title: Closed-loop phase behaviour in block copolymers.
abstract: Closed-loop phase diagrams are known in systems with specific intermolecular interactions. 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.
authors.affiliation: Department of Chemical Engineering and Polymer Research Institute, Pohang University of Science and Technology, Kyungbuk 790-784, Korea.
authors.forename: Du Yeol
authors.initials: DY
authors.lastname: Ryu
authors.forename: Unyong
authors.initials: U
authors.lastname: Jeong
authors.forename: Jin Kon
authors.initials: JK
authors.lastname: Kim
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Russell
mesh_terms: D018360:Crystallography, X-Ray / Q000379:methods*; D004548:Elasticity; D005246:Feedback; D006358:Hot Temperature; D046911:Macromolecular Substances; D008422:Materials Testing / Q000379:methods*; D008854:Microscopy, Electron / Q000379:methods; D008968:Molecular Conformation; D008970:Molecular Weight; D017711:Nonlinear Dynamics; D011108:Polymers / Q000737:chemistry; D011109:Polymethacrylic Acids / Q000737:chemistry*; D011137:Polystyrenes / Q000737:chemistry*; D012212:Rheology / Q000379:methods; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity; D033081:Shear Strength; D013314:Stress, Mechanical
chemical_list: D046911:Macromolecular Substances; D011108:Polymers; D011109:Polymethacrylic Acids; D011137:Polystyrenes; C030613:polymethacrylic acid