M98-D01 A Fundamental Investigation of the Formation and Properties of Electrospun Fibers

Transcription

1 A Fundamental Investigation of the Formation and Properties of Electrospun Fibers 1 S.B. Warner, A.Buer, M. Grimler, S.C. Ugbolue Department of Textile Sciences, University of Massachusetts Dartmouth, Dartmouth, MA G.C. Rutledge, M.Y. Shin Departments of Chemical Engineering and Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA Abstract and Objectives The objective of this project is the development of the fundamental engineering science and technology of electrostatic fiber production ( electrospinning ). Electrospinning offers unique capabilities for producing novel synthetic fibers of unusually small diameter and good mechanical performance ( nanofibers ), and fabrics with controllable pore structure and high surface area. The project aims to achieve the following goals: 1. Design and construction of process equipment for controllable and reproducible electrospinning. 2. Clarification of the fundamental electrohydrodynamics of the electrospinning process and correlation to the polymer fluid characteristics. 3. Characterization and evaluation of the fluid instabilities postulated to be crucial for producing ultrafine diameter fibers. 4. Characterization of the morphology and material properties of electrospun polymer fibers. 5. Development of techniques for generating oriented fibers and yarns by the electrospinning process. 6. Productivity improvement of the electrospinning process. Introduction Conventional fiber spinning techniques, e.g., melt spinning, dry spinning or wet spinning, rely on mechanical forces to produce fibers by extruding polymer melt or solution through a spinnerette and subsequently drawing the resulting filaments as they solidify or coagulate. Electrospinning offers a fundamentally different approach to fiber production by introducing electrostatic forces to modify the fiber formation process. Although the idea goes back at least 60 years [1], there remains extremely limited quantitative technical and scientific information available regarding the theoretical foundation of this process. Interest has renewed in recent years with the work of Reneker and co-workers, who have demonstrated electrospinning for a wide variety of polymer solutions, including rigid rod polymers [2,3]. Larrondo and St.John Manley had earlier demonstrated electrospinning for polyethylene and polypropylene fibers from the melt [4,5]; in that work, properties similar to weakly oriented conventional fibers were obtained. Most recent investigations have focused on the structure and morphology of electospun fibers [3,6,7]. Indications are that crystallites in electrospun PEO fibers are somewhat smaller than in conventional fibers, even though birefringence confirms molecular scale orientation. However, it must be emphasized that all of the fibers produced to date have necessarily been generated without regard to control of the forces driving orientation and crystallization, and have resulted in unoriented, nonwoven fabrics. Permeability studies on nonwoven electrospun fabrics indicate potential for membrane and filtration applications [8,9]. 1

2 The basic phenomenon of electrospinning is relatively simple to realize in practice. It suffices to apply a high voltage to a capillary filled with polymer fluid by means of an electrode, and then to collect the resulting fibers on a grounded plate. This basic configuration of an electrospinning process is illustrated in Figure 1. Feed of the polymer fluid to the tip of the capillary results in the formation of a droplet at the capillary tip whose size and shape are dictated by surface tension and gravitational forces. The application of a high voltage to the capillary produces a surface charge on the droplet which offsets the forces of surface tension. This results in elongation of the drop, formation of a Taylor cone and, at sufficiently high voltage, ejection of a continuous stream ( jet ) from the tip of the cone. Both electrostatic and fluid dynamic instabilities can contribute to the basic operation of the process. With low molecular weight liquids, the fluid typically breaks up into small, charged droplets. This type of process has been known as electrospraying and has enjoyed widespread commercial success in various applications such as paint spraying, ink jet printing and agricultural technologies. With polymeric fluids, viscoelastic forces stabilize the jet, permitting the formation of small diameter, charged filaments. Under certain conditions, electrostatic instabilities are believed to result in the break-up of the main filament into many smaller filaments. This process has been termed splaying [2,3,6]. Although little is currently known about this splaying process, it is thought to be responsible for the unusually small diameter fibers (as small as 40 nm [3]) which can be produced by electrospinning. The fibers are typically laid down in a random fashion on the collecting screen to form a nonwoven fabric. 2 Taylor cone Capillary Jet HV Power Supply Observed Instability Collector Figure 1 Prototypical electrospinner (point-plate configuration) Electrospinning is a unique process in that it is able to produce polymer fibers with diameters ranging over several orders of magnitude, from the micrometer range typical of conventional fibers, down to the nanometer range. Owing to the smallness of their diameters, electrospun fibers possess unusually large surface-to-volume ratios and are expected to display morphologies and material properties very different from their conventional counterparts. In addition, electrospun fibers in the form of non-woven fabrics offer unique capabilities to control pore size. Electrospinning is a fast and simple process. Since it only requires small quantities of polymer, it can rightfully be termed a microprocessing technique. The objectives of this research are to develop a quantitative understanding of the electrospinning process, to link the processing conditions and polymer fluid characteristics to the structure and properties of the final, electrospun nanofibers, to use this information to demonstrate formation of oriented yarns by electrospinning, and to increase the productivity of electrospinning so that it may be commercially viable. This 2

