Design of high antifouling pH-responsive membrane for anionic dye filtration under alkaline conditions

In this work, a novel high-antifouling poly(vinylidene fluoride) (PVDF) composite membrane was synthesized via free radical polymerization by incorporating a pH-responsive poly(N-acryloyl-L-alanine) (poly(Ala-OH)) as a functional skin layer and 2-hydroxyethyl acrylate-terminated poly(styrene-alt-maleic anhydride) (SMA-HEA) as an amphiphilic linker for effective integration onto the PVDF substrate. FT-IR and XRD analyses confirmed the successful grafting of poly(Ala-OH) through the C=C bond of the linker, leading to a significant enhancement in membrane hydrophilicity. As a result, the modified membranes exhibited a hydrophilic surface. The pH-responsive behavior of the membrane was evident under alkaline conditions (pH = 11), where deprotonation of carboxylic acid groups induced a stronger negative surface charge, causing molecular chain expansion due to electrostatic repulsion. This structural adjustment further improved membrane hydrophilicity and anionic dye rejection. Consequently, the poly(Ala-OH)-modified PVDF membrane demonstrated higher and more stable flux during cyclic filtration tests under alkaline conditions. The results highlight the critical role of the poly(Ala-OH) layer’s carboxylic acid groups and membrane charge variations in significantly enhancing overall hydrophilicity and antifouling performance, making it a promising solution for anionic dye filtration.