  We compare two theoretical approaches to dielectric diblock copolymer melts
in an external electric field. The first is a relatively simple analytic
expansion in the relative copolymer concentration, and includes the full
electrostatic contribution consistent with that expansion. It is valid close to
the order-disorder transition point, the weak segregation limit. The second
employs self-consistent field (SCF) theory and includes the full electrostatic
contribution to the free energy at any copolymer segregation. It is more
accurate but computationally more intensive. Motivated by recent experiments,
we explore a section of the phase diagram in the three-dimensional parameter
space of the block architecture, the interaction parameter and the external
electric field. The relative stability of the lamellar, hexagonal and distorted
body-centered-cubic (bcc) phases is compared within the two models. As function
of an increasing electric field, the distorted bcc region in the phase diagram
shrinks and disappears above a triple point, at which the lamellar, hexagonal
and distorted bcc phases coexist. We examine the deformation of the bcc phase
under the influence of the external field. While the elongation of the spheres
is larger in the one-mode expansion than that predicted by the full SCF theory,
the general features of the schemes are in satisfactory agreement. This
indicates the general utility of the simple theory for exploratory
calculations.
