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Scalable Liquid Shear-Driven Fabrication of Polymer Nanofibers Stoyan K. Smoukov, Tian Tian, Narendiran Vitchuli, Sumit Gangwal, Pete Geisen, Miles Wright, Eunkyoung Shim, Manuel Marquez, Jeffrey Fowler, and Orlin D. Velev* The development of scalable, rapid, and cost-effective processes for nanomaterials fabrication is one of the major challenges of nanoscience.[1,2] One class of nanomaterials of high value is nanofibers, which may have potential applications in liquid and aerosol filtration,[1,3] improved batteries,[4] biotechnology,[5] and tissue engineering.[6] This report is based on the finding that the simple process of antisolvent-induced polymer precipitation during shear-driven droplet extension “in a beaker” can be tuned precisely to produce a broad range of structures, including nano- and microscale fibers, nanoribbons, and sheets. The potential and scalability of such a simple procedure seem to have escaped much attention and detailed characterization. Fiber formation in microfluidic channels,[7–9] as well as carbon nanotube organization in flowing liquid,[10–12] have been described earlier, but the mechanism and goal of these processes differ from simple polymer precipitation in sheared bulk fluid. Polymer extension inside a second polymer, followed by a matrix dissolution,[13] as well as polymer crystallization,[14] can result in nanofibers, but the use of polymer melts in these methods imposes a number of limitations. The formation of finely dispersed materials such as nanofibers requires the creation of a large surface area (usually a highly energy-consuming process) and the ability to control and modify the characteristic sizes and shapes of the structures. Polymer nanofibers can be formed by mechanical drawing, force spinning, electrospinning, phase separation, template synthesis, and self-assembly.[1,15,16] Mechanical drawing methods such as melt blowing and melt spinning have high Prof. S. K. Smoukov, Dr. T. Tian, Prof. O. D. Velev Department of Chemical and Biomolecular Engineering North Carolina State University Raleigh, NC 27606, USA E-mail: [email protected] Prof. S. K. Smoukov Department of Materials Science and Metallurgy University of Cambridge Cambridge CB3 0FS, UK Dr. N. Vitchuli, Dr. S. Gangwal, P. Geisen, M. Wright Xanofi Inc., Raleigh, NC 27617, USA Prof. E. Shim College of Textiles North Carolina State University Raleigh, NC 27695, USA Dr. M. Marquez Ynano LLC., Midlothian, VA 23113, USA Dr. J. Fowler R&D, Syngenta Co., Greensboro, NC 27409, USA
DOI: 10.1002/adma.201404616
Adv. Mater. 2015, DOI: 10.1002/adma.201404616
productivity,[17,18] but produce fibers from melt-processable polymers with relatively large diameters (d > 0.5 µm). Electrospinning is a versatile method that produces nanofibers from a variety of polymers with uniform diameters in a wide range (d = 20–2000 nm),[19] but of modest production rate even when scaled up by parallel operation of many nozzles,[20] or by spinning from a drum[21] as commercialized by ELMARCO. The nanofabrication method that we report here can also be tuned to yield nano- and micro-ribbons and rods. The applications of such materials are less investigated, but compared with nanofibers, they have even higher surface area, stronger adhesion, and higher biodegradability. Rod-like, sheet-like, and other asymmetric particles are finding use as foams and emulsion superstabilizers,[22] permeable capsules,[23] electro-optical media, and viscosity modifiers.[24] The shear precipitation process takes place during direct injection of polymer solutions in the bulk of a viscous medium under shear. The polymer solvent is miscible with the shearing medium. A critical fourth component of the systems is a polymer antisolvent, mixed within the shear medium, which induces precipitation of the injected dissolved polymer. The typical media contained glycerin and 20–90% of antisolvent, such as water or ethanol, inducing polymer precipitation in the injected sheared solution. The initial characterization of the role of process parameters on the resulting micro- and nanomaterials was performed by using a simple benchtop batch device with concentric cylinder (Taylor–Couette) geometry, assuring controlled and easily variable uniform shear rate. A small amount of polymer solution is injected directly in the viscous fluid between the rotating cylinders and the polymer structures formed in the liquid are examined microscopically. As the polymer solution is sheared in the viscous medium, the ultralow interfacial tension between the droplets and the medium enables the formation of high surface area liquid streaks, which serve as templates for the precipitation of diverse types of polymer materials, at least one characteristic dimension of which may be on the nanoscale. As the sheared solution droplets become highly stretched, the solvent diffuses out of the polymer-containing liquid phase, while the antisolvent in the medium infuses it (Figure 1). The composition of the solvent medium changes and the polymer precipitates into solid mass. The interplay of these effects (which occur rapidly and mostly simultaneously) results in the formation of a surprisingly rich variety of structures. However, the control and judicious adjustment of the polymer and antisolvent concentrations allows facile formation of nanofibers with uniform diameter in the nanoscale range. The process and the role of the parameters are illustrated in Figure 1 with polystyrene (PS) structures synthesized under different conditions.
