Landau theory of the orthorhombic Fddd phase.

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.