Fractured artificial rock helps crack a 54-year-old mystery

by Adam Hadhazy, Princeton University

Princeton researchers have developed a technique to better understand how polymers 

flow through small channels under pressure. Credit: David Kelly Crow

Princeton researchers have solved a 54-year-old puzzle about why certain fluids strangely slow down under pressure when flowing through porous materials, such as soils and sedimentary rocks. The findings could help improve many important processes in energy, environmental and industrial sectors, from oil recovery to groundwater remediation.

The fluids in question are called polymer solutions. These solutions—everyday examples of which include cosmetic creams and the mucus in our noses—contain dissolved polymers, or materials made of large molecules with many repeating subunits. Typically, when they're put under pressure, polymer solutions become less viscous and flow faster. But when going through materials with lots of tiny holes and channels, the solutions tend to become more viscous and gunky, reducing their flow rates.

To get at the root of the problem, the Princeton researchers devised an innovative experiment using a see-through porous medium made of tiny glass beads—a transparent artificial rock. This lucid medium allowed the researchers to visualize a polymer solution's movement. The experiment revealed that the long-baffling increase in viscosity in porous media happens because the polymer solution's flow becomes chaotic, much like turbulent air on an airplane ride, swirling into itself and gumming up the works.

"Surprisingly, until now, it has not been possible to predict the viscosity of polymer solutions flowing in porous media," said Sujit Datta, an assistant professor of chemical and biological engineering at Princeton and senior author of the study appearing Nov. 5 in the journal Science Advances. "But in this paper, we've now finally shown these predictions can be made, so we've found an answer to a problem that has eluded researchers for over a half-century."

"With this study, we finally made it possible to see exactly what is happening underground or within other opaque, porous media when polymer solutions are being pumped through," said Christopher Browne, a Ph.D. student in Datta's lab and the paper's lead author.

Browne ran the experiments and built the experimental apparatus, a small rectangular chamber randomly packed with tiny borosilicate glass beads. The setup, akin to an artificial sedimentary rock, spanned only about half the length of a pinky finger. Into this faux rock, Browne pumped a common polymer solution laced with fluorescent latex microparticles to help see the solution's flow around the beads. The researchers formulated the polymer solution so the material's refractive index offset light distortion from the beads and made the whole setup transparent when saturated. Datta's lab has innovatively used this technique to create see-through soil for studying ways to counter agricultural droughts, among other investigations.

Browne then zoomed in with a microscope on the pores, or holes between the beads, which occur on the scale of 100 micrometers (millionths of a meter) in size, or similar to the width of a human hair, in order to examine the fluid flow through each pore. As the polymer solution worked its way through the porous medium, the fluid's flow became chaotic, with the fluid crashing back into itself and generating turbulence. What's surprising is that, typically, fluid flows at these speeds and in such tight pores are not turbulent, but "laminar": the fluid moves smoothly and steadily. As the polymers navigated the pore space, however, they stretched out, generating forces that accumulated and generated turbulent flow in different pores. This effect grew more pronounced when pushing the solution through at higher pressures.

"I was able to see and record all these patchy regions of instability, and these regions really impact the transport of the solution through the medium," said Browne.

Princeton researchers have developed a technique to better understand how polymers flow through small channels under pressure. Credit: David Kelly Crow

The Princeton researchers used data gathered from the experiment to formulate a way to predict the behavior of polymer solutions in real-life situations.

Gareth McKinley, a professor of mechanical engineering at the Massachusetts Institute of Technology who was not involved in the study, offered comments on its significance.

"This study shows definitively that the large increase in the macroscopically observable pressure drop across a porous medium has its microscopic physical origins in viscoelastic flow instabilities that occur on the pore scale of the porous medium," McKinley said.

Given that viscosity is one of the most fundamental descriptors of fluid flow, the findings not only help deepen understanding of polymer solution flows and chaotic flows in general, but also provide quantitative guidelines to inform their applications at large scales in the field.

