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J. Phys. Chem. B 2006, 110, 2522-2528
Electrohydrodynamic Atomization: An Approach to Growing Continuous Self-Supporting Polymeric Fibers S. N. Jayasinghe*,†,§ and A. C. Sullivan‡ Department of Materials, and Department of Biological and Chemical Sciences, Queen Mary, UniVersity of London, Mile End Road, London E1 4NS, U.K. ReceiVed: September 28, 2005; In Final Form: NoVember 28, 2005
The investigation presented in this paper illustrates a technique for growing in-situ polymerized networks, forming scaffold-like structures usually formed by means of electrospinning. The technique of jet atomization employing electrohydrodynamics is a manifestation of electrospinning. However, we show for the first time that using this technique where individual droplets are generated, a continuous self-supporting submicrometer web-like structure can be grown whereby fragments of the structure are delivered in the droplets and polymerize on the surface of the growing structure via polycondensation. The development of these growing fibers into web structures is a direct result of the processing route together with the excellent tailor-made cross-linking nature of the resin. An operational map is generated to identify a parametric space in which the stable conejet mode of electrohydrodynamic atomization prevails for generating the finest droplets. A statistical analysis on the formed fibers for a given time and electrospray condition is presented together with optical micrographs of the structure, which concludes the discussion in this paper.
Introduction Several researchers from around the world are focusing on developing novel technologies and routes for forming fibers, which are most useful in emerging nanotechnology-based applications. These applications vary from complex tissue engineering, forming composite fibers, to device development at the nanoscale.1-6 A host of different jet technologies are currently available for generating polymeric-based fibers in both the submicrometer and nanometer scales. These are fused deposition modeling,7,8 ink-jet printing9,10 (IJP), and electrospinning.11,12 In fused deposition modeling a thermoplastic polymer is extruded through a needle to form continuous fibers from which structures are prepared. The process is, however, limited to a resolution g100 µm. This technique has been employed for processing structures from either a polymer or wax. Ink-jet printing utilizes piezoelectric technology to generate and form droplets. Two types of ink-jet printers are available, namely, drop-on-demand and continuous.13,14 This route has been used for forming structures both solid and porous in 3D but has process limitations, much like that in fused deposition modeling where structural resolution is limited. This is directly related to the viscosity of the polymer solution and the internal diameters of the needles used for processing that polymer solution. The compromise limits the printing resolution; however, at best surface modification of the substrate can limit the spreading of the polymer droplets when deposited. These routes for fiber production have been explored, and the formed structures have been rather coarse. However, electrospinning, a process where a polymeric solution or melt is charged within a needle and is made to enter * Corresponding author. Tel.: ++44 (0) 2076792960. E-mail:
[email protected]. † Department of Materials. ‡ Department of Biological and Chemical Sciences. § Current address: Mechanical Engineering Department, University College London, Torrington Place, London WC1E 7JE, United Kingdom.
Figure 1. Schematic representation of the synthesized polymer solution.
a high-intensity electric field, is found to elongate from a cone to a continuous micro-thread-like viscoelastic jet later forming either a single or an umbrella of jets.15,16 This process has been extensively reported for the fabrication of metal oxide17 as well as organic-inorganic fibers18 from sol-gel precursor solutions. Another manifestation of this charged and electric-field-driven process is electrohydrodynamic atomization (EHDA), which is also famously known as electrospraying.19-21 The fundamental difference distinguishing electrospinning from electrohydrodynamic atomization is that the former generates continuous threads and the latter encourages jet breakup due to nonlinear effects that promote the formation of droplets. In both scenarios the size of fibers and droplets, respectively, can range from the micrometer to the nanometer scale.22,23 In this paper for the first time we show how the chemistry of a polymeric solution is tailored to form fibers resulting from the solution having a good cross-linking nature, when subjected to electrohydrodynamic atomization. This investigation shows that self-supporting structures in both 2D and 3D can be assembled via this jetting approach. Experimental Section Materials and Solution Preparation. Alkoxysilanes, (MeO)3Si(CH2)2PO(OMe)2 and (MeO)3SiCH(CH2PO(OMe)2)CH2CH2-
10.1021/jp0555089 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006
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Figure 2. Electrospray equipment setup.
