Pore-filled cation-exchange membranes containing poly(styrenesulfonic acid) gels

Pore-filled cation-exchange membranes containing poly(styrenesulfonic acid) gels have been prepared by a twostep procedure involving thermally initiated radical copolymerization of styrene and divinylbenzene in the pores of a microporous poly(ethylene) host followed by treatment with chlorosulfonic acid. The incorporated poly(styrenesulfonic acid) was shown to be evenly distributed throughout the thickness of the host membrane. A porecontraction of the host membrane was observed during the incorporation of the cross-linked poly(styrene); however, the sulfonation step led to increases in the thicknesses and/or other dimensions of the membranes depending on mass loading of the incorporated polyelectrolyte. The ion-exchange capacities of the resulting membranes ranged up to 5.2 meq/g, depending on the mass of poly(styrene) initially incorporated into the host as well as the sulfonation temperature. The membranes were characterized in terms of water content, fixed charge concentration, and transport numbers. The pressure-driven performance of the membranes in reverse osmosis/nanofiltration applications was examined.

The work described here shows that it is possible to fabricate gel-filled membranes in which the gel is a strong acid using a thermally initiated, in-situ polymerization route. While this route has the disadvantage that changes in the base membrane dimensions occur, the resulting cation-exchange membranes are robust and show no evidence of degradation with use. These membranes have both high ion-exchange capacities and charge concentrations. It has been shown that the properties of the membranes can be tuned by varying the mass of gel-polymer incorporated into the pores of the host. Thus it was possible to form membranes with high poly(styrenesulfonic) acid loads that had low water contents and high ion-transport selectivities. Reduction in the gel-polymer volume fraction of these membranes renders them suitable for pressure driven water treatment applications with the membranes exhibiting high NaCl rejections with reasonably good fluxes. Avoidance of the pore-contraction which occurred during the fabrication of these membranes should allow for a substantial improvement in nanofiltration performance.