Self-healing hydrogels rely on water to incorporate reversible bonds that can promote healing. Engineering self-healing properties in dry materials, such as rubber, has proven more challenging because rubber is made of polymers often connected by permanent, covalent bonds. In order to make a rubber self-healing, the bonds connecting the polymers have to be reversible so that the bonds could break and reform.
A hybrid rubber was developed with both covalent and reversible bonds. The mixing of both covalent and reversible bonds makes a tough, self-healing rubber. A molecular rope was developed 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, a transparent, tough, self-healing rubber could be created.
Typical rubber tends to crack at a 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.
The self-healing ability is appealing for a wide variety of rubber products; for example, a tire that self-heals while driving after a puncture.