Temperature and Pressure Dependence of?-Relaxation and Free Volume in Poly(vinyl acetate)

Abstract

Summary: 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, Vocc, and hole free volume, Vfh We found that Vocc shows almost no thermal expansion but is distinctly compressible (T > Tg). Due to this, Vfh = V − Vocc is not a unique function of V but 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 α-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. Vfh becomes zero at approximately 65 K below the Vogel temperature T0, . Plots of the α-relaxation frequency log ωα versus 1/Vfh can be linearised when taking into account this discrepancy by substituting 1/Vfh by 1/(Vfh − ΔV) with , or by including an energy term of the type exp[−E′/R(T − T0)] in the pre-exponential factor of the frequency-volume equation. Our results indicate that the α-relaxation in PVAc operates via the Cohen-Turnbull mechanism only when liquid-like clusters of cells of the S-S 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 β-(Johari-Goldstein) relaxation which is a non-cooperative, more local, process follows completely the free volume theory. The same seems to be true for the primitive (uncoupled) relaxation of the coupling model.