Preparation of Polyacrylonitrile Nanofibers by Solution Blowing Process

Transcription

1 Preparation of Polyacrylonitrile Nanofibers by Solution Blowing Process Xupin Zhuang, Kaifei Jia, Bowen Cheng, Ketian Guan, Weimin Kang, Yuanlin Ren Tianjin Polytechnic University, Tianjin CHINA Correspondence to: Bowen Cheng ABSTRACT Solution blowing is an innovative process for spinning nanofibers from polymer solutions using high-velocity gas flow as fiber-forming driving force. Polyacrylonitrile (PAN) nanofibers were prepared using PAN solutions in N, N-Dimethylformamide and the morphologies of the fibers were examined using a scanning electron microscope. The process parameters including solution concentration, gas pressure supplied to the gas cavity, and solution-feeding rate were varied to investigate their effects on the morphologies of the fibers. PAN fibers were successfully prepared in the form of a nonwoven web. The fiber diameters mostly ranged in hundreds of nanometers and were affected by the process parameters. Results indicated that solution blowing is an alternative technique for manufacturing nonwoven webs that consist of micro- and nanofibers. Keywords: Nanofibers; Polyacrylonitrile; Solution blowing; Nonwoven; Morphology INTRODUCTION Nanofibers exhibit special properties mainly due to their extremely high surface area-to-weight ratio compared to that of conventional fibers. Nanofibers is also becoming a very popular term. Electrospinning is the most well-known technique that allows the fabrication of continuous fibers with diameters down to nanometers [1 3]. Over the last few years, a vast array of organic, inorganic, and hybrid nanofibers have been electrospun [2, 4 8]. In electrospinning, a solid fiber is generated as a suitable viscous polymeric solution that is continuously stretched due to the electrostatic repulsions between surface charges and the evaporation of solvent [9]. Potential applications cover a wide spectrum of fields such as drug delivery systems, battery separators, information technology, reinforced materials, etc [2, 6, 10 12]. However, most published works on electrospinning were carried out using experimental laboratory-scale setups equipped with a syringe needle 0.3 mm to 1.0 mm in diameter. The nanofibers are obtained in very low yields with 1.0 ml h 1 to 5.0 ml h 1 solution flow rate, which limits the industrial applications of electrospinning [13]. To date, several methods have been developed to enhance electrospinning productivity, including multi-needle [14], needleless electrospinning [15], and gas flow-assisted electrospinning [9, 16, 17]. In addition to enhancing productivity, gas flow is also used to stabilize the process [16], to provide additional drawing force [16, 17], and to assist nanofiber alignment [9] in the gas flow-assisted electrospinning process. Gas flow is widely used in several current fiber-spinning processes. Melt blowing is a commercial one-step process for spinning polymer resin directly into a nonwoven web of fibers [18]. In the process, molten polymer is pressurized through a fine capillary and then rapidly attenuated into the fibers by the drawing force of a high-velocity gas flow. The fibers are then collected on an open screen to form a nonwoven mat [19]. The primary difference between melt-blowing and conventional melt spinning is that a gas flow, rather than a draw roll, provides the attenuating force in the former. Without a draw roll, the melt-blowing process can operate at high production rates. Commercial melt-blowing processes often operate at maximum rates of 6,000 m/min of gas flow and produce fiber with diameters in the range of 0.5 mm to 5 mm [20]. In the dry spinning process, another important fiber-spinning method, the gas is blown into a spinning cabinet where it meets with the polymer solution streams [21]. Unlike in a wet spinning process, solvent exchange does not occur in dry spinning. The solvent is evaporated with the help of a hot gas flow, separated from the air in a closed loop system, and recycled. Journal of Engineered Fibers and Fabrics 88

