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The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

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POLY (VINYL ALCOHOL)/POLYANILINE (PVA/PANI) CONDUCTIVE NANOFIBERS BY ELECTROSPINNING

N. KIZILDAG1, N. UCAR1, M.E.OZTOKSOY2, H.GARMESTANI3, Y.WANG3, K.DAHMEN3

1Istanbul Technical University, Faculty of Textile Technologies and Design, Inonu Str. No: 65 Gumussuyu - Istanbul, TURKEY

2 Istanbul Technical University, Polymer Science, Maslak - Istanbul, TURKEY 3Georgia Institute of Technology, Metarial Science & Engineering, Atlanta,GA, USA

[email protected]

Abstract: Conductive nanofibers are attracting a growing interest due to the wide range of potential applications such as electromagnetic interference shielding, electrostatic dissipating, antistatic applications, gas sensors, tissue engineering scaffolds, biomedical applications, nanoelectronic devices etc. Polyaniline (PANi), which is one of the most widely studied conducting polymers because of its simple synthesis, doping/dedoping chemistry, low cost, environmental stability, controllable electrical conductivity (between 10–2 to 102 S/cm. in its doped state) is frequently used to impart conductivity to polymeric materials. In this study, conductive nanofibers of poly (vinyl alcohol) and camphorsulfonic acid doped polyaniline (PVA/PANi) were successfully produced by electrospinning. Scanning Electron Microscope (SEM) ensured the nanofiber formation. Fourier-Transform Infrared Spectroscopy (FTIR) analysis confirmed the presence of both PVA and PANI in the nanofiber structure. Conductivity measurements showed that PVA/PANi nanofibers had an electrical conductivity of 2.3x10-3 S/cm. which is in the semiconductivity range. Keywords: conductive, electrospinning, nanofiber, polyaniline, polyvinyl alcohol. 1. Introduction Poly(vinyl alcohol) (PVA) is a hydrophilic, semicrystalline polymer that attracts much attention because of its good chemical resistance, good thermal stability, good physical properties, excellent biocompatibility, and inexpensiveness [1]. Several different forms of PVA such as gels, films, and nanowebs have been produced through different methods. Especially the availability of PVA nanofibers with high surface area to volume ratio and highly porous three dimensional structure through a simple process of electrospinning introduced a new set of potential uses such as immobilization membranes for cellulase [2], drug delivery membranes [3,4], wound-dressings [5,6], filtration medium for oil/water emulsion [7], and scaffolds for tissue engineering applications [8]. On the other hand, polyaniline (PANi) is one of the most intensively investigated conductive polymers due to its environmental stability, low cost of raw material, ease of synthesis and good compatibility with other polymer supports [9-13]. It is an electrically conducting polymer having a spatially extended π bonding system, which accounts for their intrinsic semi-conducting nature [14]. Besides, it is concluded in the literature that PANi is biocompatible showing cell and tissue compatibility in vivo and in vitro systems [15,16]. Since the use of PANi is restricted by its’ poor mechanical properties, poor solubility and inability to process it by conventional methods; many attempts have been made to prepare composites with improved processability and mechanical properties while maintaining the inherent properties of the conducting polymer [10]. There are many studies in the literature about the use of PANi with other polymers to obtain conductive polymer composites. Pan et al. [9] developed polyacrylonitrile (PAN) and PANi composite films and investigated the structural and electrical properties. When the mass fraction of dodecylbenzenesulphonic acid (DBSA) doped PANi (PANi.DBSA) was 5%, the conductivity of the composite increased from 10-13S/cm to 10-3S/cm. Zhang et al. [17] fabricated uniform composite films of nanostructured polyaniline (PANi) by blending with PVA as a matrix. The composite film with 16% PANi–b-NSA had a conductivity in the range of 10-2S/cm. Bhadra et al. [18] prepared PVA-PANi composite films chemically in inorganic acid medium at different ratios of their monomer units. The results showed the definite dependence of various parameters on the PANi-PVA ratio in the blend. FTIR spectra showed some peaks which ascertained chemical interlinking of PANi and PVA in the blends. The conductivity of the films were found to be between 0.90 to 1.80 S/cm.


