Synthesis, sequential crystallization and morphological evolution of well-defined star-shaped poly(ε-caprolactone)-b-poly(L-lactide) block copolymer

Well-defined star-shaped poly(e-caprolactone)b-poly(L-lactide) copolymers (PCL-b-PLLA) were synthesized via sequential block copolymerization, and their molecular weights and arm length ratio could be accurately controlled. Both differential scanning calorimetry and wide angle X-ray diffraction analysis indicated that the crystallization of both the PLLA and PCL blocks within the star-shaped PCL-b-PLLA copolymer could be adjusted from the arm length of each block, and both blocks mutually influenced each other. The sequential isothermal crystallization process of both the PLLA and PCL blocks within the PCL-b-PLLA copolymers was directly observed with a polarized optical microscope, and the isothermal crystallization of the PCL segments was mainly templated by the existing spherulites of PLLA. Moreover, the PLLA blocks within the star-shaped PCL-b-PLLA copolymer progressively changed from ordinary spherulites to banded spherulites when the arm length ratio of PCL to PLLA was increased while concentric spherulites were observed for the linear analog. Significantly, these novel spherulites with concentric or banded textures and the morphological evolution of the spherulites have been observed for the first time in the PCL-b-PLLA block copolymers.