APPLICATION OF SUPERCRITICAL CARBON DIOXIDE IN THE PREPARATION OF BIODEGRADABLE POLYLACTIDE MEMBRANES

摘要

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 [1]. Supercritical fluid technology referred as "Green 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) [2,3], a microcellular foaming process [4,5] and a precipitation with compressed fluid anti-solvent (PCA) [6-8]. Most of the polymer membranes in use were prepared by the immersion precipitation process. And the application of SCCO_2 has been attracting attentions in the preparation of polymer membranes [9-11]. The advantages are as follows [9,12,13]. First, CO_2 is inexpensive, environmentally benign, nonflammable, and can be completely and easily removed from products. At the same time, the moderate critical conditions (Tc=31.1?, Pc=73.8 bar) of CO_2 allow CO_2 to be used within safe commercial and laboratory operation conditions. Another advantage is that the polymer membrane can be dried rapidly with SCCO_2, but without collapse of the structure. Furthermore, after the pressure is diminished, the solvent dissolved in the SCCO_2 can be removed from the gaseous CO_2. 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. In this paper, PLA membranes were first prepared with SCCO_2 which replaced the conventional toxic nonsolvent methanol. Moreover, we chose another solvent tetrahydrofuran (THF) to replace the traditional toxic solvent chloroform.