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 °C, which leads to growth of α′‐crystals. The maximum melting temperature of the α′‐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 α′‐crystals requires suppression of formation of α‐structure, which is achieved by fast heating using a fast scanning chip calorimeter. For the PLLA homopolymer, the critical heating rate above which the α′/α‐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.