Electrospinning of polymer nanofibers: Effects on oriented morphology, structures and tensile properties

Electrospinning of polymer nanofibers: Effects on oriented morphology, structures and tensile properties

Composites Science and Technology 70 (2010) 703–718 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: ww...

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Composites Science and Technology 70 (2010) 703–718

Contents lists available at ScienceDirect

Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech

Review

Electrospinning of polymer nanofibers: Effects on oriented morphology, structures and tensile properties Avinash Baji a, Yiu-Wing Mai a,b,*, Shing-Chung Wong c, Mojtaba Abtahi a, Pei Chen c a

Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia Department of Mechanical Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong, China c Department of Mechanical Engineering, The University of Akron, Akron, Ohio 44325, USA b

a r t i c l e

i n f o

Article history: Received 19 October 2009 Received in revised form 12 January 2010 Accepted 14 January 2010 Available online 20 January 2010 Keywords: A. Fibers A. Nano composites B. Mechanical properties D. X-ray diffraction (XRD) E. Electro-spinning

a b s t r a c t The interest in fabrication of nanofibers using electrospinning method has attracted considerable attention due to its versatile maneuverability of producing controlled fiber structures, porosity, orientations and dimensions. Although the process appears to be simple and straightforward, an understanding of the technique and its influence on the morphology, structural and mechanical properties is still not completely clear. Recently, the size effect on the mechanical properties was reported for fibers across different length scales. Both modulus and strength of poly(e-capro-lactone) (PCL) fibers were found to increase significantly when the diameter of the fibers was reduced to below 500 nm. In this article, for the first time, we critically review and evaluate the role of the microstructures on the fiber deformation behavior and present possible explanations for the enhanced properties of the nanofibers. Our discussions are focused on the techniques to obtain controlled structures and the mechanisms behind the size effect in electronspun fibers are given. In-depth understanding of these mechanisms can provide fruitful outcomes in the development of advanced nanomaterials for devices and miniaturized load-bearing applications. Ó 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4.

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7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrospinning theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control of fiber diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alignment of fibers and fiber collection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Rotating drum collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Rotating disk collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Static parallel electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural properties of electrospun fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Molecular orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Effect of fiber diameter on structural properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Effect of collector on the structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical properties of the fibers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Effect of structural morphology on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Effect of collector type on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. Stationary collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Rotational collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Effect of fiber diameter on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prospective applications of electrospun fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Fiber composites for tissue engineering applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

704 705 706 706 707 708 708 709 709 710 710 710 711 711 712 712 712 712 713 713

* Corresponding author. Address: Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia. Tel.: +61 2 9351 2290; fax: +61 2 9351 3760. E-mail address: [email protected] (Y.-W. Mai). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.01.010

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7.2. Electrospun fiber reinforced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Conductive fiber composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Filler reinforced fiber systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The drive for ultra-lightweight yet strong structures for devices and miniaturized applications has motivated novel designs using polymer nanofibers [1]. Electrospinning has emerged as a powerful technique for producing high strength fibers due to its versatility, ease of use, ability to align structures and control fiber diameters [2–9]. Some of these unique features cannot be otherwise achieved by conventional fiber processing techniques. Another merit is that under the influence of an electric field, electrospinning self-assembles dispersed fillers along the axial direction such that composites can be formed by imposing additional spatial confinement to the polymer chains [5,10,11]. These reinforced fibers display superior properties and function as basic building blocks for the fabrication of high strength structures using a bottom-up approach. For example, carbon nanotubes (CNTs) and carbon black (CB) particles are among the commonly used fillers which are dispersed within the fibers to mimic the functionality of silk fibers for high strength and toughness applications [1]. This feature of dispersing filler materials can be easily extended to other applications such as filtration [12,13], tissue engineering [9,14,15], precursor for fabricating nanofiber composites [16–18] and advanced nanomaterials [4,19] etc. Despite possessing these unique features, one of the main challenges in this area is to characterize the tensile behavior of the nanofibers. This could be due to the difficulty in handling the nanofibers and also due to the low load required for the deformation. Hence, in most cases, the mechanical integrity of the fibers and fiber network structures is least understood and an understanding of the phenomenon is urgently needed. Few researchers actively pursued to characterize the mechanical deformation characteristics of the fibers by recording the stress–strain behavior of

714 714 715 715 716 716 716

the electrospun non-woven fabrics. However, this method cannot be deemed suitable because the tensile response of the non-wovens are greatly influenced by the fiber size distribution in the mats, porosity, individual fiber orientation in the mat, fiber–fiber interaction and entanglement of the fibers [20]. These parameters cannot be easily isolated and controlled in the non-woven fabrics. Hence, there has been a remarkable growth and interest in characterizing the tensile deformation behavior of single fibers and aligned fiber bundles [5–8]. More recently, it was demonstrated that the size effect is critical in influencing the fiber properties and an abrupt increase in tensile properties is observed at a given average fiber diameter [5–8]. The size effect in the fibers is attributed to the process of electrospinning that results in the formation of unique intrinsic structures within the fiber geometry [5–8]. Hence, the focus of this study is to review recent articles that characterize the intrinsic structural properties of the electrospun fibers and present possible explanations for the enhanced tensile behavior of the nanofibers. Recent articles on electrospinning focused on various spinnable polymeric materials, processing techniques for fabricating nanofiber assemblies, effects of processing parameters on fiber diameter and morphology, characteristics of the fibers and their applications [9,12,21–23]. However, the influences of electrospinning on the structure formation and on the tensile strength of the fibers are still lacking. To realize the full potential of the fibers it is essential to understand the microstructure formation during electrospinning, since the intrinsic structures of the fibers affect their overall mechanical deformation behavior. For example, the ordered arrangement of the polymer chains within the fiber geometry during electrospinning leads to strengthening of the fibers [5–8]. Thus, the main objective of this review is to outline possible mechanisms that lead to the fabrication of stronger fibers and thereby facilitate

Fig. 1. Schematic of the general laboratory setup used for an electrospinning experiment. The inset shows the SEM morphology of the electrospun nylon 6,6 fibers. The schematic illustrates the inverted conical path the jet travels before being collected as randomly oriented fibers as shown in the inset SEM micrograph. L represents the length of pipette containing the polymer solution and H is the distance between the tip and collector.

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(A)

(B)

705

(C)

Polymer solution

Taylor cone Pendant drop

Induced charges from electric field Jet initiation

Fig. 2. Schematic illustration of the Taylor cone formation: (A) Surface charges are induced in the polymer solution due to the electric field. (B) Elongation of the pendant drop. (C) Deformation of the pendant drop to the form the Taylor cone due to the charge-charge repulsion. A fine jet initiates from the cone.

an in-depth understanding of the electrospinning process and its role on the microstructure formation. Properties such as molecular orientation and crystallinity of the nanofibers and the factors that influence their deformation behavior are thoroughly analysed. The effects of fiber size on the tensile strength and elastic modulus of the fibers are also discussed. The review is organized as follows: Sections 2 and 3 discuss the basic concept behind electrospinning and control of fiber diameter. Common techniques used to collect controlled morphology of the fibers are described in Section 4. The mechanisms which yield controlled morphology, structures and spatial arrangement of the nanofibers are critically reviewed. The effects of electrospinning, fiber diameter and type of collector used to control the structures, such as crystallinity and molecular orientation, are discussed in Section 5. Various factors that determine the mechanical deformation behavior of the fibers are presented in Section 6. Here, the orderly arrangement of fiber structures as a function of fiber size is emphasized. Finally, potential applications of these fibers are given in Section 7.

