Effect of Growing Crystalline Phase on Bubble Nucleation in Poly(L-Lactide)/CO2 Batch Foaming

Creating a high-efficiency bubble nucleation agent for a polymer foaming process is essential for increasing bubble density as well as decreasing bubble size. Spherulites (crystalline phase) grown in a semicrystalline poly(L-lactide) (PLLA) matrix enhanced carbon dioxide (CO2) bubble nucleation in a batch foaming process. Several sites for PLLA spherulite formation were observed at a hold-temperature of 110 °C after CO2 saturation in molten PLLA, which was heated to 180 °C at 11 MPa. By depressurizing the system from 11 MPa to atmospheric pressure, the dissolved CO2 in the PLLA matrix became supersaturated, and CO2 bubbles nucleated around growing PLLA spherulites. The number of nucleating bubbles increased as a function of increasing spherulite quantity and area. A faster linear spherulite growth rate and a lower hold-temperature created more bubbles around the spherulites. From these observations, it was concluded that the growing spherulites expelled CO2 from the advancing spherulite−amorphous phase interface and that CO2 accumulated at the interface. Then, the increase in the concentration of CO2 led to an increase in the nucleation of bubbles around the spherulites.