Twofold Facet of Kinetics of Glass Aging.

We employ fast scanning calorimetry to monitor the isothermal aging kinetics in glassy polymers, complemented with measurements on other glasses. Apart from following the time evolution of the glass enthalpic state, we monitor the aging kinetics of the devitrification width on heating, ΔT_{dev}. We find that significantly below the glass transition temperature, T_{g}, the glass enthalpy attains equilibrium earlier than ΔT_{dev}, which evolves at long aging times toward enhanced heterogeneity. Hence, our results indicate that the description of time dependent evolution in glassy materials requires information beyond the mere description of its enthalpic state.