Self-healing materials aren’t new — researchers at SEAS, Harvard, have developed self-healing hydrogels, which rely on water to incorporate reversible bonds that can promote healing. However, engineering self-healing properties in dry materials — such as rubber — has proven more challenging. That is because rubber is made of polymers often connected by permanent, covalent bonds. 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. 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.
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 homogeneously on a molecular scale. In doing so, they were able to create a transparent, tough, self-healing rubber.
Typical rubber tends to crack at certain stress point when force is applied. When stretched, hybrid rubber develops so-called crazes throughout the material, a feature similar to cracks but connected by fibrous strands. These crazes redistribute the stress, so there is no localized point of stress that can cause catastrophic failure. When the stress is released, the material snaps back to its original form and the crazes heal. For materials science, it is not fully understood why this hybrid rubber exhibits crazes when stretched. 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. Imagine that it could be used as one of the components to make a rubber tire. If you have a cut through the tire, this tire wouldn’t have to be replaced right away. Instead, it would self-heal while driving enough to give you leeway to avoid dramatic damage!