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The diffusion constants for water vapor in poly (vinyl chloride) and poly(oxymethylene) have been determined by the time-lag method. In addition the sorption isotherms have been measured by direct weighing. The values of the solubility coefficient obtained gravimetrically have been compared with those calculated from the quotient of the permeability and diffusion constants obtained from the time lags. Quite different patterns of behavior were exhibited by the two polymers. In the case of poly(oxvmethylene) the two sets of values of the solubility coefficients were quite similar and fitted a normal Brunauer—Emmett-Teller (BET) type-III sorption isotherm. With poly(vinyl chloride), however, the solubilities calculated from P/D obeyed Henry's law and were considerably less than those obtained by direct weighing (except at low activities) which again were of the BET type III. These results were compared to those found with other polymers and have been interpreted in terms of the different degrees of clustering of water molecules in the polymer. A relationship was demonstrated between the Zimm-Lundberg clustering integral and the variation of the Flory-Huggins χ with activity.
Williams et al. (1969) studied this question.