With fuzzy nanoparticles, researchers reveal a way to design tougher
ballistic materials

Date:
December 13, 2021
Source:
National Institute of Standards and Technology (NIST)
Summary:
Researchers have discovered a new method to improve the toughness
of materials that could lead to stronger versions of body armor,
bulletproof glass and other ballistic equipment. The team produced
films composed of nanometer-scale ceramic particles decorated with
polymer strands (resembling fuzzy orbs) and made them targets in
miniature impact tests that showed off the material's enhanced
toughness. Further tests unveiled a unique property not shared by
typical polymer-based materials that allowed the films to dissipate
energy from impacts rapidly.



FULL STORY ========================================================================== Researchers at the National Institute of Standards and Technology (NIST)
and Columbia Engineering have discovered a new method to improve the
toughness of materials that could lead to stronger versions of body armor, bulletproof glass and other ballistic equipment.


==========================================================================
In a study published today in Soft Matter, the team produced films
composed of nanometer-scale ceramic particles decorated with polymer
strands (resembling fuzzy orbs) and made them targets in miniature impact
tests that showed off the material's enhanced toughness. Further tests
unveiled a unique property not shared by typical polymer-based materials
that allowed the films to dissipate energy from impacts rapidly.

"Because this material doesn't follow traditional concepts of toughening
that you see in classical polymers, it opens up new ways to design
materials for impact mitigation," said NIST materials research engineer
Edwin Chan, a co- author of the study.

The polymers that constitute most of the high-impact plastics today
consist of linear chains of repeating synthetic molecules that either physically intertwine or form chemical bonds with each other, forming
a highly entangled network. The same principle applies to most polymer composites, which are often strengthened or toughened by having some
nonpolymer material mixed in. The films in the new study fall into this category but feature a unique design.

"Mixing together plastics with some solid particles is like trying to
mix oil and water. They want to separate," said Sanat Kumar, a Columbia University professor of chemical engineering and co-author of the
study. "The realization we've made in my group is: One way to fix that
is to chemically tether the plastics to the particles. It's like they
hate each other but they can't get away." The films are made of tiny
glass spheres, called silica nanoparticles, each covered with chains of a polymer known as polymethacrylate (PMA). To produce these polymer-grafted nanoparticles (PGNs), Kumar's lab grew PMA chains on the curved surface
of the nanoparticles, rendering one end of each chain stationary.



========================================================================== Shorter, or lower molecular mass, chains on the PGNs are constrained
by neighboring chains. The lack of motion means they do not interact
much. But higher molecular mass polymers, which fan out farther from
the spherical nanoparticles, have more elbow room to move, until they
become entangled with other chains. Between these two lengths, there is
an intermediate molecular mass where polymers are free to move but are
also not long enough to knot up.

This phenomenon was useful for the material's initial purpose, which was permitting gases to move through it quickly. But Chan and others at NIST
sought to find out how this unique property would affect toughness. With
the help of Kumar's lab, the researchers tested samples of varying
molecular masses.

"We grew polymeric hair off of the particles from a really short,
brush-cut regime to a very long, hippie regime," said NIST materials
research engineer and co-author Chris Soles. "The brush-cut nanoparticles
don't entangle and can pack together, but as the polymers get longer, the distance between nanoparticles expands and the chains between particles
start to entangle and form a network." At NIST, the researchers
opened fire on the PGN composite films of different molecular masses
with a technique known as Laser-Induced Projectile Impact Testing,
or LIPIT. These high-velocity impact tests involved propelling 10- micrometer-wide (about four-thousandths of an inch) spherical projectiles toward the targets at velocities of nearly 1 kilometer per second (more
than 2,200 miles per hour) with a laser.

They determined the velocity of the projectile in transit and on impact
through images captured with a camera and strobe light flashing every
100 nanoseconds (billionths of a second). From there, the team had what
it needed to calculate the energy it took to tear through the film,
a quantity directly tied to toughness.



==========================================================================
The authors of the study found that the PGN composite films were generally tougher than solely PMA. But what was perhaps more interesting was that intermediate molecular mass yielded the toughest film.

In purely polymeric materials, longer chains tend to create a greater
number of tangles. And more tangles translate to greater toughness,
up to the point where the material is completely tied up. However,
the LIPIT tests revealed that the films could defy traditional polymer behavior. The toughest samples had chains far shorter than the length
for full entanglement, meaning that tangles were not the only factor
driving toughness.

Soles and his colleagues suspected that the reason was the decreased
packing between the chains at the intermediate molecular masses, which
could have created a situation where polymers could wriggle about more
freely and create friction with neighboring chains -- a potential avenue
for dissipating energy from a high impact.

Seeking to pin down the underlying source of the toughness and test
their hypothesis, the team members used equipment at the NIST Center
for Neutron Research to assess the motion of the polymers.

These tests confirmed that the intermediate molecular mass chains attached
to the nanoparticles displayed an ability to move and then reach a
relaxed state in just a few picoseconds (trillionths of a second). These enhanced movements of the intermediate chains dissipated energy more
readily than either the short (no tangles) or long (highly entangled)
PMA chains. This finding backed the team's intuition, especially when
taken along with the LIPIT tests.

"Right at that molecular mass where the PGN composite films showed the
highest impact resistance, the grafted PMA chains showed the highest
mobility and energy dissipation," Soles said.

The results of this study hint at the existence of a sweet spot with
respect to the length of polymers fixed to the curved surface of particles
that could boost material toughness. The finding may not be limited to
PMA either.

"Based on this kind of platform, the grafted nanoparticle
concept, you can start experimenting with more classic
high-impact polymers such as the polycarbonates used in
bulletproof windows," Chan said. "There's just so much to
explore. We're only just scratching the surface of these materials." ========================================================================== Story Source: Materials provided by National_Institute_of_Standards_and_Technology_(NIST).

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shawn H. Chen, Amanda J. Souna, Stephan Jeffrey Stranick, Mayank
Jhalaria, Sanat Kumar, Christopher L Soles, Edwin Chan. Controlling
Toughness of Polymer-grafted Nanoparticle Composites for Impact
Mitigation. Soft Matter, 2022; DOI: 10.1039/D1SM01432C ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211213142223.htm

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