3 research was initiated in May of 1998 through collaboration between groups at the University of Massachusetts Dartmouth (UMD) and the Massachusetts Institute of Technology (MIT). 3 Experimental 1. Equipment Design Electrohydrodynamic Testing. In order to realize the goal of developing a quantitative description of the electrohydrodynamics of electrospinning (items 2 and 3 of Objectives), it is necessary to design an apparatus with good control over key operating variables, in particular flow rate (material transport), applied voltage, and charge transport. This has been accomplished by the MIT group, as follows. Flow rate is controlled using a digitally controlled, positive displacement syringe pump (Harvard Apparatus PHD 2000, flow rates: µl/hr ml/min), which delivers fluid to the spinnerette via flexible teflon tubing. The spinnerette is a stainless steel tube with an outer diameter of 1/16 inch and an inner diameter of 0.04 inch. Applied voltage is regulated up to 30 kv, using a Gamma High Voltage Research ES30-P power supply. Lastly, electric field equations are simplified by implementing a parallel-plate electrospinner design, as illustrated in Figure 2. Typical operating regimes are flow rates between 0.2 and 1 ml/min, voltages between 10 and 20 kv and a plate distance of 15 to 20 cm. The parallel plates are 10 cm diameter aluminum disks. Charge transport is measured by detecting the current to ground from the bottom plate (see below). It has been our experience that process operation is sensitive to the details of the electric field, and hence the geometry of the spinning apparatus. Most previous studies in other labs have used a metal or glass capillary as the spinnerette and either a grounded flat metal plate or rotating drum as the collector. This set-up represents a point-plate configuration, which exhibits curved electric field lines near the capillary and results in a nonuniform electric field. Such non-uniformity is thought to affect charge transport and introduces significant complications into the mathematical description of the process. By adjusting the protrusion of the spinnerette tip from the upper plate, we can vary the electric field curvature near the spinnerette, independent of other parameters. Syringe Pump HV Power Supply Figure 2 Electrostatic fiber spinner with parallel-plate geometry. Productivity Enhancement. A point-and-plate apparatus has been used for initial testing at UMD. However, in order to realize the parallel goal of increased productivity (item 6 of Objectives), it is necessary to design an apparatus to obtain higher throughput of polymer. In the absence of a mathematical description of the process, this is quite difficult, since it is impractical to ramp up flow rates without altering the characteristics of the resulting fiber, or otherwise upsetting 3

4 process operation. To overcome this problem, the UMD group has designed a rotary electrospinning apparatus, capable of feeding solution to multiple spinnerettes simultaneously. This apparatus is illustrated in Figure 3. The radial geometry minimizes the effects of one threadline on another, in particular charge repulsion between jets. The rotating collector provides a mechanism for continuous removal of the non-woven fabric web as a yarn. In preliminary trials this year with PEO solutions (see below), some problems were encountered with maintaining consistent feed rates to all spinnerettes simultaneously. However, this problem should be easily resolved through a change in the feed system to the rotorspinner. 4 Figure 3 Rotor Electrospinning Unit 2. Fluids Although it has been demonstrated that a wide range of fluids can be spun by the electrospinning technique, the UMD/MIT team has focused on a limited set of fluids. At MIT, four sets of fluids have been investigated as model systems for the study of electrohydrodynamics over the past year. The first two are based on glycerol, a well-characterized Newtonian fluid that is miscible with water in all proportions. This fluid was chosen primarily for elucidating the fundamental electrohydrodynamics of the electrospinning process. Viscosity, permittivity and conductivity are variable over a wide range by mixing with water. Solutions of polyethylene oxide (PEO) in water and chloroform are also being used. KBr is also used to vary the conductivity of these solutions. At UMD, both PEO in 6:1 isopropanol/water and polyacrylonitrile (PAN) in dimethylformamide (DMF) have been spun. The use of isopropanol mixed with water allows modification of both the conductivity and evaporation rate of the spin dopes. Standardized experiments using PEO solutions facilitates comparison of experimental results with results in the literature [6,10] as well between the MIT and UMD groups. 3. Characterization Methods Quantitative analysis of the electrospinning process requires characterization of several important operating variables which influence both the stability of the spinning process and the quality of the fibers produced. To this end, several experimental techniques have been developed and implemented at UMD and MIT. 4

