effect of licl and non-ionic surfactant on morphology of polystyrene electrospun nanofibers

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e-Polymers 2008, no. 056 http://www.e-polymers.org

ISSN 1618-7229

Effect of LiCl and non-ionic surfactant on morphology of polystyrene electrospun nanofibers Delaram Fallahi 1 Mehdi Rafizadeh,1 Naser Mohammadi,1 Behrooz Vahidi 2 1Polymer Eng. Department, Amirkabir University of Technology, P.O.Box 15875-4413, Tehran, Iran; e-mail: [email protected]. 2Electrical Eng. Department, Amirkabir University of Technology, P.O.Box 15875-4413, Tehran, Iran. (Received: 1 January, 2008; published: 14 April, 2008)

Abstract: Polystyrene fibers were produced by the electrospinning technique. The effects of solution conductivity, surface tension and concentration on morphology and average diameter of electrospun fibers were investigated by scanning electron microscopy (SEM). Solutions of 12, 10, 8, 6% (w/v) polystyrene in dimethylformamide were prepared. Lithium Chloride and a non-ionic surfactant were used to change the conductivity and surface tension of the solutions, respectively. The results indicate that increasing the solution conductivity eliminates the bead formation and increases the fiber diameters. By addition of salt, fine and consistent fibers could be produced from electrospinning of 8% (w/v) PS/DMF solution. Adding 0.1% surfactant reduces the solution surface tension and results in smaller beads and higher fiber diameters. By increasing the amount of surfactant to 0.3%, big beads and thinner fibers are produced.

Introduction Electrospinning is a successful process in producing continuous fibers with a diameter ranging from a few nanometers to microns. Within the last decade, these fibers have attracted great attention due to their potential applications in nanocomposites [1], tissue engineering [2-3], sensors [4] and filters [5]. A typical electrospinning setup consists of a high voltage power supply, a syringe and a metal collector. A polymer solution is injected via the syringe, which is located at a distance from the collector. A high voltage is applied to the solution which deforms the drop of liquid at the tip of the syringe. Above a critical voltage, the electrical forces at the surface of the drop overcome the solution surface tension and an electrically charged jet of polymer solution ejects and travels toward the grounded collector. The jet is stretched while traveling and its solvent evaporates, forming ultra fine fibers on the collector. In general, the fibers are collected as a non-woven mat. Electrospining of a polymer solution may result in three structures: beads, beaded fibers and uniform fibers. The observed structure depends on various factors. It has been found that electrostatic, viscoelastic and surface tension forces have the most effects on morphology of electrospun fiber [6-7]. The electrostatic force depends on applied voltage, solvent dielectric constant and conductivity [8-9]. The viscoelastic force depends on solution concentration, viscosity and viscoelastic behavior [8-10]. The surface tension force depends on solution surface tension [8-9]. The effect of concentration and viscosity on bead formation and fiber diameters are discussed in many researches [11-14]. The solution viscosity should be high enough

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to form a stable jet. In a dilute solution with low viscosity, jet breaks-up into droplets and just beads are produced. In other words, electrospraying takes place. Increasing the concentration reduces the number of beads and increases the fiber diameters [6, 11, 14-15]. There have been some efforts to express the relation between polymer concentration and bead formation by considering the molecular entanglements in solution [10, 16-18]. Mckee et al. [16] reported that the solution entanglement concentration (Ce) is the minimum concentration required for electrospinning of beaded nanofibers, and to produce uniform fibers, 2-2.5 times Ce is needed. Ce is the boundary between the semidilute unentangled and semidilute entangled regimes in the solution. The necessity of entanglements in polymer solution for fiber formation is indicated by other researchers too [17-18]. They used the dimensionless quantity known as Berry number (C×[η], where C is the polymer concentration in solution and [η] is the intrinsic viscosity), to describe fluid elasticity and effect of entanglements. Shenoy et al. [10] introduced a simple method to estimate the morphology of electrospun fibers by calculating the number of entanglements per chain in a good solvent. It is known that bead formation in electrospinning is very similar to laminar jet break-up due to surface tension [19]. Surface tension tends to make the surface area per unit mass smaller, and tries to change the jet into spheres, which results in bead formation [20]. The effect of surface tension on fiber morphology is investigated in some papers [11, 13, 19-21]. Jet surface charge density is one important factor affecting the morphology of electrospun fibers [22-24]. In the one dimensional steady model developed for an electrospinning jet [25], the effect of surface charge density is evident in both conservation of charge and the force balance relations (equations 2 and 3 below).

