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Effect of wheel rotating speed and LiCI additives on electrospun aligned polyacrylonitrile nanofiber.

INTRODUCTION

Electrospinning is an efficient method to produce polymer fibers in nanometers to a few microns diameter range using an electrically driven jet. During electrospinning due to the chaotic oscillation of the electrospinning jet and a lack of control over the forces driving fiber orientation, generally randomly oriented and isolropic structures in the form of nonwoven nanofiber mats or webs are often generated. Because of large surface to volume ratios, large lenglh-lo-diameter ratios and high porosity, randomly oriented nanofiber mats show a wide application in filtration materials and artificial organs (1, 2). But electrospun nanofiber mats with well-aligned and highly ordered structure are often required in many applications for enhancing mechanical performance, electrical performance or cell attachment and proliferation (3-7).

Several techniques have been proposed to realize the orientation of electrospun fibers. Murata and coworkers (3) produced uniaxially aligned electrospun fibers with controlling their number and diameter by using two pieces of stainless steel collectors and a mechanical switch. Theron et al. (8) produced aligned nanofibrous scaffolds by electrospinning process using a rotating disk collector. Yu et al. (9) fabricated aligned helical polycaprolactone nanofibers by using a wooden board placed at an angular distance from the syringe nozzle and a tilted glass slide located between the ground electrode and the syringe needle. Pan et al. (10) used two oppositely placed metallic needles to manufacture uniaxially aligned electrospun fibers with diameter of submicrometers. Lee et al (11) obtained aligned nanofiber by using two pieces of Si substrates separated by a gap of 1.5 mm. Using rotating wheel as the collector to fabricate aligned nanofiber is a cost saving and facile realization method. This method has been employed by some researchers in electrospinning and some general results were gotten by them (4, 12, 13). In this work, the dependence of electrospun nanofiber diameter and orientation on the rotating speed was analyzed and an optimum rotating speed was found to fabricate highly aligned fibers.

Additionally the added salts are propitious to electrospinning due to enhancement of solution conductivity (14, 15). The effect of added salts on the morphology of disordered nanofiber has been investigated in some contributions (16-20). Son et al. (17, 18) reported that the average diameters of electrospun fibers were decreased and their distributions were narrowed by adding salt due to the increased charge density in solution. Lee et al. (19) investigated the influence of added ionic salts on nanofiber uniformity.

In this work, different rotating speed of the wheel was evaluated systemly in regards to the preparation of well aligned polyacrylonitrile (PAN) nanofiber. Meanwhile the effect of LiCI (Lithium Chloride) concentration on fiber morphology and mechanical behavior of aligned nanofiber mat was examined for the first time.

Preparation of Polymer Solution

PAN with a weight average molecular weight of 140,000 was purchased from Hangzhou Acrylon Co., DMF (N, N-Dimethylformamide, analytical grade) with purity of [greater than or equal to] 99.5% was purchased from Hangzhou Gaojing Fine Chemical Industry Co., LiCI (analytical grade) with purity of [greater than or equal to] 95% was purchased from Tianjin Tianda Chemical Co., 12 wt% solutions of PAN polymer with 0.2, 0.4, 0.6, 0.8, and 1 wt% LiCI were prepared respectively, which was similar to that used in our previous work (21).

Electrospinning

Figure 1 shows a schematic diagram of the homemade electrospinning set-up. Compared with the conventional electrospinning setup, a rotating wheel covered with aluminum foil was used instead of stationary collector to collect aligned nanofiber. The wheel radius is 12 cm and the width is 40 cm. During electrospinning the speed of the wheel ranged from 0 (round per minute) to 300 rpm. The experiments were conducted with the following experimental parameters unchanged: the applied voltage was 12 kV, the distance between the needle and the collector was 12 cm, the flow rate of solution was 0.01 ml/min and the diameter of needle was 1 mm. All experiments were performed at room temperature.

Characterization

The morphology of electrospinning nanofibers was investigated by FE-SEM (field emission scanning electron microscopy, Hitachi S4800), and fiber diameter and fiber angle were measured by Image-Pro [R] Plus6.2 Software. In this work, fiber angle was signed by the angle between fiber aligned direction and wheel rotating direction. Fifty comparatively homogeneous fibers were measured for di-ameter and angle in each SEM figure and their mean values were obtained. Tensile properties of nanofibrous mats were determined using an Instron 5543 Microtester (USA), at a cross-head speed of 5 mm/min. The length and the width of the specimens cut from the electrospun mats were 40 mm x 5 mm and each specimen was measured six times.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Effect of Rotating Speed of the Wheel on the FiberMorphology

