Polyaniline leucoemeraldine

It has recently been observed31 that the completely reduced base form of polyaniline, (leucoemeraldine), spontaneously reduces, and is itself oxidized, when its solution in NMP is mixed with an NMP solution of the completely oxidized base form of polyaniline, (pernigraniline). This reaction may be studied by "titrating" a solution of the leucoemeraldine with a solution of pernigraniline and by following the course of the reaction by the changes in the electronic spectrum of the reaction mixture. When equimolar solutions of the reactants are mixed this redox reaction is found to approach equilibrium after 20 hours at room temperature, to produce the emeraldine base, viz.,... 

The emeraldine salt of polyaniline which is one of the three different forms of polyaniline—leucoemeraldine (the fully reduced state], emeraldine (the half oxidized state] and pernigraniline (the fully oxidized state] (Fig. 8.1]—is electrically conductive while the others are insulators. Conventional chemical synthesis of polyaniline is based on an oxidative polymerization of aniline by using an oxidant in the presence of a strong acid dopant in the reaction mediunL ... 

Figure 4-1. Chemical structures of conducting polymers, (a) Frans-polyacetylene (b) cw-polyacetylene (c) poIy(/i-phenylene) (d) polypyrrole (e) polythiophene (f) poly(/>-phenylenevinylene) (g) poly(2,5-thienylenevinylene) (h) polyaniline (leucoemeraldine base form) (i) polyisothianaphthene.

The instability of PANI above 0.8 V aside, the redox behavior of the first couple of polyaniline, leucoemeraldine to emeraldine, is stable and has been exploited in EC systems (9,13,24). Thin layer electrochromic devices have been reported by a number of groups, however despite the seemingly good results derived it would be desirable to address the pemigraniline (blue) form as well. If it were possible to access this state reversibly, then PANI or related materials could find utility in glare reduction applications and in smart windows since the band associated with this form best matches the solar spectrum (25). 

B as repeat units, where the oxidation state of the polymer increases with decreasing values ofy (0 [Pg.89]

The viscoelastic characteristics of polyaniline (emeraldine base)/DMPU [PANI(EB)/DMPU] solution (14% w/w) and polyaniline (leucoemeraldine base)/DMPU [PANI(LEB)/DMPU] (14% w/w) were studied using a controlled stress rheometer in the frequency range of 1-100 Hz. The polyaniline solution was subjected to an... 

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. 

Polyaniline can also be prepared electrochemically, e.g. by oxidation of aniline in acid media (MacDiarmid and Maxfield, 1987). An interesting aspect of this polymer is that by doping, the oxidation state of the polyaniline may be continuously varied from the totally reduced insulator, leucoemeraldine (oxidation state y = 1) ... 

Polymers (CH), PPA Poiy(o-Me3SiPA) PPY DP-PPY PPY PAN PNA EM LM NA PTH PPS PBT PBiT PMeT PHeT PPV PPP P2VP P4VP PVK polyacetylene polyphenylacetylene poly[[o-(trimethylsilyl)phenyl]acetylene] polypyrrole deprotonated (25%) polypyrrole fully reduced polypyrrole polyaniline pernigraniline emeraldine leucoemeraldine nigraniline polythiophene poly(phenylene sulfide) polybenzothiophene poly(2,2 -bithiophene) poly(3-methylthiophene) poly(3-hexylthiophene) poly(phenylene vinylene) poly( p-phenylene) poly(2-vinylpyridine) poly(4-vinylpyridine) poly(A-vinylcarbazole)... 

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]. 

FIGURE 1.2. Molecular structure of widely used it-conjugated and other polymers (a) poly(para-phenylene vinylene) (PPV) (b) a (solid line along backbone) and it ( clouds above and below the a line) electron probability densities in PPV (c) poly(2-methoxy-5-(2 -ethyl)-hexoxy-l,4-phenylene vinylene) (MEH-PPV) (d) polyaniline (PANI) (d.l) leucoemeraldine base (LEB), (d.2) emeraldine base (EB), (d.3) pernigraniline base (PNB) (e) poly(3,4-ethylene dioxy-2,4-thiophene)-polystyrene sulfonate (PEDOT-PSS) (f) poly(IV-vinyl carbazole) (PVK) (g) poly(methyl methacrylate) (PMMA) (h) methyl-bridged ladder-type poly(jf-phenylene) (m-LPPP) (i) poly(3-alkyl thiophenes) (P3ATs) (j) polyfluorenes (PFOs) (k) diphenyl-substituted frares -polyacetylenes (f-(CH)x) or poly (diphenyl acetylene) (PDPA). 

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. 

Polyanilines, ([(-NH-C6H4-NH-C6H4)x-(N=C6H4=N-C6H4-)2-x] ), like so many other polymers, are stoichiometrically ill-defined. The three stable polyaniline forms are leucoemeraldine (x = 2), emeraldine (x = 1) and pemigraniline (x = 0). Emeraldine is the most important to the chemical and materials science communities because it is... 

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