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The mechanism of the order−order transition in a block copolymer between spheres in a body-centered cubic lattice (bcc-spheres) and hexagonal cylinders was investigated by means of small-angle X-ray scattering (SAXS). The block copolymer studied is a polystyrene-block-polyisoprene diblock copolymer having a molecular weight of 4.40 × 10⁴ and a weight fraction of polystyrene of 0.20. Cylindrical microdomains on a hexagonal lattice were observed up to 114.7 °C and changed to bcc-spheres above 116.7 °C. The OOT transition between the two microdomains reversibly occurred. Moreover, the repetitive OOT process (defined in the text) occurred with conservation of the grain structures and of orientation of the crystallographic axes in the grains. When the SAXS pattern from a single grain was examined, it was found that the cylinders are transformed into a series of spheres with the cylindrical axes …