Shear-Induced Crystallization of Star and Linear Poly(L-lactide)s

The influence of macromolecular architecture on shear-induced crystallization of poly(L-lactide) (PLLA) was studied. To this aim, three star PLLAs, 6-arm with M_w of 120 and 245 kg/mol, 4-arm with M_w of 123 kg/mol, and three linear PLLAs with M_w of 121, 240 and 339 kg/mol, were synthesized and examined. The PLLAs were sheared at 170 and 150 °C, at 5/s, 10/s and 20/s for 20 s, 10 s and 5 s, respectively, and then cooled at 10 or 30 °C/min. Shear-induced crystallization during cooling was followed by a light depolarization method, whereas the crystallized specimens were examined by DSC, 2D-WAXS, 2D-SAXS and SEM. The effect of shear depended on the shearing conditions, cooling rate and polymer molar mass but it was also affected by the macromolecular architecture. The shear-induced crystallization of linear PLLA with M_w of 240 kg/mol was more intense than that of the 6-arm polymer with similar M_w, most possibly due to its higher M_z. However, the influence of shear on the crystallization of the star polymers with M_w close to 120 kg/mol was stronger than on that of their linear analog. This was reflected in higher crystallization temperature, as well as crystallinity achieved during cooling.