Solubility and diffusion of small molecules in PVC

Recent studies have shown that the amount and rate of absorption of gases and organic liquids and vapors in rigid and plasticized PVC can be correlated with their molecular dimensions, solvent power, and thermodynamic activity. The amount absorbed at equilibrium depends upon the solvent power of the diffusant, expressed in the Flory‐Huggins interaction parameter, x , and upon the vapor pressure or activity of the diffusant in the environment of the PVC. The rate and kinetic form of the approach to equilibrium vary with the ratio of the final uptake to C g , the diffusant concentration which produces the glass‐rubber transition at the experimental temperature. At concentrations far below C g , absorption follows ideal Fickian kinetics with diffusion coefficients steeply dependent upon the diffusant molecular size and shape. When the concentration approaches or exceeds C g , absorption is faster, kinetics are non‐Fickian, and the dependence upon molecular size is diminished. Carbon dioxide at high pressure strongly accelerates the absorption of other substances because it uniquely combines the high diffusivity of a small gas molecule with the solubility and plasticizing action of an organic swelling agent.