LIGHT-EMITTING PLASTICS OFFER A RAINBOW OF POSSIBILITIES

COLUMBUS -- Imagine changing the color of a wall in a home, without having to paint or strip wallpaper. Research done at Ohio State University could make this interior decorating dream a reality by using light-emitting polymers.

These plastics -- which can be made in layers that are less than the thickness of a strand of human hair -- are coated with a layer of electrodes. The voltage from the electrodes cause the polymers to emit different colors of light, depending on how much voltage is passed over the plastic. One application could be a paint-like coating for walls that can change colors at the flip of a switch.

Light-emitting polymers (LEPs) were discovered at Cambridge University in England in 1990. But the polymers studied there were very unstable when exposed to oxygen and required a high-level of voltage to function. A team of researchers at Ohio State has discovered new LEPs that do not degrade in the presence of oxygen and can operate on the amount of electricity produced

by two AA batteries.

Other applications for LEPs may include television screens so thin they can be rolled up and stored in a living room corner, brilliant signs, and video displays small enough for use on a cellular phone or large enough to list incoming flights at a busy airport, said Arthur Epstein, director of the Center for Materials Research and professor of physics and chemistry at Ohio State.

Epstein collaborated on the work with Yunzhang Wang and James Blatchford, former doctoral students at Ohio State. The findings were presented March 18 and 21 in St. Louis at the annual meeting of the American Physical Society.

Poly(pyridyl vinylenes), the LEPs the researchers have studied, are made up of conjugated bonds that allow light to be emitted in every color of the rainbow. The polymers are different from basic LEPs, which have a benzene-like ring containing six carbon atoms, alternating with a pair of carbon atoms. When mixed with oxygen, pockets are formed within the polymer where light cannot be emitted.

Working with chemistry professors Timothy Swagger and Alan MacDiarmid of the University of Pennsylvania, Epstein's team substituted one of the carbon atoms of some of the benzene-like rings with nitrogen atoms, which do not react with oxygen.

"A few tenths of a percent of oxygen are very effective at preventing light emission from the entire sample," Epstein said. "But we do not have this problem when we use nitrogen in the polymer."

The researchers also found a way to lower the amount of voltage needed to make the polymers emit light. By adding a layer of an electrical conducting polymer to the polymers that emit light, the scientists were able to reduce the voltage needed to operate them.

"This is important so the polymers can be used for displays in portable devices, such as laptop computers and cellular telephones," Epstein said.

LEPs can be produced cheaply and easily, maybe even more so than the semi-conductors used in present-day light emitting diodes (LEDs), Epstein said. However, more work needs to be done to make the LEPs as reliable as they need to be for devices that require a high-level of accuracy in light-emitters, such as high-resolution computer displays that require thousands of very small pixels.

"Initially, we expect markets that don't require the same reliability as a computer display -- such as in large displays used in airports or supermarkets -- to be interested in these devices," Epstein said. "But as research develops in this area, we believe more markets will be interested in these types of light-emitting devices."

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Contact: Arthur Epstein, (614) 292-1133

Written by Kelli Whitlock, (614) 292-9475