Nanocellular and needle-like structures in poly(l-lactic acid) using spherulite templates and supercritical carbon dioxide

Abstract

Poly(L-lactic acid) (PLLA) foams with unique nanocellular and needle-like morphology were successfully prepared by combining spherulite templating and supercritical carbon dioxide (CO₂) foaming. The corresponding crystalline morphology formed under supercritical CO₂ in PLLA was illustrated to investigate the foaming behavior of spherulites in detail. It was found that not only the degree of crystallinity but also the crystalline morphology played a vital role in cell nucleation and growth, thus the foam morphology. Nanocellular structure was primarily generated for the PLLA foamed at 100 °C and 12–24 MPa. Moreover, the morphological transition from approximate circular cells to needle-like cells occurred around 16 MPa at 100 °C because of the constraint of lamellae, and the two different structures coexisted at 100 °C and at pressures ranging from 16 to 24 MPa. The results indicated that the expansion ratio of spherulite was bigger than that of PLLA foam.