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Figure 1. Schematics of the interplay of processes taking place during shear of polymer solution in viscous medium that contains polymer antisolvent (precipitant). The sheared droplets are extended in long, thin liquid strands. As the solvent diffuses out and antisolvent diffuses into the proto-fibers, the interplay of mass transport, polymer precipitation and molecular entanglement determine the structure formed as follows. Path A: If the polymer MW or concentration is too low to achieve molecular entanglement, the strands break off in droplets or rods. Path B: If the antisolvent influx is too high the rapidly forming polymer skin will precipitate in various sheet or ribbon-like structures. Path C: Balancing the fluxes and operating at the lowest antisolvent concentration leads to formation of very thin uniform nanofibers. These material morphologies are exemplified by SEM micrographs of PS structures formed as follows: Path A – short polymer rods from 5.78 kDa MW PS solution; Path B – sheet- and ribbon-like, and Path C – long fibers from 230 kDa MW PS solution after shearing at 2000 rpm in glycerol mixtures with varying ethanol content. The effects involved are described quantitatively in Figure 2a,b. All scale bars in these images = 10 µm (note the much smaller characteristic cross-section of the fibers in Path C).
A mostly trivial case is realized when the polymer concentration is too low to affect the common capillary fragmentation of droplets under shear. The outcome is polymer spheres, similar to many presently used colloids. However, solution droplets of higher polymer concentration resist capillary fragmentation and get sheared into anisometric structures. The resulting liquid fingers or strands begin solidifying into long continuous polymer structures. These fibrous precipitates may be subject to secondary fragmentation (Figure 1, Path A) yielding polymer microrods, similar to ones reported by us earlier.[25,26] We now established that the fragmentation of the protofibers can be avoided by using polymers with higher molecular weight (MW) or solutions above certain critical concentration. In the
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examples in Figure 1, solutions of two batches of PS of MW = 5.78 and 230 kDa, sheared under nearly identical conditions, yielded short rods for the low MW polymer, and long fibers for the high MW one. The polymer concentration also had to exceed a critical value before high-aspect ratio precipitated structures were formed. It had a strong impact on the morphology above this critical threshold. The existence of a critical concentration can be correlated to the molecular entanglement of the polymer chains in the semiliquid protofibers, a well-documented effect in wetspinning and electrospinning.[27] The molecular chain entanglement can be characterized by the functional dependence of the viscosity on the polymer concentration. In Figure 2a, we
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present a log–log plot of the zero shear viscosities of poly(lactic acid) (PLA) solutions and correlate them to the type of structures formed while shearing these solutions in glycerol/ethanol mixture. The plot shows a clear change in power-law line slopes between the semidilute unentangled, and the entangled regimes just below 2% w/w PLA The power line slope in the unentangled region matches theoretical expectations, although due to solution nonideality and polymer polydispersity, the slope of the entangled PS solution line is lower than Rouse model prediction for neutral polymers in good solvents.[28,29] In excellent correlation, the formation of fibers and fibrous
the intercylinder gap (Figure S1, Supporting Information). The monotonic decrease both in the average fiber diameter and its polydispersity (Figure 3b) can further be correlated to the characteristic fluid domain size, a, resulting from the process of γ shear-driven deformation, which can be evaluated by a = Cacr , τ where γ is the interfacial tension and Cacr is the dimensionless critical capillary number that depends on the ratio of viscosities of the polymer droplets and the media, and for this system may be in the range 0.3–20.[30–32] The range of rpm in Figure 3b corresponds to shear stresses τ ≈ 34–204 Pa. The interfacial tension between the two miscible liquid
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Figure 2. Effects of polymer and antisolvent concentration on the type of structure formed. a) Effect of polymer concentration on the viscosity of PLA (MW = 62.7 kDa) solution in CHCl3. The entanglement concentration is determined by the change in the slope of the log–log plot of the zero shear viscosity versus polymer concentration. The SEM images in the insets demonstrate the result of shearing these PLA solutions at 2000 rpm in glycerin containing 50% v/v ethanol. Discontinuous aggregates from spheres are observed when the sheared solutions are below the entanglement concentration (Path A in Figure 1). Fibers are formed above the entanglement level. b) Effect of the concentration of antisolvent (ethanol) added to the glycerin medium in 10 wt% PLA, sheared at 2000 rpm. Overall, both a critical polymer and a critical antisolvent concentration (20% in this case) are needed in order to precipitate thin, continuous fibers along Path C in Figure 1.
structures was first observed at concentrations around this entanglement point. While more complex effects, related to the viscoelasticity of the protofibers and the fragility of the solvent-swollen polymer, may be potentially involved, the polymer entanglement during the shearing process proved essential for the formation of fibers and other continuous structures. The shearing of polymer solutions above the entanglement threshold yielded a variety of nanofibers, nanoribbons, and nanosheets (Figure 1, Paths B and C). The specific morphology is dependent on the pattern of polymer precipitation into the sheared liquid fingers, controlled by solvent efflux into and antisolvent influx from the medium. The rate of antisolvent infusion is strongly dependent on its concentration in the medium, and critically affects both the morphology and characteristic dimensions of the resulting structures. The typical effect of the antisolvent concentration on the formation of nanofibers and other morphologies from polylactic acid is illustrated in Figure 2b. Ethanol concentrations up to ≈20% v/v produced spherical particles of increasing diameter (as this antisolvent concentration is insufficient to solidify the polymer before the liquid streaks break up into droplets by capillarity). Above