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    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, Australiab Department of Mechanical Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong, Chinac Department of Mechanical Engineering, The University of Akron, Akron, Ohio 44325, USA

    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 atten-

    tion 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 com-

    pletely clear. Recently, the size effect on the mechanical properties was reported for fibers across differ-

    ent 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 out-

    comes in the development of advanced nanomaterials for devices and miniaturized load-bearing

    applications.

    2010 Elsevier Ltd. All rights reserved.

    Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704

    2. Electrospinning theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705

    3. Control of fiber diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706

    4. Alignment of fibers and fiber collection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706

    4.1. Rotating drum collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707

    4.2. Rotating disk collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708

    4.3. Static parallel electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708

    5. Structural properties of electrospun fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709

    5.1. Molecular orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709

    5.2. Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710

    5.3. Effect of fiber diameter on structural properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7105.4. Effect of collector on the structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710

    6. Mechanical properties of the fibers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711

    6.1. Effect of structural morphology on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711

    6.2. Effect of collector type on tensile properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712

    6.2.1. Stationary collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712

    6.2.2. Rotational collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712

    6.3. Effect of fiber diameter on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712

    7. Prospective applications of electrospun fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713

    7.1. Fiber composites for tissue engineering applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713

    0266-3538/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2010.01.010

    * 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).

    Composites Science and Technology 70 (2010) 703718

    Contents lists available at ScienceDirect

    Composites Science and Technology

    j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p s c i t e c h

    http://dx.doi.org/10.1016/j.compscitech.2010.01.010mailto:[email protected]://www.sciencedirect.com/science/journal/02663538http://www.elsevier.com/locate/compscitechhttp://www.elsevier.com/locate/compscitechhttp://www.sciencedirect.com/science/journal/02663538mailto:[email protected]://dx.doi.org/10.1016/j.compscitech.2010.01.010
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    7.2. Electrospun fiber reinforced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714

    7.3. Conductive fiber composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714

    7.4. Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715

    7.5. Filler reinforced fiber systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715

    8. Concluding remarks and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716

    Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716

    References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716

    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

    [29]. Some of these unique features cannot be otherwise achieved

    by conventional fiber processing techniques. Another merit is thatunder the influence of an electric field, electrospinning self-assem-

    bles 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 exam-

    ple, 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 fil-

    tration [12,13], tissue engineering [9,14,15], precursor for fabricat-

    ing nanofiber composites [1618] and advanced nanomaterials

    [4,19] etc.

    Despite possessing these unique features, one of the main chal-

    lenges 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 deforma-

    tion. Hence, in most cases, the mechanical integrity of the fibers

    and fiber network structures is least understood and an under-

    standing of the phenomenon is urgently needed. Few researchers

    actively pursued to characterize the mechanical deformation char-

    acteristics of the fibers by recording the stressstrain behavior of

    the electrospun non-woven fabrics. However, this method cannot

    be deemed suitable because the tensile response of the non-wo-

    vens are greatly influenced by the fiber size distribution in the

    mats, porosity, individual fiber orientation in the mat, fiberfiber

    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 [58]. More recently, it was demon-strated that the size effect is critical in influencing the fiber prop-

    erties and an abrupt increase in tensile properties is observed at

    a given average fiber diameter [58]. 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

    [58]. 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 nanofi-

    ber assemblies, effects of processing parameters on fiber diameter

    and morphology, characteristics of the fibers and their applications

    [9,12,2123]. 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 electrospin-

    ning, 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 dur-

    ing electrospinning leads to strengthening of the fibers [58]. 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

    schematicillustrates theinvertedconical path thejet travels before being collectedas randomly oriented fibers as shown in theinset SEMmicrograph. L represents the lengthof pipette containing the polymer solution and H is the distance between the tip and collector.

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    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 thatinfluence 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 con-

    trolled morphology, structures and spatial arrangement of the

    nanofibers are critically reviewed. The effects of electrospinning, fi-

    ber 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 deforma-

    tion behavior of the fibers are presented in Section 6. Here, the or-

    derly arrangement of fiber structures as a function of fiber size isemphasized. Finally, potential applications of these fibers are given

    in Section 7.

