Investigation of unique interactions between cellulose acetate and ionic liquid [EMIM]SCN, and their influences on hollow fiber ultrafiltration membranes

Abstract

This study investigates the molecular interactions between ionic liquid, 1-ethyl-3-methylimidazolium thiocyanate ([EMIM]SCN) and cellulose acetate (CA), employing not only experimental characterizations including FTIR and rheological tests, but also molecular dynamics simulations. Due to the electrostatic nature of ionic liquids, [EMIM]SCN interacts intensely with CA molecules through pronounced hydrogen bonding, Coulombic forces and van der Waals interactions, which play an important role in dissolving CA and also greatly contribute to the unique rheological characteristics of CA/[EMIM]SCN solutions. The competition between the charge-ordered structure and polymer chain entanglement in the CA/[EMIM]SCN solutions leads to a three-region flow curve under shear stress. The charge-ordered network in CA/[EMIM]SCN solutions as well as the affinity and unique solvent exchange characteristics between non-solvents and [EMIM]SCN are found to greatly influence the phase inversion paths of membranes. In addition, the effects of dope flow rate, dope temperature and air-gap distance on hollow fiber formation have been elucidated and correlated to the interactions between CA and [EMIM]SCN and the phase inversion mechanisms. By tuning the spinning conditions, CA hollow fiber membranes are successfully fabricated for ultrafiltration with a PWP value of 90.10 (L/m 2 bar h) and a mean effective pore diameter of 16.68 nm.