Structural relaxation near the glass transition temperature

Enthalpy relaxation processes on a model bulk glass, As2Se3, were studied below the glass transition temperature, T-g, as well as at temperatures close to the glass transition overshoot. Extents of relaxation processes below T-g were measured by enthalpy of endothermic relaxation peak. Relaxation processes above T-g were determined by melting enthalpies of partly crystallized glass during relaxation. We found an exponential temperature dependence of physical aging. Completely annealed glasses reach the equilibrium having excess of enthalpy compared to the equilibrium supercooled liquid at the same temperature. Using the Tool-Narayanaswamy-Moynihan (TNM) model, we found that the fictive temperature, T-r, decreased during isothermal annealling toward aging temperature, T-a, and at equilibrium is equal to annealing temperature, T-a. When the glass comes to equilibrium the distribution of relaxation times decreases. The description of relaxation processes at the temperatures slightly above T-g is more complicated. The nuclei are created in the first step. In the second one these nuclei can create thermodynamically non-equilibrium agglomerates, which consequently can decrease their energy by becoming amorphous. (C) 2001 Elsevier Science B.V. All rights reserved.