    2. Electrospinning theory

    Electrospinning or electrostatic spinning is a simple technique

    which utilizes high electrostatic forces for fiber production. Elec-

    trospinning, first introduced by Formhals [24] and later revived

    by Reneker [3,4], uses high voltage (about 1020 kV) to electrically

    charge the polymer solution for producing ultra-fine fibers (diam-

    eters 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 poly-

    mer solution, a high voltage source and a grounded conductive col-lector screen. In addition, a metering syringe pump can be used to

    control the flow rate of the polymer solution. The needle of the syr-

    inge typically serves as an electrode to electrically charge the poly-

    mer 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 to-

    wards 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 solu-

    tion [2527]. 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 thependant drop into a conical shape and serves as an initiating sur-

    face [2931]. A schematic of the process is shown in Fig. 2. When

    the critical voltage is reached, the equilibrium of the forces is dis-

    turbed and a charged jet emanates from the tip of the conical drop.

    The discharged jet diameter decreases in size with concomitant in-

    crease 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 axisymmetricand 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 entan-

    glement 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 concen-

    tration 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 thin-

    ning and elongation of the jet [25,26]. At high electric forces, thejet is dominated by bending (axisymmetric) and whipping instabil-

    Pendant

    drop

    Polymer

    solution

    Induced charges

    from electric field

    Taylor cone

    Jet initiation

    (B) (C)(A)

    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.

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

    (B) SEM micrograph of the flat ribbon structure.

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    ity (non-axisymmetric), causing the jet to travel in an inverse con-

    e 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 bend-

    ing) instabilities are suppressed and the non-axisymmetric insta-

    bility 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 [3335]. 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 ef-

    fect 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 differ-

    ences 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 ten-

    sion and tension resisting the bending of electric field lines in the

    jet, and (c) surface charge repulsion. Bending and stretching is a di-

    rect 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 bend-

    ing of the jet. At this stage, the current is constant and consists

    of conduction and advection current. At the later stage of whip-

    ping, 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 deter-

    mined to be a function of surface tension, electric current and sur-

    face charge repulsion. The equation for the diameter is:

    D cnQ

    2

    I22

    p 2 ln ld 3

    !1

    3

    1

    where c is surface tension of the solution, n dielectric constant, Qflow 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. Themodel, however, neglects the elastic effect due to solvent evapora-

    tion and considers the solution Newtonian. The model also ne-

    glects 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-con-ductive 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 can-

    not 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. None-

    theless, 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 col-

    lector is stationary. However, further thinning of the fibers can beobtained 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 charac-

    teristics of the spun fibers [38,39]. The density of the fibers per unit

    area on the collector and fiber arrangement are affected by the de-gree of charge dissipation upon fiber deposition. The most com-

    Fig. 4. Schematic of the rotating drum used for fiber collection. The inset SEM

    micrograph shows the aligned fibers obtained using the rotating drum.

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    monly used target is the conductive metal plate that results in col-

    lection of randomly oriented fibers in non-woven form as shown in

    Fig. 3a. Liu et al. [38] electrospun cellulose acetate fibers on copper

    mesh, aluminumfoil, 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 col-

    lectors 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-con-

    ductive 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 byusing mechanical and electrostatic methods to control the elec-

    trospinning process. Aligned fibers have found importance in many

    engineering applications, such as tissue engineering, sensors,

    nanocomposites, filters, electronic devices [4043]. Some com-

    monly used techniques to align the fibers are discussed in the sub-

    sections 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 [4042]. 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 drumshould match the evaporation rate of the solvent, such that the fi-

    bers are deposited and taken up on the surface of the drum. The

    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.

    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 de-

    gree 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 cen-

    trifugal force is developed near the vicinity of the circumference of

    the rotating drum, which elongates the fibers before being col-

    lected 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 pat-

    terned nanofibers [23,4346].

    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 at-

    tracted to the edge of the disk are wound around the perimeter

    of the disk owing to the tangential force acting on the fibers pro-

    duced from the rotation of the disk. This force further stretches

    the fibers and reduces their diameter. The quality of fiber align-

    ment obtained using the disk is much better than the rotating

    drum; however, only a small quantity of aligned fibers can be ob-

    tained since there is only a small area at the tip of the disk.

    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 [5,4749]. 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

    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 andillustrates the conformation of the helical structure of the chain.

    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.

    Fig. 9. Transmission electron micrograph of nylon 6,6 fiber displaying preferred

    orientation of the polymer chains.

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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 volt-

    age between the blades, resulting in the deposition of fibers be-

    tween the blades.

    5. Structural properties of electrospun fibers

    Typically, in most semi-crystalline polymers, the fibers pro-

    duced 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 [5052]. In the presence of shear

    and elongation forces, the lamellae are organized to form fibrils

    and the tie chain molecules pass through the neighboring crystal-

    lites 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 polyoxymethyl-

    ene (POM) fiber and found that each fiber consists of nanofibrils

    which are aligned parallel to the fiber axis. The fibrils consist of14 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 be-

    tween the stacks of lamellae are the relaxed amorphous tie

    molecules.

