BODY ARMORFuzzy Nanoparticles: New Way to Design Tougher Ballistic Materials

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.