Cold Crystallization of PDMS and PLLA in poly(L-lactide-b-dimethylsiloxane-b-L-lactide) Triblock Copolymer and their effect on nanostructure morphology

Abstract

ABSTRACT: Poly(L-lactide-b-dimethylsiloxane-b-L-lactide) (PLLA-b-PDMS-b-PLLA) triblock copolymer was synthesized by ring-opening polymerization of L-lactide using bis- (hydroxyalkyl)-terminated PDMS as a macroinitiator. The block copolymer was immiscible in the melt, and the melt morphology was preserved upon cooling the melt to −120 °C. It was also observed that at moderate cooling rates PDMS and PLLA blocks remain in the amorphous phase at temperatures below −120 °C. The breakout and preservation of the nanostructure morphology of the triblock copolymer have been investigated by variable temperature small-angle and wideangle X-ray scattering (SAXS and WAXS) during heating. The crystallization and melting of PDMS block occurred within the confined space entrenched by the amorphous PLLA block in the temperature range from −120 to 40 °C by preserving the microphase-separated morphology. At around the glass transition temperature of PLLA (∼45 °C), chain stretching of PLLA and PDMS took place, resulting in the sudden increase of the lamellar long period. In this temperature region, amorphous PLLA transiently transformed to the mesophase structure that resulted in the increase of PLLA domain thickness. At higher temperature, the morphological perturbation was induced by the breakout crystallization of PLLA block. In addition, the effect of isothermal cold crystallization temperatures on the polymorphic behavior of PLLA in PLLA-b-PDMS-b-PLLA triblock copolymers was also investigated. It was found that the polymorphic behavior of PLLA was not influenced much by the presence of PDMS blocks. The disordered (α′) and ordered (α) forms are formed at low and high cold crystallization temperatures, respectively, similar to the PLLA homopolymer.