Friction of fuzzy nanoparticles creates tough materials

Researchers at the US National Institute of Standards and Technology (NIST) and Columbia Engineering have discovered a new method for improving the toughness of materials, which could lead to stronger versions of body armor, bulletproof glass and other ballistic equipment.

In a paper in Soft Matter, the researchers report producing films composed of nanometer-scale glass particles decorated with polymer strands (resembling fuzzy orbs), and conducting miniature impact tests on these films that showed off their 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 paper.

The polymers that constitute most of today's high-impact plastics consist of linear chains of repeating synthetic molecules that either physically intertwine or form chemical bonds with each other, creating a highly entangled network. The same principle applies to most polymer composites, which are often strengthened or toughened by having some non-polymer 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 professor of chemical engineering at Columbia University and co-author of the paper. “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, 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 film’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 the film's 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 PGN composite films of different molecular masses with a technique known as laser-induced projectile impact testing (LIPIT). In these high-velocity impact tests, 10µm-wide spherical projectiles are propelled by a laser toward the films at velocities of nearly 1km per second (more than 2200 miles per hour).

The researchers determined the velocity of the projectile in transit and on impact through images captured with a camera and a strobe light flashing every 100 nanoseconds. From this information, they could calculate the energy it took to tear through the film, a quantity directly tied to toughness.

They found that the PGN composite films were generally tougher than PMA on its own. But what was perhaps more interesting was that the 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 other factor was the decreased packing between the chains at the intermediate molecular masses. This would 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.

The 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. 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.”

This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.