A method of controllable positive-charged modification of PVDF-CTFE membrane surface based on C–Cl active site

There is a great market demand for positive-charged membrane used to treat wastewater containing positive-charged pollutants. Based on the fact that the C–Cl bond energy in the molecular chain of the PVDF-based poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) copolymer membrane is significantly lower than the C–F bond. Herein, the method of controllable positive-charged modification of PVDF-CTFE membrane surface has been successfully realized by using diethylenetriamine (DETA) a small molecule polyamine compound under mild conditions. The modified membranes were characterized by X-ray photoelectron spectroscopy (XPS), zeta potential and water contact angle, and the best condition for positive-charged modification were explored. The results show that the modified membrane has an excellent positively charged effect, changing from an original membrane which is completely negative to the positive-charged membrane whose isoelectric point value is 7.63, the content of N element is as high as 6.31 at% and the water contact angle decreased by 42.8%. After 7 cycles of treatment of polycationic acrylamide solution, the recovery rate of feed-liquid flux is still above 97.5%, indicating that the ability to resist positively charged pollutants is significantly improved. More importantly, the mechanical strength is not adversely affected. The method proposed in this paper may provide a good idea for the graft modification of chlorinated polymers.