Emeraldine 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. 

For example, the investigations of the current-generating mechanism for the polyaniline (PANI) electrode have shown that at least within the main range of potential AEn the "capacitor" model of ion electrosorption/ desorption in well conducting emeraldine salt phase is more preferable. Nevertheless, the possibilities of redox processes at the limits and beyond this range of potentials AEn should be taken into account. At the same time, these processes can lead to the fast formation of thin insulation passive layers of new poorly conducting phases (leucoemeraldine salt, leucoemeraldine base, etc.) near the current collector (Figure 7). The formation of such phases even in small amounts rapidly inhibits and discontinues the electrochemical process. 

Zengin et al. [41] characterized a polyaniline (PANI)/MWNT composite. The FTIR spectra of the composite film show benzoid and quinoid ring vibrations at 1500cm-1 and 1600 cm-1, respectively, which indicate the presence of emeraldine salt (ES) of polyaniline. A weak broad band near 3400 cm-1 is assigned to the N—H stretching mode. The strong band at 1150cm-1 is characteristic of PANI conductivity. The FTIR spectrum of PANI/MWNT composite in the ES form exhibits several clear differences from the spectrum of neat ES PANI (1) the composite spectrum shows an inverse... 

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 ... 

Also the case of polyaniline is somewhat different from that of heterocyclic polymers. It has been proposed (MacDiarmid and Maxfield, 1987) that the doping process does not induce changes in the number of electrons associated with the polymer chain but that the high conductivity of the emeraldine salt polymers is related to a highly symmetrical 7r-delocalized structure. 

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). 

The high reactivity of the 6/Pd-catalyst was exploited in the preparation of high molecular weight polyaniline, Eq. (93) [99]. After thermolytic deprotection of the Boc protecting groups and air oxidation, emeraldine, the conductive form of polyanihne was obtained. 

For polypyrroles and polythiophenes, n is usually ca. 3 for optimal conductivity, ie. there is a positive charge on every third or fourth pyrrole or thiophene along the polymer chain, near which the dopant anion A is electrostatically attached. For polyanilines, the ratio of reduced (amine) and oxidised (imine) units in the polymer is given by the y/( 1 - y) ratio. The conducting emeraldine salt form of polyaniline has y = 0.5, i.e. there are equal numbers of imine and amine rings present. 

For organic solvent solubility, an alternative approach to solubilising polyanilines and polypyrroles, without sacrificing high electrical conductivity, is the use of surfactant-like dopant anions. With polypyrrole this has recently been achieved via oxidation of the pyrrole monomer with ammonium persulfate in the presence of dodecylbenzene sulfonate [128,129]. Similarly, the conducting emeraldine salt form of PAn.HA can be readily solubilised in a range of organic solvents via the use of camphorsulfonic acid or dodecylbenzenesulfonic acid as the dopant, HA [130,131]. 

Unsaturated heterocycles and aniline have been polymerized either chemically or electrochemically on electrode surfaces. The systems are attractive as they are often significantly more stable than PA under atmospheric conditions. The importance of quinoid vs. aromatic structure becomes apparent if the chemistry and conductivity of polyaniline fPANIl is examined. The initial emeraldine salt product of PANI is believed to have the following... 

A synthetic protocol for the formation of conducting filaments of polyaniline in the 3 nanometer wide channels of the aluminosilicate MCM-41 was developed (Figure 11).1°° Aniline vapor was allowed to diffuse into the dehydrated channels of the host at room temperature, followed by immersion into an aqueous solution of peroxydisulfate at 273 K. This reaction produced encapsulated polyaniline filaments. Spectroscopic evidence including UV-VIS and infrared data showed that the filaments are in the protonated emeraldine salt form (for example, Raman spectra exhibit modes indicative of the protonated quinone radical cation structure). A single, rather broad (8 G) electron spin resonance line (for an evacuated sample), at g = 2.0032 suggested slightly lower... 

The formation of -aminodiphenylamine is supposed to be the key intermediate in the formation of a dark green precipitate at the electrode surface during continued electrolysis of acidic aniline solutions. This has been characterized as an oligomer of aniline, for example, as the octamer emeraldine formed by a cascade of head-to-tail condensations [38,39]. Nelson, however, explained it as a mixture of mainly quinhydrone with a small amount of benzidine salt [37]. Today the electropolymerization of aniline under strongly acidic conditions is intensively studied as an important way to form the conducting polymer polyaniline [40] (see Chapters 31 and 32). 

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