Today's KNOWLEDGE Share
Material scientists at Texas A&M have developed a dynamic material that self-heals after puncturing by changing from solid to liquid and back.
What if there were a fabric that, like Superman, could take a bullet and self-heal? Such a super-dynamic, action-powered polymer might actually help protect real-life flyers in space.
Material scientists at Texas A&M University have developed just such a polymer with a unique self-healing property never before seen at any scale. When struck by a projectile, this material stretches so much that when the projectile manages to pass through, it takes only a small amount of the polymer with it. As a result, the hole left behind is much smaller than the projectile itself.
However, for now, this effect has only been observed under extreme temperatures and at the nanoscale.
“This is the first time a material at any scale has displayed this behavior,” said Dr. Svetlana Sukhishvili, a professor in the Department of Materials Science and Engineering, who has been working on development of this polymer film with materials science and engineering professor Dr. Edwin (Ned) Thomas, and then-graduate student Dr. Zhen Sang. Their findings were published in the March/April issue of Materials Today.
“Besides being very cool, the new polymer will likely have many applications, including making the windows of space vehicles more resilient to the onslaught of micrometeoroids,” Thomas said. Space vehicles are frequently bombarded with micrometeoroids traveling at speeds of 10 kilometers per second. A micrometeoroid can create a hole in the window that, while small, is visible to the human eye. However, a window manufactured with a layer of this polymer could potentially sustain damage tinier than the meteoroid itself.
Thomas, who first suggested subjecting the polymer to ballistic testing, said a key goal of the research is to design a material that will protect structures such as orbiting satellites and vehicles in space, with applications for military equipment and body armor on Earth.
The phenomenal behavior occurs in the new solid polymer film as it melts when impacted by a laser-launched high-speed projectile, and snaps back to its original shape when cooled. The polymer does this by absorbing much of the kinetic energy generated by the projectile, causing the film to stretch and liquify as the projectile continues its journey, finally piercing the film. Once pierced, the polymer quickly cools, its covalent bonds reform, and it returns to its original solid state, leaving a tiny hole.
“A major goal of our work was to see if we could simultaneously provide a material that would absorb a lot of kinetic energy per unit target mass from the high-speed projectile and be capable of very rapid healing of the punctured region,” Thomas said. “We wanted the post-impact material to still be capable of performing its intended function, such as carrying air or liquids and remaining sealed against the loss of such fluids across the material membrane.”
The material is a Diels-Adler Polymer or DAP, so-named by the researchers for its dynamic covalent bond networks that can be broken and reformed. It belongs to a class of materials called Covalent Adaptative Networks or CANs. While other Diels-Adler networks have been reported in the scientific literature, DAP’s specific chemistry, topology and self-healing quality are novel. The DAP acronym could also refer to their polymer as a Dynamic Action-Powered material for its ability to self-heal.
Besides being very cool, the new polymer will likely have many applications, including making the windows of space vehicles more resilient to the onslaught of micrometeoroids
Dr. Edwin (Ned) Thomas
“When we were synthesizing DAPs, we aimed to do it in such a way that the polymers would turn to liquids upon temperature increase,” Sukhishvili said. “Although this feature was introduced to facilitate 3D printing, we thought that due to its ability to liquify upon heating, our polymers could show improved ballistic healing characteristics.
“Polymers are amazing materials, especially DAP materials,” Thomas explained. “Because at low temperatures, they are stiff and strong; then at higher temperatures, they become elastic; and at still higher temperatures, they become an easily flowing liquid. That’s a huge range of property behavior.” What’s more, he said, the process reverses itself. “Nothing else on the planet can do that!
The DAP structure is of long polymer chains containing double carbon bonds that break when severe strain and heat are applied, but quickly reform when cooled, albeit not necessarily in the same configuration.
“Think of the long polymer chains in the fabric as being like a bowl of Ramen noodle soup,” said Sang, who worked on this project for his doctoral research and is first author on the paper. “You can stir it with chopsticks, then freeze it. When you unfreeze it, you can stir it, then refreeze. It will have the same ingredients as before, just in a slightly different appearance.
Sang, who is now an engineer at Apple, Inc., said it wasn’t easy to do ballistic testing at such a small scale until he came across a new research methodology called LIPIT (laser-induced projectile impact testing), recently developed by Thomas and colleagues at MIT. Sang used LIPIT to laser-launch a tiny silica projectile 3.7 micrometers in diameter from a glass slide covered with a thin gold film resting on a one-square inch platform. His target consisted of a thin layer (75 to 435 nanometers) of the super DAP.
An ultrahigh-speed camera with a 3-nanosecond exposure time at 50 nanosecond intervals recorded the action. The research team then used scanning electron microscopy, laser scanning confocal microscopy and an infrared nano spectrometer to view the holes and assess the covalent bonding in the super polymer.
The results were puzzling at first, Sang said, because he could find no holes in the targeted polymer.
“Was I not aiming correctly? Were there no projectiles? What’s wrong with my experiment, I asked myself,” he said. However, when he placed the DAP sample under the infrared nano spectrometer, which combines chemical analysis with high-scale resolution, he was able to see the tiny perforations. “This was actually a surprising, surprising finding,” Sang said. “A very exciting finding!
He explained this behavior can’t yet be recreated at the macro level because the strain rate during perforation of a very thin target material under impact is so much larger than at the nanoscale. “If this strain rate is really high, materials often have unexpected behavior that people don’t usually see under normal circumstances,” Sang said. “With the LIPIT apparatus that we’re using, we’re talking about a strain rate many orders of magnitude higher than for conventional scale bullets and targets. At that perspective, materials behave very differently.
Other coauthors on the paper are materials science doctoral student Hongkyu Eoh; former postdoctoral researchers Drs. Kailu Xiao, Wenpeng Shan and Jinho Hyon; and Dr. Dmitry Kurouski, associate professor in the department of biochemistry and biophysics at Texas A&M.
Sukhishvili and Thomas plan to continue researching the super DAP using different polymer compositions, temperature- and stress-responses.
“One could even imagine designing DAPs with characteristics such that it would be possible to absorb kinetic energy by breaking DAP bonds, then some of these broken bonds could very rapidly reform – by perhaps having just the right ‘bond reform catalyst’ present in the material – whereby the projectile would have to break these bonds a second (or even multiple times) before the material ultimately heals itself, and is ready for the next ballistic event.
“To date, no material has the requisite time response to deform, break, reform; and then deform, break and reform again during the sub-microsecond interval of a ballistic event,” Thomas said
source: Texas A&M University
