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Landau theory of the orthorhombic Fddd phase
1Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Abstract:
Numerical self-consistent-field theory calculations by Tyler and Morse [Phys. Rev. Lett. 94, 208302 (2005)] predict a stable orthorhombic network phase with space group in very weakly segregated diblock copolymer melts. Here, we examine the predicted stability of this phase within a simple Landau theory of weakly ordered crystals, and within a straightforward extension of Leibler's theory of weakly segregated diblock copolymer melts. An Fddd structure with a ratio of unit cell parameters (a:b:c)=(1:2:2 square root 3) is found to compete very closely with the hexagonal (H) and lamellar (L) phases along the predicted H-L phase boundary, and to be stable within a very narrow range of parameters around this metastable boundary.
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