The Defect Diffusion Model and Isochoric Energy and Isobaric Enthalpy for Glass Formers

The defect diffusion model produces stretched exponential relaxation, in supercooled liquids, through the sub-diffusive motion of defects. The aggregation of the defects produces a Vogel-Fulcher type law for the divergence of the time scale at a critical temperature. The model is employed to calculate the ratio of the apparent isochoric activation energy to the isobaric activation enthalpy, $E_{V}$*/$H*$ or $E_{V}$/$E_{P}$,. This ratio measures the relative sensitivity of kinetic processes to changes in volume and temperature respectively. This ratio equation is tested using dielectric relaxation data for poly(vinyl acetate), viscosity data for glycerol and ionic conductivity data for poly(propylene glycol) containing LiCF$_{3}$SO$_{3}$. Good agreement between theory and experiment is found using model parameters previously published.