Electrospinning PVDF/EC fibre from a binary solvent system. Tingping Lei, Zhan Zhan, Wenjia Zuo, Wei Cheng, Bulei Xu, Yuanzhe Su and Daoheng Sun*

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1 294 Int. J. Nanomanufacturing, Vol. 8, No. 4, 2012 Electrospinning PVDF/EC fibre from a binary solvent system Tingping Lei, Zhan Zhan, Wenjia Zuo, Wei Cheng, Bulei Xu, Yuanzhe Su and Daoheng Sun* Department of Mechanical and Electrical Engineering, Xiamen University, Simingnanlu 422#, Xiamen, , Fujian, China ltp9851@xmu.edu.cn snowkally@yahoo.com.cn @qq.com @qq.com xubulei@126.com virus-0004@163.com sundh@xmu.edu.cn *Corresponding author Abstract: The influence of the composition of a binary component solvent on the surface morphology and diameter distribution of poly(vinylidene fluoride)/ethyl cellulose (PVDF/EC) fibres produced by electrospinning technology was investigated. Acetone (56 C) and N-methyl pyrrolidone (NMP) (203 C) binary solvent system was introduced and electrospinning was performed by changing the solution concentration, the applied voltage and the volume ratio of NMP/acetone. The results show that the morphology of PVDF/EC electrospun fibres is greatly influenced by the solution concentration and the volume ratio of NMP/acetone. PVDF/EC fibres prepared from NMP/acetone binary solvent system is much better than that prepared with a single component solvent system, and is very diverse with the volume ratios of the binary solvent system. The morphology of the fibres changed from beads (starting mass ratio of 12 wt% or pure NMP solvent) to full developed fibres as mass ratio increased to 16 wt% with the volume ratio of NMP/acetone (6/4, 5/5, 4/6, 3/7). Beadless fibres were normally obtained within the voltage applied and their average diameter decreased with increasing the applied voltage. Keywords: electrospinning; polymer fibre; poly(vinylidene fluoride)/ethyl cellulose; PVDF/EC; binary solvent. Reference to this paper should be made as follows: Lei, T., Zhan, Z., Zuo, W., Cheng, W., Xu, B., Su, Y. and Sun, D. (2012) Electrospinning PVDF/EC fibre from a binary solvent system, Int. J. Nanomanufacturing, Vol. 8, No. 4, pp Biographical notes: Tingping Lei received his BSc in Applied Physics in 2006 and Msc in Micro-electronics and Solid-electronics in 2009 from Harbin University of Science and Technology, Harbin, Heilongjiang, China. Currently, he is a PhD student in Xiamen University, Fujian Province, China. His research interests include MEMS, flexible electronics, and electrospinning, polymer nanofibres. Copyright 2012 Inderscience Enterprises Ltd.

