A groundbreaking chemistry achievement by a research team from Northwestern University has led to the creation of the first known two-dimensional (2D) mechanically interlocked material.
This innovative nanoscale material, reminiscent of chainmail’s interconnected links, boasts impressive flexibility and strength. With further development, it has potential applications in advanced, lightweight body armor and other fields that require durable, flexible materials.
Published on Friday (Jan. 17) in the journal Science, this research sets several new milestones. It marks the debut of a 2D mechanically interlocked polymer and boasts an astounding 100 trillion mechanical bonds per square centimeter—the highest density ever recorded. The researchers achieved this breakthrough through an efficient and scalable polymerization process.
“We’ve created a completely new polymer structure,” stated William Dichtel, the study’s lead author from Northwestern. “It shares characteristics with chainmail as it resists tearing effectively; each bond allows for slight movement. When pulled, it can spread the force across multiple directions, making it challenging to tear apart since one would need to break numerous bonds simultaneously. We’re continuing to investigate its properties, which we expect to study for years to come.”
Dichtel serves as the Robert L. Letsinger Professor of Chemistry at Weinberg College of Arts and Sciences and is also part of the International Institute of Nanotechnology (IIN) as well as the Paula M. Trienens Institute for Sustainability and Energy. The paper’s first author is Madison Bardot, a Ph.D. candidate in Dichtel’s lab and an IIN Ryan Fellow.
Developing a novel process
For years, scientists have struggled to create mechanically interlocked molecules utilizing polymers, finding it extremely difficult to induce mechanical bonding in polymers.
To overcome this hurdle, Dichtel’s team adopted a completely fresh approach. They began with X-shaped monomers—essentially the fundamental units of polymers—and organized them into a particularly structured crystalline arrangement. Subsequently, they reacted these crystals with another molecule to establish connections between them.
“Madison deserves a lot of credit for conceptualizing the formation of this mechanically interlocked polymer,” Dichtel noted. “This idea represented a high-risk, high-reward scenario where we had to rethink our assumptions about molecular crystal reactions.”
The resultant crystals consist of layers of interlocked 2D polymer sheets. Within these sheets, the ends of the X-shaped monomers bond with those of other X-shaped monomers, while additional monomers intertwine through the gaps. Despite its solid form, the polymer surprises with its flexibility. Dichtel’s team also discovered that dissolving the polymer results in the layers of interlocked monomers separating from one another.
“After the polymer is formed, there isn’t much that keeps the structure intact,” Dichtel explained. “Thus, when we introduce a solvent, the crystal dissolves, yet each 2D layer remains cohesive, allowing us to manipulate those individual sheets.”
To analyze the structure at a nanoscale level, collaborators from Cornell University, under the guidance of Professor David Muller, employed advanced electron microscopy methods. The images confirmed the polymer’s high crystallinity, validated its interlocked structure, and demonstrated its great flexibility.
Dichtel’s group also discovered that this new material can be produced in significant quantities. Previous polymers with mechanical bonds were often created in minuscule amounts through methods that were not easily scalable. In contrast, Dichtel’s team successfully synthesized half a kilogram of their new material, with hopes of even greater outputs as promising applications arise.
Enhancing robust polymers
Motivated by the material’s inherent strength, researchers from Duke University, led by Professor Matthew Becker, incorporated it into Ultem. Ultem, closely related to Kevlar, is highly resilient and can endure extreme temperatures along with acidic and caustic substances. The team formulated a composite material consisting of 97.5% Ultem fiber and a mere 2.5% of the 2D polymer, which significantly enhanced Ultem’s overall toughness and strength.
Dichtel envisions the potential for this new polymer to be developed as a specialized material for lightweight body armor and ballistic fabrics.
“We’re still in the analysis phase, but it’s clear that it boosts the strength of these composite materials,” Dichtel remarked. “Almost all properties measured have shown exceptional performance in some regard.”
Grounded in Northwestern’s legacy
The authors dedicated the paper to the memory of esteemed Northwestern chemist Sir Fraser Stoddart, who first introduced the concept of mechanical bonds in the 1980s. He further developed these bonds into molecular machines that could switch, rotate, contract, and expand in controlled manners. Stoddart passed away last month and was awarded the Nobel Prize in Chemistry in 2016 for his contributions.
“Molecules don’t just automatically interlock; Fraser invented clever techniques to create such structures,” said Dichtel, a former postdoctoral researcher in Stoddart’s lab at UCLA. “However, even these methods struggled to be practical for large molecules like polymers. In our work, the molecules are securely held within a crystal, which facilitates the formation of mechanical bonds around each one.
“Thus, these mechanical bonds have a rich tradition at Northwestern, and we’re excited to explore their potential in ways that were previously unattainable.”
The study, “Mechanically interlocked two-dimensional polymers,” received major support from the Defense Advanced Research Projects Agency (contract number HR00112320041) and Northwestern’s IIN (Ryan Fellows Program).
