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 Sc(T). Combination of Adam-Gibbs and Vogel-Fulcher equations yields an expression for Sc(T) which can be compared with experimental data. Good agreement is found for a range of temperatures near Tg<T<TB which depends on the fragility of the liquid and on the presence of a β relaxation. For fragile liquids, TB coincides with a qualitative change in the temperature dependence of the relaxation time scale or viscosity, with the temperature Tβ, where Johari-Goldstein-type β processes tend to merge into the α process, and with other crossover temperatures. For nonfragile liquids, TB/Tg increases, and the deviations from the Adam-Gibbs equation weaken or disappear altogether. The significance of TB, and of the Sc(T) temperature dependence implied by the Vogel-Fulcher theory fitting, are discussed in terms of the landscape paradigm.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry