Control of Inter-fiber Fusing for Nanofiber Webs via Electrospinning

Transcription

1 Control of Inter-fiber Fusing for Nanofiber Webs via Electrospinning Bharath K. Raghavan, Ph.D., Douglas W. Coffin Miami University, Oxford, Ohio UNITED STATES Correspondence to: Douglas W. Coffin ABSTRACT Electrospinning provides a viable method to produce both single fibers and mats of nonwoven fibers. For a nonwoven mat, fusing of the fibers at intersections produces an integrated structure. The ability to spin fibrous mats of nanofibers with and without fusing between the fibers is demonstrated using poly(ethylene oxide) (PEO) fibers. The fusing was controlled by adjusting the amount of water vapor in the surrounding environment. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images were used to evaluate the percent of fused fibers in the mat and the diameters of fibers. The major finding of this work is that fusing of fibers can be controlled during formation of a nanofibrous mat via electrospinning in a controlled environment. INTRODUCTION Electrospinning has proven to be an efficient and simple method to produce continuous nanofibers. 1, 2 Gaining control of the electrospinning process to develop desired features in the resulting materials is now of prime importance. For example, the ability to produce aligned arrays of nanofibers 3 or control the placement of fibers 4 has been demonstrated. In addition to controlling where and how fibers are placed, controlling the interaction between fibers is desirable. Fusing of fibers at intersecting points will improve transport properties. This is beneficial for both mechanical 5,6,7 and conductive properties 8,9 of the resulting nonwoven mats. This fusing could be obtained in post-processing 7,10 of the material, but improved process efficiency 8 and alternative forming strategies would be achieved if the fusing was controlled in the forming process. By controlling the evaporation rate one can control the state of fibers as they are collected 11, but if the fibers are too wet they tend to flatten out. Figure 1 illustrates a serendipitous fused web formed in our lab. The PEO fibers were formed via electrospinning with a solution of 5 (w/w) % of PEO was dissolved in the solvent mixture constituting distilled water and ethanol in the weight ratio of 3:2.. Figure 1 shows that the fibers are fused at crossing points, which motivated the study reported here. A search of the literature revealed some micrographs of fused webs 11, 12, 13 and it is suggested that the solvent evaporation rate is linked to the merging and flattening of fibers. 11, 13 Decreasing the solids concentration in 5, 14, 15 the forming solution can create inter-fiber fusing. The aim of the current study was to determine if the fusing of fibers could be controlled for a given concentration of solution while maintaining nanofiber morphology. FIGURE 1. A fused web of electrospun PEO fibers formed from a solution of 5 (w/w) % of PEO dissolved in solvent mixture constituting distilled water and ethanol in the weight ratio of 3:2... When forming fibers from a solution, inter-fiber merging of the materials or fusing likely occurs if the fibers come into contact before all solvent has evaporated from the solution. Then, the polymer may migrate across the fibers and form an integrated structure. In electrospinning, the solvent begins to evaporate during the spinning process. The spinning process is also what stretches the fiber to sub micron or nanometer diameters. The drying of electrospun fibers of diameters less than 1 micrometer is likely controlled by the removal of solvent from the surface rather than diffusion through the fiber 16, thus altering the environment would be a suitable method to change the rate of solidification of the fiber. Journal of Engineered Fibers and Fabrics 1

