The phase transitions in the films of an asymmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) were investigated by grazing incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM). Compared with the sequential transitions in the bulk, hexagonally perforated layer (HPL) -gyroid (GYR) - disorder (DIS) upon heating, the transitions in film geometry were dramatically changed with decreasing thickness due to the growing preferential interactions from substrate, resulting in a thickness-dependent transition diagram including four different morphologies of hexagonally modulated layer (HML), coexisting (HML and GYR), GYR, and DIS. Particularly in the films <= 10L(0), where L-0 is d-spacing at 150 degrees C, a stable HML structure was identified even above the order-to-disorder transition (ODT) temperature of the bulk, which was attributed to the suppressed compositional fluctuations by the enhanced substrate interactions. (C) 2015 Elsevier Ltd. All rights reserved.