"The new insights we have generated could help practitioners in diverse settings determine how to formulate the right polymer solution and use the right pressures needed to carry out the task at hand," said Datta. "We're particularly excited about the findings' application in groundwater remediation."

Because polymer solutions are inherently goopy, environmental engineers inject the solutions into the ground at highly contaminated sites such as abandoned chemical factories and industrial plants. The viscous solutions help push out trace contaminants from the affected soils. Polymer solutions likewise aid in oil recovery by pushing oil out of the pores in underground rocks. On the remediation side, polymer solutions enable "pump and treat," a common method for cleaning up groundwater polluted with industrial chemicals and metals that involves bringing the water to a surface treatment station. "All these applications of polymer solutions, and more, such as in separations and manufacturing processes, stand to benefit from our findings," said Datta.

Overall, the new findings on polymer solution flow rates in porous media brought together ideas from multiple fields of scientific inquiry, ultimately disentangling what had started out as a long-frustrating, complex problem.

"This work draws connections between studies of polymer physics, turbulence, and geoscience, following the flow of fluids in rocks underground as well as through aquifers," said Datta. "It's a lot of fun sitting at the interface between all these different disciplines."

Tiny polymer springs give a boost to environmental cleanup

More information: Christopher A. Browne et al, Elastic turbulence generates anomalous flow resistance in porous media, Science Advances (2021). DOI: 10.1126/sciadv.abj2619. www.science.org/doi/10.1126/sciadv.abj2619

Journal information: Science Advances 

Provided by Princeton University 

 

Closed loop for circular economy: New polymer recycling strategy ensures both high stability and complete recyclability

by Shinshu University

In a new study, researchers from Japan proposed a new recycling strategy that facilitates

 material recycling without any loss in their properties. In “closed-loop” recycling, a polymer 

film composed of polyacrylate-based microparticles is disassembled into individual 

microparticles, which can be reassembled to form the film without losing any properties.

 This process could also be applied to recycle polymer microparticles in composite 

materials. Credit: Daisuke Suzuki from Shinshu University

The ever-increasing generation of plastic solid waste has resulted in global plastic pollution both on land and in the oceans. Projections show that plastic waste will double in the next 20 years, causing further environmental problems. Large amounts of plastic waste are, at present, incinerated or deposited in landfills. This not only degrades the environment but also depletes valuable resources.

In this light, recycling plastics such as polymers is a promising sustainable alternative for waste management. But this involves the breaking of chemical bonds between monomers (building blocks of polymers), which diminishes their overall stability and quality. Addressing this concern, researchers have developed methods to recycle polymers in a "closed loop," that is, without the loss of these properties. However, these methods are complicated and expensive and require specialized monomers, necessitating further innovation.

In this direction, a group of researchers led by Daisuke Suzuki, an Associate Professor at Shinshu University, has recently proposed a closed-loop recycling process based on polymer microparticles. Their work, co-authored by Dr. Takumi Watanabe and Dr. Haruka Minato of Shinshu University, has been published in Green Chemistry.

Prof. Suzuki briefly explains the rationale behind their strategy: "Recycling materials without deterioration (closed-loop recycling) is attractive in terms of reducing anthropogenic waste. However, this currently remains very difficult given that there usually is a trade-off between mechanical stability and degradability of polymer materials."

"Our material recycling concept with microparticles enables the recycling of a huge amount of functional polymer materials that we use in our day-to-day lives and has the potential to solve the problems of resource depletion and environmental pollution."

In their study, the authors prepared polymer microparticles via the aqueous emulsion polymerization of methyl acrylate (MA) monomers in water, which resulted in polymer chains. These aggregated to form a solution containing uniform spherical poly-MA microparticles. The solution was then dried to get a thin polymer film with physical (as opposed to chemical) cross-linking among the microparticles, which could be reobtained by dissolving the film in ethanol. These recycled microparticles, in turn, could be reused to form various recycled materials.

