Optimization of the electrospinning conditions for preparation of nanofibers from polyvinylacetate (PVAc) in ethanol solvent

Optimization of the electrospinning conditions for preparation of nanofibers from polyvinylacetate (PVAc) in ethanol solvent

Available online at www.sciencedirect.com Journal of Industrial and Engineering Chemistry 14 (2008) 707–713 www.elsevier.com/locate/jiec Optimizatio...

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Available online at www.sciencedirect.com

Journal of Industrial and Engineering Chemistry 14 (2008) 707–713 www.elsevier.com/locate/jiec

Optimization of the electrospinning conditions for preparation of nanofibers from polyvinylacetate (PVAc) in ethanol solvent Ju Young Park a, In Hwa Lee a,*, Gwi Nam Bea b a

Department of Environmental Engineering, BK21 Team for Biohydrogen Production, Chosun University, Gwangju 501-759, Republic of Korea b Center for Environmental Technology Research, Korea Institute of Science and Technology, Seoul 130-650, Republic of Korea Received 9 July 2007; accepted 27 March 2008

Abstract In order to fabricate polyvinylacetate (PVAc) fiber by electrospinning, we have been examined electrospun polyvinylacetate (PVAc) under various conditions after dissolving it in ethanol solution. As the concentration of spinning solution increased, the diameter of the electrospun PVAc fiber increased. At the concentration lower than 10 wt.%, beads were formed while over the 25 wt.%, distinct fiber was not observed. At the tipcollector distance (TCD) of 7.5 cm or less, the jet of spinning solution was unstable and the fiber diameter decreased. On the other hand, at the TCD of 10 cm or more, the strength of electric field became too weak and the fiber diameter increased. As the flow rate of spinning solution increased, the fiber diameter increased and at the flow rate of 300 ml/min or more, it increased sharply. For 15 wt.% PVAc, the fiber diameter decreased as the applied voltage increased. At a high-applied voltage, however, charge acceleration caused the spinning solution not to be separated and thus the fiber diameter increased. As a result of dissolving PVAc in ethanol and electrospinning it in the following conditions, a fiber with the diameter of about 700 nm was spun: the concentration of 15 wt.%, the TCD of 10 cm, the spinning solution flow rate of 100 ml/min, and the applied voltage of 15 kV. Crown Copyright # 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. Keywords: Electrospinning; Polyvinylacetate (PVAc); Nanofiber

1. Introduction Vinyl polymers such as polyvinylacetate (PVAc), polyvinylalcohol (PVA) and PVP are widely used in the development of oral drug delivery products [1]. PVAc is a homopolymer synthesized from vinylacetate monomer via a free-radical polymerization technique. Although water-insoluble, it is slightly hydrophilic and able to absorb water to a slight extent. PVAc has been reported to be effective in controlling the release of various chemical entities, including theophylline [2,3], nifedipine [4] and chlorpromazine hydrochloride [5]. In electrospinning, high voltage is applied to polymer fluid such that electric charge is induced within the fluid. When the charge within the fluid reaches a critical amount, a fluid jet will erupt from the droplet at the tip of the needle, resulting in

* Corresponding author at: Department of Environmental Engineering, BK21 Team for Biohydrogen Production, Chosun University, Gwangju 501-759, Republic of Korea. Tel.: +82 62 230 6627; fax: +82 62 234 6627. E-mail address: [email protected] (I.H. Lee).

formation of a Taylor cone. The electrospinning jet will travel towards the region of lower potential, which in most cases is a grounded collector [6]. The parameters affecting electrospinning may be broadly classified into polymer solution parameters and processing conditions. Polymer solution parameters include molecular weight, solution viscosity, surface tension, solution conductivity and dielectric constant. The properties of the polymer solution have the most significant influence in the electrospinning process and the electrospun fiber morphology. The surface tension has a role partly in the formation of bead along the fiber length. The viscosity of solution and its electrical properties will determine the extent of elongation of the solution. This will in turn have an effect on the diameter of the electrospun fiber [6]. Processing conditions include the applied voltage, flow rate, temperature of the solution, type of collector, diameter of needle and distance between the needle tip and collector (TCD). These parameters have certain influences on fiber morphology although they are less significant than the solution parameters [6] (Table 1).

