Proc. Natl Acad. Sci. USA 110, 12555–12559 (2013)

Shape-memory materials can be deformed by an external stimulus and then returned to their original shape under certain conditions. Typically, a material is heated above a transition temperature — where it can be deformed through physical strain — and when cooled down the material adopts a different shape. Heating the material above the transition temperature again relaxes the strain and the material returns to its original shape. A better scenario, however, would be where a material can 'remember' two forms — a high-temperature and a low-temperature one — and reversibly alternate between them under repeated heating/cooling cycles.

A German-based research team led by Andreas Lendlein has now devised polymeric actuators that can first be bent into a chosen shape and then repeatedly switched between an expanded and a contracted form through heating and cooling cycles — with a tunable temperature range. Lendlein and co-workers had previously prepared reversible shape-memory materials that can alternate between two shapes, by combining in a copolymer network two polymeric segments that crystallize at different temperatures. One polymer acts as a 'skeleton': it is brought to an amorphous state on heating and when cooled down under physical strain it crystallizes and imposes a particular shape on the material. This skeleton is retained throughout the temperature range at which the other polymer — responsible for actuation — either crystallizes (on cooling) or loses crystallinity (on heating). The reversible crystallization and decrystallization of this 'actuation domain' causes macroscopic changes in the material, which alternates between two morphologies.

In the current study, the skeleton and the actuation domain have the same chemical composition — a covalently crosslinked copolymer poly[ethylene-co-(vinyl acetate)]. The network crystallizes over a broad temperature range (25–90 °C), so that part of the material serves as the skeleton and the remaining part acts as the actuation domain. Repeated cooling and heating (below 90 °C) causes the chosen shape to contract and expand. Heating the copolymer network over 90 °C erases the shape memory and the same process can be carried out with a new shape imposed through cooling under strain.