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The memory of a material

Nafion is a useful material that has been around since the 1960s, but don’t be surprised if you’ve never heard of it. It was first made by a chemist at DuPont, a company that makes chemicals, and it is a common ingredient in fuel cells. (Fuel cells, which are sometimes used to power satellites, produce energy from hydrogen.) Now, a scientist in Michigan has shown that Nafion has another nifty purpose: It can “remember” three different shapes. If you were to twist some Nafion into, say, a donut shape, it would be able to form into a donut again later.Don’t go quizzing your nearest Nafion just yet. Its memory isn’t of the usual kind: Nafion’s memory is based on temperature. Nafion is a synthetic polymer, which means it’s a manmade material of thousands of molecules linked together like a chain. Polymers come in many shapes and sizes — in fact, Silly Putty is a familiar polymer.The scientist behind this Nafion experiment is Tao Xie, who works at General Motors’s Chemical Sciences and Materials Systems Laboratory in Warren, Mich. In order to understand what Xie did, it might be easy to think of Nafion as a kind of high-tech Silly Putty. First, Xie heated a strip of the material up to 140° Celsius (284° Fahrenheit). Next, he let the Nafion cool for a bit, and then twisted it into a shape. Then, he did it again: He let the Nafion cool, and then made a shape. All in all, he made three different shapes out of the polymer as it cooled down. Then came the fun part: As he heated the Nafion back up, it changed shape on its own — again and again, as the temperature rose. And by the time it got all the way back to the highest temperature, the Nafion had changed into all three of the shapes Xie had put it in. And the material took each shape at a certain temperature, the same temperature it had the first time, when Xie had given it each shape. “We’ve shown with this material that more shapes are possible,” Xie says. Nafion is an example of a smart material. Smart materials are special because they have properties (such as shape) that change in response to changes in temperature, pressure or other external factors. (Smart materials called piezoelectrics, for example, generate a bit of electricity when pressure is applied — as a result, they’re often used in electric starters, such as those in outdoor grills.) Nafion isn’t the first material that can remember its shape, but Xie’s study is the first time this shape memory has been observed in a polymer that already exists and is used for other purposes. Usually, “materials have been tailor-made for these uses,” Andreas Lendlein told Science News. Lendlein is the director of the polymer research institute at the GKSS Research Center in Teltow, Germany. Xie says he hopes his work inspires other scientists to find new uses for Nafion.

The memory of a material
The memory of a material

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