Emeraldine Electrochemical Studies

A wide range of alkyl- and alkoxy-substituted PAn s of the general types 6 and 7 have been synthesized by the chemical or electrochemical oxidation of appropriately substituted aniline monomers.133 Such substitution imparts markedly improved solubility in organic solvents to the emeraldine salt products compared to the parent (unsubstituted) PAn/HA salts. The poly(2-methoxyaniline) (POMA) species, in particular, has been the subject of extensive studies.134 137 This species has the additional attractive feature of being soluble in water after being wet with acetone. 

PAn s formed by both the chemical and electrochemical processes have been extensively studied to establish structure-property relationships. In this section, the structural studies of PAn are reviewed the influence of structure on properties is considered in Chapter 5. The description of PAn structure is complicated by its complexity. As described earlier, PAn can exist in six different forms (the salt or base forms of leucoemeraldine, emeraldine, and pemigraniline). In addition, the proton-ated forms of PAn also have counteranions intimately associated with the positively charged PAn chains. Finally, it has also been observed that PAn s may contain considerable amounts of solvent molecules. 

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. 

This x-ray study, which is supplemented by electrochemical data, suggests that oxidation of leucoemer-aldine to emeraldine occurs in transfers of two electrons at a time. The authors also present arguments for phase separation of partly oxidized PANI in regions of leucoemeraldine and regions of emeraldine oxidation state. 

Kinetic studies by Shim and Park [132] determined the autocatalytic rate for polyaniline growth. Other reaction kinetic studies and comparison of the properties of the chemically synthesized emeraldine to the electrochemical product led Mohilner et al. [131] to conclude that the oxidation of aniline occurs via a free radical mechanism that produces the emeraldine octamer as the primary product. Emeraldine is a blue-violet base form of polyaniline that forms dark green salts as described by Green and Woodhead [133]. Emeraldine loses two protons upon ox-... 

The first electrochemical oxidation of aniline to emeraldine salt was reported by Letheby in 1862 [1] as a dark-green precipitate under aqueous acidic condition. This green powdery material soon became known as "aniline black . Almost a himdred years later interest in the electrochemistry of aniline black was revived in 1962. When Mohilner et al. [23] reported mechanistic aspects of aniline oxidations. Buvet et al. [6] studied die conductivity of prepared polyaniline and the influence of water on conductivity measurements. 

Mixed-conducting lithium-ion-doped emeraldine polyaniline (PAni)-PEO blends have been developed in order to achieve optimal electronic-ionic conductivity balance in nano-tin composite anodes. They found that the SEI impedance of the composite anodes increases with a decrease in PEO content and is much lower in pressed than in cast electrodes. Nano-Sn, AlSi , and Li Sn powders were studied by EIS to determine the electrochemical kinetics and intrinsic resistance during initial lithium insertion-extraction. It was shown that the SEI formed on particle surfaces, together with particle pulverization are responsible for the high contact resistance. 

Various chemical [1,2, 4-6] and electrochemical [3,4] procedures have been used to synthesize polyaniline by the oxidative polymerization of aniline. In no case has any complete elemental analysis been given for emeraldine base or for any of its salts although from chemical reduction studies of the base [1] it would appear that it has probably been obtained in the pure state. Leucoemeraldine base is the only polyaniline in the above series of compounds for which an elemental analysis has been reported [1]. 

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