Membranes are separation barriers, which allow a selective passage of components of single-phase and multi-phase gaseous and liquid mixtures. Membranes under the usual name filters are used for purification of water or air, but they are as well an essential component of advanced technologies ensuring, for example, an operation of nuclear reactors or extracorporeal artificial organs. Depending on the size of the pores, water purification filters eliminate solid and liquid impurities, or even salts, when desalinating seawater. Respirator membranes remove dust particles, health-threatening chemical gases, as well as micrometre-sized bacteria and even smaller nanometre-sized viruses.
The majority of contemporarily used membranes are made of traditional materials such as ceramics or cellulose and they are available with a wide range of pore sizes. Newer polymer-based membranes represent a group of special, often custom-made membranes. These are made of various types of polymers with different surface properties. Such membranes are often post-treated with chemicals, which change, for example, electrical polarity of the membranes and/or interaction with penetrating substances. This group of special membranes also includes membranes made of carbon nanotubes. When the membrane nanotubes are oriented in the direction of the flow, the average size of the pores corresponds to the diameter of the carbon nanotubes, which ranges from about 0.4 to 20 nanometres. By contrast, membranes of non-oriented nanotubes resemble a cohesive network of intertwined structure with pore diameters greater than 20 nanometres. Carbon nanotube membranes are electrically conductive and chemical properties of carbon surfaces can be readily modified similarly to the surfaces of other polymer membranes.
Research at the Institute of Hydrodynamics
At the Institute for Hydrodynamics of the Academy of Sciences of the Czech Republic in cooperation with the Centre for Polymer Systems of Tomas Bata University in Zlín, we focus on altering flow conditions of carbon nanotube membranes by means of changes in their electrical properties. In particular, we have developed a membrane made from interwoven carbon nanotubes on a polyurethane substrate with an electrically controlled selective permeability for polar and non-polar vapours of organic solvents.
An interaction of the permeating vapours with the surface of carbon nanotubes supported by the plastic substrate is the key factor, which regulates the selective permeability. In contrast to polar vapours of alcohols, non-polar hydrocarbon vapours are attracted to the surfaces of carbon nanotubes. Linear hydrocarbon molecules such as hexane may closely contact surfaces of carbon nanotubes. Their molecules can adapt their spatial structures to the longitudinal curvature of the membrane pores and slide on carbon nanotube walls at higher flow rates than alcohol vapours.
The flow of polar and non-polar organic vapours through the carbon nanotube membrane changes substantially when direct current is applied. The flow of polar alcohol vapours surpassed those of non-polar hydrocarbon vapours subjected to the same voltage. Thus, applying electric field changes the selective permeability of the carbon nanotube/polyurethane membrane towards polar and non-polar hydrocarbon vapours. Our findings show that various combinations of polarity, carbon nanotube pores size and applied voltage can selectively adjust hydrodynamic conditions enabling e.g. selective vapour purification. Such membranes offer not only a highly favourable combination of functional properties, but at the same time the low cost of both the carbon nanotubes and the production processes make such carbon nanotube membranes a viable novel alternative to traditional materials used to manufacture membranes.
Fig. 1: Nanoporous membrane of carbon nanotubes
Fig. 2: Cross-section of 50 micrometres thick membrane (dark layer) on a polyurethane mat (light layer)
Slobodian, P., Říha, P., Kondo, H., Cvelbar, U., Olejník, R., Matyáš, J., Sekine, M., Hori, M. (2020). Transparent elongation and compressive strain sensors based on aligned carbon nanowalls embedded in polyurethane. Sensors and Actuators, A: Physical. 306(May), 111946.
Slobodian, P., Říha, P., Olejník, R., Matyáš, J. (2020). Accelerated Shape Forming and Recovering, Induction, and Release of Adhesiveness of Conductive Carbon Nanotube/Epoxy Composites by Joule Heating. Polymers. 12(5), 1030.
Slobodian, P., Říha, P., Olejník, R., Sedlačík, M. (2020). Ethylene-octene-copolymer with embedded carbon and organic conductive nanostructures for thermoelectric applications. Polymers. 12(6), 1316.
Slobodian, P., Říha, P., Olejník, R., (2018). Electrically-Controlled Permeation of Vapors Through Carbon Nanotube Network-Based Membranes. IEEE Transactions on Nanotechnology 17, 332-337.
Slobodian, P., Lloret Pertegás, S., Říha, P., Matyáš, J., Olejník, R., Schledjewski, R., Kovář, M., (2018). Glass fiber/epoxy composites with integrated layer of carbon nanotubes for deformation detection. Composites Science and Technology 156, 61-69.
Slobodian, P., Cvelbar, U., Říha, P., Olejník, R., Matyáš J., Filipič, G., Watanabe, H., Tajima, S., Kondo, H., Sekine, M., Hori, M., (2015). High sensitivity of carbon nanowalls based sensor for detection of organic vapours. Royal Society of Chemistry: Advances 5, 90515–90520.