SuperArrhenius character of supercooled glass-forming liquids

We picture the viscosity and alpha-relaxation time of a supercooled liquid as Arrhenius-like at temperatures well above the melting point, and as superArrhenius, i.e., describable in terms of a temperature-dependent activation free energy, E(T), at temperatures near the glass transition temperature, T-g. We discuss this in terms of a superArrhenius factor Delta(T-g) = [E(T-g) - E-infinity]/T-g, where E-infinity is the activation free energy at high T. We relate Delta(T-g) to the property called "fragility", and discuss its relation to non-exponential relaxation and to activated dynamics. We postulate that supercooled liquids are broken into supermolecular domains, frustrated phase-related entities which are to be distinguished from species-specific local clusters. (C) 1998 Elsevier Science B.V. All rights reserved.