Direct Observation of Topologically Enhanced Interfacial Miscibility in Ring–Linear Polymers

Cyclic polymers, lacking chain ends, exhibit unique behavior when blended with linear analogues. In such cyclic/linear mixtures, cyclic polymers permit chain threading, wherein linear chains penetrate the ring-like structure, increasing entropy and enhancing interfacial diffusion relative to linear/linear or cyclic/cyclic blends. Although theory has long predicted this topological entropy-driven miscibility, experimental evidence has been scarce due to challenges in synthesis and characterization. Here, we provide the first direct experimental validation using neutron reflectivity on cyclic and linear poly(lactic acid) (PLA) analogues assembled as bilayers of higher-molecular-weight, isotopically labeled linear PLA in thin films. The results show that topological entropy markedly reduces the effective interaction parameter, χeff, promoting interfacial miscibility. This demonstrates that molecular topology alone can alter macroscopic thermodynamics, offering a fundamental route to control phase behavior in soft condensed matter.