Microstructure of HTPB-based polyurethane membranes and explanation of their low O-2/N-2 selectivity

Polyurethane (PU) membranes usually have high gas permeability but low sele ctivity. It was supposed that an increase of the hard-segment content in PU membranes would enhance the selectivity since the hard segment has a highe r oxygen affinity and could reduce the chain mobility. However, an attempt to increase the O-2/N-2 selectivity by increasing the hard-segment content failed. It was found that increase of the hard-segment content did increase the gas solubility but the solubility ratio of oxygen to nitrogen was not increased. Increase of the hard-segment content also reduced the permeabili ties of both oxygen and nitrogen, but no improvement of the O-2/N-2 diffusi vity ratio was observed. The microstructure of polybutadiene-containing PU was examined by DSC and FTIR measurements. It was found that hydrogen bondi ng between hard segments was the cause of the failure of selectivity improv ement. Avoiding the formation of hydrogen bonds between the hard segments i s the key for a successful improvement of the O-2/N-2 selectivity of the PU membranes. (C) 2001 John Wiley & Sons, Inc.