PERMEATION THROUGH POLYMER-COATINGS ON ELECTRODES - MEMBRANE VERSUS PINHOLE PERMEATION

Diffusion of substrates within polymer coatings on electrodes and crossing of the polymer-solution interface may occur through small channels and pinholes, respectively. It is shown that the dynamics of these phenomena are formally equivalent to those of membrane-type diffusion and penetration where the polymer is regarded as a homogeneous phase uniformly separated from the bathing solution. The relationships between the membrane parameters, on the one hand, and the pinhole and channel parameters, on the other, are established. Penetration into the polymer coating requires that the substrate diffuse from the solution towards the pinholes in a non-linear, constrained manner, roughly of the hemispherical type, in a thin layer adjacent to the film-solution interface. This process, albeit physically different, is kinetically equivalent to the crossing of an activation barrier at the film-solution interface in the framework of a membrane model. Although most of the analyses are performed under conditions corresponding to the plateau of the rotating disc electrode voltammograms, the competition of the various transport processes with charge transfer at the electrode surface is also discussed.