Crystal polymorphism of poly(l-lactic acid) and its influence on thermal properties

The influence of crystal polymorphism on the thermal properties of poly(l-lactic acid) (PLLA) is discussed in this contribution. Crystallization of PLLA at high temperatures yields the stable α form, whereas at low temperatures the metastable α′ modification develops, which is characterized by slightly larger lattice dimensions compared to the α counterpart, and by some degree of conformational disorder. Quantitative analysis with conventional and temperature-modulated calorimetry revealed a three-phase structure of PLLA composed of a crystal phase and two amorphous fractions with different mobility, for all the analyzed thermal histories. A higher coupling of the amorphous chain segments with the crystal phase was found in the presence of α crystals, probably due to the slightly larger lattice dimensions and the looser arrangements of PLLA chains in the α′ structure. Some peculiarities in the thermal behavior were rationalized, like an unusual frequency-dependence of the reversing apparent heat capacity upon the solid–solid transition from the α′ to the α crystals. Devitrification of the rigid amorphous segments seems also to be differently affected by the coupled crystal structure for the two analyzed crystal modifications of PLLA.

Research highlights▶ The amorphous chain segments of PLLA are largely coupled with the α crystals. ▶ The reduced coupling with α′ crystals is associated to conformational disorder. ▶ Mobilization of the RAF is affected by the coupled crystal structure (α or α′). ▶ Some peculiarities in the thermal behavior of PLLA were rationalized.