2 Electrospinning PVDF/EC fibre from a binary solvent system 295 Zhan Zhan received his Bachelors degree in Mechanical Engineering and Automations from Wuhan University of Technology, Wuhan, China in Currently, he is studying in Xiamen University for his Master of Automatic Manufacturing and Control Technology. His current research is gyroscope manufacturing. Wenjia Zuo received his Bachelors degree in Mechanical Engineering and Automation from Huazhong Agricultural University, Wuhan, Hubei Province, China in Currently, he is studying in Xiamen University for his Master of Engineering. His current research is about micro-systems manufacturing. Wei Cheng received his Bachelors degree in Mechanical Manufacturing and Automations from Xiamen University, Xiamen, Fujian Province, China in Currently, he is studying in Xiamen University for his Master of Engineering. His current research includes flexible electronics manufacturing and micro-systems manufacturing. Bulei Xu received his Bachelors degree in Measuring and Control Technology and Instrumentations from Tainjin University, Tianjin,, Hebei Province, China in Currently, he is studying for his Master of Engineering in Xiamen University, Xiamen, Fujian Province, China. His current research includes flexible electronics manufacturing. Yuanzhe Su received her Bachelors degree in Mechanical Design, Manufacturing and Automation from Xiamen University, Fujian Province, China in She is currently a Master in Xiamen University. Her main research includes preparation and testing of micro-devices, micro/nano manufacturing. Daoheng Sun received his BSc in 1987, MSc in 1990, and PhD in 1997 in Mechanical Electronics from Northeastern University, Shengyang, Liaoning Province, China. He was a Postdoctoral fellow at Southwest Jiaotong University, Chengdu, Sichuan Province, China during 1997 to He is currently a Professor of the Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen, Fujian Province, China. His research interests are MEMS/NMES, flexible electronics/printable electronics/macro electronics, micro/nano systems manufacturing technology and equipments. This paper is a revised and expanded version of a paper entitled Electrospinning PVDF/EC fibre from a binary solvent system presented at The First International Conference on Manipulation, Manufacturing and Measurement on the Nanoscale, Changchun, China, 29 August 2 September, Introduction Electrospinning is a straightforward, convenient, and effective method to produce fibres with diameters ranging from the nano- to microscale by applying a high voltage to a polymer solution or melt ejected from a micro-syringe pump. It was first patented by Formhals (1934). Since then, many polymers have been routinely electrospun, and several review articles summarising these have appeared (Huang et al., 2003; Subbiah et al., 2005; Greiner and Wendorff, 2007).

3 296 T. Lei et al. Poly(vinylidene fluoride) (PVDF) is a highly non-reactive, flexible, inexpensive, and leading polymer with excellent chemical, piezoelectric and dielectrical properties (Holmes-Siedle et al., 1984); and by electrospinning it can be utilised as a piezoelectric sensor (Xu and Li, 2010), piezoelectric generator (Chang et al., 2009), or separator (Ding et al., 2008), and so forth. Ethyl cellulose (EC) is a non-toxic, stable, compressible, inert, hydrophobic polymer with better membrane-forming ability, medium gas-separation/pervaporation capability, good flexibility, excellent durability, and low cost, and has been widely used to prepare pharmaceutical dosage forms (Li et al., 2001; Prasertmanakit et al., 2009). To the best of our knowledge, no report has been given to investigate properties of poly(vinylidene fluoride)/ethyl cellulose (PVDF/EC) blends. Therefore, it is hopeful to obtain multi-functional fibres by elecrospinning the binary polymers, which will broaden the possible uses of PVDF/EC. In this study, PVDF/EC fibres were prepared from an N-methyl pyrrolidone (NMP)/acetone binary solvent system with different boiling temperature and volatility. Apart from the solution concentration and the applied voltage, the effect of the solvent volume ratio on the morphology of electrospun PVDF/EC fibres was also investigated through the magnified images of the fibre prepared with various solvent ratios. 2 Experiments 2.1 Materials PVDF with weight-average relative molecular mass of was purchased from Shanghai Dingyue Chemical Technological Co. Ltd. (China). EC, NMP, acetone and chloroform were provided by Shantou Dahao Fine and Special Chemical Company (Guangdong, China). These solvents were in chemical grade and used without further purification. The basic properties of these solvents are summarised in Table 1. Table 1 Properties of solvents used in this study Solvent Boiling point ( C) Volatility Vapour pressure at 25 C (Pa) Polarity NMP 203 Low 45 - Acetone 56 High 24, CHCl 3 61 High 12, Source: Gregorio and Borges (2008) 2.2 Preparation of polymer solution and characterisation of its films PVDF was put into NMP solvent and magnetically stirred at a set temperature of 55 C to form PVDF transparent solution with an initial mass ratio of 16 wt%. Meanwhile, EC was dissolved in NMP solvent of a certain volume to prepare EC transparent solution with the same mass ratio of PVDF solution. The PVDF/EC mixed solution was obtained by dropping EC solution into PVDF solution in a certain ratio under stirring and spread on a glass sheet substrate which was cleaned with acetone before film casting. These samples were further dried in air, heated, or etched in CHCl 3 for the observation under an optical microscope (OM, BX51, Japan).

