Interpretation of the apparent activation energy of glass transition

Abstract

The temperature dependence of the viscosity of glass is a major concern in glass research. The apparent activation energies obtained from Arrhenius plots often show unusual values larger than bond energies, which makes the interpretation of the activation energy difficult. In this study, a reasonable interpretation of the apparent activation energy is obtained along similar lines as those adopted in solid state physics and chemistry. In contrast to the widely held view that the transition occurs at the reference temperature T0 according to the Vogel-Fulcher-Tammann formula, in this work the structural change observed at the calorimetric temperature Tg is considered as a transition from the liquid to solid phases. The energy barrier for atom rearrangements significantly changes in the transition range with width ΔTg. This change in the energy barrier alters the manner in which the apparent activation energy constitutes the Arrhenius form. Analysis of available experimental data shows that the real value of energy barrier is significantly smaller than the apparent activation energy, and the obtained values are in the reasonable range of energy expected for chemical bonds. The overestimation of the apparent activation energy depends on the ratio Tg/ΔTg, which is larger for fragile glasses than for strong glasses. Importantly, the linear term in the temperature dependence of the energy barrier does not appear in Arrhenius plots. This explains why the temperature dependence of viscosity for strong glasses obeys well the Arrhenius law, despite that the temperature dependence of energy barrier is expected for every glass.