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Chargeable polymer shows helical structure.

Rechargeable batteries, programmable billboards, windows that adjust how much sunlight beams into a room: To do their respective jobs, certain versions of these devices need specially tailored plastics that can hold a charge.

For many years, scientists have experimented with such charge-holding thin films, called polymer electrolytes, in their quest for better batteries, among other things. But the search for better charge-storing solids -- an effort, basically, to do away with the corrosive juices that top off most wet-cell batteries -- has been hamstrung by inadequate knowledge about the physical configurations of certain polymers.

Now researchers can get a clearer picture. Philip Lightfoot, a chemist at the University of St. Andrews in Fife, Scotland, and two colleagues report in the Nov. 5 SCIENCE their elucidation of the structure of a common polymer electrolyte -- [poly(ethylene oxide).sub.3][:LiCF.sub.3][SO.sub.3], which essentially a salt dissolved in a simple plastic.

"These materials have been around for 20 years, and yet we've known very little about their structure," says Peter G. Bruce, a coauthor and chemist at St. Andrews. "This missing data has held back polymer electrolyte research."

"For molecular biology to take off, people had to know the crystal structure of DNA," Bruce adds. "This is very similar. Now that we have a picture of this polymer electrolyte's crystal structure, we should be able to design better materials with higher conductivity and direct technological applications."

This material's structure remained unknown for so long because the standard technique for discerning it -- a single-crystal method -- just didn't work, the researchers state. So they tried another route. Using power X-ray diffraction, where the sample is crushed up, they found a network of molecular coils, or helixes, with lithium ions bound inside the turns of the coils.

"Seeing how ions fit inside the polymer tells us how to design better electrolytic polymers," Bruce says. The polymers fare best as conductors when they become amorphous, he adds -- that is, capable of slow deformation, like glass.

"The motion of the polymer chains helps ions move through the material. So macroscopically we want the polymer to look solid, while microscopically it's really a slow-moving liquid. If made properly, at higher temperatures it gets sticky and0stretchy -- which is part of what makes it a good conductor."

Better batteries, visual displays, and "smart" windows hinge on solid polymer electrolytes that conduct charge more quickly. "All three of these items are electrochemical cells," says Bruce. "Each has a solid polymer pressed between two electrodes. The goal is to be able to fabricate these kinds of thin films easily and cheaply, just rolling them out a few microns thick in automated production."
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Title Annotation:polymer electrolytes
Publication:Science News
Article Type:Brief Article
Date:Nov 6, 1993
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