A Morphological Study of a Semicrystalline Poly(l-lactic acid-b-ethylene oxide-b-l-lactic acid) Triblock Copolymer

Poly(l-lactic acid-b-ethylene oxide-b-l-lactic acid) (PLLA-b-PEO-b-PLLA) triblock copolymers were synthesized by a ring-opening polymerization. PLLA and PEO sequentially crystallized by slowly cooling (−2 °C/min) from the melt. In the resultant spherulitic morphology, the retardation of polarized light was additive, and the sign of the spherulite (negative) was preserved when the PEO crystallized within the framework established by the PLLA crystals. Homopolymer blends of PLLA and PEO having the same composition as the block copolymer showed similar optical behavior. However, the change in the optical retardation upon crystallization of the PEO was much greater for the triblock copolymer than for the blend. Upon heating, the small-angle X-ray scattering from both the triblock copolymer and homopolymer mixture showed a stepwise increase in the long period at ∼60 °C, i.e., when the PEO crystals melted. For comparable volume fractions (φPLLA = 0.57), the increase in long period was greater for the triblock copolymer than for the blend. Wide-angle X-ray diffraction studies on shear aligned triblock copolymers indicate that the PLLA and PEO crystals adopt the same average orientation; though, in the case of the copolymer, the orientation is more strongly coupled.