Influence of Humidity on Sulfonyl-Containing Polybenzimidazoles for Gas Separations

Due to their high chemical and thermal stability, polybenzimidazoles (PBIs) are promising candidates for use as gas separation membranes for high temperature applications.  A series of sulfonyl-containing PBIs based on the 3,3’,4,4’-tetraaminodiphenylsulfone (TADPS) monomer were recently synthesized and shown to have enhanced solubilities in common organic solvents relative to commercial PBI (Celazole^TM), thus improving their processability.  Celazole^TM and TADPS-based PBIs show a strongly hydrophilic behavior, and can uptake up to 25% water by mass at room temperature.  Due to the potential effects of water uptake on both mechanical and gas transport properties, this study seeks to understand the fundamentals of the interactions between water and PBIs. Water vapor gravimetric sorption and dilation measurements have been performed at 35 °C, and a significant hysteresis was observed during desorption experiments, due to swelling and plasticization.  The specific volume of the water-polymer mixture as well as the partial molar volume of water in the glassy polymer have also been calculated.  Furthermore, a separate FTIR analysis showed that strong hydrogen-bonding interactions occur between water molecules and the PBI backbone.