Effect of the isobaric and isothermal reductions in excess and configurational entropies on glass-forming dynamics

Dielectric relaxation times tau, over a broad range of temperatures and pressures for simple molecular glass-forming systems reveal good agreement with the pressure-extended Adam-Gibbs equation derived from the Adam Gibbs model, relating directly the dynamics to the thermodynamics through the configurational entropy S-c, assuming it to be proportional to the excess entropy S-exc of the melt with respect to the crystal. In the present study by making use of both calorimetric and expansivity measurements it was possible to make an accurate determination of the pressure and temperature dependences of S-exc over the whole investigated range, and the expected proportionality between Sc and Sexc was experimentally checked. Results show that different proportionality factors g(P) and g(T) exist between S-c and S-exc in isothermal and isobaric conditions and the ratio g(T)/g(P) of these is nearly 0.7 for all the systems.