| 초록 |
Closed-loop phase diagrams are known in systems with specific intermolecular interactions. In weakly interacting systems, however, such behavior has never been observed. Here, diblock copolymers comprised of polystyrene covalently linked to poly(n-pentylmethacrylate), P(S-b-nPMA), copolymers with only weak segmental interactions, are shown to exhibit a closed-loop phase behavior over a narrow range of molecular weight. The endothermic transitions from the disordered to ordered state and back into the disordered state, as a function of increasing temperature, are driven by delicate balances between weak segmental interactions and the entropy resulting from the mismatch in compressibility in addition to translational entropy. Morphologies and rheological properties undergo characteristic changes at the transitions. While 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. The stability limits defining the closed-loop opens new routes in the processing of multicomponent polymeric systems. Here, the very unusual phase behavior of symmetric diblock copolymers of PS and poly(n-pentylmethacrylate), denoted P(S-b-nPMA), over a narrow range of molecular weight, is reported. Small angle x-ray scattering (SAXS), transmission electron microscopy (TEM) and melt rheology studies show that P(S-b-nPMA) exhibits an LDOT. Surprisingly, at higher temperatures, P(S-b-nPMA) was found to undergo an additional transition from the ordered to the disordered state. This transition, termed an upper order to disorder transition (UODT), demonstrates the existence of a closed-loop phase diagram. |
