Green Method to Widen the Foaming Processing Window of PLA by Introducing Stereocomplex Crystallites

Polylactide (PLA) microcellular foams with well-defined cell morphology were successfully fabricated from poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) blends through a supercritical carbon dioxide (CO2) foaming method. The gaps between the melting temperature of homocrystallites (Tm(HC)) and stereocomplex crystallites (Tm(SC)) were utilized to improve PLA's melt foaming behaviors (i.e., cell nucleation and cell morphology) owing to the existence of exclusive SC crystallites when the foaming temperature fell in between Tm(HC) and Tm(SC). Thanks to the enhanced melt strength, which resulted from the introduction of SC crystallites, the cell morphology transformed from elliptical to nearly circular and the cell collapse and coalescence reduced when foamed at high temperatures and pressures. The wider foaming processing window is favorable for actual industrial manufacture such as the continuous extrusion foaming process. In addition, PLA foams possessing more SC crystallites exhibit higher heat resistance, which further broadens their applications in high-temperature conditions.