    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 chainsalong the fiber axis resulting in a high degree of molecular orienta-

    tion in the fibers [58,52]. Zong et al. [53] found the molecular

    chains in the electrospun PLLA fibers to be highly oriented com-

    pared to the random-coil shape chains in the PLLA film. Other poly-

    mer fibers such as Kevlar display similar orientated chain

    structures and is observed between the amorphous and the crys-

    talline regions of the fibers. Fig. 9 is an example using transmission

    electron microscopy (TEM) to analyse the chain orientation. The fi-

    bers are stained with ink so that upon examination with TEM,

    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 prop-

    erties 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 ori-

    entation. It quantifies the amount of extension the jet experiences

    during the electrospinning process [5]. High draw ratios experi-

    enced by the jet are capable of aligning the macromolecular chains

    along the fiber axis, thereby influencing the formation and struc-

    ture of the crystallites. The draw ratio for spun fibers can be calcu-

    lated as the ratio of the spinning velocity of the collected fiber to

    ejection velocity of the polymer solution from the pipette

    [5,43,5456]. According to the principle of mass conservation,

    the velocity of the fibers at the ground collector is given by:

    mspin wf

    100 pf prf2 t

    !2

    where mspin is spinning velocity (m/min) when fibers are collected at

    the ground electrode, wf weight (g) of polymer fibers on the groundelectrode,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 fromthe pipette

    is determined from:

    msol wsol

    100 psol prp2 t

    !3

    where msol and wsol are ejecting velocity (m/min) and weight (g) ofthe polymer solution at a given time during electrospinning, psoldensity (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 [5,43,5456]. The elongationrate of the PCL fibers during the electrospinning process is deter-

    mined from the following equation:

    e mspin msol

    H4

    where e is elongation rate and His distance between the pipette andground collector. Higher draw ratio values are expected to provide

    better chain orientation in the fibers.

    0 10 20 30 40 50 60

    Spun Sample

    Intensity

    2

    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 thepeaks to the total area of the curve [5].

    Temperature (C)

    0 50 100 150 200 250 300

    HeatFlow(W/g)

    -2.0

    -1.5

    -1.0

    -0.5

    0.0

    0.5

    Spun Nylon 6,6

    Nylon 6,6 pellets (non-spun)

    Fig. 11. DSC curves of electrospun nylon 6,6 and un-processed (non-spun) nylon6,6 pellet comparing the melting temperature and heat of fusion.

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    5.2. Crystallinity

    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 mole-

    cules that are aligned along the fiber axis have less time to realign

    themselves, leading to less favorable packing. For most semi-crys-

    talline 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 crystal-

    linity of the fibers, Lee et al. [58] and Reneker et al. [59] reported

    that the crystalline structure in fibers is developed in many polyes-

    ters 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 specifi-

    cally for rigid polymers with high glass transition (Tg) values. How-

    ever, for ductile polymers and polyesters with lower Tg values, such

    asPCL (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 en-

    thalpy 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 electro-

    spun 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 misori-

    ented 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 re-

    sults 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 mobil-

    ity of these chains at the skin is much higher than the mobility of

    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 comparedto 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 amor-

    phous chains. As the size of the fiber is reduced; the size of the

    supramolecular structure containing amorphous oriented macro-

    molecules is more significant compared to the fiber diameter. Thus,

    at the critical fiber diameter, the properties of the fiber are con-

    trolled by the oriented amorphous macromolecules in the supra-

    molecular 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 de-

    gree 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 showsthe degree of crystallinity (%) and molecular orientation (%) versus

    fiber diameter. Molecular orientation determined from WAXD in-

    creases 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

    The type of collector and the speed of the drum/disk collector

    selected influence the isotropic or anisotropic alignment of the fi-

    bers in the mats. Also, the collector type used controls the crystal

    morphology and molecular orientation [50]. In the article by Kon-

    gkhlang et al. [50], they show that when a rotational collector isused, the polymer chains in the crystalline regions are drawn fur-

    Fig. 12. WAXD pattern of aligned fibers performed on two sets of fiber diame-ter0073: (i) 250 nm and (ii) 900 nm [5].

    Fiber Diameter ( m)

    0 200 400 600 800 1000

    Crystallinity(%)

    0

    20

    40

    60

    80

    100

    Molecular

    Orientation(%)

    0

    20

    40

    60

    80

    100

    Crystallinity

    Molecular Orientation

    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].

