On Viscous Flow in Glass-Forming Organic Liquids

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.

On Structural Rearrangements Near the Glass Transition Temperature in Amorphous Silica

“…Although the transition is continuous and reversible, vitrification occurs differently when cooling the liquid down as no configuron clusters are available to form or disintegrate in the melt. At the same time, the CPT shows a continuous two-exponential function for the viscosity, which provides a rather exact description of viscosity–temperature relationships for both amorphous silica and other materials [ 24 , 44 , 75 , 99 ]. However, it should be emphasized that CPT with the equations used above operate with the volume of bonds (V 0 ) and configurons (V) and not with the so-called excess or free volume of the material, which is the specific volume of the material (the molecular weight divided by the density) minus the volume of molecules, which is typically assumed to be the volume of hard spheres representing atoms.…”

New paradigm for configurational entropy in glass-forming systems

“…4 and Fig. 5 may be considered as the argument against the 'universal' validity of the 'Stickel operator' analysis used for testing dynamic crossover phenomenon, [44][45][46][47][48][49][50] due to inherently coupling to pre-assumption of an omnipotent validity of the basic VFT relation.…”

“…(13). Its validity indicates the accidental significance of testing the dynamic crossover phenomenon via the 'Stickel operator' [44][45][46][47][48][49][50] and problems of discussions focused on fragility within frames of the VFT relation 2,5,14,24,57,[62][63][64][65] . Some discrepancies between the direct estimation of fragility and fragility strength by the use of VFT equation were raised recently.…”

“…The appearance of two lines in such a plot and their intersection related to J are indicated as a 'universal' feature of previtreous domain. [44][45][46][47] Novikov and Sokolov strengthen this 'universality', suggesting a 'magic' time scale ( J ) = 10 +K±, s, estimated empirically by the 'Stickel-operator' analysis of 30 glass-formers, including low-molecular-weight liquids, polymers, ionic systems, covalent systems and plastic crystals. 48 However, some criticism regarding this finding appeared, due to glass formers with strongly different ( J ) values.…”

The Modified Random Network (MRN) Model within the Configuron Percolation Theory (CPT) of Glass Transition

“…Finally, we note that the CPT provides the bases for the viscosity of amorphous materials being described by the Sheffield model of viscosity [51,52]. The two-exponential Sheffield equation of viscosity:…”