Polyelectrolyte Complex Tubular Membranes via a Salt Dilution Induced Phase Inversion Process

Tubular membrane geometries are key elements of many industrial filtration applications and artificial organs. Established processes to fabricate polymeric membranes require the use of organic solvents, which are prone to be phased out due to stricter regulations. In the quest of developing solvent-free alternative fabrication processes, polyelectrolyte complex (PEC) membranes are recently discovered, offering a promising alternative. While flat sheet PEC membranes are successfully fabricated, tubular or hollow fiber geometries remain a material engineering challenge. For the first time, the organic solvent-free fabrication of PEC tubular membranes in a dry-jet wet spinning process is demonstrated. The aqueous polymer solution comprising the polyanion—poly(sodium-4-styrenesulfonate) (PSS), the polycation—poly(diallyldimethylammonium-chloride) (PDADMAC), and KBr is extruded with the water-based bore fluid through a single-orifice spinneret, passing an air-gap before immersed into an aqueous coagulation bath. The phase inversion kinetics are influenced to form a defect-free lumen separation layer through the addition of glycerol to the bore fluid. The resulting tubular membranes show reproducible nanofiltration membrane properties. They have a molecular cut-off of 320 Da, are positively charged and retain salts with a characteristic salt retention hierarchy. This promising material engineering strategy motivates to continue the quest for smaller tubular dimensions toward hollow fibers.