Polymer fibers with a diameter of several ten to hundred nanometers have a very large surface area to volume ratio. This enables wide applications of polymer fibers with flexibility in surface functionalities and superior mechanical performance (Xia et al., 2004). Especially if polymer fibers offer specific structures such as porous surfaces, hollow, or core-sheath, their application can be widely extended to filters, textiles, sensors, catalysis, or in medicine and pharmacy.
Up to now, hollow fibers have been produced by template processes or using coaxial capillary (Greiner et al., 2000, Wendorff et al., 2007). The main difficulty of these processes is the limitation of applied materials. Recently, a new method is published by McHugh et al. to produce hollow fibers (McHugh et al., 2006). This process is based on the solubility of organic solvents into supercritical CO2. The unique advantage of this process is a one step process without any additives and further processes to form the hollow structure inside of fibers. Supercritical CO2 dissolves an organic solvent 1-2 order of magnitude faster than conventional liquid antisolvents (McHugh et al., 1994). In electrospinning, supercritical CO2 acts as an antisolvent which is a non-solvent for almost all polymers but miscible with most organic solvents.
A polymer solution with polymer concentrations of 3 to 10 wt% is injected by a HPLC (High pressure liquid chromatography) pump through a thin capillary with an inner diameter of 100 to 1000 µm. The autoclave consists of two stainless steel flanges and one non-conductive plastic vessel. The flanges are connected by a high voltage power supply (0 ~ 30 kV) and act as electrodes. On one of the flanges a collector is placed, to where polymer drops stretch due to the high voltage. The autoclave is pressurized by CO2 to the desired pressure. During polymer jets are flying to the collector, the organic solvent used will be extracted to the supercritical CO2.
Our investigation shows that the solubility of organic solvents in CO2 affects the morphology of fibers during electrospinning. By varying the temperature and pressure, the solubility of organic solvent can be easily modified, which enables the formation of hollow fibers. Hollow fibers by electrospinning in supercritical CO2 will provide a big potential in a wide range of applications.
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