Thin films (300 Angstrom) of homodisperse polystyrene (PS) chains tagged at one end by a short chlorinated polybutadiene (CPB) segment have been spin-coated on Si wafers covered or not by a thin (300 Angstrom) layer of amorphous carbon. These strongly asymmetric P(CB-b-S) diblock copolymers do not microphase-separate in the bulk. Depth profiles from X-ray reflectivity (XRR) and DSIMS have been obtained on thin films of this diblock before and after annealing. Whatever the surface studied, the electron density profiles derived from XRR measurements did not show any significant difference with the ones obtained with homodisperse PS. By contrast, DSIMS and XPS experiments showed a clear depletion of the CPB short block starting from the air interface when the copolymers are deposited onto the silicon substrates. Such a depletion cannot be detected when the copolymers are deposited onto the almost apolar carbon surface. This suggests a preferential accumulation of the chlorinated block toward the silicon oxide polar surface, in agreement with an estimation of the interfacial energy reduction implied by the process, indicating that segregation effects in these systems are dominated by energetic balances.