We investigated the phase transition behavior from the hexagonally perforated layer (HPL) to the gyroid (G) phase in supported thin film of a polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer (M = 34.0 kg/mol, wtp, = 0.634) by grazing-incidence small-angle X-ray scattering. After annealing at 120 'C, the PS-b-Pl thin film spin-coated on silicon wafer exhibited HPL morphology with its lamellae highly oriented parallel to the substrate up to a thickness as much as 1 um. The interface-induced orientation allowed us to obtain a well-developed diff-raction pattern in the absence of external mechanical strain to align the domains. The comparison with the computer-simulated diffraction pattern revealed that the HPL structure has mosaic grains oriented randomly in-plane with ABC stacking and undetectable amount of AB stacking. Upon heating, the HPL phase undergoes a phase transition to the G phase. The phase transition occurred epitaxially converting the HPL layers to the {1211 planes of the G structure maintaining the G 11211 plane oriented parallel to the substrate. This behavior is in contrast with the HPL to G -phase transition found from the shear-oriented HPL samples, in which the G 11211 plane is randomly oriented around the G [ill] axis.
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