Emeraldines

Figure 5-19. N(ls) XPS core level spectra of emeraldine base adsorbed on ITO. The top most spectrum corresponds to ultra-thin Him (in the mono layer regime) while the bottom spectrum corresponds to thick film.

The experimental UPS spectra of the emeraldine base form of polyaniline is compared with VEH-derived DOVS in Figure 5-18 97. The DOVS were derived from the VEH band structure calculations shown at the bottom of Figure 5-18. 

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. 

Assuming that the polymerisation does occur via a radical intermediate, then coupling is possible at all three positions on the ring (albeit with differing probabilities) and a number of alternative products to the emeraldine-based structure can be... 

In this paper we would like to propose also an additional concept of utilization of rechargeable metal-air battery with PANI/ TEG catalysts. It is necessary to note the ability of PANI based electrode to self-charge in the presence of oxygen. A deep discharge of PANI electrode leads to the transformation of emeraldine (EM) to lecoemeraldine (LEM). It is well known that LEM is chemically non-stable at the presence of oxygen due to occurance of the following reaction ... 

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... 

Phenylquinonediimine is at once further polymerised to the so-called emeraldine (in conformity with stage II on p. 322). 

According to a general rule, not only dihydric phenols, but also those diamines of the p-series which still contain one hydrogen atom attached to each nitrogen, are dehydrogenated to quinone or quinonediimine with great ease. Hence in the oxidation solution emeraldine is also immediately converted into the doubly quinonoid chain... 

PANI is usually produced by the anodic oxidation of aniline in acidic aqueous solution [5, 139], but can also be produced by chemical oxidation [138b, 140]. Hence, it is not surprising that the oxidation of PANI is pH-dependent and that, therefore, in addition to electron-transfer processes, proton-transfer reactions occur during charging. Although it is usually assumed that PANI has a chain structure (emeraldine) with head-tail connections... 

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. 

Confinement of ion-radicals considerably changes their reactivity. What is more important for practical applications is that the confinement increases the ion-radical stability. For instance, the cation-radicals of polyanilines (emeraldines) sharply enhance their thermodynamic and kinetic stabilities when they are formed encapsulated in cucurbituril (Eelkema et al. 2007). Emeraldines have electric condnctivity as high as 1 X 10 cm (Lee et al. 2006). Encapsulation of emer-... 

When 0 < y < 1 these structures are the poly(p-phenyleneamineimines), in which the oxidation state of the polymer increases with increasing content of the imine form. The fully reduced form (y = 1) is leucoemeraldine , the fully oxidized form (y = 0) is pernigraniline , and the 50% oxidized structure (y = 0.5) is emeraldine . Each structure can exist as die base or as its salt, formed by protonation, so that we can envisage four repeat units in the polymer chain, in amounts which depend on the extent of both oxidation and protonation of the structure (Fig. 5). 

Wnek 180> proposed that the structure of the oxidized insulating form of conventionally formed polyaniline is approximately a 50 % copolymer of diamine and diimine units, corresponding to the emeraldine structure and Hjertberg et al.180 obtained CPMAS NMR evidence for this conclusion. Some confirmation of the structure has also been obtained by chemical synthesis of the polymer182). However, Kitani et al.183) have suggested that the normal electrochemical synthesis leads to partially cross-linked polymers. 

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. 

Results and Discussion. The 2-ethyl polyaniline concentration in the silica gel film was determined by constructing a Beer s law calibration curve from solutions of known concentration. Assuming an average molecular weight of 5000, the 2-Et PANi concentration in the silica gel was found to be 9.6 x 10 4 M. The refractive indices of CS2 and 2-Et PANi SiC>2 were estimated to be 1.6 and 1.4 at 1.06 im, respectively. The emeraldine base doped silica gel was found to have low losses due to scatter, and exhibited good transparency at 1.06 im. Spectrophotometric measurements at 1.06 fim yielded absorption coefficients of 0.1 cm-1 (> 99% T over 1 mm pathlength) for the CS2 reference and 4 cm 1 (96% T over 1 mm pathlength) for the 2-Et PANi doped silica film. 

Since the acid-base (precipitation) reaction takes place in non-aque-ous solution (isopropanol), a glass pH electrode could not be used to follow the titration. However, PANI is known to be pH sensitive as a result of the acid-base equilibrium between the emeraldine base (EB) and emeraldine salt (ES) forms of PANI [1-3]. Interestingly, the GC/ PANI electrode was found to give a reproducible response during the titrations despite the presence of the precipitate (trimeprazine tartrate) in the stirred solution. The same GC/PANI electrodes were used repeatedly for more than 2 months without any significant changes in the... 

Bis(diphenylphosphino)propane Differential scanning calorimetry Emeraldine base Enantiomeric excess... 

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