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The literature suggest that electrical stimuli in electrospun fibers may enhance neurite extension, cell adhesion, proliferation, differentiation, and migration, etc. [19-23]. The introduction of biocompatible electroactive materials into a biological system allow the local delivery of an electrical stimulus to a specific site to foster cell growth and repair damaged tissue besides providing a physical substrate for cell growth and tissue repair [16]. In this regards, PANi-based electrically conducting polymer microfibers and nanofibers are attractive substrates for some tissue engineering applications. Prabhakaran et al. [24] fabricated a Poly-L-lactide (PLLA)/PANi scaffold which was conductive in nature by electrospinning and performed electrical stimulation of nerve stem cells seeded on the electrospun nanofibers. In vitro electrical stimulation of the nerve stem cells cultured on PLLA/PANi scaffolds resulted in extended neurite outgrowth compared to the cells grown on non-stimulated scaffolds. In addition, the potential use of electrospun nanofibers derived from the blend of camphorsulfonic acid-doped PANi (PANi.CSA) and gelatin in tissue engineering was also investigated by Li et al. [21]. The conductivity values obtained with the incorporation of PANi into gelatin with the blend ratios of 15:85, 30:70, 45:55, 60:40 (PANi:gelatin) were between 1.0×10-2 to 2.0×10-2S/cm. Cell culture results showed that the nanospun fibers were biocompatible, supported cell attachment and proliferation. McKeon et al. [25] developed several poly(D,L-lactide)(PDLA) and PANi blends with different blend ratios and electrospun to create a biodegradable, biocompatible, and electrically conductive nanofibrous scaffolds. 75:25 PDLA/PANi scaffold had a conductivity of 4.37×10-2 S/cm. Cellular data provided information that all the scaffolds supported cell adhesion and proliferation. Another area that make use of the conductive electrospun nanofibers is filtration. Antistatic filter media are used in a wide range of industrial, chemical, metallurgical, mineral and agricultural applications where the dust and processes tend to build static. Antistatic filter media not only remove the static electricity generated but also ensure the complete dust cake release [26]. As can be seen from the literature, the studies on the blends of PANi with PVA have generally been carried out with a focus on film preparation and no studies regarding the production of electrospun PVA/PANi nanofiber mats were seen. In this paper, we describe our study on electrospinning a novel blend of conductive camphorsulfonic acid doped polyaniline and poly (vinyl alcohol) (PANi/PVA). 2. Experimental PVA, distilled water, PANi, camphorsulphonic acid (CSA), N,N-dimethylformamide (DMF), were used in this study. Electrospinning solutions were prepared according to the following procedures: PVA electrospinning solution was prepared by dissolving 8g PVA in 92g distilled water (8 w/w%) using magnetic stirrer at 70ºC for 3 days. For the preparation of PVA/PANi electrospinning solution, firstly, PVA solution was prepared according to the above procedure. Separately, 100 mg PANi and 160 mg CSA were dissolved in 10 g DMF by a magnetic stirrer at 40 ºC for 2 days, allowing the protonation of polyaniline emeraldine base (PANi-EB) to a polyelectrolyte form. PANi-EB gets doped to polyaniline emeraldine salt (PANi-ES) form, threby the polymer undergoes an insulator to metal transition with change in conformation of the polymer backbone accommodating this electronic transformation. Then the solution was filtered to remove the undissolved materials and impurities. Subsequently the PVA solution was added into the PANi solution to obtain the required ratio of 80:20 w/w% PVA solution: PANi solution (aproximately 92:8 w/w% PVA: PANi). Similar procedure was followed for the preparation of 60:40 w/w% PVA solution:PANi solution (aproximately 84:16 w/w% PVA: PANi). Electrospinning of nanofibers was performed on a horizontal electrospinning setup a schematic design of which is illustrated in Figure 1.

