Qiu, GM; Qiu, Y; Tian, Rui; Xu, YouYi; ,
The supercritical fluids (SCFs) have dissolving power comparable to that of liquids, are much more compressible than dilute gases, and have transport properties intermediate between gas- and liquid-like. This unusual combination of physical properties can be advantageously exploited in environmentally benign separation and reaction processes, as well as for new kinds of materials processing. This chapter summarizes the modifications and preparations of membranes that have been conducted in SCFs, with primary emphasis on modifications and preparations of PVDF membranes that have been conducted in supercritical carbon dioxide (SCCO2) in our laboratories. SCCO2 has strong ability to dissolve small organic compounds. It can also swell many polymers, but most polymers are not soluble in SCCO2. These unique properties of CO2/polymer systems have been applied to prepare and modify the membranes for separation. The SCCO2-induced phase separation was firstly used to prepare poly (vinylidene fluoride) (PVDF) microporous membrane. The membranes held network structure and average pore size was several microns. Different from immersion precipitation method using water as non-solvent, the SCCO2-induced phase separation is dry process, in which the dry membranes were directly obtained after CO2 pressure was diminished without post-treatment. Compared with traditional immersion precipitation method, membranes prepared with SCCO2-induced phase separation have the higher pore size and porosity and excellent thermal stabillity. SCCO2, instead of organic solvent, was used to extract the pore-forming agent and diluents in the extraction-activated method and thermal induced phase separation method for preparing membrane. Compared with the extractants having high surface tension and boiling point, SCCO2 was not only environmentally sound and inexpensive, but also eliminated the shrinkage of membranes after extraction and improved the membrane performance. There was a close relationship between membrane performance and the extraction conditions of SCCO2, and it is possible to tailor membrane performance through the choice of extraction conditions. SCCO2, having unique mass-transport properties, are used to modify polymer membranes without thermal stress by impregnation in SCCO2. The polymerization of monomers were performed inside the membrane matrix with SCCO2 as solvent and carrier. After pressure release, the polymers with high molecular weight were successfully impregnated in the nascent membrane to produce polymer blend membrane. The partitioning of monomers between the CO2-swollen membrane matrix and SCCO2 solution could be controlled by the CO2 density and also by the pressure of SCCO2. The modification results thereby were controlled by the impregnation and reaction parameters. In this way, we produced blend membrane of poly(styrene-maleic anhydride) (SMA) and poly (vinylidene fluoride) (PVDF) by impregnating PVDF porous membrane with styrene/maleic anhydride. The modification conditions could be chosen to render blend membrane containing SMA throughout the entire thickness of the PVDF membrane substrate. The SMA-PVDF blend membrane retained the basic geometry of PVDF membrane matrix and had excellent thermal stability and hydrophilicity.Functional monomers can be easily diffused into and grafted onto the surface of microporous membrane, pre-irradiated byelectron beam, plasma and etc, using SCCO2 as solvent and carrier, resulting in a dry uniformly grafted membrane after pressure release. The surface grafting copolymerization can significantly change the surface properties of membrane and retain the basic porous structure of the membrane matrix. This method is simple and has no post-treatment, and the grafting percentage can be controlled by altering the monomer concentration, pressure, and temperature during the soaking process.