An electron microscope image shows the effects of a silica microsphere on a block copolymer. The sphere was fired at the copolymer at 1,500 meters per second to study the effect of such impacts on advanced materials being developed to protect soldiers, aircraft and spacecraft. Compressed layers of the copolymer appear in red and orange. Image credit: Thomas Lab/Rice University. Click image to enlarge 

Chain mail went out of style with the knights of old, but body armor still weighs down our modern military. Nanomaterials that can withstand high force impacts could be the key to lightweight, high performance protection for soldiers.

To test and analyze the impact resistance properties of nanomaterials requires a ballistics test, but on a very small scale. A team of researchers from Rice and MIT designed an experiment to do just that, using lasers to fire microbullets at small samples of nanomaterials. Rice scientist Jae-Hwang Lee is the lead author on the paper that details the results of their first tests last week in the journal Nature Communications.

“One of our motivations is to miniaturize the tests so we can get accurate results from small samples,” Ned Thomas, a professor of engineering at Rice and one of the study’s authors says. “Then, we can pick materials that look promising to scale up.”

One such promising material is polyurethane. A clear, 35-mm-thick sheet of polyurethane can arrest a 9mm bullet without cracking (see accompanying photo)-- an ideal material for a bullet-proof windshield.  However, Thomas explained that no one really understood what structural properties gave polyurethane the ability to absorb the impact of a bullet.

Polyurethane is a complex long-chain polymer built of blocks of both rubbery and glassy segments. For laboratory analysis, the researchers decided to use a model polymer called polystyrene-polydimethylsiloxane (PS-PDMS) that has a simpler nanoscale structure that consists of distinct layers of the rubbery and glassy segments.

The bullets fired at the PS-PDMS are glass spheres, about 3 microns in diameter. To fire these microbullets, they use a laser launching system. The microbullets rest on a sheet of glass that is coated with a dyed polymer. When a laser pulse is directed at the glass, the energy in the light causes the dyed polymer to vaporize with enough force that it can launch the microbullets at velocities of a kilometer a second.

“When we first started, we were shooting lots of beads,” Thomas says. “Now, we’ve perfected this thing so we can shoot a single bead or a cluster of beads. Once you have it up and going, you can do hundreds in an afternoon.”

The test results show that small amounts of PS-PDMS appear to melt in the path of the projectile. These layers morph into a more homogenous fluid, instead of breaking on impact, which seals around the microbullets, much like how the polyurethane samples  catch the 9mm bullets. Further from the microbullet, the layers kink and deform as they dissipate the impact’s force. 

They tested how the PS-PDMS material responded to projectile impact for two different orientations - microbullets hit both parallel and perpendicular to the material’s layered structure.  In the perpendicular impact tests, the material was better able to absorb the impact with less deformation of the overall material structure than the parallel layers.

“I think the experiments will provide needed understanding of the mechanisms governing how a projectile penetrates protective vests and helmets,” says Donald Shockey, the director of the Center for Fracture Physics at SRI International, who was not involved in this research project.

It’s important to note that all materials are going to fail at some point, but Shockey says that the most important part of the research is the detailed understanding of how the nanostructure influences failure mechanisms.

“The thing I really like is that he’s going to get quantitative data on deformation and fracture,” Shockey says. Modeling that data, he explained could produce a “pathway to better materials and a pathway to better predictions of ballistic performance.”

Next, Thomas says his team hopes to use this testing mechanism on a variety of other nanomaterials and to demonstrate that miniaturized tests can reliably predict and validate macro-scale tests.

Read the abstract in Nature Communications  here.

Watch a video from Rice University here.