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.