Fabrication of ultrafine fibrous polytetrafluoroethylene ...

Fabrication of ultrafine fibrous polytetrafluoroethylene porous membranes by electrospinning Jie Xionga) Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China

Pengfei Huo Department of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China

Frank K. Ko Advanced Fibrous Materials, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada (Received 19 February 2009; accepted 17 June 2009)

Poly(vinyl alcohol) (PVA) and poly(tetrafluoroethylene) (PTFE) emulsion were blended with different mass concentrations and the blended spinning solutions were electrospun into composite nanofibers. The influence of the blend ratio of PVA to PTFE and electrospinning technical parameters on the morphology and diameter of the composite nanofibers were investigated. According to the result of thermogravimetric analyzer analysis, the composite membrane was sintered at 390 C. The membranes were then characterized by differential scanning calorimetry, attenuated total reflection-Fourier transform infrared (ATR-FTIR), and scanning electron microscopy, respectively. The mechanical properties of the membranes before and after sintering were analyzed through tensile testing. The results show that the PTFE porous membranes could be electrospun effectively, thus demonstrating their potential application as filter media.

I. INTRODUCTION

Electrospinning differs from conventional fiber spinning in that it makes use of the action of a high-voltage electrical field on polymer solution with an objective to form nano/ submicrofibers. In the past decade, electrospinning has gained increasing interest in the field of nanotechnology. Many polymers have been electrospun into nano/submicrofibers successfully. Electrospun nano/submicrofiber assemblies with porous structure can be applied to many applications, such as tissue engineering scaffolds,1–3 drug delivery,4 enzyme immobilization,5 battery membrane,6 and filtration membrane.7 For example, the use of nanofibrous porous membrane in filtration has many advantages. These nanofibrous filters are characterized by a high surface-area-to-volume ratio, high porosity, pore sizes ranging from tens of nanometer to several micrometers characterized by interconnected open pore structure, light weight, and ease of functionalization. Nanofibrous porous membrane has been applied successfully in high-efficiency air filters,8–10 and they are being produced commercially.11 Preliminary research on coalescent filter12–16 and highflux ultrafiltration membrane17–19 have also been successfully demonstrated in recent years. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0347 J. Mater. Res., Vol. 24, No. 9, Sep 2009

Poly(tetrafluoroethylene) (PTFE) is a fluorocarbon polymer produced by tetrafluoroethylene polymerization. The strong bonds exist between fluorine and carbon atoms and the specific molecular structure give PTFE a good combination of chemical, physical, electric, antifriction, and other properties that are hard to find in any other material. PTFE can be used in a wide temperature range with excellent thermal and chemical resistance as well as good antifriction, electrical insulation, and mechanical properties.20,21 Therefore, PTFE is widely used as high-temperature filter material. However, PTFE has an unusually high melt viscosity, which makes it difficult to process using traditional processing technologies.22–25 For this reason, the processing of PTFE, which is similar to powder metallurgy, has a complex manufacturing process and excessive influence factors. Nowadays, the fabrication of PTFE porous membranes is usually by uniaxial and biaxial stretching.26–29 Many lubricating additives would have to be used in the production process, causing considerable environmental pollution. In this study, we demonstrate an alternate processing route to fabricate PTFE porous membrane. We specifically use poly(vinyal alcohol) (PVA) and PTFE emulsion to produce nano/submicrofibrous PTFE porous membranes that have high-temperature resistance and corrosion resistance characteristics through electrospinning and sintering. This porous membrane has the potential to be used widely as a candidate material for high-temperature filters. © 2009 Materials Research Society

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J. Xiong et al.: Fabrication of ultrafine fibrous polytetrafluoroethylene porous membranes by electrospinning

II. EXPERIMENTAL A. Materials

PTFE emulsion (SFN-1) was purchased from Chenguang Research Institute of Chemical Industry (Zigong, Sichuan). PVA powder (2488) was purchased from Shanghai Goldentree Resin Powder Co., Ltd. (Jinshan, Shanghai). These materials were used as-received without further purification. B. Preparation

