In this work, we have studied the structural anisotropy of polyaniline and polypyrrole films depending on the type of conductive carbon substrates, including highly oriented pyrolytic graphite (HOPG) and vitreous carbon being a fullerene-like material in its structure. The NEXAFS spectra were measured at both carbon and nitrogen K absorption edges. To analyze the chemical structure of the polymers, X-ray photoelectron spectra were additionally measured. The measurements were performed at the Russian-German beamline at BESSY II (HZB, Berlin).
It has been found that polyaniline films are predominantly disordered, regardless of the substrate material used. Most polypyrrole films have shown a pronounced dichroism of the NEXAFS spectra, indicating their structural anisotropy. The nature and intensity of the changes observed in the NEXAFS spectra when varying the measuring geometry turned out to be the same for the HOPG and vitreous carbon for most of thin films, which indicates a weak effect of the substrate structure on the molecular arrangement of polypyrrole films. The spectra measured at both edges have shown a significant decrease in π* resonances observed on going from grazing incidence to normal one. Based on the data obtained, the tilt angle has been estimated to be approximately 40 degrees for the pyrrole rings of the polymer.
It has been found that the substrate affects molecular arrangement only in thick long-term-deposited films. In contrast to HOPG, the films deposited on vitreous carbon have shown greater fraction of charge carriers, which are bipolarons and quaternary nitrogen derivatives, in the polymer structure. Such films have demonstrated a relatively weak linear dichroism of the spectra, which may indicate a disordering of the films due to the indicated change in the chemical structure of the polymer.
This work was supported by the Russian Foundation for Basic Research (No.16-43-180228) and bilateral Program "Russian-German Laboratory at BESSY II".