A phenomenon known as "mixing" between layers of plastic—previously only predicted in theory—has now been experimentally confirmed. A team of researchers, led by Professor So Youn Kim from the Department of Chemical and Biological Engineering at Seoul National University (SNU), and Professor Kyoung Taek Kim from the Department of Chemistry at SNU, have successfully induced mixing between two poorly mixing plastic layers by simply changing the shape of a polymer from a straight, thread-like structure into a ring-shaped one, without altering its chemical composition. This marks the first time the "topological entropy" effect—proposed in theoretical models 40 years ago—has been directly observed at a polymer interface. The study was published in ACS Central Science, a leading international journal in the field of chemistry. Plastic products often consist of multiple layers of different polymers, each with specific functions, such as food packaging, functional films, and display materials. The ability of these layers to mix at their interface is crucial for their adhesion and stability. If they don't mix well, the layers may separate, reducing the product's effectiveness and durability. Traditionally, chemical additives called compatibilizers have been used to improve adhesion between layers. However, the new research shows that changing the molecular shape of one polymer can enhance mixing without the need for additional chemicals. The researchers focused on ring-shaped polymers, where the chain ends are connected in a loop. In 1986, scientists predicted that when a ring-shaped polymer interacts with a linear one, a "threading" effect—where the linear chain passes through the ring—could increase the number of possible molecular configurations, thereby improving the mixing at the interface. This is known as the topological entropy effect. Until now, however, this phenomenon had not been experimentally confirmed due to the difficulty of synthesizing pure ring polymers and isolating the effect from other factors. In the study, the researchers used a precise method to create both linear and ring-shaped polylactide (PLA), a biodegradable plastic, with identical molecular weights. They then deposited these polymers onto a film of a higher-molecular-weight version of PLA and measured the mixing at the interface using a neutron reflectometer. They found that the mixing width increased significantly—by about 2.6 times—when one polymer was ring-shaped. This result not only confirms the theoretical prediction but also suggests that changing a polymer’s shape can have a much greater effect on interfacial mixing than simply increasing its chain length. The findings could lead to new design principles for creating stable and well-mixed polymer interfaces without the need for additional chemical additives. If the principle is validated across a broader range of polymers and conditions, it could be applied to improve the performance of packaging, functional films, and biodegradable plastics. However, further research is needed to confirm the effect's reproducibility in industrial settings and to enhance adhesion strength and long-term durability. Researchers are now exploring how this molecular design principle can be extended to other materials, such as thermal interface materials used in electronics.