Crystallization in the Binary Blends of Crystalline-Amorphous Diblock Copolymers Bearing Chemically Different Crystalline Block

The crystallization behavior of the blends of two crystalline amorphous diblock copolymers hearing chemically different crystalline block has been studied. A series of lamellae-forming blends of a polystyrene-block-poly(ethylene oxide) (SEO) with shorter block lengths and a polystyrene-block-poly-(L-lactide) (SLLA) with longer block lengths were prepared. Over the composition range investigated, the PS blocks from these two copolymers were found to mix intimately in the PS lamellar microdomains in the melt; moreover, the PEO and PLLA blocks also formed a miscible mixture in their domains. Because of the constraint imposed by the junction points localized at the lamellar interface and the nanoscale confinement effect, the crystallization behavior of PEO and PLLA blocks within the lamellar microdomains was found to deviate from that in the corresponding homopolymer blends. In the SLLA-rich blends, the junction point constraint coupled with the poor chain mobility at low crystallization temperature (<=similar to 45 degrees C) hampered the relatively long-range transport of PLLA segments for crystal growth. In this case, a local demixing between a fraction of PEO and PLLA chains took place, yielding the PLLA crystalline domains in which the PLLA crystalline stems were intervened by the PEO chains. This crystalline species gave rise to a relatively broad peak at 2 theta = 15.92 degrees in the wide-angle X-ray scattering profile and displayed a much lower melting point of ca. 100 degrees C compared to that of the typical alpha-form crystal of PLLA. It was suggested that the inserted PEO chains served as the molecular defects which induced an expansion of the a-axis and b-axis of the alpha-form PLLA unit cell and lowered the crystal melting point due to introduction of defect free energy.