Baruch Zinger

Electrically conducting polyethylene/polypyrrole films

Abstract The preparation and physical properties of conducting ionomers were investigated. A two-step process was used to prepared polyethylene based film grafted with polystyrene sulphonic acid. Two methods, both based on chemical polymerization of pyrrole using ferric chloride, were conducted to form the conducting ionomers in aqueous solutions. In one method the polymerization agent was electrochemically recycled. The conductivity, structure, composition and mechanical properties of the conducting ionomers prepared in the two methods were evaluated and compared. Films having high conductivities and improved mechanical properties were obtained.

Catalytic electrosynthesis of conducting polymers

Abstract A novel catalytic electrodeposition of several kinds of conducting polymers in aqueous solutions is described. The anodic deposition potentials of pyrrole, N-methylpyrrole and aniline were shifted in the negative direction by 140–200 mV in the presence of tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) as a catalyst in the reaction mixture. The catalytic depositions of polypyrrole and polyaniline, and the compositions of the produced films, were investigated. The conductivity of the catalytically prepared free-standing polypyrrole films was smaller than that of the directly prepared films. This one-step process was used to produce films holding anions (such as carboxylates) which cannot play the role of dopant in the direct oxidation of the monomers, and thus to simplify the preparation of drug-loaded conducting polymers for a drug release device.

Electrochemistry of quinoid dopants in conducting polymers

Abstract The preparation and electrochemical properties of polypyrrole (PPy) films, in which sulfonated hydrobenzoquinone, naphtho- and anthraquinone are intercalated as dopants, are described. The redox characteristics of the quinoid dopant are similar to the reported voltammetry of adsorbed quinones. Unlike the limited electroactive coverage of the quinone in non-conducting coatings, the apparent electroactive coverage of the quinoid dopant can be controllably varied, and high electroactive coverage can easily be obtained within a few minutes. PPy/NQS exhibits catalytic oxidation of ascorbic acid, which is controlled by the diffusion of the substrate to the electrode. Catechol derivatives are used as electrocatalysts for the formation of PPy films. A common dependency of the quinone/hydroquinone redox on the solution pH is observed. These phenomena are attributed to changes in the PPy, rather than in the dopant, which may be caused by the low p K a value of the pyrrole ring in the film, pH-dependent structural changes in the films and different interactions of the PPy backbone with the two oxidation states of the quinones.

The effects of cross-linking and anodic surface roughening on quinone polymer/carbon electrodes

A polymer in which anthraquinone-2-carbonyl groups were bound to polyethyleneimine was coated onto a glassy carbon electrode. Electrodes of this kind were studied using cyclic voltammetry and pH 7 aqueous solutions. At pH <10 only those quinone units in contact with the carbon surface are electroactive. It was shown that anodic surface roughening increased the limited number of electroactive groups in the polymer film and gave more stable activity and narrower voltammetric peaks. Above pH 10 redox propagation through the layer is more rapid but the anionic product desorbs. This desorption was inhibited by cathodically cross-linking a layer of mixed polymers on a polyethyleneimine backboned polymer containing fluorenone units as well as anthraquinone units.

Flexible polypyrrole/ClO4 films formed in aqueous solutions

Abstract The preparation of high quality polypyrrole film in aqueous solutions is described. The new polypyrrole films were produced in a phthalate buffer solution (pH 4) with perchlorate as the electrolyte. Very smooth and remarkably flexible free-standing films were grown electrochemically. The conductivity of the polypyrrole film exhibits an inverse dependency on the film thickness. The mechanical properties of these films, such as tensile strength and the modulus of elasticity, are better than the reported properties for polypyrrole/tosylate films prepared in organic solutions. In comparison with the polypyrrole films prepared in phosphate buffer solution, the stability of the polypyrrole films prepared in phthalate buffer are improved considerably. The morphology and composition of the conducting film is described and a mechanism for their formation is proposed.

New multi-charged viologen derivatives: Part 1. Electrochemical behaviour in Nafion films

Abstract The electrochemical behaviour of new viologen derivatives, C 5 V 3+ and C 5 V 4+ , has been studied in Nafion ® films on gold electrodes. The new viologens bear extra positive charges with respect to methyl viologen (MV 2+ ). They remain positively charged upon two-electron reduction, while MV 2+ is reduced to a neutral form under similar conditions. This fundamental distinction is shown to induce a substantial difference in the voltammetric behaviour and stability of C 5 V 3+ and C 5 V 4+ in Nafion films compared with MV 2+ . These differences are discussed in terms of a model suggested previously for the electrochemical behaviour of electrode couples comprising neutral/cationic forms in Nafion. The model considers the influence of the inner structure of the polymer on electrochemical reactions occurring therein.

Electrochemistry of quinizarine adsorbed on a glassy carbon electrode in aqueous solutions

Abstract The electrochemistry of quinizanne in aqueous solutions, adsorbed on electrochemically pretreated glassy carbon electrodes, is described. The anodic and cathodic reversible reactions are followed by physical and chemical processes, respectively. The desorption of the fully oxidized species from the electrode was assigned to the follow-up process in the anodic region. The follow-up reaction at the more negative potentials is ascribed to tautomerization. in which the central hydroxyl group(s) participate. When the range of the reduction is expanded, two additional irreversible reduction waves are observed. A full mechanism of the electrode reactions is proposed. Intramolecular hydrogen bonds and the pH of the aqueous solutions play an important role in the rationalization of the overall mechanism. The “electro-chemical label” concept is used to study the progress of the electrode reactions, and to evaluate the intermediates and the products which are confined to the electrode surface.

Mediated release of glutamate and gaba by cathodic reduction of cobalt complexes

Abstract The electrochemical reduction of cobalt (III) complexes containing the neurotransmitter GABA or GLU as ligands were investigated using aqueous solutions as a model sytem for drug release. The release of the neurotransmitter was found to be efficient. In one case the reduction proceeded directly, in another it was mediated by methyl viologen. The kinetic of the catalyzed process were analyzed using electrochemical techniques. In addition, unusual trace crossings in the cylic voltammograms of the catalytic system, as well unusual chronoamperometric curves, were observed. These phenomena wwere attributed to the electrodeposition of cobalt metal.

Quinonoid modified electrodes amplify the current from certain soluble one-electron redox reagents

Abstract Chemically modified carbon electrodes are prepared which hold polymeric layers of anthraquinone or dopamine units on the surface. In each case only those quinonoid units nearest the carbon are electroactive in neutral solution. Other units can be activated by one-electron redox shuttles dissolved in the solution phase which help propagate the redox process through the film. Cobalticinium dicarboxylic acid and methyl viologen are used to catalyze reductive reactions and ferrocene carboxylic acid catalyzes the oxidative propagation.

New multi-charged viologen derivatives: Part 2. Unusual electrochemical behaviour in solution

Abstract The electrochemical behaviour of a new viologen derivative, C 5 V 3+ , has been studied at gold electrodes. C 5 V 3+ displays an unexpected voltammetric behaviour, manifested as a tremendous difference between the peak currents of the two (reversible) reduction waves. A variety of electrochemical and spectroscopie techniques have been applied in order to resolve this unusual behaviour. The results indicate that C 5 V 3+ undergoes an overall two-electron reduction, but, unlike other viologens (e.g. methyl viologen), the charge partition between the two reduction steps is 0.5:1.5. A mechanisin is suggested which includes charge transfer accompanied by radical/parent ion dimerization. The mechanism is discussed in detail and shown to be in good agreement with the experimental results.