Effect of the casting solvent on the crystal characteristics and pervaporative separation performances of P(VDF-co-HFP) membranes

In this paper, two different crystalline phase compositions appear in the isothermally dried solvent-cast poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) membranes, which was explained based on the different solvent–polymer interactions. The dipolar intermolecular interaction between P(VDF-co-HFP) and dimethylacetamide (DMAc) induces polar γ phase with trans conformation, which is absent in the P(VDF-co-HFP)–acetone system. The different pervaporation behavior of the two solvent-cast membranes in separating ethyl acetate (EtAc) from water was explained based on the different permeant–polymer interactions which were further deduced from the affinity differences. The difference of polarity between EtAc molecules and the acetone-cast P(VDF-co-HFP) membrane is smaller than that between EtAc molecules and the DMAc-cast P(VDF-co-HFP) membrane, which makes for the different solubilities and diffusivities of EtAc in the two different solvent-cast membranes. And the separation properties of them in pervaporation of EtAc/water mixture are inevitably varied.