Solubility and transport of water vapor in some 6FDA‐based polyimides

Permeability, diffusion, and solubility coefficients for H 2 O vapor in four different 6FDA‐based polyimides were determined at temperatures between 25 and 45°C and over a wide range of relative humidities. The solubility of H 2 O vapor in some of the polyimides studied can be described by the “dual‐mode sorption” model whereas in other polyimides it is represented by the Flory‐Huggins equation, which suggests that the latter polymers are plasticized by H 2 O. The solubility of H 2 O vapor in the polyimides decreases as the temperature is raised and increases with increasing polarity of the polymer. The diffusion coefficients for H 2 O in the polyimides studied either increase or pass through a weak maximum with increasing H 2 O activity, or concentration in the polymers. The latter behavior is probably due to a clustering of H 2 O molecules in the polyimides at higher H 2 O activities or concentrations. The diffusion coefficients for H 2 O decrease as the chain‐packing density of the polyimides increases. The permeability coefficients for H 2 O vapor in 6FDA‐based polyimide membranes either increase slightly or are constant as the H 2 O activity is increased. The experimental values of the permeability coefficients are consistent with the values determined from diffusion and solubility coefficients. The permeability of the polyimides to H 2 O vapor appears to be controlled by the solubility of H 2 O in the polymers. The polyimides studied exhibit a very high selectivity for H 2 O vapor relative to CH 4, and therefore are potentially useful membrane materials for the dehydration of natural gas.