Defect diffusion and temperature vs. density effects for glass formers

The ratio of the apparent isochoric activation energy to the isobaric activation enthalpy, EV∗/H∗ or EV/EP, is treated within the framework of the defect diffusion model (DDM). This ratio is a quantitative measure of the relative sensitivity of kinetic processes to changes in volume and temperature, respectively. The new equation is tested using dielectric relaxation data for poly(vinyl acetate) and predictions are made for polycarbonate, propylene carbonate and glycerol. Good agreement between theory and experiment is found. Finally, the physics of the ratio is identified. It is shown that the volume change upon clustering is responsible for the magnitude of the ratio.