The effects of chemical structure on gas transport properties of poly(aryl ether ketone) random copolymers

The physical properties and gas permeation behavior of a series of homo/random copolymers of 4,4′-difluorobenzophenone (DFBP)-2,2-bis(4-hydroxy-phenyl)propane (BPA)/2,2-bis(4-hydroxy-3,5-dimethyl-phenyl)propane (TMBPA) have been investigated by systematically varying the diol ratios. The tetramethyl substitution group on the phenyl rings simultaneously increases polymer free volume and chain stiffness. These were confirmed by experimental and simulated methods. With the increase in TMBPA content, the gas permeation coefficients, diffusion and solubility coefficients of H₂, O₂, N₂, CO₂ and CH₄ were found to increase. The gas transport coefficients of the copolymers predicted from the additional rule were compared with the experimental results and the results obtained were within the expectations. In addition, the logarithm of gas permeation coefficients and the reciprocal of fractional free volume (1/FFV) also exhibited a good correlation. However, with the incorporation of TMBPA moiety, the permselectivity of gas pairs such as H₂/N₂, O₂/N₂ and CO₂/CH₄ remains reasonably high. As a result, the gas separation performance of TMBPA modified poly(aryl ether ketone) approaches the upper bound of the corresponding gas pairs. After comparing this work with gas separation performance of other methyl substituted polymers, one may conclude there is a general phenomenon that methyl substitution increases the gas permeability of the modified polymer with a small loss in gas selectivity for small gas pairs.