Dynamics of glass-forming liquids. I. Temperature-derivative analysis of dielectric relaxation data

We have measured the dielectric relaxation times of the a process of phenyl salicylate (salol) covering 11 decades in frequency. Being representative for the class of low molecular weight organic glass forming materials, the highly resolved temperature dependence of the dynamics in salol does not follow a particular function like the Vogel-Fulcher-Tammann (VFT) law over the entire accessible range of temperatures. In order to conduct a detailed and unambiguous analysis of the temperature dependence, we take advantage of the derivatives of the experimental log(f_max) values with respect to temperature, which allow us to either linearize the frequently used temperature laws or to resolve subtle changes in f_max(T) by decreasing the number of free parameters. In this manner we observe that none of the common routes for rationalizing the dynamics like Arrhenius, VFT, Souletie scaling, and idealized mode-coupling theory account for the experimental findings properly. However, we do observe a VFT behavior within the limits 265 K≤T≤320 K, i.e., for temperatures ranging from significantly above the glass transition at T _g= 220 K to far above the melting point.