The bond strength–coordination number fluctuation model of viscosity: Concept and applications

Abstract

The viscous flow of materials above the glass-transition temperature is a dissipative process which occurs accompanied by the internal friction and thus involves many-body interactions among the constituent elements. To understand the structural relaxation of supercooled liquids, we have proposed the Bond Strength–Coordination Number Fluctuation (BSCNF) model that describes the temperature dependence of the viscosity in terms of the mean values of the bond strength E0 and the coordination number Z0, and their fluctuations ΔE and ΔZ of the structural units that form the melt. In this work, firstly, after reviewing the concept and formulations of the BSCNF model, we show some applications of the model to polymers and ionic liquids. We also discuss on a theoretical foundation of the VFT law in terms of the BSCNF model, and its relation with the WLF equation is presented. A directionality in the study of structural relaxation is pointed out based on the statistical aspect of the BSCNF model. Finally, in connection with the decoupling mechanism which is crucial to understand the properties of superionic polymer conductors, we mention on the Arrhenius crossover phenomenon in the ionic conduction.