Liquid-glass transition: Dependence of the glass transition on heating and cooling rates

The free-volume model, recently extended by the present authors to account for the equilibrium thermodynamic behavior of dense liquids and glasses, is used as a basis for a relaxation theory of the kinetic effects observed in the heat capacity Cp near the glass transition temperature Tg. Most importantly, we have included the effects of the very strong temperature dependence of structural relaxation rates near Tg. Results for the value of Tg and the shape of Cp are dependent on the heating and cooling rates of the measurements, in good qualitative agreement with experiment. In particular, Tg depends linearly on the logarithm of the heating rate, with a value of the slope in excellent agreement with experiment for 0.60KNO3-0.40Ca(NO3)2.