Solution electrospinning of nanofibers

Solution electrospinning of nanofibers

4

C. Salas North Carolina State University, Raleigh, NC, United States

4.1

Introduction

The electrospinning of polymer solutions into fibers has drawn a lot of attention in recent years for applications in different fields, namely tissue engineering, filtration, drug delivery, and bio sensing, among others. The electrospinning process can be carried out from a polymer melt (already discussed in Chapter 2 of this volume) or from a polymer solution. The first electrospinning device was initially patented in 1934 by Formhals (1934); it included the spinning of cellulose acetate and cellulose esters from a solution containing equal parts of acetone and alcohol. Later in the 1990s, the pioneer work of Doshi and Reneker (1995) revisited the interest in electrospinning as an effective way to produce nanofibers for different applications. Since then, different experimental setups have been proposed for the electrospinning of polymer solutions, and many different polymers have been successfully electrospun into fibers, as reported in several review articles (Agarwal et al., 2013; Ahmed et al., 2015; Baji et al., 2010; Balamurugan et al., 2011; Bhardwaj and Kundu, 2010; Boudriot et al., 2006; Doyle et al., 2013; Frenot and Chronakis, 2003; Ghorani and Tucker, 2015; Greiner and Wendorff, 2007; Huang et al., 2003; Hunley et al., 2006; Junoh et al., 2015; Khorshidi et al., 2015; Krishnan et al., 2013; Liang et al., 2007; Reneker and Yarin, 2008; Rutledge and Fridrikh, 2007; Sill and von Recum, 2008; Subbiah et al., 2005; Teo and Ramakrishna, 2006). Several parameters play a role in the electrospinning of polymer solutions, including those that are directly related to the device (distance to collector, applied voltage, single or multiple channels, etc.) as well as parameters related to the polymer solution: viscosity, surface tension, and electrical conductivity.

4.2

Solution electrospinning versus other electrospinning techniques

Electrospinning from polymer solutions differs from electrospinning from the melt and electrospinning from emulsions.

Electrospun Nanofibers. http://dx.doi.org/10.1016/B978-0-08-100907-9.00004-0 Copyright © 2017 Elsevier Ltd. All rights reserved.

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4.2.1

Electrospun Nanofibers

Solution electrospinning compared to melt electrospinning

In melt electrospinning, as its name indicates, there is not a solvent involved, but a molten polymer; this has been pointed out as one of the main advantages of that technique. Melt electrospinning requires the heating of the polymer to adequate temperatures to facilitate stretching under the electrical field. The effect of different processing variables on melt electrospinning is similar to solution electrospinning; however, the absence of solvent excludes the effect of solvent properties such as dielectric constant, conductivity, and volatility. However, it also limits the availability of charges. Similar to solution electrospinning, in melt electrospinning, the molecular weight of the polymer plays an important role. The increase in molecular weight of the molten polymer makes the stretching of the polymer molecules by electrostatic forces difficult, and this results in thicker fibers. In terms of process parameters, the distance to the collector is smaller in melt electrospinning because of the fast solidification of the stretched polymer jet.

4.2.2

Solution electrospinning compared to emulsion electrospinning

When trying to mix two immiscible fluids, droplets of one fluid will disperse in the other, but over time, the system will eventually separate into two phases. Emulsions are dispersed systems in which droplets of one liquid are dispersed into other liquids (immiscible) and stabilized by the presence of a surface active agent (surfactant). The droplets represent the internal phase, and the liquid in which they disperse are the continuous phase. If the internal phase is oil, the emulsions are oil in water (O/W); likewise, if the internal phase is water, the emulsions are water in oil (W/O). Several variables affect the emulsification process, the size of the droplets, and properties of the produced emulsion. Variables such as the relative volumetric ratio among the phases, the type and concentration of surfactant, the amount of mechanical energy to mix the phases, and the presence of other components such as salts, alcohols, or solid particles. Emulsions have applications in many commercial and household products. The emulsion electrospinning process was first reported by Sanders et al. (2003) who studied the electrospinning of bovine serum albumin dissolved on a phosphate buffer emulsified on a poly(ethylene-co-vinylacetate) in dichloromethane. Similarly, Xu et al. used the emulsion electrospinning to produce drug encapsulated nanofibers (Xu et al., 2005). In that report, a hydrophilic drug dissolved in water was emulsified on a poly(ethylene glycol)-poly(L-lactic acid) diblock copolymer dissolved in chloroform (Xu et al., 2005). Emulsion electrospinning has been used to prepare core-shell fibers as well (Xu et al., 2006; Bazilevsky et al., 2007). The main difference of emulsion electrospinning with solution electrospinning is that in solution electrospinning, a homogeneous solution of the polymer is formed, whereas in the emulsion electrospinning, the components are immiscible. The morphology of the obtained fibers is also different, as the emulsion electrospinning allows the production of coaxial composite fibers.

Solution electrospinning of nanofibers

4.3

75

Parameters that affect solution electrospinning

The electrospinning of polymer solutions is affected by different variables, classified as solution parameters: process parameters and ambient parameters. These variables are listed in Table 4.1.

Table 4.1 Parameters that affect electrospinning from polymer solutions Solution parameters l

l

l

l

l

l

Molecular weight of the polymer Concentration of solution Viscosity Surface tension Conductivity Solvent properties

4.3.1

Process parameters

Ambient parameters

l

Applied voltage

l

Temperature

l

Distance between collector and spinneret Type of collector (static or moving) Flowrate of polymer solution

l

Humidity

l

l

Solution parameters

The solution parameters are interrelated. The molecular weight of the polymer is a key property that determines the viscosity of the polymer solution and affects the other properties, such as surface tension, conductivity, etc.

4.3.1.1 Solvent properties The electrospinning from polymer solutions (solution electrospinning) requires a solvent to solubilize the polymer of interest. The correct selection of the solvent is critical to obtain a homogeneous solution of the polymer. One approach that has been used on the identification of a suitable solvent for a given polymer is the solubility parameter. The finding of a suitable solvent for the polymer of interest can be facilitated by using the solubility parameter. The Hansen solubility parameter (Hansen, 2007) accounts individually for all the molecular interactions in a mole of material, namely dispersion forces, polar interactions (dipole–dipole), and specific interactions such as hydrogen bonding. The cohesive energy, expressed as E ¼ △H  RT (ΔH refers to the latent heat of vaporization, T is the absolute temperature, and R is the universal gas constant), is expressed as a sum of each contribution (Hansen, 2007). E ¼ ED + EP + EH

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Electrospun Nanofibers

Which after dividing by the molar volume are: E ED EP EH + + ¼ V V V V δ2 ¼ δ2d + δ2p + δ2h δ is the Hansen solubility parameter, δd represents the dispersive component, δp the polar, and δh the hydrogen bonding. A good solvent for a given polymer should have a solubility parameter close to that of the polymer. One way to identify a good solvent for a polymer is described by calculating the Hansen solubility parameter distance given for two substances 1 and 2 by:
2 R2a ¼ 4ðδd2  δd1 Þ2 + δp2  δp1 + ðδh2  δh1 Þ2 Accordingly, a good solvent for the polymer should have a small value for Ra. In addition to Ra, the relative energy difference (RED) has been defined as RED ¼

Ra Ro

Ro is the radius of a sphere with coordinates δd, δp, δh and good solvents will have RED values less than one (Hansen, 2007).

