Harvard researchers develop self-healing rubber
A team of engineering researchers at Harvard University has produced a new type of rubber that it says is tough like natural rubber and self-healing, creating potential for more durable tyres, wearable electronics and medical devices.
Developing self-healing properties in dry materials, such as rubber, has proven challenging because rubber is made of polymers often connected by permanent covalent bonds, according to the team. While these bonds are incredibly strong, they will never reconnect once broken. In order to make a rubber self-healable, the team needed to make the bonds connecting the polymers reversible, so that the bonds could break and reform.
Li-Heng Cai, a postdoctoral fellow at Harvard’s School of Engineering and Applied Science and corresponding author of the paper, said: “Previous research used reversible hydrogen bonds to connect polymers to form a rubber but reversible bonds are intrinsically weaker than covalent bonds. This raised the question, can we make something tough but can still self-heal?” Cai and Jinrong Wu, a visiting professor from China’s Sichuan University, and author and physics professor David Weitz developed a hybrid rubber with both covalent and reversible bonds.
The concept of mixing both covalent and reversible bonds to make a tough, self-healing rubber was proposed in theory by Cai but never shown experimentally because covalent and reversible bonds don’t like to mix. Cai said: “These two types of bonds are intrinsically immiscible, like oil and water.” So, the researchers developed a molecular rope to tie these two types of bonds together. This rope, called randomly branched polymers, allows two previously unmixable bonds to be mixed on a molecular scale.
In doing so, the team was able to create a transparent, tough and self-healing rubber. Wu added: “Imagine that we could use this material as one of the components to make a rubber tyre. If you have a cut through the tyre, this tyre wouldn’t have to be replaced right away. Instead, it would self-heal while driving enough to give you leeway to avoid dramatic damage.” Harvard’s Office of Technology Development has filed a patent application for the technology and is seeking commercialization opportunities. The self-healing ability is appealing for a wide variety of rubber products, the team said.