Temperature and pressure dependence of α-relaxation and free volume in poly(vinyl acetate)

We analysed reported specific volume V from pressure-volume-temperature (PVT) experiments for poly(vinyl acetate) (PVAc) using the Simha-Somcynsky equation of state (S-S eos) and calculated the specific occupied volume, V-occ, and hole free volume, V-fh We found that V-occ shows almost no thermal expansion but is distinctly compressible (T > T-g). Due to this, V-fh = V - V-occ is not a unique function of Vbut can depend on the specific values of P and T giving rise to the particular value for V We observed that with decreasing temperature the g-relaxation detected via dielectric measurements slows down faster than the shrinkage of hole free volume Vfh would predict on the basis of the free volume theory of Cohen and Turnbull. V-fh becomes zero at approximately 65 K below the Vogel temperature To, T-0(') = T-0 - 65 K. Plots of the a-relaxation frequency log omega(alpha) versus 1/V-fh can be linearised when taking into account this discrepancy by substituting 1/V-fh by 1/(V-fh - Delta V) with Delta V = E-fh(T-0 - T-0(')), or by including an energy term of the type exp[-E'/R(T- T-0)] in the pre-exponential factor of the frequency-volume equation. Our results indicate that the a-relaxation in PVAc operates via the Cohen-Turnbull mechanism only when liquid-like clusters of cells of the SS lattice appear which contain a hole free volume of three or more empty S-S cells. For regions with a smaller number of empty cells an activation energy is required for allowing segmental motion via the cooperative rearranging of cells. The beta-(Jobari-Goldstein) relaxation which is a noncooperative, more local, process follows completely the free volume theory. The same seems to be true for the primitive (uncoupled) relaxation of the coupling model.