Dartmouth researchers explain how a mysterious process can drive physical aging in some materials—unlocking new ways to design and manufacture plastics for wide-ranging use.
Polymers are all around us—from PVC pipes to polystyrene cups to polyethylene milk jugs. They may seem eternal, but like all matter, they degrade with time and can deform under stress through a process called relaxation. This can cause polymers to shrink and become brittle, as the molecules within shift to move toward a state of equilibrium.
For years, scientists have struggled to explain why some polymers age faster than expected according to the dominant theory of the "alpha" relaxation process. In a new study in the journal Physical Review Letters, researchers at Dartmouth and the Université Libre de Bruxelles (ULB) show that another relaxation mechanism they call the slow Arrhenius process (or SAP) may influence the aging of some materials.
This process happens, they hypothesize, through molecules collectively packing closer together, causing the material to contract. They further show that the relaxation rate of any material undergoing this slow-moving process can be predicted if you know how its volume depends on temperature and pressure at equilibrium.
"We can suddenly make predictions about a whole range of things that hadn't really been noticed before," says Jane Lipson, Albert W. Smith Professor of Chemistry and the study's senior author.
The rate at which materials relax as they transition to stability depends on temperature. But for years, scientists had detected temperature-dependent rate changes at the molecular level that didn't make sense if the alpha process alone was driving relaxation.
A research team in Spain was among the first to propose that an alternative, less temperature-sensitive mechanism to the alpha process was at work. In a 2013 study, they showed that some amorphous "glassy" polymers aged in two discrete stages, one following the alpha process, and another driven by an unknown process. The finding prompted Simone Napolitano, an experimental physicist at ULB, to start searching for alternate relaxation mechanisms.
Napolitano measures the nanoscale movement of molecules through a technique called dielectric spectroscopy, which involves exposing a material to an oscillating electric field and recording its response at different frequencies. This spectral information provides insight into how molecules that make up the material move internally and interact with their environment.
A turning point for Napolitano, a co-author of the current study, came after he figured out how to capture a faint molecular signal partially hidden in the low frequency region of the spectrum. In 2022, he and his colleagues became the first to systematically document a new mechanism driving relaxation in a variety of polymers.
They named it the "slow" Arrhenius process because, though its rate tracked inversely with temperature ("Arrhenius behavior"), unlike other Arrhenius processes, it was often slower than the non-Arrhenius alpha relaxation process. However, their work also revealed that at colder temperatures, the SAP ultimately becomes the faster of the two.
From sketch to breakthrough model
Napolitano reached out to Lipson and Ron White, a research associate in her lab, and together, they tried to work out an explanation for why the "slow" process worked so well at lower temperatures.
They sketched a group of molecules subtly shifting position, like people on a crowded subway car squeezing closer together to let a passenger off the train. Under their so-called "collective small displacements" model, less energy was needed to power these micro movements, in sharp contrast to the relatively large "hops" that molecules in the alpha process had to take.
The team submitted a paper based on their sketch to a journal for publication but were turned down. Longtime collaborators who often finish each other's sentences, Lipson and White recall their response to being rejected with excitement.
"You're calling this a model, the reviewers said, but there are no equations!" recalls White.
"The reviewers said to come back when you have a real theory!" adds Lipson. "So, we did."
This time, they proposed a mathematical description for the collective movements they believe were driving the SAP. They also provided a pair of equations for predicting, for any material, the energy required to activate the process, provided you have its pressure, volume, and temperature data at equilibrium.
One of the premier journals in their field not only accepted the paper, but designated it an "editor's suggestion."
"This breakthrough has extraordinary implications for both fundamental science and practical applications, including the prediction of how some materials age deep in the glassy state," says Daniele Cangialosi, a research scientist at Spain's Materials Physics Center in San Sebastian who was among the first to show that the alpha process had company.
In addition to physical aging, the new model could explain and predict many other temperature-dependent material transformations, from crystallization to material deformation and flow, to the adhesion, or peeling away, of material coatings.
"Now that we know what to look for, this process is showing up everywhere," says White. "That proves it's important. It's the molecular-level changes that are driving many macroscopic material changes."
The work could influence how plastics are designed and manufactured, giving engineers another knob to turn to get a desired property. "When engineers change the temperature and pressure, they want to know things like the viscosity of the material flowing through their molds," says White. "They also want to know when they're designing the material, how it will densify over its lifetime."
Aging is just one of the processes that change a material structurally as it moves toward its natural equilibrium state. Through the new model, scientists can now calculate how quickly these changes will unfold.
"Equilibrium is the goal," says Lipson. "How to get there is a big question. Our work draws a bright new line between the two."