Pressure-temperature dependence of polymer segmental dynamics.: comparison between the Adam-Gibbs approach and density scalings

By means of dielectric spectroscopy, the segmental dynamics (R-relaxation) of four different polymers ( poly( vinyl acetate), poly( vinyl methyl ether), poly( vinyl chloride) and poly(o-chlorostyrene)) was measured over a broad range of frequencies (10(-1)- 10(7) Hz), pressures (0- 300 MPa), and temperatures (240-460 K). Two different approaches were used to analyze the temperature-pressure dependence of the relaxation time: a pressure-extended Adam-Gibbs equation (tau(T, P)) = tau(0) exp[C/(TSc(T, P))]) and a particular density scaling (log(tau) proportional to f (TV gamma)), both recently proposed. Both approaches give an excellent description of the experimental data; however, an inconsistency between them was found. We show that other scaling laws can give an equally good description of the experimental data, being therefore possible to improve the consistency between the density scaling and the AG framework.