Hydrophilic polyurethaneurea membranes: influence of soft block composition on the water vapor permeation rates

High molecular weight segmented polyurethaneurea (PUU) copolymers based on an aliphatic diisocyanate, bis(4-isocyanatocyclohexyl)methane and mixed hyd rophilic and hydrophobic soft segments were prepared. Hydrophilic blocks co nsisted of poly(ethyleneoxide) (PEO) of molecular weight 1450 g/mol, wherea s the hydrophobic blocks were poly(tetramethylene oxide) of molecular weigh t 2000 g/mol. Ethylene diamine was used as the chain extender. Hard segment contents of the copolymers were kept constant at 18%, whereas PEO contents were varied between 0% and 50% by weight. Water vapor permeation rates (WV PR) of thin films (23-178 mu m) cast from dimethylformamide solutions were determined. In studies performed at 23 degrees C and 50% relative humidity, the relationship between PEO content and WVPR followed an S-shaped curve. For copolymers containing up to about 15% by weight of PEG, WVPR were fairl y low. This was followed by a region where WVPR increased continuously for membranes containing between 15% and 30% PEG. Further increase in PEO conte nt above 30% did not influence the WVPR substantially. There was also a dra matic increase in WVPR with an increase in temperature from 23 degrees C to 37 degrees C. Activation energy of permeation was determined to be 91.5 kJ for PUU containing 22.0% by weight of PEG. Equilibrium water absorption le vels of PUU containing different levels of PEO in their backbone structures followed a similar trend to that of WVPR. Hydrophilic PUUs showed good ten sile properties and mechanical integrity even at very high levels of water absorption. (C) 1999 Elsevier Science Ltd. All rights reserved.