Triggering the gas transport in PVdF-HFP membranes via imidazolium ionic liquids

Abstract

Ionic Liquids (ILs) are attractive additives for developing advanced polymeric membranes owing to their low volatility, high chemical, thermal stability, affinity for specific gases and ability to influence membrane morphology. Self-standing poly(vinylidenefluoride-co-hexafluoropropylene)-based films were prepared by solution casting and solvent evaporation, incorporating selected imidazolium ILs up to 60 wt%. The films were characterized for morphology, thermal properties and stability, and spectral properties by scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy.Blend membranes resulted non-porous, with a spherulitic structure. ILs trigger the conversion of the polymer matrix from a predominantly apolar α-phase to polar crystalline phases, as also shown by the FT-IR results. The polymer blends resulted heterogeneous as shown by DSC and TGA analysis. The thermogravimetric analysis showed a reduced onset of degradation for the blends, with no evidence of complexation between polymer and the ILs. The calorimetric analysis suggested a plasticizing action due to the ionic liquids. This behaviour agrees with the experimental and modelling study on gas permeability. Indeed, the blend membranes displayed a significant enhancement of gas permeability (up to three order of magnitude) at the highest ionic liquid content. The ideal CO2/N2 selectivity was simultaneously improved more than 200% upon the addition of the BF4–based IL to the polymer. The observed gas transport properties were correlated to the anion chemistry and to the specific polymer/IL interactions.