The PVA solution was prepared by dissolving PVA powder in deionized water at 90 C under constant stirring for at least 6 h. When the solution was cooled down to room temperature, the PTFE emulsion was added to PVA solution with constant stirring for 3 h to form spinning solutions of different proportions and different concentrations. Figure 1 shows a schematic diagram of the electrospinning process. It consists of a syringe feeder system, a metallic ground fiber collector, and a high-voltage power supply. A high electric field is generated between the polymer fluid contained in a glass syringe and the fiber collector. At a sufficiently high level of electric field the polymer solution jet travels through the air. As the solvent evaporates, the solid polymer fibers are collected on the electrically grounded target. In this study, spinning experiments were carried out with applied voltage of 7–15 kV, at a spinning distance of 15 cm and fluid flow rate of 0.01 mL/min. The electrospun nanofibrous porous membrane was then sintered in a muffle furnace less than 390 C for 3 min to obtain the PTFE porous membrane. C. Characterization

The morphology of the gold-sputtered electrospun nanofibrous porous membrane was examined using scanning electron microscopy (SEM; JSM-5610LV, JEOL, Tokyo, Japan). The SEM images of the membrane were analyzed using the Image-Pro Plus image analysis program. Based

on the result of thermogravimetric analyzer (TGA; PYRIS 1, Perkin Elmer, Fremont, CA), the sintering temperature was defined. Then, a differential scanning calorimeter (DSC; Pyris Diamond, Perkin Elmer), attenuated total reflection-Fourier transform infrared spectroscopy (ATRFTIR; Nicolet5700, USA), and SEM were used to analyze the composition and structure of the membrane. The mechanical properties of membrane before and after sintering were determined using an Instron (5543) tensile tester (Norwood, MA) at ambient temperature. The specimens were cut into 25 mm (length) 5 mm (width) test strips. III. RESULTS AND DISCUSSION A. Effects of different PVA/PTFE ratios on hybrid(composite) fiber morphology

The PVA/PTFE solution, with the mass ratios of 10:90, 20:80, 30:70, 40:60, and 50:50, was electrospun at a concentration of 26 wt% using a constant spinning distance of 15 cm, at a voltage of 11 kV and a flow rate of 0.01 mL/min. Morphological change of the electrospun nanofibers are shown in Fig. 2. Because of its insolubility in common solvents, pure PTFE could not be readily electrospun into nanofibers. To obtain pure PTFE porous membranes, a sacrificial matrix polymer and post-treatment were introduced into the processing of PTFE nanofibers electrospinning. PVA, a water-soluble polymer, has good fiberizability and it can be electrospun into nanofibers easily. It was demonstrated that the spinnability of the chitosan, hydroxyapatite, and zinc oxide, which cannot be electrospun readily by themselves, could be achieved by blending with PVA.30–32 In this experiment, we demonstrated that the polymer blends transformed progressively from a beadlike structure to a fibrous structure as the content of the PVA in the system increases. As shown in Fig. 2, when the mass ratio of the PVA/PTFE was 10:90, only beadlike structure could be obtained. As the content of the PVA increases, a mixture of fibers and beadlike structure began to appear. When the PVA/PTFE mass ratio reaches 30:70, fibers of about 300 nm in diameter were formed, but the fibers were not uniform exhibiting large nodules along the fiber. As the PVA/PTFE mass ratio increases further, the fiber diameters were increased with uniform fiber surface. To obtain porous membranes with higher PTFE content, the content of PVA must be kept to a minimum while assuring fiber formation. In this study, a PVA/PTFE mass ratio of 30:70 was selected for further experiments. B. Effects of electrospinning parameters on composite fiber morphology

FIG. 1. Schematic diagram of the electrospinning apparatus. 2756

The electrospinning parameters have a strong influence on fiber morphology. The concentration of the polymer

J. Mater. Res., Vol. 24, No. 9, Sep 2009


FIG. 2. SEM images of the composite fibers with different mass ratios of PVA to PTFE: (a) 10:90, (b) 20:80, (c) 30:70, (d) 40:60, and (e) 50:50.

