Surprising states of order for linear diblock copolymers

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Science  05 May 2017:
Vol. 356, Issue 6337, pp. 487-488
DOI: 10.1126/science.aan1488

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Two polymers that are immiscible would separate on a macroscopic scale—like oil and water—to minimize interfacial area. However, when the different polymer chains are linked with a covalent bond to form a linear diblock copolymer, the forced interaction drives a self-assembly into periodic domains at the scale of 10 to 100 nm (1). The shapes and arrangements of these domains are often very simple, such as spherical micelles in a body-centered cubic (bcc) lattice. One notable exception was the discovery that spherical micelles can assemble into a Frank-Kasper (FK) σ phase (2), a complex low-symmetry structure common in metals and alloys. Many other metallurgical FK structures are known, so if similar principles govern lattice selection in these distinct materials classes, then it is possible that many linear diblock copolymer phases remain undiscovered. On page 520 of this issue, Kim et al. (3) predict that several FK phases have nearly degenerate free energies, and show how clever thermal treatments can control the ordering pathway to access two of these previously undocumented structures.