Theoretical analysis of the temperature and pressure dependences of the viscosity in supercooled liquids

In this paper, a method to analyze the temperature and pressure dependences of the viscosity or of the structural relaxation time in supercooled liquids is shown from the point of view of chemical bonding. The model used in this analysis is the Bond Strength-Coordination Number Fluctuation (BSCNF) model originally proposed by one of the authors. A theoretical relationship between the Vogel temperature T-0 of the well-known Vogel-Flucher-Tammann (VFT) equation and our model parameters which contain some microscopic information on bonding connectivity among the structural units that form the melt has been obtained. Using the theoretical relationship, we analyzed the dynamics of two kinds of molecular glass forming liquids under pressure, (a) 4,4'-methylenebis(N,N-diglycidylaniline), and (b) BMPC: 1,1'-di(p-methoxyphenyl)cyclohexane. The result obtained suggests that by applying pressure in these materials, the intermolecular cooperativity is suppressed due to increases in the mean total bond strength and the fluctuations of the structural units.