Effect of Solvent and Processing Parameters on Electrospun Polyvinylpyrrolidone Ultra-fine Fibers

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1 436 Chiang Mai J. Sci. 2015; 42(2) Chiang Mai J. Sci. 2015; 42(2) : Contributed Paper Effect of Solvent and Processing Parameters on Electrospun Polyvinylpyrrolidone Ultra-fine Fibers Thammasit Vongsetskul*[a], Thanaphorn Chantarodsakun [a], Preeyaphorn Wongsomboon [a], Ratthapol Rangkupan [b,c] and Pramuan Tangboriboonrat [a] [a] Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand. [b] Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330, Thailand. [c] Center of Innovative Nanotechnology, Chulalongkorn University, Bangkok 10330, Thailand. *Author for correspondence; thammasit.von@mahidol.ac.th Received: 4 March 2013 Accepted: 20 October 2014 ABSTRACT Electrospun polyvinylpyrrolidone (PVP) (MW ~ g/mol) ultra-fine fibers with diameters from 200 to 1400 nm were successfully fabricated from the 20 %w/w PVP in the mixtures of ethanol and water. The effects of solvent compositions, the applied voltages (10, 12, 14 and 16 kv) and the electrical polarities (both negative and positive) on their diameters and morphology were investigated. Scanning electron micrographs revealed that, to obtain neat fibers with small diameters, the positive polarity and high potential should be applied. In addition, the fiber diameter increased with increasing the amount of ethanol. Keywords: electrospinning, ethanol, fiber, polyvinylpyrrolidone, water 1. INTRODUCTION Electrospinning, which literally means spinning the fibers using the electrostatic force, is one of the simplest methods for fabricating ultra-fine fibers from either polymer solution or polymer melt. This technique possesses several advantages such as repeatability and controlling on fiber dimension with low fabricating cost. In this process, a continuous strand of a polymer fluid was drawn through a spinneret by a high electrostatic force. The obtained fibers are randomly deposited on a grounded collector as a non-woven mat. The interesting characteristics of ultra-fine fibers are high surface area to mass or volume ratio and high porosity. Since ultra-fine fibers are easy to functionalize, they are suited for a wide variety of applications [1-3]. Among various types of polymers used in the fabrication of electrospun nanofibers [4-10], polyvinylpyrrolidone (PVP), a vinylbased polymer in which the pendant group is the five-membered amide ring, is frequently chosen. The chemical structure of PVP is shown in Figure 1. PVP was first patented with good adhesion, good complexation, low toxicity, biocompatibility and good solubility in non polar and polar solvents. Hence, it has been used in various fields such as food, cosmetics, pharmaceuticals, adhesives, paints, detergents and energy storages [11].

2 Chiang Mai J. Sci. 2015; 42(2) 437 Figure 1. Chemical structure of PVP. The electrospun PVP nanofiber was successfully prepared for the first time in 2001 [12]. Despite the simplicity of this technique, several factors such as sol, processing and ambient parameters affect the morphology and diameters of PVP fibers and were investigated to understand the relation between them. For example, the relationship between the solvent compositions (ethanol/n,n-dimethylformamide (DMF), dichloromethane/dmf mixtures, and ethanol) and the morphology of the obtained PVP nanofibers was studied [13]. The PVP fibers prepared in mixed dichloromethane/ DMF solvents had a bead-on-a-string shape whereas smooth fibers were obtained with ethanol. Cylindrical PVP fibers with a diameter of 20 nm were fabricated in 50:50 (w/w) mixed DMF/ethanol solvent; on the other hand, helical-shaped ones were produced from 20 wt % PVP solution in a 50:50 (w/w) mixed ethanol/dmf solvent. Five years later, the effect of selected solvents (methanol, ethanol, 2-propanol, 1, 2-dichloroethane, water, chloroform, and dichloromethane) on the diameter of the electrospun PVP nanofibers was investigated [14]. The results revealed that methanol was the best solvent, providing fibers with an optimal morphological appearance and a small fiber diameter. Mixing an unspinnable solvent with a solvent having high dielectric constant, low surface tension and low viscosity could increase the electrospinnability of the solution. In the same year, the influence of ambient parameters (both temperature and humidity) on the average fiber diameter was reported that the increase in the humidity resulted in the decrease in the average diameter of the PVP nanofibers [15]. Also, by varying the rotation speed of the collector made of thin upright copper needle from round/min, the morphology of electrospun PVP (molecular weight; MW of g/mol) nanofibers prepared from 10% w/v PVP ethanolic solution and 25 kv (applied voltage) was changed from non-helical to the tight-helical fibers [16]. Moreover, some researches focused on the application of electrospun PVP fibers such as a template for producing aligned ultralong and diameter-controlled silicon nanotubes by hot wire chemical vapor deposition [17]. The length of the tubes was about 4 mm whereas their inner diameter and wall thickness were and nm, respectively, depending on the coating time. The tube wall comprised nanoparticles or nanopillars. The inner surface of the wall was smoother than the outer surface of the wall. The PVP ultrafine fibers were produced from concentrated PVP solutions (35% w/v PVP in ethanol) by a coaxial electrospinning process [18]. The results revealed that the type of sheath solvent (mixtures of ethanol and dimethylacetamide (DMAc)) and the sheath/core solvent flow rate ratio were two critical factors that determine the quality of produced fibers. High quality PVP nanofibers with diameters of 130±10nm with homogeneous structures and smooth surfaces were also fabricated using 12% w/v

