Electropolymerized Emeraldine Salt

The mechanical properties of PAn differ considerably between the electrochemi-cally prepared polymer and that produced from solvent casting. As described earlier, electropolymerized emeraldine salts are highly porous and, consequently, have low mechanical strength. Freestanding films may be prepared electrochemically, but their poor mechanical properties limit their usefulness. In contrast, the polymers made from solution are much less porous and are widely used as freestanding films and fibers. The effect of polymer structures and morphology on PAn mechanical properties are described in the following text. 

A remarkable recent example of the influence of electropolymerization temperature on the properties of PAn products is observed in the potentiostatic polymerization of aniline in the presence of chiral (+)-HCSA to give optically active PAn/(+)-HCSA films. The CD spectra of the polymers electrodeposited at < 25°C were inverted compared to the spectra of analogous emeraldine salts deposited at > 40°C, indicating an inversion of the preferred helical hand for PAn chains.38 The observations may be rationalized in terms of a temperature-induced interconversion between two initially (kinetically) formed diastereomeric PAn products. 

The first reported optically active PAn s were the PAn/(+)-HCSA and PAn/(-)-HCSA salts (HCSA = 10-camphorsulfonic acid), prepared in our laboratories by the electropolymerization of aniline in the presence of either (+)- or (-)-HCSA.68 69 The optically active dopant anions are believed to induce a preferred one-handed helical arrangement in the PAn chains in these emeraldine salts 4, giving rise to intense CD bands in the visible region. Recent studies by Li and Wang70 have shown that PAn/(+)-HCSA films of exceptionally high optical activity can be electrochemically deposited by polymerizing the aniline monomer in the presence of small amounts of aniline oligomers. 

The first reported optically active polyanilines were the emeraldine salts PAn.(+)-HCSA (+)-HCSA and PAn.(-)-HCSA (HCSA = 10-camphorsulfonic acid), prepared in our laboratories by the electropolymerization of aniline in the presence of either (+)- or (-)-HCSA. ... 

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). 

Usually the functional moiety is covalently linked to the conjugated backbone, but it can be sometimes added to ECP as a dopant when it is under an ionic form e.g., sulfonated P-cyclodextrins have been successfiilly incorporated in one step as anionic dopants in PPy by electropolymerization [230]. In some cases, electrochemistry can be combined to chemical reactions to derive functional ECPs functional PANI materials can be obtained from the reduction of the emeraldine form by alkylthiols [255] and the functionality degree on the PANI backbone can be monitored by successive oxidation-reduction cycles various functionalized PANIs can also be synthesized through electrophilic substitution or nucleophilic addition reactions [256]. For example. Figure 18.10 shows the similar electrochemical behaviors of sulfonated PANI made from reduction of emeraldine by sulfite salt followed by reoxidation, compared to thin films deposited from solution of highly sulfonated PANIs. 

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