this critical concentration the sheared material solidified into uniform long fibers of sub-micrometer diameter (Path C in Figure 1, center image in Figure 2b). Further increasing the EtOH concentration led to increase of the fiber cross-section and deformation from circular to oval cross-section, forming ribbons (right image in Figure 2b). The deformation is likely a consequence of the rapid formation of a thick polymer skin before all of the solvent has left the protofiber core. Rapid skin formation at low polymer concentrations just above the entanglement threshold could also lead to synthesis of very thin sheet-like structures (Figure 1, Path B). The polymer ribbons and nanosheets can be valuable materials on their own, however, we focus further on the most widely used material presently, which is uniform liquid-dispersed nanofibers. Nanofibers are readily and reproducibly formed at balanced precipitation conditions, under moderate polymer and antisolvent concentrations following Path C in Figure 1. The diameter of the fibers formed in the concentric cylinder device (Figure 3a) could be controlled by several parameters, starting with the shear stress. The dependence of the fiber diameter on the angular velocity of the Couette flow device is illustrated in Figure 3b. The uniform shear stress, τ, generated by the flow between the rotating concentric cylinders can be evaluated as μω i ri , where µ is the fluid viscosity, ri and ωi are the radius τ≈ Δr and angular velocity of the inner cylinder, and Δr = ro – ri is
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Figure 3. Effect of the shear rate and scaling up the process to kilogram scale production. a) Schematic of the lab-scale concentric cylinder device for droplet shearing at a rate controlled through the angular velocity ω. b) Relation between the shear rate and fiber diameter in the concentric cylinder device. The bars represent the d10–90% range of distributions within each sample. The average fiber diameter and distribution decrease to sub-micrometer values with increasing angular velocity. c) Schematic of the scaled-up continuous flow process. The nanofibers are produced in a continuous stream of fluid with antisolvent flowing in a pipe, while the polymer solution is injected from a side opening and immediately sheared into liquid-borne nanofibers. The process is scalable and parallelizable to nanofiber production in kilograms or even tons/hour. d) Examples of the size distribution and SEM micrographs of typical PS nanofibers produced in a continuous flow shear device by injection of 12% w/w PS-THF solution in glycerin containing 25% water flowing at a rate of 55 mL s–1 through a pipe of 3 mm diameter. The median and average PS nanofiber diameters are 149 nm and 168 ± 89 nm correspondingly. The variability of the average and median fiber diameters between 1–5 kg batches of nanofibers produced in separate runs by the device is less than 5%.
phases is not well defined, but assuming an effective γ ≈ 0.01– 0.1 × 10−3 N m–1, we estimate that the dispersed polymer solution will be sheared into micrometer- and sub-micrometersized liquid fibers, which is in good correspondence with the experimental outcomes. We next developed a continuous liquid high shear fabrication device, which has a number of advantages to the batch unit described above and was used for making of the smallest size fibers reported in this paper. We found that simple injection of polymer solution into a stream of viscous medium flowing rapidly in a tube can be used for continuous large scale fabrication of nanofibers through Poiseuille shear flow. The scaled up flow device uses a circular pipe with flowing antisolvent medium, while the polymer solution was injected through a side opening (Figure 3c). A simple desktop-sized flow shearing device with a flow rate of 1.8 m3 h–1 through a tube of inner diameter 4.57 mm produces up to 10 kg h–1 nanofibers, an extraordinary high rate for such a small and inexpensive apparatus (full details of the shear devices and procedures are provided in the Experimental Section and the Supporting Information).
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Due to the easily controlled and stable laminar flow operation, the device reliably produces nanofibers of average diameters smaller than 200 nm and tight size distribution. Examples of the size distribution of typical PS nanofibers are presented in Figure 3d, and of ones from cellulose acetate are shown in the Supporting Information. The size distribution is easily reproduced in the subsequent fabrication of multikilogram size batches (see Supporting Information). The method could also be easily modified for the production of nanofibers from a large number of solution-processable and even some reactive polymers, including polylactic acid (Figure 2), PS (Figure 3), cellulose acetate, poly(methyl methacrylate), Nylon 6, polyvinyl alcohol, polyvinylidene fluoride, polypropylene, and cellulose (a few examples are given in Figure 4). The method also allows facile fabrication of fibers with embedded particles such as conductive composite PS fibers containing high volume fractions of carbon black aggregates (Figure 4c), antibacterial silver nanoparticles, magnetite nanocubes (Figure 4d,e), and many others. Finally, the method could be adapted for the formation of metal oxide (such as titania, TiO2) nanofibers, by
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greatly the potential areas of application of these nanofibers and allows making new composite and nonwoven materials. Streams of liquid-borne staple nanofibers can be easily integrated with industrial wet-laying lines and spraying devices for large-volume production of filters, coatings, mats and nonwovens. Before wet-laying or spraying, the suspended nanofibers can be mixed with commercial microfibers, resulting in intermixed micro/nanofiber materials (illustrated in Figure 4f) with very highly developed area and high particle capturing capability. Such liquid-deposited nanofiber/microfiber composites (Figure 4f) could also serve as excellent cell scaffolding and growth substrates, which are being presently investigated. The ability to control morphologies, together with the multitude of functionalization possibilities by particle additives, could in the future open the way to the large-scale fabrication of diverse classes of other nanomaterials. The liquid-shear fabrication of nanofibers and nanoribbons at unprecedented rates and volumes could enable their use as bulk components of products ranging from filters to bioscaffolds and could have a transformative impact on the emerging area of nanomanufacturing.
