Structural Regulation and Gas Separation Performance of Thermally Rearranged Membranes Derived From Spirocyclic/Fluorenyl‐Containing Copolyimides

Herein, two diamine monomers 2‐amino‐3′,6′‐bis(4‐amino‐3‐hydroxyphenoxy)spiro[isoindoline‐1,9′‐xanthen]‐3‐one (ABAHS) and 9,9′‐bis(3‐amino‐4‐hydroxyphenyl)fluorene (BAHPF) were copolymerized with 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) to prepare copolyimide (CPI) precursors and corresponding copolymerized thermally rearranged (CTR) membranes. Two rigid skeletons (ABAHS spiro heterocycle and BAHPF fluorene Cardo moiety) were synergistically introduced to tailor chain stacking. ABAHS ether groups offer superior film‐forming and mechanical strength, and intermolecular hydrogen bonds from phenolic hydroxyls and polar heteroatoms adjust chain packing density. The as‐fabricated CPI precursors exhibit prominent thermal stability with glass transition temperatures ranging from 348°C to 359°C. The precursor with an ABAHS/BAHPF molar ratio of 9:1 achieves optimal mechanical properties, with a tensile strength of 123 MPa and an elongation at break of 3.0%. After thermal rearrangement, the gas permeability of CTR membranes is drastically enhanced. The CTR‐5:5 membrane shows permeability coefficients of 72, 1408, 425 and 1537 Barrer for N 2 , H 2 , O 2 and CO 2 respectively. In contrast, the CTR‐9:1 membrane with high ABAHS content retains nitrogen polar sites and forms well‐defined molecular‐sieving channels with favorable CO 2 affinity. It delivers ideal H 2 /N 2 and CO 2 /N 2 selectivities of 27.60 and 26.16, both exceeding the 2008 Robeson upper bound, and its O 2 /N 2 selectivity reaches 7.80 above the 2015 upper bound.