Absence of the nematic phase in symmetric diblock copolymers

We examine the possibility of nematic order in systems described by the Brazovskii model [Sov. Phys. JETP 41, 85 (1975)] of the isotropic-lamellar phase transition, including symmetric diblock copolymer melts and bilayer-forming surfactant systems. We introduce a nematic order parameter and look for states that break rotational symmetry that is, which have a well-defined direction for interfacial normals, but no phase coherence between interfaces. We find an isotropic-nematic phase transition, but it is always precluded by an isotropic-lamellar phase transition occurring at higher temperature. We show that the lamellar phase is stable to dislocation-mediated melting to a nematic phase in the limit of long copolymer chains, even though the isotropic-lamellar phase transition becomes second order in this limit.