Issue 18/2021 - High energy density shape memory polymers are of high quality and low price

Stretched or deformed shape memory polymers that return to their original shape when heated or illuminated have shown great potential
in flexible robotics, intelligent biomedical devices, and deployable spatial structures.
But at present, the energy they store is not enough to meet the demand
.

Researchers have now developed a shape-memory polymer that stores almost six times more
energy than previous versions, as reported Sept.
8 in the journal American Chemical Society Center for Science.

Shape memory polymers can alternate
between the original undeformed and secondary deformed states.
The deformation state is formed by stretching the polymer and held in place by molecular changes, such as dynamic bonding networks or strain-induced crystallization, which can be reversed by heat or light, and the polymer can be restored to its original state
by releasing stored entropy.
But this requires polymers to store large amounts of energy
.
Instead, the researchers wanted to develop a new type of shape memory polymer that could be stretched into a stable, highly elongated state, allowing it to release a lot of energy
when it returned to its original state.

The research team led by scientists at Stanford University in the United States combined the 4,4'-methylenebis(phenylurea) unit into the
propylene glycol polymer backbone.
In its original state, the polymer chains are tangled and
disordered.
Stretching causes strands to align and form hydrogen bonds between urea groups, resulting in a stable supramolecular structure
in a highly elongated state.
Heating causes the bond to break and the polymer to shrink to its initial disordered state
.

In tests, the polymer stretched to 5 times its original length and stored up to 17.
9 joules/g of energy – almost 6 times
the previous shape memory polymer.
The team demonstrated that the stretched material can use this energy to lift objects
up to 5,000 times their own weight when heated.

The researchers also attached the pre-stretched polymer to the upper and lower arms of wooden mannequins to use as artificial muscles
.
When heated, the material shrinks causing the mannequin to bend its arms
at the elbows.
In addition to the record-high energy density, shape memory polymers are cheap, costing about $11 per pound in raw materials, and are easy to manufacture
, the researchers said.