Soluble Hydroxy-Containing Poly(benzoxazole-benzimidazole-imide)s for Membrane-Based H2/CO2 Separation

Conventional polyimide (PI) membranes exhibit low H2/CO2 selectivity (≤3.1). To address this limitation, this study synthesized a novel diamine monomer, 5-amino-2-(4-hydroxy-3-aminobenzene)-benzoxazole (OH-BOA), specifically designed with benzoxazole and hydroxyl functionalities. Subsequently, OH-BOA and its structural counterpart 5-amino-2-(4-hydroxy-3-aminobenzene)-benzimidazole (OH-BIA) were copolymerized with pyromellitic dianhydride (PMDA) to yield four hydroxy-functionalized poly(benzoxazole-benzimidazole-imide)s (OH-CoPIs). These OH-CoPIs exhibited good solubility, robust mechanical properties (tensile strength ≥ 114.9 MPa), high glass transition temperatures (Tg > 400 °C), and low coefficients of thermal expansion (CTE ≤ 28.3 ppm/K). Notably, they achieved a significantly enhanced H2/CO2 selectivity of up to 12.2, surpassing the commercial Matrimid 5218 (3.1) by 4-fold. Systematic evaluation revealed that increasing benzoxazole content reduced the CTE while enhancing tensile strength, Young's modulus, and H2/CO2 selectivity. The thermally rearranged membrane (TR-40CoPI-400, 400 °C) achieved high H2 permeability (26.00 Barrer) with optimal selectivity (7.6) and exceptional CO2 plasticization resistance. The synergistic integration of benzimidazole, benzoxazole, and hydroxyl groups enables high-performance PI membranes for H2/CO2 separation.