Conductive polymers: their discovery and applications

By Associate Professor Allan Blackman

This article was orignally published in the Otago Daily Times on Monday 2 April 2001.

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Chemistry is the study of matter, and matter is generally defined as “anything that occupies space”. Thus chemistry is ubiquitous. It is all around us, every minute of every day, in absolutely everything we do. Unfortunately, Chemistry is often unappreciated or misunderstood by those who haven’t studied it (and sometimes by those who have!) and many people find it intimidating. This monthly column aims to demystify Chemistry and will attempt to explain, in non-technical language, some of the ideas and developments in modern Chemistry. It is appropriate that this first column should highlight the extraordinary achievement of New Zealand-born Professor Alan MacDiarmid, who was awarded the Nobel Prize for Chemistry jointly with Professors Alan Heeger and Hideki Shirakawa at a ceremony in Stockholm on December 10th, 2000. He was the second New Zealand-born chemist to be awarded this honour, the first being Ernest Rutherford in 1908. Professor MacDiarmid was awarded the prize “for the discovery and development of conductive polymers”. Below, we’ll tell you what this means and why it is so important.

We are all familiar with the idea that some materials (e.g metals) conduct electricity and some (e.g rubber) do not. Rubber is an example of a polymer, a material that is made up of a vast number of small repeating units. In the case of pure rubber, these repeating units contain only carbon and hydrogen atoms, and it is the way in which these atoms are joined together that determines whether or not a material will conduct electricity. An electric current consists of a flow of electrons, and hence the requirement for a material to act as a conductor is the presence of electrons that are free to move throughout the material. Metals are excellent conductors because they contain electrons that are free to move – metals in fact appear shiny as a result of these free electrons. On the other hand, plastics, rubber and many other polymers which are based on carbon and hydrogen atoms do not contain mobile electrons and are hence excellent insulators. This is the reason that electrical flexes are covered in rubber and circuit breakers are made of plastic. The brilliant advance that Professor MacDiarmid made was to discover a way to turn plastics from insulators into conductors. He used a polymer called polyacetylene – this is a polymer based on the repeating unit acetylene, the gas used in welding torches. Polyacetylene itself is essentially an insulator, but in 1977 Professor MacDiarmid and coworkers developed a simple method to turn it into a conductor – they treated it with iodine (perhaps familiar to you as the stuff in the medicine cabinet). What this did was to remove a certain number of electrons from the polyacetylene, leaving “holes” in the material that the electrons once occupied. Neighbouring electrons could then “jump” into these holes, leaving holes themselves which were then filled by other neighbouring electrons, and so on. The net result of this was that the electrons were free to move in the material and it was now a conductor. Thus the first conducting polymer was prepared.

But why was this such a big deal?

Plastics are cheap and easy to make, and one of their most desirable features is that they can be easily moulded into particular shapes. The development of conducting plastics has made the possibility of new electronic devices a reality. For example, research is now underway into polymers which can emit light – such materials could be moulded into any shape desired and used as replacements for the light bulbs that we currently use. There is also intense interest in synthesising plastic materials which will replace the liquid crystal technology currently used in electronic calculators, mobile phone displays and digital watches. Perhaps the most exciting potential development in the field of conducting polymers is the replacement of the current silicon chip-based technology. At present, there is a limit to the minimum size that a silicon chip can be fabricated. The advent of polymer-based technology will allow manufacture of atomic-scale integrated circuits, which will be the basis of the next generation of unimaginably fast computers.

And all this thanks to a boy born in Masterton in 1927.