Foaming of poly(lactic acid) with supercritical CO2: The combined effect of crystallinity and crystalline morphology on cellular structure

Using a batch foaming technique, the effects of supercritical carbon dioxide on the formation of crystals under different saturation conditions (i.e. temperature or pressure) and subsequently on the cellular structure of the PLA foams were investigated. The crystallinity decreased with increasing annealing temperature or pressure, meanwhile, crystal morphologies were varied by controlling the temperature or pressure. It was found that a uniform closed cell structure was obtained by specifically controlling the foaming temperature in the range of (80-100) degrees C, in which only the ring-banded spherulites were formed. A suitable foaming window as a function of temperature was proposed. A number of stamen-like cell structure was observed because of formation of irregular-banded spherulites under 8 MPa. However, an opened and interconnected cell structure was obtained at 24 MPa. The results indicated that not only the crystallinity but also the crystalline morphology played an important role in the cellular structure.