Plastics don’t conduct electricity? Nonsense – they certainly do!
For a long time, the difference was clear-cut and easy to remember: metals conduct electricity, plastics do not. Today, there is a general consensus that the reality is more complicated. Under certain conditions, plastics are indeed capable of conducting electricity. For this discovery, the New Zealand chemist Alan MacDiarmid, together with his Japanese colleague Hideki Shirakawa and the American scientist Alan J. Heeger, was awarded the Nobel Prize in Chemistry in 2000. Ten years ago, on 7 February 2007, Alan MacDiarmid died at the age of 79.
As is so often the case, chance is said to have played a part in the discovery of conductive polymers: In his laboratory in Tokyo in the early 1970s, Hideki Shirakawa was in the process of producing polyacetylene (PAC), a plastic used, amongst other things, for electrical insulation. However, the Japanese scientist made a minor error during the synthesis.
To cross-link acetylene, a catalyst is required – that is, the addition of a substance capable of initiating or influencing a chemical reaction whilst remaining unchanged itself. Shirakawa used a thousand times the usual amount of catalyst and, instead of a white powder, obtained a silvery, shiny material.
At a seminar, Shirakawa met Alan MacDiarmid, a professor at the University of Pennsylvania, and Alan J. Heeger from the University of California, Santa Barbara, who reported that they, too, had produced a metallic-looking material during the polymerisation of acetylene. The scientists put their heads together, discussed their experiments at length and agreed to collaborate on further research into their materials. It proved to be a worthwhile endeavour, as it was ultimately honoured with the Nobel Prize in Chemistry – the highest accolade a scientist can receive during their lifetime for their work.
By adding iodine to the polymer, the scientists succeeded in altering – or rather, increasing – the plastic’s conductivity. When asked about the reasons for this in an interview with the New York Times following the award of the Nobel Prize, MacDiarmid replied, in essence, as follows: iodine, which is by nature a halogen and a strong oxidising agent, exerts a strong attractive force on the electrons in the polymer. This means that the electrical charge carriers contained within the polymer are less tightly packed, gain greater mobility and – as in metal – are able to flow.
Polyacetylene was thus the first plastic to make a name for itself as a conductor of electric current. To briefly examine the physical mechanisms involved, a key factor in its conductivity was the addition of oxidising agents, which readily reacted with atmospheric oxygen, leading to a significant reduction in what was initially very good conductivity.
Research has since overcome this challenge. Today, various highly stable conductive polymers are available on the market, which possess electrical conductivity on a par with that of copper. The range of applications for conductive polymers is vast, including in LED technology, the manufacture of efficient displays and the production of solar cells. On closer inspection, the Nobel Prize in Chemistry honours not only the outstanding scientific achievements of MacDiarmid, Heeger and Shirakawa. Moreover, the prize also highlights the great significance of conductive plastics.
To return once more to Alan MacDiarmid. The Nobel laureate had been suffering from cancer for some time and did not have long to live. To say goodbye to his family, he wanted to travel from the USA to New Zealand. MacDiarmid was in a hurry and did not want to miss his flight. In the rush on the day of his departure, he fell and sustained fatal injuries. GDeussing