Thursday, March 1, 2012

The world's strongest fibres

1 February 2012

The toughest polymer yarn of all time has been made by mixing a polymer with sheets of reduced graphene oxide (RGOF) and carbon nanotubes (CNTs) during spinning. The yarns are much cheaper than those using CNTs as the only additive, producing fibres that can be sewn like threads and coiled into springs.

One of the most common ways to strengthen polymeric fibres is to mix them with additives like CNTs during wet spinning. The composite materials formed have superior toughness to the neat polymers. One type of fibre made using poly(vinyl alcohol) (PVA) and CNTs has a toughness of up to 870J/g, which is far stronger than spider silk (165J/g) and the synthetic para-aramid Kevlar (78J/g) used in body armour.

More recent studies have replaced CNTs with flakes of RGOF, which has a two-dimensional structure like graphene. The resultant PVA composites were not as strong as those with CNTs but were cheaper to make due to the lower cost of RGOFs.

World's toughest material
Hydrogen bonds between the graphene oxide and functionalised carbon nanotubes increase the material's strength

Now, Seon Jeong Kim, at Hanyang University, Korea, and colleagues have taken things a step further by mimicking the structure of spider silk and adding a combination of functionalised CNTs (strands) and RGOFs (sheets) to the polymer. 'In nature, spider silk is very strong and flexible, and it is composed of two types of proteins: sheet type (beta sheet nanocrystals, 2D structure) and strand type (beta strands, 1D structure),' Kim says. 'Their combination is very important for enhancing the toughness of the spider silk.'

A solution of single-wall CNTs and wrinkled RGOFs was injected into an aqueous solution of PVA as it was being spun and the fibres were treated with methanol to increase the crystallinity of the material to create robust fibres. Scanning electron microscopy showed that the RGOFs and CNTs form an interconnecting network during spinning, which is helped by the wrinkled nature of the RGOF sheets that keep them well dispersed inside the polymer.

Different proportions of CNTs and RGOFs were tried out but the best combination was a 1:1 ratio which produced a high degree of spontaneous alignment of the CNTs and RGOFs along the fibre direction. The CNT bundles also attached themselves to the edges and surfaces of the RGOF sheets. This alignment led to toughness values up to 970J/g, which are greater than those reported so far for any material.

The yarns can be twisted without breaking or sewn like thread. They can also be coiled into spring shapes which are fixed by heating and return to their coil shape after complete elongation or compression. 'We think that our tough fibres can be applied soon for specialised applications like bulletproof vests,' Kim says. 'Our fabrication process is quite simple and suitable for industry. We used cheap and mass produced graphene oxide to replace some of the more expensive single-walled CNT.'

Soon Hyung Hong from the Korea Advanced Institute of Science and Technology says he would like to see further improvements in the fibres. 'They have fabricated very tough fibers, however their strength is not high enough. If they can be made strong and tough, the application could be extended as light and strong cables, high pressure vessels, etc.'

CNTs cost $25,000-90,000/kg (£15,700-56,800) but industrial grade graphene oxide costs just $450/kg. This could be reduced further by replacing the remaining single wall nanotubes with multi-wall CNTs, which are even cheaper, although their effectiveness has yet to be tested.

Source: Royal Society of Chemistry

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