Impact of humidity on gas and water vapor transport in polybenzimidazole gas separation membranes

Understanding the effects of humidity on gas transport in polymer membranes is crucial for designing high selectivity materials.  Many commercial gas separation processes operate with feed streams that contain water vapor or other plasticizing vapors that can swell polymer membranes and compromise their permeability selectivity. Polybenzimidazoles (PBIs) are an excellent class of polymers to investigate the effects of humidity on gas transport due to their unusually high water uptakes (up to 25 wt%) compared to other aromatic polymers commonly used in gas separations. PBIs have recently been investigated for use as gas separation membranes for high temperature H2/CO2 separations due to their thermal stability and good H2/CO2 separation performance at elevated temperatures.  However, the effects of water vapor on PBI separation performance are generally unexplored. My work examines the relationship between humidity, polymer swelling, and gas and water vapor diffusion in water-plasticized PBIs.  As humidity increases, sorbed water fills “free volume” cavities between polymer chains and swells the polymer. This leads to an increase in water diffusion coefficients in the polymer due to enhanced chain mobility. Gas permeabilities exhibit a complex relationship with humidity due to a trade-off in the effects of filling free volume and plasticization.