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Plastics that Glow

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[Photo of special  indicator housings]

Three types of specially designed indicator housings. The large indicator products were designed to be replacements for older incandescent lamps, and last 50 to 100 times longer. This type of indicator is now used in many applications including mass transit, heavy equipment, and as marker lights on trucks. Source: DiaLight Corporation


Ever since the discovery of nylon in the first half of this century, plastics have steadily found more and more uses. Now they are poised to invade one of the hottest areas of electronics-- light-emitting diodes. That could create lucrative new markets for such things as computer screens, advertising displays, and even wall-sized video screens.

Intriguingly, however, these advances have not come from the electronics industry, as innovative as it is. Instead they come from fundamental university research into the properties of polymers--the long-chain organic molecules formed of repeated units that are the basis of most plastics. This long-chain structure gives polymer plastics the flexibility that makes them so valuable in dozens of applications--from stretch tights to bulletproof vests to kitchenware that doesn't shatter when dropped.

Most polymers don't conduct electricity. But in the late 1970s, researchers at the University of Pennsylvania discovered a plastic that, when "doped" with small amounts of impurities, could conduct. The technique is analogous to the doping of semiconductors that makes possible transistors. Further research led to other conducting polymers, but none found so far conduct well enough to replace metals. In exploring the basic properties of these materials, however, researchers did find something else-- plastics that emit light when an electric current passes through them.

The discovery came from investigations of luminescence--the property that makes a luminous watch dial glow after it has been exposed to light--in conducting polymers. Studies of the electronic properties of these materials suggested the possibility of triggering such glows electrically, just as in the light- emitting diodes (LEDs) that form the little red on/off lights on many electronic appliances. Not only was it possible, but scientists rapidly found plastics that would emit in virtually all colors--from red to yellow to blue. Further research has improved efficiencies, boosting the first feeble glows into a source of light potentially bright enough to power a display screen. The key remaining problem is that plastic LEDs don't last long enough for most applications, but researchers are optimistic that this problem, too, will be solved.

Light-emitting plastics have become an area of intense scientific and commercial interest. Scientific articles on the subject were among the most-cited in physics in 1993. Two startup companies have already been formed, and larger electronic companies are paying close attention.

The appeal is that light-emitting plastics are thin and flexible and can even be bent around corners--it is possible to imagine low-level lighting fixtures that would fit any location, or even clothing that would glow. Plastic LEDs could also be made in large sheets, potentially as wall-size, flat display screens--long a key commercial target in consumer electronics. Such potential applications could not have been anticipated in the initial investigations into conducting polymers, but such surprises are not an uncommon outcome of fundamental research.

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