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UVA researchers challenge long-held understanding of associative polymers

by News Staff
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A recent study led by researchers at the University of Virginia challenges the conventional understanding of associative polymers, a class of materials known for their unique self-healing and flow properties. The study, published in the journal Physical Review Letters, has significant implications for various applications of these materials, including recyclable plastics, human tissue engineering, and paint consistency control.

The breakthrough was made possible through the development of new associative polymers in the lab of Liheng Cai, an assistant professor at UVA’s School of Engineering and Applied Science, by postdoctoral researcher Shifeng Nian and Ph.D. student Myoeum Kim. Cai’s prior work on the theory laid the foundation for this discovery.

By creating a novel experimental platform, Nian and Kim provided a fresh perspective on the behavior of associative polymers, leading to a better understanding of complex aspects of polymer science. Moreover, this research contributes to the advancement of self-healing materials with customized properties.

Polymers, which are large molecules made up of repeating units called monomers, can be designed with specific characteristics by manipulating their structure and bonds. Associative polymers, in particular, possess reversible bonds that allow their moieties (molecular subunits) to break apart and reform, resulting in macroscopic properties that traditional polymers cannot achieve. These materials offer solutions to various sustainability and health challenges, such as serving as viscosity modifiers in fuels, creating resilient self-healing polymers, and engineering biomaterials for tissue regeneration.

Overcoming a longstanding obstacle in this field, the UVA team addressed the tendency of moieties to aggregate into small clusters during experiments, hindering precise study of the relationship between reversible bonds and polymer behavior.

This study opens up new avenues for research and innovation in the field of associative polymers, expanding our understanding of their dynamics and facilitating the development of advanced materials with tailored properties.

Cai’s collaborators at the National Synchrotron Light Source II conducted experiments using a sophisticated X-ray tool — the soft matter interfaces beamline shown here — to reveal the inner makeup of the polymers without damaging the samples. Credit: University of Virginia

The team led by Cai successfully developed novel types of associative polymers that exhibit evenly distributed bonds throughout the material at various densities. To verify that their materials do not form clusters, they collaborated with Mikhail Zhernenkov from the U.S. Department of Energy’s Brookhaven National Laboratory. Using the advanced X-ray tool called the soft matter interfaces beamline at the National Synchrotron Light Source II, they were able to examine the inner structure of the polymers without causing any damage.

These new associative polymers allowed Cai’s team to conduct precise studies on the impact of reversible interactions on the dynamics of these materials.

The dynamics and behavior of polymers refer to properties such as the temperature at which molecular movement transitions to a rigid “glassy” state, viscosity (the flowability of a material), and elasticity (the ability to bounce back after deformation). A combination of these traits is often desirable, especially when designing biomaterials that can be injected into human tissue and self-reconstitute.

For the past 30 years, it was widely accepted that intact reversible bonds acted as crosslinkers, resulting in a rubbery material. However, the UVA-led team discovered something different.

Teaming up with Shiwang Cheng, an expert in flow dynamics and an assistant professor at Michigan State University, the researchers accurately measured the flow behavior of their polymers across a wide range of time scales.

Cheng stated, “This requires careful control over the local environment, such as temperature and humidity of the polymers. Over the years, my lab has developed a set of methods and systems for doing so.”

The team made the unexpected finding that the bonds can slow down polymer movement and dissipate energy without creating a rubbery network. Surprisingly, the research showed that reversible interactions influenced the glassy properties of the polymers rather than their viscoelastic range.

“Our associative polymers provide a system that allows for investigating separately the effects of reversible interactions on [polymer] movement and glassy behavior,” explained Cai. “This may offer opportunities to improve the understanding of the challenging physics of glassy polymers like plastics.”

Based on their experiments, Cai’s team also developed a new molecular theory that elucidates the behavior of associative polymers, potentially revolutionizing the way these materials are engineered to possess optimized properties such as high stiffness and rapid self-healing capabilities.

In addition to Nian, Kim, Cheng, and Zhernenkov, the research involved collaboration with Ting Ge, an expert in computational simulations and assistant professor of chemistry and biochemistry at the University of South Carolina, as well as Quan Chen from the State Key Lab of Polymer Physics and Chemistry at the Changchun Institute of Applied Chemistry, who contributed the initial code for analyzing polymer flow behavior.

The study titled “Dynamics of Associative Polymers with High Density of Reversible Bonds” was published in the June 2 issue of Physical Review Letters and has been featured as an Editors’ Suggestion.

Source: University of Virginia

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