Tough, self-healing hybrid rubber developed

Car tyres that automatically repair after being punctured may soon become a reality, thanks to a tough, self-healing hybrid rubber developed by scientists.
Researchers from the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS) in the US created self-healing hydrogels, which 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, researchers said.
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.

"Previous research used reversible hydrogen bonds to connect polymers to form a rubber but reversible bonds are intrinsically weaker than covalent bonds," said Li-Heng Cai, a postdoctoral fellow at SEAS.

"This raised the question, can we make something tough which can still self-heal?" said Cai.
Cai, along with Jinrong Wu, a visiting professor from Sichuan University, China, and colleagues developed a hybrid rubber with both covalent and reversible bonds.
"These two types of bonds are intrinsically immiscible, like oil and water," said Cai.
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 localised 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, researchers said.
"Imagine that we could use this material as one of the components to make a rubber tyre," said Wu.

"If you have a cut through the tyre, this tire would not have to be replaced right away. Instead, it would self-heal while driving enough to give you leeway to avoid dramatic damage," Wu said.