Reversible Electrochemical\nSwitching of Polymer Brushes\nGrafted onto Conducting Polymer Films

Abstract

We demonstrate the electrochemical switching of conformation\nof\nsurface-bound polymer brushes, by grafting environmentally sensitive\npolymer brushes from an electrochemically active conducting polymer\n(ECP). Using atom transfer radical polymerization (ATRP), we grafted\nzwitterionic betaine homopolymer and block copolymer brushes of poly­(3-(methacryloylamido)­propyl)-<i>N</i>,<i>N</i>′-dimethyl­(3-sulfopropyl)­ammonium\nhydroxide) (PMPDSAH) and poly­(methyl methacrylate)-<i>b</i>-PMPDSAH, from an initiator, surface-coupled to a poly­(pyrrole-<i>co</i>-pyrrolyl butyric acid) film. The changes in ionic solution\ncomposition in the surface layer, resulting from oxidation and reduction\nof the ECP, trigger a switch in conformation of the surface-bound\npolymer brushes, demonstrated here by electrochemical impedance spectroscopy\n(EIS) and in a change of wettability. The switch is dependent upon\ntemperature in a way that is analogous to the temperature-dependent\nsolubility and aggregation of similar betaine polymers in aqueous\nsolution but has a quite different dependence on salt concentration\nin solution. The switch is fully reversible and reproducible. We interpret\nthe switching behavior in terms of a transition to a “supercollapsed”\nstate on the surface that is controlled by ions that balance the charge\nstate of the ECP and are adsorbed to the opposite charges of the zwitterionic\ngraft, close to the graft–ECP interface. The behavior is significantly\nmodified by hydrophobic interactions of the block copolymer graft.\nWe speculate that the synergistic combination of properties embodied\nin these “smart” materials may find applications in\nelectrochemical control of surface wetting and in the interaction\nwith biomolecules and living cells.