Enthalpy recovery on thermal cycling within the non-equilibrium state of a glass

Abstract

The so-called reversible structural relaxation of a Ni-based glassy metal-alloy, two organic polymers and As 2Se 3 glass has been studied by differential scanning calorimetry (DSC) on their thermal cycling below T g. The similarity of the relaxation in these glasses creates a difficulty in reconciling the earlier conclusion that reversible relaxation in glassy metal-alloys involves changes in both chemical (i.e., a preference for unlike neighbors) and topological short range orders. Here, its origin is seen in the time- and temperature-dependent regain of the equilibrium state. Groups of atoms diffuse fast enough to allow local loss of configurational enthalpy and entropy on annealing. On heating, they absorb energy and reach their new equilibrium configurations of higher enthalpy and entropy while still below T g. Hence each mode of atomic diffusion in the structure has its own `mini glass-softening endotherm', and reversible relaxation is a reflection of a broad distribution of diffusion times arising from temporal and spatial variations in the atom’s environment. The relaxation is modeled in terms of the stretched-exponential, non-linear character of glass relaxation. An experimentally testable consequence is that reversible relaxation will be minimum in a glass with a narrow distribution of diffusion times and low contribution from the sub- T g relaxation processes.