|
|
Seminarium ZTOCCrystal polymorphism of poly(L-lactic acid) and its influence on thermal, mechanical and barrier propertiesMaria Laura Di Lorenzo (Istituto di Chimica e Tecnologia dei Polimeri, CNR Pozzuoli, Włochy) |
Poly(L-lactic acid) (PLLA) is a biodegradable, biocompatible polyester produced from renewable resources. Crystallization rate vs. temperatures plots of PLLA show deviation from the usual bell-shaped curve caused by a polymorphism. Crystallization at high temperatures yields stable alpha form, whereas at low temperatures metastable alpha' modification develops characterized by larger lattice dimensions and some degree of conformational disorder. Influence of crystal polymorphism on thermal, mechanical, and barrier properties of PLLA films was investigated. The polymorphic structure was analyzed by X-ray diffraction. Quantitative analysis with conventional and temperature-modulated calorimetry revealed a three-phase structure, crystalline phase and two amorphous fractions, for all the analyzed thermal histories. Mechanical properties and permeability to water vapor of PLLA were investigated vs. degree of crystallinity and related to the crystal polymorphism. The polymorphic structure significantly affects mechanical and barrier properties. The alpha crystal polymorph provides better barrier to water vapor and higher Young's modulus, but a lower elongation at break. The varied barrier and mechanical properties were correlated to the different packing of the polymer chains in the polymorphs, as well as to the morphology of PLLA spherulitic superstructures. The conformational disorder of the alpha' form makes a mesophase condis crystal structure with remarkable influence on the material properties.