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

The crystallization behavior of a series of lamellae-forming blends of a shorter PS-$b$-PEO (SEO) and a longer PS-$b$-PLLA (SLLA) has been studied. In SLLA-rich blends, the junction point constraint coupled with the poor chain mobility at low $T_{c}$ ($\le $45 $^{\circ}$C) hampered the formal crystallization of PLLA. 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 $= 15.92$^{\circ}$ in the WAXS profile and displayed a much lower melting point of ca. 100 $^{\circ}$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.