DSC to explore activation energy landscape of glass relaxation

Abstract

This review paper discusses the methods of estimating the activation energy of the glass relaxation from DSC experiments that include heating through the glass transition, short time annealing (physical aging) far below T_g, and annealing into equilibrium just below T_g. Isoconversional analysis of the data obtained on heating through the glass transition yields large (hundreds kJ/mol) activation energy that decreases with increasing temperature as expected for the alpha relaxation. Short time annealing runs conducted around 0.8T_g give rise to small (tens kJ/mol) activation energies, whose values are similar to those of the beta relaxation. Increasing the annealing temperature in the region 0.8T_g – T_g causes the activation energy to rise toward the values characteristic of the alpha relaxation that suggests that the overall relaxation in the respective temperature region is driven by the mixed dynamics that comprises the alpha and beta processes. As temperature rises from well below to well above T_g the activation energy passes through a maximum around T_g. The mixed relaxation dynamics is also observed on annealing into equilibrium that demonstrates that with the progress of relaxation the activation energy increases between the values characteristic of the beta and alpha relaxation.