Smart materials don’t have brains, but they do have memories. These plastics and textiles can “remember” their shapes, metamorphosing under changing physical conditions and then reverting to their earlier forms.
Now, research reveals that a material commonly used in fuel cells and other devices can remember three previous shapes. The work, described in the March 11 Nature, is the first discovery of such shape-shifting skills in a widely available polymer, and suggests the material could be exploited for use in gadgets or smart fabrics.
“So far, materials have been tailor-made for these uses,” comments Andreas Lendlein, director of the polymer research institute at the GKSS Research Center in Teltow, Germany. “It is surprising that this commercially available material shows such impressive shape-memory properties.”
The new research unmasks the versatility of Nafion, a synthetic polymer made by DuPont that is commonly used in fuel cells. Through cycles of heating and cooling, Nafion remembered three previously held shapes, reports Tao Xie of General Motors’s Chemical Sciences and Materials Systems Laboratory in Warren, Mich. This versatility is due to Nafion’s exceptionally broad transition phase, the window of physical conditions within which a material transitions between states such as solid and liquid.
Typically, memory-shape polymers are made with ingredients that have much smaller transition windows. For example, combining three sets of ingredients, each with a transition window at a different temperature, can lead to a material that can remember three different shapes. The new work suggests that scientists might be able to harness other materials, such as Nafion, that have one broad transition window.
Xie began by heating a small strip of the polymer to 140° Celsius. As the material cooled he deformed it, fixing it in a particular position. After further cooling, he fixed the material in another configuration, and after more cooling, he set a third position. Then Xie reheated the material. As the Nafion reached the temperatures of the previous fixed contortions it “remembered,” reverting to each shape. “We’ve shown with this material that more shapes are possible,” Xie says.
Xie hopes the research will prompt scientists to investigate similar materials and lead to new uses for Nafion. He speculates that the material’s memory capabilities might be useful for automated devices, such as an antenna on a space mission that needs to unfold under particular conditions.