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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. [ABSTRACT FROM AUTHOR]

Ionic liquids have gained worldwide attention as green solvents in the past decade. This study explores, for the first time, the fundamental science and engineering of using ionic liquids as a new generation of solvents to replace traditional organic solvents for the fabrication of flat sheet membranes and hollow fiber membranes. The fundamentals and characteristics of membrane formation of cellulose acetate (CA) membranes have been investigated using 1-butyl-3-methylimidazolium thiocyanate ([BMIM]SCN) as the solvent via phase inversion in water. For elucidation, other solvents, i.e., N-methyl-2-pyrrolidinone (NMP) and acetone, were also studied. It is found that [BMIM]SCN has distinctive effects on phase inversion process and membrane morphology compared to NMP and acetone because of its unique nature of high viscosity and the high ratio of [BMIM]SCN outflow to water inflow. Membranes cast or spun from CA/[BMIM]SCN have a macrovoid-free dense structure full of nodules, implying the paths of phase inversion are mainly nucleation growth and gelation, followed possibly by spinodal decomposition. The recovery and reuse of [BMIM]SCN have also been demonstrated and achieved. The derived flat sheet membranes made from the recovered [BMIM]SCN show morphological and performance characteristics similar to those from the fresh [BMIM]SCN. It is believed that this study could enrich the understanding of membrane formation using environmentally benign ionic liquids. [ABSTRACT FROM AUTHOR]