Anharmonicity of lattice vibrations and the fluctuation volume of amorphous substances near the glass transition temperature

Abstract

The fraction of the fluctuation volume (in a model of the excited state) at the glass transition tem perature depends linearly on the Gruneisen parameter, i.e., the degree of the anharmonicity of lattice vibra tions in amorphous polymers and glasses.

Model of viscous flow of glass-forming liquids and glasses

“…The constancy of f g is maintained better in glasses of the same structural type [14]; e.g., in metallic glasses, f g ≈ 0.026 (Table 2). Thus, a local configurational change in the struc ture is a necessary condition for an elementary event of viscous flow of glass forming liquids.…”

Criterion of vitrification and cooling rate of vitreous melts

“…inorganic glass, and metal amorphous alloys-metal glass ( Table 2) [9]: (14) This equation can be considered an approximate criterion of glass transition: liquid-glass transition is caused by the decrease in the fraction of fluctuation volume to the minimum value (14), at which the molecular mobility is frozen, which is intrinsic for delocalized atoms.…”

“…From gen eral considerations, the definite relationship of the fraction of fluctuation volume f g and Gruneisen parameter γ D should be anticipated: the stronger the anharmonicity the higher should be the fluctuation volume of an amorphous medium. For amorphous polymers and inorganic glass, there is a linear correla tion between f g and γ D [14], though these values vary in small ranges.…”

“…Thus, the universal character of the ratio of the parameters of Bartenev's equation (2) is the conse quence of the approximate criterion of vitrification (14) in the model of delocalized atoms.…”

“…Because the criterion of vitrification (14) was related earlier to the free volume of amorphous sub stances [10,11], let us focus on the discussion of this phenomenon within the developed approach.…”