Theoretical interpretation of activation energies associated with the glass transition, obtained from td or DSC experiments

Abstract

Thermodilatometry (TD) and differential scanning calorimetry (DSC) experiments in the glass-transition region of different kinds of glasses have been simulated on a computer by means of a previously proposed theoretical free volume model. From the results obtained, the empirical procedures often used to obtain the values of "activation energies" associated with the glass transition are theoretically justified. Moreover, these "activation energies" are interpreted, in the framework of the model used, as average energies for the structural rearrangements in the glass.The activation energy for structural rearrangement in a glass is generally widely distributed [1-3]. However, it is possible to define an average energy characteristic of these processes, starting from macroscopic measures such as those of viscosity. Like other physical magnitudes, this energy changes strongly in the region of temperature known as the glass transition. This region, which can be characterized by a temperature To, separates two possible states of the material. For temperatures higher than Tg the glass is in a metastable supercooled liquid state, while for temperatures lower than Tg a recently formed glass is in a solid-like unstable state. The former state is characterized by an excess of enthalpy and volume in relation to the metastable supercooled liquid state at the same temperature. The solid-like state can therefore undergo relaxation phenomena towards the metastable state corresponding to the supercooled liquid.In the supercooled liquid state, structural rearrangements have a marked cooperative character, which is typical of the liquid state in general. Further, the temperature-dependence of magnitudes such as viscosity, 7, the diffusion coefficient, D, or the characteristic time for structural rearrangements, ~, defined from t/or D, are fitted by the Vogel-Fulcher [4] or Doolittle [5] expressions, i.e.