TABLE 1: Solution Properties Effecting Stable Cone-jet Mode of Electrospraying25 electrical conductivity (S m-1)
density (kg m-3)
viscosity (mPa s)
surface tension (mN m-1)
relative permittivity
1.4 × 10-5
1127
33
37
30
The ground electrode has an external and internal diameter of ∼12 mm and ∼10 mm, respectively, and is made of copper. The needle is connected to a precision high-voltage power supply (model FP-30, Glassman Europe Ltd., Tadley, U.K.)
described.24
Si(OMe)3, were prepared as previously A mixture consisting of alkoxysilanes (MeO)3Si(CH2)2PO(OMe)2, [X], 6.48 mmols and (MeO)3SiCH(CH2PO(OMe)2)CH2CH2Si(OMe)3, [Y], 6.48 mmols, ethanol (not dried), 685 mmols, HCl, 0.8 mmols, and water 44.4 mmols was placed in an open polypropylene bottle and left standing at room temperature for 12 h and then at 60 °C for 2 days. The clear slightly viscous sol (Figure 1) obtained was then cooled to room temperature and had the properties described in Table 1. Later it was subjected to EHDA as described. The fibrous macroporous material obtained after EHDA was insoluble in common organic solvents and water. The average degree of condensation in the fibrous material was found to be 79% (by 29Si MAS NMR) so that the cross linking here compares favorably with the solgel-derived material.24 Electrospraying. The investigation was carried out using the electrospray setup shown in Figure 2. The electrospray equipment consists of a stainless steel needle with inner diameter ∼500 µm held ∼15 mm above a ring-shaped ground electrode.
Figure 3. High-speed camera image illustrating electrohydrodynamic atomization in the unstable mode. Scale bar represents ∼810 µm.
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Jayasinghe and Sullivan
Figure 5. Multi-jets formed at an applied voltage of ∼7.7 kV in a flow rate regime of 10-9 m3 s-1. Scale bar represents ∼810 µm.
The media properties electrical conductivity, viscosity, surface tension, relative permittivity, and density (Table 1) promoting stable cone-jet mode electrospraying of the polymer were measured, using a conductivity meter (HACH SensION 156 probe), Visco-Easy rotational viscometer, Kruss Tensiometer K9 (Du Novy’s ring and plate method), a calibrated cell connected to a precision multimeter, and a standard 25-mL density bottle, respectively. Electrospraying was carried out to a maximum applied voltage of ∼8 kV to a corresponding flow rate in the order of 10-9 m3 s-1. An operational map was generated illustrating an optimum applied voltage-to-flow rate where the the finest of threads are formed in the stable cone-jet mode of atomization. Collecting of these fibers took place on microslides for optical microscopy. Results and Discussion Mode of Atomization and Operational Map. For stable EHDA jetting to take place, te, the electrical relaxation time (te ) β0/K), must be much smaller than th, the hydrodynamic time (th ) LD2/Q).26 The inequality can be established as follows:
β0 LD2 , K Q
Figure 4. Electrospraying in the stable cone-jet mode at an applied voltage and flow rate of 7.5 kV and 10-9 m3 s-1, respectively. Scale bar represents ∼810 µm. Dotted line represents the exit of the needle.
capable of delivering an applied voltage of 30 kV to the needle. The inlet of the needle is connected via silicone tubing to a syringe having a capacity of 2.5 mL. The syringe fits firmly on a precision syringe pump cradle (model type PHD 4400, HARVARD Apparatus Ltd., Edenbridge, U.K.), which is capable of delivering low flow rates up to 10-17 m3 s-1. A highspeed camera (Phantom V7, Photosonics International Ltd., Oxford, U.K.) with a long-distance microscope lens (Nikon 50 mm, Oxford Lasers Ltd., Oxford, U.K.) capable of taking 150000 fps was held in line with a diode laser system (HSI5000, Oxford Lasers Ltd., Oxford, U.K.). The high-speed camera triggers simultaneously with the diode laser system to image the electrospraying process.
