Using commonplace monomers that are popular components of acrylic paint, chemists have created a polymer that heals itself via van der Waals interactions—the weakest of weak bonds. Because it has the ability to repair scratches and small cuts without external intervention, the material could be used to make longer-lived coatings and hardier plastic items.
The material is a co-polymer made from a mix of methyl methacrylate and n-butyl acrylate. The co-polymer’s molecular arrangement and composition are critical to achieving self-healing, says Marek Urban, a Clemson University polymer chemist who led the research project. The van der Waals forces—attractive and repulsive forces between molecules caused by temporary dipoles—heal the polymer by forcing its components to interdigitate, like fingers on clasping hands. To achieve this interaction, the monomers must both alternate positions and be present in approximately equal proportions (Science 2018, DOI: 10.1126/science.aat2975).
It takes about 14 hours for the co-polymer to fix a scratch and 80 hours to repair a cut. Urban acknowledges that healing time is long compared to some other self-healing polymers, such as those that heal instantly upon exposure to ultraviolet light. It’s a trade-off, he says, between speed and cost. Most other self-healing polymers require expensive additives whereas this one is made from cheap, readily available monomers.
Urban says his team first made the self-healing polymer about eight years ago, but it took them a long time to convince themselves that the material was healing itself with van der Waals interactions. “Those weak interactions are not easily measurable,” he says.
Compared with hydrogen bonds—which are up to 20 times stronger—van der Waals interactions are puny. But when they’re packed into a polymer and aligned in the same direction, Urban says, they cumulatively become powerful enough to make the material repair itself.
“The use of van der Waals forces, as opposed to supramolecular and dynamic covalent interactions, for self-healing polymer design is exciting because van der Waals forces are ubiquitous and more universal,” says Zhibin Guan, an expert in self-healing polymers at the University of California, Irvine, who was not involved in the research. If this approach proves applicable to other polymers that the chemical industry produces on a large scale, he adds, “it would contribute greatly to the development of more sustainable polymers for our society.”
Urban patented the co-polymer and plans to see how it holds up in real-world situations. He also wants to explore using van der Waals interactions to create self-healing polymers from other commonplace monomers, such as esters, styrenes, and urethanes. “The concept is there,” he says, “it’s a matter of tweaking the composition.”
UPDATE: This story was updated on Oct. 11, 2018, to include colored text explaining the copolymer structure and to remove redundant images.