Measurement and interpretation of equilibrium organic vapor sorption in polyurethane membranes

Abstract

The equilibrium sorption of benzene, tetrachloroethylene, and hexane vapor in four commercially available polyurethanes was measured as a function of temperature (30–60°C) and organic activity with a McBain vapor uptake apparatus. The four polyurethanes were composed of the same polyether soft segments, but contained different amounts of phase-separated, hydrogen-bonded hard segment domains. For all three organic penetrants, polymer swelling was inversely proportional to the polyurethane's hard segment content. Vapor sorption data were modeled over the entire organic activity range for all four polyurethanes using the Flory–Rhener equation (the Flory–Huggins equation did not fit the experimental data at high organic vapor activities). The morphology of each of the polyurethanes was analyzed by FTIR (in terms of the hydrogen bonding index, HBI), elemental analysis (to calculate the total volume of hard segments in a polyurethane), and small-angle X-ray scattering (to determine the long spacing between hard segment domains). The Flory–Huggins interaction parameters, χ, in the Flory–Rhener equation for the three organic vapors were found to increase linearly with HBI. The M c parameter in the Flory–Rhener equation for each polymer, defined as the effective molecular weight of soft segments between hard domains, was correlated empirically with a combination of the HBI, polymer hard segment content, and soft segment long spacing.