  Morphological transformations of amphiphilic AB diblock copolymers in
mixtures of a common solvent (S1) and a selective solvent (S2) for the B block
are studied using the simulated annealing method. We focus on the morphological
transformation depending on the fraction of the selective solvent CS2, the
concentration of the polymer CP, and the polymer-solvent interactions
{\epsilon}ij (i = A, B; j = S1, S2). Morphology diagrams are constructed as
functions of CP, CS2, and/or {\epsilon}AS2. The copolymer morphological
sequence from dissolved -> sphere -> rod-> ring/cage-> vesicle is obtained upon
increasing CS2 at a fixed CP. This morphology sequence is consistent with
previous experimental observations. It is found that the selectivity of the
selective solvent affects the self-assembled microstructure significantly. In
particular, when the interaction {\epsilon}BS2 is negative, aggregates of
stacked lamellae dominate the diagram. The mechanisms of aggregate
transformation and the formation of stacked lamellar aggregates are discussed
by analyzing variations of the average contact numbers of the A or B monomers
with monomers and with molecules of the two types of solvent, as well as the
mean square end-to-end distances of chains. It is found that the basic
morphological sequence of spheres to rods to vesicles and the stacked lamellar
aggregates result from competition between the interfacial energy and the chain
conformational entropy. Analysis of the vesicle structure reveals that the
vesicle size increases with increasing CP or with decreasing CS2, but remains
almost unchanged with variations in {\epsilon}AS2.
