Closed-loop phase diagrams are known in systems with specific intermolecular interactions1,2,3,4. In weakly interacting systems, however, such behaviour has never been observed. Here, diblock copolymers formed from polystyrene covalently linked to poly (n-pentylmethacrylate), P(S-b-nPMA), which have only weak segmental interactions, are shown to exhibit a closed-loop phase behaviour over a narrow range of molecular weight. The endothermic transitions from the disordered to ordered and back to the disordered state, as a function of increasing temperature, are dominantly entropic in origin. The morphology and rheological properties of P(S-b-nPMA) undergo characteristic changes at the transitions. Whereas the disorder-to-order transition temperature increases with decreasing molecular weight, the order-to-disorder transition temperature decreases. At a limiting molecular weight, the closed-loop vanishes and no ordering occurs. These findings provide quantitative insight into an elusive transition in weakly interacting multicomponent systems.
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This work was supported by Teralevel Nanodevice (TND) projects (supported by the Korean Ministry of Science and Technology (MOST), the Applied Rheology Center governed by the Korean Organization of Science and Engineering Foundation (KOSEF)), and by the Department of Energy, the Office of Basic Energy Science and The National Science Foundation-supported Materials Research Science and Engineering Center, at the University of Massachusetts. Small-angle X-ray scattering measurements were performed at the Pohang Light Source, Korea, and were supported by MOST and POSCO.
The authors declare no competing financial interests.
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Yeol Ryu, D., Jeong, U., Kon Kim, J. et al. Closed-loop phase behaviour in block copolymers. Nature Mater 1, 114–117 (2002). https://doi.org/10.1038/nmat724
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