Novel thin film composite membrane containing ionizable hydrophobes: pH-dependent reverse osmosis behavior and improved chlorine resistance

New polyamide thin film composite membranes were prepared by interfacial polymerization of hexafluoroalcohol (HFA)-containing aromatic diamine and trimesoyl chloride (TMC) on a porous polysulfone support. The surface properties of the resulting membranes were characterized by water contact angle, XPS, and SEM. Additionally, the desalination separation performance was evaluated by the cross-flow filtration of 2000 ppm NaCl solution. Water contact angle and XPS analyses indicated that the HFA-containing polyamide membrane is relatively hydrophobic at neutral conditions but becomes hydrophilic at basic conditions due to ionization of the HFA groups, so we refer to this group as an “ionizable hydrophobe” or “i-phobe”. The membrane showed strongly pH-dependent reverse osmosis behavior with enhanced performance (high water flux and high salt rejection) at high pH (ca. 10). Both the electron withdrawing nature and the steric bulkiness of the HFA functionality are also advantageous in protecting the polyamide membrane from chlorine attack. Based upon NMR studies of model polymers (linear polyamides with and without the HFA functionality) and the membrane performance measured before and after chlorine exposure, the HFA-containing polyamide has improved chlorine stability compared to the reference polyamide made from m-phenylenediamine and TMC.