Fluorinated cardo-based polyimide membranes for helium extraction from natural gas

Abstract

Membrane separation technology exhibits considerable potential for helium recovery from natural gas. However, overcoming the inherent permeability–selectivity trade-off through molecular design strategies remains a fundamental challenge. This study introduces a fluorine-enhanced chain packing strategy to fabricate high-performance polyimide membranes via one-step polycondensation of 6FDA and fluorinated cardo diamine monomers. The incorporation of polar C–F bonds strengthens weak interfacial interactions between helium molecules and the membrane surface, substantially enhancing sieving performance. For comparison, the reference membrane M₀ (non-fluorinated polyimide) was synthesized under identical polycondensation conditions, with its structure devoid of C–F bonds. Permeation experiments with a mixed gas (He/CH₄ = 50/50, vol%) revealed that the fluorinated membrane achieved a 50.16% enhancement in He/CH₄ selectivity compared to the M₀ membrane, while maintaining a helium permeability of 154.13 Barrer. The feasibility of the experimental data was further corroborated by molecular dynamics simulations. This work illustrates that optimized separation membrane performance can be achieved by modifying polymer microstructures.