SUPPRESSION OF CO2-PLASTICIZATION BY SEMIINTERPENETRATING POLYMER NETWORK FORMATION

CO2-induced plasticization may significantly spoil the membrane perfor mance in high-pressure CO2/CH4 separations. The polymer matrix swells upon sorption of CO2, which accelerates the permeation of CH4. The pol ymer membrane looses its selectivity. To make membuanes attractive for , for example, natural gas upgrading, plasticization should be minimiz ed. In this article we study a polymer membrane stabilization by a sem iinterpenetrating polymer network (s-ipn) formation. For this purpose, the polyimide Matrimid 5218 is blended with the oligomer Thermid FA-7 00 and subsequently heat treated at 265 degrees C. Homogeneous films a re prepared with different Matrimid/Thermid ratios and different curin g times. The stability of the modified membrane is tested with permeat ion experiments with pure CO2 as well as CO2/CH4 gas mixtures. The ori ginal membrane shows a minimum in its permeability vs. pressure curves , but the modified membranes do not indicating suppressed plasticizati on. Membrane performances for CO2/CH4 gas mixtures showed that the pla sticizing effect indeed accelerates the permeation of methane. The mod ified membrane clearly shows suppression of the undesired methane acce leration. It was also found that just blending Matrimid and Ther mid w as not sufficient to suppress plasticization. The subsequent heat trea tment that results in the s-ipn was necessary to obtain a stabilized p ermeability. (C) 1998 John Wiley & Sons, Inc.