Year

1999

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

The widespread use of conducting polymers has been hindered by their lack of processability. Therefore, an ongoing challenge has been to find processing methods for producing large quantities of these materials, with properties suitable for industrial applications. This thesis focuses on the electrosynthesis and characterisation of conducting polymer colloidal dispersions in order to achieve this objective. The synthesis of colloidal conducting polymers allows for post-synthesis processability while retaining reasonable conductivity.

Two types of conducting polymers were prepared in this project, namely colloidal polypyrrole and polyaniline. These were synthesised via the electrochemical polymerisation of the monomer in a hydrodynamic electrochemical flow-through cell. Various designs of the hydrodynamic electrochemical cell were investigated for their efficiency in terms of monomer conversion into polymer product. A three-compartment electrohydrodynamic cell with a reticulated vitreous carbon working electrode using a moderate flow rate of 40 mL/min was found to be the most efficient.

Various parameters were optimised to produce a more efficient colloidal polymer synthesis. The steric stabiliser concentration was one such variable. An optimum stabiliser concentration of 3 g/L was found to produce stable polypyrrole colloids whilst retaining the electrical behaviour of the polymer. Optimum yield of colloid was obtained using a large polymerisation solution volume, low monomer concentration, extended polymerisation time and a high porosity working electrode. The maximum amount of colloid was obtained under potentiostatic conditions, since the potential for the polymerisation can be chosen so as to avoid over-oxidation of the polymer. The colloidal particle size and morphology were also found to be dependent on the polymerisation conditions, for instance the steric stabiliser employed to stabilise the forming particles.

The main limitation of this processing method for polypyrrole colloids was that under most conditions more than 90 % of the colloid mass was lost on the working electrode surface. Some success in decreasing this polymer deposition was achieved by modifying the electrode surface with a polyaniline outerlayer.

Electrochemical polymerisation allows the incorporation of unusual dopants such as proteins into the conducting polymer. In this project electrosynthesis was successfully utilised for the synthesis of polypyrrole colloidal particles incorporating the biological dopant, iactoferrin. The protein was found to retain its biological activity. This indicates that electrosynthesis provides sufficient control over the oxidation potential so as to protect the delicate protein structure necessary to ensure bioactivity.

The thesis also explores the electrosynthesis of novel, chiral conducting polymer colloids. Chiral conducting polymers have attracted considerable recent interest, owing to their potential as chiral electrodes in asymmetric electrochemical synthesis or as electroactive membranes/ion-exchange materials for the separation of enantiomeric anions. In the present study, the chiral dopants (1S)-(+)- or (1R)-(-)-10- camphorsulfononic acid (HCSA) were incorporated into colloidal polyaniline to examine whether the chiral dopant can induce optical activity into the polymer chain. Initially, chiral polyaniline colloids were successfully prepared in the presence of poly(styrenesulfonate) (PSS) as steric stabiliser. A PSS concentration of > 6 g/L was required in order to avoid over-oxidation after 60 min of electropolymerisation, with the maximum optical activity of the stable emeraldine salt colloids being achieved with a PSS concentration of 10 g/L.

In order to overcome the problem of competitive doping between PSS- and CSA- anions (curtailing the optical activity of the colloidal polyaniline) when using the poly(styrenesulfonate) stabiliser, analogous electropolymerisation of aniline using fine colloidal silica particles (20 nm) as dispersant was investigated. Stabilisation was achieved by the formation of "raspberry" polyaniline.HCSA/silica nanocomposites. Silica-stabilised polyaniline/HCSA dispersions showed higher optical activity than those prepared using PSS as a steric stabiliser. The optically active fractions of the polyaniline colloids were found to be remarkably inert to oxidation, reduction and alkaline de-doping. In contrast, the optically inactive polyaniline components of these colloids underwent facile redox and pH switching. The exceptional chemical stability of the optically active polyaniline silica colloids suggests that they may provide useful conducting materials in harsh chemical environments where other achiral polyaniline salts would be converted into insulators.

A "core-shell" morphology approach to colloidal stabilisation was then investigated. Polyurethane latex particles with poly(ethyleneoxide) chemically grafted on the surface were utilised for electrochemical chiral polyaniline production. This proved to be the most efficient method for colloid stabilisation and maximum optical activity since the particle core is the latex particle, while the surface constitutes the chiral polyaniline component.

Finally, the preparation of optically active polypyrroles potentiostatically deposited on indium-tin-oxide films incorporating the chiral dopant anions (1S)-(+)- or (1R)-(-)-10- camphorsulfonate was investigated. No optical activity was observed indicating that, in contrast to a recent report, the polypyrrole chains do not exhibit induced chirality. However, similarly deposited PPy.(+)-tart, PPy.(-)-tart, PPy.dibenzoyl-D-tart, PPy.(+)- mand and PPy.(-)-mand films were weakly optically active, confirming partial chiral induction in their polypyrrole backbones. The considerably weaker chiral induction observed compared to that previously found in related electropolymerisations of aniline may arise from the absence of polar substituents on the pyrrole backbone necessary for strong interaction with the chiral dopant anions.

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