4 Electrospinning PVDF/EC fibre from a binary solvent system Electrospinning A schematic diagram of the electrospinning apparatus is shown in Figure 1. The polymer solution was placed in a 2 ml syringe attached to a precision syringe pump (11 Pico Plus, Harvard Inc., USA) in a vertical mount. The potential was applied to the dispensing on the end of the syringe using a high voltage power supply (DW-P403-1AC, Tianjing Dongwen High Voltage Power Supply Plant China). A grounded aluminium foil was placed on the surface of a movable jack and used as the collector. All electrospinning were carried out at room temperature. If not specified in our electrospinnining experiment, the PVDF/EC mass ratio was 16 wt%, the NMP/ acetone volume ratio was 6/4, the applied voltage was 6.5 kv, the working distance was 10 cm, the flow rate was 40 μl/h, and the inner diameter of needle was 0.25 mm. The morphology of PVDF/EC fibres electrospun from NMP/acetone binary solvent system at different volume ratios was characterised under a scanning electron microscope (SEM, LEO 1530, Germany) after gold coating. The average fibre diameter (AFD) was calculated from SEM images using Adobe Photoshop 7.0 without taking the bead size into account. Figure 1 Schematic diagram of electrospinning apparatus (see online version for colours) 3 Results and discussion When EC solution was dropped into PVDF solution under stirring, homogeneous transparent solution could be formed. The solution was spread on a glass sheet substrate and dried in air, bubble-like shapes of different size embedded in semi-dried film could be observed, however, when heated to 100 C, cake-like structures formed and increased

5 298 T. Lei et al. after acceleration of the solvent evaporation, as shown in Figures 2(a) and 2(b), respectively. Since the crystallisation of PVDF is much easier than that of EC in the solution and the incomplete evaporation of solvent only dried in air makes the film with two phases, which produces bubble-like shapes and further evaporation of solvent can be realised by heating for a suitable time leading to the formation of cake-like structures. Figure 2(c) shows that bubble-like shapes grow into mushroom-like patterns after the air-dried film etched in CHCl3 for about 1 h, which was followed by a successive heating up to 100 C and more mushroom-like patterns were observed as shown in Figure 2(d). This can be explained by the preferential solubility of EC in CHCl3 and the insolubility of PVDF in CHCl3, which causes a phase separation between PVDF and EC. Figure 2 OM micrographs of the cast films treated under different conditions, (a) dried in air (b) heated to 100 C (c) dried in air after etched in CHCl3 (d) heated to 100 C after etched in CHCl3 (see online version for colours) From the above experiment, it was found that NMP is difficult to evaporate completely in spite of long time dried in air, which is undesirable in the electrospinning especially at ambient atmosphere. Due to NMP has a high boiling point and a low vapour pressure (see Table 1), the addition of a polar solvent with a low boiling point and high volatility is required to improve its properties, which not only accelerates evaporation of the binary solvents, but also brings down the boiling point of the mixture. Here, acetone was selected for the candidate. Therefore, apart from the solution concentration and the applied voltage, the NMP/acetone volume ratio was also investigated in the experiment and will be discussed below.

6 Electrospinning PVDF/EC fibre from a binary solvent system Figure OM micrographs of PVDF/EC fibres electrospun from NMP/acetone binary solvent system at different mass ratios, (a) 12 wt% (b) 14 wt% (c) 16 wt% (d) 18 wt% (e) 20 wt% (see online version for colours) The mass ratio of PVDF/EC in NMP/acetone binary solvent system was varied from 12 to 20 wt% in order to study the influence of concentration on the fibres and choose the best for the subsequent experiments. OM micrographs show an increase in average fibre diameter (AFD) with increasing solution concentration (see Figure 3). Starting from a concentration of 12 wt%, large numbers of beads are formed on the PVDF/EC fibres. With increasing concentration, the beads gradually decrease and when the concentration reaches 16 wt%, the beads disappear and uniform fibres are obtained. However, when

