Ellipsometric study of the growth of thin organic polymer films

Abstract

AbstractEllipsometry has been used to study the contrasting electrochemical growth mechanisms and properties of two organic redox polymer films on Pt. The first part of the work is a study of the anodic polymerisation of thionine from aqueous solution. Initially, a monolayer of PtO is formed on the electrode. Thereafter uniform growth of polythionine proceeds under diffusional control, as evidenced by the square root dependence of film thickness on time. Quantitation of this relationship shows that the film contains ∼50% (by volume) organic polymer, ∼25% counterions and ∼25% H2O. Reduction requires additional counterion ingress, but the absence of significant expansion implies some solvent loss. Optical properties of the film are rather similar to those of the monomeric redox system and show a Nernstian potential dependence, in agreement with coulometric data. The second part of the work is an analogous study on the polymerisation of thiophene from CH3CN solutions. This process is kinetically controlled. At short times there is evidence for nucleation. At longer times, nuclei overlap and uniform growth is indicated by a linear increase of film thickness with time. The evolution of polymer optical properties during this process was monitored. Optical properties of both redox states were determined at several wavelengths and contrast markedly with parameters for the monomer. Switching between redox states is rapid and involves swelling via solvent ingress on reduction. Ellipsometric data acquired during voltammetric scans reveal, however, that the sequence of events in this process is complex.