Dynamics of glass-forming liquids. V. On the link between molecular dynamics and configurational entropy

We compare dielectric relaxation τ(T) data of several low molecular weight glass-forming liquids with the predictions of the Adam-Gibbs theory using experimental data for the configurational entropy S_c(T). Combination of Adam-Gibbs and Vogel-Fulcher equations yields an expression for S_c(T) which can be compared with experimental data. Good agreement is found for a range of temperatures near T_g<T<T_B which depends on the fragility of the liquid and on the presence of a β relaxation. For fragile liquids, T_B coincides with a qualitative change in the temperature dependence of the relaxation time scale or viscosity, with the temperature T_β, where Johari-Goldstein-type β processes tend to merge into the α process, and with other crossover temperatures. For nonfragile liquids, T_B/T_g increases, and the deviations from the Adam-Gibbs equation weaken or disappear altogether. The significance of T_B, and of the S_c(T) temperature dependence implied by the Vogel-Fulcher theory fitting, are discussed in terms of the landscape paradigm.