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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.

ABSTRACT:The morphology formed in asymmetric poly(E-caprolactone)-block-polybutadiene (PCL-b-PB) copolymers has been investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) as a function of total molecular weight M,, (8000 s M,, s 62000) and crystallization temperature T, ( -20 s Tes 45°C). All the copolymers have a cylindrical or spherical microdomain structure in the melt with the highly crystalline PCL block inside. In PCL-b-PB copolymers with M,, s 19000, the SAXS result showed the morphological transition (microdomain structure -lamellar morphology) at each Te by the crystallization of PCL blocks. The repeating distance of the lamellar morphology increased significantly with increasing T" as usually observed in the crystallization of homopolymers. The PCL crystallinity Xe (i.e., the weight fraction of crystallized PCL blocks against total PCL blocks in the system) was 0.58-0.79, comparable to that of PCL homopolymer. In PCL-b-PB copolymers with M,, 2'. 44000, on the other hand, the morphology did not change by the crystallization at any T" and Xe was extremely reduced

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