Blended polyhalogenated membranes exhibiting a wide dynamic range for He/CH4 separation

In this study, a polymer blending approach is proposed to fabricate membranes exhibiting wide dynamic range for helium/methane separation. Poly(PFMMD-co-CTFE-50), a 50:50 mol.% copolymer of perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) and chlorotrifluoroethylene (CTFE) exhibiting high helium/methane selectivity and low helium permeability, was blended with Teflon AF2400, a commercial perfluoropolymer exhibiting low helium/methane selectivity and ultra-high helium permeability. Remarkably, by simply changing the blend composition, the pure- and mixed-gas helium permeability and helium/methane selectivity of the blended membranes move along the 2008 Robeson upper bound while spanning multiple orders of magnitude, which makes these membranes capable of operating with a wide dynamic range, a feature that could enable harvesting helium from streams exhibiting different compositions. As shown by thermal analysis, atomic force microscopy (AFM), interfacial energy measurements and Flory-Huggins modeling, all blends exhibit a phase-separated morphology, irrespective of their composition and fabrication protocol. It was demonstrated that the larger polarity of the C–Cl bond relative to the C–F bond and the much larger surface energy of poly(PFMMD-co-CTFE-50) relative to Teflon AF2400 renders the interaction between the two polymers highly unfavorable. Theoretical analysis of permeability data with the generalized Maxwell model reveals additional details regarding the blend microstructure and morphology, which are in reasonable agreement with AFM results.