2 Recently, a novel solution blowing process was proposed using the elements of both dry spinning and melt-blowing technologies to spin ultrathin fibers [22, 23]. In the process, polymer solutions are blown into ultrathin fibers by streams of gas flow with the volatilization of the solvents. The fibers of poly (methyl methacrylate), polystyrene, and poly (lactic acid) were successfully spun, and their fiber diameters were close to those produced by electrospinning. Moreover, high-voltage equipment is not required, making the process convenient to operate [22]. Polyacrylonitrile (PAN), an important fiber material, is made into fibers by wet or, more often, dry spinning. In this work, PAN nanofibers were spun using a solution blowing process and the effect of various processing parameters on fiber diameter and morphology were demonstrated. EXPERIMENTAL Materials PAN (Mw = ) was supplied by Sinopec Qilu Company Ltd. (Shandong, China). N,N-Dimethylformamide (DMF) was analytical grade and used without further purification. Solutions of different PAN concentrations were prepared by dissolving PAN powder in DMF. Solution Blowing of PAN Nanofibers The solution blowing apparatus is similar to the previously described apparatus [23]. It is equipped with an extra cylindrical spinning cabinet and an air exhaust device, which allows the possibility of recycling the solvents. Figure 1 illustrates the schematic of the solution blowing apparatus. A single annular die, including a spinning nozzle with an annular gas cavity surrounding it, was used. The polymer solution was supplied to the nozzle with the control of a peristaltic pump, and compressed air was delivered to the air gas cavity by controlling the pressure regulator. As soon as the polymer solution stream was pressed out of the nozzle, it was subsequently blown into fibers by the streams of gas flow to the mesh-like collector through a cylindrical spinning cabinet. The evaporated solvent was removed with air by an exhaust blower through a groove under the collector screen. FIGURE 1. Schematic of the solution blowing apparatus. A series of experiments was conducted by varying the process variables, including polymer solution concentration (c), solution-feeding rate (r), gas pressure (p) supplied to the gas cavity, and so forth. The inside diameter of the nozzle is 0.38 mm and the inside diameter of the annular gas cavity is 1.25 mm. The distance between the nozzle tip to the collector screen is 45 cm. Characterization A thin layer of PAN fiber mats was carefully taken off from the collector to observe their morphology using a JEOL JSM-5800LV scanning electron microscope (SEM) after gold coating. The fiber diameters of the blown fibers were measured using a Java NIH Image J 1.29 image processing software. Pore sizes were measured on a capillary flow analysis (Porous Materials, Inc.). A wetting liquid with a surface tension of 15.9 dynes/cm was used as the wetting agent. RESULTS AND DISCUSSION In solution blowing, as the solution exits the nozzle, it is stretched by the drag forces of the high gas velocity accompanied by solvent evaporation and fiber formation. The process is similar to electrospinning, except with different fiber formation driving forces. In electrospinning, the electrical field provides the forces needed to stretch the fibers, whereas the aerodynamic drag forces of the gas jets are responsible for the polymer stretching in solution blowing. Journal of Engineered Fibers and Fabrics 89

3 Effect of Solution Concentration A series of experiments was conducted with different PAN concentrations at a fixed air pressure of 0.25 MPa and a PAN solution feed rate of 0.21 ml/min. The SEM photos of the fibers are shown in Figure 2. At 5% concentration, the ultrafine PAN fibers were spun containing numerous beads. The morphology was changed from beaded fiber to uniform-fiber structure with an increase in concentration from 8% to 16%. As the concentration was increased to 20%, only a small quantity of fibers was observed and big droplets and bulk formed. The fiber diameter distribution and the average diameter for the concentration of 8% to 16% were measured. From the results, the average diameter increased with an increase of concentration, and the average fiber diameters of 8%, 12%, and 16% solution concentrations were 326, 386, and 701 nm, respectively. Fiber strands were observed from the SEM photo of the 16% solution, which led to a significant increase of the average fiber diameter. The experimental phenomenon is similar to that of electrospinning. Below a certain concentration, chain entanglements are insufficient to stabilize the jet, and the contraction of the diameters of the jet driven by the surface tension causes the solution to form beads [24]. At a higher concentration, the viscoelastic force of the polymer solution, which resists rapid changes in fiber shape, results in uniform fiber formation [24]. The increase of solution concentration broadened the fiber distribution. FIGURE 2. Effect of solution concentration (c) on morphology and fiber diameter distribution: (p=0.25 MPa, r=0.21 ml/min). (a) 5%; (b) 8%; (c) 12%; (d) 16%; (e) 20%. Journal of Engineered Fibers and Fabrics 90

