Gas transport properties and pervaporation performance of fluoropolymer gel membranes based on pure and mixed ionic liquids

Ionic liquid polymer gel membranes based on poly(vinylidene fluoride-co-hexafluoropropylene) (p(VDF-HFP)) were prepared by solvent casting from a solution in acetone. The membranes contain from 20 wt% to 80 wt% of pure or mixed ionic liquids based on the imidazolium cation: 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and 1-hexadecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HdMIM][TFSI]). Both ionic liquids have the fluorinated anion bis(trifluoromethylsulfonyl)imide, which should make them more compatible with a fluoropolymer matrix, while the different tail lengths in the cation give significantly different melting points. The thermal and mechanical properties of the blend membranes were evaluated and compared with the neat polymer. The performance of the membranes was tested in terms of pure gas permeability and selectivity, separation of limonene/carvone mixtures by pervaporation, and influence of a low water content in the feed on separation by the blended membrane. DSC analysis shows a gradual decrease of the melting point of the gel and a decrease in the overall melting enthalpy with increasing IL content, whereas the melting enthalpy normalized for the polymer fraction shows an initial drop and then a gradual increase. In the presence of the ionic liquid, the elastic modulus and break strength decrease dramatically, while the maximum deformation at first increases due to higher flexibility of the plasticized polymer and then rapidly decreases above 40 wt% of IL as a consequence of the progressive decrease of the number of entanglements. The gas transport through the membranes prepared in this study was evaluated in the temperature range of 20-60 °C, which includes the melting point of the high temperature ionic liquid [HdMIM][TFSI]. These permeation measurements show a significant increase of the permeability in the presence of the IL mixture, especially for carbon dioxide. This suggests a potential application in gas separation membranes, for instance for natural gas treatment or for CO2 sequestration from flue gas.