1226-086X/$ – see front matter. Crown Copyright # 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2008.03.006

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Table 1 The properties of solvent. Type of solvent Solubility bp (8C) Dielectric constant Dipole moment in Debyes Type

Ethanol Poor solvent 78.3 24.55 1.71 Protic

In this study, PVAc dissolved in ethanol solution was electrospun under various conditions. In this paper, we investigate the morphology of the PVAc fibers made by electrospinning process. The important parameters in the morphology of electrospun PVAc fibers such as concentration, applied voltage, tip-collector distance (TCD) and flow rate are investigated experimentally. Also, from the results, we determined the optimal conditions for electrospinning of PVAc to prepare homogeneous and fine fibers. 2. Experiments 2.1. Materials Polyvinylacetate (PVAc) (Mn 140,000) was purchased from Aldrich, and ethanol (EP grade) was purchased from Junsi and used without further purification.

Fig. 1. Schematic and photo of electrospinning apparatus.

Fig. 2. Morphology of electrospun fibers at various PVAc concentrations: (a) 5 wt.%, (b) 8 wt.%, (c) 10 wt.%, (d) 15 wt.%, (e) 20 wt.%, (f) 25 wt.%, and (g) 30 wt.% (applied voltage: 15 kV; flow rate: 100 ml/min; TCD: 10 cm) (1000).

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Fig. 3. The chemical properties of PVAc solutions dissolved in ethanol at various concentrations. Fig. 6. Average diameters of 15 wt.% PVAc electrospun fibers at various TCDs (applied voltage: 15 kV; flow rate: 100 ml/min).

2.2. Measurement The viscosity of the electrospinning solution was measured with the viscometer (LVDV II+, Brookfield Co., USA) and the conductivity was measured with the conductivity meter (CM11P, TOA Electronic Ltd.). The morphology and diameter of electrospun fiber were determined with the field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi Ltd., Japan). 2.3. Electrospinning process Fig. 4. Average diameters of PVAc electrospun fibers at various concentrations (applied voltage: 15 kV; flow rate: 100 ml/min; TCD: 10 cm).

The schematic of experimental systems used for the electrospinning process is shown in Fig. 1. In the electrospinning process, syringe pump (200 series, KD Scientific Inc., USA) was used as an injector having plat capillary tip with the

Fig. 5. SEM Photographs of 15 wt.% PVAc electrospun fiber at various TCDs: (a) 7.5 cm, (b) 10 cm, (c) 12.5 cm, (d) 15 cm, and (e) 17.5 cm (flow rate: 100 ml/min; applied voltage: 15 kV).

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Fig. 7. SEM photographs of a 15 wt.% PVAc electrospun fiber at various flow rates: (a) 50 ml/min, (b) 100 ml/min, (c) 300 ml/min, (d) 500 ml/min, (e) 800 ml/min, and (f) 1000 ml/min (applied voltage: 15 kV; TCD: 10 cm).

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Fig. 7. (Continued ).

diameter of 0.8 mm and connected to high-voltage DC power supplier generating positive DC voltage up to 50 kV (DC power supply PS/ER 50R06 DM22, Glassman High Voltage Inc., USA). Grounded counter electrode was connected to the collector which was covered with aluminum foil. Positive voltage was applied to the polymer solution between 10 kV and 20 kV with stepwise increase. 3. Results and discussion 3.1. Effect of polymer concentration Fig. 2 shows SEM image of electrospun PVAc fiber according to the concentration of 5 wt.% to 30 wt.% solution. Electrospinning was performed under such conditions as the applied voltage of 15 kV and the spinning solution flow rate of 100 ml/ min, and the TCD of 10 cm. The concentration of polymer solution affects bead formation, bead density, morphology of the electrospun fiber, and diameter of the fiber [6,7]. As shown in the pictures (Fig. 2), beads disappear as the concentration increases. The reason why beads are formed is mainly because the viscosity of the solution at low concentration is relatively lower than the surface tension. Beads are products of instability of the jet under electric field. The viscosity of the solution, charge density and surface tension are main factors known to produce beads [8]. In general, the higher the charge density and the lower the surface tension, bead formation is suppressed [9]. The diameter of the PVAc fiber increased as the concentration increased. At 10 wt.% or lower concentration, bead fibers