solution and the spinning voltage were found to be especially significant. In this study, the effects of concentration and spinning voltage on fiber morphology for PVA/ PTFE with a blend ratio of 30:70 were observed. 1. Effect of polymer concentration

The PVA/PTFE solution, with the mass ratio of 30:70, was electrospun at a concentration of 18, 20, 22, 24, 26, and 28 wt%, at a constant spinning distance of 15 cm, a voltage of 11 kV, and a flow rate of 0.01 mL/min. The morphological changes of the electrospun fibers are shown in Fig. 3. It can be seen that the viscosity of the solution and the spinnability increased with the increase of concentration of the solution, which is consistent with other published results.33–36 When the concentration of the polymer solution is lower than 22 wt%, the fibers tend to form

beadlike structures. Further increase in the concentration of solution resulted in the formation of continuous uniform nanofibers with large diameter. The changes of diameter with polymer concentration are shown in Fig. 4. It is of interest to note that there is no obvious change of diameter of fibers for those obtained from a solution with concentrations of 22 to 26 wt%. This can be attributed to the presence of surfactant in the PTFE emulsion. The existence of surfactant in the solution tends to increase the solution conductivity and decrease the surface tension, thus facilitating the formation of finer fibers.37 Since surfactant is a part of the PTFE emulsion, the surfactant content changes with solution concentration. At lower concentration, there was little surfactant presence thus polymer solution is the main influencing factor on fiber diameter. When the polymer concentration is between 22 and 26 wt%, the influence of surfactant begins to become effective, and thus reducing


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FIG. 3. SEM images of the composite fibers with different concentrations: (a) 18 wt%, (b) 20 wt%, (c) 22 wt%, (d) 24 wt%, (e) 26 wt%, and (f) 28 wt%.

FIG. 4. Changes in electrospun composite fibers diameters with solution concentrations.

the influence of polymer concentration. As a result, the diameter of fiber only increases slightly in this concentration range. At higher concentration, the effect of surfactant tends to be stable, whereas polymer concentration remains as the main influencing factor on fiber diameter, with fiber diameter increasing rapidly as polymer concentration increases. 2. Effect of voltage

Considering the fact that voltage is the most influential parameter in the electrospinning of PVA/PTFE once 2758

the concentration of spinning solution is kept constant, we concentrated our observation on the effect of voltage on fiber diameter. For our experiment a PVA/PTFE solution, with mass ratio of 30:70, was prepared for electrospinning at a concentration of 26 wt%, at voltage levels of 7, 9, 11, 13, and 15 kV. The spinning distance was 15 cm with a constant flow rate of 0.01 mL/min. Figure 5 shows the relationship between mean fiber diameters and spinning voltages. When the spinning voltages are within the range of 711 kV, the diameter decreased from 550 to 420 nm. This phenomenon relates to the fact that increasing the voltage tends to increase the electrostatic stress and thus a higher degree of drawing on the polymer jet during spinning. At higher spinning voltages from 11 to 15 kV, fewer changes in fiber diameter were found. C. Determination of sinter temperature

To obtain pure PTFE porous membrane, proper heat treatment should be applied. The TGA curve obtained by thermogravimetric analyzer for the heat-treatment process is shown in Fig. 6. It can be seen that there are two weight-loss steps in the TGA curve. The first weight loss of about 30 wt%, is located at 300–400 C, which corresponds to traces of PVA in the composite membrane.38 The second weight loss begins at 600 C. This can be attributed to decomposition of PTFE. In the range of 400–600 C, there is



FIG. 5. Changes in electrospun composite fiber diameters with voltages.

FIG. 7. DSC curves of porous membrane (a) before and (b) after sintering.

