Ultra-Selective Molecular-Sieving Gas Separation Membranes Enabled by Light–Heat Dual Crosslinking of Polyimide for Helium Extraction From Natural Gas

Helium extraction from natural gas demands ultra-high He/CH 4 selectivity (>1000), yet this benchmark remains largely unmet by polymeric membranes due to the permeability-selectivity trade-off and the challenge of constructing sub-4 Å sieving channels. Here, we report an orthogonal light–heat dual-crosslinking strategy to program the free-volume topology of polyimide membranes. A photoreactive precursor (6FCC-X) was synthesized via a click reaction by incorporating photosensitive units (C═C─S) and varying amounts of thermally labile─COOH groups into the backbone. UV irradiation first triggers [2+2] cycloaddition to construct a dense and uniform super-crosslinked network, which defines the primary molecular-sieving framework. Subsequent thermal decarboxylation generated rigid biphenyl linkages that are proposed to function as molecular-strut-like structural motifs, enhancing chain rigidity, improving micropore connectivity, and refining sub-4 Å sieving constrictions. This sequential topology programming decouples helium transport from methane exclusion, which enables the optimized 6FCC-3-CR membrane to achieve a He permeability of 33.9 Barrer and an exceptional He/CH 4 selectivity of 3767, outperforming most reported polymer-based membranes and rivaling advanced carbon molecular sieve membranes. This work establishes a molecular-strut-mediated topology-programming principle for polymer molecular sieves, offering a general route to reconcile fast transport with precise molecular exclusion.