Spinning Continuous Fibers for Nanotechnology
Nanotubes of carbon and other materials are arguably the most fascinating nanomaterials playing an important role in nanotechnology today. Their unique mechanical, electronic, and other properties are expected to result in revolutionary new materials and devices. However, these nanomaterials, produced mostly by synthetic bottom-up methods, are discontinuous that leads to difficulties with their alignment, assembly, and processing into applications. Partly because of this, and despite considerable effort, a viable CNT-reinforced supernanocomposite is yet to be demonstrated. Advanced continuous fibers produced a revolution in the field of structural materials and composites in the last decades. Fiber properties are known to substantially improve with a decrease in their diameter. However, conventional mechanical fiber spinning techniques cannot produce fibers with diameters smaller than about 2 micrometers. Most commercial fibers are several times that diameter, owing to the trade-offs between the technological and economic factors.
(A) Comparison of commercial advanced carbon fiber, one of the smallest advanced fibers available, and electrospun continuous nanofiber.
Electrospinning technology enables production of continuous nanofibers from polymer solutions or melts in high electric fields. A thin jet of polymer liquid is ejected, elongated, and accelerated by the electric forces. The jet undergoes a variety of instabilities, dries, and is deposited on a substrate as a random nanofiber mat. There interest in the electrospinning and electrospun nanofibers has been growing steadily since the mid-1990s, triggered by potential applications of nanofibers in the nanotechnology. Over a hundred synthetic and natural polymers were electrospun into fibers with diameters ranging from a few nanometers to micrometers (panel A).
Comparison of (B) vapor grown commercial carbon nanofibers and (C) electrospun carbon nanofibers showing substantially better nanofiber uniformity and sample purity.
The main advantage of this top-down nanomanufacturing process is its relatively low cost compared to that of most bottom-up methods. The resulting nanofiber samples are often uniform and do not require expensive purification (panels B, C). Unlike submicron-diameter whiskers, nanorods, carbon nanotubes, and nanowires, the electrospun nanofibers are continuous. As a result, this process has unique potential for cost effective electromechanical control of fiber placement and integrated manufacturing of two- and three-dimensional nanofiber assemblies. In addition, the nanofiber continuity may alleviate, at least in part, concerns about the properties of small particles, which have begun to catch the attention of the public [Washington Post, “For Science, Nanotech Poses Big Problems” by Rick Weiss, Sunday, Jan. 31, 2004]. Nanofibers are expected to posses high mechanical properties combined with extreme flexibility. The nanofiber assemblies may feature very high open porosity coupled with remarkable specific surface area. Uses of nanofibers in composites, protective clothing, catalysis, electronics, biomedicine (including tissue engineering, implants, membranes, and drug delivery), filtration, agriculture, and other areas are presently being developed. Clearly there is a growing interest in the process, but the results reported to date are centered mostly on the empirical production and the proposed uses of polymer nanofibers. At the same time, thorough understanding of the mechanisms of jet formation and motion is needed for the development of robust methods of process control. Analysis of the electrospinning process is complicated by electromechanical coupling, non-linear rheology, and unusual jet instabilities. Some progress was recently made on modeling of jet initiation . Steady-state spinning was modeled in the non-linear rheologic regime important for polymer jets . Experimental observations and modeling of bending (or whipping) instability  produced a major breakthrough in process analysis and understanding. These instabilities are responsible for both rapid jet thinning in this process and the resulting random nanofiber orientation.
(D and E) Examples of highly aligned and spaced linear and orthogonal assemblies of continuous nanofibers produced by the gap method of alignment developed by the PIs group at UNL.
More recently, three major breakthroughs were made that are expected to have lasting impact on the quality and scope of the applications. First, several methods of nanofiber alignment were developed that can be roughly classified into methods ’directing’ or suppressing jet bending instabilities . The methods need to be further improved because most produce only partial alignment, but the results show promise. Alignment can revolutionize existing and help develop entirely new applications of nanofibers. The modified methods demonstrate the feasibility of integrated nanofiber manufacturing and placement or assembly (panels D, E) that can be extremely economic when compared to the postprocessing methods that are currently being developed for carbon nanotubes. In the second breakthrough, the original process used with high polymers has been modified and applied, in combination with sol-gel chemistry, to produce continuous ceramic nanofibers . These nanofibers can be beneficial in the areas of catalysis, tough and high-temperature ceramics, active and sensing materials, and many others. Nanocrystalline nanofibers such as the one shown in panel F may lead to supertough ceramics. Along with the polymer and polymer-derived carbon and ceramic nanofibers, the sol-gel derived ceramic nanofibers provide a comprehensive nanomaterial and nanomanufacturing platform for extremely broad variety of applications. In the third breakthrough, a method of co-axial electrospinning was developed and used to produce continuous coated and hollow nanofibers . This method allows a single step production of continuous nanotubes or nanopipes that complements the multiple-step template methods of tube production demonstrated earlier.
