Abstract
The two-exponential Sheffield equation of viscosity η(T) = A1·T·[1 + A2·exp(Hm/RT)]·[1 + C·exp(Hd/RT)], where A1, A2, Hm, C, and Hm 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(Hm/RT)]·[1 + C·exp(Hd/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 Tg 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 Hm for both salol and α-phenyl-o-cresol.
Highlights
The salient feature characterizing a supercooled liquid is the dramatic increase of viscosity η(T)with decreasing temperature T, which may encompass some 15 orders of magnitude over a temperature range of almost several hundred K [1,2,3,4,5,6,7]
Apart from the clear physical parameters used in the models, one of important features of the models is the asymptotic description of viscosities far from the transformation range, e.g., near the glass transition temperature Tg
Utilization of the Sheffield equation of viscosity for glass-forming organic materials is successfully demonstrated for two cases—salol and α-phenyl-o-cresol
Summary
The salient feature characterizing a supercooled liquid is the dramatic increase of viscosity η(T)with decreasing temperature T, which may encompass some 15 orders of magnitude over a temperature range of almost several hundred K [1,2,3,4,5,6,7]. The interest in analyzing the viscous flow in glass-forming materials is not diminishing, with many novel findings having occurred over the last decade [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. The deviation from the Arrhenius-type behavior can be described by the activation energy of the viscous flow Q(T), dependent on temperature T. The typical variation of the activation energy of the flow with temperature is illustrated by Figure 1
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