| It is well known that supercritical fluid has some distinctive properties like gaslike viscosity and diffusity, liquidlike density and dissolvability. Small changes in temperature and pressure could cause dramatic changes in the density, viscosity and dielectric properties of the fluid, which makes it a tunable solvent. Supercritical fluid technology referred as “Gren Chemistry Technology” has been developed to form polymeric materials such as microspheres, porous fibers, and porous membranes. Typical processes are a rapid expansion of supercritical solutions (RESS), a microcellular foaming process and a precipitation with compressed fluid anti-solvent (PCA).
Most of the polymer membranes in use were prepared by the immersion precipitation process. And the application of SCCO2 has been attracting attentions in the preparation of polymer membranes. The advantages are as follows. First, CO2 is inexpensive, environmentally benign, nonflammable, and can be completely and easily removed from products. At the same time, the moderate critical conditions of CO2 allow CO2 to be used within safe commercial and laboratory operation conditions. Another advantage is that the polymer membrane can be dried rapidly with SCCO2, but without collapse of the structure. Furthermore, after the pressure is diminished, the solvent dissolved in the SCCO2 can be removed from the gaseous CO2. In addition, it is convenient to control the morphology of the products by altering the pressure. Polylactide (PLA) is a kind of “Green Materials” because of its biodegradability and biocompatibility. Recently, the use of polylactide has been focusing on medicine and biology. Traditionally, PLA membranes were prepared by the immersion precipitation process with chloroform/methanol as the solvent/nonsolvent. Here, PLA membranes were first prepared with SCCO2 which replaced the conventional toxic nonsolvent methanol. Moreover, we chose another solvent tetrahydrofuran (THF) to replace the traditional toxic solvent chloroform.
When supercritical carbon dioxide was used as nonsolvent, PLA membranes prepared with different solvent were studied by SEM, WAXD and polarizing microscope. It was found that the solvent had a profound effect on the cross-section structure of PLA membranes. The solvent and the preconditioning had some effect on the crystalline morphology of PLA membranes with SCCO2 as nonsolvent. Small spherulites were found in the membrane prepared from PLA/chloroform, but rod-like crystalline formed in the membrane prepared from PLA/THF. The rod-like morphology was changed into leafy structure for the PLA membrane with preconditioning. Therefore, compared to the traditional membrane formation, SCCO2 can be used as convenient driving force to prepare membranes with different morphology. |