Qur =ρπ 2 (1)

IkErur2 2 =+πσπ (2)

z21

∂∂

++∂∂

−=∂∂ τ

ρσ

ρ rE

zP

zuu (3)

Q is the mass flow rate, u the velocity, ρ the density, E the applied voltage, I the current, P the internal pressure of the fluid, τ the viscose force, σ the surface charge density and r the radius of the jet at axial coordinate z . Qin et al [26] has developed an allometric scaling law in electrospinning in which the effect of adding salt on the diameter of the stable part of the jet is shown. In their relation the radius of the stable part of the jet (r) scales the distance from the nozzle (z) as following:

)/(z~r αα +− 1 (4) α is a surface charge parameter. When 0=α , there is no charge on jet surface and in case 1=α , the jet is fully surface charged. The value of α depends upon the salt concentration added in the solution. Their theoretical prediction agreed quite well with their experimental data [26]. Others have shown that by increasing the charges that are carried by the jet, higher elongation forces are imposed to the jet under the electric field which results in less or smaller beads [14,22].


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Polystyrene is a well known polymer and electrospinning of polystyrene solutions have been reported in some papers. Effect of solvents [6, 27-29, 31], polymer molecular weight [30], polymer concentration [6, 28-31], addition of salt [31], addition of ionic surfactant [24], and operating parameters [32] on electrospinning of polystyrene solutions have been investigated. However, there are some aspects that need further exploration. In this research, the effects of solution conductivity and surface tension on morphology of electrospun polystyrene nanofibers are investigated. Solutions of polystyrene (PS) in dimethylformamide (DMF) were prepared. Lithium chloride (LiCl) and a non-ionic surfactant were used to change the conductivity and surface tension of the solutions, respectively. We used LiCl according to Qin et al [33]. They examined different inorganic salts and showed that LiCl has the most effect on increasing the electrical conductivity of solutions of polyacrylonitrile in DMF. To investigate the effect of conductivity and surface tension separately, we selected a non-ionic surfactant which had no significant effect on solutions conductivity. Results and discussion To investigate the effect of conductivity and surface tension separately, we had to make sure that addition of the surfactant, makes no change in the solution conductivity, and by adding salt, solution surface tension remains unchanged. Solutions of polystyrene in DMF containing different amounts of LiCl and surfactant were prepared and used for electrospinning. Table 1 summarizes the characterization of these solutions as well as the diameter of obtained electrospun fibers. Tab. 1. Characterization of solutions and electrospun fibers.

PS conc.

(w/v %)

LiCl (w/v%)

Surfactant (w/v%)

C[η] Surface*Tension (mN/m)

Conductivity(μS/cm)

Avg. Diam. (nm)

Beads

12 0 0 5.88 34.2 1 250±13 Beaded fibers12 0.1 0 5.88 34.8 561 343±13 No bead 12 0.3 0 5.88 34 1660 453±31 No bead 12 0 0.1 5.88 23.8 1.3 368±25 Few small

beads 12 0 0.3 5.88 23.6 1.3 264±25 Big beads 12 0.2 0.3 5.88 24.1 1110 302±19 No bead 10 0.1 0 4.9 33.8 578 253±13 No bead 8 0.1 - 3.92 34.9 573 163±8 No bead 6 0.1 - 2.94 35 581 112±8 No bead

(big particles)* The instrumental error is within ± 0.8 mN/m. As shown in Table 1, adding 0.1% surfactant to the solution decreases the surface tension significantly. The Critical Micelle Concentration (CMC) of the used surfactant in DMF is below 0.01%, so by increasing the amount of surfactant to 0.3%, the surface tension remains unchanged. Addition of salt and surfactant do not change the solution surface tension and conductivity, respectively.


