Emeraldine state, polyaniline

FIGURE 3.30. Polyaniline in the emeraldine state (PANI-E) in equilibrium with an HCl solution. Partitioning the protons into the polymer (H as well as the protonation reaction (see Eqn. 35) are shown. The A< o is the Donnan potential. 

FIGURE 3.32. Conductivity of polyaniline in the emeraldine state (PANI-E) with and without poly(vinyl sulfonate) (PVS) incorporated into the polymer matrix, as a function of the composition of the electrolyte with which the polymer was equilibrated. 

Polyaniline in the emeraldine base state doped with di(butoxyethoxyethyl) ester of sulphosuccinic acid had high film conductivity and an elongation at break of 195%. This high flexibility is particularly needed for elastomer coatings to impart elasticity on conductive materials. 

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. 

The composition of polyaniline (emeraldine base) is shown in Fig. 12, and consists of alternating reduced and oxidized repeating units. Polyaniline can be switched back and forth from its insulating state to the conducting state by doping with HCl... 

Polyaniline (PANI) has complicated chemical structures that arc postulated [90,91]. To study such postulated structures Kaplan et al. [92] and Richter et al. [93] made solid state " C and NMR [92] and N NMR [93] and an emeraldine base (EB), a leucoemer-aldine base (LEB) and an emeraldine hydrochloride (ES), and revealed that EB is an alternating co-polymer... 

Of these, protoemeraldine and emeraldine are conducting forms. However, they are conducting only when they are protonated. This is called the protonic doping and Chiang and MacDiarmid [147] discovered for the protonic doping of these states that the pKa of polyaniline is linearly related to pH of the medium as... 

Whatever is the y value, the polyaniline is always an insulator its conducting properties only appear when the half-reduced-half-oxidized form (emeraldine) is treated with an acidic solution. The polymer chain is then protonated. The optical and magnetic properties of polyaniline also both depend on the oxidation and protonation states. ... 

In a first step, the electronic structure of the polyaniline base form has been followed versus the oxidation state. After that, we have proceeded further with the protonated emeraldine form. In all cases, the binding energy scale has been calibrated by setting the Cls core level main peak at 284.4 eV. 

Polyaniline was first prepared at the turn of the century. Several oxidation states are known. The conductivity and the color of the material vaiy progressively with oxidation. Only one form, however, known as the emeraldine salt, is truly conducting. The material can be prepared readily by either electrochemical or chemical oxidation of aniline in aqueous acid media. Common oxidants, such as ammonium peroxydisulfate, can be used. Flexible emeraldine films can be cast from solutions of A methylpyrrolidone and made conductive by protonic doping. This is done by dipping the films in acid or exposing them to acid vapors. The process results in protonation of the imine nitrogen atoms ... 

---