(499h) Self-Cleaning, Photoresponsive Thin Film Composite Water Filtration Membranes

Materials with reversibly switchable properties hold great interest in the development of membranes with new functions such as fouling resistance and self-cleaning, adjustable pore size, and tunable flux. Photoresponsive membranes can show these changes without the need for changes to the feed composition. They can also show orthogonal response when combined with pH- or ionic strength responsive functionalities. This research focuses on light-responsive membranes that can self-clean to remove foulants upon exposure to light. We show the synthesis of new comb-shaped polymers with photo-regulated zwitterion formation and their use as responsive membrane selective layers. Zwitterions are charged molecules that comprise equal numbers of covalently bound cations and anions. The unique co-existence of opposite charges bridged through alkylene spacers provides zwitterions extremely high polarity. This feature leads to the exceptional anti-fouling properties for zwitterionic materials. This has exciting applications in the membrane field. We synthesized novel comb-shaped graft copolymers at different side-chain lengths with polyacrylonitrile (PAN) backbones and photoreactive side-chains using atom transfer radical polymerization (ATRP). The side-chains undergo a light-induced transition between a hydrophobic neutral state and a zwitterionic state, allowing photo-regulated control over membrane features. We used these graft copolymers to modify a commercially available UF membrane by coating a thin layer of the copolymer solution followed by precipitation in a non-solvent. Prior to any photo-treatment, coated membranes consist mainly of hydrophobic groups that dominate the membrane surface, and they increase upon visible light irradiation. We postulate that the hydrophobic groups promote the adsorption of hydrophobic solutes on channel walls, reducing flow rate. Upon reversing the photochemical response by irradiation with UV light, the hydrophobic groups are converted to zwitterionic groups that release the adsorbed molecules and permit water passage once again. This â??self-cleaningâ? behavior has been shown by measuring pre- and post-UV water permeability after fouling with bovine serum albumin. Short chain copolymer shows essentially complete self-cleaning performance, as indicated by the fact that even after three cycles of protein fouling, the water permeability can be increased back to its initial value using UV exposure. In addition, the short chain membrane surface proves antifouling at UV-irradiated state, which is shown by the absence of permeability decline after 2 hours of BSA filtration. The following visible light exposure does not change the permeability, however the visible-irradiated surface shows a significant permeability decline after 2h BSA filtration. Changes in fingerprint IR peaks pertinent to neutral and zwitterionic forms upon light treatment have further confirmed the structural difference between the two forms at a molecular level. This finding is also in agreement with the hydrophilicity and surface roughness results; the UV-irradiated surface has a smoother surface with enhanced hydrophilic wettability. Additional work focuses on determining optimal phototreatment regimes for self-cleaning behavior with minimal energy use, and developing a better understanding of how polymer self-organization controls this responsive behavior.