FIG. 6. TGA curve of composite fibrous porous membrane.

no weight change. This means that PTFE is preserved well and PVA decomposed completely. From the analysis of the TGA curve, we concluded that 300–400 C is a proper sintering temperature range. In this study, a sintering temperature of 390 C was chosen. D. Structural analysis of porous membrane

To further confirm the chemical composition of the electrospun fibers, the DSC curves of porous membrane before and after sintering were examined, as shown in Fig. 7. There is an endothermic peak that begins at 180 C at the DSC curve before sintering. After the sintering process the peak almost disappeared. The result confirms that PVA has been decomposed completely; only the pure PTFE porous membrane is left. The ATR-FTIR spectra provide additional confirmation of this result. Figure 8 shows the ATR-FTIR spectra of porous membrane before and after sintering. The strong absorption at 3334 cm 1 is associated with the

FIG. 8. ATR-FTIR spectra of porous membrane (a) before and (b) after sintering.

stretching of O–H bonds, whereas the absorption at 2911 cm 1 is due to C–H stretching. The absorption corresponding to the CH2 appears at about 1442 cm 1, and the absorption at 1098 cm 1 attributes to the C–O stretching. These are characteristic infrared absorption peaks of PVA in the composite nanofibrous porous membrane.39 It is evident that all the absorption peaks associated with PVA bonds disappear after sintering, and only the strong absorption of the C–F bond at 1203 and 1147 cm 1 remain.40 E. Morphology of PTFE porous membrane

As shown in Fig. 9, the SEM images of porous PTFE membrane reveal many differences between the membranes before and after sintering. Before sintering, the membrane is formed by randomly oriented nanofibers


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FIG. 10. Stress–strain curves of porous membrane (a) before and (b) after sintering.

were no fibers sliding during the tensile testing thus resulting at high strength and Young’s modulus for the sintered PTFE membranes. IV. CONCLUSION FIG. 9. SEM images of porous membrane (a) before and (b) after sintering.

with well-defined fiber morphology. After sintering, the PVA fibers were decomposed and the remaining PTFE fibers were fused on the points of fiber crossovers to form an interconnected fibrous network. F. Mechanical properties of the porous membrane

The tensile stress–strain curves of the porous membrane before and after sintering are shown in Fig. 10. It can be seen that the mechanical properties of porous membrane improved significantly after sintering. The strength of the sintered fiber network increases from 4 to 10 MPa while the modulus increases from 22 to 260 MPa. Failure strain increases from 63–69%. This can be explained in terms of the structural and compositional changes due to sintering. Before sintering, the membrane was formed by multiple layers of randomly oriented composite nanofibers of PTFE particles in a PVA matrix. During the tensile testing process, the fibers are free to slide along the direction of applied load, until the breakage of the fibers. Moreover, the PTFE particles were separated in the fibers. The PVA between PTFE particles became a weak link in the fibers when the fibers were pulled. After sintering, the PVA matrix was decomposed and the molten PTFE particles were bonded together41 to form an integrated fibrous network. As a result, there 2760

A novel method for the production of PTFE porous membrane through PVA-assisted PTFE emulsion electrospinning was demonstrated. The PVA and PTFE emulsion were blended in water with different mass ratios to form spinning solutions. As the content of the PVA increases, the spinnability of solution was found to improve. When the mass ratio of PVA to PTFE reaches a ratio of 30:70, uniform fibers with low PVA content was obtained. When other electrospinning parameters are kept constant, the diameter of fiber was found to increase with increasing concentration of the spinning solution, whereas the diameter of the fibers tends to decrease with increasing voltage. After sintering, complete decomposition of PVA was confirmed through the analysis of DSC and ATR-FTIR spectra. On sintering, the PTFE particles were fused to form an interconnected porous network of fibers in the micro- and nanoscale with substantial improvement of mechanical properties. This study demonstrates the feasibility of the formation of high-temperature PTFE nanofiber-based filter by the electrospinning process. ACKNOWLEDGMENTS

This work was supported by a Pre-973 Program (2008CB617506), a Program for Changjiang Scholars and Innovative Research Team in University (IRT 0654), and Open Fund of Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education (2006003). We thank Dr. Ni Li for her experimental assistance with electrospinning.



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