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9th January 2018, Cambridge, MA

Is this the fibre the composites industry has been waiting for?

A low-cost alternative to carbon, ceramics and other high performance fibres appears to be on the horizon, following the development of a new process called gel electrospinning at MIT (the Massachusetts Institute of Technology).

MIT professor of chemical engineering Gregory Rutledge and postdoc Jay Park will publish details of their breakthrough in turning inexpensive polyethylene into ultrafine fibres with extremely high performance properties in the February edition of the Journal of Materials Science.

Trade-offs

“In materials science, there are a lot of trade-offs,” Rutledge explains. “Typically, researchers can enhance one characteristic of a material but will see a decline in a different characteristic. Strength and toughness are a pair like that – usually when you get high strength, you lose something in the toughness. The material becomes more brittle and therefore doesn’t have the mechanism for absorbing energy and tends to break.”

The new ultra-fine fibres created by the MIT team in a Scanning Electron Microscope (SEM) image. © MIT

In the fibres made by the new gel electrospinning process, those trade-offs are eliminated.

“It’s a big deal when you get a material that has very high strength and high toughness and that’s the case with this process,” Rutledge says. “We started off with a mission to make fibres in a different size range, namely below one micron, because those have a variety of interesting features in their own right, and we’ve looked at such ultrafine fibres for many years. But there was nothing in what would be called the high-performance fibre range such as aramids like Kevlar, and gel spun polyethylenes like Dyneema and Spectra. There hasn’t been a whole lot new happening in that field in many years, because they have very top-performing fibres in that mechanical space. What really sets those fibres apart is what we call specific modulus and specific strength, which means that on a per-weight basis they outperform just about everything.”

Strength and toughness

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

In creating the ultrafine material, the team had aimed to just match the properties of existing microfibres, and demonstrating that would have been an accomplishment.

In fact, the fibres turned out to be better in significant ways. While the test materials had a modulus not quite as good as the best existing fibres, they were quite close – enough to be competitive.

“The strengths are about a factor of two better than the commercial materials and comparable to the best available academic materials,” Rutledge says. “And their toughness is about an order of magnitude better. We are still investigating what accounts for this impressive performance but it seems to be something that we received as a gift, with the reduction in fibre size, that we were not expecting.”

A diagram of the device used to produce the fibres shows a heated syringe (left) through which the solution is extruded, and a chamber (right) where the strands are subjected to an electric field that spins them into the highest performing polyethylene fibres ever made. © MIT

“Most plastics are tough, but they’re not as stiff and strong as what we’re getting, and glass fibres are stiff but not very strong, while steel wire is strong but not very stiff. The new gel-electrospun fibres seem to combine the desirable qualities of strength, stiffness, and toughness in ways that have few equals.”

Single-stage process

Using the gel electrospinning process is essentially very similar to the conventional gel spinning process in terms of the materials, but the use of electrical forces in a single-stage process rather than the multiple stages of the conventional process have resulted in more highly drawn fibres, with diameters of a few hundred nanometres rather than the typical 15 micrometres.

The process combines the use of a polymer gel as the starting material, as in gel spun fibres, but uses electrical forces rather than mechanical pulling to draw the fibres out. The charged fibres induce a “whipping” instability process that produces their ultrafine dimensions. It is these narrow dimensions which result in the unique properties of the fibres.

The development may lead to protective materials that are as strong as existing ones but less bulky, making them more practical.

“They may have applications we haven’t thought about yet, because we’ve only now learned that they have this level of toughness,” Rutledge concludes.

The research was supported by the US 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.

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