Langmuir 2007, 23, 2311-2314
2311
Co-electrospinning of Core-Shell Fibers Using a Single-Nozzle Technique Alexander V. Bazilevsky, Alexander L. Yarin,* and Constantine M. Megaridis Department of Mechanical and Industrial Engineering, UniVersity of Illinois at Chicago, Chicago, Illinois 60607-7022 ReceiVed NoVember 1, 2006. In Final Form: January 11, 2007 Co-electrospinning is ideally suited for fabricating continuous fibers encasing materials within a polymer sleeve, but requires relatively complex coannular nozzles. A single-nozzle co-electrospinning technique is demonstrated using blends of poly(methyl methacrylate) (PMMA)/polyacrylonitrile (PAN) solutions in dimethylformamide (DMF). The as-spun fibers have outer diameters in the range of 0.5-5 µm and possess a core-shell structure similar to that attained via coannular nozzles. The technique relies on the precipitation of PMMA solution droplets, which become trapped at the base of the Taylor cone issuing the PAN solution jet from its tip. A theoretical analysis shows that the outer shell flow is sufficiently strong to stretch the inner droplet into the Taylor cone, thus forming a core-shell jet. The method seems attractive for technological applications involving macroscopically long and radially inhomogeneous or hollow nano/micro fibers.
Core-shell polymer nano- and microfibers were first manufactured in a two-stage process, which started with ordinary (single-nozzle) electrospinning of the core polymer (stage 1) and was followed by the coating deposition of the shell polymer (stage 2).1-3 A reverse process of filling carbon nanotubes with pure liquids and suspensions also has resulted in core-shell nanostructures.4,5 More recently, a single-stage process, called co-electrospinning, was introduced; it employed a compound coannular nozzle issuing core polymer solution from the inner tube and an annular coflow of a shell polymer solution.6-11 Nano/ micro fibers were produced with this process. In a sense, coelectrospinning may be viewed as a reincarnation or modification of established techniques in textile and optical fiber spinning, or ink-jet printing.12-15 Co-electrospinning requires a polymer solution in the shell and either a polymer solution or a nonpolymeric Newtonian liquid or even a powder to fill the inner core.6-8 The physical pattern of co-electrospinning comprises a compound droplet sustained at the edge of a coreshell nozzle; this compound droplet transforms into a compound Taylor cone with a core-shell jet issuing from its tip.16 As in the ordinary electrospinning process (reviewed in refs 17-23), the jet is simultaneously pulled, stretched, elongated, and bent * To whom correspondence should be addressed. E-mail: ayarin@ uic.edu. Phone: (312) 996-3472. Fax: (312) 413-0447. (1) Bognitzki, M.; Hou, H.; Ishaque, M.; Frese, T.; Hellwig, M.; Schwarte, C.; Schaper, A.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2000, 12, 637-640. (2) Bognitzki, M.; Czado, W.; Freze, T.; Schaper, A.; Hellwig, M.; Steinhart, M.; Greiner, A.; Wendorff, J. H. AdV. Mater. 2001, 13, 70-72. (3) Liu, W.; Graham, M.; Evans, E. A.; Reneker, D. H. J. Mater. Res. 2002, 17, 1-8. (4) Kim, B. M.; Sinha, S.; Bau, H. H. Nano Lett. 2004, 4, 2203-2208. (5) Kim, B. M.; Qian, S.; Bau, H. H. Nano Lett. 2005, 5, 873-878. (6) Sun, Z.; Zussman, E.; Yarin, A. L.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2003, 15, 1929-1932. (7) Li, D.; Xia, Y. Nano Lett. 2004, 4, 933-938. (8) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151-1170. (9) Zhang, Y.; Huang, Z. M.; Xu, X.; Lim, C. T.; Ramakrishna, S. Chem. Mater. 2004, 16, 3406-3409. (10) Loscertales, I. G.; Barrero, A.; Marquez, M.; Spretz, R.; Velarde-Ortiz, R.; Larsen, G. J. Am. Chem. Soc. 2004, 126, 5376-5377. (11) Yu, J. H.; Fridrikh, S. V.; Rutledge, G. C. AdV. Mater. 2004, 16, 15621566. (12) Ziabicki, A. Fundamentals of Fibre Formation; Wiley: London, 1976. (13) Hertz, H; Hermanrud, B. J. Fluid Mech. 1983, 131, 271-287. (14) Doupovec, J.; Yarin, A. L. J. LightwaVe Technol. 1991, 9, 695-700. (15) Yarin, A. L. J. Fluid Mech. 1995, 286, 173-200. (16) Reznik, S. N.; Yarin, A. L.; Zussman, E.; Bercovici, L. Phys. Fluids 2006, 18, 062101.