Fig. 3. (A) SEM micrograph of the fibers showing typical circular morphology and (B) SEM micrograph of the flat ribbon structure.

2. Electrospinning theory Electrospinning or electrostatic spinning is a simple technique which utilizes high electrostatic forces for fiber production. Electrospinning, first introduced by Formhals [24] and later revived by Reneker [3,4], uses high voltage (about 10–20 kV) to electrically charge the polymer solution for producing ultra-fine fibers (diameters ranging from a few nanometer to larger than 5 lm) [3]. Fig. 1 shows a schematic illustration of the basic electrospinning setup, which essentially consists of a pipette or a syringe filled with polymer solution, a high voltage source and a grounded conductive collector screen. In addition, a metering syringe pump can be used to control the flow rate of the polymer solution. The needle of the syringe typically serves as an electrode to electrically charge the polymer solution and the counter-electrode is connected to the conductive collector screen. Under the influence of a strong electrostatic field, charges are induced in the solution and the charged polymer is accelerated towards the grounded metal collector. At low electrostatic field strength, the pendant drop emerging from the tip of the pipette is prevented from dripping due to the surface tension of the solution [25–27]. As the intensity of the electric field is increased, the induced charges on the liquid surface repel each other and create shear stresses. These repulsive forces act in a direction opposite to the surface tension [28], which results in the extension of the pendant drop into a conical shape and serves as an initiating sur-

face [29–31]. A schematic of the process is shown in Fig. 2. When the critical voltage is reached, the equilibrium of the forces is disturbed and a charged jet emanates from the tip of the conical drop. The discharged jet diameter decreases in size with concomitant increase in length before being deposited on the collector. This process can be explained by the three types of physical instabilities experienced by the jet [25,26]. These instabilities influence the size and geometry of the deposited fibers. The first instability, also known as the Rayleigh instability is axisymmetric and occurs when the strength of electric field is low or when the viscosity of the solution is below the optimum value. Use of very low viscosity solutions causes jet break-up and leads to the bead-on-fiber morphology. It is attributed to the poor chain entanglement density in the solution and insufficient resistance to the electrostatic field [31,32]. Rayleigh instability is suppressed at high electric fields (high charge densities) or when using higher concentration of polymer in the solution. Following the initial straight path of the jet, which is controlled by the Rayleigh instability, the polymer jet is influenced by two other instabilities: the bending and whipping instabilities. These instabilities arise owing to the charge-charge repulsion between the excess charges present in the jet which encourages the thinning and elongation of the jet [25,26]. At high electric forces, the jet is dominated by bending (axisymmetric) and whipping instabil-

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ity (non-axisymmetric), causing the jet to travel in an ‘‘inverse cone” manner. It produces wave or dumb-bell shaped patterns in the jet as shown in Fig. 1. At higher electric fields and at sufficient charge density in the jet, the axisymmetric (i.e., Rayleigh and bending) instabilities are suppressed and the non-axisymmetric instability is enhanced. The whipping instability produces a bending force on the jet, resulting in a high degree of elongation of the jet [32]. During these processes, the solvent evaporates and finally leads to the deposition of ultra-fine fibers on the conductive ground electrode. 3. Control of fiber diameter Systematic investigations on the effect of electrospinning parameters on the diameter and the morphology of the fibers have been reported by several researchers [33–35]. Major factors that control the diameter of the fibers are: (1) concentration of polymer in the solution, (2) type of solvent used, (3) conductivity of the solution, and (4) feeding rate of the solution. An example of the effect of parameters on fiber geometry is shown in Fig. 3. Fig. 3a shows typical circular fibers and Fig. 3b shows flat fiber belts that are obtained because of the rapid evaporation of the solvent. The flattened fibers are obtained when a fraction of the solvent is trapped inside the fiber. When the solvent evaporates, the fiber collapses, resulting in flat fiber belts. Clearly, there is a critical need to produce fibers of uniform diameters so that the electrospinning process can be rendered reproducible for scientific modeling and industrial applications. Fridrikh et al. determined the parameters that control the fiber diameter using an analytical model [33] that is based on the differences between surface tension of the solution and the electrostatic charge repulsion in the jet. At high electrical field, the motion of the jet is influenced by three main forces, namely: (a) external electric field, (b) normal stresses, which comprise the surface tension and tension resisting the bending of electric field lines in the jet, and (c) surface charge repulsion. Bending and stretching is a direct effect of normal stresses, which originate from the bending of the centerline of the jet. Hence, the normal stress gives rise to the whipping instability. When the surface charge repulsion exceeds the surface tension, it leads to the whipping instability and bending of the jet. At this stage, the current is constant and consists of conduction and advection current. At the later stage of whipping, the bulk current is dominated by the advection current and the surface charge repulsion is balanced by the surface tension. At this stage, the stretching of the jet is ceased and a constant diameter of the jet is obtained. The developed model predicts the diameter of this terminal jet, assuming that no further thinning of the jet occurs. Thus, the final diameter of the fiber (D) is determined to be a function of surface tension, electric current and surface charge repulsion. The equation for the diameter is:



cn

Q2 I2



2 l

!13 

p 2 ln d  3

ð1Þ

where c is surface tension of the solution, n dielectric constant, Q flow rate of the solution, I current carried by the jet, l initial jet length and d diameter of the nozzle. Primarily, flow rate, electric current and surface tension of the solution control the whipping jet diameter. For instance, increasing the current carrying ability of the jet by 32 times or reducing the flow rate by 32 times results in a ten-fold fiber diameter reduction [33,36]. The flow rate of the solution to the nozzle can be easily controlled by using a flow meter. This model is certainly not comprehensive, considering the number of parameters that would control fiber diameters. The model, however, neglects the elastic effect due to solvent evapora-

Fig. 4. Schematic of the rotating drum used for fiber collection. The inset SEM micrograph shows the aligned fibers obtained using the rotating drum.

tion and considers the solution Newtonian. The model also neglects the volatility of the solvents and charge carrying ability of the polymers. The accuracy of predicting the diameter of the fiber depends on the charge carrying ability of the jet. When non-conductive polymers such as PCL are used for electrospinning, the charges are solely accommodated by the volatile solvent. The charges from the evaporated solvent may reach the collector, which contributes to the measured current [33], and which leads to over-predicting the stretching of the jet. Hence, the model cannot predict the fiber diameters accurately for the polymers in a highly volatile solvent. However, theoretical fiber diameters of conductive polymers agree well with experimental values. This is due to the fact that the charges stay with the jet until it reaches the collector and drying occurs after the stretching of the jet. Nonetheless, the model provides a simple analytical method to estimate the diameter of the fibers with convincing agreement. Eq. (1) evaluates the terminal diameter considering that the collector is stationary. However, further thinning of the fibers can be obtained when rotating collectors such as a rotating drum or a rotating disk collector is used [8]. Kotaki et al. [37] showed that the speed of the rotating collector induced tensile stresses on the fibers before being wound around the collector. The tensile stresses are responsible for further thinning of the fiber diameter, which is not predicted by Eq. (1).

