Maximizing membrane antifouling potential: The impact of fluoride positioning in multifunctional designs

Fluoropolymers with low surface energy demonstrate outstanding potential for fouling release. However, their limited effectiveness in practical antifouling applications requires integration with other strategies. This study explored the significant impact of fluoropolymers in a multifunctional approach that combines antiadhesion (S), antibacterial (M), and fouling release (H) properties to enhance the performance of thin-film-composite (TFC) membranes for controlling biofouling (The letters S, M and H originate from the initial letters of the corresponding functional monomers). We constructed membrane surface functionalities with fluoropolymers placed in different layers: p(H-M-S), which incorporates fluoropolymers in the innermost layer as a release-antibacterial-antiadhesion membrane; p(M-H-S), where fluoropolymers are in the middle layer as an antibacterial-release-antiadhesion membrane; and p(M-S-H), with fluoropolymers in the outermost layer as an antibacterial-antiadhesion-release membrane. This multifunctional approach resulted in superior membrane transport properties and varying resistance to biofouling. During repeated filtration cycles, the p(H-M-S) membrane showed the most effective biofouling mitigation and long-term durability, achieving an 82 % flux recovery in the third cycle due to the synergistic effects of its three combined functions. The p(M-H-S) membrane displayed strong antiadhesion performance in the early stages but had limited durability over time. In contrast, the p(M-S-H) membrane revealed the weakest fouling resistance, likely because of the hydrophobic nature of the fluorinated components in the outermost layer. Bacterial adhesion assay and protein release tests further demonstrated that the p(M-H-S) membrane reduced bacterial adhesion by 66 % and released 23 % of the protein foulants. This effectiveness is attributed to the antifouling activity provided by the hydrophilic zwitterions and bactericidal quaternary ammonium compounds, as well as the fouling-release capability of fluoropolymers, which facilitates the detachment of foulants under hydraulic forces. Microscopic analysis, coupled with interfacial energy evaluations, confirmed the presence of various multi-defense mechanisms based on the different functional architectures of the membranes. These findings offer valuable insights for designing optimized multifunctional antifouling membranes with improved performance and stability.