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, χ_eff, 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.