Effects of different ionic liquids as green solvents on the formation and ultrafiltration performance of CA hollow fiber membranes

In this work, we have examined and compared the interactions between cellulose acetate and different ionic liquids at a molecular level and explored their effects on dope rheology, hollow fiber morphology, and performance. Ionic liquid, 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc), was found to interact with cellulose acetate (CA) more closely with intensive hydrogen bonds than 1-ethyl-3-methylimidazolium thiocyanate ([EMIM]SCN). Thus, the CA/[EMIM]OAc solution exhibits a more pronounced charge-ordered network than the CA/[EMIM]SCN solution. In addition, the former does not obey the Cox-Merz rule, whereas the latter obeys the rule with its shear viscosity η identical to its complex viscosity |η∗| at the equivalent shear rate and angular frequency. These dissimilar factors have contributed to an instantaneous liquid-liquid demixing and resulted in a dense outer skin surface and a porous cross-sectional structure comprising macrovoids for the CA/OAc membrane. In contrast, the CA/SCN membrane has a looser interconnected nodular structure resulting from the delayed liquid-liquid demixing. The effects of spinning conditions on membrane properties have been determined. The higher dope flow rate and take-up speed result in smaller pore size. [EMIM]OAc is a more practical solvent than [EMIM]SCN to fabricate CA hollow fibers with a broader choice of spinning parameters. The newly developed CA/OAc membranes have PWP values of 230 and 260 L/(m² bar h) and pore sizes of 10-27 nm. This work may provide useful insights to develop polymeric membranes using ionic liquids as solvents and facilitate a greener fabrication method in the membrane industry.