(518b) Impact of Humidity on Gas Transport in Polybenzimidazoles

Polybenzimidazoles (PBIs) have recently been investigated for use as gas separation membranes due to their thermal stability and good H₂/CO₂ separation performance at elevated temperatures. However, PBIs are hydrophilic, and effects of humidity on PBI gas transport properties are generally unexplored. Recent studies have shown that water binds strongly to PBIs and that the high water uptake of PBIs (up to 25 wt%) causes significant swelling and changes in volumetric properties such as polymer fractional free volume.

The effect of humidity on gas transport in both commercial PBI and novel sulfone-containing PBIs has been examined using single gas humidified permeation measurements. Samples were pre-equilibrated with water at relative humidities ranging from 2 to 60% at 35 °C prior to permeating a single gas species (e.g., H₂ or CO₂). Gas permeabilities decreased by up to an order of magnitude at low humidities, which is believed to be due primarily to competitive sorption and antiplasticization. Gas permeabilities increased at higher humidities where more significant swelling and plasticization occurred. Humidified gas permeabilities can be correlated with changes in fractional free volume that result from water uptake and swelling.