4.3.1.2

Effect of viscosity

The effect of viscosity on the ability of a polymer solution to form fibers by electrospinning was already noted on the early work by Doshi and Reneker (1995), where solutions of poly (ethylene oxide) (Mw ¼ 1,450,000 g/mol) only formed fibers at viscosities between 800 and 4000 cP. It is important to introduce some concepts of polymer physics that will be used in further discussion. The concentration of polymer at which the viscosity of solution changes abruptly is known as the overlap concentration, c*. The value of c* can be determined from the chain dimensions of the polymer, which can be measured experimentally, and it can be estimated from the values of intrinsic viscosity as c*  1[η] (Gupta et al., 2005). The value of c* depends on the chemical structure of the polymer, molecular weight, temperature, and type of solvent. Different regimes have been identified for polymers dissolved in good solvents. The model of de Gennes (1979) includes three concentration regimes: dilute (c < c*), semidilute (c > c*), and concentrated solutions (c ≫ c*). It predicts a scaling law of viscosity of the polymer solution with concentration given by: η ¼ ηs

 c 3=ð3ν1Þ c*

ν is the Flory exponent of value 0.5 for theta solvent and 0.6 for good solvents.

Relative viscosity

h hrel = h s

Specific viscosity hsp =

h − hs −1 = hrel hs

Intrinsic viscosity

Viscosity dependence with concentration

Mark Houwing–Sakurada equation viscosity dependence with molecular weight

Solution electrospinning of nanofibers

Viscosity of polymer solutions

hsp

[h]

[h] = lim

hsp

c→0

c

hsp c

[h] + kH

[h]2

c+…

Huggins equation Concentration (c)

[h] = K × Mνa

Mν =

ΣNi Mi1+a

1 a

ΣNi Mi

Mark Houwing–Sakurada equation

Fig. 4.1 Expressions used to determine viscosity of polymer solutions.

77

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Electrospun Nanofibers

The model of Colby and Rubinstein (1990) defines four concentration regimes, dividing the semidilute regime into two subregimes, the semidilute unentangled and semidilute entangled, the transition between the regimes occurs at a concentration defined as the entanglement concentration (ce). In this model, the scaling law for neutral, linear chain polymers in a good solvent is predicted as ηsp  c1.0 in the dilute regime; ηsp  c1.25 in the semidilute unentangled regime and ηsp  c4.25–4.5 in the semidilute entangled regime. These concepts have been useful in studying solution electrospinning and the range of conditions that would yield uniform fibers from different polymers. McKee et al. (2004b) studied the influence of solution rheology on the electrospinning of solutions of linear and branched polyesters. The electrospinning was carried out at conditions in the semidilute unentangled, semidilute entangled, and concentrated solutions (see Fig. 4.1). Bead-free fibers were obtained at concentrations above the critical entanglement concentration. It was also observed that the fiber diameter increased with solution viscosity and scaled with normalized concentration (c/ce) as Diameter (μm)  (c/ce)2.6 (Fig. 4.2). Gupta et al. (2005) studied the relationship between viscosity and concentration on electrospinning of seven linear homopolymers of poly(methylmethacrylate) with different molecular weights (12,470–365,700 g/mol) using dimethylformamide (DMF) as solvent. Three regimes were identified: dilute, semidilute unentangled, and semidilute entangled, with the onset between the two regimes at c/c* ¼ 3 (c*: critical chain overlap concentration). The results showed that the electrospinning of the polymethyl metacrylate (PMMA) of low molecular weight (12,470) at concentrations in the semidilute unentangled regime (c/c* < 3) and semidilute entangled (3 < c/c* < 6) regime only produced beads. For the PMMA with molecular weight of 125,900 in the semidilute unentangled regime, beads were produced, but at concentrations in the semidilute entangled regime and higher (c/c* > 6) uniform fibers were obtained. The high molecular weight PMMA (205,800) produced fibers with some beads at concentrations in both the semidilute unentangled and semidilute entangled regimes and uniform fibers at higher concentrations (Gupta et al., 2005). The effect of solvent on the polymer–polymer interactions in solution hydrogen bonding on fiber formation was reported by McKee et al. (2004a). They used poly (alkyl metacrylate) polymers containing pendant carboxylic acid and selfcomplementary hydrogen bonds groups. The polarity of the solvent was changed by using different amounts of DMF and chloroform. In the case of PMMA, the critical entanglement concentration did not change with solvent polarity and the obtained fiber diameter scaled with the normalized concentration as Diameter (μm)  (c/ce)2.7. However, for the copolymers with self-complementary hydrogen bonds groups, the transition from semidilute unentangled to a semidilute entangled regime was dependent on solvent polarity. Specifically, the critical entanglement concentration reduced from 7 wt% in pure DMF to 5 wt% in a mixture 20/80 DMF/chloroform. This change in solvent polarity was reflected on the diameter of the electrospun fibers, reportedly due to the enhanced intermolecular hydrogen bonding between the selfcomplementary hydrogen bonds groups in chloroform (McKee et al., 2004a). Shenoy et al. (2005) studied the role of chain entanglement on fiber formation during electrospinning used the concept of a solution entanglement number.

7a: 8 wt% C < Ce

7b: 10 wt% C = Ce

7c: 12 wt% C > Ce

7d: 16 wt%

7e: 18 wt%

7f: 20 wt% C = 2Ce

1000

3.73 100 hsp

C•• = 16 wt% 2.73 10 Ce = 7 wt% 1.39 1 1

10

100

Fig. 4.2 Left: Specific viscosity versus concentration for branched poly (ethylene terephthalate-co-ethylene isophthalate) (PET-co-PEI, Mw ¼ 46,000 g/mol and branching index ([η]branched/[η]linear) of 0.8). Right: Field emission scanning electron microscopy (FESEM) images of electrospun fibers of branched PET-co-PEI (Mw ¼ 76,000 g/mol, branching index ¼ 0.43, ce ¼ 10 wt%) at several concentrations (8–20 wt%). Reprinted (adapted) with permission from McKee, M.G., Wilkes, G.L., Colby, R.H., Long, T.E., 2004. Correlations of solution rheology with electrospun fiber formation of linear and branched polyesters. Macromolecules 37(5), 1760–1767. Copyright (2004) American Chemical Society.

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Electrospun Nanofibers

ðηe Þsoltn ¼

ϕp M w M ¼ ðMe Þsoltn Me

ϕp is the polymer volume fraction. The application of the model to different polymer/good solvent pairs indicates that the onset of fiber formation occurs at (ηe)soltn ¼ 2 and complete formation of fibers occurs at (ηe)soltn  3.5. The model was applied to different polymer systems already reported in the literature. For instance for polystyrene (PS) of 190 kDa molecular weight in tetrahydrofuran (THF), the model predicts concentrations of 20 wt% for the onset of fiber formation ((ηe)soltn ¼ 2) and 34 wt% as the concentration to obtain uniform fibers ((ηe)soltn  3.5), with the experimental values reported as 18 wt% and 30–35% respectively for these two limits (see Fig. 4.3). To use the model, it is sufficient to know the molecular weight entanglement and the weight average molecular weight, then the volume fraction can be obtained. It was pointed out that for low molecular weight polymers, it is difficult to obtain fibers, even if they are at high concentrations (high (ηe)soltn) (Shenoy et al., 2005). 7 300 k 6

(he)soltn calculated

5 190 k

4

34 wt%

Fibers only

3

Fibers + beads

2 Beads

100 k

20 wt%

1 50 k 0 0

5

10

15

20 wt% PS

25

30

35

40

Fig. 4.3 Plot of the calculated entanglement number (ηe)soltn as a function of concentration for polystyrene dissolved in tetrahydrofuran (PS/THF). The dashed line at (ηe)soltn  3.5 indicates the transition for complete fiber formation. The dotted line indicates the boundary between beads and a mixture of fibers and beads. Different weight-average molecular weight (Mw): 50, 100, 190, and 300 k are plotted. The arrows indicate the onset of fiber formation (20 wt%) and complete fiber formation (34 wt%) for a sample of Mw ¼ 190 kDa. Reprinted from Shenoy, S.L., Bates, W.D., Frisch, H.L., Wnek, G.E., 2005. Role of chain entanglements on fiber formation during electrospinning of polymer solutions: good solvent, non-specific polymer–polymer interaction limit. Polymer 46(10), 3372–3384. Copyright (2005), with permission from Elsevier.

