Rational Design of Intrinsically Ultramicroporous Polyimides Containing Bridgehead-Substituted Triptycene for Highly Selective and Permeable Gas Separation Membranes

Highly ultramicroporous, solution-processable polyimides bearing 9,10-bridgehead-substituted triptycene demonstrated the highest BET surface area reported for polyimides (840 m² g^–1) and several new highs in gas selectivity and permeability for hydrogen (1630–3980 barrers, H₂/CH₄ ∼ 38) and air (230–630 barrers, O₂/N₂ = 5.5–5.9) separations. Two new dianhydrides bearing 9,10-diethyl- and 9,10-dipropyltriptycenes indicate that the ultramicroporosity is optimized for fast polymeric sieving with the use of short, bulky isopropyl bridgeheads and methyl-substituted diamines (TrMPD, TMPD, and TMBZ) that increase intrachain rigidity. Mechanically, the triptycene-based analogue of a spirobisindane-based polyimide exhibited 50% increases in both tensile strength at break (94 MPa) and elastic modulus (2460 MPa) with corresponding 90% lower elongations at break (6%) likely due to the ability of highly entangled spiro-based chains to unwind. To guide future polyimide design, structure/property relationships are suggested between the geometry of the contortion center, the diamine and bridgehead substituent, and the mechanical, microstructural, and gas transport properties.