Solid-state NMR of polyaniline nanofibers

Abstract

The structure of nanofibers of polyaniline (PANI) formed by oxidation of aniline with ammonium persulfate in the presence of HCl have been determined by solid-state 13C and 15N NMR experiments. The nanofibers synthesized in this way by rapid mixing and in the presence of strong acid resemble standard PANI in structure. Due to rapid mixing the reactants are consumed very quickly at the very beginning of the reaction, preventing secondary growth. Solid-state 15N and 13C CP MAS NMR results suggest that the emeraldine base form of the nanofibers exists mainly as an alternating copolymer of reduced and oxidized repeat units. A broad shoulder between 80 and 150 ppm in the 15N spectra is attributed to the presence of positively charged radical centers distributed along the polymer backbone, due to protonation of the imine nitrogens. Removal of the shoulder by dedoping with LiOH confirms that it is due to the positively charged imine nitrogens. Non-quaternary suppression 15N NMR experiment indicates the cross-linking and the presence of tertiary nitrogens in the nanofibers. The imine to amine ratio obtained from a variable contact time experiment is 0.8.