This illustration shows how X-shaped monomers are interlinked to create the first 2D mechanically interlocked polymer. Similar to chainmail, the material exhibits exceptional strength. Credit: Mark Seniw, Center for Regenerative Nanomedicine, Northwestern University

Northwestern University researchers have achieved a groundbreaking development in materials science with the creation of a two-dimensional, mechanically interlocked polymer that mirrors the robust, flexible properties of chainmail.

This novel material, boasting the highest density of mechanical bonds ever recorded, promises revolutionary applications in lightweight body armor and other high-demand fields.

2D Interlocked Materials

A Northwestern University-led research team has achieved a remarkable feat of chemistry by creating the first two-dimensional (2D) mechanically interlocked material.

This nanoscale innovation, resembling the interlocking links of chainmail, boasts exceptional flexibility and strength. With further refinement, it shows great potential for applications in lightweight, high-performance body armor and other demanding uses that require materials to be both tough and flexible.

Published today (January 16) in the journal Science, the study establishes several key firsts in the field. This is not only the first-ever 2D mechanically interlocked polymer but also a material with an unprecedented density of 100 trillion mechanical bonds per square centimeter — the highest ever achieved. The research team accomplished this using a novel, efficient, and scalable polymerization process, paving the way for large-scale production.

“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.”

Dichtel is the Robert L. Letsinger Professor of Chemistry at the Weinberg College of Arts and Sciences and a member of the International Institute of Nanotechnology (IIN) and the Paula M. Trienens Institute for Sustainability and Energy. Madison Bardot, a Ph.D. candidate in Dichtel’s laboratory and IIN Ryan Fellow, is the study’s first author.

Challenges and Innovations in Polymerization

For years, researchers have attempted to develop mechanically interlocked molecules with polymers but found it near impossible to coax polymers to form mechanical bonds.

To overcome this challenge, Dichtel’s team took a whole new approach. They started with X-shaped monomers — which are the building blocks of polymers — and arranged them into a specific, highly ordered crystalline structure. Then, they reacted these crystals with another molecule to create bonds between the molecules within the crystal.

“I give a lot of credit to Madison because she came up with this concept for forming the mechanically interlocked polymer,” Dichtel said. “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 crystals comprise layers and layers of 2D interlocked polymer sheets. Within the polymer sheets, the ends of the X-shaped monomers are bonded to the ends of other X-shaped monomers. Then, more monomers are threaded through the gaps in between. Despite its rigid structure, the polymer is surprisingly flexible. Dichtel’s team also found that dissolving the polymer in solution caused the layers of interlocked monomers to peel off each other.

“After the polymer is formed, there’s not a whole lot holding the structure together,” Dichtel said. “So, when we put it in solvent, the crystal dissolves, but each 2D layer holds together. We can manipulate those individual sheets.”

Exploring the Nanoscale and Industrial Scalability

To examine the structure at the nanoscale, collaborators at Cornell University, led by Professor David Muller, used cutting-edge electron microscopy techniques. The images revealed the polymer’s high degree of crystallinity, confirmed its interlocked structure, and indicated its high flexibility.

Dichtel’s team also found the new material can be produced in large quantities. Previous polymers containing mechanical bonds typically have been prepared in very small quantities using methods that are unlikely to be scalable. Dichtel’s team, on the other hand, made half a kilogram of their new material and assume that even larger amounts are possible as their most promising applications emerge.

Enhancing Composite Materials

Inspired by the material’s inherent strength, Dichtel’s collaborators at Duke University, led by Professor Matthew Becker, added it to Ultem. In the same family as Kevlar, Ultem is an incredibly strong material that can withstand extreme temperatures as well as acidic and caustic chemicals. The researchers developed a composite material of 97.5% Ultem fiber and just 2.5% of the 2D polymer. That small percentage dramatically increased Ultem’s overall strength and toughness.

Dichtel envisions his group’s new polymer might have a future as a specialty material for lightweight body armor and ballistic fabrics.

“We have a lot more analysis to do, but we can tell that it improves the strength of these composite materials,” Dichtel said. “Almost every property we have measured has been exceptional in some way.”

Honoring a Legacy in Chemistry

The authors dedicated the paper to the memory of former Northwestern chemist Sir Fraser Stoddart, who introduced the concept of mechanical bonds in the 1980s. Ultimately, he elaborated these bonds into molecular machines that switch, rotate, contract, and expand in controllable ways. Stoddart, who passed away last month, received the 2016 Nobel Prize in Chemistry for this work.

“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.”

Reference: “Mechanically interlocked two-dimensional polymers” by Madison I. Bardot, Cody W. Weyhrich, Zixiao Shi, Michael Traxler, Charlotte L. Stern, Jinlei Cui, David A. Muller, Matthew L. Becker and William R. Dichtel, 16 January 2025, Science.
DOI: 10.1126/science.ads4968

The study was primarily supported by the Defense Advanced Research Projects Agency (contract number HR00112320041) and Northwestern’s IIN (Ryan Fellows Program).

By Northwestern University