Study on the Interconversion of Unit Structures in Polyaniline by X-ray Photoelectron Spectroscopy

XPS N_1S and valence spectra of various forms of polyaniline were studied and the compositions of four unit structures in some films were determined. A proposed scheme of interconversion of the unit structures by reduction/oxidation and acid/base treatment is useful to understand the interconversion mechanism of polyaniline. Reduced-base-treated polyaniline (1A, white) is taken as a standard material because it consists solely of imino-l,4-phenylene (IP) units (—NHC₆H₄—). Electrochemical oxidation of 1A in nonaqueous medium converts part (46%) of IP into its radical cation (IP^.+) (doped 1A). Acid treatment of 1A changes some IP (28%) to its cation (IP⁺) (IS). Base treatment of doped 1A converts two IP^.+-IP parts into two IP parts and one nitrilo-2,5-cyclohexadiene-l,4-diylidenenitrilo-l,4-phenylene (NP) (—N=C₆H₄= N—C₆H₄—) part to give a sample which is practically the same as base-treated polyaniline (2A). Washing of as-polymerized polyaniline changes some IP⁺to IP and some IP*⁺to NP (2S(H₂0)). An earlier conclusion that the radical cation of imino-l,4-phenylene plays an important role in electrical conduction in polyaniline has been supported, and it is pointed out that a highly conducting form of polyaniline exhibits a finite density of state at the Fermi energy. A large change in the valence structure is also observed at 8.7 eV in polyaniline containing IP*⁺. The advantage of an electrochemical redox process in nonaqueous media over that in aqueous media for practical purposes is explained.