Experimental Section
Figure 4. Examples of diverse polymer and composite fibers produced by shear-driven liquid fabrication. All samples are synthesized in glycerol medium. SEM micrographs of: a) Very fine uniform PS nanofibers from 12% w/w PS solution in THF, antisolvent is 25% water. b) Porous, ribbonlike Nylon fibers from 12% Nylon 6 solution in formic acid, antisolvent is a mixture of 15% ethanol and 40% 0.2 N NaOH. c) Composite electrically conductive PS/carbon black fibers produced from a solution of 10.5% PS and 4.5% CB in 40% DMF, 60% CHCl3, antisolvent is 5% ethanol. The percolated CB particle aggregates are comparable in size to the fiber diameter and are protruding from the surface. d) Composite magnetically responsive CA-iron oxide fibers from a solution of 16% CA and 0.2% iron oxide particles in THF with 25% water as antisolvent. e) TEM image of composite PS/Fe3O4 fiber produced from a solution of 10% PS in CHCl3 containing ≈0.5% Fe3O4 magnetite nanocubes, antisolvent is 25% ethanol; f) SEM image of CA nanofibers incorporated in a 3D wetlaid matrix mixed together with commercial polyester microfibers. The liquid-suspended staple nanofibers are specifically well-suited for direct integration in industrial processes for wet-laying of high surface area filters and nonwovens.
pyrolysis of shear-spun nanofibers in air as described in the Supporting Information. In summary, we introduce here a simple shear process that could be easily scaled up to continuous production of nanofibers in a stream of fluid medium. This continuous flow device is presently being scaled up to the commercial fabrication of nanofibers at rates that could exceed tens or hundreds of kilograms/hour. The process has a few specific features that can make it of high value to commercial nanotechnology, largely stemming from its ability to make nanofibers dispersed and carried in a flux of liquid as opposed to methods such as electrospinning and melt-blowing, where dry fibers are carried and deposited by gas phase. The use of liquid medium expands
Adv. Mater. 2015, DOI: 10.1002/adma.201404616
A benchtop Couette flow apparatus with a 2.3-mm gap (Figure 3a) was constructed with a rotating cylindrical shaft (radius ri = 5.0 mm) and a disposable, stationary polypropylene tube (ID = 14.6 mm). Shaft rotation was digitally controlled within the 150–6000 rpm range. The fibers were formed after injecting polymer solution in a sheared viscous antisolvent medium, similarly to the process previously used to fabricate anisotropic polymer rods.[25] Typically, 0.05 mL of polymer solution, injected into 6.5 mL of antisolvent, was sheared at 2000–6000 rpm for 2 min. The shearing medium was a mixture of glycerol and polymer antisolvent. Our initial experiments at higher shear rates showed significant increase in polydispersity from instabilities at the top free liquid–air interface. This instability was suppressed by introducing a top baffle made of cured PDMS with uniform thickness to prevent drawing of air into the liquid medium during shear. The continuously operating Poiseuille flow device was built around a cylindrical pipe through which the viscous antisolvent medium is pumped at high flow rates (Figure 3c). A triplex positive displacement pump (CAT Pumps, Model No. 2SF30ES) is used to push the viscous medium with antisolvent through 4 ft long 4.57 mm inner diameter stainless steel tube. The polymer solution is injected through a small side inlet into the larger pipe. The polymer solution in the desktop prototype is injected using a 20 mL syringe pump. These liquid-borne nanofibers are easily amenable to wet-laying, and thus can be easily integrated in a number of common filter and nonwoven manufacturing lines.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors are grateful to Rossitsa Alargova, Behnam Pourdeyhimi, and Alexander Yarin for helpful discussions, Joseph Tracy and Peter Krommenhoek for providing magnetite nanocubes, and to Saad Khan, Christopher Bonino, Sara Arvindson, and Arjun Krishnan for assistance
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www.MaterialsViews.com with rheometry. The authors acknowledge the NSF (Grant Nos. CMMI0927554 and IIP-1127793 to Stoyan Smoukov and Orlin Velev), as well as Syngenta, Xanofi, and Triangle MRSEC (Grant No. NSF DMR-1121107), for partial support of this study. Received: October 6, 2014 Revised: February 8, 2015 Published online:
[1] C. J. Luo, S. D. Stoyanov, E. Stride, E. Pelan, M. Edirisinghe, Chem. Soc. Rev. 2012, 41, 4708. [2] O. D. Velev, S. Gupta, Adv. Mater. 2009, 21, 1897. [3] S. Ramakrishna, R. Barhate, J. Membr. Sci. 2007, 296, 1. [4] S. Cavaliere, S. Subianto, I. Savych, D. J. Jones, J. Rozière, Energy Environ. Sci. 2011, 4, 4761. [5] Z. Ma, M. Kotaki, S. Ramakrishna, J. Membr. Sci. 2005, 265, 115. [6] Q. P. Pham, U. Sharma, A. G. Mikos, Tissue Eng. 2006, 12, 1197. [7] W. Jeong, J. Kim, S. Kim, S. Lee, G. Mensing, D. J. Beebe, Lab Chip 2004, 4, 576. [8] O. Bonhomme, J. Leng, A. Colin, Soft Matter 2012, 8, 10641. [9] J.-I. Park, A. Saffari, S. Kumar, A. Gunther, E. Kumacheva, Annu. Rev. Mater. Res. 2010, 40, 415. [10] B. Vigolo, A. Pénicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, P. Bernier, P. Poulin, Science 2000, 290, 1331. [11] A. B. Dalton, S. Collins, E. Muñoz, J. M. Razal, V. H. Ebron, J. P. Ferraris, J. N. Coleman, B. G. Kim, R. H. Baughman, Nature 2003, 423, 703. [12] C. Mercader, A. Lucas, A. Derré, C. Zakri, S. Moisan, M. Maugey, P. Poulin, Proc. Natl. Acad. Sci. USA 2010, 107, 18331.