4. Alignment of fibers and fiber collection methods Recently, it was determined that the nature of the collector influences significantly the morphological and the physical characteristics of the spun fibers [38,39]. The density of the fibers per unit area on the collector and fiber arrangement are affected by the degree of charge dissipation upon fiber deposition. The most com-

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707

Fig. 5. Schematic of the disk collector used for fiber collection. The SEM micrograph shows the alignment of the fibers obtained using the disk collector. Better alignment of the fibers is observed compared to the rotating drum.

monly used target is the conductive metal plate that results in collection of randomly oriented fibers in non-woven form as shown in Fig. 3a. Liu et al. [38] electrospun cellulose acetate fibers on copper mesh, aluminum foil, paper and water as collectors. They found the type of collector used greatly determined the arrangement and packing density of the fibers. The use of metal and conductive collectors helped dissipate the charges and also reduced the repulsion between the fibers. Therefore, the fibers collected are smooth and densely packed. Conversely, the fibers collected on the non-conductive collectors do not dissipate the charges which repel each other. Hence, the fibers are loosely packed. The fibers can also be collected on specially designed collector systems so as to obtain aligned fibers or arrays of fibers. Recently, researchers focused on achieving highly ordered aligned fibers by using mechanical and electrostatic methods to control the electrospinning process. Aligned fibers have found importance in many

engineering applications, such as tissue engineering, sensors, nanocomposites, filters, electronic devices [40–43]. Some commonly used techniques to align the fibers are discussed in the subsections below. 4.1. Rotating drum collector The schematic of the electrospinning setup with a rotating drum collector is shown in Fig. 4. This method is commonly used to collect aligned arrays of fibers. Furthermore, the diameter of the fiber can be controlled and tailored based on the rotational speed of the drum [40–42]. The cylindrical drum is capable of rotating at high speeds (a few 1000 rpm) and of orienting the fibers circumferentially. Ideally, the linear rate of the rotating drum should match the evaporation rate of the solvent, such that the fibers are deposited and taken up on the surface of the drum. The

Fig. 6. Schematic of static electrodes used for collecting aligned fiber bundles. The optical micrograph shows the aligned fibers collector using this technique [5].

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alignment of the fibers is induced by the rotating drum and the degree of alignment improves with the rotational speed [40,43]. At rotational speeds slower than the fiber take-up speed, randomly oriented fibers are obtained on the drum. At higher speeds, a centrifugal force is developed near the vicinity of the circumference of the rotating drum, which elongates the fibers before being collected on the drum [23,43,44]. However, at much higher speeds, the take-up velocity breaks the depositing fiber jet and continuous fibers are not collected. 4.2. Rotating disk collector The rotating disk collector is a variation setup of the rotating drum collector and is used to obtain uniaxially aligned fibers. Fig. 5 shows the common setup. The advantage of using a rotating disk collector over a drum collector is that most of the fibers are deposited on the sharp-edged disk and are collected as aligned patterned nanofibers [23,43–46]. The jet travels in a conical and inverse conical path with the use of the rotating disk collector as opposed to a conical path obtained when using a drum collector. During the first stage, the jet follows the usual envelope cone path which is due to the instabilities influencing the jet. At a point above the disk, the diameter of the loop decreases as the conical shape of the jet starts to shrink. This results in the inverted cone appearance, with the apex of the cone resting on the disk. The electric field applied is concentrated on the tapered edge of the disk and hence the charged polymer jet is pulled towards the edge of the wheel, which explains the inverted conical shape of the jet at the disk edge. The fibers that are attracted to the edge of the disk are wound around the perimeter of the disk owing to the tangential force acting on the fibers produced from the rotation of the disk. This force further stretches the fibers and reduces their diameter. The quality of fiber alignment obtained using the disk is much better than the rotating drum; however, only a small quantity of aligned fibers can be obtained since there is only a small area at the tip of the disk.

Fig. 8. Structural morphology of electrospun fibers displaying the densely packed lamellae and fibrillar structure. Reprinted with permission from [51]. Copyright [2010], American Institute of Physics.

4.3. Static parallel electrodes The advantage of using this technique lies in the simplicity of the setup and the ease of collecting single fibers for mechanical testing. Good alignment has been obtained with this technique. The air gap between the electrodes creates residual electrostatic repulsion between the spun fibers, which helps the alignment of the fibers [5,47–49]. Two non-conductive strips of materials are placed along a straight line and an aluminum foil is placed on each of the strips and connected to the ground as shown in Fig. 6. This technique enables fibers to be deposited at the end of the strips so that the fibers adhere to the strips in an alternate fashion and

Fig. 9. Transmission electron micrograph of nylon 6,6 fiber displaying preferred orientation of the polymer chains.

collected as aligned arrays of fibers. A similar technique by Teo and Ramakrishna [47,49] used double-edge steel blades along a

Fig. 7. Schematic representation of the nanofibril in a single POM nanofiber. The schematic representation of the crystal orientation of 700 nm POM fiber is shown and illustrates the conformation of the helical structure of the chain.

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line to collect aligned arrays of fibers. The fibers were deposited at the gap between the electrodes, however, few fibers were found to deposit on the blades. It was resolved by applying a negative voltage between the blades, resulting in the deposition of fibers between the blades. 5. Structural properties of electrospun fibers Typically, in most semi-crystalline polymers, the fibers produced by electrospinning display structural hierarchy. During the fiber formation process, a fraction of the chains crystallizes to form lamellae consisting of small crystals and the remaining fraction forms the amorphous phase [50–52]. In the presence of shear and elongation forces, the lamellae are organized to form fibrils and the tie chain molecules pass through the neighboring crystallites to form small-sized bundles. The general structure in the fiber is expected as shown in Fig. 7. Due to the shear forces experienced by the jet during electrospinning, the chain orientation (see Fig. 7) aligns along the fiber axis [50]. Konkhlang et al. [50] examined the crystal morphology and molecular orientation of polyoxymethylene (POM) fiber and found that each fiber consists of nanofibrils which are aligned parallel to the fiber axis. The fibrils consist of 14 polymer chains and 40 monomeric units. Similar observations have been found by Lim et al. [51]. They visualize the structural morphology of the fiber and found the fibers to have densely packed lamellae and fibrillar structures as shown in Fig. 8. The lamellar structures determine the crystallinity of the fibers. In between the stacks of lamellae are the relaxed amorphous tie molecules.

there is a phase contrast between the amorphous and crystalline lamellae. Some degree of chain orientation can be found on the surface regions of the fibers. Hence, it can be summarized that the process of electrospinning alters the intrinsic structural properties of the material. The orientation extent can be quantified by using X-ray diffraction analysis on the samples. Alternatively, the draw ratio can be used to obtain an estimate of the molecular orientation. It quantifies the amount of extension the jet experiences during the electrospinning process [5]. High draw ratios experienced by the jet are capable of aligning the macromolecular chains along the fiber axis, thereby influencing the formation and structure of the crystallites. The draw ratio for spun fibers can be calculated as the ratio of the spinning velocity of the collected fiber to ejection velocity of the polymer solution from the pipette [5,43,54–56]. According to the principle of mass conservation, the velocity of the fibers at the ground collector is given by:

mspin ¼

ð2Þ

100  pf  pðr f Þ2  t

where mspin is spinning velocity (m/min) when fibers are collected at the ground electrode, wf weight (g) of polymer fibers on the ground electrode, pf density (g/cc) of the PCL fibers, rf average radius (cm) of the collected fibers and t electrospinning time (min). Usually, the electrospinning process is run for longer duration of time (45 min). The ejection velocity of the PCL solution from the pipette is determined from:

msol ¼

5.1. Molecular orientation The polymer jet under the influence of an electrostatic field experiences a high degree of elongation strain (104 times the draw ratio and over 106 s1 draw rate). The high elongation strains and shear forces are capable of aligning the macromolecular chains along the fiber axis resulting in a high degree of molecular orientation in the fibers [5–8,52]. Zong et al. [53] found the molecular chains in the electrospun PLLA fibers to be highly oriented compared to the random-coil shape chains in the PLLA film. Other polymer fibers such as Kevlar display similar orientated chain structures and is observed between the amorphous and the crystalline regions of the fibers. Fig. 9 is an example using transmission electron microscopy (TEM) to analyse the chain orientation. The fibers are stained with ink so that upon examination with TEM,

!

wf

!

wsol

ð3Þ

100  psol  pðr p Þ2  t

where msol and wsol are ejecting velocity (m/min) and weight (g) of the polymer solution at a given time during electrospinning, psol density (g/cc) of PCL solution, rp diameter (cm) of the pipette used and t electrospinning time (in minutes). Using Eqs. (2) and (3), the draw ratio is the ratio of mspin/msol [5,43,54–56]. The elongation rate of the PCL fibers during the electrospinning process is determined from the following equation:





mspin  msol

 ð4Þ

H

where e is elongation rate and H is distance between the pipette and ground collector. Higher draw ratio values are expected to provide better chain orientation in the fibers.

