Hierarchical structure control for ultra-high-flux helium separation membranes based on fluorinated polyimide materials

Abstract

Polyimide materials with multiple trifluoromethyl groups in repeating units, e.g., 6FDA-TFMB, have excellent intrinsic performance for membrane-based helium enrichment. Nevertheless, these fluorinated groups would seriously weaken cohesive energy density and then decelerate molecular chain aggregation in supersaturated solution. For asymmetric membrane fabrication using dry-wet phase inversion technology, this variation is negative to form dense and selective skin layer during solvent evaporation. In this research, EtOH, because it possesses low boiling point and little effect on solution stability, is utilized to enhance surface evaporation and accelerate polymer inspissation locally to construct defect-free skin layer. Meanwhile, formamide, because it could simultaneously motivate gelation and increase viscosity for polymer solution, is used to speed solidification during nonsolvent induced phase inversion, so that the transitional layer next to the surface could be formed instantaneously with artful sponge structure to consolidate and protect the nascent dense layer. The membrane casting solution is optimized with 22.0 ​wt% 6FDA-TFMB, 30.0 ​wt% EtOH, 3.0 ​wt% formamide, 31.5 ​wt% NMP, and 13.5 ​wt% THF, approximately. The defect-free dense layer could be thinner than 150 ​nm through this favorable formula. Besides, helium permeance is tested to be 760 GPU, the selectivity between helium and methane is tested to be 157, and the allowable operation pressure is higher than 4.0 MPaG, respectively. After adequate aging treatment, helium permeance is stabilized at 398 GPU, and the selectivity is increased to 239.5. On the whole, this work supplied an effective strategy to overcome the weakness in low cohesive energy density and construct highly fluorinated polyimide materials into excellent membranes with ultrathin and defect-free selective layer.