Emeraldine salt

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... 

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. 

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... 

Electron paramagnetic resonance spectroscopy Emeraldine salt Fourier-transform infrared Gel-permeation chromatography Head-to-head... 

More recently, Buchwald has reported the polymerization of the monomer in Eq. (38) [220]. This monomer was polymerized at 80 °C for 24 h in the presence of a catalyst comprised of Pd2(dba)3 and ligand 14. The polymers generated from this monomer bearing a Boc group are soluble in THF and chloroform with the aid of sonication. After isolation, the Boc group could be removed by thermolysis at 185 °C or by protonolysis in air. Emeraldine or the emeraldine salt forms of polyaniline result. 

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, 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. 

FIG. 11.19 Oxidative doping (p-doping) of leuco-emeraldine base and protonic acid doping of emeraldine base, leading to the same final product, emeraldine salt. Reproduced from Fig. 34.3 in Mark (1996). Courtesy Springer Verlag. 

Figure 1 Typical diffraction patterns from undoped CPs (a) frms-polyacetylene, Shirakawa-type, from Ref. 34 (b) polyaniline, emeraldine salt form II, from Ref. 24 (c) polythiophene, from Ref. 22.

Figure 12 Formation of a polaron lattice according to Ref. 89 (a) emeraldine salt in bipolaron form (b) dissociation of the bipolaron into two polarons (c) rearrangement of the charges into a polaron lattice. ...

A mixture at 27°C consisting of freshly distilled aniline (0.1097 mol), 85 ml of 3M HCl, 95 ml of ethanol, and LiCl (16 g) was treated with ammonium persulphate (0.0274 mol), 60 ml of 2M HCl, and LiCl (8 g) also at 27°C. The mixture was reacted for roughly 2 hours while the potential of the reaction mixture was controlled by a standard calomel electrode. It was then treated with FeCL (0.0183 mol), LiCl (5 g), and 50 ml of 2M HCl. After an additional hour the reaction was terminated, and the polymer could be isolated by either filtration or by centrifuging. It was then washed with distilled water, dried, and converted to the emeraldine salt using 2M HCl. This salt was then converted to the emeraldine base by treatment with 2 liter of 0.3 M aqueous... 

Polyaniline provides the prototypical example of a chemically distinct doping mechanism [33,34], Protonation by acid-base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base) to metal (the emeraldine salt). The doping mechanism is shown schematically in Fig. II-2. The chemical structure of the semiconducting emeraldine base form of polyaniline is that of an alternating copolymer, denoted as [(1A)(2A)] , with... 

The emeraldine salt, (II-7), can also be obtained directly from leucoemeraldine by charge transfer doping, with reactions analogous to (II-1) and (II-3). Thus, the doping of polyaniline has been described as two-dimensional in a space where one axis describes the charge transfer chemistry and the second, orthogonal axis describes the acid/base (protonation) chemistry [37]. 

Although the emeraldine salt is formally a metal with one unpaired electron per formula unit, disorder can cause localization of the wave functions and a transition from metal to insulator (see Section VI). 

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... 

Alternative oxidants such as potassium iodate were also explored for the intrazeolite polymerization of aniline in NaY and acidic forms of Y zeolite. With peroxydisulfate, the polymerization proceeded only if a sufficient supply of intrazeolite protons was available. No polymer formed in either NaY or in acid zeolites with neutral iodate solution, but at low pH polyaniline was obtained in all hosts. The open nature of the zeolite host, even when partially filled with polymer, permits the introduction of base (such as ammonia). On admission of ammonia into the emeraldine salt-containing zeolite, the protonated polymer was converted into the neutral emeraldine base form. 

Encapsulation chemistry similar to that described above (exchange of anilinium, followed by oxidation with peroxydisulfate) was foimd to produce polyaniline not only in zeolite Y, but also in montmorillonite clay. 5 Spectral features (UV-VIS, IR and EPR) of the products were indicative of emeraldine salt and base formation, respectively. The change in basal spacing of the montmorillonite upon intercalation provided additional evidence for the inclusion polymerization. 

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... 

Figure 1. Structures of PANI oxidation states (a) leucoemeraldine, (b) emeraldine salt and (c) pemigraniline.

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. 

Figure 6. The structures of emeraldine base and emeraldine salt forms of PANI. A represents the dopant anion.

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