0.5 Spun Sample

Intensity

Heat Flow (W/g)

0.0

-0.5

-1.0 Spun Nylon 6,6 Nylon 6,6 pellets (non-spun)

-1.5

0

10

20

30

40

50

60

2

-2.0

0

50

100

150

200

250

300

Temperature (ºC) Fig. 10. Typical ‘‘intensity versus 2h” plot obtained from the WAXD analysis on PCL fibers. Crystallinity of the fibers can be determined as the ratio of the area of the peaks to the total area of the curve [5].

Fig. 11. DSC curves of electrospun nylon 6,6 and un-processed (non-spun) nylon 6,6 pellet comparing the melting temperature and heat of fusion.

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The rapid evaporation of the solvent from the jet accompanied by the rapid structure formation, which occurs within milliseconds (50 ms) leads to less developed structures in the fibers. The rapid solvent evaporation reduces the jet temperature. Thus, the molecules that are aligned along the fiber axis have less time to realign themselves, leading to less favorable packing. For most semi-crystalline polymers, the stretched chains under high elongation rate do not get enough time to form crystalline lamellae, which yields lower crystallinity. Hence, the crystallinity in the fibers is thereby influenced by the rate of solvent evaporation [23,57]. The most common technique used to determine the degree of crystallinity is wide angle X-ray diffraction (WAXD) analysis. The ratio of the area under the peaks to the total area under the curve of intensity versus 2h plot is shown in Fig. 10 and it gives the sample crystallinity. Contrary to the theory that electrospinning reduces the crystallinity of the fibers, Lee et al. [58] and Reneker et al. [59] reported that the crystalline structure in fibers is developed in many polyesters and ductile materials. Moreover, the crystallinity can be even higher than the un-processed polymer pellets. They argue that electrospinning inhibits the development of crystallinity specifically for rigid polymers with high glass transition (Tg) values. However, for ductile polymers and polyesters with lower Tg values, such as PCL (Tg  60 °C), this takes longer time to crystallize. Therefore, ductile polymers have the possibility of crystallization during the jet drawing/elongation process, even after the fibers are solidified. Fig. 11 shows the DSC analysis of nylon 6,6 fibers compared with the un-processed nylon 6,6 pellets. The results are consistent with the results of Lee et al. [58] and Reneker et al. [59]. The melting enthalpy of electrospun nylon 6,6 is calculated as 107 J/g compared to 91 J/g for the unspun sample, suggesting an increase in the degree of crystallinity.

5.3. Effect of fiber diameter on structural properties Zussman et al. [60] in their study demonstrate that the electrospun fibers possess skin-core morphology. The skin region of the fibers contains oriented layered planes that are parallel to the fiber axis and contain few crystallites. But the crystallites are misoriented with respect to the fiber axis. The properties of the skin differ from those of the core region for the fibers as the skin layers are essentially characterized by the oriented layered planes whereas, the core region is characterized by random-coil chains. These results are substantiated by the molecular dynamic simulations of Curgul [61] who has demonstrated that the molecules are oriented preferentially parallel to the surface for the nanofibers. The mobility of these chains at the skin is much higher than the mobility of

Crystallinity (%)

5.2. Crystallinity

100

100

80

80

60

60

40

40

Crystallinity Molecular Orientation

20

0

0

200

400

600

800

20

Molecular Orientation (%)

710

0 1000

Fiber Diameter ( m) Fig. 13. Plots of crystallinity (%) and molecular orientation (%) versus fiber diameter for aligned fibers. The degree of crystallinity and molecular orientation increases gradually as the fiber diameter is reduced [5].

the chains present in the core region [62,63]; hence the chains at the skin are easily oriented under the influence of an electric field. As the fiber diameter is reduced, at some critical fiber diameter, the size of the skin region becomes comparable to the overall diameter of the fiber [6]. Moreover, the oriented layered planes on either side of the fiber wall are coupled together and influence the overall properties of the fibers. In contrast, when the fiber diameter is increased, orientation of the chains at the surface of the fiber walls becomes less compared to the majority of the chains in the fiber core region. These results are in agreement with those reported later by Arinstein et al. [6] who have shown that the fibers consist of supramolecular region which consists of oriented amorphous chains. As the size of the fiber is reduced; the size of the supramolecular structure containing amorphous oriented macromolecules is more significant compared to the fiber diameter. Thus, at the critical fiber diameter, the properties of the fiber are controlled by the oriented amorphous macromolecules in the supramolecular region. This work [6] is very important and more detailed investigations are still lacking. In a study conducted in our laboratory, we have established the relationship between the microstructure of the PCL fibers and their diameter [5]. The degree of crystallinity and molecular orientation in the fibers is determined using wide angle X-ray diffraction (WAXD) analysis. Fig. 12 shows the WAXD pattern of the fibers with diameters 250 and 900 nm, respectively. The arc width of the strongest equatorial reflection provides an indication of the degree of orientation within the samples. It is clear from the WAXD patterns (Fig. 12) that 900 nm-wide fibers have lesser degree of orientation compared to 250 nm-wide fibers, that is, the wider the fibers the less molecular orientation is exhibited. Fig. 13 shows the degree of crystallinity (%) and molecular orientation (%) versus fiber diameter. Molecular orientation determined from WAXD increases with decreasing fiber diameter. Therefore, it confirms that as the fiber diameter is reduced, the alignment of the molecules in the direction of fiber axis is improved.

5.4. Effect of collector on the structural properties

Fig. 12. WAXD pattern of aligned fibers performed on two sets of fiber diameter0073: (i) 250 nm and (ii) 900 nm [5].

