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Nobel-Winning Chemists Made Plastics Sizzle
10 October 2000 7:00 pm
Like the plastics that coat electrical wires, nearly all polymers are insulators. But some plastics can be made to conduct electricity, and that discovery has earned this year's Nobel Prize in chemistry for Alan Heeger of the University of California, Santa Barbara; Alan MacDiarmid of the University of Pennsylvania in Philadelphia; and Hideki Shirakawa of the University of Tsukuba in Japan. Their innovation paved the way for high-tech gadgets such as full-color displays on cellular phones and plastic electronics for computerized merchandise tags. Applications on the horizon may include molecular computers and cheap, wide solar cells.
Before the mid-1970s, all plastics were thought to be insulators. Then, in a wonderful bit of serendipity, Shirakawa accidentally added excess catalyst to a brewing batch of plastic called polyacetylene. The result was a silvery film. Shirakawa told MacDiarmid of his discovery during a coffee break at a seminar in Tokyo in 1975. MacDiarmid was instantly interested, as he and Heeger--who was also at the University of Pennsylvania at the time--had been experimenting with metallic-looking films from polymers made from inorganic building blocks.
MacDiarmid invited Shirakawa to visit the University of Pennsylvania, and the researchers quickly set about modifying and testing the polymers. In one case, they used an iodine vapor to oxidize the film, a treatment they knew could change the optical properties of the film. Yet when they measured the result, they found another change: The conductivity had increased 10 million times. How had that happened? The polyacetylene the researchers were studying is a chainlike molecule with alternating double and single bonds. They discovered that when excess charges are added to the molecule--as happens during oxidation--these charges can then hop along the alternating bonds with relative ease.
This conductive structure is also at the heart of similar polymers that have come to dominate commercial applications of the field, notably silicon and other semiconductors. And that makes it "rather controversial" that the Nobel committee chose to overlook discoverers of these materials, says Stephen Forrest, a materials scientist at Princeton University. Still, he and most others call the selection appropriate. "Now the research has more emphasis on semiconducting polymers," says Zhenan Bao, a chemist at Bell Laboratories, the research arm of Lucent Technologies in Murray Hill, New Jersey. "But it's all based on their early concepts."