Universal representation of viscosity in glass forming liquids

Temperature dependence of the viscosity eta(T) of fragile and strong polymer and non-polymer glass formers was analyzed. A temperature T-X was introduced which marks the boundary between a high-and low-temperature regime, where different approaches are applied to interpolate eta = eta(T). At T<T-X a master curve is obtained for polymer and non-polymer glass formers by scaling the quantity (T-g/T-1) with a factor F = T-X/(T-X-T-g) where T-g is the glass transition temperature. F is related to the steepness parameter m via F similar or equal to 0.079 m. A continuous transition from the master curve of non-polymer systems to the one of polymers is found if molecular weight is systematically increased. T-X is found to be close to the critical temperature T-c reported from analyses of the dynamic susceptibility by mode coupling theory. It is tentatively concluded that the magnitude of the interval between T-X and T-g defines the degree of fragility. At T>T-X deviations from the master curves occur, and the Vogel-Fulcher-Tammann equation provides a very good fit to highest temperatures. The two master curves at T<T-X can be interpolated by several formulae. (C) 1998 Elsevier Science B.V.