Newswise — DURHAM, N.C. --When considering the environmental effects of automobiles, a significant amount of attention has been dedicated to tailpipe emissions. However, there is an additional ecological concern stemming from cars that might not immediately come to mind: the pollution caused by microplastics.
Car tires consist of a combination of rubber, plastic polymers, and various other substances. When tires come into contact with the road, minuscule particles of these materials, often smaller than a grain of sand, detach and disperse. These particles can find their way into soils and waterways through runoff, while others become airborne, posing uncertain long-term consequences for human and environmental well-being.
According to a recent study published in the journal Science on June 22, Duke chemistry professor Stephen Craig and his team propose a promising solution for improvement. They outline a method to significantly enhance the toughness of rubbery materials by an entire magnitude, while maintaining their overall performance and functionality.
Craig is a member of a collaborative group comprising researchers from Duke University and MIT. Their focus has been on investigating the molecular reactions occurring in a group of versatile polymer-based materials known as elastomers. Elastomers are commonly found in everyday products such as rubber tires, the nitrile used in medical gloves, and the silicone present in soft contact lenses. These materials possess a remarkable property of being able to undergo repeated stretching and compression while effortlessly reverting back to their original form.
However, despite their resilience, elastomers are not impervious to damage. Excessive strain can cause them to develop cracks. According to Craig, conventional approaches aimed at enhancing the durability of these materials often come with a compromise. For instance, increasing toughness may result in reduced elasticity, and vice versa.
The recent study indicates that compromising between toughness and elasticity may not always be necessary. The key lies in the presence of weak bonds integrated within the material, which surprisingly contribute to its overall strength.
Upon closer examination, elastomers can be visualized as a tangled arrangement of loosely coiled strings or strands, resembling a chaotic cluster of spaghetti. These individual strands are elongated, chain-like molecules referred to as polymers, and adjacent strands are held together by covalent bonds known as cross-links.
The presence of cross-links is what enables these materials to maintain their form. When force is applied and the material is pulled, the intertwined polymer chains within it elongate and become straight. Releasing the force allows the chains to revert to their original, coiled, and densely packed state.
In the context of the recent study, the team's concept involved linking certain polymer chains together using intentionally designed weak cross-links that are prone to breaking.
During their investigation, the researchers meticulously planned and produced two identical elastomers using polyacrylate, a flexible polymer commonly employed in the manufacturing of items such as hoses, seals, and gaskets. They proceeded to replace the original cross-links in one of the elastomers with weaker alternatives that were approximately five times less robust. This was achieved by incorporating a specific molecule, cyclobutane, which possesses a ring-shaped structure and is susceptible to breaking when subjected to strain.
In light of other factors remaining constant, Craig explained that one would expect materials with linkers that are more prone to breaking to be more susceptible to tearing.
Contrary to their expectations, the researchers made a surprising discovery. Instead of weakening the material, the incorporation of weaker linkers resulted in a significantly stronger overall network. Craig emphasized the unexpected outcome, stating that the material exhibited enhanced strength rather than the anticipated weakening.
To assess the mechanical properties, the researchers subjected thin sheets of both materials to a series of tests using a machine designed to measure the force required to tear the samples apart.
Although both materials displayed similar levels of stiffness and elasticity, the material incorporating weak cross-linkers exhibited a remarkable resistance to tearing. In fact, it proved to be up to nine times more challenging to tear compared to the material cross-linked with stronger bonds.
According to Craig, the improvement in toughness was achieved without any substantial alterations in other measurable physical properties of the material. This enhancement was observed by replacing only a small portion of the overall material with the weaker cross-linkers.
Shu Wang, the first author of the study, explained that tearing in a polymer material can be viewed as a chemical reaction. Wang conducted this research as part of his Ph.D. dissertation under the guidance of Craig and Duke polymer theorist Michael Rubinstein.
Generally, for a crack to propagate, the polymer strands spanning the forefront of the tear typically need to rupture.
However, in their design, the weaker cross-links are the ones that break first, leaving the primary polymer threads unbroken but untethered. This unique arrangement enables the material to withstand further degradation, even when small nicks and imperfections begin to emerge.
The team has taken steps to protect their approach by filing a patent for their findings. However, there is still considerable work ahead in leveraging these insights to develop more resilient synthetic rubber, akin to the type commonly used in tires. Craig emphasized that further research and development are necessary to achieve this goal.
"But that's the long-term application that excites me the most," Craig expressed with enthusiasm.
According to previous research, it is estimated that globally, tires generate approximately 6 million metric tons of dust and debris annually. This tire-related pollution is responsible for around 10% of the microplastics that find their way into the oceans and contributes to 3-7% of the particulate matter present in the air we breathe.
Craig highlighted the significant impact of tire tread wearing down on roads, leading to the release of microplastics. He emphasized that even a modest 10% reduction in this release would result in preventing 600,000 tons of microplastics from entering the environment.
Craig expressed his enthusiasm about the potential translation of these ideas to address the tire-related microplastic issue. He eagerly awaits further exploration of how these concepts could be applied to tackle this problem effectively.
This research was supported by the Center for the Chemistry of Molecularly Optimized Networks, or MONET, which is funded by the National Science Foundation (CHE-2116298).
CITATION: "Facile Mechanochemical Cycloreversion of Polymer Cross-Linkers Enhances Tear Resistance," Shu Wang, Yixin Hu, Tatiana B. Kouznetsova, Liel Sapir, Danyang Chen, Abraham Herzog-Arbeitman, Jeremiah A. Johnson, Michael Rubinstein, Stephen L. Craig. Science, June 23, 2023. DOI: 10.1126/science.adg3229
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