Gas transport properties of poly(1,5-naphthalene-2,2′-bis(3,4-phthalic) hexafluoropropane) diimide (6FDA-1,5-NDA) dense membranes

The intrinsic gas transport properties of permeation, diffusion and sorption for He, O2, N2, CH4 and CO2 in aromatic polyimide, poly(1,5-naphthalene-2,2′-bis(3,4-phthalic) hexafluoropropane) diimide (6FDA-1,5-NDA) dense membranes were investigated. The permeation of pure gases of He, O2, N2, CH4 and CO2 was measured with a temperature-controlled permeation cell while the sorption isotherm was obtained from the Cahn 2000 microbalance sorption cell. The 6FDA-1,5-NDA membrane has a selectivity of 49 for CO2/CH4 with a permeability of 22.6 Barrers for CO2 under 10 atm at 35 °C. The Henry’s diffusivity holds a dominating effect over the Langmuir diffusivity and decreases in the order of O2>CO2>N2>CH4, in fair agreement with the apparent diffusivity. The activation energies of permeation and diffusion increase with increasing gas kinetic diameters in the order of CO2, O2, N2 and CH4. The solubility of gases tested adopts a parallel trend with their critical temperatures. Upon pressure acceleration, the gas diffusivity increases while the solubility decreases. The overall gas permeability of 6FDA-1,5-NDA decreases with increasing pressure, which can be explained by using the dual-mode sorption model and the partial immobilisation model. More than 50% of the entire total gas sorbed is distributed in the Langmuir environment when pressure is less than 25 atm. The fractional mobility of Langmuir species decreases while the fractional mobility of the Henry species increases when the feed pressure increases. CO2 exhibits the most significant pressure-dependent properties due to its the strongest interaction and highest condensability.