Water Vapor Sorption, Diffusion, and Dilation in Polybenzimidazoles

Polybenzimidazoles (PBIs) have been investigated for use as gas separation membranes due to their thermal stability and good H 2 /CO 2 separation performance at elevated temperatures. However, PBIs are hydrophilic, and effects of water vapor on PBI separation performance are essentially unexplored. This study investigates water sorption, diffusion, and dilation of a commercial PBI, Celazole, and a recently synthesized sulfonyl-containing PBI, TADPS-TPA. Water binds strongly to both polymers and requires elevated temperatures for removal. Water vapor sorption in Celazole follows dual-mode behavior up to unit activity, while upward concavity is observed at high activities in the TADPS-TPA sorption isotherm. Both PBIs swell significantly in water, and desorption hysteresis suggests water-induced conditioning of the PBI matrix. Model fits of dilation behavior enable estimation of partial molar volume and water-accessible fractional free volume (FFV) as a function of water activity. Water vapor diffusion coefficients increase with increasing vapor activity, which correlates with water-accessible FFV.