LIMITATIONS OF THE X-RAY PHOTOELECTRON-SPECTROSCOPY TECHNIQUE IN THE STUDY OF ELECTROACTIVE POLYMERS

Polyaniline (PAN) and polypyrrole (PPY) were analyzed by X-ray photoel ectron spectroscopy (XPS) and bulk analytical techniques. A systematic comparison of the results was carried out to determine the effects of anion size, synthesis methods, and the nature of the polymer (powder or film) on the variation between the surface and bulk compositions. G ood agreement between the XPS and bulk analytical results is generally obtained for chemically synthesized polyaniline (in powder form). How ever, these two methods may give substantial discrepancies in the anal ysis of anion concentration in as-cast polyaniline base films subjecte d to re-doping by acids. Such discrepancies are also found in the anal ysis of as-synthesized polypyrrole doped by large anions and polypyrro le base films and powders after re-doping by acids. In these cases the XPS technique tends to give anion concentrations that are significant ly higher than the bulk values. In some of these cases the high surfac e anion concentration supports a high proportion of positively charged nitrogens (>50% for polyaniline, >30% for polypyrrole), which must in volve the protonation of some amine units in addition to all of the im ine units. However, in actuality, the re-doping process can only reple nish the anions to less than half of the original bulk concentration i n polypyrrole.