Hexbyte Glen Cove Elastic polymer that is both stiff and tough, resolves long-standing quandary

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A highly entangled hydrogel (left) and a regular hydrogel (right). Credit: Suo Lab/Harvard SEAS

Polymer science has made possible rubber tires, Teflon and Kevlar, plastic water bottles, nylon jackets among many other ubiquitous features of daily life. Elastic polymers, known as elastomers, can be stretched and released repeatedly and are used in applications such as gloves and heart valves, where they need to last a long time without tearing. But a conundrum has long stumped polymer scientists: Elastic polymers can be stiff, or they can be tough, but they can’t be both.

This stiffness-toughness conflict is a challenge for scientists developing polymers that could be used in applications including tissue regeneration, bioadhesives, bioprinting, wearable electronics, and soft robots.

In a paper published today in Science, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have resolved that long-standing conflict and developed an elastomer that is both stiff and tough.

“In addition to developing polymers for emerging applications, scientists are facing an urgent challenge: Plastic pollution,” said Zhigang Suo, the Allen E. and Marilyn M. Puckett Professor of Mechanics and Materials, the senior author of the study. “The development of biodegradable polymers has once again brought us back to fundamental questions—why are some polymers tough, but others brittle? How do we make polymers resist tearing under repeated stretching?”

Polymer chains are made by linking together monomer building blocks. To make a material elastic, the are crosslinked by . The more crosslinks, the shorter the chains and the stiffer the material.

“As your polymer chains become shorter, the energy you can store in the material becomes less and the material becomes brittle,” said Junsoo Kim, a at SEAS and co-first author of the paper. “If you have only a few crosslinks, the chains are longer, and the material is tough but it’s too squishy to be useful.”

To develop a polymer that is both stiff and tough, the researchers looked to physical, rather than to link the polymer chains. These physical bonds, called entanglements, have been known in the field for almost as long as polymer science has existed, but they’ve been thought to only impact stiffness, not toughness.

But the SEAS research team found that with enough entanglements, a polymer could become tough without compromising stiffness. To create highly entangled polymers, the researchers used a concentrated monomer precursor solution with 10 times less water than other polymer recipes.

Each polymer chain has a large number of entanglements along its length (left) and a cross-link at each end. A stretched polymer (middle) showing transmission of the tension to other chains. Credit: Suo Lab/Harvard SEAS

“By crowding all the monomers into this solution with less water and then polymerizing it, we forced them to be entangled, like tangled strings of yarn,” said Guogao Zhang, a postdoctoral fellow at SEAS and co-first author the paper. “Just like with knitted fabrics, the polymers maintain their connection with one another by being physically intertwined.”

With hundreds of these entanglements, just a handful of chemical crosslinks are required to keep the polymer stable.

“As elastomers, these polymers have high toughness, strength, and fatigue resistance,” said Meixuanzi Shi, a visiting scholar at SEAS and co-author of the paper. “When the polymers are submerged in water to become hydrogels, they have low friction, and high wear resistance.”

That high fatigued resistance and high wear resistance increases the durability and lifespan of the polymers.

“Our research shows that by using entanglements rather than crosslinks, we could decrease the consumption of some plastics by increasing the durability of the materials,” said Zhang.

“We hope that this new understanding of polymer structure will expand opportunities for applications and pave the way for more sustainable, long-lasting polymer materials with these exceptional mechanical properties,” said Kim.

More information:
Zhigang Suo, Fracture, fatigue, and friction of polymers in which entanglements greatly outnumber crosslinks, Science (2021). DOI: 10.1126/science.abg6320

Elastic polymer that is both stiff and tough, resolves long-standing quandary (2021, October 7)
retrieved 8 October 2021
from https://phys.org/news/2021-10-elastic-polymer-stiff-tough-long-standing.html

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Hexbyte Glen Cove Solving a long-standing mystery about the desert's rock art canvas thumbnail

Hexbyte Glen Cove Solving a long-standing mystery about the desert’s rock art canvas

Hexbyte Glen Cove

Petroglyphs at Mesa Verde National Park, Colorado. Credit: Christine Fry & Peter Russo

Wander around a desert most anywhere in the world, and eventually you’ll notice dark-stained rocks, especially where the sun shines most brightly and water trickles down or dew gathers. In some spots, if you’re lucky, you might stumble upon ancient art—petroglyphs—carved into the stain. For years, however, researchers have understood more about the petroglyphs than the mysterious dark stain, called rock varnish, in which they were drawn.

In particular, science has yet to come to a conclusion about where , which is unusually rich in manganese, comes from.

Now, scientists at the California Institute of Technology, the Department of Energy’s SLAC National Accelerator Laboratory and elsewhere think they have an answer. According to a recent paper in Proceedings of the National Academy of Sciences, rock varnish is left behind by microbial communities that use manganese to defend against the punishing desert sun.

The mystery of rock varnish is old, said Usha Lingappa, a graduate student at Caltech and the study’s lead author. “Charles Darwin wrote about it, Alexander von Humboldt wrote about it,” she said, and there is a long-standing debate about whether it has a biological or inorganic origin.

But, Lingappa said, she and her colleagues didn’t actually set out to understand where rock varnish comes from. Instead, they were interested in how microbial ecosystems in the desert interact with rock varnish. To do so, they deployed as many techniques as they could come up with: DNA sequencing, mineralogical analyses, , and—aided by Stanford Synchroton Radiation Lightsource (SSRL) scientist Samuel Webb—advanced X-ray spectroscopy methods that could map different kinds of manganese and other elements within samples of rock varnish.

“By combining these different perspectives, maybe we could draw a picture of this ecosystem and understand it in new ways,” Lingappa said. “That’s where we started, and then we just stumbled into this hypothesis” for rock varnish formation.

Among the team’s key observations was that, while manganese in desert dust is usually in particle form, it was deposited in more continuous layers in varnish, a fact revealed by X-ray spectroscopy methods at SSRL that can tell not only what make up a sample but also how they are distributed, on a microscopic scale, throughout the sample.

That same analysis showed that the kinds of manganese compounds in varnish were the result of ongoing chemical cycles, rather than being left out in the sun for millennia. That information, combined with the prevalence of bacteria called Chroococcidiopsis that use manganese to combat the oxidative effects of the harsh desert sun, led Lingappa and her team to conclude that rock varnish was left behind by those bacteria.

For his part, Webb said that he always enjoys a project—”I’ve been a mangaphile for a while now”—and that this project arrived at the perfect time, given advances in X-ray spectroscopy at SSRL. Improvements in X-ray beam size allowed the researchers to get a finer-grained picture of rock varnish, he said, and other improvements ensured that they could get a good look at their samples without the risk of damaging them. “We’re always tinkering and fine-tuning things, and I think it was the right time for a project that maybe 5 or 10 years ago wouldn’t really have been feasible.”

More information:
Usha F. Lingappa et al, An ecophysiological explanation for manganese enrichment in rock varnish, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073/pnas.2025188118

Solving a long-standing mystery about the desert’s rock art canvas (2021, July 2)
retrieved 3 July 2021
from https://phys.org/news/2021-07-long-standing-mystery-art-canvas.html

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