Spectro-Electro-Gravimetry of the Compactation Effect of Poly-(ortho-toluidine)
Authors: Ferrer Amparo , Agrisuelas Jeronimo , Frau Antonello F , García Jose , Vicente Francisco 
Published: February 2020

An example of an electroactive polymer is poly(ortho-toluidine) (POT), whose monomer has been widely used as aniline derivative for dyestuff, pesticides, pharmaceutical drugs and organic intermediates. POT’s properties can be looked into by spectroelectrochemical means, in that they are directly related to the variations of colour associated to the oxidation states of its active sites [1-4]. POT displays three main oxidation forms: fully reduced state (leucoemeraldine, LE), partially oxidized (emeraldine, EM), and fully oxidised (pernigraniline, PN). Among these, only the EM form is conductive.

The 100 CV cycles obtained during electropolymerisation of o-toluidine are shown in Figure 1. At about 0.85 V, the sharp peaks observed correspond to the oxidation of o-toluidine and the fact that their maximum current density increases as cycling increases suggests that the polymer is forming.

In this work, the behavior of POT during compaction and subsequent swelling has been reported on. Basically, electropolymerisation of a monomer, ortho-toluidine, was executed on a flat electrode. Then, Electrochemical Quartz Crystal Microbalance (EQCM) coupled with in situ Vis-NIR spectroscopy were employed to characterize the as-formed polymer.

The compaction-decompaction process of poly(o-toluidine) (POT) films synthesized from H2SO4 solution by cyclic voltammetry has been looked into. We present a detailed electrochemical mechanism for the insulating-to-conductor change of POT films. Oxidation of POT films involves their transformation into the conducting (i.e. emeraldine) form, with simultaneous anion insertion into the polymeric matrix. Coupling of electrochemical quartz crystal microbalance (EQCM) with vis-NIR spectroscopy provided singular information about the global response of current during the doping processes. Isolated polarons (P*) could form because of trapped anions when the film is reducing and shrinking (compaction). During the decompaction (i.e. relaxation), protons are expulsed when the isolated polarons detected at 420 nm are oxidized. Anions insert during the formation of conducting polarons (PC), which consist of coil structures due to the strong interaction of anions with the C-NH+ -C group of polymer detected at 840 nm. This mechanism can be used to understand the relaxation process in other polyaniline-derived films.

During the LE → EM change, POT can show two different intermediate states, known as polaron and bipolaron forms, which are considered as well-defined species with their own characteristics.

The transition mechanism between such intermediate states of POT, with corresponding intake or loss of ions A- from the medium. It can be observed that POT presents two successive redox processes, the first of which implies an anion transfer, whilst the second one is brought about by proton transfer [5].

the wavelength centered at 840 nm is suitable for studying the so-called isolated polaron. This species differs from others in that, during polymer oxidation, it forms somewhere in the polymer chain along with a trapped anion. As this polaron-anion pair does not conduct, its presence diminishes the conductivity of the polymer and, actually, if it gets to interact with other charges, therefore, prevents electron flow across polymer chains.

The electrochemical behavior of POT is reproducible and the polymer can be stabilized through a compaction process during a given time. As demonstrated before, the electrochemical activity depends upon the compaction time. From analysis of the F(dm/dQ) function, it can be inferred that POT can be reduced and shrink with either concomitant expulsion of anions or intake of H3O+ cations, or both processes simultaneously. When POT underwent relaxation, the values of current, peak potential charge, as well as the variations of mass and absorbance (at 840 nm) with respect to time showed linear response.

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Part of this work was supported by MINECO-FEDER CTQ2015-71794-R.