Origin of glass fragility and Vogel temperature emerging from Molecular dynamics simulations

Vitreous silica is modelled by Molecular dynamics. Glass structure is transferred into undirected graph and decomposed into disjoint structural units that are ideally mixed to calculate configuration entropy. A good agreement of the approach with experimental heat capacity drop at Tg is demonstrated. Entropy is related with structural evolution of the obtained units; among them dangling oxygen dominantly effects low-temperature course of entropy. Configuration entropy is fitted by two parabolas; the one, corresponding to lower temperatures, introduces temperature T⁎, at which structure is completely frozen. It is proposed T⁎ is a counterpart of Vogel temperature in Vogel-Fulcher-Tammann (VFT) equation. A new model of viscosity is introduced as the combination of the quadratic dependence of configuration entropy and Adam and Gibbs equation. The model removes the singularity at Vogel temperature, predicts existence of strong and fragile glasses, and provides a base for fitting viscosities of real glasses. Fragility is associated with quickness of structural response to temperature changes and the distance between Tg and T⁎. The validity of the model is demonstrated on experimental data of viscosity for SiO2 and B2O3 glasses.