Miah, Mijan (2000) The high frequency electromagnetic properties of conducting polymers. (PhD thesis), Kingston University, .
Abstract
Organic semiconductors such as polypyrrole, polyalkylthiophenes and polyanilines are being used as alternatives to current materials for electromagnetic interference (EMI) technology. Their production, processability, lightweight construction and cost compare very favourably with those of materials in more established technologies. The project involves the synthesis and purification of the conducting polymers, and the selective use of dopant additives to alter the local structure of the polymer chain, termed as 'doping' in order to produce desired interactions with electromagnetic radiation. Conducting polymers are effectively a new class of microwave absorbing material, and in order to optimise the use of such materials, correlations have been made between the structural variables (e.g. molecular weight, crystal structure, counter-ion size, side-group functionality), the electrical properties ([sigma][sub]dc, [epsilon]*) and the magnetic properties ([mu]*) Polythiophenes and polypyrroles were synthesised using both chemical and electrochemical methods; however, it was found that the chemical methods were more effective, as more processable, soluble and therefore lower molecular weight materials were produced as opposed to the brittle insoluble high molecular weight materials. This allows the manipulation of subtle properties of the conducting polymer such as [epsilon]', [epsilon]", [mu]' or [mu]" in order to enhance the lossy behaviour. These materials were doped to a wide range of conductivities, ranging from the undoped insulating state usually associated with polymers, through a semiconducting up to a metallic state, with conductivities comparable to that of copper. As the conductivity changed, it was found that the electromagnetic radiation could either be transmitted through the polymer material in the insulating state, or be reflected from the material in the conducting state. The intermediate semiconducting form had a maximum absorption of the electromagnetic radiation. The real and imaginary dielectric and magnetic constants of the conducting polymer were measured at microwave frequencies, using a vector network analyser. These conducting polymers were also arranged in a sandwich structure, together with other components with the aim of providing a lightweight, durable and portable device that may be switched 'on' or 'off' under the potentiostatic control, providing a method of controlling the radiation throughout of the device at optical and microwave frequencies. The rate of switching the reflective and transmissive states more conventionally known as the doped and undoped states respectively was controlled by the ionic volume of the dopant ions. The ionic value of dopant ions was determined to be < 1.8x10[sup]-27m[sup]3 based on using a variety of dopant ion volumes and measuring the diffusion coefficients. This device was constructed using a variety of techniques to determine which arrangement would confer the fastest switching time and give the best 'radiation absorbing characteristics'. A variety of conducting polymers, electrode and electrolyte combinations were used in order to fabricate a lightweight and portable device that could be made to interact with the incident electromagnetic radiation. This project investigates the effect of electromagnetic absorption with paramagnetic metal ion complexation to conducting polymers. Current electromagnetic absorption technology addresses either the electric component or the magnetic component of the electromagnetic wave. It has been established that an oscillating electromagnetic field is absorbed to some extent by conducting polymers by the excitation of the mid-gap states at intermediate doping levels. However, this addresses only the electric component of the electromagnetic wave. By complexing paramagnetic ions within the polymer chain, it is expected that this will provide additional electromagnetic absorption. With the electrochromic properties of conducting polymers being well understood, it is also the aim of this project to use the complexed conducting polymer to switch between the conducting and insulating state under the user's control. It is therefore of interest to find the limiting factors affecting the rate of switching. It was found that the complexation of magnetic ions into the polymer contributed to electromagnetic absorption by providing additional loss mechanisms for the incident radiation. Low conductivity materials that were doped to 3-5 mol% and were governed by DC conduction processes were the best absorbers of the electromagnetic radiation.
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