40-year-old theory of how plastics ‘mix’ confirmed for the first time by SNU professors So Youn Kim and Kyoung Taek Kim’s joint research team
Changing only the polymer architecture from linear to ring-shaped increases interfacial mixing width by 2.6-fold without altering chemical composition / Study published in the international journal ACS Central Science
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Even when their chemical compositions are identical, two linear polymers do not mix at the interface (left). When one is changed into a ring polymer, however, the interface mixes over a much broader region through a “threading” effect in which the linear chain passes through the ring (right). The only difference between the two cases is the molecular topology.
view moreCredit: © ACS Central Science, originally published in ACS Central Science
A “mixing” phenomenon between plastic layers that had been predicted only in theory for 40 years has now been experimentally confirmed.
A research team led by Professor So Youn Kim of the Department of Chemical and Biological Engineering at Seoul National University College of Engineering, together with a team led by Professor Kyoung Taek Kim of the Department of Chemistry at SNU College of Natural Sciences, has succeeded for the first time in inducing mixing at the interface between two plastic layers that otherwise mix poorly, simply by changing a thread-like linear polymer into a ring-shaped polymer without altering its chemical composition.
The researchers compared the interfacial miscibility of linear and ring polymers under conditions in which their molecular weights—and therefore their chain lengths—were the same. They found that the interfacial mixing width was 1.9 nm when two linear polymers were combined, but increased to 5.0 nm, or approximately 2.6 times greater, when one of the polymers was changed to a ring architecture. The results experimentally demonstrate that interfacial mixing can be controlled solely through polymer topology, or polymer architecture.
The study marks the first direct observation at an actual polymer interface of the “topological entropy” effect, which was proposed through theory and simulations approximately 40 years ago. The findings were published in ACS Central Science, an international journal published by the American Chemical Society (ACS).
Plastic products are often made by stacking multiple polymer layers with different functions. Food packaging, functional films, and display materials are representative examples. How well polymer layers mix at their interface affects interlayer adhesion and stability, and poor interfacial miscibility can cause layers to detach or peel apart, reducing barrier performance and durability.
Even polymers that are chemically identical may not mix well when there is a large disparity in molecular weight. This is because when shorter polymer enter into longer polymer, they lose conformational entropy—the freedom to have different configurations—and this penalty can instead make mixing unfavorable. As a result, multilayer films and coatings have traditionally relied on additional compatibilizers, or adhesion agents, to ensure sufficient adhesion between layers.
The researchers focused on ring polymers, whose chain-ends are connected. In 1986, Cates and Deutsch predicted that when a ring polymer encounters a linear polymer, a “threading” phenomenon—in which a long chain passes through a ring like a thread—could increase the number of possible molecular configurations and thereby enhance interfacial miscibility. This is known as the topological entropy effect. Until now, however, there had been no direct experimental validation of that topological effect because ‘pure’ ring polymers are difficult to synthesize and isolating the entropy effect itself is challenging experimentally.
Using Professor Kyoung Taek Kim’s team’s precise synthesis method, the researchers synthesized linear and ring polylactide (PLA), a biodegradable plastic, with discrete and pure molecular distribution. They deposited these polymers onto a thin film of higher-molecular-weight deuterated PLA to form bilayer films and measured their interfacial mixing at the REF-V neutron reflectometer at the HANARO research reactor of the Korea Atomic Energy Research Institute.
To isolate and identify the effect produced solely by the “architecture” of the polymer, rather than by chemical composition or measurement conditions, the researchers compared linear and ring polymers with identical chain lengths and verified that interfacial diffusion had reached an equilibrium state in which it no longer changed over time. They also conducted separate wetting experiments to determine whether the hydrogen/deuterium substitution used for neutron reflectivity have affected the results, confirming that the differences originated from molecular topology.
The experimental results were more pronounced than expected. Although the linear polymers had the same chemical composition, they barely mixed at the interface. When one polymer was changed to a ring architecture, however, the interfacial mixing width increased from 1.9 nm to 5.0 nm, approximately a 2.6-fold increase. Analysis of the effective interaction parameter (χeff), which represents interfacial miscibility, further showed that the effect produced by the ring architecture was approximately 22 times greater than the effect of simply doubling the polymer chain length. The researchers presented this value as a semi-quantitative metric for comparing the relative magnitudes of the two effects.
The team also confirmed the topological effect in “autophobic dewetting,” a phenomenon in which even chemically identical polymers can become unstable because the shorter polymer layer fail to cover the longer polymer layer and instead contracts or dewettings. Short linear polymers became unstable on high-molecular-weight PLA, with the film dewetting, whereas ring polymers of the same chain length maintained a stable film.
The significance of the study lies in proposing a new design principle for controlling the miscibility and stability of plastic interfaces solely through molecular “architecture,” while leaving the chemical composition and material properties unchanged. The researchers expect that if the principle is confirmed across a wider range of polymers and processing conditions, it could provide a new approach for stabilizing such interfaces without adding separate chemical compatibilizers.
Recent advances have also made it possible to synthesize ring architectures of widely used polymers such as polyethylene and polypropylene. If the principle demonstrated in this study can be applied to a broader range of plastic materials, it could potentially be used to improve interfacial performance in packaging materials, functional films, recycled plastics, and biodegradable plastics. However, because the present study represents a fundamental demonstration of the topological effect using ring PLA, further research will be required before industrial application to confirm the reproducibility of the effect under diverse polymer and processing conditions, as well as improvements in adhesion strength and long-term durability.
Professor So Youn Kim, who supervised this research, said, “This study is particularly significant because it provides the first experimental confirmation of a 40-year-old theoretical prediction that an immiscible interface can be made miscible simply by changing molecular shape, without altering the chemical structure at all.” She added, “In this study, the neutron reflectometer at the Korea Atomic Energy Research Institute’s HANARO reactor played an important role in directly identifying the topological effect.”
Professor Kyoung Taek Kim said, “Because we had techniques to synthesize topological ‘pure’ ring polymers with precisely controlled molecular weights, we were able to isolate and observe the topological effect.” He added, “We plan to expand this research to commodity polymers and develop the approach into a new tool for interface engineering.”
First author Dr. Seong Eun Kim was selected for the National Research Foundation of Korea’s Domestic Postdoctoral Fellowship Program and is currently working as a postdoctoral researcher at the Korea Institute of Science and Technology (KIST), where she conducts research on thermal interface materials (TIMs). Going forward, she plans to extend to thermal-transfer materials the perspective developed through this study—that “the molecular structure of an interface determines a material’s macroscopic performance”—along with her experience in interfacial analysis. In particular, she plans to investigate how the structures and interactions at interfaces between heat-transfer materials in electronic devices affect thermal-transfer performance and stability, and, based on these findings, continue research aimed at developing interface-design strategies for high-performance thermal interface materials.
This research was supported by the National Research Foundation of Korea (NRF) Mid-Career Researcher Program (NRF-2021R1A2C2007339, RS-2026-25473503) and the Samsung Science & Technology Foundation (SRFC-MA2201-02). Neutron reflectivity experiments were conducted using the REF-V reflectometer at the HANARO research reactor of the Korea Atomic Energy Research Institute.
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