Relation between configurational entropy and relaxation dynamics of glass-forming systems under volume and temperature reduction

The structural relaxation dynamics of two Molecular glass-forming systems have been analyzed by means of dielectric spectroscopy, under cooling and compression conditions. The relation of the dynamic slowing down with the reduction of the configurational entropy, S(c), as predicted by Adam and Gibbs (AG), was also investigated. As S(c) is not directly accessible by experiments. it was estimated, following a common procedure in literature, from the excess entropy S(exc) of the supercooled liquid with respect to the crystal, determined from calorimetric and expansivity measurements over the same T-P range of dynamics investigation. The AG relation, predicting linear dependence between the logarithmic of structural relaxation time and the reciprocal of the product of temperature with configurational entropy, was successfully tested. Actually, a bilinear relation between S(exc) and S(c) was found, with different proportionality factors in isothermal and isobaric conditions. Using Such results, we derived an equation for predicting the pressure dependence of the glass transition temperature, in good accordance with the experimental values in literature. (C) 2009 Elsevier B.V. All rights reserved.