Figure 2 revealed that when the wheel was stationary, fiber average diameter was nearly 0.5 [micro] m. Increasing wheel rotating speed to 50 rpm, the fiber diameter decreased significantly and it came to 282 nm. There was an almost twofold decrease in the diameter. Continuing to increase the rotating speed, the fiber diameter decreased continuously, but the decrease was not significant. It can be concluded that electrospinning fiber diameter was influenced greatly by collector state and was not sensitive to the speed of the rotating wheel. The fibers deposited on the stationary collector were unconstrained and tended to relax (13). Because of wheel rotation the fibers were constrained and not allowed to relax. Fiber average diameter collected on the rotating target was smaller than that collected on the stationary target. On the other hand, Shivkumar and coworkers (22) thought that for an evolving solution jet ejected from the needle it could be divided into three regions: original straight segment, following jet solidification segment and residual part. The length of each segment varied under different electrospinning conditions. Final fiber diameter was mainly determined by jet solidification segment due to extensive whipping. In our experiment, because DMF is a high boiling solvent and jet trajectory was changed by the rotating wheel, it was expected that DMF did not evaporate completely during the electrospinning and the jet was still wet when it reached the wheel. That was to say in the same needle-to-collector distance the jet solidification segment was long and the third segment was very short, even disappeared. Then the wet jet was stretched greatly by the rotating wheel and twofold decrease of fiber diameter was obtained. Meanwhile, because of very rapid traction exerted on the jet by the rotating wheel and jet submicroscale diameter, the jet solidified as soon as it contacted the wheel and the rotating speed had not obvious influence on the fiber diameter.

The typical morphology of electrospun nanofibers is collected in the form of disordered arrangement on a stationary collector. At this time the mat is anisotropic and has high porosity. Figure 3 indicated that fiber average angle decreased with the increase of wheel rotating speed. In Fig. 3 picture A was the fiber morphology when the rotating speed was 50 rpm and picture B was the fiber morphology when the speed was 300 rpm. The white arrow in the picture means the rotating direction of the wheel. When the speed increased from 50 to 150 rpm, the angle decreased rapidly from 34 [degrees] to 18 [degrees], which implied that nanofiber orientation was greatly improved. A further increase of the rotating speed to 300 rpm, the angle changed slightly.

When the jet reached the rotating wheel, the contacting point between the jet and the wheel was marked as C (Fig. 4a). One part of the jet just near the point C was carried out to study the forces acted on it (Fig. 4b). The gravity was neglected. At this time, because of very small amount of residual charges in the jet, electric field force acted on the jet was also neglected. [f.sub.1], mainly resulted from the rotating wheel and [f.sub.2] came from the jet other part. In rectangular coordinate, when [f.sub.1y] [Greater than] [f.sub.2y], except point C the jet moved to the positive of axis y and formed the fiber arranging along wheel rotating direction. When wheel rotating speed was equal to or larger than 150 rpm, under these cases [f.sub.1y] = [f.sub.2y], the angle was already small and did not change obviously.

During the electrospinning process, it was found that high rotating speed of the wheel made the jet very unstable. Meanwhile, low rotating speed could save energy. So in the following discussion in this article the results were obtained with the rotating speed of the wheel fixed at 150 rpm. At this time, the fiber diameter was 263 nm, which was desirable for many kinds of application such as filtration, biomaterial and so on.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Scaling and dimensional analysis actually started with Newton (23) and was developed independently some what later in the biological sciences, turbulence, and economics (23-25). By fitting experimental data (the solid lines in Figs. 2 and 3), we found that fiber diameter (d) and fiber angle ([theta]) both have exact exponential relationship with wheel rotating speed ([omega]). These dimensional relationships can be expressed as:

d ~ [beta.sub.1] [e.sup.-[lambda.sub.1] [omega]] (1)

[theta] ~ [beta.sub.2] [e.sup. -[lambda.sub.2] [omega]] (2)

[beta.sub.1], [beta.sub.2], [theta.sub.1], [theta.sub.2] are coefficients. They change with processing parameters.

Effect of LiCI Concentration on the Aligned Fiber

In our previous experiments, when electrospinning 12 wt% PAN/DMF solution with LiCI by using a stationary plate as the collector, the following phenomenon appeared: firstly the jets ejected from the needle, then buckled together and finally spinning process was terminated. The typical envelope cone formed by jet spiraling loops because of whipping instability was not observed during electrospinning process and nanofibers were not observed on the stationary plate either. However under the same ambient condition and other electrospinning parameters these buckling jets did not exist when rotating wheel was used and nanofibers were collected arranging along wheel rotating direction in our experiments. If stationary collector was used, the enhancing inter-repulsive force coming from residual charges because of salt ions resulted in jet unconventional evolution and wet jet buckled together easily (26). The airflow formed by rotating wheel could weaken the inter-repulsive force. Meanwhile electrospun jet was stretched and taken away continuously by the rotating collector. Thus rotating collector can collect the nanofiber which can not be collected by traditional stationary collector.

