Microcellular Foaming of Poly(lactic acid)/Silica Nanocomposites in Compressed CO2: Critical Influence of Crystallite Size on Cell Morphology and Foam Expansion

During the solid state foaming, the CO2 saturated poly(lactic acid) (PLA) sample at 5 MPa and 20 °C has a high crystallinity of 23.2%, and the prepared PLA foams exhibits low foam expansion and nonuniform cell structure. This study presents an interesting effect of nanosilica addition on the cell morphology and expansion ratio of PLA foams. It was found that the presence of nanosilica increased the induced crystallinity of PLA up to 29.7% at 5 MPa. The resultant PLA/silica foams exhibited significant and concurrent increase in cell structure uniformity and cell density: the cell density increased about 5–10 times, the expansion ratio increased 1.4–2.1 times, and the crystallinity of foams increased 1.3 times, compared to pure PLA foams. Further investigation suggested that the formation of the tiny crystallite size and the well dispersed nanosilica aggregates were thought as the main reasons to explain the interesting effect of nanosilica addition on the foaming behavior of PLA.