EFFECT OF METHYL SUBSTITUENTS ON PERMEABILITY AND PERMSELECTIVITY OF GASES IN POLYIMIDES PREPARED FROM METHYL-SUBSTITUTED PHENYLENEDIAMINES

Permeability and solubility coefficients for H-2, CO2, O2, CO, N2, and CH4 in polyimides prepared from 6FDA and methyl-substituted phenylenediamines were measured to investigate effects of the substituents on gas permeability and permselectivity. The methyl substituents restrict internal rotation around the bonds between the phenyl rings and the imide rings. The rigidity and nonplanar structure of the polymer chain, and the bulkiness of methyl groups make chain packing inefficient, resulting in increases in both diffusion and solubility coefficients of the gases. Polyimides from tetramethyl-p-phenylenediamine and trimethyl-m-phenylenediamine display very high permeability coefficients and very low permselectivity due to very high diffusion coefficients and very low diffusivity selectivity, as compared with the other polyimides having a similar fraction of free space. This suggests that these polyimides have high fractions of large-size free spaces.