Fluorinated biphenyl aromatic polyimides for gas separation applications: Real gas mixture study

Three aromatic polyimides (PIs) based on diamines containing a biphenyl unit and CF₃-groups in the m-position to amines and 6FDA of high-molecular weights were synthesized. The polymers differed by number of methyl-substituents in the biphenyl moiety, as 6FDA-BFAPB, PI without methyl substituents, 6FDA-4MeBFAPB, PI with four methyl groups, and 6FDA-4MeBFAPB, PI with six methyl groups. The comparative analysis of the polymer properties was performed with focus on their gas transport properties. All PIs demonstrated high thermostability, with decomposition temperature above 450 °C in an inert atmosphere and char yield of 45-50% but were readily soluble at room temperature even in common organic solvents. The T_g’s of PIs bearing methyl groups were notably higher than that of 6FDA-BFAPB, indicating their more rigid backbone chains. Detailed investigation of the PIs’ gas transport properties was also made for pure gases and real gas mixture in the wide interval of pressures. It was shown that introduction of the methyl substituents in the main polymer chain resulted in an important improvement of the gas transport properties; both methylated PIs were located much closer to the Robeson limit.