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81

Eda and Shivkumar (2007) applied the model to predict the transition from beads to fiber on the electrospinning of PS using THF and N,N-dimethylformamide as solvents. For the high molecular weight PS, the concentrations obtained experimentally matched those predicted by the number of entanglements, with some discrepancies between the predicted values and the experimental results at low molecular weight. Likewise, Munir et al. (2009) studied the effect of molecular weight and concentration on the electrospinning of polyvinylpyrrolidone (PVP). When comparing the predicted values of entanglement numbers with the experimental results, discrepancies were observed for the low molecular weight (Mw of 55, 29, and 10 kg/mol) PVP. For instance, bead formation was predicted for the 55 kDa molecular weight PVP below 33%, and beads + fibers above that value; however, the results indicate uniform fibers were formed at 35 wt% (see Fig. 4.4). This result was explained on the basis of solidification of the jet (Munir et al., 2009). The influence of rheology of polyelectrolytes on electrospinning has also been studied. For instance, the study of poly (2-dimethylamino ethyl methacrylate) hydrochloride (PDMAEMAHCl) at different sodium chloride concentrations indicates that the entanglement concentration increases with the addition of NaCl (McKee et al., 2004a). Because of the screening of electrostatic charges, the polymer solution at a higher concentration of NaCl resembles the behavior of a neutral polymer solution. It was observed that the electrospinning of the PDMAEMAHCl from solutions in 80/20 water/methanol produced uniform fibers at concentrations well above critical entanglement concentration (ie, c ¼ 8ce). The fibers with solutions from NaCl formed at concentrations four times higher than the critical entanglement, and exhibit an increased diameter up to 20% NaCl, above that concentration the addition of NaCl did not affect the fiber diameter considerably (McKee et al., 2004a). Hemp et al. (2012) synthesized adenine-containing polyelectrolytes and studied their behavior in electrospinning. It was found that the specific viscosity scaled up with concentration as ηsp  c0.6, ηsp  c1.6, and ηsp  c5.6 at the semidilute unentangled, semidilute entangled, and concentrated regime. Polymers with a different concentration of vinylbenzyl adenine (VBA) were electrospun at concentrations above the entanglement concentration. The diameter of the fibers increased with molecular weight at concentrations equal to 4.5ce and 5.3ce (Hemp et al., 2012). The diameters of the fibers increased with the concentration according to a power law as shown in Table 4.2 (Hemp et al., 2012). Similarly, scaling laws have been reported for natural polymers, such as alginate, an anionic polysaccharide (Saquing et al., 2013). The specific viscosity of alginate solutions with and without surfactant (Triton X) was determined as a function of alginate concentration and the spinnability of these solutions was studied. An entanglement concentration of 0.45 wt% was found. The specific viscosity scaled with concentration in the three regimes as ηsp  c0.79 (dilute), ηsp  c1.6 (semidilute unentangled), and ηsp  c3.3 dilute (semidilute entangled) as shown in Fig. 4.5. In this system, the alginate did not form fibers, even at concentrations well above the entanglement concentration (13ce), indicating other contributions rather than entanglement to the electrospinnability. It was necessary to add a coadjutant polymer (polyethylene oxide) to produce fibers, which in turn shifted the behavior of the alginate toward a neutral polymer solution. Furthermore,

(he)soltn (calculated)

3

Beaded fibers

Mw = 55 k

Beads

Mw = 29 k

2 5 wt%

33 wt%

b

c

d

e

f

g

h

i

j

k

l

82

Mw = 350 k fibers

10 wt%

4

a

1 Mw = 10 k

0 0

5

10

15 20 25 30 35 PVP concentration (wt%)

40

45

Electrospun Nanofibers

Fig. 4.4 Left: Plot of the calculated entanglement number (ηe)soltn as a function of concentration for PVP/(water/ethanol) systems for various molecular weights. Right: SEM images showing the different morphology and structural regimes during bead-fiber transition at a flow rate of 8 μL/min for the system shown in left. Note the predicted value of bead formation for Mw ¼ 55 kDa differs from the experimental results (image f second row at right). Reprinted from Munir, M.M., Suryamas, A.B., Iskandar, F., Okuyama, K., 2009. Scaling law on particle-to-fiber formation during electrospinning. Polymer 50(20), 4935–4943. Copyright (2009), with permission from Elsevier.

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83

Table 4.2 Onset of electrospinning and scaling factors for PDMAEMAHCl and the adenine-containing polyelectrolytes VBA (mol %)

c/ce at onset of fiber formation

η0 (cP) at onset of fiber formation

Scaling factor for fiber diameter versus c/ce

Scaling factor for fiber diameter versus zero-shear viscosity

0 11 22 35

4.5 4.5 3.8 2.9

312 415 128 116

6.8 5.1 3.6 3.4

1.8 1.3 0.7 0.6

Adapted from Hemp, S.T., Hunley, M.T., Cheng, S., DeMella, K.C., Long, T.E., 2012. Synthesis and solution rheology of adenine-containing polyelectrolytes for electrospinning. Polymer 53(7), 1437–1443. Copyright (2012), with permission from Elsevier.

103 Alginate Alginate/PEO (70:30) PEO

hsp

102

101

100 10−1

100 Total polymer concentration (wt%)

Fig. 4.5 Comparison of the viscosity scaling relationships of alginate, alginate/PEO (70/30 wt ratio), and PEO systems in water using low-viscosity alginate. The arrows highlight the shift in the apparent entanglement concentration with PEO content. Reprinted (adapted) with permission from Saquing, C.D., Tang, C., Monian, B., Bonino, C.A., Manasco, J.L., Alsberg, E., Khan, S.A., 2013. Alginate– polyethylene oxide blend nanofibers and the role of the carrier polymer in electrospinning. Ind. Eng. Chem. Res. 52(26), 8692–8704. Copyright (2013) American Chemical Society.

the increase in molecular weight of PEO used determines the formation of fibers. For instance, uniform fibers were obtained with PEO of 600 kDa and above, below that 600 kDa only beads were observed at the different ratios of Alginate/PEO studied (30/70, 50/50, or 70/30 alginate/PEO) (Saquing et al., 2013). In a different study, the polyelectrolyte nature of polyamide 6 was changed by dissolving the polymer in formic acid, adding water as a co-solvent (Tsou et al., 2013). The addition of water shifted the behavior of polyamide 6 in solution toward that of a neutral polymer, with a decrease in the viscosity of the polymer solution (at concentrations above 10 wt%), and the fiber diameter decreasing with the amount of water in the solvent (see Fig. 4.6). The specific viscosity scaled with the polymer concentration as a power law, with the exponent changing depending on the molecular weight of the polymer ηsp  c0.5–0.54, ηsp  c1.5–1.8, and ηsp  c3.5–3.7, the lower value exponent corresponds to the higher molecular weight polymer.

84

Electrospun Nanofibers

Fig. 4.6 SEM images of fibers electrospun from 8 wt% (left column) and 15 wt% solution (right column) using different formic acid (FA)/H2O ratios; (A, B) 99/1, (C, D) 90/10, and (E, F) 85/15. Reprinted from Tsou, S.-Y., Lin, H.-S., Cheng, P.-J., Huang, C.-L., Wu, J.-Y., Wang, C., 2013. Rheological aspect on electrospinning of polyamide 6 solutions. Eur. Polym. J. 49(11), 3619–3629. Copyright (2013), with permission from Elsevier.