The type of collector and the speed of the drum/disk collector selected influence the isotropic or anisotropic alignment of the fibers in the mats. Also, the collector type used controls the crystal morphology and molecular orientation [50]. In the article by Kongkhlang et al. [50], they show that when a rotational collector is used, the polymer chains in the crystalline regions are drawn fur-

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ther in the draw direction compared to the polymer chains in the non-woven fabrics that are obtained using a stationary collector. The force due to the rotational speed of the collector along with the shear and elongation forces may contribute to the alignment of the polymer chains in the direction of the fiber axis. Thus, it is expected that the crystal orientation in the fibers improves with the speed of the collector [64]. The use of high speed rotational collectors leads to a ‘‘fanning” effect and the evaporation of the solvent is much quicker compared to the stationary collectors [37]. The speed creates a high-viscosity environment for the polymer chains in the electrospinning jet and leads to the transfer of the tensile stress onto the polymer chains during the fiber deposition. Thus, the crystallization in the fibers occurs due to the ‘‘sliding diffusion” which facilitates formation of extended chain crystals (ECC) from the folded chain crystals by lamellar thickening [37]. It leads to increases in crystal size and crystallinity. This is caused by a more perfect planar zig–zag conformation of the ECC structures under the influence of an applied tensile stress. Also, as the rotational force contributes towards the stretching of the polymer jet, higher rotational speed decreases the diameter of the fibers. This explains the ordering of the crystals at higher collector speeds. Furthermore, when the static parallel electrodes are used to obtain aligned fiber arrays, the extended chain crystals are not observed from WAXD and infrared spectroscopic analyses. Also, the crystal orientation is expected to be inferior compared to the rotational collectors. Hence, we did not find a significant increase in the degree of crystallinity using the parallel electrodes method even though the fiber diameter was reduced [5]. 6. Mechanical properties of the fibers Polymer nanofibers are treated as 1-dimensional systems and have found to possess unusual mechanical properties. The mechanical deformation behavior displayed by the fibers is unique and can be significantly different from their macroscopic counterparts [5,43,37,57,65]. The unique features of the fibers are attributed to the process of electrospinning. 6.1. Effect of structural morphology on tensile properties The lamellar and amorphous fractions of the chains within the fibers influence the strength and elastic modulus of the fibers. Changes in the structural formation taking place in the fibers dur-

711

ing electrospinning, specifically crystallinity and molecular orientation, impart physical uniqueness to the material and play an important role in the deformation behavior of the fibers [5–8]. Hence, knowledge of their intrinsic structures is essential to understand their effects on mechanical properties. The amorphous phase of the fibers provides the elastomeric properties and the crystalline phase imparts dimensional stability to the array of molecules [5]. Thus, the mechanical deformation characteristics of the fiber is influenced by the random or/and ordered arrangements of the crystalline and amorphous phases in the fiber [5–8,50,51]. According to Curgul et al. [61], the mechanical deformation of the fibers is affected by the skin and core morphologies of the fiber. The mass density in the core region is similar to the bulk polymer density. Thus, the core region of the fiber exhibits bulk-like structure and physical properties, whereas, the property exhibited by the surface region is entirely different. This is attributed to the significantly lower density and increased mobility of the chains at the surface/skin of the fiber compared to the core region [62,63]. Hence, the overall deformation of the fiber is determined by the number of oriented fragments present in the surface regions. This theory is also confirmed by Arienstein et al. [6]. In their study, they concluded that the orientation of the amorphous chains in the supramolecular region of the fibers influences the deformation process of the fibers. If this understanding is applied to study the effect of fiber diameter on tensile strength, it should result in an exponential increase, or an abrupt shift, in properties as the fiber diameter is reduced. This is because the effect of fiber surface/skin on the overall nanofiber is increasingly dominant as (a) the fiber surface dimension approaches the radius of gyration of polymer chains, thus constraining the segmental motion, and (b) the fiber core region diminishes when the fiber diameter decreases. In our previous study [5], we also evaluated the effect of fiber diameter on the tensile deformation. The tensile response of the fiber was compared with the tensile properties of the bulk polymer system prepared using injection molding. Representative stress– strain curves of spun and bulk systems are shown in Fig. 14. There is a significant difference in the tensile strength and tensile behavior. The stress–strain curve of the spun sample is consistently found not to display the necking phenomenon whereas the bulk sample shows clear necking. This is attributed to the oriented and stretched polymer chains in the spun fibers [66,67]. Similar results were obtained by Lu et al. [66].

60

Stress (MPa)

50

1

40 2

30 Necking

20

10

0 0

2

4

6

Strain (mm/mm) Fig. 14. Stress–strain curves obtained from tensile tests performed on electrospun PCL and non-spun PCL samples. Curve 1 represents the electrospun sample and Curve 2 represents the non-spun sample [5].

Fig. 15. SEM micrograph of randomly oriented fibers with fiber–fiber fusion points.

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6.2. Effect of collector type on tensile properties 6.2.1. Stationary collector The morphology of the mats obtained using a stationary collector is shown in Fig. 3a. Their mechanical deformation depends greatly on the degree of alignment of the fibers within the mat, fiber lay-ups and interface properties of fiber–fiber contact [19,45,66]. Typically, the tensile strength and modulus of the non-woven fabrics are lower than the mats with uniaxially oriented fibers. This is attributed to the highly porous nature of the non-woven fabrics. Moreover, during tensile loading, only the fibers oriented along the loading direction experience the stretching force, while the fibers that are oriented perpendicular to the loading direction do not experience any force. The fibers tend to orient in the direction of loading before the non-linear region in the stress–strain curves. After the non-linear point, the fiber mesh structure is damaged and better orientation of the fibers along the loading direction is observed. This stage is followed by the high orientation of the fibers at the maximum stress point and fiber breakage at several points is noticed. Such deformation behavior in non-woven fabrics is usually observed when there is no fiber-to-fiber bonding. The lack of bonding between the fibers facilitates easy orientation and stretching of the fibers when loaded and can give high degree of elongation before failure [19]. The use of highly volatile solvents during electrospinning can produce non-wovens with little or no fiber fusions. Fibers cannot fuse together when the solvent evaporation is high and this also results in weak intermolecular interaction. However, when there is fusion between fibers as shown in Fig. 15, the modulus of the non-wovens increases and the elongation to break decreases. The fusion of fibers is obtained if the solvent is not completely evaporated during the fiber forming process. 6.2.2. Rotational collector Macroscopically aligned fibers obtained by modifying the fiber collecting system are found to have anisotropic properties [68,69] which can be potentially useful in a variety of optical, mechanical and bio-medical applications. Uniaxial, aligned fibers are found to possess anisotropic tensile properties. The tensile strength and modulus of these samples are higher than randomly oriented fibers [69]. When the fibers are oriented in the loading

400

300 40

200 20

Tensile Modulus (MPa)

Tensile Strength (MPa)

60

Strength Modulus

0 0.0

0.5

1.0

1.5

2.0

Fiber Diameter (

2.5

100 0 3.0

m)

Fig. 16. Plot of tensile modulus and tensile strength versus fiber diameter. Tensile modulus increases with decreasing fiber diameter. These results can be attributed to the better molecular orientation and crystallinity in smaller fiber diameters. Larger than 2 lm, both tensile modulus and tensile strength appear invariant with fiber diameter [5].

