How hot was it?
At the heart of the sensitive polymers are photoluminescent dyes, explains Chris Weder, a polymer chemist at Case Western Reserve University in Cleveland. Individually, each fluorescent-dye molecule emits a specific visible wavelength--a color--when zapped with UV light. When two or more dye molecules are close enough together, however, they redistribute this incoming energy among themselves and emit a color different from that of any individual molecule.
To create the sensor, Weder's team started with a hot solution of a polymer and a dye. If the liquid is cooled rapidly, the dye molecules remain evenly dispersed throughout the resulting solid.
This solid functions as a temperature sensor because reheating it above a certain threshold loosens the polymer's microstructure and the dye molecules, in Weder's words, "move through the polymer and find each other." As a result, a sample that has been exposed to a temperature above the threshold will under UV light appear a color different from that of the original.
The sensor can also indicate how long an object bearing it--a milk carton, for example--had been above a threshold temperature. A short trip from a delivery truck to a refrigerator results only in small color changes. But if that milk carton had been left in the hot sun all day, the color change would be striking. This gives a much better picture of the "thermal history of the product," says Weder.
The group is working on sensors that won't require a UV light. Weder notes that, as anyone who has ever bought spoiled food knows, "there are many applications where you'd like the end customer to see the color changes."--A.C.
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|Title Annotation:||POLYMERS; research on heat-sensing polymers|
|Article Type:||Brief Article|
|Date:||Sep 17, 2005|
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