Temperature and volume effects on local segmental relaxation in poly(vinyl acetate)

Dielectric data on the local segmental relaxation of amorphous poly(vinyl acetate) (PVAc) are analyzed. At atmospheric pressure, the fragility (T-g-normalized temperature dependence) is invariant to molecular weight. Since the fragility is also independent of pressure, the combined temperature and pressure dependences of the relaxation times can be analyzed using the Avramov model of structural relaxation. This entropy model gives a satisfactory description of the data and yields an expression for the pressure dependence of the dynamic glass transition identical to the empirical Andersson equation. Although in principle the Avramov model parameters can be deduced from measurable thermodynamic properties of the glass-former, the results are at odds with the values obtained from fitting the relaxation times for PVAc. Using pressure-volume-temperature data, the respective contributions of thermal energy and volume to the local dynamics in PVAc were assessed. Specifically, we calculate the ratio of the isochoric to isobaric apparent activation energies. The obtained value, approximate to0.6 at atmospheric pressure just above T-g indicates that temperature and volume both govern to a significant degree the relaxation times. This result is supported by a comparison of the relative magnitude of the isobaric and isochronal thermal expansion coefficients. These properties of PVAc are similar to those of other nonassociated polymers and glass-forming liquids and can be shown to be weakly correlated with the degree of super-Arrhenius behavior.