Conductive polymer pioneers receive Nobel Prize

Until about 25 years ago, students were routinely taught that plastics do not conduct electricity.

This year's Nobel Prize winners for chemistry -- Alan J. Heeger (University of California at Santa Barbara and a founder of Uniax Corp.), Alan G MacDiarmid (University of Pennsylvania), and Hideki Shirakawa (University of Tsukuba) -- stood that axiom on its head with the discovery and development of conductive polymers.

Today, conductive and semiconductive polymers have entered the mainstream for uses ranging from cell phone displays to static dissipation for photographic films, electromagnetic radiation shields in computer monitors, and smart, self-darkening windows.

Serendipity
The conductive polymer story begins in the early 1970s, when Shirakawa began synthesizing polyacetylene to control its proportion of cis- and trans-isomers. A student mistake -- adding 1000 times too much catalyst -- transformed the black polymer into a silvery film.

Shirakawa identified the silvery film as trans-polyacetylene, and found the same catalytic reaction at another temperature could produce copper-colored cis-polyacetylene. The use of temperature and catalyst concentration to control structure would prove critical in the future.

At the same time, US chemistry MacDiarmid and physicist Heeger were experimenting with metallic-looking sulfur nitride films. MacDiarmid referred to the work at a seminar in Tokyo, where he invited Shirakawa to his laboratory at University of Pennsylvania to look at the silvery film.

In the US, the researchers hit upon oxidizing polyacetylene with iodine vapor. Shirakawa knew oxidation changed the polymer's optical properties. MacDiarmid suggested that Heeger assess the films.

One of Heeger's students found the iodine-doped trans-polyacetylene had 10 million times more electrical conductivity than conventional polyacetylene. The researchers published the first paper describing a conductive polymer in the summer of 1977.

As researchers later found, polyacetylene conducts electricity differently than metals. The molecule consists of long chains of alternating single and double bonds, called conjugated double bonds.

Doping the molecule to insert electrons and create holes (electron vacancies) enables electrons to flow over those bonds. Packing the molecules into ordered rows makes it easier for electrons to move along the molecular chain. (For a longer, more technical description of the science behind conductive polymers, click here).

Applications
Conductive polymers have many potential uses. They are electroluminescent, meaning a thin layer of polymer will emit light when excited by an electrical field. Dow Chemical Co. has already commercialized polyfluorene copolymers for use in green polymeric light-emitting diodes (pLEDs). It expects to introduce red and blue pLEDs during the first half of 2001.

Polymeric LEDs generally consist of a semiconductive polymer sandwiched between a metal foil electrode on one side and a thin film electrode that allows to pass through it on the other. Potential uses include flat television screens and luminous traffic and information signs.

Since pLEDs could be extruded, visionaries imagine such science-fiction stables as full-wall televisions and light-emitting wallpaper.

Other existing and potential conducting polymer applications include:

• Polythiophene derivatives are now used in antistatic treatment of photographic film. They could also be used for remote supermarket checkout.
• Doped polyaniline controls static buildup on carpets used in offices and operating theatres, as well as electromagnetic radiation from computer screens. In coatings, it acts as a corrosion inhibitor.
• Polyphenylenevinylene could be used in mobile phone displays.
• Polydialkylfluorenes are being tested in color screens for video and TV.

One reason for industry excitement is that it is significantly easier to produce LEDs and other electronic products by chemically manipulating a polymer than by painstaking deposition of highly perfect micron-scale thin films.

Polymer-based LEDs and electronic semiconductor circuitry promise to reduce the cost of many consumer products.

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