Revealing the nature of glass by the hyperquenching-annealing-calorimetry approach

C. Austen Angell has been a great inspiration for liquid and glass scientists including myself. He was a close collaborator of mine on glass dynamics and thermodynamics, particularly on the sub-T_g relaxation in glass. Here, in memory of his seminal contributions to science and our collaboration, I revisit our published data and analyze our unpublished data, as well as review some of important results reported recently in the literature, concerning the sub-T_g relaxation in glass, a field on which Austen has profoundly impacted. I highlight new insights into the dynamics, thermodynamics, and structural heterogeneity in glass far from equilibrium, based on the hyperquenching-annealing-calorimetry (HAC) experiments. I describe key extensions and applications of the HAC approach in glass research. Finally, I present the perspectives and challenges regarding the sub-T_g relaxation in glasses with fictive temperatures well above T_g.