MOVING FRONT PHENOMENA IN THE SWITCHING OF CONDUCTIVE POLYMERS

A theoretical analysis of mass transport phenomena in conductive polym er-modified electrodes is presented. In the first part, the electronic transport properties of such polymers are described by two different diffusion coefficients: one relates to the electron mobility in the co nductive state, the other describes electronic transport in the non-co nductive state. Thus, this approach postulates a discontinuity for the electron diffusion coefficient with the local concentration of oxidiz ed states within the film. It is shown that this hypothesis leads to t he concept of a moving front which separates an area where the film is in its conductive state from one where it is in its insulating state. The same conclusions an drawn when D increases steeply with the conce ntration of oxidized sites, i.e. the doping level of the polymer. Thus , moving front phenomena appear to be intrinsically linked to the spec ificity of conductive polymers, i.e. the dramatic change in electronic conductivity upon switching. Assuming that the electrochemical proces s, for a chronoamperometric experiment, is controlled by electron diff usion leads to a front velocity proportional to t(-1/2) and to the dif fusion coefficient of the electron in the conductive zone. When this c oefficient tends towards infinity a contradiction to the assumption of a process controlled by electron movements occurs. In this case, the electrochemical process can be controlled either by counter-ion moveme nts or by the rate of the electrochemical reaction that takes place at the moving boundary; the velocity of the front is not proportional to t(-1/2) and the chronoamperometric response can deviate from the usua l Cottrell behaviour. In the second part of this work, the counter-ion movement is analysed. It is proposed that the conducting properties o f the material might lead to a marked enhancement of the migrational a spect of ion transport within the internal structure of the film. It i s then shown that describing ion transport as a migration phenomenon i nstead of a diffusion phenomenon leads again to the concept of a movin g front. Several propagation equations are demonstrated: the first des cribes the concentration profile of counter-ions and the second descri bes the potential profile within the film. These two equations indicat e that both concentration and electric potential propagate in the mate rial at the same velocity. This velocity is proportional to the drivin g electric field, i.e. the potential drop that develops at the conduct ive\insulating interface or within the insulating zone of the material and varies with anion mobility. (C) 1998 Elsevier Science S.A. All ri ghts reserved.