Polymers With Properties that Respond to Light

Light-controlled polymers can switch between rigid and soft states, paving the way for new applications.

Researchers have been experimenting with how light affects certain materials to transform and change their properties. Some of the latest work is from researchers at MIT, who have designed a polymer that can change in response to light from a rigid material to one that’s softer and can self-heal.

A team in MIT’s Koch Institute for Integrative Cancer Research and the Program in Polymers and Soft Matter developed the material. It comprises polymers attached to a light-sensitive molecule that researchers used to change the bonds formed within the material, said Jeremiah Johnson, an associate professor of chemistry who led the research, in an MIT news release.

Switching States

“You can switch the material states back and forth, and in each of those states, the material acts as though it were a completely different material, even though it’s made of all the same components,” he said in the release.

Researchers anticipate the material can be used in a variety of applications, from coatings for cars and satellites to novel techniques for delivering medications to patients, they said. Typically, the topology—or how the material’s components are arranged—controls the properties of polymers. Usually, once a material is formed, no one can change its topology reversibly—that is, to the other extreme of the property, such as from elastic to brittle—without changing its chemical composition.

Johnson’s team aimed to try to develop a material that could defy this usual condition—one that could reversibly switch between two different topological states with no change in chemical make-up. To do so, they chose a type of material called polymer metal-organic cages, or polyMOCs, that they designed a few years ago.

reversible polymer

MIT chemists have designed a polymer that can reversibly switch in response to light from a large structure (orange spheres) to the smaller blue shapes. (Image source: Demin Liu/Molgraphics)

Cage-Like Structures

PolyMOCs consist of cage-like structures that contain metal and are joined together by flexible polymer linkers, researchers said. They created these materials by mixing polymers attached to groups called ligands, which can bind to a metal atom. 

Researchers used palladium as their metal, with each atom forming bonds with four ligand molecules, they said. This created rigid cage-like clusters with varying ratios of palladium to ligand molecules, which determined the size of the cages.

To get the PolyMOCs to achieve topological reversibility, researchers had to make the material reversibly switch between two different-sized cages: one with 24 atoms of palladium and 48 ligands, and one with three palladium atoms and six ligand molecules.

UV Exposure

Researchers incorporated a light-sensitive molecule called DTE into the ligand to achieve this effect. When exposed to ultraviolet (UV) light, it caused a change in the ligand that affected the bonds with palladium in a way that reverses the topology, a process that takes about five hours to complete, they said.

In experiments, the team performed the reversal up to seven times. However, eventually the material falls apart because with each reversal, a small percentage of the polymers doesn’t switch back.

When the material is in the small-cluster state, it can become up to 10 times softer and more dynamic, which is responsible for its self-healing capability, Johnson explained. “They can flow when heated up, which means you could cut them and upon mild heating, that damage will heal,” he said.

No Trade-off

This approach overcomes the trade-off that usually occurs with self-healing materials, which typically are structurally weak. In the case of the material developed by the MIT team, it can switch between the softer, self-healing state and a more rigid state. Researchers published a paper on their work in the journal Nature.

For this particular project, researchers used the polymer polyethylene glycol (PEG) to fabricate their material, but they say any kind of polymer can be used. The palladium used also would likely have to be replaced for mass production, as it’s a rare and expensive metal, Johnson said.

“Anything made from plastic or rubber, if it could be healed when it was damaged, then it wouldn’t have to be thrown away,” he said. “Maybe this approach would provide materials with longer life cycles.”

Researchers plan to continue their research, which includes working on creating materials that can reversibly switch from a solid state to a liquid state. They also aim to use light to create patterns of soft and rigid sections within the same material, Johnson added.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

 

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