Crystallization of polylactide during impregnation with liquid CO2
The evolution of the morphology and degree of crystallinity was investigated postmortem in initially amorphous specimens of a commercial poly(dl-lactide) with a relatively low d-lactide content, after different immersion times in liquid CO2 at 10 A degrees C and 5 MPa. Relatively high concentrations of CO2 induced a crystalline phase that remained stable at room temperature after desorption of the CO2, but was distinct from those generally associated with melt crystallization of polylactides (PLA), as demonstrated by transmission electron microscopy and wide-angle X-ray diffraction, consistent with previous observations. Crystallinity developed at the surface of the specimens within relatively short times compared with those necessary for the overall CO2 content to reach saturation, resulting in a well-defined semicrystalline layer, whose thickness increased with immersion time. This behaviour was argued to be consistent with the existence of a well-defined diffusion front, associated with a step-like CO2 concentration gradient that reflected a strong increase in the diffusivity of the CO2 with the local CO2 content. Crystallization led to a reduction in both the rate of CO2 uptake and the CO2 concentration at saturation compared with that observed for a poly(dl-lactide) with a significantly higher d-lactide content and little tendency to crystallize in the presence of liquid CO2. Assuming the CO2 to be concentrated in the amorphous regions of semicrystalline PLA, a simple model for non-linear Fickian diffusion based on data from previous desorption measurements was used to show that diffusion through the semicrystalline surface layer should dominate impregnation kinetics in initially amorphous specimens that undergo rapid crystallization above a certain critical CO2 concentration, consistent with the observed rates of CO2 uptake.