A thermodynamic approach to the viscous behaviour of glass-forming liquids

The temperature dependence of the viscosity of undercooled melts has an important role in the study of the nucleation, crystal growth and the glass-forming ability of materials. A novel expression for the Gibbs free energy difference ΔG between the undercooled liquid and the corresponding equilibrium solid has been reported by the authors and their coworkers using the hole theory of liquids as well as by expanding the free energies of the liquid and solid phases using Taylor series. The present paper elucidates how these expressions for ΔG are used to derive the temperature dependence of the viscosity of glass-forming melts. The temperature dependence of relaxation phenomena is also reviewed in the framework of the Adam and Gibbs approach based on the configurational entropy concept. The importance of the ideal glass transition temperature in the viscous behaviour of glass-forming liquids is also discussed.