Isoporous membranes made from diblock copolymers have numerous applications, including water treatment and protein separation, and are successfully produced at a laboratory scale under controlled conditions. However, achieving optimal conditions for membrane preparation remains a challenge due to the complexity of the involved phenomena. Experimental studies have shown that the self-assembly of block copolymers in solution significantly affects the final membrane structure. Despite extensive research, understanding the multiscale phenomena leading to the characteristic morphology is still elusive. We address this gap by using mesoscale computational simulations to investigate the self-assembly of block copolymers in selective solvents, consistent with isoporous membrane preparation. We focus on the interplay between entropic and enthalpic interactions and their effects on the morphology of the micellar aggregates in solution. Our computational results are consistent with experimental evidence, revealing a morphological transition of the aggregates as the polymer concentration and solvent affinity change. We propose different phase parameters to characterize the emergence of monodisperse-spherical micelles in solution and describe the order of crew-cut micelles using a rigid-sphere approximation. Our study provides valuable insights into the self-assembly of diblock copolymers to optimize the preparation of isoporous membranes.