  We consider orientation mechanisms for block copolymers in an electric field.
Theoretical and experimental studies have shown that nonuniformity of the
dielectric constant gives rise to a preferred orientation of the melt with
respect to the applied field. We show that the presence of ions, as found in
anionically prepared copolymers, may increase the alignment effect markedly.
Time-varying (ac) and static (dc) fields are considered within a unified
framework. We find that orientation of block copolymers can in principle be
achieved without a dielectric contrast if there is a mobility contrast. The
presence of ions is especially important at small field frequencies, as is in
most experiments. Unlike the no-ions case, it is found that orienting forces
depend on the polymer chain lengths. The mobile-ions mechanism suggested here
can be used to reduce the magnitude of orienting fields as well as to
discriminate between block copolymers of different lengths.
