Optimization of Polyelectrolyte Coacervate Membranes via Aqueous Phase Separation

Polymeric membranes fabricated via the nonsolvent-induced phase separation process rely heavily on toxic aprotic organic solvents, like N-methyl-pyrrolidine (NMP) and dimethylformamide. We suggest that the "saloplastic" nature of polyelectrolyte complexes (PECs) makes them an excellent candidate for fabricating next-generation water purification membranes that use a more sustainable aqueous phase separation process. In this study, we investigate how the properties of PECs and their interactions with salt can form pore-containing membranes from the strong polyelectrolytes poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). How the single-phase polymer-rich (coacervate) dope solution and coagulation bath impacted the formation, morphology, and pure water permeance (PWP) of the membranes was systematically evaluated by using scanning electron microscopy and dead-end filtration tests. The impact of a salt annealing post-treatment process was also tested; these treated PEC membranes exhibited a PWP of 6.2 L m^-2 h^-1 bar^-1 and a dye removal of 91.7% and 80.5% for methyl orange and methylene blue, respectively, which are on par with commercial poly(ether sulfone) nanofiltration membranes. For the first time, we have demonstrated the ability of the PEC membranes to repel Escherichia coli bacteria under static conditions. Our fundamental study of polyelectrolyte membrane pore-forming mechanisms and separation performance could help drive the future development of sustainable materials for membrane-based separations.