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[13] D. Wang, G. Sun, B.-S. Chiou, Macromol. Mater. Eng. 2007, 292, 407. [14] A. Pennings, A. Kiel, Colloid Polym. Sci. 1965, 205, 160. [15] R. Kotek, Polym. Rev. 2008, 48, 221. [16] B.-S. Kim, I.-S. Kim, Polym. Rev. 2011, 51, 235. [17] C. J. Ellison, A. Phatak, D. W. Giles, C. W. Macosko, F. S. Bates, Polymer 2007, 48, 3306. [18] N. Anantharamaiah, S. Verenich, B. Pourdeyhimi, AATCC Rev. 2008, 3, 1. [19] D. Li, Y. Xia, Adv. Mater. 2004, 16, 1151. [20] S. A. Theron, A. L. Yarin, E. Zussman, E. Kroll, Polymer 2005, 46, 2889. [21] O. Jirsak, S. Petrik, Int. J. Nanotechnol. 2012, 9, 836. [22] R. G. Alargova, D. S. Warhadpande, V. N. Paunov, O. D. Velev, Langmuir 2004, 20, 10371. [23] P. F. Noble, O. J. Cayre, R. G. Alargova, O. D. Velev, V. N. Paunov, J. Am. Chem. Soc. 2004, 126, 8092. [24] J. S. Guevara, A. F. Mejia, M. Shuai, Y.-W. Chang, M. S. Mannan, Z. Cheng, Soft Matter 2013, 9, 1327. [25] R. G. Alargova, K. H. Bhatt, V. N. Paunov, O. D. Velev, Adv. Mater. 2004, 16, 1653. [26] R. G. Alargova, V. N. Paunov, O. D. Velev, Langmuir 2006, 22, 765. [27] S. L. Shenoy, W. D. Bates, H. L. Frisch, G. E. Wnek, Polymer 2005, 46, 3372. [28] R. H. Colby, Rheol. Acta 2010, 49, 425. [29] M. G. McKee, G. L. Wilkes, R. H. Colby, T. E. Long, Macromolecules 2004, 37, 1760. [30] C. Mabille, F. Leal-Calderon, J. Bibette, V. Schmitt, Europhys. Lett. 2003, 61, 708. [31] S. Sugiura, M. Nakajima, N. Kumazawa, S. Iwamoto, M. Seki, J. Phys. Chem. B 2002, 106, 9405. [32] J. Rallison, Annu. Rev. Fluid Mech. 1984, 16, 45.
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Scalable Liquid Shear-Driven Fabrication of Polymer Nanofibers Stoyan K. Smoukov, Tian Tian, Narendiran Vitchuli, Sumit Gangwal, Pete Geisen, Miles Wright, Eunkyoung Shim, Manuel Marquez, Jeffrey Fowler, and Orlin D. Velev* The development of scalable, rapid, and cost-effective processes for nanomaterials fabrication is one of the major challenges of nanoscience.[1,2] One class of nanomaterials of high value is nanofibers, which may have potential applications in liquid and aerosol filtration,[1,3] improved batteries,[4] biotechnology,[5] and tissue engineering.[6] This report is based on the finding that the simple process of antisolvent-induced polymer precipitation during shear-driven droplet extension “in a beaker” can be tuned precisely to produce a broad range of structures, including nano- and microscale fibers, nanoribbons, and sheets. The potential and scalability of such a simple procedure seem to have escaped much attention and detailed characterization. Fiber formation in microfluidic channels,[7–9] as well as carbon nanotube organization in flowing liquid,[10–12] have been described earlier, but the mechanism and goal of these processes differ from simple polymer precipitation in sheared bulk fluid. Polymer extension inside a second polymer, followed by a matrix dissolution,[13] as well as polymer crystallization,[14] can result in nanofibers, but the use of polymer melts in these methods imposes a number of limitations. The formation of finely dispersed materials such as nanofibers requires the creation of a large surface area (usually a highly energy-consuming process) and the ability to control and modify the characteristic sizes and shapes of the structures. Polymer nanofibers can be formed by mechanical drawing, force spinning, electrospinning, phase separation, template synthesis, and self-assembly.[1,15,16] Mechanical drawing methods such as melt blowing and melt spinning have high Prof. S. K. Smoukov, Dr. T. Tian, Prof. O. D. Velev Department of Chemical and Biomolecular Engineering North Carolina State University Raleigh, NC 27606, USA E-mail: [email protected] Prof. S. K. Smoukov Department of Materials Science and Metallurgy University of Cambridge Cambridge CB3 0FS, UK Dr. N. Vitchuli, Dr. S. Gangwal, P. Geisen, M. Wright Xanofi Inc., Raleigh, NC 27617, USA Prof. E. Shim College of Textiles North Carolina State University Raleigh, NC 27695, USA Dr. M. Marquez Ynano LLC., Midlothian, VA 23113, USA Dr. J. Fowler R&D, Syngenta Co., Greensboro, NC 27409, USA
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productivity,[17,18] but produce fibers from melt-processable polymers with relatively large diameters (d > 0.5 µm). Electrospinning is a versatile method that produces nanofibers from a variety of polymers with uniform diameters in a wide range (d = 20–2000 nm),[19] but of modest production rate even when scaled up by parallel operation of many nozzles,[20] or by spinning from a drum[21] as commercialized by ELMARCO. The nanofabrication method that we report here can also be tuned to yield nano- and micro-ribbons and rods. The applications of such materials are less investigated, but compared with nanofibers, they have even higher surface area, stronger adhesion, and higher biodegradability. Rod-like, sheet-like, and other asymmetric particles are finding use as foams and emulsion superstabilizers,[22] permeable capsules,[23] electro-optical media, and viscosity modifiers.[24] The shear precipitation process takes place during direct injection of polymer solutions in the bulk of a viscous medium under shear. The polymer solvent is miscible with the shearing medium. A critical fourth component of the systems is a polymer antisolvent, mixed within the shear medium, which induces precipitation of the injected dissolved polymer. The typical media contained glycerin and 20–90% of antisolvent, such as water or ethanol, inducing polymer precipitation in the injected sheared solution. The initial characterization of the role of process parameters on the resulting micro- and nanomaterials was performed by using a simple benchtop batch device with concentric cylinder (Taylor–Couette) geometry, assuring controlled and easily variable uniform shear rate. A small amount of polymer solution is injected directly in the viscous fluid between the rotating cylinders and the polymer structures formed in the liquid are examined microscopically. As the polymer solution is sheared in the viscous medium, the ultralow interfacial tension between the droplets and the medium enables the formation of high surface area liquid streaks, which serve as templates for the precipitation of diverse types of polymer materials, at least one characteristic dimension of which may be on the nanoscale. As the sheared solution droplets become highly stretched, the solvent diffuses out of the polymer-containing liquid phase, while the antisolvent in the medium infuses it (Figure 1). The composition of the solvent medium changes and the polymer precipitates into solid mass. The interplay of these effects (which occur rapidly and mostly simultaneously) results in the formation of a surprisingly rich variety of structures. However, the control and judicious adjustment of the polymer and antisolvent concentrations allows facile formation of nanofibers with uniform diameter in the nanoscale range. The process and the role of the parameters are illustrated in Figure 1 with polystyrene (PS) structures synthesized under different conditions.