3 438 Chiang Mai J. Sci. 2015; 42(2) PVP K60 and a mixed solvent of acetone, ethanol and DMAc in the ratio of 3:1:1(v/v/ v) by a modified coaxial electrospinning process [19]. Lastly, the structural nanocomposites providing dual drug release were generated using a combination of electrospraying and electrospinning. Ketoprofen (KET), PVP and Eudragit L were used as a model drug, the fiber sheath and the fiber core. It was found that the uniform nanofibers with distinct core-sheath nanostructures and diameters of 0.64 ± 0.21 μm were produced. KET was homogeneously distributed in both the sheath and core of the fibers. In vitro dissolution tests showed that the core-sheath fibers provided dual drug release profiles with an immediate release of 35.1% in acidic solutions and sustained release of 62.2% in a ph 6.8 phosphate buffer [20]. Even though the effects of solvents on the morphology and diameter of electrospun PVP ultra-fine fibers were studied [13-15], there is no report on the preparation of PVP ultra-fine fibers when the mixed solvent comprises of water and ethanol, common solvents with low toxicity. Hence, in this contribution, the effect of the solvent compositions, applied voltages and electrical polarity on the morphology and diameter of electrospun PVP ultra-fine fibers, prepared from ethanol/water mixtures, was studied and reported. 2. MATERIALS AND METHODS 2.1 Materials PVP with MW of g/mol from Sigma-Aldrich (USA) and absolute ethanol from LabScan (Thailand) were used without further purification. Deionized water (2A grade) was applied throughout this work. 2.2 Preparation of PVP Spinning Mixtures and Fiber Mats PVP powders (2 g) were dissolved in absolute ethanol:deionized water (10 ml) with the ratios of 100/0, 25/75, 50/50, 75/25 and 0/100 %v/v, under magnetic stirring for 1 h at room temperature. Electrospinning of the as-prepared 20% w/w PVP solutions was carried out in room conditions (25 ± 1 C, relative humidity of 71 ± 3%) by connecting the emitting electrode of positive or negative polarities from a Gamma high-voltage power supply (D-ES50PN-20W/QAM, FL, USA). Each solution was filled in a standard 25-ml syringe. The open end was attached to a nozzle, a blunt guage-18 stainless steel needle (OD = 0.91 mm). The grounding electrode and fiber collector were an aluminum foil. The schematic diagram for electrospinning apparatus used in this experiment is shown in Figure 2. Figure 2. Experimental setup for electrospinning process. The applied voltages were varied from 10, 12, 14 to 16 kv. A distance between the nozzle tip and the ground collector was 10 cm. The feed rate of the solutions was 1.2 ml h -1. The collection time was 15 min for observing the as-spun fiber morphology. 2.3 Characterizations of PVP Fiber Mats and Spinning Solutions Fiber morphology was observed under a scanning electron microscope

4 Chiang Mai J. Sci. 2015; 42(2) 439 (SEM) (S-2500, Hitachi, Japan). Each mat was sputtered (E-102, Hitachi, Japan) with a thin gold layer prior to SEM observation. Diameter of the individual fiber was measured directly from the SEM images using a Photoshop 5.0 (n 50). The solution viscosity was determined by a Brookfield digital viscometer (DV-II, Brookfield Engineering Laboratories Vertriebs, Germany) at 25 C. The mixture conductivity was determined with a conductivity meter (ThermoOrion 105, Orion Research Inc., USA). 920, 620, 550 and 490 nm when the applied potentials were 10, 12, 14 and 16 kv, respectively. The relationship between the fiber diameters and the applied potentials is shown in Figure RESULTS AND DISCUSSION 3.1 Effect of Applied Electrical Potentials To study the effect of applied potentials on the fiber morphology, the PVP solution in 50/50 ethanol/water and positive polarity were used. The SEM images of the fibers as a function of the applied potentials varying from 10, 12, 14 to 16 kv are presented in Figure 3. The SEM images reveal that, besides the smooth surface, the fiber diameters were Figure 3. SEM images of PVP fibers prepared from 20 wt % PVP in 50/50 ethanol/water and positive polarity. Figure 4. The relationship between the fiber diameters and the applied potentials for the fibers fabricated from 20 wt % PVP in 50/50 ethanol/water and positive polarity. The results showed that, with increasing the applied potentials, the fiber diameters decreased from 920 to 490 nm due to the increase in the electrostatic force, pulling the polymer strand from the spinneret to the fiber collector and the Coulombic repulsion force, stretching the polymer charged jet. 3.2 Effects of The Composition of Mixed Solvents and Electrical Polarity The average diameters of the fibers fabricated at ±16 kv as a function of %ethanol in the mixed solvent are displayed in Figure 5. Figure 5 indicates that the average fiber diameter linearly increased with increasing the amount of ethanol in the mixed solvent. The increasing rate of fiber diameter under the positively applied potential was approximately 7 nm/%ethanol. This effect

