The peculiarities of relaxation processes at heating of glasses in glass transition region according to the data of mechanical relaxation spectra (Review)

Abstract

The results of study of mechanical losses by dynamic methods for silicate, borate, and chalcogenide glasses, metallic glass and glycerol at heating in the glass transition region were analyzed. Essential differences between the dynamic viscosity values η* calculated by means of the Maxwell equation based on the relaxation time at the maximum of losses (ωτ = 1) for labile states of glass, and the experimental values of η for the metastable liquid at the same temperature were revealed. The ratio η*/η was systematized in the framework of kinetic theory of glass transition and thermodynamics. An interpretation of the regularities was proposed based on the theory of dynamic properties of liquids. It was shown that different widths of spectra of relaxation times were the most probable reason of the difference between η* and η. The width of spectrum is determined by the degree of ordering of states of compared metastable liquid and glass at the same temperature; it depends on the thermal prehistory of each state. A wider spectrum of relaxation times corresponds to a more ordered state. For the considered glasses, the ratio of the temperature corresponding to viscosity value η* of the metastable liquid to the temperature of α-relaxation maximum (T_α) is 1.03 ± 0.01 at T_α variation from 190 to 1550 K. It is the evidence that all the relaxation frequencies, constituting both “narrow” and “broad” spectrum are associated with one and the same molecular mechanism. Mechanical losses in the metastable supercooled glycerol are described by the Maxwell equation with high precision for η values from 10¹³ to 10⁵ Pa s.