Solution-grown composite single crystals of poly(L-lactic acid)-b-polystyrene block copolymers and poly(L-lactic acid) homopolymers

Abstract

In the present work, composite single crystals (CSCs) composed of poly (L-lactic acid) (PLLA) homopolymer and poly (L-lactic acid)-b-polystyrene (PLLA-b-PS) block copolymers (BCPs) were prepared by solution co-crystallization. The effects of crystallization temperature (Tc), mass ratio of BCP/homopolymer and PS block length on the morphology and growth habit of the CSCs were investigated. Particularly, parallelogram CSCs with four {110} crystal planes as edges tend to be formed at a low Tc and a small BCP/homopolymer mass ratio for the BCP with a short PS block. The parallelogram shape of the crystals indicates the growth rates along four {110} crystal planes are different in two directions. It is proposed that the folded surface free energy is reduced after incorporation of PLLA-b-PS BCP into the crystals, leading to accelerated growth and concave surface structure along the long side direction. The chain folding of the PLLA block in the CSCs was also discussed. It is found that, in order to match the chain folding of PLLA homopolymer, the PLLA block in the BCPs adopts more chain folds in the CSCs than that in the single crystals formed by neat BCPs.