Tg, such glassy state relaxes toward the equilibrium state. There is growing evidence that such relaxation process is quite complex, influenced by the complexity of the potential energy landscape. Here we report the observation of a hierarchical aging pathway in the process of the glass-to-supercooled liquid transition in various molecular glass forming systems. Differential scanning calorimetry reveals that the glassy state upon annealing below Tg firstly enters a transient metastable supercooled liquid state with slightly higher enthalpies than that of the equilibrium supercooled liquid state when extrapolated to below Tg. This observation is also confirmed by structural measurements via Raman scattering. The dynamics of the transient metastable-to-stable supercooled liquid transition exhibits characteristics of thermodynamic transition at spinodal temperature Tsp slightly above Tg, leading to anomalous behaviors such as the appearance of a transition-like behavior in the plot of fragility measurements. These observations imply that the free energy landscape of the supercooled liquid develops complexity with qualitative changes approaching Tg and could have strong influence on the process of the glass transition.