Reversible isopentane-induced depression of carbon dioxide permeation through polycarbonate

Permeability data are reported for carbon dioxide in Lexan polycarbonate at 35°C. Measurements were made for both pure carbon dioxide and for a mixed feed consisting of carbon dioxide with a 117.8-torr (0.155-atm) Partial pressure of isopentane. The effects of varying upstream CO₂ driving pressure from 1 up to 20 atm were studied. The permeability to CO₂ is reduced significantly in the presence of isopentane; however, the fractional depression of the CO₂ permeability due to the isopentane at low driving pressures is much more significant than at high CO₂ driving pressures. The well-known pressure dependence of carbon dioxide permeabilities in glassy polymers, therefore, is largely diminished by introducing isopentane to the pure carbon dioxide feed. These observations are consistent with a model for transport in glassy polymers which explains the observed trends in terms of competition between the two penetrants for microvoid sorption sites existing in the non-equilibrium glassy polymer. Exclusion of carbon dioxide from microvoid sorption sites by the more condensable isopentane preempts transport through the microvoid regions, resulting in the observed depression of the CO₂ permeability.