Ultra-fine fibres have exceptional strength

Researchers at the Massachusetts Institute of Technology (MIT) have developed a process that can produce ultra-fine fibres with diameter measured in nanometres, or billionths of a metre; that are exceptionally strong and tough.

The researchers use a variation of a traditional method called gel spinning but adds electrical forces to come up with a new process called gel eletrospinning. The result is ultra-fine fibres of polyethylene 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.

In materials science, MIT professor of chemical engineering Gregory Rutledge explains, "There are a lot of tradeoffs." 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," he says. "The material becomes more brittle and therefore doesn't have the mechanism for absorbing energy, and it tends to break."

But in the fibres made by the new process, many of those tradeoffs are eliminated.

"We started off with a mission to make fibres in a different size range, namely below 1 micron, because those have a variety of interesting features in their own right," Prof Rutledge says. "And we've looked at such ultra-fine fibres, sometimes called nanofibres, 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.

Outperforming everything

"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," Prof Rutledge says. But this new material, he says, exceeds all the others. "What really sets those apart is what we call specific modulus and specific strength, which means that on a per-weight basis they outperform just about everything."

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

In creating this ultra-fine material, the team had aimed just to match the properties of existing microfibres, "so demonstrating that would have been a nice accomplishment for us," Prof Rutledge says. In fact, the material 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, he says. Crucially, he 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 researchers are still investigating what accounts for this impressive performance. "It seems to be something that we received as a gift, with the reduction in fibre size, that we were not expecting," Prof Rutledge says.

He explains that "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.

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 fibres," with diameters of a few hundred nanometres rather than the typical 15 micrometres, he says. The researchers' 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 ultra-fine dimensions. And those narrow dimensions, it turns out, led to the unique properties of the fibres.

These results might lead to protective materials that are as strong as existing ones but less bulky, making them more practical. And, Prof Rutledge adds, "they may have applications we haven't thought about yet, because we've just now learned that they have this level of toughness."

New ultra-fine fibres created by the MIT team are seen in a scanning electron microscope image. Image: MIT


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. Image: MIT




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