Crystal nucleation in random L/D-lactide copolymers

Poly (L-lactic acid) (PLLA) and its random copolymers with small amount of d-isomer co-units but identical mass-average molar mass of 120 kDa were analyzed regarding the non-isothermal and isothermal crystallization and nucleation behaviors. First, it was confirmed that the gross crystallization rate and crystal growth rate decrease with increasing d-isomer content in the chains. In extension to earlier work, the isothermal analysis of the crystallization rate was completed by determination of critical cooling rates to suppress ordering of the melt. While in case of the PLLA homopolymer the melt needs to be cooled faster than 0.5 K s-1 in order to suppress crystallization, the critical cooling rate is lowered by about one order of magnitude to 0.05 K s-1 in case of a copolymer with 4% d-isomer co-units. However, even if the melt is cooled faster than the critical cooling rate to suppress crystallization, nuclei formation still occurs in a specific cooling-rate range depending on the d-isomer content, ceasing only on cooling about two orders of magnitude faster than the critical cooling rate to suppress crystallization. The observation of slower nuclei formation at non-isothermal conditions in PLLA containing d-isomer co-units is confirmed by isothermal nucleation experiments.