Gas permeability of polypropylene oxide/polybutadiene bi-soft segment urethane/urea membranes

Cross-linked urethane/urea membranes with two soft segments, polypropylene oxide and more hydrophobic polybutadiene, were prepared by extending a polypropylene oxide-based tri-isocyanate-terminated prepolymer (PU) with polybutadiene diol (PBDO). The membranes obtained were analysed with relation to the structure of phaseseparation, degree of cross-linking and gas permeation. The PU/PBDO membranes showed different degrees of crosslinking and different degrees of phase separation according to the variation of the ratio of PU and PBDO. It was found that the CO2 permeability of the membranes is dependent on the feed pressure and the ratio of PU/PBDO. A 67 wt.% PBDO membrane has a lower degree of cross-linking and displays a linear behaviour of CO2 permeability vs. pressure, ranging from 90 to 550 barrer. A 20 wt.% PBDO membrane has a higher degree of cross-linking and displays phase separation, leading to higher CO2 permeability, ranging from 150 to 950 barrer.

The variation of the ratio of PBDO and PU in the membranes changes the properties such as the degree of cross-linking and the degree of phase separation and thus influences the gas permeability. The membranes with lower PBDO content (20 wt.%) exhibit a higher CO2 permeability. This result was not expected since this membrane presents a higher degree of crosslinking. This can be explained by the higher solubility of CO2 in polypropylene oxide segments and by the phase separation morphology. In fact, in the membrane with 20 wt.% of PBDO, the two soft segments are phase- separated, while in the membrane with 67 wt.% of PBDO, the two soft segments are highly dispersed in each other.