Flexibility-regulated ultramicroporous transport in rigid-flexible polysilsesquioxane membranes for helium separation

Helium recovery from natural gas-related streams requires membranes with sub-angstrom-level control over ultramicroporous transport pathways, because the kinetic diameter difference between He and larger gas molecules is extremely small. However, conventional membranes often suffer from the permeance–selectivity trade–off, leading to insufficient He separation from N₂ and CH₄. Herein, we report a bio-inspired rigid-flexible polysilsesquioxane (PSQ) membrane prepared by the co-condensation of a rigid precursor, 1,2-bis(triethoxysilyl)ethane (BTESE), and a flexible precursor, N-(trimethoxysilylpropyl)imidazole (TMSPI). The BTESE-derived rigid siloxane framework provides an ultramicroporous backbone, while the TMSPI-derived imidazole-containing side chains introduce locally flexible and sterically active units into the pore environment. By optimizing the TMSPI content, the effective transport aperture and pore-wall flexibility were simultaneously regulated. The BTESE-T20% membrane with a TMSPI content of 20 mol% achieved a He permeance of 2.54 × 10⁻⁸ mol m⁻² s⁻¹·Pa⁻¹ (75 GPU) coupled with an ideal He/N₂ selectivity of approximately 300, which is an order of magnitude higher than that of conventional PSQ membranes. Gas permeation analysis and molecular simulations revealed that TMSPI incorporation narrowed the effective transport aperture to approximately 0.39 nm, close to the critical sieving window for He/N₂ and He/CH₄ separation. The enhanced separation performance is attributed to flexibility-regulated steric sieving: moderately flexible TMSPI-derived side chains impose limited resistance to He transport while substantially increasing the diffusion resistance of larger N₂ and CH₄ molecules. This work provides a rational pore-wall flexibility regulation strategy for designing high-selectivity PSQ membranes for helium separation, and this design concept may be extended to other ultramicroporous and hybrid membrane systems where sub-angstrom transport control is required.