You’ve heard of self-healing concrete before, but what about rubber that repairs itself? It now exists, thanks to researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). In a new study published in Advanced Materials, the research team reveals how they developed a hybrid rubber with both covalent and reversible bonds that is capable of repairing itself.
While self-healing materials aren’t new, this is the first time engineers have created a self-healing rubber. The task was difficult, as rubber is made of polymers often connected by permanent, covalent bonds. Because the bonds are strong, they never reconnect once broken. The researchers overcame this by making the bonds connecting the polymers reversible, so the material could break and reform.
To mix covalent and reversible bonds, the researchers developed a molecular rope (called randomly branched polymers) which tied the two types of bonds together. This rope allowed two previously unmixable bonds (“like oil and water,” according to Li-Heng Cai, a corresponding author) to be mixed homogeneously on a molecular scale. It was this step that produced the self-healing rubber.
Unlike typical rubber, the self-healing variety redistributes stress so there is no localized point of trauma that results in cracking. When the stress is released, the material “snaps back” to its original form and the cracks repair themselves. Harvard’s Office of Technology Development has already filed a patent for the technology and is seeking commercialization opportunities. This means that in the very near future, objects that utilize rubber are likely to become more durable.
Cai, a postdoctoral fellow at SEAS, Jinrong Wu, a visiting professor from Sichuan University, China, and author David A. Weitz, Mallinckrodt Professor of Physics and Applied Physics, developed the hybrid rubber as a team. Their research was supported by the National Science Foundation, Harvard Materials Research Science and Engineering Center (MRSEC) and the National Institute of Health/National Heart, Lung and Blood Institute.
“There is still a lot more to do. For materials science, it is not fully understood why this hybrid rubber exhibits crazes when stretched,” Weitz said. “For engineering, the applications of the hybrid rubber that take advantage of its exceptional combination of optical transparency, toughness, and self-healing ability remain to be explored. Moreover, the concept of using molecular design to mix covalent and reversible bonds to create a homogenous hybrid elastomer is quite general and should enable development of tough, self-healing polymers of practical usage.”