Oxidation of Aniline: Polyaniline Granules, Nanotubes, and Oligoaniline Microspheres

Abstract

Aniline was oxidized with ammonium peroxydisulfate in solutions of strong acid (0.1 M sulfuric acid), weak acid (0.4 M acetic acid), or alkali (0.2 M ammonium hydroxide). The properties of the oxidation products and their morphology are controlled by the initial acidity of the medium and the acidity profile during the oxidation; the acidity increases because sulfuric acid is a byproduct. Conducting polyaniline nanogranules, nanotubes, or nonconducting oligoaniline microspheres were obtained, respectively. FTIR spectra suggest that the oligomers produced by the oxidation of neutral aniline molecules at the beginning of oxidation are similar, regardless of the acidity of the medium. Neutral aniline molecules, prevailing under alkaline conditions, are easily oxidized to aniline oligomers composed of ortho-and para-coupled aniline constitutional units. Orthocoupled units are further converted by oxidative intramolecular cyclization to phenazines. It is proposed that, in acidic media, N-phenylphenazine units constitute the initiation centers for the subsequent polymerization of aniline, which takes place at pH < 2 when the intermediate pernigraniline chains become protonated. Anilinium cations, which dominate in strongly acidic media, are difficult to oxidize to oligomers, but they easily participate in the formation of polymer chains once their growth has started. The self-organization of phenazine units is responsible for the generation of polyaniline nanotubes. Partial sulfonation of aromatic amines occurs at higher pH, especially in alkaline media. The sulfonated oligomers stabilize aniline emulsions and enable the formation of oligoaniline microspheres, when the miscibility of aniline with aqueous medium is limited. The final oxidation products obtained in alkaline conditions contain only low-molecular-weight oligomers; the polymeric component is the dominating product only in strongly acidic media. Both components are present in various proportions when the oxidation takes place at intermediate pH ranges.

Polymerization of aniline under various concentrations of APS and HCl

“…As the pH reaches around 3.0-3.5, a second reaction associated with polymerization is triggered. 11,14,18,20 As depicted in Figure 3, the starting and ending of polymerization are determined practically at the inflection point (as indicated by a dotted line) and at the turning point to a steady state (by a double arrow line) from each pH profile. Accordingly, the temperature profile increases at the start and reaches a local maximum value at the end of polymerization.…”

“…[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.…”

“…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. [5][6][7][10][11][12][13][14][15][16][17][18][19][20] For the third approach, by choosing [APS]/[AN]¼0.25-1. 25, effects of terminal pH on the final products were analyzed.…”

“…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 …”