(F) Cross-section of pioneering continuous nanocrystalline zirconia nanofiber produced at UNL for potential applications in supertough ceramics.
Despite considerable recent progress, serious challenges remain. Improved models of the process are needed to achieve better understanding of the mechanisms of jet thinning. In particular, thermal and mass transport within the jet in conjunction with solvent evaporation is crucial for jet thinning, solidification, and formation of nanofiber molecular structure. The evaporation leads to inhomogeneous transient concentration and temperature profiles that effect local rheological and other properties of the fluid, and, therefore, jet thinning. The major experimental challenge is to develop robust methods for manufacturing extremely small nanofibers. Although diameters as small as 3 to 5 nanometers were reported, nanofibers smaller than about 50 nanometers in diameter cannot currently be produced uniformly and repeatedly for most materials systems. The effects of solvent evaporation and jet instabilities on the diameter reduction need to be studied more carefully. Thorough experimental analysis of the composition of the electrospinning instability zone may provide insight on the temporal and spatial evolution of jet instabilities and jet thinning. Another need is to model nanofiber deposition on substrates, both stationary and moving, which can help improve the nanofiber alignment using the newly developed methods and may also lead to the development of novel techniques. Multiple jet electrospinning should also be analyzed and jet interactions with each other as well as with the modified field configurations used in the alignment methods should be characterized and modeled. Multiple jets are critical for the scale-up of the nanofiber production process. New elegant processes yielding high aerial jet densities are expected to be developed. Analysis and implementation of the combined methods involving ultrasmall diameter jets, alignment, and multiple jet spinning are expected to be especially challenging.
Electrospun continuous polymer, carbon, and ceramic nanofibers have considerable advantages when compared to the discontinuous carbon or other nanotubes or nanorods in terms of the cost, health concerns, and the possibility of integrated one-step manufacturing of assemblies. A fundamental experimental and theoretical analysis of the process is needed to develop flexible and reliable methods of fabrication of nanofibers and their assemblies and composites.
References and Notes
1. A. F. Spivak and Y. A. Dzenis, J. Appl. Mech. 66, 1026 (1999); A. L. Yarin, S. Koombhongse, and D. H. Reneker, J. Appl. Phys 90, 4836 (2001)
2. A. F. Spivak, Y. A. Dzenis, and D. H. Reneker, Mech. Res. Comm. 27, 37 (2000); J. J. Feng, J.Non-Newtonian Fluid Mech. 116, 55 (2003)
3. A. L. Yarin, S. Koombhongse, and D. H. Reneker, J. Appl. Phys 89, 3018 (2001); Y.M. Shin et al, Appl. Phys. Letters 78, 1149 (2001)
4. A. Theron, E. Zussman, and A. L. Yarin, Nanotechnology 12, 384 (2001); R. Dersch et al, J. Pol. Sci: A: Pol. Chem. 41, 545 (2003); D. Li, Y. Wang, and Y. Xia, Nano Letters 3, 1167 (2003); An alignment method similar to the one described by Dersch and Li has been used in Dzenis’ laboratory at UNL since 2001
5. Y. A. Dzenis and G. Larsen, U.S. patent pending (2001); G. Larsen et al, J. A. Chem. Soc. 125, 1154 (2003); H. Dai et al, Nanotechnology 13, 674 (2002); S.-S. Choi et al, J. Mat. Sci. Letters 22, 891 (2003)
6. Z. Sun et al, Adv. Materials 15, 1929 (2003); I. G. Loscertales et al, J. Am. Chem. Soc. 126, 5376 (2004); D. Li and Y. Xia, Nano Letters 4, 933 (2004)
7. I thank Y. Wen for providing the SEM images. This article resulted from research supported by the National Science Foundation and other agencies.
FIGURES (adapted from the perspective on materials science, Y. Dzenis, Science, Vol. 304, 25 June 2004, 1917)