This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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, 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 polymers enter longer polymers, they lose conformational entropy—the freedom to have different configurations—and this penalty can 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.
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