(1)
where β is the relative permittivity, 0 is the permittivity of free space (8.854 × 10-12 F m-1), K is the electrical conductivity, L and D are the axial length and jet diameter, respectively, with Q representing the flow rate. Substitution of the values from Table 1 into the inequality (eq 1) illustrates that the process is undergoing stable cone-jet mode electrospraying. The viscous dimensionless parameter, δm is ,1 for liquids having high viscosities. Conversely for liquids having lower viscosities δm . 1, which is a commonly faced scenario in EHDA. The viscous dimensionless parameter is defined as
δm )
[ ] F0γ2 Kη3
1/3
(2)
where F, γ, and η are the density, surface tension, and viscosity, respectively, of the solution. Substituting the values found in Table 1 into eq 2 shows that this liquid is undergoing electrospraying with δm , 1.
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Figure 6. (a-f) High-speed images depicting the growing and polymerizing “fir-tree” structures. Scale bar represents 500 µm.
In this condition we can estimate the jet diameter (dj) which is calculated employing eq 3:
[
dj ) (β - 1)1/2 ×
]
Q0 K
1/3
(3)
Therefore, dj by substitution is estimated as 16.8 µm. In reality,
the jet diameter varies as a function of applied voltage, flow rate, and media properties. However, the average measured jet diameter is 16 µm, which corresponds remarkably well with the estimated jet diameter. The siloxane-resin mixture was set to initially flow through the needle at the constant flow rate of 10-9 m3 s-1. Applying a voltage of 3 kV, corresponding to an applied electric field
2526 J. Phys. Chem. B, Vol. 110, No. 6, 2006
Jayasinghe and Sullivan
Figure 7. Formed polymer webs at (a) ∼5.5 kV and (b) ∼6.5 kV for a constant flow rate of 10-9 m3 s-1.
strength of 0.2 kV/mm, generated a potential difference between the needle and ring electrode. The polymer solution was seen to undergo near-microdripping mode. On increasing the applied voltage to ∼4 kV, microdripping was observed with an increase in its droplet production frequency with a corresponding increase in electric field strength to ∼0.32 kV/mm. At an applied voltage of 5.0 kV, near-cone-jet mode (unstable) of atomization was observed (Figure 3). On reaching an applied voltage of 5.5 kV, the cone-jet mode of atomization prevailed. Characteristics of the ensuing cone and jet dimension were estimated (Table 2). On increasing the applied voltage to 6.5 kV, corresponding to an applied electric field strength of 0.43 kV/mm, the cone area became smaller along with the jet diameter and cone depth. These changes were more significant at the applied voltage of 7.5 kV (Figure 4, parts a and b). On increasing the applied voltage >7.5 kV, multi-jet mode prevailed (Figure 5).
TABLE 2: Electrohydrodynamic Atomization Characteristics applied voltage (kV)
jet length (µm)
jet diameter (µm)
cone depth (µm)
cone area (µm2)
5.5 6.5 7.5
50 21 6
26 15 7
1211 979 890
490 455 396 495 360 450
Microslides (∼35 mm × ∼25 mm) were placed on the ground electrode for each applied voltage for a given time of ∼600 s and micrographed to characterize the polymerized networks. Table 3 summarizes the fiber characteristics for a given applied voltage. Collection in this method was carried out five times for each applied voltage for a constant flow rate in the 10-9 m3 s-1 regime. Figure 6, parts a-f, depict high-speed photographs showing the fir-tree-like structures growing with respect to exposed time
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Figure 8. Polymer network grown (a) at an applied voltage of ∼7.5 kV and flow rate of 10-9 m3 s-1 and (b) a characteristic high-magnification micrograph depicting finer threads within the dark clustered regions.