by the electric forces. Solvent evaporates rapidly, causing the shell jet to solidify, thus producing compound core-shell nano/ micro fibers. To eliminate the inner core and thus produce hollow tubes, co-electrospinning should be followed by selective removal of the core material (via solvents or heat treatment, for example). Calcination and pyrolysis of metal-containing polymers transform nanofibers and nanotubes into ceramic or metal structures (sol-gel technique).3,8,24,25 Carbonization of core-poly(methyl methacrylate) (PMMA)/shell-polyacrylonitrile (PAN) co-electrospun fibers results in the elimination of the PMMA core and the formation of turbostratic carbon walls forming macroscopically long nanotubes from the PAN shell.26 Coannular nozzles used in co-electrospinning experiments have a number of drawbacks. First, it is difficult to achieve good concentricity of the core and shell materials in the as-spun fibers; this may result in protrusions of long segments of the core material from the shell. Core entrainment is also not automatically guaranteed; in order to facilitate it, a protrusion of the inner nozzle from the outer shell nozzle has been found to help.16,26 It is thus apparent that eliminating the coannular nozzle feature in co-electrospinning would accelerate progress in this area. As reported in the following, electrospinning of core-shell fibers from a single nozzle is possible. Experimental Methods and Results The experimental setup is sketched in Figure 1. The working fluid is supplied by a syringe pump and is fed into a stainless-steel needle subjected to an electric potential of 7-9 kV relative to a (17) Frenot, A.; Chronakis, I. S. Curr. Opin. Colloid Interface Sci. 2003, 8, 64-75. (18) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223-2253. (19) Dzenis, Y. Science 2004, 304, 1917-1919. (20) Ramakrishna, S.; Fujihara, K.; Teo, W. E., Lim, T. C.; Ma, Z. An Introduction to Electrospinning and Nanofibers; World Scientific: Singapore, 2005. (21) Subbiah, T.; Bhat, G. S.; Tock, R. W.; Pararneswaran, S.; Ramkumar, S. J. Appl. Polym. Sci. 2005, 96, 557-569. (22) Reneker, D. H.;Yarin, A. L.; Zussman, E.; Xu, H. In AdVances in Applied Mechanics; Aref, H., van der Giessen, E., Eds.; Elsevier: New York, 2006; Vol. 41, pp 43-195. (23) Li, D.; McCann, J. T.; Xia, Y.; Marques, M. J. Am. Chem. Soc. 2006, 89, 1861-1869. (24) Li, D.; Wang, Y.; Xia, Y. AdV. Mater. 2004, 16, 361-365. (25) Li, D.; McCann, J. T.; Xia, Y. Small 2005, 1, 83-86. (26) Zussman, E.; Yarin, A. L.; Bazilevsky, A. V.; Avrahami, R.; Feldman, M. AdV. Mater. 2006, 18, 348-353.
10.1021/la063194q CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
2312 Langmuir, Vol. 23, No. 5, 2007
Letters
Figure 1. Schematic of co-electrospinning of PMMA/PAN blend in DMF using a single circular nozzle. The inset (right) shows the magnified detail of the needle orifice, which is shown in black. In the inset, the PMMA/DMF droplets are shown in dark gray, while the PAN/DMF liquid is lighter. The liquid wets the nozzle. The core-shell fibers emanating from the Taylor cone tip are collected on the edge of the rotating wheel.