The films synthesized in this work exhibit several desirable properties, which they retain upon recycling. They have high mechanical stability and fracture energy, which is an indicator of their toughness. The latter property increases with the interfacial thickness between the poly-MA microparticles. This, in turn, decreases with the degree of interparticle cross-linking but increases upon heating the film.

The researchers further enhanced the fracture energy of the polymer films by mixing the microparticles with silica nanofillers. Moreover, adding colored pigments gave the resulting composite films tunable optical properties, which did not diminish upon recycling. These results suggest that closed-loop recycling based on polymer microparticles will enable resource circulation for polymers as well as numerous other composite materials that contain polymer microparticles to create adhering interfaces between their different layers.

Prof. Suzuki says, "Our concept can lead to the production of fully recyclable films with high fracture energy. Therefore, it will enable the recycling of huge amounts of various polymer materials, thus reducing plastic waste and potentially solving the problems of environmental degradation and plastic pollution."

The "closed"-loop recycling strategy certainly "opens" new doors for the efficient and sustainable recycling of polymer materials!

More information: Takumi Watanabe et al, Closed-loop recycling of microparticle-based polymers, Green Chemistry (2023). DOI: 10.1039/D3GC00090G

Journal information: Green Chemistry 

Provided by Shinshu UniversityTechnology transforms plastic waste bottles into polymers for lithium-ion batteries

 

New self-healing polymer possesses a quality never before seen at any scale

Material scientists at Texas A&M have developed a dynamic material that self-heals after puncturing by changing from solid to liquid and back

Texas A&M University

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An illustration of a potential use for the new material. A key goal of the research is to design a material that will protect structures such as orbiting satellites and vehicles in space, with applications for military equipment and body armor here on Earth.

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Credit: Texas A&M Engineering

What if there were a fabric that, like Superman, could take a bullet and self-heal? Such a super-dynamic, action-powered polymer might actually help protect real-life flyers in space.

Material scientists at Texas A&M University have developed just such a polymer with a unique self-healing property never before seen at any scale. When struck by a projectile, this material stretches so much that when the projectile manages to pass through, it takes only a small amount of the polymer with it. As a result, the hole left behind is much smaller than the projectile itself.

However, for now, this effect has only been observed under extreme temperatures and at the nanoscale.

“This is the first time a material at any scale has displayed this behavior,” said Dr. Svetlana Sukhishvili, a professor in the Department of Materials Science and Engineering, who has been working on development of this polymer film with materials science and engineering professor Dr. Edwin (Ned) Thomas, and then-graduate student Dr. Zhen Sang. Their findings were published in the March/April issue of Materials Today.

“Besides being very cool, the new polymer will likely have many applications, including making the windows of space vehicles more resilient to the onslaught of micrometeoroids,” Thomas said. Space vehicles are frequently bombarded with micrometeoroids traveling at speeds of 10 kilometers per second. A micrometeoroid can create a hole in the window that, while small, is visible to the human eye. However, a window manufactured with a layer of this polymer could potentially sustain damage tinier than the meteoroid itself.

Thomas, who first suggested subjecting the polymer to ballistic testing, said a key goal of the research is to design a material that will protect structures such as orbiting satellites and vehicles in space, with applications for military equipment and body armor on Earth.

The phenomenal behavior occurs in the new solid polymer film as it melts when impacted by a laser-launched high-speed projectile, and snaps back to its original shape when cooled. The polymer does this by absorbing much of the kinetic energy generated by the projectile, causing the film to stretch and liquify as the projectile continues its journey, finally piercing the film. Once pierced, the polymer quickly cools, its covalent bonds reform, and it returns to its original solid state, leaving a tiny hole.

“A major goal of our work was to see if we could simultaneously provide a material that would absorb a lot of kinetic energy per unit target mass from the high-speed projectile and be capable of very rapid healing of the punctured region,” Thomas said. “We wanted the post-impact material to still be capable of performing its intended function, such as carrying air or liquids and remaining sealed against the loss of such fluids across the material membrane.”