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Figure 1. Schematics of the interplay of processes taking place during shear of polymer solution in viscous medium that contains polymer antisolvent (precipitant). The sheared droplets are extended in long, thin liquid strands. As the solvent diffuses out and antisolvent diffuses into the proto-fibers, the interplay of mass transport, polymer precipitation and molecular entanglement determine the structure formed as follows. Path A: If the polymer MW or concentration is too low to achieve molecular entanglement, the strands break off in droplets or rods. Path B: If the antisolvent influx is too high the rapidly forming polymer skin will precipitate in various sheet or ribbon-like structures. Path C: Balancing the fluxes and operating at the lowest antisolvent concentration leads to formation of very thin uniform nanofibers. These material morphologies are exemplified by SEM micrographs of PS structures formed as follows: Path A – short polymer rods from 5.78 kDa MW PS solution; Path B – sheet- and ribbon-like, and Path C – long fibers from 230 kDa MW PS solution after shearing at 2000 rpm in glycerol mixtures with varying ethanol content. The effects involved are described quantitatively in Figure 2a,b. All scale bars in these images = 10 µm (note the much smaller characteristic cross-section of the fibers in Path C).
A mostly trivial case is realized when the polymer concentration is too low to affect the common capillary fragmentation of droplets under shear. The outcome is polymer spheres, similar to many presently used colloids. However, solution droplets of higher polymer concentration resist capillary fragmentation and get sheared into anisometric structures. The resulting liquid fingers or strands begin solidifying into long continuous polymer structures. These fibrous precipitates may be subject to secondary fragmentation (Figure 1, Path A) yielding polymer microrods, similar to ones reported by us earlier.[25,26] We now established that the fragmentation of the protofibers can be avoided by using polymers with higher molecular weight (MW) or solutions above certain critical concentration. In the
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examples in Figure 1, solutions of two batches of PS of MW = 5.78 and 230 kDa, sheared under nearly identical conditions, yielded short rods for the low MW polymer, and long fibers for the high MW one. The polymer concentration also had to exceed a critical value before high-aspect ratio precipitated structures were formed. It had a strong impact on the morphology above this critical threshold. The existence of a critical concentration can be correlated to the molecular entanglement of the polymer chains in the semiliquid protofibers, a well-documented effect in wetspinning and electrospinning.[27] The molecular chain entanglement can be characterized by the functional dependence of the viscosity on the polymer concentration. In Figure 2a, we
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present a log–log plot of the zero shear viscosities of poly(lactic acid) (PLA) solutions and correlate them to the type of structures formed while shearing these solutions in glycerol/ethanol mixture. The plot shows a clear change in power-law line slopes between the semidilute unentangled, and the entangled regimes just below 2% w/w PLA The power line slope in the unentangled region matches theoretical expectations, although due to solution nonideality and polymer polydispersity, the slope of the entangled PS solution line is lower than Rouse model prediction for neutral polymers in good solvents.[28,29] In excellent correlation, the formation of fibers and fibrous
the intercylinder gap (Figure S1, Supporting Information). The monotonic decrease both in the average fiber diameter and its polydispersity (Figure 3b) can further be correlated to the characteristic fluid domain size, a, resulting from the process of γ shear-driven deformation, which can be evaluated by a = Cacr , τ where γ is the interfacial tension and Cacr is the dimensionless critical capillary number that depends on the ratio of viscosities of the polymer droplets and the media, and for this system may be in the range 0.3–20.[30–32] The range of rpm in Figure 3b corresponds to shear stresses τ ≈ 34–204 Pa. The interfacial tension between the two miscible liquid
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Figure 2. Effects of polymer and antisolvent concentration on the type of structure formed. a) Effect of polymer concentration on the viscosity of PLA (MW = 62.7 kDa) solution in CHCl3. The entanglement concentration is determined by the change in the slope of the log–log plot of the zero shear viscosity versus polymer concentration. The SEM images in the insets demonstrate the result of shearing these PLA solutions at 2000 rpm in glycerin containing 50% v/v ethanol. Discontinuous aggregates from spheres are observed when the sheared solutions are below the entanglement concentration (Path A in Figure 1). Fibers are formed above the entanglement level. b) Effect of the concentration of antisolvent (ethanol) added to the glycerin medium in 10 wt% PLA, sheared at 2000 rpm. Overall, both a critical polymer and a critical antisolvent concentration (20% in this case) are needed in order to precipitate thin, continuous fibers along Path C in Figure 1.
