Chemical and mechanical properties of redox polymer-modified electrodes. Part III. Redox thermodynamics on linear polyvinylferrocene electrodes

The redox thermodynamics of thin films (< 200 nm) of linear polyvinylferrocene (LPVF) deposited by spincoating on polished electrode substrates have been studied in tetra-n-butylammonium perchlorate + acetonitrile solutions. Three distinct regions of thermodynamic behavior were revealed through analysis of Nernst plots (E vs. log(c_Fc⁺/c_Fc⁰)) constructed from controlled potential coulometric data. For films which were 0-3% oxidized, super-Nernstian slopes with values dependent upon electrolyte concentration were attributed to Donnan equilibrium contributions to the electrode potential. This form of LPVF thus appears to function as a simple anion exchanger. Further oxidation (up to ca. 50%) is accompanied by a transition to a second thermodynamic region characterized by sub-Nernstian behavior which is interpreted in terms of an increasing association of ferrocenium/perchlorate ion pairs into dimers or higher order aggregates. Oxidation above 50% results in an abrupt return to super-Nernstian behavior, for which a mechanical/electrochemical model is proposed. In this third region, the LPVF film is envisioned to be a homogeneous network structure characterized by a high concentration of ionic crosslinks and a capability for sustaining electrochemically induced stress arising from the forced accommodation of neutralizing counterions. The transitions from one thermodynamic region to another are consistent with increasing film ion content as oxidation proceeds. The complexity in thermodynamic redox behavior of LPVF results from the substantial potential-dependent changes in film ion concentration (0 to 6 M) along with a relatively high degree of structural freedom due to the lack of covalent crosslinks.