Temperature-Dependent Transport Properties of Poly[2-(methacryloyloxy)ethyl]trimethylammonium Chloride Brushes Resulting from Ion Specific Effects

Combined use of electrochemical techniques (electrochemical impedance spectroscopy and cyclic voltammetry) and quartz crystal microbalance with dissipation allowed to resolve separately the thermal effects on diffusion and electron-transfer steps of the electrochemical reaction of the [Fe(CN)6]3–/4– redox couple at a Au electrode modified with poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PMETAC) brushes. Arrhenius-type dependences of the kinetic constant and the diffusion coefficient with temperature were observed in different electrolytes. Ion-paired collapsed polyelectrolyte brushes in NaClO4 result in compact stiff structures with less amount of entrapped water and markedly different from the same brushes with a collapse driven by pure Coulombic screening in NaCl. A remarkable difference related to the type of counterion is the occurrence of a thermal transition for the polyelectrolyte brush in the presence of ClO4– ions at near-ambient temperature ( 17 °C). Activation energies for electron-transfer and diffusion processes become twice as large as those for temperatures above the thermal transition. These electrochemical studies demonstrate not only the critical role of ion-pairing interactions in determining the physicochemical properties of the macromolecular system but also provide experimental evidence of counterion-induced thermocontrolled transport functionality in the polyelectrolyte brush layer.