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

We compare dielectric relaxation $\mathrm{\tau (T)}$ data of several low molecular weight glass-forming liquids with the predictions of the Adam–Gibbs theory using experimental data for the configurational entropy $S_c\mathrm{(T).}$ Combination of Adam–Gibbs and Vogel–Fulcher equations yields an expression for $S_c\mathrm{(T)}$ which can be compared with experimental data. Good agreement is found for a range of temperatures near $T_g{\mathrm{<T<T}}_B$ 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 $T_{\beta },$ where Johari–Goldstein-type β processes tend to merge into the α process, and with other crossover temperatures. For nonfragile liquids, $T_B{\mathrm{/T}}_g$ increases, and the deviations from the Adam–Gibbs equation weaken or disappear altogether. The significance of $T_B,$ and of the $S_c\mathrm{(T)}$ temperature dependence implied by the Vogel–Fulcher theory fitting, are discussed in terms of the landscape paradigm.