Reverse Osmosis Membranes Made of Novel Sulfonated Poly(arylene ether sulfone) Bearing Carboxylic Acid Group

Novel sulfonated poly(arylene ether sulfone) bearing pendant carboxylic acid group polymer (C-SPAES) was synthesized by direct copolymerization of 4,4′-bis(4-hydroxyphenyl) valeric acid , 4,4'-dichlorodiphenyl sulfone and disodium 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone. The expected structure of the polymer was confirmed by ¹H NMR. The ion exchange capacity (IEC) of the C-SPAES could be controlled by varying the sulfonated monomer content. To achieve improved performance of the composite reverse osmosis membrane, the polymer with an IEC value of 1.31mmol/g was selected for the functional material. Reverse osmosis membranes were prepared by brush coating dilute polymer solutions in 96% formic acid onto polysulfone ultrafiltration membrane, which were subsequently air-dried to remove solvent. The polysulfone ultrafiltration membrane was obtained by coating onto a non-woven polyester paper  a solution of bisphenol-A polysulfone and additives in DMF, and gelating the solution in water. The MWCO of the polysulfone ultrafiltration membrane can be controlled by changing the concentration of the polysulfone and additives. In this study, a MWCO of ca. 20000 for the ultrafiltration membrane was selected to avoid the C-SPAES molecule entering the pores of the support-membrane. The SEM results indicated that the obtained reverse osmosis membranes had smooth surface, and no defect was observed. Meanwhile, the functional layer thickness obtained from the SEM results was about 200nm. The compact and no defect functional layer structure of the membrane resulted in the improved separation properties, and the thin functional layer was benefit for water molecule passing through the membranes. On the other hand, carboxylic acid group was hydrophilic, which increased the hydrophilicity of the reverse osmosis membrane, resulting in the increasing of the water flux. The C-SPAES reverse osmosis composite membranes exhibited increased salt rejection with increasing C-SPAES concentration, and the water flux decreased with the increase of C-SPAES concentration. Notably, C-SPAES-1.0, with C-SPAES content of 1.0%, exhibited a salt rejection of 90.2% and water flux of 11.4L/(m²h)under a feed pressure of 2.0 MPa. After exposing in 2000 ppm NaClO for 24 h, the salt rejection of the membranes was still retained above 88%, which was much higher than the commercial PA composite reverse osmosis membrane under the same condition.