A study into the use of ion beam analysis for the quantitative and qualitative analysis of conducting polymers : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand
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Date
2004
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Massey University
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Abstract
Since their discovery in the late 1970s conducting polymers have become increasingly used materials in many applications. They are utilised for their conductivity and/or their electroactive properties. These applications include sensor technologies, actuators, and battery materials. The properties of conducting polymers rely on the extent of the reduction / oxidation or redox state, and hence the dopant levels, of the materials. The aim of this work was to investigate the use of the Ion Beam Analysis (IBA) techniques Rutherford Backscattering Spectroscopy (RBS), and Proton Induced X-ray Emission (PIXE) for the analysis of 'soft' organic materials, in particular, conducting polymers. These IBA techniques are not new, as they have been extensively used for the characterisation of many inorganic, 'hard', materials such as aluminium oxide and silicon oxynitride. While they have been used to alter the molecular structure, and hence the properties of conducting polymers in the past, little to no research has explored the use of ion beams as a tool for the characterisation of these materials. Conducting polymers can either be prepared chemically or electrochemically. They are predominantly prepared in an oxidised state and this charge is balanced by negatively charged counter ions. In this work, the conducting polymers were formed electrochemically by deposition onto support materials at constant electrode potential. The number of counter ions required to balance the polymer chain depends on the type of conducting polymer formed and extent of oxidation. Issues such as the influence of the support material and extent of polymer oxidation on the extent of counter ions through the polymer films are of importance. Gaining knowledge of the dispersion of counter ions may provide new insights into the redox mechanisms for conductive polymers. Complex bis terthiophene porphyrin conducting polymers were prepared and investigated for the uptake of zinc into the freebase porphyrin unit after polymerisation by acquiring elemental depth profiles using RBS analysis. Issues such as the influence of the support material and extent of polymer oxidation on the extent of counter ions through the polymer films were found to be of importance. Gaining knowledge of the extent of counter ions provides new insights into the redox mechanisms for conductive polymers. The results were compared to those obtained for a sample where zinc was coordinated to the porphyrin prior to the polymerisation process. Unexpected high concentrations of both nitrogen and oxygen were found, which were interpreted to be due to entrapped cations originating from the electrolyte ((Bu)4N+), together with trapped water molecules, within the polymer films. The chlorine depth profiling assisted with understanding the extent of the perchlorate counter ion throughout the polymer films. The combination of both RBS and PIXE demonstrated that trace element impurities can be detected using ion beam analysis, which other analytical techniques are unable to do. A series of polypyrrole films incorporating a range of counter ions were prepared as model compounds for study in the second section of this work. RBS and PIXE techniques were used to evaluate film homogeneity with respect to depth and to infer the counter ion / pyrrole unit ratio for each of the six PPy film formed. RBS was also used to characterise a series of terthiophene-ferrocene based conducting co-polymers. The ratio of co-polymer monomer to terthiophene-ferrocene monomers and the dopant levels for the polymers were determined using a RBS deconvolution method developed in this study. This new method can be extended for characterization of a wide range of organic polymers. The limitations of RBS for the analysis of these soft materials were identified. The advantage that RBS offers over other analytical techniques is that it provides a means for low atomic number element depth profiling in these materials.
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Counter ions, Polymerisation