Relaxation Kinetics in Temperature Jump Experiments Above Calorimetric Glass Transition via Fast Scanning Calorimetry

ABSTRACT We investigate the kinetics of physical aging in single‐ and mixed‐modifier phosphate glasses following temperature down‐jumps to aging temperatures above their calorimetric glass transition temperature but below their fictive temperature T f , using fast scanning calorimetry (FSC). This technique enables ultrafast heating and cooling (up to ∼30 000°C/s), enabling structural relaxation experiments over very short timescales on a single sample by repeatedly resetting its thermal history. Our results indicate that the non‐exponentiality (Kohlrausch–Williams–Watts) parameter remains nearly temperature‐independent for both aging and shear relaxation. Modeling of the aging kinetics with the Tool–Narayanaswamy–Moynihan formalism revealed negligible nonlinearity, indicating that the role of the fictive temperature in governing aging kinetics diminishes at these elevated aging temperatures above . This reduction in nonlinearity can be understood as a consequence of greater spatial homogeneity in structural rearrangements during aging, as more of the energy landscape becomes accessible at T > . Additionally, the average timescales for enthalpy and recovery are closely coupled to that of shear relaxation in the corresponding supercooled liquid for the single‐modifier system, whereas a large temporal decoupling is observed for the mixed‐modifier case. These findings may have important implications for understanding and bridging the behavior between the glassy and supercooled liquid states, and for elucidating mixed‐modifier effects on different relaxation timescales.