  We consider the coupling between an external electric field and dissociated
ions embedded in anisotropic polarizable media such as block copolymers. We
argue that the presence of such ions can induce strong morphological changes
and even lead to a structural phase transition. We investigate, in particular,
diblock copolymers in the body centered cubic (bcc) phase. In pure dielectric
materials (no free charges), a dielectric breakdown is expected to occur for
large enough electric fields, preempting any structural phase transition. On
the other hand, dissociated ions can induce a phase transition for fields of
about 10 V/$\mu$m or even lower. This transition is from an insulator state,
where charges are preferentially localized inside bcc spherical domains, to a
conducting state where charges can move along the long axis of oriented
cylinders (forming a hexagonal phase). The cylinders diameter is small and is
determined by the volume fraction of the minority copolymer component. The
strength of this mechanism can be tuned by controlling the amount of free ions
present as is observed by our experiments. These theoretical predictions
support recent experimental findings on the bcc to hexagonal phase transition
in copolymer systems.
