Impact of humidity on gas transport in polybenzimidazole membranes

Polybenzimidazoles (PBIs) are promising materials for high temperature H-2/CO2 separation in applications such as steam reforming and pre-combustion carbon capture where significant amounts of water are often present. However, PBIs are hydrophilic, and the impact of humidity on PBI gas separation properties is relatively unexplored. Furthermore, opportunity exists to elucidate the interplay between plasticization, free volume, and gas transport in glassy polymer membranes such as PBIs. This study investigates the effect of humidity on H-2, O-2, and CO2 permeabilities at 35 degrees C in a commercial PBI and two sulfone-containing PBIs. Water uptake significantly reduces PBI gas permeabilities at low humidities due to competitive sorption and antiplasticization. At high humidities, plasticization increases the permeabilities of larger gases in more hydrophilic PBIs. Effective fractional free volumes evaluated from gas permeation data and previously reported water sorption and dilation data suggest water plasticizes PBIs by increasing accessible free volume via enhanced molecular dynamics rather than by creating new free volume cavities.

Automated summary

Polybenzimidazoles (PBIs) have promising potential for high temperature H-2/CO2 separation applications, but their gas separation properties are affected by humidity. Water uptake reduces PBI gas permeabilities at low humidities through competitive sorption and antiplasticization, while at high humidities, plasticization increases permeabilities of larger gases in more hydrophilic PBIs. Water plasticizes PBIs by enhancing molecular dynamics to increase accessible free volume rather than creating new free volume cavities.