Year

2005

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Chemistry - Faculty of Science

Abstract

Electrochemical actuators based on conducting polymers and conducting polymer/carbon nanotube composites have been constructed and characterised. The actuation performance was optimised in terms of the actuation strain, cycle life, stability and work-per-cycle. Different conducting polymers, such as polypyrrole, polythiophene, and polyaniline were investigated as actuator materials. More reproducible and stable strain response from polypyrrole actuators has been achieved by the combination of current pulsing control for electrochemical stimulation and ionic liquid for the electrolyte. Current pulsing ensures that the oxidation and reduction processes are equal in their magnitude and prevents the slow net oxidation (or reduction) that typically occurs during symmetrical voltage cycling. Ionic liquid electrolytes permit higher current densities to be applied to the polymer because the polymer can endure a wider potential window in ionic liquids. Also, operation of polypyrrole actuators in ionic liquids produces much smaller changes in elastic modulus, thus resulting in a more stable isotonic strain with increasing applied loads and a higher work-per-cycle. The effect of dopants, potential scan rate and electrolyte temperature on actuation performance is also investigated for polypyrrole actuators. For polythiophene actuators, an increasing strain response was observed under increasing applied loads when operated in ionic liquids as electrolytes. This is the first time that conducting polymer actuators have been reported to exhibit an increasing strain with increasing applied loads. Using the phase inversion technique, polyaniline actuators were prepared in different geometrical configurations such as films, tubes and tubes with a platinum wire as helix. The actuation strain of the polyaniline actuator was significantly increased by incorporating a platinum wire as helix configuration into the polymer in acid solution electrolyte. However, the porous nature and subsequent brittleness of the polyaniline actuators prepared using the phase inversion technique has restricted their actuation performance and limited practical application. The use of carbon nanotubes as reinforcement fillers and electrical conductors in polyaniline was investigated due to the remarkable properties of carbon nanotubes such as high tensile strength, Young�s modulus, and electrical conductivity. Polyaniline/carbon nanotube composite actuators were fabricated aiming to improve the actuation behaviour. Layered composite actuators consisting of polypyrrole, polyaniline and carbon nanotubes were fabricated by coating a polyaniline/carbon nanotube solution onto the polypyrrole tube and drying on a hotplate. Polyaniline/carbon nanotube composite fibre actuators were prepared using a wet-spinning process. The presence of carbon nanotubes allowed high stresses to be applied to the composite actuators. The actuation performance of the composite actuators is enhanced by the carbon nanotubes as reinforcement fillers in conducting polymers.

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