STRUCTURE PERMEABILITY RELATIONSHIPS OF POLYIMIDES WITH BRANCHED OR EXTENDED DIAMINE MOIETIES

Mean permeability coefficients for CO2, O2, N2, and CH, in seven types of 6FDA polyimides with branched or extended diamine moieties were de termined at 35.0-degrees-C (95.0-degrees-F) and at pressures up to 10. 5 atm (155 psia). In addition, solubility coefficients for CO2, O2, N2 , and CH4 in six of these polyimides were determined at 35.0-degrees-C and at 6.8 atm (100 psia). Mean diffusion coefficients for the six ga s/polyimide systems were calculated from the permeability and solubili ty data. The relationships between the chemical structure of the polyi mides, some of their physical properties (glass transition temperature , mean interchain spacing, specific free volume), and their gas permea bility, diffusivity, and solubility behavior are discussed. The 6FDA p olyimides studied here exhibit a considerably lower selectivity for th e CO2 / CH4 and O2 / N2 gas pairs than 6FDA polyimides with short and stiff aromatic diamines with comparable CO2 and O2 permeabilities.