The apparent activation energy and relaxation volume from the point of view of Adam$ndash$Gibbs theory

The temperature and pressure dependence of the glass-transition process is discussed on the basis of the molecular Adam–Gibbs equation for the relaxation time. An equation for measuring the temperature and pressure dependence of the Adam–Gibbs size of the cooperative rearranging region is suggested. The apparent activation energy and the apparent relaxation volume by this equation are presented as the product of the activation energy and the activation volume per basic molecular kinetic unit respectively with the size of the cooperative unit to the second power. In this way, for the first time, an equation for measuring the activation volume of a basic molecular kinetic unit involved in the cooperative molecular dynamics is obtained. The isobaric and the isothermal activation volume of a basic molecular species (BMS) measured by this equation correlate with the molar volume of the liquid. From the experimental results it follows that the size of the cooperatively rearranging region at isothermal conditions increases with pressure until at the glass-transition pressures it reaches an apparent limit. On the basis of the activation volume measured per BMS, a method for evaluating their volume in liquids and their number in molecules or in repeated units in polymers is suggested. By this method, for the first time, from a relaxation experiment, the basic kinetic units in several molecular glass-forming liquids are found to be close to the fragments of the monomer segments or molecules known in thermodynamics as Wunderlich's beads.