Structural relaxation near the glass transition temperature

Enthalpy relaxation processes on a model bulk glass, As2Se3, were studied b elow the glass transition temperature, T-g, as well as at temperatures clos e to the glass transition overshoot. Extents of relaxation processes below T-g were measured by enthalpy of endothermic relaxation peak. Relaxation pr ocesses above T-g were determined by melting enthalpies of partly crystalli zed glass during relaxation. We found an exponential temperature dependence of physical aging. Completely annealed glasses reach the equilibrium havin g excess of enthalpy compared to the equilibrium supercooled liquid at the same temperature. Using the Tool-Narayanaswamy-Moynihan (TNM) model, we fou nd that the fictive temperature, T-r, decreased during isothermal anneallin g toward aging temperature, T-a, and at equilibrium is equal to annealing t emperature, T-a. When the glass comes to equilibrium the distribution of re laxation times decreases. The description of relaxation processes at the te mperatures slightly above T-g is more complicated. The nuclei are created i n the first step. In the second one these nuclei can create thermodynamical ly non-equilibrium agglomerates, which consequently can decrease their ener gy by becoming amorphous. (C) 2001 Elsevier Science B.V. All rights reserve d.