[FIGURE 5 OMITTED]

The following SEM micrographs showed that there was a trend that fiber assembling together on the rotating wheel with the increase of LiCI concentration (Figs. 5a-e). When LiC1 concentration came to 1 wt%, the aligned nanofiber separated from each other again (Fig. 5f). We thought that fiber assemblage was because of jet buckling described above and the separation was because of the increase of ion concentration with the increase of LiCI concentration which would lead to excessive residual charges on the nanofiber and enhance the nanofiber inter-repulsive force (27, 28). Just as the investigation we made before that when LiCI concentration was equal to 1 wt%, surface charge became saturated and full charge on jet surface was obtained (29). Fiber assemblage was expected to improve mechanical performance of electrospun nano-fiber mat. Meanwhile because of this kind of fiber assemblage, LiCI adding into electrospun polymer solution provided us some new ideas about fabricating electrospun nanofiber yarn. There will be an in-depth investigation about electrospun yarn fabrication with salts additives in our future work.

[FIGURE 6 OMITTED]

When wheel rotating speed was 150 rpm, fiber average diameters under different LiCI concentration were measured and the result was listed in the Table 1. It showed that LiCI adding increased fiber diameter and with the increase of LiCI content fiber average diameter increased, which was consistent with our previous work (26). When LiCI concentration was 1 wt%, the diameter decreased. Figure 5f showed that the fiber separated from each other on the collector, which implied that the jet did not buckle during electrospinning process, the jet trajectory was conventional spiraling loop and the jet was stretched sufficiently during electrospining. Table 1 also indicated that when the speed of rotating wheel was 150 rpm, fiber average angle was not sensitive to the LiCI concentration. The degree of fiber orientation was mainly influenced by the tangential force exerted on the polymer jet by the rotating wheel.

The mechanical properties of electrospun mats with different LiCI concentration were shown in Fig. 6. The stress of the mats electrospun from 12% PAN/DMF with LiCI additives was smaller than that from 12% PAN/ DMF without salts. It is well known that fiber mechanical properties were determined by the crystallinity degree, the degree of orientation of the macromolecules along the fiber axes, and the residual compressive stress of the fiber surface (30). Qin and Wang (14) thought that the crystallinity of nanofibers with LiCI was lower than that of nanofibers without LiCI and the chemical composition of PAN was unchanged. In this article stress decrease for with and without salts was explained from the difference of mat macroscopical morphology. Under similar fiber angle (Table 1), fiber assemblage in Figs. 5c-e was propitious to the stress of PAN mat and bigger fiber diameter (Table 1) was also propitious to the stress of PAN mat, but the efficiency of electrospinning 12 wt% PAN/DMF with no salt was high and the nanofibers crossed closely (Fig. 5a), which resulted in more fibers devoting to bear the load. Meanwhile in Fig. 5a there were still many intercrossing dots, which also enhanced fiber interconnection. Additionally in Figs. 5c-e, some fibers just were assembled together, but not twisted. The fiiction between fibers was small. So the stress of the mat with no salt was bigger. It could be found from Fig. 6 that because of notable fiber assemblage the stress of the mat with 0.6% LiCI additives was most close to that of the mat without LiCI and the failure strain was the biggest.

CONCLUSION

Rotating speed of the wheel was optimized to 150 rpm to prepare unidirectional aligned PAN fibers with diameters in the nanoscale range. Electrospun fiber diameter and fiber angle both showed exponential relationship with wheel rotating speed. Inorganic salt LiCI was selected to enhance the solution conductivity and the effect of LiCI on the aligned nanofiber was examined. As electrospin-ning 12% PAN/DMF solution with LiCI, fiber assemblage was observed on the rotating wheel, which will result in easy fabrication of electrospun yarn. Compared with the mechanical performance of the mat without LiCI additive, the mat with 0.6 wt% LiCI additives showed best stress and strain.

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TABLE 1. Fiber average diameter and fiber average angle under
different LiCI concentration.

LiCI concentration  Fiber diameter  Fiber angle
      (wt%)              (nm)       ([degrees])

No salt                        263           18
0.2                            360           27
0.4                            371           22
0.6                            373           17
0.8                            397           16
1                              285           11


Ni Li, Jie Xiong, Hua Xue Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

Correspondence to: Ni Li; e-mail: muzilini@mail.dhu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 10902099; contract grant sponsor: Science Foundation of Zhejiang Sci-Tech University (ZSTU); contract grant number: 0901802-Y.

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Title Annotation:lithium chloride
Author:Li, Ni; Xiong, Jie; Xue, Hua
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Nov 1, 2011
Words:3073
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