The pressure dependence of CO2 sorption and permeation in poly(acrylonitrile)

Abstract Sorption isotherms for carbon dioxide in glassy poly(acrylonitrile) (PAN) microspheres and films were measured at temperatures between 25°C and 65°C and pressures up to 30 atmospheres. The transient and steady-state permeation of carbon dioxide through glassy PAN films was measured at 35, 45 and 55°C using upstream pressures between 5 and 20 atmospheres. p]Sorption isotherms for films and microspheres were nonlinear and indicative of the dual modes of sorption characteristic of gas/glassy polymer systems. Annealing the microspheres resulted in a decreased level of sorption; microsphere sorption capacities after annealing remained consistently higher, however, than sorption determined for film samples. The difference in sorption capacities may be associated with surface adsorption on the high specific surface area microspheres. p]Steady-state CO 2 permeability coefficients for the PAN films decreased with increasing upstream pressure. The permeability coefficients were in the range of 10 −13 cm 3 (STP)cm/cm 2 -sec-cmHg, which is nearly four orders of magnitude smaller than CO 2 permeability coefficients found for polycarbonate and two orders of magnitude smaller than permeabilities of poly(ethylene terephthalate) to CO 2 . p]The steady-state measurements were analyzed according to the Paul—Koros partial immobilization transport model. Diffusion coefficients for both the Henry's law and Langmuir CO 2 populations were in the range of 10 −11 cm 2 /sec. Over the temperature range studied, the Langmuir population had a relative mobility of about 0.6 of the mobility of the Henry's law population. p]Permeation time-lags were 15—20 times larger than would be expected from the independently-determined sorption and steady-state permeation measurements assuming Fick's law applies. A possible explanation for the anomalously long time-lags invokes a very slow relaxation in PAN during the transport process which results in a protracted approach to steady-state permeation.