5 440 Chiang Mai J. Sci. 2015; 42(2) Figure 5. The average diameters of PVP fibers fabricated at ±16 kv as a function of the amount of ethanol in the mixed solvent. was due to the rate of solvent evaporation and the conductivity and viscosity of the spinning mixtures. Since ethanol evaporated faster than water (boiling points of water and ethanol = 100 and 78 o C, respectively), the spinning mixtures containing higher amount of ethanol had less time for the electrostatic force to pull and stretch the polymer strand before reaching the fiber collector. In addition, the fiber diameter fabricated by the positive polarity was less than that done by the negative one when the composition of the mixed solvent and the applied potential were fixed because of the difference in the electrostatic forces acting on the polymer droplet. When either negative or positive polarities were applied to the spinneret, the number of electrons in the polymer solution would be either increase or decrease, respectively [21]. Thus, PVP chains, the electron-rich species, were attracted to the positively charged spinneret rather than the negatively one. Therefore, the Coulombic force could interact on the polymer charged jet more when the positive voltage was applied. The effect of the mixed solvent composition on the mixture conductivity is depicted in Figure 6. Figure 6. The conductivity of PVP spinning mixtures versus the amount of ethanol in the mixed solvents. Figure 6 reveals that increasing the ethanol concentrations led to the conductivity reduction. This suggests that the number of free ions decreased with increasing the amount of added ethanol. Hence, the electrostatic force, responsible for pulling the polymer strand to the fiber collector, interacted on the highly conductive solution rather than the lowly one. The influence of the mixed solvent compositions on the mixture viscosity is shown in Figure 7. Figure 7. The viscosity of PVP spinning mixtures plotted with the amount of ethanol in the mixed solvents.

6 Chiang Mai J. Sci. 2015; 42(2) 441 Figure 7 shows that increasing the ethanol concentrations led to the increase in the viscosity. However, when the absolute ethanol was the solvent, the viscosity decreased again. Since the viscosities of water and ethanol at 25 C are 8.99 and 12.6 cps, respectively, therefore, the mixture viscosity increased with increasing the amount of ethanol except the absolute ethanol mixture. Another reason concerns the hydrogen bonding between PVP chains and solvent molecules. Water could form the hydrogen bond with the nitrogen and hydrogen atoms of the amide pendant groups much better than ethanol could. Hence, the strong hydrogen bonds existed in all the spinning mixtures except the pure ethanol one. Therefore, the viscosity of the pure ethanol mixtures was exceptionally low. 4. CONCLUSION Electrospun PVP ( g/mol) ultra-fine fibers with diameters of nm were successfully fabricated from the 20 %w/w PVP in the mixtures of ethanol and water. The positive polarity was more efficient to produce the PVP fibers than the negative one. The increase in the applied voltage and the amount of ethanol in the mixed solvent resulted in the decrease and increase of fiber diameters, respectively. As prepared from the biocompatible polymer and low toxic solvents, these fibers have potentials for the applications in medicine and biology in the future. ACKNOWLEDGEMENTS Research grants from The Thailand Research Fund (TRF)/Mahidol University (TRG ); 317 Thailand Ministry of Science and Technology; Faculty of Science, Mahidol University to T.V.; and from TRF/Commission on Higher Education to P.T. (RTA ) are gratefully acknowledged. REFERENCES [1] Zeng J., Xu X., Chen X., Liang Q., Bian X., Yang L. and Jing X., Biodegradable electrospun fibers for drug delivery, J. Control. Release, 2003; 92: DOI /S (03) [2] Wang X., Kim Y.G., Drew C., Ku B.C., Kumar J. and Samuelson L.A., Electrostatic assembly of conjugated polymer thin layers on electrospun nanofibrous membranes for biosensors, Nano Lett., 2004; 4(2): DOI /nl034885z [3] Klein E., Affinity membranes: A 10- year review, J. Membr. Sci., 2000; 179: DOI /S (00) [4] Krissanasaeranee M., Vongsetskul T., Rangkupan R., Supaphol P. and Wongkasemjit S., Preparation of ultra-fine silica fibers using electrospun poly(vinyl alcohol)/silatrane composite fibers as precursor, J. Am. Ceram. Soc., 2008; 91(9): DOI /j x. [5] Sutjarittangtham K., Jaiturong P., Intatha U., Pengpat K., Eitssayeam S. and Sirithunyalug J., Fabrication of natural tapioca starch fibers by a modified electrospinning technique, Chiang Mai J. Sci., 2014; 41(1): [6] Kampeerapappun P. and Phanomkate N., Slow release fertilizer from coreshell electrospun fibers, Chiang Mai J. Sci., 2013; 40(4): [7] Suwantong O., Pankongadisak P., Deachathai S. and Supaphol P., The potential of electrospun poly(l-lactic Acid) fiber mats containing a crude Garcinia dulcis extract for use as wound dressings, Chiang Mai J. Sci., 2013; 40(3): [8] Kampeerapappun P., Preparation characterization and antimicrobial activity of electrospun nanofibers from

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