Figure 3. Optical images of as-spun core-shell (PMMA-PAN) microfibers collected on a glass slide placed between the nozzle and the collector wheel. The fiber shown in panel a has relatively uniform inner/outer diameters compared to the one shown in panel b. Both fibers have diameters at the upper end of the range covered by this method, because fiber stretching at the location of the collecting glass slide had not terminated yet. These large fibers were selected to show enhanced morphological detail.
Figure 2. Optical appearance of a PMMA/PAN emulsion about 1 day after mixing equal amounts of each polymer in DMF to create a homogeneous blend containing 6 wt % PMMA and 6 wt % PAN. The PMMA/DMF droplets are dispersed in the surrounding PAN/ DMF matrix. vertically rotating grounded collector wheel,27 whose sharp edge is positioned 12 cm from the needle orifice. The outer and inner diameters of the needle were 0.82 and 0.51 mm, respectively. Flow rates through the needle varied between 0.3 and 0.6 mL/h. The wheel rotated at a constant speed of 1000 rpm, collecting loops of the as-spun fiber on its sharpened edge. The fiber bundles were peeled off for subsequent morphological analysis. The working fluid was a blend of PAN (Mw ) 150 kDa) and PMMA (Mw ) 996 kDa). The blend content in PAN/PMMA was 12 wt % overall (6% + 6%). Dimethylformamide (DMF) was used as the solvent. Two different procedures of blend preparation were followed. In the first, dry polymer powders were mixed and then dissolved in DMF. In the second procedure, PAN and PMMA solutions in DMF were prepared and then mixed. It was found that both polymer blends were metastable and decomposed into emulsions. Within 1 day after mixing, the emulsions consisted of 100-200 µm-diameter drops of PMMA/DMF solution in PAN/DMF (Figure 2). In about one more day, the emulsions further separated into two distinct layers. The lighter PAN solution layer floated on top of the heavier PMMA layer. The thickness of the bottom PMMA/DMF layer was about one-half of that of the top PAN/DMF layer. This indicates that DMF was redistributed between the two polymers, and, in the separated layers, polymer concentrations were ∼8 wt % for PAN and ∼21 wt % for PMMA, instead of the initial 6 and 6 wt %. (27) Theron, A.; Zussman, E.; Yarin, A. L. Nanotechnology 2001, 12, 384390.
Electrospinning of the 1-day-old PMMA/PAN blend in DMF (containing 100-200 µm PMMA/DMF drops) revealed a surprising outcome. The as-spun fibers had a core-shell structure (Figure 3) similar to that of fibers co-electrospun from a coannular nozzle. The outer diameters were in the range of 0.5-5 µm, while the wall thickness varied from 200 nm to 1 µm. Most of the fibers were almost uniform along their length (Figure 3a), whereas some fibers showed undulations (Figure 3b), probably resulting from capillary instability of the shell fluid. Heating of the as-spun fibers in nitrogen at 750 °C, following the procedure described in ref 26, revealed the formation of carbonized tubes (Figure 4), as in the case of coreshell fibers co-electrospun from coannular nozzles. Note that PMMA-PAN core-shell fibers co-electrospun from coannular nozzles under similar conditions26 had sizes similar to the present fibers. On the other hand, monolithic PAN fibers electrospun from DMF solutions (also containing single- and multiwalled carbon nanotubes) were in the range of 50-200 nm.28,29 The latter, however, were electrospun under a much higher voltage of 25 kV and did not involve any high molecular weight PMMA core posing higher viscoelastic resistance to stretching. It is also emphasized that occasional core-shell fibers in the present work had diameters below 500 nm, but their inner structure was not visible in optical images similar to those in Figure 3. In principle, there are no physical restrictions for reducing the diameters of PMMA-PAN core-shell fibers to the range of 100 nm. This could be achieved by using PMMA and PAN solutions with different molecular weights and concentrations (cf. the theoretical section below) and higher speeds of the rotating wheel. The mechanism suggested to be responsible for the formation of core-shell fibers in the electrospinning of emulsions is sketched in the inset of Figure 1. While the continuous outer phase (PAN/DMF) (28) Ko, F.; Gogotsi, Y.; Ali, A.; Naguib, N.; Ye, H.; Yang, G.; Li, C.; Willis, P. AdV. Mater. 2003, 15, 1161-1165. (29) Ge, J. J.; Hou, H.; Li, Q.; Graham, M. J.; Greiner, A.; Reneker, D. H.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2004, 126, 15754-15761.
