New
technique developed at MIT could produce strong, resilient nanofibers for many
applications.
By David L. Chandler | MIT News Office
By David L. Chandler | MIT News Office
January 4, 2018 -- Researchers
at MIT have developed a process that can produce ultrafine fibers — whose
diameter is measured in nanometers, or billionths of a meter — that are
exceptionally strong and tough. These fibers, which should be inexpensive and
easy to produce, could be choice materials for many applications, such as
protective armor and nanocomposites.
The new process, called gel electrospinning,
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.
In materials science, 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 fibers made by the new process, many
of those tradeoffs are eliminated.
“It’s a big deal when you get a
material that has very high strength and high toughness,” Rutledge says. That’s
the case with this process, which uses a variation of a traditional method
called gel spinning but adds electrical forces. The results are ultrafine fibers
of polyethylene that match or exceed the properties of some of the strongest
fiber materials, such as Kevlar and Dyneema, 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 Dyneema and
Spectra, are also used in ropes for extreme uses, and as reinforcing fibers in
some 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, 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.” Modulus refers to how stiff a fiber is,
or how much it resists being stretched.
Compared to carbon fibers and
ceramic fibers, which are widely used in composite materials, the new
gel-electrospun polyethylene fibers 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, Rutledge says.
In creating this ultrafine
material, the team had aimed just to match the properties of existing
microfibers, “so demonstrating that would have been a nice accomplishment for
us,” 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 fibers, 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 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 stiff but not very strong, while steel wire is strong but not very
stiff. The new gel-electrospun fibers 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 fibers,” with
diameters of a few hundred nanometers rather than the typical 15 micrometers,
he says. The researchers’ process combines the use of a polymer gel as the
starting material, as in gel spun fibers, but uses electrical forces rather than
mechanical pulling to draw the fibers out; the charged fibers induce a
“whipping” instability process that produces their ultrafine dimensions. And
those narrow dimensions, it turns out, led to the unique properties of the
fibers.
These results might lead to
protective materials that are as strong as existing ones but less bulky, making
them more practical. 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 research was supported by the
U.S. Army through the Natick Soldier Research, Development and Engineering Center
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