5 On-line Photography. To image the fluid jets online, two photographic techniques are employed at MIT: macrophotography and strobe photography. Macrophotography refers to the use of short distance, high magnification imaging, suitable for capturing quantitative images of the jet profile. For this purpose, we use a long distance microscope or extension tubes connected to a conventional 35 mm camera, providing magnifications of 10x or higher. Area backlighting is used for highest contrast and resolution of the jet. IDL software is used for image analysis. Strobe photography refers to the use of strobe lighting to capture instantaneous images of rapidly varying jets when they exhibit instabilities. For this purpose, a conventional 35 mm camera is equipped with a strobe (GenRad 1538A) which is capable of producing flashes with durations as short as 0.5 µs. High intensity, short duration flashes are useful for still images, while high flash frequencies aid in the dynamic visualization of the process. 5 Operating Parameters. Three important operating parameters identified for process characterization are flow rate, electric field strength, and electric current. The first two are controlled primarily through equipment design, as described above for the parallel plate configuration at MIT. Electric current, on the other hand, is a parameter which must be measured during operation. The total current contains contributions from both convection and conduction currents. From measurements of the current as a function of flow rate, the surface charge density of the jet may be extracted. Surface charge density plays a major role in determining jet stability. There are two common techniques for measuring electric currents in electrospinning. In the first method, one inserts a resistor in line between the collector and ground and then measures the voltage drop across it. From this, the current can be calculated using Ohm s law. Since the jet currents are conjectured to be small, high impedances are required to observe a signal. In the second method, one uses a sensitive current meter (e.g. Fluke 88 digital multimeter) between the collector and ground to measure current directly. In electrospinning applications, the current meter should be able to measure currents in the microampere range. In the investigations at MIT, both protocols have been used. Velocimetry. At UMD, the spinline geometry of the electrospun fibers was studied using Laser Doppler Velocimetry (LDV). The Dantec LDA EduSys 3 system, designed for forward scatter operations, and fitted with a 10 mw coaxial He-Ne laser and a wavelength of 632 nm was used. The transmission optics included a neutral beam-splitter, a Bragg Cell, a beam displacer, and a 300 mm focal length front length. The collecting optics, set-up in a forward scattering position, included a 300-mm close-up lens and a photomultiplier. The signal processing system consisted of a frequency tracker and a counter. A preliminary experiment, wherein velocity was measured simultaneously using both the original system equipment and a FFT spectrum analyzer, confirmed the high reliability of the frequency tracker/counter; however, the length of time necessary to perform a measurement with the frequency tracker/counter was an order of magnitude longer than with the FFT spectrum analyzer. The velocity measurements were taken once a good confidence level was indicated by the filtering electronics, which was indicated on the oscilloscope by a constant burst signal. For velocity measurement, the UMD point-plate configuration was used with a 10 cm diameter wire stabilizer ring place around the threadline 3 cm downstream from the spinnerette tip. The feed system was set on an X-Y table to permit precise alignment of the electrospinning jet in the LDV measuring volume. Five sets of 5,000 or 10,000 counts were recorded for each of the positions. Fiber Characterization. Samples of the nonwoven web obtained by cut and placed onto a specimen holder covered with adhesive. A few drops of silver paint were added on the side to insure electrical grounding of the sample. The samples were coated with gold-palladium using a 5

