Temperature and Pressure Dependence of α ‐Relaxation and Free Volume in Poly(vinyl acetate)

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, 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 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 V fh 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 T 0 , $T'_0 = T_0 - 65\,{\rm K}$ . Plots of the α ‐relaxation frequency log  ω α versus 1/ V fh can be linearised when taking into account this discrepancy by substituting 1/ V fh by 1/( V fh  − Δ V ) with $\Delta V = E_{{\rm fh}} (T_0 - T'_0 )$ , 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 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.