  We study thin films of A/B diblock copolymer in a lamellar phase confined
between two parallel plates (electrodes) that impose a constant voltage across
the film. The weak-segregation limit is explored via a Ginzburg-Landau-like
free-energy expansion. We focus on the relative stability of parallel and
perpendicular orientations of the lamellar phase, and how they are affected by
variation of four experimental controllable parameters: addition of free ions,
the difference in ionic solubilities between the A and B blocks, the dielectric
contrast between the A/B blocks, and the preferential interaction energy of the
plates with the blocks. It is found that, in general, the addition of ions
lowers the critical voltage needed to reorient the lamellae from being parallel
to the plates, to being perpendicular to them. The largest reduction in
critical voltage is obtained when the ions are preferentially solubilized in
the block that is not preferred by the plates. This reduction is even further
enhanced when the dielectric constant of this block has the higher value. These
predictions are all subject to experimental verification.