direction, the uniaxial orientation of the fibers helps the tensile force distribute equally to all fibers. Further, since the molecular chains in the fibers are aligned along the fiber axis, which is in the loading direction, the samples display enhanced strength and modulus. The tensile strength of the aligned fiber array samples also depends on the technique used to collect these arrays. When a rotating disk or a drum is used to collect the fibers, mechanical forces may be applied to the jet due to the rotational speed of the collector. This force along with the shear and elongation forces entice the alignment and stretching of the polymer chains in the fiber axis direction. In addition, the rotational speed of the collector determines the stacking density of the fibers. At higher rotational speeds, the deposited fibers have a denser lateral packing and minimum inter-fiber spacing. Also, the fibers tend to have uniform morphology and diameter at higher rotational speeds, which contribute towards the strength of the samples [39–46]. 6.3. Effect of fiber diameter on tensile properties Fiber structure, geometrical arrangement of the fibers, individual fiber properties and interaction between fibers greatly influence the mechanical properties of fiber mats. These features are difficult to control during the electrospinning process. Therefore, determining the tensile deformation of the single fiber is of fundamental importance. Recently, researchers determined the tensile deformation of single fibers and demonstrated that the size of the fiber influenced their tensile response. An enhanced behavior is observed by researchers when the diameter of the fibers is reduced below the critical diameter [5–8]. Arinstein et al. [6] in their study demonstrate that the size of the fiber has an effect on its deformation behavior. At some critical diameter, the fibers display almost an exponential increase in tensile strength. This phenomenon prevails when the size of the supramolecular structures of the fibers is comparable with the overall fiber diameter. The orientation of macro molecules present in the supramolecular structures of the amorphous phase plays a dominant role to increase the fiber mechanical properties. Upon increasing the fiber size, both tensile strength and tensile modulus decreases and the larger diameter fibers tend to display bulk-like properties. This observation is extremely important for conceivable applications of eletrospun nanofibers. Instead of considering such polymers as fibers, they can be used as miniaturized high aspect ratio components for devices and sensors. Hence, by acknowledging the abrupt changes in strength and modulus as fiber diameter decreases, we cannot use measurements obtained, or extrapolated, from bulk specimens to model devices at nanometer length scale. Fig. 16 shows the tensile strength and tensile modulus versus fiber diameter seen in individual PCL fibers. The fibers with diameters greater than 2 lm do not affect the modulus or tensile strength and can be thought to have bulk-like properties. The enhanced properties of finer diameter fibers are attributed to the gradual ordering of the molecular chains and modest increase in the crystallinity of the fibers. The size effect can also be due to the densely packed lamellae and fibrillar structures. In finer diameter fibers, the lamellae and fibrillar structures align themselves along the fiber axis, which plays a critical role in enhancing the mechanical properties of the fibers. The fibrillar structure has a high degree of molecular orientation and provides high resistance to the axial tensile force. When the fiber diameter is increased, the lamellae tend to re-orientate and the presence of alignment and fibrillar lamellae structure is decreased, resulting in reduced mechanical properties. Dzenis [67] and others [70,71] modeled this size effect in polymer nanofibers and considered the surface energy of the fibers to contribute towards the axial tensile force. The assumption made in these studies is that fibers of different sizes have similar mor-

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Fig. 17. Schematic illustrating the hierarchical organization of bone. It also shows the self-assemble process of mineralization observed in natural composites.

phology and can be considered isotropic. However, the size effect seen in experiments attributes this effect to the electrospinning process and the macromolecular orientations in the fibers. The use of an electric field during electrospinning spontaneously aligns highly mobile molecular chains in the direction of the fiber axis, resulting in a higher degree of molecular orientation. The large shearing imposed on the electrospinning jet and the high draw ratio seen for finer fibers suggest better chain orientation for the thinner fibers. Some studies attribute this effect to the orientation of the chains in the outermost region of the fibers [6,7]. Because of the large shear stress and surface tension influencing the electrospinning jet, it results in a higher number of monomers aligned on the fiber surface and in contact with the surface. Besides, as the fiber diameter is reduced, the chains orient along both surfaces of the fiber and are considered to be physically coupled which enhances the properties. Hence, when the thickness of these surface layers is comparable to the overall fiber size, it plays an important role in influencing the mechanical properties of the fibers. In contrast, as the fiber size is increased, misorientation of the polymer chains along the surface occurs and the surface layer is no longer comparable to the overall fiber size. The higher degree of misorientation present in the fiber core region yields bulk-like properties [60,72,73] and dictates the overall fiber properties. 7. Prospective applications of electrospun fibers 7.1. Fiber composites for tissue engineering applications Ultra-fine fibers of biodegradable polymers produced by electrospinning have found potential applications in tissue engineering due to their high surface area to volume ratios and high porosity of the fibers [9,14,65,73,74]. Moreover, the flexibility of seeding stem cells and human cells on the fibers makes electrospun materials most suited for tissue engineering applications [75,76]. The fibers produced can be used systematically to design the structures such that they do not only mimic the properties of the extracellular ma-

Fig. 18. SEM micrograph of fracture surface showing the presence of electrospun nanofibers in the matrix resin. Reprinted from [16], with permission from Elsevier.

trix (ECM), but also possess high strength and high toughness. For instance, non-woven fabrics exhibit isotropic properties and support neo-tissue formation. These mats resemble the ECM matrix and can be used as skin-scaffold and wound dressing materials where the materials are required to be more elastic than stiff [14,77–82]. When anisotropic properties are desired for load-bearing applications, such as musculoskeletal tissues (tendons and ligaments), aligned electrospun fibers can be used to mimic the structural anisotropy of the tissues. Many natural polymers (collagen, starch, chitin and chitosan) and synthetic biodegradable polymers (poly(e-caprolactone) (PCL), polylactide (PLA), poly(D, Llactide-coglycolide) (PLGA)) have been widely investigated for potential applications in developing tissue scaffolds [77,83–86]. This suggests a thorough understanding of the mechanical behavior of electrospun nanofibers is essential. For example, the fracture toughness of synthetic electrospun scaffolds has not been addressed at all and this is a critical factor for assessing the mechanical integrity of the scaffolds. The natural tissues due to

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their hierarchical structural arrangement possess superior fracture toughness values compared to any of the synthetic scaffold materials that are currently used [83]. To match the mechanical integrity of natural composites, the design of the scaffolds developed should mimic the architectural design used by natural tissues. Fig. 17 shows the self-assembly design mechanism seen in bones. Natural tissues at the structural level essentially consists of collagen fibrils and bone ceramic in the form of hydroxyapatite (HAP). Mineralization of the tissue takes places by the mechanism known as protein fiber-guided mineralization. Collagen fibers (300 nm in length and 1.23 nm in width) are self-assembled in an orderly fashion and generate channels or grooves. Mineral particles originate and develop at the grooves grow in length and width as sheets of mineral platelets. The mineral platelets are placed parallel to each other and provide strength to the composite. Thus, the arrangement of collagen and bone crystals at the structural level can critically affect the mechanical integrity of the whole system [88–90]. The strength of the tissue scaffolds processed using the conventional techniques lack the architecture design seen in the natural composites and hence their mechanical characteristics are drastically different from natural bone composites. However, the electrospinning technique is capable of mimicking the protein guided mechanism and can potentially align the HAP particles in the fiber direction. Further, the strength of the fibers can be controlled by the loading of HAP fillers. Shields et al. [65] showed that electrospun collagen fibers with diameters 100 nm intended for articular cartilage repair have modulus 170 MPa and maximum stress of 3.3 MPa. These values closely match the cartilage mechanical properties of Young’s modulus of 130 MPa and maximum stress of 20 MPa. Stanishevsky et al. [87] fabricated composites of hydroxyapatite (HAP)/collagen using electrospinning for hard tissue scaffold applications and demonstrated that the properties of the electrospun material can be easily controlled by the HAP loading in the fibers. These results confirm that electrospinning of natural or synthetic polymers for tissue engineering applications are very promising. 7.2. Electrospun fiber reinforced composites Although electrospun fiber reinforced polymer composites have significant potential for development of high strength/high toughness materials and materials with good thermal and electrical con-

ductivity, very few studies have investigated the use of electrospun fibers in composites [16–18,91]. Fig. 18 shows a SEM micrograph of an electrospun fiber reinforced composite. Traditional reinforcements in polymer matrices can create stress concentration sites due to their irregular shapes and cracks propagate by cutting through the fillers or travelling up, down and around the particles. However, electrospun fibers have several advantages over traditional fillers [17]. The reinforcing effect of the fibers is influenced essentially by the fiber size. Smaller size fibers give more efficient reinforcement. Also, as discussed in the previous sections, fibers with finer diameters have preferential orientation of the polymer chains along the fiber axis. The orientation of macromolecules in the fibers improves with the reduction in diameter, making finer diameter fibers very strong. Hence, the use of nanometer-sized fibers can significantly enhance the mechanical integrity of the polymer matrix compared to micron-sized fibers. Moreover, the high percentage of porosity and irregular pores between the fibers can lead to an interpenetrated structure when dispersed in the matrix, which also enhances the mechanical strength due to the interlocking mechanism. These characteristic features of nanofibers enable the transfer of applied stress to the fiber–matrix interface in a better fashion than most of the commonly used filler materials [16]. Current issues related to the use of electrospun nanofibers as reinforcement materials are the control of dispersion and orientation of the fibers in the polymer matrix. To achieve better reinforcement, electrospun nanofibers may need to be collected as a highly aligned yarn instead of a randomly distributed felt so that the post-electrospinning stretching process could be applied to further improve the mechanical properties. Further, if crack growth is transverse to the fiber orientation, the fracture toughness of the composite can be optimized. Hence, the interfacial adhesion between fibers and matrix material needs to be controlled such that the fibers are capable of deflecting the cracks by fiber–matrix interface debonding and fiber pull-out. The interfacial adhesion should not be too strong or too weak. Optimal control can only be attained by careful selective fiber surface treatment. The dispersion of electrospun mats in the matrix can be improved by trimming the fibers to shorter fragments. This can be achieved, if the electrospun fibers are collected as aligned bundles (instead of non-woven network), which can then be optically or mechanically trimmed to obtain fiber fragments of several 100 nm in length. 7.3. Conductive fiber composites

Fig. 19. TEM micrograph of nylon 6,6-CNT fiber. The CNTs are embedded in the core region of the fiber and are aligned along the fiber axis.

Electrospinning has found applications in developing flexible and compliant conductive nanofibers for applications in miniaturized devices [23,92–94]. Researchers seek to develop compliant electrodes for electroactive polymer actuators. Use of electrospinning to produce fibers from conductive polymer matrices can be useful for these applications. Moreover, electrospinning can be used to disperse carbon nanotubes (CNT) in fibers to improve the mechanical, electrical and conductive properties of the matrix material [10,11]. Due to the high elongation of the polymer jet during electrospinning, the CNTs tend to orient along the fiber axis and are embedded in the fiber core as shown in Fig. 19. Application of CNTs and carbonaceous fillers in polymers is known to improve the electrostatic charge dissipation and electromagnetic shielding efficiency, thus improving the overall conductivity of the polymers. Accordingly, many polymers are being investigated that can be easily electrospun and used as matrix material for CNTs. Another advantage of using electrospun fibers for developing electrodes is the surface area to volume ratio of the fibers. Since the rate of electrochemical reactions is affected by the surface area of the electrode, the high surface area of the fibers for the development of porous electrodes can be exploited.

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Norris et al. [92] fabricated ultrafine electrically conductive polyaniline/polyethylene oxide (PAN/PEO) fiber blend using electrospinning. The standard four-point probe method was used to determine the conductivity of non-woven fibers and cast films. By controlling the PAN/PEO ratio in the blend, they improved the conductivity of non-woven mats comparable to that of the cast film. Ko [95] determined that the size of the fiber obtained from conductive polymers has important effects on system response time to electronic stimuli and the current carrying capability of the fiber. Using poly(3,4-ethylenedioxythiophene) (PEDOT) fibrous mats it was demonstrated that the conductivity of the mats increased exponentially as the fiber diameter was decreased. Packing density of the molecules in finer diameter fibers could be a possible reason for the enhanced conductivity in the fibers. It might also be attributed to the intrinsic fiber conductivity effect or the geometric surface effect resulting from the reduction in fiber diameters. 7.4. Filtration Fig. 20. TEM micrograph of electrospun fiber filled with magnetite particles.

Electrospun fibers with micron-sized diameters have found extensive functions in filtration applications [23,96–98]. Electrospun non-woven fabrics used for filtration provide the advantages of high surface area to volume ratio, low air resistance, lower filter mass and flexibility of adding surface functionality on the fibers by blending or incorporating nanofillers [97]. Electrospun fibers are being widely investigated for aerosol filtration, air cleaning applications in industry and for particle collection in clean rooms. Typically, aerosol particles are filtered due to the electrostatic attraction that exists between the filter media and aerosol particles. Electrospun fibers used in filtration media can improve the efficiency of filtration as the static charge used to produce the electrospun fibers may remain in the fibers and help in the filtration of aerosol particles. It is seen that the filtration efficiency of the electrospun mats is comparable to the commercially available filters but the advantage lies in the filter mass which is substantially lower for the former than the latter [23,98]. It is well-known that as the surface area of the fibers is increased, the surface adhesion properties of the fibers improve. Hence, by decreasing the diameter of the fiber in the filter media, the efficiency of capturing sub-micron sized particles can be significantly improved compared to the larger fibers. For efficient filtration, the sizes of the structural elements in the filter media have to match the size of the particles of droplets that are to be captured by the filter media. The advantage of using electrospun fibers in the filtration media is that the fiber diameters can be easily controlled and can make an impact in high efficiency particulate air filtrations.

larly useful for miniaturized electronic components and load-bearing applications. The key factors that influence the reinforcing effects of the filler in electrospun fibers are the dispersion, distribution and alignment of the fillers in the fiber matrix. Electrospinning offers an efficient route for obtaining homogeneous dispersion and distribution of the nanoscale reinforcements [10,11]. Moreover, it is demonstrated that the high electrostatic forces and shear force experienced by the jet during electrospinning align the fillers along the fiber axis. Good dispersion of the fillers is essential for constraining the segmental motion of the molecular chains. Further, the use of fillers such as CNTs in electrospun fibers is seen to align itself along the fiber axis (see Fig. 19). The embedded CNT reduces the overall mobility of the polymer chains and provides the confinement effect to the neighboring molecules. Thus, orientation of polymer chains during electrospinning and the presence of hard fillers within the fibers strengthen the fibers. In our previous work [5], the reinforcing effect of the HAP filler on a PCL matrix is verified by comparing the tensile strengths of the electrospun fiber composites with those of the melt-processed composites. Electrospinning is found to be far superior to meltprocessing. More interestingly, for electrospun fiber samples, filler addition increases the tensile strength. However, filler addition decreases the tensile strength of melt-processed composites (bulk). In the electrospun fibers, HAP particles are contained in the nanof-

30

7.5. Filler reinforced fiber systems

Magnetic Fiber

Nanoscale reinforcements have been often used by researchers to fabricate multi-functional high strength composites. Therefore, novel fibrous composites can be obtained by incorporating high strength and high aspect ratio fillers in fiber matrices [1,10,11,87,99]. For instance, electrospinning is studied for fabricating lightweight fibrous composite with unique properties for protective clothing and body armor applications [1]. Commonly used fillers are carbon nanotubes (CNTs), organoclay, hydroxyapatite (HAP), silica and titania particles. Filler reinforced fibers have many potential applications: ultra-strong wires, nanocomposites, nanoprobes, electronic devices, tissue replacement materials [10,11,87] etc. For example, addition of HAP particles to biodegradable polymer fibers shows potential for bio-medical applications. Similarly, CNT inclusion in electrospun fibers enhances the overall strength and conductivity. Such CNT reinforced fibers are particu-

M (emu/g)

20 10 0 -10 -20 -30 -10000

-5000

0

5000

10000

H (Oe) Fig. 21. Magnetic hysteresis loop of magnetic filler reinforced electrospun fiber.

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ibers and serve to constrain the segmental motion of the polymer chains. Hence, the fiber-guided composites are seen to have enhanced tensile strength. It can be concluded that the fiber-guided architecture creates a more effective reinforcement compared to the filler-dispersion approach. Thus, the overall mechanical performance of filler reinforced electrospun fibers is influenced by the dispersion and orientation of the fillers within the fibers. This observation can also be attributed to the re-ordering of molecular chains in electrospun systems which is not seen in polymers processed by melt flow [5].