4.3.1.3

Effect of electric charge

The excess charges in the liquid play an important role in the electrospinning process. The application of an electrical field induces ion movement, with the ions moving to the oppositely charged electrolytes (Reneker and Yarin, 2008; Reneker et al., 2000).

Solution electrospinning of nanofibers

85

The electrical charge of the liquid on electrospinning is the main driving force for the flow of fibers from the spinneret toward the collector. The work by Deitzel et al. (2001), which used aqueous solutions of PEO, indicates an increase in the current with the applied voltage, with the current values in the range of 7–11 nA. Typical current measured for the preceding situation was about 100 nA. The strength of the electrical field is expressed as (Doshi and Reneker, 1995): rffiffiffiffiffiffiffi 4γ E¼ εo R E is electric field strength, γ is the surface tension of polymer solution, εo is the permittivity of the free space, and R is the radius of the curvature of the rounded-off cone apex. The surface charge energy is defined as σ o ¼ εEo . Several models have been proposed for the evaluation of stretching of the viscoelastic jet in electrospinning (Spivak et al., 2000; Reneker et al., 2000; Feng, 2002, 2003; Carroll and Joo, 2008). For a thorough discussion on the instabilities of jets produced during electrospinning, see the review of Reneker and Yarin (2008). The forces that act on the liquid are surface tension, coulomb forces, viscoelastic effects, and electrical forces from the electrical field created between the pendant droplet and the collector by the imposed potential difference (see illustration in Fig. 4.7). The surface tension is a property of the liquid that indicates the strong cohesiveness of the liquid molecules, or in other words, an indication of the dissimilarity of the phases at the interface. The other forces acting on the system are the electrostatic forces, or Coulomb’s forces. These forces vary inversely with the distance between the charges. The viscoelastic behavior of polymeric materials can be modeled according to well-known and established models. For instance, the Maxwell model considers the sum of elastic and viscous effect as a dashpot and spring. It has been used to model the behavior of polymer jets in electrospinning, assuming that the polymer molecules are composed of beads linked by springs. The figure illustrates the different forces that intervene on electrospinning of a polymer solution. Surface tension effects

Electrostatic effects r

R

F=k

ΔP =

2g R

q1q2 r2

Viscoelastic effects

de s 1 ds = + dt h E dt

Dashpot

e : strain, s : shear stress, h: viscosity, E: elastic modulus

Spring

F

Fig. 4.7 Idealized illustration of forces that play a role on electrospinning of polymer solutions.

86

Electrospun Nanofibers

When an electrical field is applied, the liquid jet expelled from the spinneret travels on a straight line for a short distance, then bending instabilities appear and spiraling occurs, then the jet forms a cone with the vertex located at the tip. Reneker et al. (2000) and Reneker and Yarin (2008) pointed out that in the rectilinear part of the electrified jet, the electrical charges can be considered a static system, interacting mainly by Coulomb’s law. Bending instabilities in electrospun jets were modeled according to the Maxwell viscoelastic model by a system of connected dumbbells. The beads interacted with each other according to Coulomb’s law. The polarity of the electrical field has been shown to induce changes in the surface energy of electrospun fiber of nylon 6 (Stachewicz et al., 2012). The results of XPS analysis indicate a different content of oxygen and nitrogen for fibers produced from positive or negative voltage (Stachewicz et al., 2012).

4.4

Equipment for solution electrospinning

Different experimental setups have been reported in the literature for the electrospinning of polymers from solution. The electrospinning process requires a power supply that will provide the required voltage to attain the desired electrical field, the solution dispenser (typically a pump, or hydrostatic pressure), which will provide the required solution flowrate, the container for the polymer solution (typically one or multiple needles or others), and the collector. The difference in voltage between the nozzle or tip and the collector determines the strength of electric field. The collector can be static, which consists of a metallic plate or dynamic that includes a rotating metallic drum or disk. The review article of Teo and Ramakrishna (2006) already described the different types of collectors that are depicted in Fig. 4.8. Different experimental setups have been described over the years for the electrospinning process. For instance, the work of Doshi and Reneker (1995) described the use of a capillary tube with a 1.5 mm tip with the fluid impulse by hydrostatic pressure; the collectors were different metallic screens. Yarin et al. (2001) described the electrospinning from both a pendant drop and sessile drop experimental setup (see Fig. 4.9). Shin et al. (2001) proposed the use of two aluminum plates arranged in parallel to gain control over the electric field, as shown in Fig. 4.10. In their setup, the top plate is connected to the power supply, whereas the bottom plate is insulated (Shin et al., 2001).

4.4.1

Electrospinning setup used for fiber alignment

Li et al. (2003) proposed the use of two conductive silicon stripes arranged in parallel and separated by a given gap as collectors to obtain uniaxially aligned fibers of PVP. The setup is shown in Fig. 4.11. A similar approach was proposed (Wong et al., 2008), but using two non-conductive strips wrapped with aluminum foil as the electrodes.

Rotating drum

Rotating wire drum

Rotating drum with wrapped wire

Rotating drum with sharp pin inside

Sharp pin

Translational movement

Rotating drum with knife edge electrodes

Rotating drum Multiple knife-edge electrodes Knife-edge blade

Knife-edge blade

Parallel ring collector

Rotating disk

Yarn collection using water bath

Knife-edge blades

Parallel electrodes

Array of counterelectrodes

Blade electrodes in line

Fig. 4.8 Different fiber collectors described in the literature. Reproduced (adapted) from Teo, W.E., Ramakrishna, S., 2006. A review on electrospinning design and nanofibre assemblies. Nanotechnology 17, R89. Copyright (2006) IOP Publishing, Ltd.

88

Electrospun Nanofibers

Lamp

Lamp

Camera High voltage

High voltage Camera

Pump

Fig. 4.9 Electrospinning setup using a pendant drop (left) and a sessile drop (right). Reproduced from Yarin, A.L., Koombhongse, S., Reneker, D.H., 2001. Taylor cone and jetting from liquid droplets in electrospinning of nanofibers. J. Appl. Phys. 90, 4836–4846 with permission from AIP Publishing LLC. Copyright (2001) AIP Publishing LLC.

Syringe pump

HV

Fig. 4.10 Parallel-plate electrospinning setup. The electric field is generated by the difference in voltage between two parallel plates. The polymer fluid is delivered to a metal capillary in the center of the top plate at a constant flow rate. Reprinted from Shin, Y.M., Hohman, M.M., Brenner, M.P., Rutledge, G.C., 2001. Experimental characterization of electrospinning: the electrically forced jet and instabilities. Polymer 42(25), 09955–09967. Copyright (2001), with permission from Elsevier.

R

A different electrospinning setup was proposed for the electrospinning of collagen and gelatin with polyethylene oxide (Kidoaki et al., 2005). The device consisted of a disk electrode in which the needles were inserted. Two techniques were tested, namely mixed electrospinning and multilayer electrospinning. For the mixing electrospinning, the collector was moving along one direction back and forth along the line of contact with the fibers jet, this allowed the formation of multiple layers (Kidoaki et al., 2005). Tubular-shaped nanofiber structures were prepared by a rotating mandrel as shown in Fig. 4.12.