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0.01

0.1

1

0.1 1 10 100 1000

shear rate (1/s)

Shea

r Vis

cosi

ty (P

a.s)

no salt , no surfactant

0.3% LiCl

0.3% surfactant

0.1% surfactant

Fig. 1. Shear viscosity of solutions of PS in DMF containing different amounts of surfactant and salt. Fig. 1 shows the shear viscosity of different PS solutions. This graph indicates that addition of salt and surfactant has no effect on the solution shear viscosity. Effect of salt addition The addition of inorganic salt increases conductivity of the PS solutions dramatically (Table 1). The SEM images of fibers electrospun from solutions of 12% (w/v) PS in DMF containing different amounts of LiCl are shown in Fig. 2. Addition of salt eliminates the bead formation and increases the average diameter of fibers. The characterization of electrospun fibers are shown in Table 1. The elimination of bead formation and reduction in fiber diameters by increasing the conductivity was reported in some papers [24, 34-35]. It was suggested that by increasing the conductivity, net charge density of the jet was increased and the jet was stretched under a stronger force, resulting in bead-free and thinner fibers. Li et al. [36] found that adding sodium chloride (NaCl) to poly(acrylic acid) (PAA) solutions, did not affect the average fiber size, whereas Kim et al. [23] showed that in PAA nanofibers, the minimum diameter of nanofibers was observed when 0.01 M NaCl was added to the solution. By increasing the amount of salt, the diameter of PAA nanofiber increased. The diameter change was attributed to the chain conformation change of PAA and rapid increase of the conductivity with increasing the ionic strength. Mit-uppatham et al. [15] reported an increase in fiber diameters by adding inorganic salts to nylon 6 solutions in formic acid. Manee-in et al. [31] investigated the effect of inorganic salt by addition of 1% LiCl or KCl to 30% (w/v) PS solutions in different solvents including DMF. The obtained fibers were bead-free and in comparison with those obtained from neat solutions (without the salt), the addition of salts increased the size of the fibers. They have also reported that electrospinning of 30% (w/v) PS solution in DMF (without the salt) resulted in bead-free fibers due to high concentration of the solution. So they did not investigate the effect of salts on beads and bead formation.


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(a) (b)

(c)

Fig. 2. SEM images of fibers electrospun from solutions of 12% (w/v) PS in DMF containing a) 0% , b) 0.1% and c) 0.3% LiCl. All magnifications are 3000x. In this work, the elimination of bead formation by adding salt can be attributed to higher conductivity and higher net charge density on the jet, which strengthens the elongation force and eliminates the beads. However, a jet with higher net charge density carries more volume of solution toward the collector and produces fibers with higher diameter. The increase in fiber diameters by amount of salt is shown in Table 1. The increase in flow rate by adding a salt to the polymer solution is reported by others [13, 31]. Effect of surfactant The SEM images of fibers electrospun from solutions of 12% (w/v) PS in DMF containing different amounts of surfactant are shown in Fig. 3. Addition of 0.1% surfactant to the solution reduces the formation of beads, significantly. Surprisingly, addition of 0.3% surfactant increases the number and size of the beads. It is accepted that surface tension is one of the key factors that affect bead formation [11,19-20], although Demir et al. [13] reduced the surface tension of polyurethaneurea solutions by adding polydimethylsiloxane, and observed no significant effect in fiber morphology. Surface tension of different PS solutions is shown in Table 1. Adding 0.1% surfactant decreases the solution surface tension from 34.2 to 23.8 mN/m and decreases the bead formation. The decrease in bead formation may be attributed to the decrease in surface tension. Surface tension is the main factor in laminar jet break-up and is known as the main driving force for bead formation [19-20]. Decreasing the surface tension can result in smaller beads in electrospun fibers. However, by increasing the amount of surfactant to 0.3%, number and size of the beads are increased,


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unexpectedly. Taking into consideration that increasing the amount of surfactant from 0.1% to 0.3% do not change the surface tension any more (Table 1), it may be concluded that another mechanism besides surface tension reduction could affect bead formation. In a polymer-surfactant system, depending on nature of the polymer and surfactant, some interactions may exist that can result in adsorption of polymer molecules onto micelles or occurrence of polymer–surfactant complexes [37]. These interactions may affect the viscoelastic properties of the solution and hence affect the bead formation in electrospinning process. However, addition of 0.2% LiCl to solution containing 0.3% surfactant suppresses the bead formation (Fig. 3-d).

