SCIENTIST DEVELOPED AN ULTRA-STRONG NANO-FIBRE

Researchers at MIT have developed a process that can produce ultrafine fibres – whose diameter is measured in nanometers, or billionths of a meter – that are exceptionally strong an tough. These fibres, which should be inexpensive and easy to produce, could be choice materials for many applications, such as protective armour and nanocomposites.

The process is called gel electrospinning and is described in a paper by MIT professor of chemical engineering, Gregory Rutledge, and postdoc Jay Park. The paper appears online and will be published in the February edition of the Journal of Materials Science.
"It’s a big deal when you get a material that has a very high strength and high toughness," says Rutledge. That is the case with this process, which uses a variation of a traditional method called gel spinning but adds electrical forces. The results are ultrafine fibres of polyethene that match or exceed the properties of some of the strongest fibre materials, such as Kevlar and Dyneema, which are used for applications including bullet-stopping body armour.

"We started off with a mission to make fibres in a different size range, namely below 1 micron [millionth of a meter] because those have a variety of interesting features in their own right," Rutledge says. "And we’ve looked at such ultrafine fibres, sometimes called nanofibers, for many years. But there was nothing in what would be called the high-performance fibre range." High-performance fibres, which include aramids such as Kevlar, and gel spun polyethylenes like Dyneema and Spectra, are also used in ropes for extreme uses, and as reinforcing fibres in some high-performance composites.

This new material exceeds all the others, and what really sets it apart is "what we call specific modulus and specific strength, which means that on a per-weight basis they outperform just about everything" adds Rutledge. Modulus refers to how stiff a fibre is, or how much it resists being stretched.

Compared to carbon fibres and ceramic fibres – which are widely used in composite materials, the new gel-electrospun polyethene fibres have similar degrees of strength but are much tougher and have a lower density, which basically means that pound for pound, they outperform the standard materials by a wide margin, says Rutledge.

By creating this ultrafine material, the team had aimed just to match the properties or existing microfibers, "so demonstrating that would have been a nice accomplishment for us," adds Rutledge. The material turned out to be better in significant ways. The test materials had a modulus that was not as good as the best-existing fibres, they were quite close; enough to be "competitive," he says. Crucially, Rutledge adds "the strengths are about a factor of two better than the commercial materials and comparable to the best available academic materials. And their toughness is about an order of magnitude better."

The team of researchers are investigating what accounts for impressive performance. "It seems to be something that we received as a gift, with the reduction in fibre size, that we were not expecting," says Rutledge.
Using the gel electrospinning process "is essentially very similar to the conventional, gel spinning process in terms of the materials we’re bringing in but, because we’re using electrical forces," and using a single-stage process rather than the multiple stages of the conventional process, "we are getting much more highly drawn fibers," with diameters of a few hundred nanometers rather than the typical 15 micrometers, he says.

The research was supported by the U.S. Army through the Natick Soldier Research, Development and Engineering Center, and the Institute for Soldier Nanotechnologies, and by the National Science Foundation’s Center for Materials Science and Engineering.