Precise Surface Engineering of Hairy Nanoparticles Promoted Packing Frustration for Gas Perm‐Selectivity Beyond Empirical Upper Bound

Abstract Polymers have been broadly applied in the gas separation industry for their excellent processability and mechanical performance; however, the intrinsic tradeoff between gas permeability and selectivity defines their low efficiency for gas separation. Herein, the frustrated packing of hairy nanoparticles (HNs) is applied for the fabrication of gas separation membranes. Poly (propylene glycol) (PPG) is covalently grafted on the surface of the 2.5 nm microporous coordination nanocages (CNCs) with predefined high grafting densities. By precisely tunning the M _w , the grafted PPG demonstrates enhanced rigidity due to the spatial nanoconfinement from the grafted neighbors, and this imposes the frustrated packing of the polymer fragments in the interstitial area among the HNs. The generated extra free volume and extrinsic microporosity can be confirmed from positron annihilation life spectroscopic studies, and it contributes to the interconnection of the intrinsic micropores of CNCs for a gas permeation framework. Meanwhile, the HN unit inherits the mechanical property from the grafted polymers and can be processed into flexible membranes that finally achieve H ₂ /CO ₂ separation performance above the Robeson upper bound. The work reveals the microscopic mechanism for the control of frustrated packing and moreover, opens new avenues to the feasible fabrication of high‐performance gas separation membranes.