Electrically Mediated Membrane Pore Gating via Grafted Polymer Brushes

Stimuli-responsive membranes that enable the smart control of membrane transport properties represent an interesting class of materials for a wide range of separation processes. In this work, surface-initiated, electrochemically mediated atom transfer radical polymerization was used to grow poly(acrylic acid) brushes from the surface of a percolating network of carbon nanotubes coating an ultrafiltration membrane. Brush molecular weights as high as 930 kDa were achieved, with very low dispersivity. When no potential was applied, the material was able to reject 88% of 12 kDa dextran in 50 mM NaCl solution. Membrane rejection and flux were readily changed upon the application of electrical potentials, with positive potentials increasing flux by 85% and reducing rejection by 56%, while negative potentials reduced flux by 30% and increased rejection by 5%. We show evidence that counterion migration, moving in response to the applied electric field, is responsible for the expansion or collapse of the grafted charged polymer brushes, which induced or eliminated pore gating. In addition, atomic force microscopy was used to probe the forces exerted by brush conformational changes in response to the applied field. These materials can find applications in industrial separations where gating is needed and may possess interesting anti-fouling properties.