Solution electrospinning of nanofibers

89

(A)

Needle

Si

V Power supply Si

Fiber

100 μm (B)

Collector Si

Si 2 μm

Fig. 4.11 Images showing the orientation of PVP nanofibers on a collector containing a two parallel silicon electrodes separated by a gap. (A) Dark-field optical micrograph of PVP nanofibers collected on top of the gap formed between the electrodes. (B) SEM images taken from the same sample, showing aligned nanofibers deposited across the gap. Reprinted (adapted) with permission from Li, D., Wang, Y., Xia, Y., 2003. Electrospinning of polymeric and ceramic nanofibers as uniaxially aligned arrays. Nano Lett. 3(8), 1167–1171. Copyright (2003) American Chemical Society.

Polymer solutions from syringes Plus terminal of power supply

Insulated ceramic pole

Needles Mandrel Rotation

Insulated acrylic board

Hole

Rotation motor Traverse

Ground Traverse motor

Fig. 4.12 Left side: Setup for mixing electrospinning. Right side, upper row: SEM images of the tubular bilayered constructs produced (upper row) and magnification of area 3 on image showing the layer of collagen and the layer of segmented polyurethane (SPU). Reprinted from Kidoaki, S., Kwon, I.K., Matsuda, T., 2005. Mesoscopic spatial designs of nano- and microfiber meshes for tissue-engineering matrix and scaffold based on newly devised multilayering and mixing electrospinning techniques. Biomaterials 26(1), 37–46. Copyright (2005), with permission from Elsevier.

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Electrospinning setups with multiple jets have also been proposed, for instance Theron et al. (2005) studied an array of nine needles arranged on both a squared matrix and a linear matrix (Theron et al., 2005). A cross-electrospinning method with multiple needles was used to prepare PVDF-HFP/PI composite nanofiber membranes, the setup consisted on a reciprocally moving multiple needle spinneret and a rotating drum as collector as shown in Fig. 4.13 (Chen et al., 2014).

PI

PVDF-HFP

High voltage Power supply

Collector

Fig. 4.13 Four-needle electrospinning setup. The needles are attached to a plate with reciprocant movement. Reprinted (adapted) from Chen, W., Liu, Y., Ma, Y., Liu, J., Liu, X., 2014. Improved performance of PVDF-HFP/PI nanofiber membrane for lithium ion battery separator prepared by a bicomponent cross-electrospinning method. Mater. Lett. 133, 67–70. Copyright (2014), with permission from Elsevier.

Recently, a centrifugal electrospinning setup was proposed for high throughput production of aligned nanofibers (Edmondson et al., 2012). The system consists of a spinneret attached to a rotating hub, which is driven by a variable speed motor. The collector consists of a series of metallic (aluminum) plate electrodes that were attached to a grounded metallic rod, a schematic of the system is shown in Fig. 4.14. This system was used to prepare aligned nanofibers from 20% solutions of polyvinylidene fluoride (PVDF) in dimethyl formamide/acetone mixture. It was observed that the degree of fiber alignment was improved by the centrifugal speed up to a maximum value of 300 rpm. Further increase on the rotating speed did not produce fibers. In addition, the fiber diameter was observed to decrease with the rotating speed, from 345 nm (for the static system, no rotation) to 224 nm at rotation speed of 300 rpm. The degree of alignment was also reduced as the gap distance between electrodes increased (Edmondson et al., 2012).

4.4.2

Hollow fibers: Coaxial electrospinning

A coaxial electrospinning setup was used to produce hollow fibers (Loscertales et al., 2004). The hollow fibers formed when two jets of immiscible liquids were electrospun at the same time. The fluid that goes on the inside works as an immiscible template for the

Solution electrospinning of nanofibers

91

Centrifugal electrospinning setup

(B) Syringespinneret with motor

Nonconductive chamber (2-4 ft diam.)

(A)

Grounded electrodes

Regulated pressure control (0–120 psi)

(C)

4 in.

Fig. 4.14 Centrifugal electrospinning system for large-area production of aligned polymer nanofibers. (A) Schematic illustration of the system configuration. (B) Photograph of the system with deposited PVDF nanofibers. (C) Electrospun PVDF fibers deposited across a 4-in. gap between two grounded electrodes. Reproduced with permission from Edmondson, D., Cooper, A., Jana, S., Wood, D., Zhang, M., 2012. Centrifugal electrospinning of highly aligned polymer nanofibers over a large area. J. Mater. Chem. 22, 18646–18652. Copyright (2012) The Royal Society of Chemistry.

formation of hollow fibers (see Fig. 4.15). This approach has been applied in the production of hollow fibers for other systems, for instance Li et al. (2005) produced hollow fibers of PVP with titanium alkoxide and using mineral oil as the immiscible internal phase. A slit-surface technique to obtain core-sheath fibers was also reported recently, the system can be observed in Fig. 4.16 (Sharma et al., 2013; Yan et al., 2015).

4.4.3

Electrospinning at high temperatures

Other variations of the experimental electrospinning setup are the use of a heating element or jacket along the solution dispenser; this will keep the temperature high to avoid gel formation at the tip. For instance, a combination of a heating jacket and the use of a laser source to keep the temperature high at the needle tip were used in the electrospinning of isotactic polypropylene in ortho-dichloro benzene, as illustrated in Fig. 4.17 (Wang et al., 2010). Other versions of this setup include the use of an infrared heater for the needle instead of heated oil (Liu et al., 2013).

4.4.4

Needle-less electrospinning

Needle-less electrospinning setups have also been proposed. In this case, the jets form at the surface of the liquid, which is immersed in the electrode. For instance, electrospinning from bubbles and foams has been reported recently (Varabhas et al., 2009). A simple

92

Electrospun Nanofibers

Sol or polymer

Inner fluid (template)

Detail of compound cone

Detail of compound jet

Inner cone Compound jet

Fig. 4.15 Electrospinning setup to produce hollow fibers. Two immiscible liquids (shown as red and blue) are injected through two concentric electrified needles. A compound Taylor cone is developed, from whose tip a coaxial nanojet is emitted. Reprinted (adapted) with permission from Loscertales, I.G., Barrero, A., Ma´rquez, M., Spretz, R., Velarde-Ortiz, R., Larsen, G., 2004. Electrically forced coaxial nanojets for one-step hollow nanofiber design. J. Am. Chem. Soc. 126, 5376–5377. Copyright (2004) American Chemical Society.

Fig. 4.16 Left: Image of the slit fixture. Center: Example of multiple electrospinning cone-jets formed across a slit-surface using the system at the left. Right: Representative scanning electron images depicting different types of core-sheath fibers fabricated using slit-surface electrospinning. Reproduced from Yan, X., Marini, J., Mulligan, R., Deleault, A., Sharma, U., Brenner, M.P., Rutledge, G.C., Freyman, T., Pham, Q.P., 2015. Slit-surface electrospinning: a novel process developed for high-throughput fabrication of core-sheath fibers. PLoS One 10, e0125407.

Solution electrospinning of nanofibers

93

Heating oil

Heating oil

Jacket er Las m bea

Laser source

Collector

Fig. 4.17 Illustration of electrospinning setup at high temperature.

setup was described to produce nanofibers from bubbles to form on the surface of a solution of PVP in ethanol. Similar to the electrospinning from droplets, the formation of a Taylor cone was observed (see Fig. 4.18).

Fig. 4.18 Electrospun jets launched from polymeric bubbles. Reproduced from Varabhas, J.S., Tripatanasuwan, S., Chase, G.G., Reneker, D.H., 2009. Electrospun jets launched from polymeric bubbles. J. Eng. Fibers Fabr. 4, 46–50. Copyright (2009).

Higham et al. (2014) used a multiple jet needleless setup to produce electrospun nanofibers from polyvinyl alcohol and polyethylene oxide foamed solutions. The setup is shown in Fig. 4.19. Pressurized carbon dioxide flowed through a porous plate into the polymer solution in contact with a copper wire that served as the electrode. Interestingly, the surface of the bubbles developed a conical shape, similar to the Taylor cone found in syringe electrospinning.