structures was first observed at concentrations around this entanglement point. While more complex effects, related to the viscoelasticity of the protofibers and the fragility of the solvent-swollen polymer, may be potentially involved, the polymer entanglement during the shearing process proved essential for the formation of fibers and other continuous structures. The shearing of polymer solutions above the entanglement threshold yielded a variety of nanofibers, nanoribbons, and nanosheets (Figure 1, Paths B and C). The specific morphology is dependent on the pattern of polymer precipitation into the sheared liquid fingers, controlled by solvent efflux into and antisolvent influx from the medium. The rate of antisolvent infusion is strongly dependent on its concentration in the medium, and critically affects both the morphology and characteristic dimensions of the resulting structures. The typical effect of the antisolvent concentration on the formation of nanofibers and other morphologies from polylactic acid is illustrated in Figure 2b. Ethanol concentrations up to ≈20% v/v produced spherical particles of increasing diameter (as this antisolvent concentration is insufficient to solidify the polymer before the liquid streaks break up into droplets by capillarity). Above this critical concentration the sheared material solidified into uniform long fibers of sub-micrometer diameter (Path C in Figure 1, center image in Figure 2b). Further increasing the EtOH concentration led to increase of the fiber cross-section and deformation from circular to oval cross-section, forming ribbons (right image in Figure 2b). The deformation is likely a consequence of the rapid formation of a thick polymer skin before all of the solvent has left the protofiber core. Rapid skin formation at low polymer concentrations just above the entanglement threshold could also lead to synthesis of very thin sheet-like structures (Figure 1, Path B). The polymer ribbons and nanosheets can be valuable materials on their own, however, we focus further on the most widely used material presently, which is uniform liquid-dispersed nanofibers. Nanofibers are readily and reproducibly formed at balanced precipitation conditions, under moderate polymer and antisolvent concentrations following Path C in Figure 1. The diameter of the fibers formed in the concentric cylinder device (Figure 3a) could be controlled by several parameters, starting with the shear stress. The dependence of the fiber diameter on the angular velocity of the Couette flow device is illustrated in Figure 3b. The uniform shear stress, τ, generated by the flow between the rotating concentric cylinders can be evaluated as μω i ri , where µ is the fluid viscosity, ri and ωi are the radius τ≈ Δr and angular velocity of the inner cylinder, and Δr = ro – ri is
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Figure 3. Effect of the shear rate and scaling up the process to kilogram scale production. a) Schematic of the lab-scale concentric cylinder device for droplet shearing at a rate controlled through the angular velocity ω. b) Relation between the shear rate and fiber diameter in the concentric cylinder device. The bars represent the d10–90% range of distributions within each sample. The average fiber diameter and distribution decrease to sub-micrometer values with increasing angular velocity. c) Schematic of the scaled-up continuous flow process. The nanofibers are produced in a continuous stream of fluid with antisolvent flowing in a pipe, while the polymer solution is injected from a side opening and immediately sheared into liquid-borne nanofibers. The process is scalable and parallelizable to nanofiber production in kilograms or even tons/hour. d) Examples of the size distribution and SEM micrographs of typical PS nanofibers produced in a continuous flow shear device by injection of 12% w/w PS-THF solution in glycerin containing 25% water flowing at a rate of 55 mL s–1 through a pipe of 3 mm diameter. The median and average PS nanofiber diameters are 149 nm and 168 ± 89 nm correspondingly. The variability of the average and median fiber diameters between 1–5 kg batches of nanofibers produced in separate runs by the device is less than 5%.
phases is not well defined, but assuming an effective γ ≈ 0.01– 0.1 × 10−3 N m–1, we estimate that the dispersed polymer solution will be sheared into micrometer- and sub-micrometersized liquid fibers, which is in good correspondence with the experimental outcomes. We next developed a continuous liquid high shear fabrication device, which has a number of advantages to the batch unit described above and was used for making of the smallest size fibers reported in this paper. We found that simple injection of polymer solution into a stream of viscous medium flowing rapidly in a tube can be used for continuous large scale fabrication of nanofibers through Poiseuille shear flow. The scaled up flow device uses a circular pipe with flowing antisolvent medium, while the polymer solution was injected through a side opening (Figure 3c). A simple desktop-sized flow shearing device with a flow rate of 1.8 m3 h–1 through a tube of inner diameter 4.57 mm produces up to 10 kg h–1 nanofibers, an extraordinary high rate for such a small and inexpensive apparatus (full details of the shear devices and procedures are provided in the Experimental Section and the Supporting Information).
