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

Abstract

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.

Testing the self-cleaning properties of a coordination polymer surface

“…Recently some new self-cleaning systems have been proposed utilizing the photocatalytic effect for the degradation of organic pollutants adsorbed on a surface (Yusof et al 2018). Moreover some "smart" surfaces have been also developed and the self-cleaning effect can be UV (Kaner et al 2017;Pifferi et al 2015) or laser-induced (Zheng et al 2016).…”

Light‐Responsive Polymer Membranes

“…For example, the switchability of SP‐based materials, which become zwitterionic and thus more hydrophilic under UV light, can be used to generate antifouling surfaces …”

“…Recently, Kaner et al prepared thin film membranes with antifouling properties due to light induced changes in surface morphology at constant pore size and water permeability. The self‐cleaning membranes were obtained by coating a porous PVDF membrane with a thin layer of comb‐shaped graft copolymers at two side‐chain lengths featuring polyacrylonitrile backbones and photoreactive SP methacrylate side chains.…”

Recent Advances in the Fabrication of Membranes Containing “Ion Pairs” for Nanofiltration Processes

“…Furthermore, there is a significantly lower water flux decline of the zwitterionic polyamide membrane compared to the pristine polyamide membrane in filtrating a mixed feed solution containing organic foulants, such as bovine-serum albumin, alginate, and natural organic matter. Kaner et al [60] reported a novel self-cleaning, photo-responsive membrane, and found that the pre-deposited foulant could be removed from the membrane surface in response to UV or visible light irradiation while maintaining stable pore size and water permeance. This is because the hydrophobic spiropyran groups within the membrane are converted to hydrophilic zwitterionic merocyanine groups by irradiation with UV light, resulting in the release of adsorbed molecules and the full recovery of the initial water flux.…”