Relationship between the mobility in glass forming liquids and excess thermodynamic quantities

The key characteristic of the glass is the dramatic decrease in mobility as the temperature is decreased towards the glass transition temperature, where this decrease in mobility can be ten or more orders-of-magnitude more than expected from the Arrhenian response exhibited at temperatures well above Tg. One model that has been proposed to describe this super-Arrhenian behavior is due to Adams and Gibbs (J. Chem. Phys., 1965), where the logarithm of the mobility is postulated to be proportional to 1/TSc where Sc is the configurational entropy. Alternatively, in order to describe the nonlinear viscoelastic behavior of glassy polymers constitutive models have been developed (Caruthers, et al., Polymer, 2004; Medvedev and Caruthers, J. Rheology, 2013) where the logarithm of the mobility is postulated to be proportional to 1/Uc where Uc is the configurational internal energy. In this talk we will critically compare the ability of 1/TSc, 1/Uc and other configurational and excess thermodynamic quantities to quantitatively describe the mobility of equilibrium liquids above Tg as a function of temperature and pressure.