For many years, electrochemically deposited polypyrrole has found application in a host of technologically significant areas. Popular applications include use in rechargeable batteries, electrochromic displays and artificial muscles. However, perhaps the most significant application of polypyrrole is as a gas sensing material. The relatively low selectivity of polypyrrole has led to it seldom being used as a 'stand alone' sensor; the ease by which the properties of polypyrrole may be subtly modified during electrochemical deposition (resulting in subtly different sensor responses) makes it ideally suited for incorporation into sensing 'arrays'. The level of understanding concerning the growth dynamics and structural characteristics of electrochemically deposited polypyrrole was poor prior to the commencement of the work presented; this thesis describes research undertaken in order to elucidate the properties of this material. As variation of the dopant group used during electrochemical deposition has been shown to result in significant structural and operational variations, the work presented focuses on polypyrrole doped with sodium benzene sulfonate (benzene sulfonic acid, sodium salt). The effects of deposition parameter variation have been studied (such as deposition potential and dopant concentration); repeatable relationships were found between deposition parameters and [a] sensor electrical conductivity, and [b] the surface morphology of the films formed. The influence of sensor substrate design is also considered; dissimilarities were found between the consistency and resistance temporal stability of elements deposited on simple 'boot' electrodes and interdigital microelectrodes. A significant proportion of the work presented concerns the study of the macrostructure of electrochemically deposited polypyrrole films. Several novel structural features have been presented, all of which have been documented in the scientific press. These include: The formation of 'tendrillar' morphology (as opposed to the commonly observed polypyrrole 'nodular' morphology) during electrochemical deposition from aqueous electrolyte. Tendril formation has been shown to be the result of the accumulation of impurities at the advancing growth face; a model has been presented which relates impurity accumulation to tendrillar polymer morphology; Demonstration of the evolution of gas at the polymer/substrate interface during aqueous electrolytic deposition. It is suggested that gas evolution is the result of the catalysed disassociation of the (aqueous) supporting electrolyte, and shown that the production of gas at the substrate/polymer interface results in the formation of discrete pockets, the positions of which relate strongly to the positions of nodules on the upper film surface. Demonstration of the recrystallisation of ionic dopant trapped within the polymer films during maturation. Dopant recrystallisation has been verified by SEM and ED AX; crystal growth has been demonstrated by XRD.Finally, the microstructures of a range of subtly different polypyrrole films have been considered. Repeatable relationships were observed between deposition potential, electrolyte solution concentration and microstructure. Characteristic features of X-Ray diffractograms have been related to the theoretical spacing between adjacent pyrrole rings (=3.6A), we believe for the first time.

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