THERMODYNAMICS AND KINETICS OF REDOX SWITCHING OF POLYVINYLFERROCENE FILMS IN PERCHLORATE SOLUTIONS

Cyclic voltammetry was used to create nonequilibrium populations of di fferent solvation and configurational states of partially oxidized pol yvinylferrocene (PVF). Oxidation levels were established by scanning e ither from a fully reduced state to the desired oxidation level or fro m a fully oxidized state to the desired level. Coulostatic conditions were then established by opening the external circuit, and subsequent mass and potential changes were followed. The film's approach to equil ibrium involves configurational changes within the polymer and simulta neous and subsequent solvent transfer. At very short times (t less tha n or equal to 0.2 s) the approach to equilibrium is limited by both so lvation and reconfiguration processes. For a short time afterward (0.2 < t/s < 1.0) reconfiguration alone is rate Limiting. At intermediate times (1 < t/s < 5) both processes play comparable roles. At long time s (t > 5 s) solvation is the controlling step. The electroactive polym er film does not completely reach equilibrium even after 1 h at open c ircuit as evidenced by continuous small mass changes. These mass chang es are the result of water transfer between the polymer film and the b athing electrolyte. A scheme of cubes model rationalizes mechanistic p athways leading to equilibrium. In particular, the observed extrema in mass (solvent population) are predicted. The electrode potential, aft er 1 h at open circuit, shows nearly Nernstian dependence on the redox composition for film states produced by either anodic or cathodic cyc lic voltammetric scans. These Nernst plots are displaced by only a few millivolts because only a weak Nernstian dependence on film water con tent exists. Films that are 50% oxidized exhibit a sub-Nernstian respo nse with respect to the perchlorate concentration in the bathing solut ion under permselective conditions.