(595g) Hollow Fibers by Electrospinning in Supercritical CO2

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.

Bognitzki, M., Schattka, H., Greiner,
A., (2000). Polymer, Metal, and Hybrid Nano- and
Mesotubes by Coating Degradable Polymer Template Fibers (TUFT Process). Adv. Mater., 12, 637-640

Dror, Y., Salalha, W., Avrahami, R.,
Zussman, E., Yarin, A. L., Dersch, R., Greiner, A., Wendorff, J. H., (2007). One-Step Production of Polymeric Microtubes by
Co-electrospinning. small, 3, 1064-1073

Li, D., Xia, Y., (2004). Electrospinning of
nanofibers: Reinventing the wheel? Adv. Mater., 16, 1151-1170

McHugh, M. A., Krukonis, V. J., (1994). Supercritical
fluid extraction, Butterworth-Heinemann, Stoneham

Shen, Z., Thompson, B. E., McHugh,
M. A., (2006) Electrospinning in Near-Critical CO₂. Macromolecules,
39, 8553-8555