Crystals in Situ Induced by Supercritical CO2 as Bubble Nucleation Sites on Spherulitic PLLA Foam Structure Controlling

The CO2-induced crystals were successfully utilized to regulate the structure of the foamed poly(L-lactic acid) (PLLA) spherulites with large numbers of cells ranging from nanoscale to microscale in three temperature regions. Region I (60-80 degrees C) was characterized by the double melting peaks and the nanocells with the dramatically improved cell density. By increasing the temperature to Region II (90-110 degrees C); more highly perfect crystals and the unique spherulitic foam morphology were developed. In Region III (115-120 degrees C) double melting endotherms emerged again, and microcells were formed. The double melting endotherms in Regions I and III originated from the imperfect crystals created during the saturation and cooling stage, respectively. A bimodal structure with microcells surrounded by nanocells occurred at 115 degrees C and 20 MPa. It was shown that cells could nucleate and grow in between the neighboring lamellar stacks and that the cells interacted with the spherulites.