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Due to the easily controlled and stable laminar flow operation, the device reliably produces nanofibers of average diameters smaller than 200 nm and tight size distribution. Examples of the size distribution of typical PS nanofibers are presented in Figure 3d, and of ones from cellulose acetate are shown in the Supporting Information. The size distribution is easily reproduced in the subsequent fabrication of multikilogram size batches (see Supporting Information). The method could also be easily modified for the production of nanofibers from a large number of solution-processable and even some reactive polymers, including polylactic acid (Figure 2), PS (Figure 3), cellulose acetate, poly(methyl methacrylate), Nylon 6, polyvinyl alcohol, polyvinylidene fluoride, polypropylene, and cellulose (a few examples are given in Figure 4). The method also allows facile fabrication of fibers with embedded particles such as conductive composite PS fibers containing high volume fractions of carbon black aggregates (Figure 4c), antibacterial silver nanoparticles, magnetite nanocubes (Figure 4d,e), and many others. Finally, the method could be adapted for the formation of metal oxide (such as titania, TiO2) nanofibers, by
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greatly the potential areas of application of these nanofibers and allows making new composite and nonwoven materials. Streams of liquid-borne staple nanofibers can be easily integrated with industrial wet-laying lines and spraying devices for large-volume production of filters, coatings, mats and nonwovens. Before wet-laying or spraying, the suspended nanofibers can be mixed with commercial microfibers, resulting in intermixed micro/nanofiber materials (illustrated in Figure 4f) with very highly developed area and high particle capturing capability. Such liquid-deposited nanofiber/microfiber composites (Figure 4f) could also serve as excellent cell scaffolding and growth substrates, which are being presently investigated. The ability to control morphologies, together with the multitude of functionalization possibilities by particle additives, could in the future open the way to the large-scale fabrication of diverse classes of other nanomaterials. The liquid-shear fabrication of nanofibers and nanoribbons at unprecedented rates and volumes could enable their use as bulk components of products ranging from filters to bioscaffolds and could have a transformative impact on the emerging area of nanomanufacturing.
Experimental Section
Figure 4. Examples of diverse polymer and composite fibers produced by shear-driven liquid fabrication. All samples are synthesized in glycerol medium. SEM micrographs of: a) Very fine uniform PS nanofibers from 12% w/w PS solution in THF, antisolvent is 25% water. b) Porous, ribbonlike Nylon fibers from 12% Nylon 6 solution in formic acid, antisolvent is a mixture of 15% ethanol and 40% 0.2 N NaOH. c) Composite electrically conductive PS/carbon black fibers produced from a solution of 10.5% PS and 4.5% CB in 40% DMF, 60% CHCl3, antisolvent is 5% ethanol. The percolated CB particle aggregates are comparable in size to the fiber diameter and are protruding from the surface. d) Composite magnetically responsive CA-iron oxide fibers from a solution of 16% CA and 0.2% iron oxide particles in THF with 25% water as antisolvent. e) TEM image of composite PS/Fe3O4 fiber produced from a solution of 10% PS in CHCl3 containing ≈0.5% Fe3O4 magnetite nanocubes, antisolvent is 25% ethanol; f) SEM image of CA nanofibers incorporated in a 3D wetlaid matrix mixed together with commercial polyester microfibers. The liquid-suspended staple nanofibers are specifically well-suited for direct integration in industrial processes for wet-laying of high surface area filters and nonwovens.
pyrolysis of shear-spun nanofibers in air as described in the Supporting Information. In summary, we introduce here a simple shear process that could be easily scaled up to continuous production of nanofibers in a stream of fluid medium. This continuous flow device is presently being scaled up to the commercial fabrication of nanofibers at rates that could exceed tens or hundreds of kilograms/hour. The process has a few specific features that can make it of high value to commercial nanotechnology, largely stemming from its ability to make nanofibers dispersed and carried in a flux of liquid as opposed to methods such as electrospinning and melt-blowing, where dry fibers are carried and deposited by gas phase. The use of liquid medium expands
Adv. Mater. 2015, DOI: 10.1002/adma.201404616
A benchtop Couette flow apparatus with a 2.3-mm gap (Figure 3a) was constructed with a rotating cylindrical shaft (radius ri = 5.0 mm) and a disposable, stationary polypropylene tube (ID = 14.6 mm). Shaft rotation was digitally controlled within the 150–6000 rpm range. The fibers were formed after injecting polymer solution in a sheared viscous antisolvent medium, similarly to the process previously used to fabricate anisotropic polymer rods.[25] Typically, 0.05 mL of polymer solution, injected into 6.5 mL of antisolvent, was sheared at 2000–6000 rpm for 2 min. The shearing medium was a mixture of glycerol and polymer antisolvent. Our initial experiments at higher shear rates showed significant increase in polydispersity from instabilities at the top free liquid–air interface. This instability was suppressed by introducing a top baffle made of cured PDMS with uniform thickness to prevent drawing of air into the liquid medium during shear. The continuously operating Poiseuille flow device was built around a cylindrical pipe through which the viscous antisolvent medium is pumped at high flow rates (Figure 3c). A triplex positive displacement pump (CAT Pumps, Model No. 2SF30ES) is used to push the viscous medium with antisolvent through 4 ft long 4.57 mm inner diameter stainless steel tube. The polymer solution is injected through a small side inlet into the larger pipe. The polymer solution in the desktop prototype is injected using a 20 mL syringe pump. These liquid-borne nanofibers are easily amenable to wet-laying, and thus can be easily integrated in a number of common filter and nonwoven manufacturing lines.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors are grateful to Rossitsa Alargova, Behnam Pourdeyhimi, and Alexander Yarin for helpful discussions, Joseph Tracy and Peter Krommenhoek for providing magnetite nanocubes, and to Saad Khan, Christopher Bonino, Sara Arvindson, and Arjun Krishnan for assistance
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www.MaterialsViews.com with rheometry. The authors acknowledge the NSF (Grant Nos. CMMI0927554 and IIP-1127793 to Stoyan Smoukov and Orlin Velev), as well as Syngenta, Xanofi, and Triangle MRSEC (Grant No. NSF DMR-1121107), for partial support of this study. Received: October 6, 2014 Revised: February 8, 2015 Published online:
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