Epitaxial phase transition of polystyrene-b-polyisoprene from hexagonally perforated layer to gyroid phase in thin film

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_n = 34.0 kg/mol, wt_PI = 0.634) by grazing-incidence small-angle X-ray scattering. After annealing at 120 °C, the PS-b-PI 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 μm. The interface-induced orientation allowed us to obtain a well-developed diffraction 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 {121} planes of the G structure maintaining the G {121} 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 {121} plane is randomly oriented around the G [111] axis.