Effect of hydrophilicity and free volume on the dehumidification performance and hydrolytic stability of fluorinated polyamide membranes

A series of fluorinated polyamides (PAs) air dehumidification membranes containing hydrophilic, bulky groups have been designed to balance H₂O permeability, H₂O/N₂ selectivity, and hydrolytic stability. As expected, the polyamide membrane incorporating hydrophilic carboxylic acid groups increases dehumidification efficiency but with a significant sacrifice of hydrolytic stability. The incorporation of large free volume groups could increase the H₂O permeability, while H₂O/N₂ selectivity is lower. The fluorinated polyamides with excellent air dehumidification performance and hydrolytic stability were obtained successfully by fine-tuning the type of hydrophilic groups. The co-PA-PABZ membranes with hydrophilic benzimidazole groups exhibited an excellent H₂O permeability of ∼4142 Barrer and a H₂O/N₂ selectivity of around 59171. The dehumidification performance is much higher than without hydrophilic groups co-FPA-70 polyamide membrane (P_H2O = 3943 Barrer, H₂O/N₂ = 9388). Importantly, they also demonstrated outstanding hydrolytic stability at 80 °C water, retaining 91 % molecular weight and 97 % tensile strength over 5000 h, as confirmed by ¹H NMR, molecular weight and mechanical tests. The dehumidification aging test of more than 2000 h on this membrane further indicated its excellent durability under operation conditions. This efficiency-design strategy provides a universal method for developing aromatic polyamides with alkaline hydrophilic functionalities, facilitating the creation of high-performance, hydrolytically stable dehumidifying membranes.