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).

Fifteen years of a persistent, devastating megadrought — the longest lasting in a thousand years — have nearly dried out Chile’s water reserves, even affecting the country’s vital mining output. This is but one blatant example of how the warming climate is causing multi-year droughts and acute water crises in vulnerable regions around the globe. However, droughts tend only to be noticed when they damage agriculture or visibly affect forests. Thus, some pressing questions arise: Can we consistently identify extreme multi-year droughts and examine their impacts on ecosystems? And what can we learn from the drought patterns of the past forty years?

To answer these questions, researchers from the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) and the Institute of Science and Technology Austria (ISTA) have analyzed global meteorological data and modeled droughts between 1980 and 2018. They demonstrated a worrying increase in multi-year droughts that became longer, more frequent, and more extreme, covering more land. “Each year since 1980, drought-stricken areas have spread by an additional fifty thousand square kilometers on average — that’s roughly the area of Slovakia, or the US states of Vermont and New Hampshire put together — , causing enormous damage to ecosystems, agriculture, and energy production,” says ISTA Professor Francesca Pellicciotti, the Principal Investigator of the WSL-funded EMERGE Project, under which the present study was conducted. The team aims to unveil the possible long-lasting effects of persistent droughts around the globe and help inform policy preparing for more frequent and severe future megadroughts.

Unveiling extreme droughts that flew under the radar

The international team used the CHELSA climate data prepared by WSL Senior Researcher and study author Dirk Karger, which goes back to 1979. They calculated anomalies in rainfall and evapotranspiration — water evaporation from soil and plants — and their impact on natural ecosystems worldwide. This allowed them to determine the occurrence of multi-year droughts both in well-studied and less accessible regions of the planet, especially in areas like tropical forests and the Andes, where little observational data is available. “Our method not only mapped well-documented droughts but also shed light on extreme droughts that flew under the radar, such as the one that affected the Congo rainforest from 2010 to 2018,” says Karger. This discrepancy is likely due to how forests in various climate regions respond to drought episodes. “While temperate grasslands have been most affected in the past forty years, boreal and tropical forests appeared to withstand drought more effectively and even displayed paradoxical effects during the onset of drought.” But how long can these forests resist the harsh blow of climate change?

Contrasting impacts on ecosystems

The persistently rising temperatures, extended droughts, and higher evapotranspiration ultimately lead to dryer and browner ecosystems, despite also causing heavier precipitation episodes. Thus, scientists can use satellite images to monitor the effect of drought by tracking changes in vegetation greenness over time. While this analysis works well for temperate grasslands, the changes in greenness cannot be tracked as easily over dense tropical forest canopies, leading to underestimated effects of drought in such areas. Thus, to ensure consistent results worldwide, the team developed a multistep analysis that better resolves the changes in high-leaf regions and ranked the droughts by their severity since 1980. Unsurprisingly, they showed that megadroughts had the highest immediate impact on temperate grasslands. ‘Hotspot’ regions included the western USA, central and eastern Mongolia, and particularly southeastern Australia, where the data overlapped with two well-documented multi-year ecological droughts. On the other hand, the team shed additional light on the paradoxical effects observed in the tropical and boreal forests. While tropical forests can offset the expected effects of drought as long as they have enough water reserves to buffer the decrease in rainfall, boreal forests and tundra react in their distinct way. It turns out that the warming climate extends the boreal growth season since vegetation growth in these regions is limited by lower temperatures rather than water availability.

Droughts evolve in time and space

The results show that the trend of intensifying megadroughts is clear: The team generated the first global — and globally consistent — picture of megadroughts and their impact on vegetation at high resolution. However, the long-term effects on the planet and its ecosystems remain largely unknown. Meanwhile, the data already agrees with the observed widely greening pan-Arctic. “But in the event of long-term extreme water shortages, trees in tropical and boreal regions can die, leading to long-lasting damage to these ecosystems. Especially, the boreal vegetation will likely take the longest to recover from such a climate disaster,” says Karger. Pellicciotti hopes the team’s result will help change our perception of droughts and how to prepare for them: “Currently, mitigation strategies largely consider droughts as yearly or seasonal events, which stands in stark contrast to the longer and more severe megadroughts we will face in the future,” she says. “We hope that the publicly available inventory of droughts we are putting out will help orient policymakers toward more realistic preparation and prevention measures.” As a glaciologist, Pellicciotti also seeks to examine the effects of megadroughts in the mountains and how glaciers can buffer them. She leads a collaborative project titled “MegaWat — Megadroughts in the Water Towers of Europe — From Process Understanding to Strategies for Management and Adaptation.”

Project and funding information The present study was conducted within the scope of the EMERGE Project of the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) with Professor Francesca Pellicciotti from the Institute of Science and Technology Austria (ISTA) serving as its Principal Investigator. The research was supported by funding from the Extreme Program of the WSL for the EMERGE project.