Interpretation of the apparent activation energy of glass transition

Abstract

The temperature dependence of the viscosity of glass is a major concern in the field of glass research. Strong deviations from the Arrhenius law make the interpretation of the activation energy difficult. In the present study, a reasonable interpretation of the apparent activation energy is demonstrated along similar lines as those adopted in solid-state physics and chemistry. In contrast to the widely held view that phase transition in glass occurs at the reference temperature $T_{0}$ according to the Vogel--Fulcher--Tammann formula, in the present work the transition observed at the glass-transition temperature $T_{g}$ is regarded as a phase transition from the liquid to solid phases. A distinct feature of glass is that the energy barrier significantly changes in the transition range with width $\Delta T_{g}$. This change in the energy barrier alters the manner in which the apparent activation energy constitutes the Arrhenius form. Analysis of available experimental data showed that the actual energy barrier is significantly smaller than the apparent activation energy, and importantly, the values obtained were in the reasonable range of energy expected for chemical bonds. The overestimation of the apparent activation energy depends on the ratio $T_{g}/\Delta T_{g}$, which explains the existence of two types of glasses strong and fragile glasses. The fragility can be re-interpreted as an indication of the degree of increase in the energy barrier when approaching $T_{g}$ from high temperatures. Since no divergence in viscosity was observed below $T_{g}$, it is unlikely that a transition occurs at $T_{0}$.