Solid heat-expandable polylactide-poly(methyl methacrylate) foam precursors prepared by immersion in liquid carbon dioxide
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 (CO2). The phase behavior of these blends was strongly dependent on the processing steps, but impregnation with liquid CO2 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 CO2. It was therefore necessary to reduce the overall CO2 content by allowing it to desorb partially at 10 A degrees 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 heat-expandable polylactide foams, as well as improved heat stability.