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 Tg. 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.

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Poly(L-lactic acid) (PLLA) was immersed in a simulated body fluid (SBF) solution at 37.58C for distinct times. The variation of the surface mechanical properties of PLLA samples with immersion time was followed by microhardness. These measurements showed that PLLA microhardness decreased significantly ($ 60%) after only 30 days of immersion. The results were explained in terms of hydrolytic degradation of the samples. The dependence of microhardness with the applied dwell time was also analyzed. The creep curves were successfully described by a power law. A decrease of the creep constant k as the immersion time increased was found. Differential scanning calorimetry was also used to analyze the changes in the physical properties of PLLA, namely in crystallinity degree (X c ) and glass transition temperature (T g ), as a function of the immersion time in SBF. A significant variation in the crystallinity degree of PLLA, initially nearly amorphous (X c 5 9%), was detected after only 3 days of immersion (X c 5 37%). The interpretation of this behavior was based on the hydrolysis process suffered by PLLA.