With fuzzy nanoparticles, researchers reveal a way to design tougher ballistic materials – sciencedaily
Researchers from the National Institute of Standards and Technology (NIST) and Columbia Engineering have discovered a new method to improve the strength of materials that could lead to stronger versions of bulletproof vests, bulletproof glass and others. ballistic equipment.
In a study published today in Soft material, the team produced films composed of nanoscale ceramic particles decorated with polymer strands (resembling fuzzy orbs) and made them targets in miniature impact tests that showed the material’s improved toughness . Further testing uncovered a unique property not shared by typical polymer-based materials that allowed films to rapidly dissipate impact energy.
“Because this material does not follow the traditional hardening concepts you see in conventional polymers, it opens up new ways of designing materials for impact mitigation,” said the NIST Materials Research Engineer. , Edwin Chan, study co-author.
The polymers that make up most high-impact plastics today are made up of linear chains of repeating synthetic molecules that physically intertwine or form chemical bonds with each other, forming a tightly entangled network. The same principle applies to most polymer composites, which are often reinforced or cured by mixing a non-polymeric material. The films in the new study fall into this category but feature a unique design.
“Mixing plastics with solid particles is like trying to mix oil and water. They want to come apart,” said Sanat Kumar, professor of chemical engineering at Columbia University and co- author of the study. “The realization that we made in my group is this: One way to solve this problem is to chemically attach the plastics to the particles. It’s like they hate each other but they can’t get away with it. “
The films are made up of tiny glass spheres called silica nanoparticles, each covered with chains of a polymer called polymethacrylate (PMA). To produce these polymer grafted nanoparticles (PGNs), Kumar’s lab developed chains of PMAs on the curved surface of the nanoparticles, making one end of each chain stationary.
The shorter or lower molecular weight chains on PGNs are constrained by neighboring chains. The lack of movement means they don’t interact much. But the higher molecular weight polymers, which expand further away from the spherical nanoparticles, have more leeway to move, until they become entangled with other chains. Between these two lengths, there is an intermediate molecular mass where the polymers are free to move but are also not long enough to knot.
This phenomenon was useful for the material’s original purpose, which allowed gases to pass through it quickly. But Chan and others at NIST investigated how this unique property would affect toughness. With help from Kumar’s lab, the researchers tested samples of varying molecular weights.
“We grew polymer hair from the particles on a very short bunch and brush cut on a very long, hippie bunch,” said Chris Soles, materials research engineer and co-author of NIST. “Brush-cut nanoparticles do not tangle and can pile up, but as the polymers lengthen the distance between the nanoparticles increases and the chains between the particles begin to tangle and form a network. . “
At NIST, researchers opened fire on PGN composite films of different molecular weights with a technique known as Laser-Induced Projectile Impact Testing, or LIPIT. These high-speed impact tests involved firing spherical projectiles 10 micrometers wide (about four thousandths of an inch) towards targets at speeds of nearly 1 kilometer per second (over 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 a strobe light flashing every 100 nanoseconds (billion seconds). From there, the team had what it took to calculate the energy required to rip the film, an amount directly related to toughness.
The study authors found that PGN composite films were generally stronger than PMA alone. But what was perhaps more interesting was that the intermediate molecular weight gave the hardest film.
In purely polymeric materials, longer chains tend to create more tangles. And more tangles translate into greater toughness, to the point where the material is completely bonded. However, LIPIT testing revealed that the films could challenge the traditional behavior of polymers. The strongest samples had chains much shorter than the length for full entanglement, meaning that tangles were not the only factor determining toughness.
Soles and his colleagues suspected that the reason for this was the decrease in packing between the chains at the intermediate molecular weight level, which could have created a situation in which the polymers could wiggle more freely and create friction with neighboring chains – a potential way to dissipate the energy of a high impact.
Seeking to identify the underlying source of the toughness and test their hypothesis, the team members used equipment from the NIST Center for Neutron Research to assess the movement of the polymers.
These tests confirmed that chains of intermediate molecular masses attached to nanoparticles displayed an ability to move and then reach a state of relaxation within picoseconds (trillionths of a second). These improved movements of the intermediate chains dissipated energy more easily than short (no entanglement) or long (heavily entangled) PMA chains. This discovery reinforces the team’s intuition, especially when it is associated with LIPIT tests.
“Just at that molecular weight 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 suggest the existence of a sweet spot relative to the length of the polymers attached to the curved surface of the particles which could increase the toughness of the material. The finding may not be limited to PMA either.
“Based on this kind of platform, the concept of grafted nanoparticles, you can start experimenting with more conventional high-impact polymers such as polycarbonates used in bulletproof windows,” Chan said. “There is so much to explore. We are only scratching the surface of these materials.”