Self-Cleaning Membranes from Comb-Shaped Copolymers with Photoresponsive Side Groups

In this study, we present a novel self-cleaning, photoresponsive membrane that is capable of removing predeposited foulant layers upon changes in surface morphology in response to UV or visible light irradiation while maintaining stable pore size and water permeance. These membranes were prepared by creating thin film composite (TFC) membranes by coating a porous support membrane with a thin layer of novel comb-shaped graft copolymers at two side-chain lengths featuring polyacrylonitrile (PAN) backbones and photoreactive side chains, synthesized by atom transfer radical polymerization (ATRP). Photoregulated control over membrane properties is attained through a light-induced transition, where the side chains switch between a hydrophobic spiropyran (SP) state and a zwitterionic, hydrophilic merocyanine (MC) state. The light-induced switch between the SP and MC forms changes surface hydrophilicity and causes morphological changes on the membrane surface as evidenced by atomic force microscopy (AFM). Before any phototreatment, the as-coated membrane surface comprises mostly hydrophobic SP groups that allow the adsorption of organic solutes such as proteins the membrane surface, reducing flow rate. Once exposed to UV light, conversion of the SP groups to hydrophilic MC groups leads to the release of adsorbed molecules and the full recovery of the initial water flux. A fouled membrane in the more hydrophilic MC form is also capable of self-cleaning upon conversion to the less hydrophilic SP form by visible light irradiation. The self-cleaning behavior observed for this system, where the surface became less hydrophilic but also experienced a morphological change, demonstrates a novel mechanism that has a mechanical component in addition to the changes in hydrophilicity. It is also the first report, to our knowledge, of self-cleaning performance accompanied by a decrease in hydrophilicity.