Figure 1. Schematic of electrospinning setup used to producing nanofiber mats.

It consisted of a precisely-controlled syringe pump, a high voltage power supply capable of 0–25 kV, and a grounded collector. Upon applying a high voltage, a fluid jet was ejected from the tip of the nozzle. As the jet accelerated toward the collector, which was placed at 10 cm from the nozzle, the solvent evaporated and nanofibers were collected on the conductive collector. The electrospinning of PVA was conducted with a applied voltage of 23 kV and a feeding rate of 1.1 mL/h using a stationary collector. The feed rate was set to 1.0 ml/h for the elctrospinning of 92:8 PVA:PANi nanofibers. Electrospinning of 84:16 PVA:PANi was conducted with a applied voltage of 15 kV, a distance of 10 cm, feed rate of 0.9 mL/h and with rotating collector. The average diameter of the electrospun fibers was observed with a scanning electron microscope (SEM). The diameter of 50 randomly selected nanofiber mats were measured on SEM photomicrographs


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and analyzed using Image Analysis Software to obtain the average fiber diameter. Fourier transform infrared (FTIR) spectrum was recorded in the spectral region of 400–4,000 cm-1 for pure PVA, 92:8 w/w% PVA/PANi and 84:16 w/w% PVA/PANi on a FTIR spectrophotometer. Conductivity measurements were carried out by GW INSTEK conductivity meter which had a special prob for testing textile surfaces. 3. Results and Discussion Results obtained are discussed below with relevant figures and tables. Morphology Figure 2 shows the SEM images of electrospun PVA, 92:8 w/w% PVA/PANi and 84:16 w/w% PVA/PANi nanofibers produced by electrospinning.

a. b. c.

Figure 2. SEM images of electrospun a. PVA; b. 92:8 w/w% PVA/PANi; c. 84:16w/w% PVA/PANi nanofibers The effect of PANi addition to fiber diameter is presented in Table 1. Table 1. Average nanofiber diameter and coefficient of variation values

Diameter (nm) CV%

100 % PVA 266.57 35.09

92:8 w/w% PVA/PANi 109.76 30.47

84:16 w/w% PVA/PANi Spraying together with very few fiber The fiber diameter decreased with the addition of PANi in agreement with previous studies [13,24,27]. By incorporating PANi to PVA, the net charge density of the solution might have increased favoring the formation of fibers with smaller diameters. The presence of PANi has a similar effect to the additon of salt to electrospinning solutions. It not only affects the viscosity but also the ionic conductivity and the dielectric constant of the solution [28]. Higher net charge density results in higher stretching of fibers, thus reducing the fiber diameter of PVA/PANi fibers. Li et al. produced gelatin nanofibers with diameters of 803±121 nm and gelatin/PANi (60/40) with much smaller diameters of 61±13 nm [13]. 8% PANi addition resulted in the formation beaded nanofibers. When the literature is examined [24], it can be said that the properties of matrix (main) polymer such as molecular weight, viscosity etc. are also important for the percentage value of PANi that can be added into the blend which the bead free nanofiber will be obtained. FTIR Spectra FTIR spectra of electrospun PVA, 92:8 w/w% PVA/PANi and 84:16 w/w% PVA/PANi nanofibers are presented in Figure 3. On FTIR spectrum, the characteristic peaks around 3300 cm-1 arising from the stretching vibration of N-H group of PANI and O-H group of PVA [29,30]. As can be seen from the spectra, peak intensity around 3300 cm-1 change as the amount of PVA solution decreases due to an increase of PANI solution and this change shows the presence of PANI. The characteristic peaks around 2940 cm-1 arising from the stretching vibration of aromatic C-H group of PANi and aliphatic C-H group of PVA can be seen from this FTIR graph [29,30]. The peaks at about 1580 and 1425 cm-1 (marked as a and b on the figure) are the absorption of quinon and benzene rings of PANI, respectively [30-32]. It is notable that the intensity of these peaks increased as PANi content in the composite nanofiber increased. From FTIR data it is clear that new peaks corresponding to PANi are observed in the PVA/PANi blend nanofibers. The appearance of new peaks along with changes in existing peaks directly indicated the blend formation in aggreement with previous studies [30].