7 300 T. Lei et al. further increasing concentration up to 18 wt%, the fibres are different in size and some are greatly thickened. At a higher concentration, fibres exhibit curly, wavy, and straight structures. Figure 3(e) shows identical knots with regular distance when the concentration is 20 wt%. The formation of the beaded fibres can be regarded as low concentration resulting a relatively low viscosity and high surface tension, because surface tension becomes dominant over viscosity at low concentration, and high surface tension tends to make the surface area per unit mass smaller by changing the jets into spheres. But with increasing the concentration, the solution will increase its viscosity and decrease the surface tension, where the viscosity becomes dominant which favours the formation of larger diameter fibres (Fong et al., 1999). The relationship between AFD and PVDF/EC mass ratios is plotted in Figure 4, which indicates that an ultra-fine fibre might be obtained if mass ratio less than 12 wt%, but according to Figure 3, the beads will proliferate and no ultra-fine fibres could be formed. It also should be emphasised that electrospinning from the solution with high concentration is more likely to form curly, wavy structures on fibres. Both beads and curled structures have negative influence on surface area to volume ratio of electrospun fibres, which will degrade the performance of the fibres. Therefore, it is of great significance to generate fibres without beads or curled structures. Considering the effect of solution concentration on morphologies and AFD of the fibres, mass ratio of 16 wt% is preferred to the others. Figure 4 Effect of PVDF/EC mass ratios on AFD of the electrospun fibre from NMP/acetone binary solvent system The voltage applied during electrospinning was also varied and its influence on the PVDF/EC fibres was investigated. Figure 5 shows the effect of the applied voltage on AFD of 16 wt% PVDF/EC from NMP/acetone binary solvent system. The beadless fibre size decreases from about 1.5 to 0.8 μm with increasing spinning voltage ( kv), keeping all other conditions constant. When the applied voltage approaches 5.8 kv, the change of AFD slows down and becomes steady from 6.5 kv (see marks in Figure 5). Increasing the applied voltage would increase the electric field force that overcomes the surface tension of the polymer solution, which extrudes more polymer during the electrospinning process (Nasir et al., 2006). At the same time, increasing the applied voltage would also increase electrostatic repulsion force for fluid jet, which favours

8 Electrospinning PVDF/EC fibre from a binary solvent system 301 thinner-fibre formation (Park et al., 2007). As a result, the diameter of electrospun fibres would increase or decrease with the increase of the applied voltage, depending on the combining effect between the electrostatic field force and the repulsion force arising from surface tension and viscosity force. Figure 5 Effect of the applied voltage on AFD of PVDF/EC fibre electrospun from 16 wt% solution using NMP/acetone binary solvents At the very beginning, we found that PVDF/EC electrospun from each pure solvent (NMP or acetone) could not form good fibres as expected. Either too much solvent surrounds the fibres or masses of pellets scatter on the collector as shown in Figure 6. The difference arising from these two solvents could be attributed to the vapour pressure as listed in Table 1, where NMP with a very low vapour pressure allows the polymer jets to experience long-time stretching with large amounts of solvent remained, however little stretching occurs to the solution used pure acetone for rather high evaporation rate in acetone. Still, it was possible to produce good fibres by mixing these two solvents and adjusting their volume ratios. Figure 7 shows the fibre morphologies prepared from 16 wt% PVDF/EC solution with NMP/acetone volume ratios of 9/1, 8/2, 7/3, 6/4, 5/5, 4/6, 3/7, 2/8, and 1/9. When the volume ratio of NMP/acetone was 9/1, 8/2, and 7/3, thread and bead morphology of electrospun fibres could be observed [see Figures 7(a) to 7(c)]. However, with increasing the amount of acetone, the fibres were found beadless for the volume ratios of 6/4, 5/5, 4/6, and 3/7 [see Figures 7(d) to 7(g)]. Further increasing acetone resulted in beads reproducing in the solution with NMP/acetone volume ratios of 2/8 and 1/9 [see Figures 7(h) to 7(i)]. Because the boiling point of acetone (56 C) is lower than that of NMP (203 C) and the vapour pressure of the mixed solvent is decreased by increasing the amount of acetone, which is easier to evaporate during electrospinning. Meanwhile, more charges could be accumulated onto fibre surface, which is good for the uniform fibre formation under the higher electrostatic repulsion. Thus, the addition of acetone to PVDF/EC solution is quite necessary and the amount should be well controlled; in this way, we can prepare uniform fibres electrospun from the NMP/acetone binary solvent system.