6 Denton Vacuum Desk II sputtering machine, and observed using an AMRAY 1200B SEM. To measure the diameter of the fibers viewed on a photomicrograph, a line was drawn across the image and a diameter distribution obtained from the sample of fibers intersecting the line. The results were used to compile fiber diameter distribution profiles. The electrospun web samples were also observed using a Leica DMRX polarized optical microscope fitted with a Sernamont compensator, a green filter of wavelength 546nm, and a Zeiss Microfilar Microcode II eyepiece. The microfilar eyepiece was calibrated using a microscopic ruler. The path difference between the ordinary and extraordinary rays was measured using the Sernamont compensator. The diameter of the fibers was estimated using the Microfilar Microcode. The refractive index of the mounting liquid used for the measurement was Single fiber tenacity was measured using the cantilever technique. A cantilever consisting of a 30 µm glass fiber was glued at one end onto a microscope slide and a 15-µm nylon fiber was attached at the free end of the glass fiber. The electrospun test fiber was glued with epoxy resin to the free end of the nylon fiber. A part of the same fiber was cut and deposited on a SEM specimen holder for diameter measurement using SEM. As the sample fiber was stretched, the deflection of the cantilever was measured under light miscroscopy using a calibrated eyepiece. A chart was used to convert deflection into actual values. The elongation-to-break of electrospun PAN fibers was estimated using a caliper. Results Jet Profiles. A representative image of an electrified fluid jet is shown in Figure 4. (The jets in the experiments were flowing in a vertical direction but the images have been rotated by 90 degrees here.) The z-axis is along the jet direction. As this images shows, the two edges of the jet are clearly visible. This permits accurate determination of the jet profile as a function of the distance (z) from the spinnerette. The image is digitized and subsequently analyzed for edge detection using IDL, as shown in Figure 5. In addition to single images such as the one shown here, composite images covering a longer axial distance have also been obtained, in an attempt to assess asymptotic behavior. From the radius vs. distance relationship, one can in principle determine the scaling behavior for the jet radius as a function of the axial distance. An accurate scaling analysis should provide important information about the competition between forces operative on the fluid jet during electrospinning. Figure 4 Electrified jet of 2 wt. % PEO (MW=2,000,000) in water. The diameter of the capillary is 1.6 mm. The experimental conditions were Q=0.5 ml/min, V=22 kv and d=15 cm. 6

7 7 Figure 5 Edge profile of jet shown in Figure 4. Operating diagrams. From accurate measures of the relevant operating parameters, we construct operating diagrams which serve to organize the apparently complex and several phenomena which may be operative during electrospinning. For a given fluid, the operating diagram delineates the different regions of jet behavior in a coordinate system of process variables, for example the electric field and the flow rate. In general, for low flow rates and voltages, dripping is observed (Rayleigh instability). As the flow rate or voltage is increased, stable jets are obtained. Increasing the voltage further can then lead to destabilization of the jet. Below a certain flow rate, it is possible to transition from one region of instability to another without passing through a stable jet regime. Higher flow rates tend to stabilize jets. This behavior is illustrated in Figure 6, showing a typical operating diagram projected onto a 2D plot of electric field strength vs. flow rate. The nearly-parallel Dripping-Jet and Jet-Dripping curves illustrate hysteresis in the lower transition. Jet Current. Jet current measurements were carried out for 2 wt. % solution of PEO (MW=2,000,000) in water. At the high voltages operative during electrospinning, even small fluctuations (ca. 1%) in the power supply are sufficient to produce a measurable AC signal over and above the DC current of relevance to determining surface charge density in the jet. For low conductivity fluids, the AC signal (displacement current) can be a significant fraction of the measured current, and care must be taken to correct for it. Measurements in the MIT lab indicate that the resultant error is higher for the point-plate configuration than for the parallel plate configuration. Previous investigations have failed to take this effect into account. An example of the current-voltage relation for various flow rates of PEO solutions, after correction, is shown in Figure 7. 7

8 8 E (k V /c m ) Figure Drippin Jet g Q (ml/mi n ) Jet - ripping D Sta le- b U stable n Jet Stability diagram for 2 wt. % PEO (MW = 2,000,000) in water. The boundaries for the transitions in jet behavior are delineated Q=1.5 ml/min Q=1ml/min R 2 = Q=0.5 ml/min 70 Linear Fit Linear Fit Linear Fit R 2 = R 2 = Applied Potential (kv) Figure 7 Current-voltage relation for 2 wt. % PEO (MW = 2,000,000) in water. The plate distance was 15 cm. 8

