Teaching an Old Polymer Memory Tricks

A polymer takes on four shapes depending on the temperature.

Researchers have uncovered shape-memory properties in a commercially available polymer that’s widely used to make fuel-cell membranes. The polymer, Nafion, can take on four different shapes in response to temperature changes–researchers have made triple-shape polymers before. “It’s arguably the most versatile smart polymer ever discovered,” says Tao Xie, a polymer scientist at the GM Research and Development Center in Warren, MI, who published his findings in this week’s Nature.

Morphing magic: Researchers have trained the polymer Nafion to take on different shapes at different temperatures. Clockwise from top left: the polymer’s original shape; at 140 ºC; at 107 ºC; and at 68 ºC.

Shape-memory polymers go from a predefined shape to another in response to triggers such as temperature, light, or magnetic field. Most of these materials have one temporary shape and a permanent state they revert to when triggered. The polymers are being developed for self-adjusting orthodontic braces, self-tying sutures for laparoscopic surgery, medical devices such as blood vessel stents and bone implants, and aircraft with wings that morph during flight.

The versatility of Nafion as a multishape polymer is surprising, as it was not initially developed for this purpose, says Andreas Lendlein, a shape-memory polymer pioneer at GKSS Research Center in Teltow, Germany. Xie says his findings suggest that a broad range of polymers with similar properties might be able to change shape multiple times.

Nafion’s shape-shifting properties also indicate that it could take on more than four shapes. The polymer can be programmed to morph at arbitrary temperatures within a broad range as long as these temperatures are well separated. Xie was able to program three transitions, giving a total of four shapes. “You wonder how many they can remember,” he says.

“The more shapes you can memorize, the more complex you can make the structure and function,” says Christoph Weder, a polymer chemist at the University of Fribourg in Switzerland. It could, for one, expand current applications: aircraft wings that take on four different forms; spacecraft antenna that expand to different widths; and adjustable-width stents that could be moved from a wider blood vessel to a narrower one or that could be placed at the branch of an artery.

In 2006, Lendlein and his colleagues developed a material that can take on three shapes in response to heat changes. The key was to develop two types of polymers with distinct melting points. The material kept one shape at room temperature, shifted to a second shape when heated above one melting point, and a final shape when heated to an even higher temperature.

With Nafion, Xie says, “we don’t have to do anything fancy.” It can be programmed to hold multiple shapes because it has a broad temperature range, 55 °C to 130 °C, where it stays soft and rubbery and can be deformed. Most polymers have one such glass transition temperature; they are rubbery above it and brittle below.

Programming the polymer involves heating it to a high temperature within the glass transition range, deforming and then cooling it to a lower temperature while maintaining the deforming force. “After the [cooling] event, that shape is locked,” Xie says. Deforming the polymer and cooling it again will program additional shapes.

Xie was able to program Nafion three times. The polymer associates the three temporary shapes–long, longer, and bent–with the temperatures at which it was deformed. When it’s heated, it cycles through the temporary shapes, going from bent to straight and then shorter at those preset temperatures until it reaches its permanent shape.

Neither the material nor the concept are new, Weder says, but this is interesting work and novel since it is the first four-shape memory material. “Xie takes a piece of polymer that everyone knows and plays around with and he shows a new trick with this polymer,” he says. “In principle, I don’t see why it can’t take on more shapes.”

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