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Shear-induced network-to-network transition in a block copolymer melt.
Flow induced alignment of one- or two-dimensional block copolymer microdomains, such as cylinders and lamellae, has been known for more than three decades. This can produce materials with inherent anisotropic physical properties, including thermal expansivity, elasticity, fracture toughness, and various transport properties including diffusivity. In contrast, three-dimensional periodic block copolymer structures, such as spheres organized on a cubic lattice or the bicontinuous cubic gyroid morphology, are isotropic and do not accommodate large shear strains through domain alignment. The UMN Microstructured Polymers group previously discovered a new class of ABC triblock copolymers with triply periodic and triply continuous network morphologies, including an anisotropic orthorhombic phase denoted O70 that represented the first example of a noncubic network structure in soft self-assembled materials (which include surfactants, soaps, lipids, and block copolymers). Building on this discovery, MRSEC investigators Eric Cochran and Frank Bates have now demonstrated that the O70 phase (space group Fddd) exhibits anisotropy in all physical properties, including response to flow and deformation, which produced an unprecedented long-range topological restructuring to a different orthorhombic symmetry. The MRSEC investigators synthesized a poly(cyclohexylethylene-b-ethylethylene-b-ethylene) (CEEE) triblock copolymer, which formed the O70 phase. When this material was subjected to dynamic reciprocating shear and cooled to room temperature, the poly(cyclohexylethylene) blocks vitrified as a glass, which fixed the multicontinuous morphology of the triblock copolymer. Small angle x-ray scattering (SAXS), performed in the UMN Institute of Technology Characterization Facility, revealed a remarkable single crystal state of order, as evinced by the scattering patterns acquired with the beam directed along the three principal coordinates established by the shearing geometry. Notably, scattering peak positions from all three patterns revealed that the dynamic shearing transformed the O70 phase into another (metastable) orthorhombic network structure with Pnna (No. 52) space group symmetry, denoted O52. This kind of field-induced network-to-network phase transition in soft condensed matter is exceptional. In addition to the aforementioned impact of anisotropic structures on mechanical properties, these experiments are significant because they demonstrate a remarkable and fundamentally significant tendency for network forming linear block copolymer melts to maintain local threefold connector symmetry, even when subjected to massive deformation. Furthermore, orthorhombic crystals are birefringent, suggesting that O70 and O52 materials may be useful as photonic materials, particularly given their high degree of single crystal order. Perhaps most importantly, these findings prove that network formation in ABC triblock copolymers is universal and not restricted to a single model system. [Cochran E.W.; Bates, F.S. Phys. Rev. Lett. 2004, 93, 087802]
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modified on April 3, 2008