Diffusion of CO 2 in polylactide was modelled by assuming the diffusion coefficient to depend on CO 2 concentration, c, according to D[c] ¼ D[0]exp [Ac], where D[0] and A are empirical constants, with the aim of optimizing impregnation of nominally amorphous and semicrystalline polylactide/CO 2 -based precursors for physical foaming. Numerical simulations provided a consistent description of desorption at different temperatures, T, from polylactide impregnated with liquid CO 2 at 10 C and 5 MPa, and D[0, T] could be represented analytically using Arrhenius or Williams-Landel-Ferry-type expressions, allowing interpolation and extrapolation. Sorption was argued on this basis to involve a step-like diffusion front, such that the CO 2 content of a plate of thickness l increased as (D[0]t) 1/2 l À1 F[Ac o ], where c o is the value of c at saturation and F is a function of Ac o only. A major practical concern with polylactide/ CO 2 precursors is that the glass transition temperature, T g , decreases strongly with c, so that amorphous polylactide saturated with CO 2 at 10 C and 5 MPa degasses spontaneously at room temperature and pressure. However, it was inferred from the models and confirmed experimentally that partial impregnation in liquid CO 2 for relatively short times could provide a relatively rapid means of preparing precursors with a roughly uniform CO 2 content of around 0.1 g/g that were stable with respect to rapid CO 2 loss on heating to room temperature. The resulting precursors gave satisfactory foam morphologies and densities on foaming at 100 C. Moreover, it was also possible to adapt the impregnation conditions so as to obtain partially foamed structures from semicrystalline polylactide under these conditions, in spite of its

The evolution of the morphology and degree of crystallinity was investigated postmortem in initially amorphous specimens of a commercial poly(DLlactide) with a relatively low D-lactide content, after different immersion times in liquid CO 2 at 10°C and 5 MPa. Relatively high concentrations of CO 2 induced a crystalline phase that remained stable at room temperature after desorption of the CO 2 , but was distinct from those generally associated with melt crystallization of polylactides (PLA), as demonstrated by transmission electron microscopy and wideangle 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 CO 2 content to reach saturation, resulting in a welldefined 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 CO 2 concentration gradient that reflected a strong increase in the diffusivity of the CO 2 with the local CO 2 content. Crystallization led to a reduction in both the rate of CO 2 uptake and the CO 2 concentration at saturation compared with that observed for a poly(DL-lactide) with a significantly higher Dlactide content and little tendency to crystallize in the presence of liquid CO 2 . Assuming the CO 2 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 CO 2 concentration, consistent with the observed rates of CO 2 uptake.

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Solid heat-expandable foam precursors were prepared by impregnating melt-blended poly(D,L-lactide) (PDLLA)-poly(methyl methacrylate) (PMMA) blends with liquid carbon dioxide (CO 2 ). The phase behavior of these blends was strongly dependent on the processing steps, but impregnation with liquid CO 2 led to phase separation regardless of the prior thermomechanical history, and crystallization in blends containing a low-D grade of PDLLA suppressed subsequent expansion. On the other hand, blends containing nominally amorphous high-D PDLLA were found to be unstable with respect to expansion under ambient conditions when saturated with CO 2 . It was therefore necessary to reduce the overall CO 2 content by allowing it to desorb partially at 10 °C immediately after impregnation. Under these conditions, the amorphous PDLLA-50 wt% PMMA precursors were stable at ambient temperature and pressure, and showed peak expansion ratios at significantly higher temperatures than pure PDLLA, thanks to the increase in effective glass transition temperature with increasing PMMA content. It was hence demonstrated that blending with PMMA may provide a convenient means of tailoring the process window for heatexpandable polylactide foams, as well as improved heat stability.

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