Synthesis of a novel next-generation positively charged polymer and its in-situ grafting into thin film composite membranes to enhance the performance for desalination

High efficiency of the polyamide membranes can be achieved by rationally tailoring the active layer, which empowers the membrane with better anti-fouling characteristics, high flux, and separation efficiencies. Herein, a next-generation anti-fouling positively charged copolymer of diallyldimethylammonium chloride (DADMAC) and N¹, N¹-diallyldodecane-1,12-diammonium chloride (DADAC) was synthesized and thoroughly characterized by TGA, FTIR, proton (¹H) and the carbon (¹³C) NMR. The synthesized poly(DADMAC-co-DADAC), after conversion to poly(DADMAC-co-DADA) having free NH₂ group, was in-Situ grafted while forming the active layer due to its rationally designed anchoring points of NH₂ group. The in-Situ grafting resulted in a cross-linked polyamide active layer containing positively charged brushes of poly (DADMAC-co-DADA) at regular intervals. The AFM analysis has shown that the polyamide layer's surface roughness increased with the poly-(DADMAC-co-DADA) grafting concentration, which positively impacts the membranes' flux. When exposed to the water, the grafted poly-(DADMAC-co-DADA) expanded and spread a positively quaternary ammonium network on the membrane surface, effectively repelling the cations. The introduced membrane has shown high performance compared to the pristine polyamide membrane. The functionalized membranes have displayed a 3.5 times flux for 2000 ppm of NaCl while maintaining a good rejection at 93.1%. The anti-fouling behavior of the membranes was evaluated against the model positively charged CTAB (cetyltrimethylammonium bromide) foulants (2000 ppm). Poly- (DADMAC-co-DADA) grafted membranes have shown a high anti-fouling tendency. After 600 min (10 h) of continuous operation, the flux declined less than 15%, whereas the pristine polyamide membrane lost 99%. The poly-(DADMAC-co-DADA) grafted membranes recovered more than 93% of their maximum flux. Thus, the in-Situ grafting of positively charged polymer is a possible opportunity to develop the next generation of high-performance polyamide membranes.