Stability and Reorganization of alpha'-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 °C, which leads to growth of ??-crystals. The maximum melting temperature of the ??-crystals is around 150 °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 ??-crystals requires suppression of formation of ?-structure, which is achieved by fast heating using a fast scanning chip calorimeter. For the PLLA homopolymer, the critical heating rate above which the ??/?-transition is completely suppressed is around 30 K s -1 , which signifi cantly 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.