21 October 2005
21 October 2005
Researchers from the Paul Pascal Research Centre and University of South Paris, France, have used hot-drawing to improve the mechanical properties of fibres made from carbon nanotubes.
The process increased the fibre's toughness at low strains and made them more water resistant.
""By using hot stretching treatments, we could obtain nanotube fibres with a toughness greater than that of Kevlar - 60 J/g versus 35 J/g - and with a strain-to-failure of only 10%,"" Philippe Poulin told nanotechweb.org. ""Even though the toughness of these fibres is not as high as the toughness of untreated fibres, hot-stretched fibres are much closer to meeting the requirements of several applications.""
The researchers made their fibres by wet-spinning surfactant-dispersed carbon nanotubes from a solution of aqueous polyvinyl alcohol (PVA) at a speed of 6 m/min. The resultant fibres contained equal fractions of PVA and nanotubes by weight. After drying the fibres, the researchers ""hot-drew"" them to an elongation of 850% at 180 °C. The team prepared fibres from both single-walled and multi-walled carbon nanotubes.
The hot-drawing process improved the alignment of the nanotubes and PVA chains with the axis of the fibre, as well as increasing the PVA's crystallinity. The scientists say that this enhanced crystallinity boosts stress transfer between the nanotubes and PVA, and prevents the PVA from swelling in water.
After hot-drawing, the nanotube fibres had tensile strengths of 1.4 - 1.8 GPa, failure strains of 6 - 12% and a toughness of 40 - 60 J/g. Fibres made from single-walled nanotubes were generally slightly stronger and stiffer than fibres made from multiwalled nanotubes.
The fibres could have applications in bullet-proof vests, helmets, safety gloves and clothes, and composites - anywhere needing materials that can resist impacts and absorb energy.
""We have demonstrated promising properties on the laboratory bench and we would now like to validate applications,"" said Poulin. ""This will require the production of much more fibre so that we can weave textiles or make cables. We are convinced that the simplicity of our method is a major feature in achieving a future scale-up.""
Before hot-drawing, many of the fibres had failure strains that were more than 250%. The best-performing fibre contained single-walled carbon nanotubes and had a failure strain of 430%, more than four times higher than previous results. The fibre had a toughness of around 870 J/g, which the researcher believes is the highest toughness reported for any material.
The best-performing multiwalled nanotube fibre had a failure strain of 340% and a toughness of 690 J/g. The researchers say they are the first to show that supertough fibres can be spun from multiwalled nanotubes.
""The possibility of using multiwalled nanotubes offers more opportunities to use different kinds of materials,"" said Poulin. ""These 'super tough' fibres are interesting from a conceptual point of view but they are not so useful for applications. This is because the super-toughness is associated with an extremely large strain-to-failure. It is nice to stop a bullet, but the bullet has to be stopped not too far [along its trajectory] ... this is why fibres with lower strain-to-failure and greater toughness are required.""
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