We examine the effect of composition fluctuations on the stability of the orthorhombic Fddd network phase in the diblock copolymer melt phase diagram within the self-consistent Hartree approximation to the Landau-Brazovskii theory. For weak fluctuations, the Fddd structure is an equilibrium phase; however, stronger fluctuations render this phase metastable. These results suggest a reinterpretation of a recent experiment beyond mean-field theory. Fluctuations may also explain why an equilibrium Fddd phase is not generally observed in analogous self-assembling systems.

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