Shape-memory plastics

In this model of a window blind, temperature-memory polymer actuators are used to open and close the slats over a temperature range, which can be programmed.

IN a breakthrough research, biomaterial scientists in Teltow, Germany, have developed plastics that can repeatedly change from one shape to another and then back again when temperatures are changed within a selected range. The material, which the inventors have dubbed “polymer actuators”, thus overcomes a major limitation that affects similar materials. Up until now, polymers with temperature-controlled shape memory could only change form once. The new material, the researchers say, will open up a wide variety of applications—from automatic blinds that function without electricity to new kinds of heat engines. This finding from the Institute of Biomaterial Science in Teltow, which is a part of the Helmholtz-Zentrum Geesthacht, has been reported in the latest online edition of Proceedings of the National Academy of Sciences.

“Once the ambient temperature goes above and then below certain threshold values, our actuators can change shape many hundreds of times,” says Andreas Lendlein, director of the institute.

According to him, plastics provide a relatively large amount of freedom for choosing the threshold values and the kind of shape change.

Tilman Sauter, a doctoral student at the institute, gives an everyday example of its potential applications: “The material could be used to make sunblinds that need no external electricity supply and are able to darken a room purely on the basis of how warm they are.”

Another application envisaged is a heat engine, where the actuators would form the basis of a drive element.

To demonstrate how the engine would work, researchers at the institute use an experiment that involves heating a plastic, which then opens up and moves a drive unit. When the temperature drops, the plastic returns to its original state. The temperature-memory effect makes it possible to control the rotational speed of the drive unit.

At a molecular level, the active polymers are built of structural elements whose mobility undergoes changes within a very wide temperature range. To translate this activity at the nano level to the macroscopic level, some of these structural elements are assigned to an internal scaffold that determines the geometry of the motion and orients the motion.

“We can vary the proportion of motion elements to shape-giving elements, and this allows us to control the movement,” explains Marc Behl of the institute who was associated with the work.

R. Ramachandran