2 Increasing the relative humidity (RH) of the surrounding environment has been shown to decrease the diameter of fibers formed from PEO, when the setup is prepared so that the fibers are dry before hitting the collector plate. 17 De Vrieze et al 18 have shown that controlling the temperature and humidity of the environment can influence the diameter of fibers depending on the solvents used. In addition, fusing occurred at higher humidity 18 for some solutions. The objectives of stretching fibers to diameters in the submicron range and slowing down solvent evaporation to form fused nanofibers can be opposing goals. For example, slowing down the evaporation rate, could allow more time to draw the fibers to smaller diameters, but as the diameter decreases the evaporation rate will likely increase. The fibers shown in Figure 1 are microfibers as were the previous reported fused fiber webs 5,7,10,11,14,15,18. By controlling the spinning process and the surrounding environment, our results demonstrate that it is feasible to form fused nanofiber webs. EXPERIMENTAL METHODS A single port electrospinning apparatus was used for this study. The polymer solution was supplied with a 5 ml syringe with needle sizes in the range of 21 to 26 gauges. A syringe pump with a flow rate of 5 µl/min was used to keep the solution under pressure during the spinning process. The positive lead of the voltage potential was connected to the needle of the syringe and the negative lead was connected to a metal collector. A piece of carbon paper was placed over the metal plate to collect the nanofibers. Poly (ethylene oxide) (PEO, molecular weight: 400,000 g/mol, Aldrich) and ethanol (200 proof, absolute, ACS reagent, 99.5%, Sigma-Aldrich) were used. The solutions of PEO with different polymer concentrations were studied. At lower polymer concentrations (3 (w/w) % of PEO dissolved in a solvent mixture constituting distilled water and ethanol in the weight ratio of 3:2) many beads had formed and at higher polymer concentrations (7 (w/w) % and greater (w/w) % of PEO dissolved in the solvent mixture of distilled water and ethanol in the weight ratio of 3:2) microfibers were formed. The results presented here all correspond to a solution of 5 (w/w) % of PEO dissolved in a mixture of distilled water and ethanol in the weight ratio of 3:2. The distance between the syringe tip and collector was varied from cm. The position of the syringe was placed both in horizontal as well as vertical positions. The DC voltage was varied over the range of kv, but only results at 10.8 kv are presented. Images were obtained for the samples with either Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM). TEM were more efficient in visualizing the samples of nanofibers less than 100 nm compared to SEM. For SEM, samples were prepared by adhering the nanofibers onto an aluminum stub using carbon tape. A gold coating of about 13.3 nm was sputtered onto the sample. The morphology of samples was observed using a high-resolution fieldemission gun scanning electron microscopy FEG-SEM (Supra 35-VP, Carl Zeiss, Germany) at different magnifications. For TEM analysis, the nanofibers were placed on 200 mesh copper grid and was observed using a Zeiss-10C TEM operating at an accelerating voltage of 60 kv. To assess bonding, each sample was viewed at three different areas and each area was viewed at five different magnifications. From 15 micrographs for each sample, diameter of 100 fibers was measured using ImageJ software and standard deviation was calculated. At lower magnification, the uniformity of the fiber formations was assessed. At higher magnifications, the amount of inter-fiber fusing could be assessed and each specimen was assigned a degree of fusing number (FN) number from 1 to 5 as outlined in Table I. For the results presented here, the measurements were made in triplicate. All replicates were performed at similar operating conditions and process parameters such as relative humidity, voltage, concentration of polymer solution, distance between the collector and syringe pump and position of the syringe. Statistical significance was investigated using the Welch t-test and an F-Test. The spinning was conducted in a closed chamber. Humidity and temperature sensors were placed inside the chamber and monitored during the spinning. An infra-red heater was used to change temperature and heated water was used to introduce water vapor into the chamber. For results presented here, the temperature was 21 o to 22 o C, and the relative humidity of the air with respect to water vapor was varied between 28% and 56%. Journal of Engineered Fibers and Fabrics 2

