Int J Plast Technol DOI 10.1007/s12588-013-9053-9 R E S E A R C H A RT I C L E
Solution electrospinning of styrene-acrylonitrile random copolymer from dimethyl sulfoxide T. Senthil & S. Anandhan
Received: 9 September 2012 / Accepted: 21 October 2013 # Central Institute of Plastics Engineering & Technology 2013
Abstract Electrospinning is an efficient and versatile technique for the fabrication of ultrafine fibers having diameters ranging from nano to sub-micron level for various potential applications. In this study, we have investigated the influence of process and solution parameters, such as solution concentration, flow rate and applied voltage, on the morphology of electrospun poly(styrene-co-acrylonitrile) (SAN) fibers. Morphology and average diameter (Davg.) of the electrospun SAN fibers were characterized by scanning electron microscopy (SEM). The SEM results reveal that concentration, applied voltage and flow rate of solution are strongly associated with formation of defects, such as beads, in the fibers. Ultrafine SAN fibers with Davg. in the range of 96–872 nm were obtained by controlling the experimental parameters. The Davg. of electrospun fibers increased with increasing solution concentration, applied voltage and flow rate. Also, the Davg. exhibits a power law relationship with the solution concentration. Keywords Processing . Copolymer . Fibers . Morphology . Nanotechnology
Introduction Electrospinning is the simplest process for the production of ultrafine polymeric fibers with diameters ranging from 3 nm to 10 μm [1]. It is an effective and inexpensive method, whereby ultrafine polymeric nanofibers are fabricated by applying a high voltage to a polymer solution or melt [2]. This process significantly improves the properties of ultrafine fibers due to their size reduction to nanometer level. Electrospun nanofibers possess excellent characteristics, including superior mechanical performance, very large surface area to volume ratio, flexibility in surface functionalities T. Senthil : S. Anandhan (*) Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Srinivas Nagar, Mangalore 575025, India e-mail:
[email protected] S. Anandhan e-mail:
[email protected]
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and increased aspect ratios [2]. Nanofibers have great promises and variety of functional applications, such as in tissue engineering scaffolds, filtration membranes, protective clothing, sensors, nano-electronic machines and catalysts [3]. An electrospinning setup typically consists of three major components: a high voltage DC power supply, which creates a positive charge on the polymer solution; a controllable syringe pump, which pumps a polymer solution or melt through the spinning needle at a constant flow rate; and a static or rotating collector (Fig. 1). The positive electrode is usually connected to the spinneret and the negative one is attached to the collector. As the voltage is raised on a droplet formed from polymeric solution at the tip of the needle, it stretches and assumes a conical shape (Taylor cone). Once a threshold voltage is applied to the polymer solution, a critical value is obtained with which the electrostatic forces overcome the surface tension and the charged polymer jet evenly spreads over the collector plate [4]. The electrospun fiber diameter, distribution and morphology can be affected either by the polymer solution parameters or process parameters. Process parameters include flow rate, applied voltage, and distance between needle tip and collector; solution parameters include concentration, viscosity and conductivity [5, 6]. Lot of research has been done on the effect of various parameters on the morphology of fibers and spinning process. Zong et al. reported that diameter of electrospun poly(L-lactic acid) (PLLA) nanofibers significantly changed over a wide range of concentration and applied voltage [7]. Lee et al. have also found that polystyrene (PS) fibers formed were without beads at polymer concentration higher than 15 wt.% and the morphology of electrospun polymer fibers depended on the strength of the electric field and needle tip-to-collector distance (TCD) [8]. Choktaweesap and coworkers studied nanofibers from gelatin solutions dissolved in either single or mixed solvent system [9]. Several research groups have studied a number of systems, including poly(methyl methacrylate) (PMMA) [10], poly(ethylene oxide) (PEO) [11] and nylon–11 [12]. They
Fig. 1 Schematic diagram of a typical setup for electrospinning
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observed that the morphology of electrospun fibers were correlated strongly with the nature of solvent, solution concentration and applied voltage. Neo et al. demonstrated the effects of electrospinning parameters on the size and morphology of electrospun zein fibers [13]. They found that solution concentration had the most significant impact on Davg. compared to the others. Recently, Wang et al. systematically investigated the influence of fabrication parameters, such as solution concentration, applied voltage, flow rate, ambient humidity and TCD to fabricate uniform poly(γ-glutamic acid) (PGA) nanofibers [14]. They observed that by changing the solution concentration, the diameter of the electrospun nanofibers can be controlled within the range of 186– 603 nm. Anandhan et al. reported the preparation of ultrafine polybenzimidazole (PBI) nanofibers by electrospinning [15]. They observed a significant change of fiber diameter and morphology by changing the electrospinning parameters. However, most of the studies focused on the effect of experimental conditions, solvent, applied voltage and the factors significantly affecting the fiber stability and distribution [16]. Obtaining defect-free fibers has been the most challenging problem. On the other hand, electrospinning parameters should be optimized for every polymer system based on the nature of solvent used. Numerous studies have shown that electrospun PS nanofibers have attractive applications in areas such as filtration, insulation, sensors and catalysis immobilization because of their excellent properties [17]. Uyar et al. studied the effect of solution conductivity on the electrospinning of bead-free PS fibers from N,Ndimethylformamide (DMF) solutions. It has been found that a slight variation in the conductivity of the solution can significantly affect the fiber morphology and a higher conductivity yielded bead-free fibers [18]. Several research groups have demonstrated that polyacrylonitrile (PAN) nanofibers can be fabricated by electrospinning technique and they exhibit superior characteristics, which include high thermal stability, resistance to bacteria and photo irradiation, high mechanical strength and resistance to most solvents [19]. Electrospun PAN fibers find interesting applications such as filtration, tissue engineering and flame retardant membranes. SAN is a random copolymer of styrene and acrylonitrile and has many useful properties that are a combination of those of PS and PAN, such as excellent gloss, good chemical and oil resistance, high transparency and high mechanical strength [20]. On the basis of these unique properties, SAN nanofibers could also be used for various applications that demand chemical and oil resistance, such as ultrafiltration of corrosive chemicals, catalyst immobilization, etc. Yu and Liu electrospun SAN solution in DMF that contained PdCl2 [21] and obtained fibers of diameter 200 nm that contained nanoparticles of Pd. They used those fibers for catalyzing hydrogenation of some olefins. Kim et al. used ionic liquid/SAN electrospun fibers for detection of alcohol vapors [22]. However, a detailed report on effect of electrospinning conditions on SAN fiber diameter and morphology is not found in literature. Our previous work has demonstrated the effects of solution and process parameters on the electrospun SAN webs morphology and fiber diameter [23]. In this study, SAN dissolved in dimethyl sulfoxide (DMSO) was electrospun under different conditions and the effects of solution and processing parameters on morphology and size of electrospun SAN fibers were studied. Our main focus was on optimizing spinning conditions for SAN to obtain bead-free uniform fibers, by adjusting these variables. We have also found that diameter, size distribution and
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morphology of the electrospun SAN fibers could be controlled by carefully selecting process and solution parameters.
Experimental Materials SAN (Grade: Santron IMS 1000, acrylonitrile content: 30 %, specific gravity: 1.07, MFI: 35 g/10 min at 220 °C under a load of 10 kg, ASTM D-1238) was supplied by Bhansali Engineering Polymers, Rajasthan, India; analytical-grade DMSO (purity >99 %) was obtained from HPLC private ltd., Mumbai, India and used without further purification. The viscosity average molecular weight (Mv ) of SAN was determined by using an Oswald viscometer and its value is 2.46×106 g.mol−1. The values of the constants K and a in the Mark-Houwink equation, at 25 °C for SAN in n-butanone, are 13.6×10−4 and 0.62, respectively [24]. Preparation of nanofibers SAN solutions of varying concentration, i.e., 8, 10, 12, 15 and 20 % (w/v) in DMSO were prepared. The solutions of SAN were loaded into hypodermic syringes and then they were electrospun under a varied DC voltage of 10–25 kV, onto aluminum foil wrapped static collector. The solutions were delivered at flow rates of 0.8, 0.9 and 1 mL.h−1 through a needle of 0.5 mm inner diameter with a beveled tip; needle tip-tocollector distance was 23 cm. The electrospinning setup (model: E–spin Nano) was procured from Physics Equipments Co., Chennai, India. Measurement and characterization Viscosity of polymer solutions was measured by a Brookfield digital viscometer (Model DV II, USA) at 30 °C. The morphology and diameter of the gold sputtered electrospun fibers (JEOL JFC 1600 auto fine coater, Japan) were determined by a scanning electron microscope (JEOL-JSM-6380 LA, Japan) at an accelerating voltage of 20 kV, using secondary electrons for imaging. Smile Shot™ software was used for the analysis of images and measurement of fiber diameters. Diameter of an individual fiber is an average of 3 measurements taken along the fiber axis. For calculating the Davg of the electrospun fibers obtained under a particular spinning condition, diameters of at least 50 fibers were measured and their average values along with standard deviations have been reported.
Results and discussion Effect of polymer solution concentration Polymer solution viscosity has been found to be one of the greatest influences on ultrafine fibers’ morphology and diameter [18]. Viscosities of the SAN solutions increase substantially with increasing polymer concentration (Table 1).
