When polymers conduct electricity

When polymers conduct electricity
The now-famous diode, used in many screens, is one of the numerous applications connected to the development of conductive polymers. But how can a simple piece of plastic conduct electricity when this same material is used to insulate metal wires carrying electricity?

The story goes that it all began in 1974, when a young Korean student at Tokyo Metropolitan University in Japan, on the point of preparing polyacetylene (an organic polymer), made a mistake with the dose. Still, with a poor understanding of the subtleties of the Japanese language, he misinterpreted the instructions and made a mistake (by a factor of 1 to 1,000, no less!) in the dose for one of the reagents.
Result: instead of a black powder appearing, the now-familiar form of polyacetylene, a pretty silver film emerged. The first polyacetylene film was born!

Somewhat confused by his error, the young student was still a long way from knowing that his "discovery” would give rise to a host of reports in scientific reviews and, some thirty years later, would lead to a Nobel Prize for chemistry.

The sharp eyes of Hideki Shirakawa

Luckily, this university laboratory was headed by Hideki Shirakawa, who decided to study this new kind of plastic film a little more closely rather than throw it in the bin. He therefore presented this "find” at different congresses, which aroused the curiosity of chemist Alan MacDiarmid and physicist Alan Heeger, both teachers at the University of Pennsylvania (Philadelphia) who had been working for some time on the electrical conductivity of non-metallic materials.

Supposing that this new material could have interesting properties in the field of electrical conduction (particularly due to the particular chemical structure of polyacetylene), Alan MacDiarmid proposed to Hideki Shirakawa that he join his team. And the path to a Nobel Prize opened up…The three scientists were awarded the prize in 2000 for their discovery of conductive polymers.

How does it work?

To become an electrical conductor, a polymer must be conjugated. In other words, the chain of carbon atoms must be strung together around single and multiple bonds making the structure of the material fairly comparable to that of silicon.

However, this is a long way from being able to produce a sufficiently conductive material.

Another operation is necessary: doping with iodine vapour. Basically, the iodine captures some of the electrons, leaving room for positive charges which allow the electric current to circulate. In this way, "gaps” are formed which can travel along the polymer chain, which thus becomes an electrical conductor. QED!

The principle was therefore established. After years of research, adaptation and fine-tuning, but also false trails, this new technology finally matured and the first industrial developments soon saw the light of day.

One of the most important (if not one of the most spectacular) concerns optoelectronics and more precisely the manufacture of electroluminescent diodes which, once electrically excited, emit light.

This new technology is applicable to a wide variety of polymers on the market. The diversity of their intrinsic properties enables manufacturers to offer a range of specific products (lighting, screens, gadgets of all kinds, etc.).

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