Monte Carlo simulations have been performed with a lattice model that
represents systems of symmetric diblock copolymer chains immersed in a
nonselective solvent of varying thermodynamic quality. Different valu
es for the polymer concentration and the energy parameter that gauges
solvent quality have been set. This way, the chain dimensions and the
solvent segregation and microphase separation transitions have been nu
merically characterized for two different chain lengths. The results a
re discussed and compared with existing theoretical predictions (mean-
field and self-consistent-field treatments for Gaussian chains).