Stability and Reorganization of α′-Crystals in Random L/D-Lactide Copolymers

The isotropic melt of poly(L-lactic acid) (PLLA) and random L/D-lactide copolymers with 2% and 4% D-isomer co-units in the chain has been crystallized at 90 degrees C, which leads to growth of alpha'-crystals. The maximum melting temperature of the alpha'-crystals is around 150 degrees C in case of the homopolymer, and decreases by about 10 and 15 K in the random copolymers containing 2% and 4% D-isomer co-units, respectively. Analysis of the stability of the alpha'-crystals requires suppression of formation of alpha-structure, which is achieved by fast heating using a fast scanning chip calorimeter. For the PLLA homopolymer, the critical heating rate above which the alpha'/alpha-transition is completely suppressed is around 30 K s(-1), which significantly decreases in the copolymers due to the presence of D-isomer co-units. The slower kinetics of the melting-recrystallization process in such random L/D-lactide copolymers is explained by the required segregation of the chain defects.