Effect of the upstream pressure on gas transport in poly(ether-imide) films

Values at several pressures and temperatures of the permeability and diffusion coefficients of oxygen, nitrogen and carbon dioxide through films of poly(bisphenol-A-co-4-nitrophthalicanhydride-co-1,3-phenylenediamine) (PBNPI), a polietherimide marketed under the trade name ULTEM®, are reported. The variation with pressure of the concentration of the gases in the films is described in terms of the dual-mode model. The plot of the concentration of CO2 in Langmuir sites against Tg − T lies in the straight line roughly intercepting the origin defined by other glassy polymers. The permeability coefficient is independent on pressure suggesting that gas diffusion through Langmuir sites is negligible. The interpretation of gas solution in the continuous phase in terms of the Flory-Huggins theory gives reasonable values for the gas-polymer interaction enthalpic parameter of carbon dioxide, oxygen and nitrogen. The permselectivity coefficient of oxygen with respect to nitrogen, α (O2/N2), is 7.1 ± 3.5 at 30 °C, the diffusive process being mainly responsible for the relatively good permselectivity of the films.