Anharmonic Oscillations and Tg, Tm Transitions

Abstract

A working hypothesis has been developed to account for a change in character of thermal motion at glass transition. According to this hypothesis the pronounced onset of anharmonic vibrations is responsible for a stepwise increase in the thermal expansion coefficient alpha as well as in the similar temperature change in specific heat coefficient cp. In this paper the both transitions (the first order transition at melting point and the second order transition at Tg) are investigated on the basis of the viewpoint connected with the change in characteristics of motion of the particles, so typical for an onset of a liquid state. At present, two different definitions for the coefficient of thermal expansion are used. One is usually adopted in polymer physics, where also the statistical approach to polymer chains configurations plays the major role in Tg definition. However, the statistical configurational approach of polymer physics cannot be applied directly to the inorganic glasses and also does not provide any explanation of relatively small changes in cp values at melting point transition which sometime occur. Using the interpretation of solid state physics, the present paper intends to make a first step and bridge over the gap between these two approaches and to explain the Tg transition in dynamic terms, common to the polymers as well as to the low molecular weight substances.