Year

1992

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

electrochemical properties of 5-membered heterocyclic conducting polymers have been investigated. Work has centred on polypyrrole, although poly-3-methylthiophene has also been considered. The redox nature of these polymers makes them ideally suited for characterisation by electrochemical (EC) techniques. Conventional EC methods such as cyclic voltammetry, chronoamperometry, chronopotentiometry and chronocoulometry have been employed. In addition, a new technique, Resistometry, has been used to study the resistance changes that occur upon oxidation and reduction of these materials. Microelectrodes have also been employed throughout this work. The electrochemical characteristics of conducting polymers at these electrodes have been compared to the characteristics at electrodes of conventional size.

The two major aspects of heterocyclic conducting polymer electrochemistry that were addressed were: (i) Electrochemical synthesis/deposition at electrode surfaces, and (ii) Electrochemical doping and dedoping.

The electropolymerisation of heterocyclic monomers of (3-methylthiophene and pyrrole) was found to be dependent on a number of factors. These included: the solvent; the monomer concentration; the electolyte concentration; the chemical nature of the electrolyte (particularly the anion); the electrochemical method used to initiate the polymerisation reaction; the nature of the electrode and the size of the electrode. Polymer deposition at microcroelectrodes was particularly dependent on these factors where it was found that, under certain conditions, no deposition occurred. This was despite the fact that the monomer was oxidised and that deposition occurred at conventionally sized electrodes under identical conditions. Further investigation of this phenomenon revealed that the electropolymerisation process involved the formation of soluble intermediates and that polymer deposition was the result of the continual precipitation of these species.

The factors influencing the doping and dedoping (oxidation and reduction) of polypyrrole were investigated. It was found that the thickness of the polymer, the solvent from which the polymer was synthesised, the dopant ion and the electrolyte ions affected this process. The nature of the processes that accompany the doping and dedoping of polypyrrole were also investigated. The transport of anions in and out of the polymer during doping/dedoping has been well documented, but this work revealed the importance of cation transport in this respect. Reaction order experiments at microelectrodes and resistometry were used to confirm this. It was also shown that the capacitance like behaviour of polypyrrole in its oxidised form was not due to double layer charging effects (as originally thought) but was an extension of the doping process.

Finally, a practical application of the doping/dedoping processes of polypyrrole were investigated with a view to determining electroinactive ions using flow injection analysis. It was found the the electrochemical signal produced upon injection of an ion containing sample into a glycine eluent was not the result of doping/dedoping (as assumed), but was due largely to changes in solution conductivity.

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