Pressure and temperature dependences of the dynamics of glass formers studied by broad-band dielectric spectroscopy

Abstract

The pressure and temperature behaviours of the structural relaxation time in glass-forming systems can be well described by a new equation, based on the Adam-Gibbs model, where the configurational entropy has been written in terms of the excess heat capacity and the molar thermal expansion. An experimental check of this equation was performed on the dynamics of an epoxy compound, investigated by wide-band dielectric spectroscopy (10⁻²–10¹⁰Hz). Six different sets of relaxation data, corresponding to two different conditions, were analysed: firstly, isobaric (atmospheric pressure and 95.5 MPa) with various temperatures down to the supercooled and glassy phases and, secondly, isothermal (at four different temperatures) with changing pressure in the range 0.1–270 MPa. Moreover, the thermal expansion was independently determined from data on the pressure and temperature variations of the volume of the material. From this analysis, the pressure-extended Adam-Gibbs model resulted, which is so suitable for describing the two-dimensional surface τ(T,P) over the whole investigated range of temperatures and pressures that a master curve can be obtained from all the relaxation times if plotted versus a variable related to the configurational entropy directly derived from thermodynamic data.