The concentration and pressure dependence of the diffusion of dichlorodifluoromethane in poly(dimethyl siloxane)

The diffusion of dichlorodifluoromethane in poly(dimethyl siloxane) as a function of concentration and pressure was measured by nuclear magnetic resonance (NMR) pulsed‐field‐gradient techniques. The pressure dependence of diffusion could be treated phenomenologically by an activation volume description where the activation volume of 20.9 cm 3 /mol is only a fraction of the penetrant's molar volume. A free‐volume theory, modified to include the effects of pressure, was used to analyze both the pressure and concentration dependence of diffusion. The functional form of the equation was obeyed very well although it was necessary to assume that the fractional freevolume of penetrant was more than twice its expected value. This discrepancy was interpreted to mean that the motions of the penetrant molecules are weakly coupled to the polymer lattice.