A team of researchers at MIT have formulated a process that can create ultrafine fibers — whose diameter is measured in nanometers, or billionths of a meter — that are remarkably strong and tough. These fibers, which should be inexpensive and easy to create, could be choice materials for number of applications, such as nanocomposites and protective armor.
The new process, termed as gel electrospinning, is explained 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 issue of the Journal of Materials Science.
In materials science, Rutledge explains, “there are a lot of tradeoffs.” Usually researchers can improve one characteristic of a material but will see deterioration 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 fibers created by the new process, many of those tradeoffs are prevented.
“It’s a big deal when you get a material that has very high strength and high toughness,” Rutledge says. That is the case with this process, which uses a variation of a traditional technique called gel spinning but incorporates electrical forces. The results are ultrafine fibers of polyethylene that match or surpass the properties of some of the strongest fiber materials, such as Dyneema and Kevlar, which are used for applications including bullet-stopping body armor.
“We started off with a mission to make fibers 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 fibers, sometimes called nanofibers, for many years. But there was nothing in what would be called the high-performance fiber range.” High-performance fibers, which include aramids such as Kevlar, and gel spun polyethylenes like Spectra and Dyneema, are also used in ropes for extreme uses, and as reinforcing fibers in certain high-performance composites.
“There hasn’t been a whole lot new happening in that field in many years, because they have very top-performing fibers in that mechanical space,” Rutledge says. But this new material, he says, surpasses 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.” Modulus denotes how stiff a fiber is, or how much it resists being stretched.
Compared to ceramic fibers and carbon fibers, which are extensively used in composite materials, the new gel-electrospun polyethylene fibers have similar degrees of strength but are a lot tougher and possess lower density. That means that, pound for pound, they outclass the standard materials by a huge margin, Rutledge says.
In developing this ultrafine material, the team had aimed just to match the properties of current microfibers, “so demonstrating that would have been a nice accomplishment for us,” Rutledge says. In reality, the material turned out to be better in major ways. While the test materials had a modulus not quite as good as the best prevailing fibers, they were fairly 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 examining what accounts for this remarkable performance. “It seems to be something that we received as a gift, with the reduction in fiber size, that we were not expecting,” Rutledge says.
He explains that “most plastics are tough, but they’re not as stiff and strong as what we’re getting.” And glass fibers are rigid but not very strong, while steel wire is strong but not very rigid. The new gel-electrospun fibers seem to integrate the required qualities of stiffness, strength, 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 instead of the multiple stages of the conventional process, “we are getting much more highly drawn fibers,” with diameters of a few hundred nanometers instead of the typical 15 mµ, he says. The researchers’ process adds the use of a polymer gel as the starting material, as in gel spun fibers, but uses electrical forces instead of mechanical pulling to draw the fibers out; the charged fibers induce a “whipping” instability process that creates their ultrafine dimensions. And those narrow dimensions, it turns out, results in the unique properties of the fibers.
These results might bring about protective materials that are as strong as current ones but less bulky, making them more useful. And, 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.”
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 supported the research.