Saturday science subject: Self-healing rubber

— 10:25 AM on February 23, 2008

One problem with synthetic objects is that they generally don't fix themselves. If there's a hole in your bike tire, there's really no chance of it spontaneously healing and letting you ride off without having to take it apart. However, a new discovery made by Ludwik Leibler from the Industrial Physics and Chemistry Higher Educational Institution in Paris could change all that. As Nature reports, Leiber managed to create a form of rubber that can "heal" itself after being snapped in two.

Leibler's approach was to use small molecular groups instead [of a single, stretchy molecule like normal rubber]: the fatty acids from vegetable oil. Reacting these molecules with urea in a two-step process stuck nitrogen-containing chemical groups (amides and imidazolidones) onto the ends of the fatty acids. The fatty acids link to each other using hydrogen bonds — a strong attractive force between hydrogen and another atom, and the bond responsible for holding water molecules to each other.

The resulting molecular system is very non-uniform: some acids have three protruding groups and some have two. This means that the compound can't crystallize into a hard, shatterable material. Instead it can be stretched to five times its original size and then return to normal, albeit slower than an elastic band would — it takes around a minute.

If rubber is cut, the end groups on the acids become exposed, and the hydrogen bonds to neighbouring groups are broken. It is in the amide group's nature to seek out a partner to link up to, and this happens when the cut surfaces are brought back in contact — the hydrogen bonds can form again. The longer the cut ends are held together, the more of these partnerships are made, and the more completely healed the rubber is.

According to Nature, a freshly cut piece of Leibler's rubber that's been held together for 15 minutes is strong enough to be stretched to twice its size without breaking. What's more, the rubber doesn't have to be mended immediately after being broken—Nature says you can wait up to 18 hours to put the pieces back together. Leibler plans to commercialize the material through French chemical company Arkema, and he expects children's toys would be a good application for it.

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