Researchers from the Massachusetts Institute of Technology and the Institute of Polymer Research at the GKSS Research Center in Germany have developed a polymer which can morph into 3 different structures, then revert to its original shape, in response to changes in temperature.
Here’s the abstract from the paper, which was published online in the Proceedings of the National Academy of Sciences earlier this week:
Shape-memory polymers represent a promising class of materials that can move from one shape to another in response to a stimulus such as heat. Thus far, these systems are dual-shape materials. Here, we report a triple-shape polymer able to change from a first shape (A) to a second shape (B) and from there to a third shape (C). Shapes B and C are recalled by subsequent temperature increases. Whereas shapes A and B are fixed by physical cross-links, shape C is defined by covalent cross-links established during network formation. The triple-shape effect is a general concept that requires the application of a two-step programming process to suitable polymers and can be realized for various polymer networks whose molecular structure allows formation of at least two separated domains providing pronounced physical cross-links. These domains can act as the switches, which are used in the two-step programming process for temporarily fixing shapes A and B. It is demonstrated that different combinations of shapes A and B for a polymer network in a given shape C can be obtained by adjusting specific parameters of the programming process. Dual-shape materials have already found various applications. However, as later discussed and illustrated by two examples, the ability to induce two shape changes that are not limited to be unidirectional rather than one could potentially offer unique opportunities, such as in medical devices or fasteners.
The prototype structure demonstrated in the film clip below is a flattened plastic tube which expands when heated, and maintains its shape after cooling. Potential applications for this structure include a stent which would expand in response to a temperature change after being inserted into a patient’s blood vessel; another temperature change would then cause the structure to collapse so that it could be removed.
The second prototype is a fastener which unfolds itself then extends two arms which secure the structure to the plastic container. This could be used on assembly lines to fasten small parts which are difficult to reach.
The structures are made of two polymers with different melting points. At room temperature, the structure retains its original shape. When heated to a specific higher temperature, parts of the material soften, causing the structure to adopt a new shape. At a yet higher transition temperature, the remaining material softens, and the structure takes on a third shape.