By Colin Pratt
Until about 30 years ago all carbon based polymers were rigidly regarded as insulators. The idea that plastics could be made to conduct electricity would have been considered to be absurd. Indeed, plastics have been extensively used by the electronics industry because of this very property. They were utilized as inactive packaging and insulating material. This very narrow perspective is rapidly changing as a new class of polymer known as intrinsically conductive polymer or electroactive polymers are being discovered. Although this class is in its infancy, much like the plastic industry was in the 30's and 50's, the potential uses of these are quite significant. In 1958, polyacetylene was first synthesised by Natta et al. as a black powder. This was found to be a semi-conductor with a conductivity between 7 x 10-11 to 7 x 10-3 Sm-1, depending upon how the polymer was processed and manipulated. This compound remained a scientific curiosity until 1967, when a postgraduate student of Hideki Shirakawa at the Tokyo Institute of Technology was attempting to synthesise polyacetylene, and a silvery thin film was produced as a result of a mistake. It was found that 1000 times too much of the Ziegler-Natta catalyst, Ti(O-n-But)4 - Et3Al, had been used. When this film was investigated it was found to be semiconducting, with a similar level of conductivity to the best of the conducting black powders. Further investigations, initially aimed to produce thin films of graphite, showed that exposure of this form of polyacetylene to halogens increased its conductivity a billion fold. Undoped, the polymer was silvery, insoluble and intractable, with a conductivity similar to that of semiconductors. When it was weakly oxidised by compounds such as iodine it turned a golden colour and its conductivity increased to about 104 Sm-1.In the 1980's polyheterocycles were first developed. Polyheterocycles were found to be much more air stable than polyacetylene, although their conductivities were not so high, typically about 103 Sm-1. By adding various side groups to the polymer backbone, derivatives which were soluble in various solvents were prepared. Other side groups affected properties such as their colour and their reactivity to oxidising and reducing agents. A Logarithmic conductivity ladder of some of these polymers are shown below .
Since then it has been found that about a dozen different polymers and polymer derivatives undergo this transition when doped with a weak oxidation agent or reducing agent. They are all various conjugated polymers. This early work has led to an understanding of the mechanisms of charge storage and charge transfer in these system. All have a highly conjugated electronic state. This also causes the main problems with the use of these systems, that of processibility and stability. Most early conjugated polymers were unstable in air and were not capable of being processed. The most recent research in this has been the development of highly conducting polymers with good stability and acceptable processing attributes.
One early explanation of conducting polymers used band theory as a method of conduction. This said that a half filled valence band would be formed from a continuous delocalized π -system. This would be an ideal condition for conduction of electricity. However, it turns out that the polymer can more efficiently lower its energy by bond alteration (alternating short and long bonds), which, introduces a band width of 1.5 ev making it a high energy gap semiconductor. The polymer is transformed into a conductor by doping it with either an electron donator or an electron acceptor. This is reminiscent of doping of silicon based semiconductors where silicon is doped with either arsenic or boron. However, while the doping of silicon produces a donor energy level close to the conduction band or a acceptor level close to the valence...