were observed and at 25 wt.% or higher concentration, large droplets were formed on the collector even though no fibers were formed. (a)–(c) in Fig. 2 show the morphology of fibers including beads at relatively low polymer concentrations; (c)– (e) show that of constant and regular fibers; while (f)–(g) show that of irregular fibers produced when the viscosity of the electrospinning solution is relatively lower than the surface tension because the polymer concentration is too high [8,10]. Fig. 3 is the graph that shows the chemical properties of PVAc solution at various concentrations. The viscosity of the solution increased as the PVAc concentration increased. It has the characteristic of intermolecular interaction of the polymer solution, so it can be considered as an important factor that affects the morphology of the fiber [11]. As the concentration of the solution increased, the conductivity and surface tension also increased. However, at 20 wt.% or higher concentration, the conductivity tended to decrease a little bit. Fig. 4 shows the average diameter of electrospun fibers at various concentrations. As shown in the picture, the diameter of PVAc fiber increased as the concentration increased. When the PVAc concentration was 10 wt.%, 15 wt.%, 20 wt.% and 25 wt.%, fibers with respective diameters of 302 nm, 711 nm, 1587 nm and 1616 nm were made. 3.2. Effect of TCD Figs. 5 and 6 show the morphology and average diameter of the electrospun fibers at different TCDs. Electrospinning was performed under the same conditions except TCD: 15 wt.% of

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becomes weak, making electrospinning hard [9,13]. When observed with the unaided eyes in this experiment, however, electrospinning was done well within the range of TCD 7.5– 17.5 cm. 3.3. Effect of flow rate

Fig. 8. Average diameter of 15 wt.% PVAc electrospun fiber at various flow rates (applied voltage: 15 kV; TCD: 10 cm).

PVAc concentration, 15 kV of applied voltage, and 100 ml/min of solution flow rate, and TCD was set between the range of 7.5 cm and 17.5 cm. When TCD was 7.5 cm or less, the distance between tip and collector was short that the instability of jet increased and the spinning solution could not be fully stretched, resulting in the increase of fiber diameter. When TCD was 10 cm, the fiber with the smallest average diameter of 711 nm was made but when TCD was bigger than 10 cm, the fiber diameter tended to increase again. It is presumed that if TCD is too big it weakens the strength of electric field, resulting in the increase of fiber diameter. If TCD is too short, electric field becomes so strong that the jet of spinning solution becomes unstable, resulting in the formation of beads. If TCD is too long, on the other hand, the strength of electric field becomes so weak that the fiber diameter increases [10,12]. Unlike the above result, however, no beads were formed in this experiment though the fiber diameter was changed by the change of TCD. In general, if TCD is too short, the solution cannot evaporate easily and thus fully developed fiber cannot be made. On the contrary, if TCD is too long, the electric field formed between tip and collector

Fig. 7 shows SEM photographs made at various solution flow rates ranging from 50 ml/min to 1000 ml/min at the concentration of 15 wt.%. It is known that electrospun fiber changes its morphology according to the flow rate of spinning solution and beads are formed at the flow rate faster than a certain rate [7]. In this experiment, no beads were formed even though the flow rate of spinning solution increased, but it was found that the distribution of fiber diameters was widened. Fig. 8 shows the comparison of average diameters of electrospun fibers made by changing the flow rate of spinning solution within the range of 50–1000 ml/min at the concentration of 15 wt.%. As the flow rate of spinning solution increased, the average diameter also increased. It is thought that as the flow rate increased electrostatic density decreased, as a consequence the fabrication could not fully developed, resulting in the increase of fiber diameter. In our experiment, it was found that the fiber diameter sharply increased at the flow rate of 300 ml/min, and the optimum solution flow rate was determined as 100 ml/min. 3.4. Effect of applied voltage Fig. 9 shows the morphology and average diameter of electrospun fiber at various levels of voltage applied to 15 wt.% PVAc. Electrospinning was done at the solution flow rate of 100 ml/min and TCD of 10 cm. It was shown that as the applied voltage increased, the fiber diameter decreased. At voltages higher than 17.5 kV, the charge could be accelerated, so that there was not enough time for the spinning solution to be developed and thus the fiber diameter increased [8,11,14].

