A research team at Northwestern University has achieved a groundbreaking milestone in chemistry by creating the world’s first two-dimensional (2D) mechanically interlocked material. This nanoscale innovation, resembling the interlocking links of chainmail, demonstrates exceptional flexibility and strength, offering great potential for applications in lightweight, high-performance body armor and other advanced uses requiring both toughness and flexibility. The findings, published on January 16 in Science, establish key firsts in the field, including the creation of the first-ever 2D mechanically interlocked polymer and the achievement of an unprecedented density of 100 trillion mechanical bonds per square centimeter.

The new material is a result of an innovative, efficient, and scalable polymerization process, opening the door for large-scale production. “We made a completely new polymer structure,” said William Dichtel, the corresponding author of the study and a professor of chemistry at Northwestern University. “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.”

For years, scientists have struggled to create polymers with mechanical bonds, as these molecules tend to form weak, disordered structures. Dichtel’s team overcame this obstacle by using a novel approach: starting with X-shaped monomers and arranging them into a highly ordered crystalline structure. They then reacted these crystalline structures with another molecule to form mechanical bonds between the molecules.

“I give a lot of credit to Madison because she came up with this concept for forming the mechanically interlocked polymer,” Dichtel said. Madison Bardot, a Ph.D. candidate in Dichtel’s laboratory, is the study’s first author. “It was a high-risk, high-reward idea where we had to question our assumptions about what types of reactions are possible in molecular crystals.”

The resulting polymer is made up of layers of interlocked 2D sheets, each containing X-shaped monomers that are bonded together. Despite its rigid structure, the material is surprisingly flexible and can be manipulated by dissolving it in solution, which causes the layers of interlocked monomers to peel apart.

Collaborators at Cornell University, led by Professor David Muller, used cutting-edge electron microscopy techniques to examine the material’s structure at the nanoscale. The analysis revealed that the polymer exhibits a high degree of crystallinity, confirming its interlocked structure and remarkable flexibility.

One of the key innovations of this material is its scalability. Unlike previous polymers that could only be made in small quantities, Dichtel’s team was able to produce half a kilogram of the material, with the potential for even larger-scale production. “We assume that even larger amounts are possible as our most promising applications emerge,” Dichtel said.

Building on the material’s strength, Dichtel’s collaborators at Duke University, led by Professor Matthew Becker, incorporated the 2D polymer into Ultem, a high-performance material known for its strength and resistance to extreme temperatures, acids, and chemicals. The researchers created a composite material consisting of 97.5% Ultem fiber and just 2.5% of the 2D polymer. Despite the small percentage, the addition of the new polymer dramatically enhanced the strength and toughness of the composite.

Dichtel envisions the new material playing a key role in the development of lightweight body armor and ballistic fabrics. “Almost every property we have measured has been exceptional in some way,” he said, noting that the new polymer could revolutionize protective materials in a variety of industries. Rewrite

The research team dedicated their paper to the memory of Sir Fraser Stoddart, a Northwestern chemist and pioneer in the field of mechanical bonds. Stoddart’s work in the 1980s laid the foundation for molecular machines, which can switch, rotate, contract, and expand in controllable ways. In 2016, Stoddart was awarded the Nobel Prize in Chemistry for his contributions.

“Molecules don’t just thread themselves through each other on their own, so Fraser developed ingenious ways to template interlocked structures,” Dichtel reflected. “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.”

This groundbreaking research, part of a long tradition of innovation at Northwestern, opens new possibilities in the design and application of materials that were previously thought to be impractical for large-scale use.

As Dichtel and his team continue to explore the material’s properties, they are excited about the potential applications that could revolutionize industries from body armor to lightweight composites, marking a new era in material science.

By Impact Lab