Coupling model interpretation of dielectric relaxation of poly(vinyl acetate) near T g

Abstract

Recent dielectric relaxation measurements on poly(vinyl acetate), PVAc, by Nozaki and Mashimo (NM) have demonstrated temperature and frequency dependences for well-annealed samples near the glass transition temperature Tg which cannot be explained by conventional theories or concepts. NM concluded that a new concept has to be introduced to interpret the dielectric behavior of PVAc near Tg. We demonstrate that the coupling model of relaxation is able to provide this new concept and can quantitatively describe the dielectric behavior at all temperatures. NM have carried out dielectric relaxation measurements between T=84.77 and 26.85 °C on a PVAc sample with Tg=31 °C. The samples were carefully annealed at each temperature to ensure that it is in equilibrium. NM found that the asymmetric dielectric loss curves become broader as temperature is lowered toward Tg with the most significant changes occurring near Tg. Furthermore the frequency of maximum loss fm was found to be described by the WLF equation (expected to be valid on the basis of free volume models) only at high temperatures. Extrapolation of the WLF curve to the glass transition region deviates significantly from the measurements. All of these phenomena are expected by the coupling model. The model predicts a relation between the shape of the dielectric loss curves and properties of the relaxation time. We demonstrate that the temperature dependence of fm is quantitatively predicted at all temperatures, including the deviations from WLF behavior near Tg. The same coupling model was previously shown to resolve several other problems in polymer physics near Tg.