Linear dichroism of X-ray absorption spectra and molecular arrangement in electrodeposited polyaniline and polypyrrole films
- Dr. Alexander SYUGAEV
- Dr. Alexander SYUGAEV (Udmurt Federal Research Center, Ural Branch of Russian Academy of Sciences 132 Kirov Street, 426000 Izhevsk, Russia)
- Dr. Alena MARARTKANOVA (Udmurt Federal Research Center, Ural Branch of Russian Academy of Sciences 132 Kirov Street, 426000 Izhevsk, Russia)
Conductive polymers are used in practice as materials for electrochemical devices (batteries, fuel cells, supercapacitors), anticorrosive protective coatings and electromagnetic radiation protective layers. The properties of such materials depend on not only chemical structure of the polymer, but also on the spatial arrangement of the chains being charge conductors. But the spatial arrangement of the chains in functional films of conductive polymers and its effect on the physicochemical properties of polymers is far from being comprehensively studied and understood. Diffraction methods turn out to be inapplicable to analysis of such materials because of their relatively weak crystallinity. In this work, molecular arrangement in electrodeposited polyaniline and polypyrrole films was studied with use of linear dichroism of x-ray absorption spectra (NEXAFS) measured at both nitrogen and carbon K absorption edges. The spectra were measured using synchrotron radiation at the Russian-German beamline at the BESSY II (HZB, Berlin). The pronounced dichroism of the x-ray absorption spectra was observed for polypyrrole films. When going from the normal to the grazing angle of the exciting radiation incidence, the π resonances grow, with σ resonance intensity simultaneously decreasing. This indicates preferential arrangement of most pyrrole rings in-plane the substrate. Dichroism is stronger for initial stages of deposition characterized by 2D growth of films, as compared with later deposition stages, at which the films have "cauliflower"-like morphology. In the case of polyaniline, the grown films have different morphology depending on the electrodeposition conditions and seen as either smooth homogeneous films (in stirring electrolyte) or rough non-homogeneous films containing polymer particles deposited on the surface (quiescent electrolyte). The pronounced dichroism was observed only in the second case. The observed dichroism was stronger for the films containing more polymer particles deposited on the film surface. For such samples, the spectra at both absorption edges exhibited weaker π* resonances, indicating predominantly up-right arrangement of the benzene rings in the particles. Changes observed in other resonances allow suggestion about in-plane orientation of the polymer chains themselves in the particles relative the substrate surface. This work was supported by Russian Foundation for Basic Research (No.16-43-180228), bilateral Program "Russian-German Laboratory at BESSY II" and partially by FASO of Russia within the state assignment No. АААА-А17-117022250038-7.