Fine pore tailoring of brominated poly (arylene ether sulfone) tight ultrafiltration membrane via solvent evaporation and nonsolvent-induced phase separation

Tight ultrafiltration (TUF) membranes exhibit great application prospects in the separation of high-value-added chemicals and biomolecules, and the pore structure of membrane is a key factor dominating the separation efficiency. However, precise control of pore structure in polymeric membranes to meet specific separation needs, especially for TUF membranes with 1–3 kDa MWCOs, remains a substantial challenge. In this study, we develop a facile and efficient approach to precisely regulate the MWCO of membranes below 3 kDa by combining solvent evaporation with non-solvent-induced phase separation. A random copolymer, PES-Br-20, was prepared by polycondensation of bis (4-hydroxyphenyl) sulfone, 3,3′,5,5′-tetramethylbiphenol, and 4,4′-difluorodiphenyl sulfone, followed by bromination to introduce bromine-functional groups, which improved the copolymer's solubility in organic solvents. By employing N-methyl-2-pyrrolidone (NMP) as the main solvent and tetrahydrofuran (THF) as a volatile co-solvent, the pore size of the resultant membranes was precisely tailored by changing the co-solvent ratio and optimizing the evaporation time. We systematically evaluated the effects of copolymer content, PVP K30 content, co-solvent ratio, and air-bath time on membrane separation performance. Increasing the THF content from 36.00 wt% to 51.43 wt% reduced the MWCO of PES-Br-20 membranes from 8.5 kDa to 1.0 kDa, surpassing the tunability limits and minimum pore size thresholds typically achievable with conventional PSf and PES membranes. Furthermore, by varying the air-bath time from 5 s to 30 s, the pure water flux decreased from 26.3 ± 1.1 to 15.7 ± 0.3 L m−2·h−1·bar−1, accompanied by a precise shift in MWCO from 1.8 kDa to 1 kDa. This work highlights the outstanding capability of PES-Br-20 membranes in achieving finely adjustable pore sizes and underscores their strong potential for low-MWCO separation applications.