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The melting behavior of poly( -caprolactone)-block-polybutadiene (PCL-b-PB) copolymers, crystallized at various temperatures T c from the melt (homogeneous state or microphase-separated state), has been studied by small-angle X-ray scattering with synchrotron radiation (SR-SAXS). The strong SR-SAXS intensity peak changed into a sharp diffraction or a diffuse peak on heating, indicating a morphological transition from the lamellar morphology (an alternating structure consisting of crystallized lamellae and amorphous layers) into the microdomain structure or the homogeneous melt. The melting process of PCL-b-PB was extremely different from that of the poly( -caprolactone) (PCL) homopolymer; the long spacing of the lamellar morphology in PCL-b-PB, which depended significantly on Tc, remained constant or decreased slightly on heating, whereas that in PCL increased steadily. The morphological change in PCL-b-PB and PCL under the different thermal treatments was quantitatively investigated by SAXS and DSC. Details of the lamellar morphology initially formed in PCL-b-PB were determined mainly by the nonequilibrium crystallization of the PCL block at T c, as in the case of PCL, but the subsequent heating did not alter this morphology, owing probably to the competition between the lamellar thickening and the contraction of the PB block existing between PCL lamellae, resulting in the characteristic melting behavior of PCL-b-PB.