INSITU SPECTROSCOPIC INVESTIGATIONS OF THE GROWTH, ELECTROCHEMICAL CYCLING AND OVEROXIDATION OF POLYPYRROLE IN AQUEOUS-SOLUTION

In this paper, we present results from in situ ellipsometry and Fourier Transform Infra Red spectroscopy on the growth, electrochemical cycling and overoxidation of polypyrrole in 1 M NaClO4. The data show that the initial charge-carrying species are polarons. At higher potentials, bipolarons are also formed, and co-exist with the polarons; strongly suggesting that there is no appreciable energy gain in forming a bipolaron (with respect to two polarons). Both species have narrow, well-defined conjugation lengths, with bipolarons being ca 9 monomer units smaller than polarons. The oxidation of the polypyrrole results in the expulsion of solvated protons from the film, via the deprotonation of water; in addition, oxidation of the film is accompanied by a reversible 30% reduction in its thickness, a quite unexpected result in a view of the necessity for charge compensation through ingress of solvated anions. Ellipsometry shows that the major changes in L occur between -0.2 V and +0.2 V, ie at potentials associated with strong growth in bipolaron rather than polaron concentration. A model is postulated in which the major cause of the contraction is electrostriction, assocaited with the formation of bipolarons. This is, in turn, initiated by the generation of a critical concentration of polarons. The kinetically fast proton expulsion that takes place in the early part of the oxidation process is superseded at higher potentials by the thermodynamically-favoured influx of anions. Overoxidation of polypyrrole in aqueous solution results in the formation of pyrrolinones, with a range of short conjugation lengths, and CO2 (from the complete oxidation of end-groups); as well as an influx of solvent into the film, which expands. Eventually, the pyrrolinone segments are also oxidised to CO2, but the mechanical integrity of the polymer is maintained.