(a) (b)

(c) (d) Fig. 3. SEM images of fibers electrospun from solutions of 12% (w/v) PS in DMF containing a) 0% , b) 0.1% , c) 0.3% surfactant and d) 0.3% surfactant, 0.2% LiCl. All magnifications are 3000x. Effect of concentration of polymer The results given above showed that increase of conductivity due to addition of salt is more effective in elimination of beads than decreasing the surface tension. By adding salt to solution of 12% PS in DMF, the beads were eliminated. On the other hand, it is accepted that reduction of the polymer concentration in solution, enhances the bead formation [11-14]. To see whether or not in lower concentrations, addition of salt could eliminate the beads, solutions of 10, 8 and 6 % PS in DMF containing 0.1% LiCl were electrospun. By decreasing the concentration of PS from 12% to 6%, the Berry number changes from 5.88 to 2.94 (Table 1). The SEM images of fibers electrospun from solutions of 6-10% (w/v) PS in DMF containing 0.1% LiCl are shown in Fig. 4.


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(a) (b)

(c)

Fig. 4. SEM images of fibers electrospun from solutions of a) 10% (w/v) b) 8% (w/v) c) 6% (w/v)PS in DMF containing 0.1% LiCl. All magnifications are 3000x. Addition of salt eliminates the bead formation in solutions of 10 and 8% PS in DMF and results in bead-free fibers. In solution of 6% PS in DMF, although the presence of salt eliminates the beads, some big particles were formed that spoils the consistency of fibers. The average diameter of fibers reduces by decrease in polymer concentration (Table 1). As a result, by addition of salt, fine, bead-free and consistent fibers could be produced from electrospinning of 8% (w/v) PS/DMF solution. Experimental part Materials General purpose polystyrene with weight average molecular weight of 220000 was supplied by Tabriz Petrochemical Inc. Dimethylformamide (DMF) and lithium chloride (LiCl) were purchased from Merck Co. A non-ionic surfactant (polyether modified polysiloxane) (Efka3030) were obtained from Ciba Specialty Chemicals. All the materials were used as received. Intrinsic viscosity The intrinsic viscosities of PS in DMF were measured by using an Ubbelohde viscometer at 25 °C. The flow time of dilute solutions and the solvent were measured and intrinsic viscosity was determined by extrapolating both Csp /η and )Cln( rel /η to zero polymer concentration:


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)C

ln(lim)

C(lim rel

0Csp

0Cηη

→→ = (5)

where spη and relη are specific and relative viscosities, respectively, and C is the PS concentration (g/dl). Electrospinning Solutions of polystyrene in DMF containing different amounts of surfactant and LiCl were prepared. The electrospinning setup consisted of a syringe and needle, a syringe pump (New Era Pump System, NE1000), a ground electrode (Aluminium sheet), and a high voltage power supply (Gamma High Voltage Research, RR60) which could generate positive DC voltages up to 60 kV. Fig. 5 shows schematics of the set up. The electrospinning was performed at room temperature and the solution was delivered at a flow rate of 1.2 ml/h. The applied electric potential was 20 kV and the distance to collector was fixed at 15 cm.

Fig. 5. Schematics of the electrospinning set-up. Measurement and characterization The surface tension and conductivity of solutions were measured using an interfacial tension meter (Kruss Model K8) and conductivity meter (Console C533) respectively, at 24 °C. The rheology tests were performed on a rheometer (MCR 300 Modular Compact Rheometer, Anton Paar-Physic) at 25 °C. The morphology of electrospun fibers were observed on a scanning electron microscope (SEM) (Cambridge LEO 1455 VP and Philips XL30). Acknowledgements The authors acknowledge the financial support of the Ministry of Science, Research and Technology of Iran.


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effect of licl and non-ionic surfactant on morphology of polystyrene electrospun nanofibers

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