Influence of crystal polymorphism on mechanical and barrier properties of poly(l-lactic acid)

The effect of crystal polymorphism on barrier and mechanical properties of PLLA is detailed in this contribution. PLLA films containing different amounts of alfa and alfa' crystal forms were prepared by annealing quenched PLLA at different temperatures. The polymorphic structure of the films was analyzed by X-ray diffraction. Mechanical properties and permeability to water vapor were investigated as a function of degree of crystallinity (wC) and related to crystal polymorphism developed during annealing. The polymorphic structure of PLLA significantly affects mechanical and barrier properties. The alfa crystal modification provides a better barrier to water vapor and a higher Young’s modulus, compared to films containing the alfa' modification, but a lower elongation at break. The varied barrier and mechanical properties were correlated to the different packing of PLLA chains in the two analyzed polymorphs. The conformational disorder of the alfa' form makes this structure a mesophase (condis crystal), with remarkable effects on material properties.