The material is a Diels-Adler Polymer or DAP, so-named by the researchers for its dynamic covalent bond networks that can be broken and reformed. It belongs to a class of materials called Covalent Adaptative Networks or CANs. While other Diels-Adler networks have been reported in the scientific literature, DAP’s specific chemistry, topology and self-healing quality are novel. The DAP acronym could also refer to their polymer as a Dynamic Action-Powered material for its ability to self-heal.

“When we were synthesizing DAPs, we aimed to do it in such a way that the polymers would turn to liquids upon temperature increase,” Sukhishvili said. “Although this feature was introduced to facilitate 3D printing, we thought that due to its ability to liquify upon heating, our polymers could show improved ballistic healing characteristics.”

“Polymers are amazing materials, especially DAP materials,” Thomas explained. “Because at low temperatures, they are stiff and strong; then at higher temperatures, they become elastic; and at still higher temperatures, they become an easily flowing liquid. That’s a huge range of property behavior.” What’s more, he said, the process reverses itself. “Nothing else on the planet can do that!”

The DAP structure is of long polymer chains containing double carbon bonds that break when severe strain and heat are applied, but quickly reform when cooled, albeit not necessarily in the same configuration.

“Think of the long polymer chains in the fabric as being like a bowl of Ramen noodle soup,” said Sang, who worked on this project for his doctoral research and is first author on the paper. “You can stir it with chopsticks, then freeze it. When you unfreeze it, you can stir it, then refreeze. It will have the same ingredients as before, just in a slightly different appearance.” 

Sang, who is now an engineer at Apple, Inc., said it wasn’t easy to do ballistic testing at such a small scale until he came across a new research methodology called LIPIT (laser-induced projectile impact testing), recently developed by Thomas and colleagues at MIT. Sang used LIPIT to laser-launch a tiny silica projectile 3.7 micrometers in diameter from a glass slide covered with a thin gold film resting on a one-square inch platform. His target consisted of a thin layer (75 to 435 nanometers) of the super DAP.

An ultrahigh-speed camera with a 3-nanosecond exposure time at 50 nanosecond intervals recorded the action. The research team then used scanning electron microscopy, laser scanning confocal microscopy and an infrared nano spectrometer to view the holes and assess the covalent bonding in the super polymer.

The results were puzzling at first, Sang said, because he could find no holes in the targeted polymer.

“Was I not aiming correctly? Were there no projectiles? What’s wrong with my experiment, I asked myself,” he said. However, when he placed the DAP sample under the infrared nano spectrometer, which combines chemical analysis with high-scale resolution, he was able to see the tiny perforations. “This was actually a surprising, surprising finding,” Sang said. “A very exciting finding!”

He explained this behavior can’t yet be recreated at the macro level because the strain rate during perforation of a very thin target material under impact is so much larger than at the nanoscale. “If this strain rate is really high, materials often have unexpected behavior that people don’t usually see under normal circumstances,” Sang said. “With the LIPIT apparatus that we’re using, we’re talking about a strain rate many orders of magnitude higher than for conventional scale bullets and targets. At that perspective, materials behave very differently.”

Other coauthors on the paper are materials science doctoral student Hongkyu Eoh; former postdoctoral researchers Drs. Kailu Xiao, Wenpeng Shan and Jinho Hyon; and Dr. Dmitry Kurouski, associate professor in the department of biochemistry and biophysics at Texas A&M.

Sukhishvili and Thomas plan to continue researching the super DAP using different polymer compositions, temperature- and stress-responses. 

“One could even imagine designing DAPs with characteristics such that it would be possible to absorb kinetic energy by breaking DAP bonds, then some of these broken bonds could very rapidly reform – by perhaps having just the right ‘bond reform catalyst’ present in the material – whereby the projectile would have to break these bonds a second (or even multiple times) before the material ultimately heals itself, and is ready for the next ballistic event.

