On Viscous Flow in Glass-Forming Organic Liquids

The two-exponential Sheffield equation of viscosity η(T) = A₁·T·[1 + A₂·exp(H_m/RT)]·[1 + C·exp(H_d/RT)], where A₁, A₂, H_m, C, and H_m are material-specific constants, is used to analyze the viscous flows of two glass-forming organic materials-salol and α-phenyl-o-cresol. It is demonstrated that the viscosity equation can be simplified to a four-parameter version: η(T) = A·T·exp(H_m/RT)]·[1 + C·exp(H_d/RT)]. The Sheffield model gives a correct description of viscosity, with two exact Arrhenius-type asymptotes below and above the glass transition temperature, whereas near the T_g it gives practically the same results as well-known and widely used viscosity equations. It is revealed that the constants of the Sheffield equation are not universal for all temperature ranges and may need to be updated for very high temperatures, where changes occur in melt properties leading to modifications of A and H_m for both salol and α-phenyl-o-cresol.