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Electrospun Nanofibers

Grounded collection plate Power supply Polymer foam column Copper electrode

(B)

+ Pressure gauge

Plate P

Holes Compressed gas tank

(A)

Rotameter

(C)

Fig. 4.19 Illustration of the foam electrospinning setup (A) and images of the actual foam during experiments utilizing a compressed gas flow rate of 310 cm at 20 psi showing, (B) multiple polymer fibers (some shown by arrows) formed on the foam electrospinning of 5 wt % PEO and 0.1 wt% Triton X-100® with 40 kV of applied voltage at a collection distance of 30 cm, and (C) bubble deformation occurring during electrospinning at 24 kV of applied voltage and 8 cm collection distance using 7 wt% PVA and 0.1 wt% rhodamine B dye for visualization. Reproduced from Higham, A.K., Tang, C., Landry, A.M., Pridgeon, M.C., Lee, E.M., Andrady, A.L., Khan, S.A., 2004. Foam electrospinning: a multiple jet, needle-less process for nanofiber production. AIChE J. 60(4), 1355–1364. Copyright (2004), with permission from John Wiley & Sons Inc.

It was also shown that by changing the shape of the copper electrode immersed in the polymer solution, the yield of the process was improved, as shown in Fig. 4.20. Sidaravicius et al. (2015) also used a foam electrospinning method for the study of the influence of solution parameters on the electrospinning of polyethylene oxide and polyvinyl alcohol solutions. Other needle-less electrospinning setups have been described. For instance, the use of a porous cylindrical tube connected to an electrode as depicted in Fig. 4.21 was used by Dosunmu et al. (2006) on the electrospinning of nylon 6.

4.4.5

Commercial and industrial electrospinning setup

The scale-up of the electrospinning process requires the understanding of the role of each of the variables that affect the process. The key for mass production is to have a high throughput in the process. To this end, the multiple nozzle electrospinning process has been studied and scaled to commercial devices. The scaling of the effect of electrical fields has been the focus of attention in the literature, with an emphasis on

Solution electrospinning of nanofibers

95

+

+

CO2

+

CO2

1-Dimensional

2-Dimensional

10 μm

(A)

CO2 Avg: 313 ± 54 nm

3-Dimensional

10 μm

(B)

Avg: 342 ± 80 nm

10 μm

(C)

Fig. 4.20 Different electrode setups (sketch at upper row) and corresponding scanning electron microscopy of the fibers obtained by electrospinning of a 7 wt% PVA in water solution. (A) 1-D electrode (ringed wire, 0.25 in.2 surface area), (B) 2-D electrode (wire mesh, 1.9 in.2 surface area), and (C) a 3-D electrode (“bird nest,” 5.8 in2 surface area). Thicker mats were obtained for 2-D and 3-D configurations. Reproduced from Higham, A.K., Tang, C., Landry, A.M., Pridgeon, M.C., Lee, E.M., Andrady, A.L., Khan, S.A., 2004. Foam electrospinning: a multiple jet, needle-less process for nanofiber production. AIChE J. 60(4), 1355–1364. Copyright (2004), with permission from John Wiley & Sons Inc.

1.7 cm Applied air pressure

Multiple jets

Porous cylindrical tube

Collector

15 cm 6 cm

Electrode

1 μm

Fig. 4.21 Left: Illustration of the cross-sectional view of the cylindrical porous tube with its axis oriented vertically within a coaxial cylindrical collector (not to scale). Right: SEM image of the obtained fibers. Reproduced from Dosunmu, O.O., Chase, G.G., Kataphinan, W., Reneker, D.H., 2006. Electrospinning of polymer nanofibres from multiple jets on a porous tubular surface. Nanotechnology 17(4), 1123. Copyright (2006) IOP Publishing Ltd.

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Electrospun Nanofibers

simulation and modeling of the behavior of the multiple jets (see Kim et al., 2006; Varesano et al., 2010; Theron et al., 2005; Kim et al., 2015). A list of industrial and lab scale electrospinning setup providers (as retrieved from ElectrospinnTech: http://electrospintech.com/espin-supplier.html#.VkZkdnarRhE) is shown in Table 4.3. The table is provided for reference purposes only and is not exhaustive, the author neither endorses nor recommends any provider. The interested reader is referred to each website for more information and capabilities of the equipment available. Table 4.3

Some commercial providers of electrospinning devices

Company name

Website

4SPIN ANSTCO Bioinicia

http://www.4spin.info/ http://anstco.com/english/indexen.html http://bioinicia.com/, Fuidnatek® http://fluidnatek. com/ http://www.espinnanotech.com http://www.electrospinz.co.nz http://www.electrospunra.com http://elixirtechnologies.com/electro_spinning.html http://www.elmarco.cz/ http://www.ehuber.de/ http://en.fnm.ir/ http://www.fuence.co.jp/en http://grafen.com.tr/ http://www.holmarc.com/nano_fiber_electrospinning_ station.php http://www.imetechnologies.nl/Electrospinningn206m266 http://www.inovenso.com http://english.keskato.co.jp/products/neu.html http://www.linaribiomedical.com/index.php/en/ http://www.mecc.co.jp/en/html/nanon/list.html http://www.mtixtl.com/ http://www.electro-spinning.com http://www.nadetech.com/ http://www.nanocat.it/ http://www.nanoflux.com.sg http://vajendra.wix.com/indiaelectrospinning http://www.nanonc.co.kr/ http://www.phyeqpt.in/ http://www.progenelink.com/ http://www.ske.it/material-technologies/e-fiberelectrospinning-platform http://www.spinbow.it/ http://www.spraybase.com http://www.spur-nanotechnologies.cz/ http://www.yflow.com/

E-Spin NanoTech Pvt. Ltd. Electrospinz Electrospunra Elixir Technologies Elmarco Erich Huber GmbH Fnm Co. Fuence Grafen Inc. HOLMARC IME Technologies inovenso KatoTech Co. Ltd Linari Engineering s.r.l MECC Co. Ltd MTI Corporation NaBond Nadetech Innovations Nano-Cat Nanoflux Nanomate Electrospinning NanoNC Physics Equipment Progene Link Sdn Bhd SKE S.r.l. SPINBOW Spraybase SPUR Yflow

Solution electrospinning of nanofibers

4.5

97

Solution electrospinning of synthetic soluble polymers

The electrospinning of synthetic organosoluble polymers has been reported since the early work of Formhals (1934), who used cellulose esters dissolved in acetone/alcohol mixtures. The advantage of organosoluble polymers is the vast amount of solvents with different properties available (Greiner and Wendorff, 2007). However, not all the organic solvents available for a given polymer will meet all the required criteria for electrospinning in terms of volatility, toxicity, and effectivity in dissolving the polymer (see previous discussion on solubility). For instance, some solvents, such as formic acid, can be very corrosive, and others, such as hexafluoroisopropanol, are toxic and very volatile. The volatility of the solvent plays an important role in the solidification of the jet, if it is too volatile, then the polymer solution will solidify inside the nozzle before the jet forms. The list of polymers that have been electrospun from solutions into fibers is extensive; it includes polyethylene terephthalate (PET), polyacrylic acid, PS, polyacrylonitrile, polycarbonate, PMMA, poly (vinyl butyral), polyvinyl chloride, PVDF, polyurethane, cellulose esters, and many others. Of particular importance is the functionality of the polymer nanofibers, which in turn depends on the intended application. For instance, polyesters and polyamides find applications in textiles and apparel, and electro spun fibers from these polymers have been proposed for applications in tissue engineering. The mechanical properties of the polymeric nanofibers are also important, and this again comes back to the inherent properties of the polymer. Some applications include the fabrication of energy harvesting devices and lightweight materials. For instance, piezoelectric PVDF fibers were produced by electrospinning using a rotating collector and near-field electrospinning (Yu and Cebe, 2009; Yee et al., 2008).

4.5.1

Polyolefins

Among the properties to consider in the final electrospun fibers are the microstructure of the polymer, in fact the tactility of the polymer affects the crystallinity and morphology of the molecule, and therefore its thermal properties. For instance, syndiotactic polypropylene (sPP) was electrospun at room temperature from solution. The solvent was a mixture of 80/10/10 cyclohexane/acetone/DMF. The formed fibers developed some “microholes” as shown in Fig. 4.22. When comparing the wide-angle X-ray diffraction (XRD) patterns of electrospun fibers and cast films, they exhibit different crystalline structures, reportedly due to strain-induced crystallization during electrospinning (Lee et al., 2009). Watanabe et al. (2011) studied the effect of different organic solvents on the morphology and mechanical properties of electrospun sPP fibers. They used a mixture of organic solvents, decalin/acetone/DMF in weight ratios 80/10/10 and cyclohexane/acetone/DMF in the ratio 80/10/10. The nanofibers prepared from the mixture with decalin exhibit better morphology (smooth uniform surface) and showed enhanced mechanical properties (higher tensile stress and elongation at break) compared to those from the mixture of cyclohexane (Watanabe et al., 2011).

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Electrospun Nanofibers

5 µm

2 µm

Fig. 4.22 SEM images of electrospun syndiotactic polypropylene (sPP) fibrous membrane from solutions at a lower temperature (about 20–25°C). Note the formation of holes on the structure of the fibers. Reprinted (adapted) with permission from Lee, K.-H., Ohsawa, O., Watanabe, K., Kim, I.-S., Givens, S.R., Chase, B., Rabolt, J.F., 2009. Electrospinning of syndiotactic polypropylene from a polymer solution at ambient temperatures. Macromolecules 42(14), 5215–5218. Copyright (2009) American Chemical Society.

The electrospinning of sPP at high temperature (>80°C) from solutions in ortho-dichlorobenzene (o-DCB) using tetra-n-butyl ammonium perchlorate (Bu4ClO4) to enhance the conductivity of the solution was also reported. Aligned fibers were obtained by using a rotating disk collector. Different crystalline forms were observed after annealing the fibers at different temperatures. For instance, the as-spun sPP fibers (at 35°C) contain a mixture of all-trans mesophase, and helical crystalline forms as shown in the 2D-WAXD results (Jao et al., 2014). Isotactic polypropylene was electrospun from a solution on o-DCB using a high temperature setup. Retardation of chain crystallization and crystal transition from meso to monoclinic in the as-spun fibers was reported at these conditions (Wang et al., 2010).The effect of solution concentration on the electrospinning of sPP from solution in o-DCB using a high temperature (80°C) setup was studied (Jao et al., 2014). It was found that solutions with a concentration close to the entanglement concentration (2.7 wt%) produced electro spraying, whereas those above the critical entanglement concentration produced uniform fibers (see Fig. 4.23). Different crystalline structures were observed for the fibers produced at different concentrations, for instance, random fibers produced from 5 wt% solution exhibit mesophase formation, whereas those from 8 wt% exhibit form I crystallites. Aligned fibers were obtained after using a rotating disk collector, these fibers exhibit mesophase formation regardless of the concentration of sPP. The average diameters of sPP fibers are 137  76, 149  58, 182  96, and 354  180 nm for electrospinning solutions of 5, 7, 8, and 9 wt%, respectively (Jao et al., 2014).

Solution electrospinning of nanofibers

99

Fig. 4.23 Effect of solution concentration on the morphology of electrospun fibers of sPP observed by SEM. sPP fibers were collected by a stationary plate, except the bottom right, in which fibers were collected by a rotating disk with a linear velocity of 15.7 m/s. The arrow points toward the collecting direction. The scale bar of the images is 20 μm and that of the inset image is 2 μm. Reprinted from Jao, C.-S., Wang, Y., Wang, C., 2014. Novel elastic nanofibers of syndiotactic polypropylene obtained from electrospinning. Eur. Polym. J. 54, 181–189. Copyright (2014), with permission from Elsevier.

100

4.5.2

Electrospun Nanofibers

Polyethylene

Linear low density polyethylene (PE) (Mw ¼ 190 kDa) was electrospun from a solution of 2–5 wt% in p-xylene using a using tetrabutyl ammonium bromide to increase the conductivity of solution and a high temperature setup, fibers with diameters between 2 and 7 μm were obtained (Givens et al., 2007). The electrospun fibers exhibit similar crystalline forms and percentages of crystallinity (56%) as the bulk material (60%) as determined by XRD analysis. Nonetheless, the morphology of the thick fibers exhibits a rough surface (see Fig. 4.24; Givens et al., 2007).

Fig. 4.24 FESEM images of linear low-density polyethylene electrospun fiber at 3 K (left), 15 K (middle), and 20 K magnification. The fibers exhibit a rough surface. Reprinted (adapted) with permission from Givens, S.R., Gardner, K.H., Rabolt, J.F., Chase, D.B., 2007. High-temperature electrospinning of polyethylene microfibers from solution. Macromolecules 40(3), 608–610. Copyright (2007) American Chemical Society.

Ultrahigh molecular weight PE (Mw ¼ 6  106 Da) was electrospun at a high temperature (100–160°C) from a solution using a mixture of equal parts of p-xylene and cyclohexanone. Aligned fibers were obtained by using a disk collector. Wide-angle X-ray scattering results indicate the presence of oriented and unoriented crystals. The presence of cyclohexanone on the solvent helped to increase the electrical conductivity of the solvent, which in turn resulted in fibers with smaller diameters (Rein et al., 2007). An orientation analysis of electrospun high density PE fibers indicates that the crystalline forms are different, depending on the diameter of the fiber. It was proposed that thicker fibers exhibit a folded-chain crystal structure, whereas thinner fibers exhibit a structure from shish-kebab to fibrillar, as shown in Fig. 4.25 (Yoshioka et al., 2010b). Similarly, a parallel-electrode collector was used to obtain aligned PE fibers that exhibit a multiple-neck deformation pattern, ie, a collection of fibers followed by a non-fibrillated part (see Fig. 4.26; Yoshioka et al., 2010a). A recent report describes the electrospinning of ultrahigh molecular weight PE (3–6  106) dissolved in a p-xylene and cyclohexanone mixture indicates an increase in the thermal conductivity (compared to bulk PE) of the electrospun fibers with the

Solution electrospinning of nanofibers

A

A⬘

(i)

f > 1 μm

A⬙

B

101

B⬘

C

(ii) 1 μm > f > 400 nm

C⬘

(iii)

B⬙

C⬙

D

D⬘

400nm > f D⬙

002 200

α

60° 110

a = 180°

A⵮

60° > a

120° > a > 60°

B⵮

Physical cross-link (micellar junction or crystallite) (Zuo et al., 2007)

C⵮

D⵮

q

Folded-chain crystal

Shish

Kebab

Fig. 4.25 Bright-field transmission electron microscopy images of electrospun high density PE fibers of different diameters (A–D), corresponding selected-area electron diffraction patterns obtained from each single fiber (A0 –D0 ), schematic reflection patterns (A00 –D00 ), and models of fiber structures for each fiber diameter (A000 –D000 ). Reprinted from Yoshioka, T., Dersch, R., Tsuji, M., Schaper, A.K. 2010. Orientation analysis of individual electrospun PE nanofibers by transmission electron microscopy. Polymer 51, 2383–2389. Copyright (2010), with permission from Elsevier.

1 μm

500 nm

Fig. 4.26 SEM images showing multiple necking fiber (left) and the magnified image (right) from the fibrillated part. The electrospinning conditions were 20 kV, 100 mm distance to collector using a parallel-electrode collector. Reprinted from Yoshioka, T., Dersch, R., Greiner, A., Tsuji, M., Schaper, A.K., 2010. Highly oriented crystalline PE nanofibrils produced by electric-field-induced stretching of electrospun wet fibers. Macromol. Mater. Eng. 295(12), 1082–1089. Copyright (2010), with permission from John Wiley and Sons.

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Electrospun Nanofibers

applied electrospinning voltage. The high thermal conductivity was ascribed to the higher degree of orientation and enhanced level of crystallinity of the PE fibers (Ma et al., 2015).

4.5.3

Polyesters

PET is one of the main polyesters commercially used for the production of fibers. Compared to polybutylene terephthalate, PET has a slower crystallization rate, which makes it difficult to mold into products. PET have been electrospun from solutions in different solvents, including a 50:50 mixture of dichloromethane and trifluoroacetic acid (Darrell and Iksoo, 1996; Veleirinho et al., 2008). Kim et al. (2004) found that the PET electrospun fibers were changed, as well as the mechanical properties being improved when the rotating velocity of the collector drum increased (Kim et al., 2004). Chen et al. (2009) used hexafluoroisopropanol as the solvent to prepared PET nanofibers filled with multiwalled carbon nanotubes (MWCN) (Chen et al., 2009). The addition of MWCN decreased the crystallization temperature of PET (by 14°C with up to 2% nanotubes). Veleirinho et al. (2008) found a concentration of 10 wt% of PET in trifluoroacetic acid/dichloromethane as the threshold concentration of PET to obtain uniform fibers. Wang et al. found that the specific viscosity of PET in trichloroacetic acid scaled with the concentration as η  ϕ3.7 and the diameter scaled with the concentration of PET in solution. The as-spun PET fibers exhibit mesomorph phase formation, which transformed into triclinic crystals after annealing at temperatures higher than 100°C. When embedded into an isotactic polypropylene, the PET fibers induced trans crystallization of the iPP (Wang et al., 2012). A similar effect was observed with fibers of syndiotactic PS fibers embedded on a matrix of iPP (see Fig. 4.27; Wang et al., 2011).

4.6

Solution electrospinning of natural polymers

Biopolymers have attracted a lot of attention from the scientific community for the development of functional materials with improved properties. Several biopolymers have been studied to produce electro spun nanofibers and the extensive literature on the topic has been reviewed (Schiffman and Schauer, 2008; Krishnan et al., 2013; Mokhena et al., 2015; Frey, 2008; Elsabee et al., 2012; Jayakumar et al., 2010; Greiner and Wendorff, 2007). These biopolymers include proteins (zein, wheat gluten, soybean proteins, silk fibroin, collagen, casein, gelatin, fibrinogen, elastin, etc.), polysaccharides, and derivatives (alginate, cellulose, hyaluronic acid, dextran, chitin, chitosan), and DNA (Fang and Reneker, 1997). Electrospun nanofibers have also been produced from lignin (Salas et al., 2014; Dallmeyer et al., 2010; Ruiz-Rosas et al., 2010; Ago et al., 2012). Lignin is present in plant cell walls, and is obtained as a byproduct from the pulp and paper industry. Similar to the electrospinning from synthetic polymers, in the case of natural polymers, the solvent of choice is very important. Because of their molecular structure,

Solution electrospinning of nanofibers

Tc = 116°C

103

Tc =118° C

Fig. 4.27 The evolution of surface-induced crystallization of iPP by the electrospun syndiotactic polystyrene fibers. The diameter of fiber on the left is 2.0 μm and that on the right is 0.2 μm. Left column shows the process at 116°C under cross-polarized light, and right column obtained at 118°C under phase contrast. The scale bar is 50 μm. Reprinted from Wang, C., Liu, F.-H., Huang, W.-H., 2011. Electrospun-fiber induced transcrystallization of isotactic polypropylene matrix. Polymer 52(5), 1326–1336. Copyright (2011), with permission from Elsevier.

many biopolymers are difficult to dissolve in water, and most organic solvents. Even when they dissolve, in some cases it is necessary to use a coadjutant polymer that can help in the adjustment of viscosity and improved mechanical properties of the electrospun nanofibers. In some instances, the addition of other components is necessary to obtain uniform bead-free nanofibers, for instance, the addition of salts to increase conductivity or addition of surfactants to help in reducing the surface tension of the system. Water soluble polymers such as polyvinyl alcohol or polyethylene

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Electrospun Nanofibers

oxide are widely used as coadjutant polymers to produce biopolymer scaffolds by electrospinning.

4.7

Highlights and concluding remarks

An overview of the solution electrospinning process is presented with a brief discussion of main parameters that affect the production of uniform fibers. The increasing amount of publications on the topic every year highlights the potential of this technique for the development of functional fibers from a variety of polymers and for different applications. The advantage of solution electrospinning is the availability of different solvents for a given polymer, which allows the tuning of the properties of the precursor solutions to achieve uniform fibers; in addition, other coadjutant molecules can be used, such as surfactants to help reduce the surface tension, or salts to increase the conductivity. There is huge interest in the use of biomaterials for developing functional materials, and electrospinning from solution represents one simple alternative. In addition, as new technological challenges emerge, the electrospinning process continues to evolve, as indicated by the different alternatives and experimental setups used to produce coaxial composite fibers, hollow fibers, and aligned fibers. Similarly, the understanding of the mechanism and forces involved in the electrospinning process has allowed the industrial scale-up of the process for the massive production of nanofibers.

References Agarwal, S., Greiner, A., Wendorff, J.H., 2013. Functional materials by electrospinning of polymers. Prog. Polym. Sci. 38, 963–991. Ago, M., Okajima, K., Jakes, J.E., Park, S., Rojas, O.J., 2012. Lignin-based electrospun nanofibers reinforced with cellulose nanocrystals. Biomacromolecules 13, 918–926. Ahmed, F.E., Lalia, B.S., Hashaikeh, R., 2015. A review on electrospinning for membrane fabrication: challenges and applications. Desalination 356, 15–30. Baji, A., Mai, Y.-W., Wong, S.-C., Abtahi, M., Chen, P., 2010. Electrospinning of polymer nanofibers: effects on oriented morphology, structures and tensile properties. Compos. Sci. Technol. 70, 703–718. Balamurugan, R., Sundarrajan, S., Ramakrishna, S., 2011. Recent trends in nanofibrous membranes and their suitability for air and water filtrations. Membranes 1, 232. Bazilevsky, A.V., Yarin, A.L., Megaridis, C.M., 2007. Co-electrospinning of core–shell fibers using a single-nozzle technique. Langmuir 23, 2311–2314. Bhardwaj, N., Kundu, S.C., 2010. Electrospinning: a fascinating fiber fabrication technique. Biotechnol. Adv. 28, 325–347. Boudriot, U., Dersch, R., Greiner, A., Wendorff, J.H., 2006. Electrospinning approaches toward scaffold engineering—a brief overview. Artif. Organs 30, 785–792. Carroll, C.P., Joo, Y.L., 2008. Axisymmetric instabilities of electrically driven viscoelastic jets. J. Non-Newtonian Fluid Mech. 153, 130–148. Chen, H., Liu, Z., Cebe, P., 2009. Chain confinement in electrospun nanofibers of PET with carbon nanotubes. Polymer 50, 872–880.

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