Fast crystallization of poly(lactic acid) by tailored blending

Poly(lactic acid) (PLLA) is a biodegradable and biocompatible thermoplastic polyester produced from renewable sources, widely used for a number of applications [1]. Unfortunately, PLLA has a low crystallization rate, which causes difficulties in industrial processing that involves fast cooling [2]. In this contribution we demonstrate that the crystallization kinetics of a commercial poly(lactic acid) (PLLA) with a molar mass of 120 kDa and containing 4% of D-isomers can be sizably improved by blending with low amount (< 10 m%) of a PLLA of high stereoregularity, made of pure L-isomer, and low molar mass. Blends of the two PLLA's are prepared by solution mixing. The blends are homogeneous in the melt and display a single, composition-dependent glass transition. Increasing concentration of the highly stereo-regular low-molar mass component in the blends results in an earlier onset of crystallization and a higher crystal growth rate. The blend composition affects the crystal polymorphism of PLLA, as increasing content of the highly stereoregular and low molar mass polymer favors growth of ?-crystals at lower temperatures than typically observed in unmodified PLLA. It is suggested that the short PLLA molecules of the low-molar mass component start to crystallize at higher temperature than the long molecules of the high-molar mass component, acting as crystal nuclei for the subsequent crystal growth that involves both species.