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

Abstract

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 CC 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.