Fig. 9. Effect of applied voltage on 15 wt.% PVAc solution at TCD of 10 cm and the flow rate of 100 ml/min (left: SEM images at low magnification (1000) and right: average diameter of the electrospun fiber).

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Fig. 10. Effect of applied voltage on 20 wt.% PVAc solution at TCD of 10 cm and the flow rate of 100 ml/min (left: SEM images at low magnification (2000) and right: average diameter of the electrospun fiber).

In general, when high voltage is applied during electrospinning, Taylor cone formation becomes stable and columbic repulsive force within the jet of spinning solution makes viscoelastic solution extended. If higher than critical voltage is applied, more charge will drop from the end of the needle due to the acceleration of charge, the Taylor cone becomes unstable [6]. Fig. 10 shows the morphology and average diameter of fiber at various levels of voltage applied to 20 wt.% PVAc. For 20 wt.% PVAc, it was shown that the fiber diameter increased as the applied voltage was increased. Tan et al. [15] have been reported that in case of the polymer concentration of spinning solution is lower region, the fiber average diameter decreased as the applied voltage increased. On the contrary, the concentration of spinning solution move to higher region, the average diameter of fiber increased as the applied voltage increased. In this experiment for the 20 wt.% of PVAc spinning solution, the average diameter of PVAc fiber increased from 1631 nm to 2121 nm as the applied voltage increased from 10 kV to 17.5 kV. 4. Conclusion As a result of electrospinning the polymer solution made by dissolving PVAc in ethanol under various conditions, the fiber with the diameter of about 700 nm was obtained at the concentration of 15 wt.%, TCD of 10 cm, the spinning solution flow rate of 100 ml/min, and the applied voltage of 15 kV.

Acknowledgement This study was supported by research funds from KIST (Korea Institute of Science and Technology). References [1] G.A.G. Novoa, J. Heinamaki, S. Mirza, O. Antikainen, A.I. Colarte, A.S. Paz, J. Yliruusi, Eur. J. Pharm. Biopharm. 59 (2005) 343. [2] W.G. Schmidt, W. Mehnert, H. Fromming, Eur. J. Pharm. Biopharm. 42 (1996) 348. [3] F. Zhang, J.W. McGinity, Drug Dev. Ind. Pharm. 26 (2000) 931. [4] A. Ali, S.N. Sharma, Indian Drugs 33 (1996) 30. [5] T. Niwa, H. Takeuchi, T. Hino, A. Itoh, Y. Kawashima, K. Kiuchi, Pharmacol. Res. 11 (1994) 478. [6] S. Ramakrishna, K. Fujihara, W.E. Teo, T.C. Lim, M. Zuwei, An Introduction Electrospinning and Nanofibers, World Scientific Publishing Company, Singapore, 2005, p. 101. [7] J.P. Jeun, Y.M. Lim, Y.C. Nho, J. Ind. Eng. Chem. 11 (2005) 573. [8] H. Fong, I. Chun, D.H. Reneker, Polymer 40 (1999) 4585. [9] J.A. Matthews, G.E. Wnek, D.G. Simpson, G.L. Bowlin, Biomacromolecules 3 (2002) 232. [10] J.M. Deitzel, J. Kleinmeyer, D. Harris, N.C.B. Tan, Polymer 42 (2001) 261. [11] G.T. Kim, Y.J. Hwang, Y.C. Ahn, H.S. Shin, J.K. Lee, C.M. Soung, Korean J. Chem. Eng. 22 (2005) 147. [12] X.H. Zhong, K.S. Kim, D.F. Fang, S.F. Ran, B.S. Hsiao, B. Chu, Polymer 43 (2002) 4403. [13] B.S. Lee, K.H. Lee, D.K. Lee, B.Y. Park, H.K. Kim, J. Korean Fiber Soc. 40 (2003) 341. [14] H.J. Sim, S.H. Lee, J. Korean Fiber Soc. 41 (6) (2004) 414. [15] S.H. Tan, R. Inai, M. Kotaki, S. Ramakrishna, Polymer 46 (2005) 6128.