TABLE 3: Statistical Data of Polymerized Fibers applied voltage fiber diameter standard deviation mean diameter (kV) (µm) (µm) (µm) 5.5 6.5 7.5
15-60 17-35 0.2-13
6 3 7
31 17 4
for an applied voltage of 7.5 kV and flow rate of 10-9 m3 s-1. During their formation, the generated droplets were seen to be extremely small. These fine charged droplets were found to be attracted to the nearest grounded structure, which in this case is the growing “fir-tree” branches, where these fine droplets loose solvent, leaving the residual desolvated siloxane aggregate attached and well placed to form cross links to the growing grounded branch. Figure 7a is a characteristic micrograph of the polymerized network generated at an applied voltage of 5.5 kV which had formed fibers in the range 15-60 µm in diameter. Similarly for applied voltages 6.5 kV (Figure 7b) and 7.5 kV (Figure 8a) the corresponding fiber diameter became significantly smaller, as expected by the formation of finer droplets during stable conejet mode of atomization. From the above, it is clearly seen that
increasing the applied voltage reduces the formed fiber diameter. Examining the polymer network formed at the applied voltage of 7.5 kV (Figure 8a), it is noted that there are regions where clusters of fibers appear to form nodes with “solid” surface texture. On close examination of these regions using optical microscopy, it was observed that these regions contain a high density of fibers resulting in a less porous structure (Figure 8b). We assume at this stage that these regions are promoted as a result of droplet densification along branches at the crown of the structure proximal to droplets emitted from the jet source. Conclusions This investigation demonstrates electrospray as a competing approach to forming continuous polymer fibers via polycondensation. The architectures formed in 3D were found to be self-supporting structures. The authors are currently developing this technique for a wide range of polymers in the hope to form even finer filaments “nanofibers”, which will bring out the limitations of this jet-based approach. Furthermore it is the intention of the authors to use this polymerization route for precision patterning surfaces in both 2D and 3D for bio-related applications.
2528 J. Phys. Chem. B, Vol. 110, No. 6, 2006 Acknowledgment. S.N.J. gratefully acknowledges funding provided by The Royal Society and The Engineering and Physical Sciences Research Council of the United Kingdom for this research at Queen Mary. References and Notes (1) Yang, F.; Murugan, R.; Wang, S.; Ramakrishna, Biomaterials 2005, 2603, 26. Bellucci, S. Nucl. Instrum. Methods Phys. Res., Sect. B 2005, 57, 234. Minett, A.; Fraysse, J.; Gang, G.; Kim, G.-T.; Roth, S. Curr. Appl. Phys. 2002, 61, 2. (2) Lozano, K.; Yang, S.; Jones, R. E. Carbon 2004, 2329, 42. Min, B.-M.; Lee, S. W.; Lim, J. N.; You, Y.; Lee, T. S.; Kang, P. H.; Park, W. H. Polymer 2004, 7137, 45. (3) Smith, L. A.; Ma, P. X. Colloids Surf., B 2004, 125, 39. (4) Dharmaraj, N.; Park, H. C.; Lee, B. M.; Viswanathamurthi, P.; Kim, H. Y.; Lee, D. R. Inorg. Chem. Commun. 2004, 431, 7. Park, Y. W.; Kaiser, A. B. Curr. Appl. Phys. 2002, 33, 2. (5) Boland, E. D.; Coleman, B. D.; Barnes, C. P.; Simpson, D. G.; Wnek, G. E.; Bowlin, G. L. Acta Biomater. 2005, 115, 1. (6) Kosminder, K.; Scott, J. Filtr. Sep. July/August 2002, Feature Article. Kim, C. J. Power Sources 2005, 382, 142. (7) Crump, S. S. Proceedings of the 2nd International Conference on Rapid Prototyping; University of Dayton: Dayton, OH, 1991; pp 354357. (8) Ziemian, C. W.; Crawn, P. M. Rapid Prototyp. J. 2001, 7, 138. (9) Le, H. P. J. Imaging Sci. Technol. 1998, 42, 49. (10) Lloyd, W. J.; Taub, H. H. In Output hardcopy deVices; Durbeck, R. C., Sherr, S., Eds.; Academic: New York/Boston, 1988; pp 311-370.
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