By means of Monte Carlo simulations of a lattice model, microphase separation transition of
symmetric diblock‐copolymer melts confined in the thin‐film geometry between parallel hard
walls is studied. We impose a surface field which acts repulsively only on one of the two
species, to stabilize lamellar order parallel to the surfaces. However, interplay between two
characteristic lengths, that is, the natural thickness of the lamellar l and the thickness of the
film D, causes complicated behavior. In case that the two lengths are compatible with each …