COEXISTENCE OF EXTERNAL PROTONATION AND SELF-DOPING IN POLYANILINE

Polyaniline which possesses two simultaneous modes of doping, external doping by ClO4- anions and self-doping by -SO3- groups, has been synthesized by three different methods. In each of the methods, the self-doping is accomplished by the copolymerization of metanilic acid and aniline using either (NH4)2S2O8 or Fe(ClO4)3.9H2O as oxidant. In method I external protonation by HClO4 occurs during the polymerization process while in methods II and III treatment by HClO4 is carried out after the polymerization step. The effects of oxidant/monomer ratio and metanilic acid/aniline ratio on the physicochemical properties of the copolymers are investigated and comparisons are made with the self-doped polymer obtained from the sulfonation of polyaniline. Experimental techniques used for characterization of the polymers include X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) absorption spectroscopy, ultraviolet-visible (UV-Vis) absorption spectroscopy, elemental analysis, thermogravimetry and four-probe measurements. Under the conditions investigated, self-doping can constitute up to about half of the doping of a 50% protonated polymer. The external dopant, ClO4- anions, are thermally less stable than the -SO3- groups of the incorporated metanilic acid units. The latter as well as the -SO3-groups covalently bonded to emeraldine base through sulfonation undergo thermal decomposition at a lower temperature than metanilic acid itself.