CARBON nanotubes have been bonded to the molecular backbone of a polymer for the first time. The feat could one day lead to a new generation of tough, lightweight composites for use in cars and aircraft.
Because carbon nanotubes are 30 times stronger than steel yet five times less dense, chemists have been keen to use them to reinforce plastics. But simply mixing them in melted plastic does not work, since no bonds form between the tubes and the long chains that make up a polymer. 鈥淚t means a force on the composite material is not transferred to the nanotubes, so they don鈥檛 provide any reinforcement,鈥� says Yurii Gun鈥檏o, a chemist at Trinity College Dublin in Ireland.
Now Gun鈥檏o and his team have worked out a way to chemically bond nanotubes to a polymer. Their trick is to use the imperfections in nanotubes as bonding points. Nanotubes, which comprise rolled-up sheets of carbon atoms arranged in a chicken-wire pattern, contain imperfections where their regular geometry is interrupted, for example where a heptagon-pentagon pair forms instead of the regular hexagon-hexagon pairing. These areas are highly reactive, Gun鈥橩o says.
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The team discovered that by mixing nanotubes suspended in an inert fluid with n-butyl lithium, the butyl carbon chain bonded strongly to the nanotube鈥檚 imperfections. The other end of the butyl group was able to attach the backbone of the polymer chlorinated polypropylene (see Graphic).
Gun鈥檏o鈥檚 method showed a fourfold increase in strength over pure chlorinated polypropylene (Journal of the American Chemical Society, vol 126, p 10226). The team has now applied this method to materials including polystyrene, soluble polypropylenes, PVC and polyamides, achieving similar strength improvements and demonstrating how widely the concept could be exploited.
Better still, the Dublin team think the technology will eventually perform much better, strength-wise. The reason? They achieved their fourfold strength boost using just 0.6 per cent nanotubes by volume. Gun鈥檏o says if the nanotube content can be increased to 10 per cent of the volume, strength could be boosted by a factor of up to 20.
鈥淭he cost of nanotubes is going down all the time,鈥� Gun鈥檏o says. 鈥淭wo years ago, high-purity nanotubes cost about 鈧�300 per gram. Today they cost 鈧�30.鈥� He estimates that such super-strengthened polymers of the future need only cost about 25 to 30 per cent more than the pure polymer.
