Charge transport in electroactive polymer films

The electrochemical quartz crystal microbalance (EQCM) and ellipsometry have been used to study directly the movement of ions and solvent into/out of electroactive polymer films. The systems studied were polyvinylferrocene (PVF), polybithiophene (PBT) and polythionine (PTh). The overall mass changes accompanying oxidation/reduction indicate that film sources of counter-ions (required to maintain electroneutrality) can be significant. The extent of participation of these species depends on the nature and concentration of the bathing electrolyte solution. In the case of PVF, optical data also indicate a structural change: reduced PVF appears to be a homogeneous compact film, whilst oxidised PVF⁺ is a more diffuse, inhomogeneous film, whose polymer content decreases with distance from the electrode. Voltammetric experiments at rapid (and in some cases even moderate) scan rates show that transport of mobile species can be quite slow. It was generally observed that ingress into the polymer was slower than egress of the same species from the polymer. Charged species, notably proton in hydrated systems, move faster than neutral species, such as solvent, due to the influence of the field.