Morphological contributions to glass transition in poly(L-lactic acid)

Poly(L-lactic acid) (PLLA) samples were prepared with different degrees of crystallinity, obtained during cooling from the melt at different scanning rates, and subjected to annealing below T_g. For intermediate crystallinities two endothermic peaks assigned to enthalpy recovery could be detected by differential scanning calorimetry (DSC) on heating, indicating the existence of two distinct glass transition dynamics. The morphology at different length scales was characterized using WAXS, SAXS, and polarized light microscopy to correlate the DSC results to the microstructure. The low-temperature process was assigned to the bulklike glass transition whereas the high-temperature one was attributed to the restricted motions of the amorphous phase confined by the crystalline structures. The position and broadness of the two endotherms were found to be essentially independent of the spherulitic content of the samples. This was related to the invariance of the lamellar morphology within a large range of degrees of crystallinity. The occurrence of the high-temperature process throughout such range allowed to attribute this process to the hindered motions of the mobile amorphous phase within the lamellar stacks.