Poly electrochemical passivation

Albery et al. [39, 49] prepared poly(3-thiopheneacetic acid) and its copolymer with thiophene by electrochemical polymerization. Bartlett et al. [50] electrochemically synthesized conducting poly(3-thiophene-acetic acid) films in dry acetonitrile containing tetraethyl ammonium tetrafluoroborate. These films are redox active in acetonitrile, however, stability was reportedly poor in comparison with poly(3-methylthio-phene) and poly(methyl 3-thiopheneacetate) due to traces of water. In dry acetonitrile, the polymer can be electrochemically oxidized and reduced. Upon oxidation in water and methanol, poly(3-thiopheneacetic acid) film converted into a passive film. Based on the electrochemistry and an FT-IR study, Bartlett et al. postulate the mechanism for the electrochemical passivation shown in the Figure 4.33. In the mechanism, passivation of the polymer involves the formation of an intermediate cyclic lactone and subsequent breakdown by reaction with solvent. This process does not destroy the conductivity of the polymer so the process can continue until all the monomer units within the film are converted to a lactone form (Figure 4.33, IV). The electrochemical passivation is not observed... 

Figure 4.33 Proposed mechanism of the electrochemical passivation of poly(3-thiopheneacetic acid) in water and methanol. Journal of Material Chemistry, 1994, 4, 1805, P. N. Bartlett, D. H. Dawson. Reproduced by permission of The Royal Society of Chemistry.)...

It has been demonstrated that hydrophobic, fluorinated, hyperbranched poly(acrylic acid) films can passivate and block electrochemical reactions on metal surfaces thus preventing surface corrosion [57]. Hyperbranched films can be synthesized on self-assembling monolayers on the metal surface via sequential grafting reactions to obtain thick and homogeneous films. 

From SECM studies of the electrochemical doping and undoping processes of poly-(didodecyl-terthiophene), the electron transfer between the polymer and a redox mediator in solution was investigated as a function of the doping state of the polymer [164], The electron transfer was found to occur at the polymer-electrolytic-solution interface and not inside the polymer film. The investigated polymer films are not permeable to redox species when placed in the neutral state and therefore behave as completely passivating films. 

For organic encapsulation we have used the electrochemical synthesis of poly(oxy-phenylene) as originally presented by Mengoli et al. It is done by the electrochemical oxidation of 2-allylphenol in water/methanol/butylcellosolve mixture to yield poly(oxyphenyl-ene). This electrooxidation is done in the presence of allylamine in order to minimize the competing passivation of the substrate and to crosslink the linear polymer during the curing step. 

Hermas, A.A., Wu, Z.X., Nakayama, M., and Ogura, K. (2006) Passivation of stainless steel by coating with poly(o-phenylenediamine) conductive polymer. J. Electrochem. Soc., 153, B199-B205. 

However, the most extensively conjugated and most conductive poly thiophenes have been prepared by electrochemical techniques. Polythiophene can be obtained by a cathodic route involving the electroreduction of the complex (2-bromo-5-thienyl)triphenylnickel bromide in acetonitrile [140]. A drawback of this method is the fact that the polymer is produced in its neutral insulating form, which leads rapidly to passivation of the electrode. Thus, only thin films (—100 nm) can be obtained. 

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