We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency fmax(T) of dielectric loss ε″(ω) and the de-conductivity σdc(T). of several glass-forming liquids, covering 12 decades in the peak frequency fmax and 9 decades in σac. Although de-conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of fmax(T). The fmax(T) and σdc(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 fmax, σdc and η-1 in an extended temperature range T≥Tm, some liquids deviate from this behavior by displaying a crossover at T=TA to an Arrhenius regime. In these cases the quantity fmax(T) decouples from the common curves for σdc(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 TB<Tm which tends to retard the glass transition. The onset of this qualitative change in the temperature dependence at TB turns out to be a characteristic temperature also in other experiments.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry