Spectroscopic evidence for intermediate species formed during aniline polymerization and polyaniline degradation

Abstract

Spectroscopic methods are used to investigate the formation of low molecular mass intermediates during aniline (ANI) oxidation and polyaniline (PANI) degradation. Studying ANI anodic oxidation by in situ Fourier transform infrared spectroscopy (FTIRS) it is possible to obtain, for the first time, spectroscopic evidence for ANI dimers produced by head-to-tail (4-aminodiphenylamine, 4ADA) and tail-to-tail (benzidine, BZ) coupling of ANI cation radicals. The 4ADA dimer is adsorbed on the electrode surface during polymerization, as proved by cyclic voltammetry of thin PANI films and its infrared spectrum. This method also allows, with the help of computational simulations, to assign characteristic vibration frequencies for the different oxidation states of PANI. The presence of 4ADA retained inside thin polymer layers is established too. On the other hand, FTIRS demonstrates that the electrochemically promoted degradation of PANI renders p-benzoquinone as its main product. This compound, retained inside the film, is apparent in the cyclic voltammogram in the same potential region previously observed for 4ADA dimer. Therefore, applying in situ FTIRS is possible to distinguish between different chemical species (4ADA or p-benzoquinone) which give rise to voltammetric peaks in the same potential region. Indophenol and CO(2) are also detected by FTIRS during ANI oxidation and polymer degradation. The formation of CO(2) during degradation is confirmed by differential electrochemical mass spectroscopy. To the best of our knowledge, this is the first evidence of the oxidation of a conducting polymer to CO(2) by electrochemical means. The relevance of the production of different intermediate species towards PANI fabrication and applications is discussed.

Mesoporous Polyaniline Films for High Performance Supercapacitors

“…30 Also, set of small multiple peaks appear between 0.4 and 0.7 V that have been indirectly assigned in the literature to the oxidation/reduction of dimers of 4-aminodiphenylamine (4ADA) and benzidine (BZ), respectively. [31][32][33] The mechanism was confirmed by recording the CV of the mixture of aniline with the pure substances of 4ADA and BZ. 31 Also, Barbero and colleagues 31 have used in situ Fourier transform infrared spectroscopy (FTIRS) while scanning the electrode potential in aniline solution to prove that dimers produced by head to tail (4ADA) and tail to tail (BZ) coupling of cation radicals.…”

“…[31][32][33] The mechanism was confirmed by recording the CV of the mixture of aniline with the pure substances of 4ADA and BZ. 31 Also, Barbero and colleagues 31 have used in situ Fourier transform infrared spectroscopy (FTIRS) while scanning the electrode potential in aniline solution to prove that dimers produced by head to tail (4ADA) and tail to tail (BZ) coupling of cation radicals. On the other hand, when a thick PANI film is produced at high potential (>1 V versus SCE), the film could degrade by oxidizing the pernigraniline salt to produce benzoquinone.…”

Polyaniline nanostructure electrode: morphological control by a hybrid template

“…The voltammogram in black (AAO template) does not show any visible oxidation process below 0.9 V, and beyond this value, an irreversible intense redox process appears which is assigned to the formation of the radical-cation anilinium (C 6 H 5 NH *+ ) which can be formed from the oxidation of the aniline monomers at the vicinity of the electrode surface. This anilinium radical is stabilized by resonance [22] and a rapid coupling process of radicals takes place which leads to the formation of the dimer p-aminodiphenylamine (ADPA), and thereafter, the polymer growth process continues to progress [21][22][23]. In contrast, the profile of the first electropolymerizing cycle of aniline in the hybrid template shows a very different shape.…”

“…It is observed that using the hybrid template for electropolymerization of aniline enhances this feature. On the other hand, the process involved in the middle peak II has been generally attributed to a side reaction (crosslinking) or degradation reactions [23,24]. The crosslinking process, involving a previous coupling reaction between ADPA-adsorbed dimer units and a monomer radical cation that takes place at early stages during the PANI growth, is associated with thin films of PANI [23].…”

Fabrication and surface characterization of poly(4-aminodiphenylamine) film modified electrode and its application for lead and cadmium determination

“…The major intermediate product was characterized as the dimer-aniline like constituent called 4-aminodiphenylamine (4-adpa) [17]. Recently, Planes et al [18] have identified 4-adpa as the Ani-dimer intermediate produced via head-to-tail coupling of Ani cation radicals using a spectroscopic method. The conductivity and molecular mass of poly(4-aminodiphenylamine) prepared in aqueous HCl solution [19] were found to be significantly less than those of Pani due to the formation of oligomeric products composed of four monomeric units.…”