“To date, no material has the requisite time response to deform, break, reform; and then deform, break and reform again during the sub-microsecond interval of a ballistic event,” Thomas said.

By Denise Brehm

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Journal

Materials Today

DOI

10.1016/j.mattod.2024.12.006 

Article Title

Supersonic puncture-healable and impact resistant covalent adaptive networks

 

Polymer editing can upcycle waste into higher-performance plastics

DOE/Oak Ridge National Laboratory

 

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To upcycle the polymers of discarded plastics, chemists at Oak Ridge National Laboratory invented a way to generate new macromolecules with more valuable properties than those of the starting material.

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Credit: Adam Malin/ORNL, U.S. Dept. of Energy

 

By editing the polymers of discarded plastics, chemists at the Department of Energy’s Oak Ridge National Laboratory have found a way to generate new macromolecules with more valuable properties than those of the starting material. Upcycling may help remedy the roughly 450 million tons of plastic discarded worldwide annually, of which only 9% gets recycled; the rest is incinerated or winds up in landfills, oceans or elsewhere.

 

ORNL’s invention may change plastic’s environmental fate by rearranging polymeric building blocks to customize the properties of plastics. Molecular subunits link to produce polymer chains that can connect through their backbones and cross-linked molecules to form multipurpose plastics. The makeup of polymer chains determines how strong, rigid or heat-resistant those plastics will be.

 

Molecular editing is so promising that it has been the basis of two Nobel Prizes in Chemistry. In 2005, the prize went to developers of the metathesis reaction, which breaks and makes double bonds between carbon atoms in rings and chains so their subunits can swap to create new molecules limited only by imagination. Similarly, in 2020, the prize went to developers of CRISPR, “genetic scissors” for editing DNA strands, biopolymers made of nucleotide subunits that carry the code of life.

 

”This is CRISPR for editing polymers,” said ORNL’s Jeffrey Foster, who led a study that was published in Journal of the American Chemical Society. “However, instead of editing strands of genes, we are editing polymer chains. This isn’t the typical plastic recycling ‘melt and hope for the best’ scenario.”

 

The ORNL researchers precisely edited commodity polymers that significantly contribute to plastic waste. In some experiments, the researchers worked with soft polybutadiene, which is common in rubber tires. In other experiments, they worked with tough acrylonitrile butadiene styrene, the stuff of plastic toys, computer keyboards, ventilation pipes, protective headgear, vehicle trim and molding, and kitchen appliances.

 

“This is a waste stream that's really not recycled at all,” Foster said. “We're addressing a significant component of the waste stream with this technology. That'd make a pretty big impact just from conservation of mass and energy from materials that are now going into landfills.”

 

Dissolving the waste polymers is the first step in creating drop-in additives for polymer synthesis. The researchers shredded synthetic or commercial polybutadiene and acrylonitrile butadiene styrene and immersed the material in a solvent, dichloromethane, to conduct a chemical reaction at a low temperature (40 degrees Celsius) for less than two hours.

 

A ruthenium catalyst facilitated the polymerization, or polymer addition. Industrial firms have used this catalyst to make robust plastics and to convert biomass such as plant oils into fuels and other high-value organic compounds with no difficulty, highlighting the potential for its use in chemical upcycling.

 

The molecular building blocks of the polymer backbone contain functional groups, or clusters of atoms that serve as reactive sites for modification. Notably, the  double bonds between carbons increase the chances for chemical reactions that enable polymerization. A carbon ring opens at a double bond to create a polymer chain that grows as each functional polymer unit directly slips in, conserving the material. The plastic additive also helps control the molecular weight of the synthesized material and, in turn, its properties and performance.

 

If this material synthesis strategy could be expanded to a broader range of industrially important polymers, then it could prove an economically viable path for reusing manufacturing materials that today can only be used in a single product. The upcycled materials might be, for instance, softer and stretchier than the original polymers or, perhaps, easier to shape and harden into durable thermoset products.

