New materials like chain mail could be the future of armor

In an amazing feat of chemistry, a Northwestern University-led research team has developed the first two-dimensional (2D) mechanically interlocked material.

Nanoscale materials, similar to the interlocking links of chain mail, exhibit extraordinary flexibility and strength. Future research could lead to use in high-performance, lightweight bulletproof vests and other applications where lightweight, flexible, and strong materials are required.

The study, published January 17 in the journal Science, is among several firsts in this field. Not only is this the first 2D mechanically bonded polymer, the new material contains 100 trillion mechanical bonds per square centimeter, which is the highest mechanical bond achieved to date. This will be the highest density.

The researchers produced this material using a new, highly efficient and scalable polymerization process.

“We created an entirely new polymer structure,” said Northwestern’s William Dichtel, the study’s corresponding author.

“It’s similar to chain mail in that it doesn’t tear easily because each mechanical bond has a little bit of freedom to glide. When you pull it, the applied force is distributed in multiple directions. And if you want to tear it, We’re going to have to break it in so many different places. We’re continuing to explore its properties and will probably study it for years.”

Dichtel is the Robert L. Lessinger Professor of Chemistry in the Weinberg College of Arts and Sciences and a member of the International Institute for Nanotechnology (IIN) and the Paula M. Trienen Institute for Sustainable Energy. Madison Bardo, Ph.D., a candidate in Dichtel’s lab and IIN Ryan Fellow, is the study’s lead author.

Inventing new processes

Researchers have been trying for years to develop molecules that mechanically bond with polymers, but they have found it nearly impossible to coax polymers into forming mechanical bonds.

To overcome this challenge, Dichtel’s team took an entirely new approach. They started with X-shaped monomers, the building blocks of polymers, and arranged them into specific, highly ordered crystal structures. These crystals were then reacted with other molecules to create bonds between the molecules within the crystals.

“I give Madison a lot of credit because she came up with this concept of forming mechanically bonded polymers,” Dichtel said. “This was a high-risk, high-reward idea, and we had to question our assumptions about what kinds of reactions are possible within molecular crystals.”

The resulting crystals constitute layers of 2D intertwined polymer sheets. Within the polymer sheet, the ends of X-shaped monomers are bonded to the ends of other X-shaped monomers. More monomer is then passed through the gap in between. Despite its rigid structure, polymers are surprisingly flexible.

Dichtel’s team also discovered that when the polymer is dissolved in solution, the entangled monomer layers peel away from each other.

“Once the polymer is formed, not much of it retains its structure,” Dichtel says. “So when you put it in a solvent, the crystals dissolve, but each 2D layer is held together. You can manipulate these individual sheets.”

To examine the structure at the nanoscale, Cornell University collaborators led by Professor David Muller used cutting-edge electron microscopy techniques. The images revealed a high degree of crystallinity of the polymer, confirming the entangled structure and indicating high flexibility.

Dichtel’s team also discovered that this new material could be produced in large quantities. Previous polymers containing mechanical bonds were typically prepared in very small quantities using less scalable methods. Meanwhile, Dichtel’s team envisions producing half a kilogram of the new material, with the possibility of producing larger quantities once the most promising applications emerge.

Adding strength to tough polymers

Inspired by the material’s inherent strength, Duke University’s Dichtel collaborators, led by Professor Matthew Becker, added this material to Ultem. Ultem, which belongs to the same family as Kevlar, is an extremely strong material that can withstand extreme temperatures and acidic and corrosive chemicals.

The researchers developed a composite material consisting of 97.5% Ultem fibers and only 2.5% 2D polymer. That small percentage dramatically increased Ultem’s overall strength and toughness.

Dichter envisions the group’s new polymers could have a future as specialty materials for lightweight bulletproof vests and bulletproof fabrics.

“We need to do more analysis, but we found that this increases the strength of these composites,” Dichtel said. “Almost every property we measured was exceptional in some way.”

history is deeply engraved

The authors dedicated this paper to the memory of Sir Fraser Stoddart, the former North West chemist who introduced the concept of mechanical bonding in the 1980s. Ultimately, he elaborated these bonds into molecular machines that switched, rotated, contracted, and expanded in controllable ways.

Stoddart, who passed away last month, won the 2016 Nobel Prize in Chemistry for this work.

“Molecules don’t pass through each other alone, so Fraser developed an ingenious way to template intertwined structures,” said Dichtel, a former postdoctoral fellow in Stoddart’s lab at UCLA. .

“However, even these methods have not reached sufficient practicality for use with large molecules such as polymers. In our current work, the molecules are tightly held in place within the crystal. held together, which templates the formation of mechanical bonds around each molecule.”

“Therefore, these mechanical connections have a deep tradition at Northwestern, and we are excited to explore their potential in ways not possible before.”