Dynamics of glass-forming liquids .2. Detailed comparison of dielectric relaxation, dc-conductivity, and viscosity data

We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency f(max)(T) of dielectric loss epsilon ''(omega) and the dc-conductivity sigma(dc)(T) of several glass-forming liquids, covering 12 decades in the peak. frequency f(max) and 9 decades in sigma(dc). Although de-conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of f(max)(T). The f(max)(T) and sigma(dc)(T) are analyzed using the temperature-derivative method and compared to the viscosity data eta(-1)(T). While most liquids reveal a common Vogel-Fulcher-Tammann (VFT) behavior for f(max), sigma(dc) and eta(-1) in an extended temperature range T greater than or equal to T-m, some liquids deviate from this behavior by displaying a crossover at T = T-A to an Arrhenius regime. In these cases the quantity f(max)(T) decouples from the common curves for sigma(dc)(T) and eta(-1)(T) and attains activation energies in excess (similar to 40% for alcohols) of those related to translational processes. For many samples a departure from the VFT behavior occurs at lower temperatures T-B < T-m which tends to retard the glass transition. The onset of this qualitative change in the temperature dependence at T-B turns out to be a characteristic temperature also in other experiments. (C) 1996 American Institute of Physics.