ANHARMONIC OSCILLATIONS AND T-G, T-M TRANSITIONS

A working hypothesis has been developed to account for a change in cha racter of thermal motion at glass transition. According to this hypoth esis the pronounced onset of anharmonic vibrations is responsible for a stepwise increase in the thermal expansion coefficient a as well as in the similar temperature change in specific heat coefficient c(p). I n this paper the both transitions (the first order transition at melti ng point and the second order transition at T-g) are investigated on t he 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 t he major role in T-g definition. However, the statistical configuratio nal approach of polymer physics cannot be applied directly to the inor ganic glasses and also does not provide any explanation of relatively small changes in c(p) values at melting point transition which sometim e occur. Using the interpretation of solid state physics, the present paper intends to make a first step and bridge over the gap between the se two approaches and to explain the T-g transition in dynamic terms, common to the polymers as well as to the low molecular weight substanc es.