8. Concluding remarks and future work Since its discovery, significant advancement has been made in R&D of electrospinning. Researchers have mostly focused on optimizing the processing parameters to obtain fibers of desired shapes and forms with little understanding of the parameters that control the enhanced microstructures and mechanical properties. Many polymers have been successfully electrospun into nanofibers. However, very few studies can be found on the macromolecular orientation and crystalline structures of the fibers. Through this review, concepts behind the structures and morphology of electrospun fibers are discussed, their effects on the mechanical properties are emphasized, and future work identified. The unique mechanical properties of the electrospun fibers described in this study demonstrate the potential of using these fibers for miniaturized polymer devices and composites applications. Recently, researchers identified the size effect of fibers on the structural and tensile properties of the fibers. For example, tensile properties such as elastic modulus and strength increase with decreasing fiber diameter. The enhanced orderliness of the amorphous phase in the supramolecular structure of the fiber plays an important role in influencing the properties of the fibers. High shear forces are seen to produce a skin and core morphology in the fibers. The heterogeneity in the skin and core regions is established due to the higher degree of chain orientation in the former compared to the latter. The core region of the fiber shows bulk-like properties and the skin region displays enhanced properties. Hence, when the skin thickness is comparable to the overall fiber diameter, both the tensile modulus and tensile strength are significantly increased. The heterogeneity in the skin and core regions of the fibers is more remarkable when the fibers are reinforced with CNTs. Inclusion of CNTs in the fiber matrix presents an additional interface for surface chain orientation [99]. Therefore, the overall chain orientation increases with the CNT loading. These oriented regions are stiffer compared to the regions of disoriented chains. Consequently, CNT reinforcement leads to stiffening and strengthening of the fibers. To-date, there are no extensive experimental studies designed to investigate the molecular structures of the core and skin regions of electrospun fibers. The surface behavior at the skin can differ very much from the core properties of the fiber. Further investigations on the strengthening mechanisms across the skin and core regions are pressingly needed. Current efforts have been mainly about the incorporation of filler materials to increase the strength of electrospun fibers [44,87,99]. However, future work should focus more on the multi-functionality of nanofiber composites with fillers for specific applications. For instance, electrospinning technique can be used to incorporate magnetic fillers within the fiber matrix to obtain super-paramagnetic nanostructured composite with controlled geometry. Moreover, fibers with homogenous dispersion and distribution of the fillers are very attractive since the composite is expected to display enhanced magnetic-field dependent superparamagnetism. Such features are seldom achieved using

other conventional techniques. We have recently demonstrated that magnetic particle electrospun fibers can display superparamagnetism at ambient temperature. A TEM micrograph of one such composite with uniformly distributed magnetic particles is shown in Fig. 20 [100] and its magnetic hysteresis loop at 300 K is plotted in Fig. 21 [100]. In addition, the composite fibers are also seen to deflect in the direction of increasing magnetic field and confirm field responsive behavior. Further, the incorporation of magnetic particles enhances the fiber elastic modulus. Thus, many attractive features like mechanical strength, magnetic and conductive properties of the nanoparticles can be utilized to obtain multi-functional composites. Developments of such nanostructured fiber composites can be especially useful in miniaturized electronic parts and for electromagnetic interference shielding. Also, the use of biodegradable polymeric fiber as the carrier matrix can enhance its usefulness in bio-medical, magnetic resonance imaging and drug-delivery applications. Acknowledgments We would like to thank the Australian Research Council for the support of this work. SCW acknowledges the financial support from the US National Science Foundation under the CAREER Award CMMI #0746703 and Award DMI #0520967. Sincere thanks are due to TA Blackledge and DH Reneker for helpful discussions and constructive comments during the preparation of this review paper. References [1] Ko FK, Sukigara S, Gandhi M, Ayutsede J. Electrospun carbon nanotube reinforced silk fibers. US patent no. 0082197; 2007. [2] Zarkoob S, Eby RK, Reneker DH, Hudson SD, Ertley D, Adams WW. Structure and morphology of electrospun silk nanofibers. Polymer 2004;11:3973–7. [3] Reneker DH, Chun I. Nanometre diameter fibres of polymer, produced by electrospinning. Nanotechnology 1996;7:216–23. [4] Doshi J, Reneker DH. Electrospinning process and applications of electrospun fibers. J Electrostat 1995;35:151–60. [5] Wong SC, Baji A, Leng SW. Effect of fiber diameter on tensile properties of electrospun poly(å-caprolactone). Polymer 2008;21:4713–22. [6] Arinstein A, Burman M, Gendelman O, Zussman E. Effect of supramolecular structure on polymer nanofiber elasticity. Nat Nanotech 2007;2:59–62. [7] Tan EPS, Ng SY, Lim CT. Tensile testing of single ultrafine polymeric fiber. Biomaterials 2005;26:1453–6. [8] Zussman E, Burman M, Yarin AL, Khalfin R, Cohen Y. Tensile deformation of electrospun nylon-6, 6 nanofibers. J Polym Sci Part B: Polym Phys 2006;44:1482–9. [9] Lannutti J, Reneker D, Ma T, Tomasko D, Farson D. Electrospinning for tissue engineering scaffolds. Mater Sci Eng (Biomim Supramol Sys) 2007;27:504–9. [10] Dror Y, Salalha W, Khalfin RL, Cohen Y, Yarin AL, Zussman E. Carbon nanotubes embedded in oriented polymer nanofibers by electrospinning. Langmuir 2003;19:7012–20. [11] Salalha W, Dror Y, Khalfin RL, Cohen Y, Yarin AL. Zussman E Single-walled carbon nanotubes embedded in oriented polymeric nanofibers by electrospinning. Langmuir 2004;20:9852–5. [12] Deitzel JM, Kleinmeyer J, Harris D, Tan NCB. The effect of processing variables on the morphology of electrospun nanofibers and textiles. Polymer 2001;42:261–72. [13] Gibson P, Schreuder-Gibson H, Rivin D. Transport properties of porous membranes based on electrospun nanofibers. Colloid Surf A-Physicochem Eng Asp 2001;187:469–81. [14] Li WJ, Laurencin CT, Caterson EJ, Tuan RS, Ko FK. Electrospun nanofibrous structure: a novel scaffold for tissue engineering. J Biomed Mater Res 2002;60:613–21. [15] Matthews JA, Wnek GE, Simpson DG, Bowlin GL. Electrospinning of collagen nanofibers. Biomacromolecules 2002;3:232–8. [16] Fong H. Electrospun nylon 6 nanofiber reinforced BIS-GMA/TEGDMA dental restorative composite resins. Polymer 2004;45:2427–32. [17] Tian M, Gao Y, Liu Y. Bis-GMA/TEGDMA dental composites reinforced with electrospun nylon 6 nanocomposite nanofibers containing highly aligned fibrillar silicate single crystals. Polymer 2007;48:2720–8. [18] Kim JS, Reneker DH. Mechanical properties of composites using ultrafine electrospun fibers. Polym Compos 1999;20:124–31. [19] Zhang M, Atkinson KR, Baughman RH. Multifunctional carbon nanotube yarns by downsizing an ancient technology. Science 2004;306:1358–61. [20] Wei X, Xia Z, Wong SC, Baji A. Modelling of mechanical properties of electrospun nanofibre network. Int J Exp Comp Biomech 2009;1:45–57.

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