9 302 Figure 6 Figure 7 T. Lei et al. SEM images of PVDF/EC fibres electrospun from pure solvent, (a) NMP (b) acetone SEM images of PVDF/EC fibres electrospun from NMP/acetone binary solvent system at different volume ratios, (a) 9/1 (b) 8/2 (c) 7/3 (d) 6/4 (e) 5/5 (f) 4/6 (g) 3/7 (h) 2/8 (i) 1/9

10 Electrospinning PVDF/EC fibre from a binary solvent system Figure SEM images of PVDF/EC fibres electrospun from NMP/acetone binary solvent system at different volume ratios, (a) 9/1 (b) 8/2 (c) 7/3 (d) 6/4 (e) 5/5 (f) 4/6 (g) 3/7 (h) 2/8 (i) 1/9 (continued) Although the solution concentration, the applied voltage and the NMP/acetone volume ratio have been investigated in this paper, parameters such as working distance (needle to the collector), flow rate, temperature and humidity, etc. however show unnoticed effects in this experiment but should be taken into consideration for other investigations.

11 304 T. Lei et al. 4 Conclusions The diameter and its distribution of the PVDF/EC fibres fabricated by electrospinning were greatly influenced by the solution concentration as well as the NMP/acetone binary solvent system. In terms of the solution concentration, the structure of PVDF/EC electrospun fibres changed from beads to full developed fibres as mass ratio increased to 16 wt%. The morphology of PVDF/EC fibres prepared from NMP/acetone binary solvent system was much better than that prepared with a single component solvent system, and was very diverse with the volume ratios of the binary solvent system because of the difference in volatilisation of the two components in the binary solvent system. As for the applied voltage, the fibres were free of beads and their diameter decreases with increasing the applied voltage. Morphology of the fibres could be well controlled if electrospinning conditions and operating parameters were correctly selected. To understand a complex electrospinning process comprehensively, more detailed investigations need to be done in order to take more specific polymer and solvent properties into consideration. Acknowledgements The project has been supported by the National Natural Science Foundation of China (contract no and ), the Important Project of Ministry of Education (contract no ). References Chang, C., Fuh, Y.K. and Lin, L. (2009) A direct-write piezoelectric PVDF nanogenerator, Transducers 2009, Dever, CO, USA, pp Ding, Y., Zhang, P., Long, Z., Jiang, Y. and Xu, F. (2008) Preparation of PVdF-based electrospun membranes and their application as separators, Science and Technology of Advanced Materials, Vol. 9, No. 1, p Fong, H., Chun, I. and Reneker, D.H. (1999) Beaded nanofibers formed during electrospinning, Polymer, Vol. 40, No. 16, pp Formhals, A. (1934) Process and apparatus for preparing artificial threads, US Patent 1,975,504. Gregorio, R., Jr. and Borges, D.S. (2008) Effect of crystallization rate on the formation of the polymorphs of solution cast poly(vinylidene fluoride), Polymer, Vol. 49, No. 18, pp Greiner, A. and Wendorff, J. (2007) Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angewandte Chemie International Edition, Vol. 46, No. 30, pp Holmes-Siedle, A.G., Wilson, P.D. and Verrall, A.P.A. (1984) PVdF: an electronically-active polymer for industry, Materials & Design, Vol. 4, No. 6, pp Huang, Z.M., Zhang, Y.Z. and Kotaki, M. (2003) A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Composites Science and Technology, Vol. 63, No. 15, pp Li, X.G., Kresse, I., Xu, Z.K. and Springer, J. (2001) Effect of temperature and pressure on gas transport in ethyl cellulose membrane, Polymer, Vol. 42, No. 16, pp

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