Synthesis of an o-Cresolphthalein-Based Polyimide and Its Derivative Cross-Linked Thermally Rearranged Polymers for Gas Separation

Phenolphthalein-based polyimides and their derivative cross-linked and thermally rearranged polymers (XTR) exhibited attractive gas transport properties. This study attempted to further improve the gas transport performance of XTR polymers by tuning the molecular structures of a phenolphthalein-based diamine, diamino-phenolphthalein (DAP). We synthesized an o-cresolphthalein-based diamine (OCA), which had a similar molecular structure to DAP except that there were two methyl groups on the ortho position to hydroxyl groups of DAP. The OCA monomer reacted with 4,4′-(hexafluoroisopropyl)phthalic anhydride (6FDA) to form a polyimide, 6FDA-OCA, which was used as a precursor to form XTR polymers by heating in argon. Molecular simulation results indicated that the methyl groups of OCA gave the 6FDA-OCA backbone a lower torsion angle and a higher rotation energy than 6FDA-DAP. This led to a higher T_g, better gas transport performance, and better plasticizing resistance of 6FDA-OCA than 6FDA-DAP. After TR reactions, the CO₂ permeability of 6FDA-OCA-450 reached 3743 Barrer, which was 60% higher than that of 6FDA-DAP-450, and the ideal selectivity of CO₂/CH₄ was 18.8, which was slightly higher than the selectivity of 18.6 to 6FDA-DAP. Therefore, the strategy of introducing methyl groups in the benzene ring of phenolphthalein was successful to enhance gas transport properties to phenolphthalein-based polyimides and TR polymers.