Int J Plast Technol Table 1 Solubility parameter, viscosity and conductivity of the solvent and SAN solutions Solvent
DMSO
Solubility parameter (MPa)1/2
Viscosity (cP)
Pure solvent
SAN
Pure solvent
8%
10 %
12 %
15 %
20 %
26.6
20.3
4.20
30
38
55
108
228
Solution concentration decides the limiting boundaries for the formation of electrospun fibers due to variations in the viscosity and surface tension. Low concentration solution forms droplets due to the influence of surface tension, while a higher concentration prohibits fiber formation due to higher viscosity. The viscosity of polymer solutions increases as the concentration increases from 8 to 20 %, because of the higher number of polymer chain entanglements, which are necessary for the formation of continuous fibers. A high viscosity of polymer solution has been associated with the formation of smooth and bead-free fibers [25, 26]. Figure 2a–e shows SEM micrographs of electrospun fibers from SAN solutions of varying concentrations under a fixed electrostatic field strength of 0.65 kV.cm−1. It can be observed from these micrographs that the diameters of the fibers increase with increasing solution concentration. Also, beads can be seen in fibers spun from solutions of lower concentrations (up to 12 %). It has been found in literature that Davg. increases proportionally to the solution concentration (C) according to a power law relationship [27, 28]. Figure 3 shows the relationship between the solution concentration (C) and Davg. under an applied voltage of 15 kV and at a flow rate of 0.9 mL.h−1. Moreover, the Davg. increases with solution concentration by the power law relationship Davg: ¼ C a
ð1Þ
Fig. 2 SEM micrographs (magnification 5,000× and scale bar=5 μm) of electrospun SAN fibers obtained from various solution concentrations a 8 %, b 10 %, c 12 %, d 15 %, and e 20 %, under a fixed electrostatic field strength of 0.65 kV.cm−1
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Fig. 3 Dependence of average fiber diameter on solution concentration
Where, C is the polymer solution concentration and a, the power law exponent. Similarly, plots of Davg. Vs C were prepared at different voltages and flow rates, and these were fit to a power law relationship. In the present study, power law dependence of Davg. on polymer concentration was determined to be Davg. ~ C2.24. These results
Fig. 4 Dependence of Davg. of electrospun SAN fibers on solution concentration, a at a flow rate of 0.8 mL.h−1, b at a flow rate of 0.9 mL.h−1, c at a flow rate of 1 mL.h−1
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illustrate the importance of using an optimum solution concentration for obtaining smooth fibers that are free from defects. However, it should be noted that the power law simply indicates the relationship between fiber diameter and solution concentration and do not represent the heterogeneous diameters that may arise from beading. Figures 4, 5 and 6 show the plots of the Davg. of SAN fibers spun at different solution concentrations under varying flow rates and applied voltages. SEM micrographs of the SAN fibers electrospun under different conditions and the fiber diameter distribution plots are shown in Figs. 7, 8, 9, 10, 11 and 12. Figure 4a–c reveals that the Davg. of the fibers increases with increasing solution concentration of polymer. Also, the amount of beads decreases when increasing the solution concentration. The formation of bead defects in electrospun fibers has been investigated in several papers [29, 30]. For a fixed applied electrical potential, only discrete droplets with large size distribution will be obtained at very low solution concentrations. Solutions with low concentrations do not have enough chain entanglements to withstand both the electrostatic and Columbic repulsion forces acting on an infinitesimal segment of an ejected, charged jet. Once the charged jet is broken up into smaller charged entities, surface tension will minimize the surface area of the broken jets and, hence, discrete spheres will be formed. Increasing the solution concentration will result in a combination of smooth and beaded fibers. At higher concentrations, the high chain
Fig. 5 Effect of applied voltage on Davg. of SAN fibers electrospun at different solution concentrations. a at a flow rate of 0.8 mL.h−1 b at a flow rate of 0.9 mL.h−1, c at a flow rate of 1 mL.h−1
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Fig. 6 Effect of flow rate on Davg. of SAN fibers electrospun at different solution concentrations. a at an applied voltage of 10 kV, b at an applied voltage of 15 kV, c at an applied voltage of 20 kV, d at an applied voltage of 25 kV
entanglements would completely prevent the charged jet from breaking up. Interestingly, increasing polymer concentration will also result in fibers with larger diameters. This increase in the Davg. is a result of the increased viscoelastic forces that counteract the Columbic repulsion forces that try to stretch the charged jet, which, in turn, cause the jet to thin down. In the present study, we observed that at low concentration (12 %) and beads form at lower concentrations (8 to 12 %). The average diameter of electrospun SAN fibers lies in the range of 96–872 nm. When the applied voltage is above 20 kV, fibers with larger diameter begin to form. It has been demonstrated that the Davg. increases with increasing applied potential (10 to 25 kV) and smooth fibers were obtained only at higher solution concentrations and higher applied voltages. At flow rate of 0.8 mL.h−1, at lower concentrations (
