  We study the cylinder to sphere morphological transition of diblock
copolymers in aqueous solution with a hydrophobic block and a charged block. We
find a metastable undulated cylinder configuration for a range of charge and
salt concentrations which, nevertheless, occurs above the threshold where
spheres are thermodynamically favorable. By modeling the shape of the cylinder
ends, we find that the free energy barrier for the transition from cylinders to
spheres is quite large and that this barrier falls significantly in the limit
of high polymer charge and low solution salinity. This suggests that observed
undulated cylinder phases are kinetically trapped structures.
