Cell Morphology and Improved Heat Resistance of Microcellular Poly(l-lactide) Foam via Introducing Stereocomplex Crystallites of PLA

The preparation of poly(l-lactic acid) (PLLA) foam with well-defined cell structure and high heat resistance is critical to broaden its applications. In this study, the stereocomplex crystallites (SC) with higher melting point and heat stability were introduced into PLLA foam by melt blending the PLLA with different amounts of poly(d-lactic acid) (PDLA). The crystal structures of pure PLLA and PLLA/PDLA blends formed during the blending and molding process were compared. It was found that no obvious crystallization was detected in pure PLLA, while SC formed in PLLA/PDLA blends. Crystal structure and morphology evolution of PLLA and PLLA/PDLA during the CO2 saturation and foaming process were investigated by combination of DSC, WAXD, FTIR, and SEM techniques. The results suggested SC had a higher melting peak, higher thermal stability, and smaller crystal domain size in relation to the homocrystal of PLLA, and it did not further develop with the CO2 saturation and the foaming processes, while the CO2 saturation induced the formation of the mesomorphic structure in PLLA and PLLA/PDLA blends. The mesomorphic structure transformed into the more stable α form in the following foaming process. The resultant PLLA/PDLA foam exhibited a significant and concurrent increase in cell density and cell structure uniformity relative to PLLA foam. The heat resistance measurement presented that thus-prepared PLLA/PDLA foams had better heat resistance than PLLA foam, which was attributed to the higher melting point, higher heat stability, and higher cell nucleation ability of SC in PLLA/PDLA foams.