Letters
Langmuir, Vol. 23, No. 5, 2007 2313
Figure 5. Schematic of the modeled PAN/DMF flow around a spherical PMMA/DMF droplet trapped over the Taylor cone issuing the coannular jet, which, upon drying, forms a continuous coreshell fiber. The Taylor cone and the issued jet are aligned with the z-axis. The needle orifice (not shown) is just above the top of the shown area.
Figure 4. Heat treatment in a nitrogen atmosphere converted coreshell PMMA/PAN fibers into turbostratic carbon tubes, as shown in these scanning electron micrographs. The defects in these tubes were caused by handling, and are shown to demonstrate the hollow character of the heat-treated tubes. undergoes ordinary electrospinning from the Taylor cone tip, occasionally a droplet of the dispersed phase (PMMA/DMF) becomes trapped at the cone base. The electric charges leave the PMMA/ PAN interface and escape to the outer fluid surface very rapidly, as compared to the residence time of the drop.16 The interfacial capillary forces are evidently very weak, as both polymers are dissolved in the same solvent (DMF). Consequently, the inner drop experiences only the forces applied to its surface from the PAN solution being sucked into the jet due to the action of the electric forces acting at the outer surface. These forces, if strong enough, will stretch a jet from the PMMA/DMF drop tip, thus resulting in the formation of a core-shell jet and, ultimately, core-shell fibers. The core-shell fiber structure is maintained until the trapped droplet is entirely consumed. This structure reappears when a new PMMA/DMF drop becomes trapped over the Taylor cone (Figure 1). As found by inspecting the as-spun fibers, the number of defects (disruptions of the core) is relatively low because of the very strong stretching of material elements in electrospinning.30 Indeed, the length of a fiber section of d ∼ 1 µm diameter spun from one trapped PMMA/DMF drop of diameter D ∼ 100 µm is about D3/d2 ∼ 1 m. Taking into account the employed flow rates, it is estimated that such a droplet is consumed within a few milliseconds. It is emphasized that, in the above estimate, the jet issued from the drop tip does not splay,30 and thus a single Taylor cone issues a single continuous core-shell jet. No indications of long fiber sections without core material were found in the present case of co-electrospinning from a single nozzle, whereas, in co-electrospinning from core-shell nozzles, such sections are rather common.16,26 The present process seems to be quite robust.
Theoretical Analysis and Results A theoretical analysis of the mechanism believed to be responsible for the formation of core-shell fibers from a single nozzle is presented below. It resembles flows characteristic of spherical journal-bearing or hydraulic suspenders (in the latter case, the flow direction is opposite that in the Taylor cone and (30) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531-4547.
