Highly Selective Soluble Polyimides Simultaneously Containing Benzimidazole and Hydroxyl Groups for Membrane-Based Gas Separation

A diamine monomer 5-amino-2-(4-hydroxy-3-aminobenzene)-benzimidazole (OH-BIA) containing both benzimidazole and hydroxyl groups was synthesized through a facile two-step reaction and polymerized with four commercially available dianhydrides to yield four polyimides. The solubility, chain packing, mechanical performance, thermal stability, and gas transport properties of the synthesized polyimides were investigated in detail. The synthesized polyimides exhibited good solubility, excellent mechanical properties (tensile strengths ≥103.3 MPa), high thermal stability and glass-transition temperatures (≥396 °C), and high gas selectivity due to the simultaneous introduction of benzimidazole and hydroxyl groups. In particular, because of their enhanced size-sieving ability, these polyimide membranes exhibited higher ideal selectivity for some gas pairs such as CO₂/CH₄ (up to 116) compared to commercial Matrimid 5218 (selectivity for CO₂/CH₄ is 36) and other reported polyimide dense membranes (selectivity for CO₂/CH₄ is up to 63). This work provides useful insights into the design of highly selective polyimides for membrane-based gas separation.