Electrochemical quartz crystal microbalance studies of electroactive polymer films in gravimetric and viscoelastic regimes

Under rigid film conditions the Electrochemical Quartz Crystal Microbalance (EQCM) acts as a gravimetric probe for mass changes occurring during electroactive film redox switching. The sensitivity of the technique allows one to monitor redox driven population changes of both ion and solvent between an electroactive film and its bathing solution. The interpretation of EQCM data to date has mainly been of a qualitative nature. The approach taken here is a quantitative one and attempts to provide kinetic information (rate constants) for specific electro(chemical) steps.;The model to be considered is an electroactive redox polymer under permselective conditions. A new 3D vector (U) representation is developed to describe the redox and compositional state of a system in terms of injected electronic charge (Q), potential (E) and a mobile species population parameter (E), which can be calculated from film mass changes (AM). Poly(vinylferrocene) is then used as a model system to validate the methodology. The kinetics of redox switching of poly(vinylferrocene) are analysed using cyclic voltammetry and quantitative data is extracted from the relative fluxes of water and counter ion during the redox cycle. Solvent populations are extracted from the data and a new theoretical model shows solvent activity parameter effects (polymer/solvent interactions) are evident during redox switching.;Crystal impedance spectra provide a diagnostic for rigid vs. viscoelastic film behaviour. The evolution from gravimetric to viscoelastic responses of poly(3-methylthiophene)-loaded thickness shear mode resonators was investigated. The relationship between ion and solvent populations (composition) and shear moduli (dynamics) was explored. Extraction of viscoelastic film characteristics, i.e. shear modulus, film thickness and film density is achieved by equivalent circuit modelling. The problem of the uniqueness of fit is tackled by a new approach in which impedance and coulometric data from the acoustically thin regime define a solvent swelling factor. Extrapolation into the acoustically thick regime using the solvent swelling factor defines the film thickness and film density. The method is validated using crystal impedance data for poly(3-methylthiophene) films exposed to propylene carbonate.