DEPENDENCE OF THE GAS-PERMEABILITY OF SOME POLYIMIDE ISOMERS ON THEIR INTRASEGMENTAL MOBILITY

The polyimide PMDA-4,4'-ODA has a higher permeability and a lower selectivity toward H-2, CO2, CH4, O2, and N2 than its isomer PMDA-3,3'-ODA. The differences in the gas permeability and selectivity are attributed to differences in the intrasegmental (rotational) mobility of PMDA-4,4'-ODA (the paraisomer) and of PMDA-3,3'-ODA (the meta-isomer). The phenyl rings in the para-isomer can rotate around their principal axis whereas such a rotation is not possible in the meta-isomer. This hypothesis is supported by dynamic mechanical analyses as well as by measurements of the CO2 diffusivity in the two isomers. Computer simulations indicate that rotation of the phenyl ring in the para-isomer is hindered by two larger rotational energy barriers of 4.0 and 2.4 kcal/mole, and two smaller ones of 1.3 kcal/mole.