Vitrimers are a class of polymers that can change their topology by thermally activated bond-exchange reactions. They are solid and strong at lower temperatures, like thermoset plastics, but can also be reshaped multiple times at higher temperatures, like thermoplastics. However, vitrimers are typically brittle and cannot be stretched far before breaking.
Now, scientists at the University of Tokyo in Japan have improved the properties of vitrimers by adding a molecule called polyrotaxane (PR) in 10% concentration. The epoxy resin vitrimer incorporated with PR (VPR) exhibited enhanced toughness, with an elongation at break 5.3 times greater than that of a vitrimer without PR, it showed self-healing properties, was chemically recycled 10 times faster, and recovered its original memorised shape twice as fast. Moreover, the PR-enriched material showed 25% biodegradation following exposure to seawater for 30 days, with the polyrotaxane breaking down into a food source for marine life. In contrast, the original vitrimer was not biodegradable at all.
“Rapid decomposition was observed during the first week of seawater biodegradability testing, after which it progressed more slowly,” the researchers wrote. “This finding is likely due to PR attracting decomposing bacteria, as it is composed of…marine-degradable components. The bacteria gathered and formed a biofilm on the VPR surface, thereby promoting gradual biodegradation, which commences with the degradation of PR,” they explained.
The material can hold its form and has strong internal chemical bonds at low temperatures. However, at temperatures above 150 C, those bonds recombine, and the material can be reformed into different shapes. Due to its improved toughness, VPR can be used to create and retain more complex shapes, as the University of Tokyo demonstrated by making a video of an origami crane being restored to its shape after being flattened.
It is also easier to dispose of or recycle VPR, which breaks down into its raw components when heated and dissolved in an acetone solvent. “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,” said Project Assistant Professor Shota Ando from the Graduate School of Frontier Sciences.
The team foresees many applications for the versatile material, from engineering to fashion, robotics to medicine. “Just to give some examples, infrastructure materials for roads and bridges are often composed of epoxy resins mixed with compounds such as concrete and carbon,” said Ando. “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. “I have always thought that existing plastics are very difficult to recover and dispose of because they are subdivided according to their uses,” said Ando. “It would be ideal if we could solve many of the world's problems with a single material like this.”
The scientists shared their findings in “Environmentally Friendly Sustainable Thermoset Vitrimer-Containing Polyrotaxane,” recently published in ACS Materials Letters.