Water Vapor Transport in Hydrophilic Polyurethanes
Water uptake and water vapor transmission rates have been obtained on two series of hydrophilic segmented polyurethanes based on MDI, butanediol, and several block copolymer polyethers with varying proportions of poly(ethylene oxide) PEO and poly(propyleneoxide) PPO. In series I the ratio of PEO to PPO is varied at fixed hard segment concentration. In series II the hard segment concentration is varied at a fixed 50/50 ratio of PEO to PPO. At constant hard block composition and decreasing amounts of PEO in series I polymers water uptake decreases from 58g/100g of polymer at pure PEO to 2g/100g of polymer for pure PPO. Transmission rates determined with water in contact with the upstream surface and 50% relative humidity downstream show a corresponding decrease. As hard block concentration is increased in II both water uptake and transmission rate decrease. The sorption isotherms are surprisingly simple concave upward curves which rise steeply at saturation. Analysis of the data on the basis of moles of water per EO unit provides a clear picture of the influence of compositional variations on the course of the sorption isotherms and shows that saturation water concentrations are not related in a simple manner to PEO concentration. The diffusion coefficients calculated from steady state transmission rates decrease with increasing water concentration in series I but increase with water concentration in series II. The apparently conflicting trends can be rationalized in terms of water clustering in the first case and the blocking effect of hard segment domains in the second. Comparisons are also made with results on a series of polyurethaneureas based on Hylene W and containing a mixture of separate PEO and PPO macroglycols which were studied by Tobolsky and coworkers for reverse osmosis.
KeywordsSorption Isotherm Hard Segment Soft Segment Water Vapor Transport Water Vapor Transmission Rate
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