Strong, stretchy, self-healing plastic developed

An innovative strong and stretchy plastic, which can be healed with heat, remembers its shape and is partially biodegradable, has been developed by researchers at The University of Tokyo. Named VPR and described in the journal ACS Materials Letters, the material can hold its form and has strong internal chemical bonds at low temperatures. At higher temperatures (above 150°C), these bonds recombine and the material can be reformed into different shapes.

The Tokyo researchers set out to create a sustainable alternative to conventional plastic, based on the epoxy resin vitrimer. Vitrimers are a relatively new class of plastics, which are solid and strong at lower temperatures (like thermoset plastics, used to make heat-resistant tableware), but which can also be reshaped multiple times at higher temperatures (like thermoplastics, used for plastic bottles). However, they are typically brittle and cannot be stretched far before breaking. By adding a molecule called polyrotaxane, the team was able to create an improved version, which they named VPR (vitrimer incorporated with polyrotaxane).

“VPR is over five times as resistant to breaking as a typical epoxy resin vitrimer,” said Professor Shota Ando. “It also repairs itself 15 times as fast, can recover its original memorised shape twice as fast, and can be chemically recycled 10 times as fast as the typical vitrimer. It even biodegrades safely in a marine environment, which is new for this material.”

Polyrotaxane has been gaining interest in science and industry for its ability to enhance the toughness of different materials. In this study, the improved toughness of VPR meant that more complex shapes could be created and retained even at low temperatures. Disposal or recycling was also easier than for vitrimers without polyrotaxane.

“Although this resin is insoluble in various solvents at room temperature, it can be easily broken down to the raw material level when immersed in a specific solvent and heated,” Ando said. “It also showed 25% biodegradation after exposure to seawater for 30 days. By comparison, vitrimer without PR did not undergo any apparent biodegradation.”

From engineering to fashion, robotics to medicine, the team foresees both practical and playful applications for VPR. “Just to give some examples,” Ando said, “infrastructure materials for roads and bridges are often composed of epoxy resins mixed with compounds such as concrete and carbon. By using VPR, these would be easier to maintain as they would be stronger and healable using heat.

“Unlike conventional epoxy resins, this new material is hard but stretchable, so it could also be expected to strongly bond materials of different hardness and elongation, such as is needed for vehicle manufacture. Also, as it has shape memory, shape editing and shape recovery capabilities, you might also someday be able to rearrange the silhouette of your favourite clothes at home with a hair dryer or steam iron.”

The team’s next step will be to work with companies to determine the feasibility of its various ideas for VPR, as well as continuing its research in the lab.

Image caption: The complex shape of an origami crane was restored using heat after being flattened. Image ©2023, Shota Ando.