Influence of a'-/a-crystal polymorphism on properties of poly(l-lactic acid)

Poly(l-lactic acid) (PLLA) is a biodegradable and biocompatible thermoplastic polyester produced from renewable sources, widely used for biomedical devices, in food packaging and in agriculture. It is a semicrystalline polymer, and as such its properties are strongly affected by the developed semicrystalline morphology. As a function of the crystallization temperature, PLLA can form different crystal modifications, namely ??-crystals below about 120 °C and ?-crystals at higher temperatures. The ?? modification is therefore of special importance as it may be the preferred polymorph developing at processing-relevant conditions. It is a metastable modification which typically transforms into the more stable ?-crystals on annealing at elevated temperature. The structure, kinetics of formation and thermodynamics of ??- and ?-crystals of PLLA are reviewed in this contribution, together with the effect of ??-/?-crystal polymorphism on the properties of PLLA.