the jet).31 The lower part of the outer PAN/DMF volume can be approximated by a sphere. In addition, the trapped PMMA/DMF (inner) volume also is assumed to initially maintain a spherical shape. Thus, we are dealing with a flow inside an outer sphere (radius R′) and around a fixed spherical drop of radius R, whose center is displaced by e in the z direction (Figure 5). The present analysis considers the inner droplet to be fixed in space, and could be relaxed, as is shown below, without major implications in the overall trends reported. The gap h (between the inner sphere and the outer liquid/gas interface) where the PAN solution flows varies approximately as h(θ) ) - e cos θ, where ) R′ - R , R, and θ is the angular deflection from the z-axis (see Figure 5). The flow of the PAN solution in this gap is axisymmetric and inertialess. The corresponding Stokes equations simplified for a narrow gap in the lubrication approximation read
∂2Vr ∂2Vθ ∂p 1 ∂p + µ 2 ) 0, + µ 2 ) 0, ∂σ R ∂θ ∂σ ∂σ ∂Vr 1 ∂(Vθ sin θ) + ) 0 (1a-c) ∂σ R sin θ ∂θ
-
where σ ) r - R (r is the radial spherical coordinate centered at the drop center O), p denotes pressure, Vr and Vθ are the radial and angular velocity components, respectively, and µ is the viscosity of the PAN solution in this flow. From the continuity equation (eq 1c), it follows that Vθ ) O(RVr/), that is, Vθ . Vr. From the dynamic equations (eqs 1a,b), it follows that [∂p/∂σ]/[∂p/R∂θ] ∼ Vr/Vθ ∼ /R, and thus p ) p(θ) in the gap. Therefore, integrating eq 1b, we obtain Vθ ) -[hσ/µR]dp/dθ + [σ2/2µR]dp/dθ, where the following boundary conditions were used: σ ) 0 (inner drop surface), Vθ ) 0 (neglecting the entrainment of PMMA/DMF by shear influence of the outer PAN/DMF flow; no flow inside the inner drop), and σ ) h (outer drop surface), ∂Vθ/∂σ ) 0 (stress free). Then, the aVerage angular velocity in the gap is
Vθ ) (1/h)
∫0h Vθdσ ) -[h2/3µR]dp/dθ
(2)
Integrating the continuity equation (eq 1c) over the gap, and using the boundary conditions at σ ) 0 and h, Vr ) 0, and the expression for Vθ, we obtain a particular form of the Reynolds (31) Loitsyanskii, L. G. Mechanics of Liquids and Gases; Pergamon Press: Oxford, 1966 (English translation of the 2nd Russian edition); Nauka: Moscow, 1970 (3rd Russian edition).
2314 Langmuir, Vol. 23, No. 5, 2007
Letters
at the drop tip. The latter yields L/l ∼ [µ(-Q)/3G]1/2. Therefore, PMMA solutions with lower G, and PAN solutions with higher µ, as well as flows with higher Q, would result in easier stretching and thinning of the PMMA core and, in turn, thinner core-shell fibers (G and µ can be varied by manipulating molecular weight and concentration). Introducing the viscoelastic relaxation time as ϑ ∼ µ/G, the stretching ratio can be written as L/l ∼ [(-Q)ϑ/ 3]1/2. Taking the estimate -Q ∼ 1 mL/h, ϑ ∼ 0.1 s, and ∼ 0.5 × 10-2 cm, we deduce L/l ∼ 10, which indicates a high stretch ratio, suggesting that the PMMA/DMF fluid can be pulled into the PAN/DMF jet, thus leading to the formation of a coreshell jet and the subsequent formation of core-shell fibers. Figure 6. Co-electrospinning of fine PMMA/PAN in DMF emulsion. The inset shows detail of the Taylor cone, which results in the issuance of a jet containing multiple highly stretched PMMA/DMF droplets (darker vertical lines) in the surrounding PAN/DMF matrix.
