The morphology and spherulite growth of PLA stereocomplex in linear and branched PLLA/PDLA blends: effects of molecular weight and structure

Abstract

Linear and three-armed poly(L-lactide)/poly(D-lactide) (PLLA/PDLA, 3PLLA/3PDLA and 3PLLA/PDLA) specimens were prepared through a solution mixing method, and the morphology and the spherulite growth of poly(lactide) stereocomplex crystallites (PLA sc) from the melt with various structures and molecular weights were investigated. Polarized optical microscopy images revealed that regular spherulites and a visible boundary developed in all of the PLLA/PDLA blends with relatively lower molecular weights (M_n < 20 kg mol⁻¹ for each branch), and irregular spherulites formed when the molecular weights of PLA polymers were higher than 60 kg mol⁻¹. The growth rate of the spherulites (G) increased at first, and then decreased as the molecular weight increased in all of the PLLA/PDLA, 3PLLA/3PDLA and 3PLLA/PDLA specimens. The highest G value in all of the specimens was achieved when the average molecular weights of each branch were ∼10 kg mol⁻¹. With similar molecular weights for each of the branches, the G reduced gradually as the three-armed PLA was added to the blend (i.e., PLLA/PDLA > 3PLLA/PDLA > 3PLLA/3PDLA).