Role of Aniline Oligomeric Nanosheets in the Formation of Polyaniline Nanotubes

Abstract

We report the phenomenon of nanosheet rolling during typical aqueous polymerization and study its implications for the self-assembly of polyaniline nanotubes. Specifically, this is done through a detailed morphological and structural characterization of products obtained after 20 min, 1 h in falling pH experiments, and after 20 h at constant pH 2.5 during the oxidative polymerization of aniline with ammonium persulfate in the presence of alanine. The chemical structure has been investigated by FTIR, UV−vis, solid-state 13C and 15N NMR, liquid NMR, and XRD, whereas the morphology was imaged using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of self-assembled nanoflakes with different thicknesses ranging from tens to hundreds of nanometers is confirmed through SEM. TEM revealed the presence of very thin layers: nanosheets with sharp and well-defined edges. The presence of hydrogen bonds is confirmed by FTIR and is consistent with XRD results. The stacking of nanosheets and the formation of thicker flakes based on π−π electron interactions have been proposed on the basis of XRD experiments, where self-assembled layers made of cross-linked oxidized aniline structures stack on each other and are stabilized by hydrogen bonds and π−π interactions. In this way, hydrophobic cross-linked oligomers (formed at the beginning of the synthesis at higher pH) minimize their surface energy, self-assembling into well-ordered macromolecular structures. On the basis of the SEM and TEM images and the information obtained from other analytical techniques applied here, the presence of PANI nanotubes formed in the reaction carried out at constant pH of 2.5 is confirmed. The role of the nanosheets in the formation of nanotubes is proposed.