lubrication equation governing the pressure distribution in the gap in the present case:
d 3 dp h sin θ )0 dθ dθ
[
]
(3)
Integrating leads to h3 sin θ dp/dθ ) C, where C is a constant. Under the present circumstances, the flow emanating from the Taylor cone tip consists only of PAN/DMF. Consequently, the volumetric flow rate Q supplied by the syringe pump is channeled in its entirety through the gap h. The overall mass balance in the gap thus yields 2πRhVθ sin θ ) Q. It is noted that Q is negative, as is Vθ due to the coordinate system chosen. With the above expression for Vθ, it is Q ) 2πRh sin θ [-h2/3µR] dp/dθ, which infers C ) -3µQ/2π. Using the expression for C, as well as h(θ) ) - e cos θ, we deduce
1 dp 3µQ )dθ 2π ( - e cos θ)3 sin θ
(4)
We are interested only in the proximity of the Taylor cone, that is, θ , 1, where cos θ ≈ 1 and sin θ ≈ θ. In this small angle limit, the dominating term in the pressure distribution given by eq 4 becomes
p)-
3µQ ln θ 2π (1 - λ)3 3
(5)
where λ ) e/ and p < 0, which means that the surface of the inner drop is pulled downward (Figure 5). It is emphasized that the shear stress τrθ in the gap is much lower than the pressure p, since τrθ/p ) O(/R) , 1. This means that the shear stress can be safely neglected, and the trapped drop will be affected only by the pulling effect of the negative pressure (traction). According to eq 5, pulling on the surface of the inner drop becomes very strong in the vicinity of the Taylor cone where θ , 1. An average pressure over the section 0 < θ < θ* , 1 is P ) -3µQ/[2π3(1 - λ)3](ln θ* - 1), where, for example, for θ* ) 1°, (ln θ* - 1) ) -5.6. The trapped PMMA/DMF drop may deform in response to the pulling effect of pressure. But the viscoelastic response of the PMMA/DMF polymeric solution opposes any such deformation. Interfacial tension is another force that would oppose droplet deformation. However, interfacial tension is too weak (same solvent on both sides) to have an effect. In the expected strong elongational flow near the tip (Figure 1), the viscoelastic response of the inner fluid should be predominantly elastic and governed by the momentum balance 2G(L/l)2 ) -P, where G is the elasticity modulus of the PMMA/DMF fluid and L/l is the stretching ratio
It is worth mentioning that the overall force pulling the PMMA/DMF drop along the z-axis is Fz ≈ 3µ(-Q)R2θ*2/3(1 - λ)3. This force tends to push the droplet downward and, in the inertialess approximation, is balanced by the viscous drag31 Fd ≈ 8πµR4U/3, where U ) de/dt is the center-of-mass velocity of the PMMA/DMF drop relative to the PAN/DMF fluid. This yields U ≈ (-Q)θ*2/R2, and, in turn, U ∼ 10-6-10-5 cm/s. This low velocity suggests that the inner drop would barely move within the few milliseconds available for it to be fully stretched and sucked into the Taylor cone. This provides an a posteriori verification of the initial assumption of no movement for the inner drop within the outer PAN/DMF fluid.
Summary and Outlook A single-nozzle co-electrospinning technique has been demonstrated using blends of PMMA/PAN solutions in DMF. The as-spun fibers featured a core-shell structure similar to that attained previously via more complex coannular nozzles. A theoretical analysis has been formulated for the mechanism believed to be responsible for the formation of core-shell fibers from a single nozzle. The utility of the single-nozzle technique extends further from the large-particle emulsion configuration shown in Figure 1. In principle, emulsions of fine PMMA/DMF droplets in PAN/DMF solution (or another polymer pair dissolved in a solvent) can be formed (Figure 6). When electrospun, such emulsions may result in fibers with multicore-shell structure where stretched PMMA filaments extend along the PAN matrix (see detail in Figure 6). Carbonization of such fibers is expected to produce multichannel carbon nanotubes. Note that proof of this concept has been demonstrated in producing multichannel porous nanofibers via co-electrospinning a mixture of PAN and poly(AN-co-MMA) in DMF, which decomposed while the solvent was evaporating from the jet in flight.32 Finally, co-electrospraying of two immiscible Newtonian liquids from coannular nozzles has been considered to form coreshell droplets.33 The results of the present work hint that coelectrospraying from a single nozzle should be possible. Acknowledgment. This work was supported in part by The Volkswagen Foundation and the National Science Foundation through grant NSF-NIRT CTS 0609062. The assistance of K. Sun with SEM micrographs is greatly appreciated. LA063194Q (32) Peng, M.; Li, D.; Shen, L.; Chen, Y.; Zheng, Q.; Wang, H. Langmuir 2006, 22, 9368-9374. (33) Loscertales, I. G.; Barrero, A.; Guerrero, I.; Cortijo, R.; Marquez, M.; Ganan-Calvo, A. M. Science 2002, 295, 1695-1698.
