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 ti mes in terms of the peak frequency f(max)(T) of dielectric loss epsilo n ''(omega) and the dc-conductivity sigma(dc)(T) of several glass-form ing liquids, covering 12 decades in the peak. frequency f(max) and 9 d ecades 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 tempera ture-derivative method and compared to the viscosity data eta(-1)(T). While most liquids reveal a common Vogel-Fulcher-Tammann (VFT) behavio r 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 sig ma(dc)(T) and eta(-1)(T) and attains activation energies in excess (si milar to 40% for alcohols) of those related to translational processes . For many samples a departure from the VFT behavior occurs at lower t emperatures 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.