Exploration of the formation mechanisms of polyaniline nanotubes and nanofibers through a template-free method

This study reports the polymerization of aniline monomers in different HCl concentrations to investigate the formation mechanisms of one-dimensional polyaniline (PANI) nanostructures. Fourier transform infrared (FT-IR) spectra indicate that the products obtained in different acidic solutions have different molecular structures. In low-acidity conditions (HCl concentration ≤0.1 M), aniline monomers form phenazine-like aniline units in the initial reaction stage. As the reaction continues, a structure consisting of a head of phenazine-like aniline units and a tail of para-linked aniline units develops. By contrast, the reaction only produces para-linked aniline units as the concentration of HCl increases to 0.2 M. PANI products with different molecular structures exhibit different shapes, including nanotubes and nanofibers. For nanotubes, electron microscopy images reveal the flake-like intermediates formed in the initial reaction stage and then curl into nanotubes as the reaction proceeds. The phenazine-like aniline units serve as the axis for PANI nanotube curling. On the other hand, the para-linked aniline units act as a template for the formation of PANI nanofibers. This study demonstrates the formation mechanisms of PANI nanotubes and nanofibers. The acid concentration in the polymerization solution is the critical factor determining whether the aniline monomers form nanotubes or nanofibers.     

Introduction of methanol in the formation of polyaniline nanotubes in an acid-free aqueous solution through a self-curling process

This study reports the synthesis of polyaniline (PANI) nanotubes with the introduction of methanol in aqueous solutions. SEM images indicate that well-dispersed PANI nanotubes are produced when the concentration of methanol increases up to 1 M. On the other hand, polymers primarily form irregular agglomerates when only monomers and oxidant are used in the absence of methanol. Transmission electron microscopy (TEM) reveals that the resulting nanotubes have an outer diameter of about 200 nm and an inner diameter of 0-50 nm. Fourier transform infrared (FT-IR) spectra indicate that the intermediate samples obtained at a reaction time of around 60 min have a structure consisting of a head of phenazine-like units and a tail of para-linked aniline units. Ultraviolet-visible (UV-vis) spectra indicate that emeraldine salt polyaniline (ES-PANI) appears in sequence as the reaction time reaches 75 min. At the time interval of 60-75 min, the self-curling behavior of the PANI intermediates first appears at 60 min, and PANI nanotubes can be observed starting at 75 min. This is the first study to observe this self-curling process and propose that it explains the formation of PANI nanotubes. Noteworthily, the nanotubes transform into irregular agglomerates after a de-doping process.