  We propose a mean-field model to explore the equilibrium coupling between
micelle aggregation and lattice choice in neutral copolymer and selective
solvent mixtures. We find both thermotropic and lyotropic transitions from
face-centered cubic to body-centered cubic ordered phases of spherical
micelles, in agreement with experimental observations of these systems over a
broad range of conditions. Stability of the non-closed packed phase can be
attributed to two physical mechanisms: the large entropy of lattice phonons
near crystal melting and the preference of the inter-micelle repulsions for the
body-centered cubic structure when the lattice becomes sufficiently dense at
higher solution concentrations. Both mechanisms are controlled by the decrease
of micelle aggregation and subsequent increase of lattice density as solvent
selectivity is reduced. These results shed new light on the relationship
between micelle structure -- "crewcut" or "hairy" -- and long-range order in
micelle solutions.
