  We study both experimentally and theoretically the self-assembly of binary
block copolymers in dilute solution, where self-assembly is triggered by
changing the solvent from the common good solvent THF to the selective solvent
water, and where the two species on their own in water form vesicles and
spherical micelles respectively. We find that in water the inter-micellar
exchange of these block copolymers is very slow so that the self-assembled
structures are in local but not global equilibrium (i.e., they are
non-ergodic). This opens up the possibility of controlling micelle morphology
both thermodynamically and kinetically. Specifically, when the two species are
first dissolved in THF before mixing and self-assembly (`premixing') by
dilution with water, the morphology is found to depend on the mixing ratio of
the two species, going gradually from vesicles via `bulbed' rods, rings,
Y-junctions and finally to spherical micelles as we increase the proportion of
the sphere-formers. On the other hand, if the two species are first partially
self-assembled (by partial exchange of the solvent with water) before mixing
and further self-assembly (`intermediate mixing'), novel metastable structures,
including nanoscopic pouches, emerge. These experimental results are
corroborated by self-consistent field theory calculations (SCFT) which
reproduce the sequence of morphologies seen in the pre-mixing experiments. SCFT
also reveals a clear coupling between polymer composition and aggregate
curvature, with regions of positive and negative curvature being stabilized by
an enrichment and depletion of sphere formers respectively. Our study
demonstrates that both thermodynamic and kinetic blending of block copolymers
are effective design parameters to control the resulting structures and allow
us to access a much richer range of nano-morphologies than is possible with
monomodal block copolymer solutions.
