In what they're calling the "highest density of mechanical bonds ever achieved," researchers created a super-strong flexible material that works very much like chainmail. The breakthrough has already demonstrated its ability to improve body armor.
In the world of chemistry, getting polymers (long chains of large molecules) to form mechanical bonds inside their structures has proven extremely challenging. Unlike chemical bonds, which involve the sharing of electrons by atoms or the effects of electrostatic forces among them, mechanical bonds involve molecules physically threading through one another.
Now however, a breakthrough technique at Northwestern University (NU) has overcome the challenge. Researchers there made two-dimensional sheets out of X-shaped monomers, which are the building blocks of polymers. (In chemistry, two-dimensional objects are those consisting of just a single layer of atoms.) The monomers were made up of molecules holding four extended aromatic groups, which gave them their X shape.
Next, they layered these sheets in a crystalline structure and coaxed the ends of all the X's to attach to each other through the introduction of a chemical known as dialkyldichlorosilane. More layers caused more monomers to spread and connect through the lattice, resulting in a series of loops all threaded together in a super-strong web akin to the metal links in chainmail.
The researchers say that the new material has 100 trillion mechanical bonds per every one square centimeter, making it the substance with the highest density of these bonds ever created.
"We made a completely new polymer structure," said Northwestern's William Dichtel, the study's corresponding author. "It's similar to chainmail in that it cannot easily rip because each of the mechanical bonds has a bit of freedom to slide around. If you pull it, it can dissipate the applied force in multiple directions. And if you want to rip it apart, you would have to break it in many, many different places. We are continuing to explore its properties and will probably be studying it for years."
Ultra Ultem
After experimenting with the material, the researchers found that, unlike previous mechanically bonded materials, it could be produced in larger quantities. In tests, they produced a kilogram of the material and believe even higher amounts are possible.
Investigating the practical nature of the new material, Dichtel's collaborators at Duke University added it to something known as Ultem, a very strong material similar to Kevlar that can resist impacts, caustic chemicals, and extreme temperatures. The Duke team found that adding just 2.5% of the new material to Ultem boosted a measure of strength known as its tensile modus by 45% .
"We have a lot more analysis to do, but we can tell that it improves the strength of these composite materials," Dichtel concluded. "Almost every property we have measured has been exceptional in some way." This, he says, might make the new material perfect for the development of new lightweight armor or other ballistic fabrics.
Dedication
The researchers dedicated their study, which has been published in the journal Science, to Sir Fraser Stoddart, a former fellow Northwestern chemist who won the Nobel Prize in Chemistry in 2016 for his work pioneering mechanical bonds.
"Molecules don't just thread themselves through each other on their own, so Fraser developed ingenious ways to template interlocked structures," said Dichtel, who was a postdoctoral researcher in Stoddart's lab at UCLA. "But even these methods have stopped short of being practical enough to use in big molecules like polymers. In our present work, the molecules are held firmly in place in a crystal, which templates the formation of a mechanical bond around each one.
"So, these mechanical bonds have deep tradition at Northwestern, and we are excited to explore their possibilities in ways that have not yet been possible."
Source: Northwestern