Evolution of Polyaniline Nanotubes:  The Oxidation of Aniline in Water

Abstract

The course of aniline oxidation with ammonium peroxydisulfate in aqueous solutions has been investigated. The reaction was terminated at various times and the intermediates collected. Besides the precipitates, the films deposited in situ on silicon windows have also been studied. The kinetic course of polymerization is controlled by the acidity level, which changes during the polymerization from pH 8 to a final value close to pH 1. It has two distinct exothermic phases. Gel-permeation chromatography indicates that aniline oligomers are produced at first at high pH, while polyaniline follows after the pH becomes sufficiently low. The growth of polyaniline nanotubes was observed by optical microscopy and confirmed by electron microscopy. The molecular structure of the reaction intermediates was studied in detail by FTIR spectroscopy. Oxidation products are markedly sulfonated, and they contain phenazine units. Aniline oligomers are more soluble in chloroform than the polymer fraction, which contains nanotubes.

Polymerization of aniline under various concentrations of APS and HCl

“…3 From a technical point of view, the above-mentioned preparation for PANI is a simple process, but the formation mechanism involves an intricate interplay of consecutive chemical and physical reactions. [3][4][5][6][7][8][9][10][11][12] To account for the formation of PANI, the following basic mechanisms were reported. The redox reaction of neutral ANs with APS produces AN radicals.…”

“…4 Under a neutral or alkaline condition, the mixed ortho-para coupling of AN radicals together with oxidative intramolecular cyclization forms AN oligomers with phenazine-like structures at the initial stage (see Figures 2a and b for phenazine and phenazine-like trimer). [9][10][11][12][13] The oxidation of AN and AN oligomers with APS both release protons, and the pH of the solution falls. [12][13][14][15][16][17][18][19][20] In a strong acidic environment, the formation of a para-coupled structure is then favored.…”

“…[9][10][11][12][13] The oxidation of AN and AN oligomers with APS both release protons, and the pH of the solution falls. [12][13][14][15][16][17][18][19][20] In a strong acidic environment, the formation of a para-coupled structure is then favored. 4,9 The subsequent polymerization processes result in emeraldine salt (ES) PANI.…”

“…[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In general, there are three experimental approaches to study the effects from the acidic environment. [5][6][7][10][11][12][13][14][15][16][17][18][19][20][21] For the first approach, using automatic titration to control a constant pH, effects of pH on the morphological evolution of PANI nanostructures were discussed. 21 For the second approach, by adopting [APS]/[AN]¼1 or 1.25 in weak acidic or alkaline mediums (that is, starting with high pH), the properties of products obtained at various moments during the strong pH falling were investigated.…”

“…9-13 The oxidation of AN and AN oligomers with APS both release protons, and the pH of the solution falls. [12][13][14][15][16][17][18][19][20] In a strong acidic environment, the formation of a para-coupled structure …”