4 Effect of Gas Pressure In the solution blowing process, high-velocity gas flow plays the role of deforming the solution streams and evaporating the solvent to solidify them into fibers. Thus, gas flow velocity (i.e., gas pressure) is an important parameter. In our experimental design, a series of experiments was conducted with different gas pressure levels at a fixed concentration of 12% and a feed rate of 0.21 ml/min, which resulted in various SEM photos, as shown in Figure 3. When the gas pressure was 0.18 MPa, the maximum diameter of the fibers was over 1.4 μm and the minimum diameter was over 300 nm. When the gas pressure was increased to 0.30 MPa, the diameters were between 100 and 800 nm and most were below 400 nm. These results indicate that increasing the gas pressure (i.e., aerodynamic drag force) is beneficial to decreasing the average fiber diameter. Effect of Feeding Rate Figure 4 shows that the diameters of the solutionblown PAN fibers changed with varying feeding rates. From the figure, it was shown that increasing the flow rate, larger diameter fibers were obtained by increasing the flow rate because increasing the feeding rate decreases the shear force on the surface per unit volume solution, which goes against thinning the solution streams and fibers. FIGURE 4. Effect of feeding rate (r) on morphology and fiber diameter distribution (c=12%, p=0.25 MPa): (a) 0.26 ml/min; (b) 0.30 ml/min; (c) 0.34 ml/min. FIGURE 3. Effect of gas pressure (p) on morphology and fiber diameter distribution (c=12%, r=0.21 ml/min): (a) 0.18 MPa; (b) 0.21 MPa; (c) 0.30 MPa. Pore-size characterization A comparison of the SEM photographs of solution-blown nanofiber mats with the electrospun ones [25 28] clearly revealed that electrospun fibers are straight and tightly compressed in bulk, whereas blown fibers are curly and three-dimensional, which results in loosely connected mats. A blown nanofiber mat corresponding to Figure 2(b) was selected and its pore size was examined using the capillary flow method, as shown in Figure 5. The mean pore size of the electrospun mats is often below 5 μm [29 30], and the pore of the melt-blown fabric is Journal of Engineered Fibers and Fabrics 91

5 often over 30 μm [31]. Thus, the pore size of the solution blown mat is larger than that of the electrospun one and smaller than that of the melt-blown fabric. The filtration and transport characteristics of the nanofiber mats depend upon the pores created by the arrangement of fibers during the manufacturing process. The solution blown nanofiber mats are expected to find applications in filtration with high-filtration efficiency and low-pressure drop. FIGURE 5. Pore size distribution of the solution blown nanofiber mat. CONCLUSIONS PAN nanofiber mats were successfully spun using a solution blowing process. The process parameters were varied to study their effects on the morphologies of the fibers. The fiber diameters mostly ranged in hundreds of nanometers and were affected by the process parameters. Results indicated that solution blowing is an alternative technique in manufacturing nonwoven webs that consist of ultrathin fibers. ACKNOWLEDGEMENTS The author is grateful for the financial support from Tianjin Natural Science Foundation (10JCYBJC02700) and National Natural Science Foundation of China ( ). REFERENCES [1] Zhou, F. and R. Gong: Manufacturing technologies of polymeric nanofibres and nanofibre yarns. Polymer International, 57, (2008). [2] Teo, W. and S. Ramakrishna: Electrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite. Composites Science and Technology, 69, (2009). [3] Greiner, A. and J.H. Wendorff: Electrospinning: a fascinating method for the preparation of ultrathin fibers. Angew. Chem. Int. Ed., 46, (2007). [4] H. T. Chiu, J. M. Lin, T. H. Cheng, S. Y. Chou: Fabrication of electrospun polyacrylonitrile ion-exchange membranes for application in lysozym, express Polymer Letters 5, (2011). [5] Cho, D., et al.: Structural properties and superhydrophobicity of electrospun polypropylene fibers from solution and melt. Polymer, 51, (2010). [6] Ding, B., et al.: Electrospun nanomaterials for ultrasensitive sensors. Materials Today, 13, (2010). [7] Yu, J.H., S.V. Fridrikh and G.C. Rutledge: Production of submicrometer diameter fibers by two-fluid electrospinning. Advanced Materials, 16, (2004). [8] Madhugiri, S., et al.: Electrospun mesoporous molecular sieve fibers. Microporous and Mesoporous Materials, 63, (2003). [9] Zhou, W., et al.: Gas flow-assisted alignment of super long electrospun nanofibers. Journal of Nanoscience and Nanotechnology, 7, (2007). [10] Chen, H.M. and D.G. Yu, An elevated temperature electrospinning process for preparing acyclovir-loaded PAN ultrafine fibers. Journal of Materials Processing Technology, 210, (2010). [11] Gopalan, A.I., et al.: Development of electrospun PVdF-PAN membrane-based polymer electrolytes for lithium batteries. Journal of Membrane Science, 325, (2008). [12] Kim, J.R., et al.: Electrospun PVdF-based fibrous polymer electrolytes for lithium ion polymer batteries. Electrochimica Acta, 50, (2004). [13] Zhou, F., R. Gong and I. Porat: Mass production of nanofibre assemblies by electrostatic spinning. Polymer International, 58, (2009). [14] Varesano, A., et al.: Multi-jet nozzle electrospinning on textile substrates: observations on process and nanofibre mat deposition. Polymer International, 59, (2010). Journal of Engineered Fibers and Fabrics 92

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