Enhancing Nanofiller Dispersion Through Prefoaming and Its Effect on the Microstructure of Microcellular Injection Molded Polylactic Acid/Clay Nanocomposites

Promoting better dispersion of nanofillers is crucial in producing high performance nanocomposite foams. For the effects of nanofillers on controlling foam structures and mechanical properties to be enhanced, prefoamed pellets were produced via supercritical fluid (SCF) extrusion foaming and subsequently microcellular injection molded. Structural, thermal, and mechanical properties of the prefoamed pellets and resultant polylactic acid (PLA)/clay nanocomposite foams were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy, and tensile testing. The diffraction peaks in the XRD plots of extruded prefoamed pellets were observed to shift to lower angles, indicating that prefoaming improved the intercalation of clay platelets. The driving forces behind the intercalation and dispersion of clay were (i) enhanced diffusion via the plasticizing effect of SCF and (ii) the phase change of SCF from the supercritical to the gaseous state within the polymer melt. In the nanocomposites made with prefoamed pellets, clay was present mostly in separate layers, and the distance between the clay platelets was usually greater than the effective radius of gyration of the polymer molecules. With the much improved nucleating effect of the dispersed clay platelets, the microcellular injection molded nanocomposite foams exhibited the smallest cell size and the highest cell density. They also displayed a desirable microstructure in which cells were well-separated, closed, and round in shape. DSC analyses showed that PLA was able to crystallize upon heating and that the prefoaming step promoted PLA crystallization. TGA analyses showed that prefoaming did not cause the molecular weight of PLA to decrease or change significantly.