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.”