 

The scientists upcycled plastic waste by employing two processes in tandem. Both are types of metathesis, which means a change of places. Double bonds break and form between carbon atoms, allowing polymer subunits to swap.

 

One process, called ring-opening metathesis polymerization, opens carbon rings and elongates them into chains. The other process, called cross metathesis, inserts chains of polymer subunits from one polymer chain into another.

 

Traditional recycling fails to capture the value in discarded plastics because it reuses polymers that become less valuable through degradation with each melt and reuse. By contrast, ORNL’s innovative upcycling utilizes the existing building blocks to incorporate the mass and characteristics of the waste material and provide added functionality and value.

 

”The new process has high atom economy,” Foster said. “That means that we can pretty much recover all the material that we put in.”

 

The ORNL scientists demonstrated that the process, which uses less energy and produces fewer emissions than traditional recycling, efficiently integrates waste materials without compromising polymer quality. Foster, Ilja Popovs and Tomonori Saito conceptualized the paper’s ideas. Nicholas Galan, Isaiah Dishner and Foster synthesized monomer subunits and optimized their polymerization. Joshua Damron performed nuclear magnetic resonance spectroscopy experiments to analyze reaction kinetics. Jackie Zheng, Chao Guan and Anisur Rahman characterized mechanical and thermal properties of final materials.

 

“The vision is that this concept could be extended to any polymer that has some sort of backbone functional group to react with,” Foster said. If scaled up and expanded to employ other additives, broader classes of waste could be mined for molecular building blocks, dramatically reducing the environmental impact of other difficult-to-process plastics. The circular economy — in which waste materials are repurposed rather than discarded — then becomes a more realistic goal.

 

Next, the researchers are interested in changing the types of subunits in the polymer chain and rearranging them to see whether they can create high-performance thermoset materials. Examples are epoxy resins, vulcanized rubber, polyurethane and silicone. Once cured, thermoset materials cannot be remelted or reshaped because their molecular structure is cross-linked. That makes their recycling a challenge.

 

The researchers are also interested in optimizing solvents for environmental sustainability during industrial processing.

 

”Some preprocessing is going to be required on these waste plastics that we still have to figure out,” Foster said.

 

The DOE Office of Science (Materials Science and Engineering program)[DL1]  and ORNL Laboratory Directed Research and Development program funded the research.

 

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. — Dawn Levy

 

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Journal

Journal of the American Chemical Society

DOI

10.1021/jacs.4c10588 

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

Polyalkenamers as Drop-In Additives for Ring-Opening Metathesis Polymerization: A Promising Upcycling Paradigm

 FORWARD TO THE PAST

New chainmail-like material could be the future of armor

First 2D mechanically interlocked polymer exhibits exceptional flexibility and strength

Northwestern University

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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.

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Credit: Mark Seniw, Center for Regenerative Nanomedicine, Northwestern University

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

Resembling the interlocking links in chainmail, the nanoscale material exhibits exceptional flexibility and strength. With further work, it holds promise for use in high-performance, light-weight body armor and other uses that demand lightweight, flexible and tough materials.

Publishing on Friday (Jan. 17) in the journal Science, the study marks several firsts for the field. Not only is it the first 2D mechanically interlocked polymer, but the novel material also contains 100 trillion mechanical bonds per 1 square centimeter — the highest density of mechanical bonds ever achieved. The researchers produced this material using a new, highly efficient and scalable polymerization process.

“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.

Inventing a new process

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.” 

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 even larger amounts are possible as their most promising applications emerge.

Adding strength to tough polymers

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 light-weight 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.”

Steeped in Northwestern history

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.”

The study, “Mechanically interlocked two-dimensional polymers,” was primarily supported by the Defense Advanced Research Projects Agency (contract number HR00112320041) and Northwestern’s IIN (Ryan Fellows Program).

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Journal

Science

DOI

10.1126/science.ads4968 

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

Mechanically interlocked two-dimensional polymers

Article Publication Date

17-Jan-2025