Quantitative model of super-Arrhenian behavior in glass forming materials

The key feature of glass forming liquids is the super-Arrheman temperature dependence of the mobility, where the mobility can increase by ten orders of magnitude or more as the temperature is decreased if crystallization does not intervene. A fundamental description of the super-Arrheman behavior has been developed; specifically, the logarithm of the relaxation time is a linear function of 1/<(U)overbar>(x), where <(U)overbar>(x) is the independently determined excess molar internal energy and B is a material constant. This one-parameter mobility model quantitatively describes data for 21 glass forming materials, which are all the materials where there are sufficient experimental data for analysis. The effect of pressure on the log a mobility is also described using the same <(U)overbar>(x)(T,p) function determined from the difference between the liquid and crystalline internal energies. It is also shown that B is well correlated with the heat of fusion. The prediction of the B/<(U)overbar>(x) model is compared to the Adam and Gibbs 1 /T<(S)overbar>(X) model, where the B/<(U)overbar>(x) model is significantly better in unifying the full complement of mobility data. The implications of the B/<(U)overbar>(x) model for the development of a fundamental description of glass are discussed.