The effect of different annealing treatments with supercritical carbon dioxide (SCCO2) on the structural and mechanical properties of semicrystalline poly-L-lactic acid (L-PLA) was investigated. 2000, 27,000, 100,000, and 350,000 g mol(-1) molecular weight L-PLA polymers were used in the study. The solid-state processing Of L-PLA at temperatures lower than the effective melting point led to solvent- and thermal-induced crystallization. Solvent-induced and isothermal crystallization mechanisms could be considered similar regarding the increase of polymer chain mobility and mass-transfer in the amorphous region; however, quite different microstructures were obtained. SCCO2 solvent-induced crystallization led to polymers with high crystallinity and melting point. On the contrary, SCCO2 thermal-induced crystallization led to polymers with high crystallinity and low melting point. For these polymers, the hardness increased and the elasticity decreased. Finally, the effect of dissolving SCCO2 in the molten polymer (cooling from the melt) was analyzed. Cooling from the melt led to polymers with high crystallinity, low melting point, low hardness, and low elasticity. Distinctive crystal growth and nucleation episodes were identified. This work also addressed the interaction of SCCO2-drug (triflusal) solution with semicrystalline L-PLA. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 291-300, 2009