Crystallization behavior, structure, morphology, and thermal properties of crystalline and amorphous stereo diblock copolymers, poly(l-lactide)-b-poly(dl-lactide)

Abstract

We investigated the crystallization behavior, structure, morphology, and thermal properties of stereo diblock poly(lactide) (PLA) copolymers of crystalline poly(L-lactide) (PLLA) and amorphous poly(DL-lactide) (PLLA-b-PDLLA) having different molecular weights of the PLLA block, as representative and typical examples of crystalline and amorphous block copolymers. PLLA-b-PDLLA polymers were crystallizable for PLLA fractions down to 25.4%. The transition temperature from the δ-form to the α-form decreased with a decrease in the PLLA fraction and finally only α-form crystallites were formed for PLLA-b-PDLLA with a PLLA fraction of 25.4%. The melting temperature (T_m), radial growth rate of spherulites, and total crystallization rate decreased with decreasing PLLA fraction. Surprisingly, the long period, the mean lamellar thickness (d_c), and the mean thickness of the amorphous layer increased with decreasing PLLA fraction. The contradiction between the dependence of T_m and d_c on the PLLA fraction can be explained by the fact that the degree of disorder in the crystalline lattice increased and the crystalline surface area along the direction perpendicular to the c-axis became small with decreasing PLLA fraction, and their synergy effect. Some fractions of PDLLA chains were confined in the amorphous regions between the crystalline regions, but the remaining parts of the PDLLA chains should have been excluded from the amorphous regions between the crystalline regions and, as a result, located outside of the alternately layered crystalline and amorphous regions. Confinement of PDLLA chains in the amorphous regions between the crystalline regions is in marked contrast with the result reported for PLLA/PDLLA blends, wherein PDLLA molecules were excluded from the amorphous regions between the crystalline regions.