Ten years ago, Emory University physicist Connie Roth upended her field of fundamental science with a discovery that polymer blends — an invention that has made modern life possible — have long-range property gradients across their internal interfaces.
Through a National Science Foundation (NSF) Special Creativity award she won to continue that research into the junction points, her work suggests that sound waves may create connections across disordered matter that are both longer and deeper than previously thought. The research could have practical implications in the design and development of everyday products from automotive parts to medical devices to food storage containers.
"Scientists don't understand how exactly the interfaces between polymers create better properties. Industry just mixes different components together until we get the best product," says Roth, chair of Emory College's Department of Physics, whose innovative study of polymers has earned her multiple NSF grants and election as Fellow of the American Physical Society.
"I'm focused on experimentally mapping material behavior near interfaces in a way that helps us solve the puzzle and has the most impact," Roth adds. "If new experiments substantiate this concept, it will change the way we understand disordered systems."
Defying existing paradigms
Historically, scientists modeled different polymer domains — or regions within a polymer — as having distinct properties due to the large molecules of a polymer chain not blending well with other polymers.
Those repeating building blocks, known as monomers, form everything from natural materials such as DNA and rubber to synthetic products, such as nylon and polyvinyl chloride (PVC).
Roth's research has long focused on how interfaces alter a polymer's properties. Because every polymer has its own glass transition temperature — the threshold at which it changes from a liquid to glass — understanding that touchpoint is key to affecting other properties and to designing better blends.
Roth's research lab uses techniques such as fluorescence and ellipsometry to try to explain the effects of forces, interfaces and other influences on polymer properties. She won a five-year NSF Faculty Early Career Development Program grant in 2012 for her research and teaching focused on understanding the underlying properties of polymers.
In 2022, she won the Special Creativity grant, a rare extension of funding to the "most creative investigators" for her research into polymer material properties at the nanoscale level.
Roth's first experiment that defied the physics paradigm of polymers not blending well involved studying how the glass transition temperature changes from one polymer domain to another. The results, locally mapped using fluorescence, showed that the local properties of polymers were strongly perturbed — and that the transition between polymer domains was much deeper and longer than expected, without the sharp distinctions of two materials that don't mix.
Her next step was to coordinate with fellow physics faculty member Justin Burton. The pair designed and conducted an experiment measuring the change in viscoelasticity across a polymer interface using a quartz crystal microbalance. By measuring the change in how a crystal vibrates at a precise frequency, they could model how the stiffness of two polymer domains merge when the interface between them forms.
Roth's findings, published last year in the Proceedings of the National Academy of Sciences, revealed that sound wave transmission across a polymer interface couples the stiffness of the materials. That, in turn, allows waves between neighboring polymers to move over long distances.
Macromolecules, a peer-reviewed journal for cutting-edge polymer science research, published a paper this week about Roth's novel approach to studying interfaces and her findings.
"The theorists wouldn't predict our results at all. That's what cool," says Burton, associate physics professor and Winship Distinguished Research Professor. "She is making waves in the community because her work is quite innovative."
Deboleena Roy, vice dean of faculty and divisional dean of the sciences at Emory College, echoes that sentiment.
"Connie's work is a perfect example of the creativity and progress that can come with interdisciplinary and collaborative scientific research," Roy says. "We are lucky to have Connie on our faculty, and not only that, to have a leader and innovator such as herself serve as the chair of our physics department."
Roth's findings are now the subject of experiments in labs around the world and a topic that theorists are tackling to rework mathematical models to explain.
In the meantime, she and her team are examining other variables to be tested, to support or change the initial interpretation. Future collaborations at Emory and abroad will help her lab design those experiments and develop new mathematical models to explain these results.
"All research is problem solving," Roth says. "Figuring out the puzzle appeals to me, especially if it means an opportunity to understand in a new way any number of things in our daily lives."
