Dynamics of glass-forming liquids. II. Detailed comparison of dielectric relaxation, de-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 ε″(ω) and the de-conductivity σ_dc(T). of several glass-forming liquids, covering 12 decades in the peak frequency f_max and 9 decades in σ_ac. 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 σ_dc(T) are analyzed using the temperature-derivative method and compared to the viscosity data η^-1 (T). While most liquids reveal a common Vogel-Fulcher-Tammann (VFT) behavior for f_max, σ_dc and η^-1 in an extended temperature range T≥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 σ_dc(T) and η^-1(T) and attains activation energies in excess (∼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.