3 RESULTS In general, when forming at low humidity, inter-fiber fusing was low and when forming fibers in a high humidity environment inter fiber fusing was high. The pictures in Table I illustrate how the fusing changed with changes in relative humidity for the case of a solution of 5 (w/w) % of PEO dissolved in the solvent mixture of distilled water and ethanol in the weight ratio of 3:2 spun in the horizontal position with a voltage of 10.8 kv using a 25 gauge needle and a temperature at 22 o C. The SEM micrographs revealed that the fiber widths increased near crossing points. The apparent diameter was determined both at fiber crossings and at centers of free-segments. The third column of Table I provides both average and standard deviation of the fiber diameter, which is plotted in Figure 2. TABLE I. Ranking system for fusing number. For FN=1, the fibers were nanofibers and no inter-fiber fusing was evident. The average diameter for FN=1 is significantly lower (t-values >10, p-value<0.01) than the sub-micron fibers with FN=2 to 5. For the submicron fibers, the average diameter for free-segments was affected little by the relative humidity. A permutation F-test with 1000 permutations gave an F(3,596)=1.96 and a p-value of indicating no significant difference in mean diameter for FN=2 to 5. For the sub-micron fibers (FN=2 to 5), the diameter at the fiber crossings was significantly larger (t-values >10, p-value<0.01) than the diameter of free segments. An F-test gave F=10 for a comparison of FN=2, 3, 4 and F=132 for FN=2 to 5, suggesting that the diameters are independent. There is a slight upward trend for FN=2 to 4 and a large increase for FN= Fiber Diameter, nm at fiber crossings at fiber free-segment centers Fusing Number FIGURE 2. Fiber diameter versus fusing number. Journal of Engineered Fibers and Fabrics 3

4 For FN=5, the distribution of diameters at fiber crossings was bimodal with one peak at about 450 nm and another peak at 850 nm. The results shown in Table I and Figure 2 displayed fusing only when microfibers were obtained. Based on experience when varying other conditions we changed to a 26 gauge needle and vertical placement of needle and collector to obtain nanofibers. At a temperature of 21 C and a spacing of 13 cm, fused nanofibers were obtained. At 35% RH, the FN was 4 and the diameter was nm, and at 40% RH the FN was 5 and the diameter was Figure 3 provides TEM image of the web formed at 40% RH. DISCUSSION The micrographs in Table I combined with the results given in Figure 2 show that for a fixed polymer solution, inter-fiber fusing increased with increased relative humidity of the environment. Therefore in addition to post-processing 7, and adjusting the polymer solution 5,14,15, adjusting the environment is a viable method to control inter-fiber fusing. The diameter of fibers at the lowest RH were less than 100 nm, but no inter-fiber fusing was evident. Changing the process to one using a higher gauge needle and vertical spinning, fused nanofibers were formed from the same solution Figure 3. This indicates that one could electrospin fused nanofiber webs without the need for postprocessing. The results obtained here are consistent with results obtained by Tripatanasuwan et al 17 who also spun PEO fibers from an aqueous solution. In their study, they used a higher mass concentration of 6%, a lower voltage of 5 kv, and a longer spacing of 18 cm. Our lower mass concentration would lead to smaller diameters, and the shorter spacing would lead to fusing. They obtained diameters as low as 63 nm, but no fusing. FIGURE 3. Nanofibers at 40% relative humidity. It was of interest to determine if the fusing could be turned on and off in one spinning session. Figure 4 provides an example of an electrospun mat produced under conditions where the humidity was varied from 35% to 56% RH. This increase in humidity corresponds to regions of Figure 4 moving from the lower right corner to the upper right corner of the image. 56% RH As the relative humidity increased, there was an increase in fiber diameter. Athough as shown in Figure 2, the increase comes more from an increase in diameters were two fibers cross rather than changes in the diameter of free fiber segments. This suggests that fibers are flattened after forming the web, rather than undergoing less drawing during spinning. The bimodal distribution of diameters of crossing fibers for FN=5 indicates that some fibers may flatten more than others depending on where they land, and how much solvent is left in the fiber when it lands. The result shown in Figure 4 illustrates the advantage that in situ inter-fiber fusing could have over posttreatment. Specific areas or layers could be formed with and without fusing to build in functionality. By changing the environment in a controlled manner one could form layers of un-fused fibers between layers of fused fibers. 1 m 35 % RH FIGURE 4. Varying the environment during electrospinning affects inter-fiber fusing. CONCLUSION Regulation of the surrounding environment appears to be a viable method to control the evaporation rate of solvents and allow inter-fiber fusing. Although, the diameter of the fiber at crossings also appears to increase as bonding increases, it was possible to form a bonded mat of PEO fibers with diameters less than 100 nm. The ability to control the environment during the spinning provides the opportunity to impart different functionality to different parts of the materials. Journal of Engineered Fibers and Fabrics 4

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