Leucoemeraldine form, polyaniline

Figure 1.1 Chemical structures for monomers of the conjugated polymers discussed in this chapter (a) vinylene (the repeat unit of polyacetylene) (b) ethynylene (polydiacetylene is alternating vinylene-ethynylene) (c) phenylene (d) thiophene (e) leucoemeraldine form of polyaniline (f) pernigraniline form ofpolyaniline (the number of protons can vary between the...

Polyaniline (PANI) is one of the most frequently investigated conducting polymers because of its ease of synthesis, its relative environmental stabUity and its abUity to exist in a range of intrinsic redox states. PANI exists in various oxidation states, ranging Irom a reduced leucoemeraldine form to the fully oxidized pemigraniline form. 

In contrast to ring-sulfonation of polyaniline, Bergeron et al. derivatized the leucoemeraldine form of poly aniline at the N-position with an alkylsulfonate group as shown in Figure 20.46 [38]. The sodium salt was achieved by reacting the leucoemeraldine base with NaH and 1,3-propanesultone. 

Polyaniline, one of the most studied conducting polymers (in general as well as for corrosion protection), is usually obtained by protonation of what is called the emeraldine base form, shown in Figure 2. The protonation reaction does not change the number of electrons in the polymer backbone. However, starting at the leucoemeraldine form of polyaniline, one would obtain the emeral e salt (conducting) form of polyaniline by an oxidation reaction. Protonic doping has also been observed in the case of alkoxy substituted poly(para-phenylene vinylene) (PPV) ... 

Polymerization mechanisms for polyaniline have been proposed in the literature [45, 46], Figure 2 illustrates some of the basic steps occurring during polymerization of aniline. The oxidation states of PANI, and of polyanilines in general, are indicated by an index for the degree of oxidation (Y). It is in its completely reduced form (leucoemeraldine) when Y = 1, and its completely oxidised form (pemigraniline) is dominant when Y = 0. At Y = 0.5, the 50% intrinsically oxidized polymer (emeraldine) is ambient [49, 50], The molecular structures of the different forms (oxidation states) of PANI are illustrated in Figure 1. 

Polyanilines, ([(-NH-C6H4-NH-C6H4)x-(N=C6H4=N-C6H4-)2-x] ), like so many other polymers, are stoichiometrically ill-defined. The three stable polyaniline forms are leucoemeraldine (x = 2), emeraldine (x = 1) and pemigraniline (x = 0). Emeraldine is the most important to the chemical and materials science communities because it is... 

We may conclude in this section that four optically different forms of polyaniline are produced depending on the applied potential. Only two forms of these, i.e., one absorbing at about 440 nm and the other at both 440 and about 800 nm, are conductive forms. We believe the former is a relatively localized form of radical cations, while the latter is a delocalized version of radical cations, i.e, polarons. The one absorbing at about 340 nm, which is fully reduced leucoemeraldine, and one at 610 nm, half of which is rigid quinoid form or pemigranaline and thus cannot be called a bipolaron, are not electrically conductive. These observations are consistent with results obtained from EPR and a.c. impedance measurements. Some disparity exists in the literature for the interpretation of the processes taking place during the first oxidation step. 

Leucoemeraldine base, the completely reduced form of polyaniline base, can be controllably oxidatively doped to the highly conducting regime/analogously to (CH)x,viz.,... 

It has recently been observed31 that the completely reduced base form of polyaniline, (leucoemeraldine), spontaneously reduces, and is itself oxidized, when its solution in NMP is mixed with an NMP solution of the completely oxidized base form of polyaniline, (pernigraniline). This reaction may be studied by "titrating" a solution of the leucoemeraldine with a solution of pernigraniline and by following the course of the reaction by the changes in the electronic spectrum of the reaction mixture. When equimolar solutions of the reactants are mixed this redox reaction is found to approach equilibrium after 20 hours at room temperature, to produce the emeraldine base, viz.,... 

The spectra of leuoemeraldine, (lA)n, and the emeraldine base, [(1 A)(2A)]n, have been extensively studied elsewhere.28 29 We include the spectra of these two important forms of polyaniline obtained from films spin-cast from sulfuric acid (see Figure 5) in order that they can be directly and quantitatively compared with those of the emeraldne salt and the bipolaron lattice. Leucoemeraldine is an insulator with a large band gap. The ti-tc transition shows an onset at approximately 3 eV, with a peak at 3.7-3.8 eV. The emeraldine base shows two principal absorption bands (see Figure 5), with maxima at 2 eV and at 3.9 eV, respectively. 

The available data provide the basis for an understanding of the electronic structure of the four principal forms of polyaniline the fully reduced leucoemeraldine, (lA)n the emeraldine base, [(lA)(2A)]n the oxidized and fully protonated emeraldine salt, [lS] (A )n and the fully oxidized bipolaron lattice, (-B-NH+=Q=NH+-)n. 

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