Shape-memory polymers flip between their normal state, where molecules are flexible and disordered, and their deformed state, where the molecules bind into chains after being stretched. Once in the stretched, deformed state, the polymer can be unstretched –resuming its ‘normal’ state – by applying heat or light.
However, traditional shape-memory polymers do not store significant amounts of energy while being stretched – meaning they don’t release much energy while unstretching, which limits their use in tasks that involve lifting or moving objects.
Zhenan Bao at Stanford University and her colleagues have now produced a shape-memory polymer that does store and release appreciable amounts of energy. Their polymer has a poly(propylene glycol) backbone to which they added 4-,4’-methylene bisphenylurea units.
When stretched, the polymer chains become physically aligned, and intra- and intermolecular bonds – called hydrogen bonds – form between the chains. These help the material maintain its highly stretched form. Then, when heated to 70 °C, the hydrogen bonds break and the material can revert to its original unstretched form, releasing energy from the bonds in the process.
Those longer fibres allow the polymer to be stretched to five times its initial length, and to store up to 17.9 Joules of energy per gram in its stretched form – six times most other shape memory polymers. “It could be used to lift weight, or to help to assist or provide additional push to someone who maybe is having difficulty walking,” says Bao.
To demonstrate its potential uses, the team turned the material into an artificial muscle between the upper and lower arm of a wooden mannequin. When the mannequin’s elbow joint was stretched, the polymer stretched too. Then, by applying heat to the polymer, it contracted again, causing the mannequin’s elbow joint to bend.
“The reliable characteristics, and high strength of the material offer a new method for designing joints that are capable of lifting multiple times the material weight,” says Jonathan Aitken at the University of Sheffield, UK. However, Aitken questions whether the temperatures the polymer needs to be heated to make it practical to use.
Bao hopes to next investigate the polymer’s properties to see if there is a molecular mechanism that can automatically elongate and shrink when changing temperature naturally.
Journal reference: ACS Central Science, DOI: 10.1021/acscentsci.1c00829
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