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Figure 3. FTIR spectra of PVA (3), 92:8 w/w% PVA/PANi (1), and 84:16w/w% PVA/PANi (2) nanofibers Conductivity The electrical conductivity of the composites depends on the extent of dispersion, geometry and interactions of the components [33]. The effect of PANI addition on the electrical conductivity of the electrospun nanowebs are presented in Table 2. Table 2. Conductivity measurements of nanowebs

Conductivity

PVA nanoweb [34] 1.25*10-15

92:8 w/w% PVA/PANi nanoweb 2.30*10-3

84:16 w/w% PVA/PANi nanoweb 1.10*10-3 From Table 1, it is seen that 92:8 w/w% PVA/PANi nanoweb showed an enhanced electrical conductivity of 2.3*10-3 S/cm compared to 1.25 ×10-15 S/cm for neat PVA nanoweb [34]. With the increase in the PANi content, from 8% to 16%, decrease in electrical conductivity was observed contrary to the expectations. As also discussed by Mohd et al., occurence of the beads might have contributed to the poor mobility of electrons [34]. Another reason might have been the structural changes occurred in the nanowebs with the increase of the PANi content. When the 84:16 w/w% PVA:PANi solution was electrospun, instead of electrospinning, electrospraying was formed and as a result of this a film-like structure was formed on the collector. For comparison, films were prepared by film casting method using the same solutions. The conductivity results of films are presented in Table 3. Table 3. Conductivity of films

Conductivity

92:8 w/w% PVA/PANi film 7.60*10-5

84:16 w/w% PVA/PANi film 6.90*10-5 When the conductivity of the films and nanowebs were compared, the conductivity of the nanowebs were seen to be higher. The possible reason of this might have been the orientation of the macromolecules due to the stretching during electrospinning. According to Wang et al. [35], who reported a higher conductivity for


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drawn fibers than undrawn fibers, chain alignment under uniaxial extension may be responsible for improvement of the conductivity in the drawn fibers. Our result was in contrary to the findings of Norris et al. [36], who reported lower conductivity for electrospun PANi/PEO nanowebs than films. They suggested that the difference between the films and fibers was due to the difficulties in measuring the conductivity of the nanoweb, which had much lower 'fill factor', with the four-probe method and that it was reasonable to expect the conductivity of a single fiber to be higher than the nanoweb and equal to the conductivity of the cast film. 4. Conclusions In this study,

• conductive nanofibers of poly (vinyl alcohol) and camphorsulfonic acid doped polyaniline (PVA/PANi) were successfully produced by electrospinning. Scanning Electron Microscope (SEM) ensured the nanofiber formation.

• Fourier-Transform Infrared Spectroscopy (FTIR) analysis confirmed the presence of both PVA and PANI in the nanofiber structure.

• 8% PANi addition resulted in the nanofiber diameter decrease from 266.57 nm to 109.76 nm. • Conductivity measurements showed that PVA/PANi nanofibers had an electrical conductivity of

2.3x10-3 S/cm. which is in the semiconductivity range. • Comparison between the conductivities of nanowebs and films showed that the conductivity of the

nanowebs were much higher than the conductivity of the films. • With the increase in the content of PANi from 8% to 16%, the conductivity of the nanowebs

unexpectedly decreased due to the increase in bead formation and formation of film-like structure as a result of electrospraying.

In future work, optimization of process parameters for the production of bead-free fibers and production of nanofibers from solutions with a wide range of concentrations for the determination of the percolation threshold will be studied. Acknowledgement The support of Istanbul Technical University through Project 36165 is acknowledged. References [1] Pitt, S. & Surawut, C.: On the Electrospinning of Poly(vinyl alcohol) Nanofiber Mats: A Revisit, Journal of

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