9 Jet Velocity. In last year s report [11] we demonstrated the utility of LDV to measure fiber velocities downstream of the spinnerette. These measurements were in reasonable accord with average values obtained on comparable systems using accurate flow rate and diameter measurements. Both velocity and the standard deviation in velocity were observed to increase significantly with distance from the spinnerette. Here, Figure 8 illustrates the distribution of fiber velocities obtained by LDV at UMD, which gives rise to the increase in standard deviation observed by that method. In the stable region near the spinnerette, an approximately Gaussian distribution is observed, and sampling rate is high. In the unstable region, rapid motion of the jet is problematic for LDV measurements, resulting in only intermittent sampling and low sampling rate. Furthermore, the thread is not often translating lateral to as well as along the axis of the fiber in this region. The transition from one region to the other was observed at an average velocity of about 15 m/s in this experiment. 9 Figure 8a. Velocity distribution for PAN in chloroform near the spinnerette (in the stable region of the jet). Figure 8b. Velocity distribution for PAN in chloroform far from the spinnerette (in the unstable region of the jet). 9

10 Fiber Characterization. As we showed in a previous report [11], the fiber diameter distribution of a typical electrospun web can be fit using the log normal distribution. The mean diameter for the electrospun fibers presented in this study can be as much as five times smaller than that of typical meltblown polypropylene fibers [11]. The diameter distribution of the electrospun fibers is also much narrower, exhibiting a coefficient of variation of about 20%, versus 40% for meltblown fibers [11]. In mechanical testing, a typical 10 mm length of electrospun PAN fiber, with diameter of 1.25 µm (assuming a round cross-section, the corresponding denier is 0.014) exhibited failure at 0.4 mm deflection, or 41 mg of force. The resulting tenacity is 2.9 g/d. The mean elongation-at-break of the same fiber was 190% with a standard deviation of 16%. 10 Conclusion and Outlook This report summarizes selected results obtained during the funding period Process equipment is now in place and tested at both UMD and MIT to achieve the stated objectives of this program. Key process variables and material properties (not discussed here) have been identified and methods implemented for their quantification. From this foundation, we have begun developing and testing mathematical models of the process, including both stable and unstable operating regimes. Future work will focus on process analysis, increased productivity, and optimization of processing operating regimes to produce fibers, yarns and nonwovens of high orientation, well-controlled size, good mechanical properties and other target characteristics. Acknowledgements. The authors are grateful to the National Textile Research Center for funding this research project, Number, under the United States Department of Commerce Grant Web site: References [1] A. Formhals, US Patent 1,975,504 (1934). [2] G. Srinivasan and D.H. Reneker, Polym. Int., 36, 195 (1996). [3] D.H. Reneker and I. Chun, Nanotechnology, 7, 216 (1996). [4] L. Larrondo and R. St. John Manley, J. Polym Sci. (Polym. Phys.), 19, 909 (1981). [5] L. Larrondo and R. St. John Manley, J. Polym. Sci, (Polym Phys.), 19, 921 (1981). [6] J. Doshi and D.H. Reneker, J. Electrostatics, 35, 151 (1995). [7] H. Fong, I. Chun and D.H. Reneker, Polymer, 40, 4585 (1999). [8] J. Deitzel, N.C.Beck Tan, J.D. Kleinmeyer, J. Rehrmann, D. Tevault, D. Reneker, I. Sendijarevic and A. McHugh, Army Research Laboratory Report ARL-TR-1999 (1999). [9] P.W. Gibson, H.L. Schreuder-Gibson and D. Rivin, AIChE Journal, 45, 190 (1999). [10] R. Jaeger, M.M. Bergshoef, C. Martin-i-Batlle, D. Schoenherr, and G.J. Vansco, Macromol. Symp., 127, 141 (1998). [11] S.B. Warner, A. Buer, S.C. Ugbolue, G.C. Rutledge and M.Y. Shin, Project, National Textile Center Annual Reports, 83 (1998). 10