Emeraldine base conductivity

The common form of polyaniline is a 1 1 combination of alternating reduced (A) and oxidized (C) units it is termed emeraldine (or emeraldine base). The emeraldine base is essentially non-conductive, but its conductivity increases by 9-10 orders of magnitude by treating with aqueous protonic acids. The conductive form of poly aniline can therefore be roughly depicted as a 1 1 combination of alternating A and D units. 

The emeraldine base form of polyaniline may also react in non-aqueous electrolytes, such as a LiClOl -propylene carbonate solution, with the formation of the conductive emeraldine hydroperchlorate salt ... 

PANI is unique in that its most oxidized state, the pernigraniline form (which can be accessed reversibly), is not conducting. In fact, it is the intermediately oxidized emeraldine base that exhibits the highest electrical conductivity. Protonic Acid Doping is the most general means by which to obtain this partially pro-tonated form of PANI [301]. Exposure of the emeraldine salt to alkali solutions reverses this process and brings a return to the insulating state. 

Figure 4.6 shows the schematic diagrams of ciclodextrins, polyaniline with emeraldine base, and inclusion complex formation of cyclodextrins and a conducting polymer chain insulated molecular wire. 

FIG. 11.18 Conductivity of emeraldine base as a function of pH of the HCl dopant solution as it undergoes protonic acid doping ( ) and ( ) represent two independent series of experiments. From MacDiarmid, 2001. Courtesy John Wiley and Sons, Inc. 

Protonation by acid-base chemistry leads to an internal redox reaction (Fig. 11.19), without change of the number of electrons (Heeger, 2001 MacDiarmic, 2001). The semiconductor (emeraldine base, emeraldine salt, 100 S/cm). Complete protonation of the imine nitrogen atoms in emeraldine base by aqueous HC1 results in the formation of a delocalised polysemiquinone radical cation. This is accompanied by an increase in conductivity of more than 12 orders of magnitude. 

Polyaniline is structurally much more complicated than PA, even if we restrict our attention to the emeraldine base (EB) and salt (ES) forms. There are two classes of base forms, to which correspond two classes of salt forms ESI and II [28], and the EB - ES interconversion does not mix the classes. This interconversion corresponds to addition or removal of a proton onto the N atom in the chain without changing the total number of electrons this causes a conductivity change by more than 10 orders of magnitude, from 10-10 S/cm to > 1 S/cm [52]. 

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. 

TABLE 1. Effect of selected dopants on the film conductivity and elongation at break for polyaniline (emeraldine base). 

FTIR spectroscopy has been used to monitor the conducting states of a conducting polymer as well as to know if a doping experiment is successful [86, 87], The FTIR and UV-Vis spectra of unsubstituted PANI is similar to that of substituted PANI though with slight band shifts. Doped PANI and its derivatives exist in the emeraldine salt forms which are essentially delocalized polysemiquinone radical cations whose stability is maintained by the presence of dopant anions. The degree of electron delocalization in the polysemiquinone forms of the doped PANI manifests itself in the form of an electronic-like band at ca. 1100 cm 1 associated with polarons [86], The structures of emeraldine base and emeraldine salt form of PANI are presented in Figure 6. 

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... 

Fig. 12 Switching between emeraldine base (insulating form) and emeraldine salt (conducting form) by HCl and ammonia, respectively...

Polyaniline is a mixture of benzenoid and quinoid (p-phenyleneamineimine) entities. The neutral compound, emeraldine base is an equal mixture of amine and imine functionalities. Treatment with a non-oxidising acid results in protonation of the imine groups to give emeraldine salt. This causes positive charge to be transferred to the backbone and is the origin of the conductivity, which increases by 10 orders of magnitude from the base to the salt form. The polymer in the base and salt forms can be described as ... 

FIGURE 5.1 The doping of emeraldine base with protons to form the conducting emeral-dine salt form of polyaniline (a polaron lattice). 

In PANI protonated with HCl (protonation is a kind of doping an imine of the polymer backbone makes a bond with a proton, resulting in a free charge on the backbone locally neutralized by a chlorine ion near the chain, keeping the number of electrons on the backbone constant), moiphology and crystalline form were found to affect the evolution of susceptibility upon protonation [261]. In a powder form of PANI, called EB-1 (emeraldine base) for the insulating base form and ES-1 (eineraldine salt) for the conducting salt form, both the Curie and Pauli susceptibilities are found to increase... 

Figure 6.47, The chemical stnictures for (a) poly-acetylenc and polyanilinc in (b) the insulating emeraldine base (EB) form (c) the conducting emeraldine salt (ES) form (d) polythiophene (e) poly(3-mcthylthiophene) and (f) polypyrrole.

In this model, the polyaniline chain is assimilated to a series network of parallel resistances and capacitances (Figure 8.21). The resistances./ (/) and the capacitances C(0 will correspond to the emeraldine salt part (the conductive part) and will depend on the emeraldine salt segments length /. The resistance / (/ ) and the capacitances C (/ ) correspond to the emeraldine base part (the insulating part) and depend on the emeraldine base segments length /. 

Emeraldine, based on treatment with 1,0 M HF, HCl, HBr or HI, produced highly conductive polymeric salts and the HBr salt was found to be the most stable material against thermal degradation [170]. 

Conductive polyaniline is observed to be quite stable material and its thermal degradation usually involves a three-step process i.e. removal of moisture, then of HCl and finally the breakdown of the polymer backbone. An emeraldine base treated with HBr is reported to be the most stable material against thermal degradation and methyl substitution enhances the thermal stability of polyaniline in inert environments as well as in air. The thermo-oxidative stability of N-substituted polymers is better than the ring-substituted polymers. 

PANI is more commonly prepared by polymerization of aniline usiag (NH 2S2 8 HCl (112,127). As prepared, it has stmcture (16) known as emeraldine hydrochloride. In this form, PANI is highly conductive but completely insoluble. When emeraldine hydrochloride is deprotonated with NH OH, the highly soluble emeraldine base (17) is produced. It is processible from oiganic solvents such as aqueous acetic acid or DMSO. It must then be treated with HCl to regenerate the again insoluble conducting form of the polymer. 

By the mid-80s it was clear to most researchers that success on the conductivity side had taken its toll on polymer processability. Attention turned back to restoring the solubility and mechanical properties of the polymer. Polyaniline received the most attention initially. The nonconductive emeraldine base form is soluble in A-methylpyrrolidone [28] and films can be cast. Subsequent doping with a protonic acid from aqueous solution, or in situ with a photo-acid generator [45], is necessary to achieve conductivity. Polyaniline is also soluble in sulfuric acid, not the most convenient of solvents. Nevertheless it proved possible to spin fibers [46], cast films and extmde sheets of conductive polyaniline sulfate, but the laboratory experiments did not make the transition into large-scale manufacmring. 

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