Disulfonated poly(arylene ether sulfone) random copolymer thin film composite membrane fabricated using a benign solvent for reverse osmosis applications

Abstract

High performance thin film composite (TFC) membranes for reverse osmosis applications were fabricated by coating solutions of highly chlorine-tolerant disulfonated directly copolymerized poly(arylene ether sulfone) random copolymers (BPS-XX, e.g., BPS-20 and 32) on a commercially available porous polysulfone (e.g., Udel (R)) support. Solvents used in the formation of the TFCs must dissolve the sulfonated polysulfones used as the skin materials, while not harming the non-sulfonated polysulfone support membrane. For this purpose, environmentally friendly solvents were selected via a systematic screening process using a triangular solubility diagram. However, these benign solvents [e.g., di(ethylene glycol)] generally have high boiling points (>similar to 190 degrees C). Thus, they necessitate the use of a special TFC formation process, since solvent evaporation at high temperatures caused pore shrinkage in the polysulfone support membrane and could lead to a catastrophic decrease in membrane water permeance. Support membranes were initially immersed in an IPA/glycerin mixture, after which the IPA was allowed to evaporate, leaving glycerin within the membrane pore structure. After a repeated coating procedure using dilute BPS-XX solutions, the TFC membranes were dried under vacuum at elevated temperatures. During this process, the glycerin reduced pore penetration of BPS-XX and prevented pore collapse during the drying procedures. Finally, water-miscible glycerin was eliminated via water treatment. The newly developed coating method formed ultra-thin and defect-free BPS-XX layers on a micro-porous Udel support membrane. For example, BPS-32 TFC membranes showed NaCl rejection (similar to 97%), similar to that of its dense membranes. Furthermore, decreasing the amount of coating solution and, therefore, the BPS-32 coating thickness, resulted in improved pure water flux. The TFC water flux was further improved and was accompanied by small reduction in salt rejection after various TFC membrane treatments (e.g., in situ acidification or IPA treatment).