Year

1994

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Membranes are the basis of many important applications including artificial kidney, purification of water, concentration of aqueous solutions and protein recovery. Much effort has been directed towards making synthetic membrane materials more inert with a view to preventing fouling and extending their lifetime. However once a conventional membrane has been fabricated, its characteristics (such as morphology) are fixed. This is a limitation of conventional membranes.

The present work is concerned with the development of smart separation systems, based on conducting electroactive polymer membranes such as polyaniline and polypyrrole, which are capable of responding to electrical stimuli. This is a novel concept in separation technology differing from conventional membranes due to the nature of the transport mechanism involved. The conducting polymer membranes are used as a working electrode in an electrochemical transport cell. Changes in the oxidation state of the polymers can be induced by application of potential to the membrane and this causes the movement of ions in and out of the polymer. This movement can be used to effect transport and separation processes. In conventional methods (e.g. electro-dialysis) an electrical field is applied across both sides of the film and separation is achieved based on the electrical field generated by passing the current across the membrane.

The major objective of the present investigation was to investigate the transport of organic/inorganic species across conducting polymers using novel electrochemical control. In order to achieve this the improvement of the mechanical properties of the materials, as a primary aim, was considered. Significant achievements obtained can be summarised as follows:

1) Preparation of robust free-standing membranes based on polyaniline, polypyrrole with a variety of counterions and composites was achieved. The effect of growth conditions such as current density, electrolyte concentration and effect of substrate on the membrane properties of the composite films was investigated.

2) As a major objective of the current investigation it was demonstrated that the conducting polymer membranes can be used to transfer species from one side of the membrane to the other with some control over selectivity and flux. This control was achieved using electrochemical devices. Such transport was found to be selective, resulting in the separation of mixtures. Using this approach the electrochemically controlled transport and separation of inorganic acids (e.g. HC1, H2S04 and HN03 ) , inorganic salts (such as NaCl and KC1) and a wide range of sulfonated organic compounds across the membranes was shown to be possible. The rate and selectivity of the transport was found to be markedly affected by the electrochemical variables. For example, by altering the electrode configuration and electrochemical waveform the transport of organic anions was significantly increased.

3) In order to study the mechanism of the mass transport in detail a electrochemical quartz crystal microbalance ( EQCM ) was employed. Using this technique it was shown that the ion movement during redox reactions of polypyrrole films was dependent on the nature of supporting electrolyte, the anion incorporated into the polymer during synthesis and the electrochemical waveform applied. This explained the conditions under which the polymer acts as an anion exchanger or a cation exchanger. Using the information provided by EQCM the transport properties of the membrane can be manipulated. A model to explain the transport mechanism was proposed.

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