Supercooled liquids and the glass transition: Temperature as the control variable

Abstract

It has long been appreciated that both temperature and density play roles in determining the extremely super-Arrhenius, low-temperature behavior of the viscosity and long α-relaxation times that characterize fragile supercooled liquids. But what has not been generally appreciated, and what we believe we have established (by focusing on a model-free analysis in terms of temperature and density, rather than upon temperature and pressure) is that over the range of densities and temperatures spanned by the experiments carried out at 1 atm pressure, temperature is the dominant control variable. This information is essential input to the formulation of a theory or model of the long-time dynamics of low-temperature fragile liquids, and it suggests a focus on activated dynamics rather than on free volume. This work indicates that, except possibly at very high densities (very high pressures), the glass transition is not a result of congestion due to a lack of free volume.