Ionic liquids as co-solvents for zwitterionic copolymers and the preparation of poly(vinylidene fluoride) blend membranes with dominated β-phase crystals

A new series of green solvents that constituted with ionic liquids (ILs) and dimethyl acetamide (DMAc) were developed as good solvents for zwitterionic copolymers, and the dissolutions of amphiphilic copolymers carrying carboxylbetaine and sulfonatebetaine in these new solvents were studied. It was determined that the commonly found ILs such as 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6) and many others were all effective co-solvents for zwitterionic copolymer with carboxylbetaine groups. However, the 1-methylimidazolium trifluoromethanesulfonic ([MIM]CF3SO3) instead of the [BMIM]BF4 was proved as an effective co-solvent for the copolymer carrying sulfonatebetaine units. These newly designed mix-solvents were also good solvents for poly(vinylidene fluoride) (PVDF) and their influences on the crystallization as well as the formation of PVDF membranes were investigated. Our results had demonstrated that blend membranes with zwitterionic additives could be facilely prepared with many new solvent candidates.