Amorphous fractions of poly(lactic acid)

Detailed knowledge of the organization of amorphous and crystalline phases in a semicrystalline polymer such as poly(L-lactic acid) (PLLA) is crucial for complete understanding and prediction of its physical properties. In semicrystalline polymers, amorphous segments located at different distances from the crystalline domains can exhibit different properties. In PLLA, there exist (1) two distinct mobile amorphous fractions (a bulk-like totally mobile amorphous fraction and a slightly constrained mobile amorphous fraction), which vitrify/devitrify in the region of the bulk glass transition temperature (Tg); and (2) a rigid amorphous fraction, located at the crystal-amorphous interface, which vitrifies/devitrifies at temperatures higher than the bulk Tg. Knowledge of these three amorphous phases was derived through the use of different experimental techniques. The results together suggested a link between the crystalline morphology and the percentage content and segmental dynamics of the three amorphous fractions of PLLA. These studies are collected and summarized in the present contribution, with special emphasis on the evolution of the different amorphous fractions during crystallization of PLLA at different temperatures. A connection between the presence of rigid amorphous fraction at the crystal-amorphous interface and the melting behavior of PLLA is discussed. In addition, some physical properties of completely amorphous PLLA and poly(lactic acid) copolymers containing different percentages of D-lactic acid co-units are presented and compared.