Transition zone theory of the glass transition

Abstract An energy-based theory of the glass transition is described and validated by evaluation of crystallization and viscosity data of sixteen diverse systems, ranging from the fragile o-terphenyl to the strong SiO2 glass formers, including glass formation in non-crystallizing polymers. Transition zone theory demonstrates an inverse temperature dependence of the entropy of activation for crystallization and viscous relaxation which results in a temperature at which their free energies of activation are equivalent. Below this temperature there is a greater probability of crystallization than of relaxation. Under such conditions, crystal-like organization can propagate only over the internal length scales for which the system has relaxed, i.e. a few nanometers. However, that structural organization makes the barrier to bulk liquid relaxation insurmountable, resulting in glass formation. The temperature of the crystallization-relaxation free energy of activation equivalence point, defines the glass transition temperature. The temperature dependent slope of the free energy of activation for relaxation at the equivalence point reflects the fragility of the system. The time-temperature dependence of viscous relaxation shifts the crystallization-relaxation free energy of activation equivalence point, consistent with the sample history dependence of Tg. Crystal-like formation over nanometer-length scales, which can still occur below Tg, is shown to account for glass aging.