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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 col-

    lectors leads to a fanning effect and the evaporation of the sol-

    vent 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 dif-

    fusion 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 zigzag conformation of the ECC struc-

    tures 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 de-

    gree 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 counter-

    parts [5,43,37,57,65]. The unique features of the fibers are attrib-uted 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-

    ing electrospinning, specifically crystallinity and molecular orien-

    tation, impart physical uniqueness to the material and play an

    important role in the deformation behavior of the fibers [58].

    Hence, knowledge of their intrinsic structures is essential to under-

    stand 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 [58,50,51].

    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 struc-

    ture and physical properties, whereas, the property exhibited by

    the surface region is entirely different. This is attributed to the sig-

    nificantly 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 fi-

    ber 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 behav-ior. The stressstrain 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 re-

    sults were obtained by Lu et al. [66].

    Strain (mm/mm)

    0 2 4 6

    Stress(MPa)

    0

    10

    20

    30

    40

    50

    60

    2

    1

    Necking

    Fig. 14. Stressstrain curves obtained from tensile tests performed on electrospun

    PCL and non-spun PCL samples. Curve 1 represents the electrospun sample andCurve 2 represents the non-spun sample [5]. Fig. 15. SEMmicrograph of randomly oriented fibers with fiberfiber 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 collec-

    tor is shown in Fig. 3a. Their mechanical deformation depends

    greatly on the degree of alignment of the fibers within the mat, fi-

    ber lay-ups and interface properties of fiberfiber contact

    [19,45,66]. Typically, the tensile strength and modulus of the

    non-woven fabrics are lower than the mats with uniaxially ori-

    ented fibers. This is attributed to the highly porous nature of the

    non-woven fabrics. Moreover, during tensile loading, only the fi-

    bers oriented along the loading direction experience the stretching

    force, while the fibers that are oriented perpendicular to the load-

    ing direction do not experience any force.

    The fibers tend to orient in the direction of loading before the

    non-linear region in the stressstrain 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 be-

    tween 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 electrospin-

    ning 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

    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 min-

    imum inter-fiber spacing. Also, the fibers tend to have uniform

    morphology and diameter at higher rotational speeds, which con-

    tribute towards the strength of the samples [3946].

    6.3. Effect of fiber diameter on tensile properties

    Fiber structure, geometrical arrangement of the fibers, individ-

    ual fiber properties and interaction between fibers greatly influ-

    ence 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 funda-

    mental 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 re-

    duced below the critical diameter [58].

    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 ten-

    sile 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. Uponincreasing 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 acknowledg-

    ing 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 fi-

    ber diameter seen in individual PCL fibers. The fibers with diame-

    ters greater than 2 lm do not affect the modulus or tensile

    strength and can be thought to have bulk-like properties. The en-

    hanced properties of finer diameter fibers are attributed to thegradual 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 diam-

    eter 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 poly-

    mer nanofibers and considered the surface energy of the fibers to

    contribute towards the axial tensile force. The assumption madein these studies is that fibers of different sizes have similar mor-

    Fiber Diameter ( m)

    0.0 0.5 1.0 1.5 2.0 2.5 3.0

    TensileStrength

    (MPa)

    0

    20

    40

    60

    TensileModulus(MPa)

    0100

    200

    300

    400

    Strength

    Modulus

    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 withfiber diameter [5].

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    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 ra-tio 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 electros-

    pinning jet, it results in a higher number of monomers aligned on

    the fiber surface and in contact with the surface. Besides, as the fi-

    ber diameter is reduced, the chains orient along both surfaces of

    the fiber and are considered to be physically coupled which en-

    hances 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 con-

    trast, 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 misorien-

    tation 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 elec-

    trospinning 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 suchthat they do not only mimic the properties of the extracellular ma-

    trix (ECM), but also possess high strength and high toughness. For

    instance, non-woven fabrics exhibit isotropic properties and sup-

    port neo-tissue formation. These mats resemble the ECM matrixand can be used as skin-scaffold and wound dressing materials

    where the materials are required to be more elastic than stiff

    [14,7782]. When anisotropic properties are desired for load-bear-

    ing applications, such as musculoskeletal tissues (tendons and lig-

    aments), aligned electrospun fibers can be used to mimic the

    structural anisotropy of the tissues. Many natural polymers (colla-

    gen, starch, chitin and chitosan) and synthetic biodegradable poly-

    mers (poly(e-caprolactone) (PCL), polylactide (PLA), poly(D, L-

    lactide-coglycolide) (PLGA)) have been widely investigated for po-

    tential applications in developing tissue scaffolds [77,8386].

    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

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

    Fig. 18. SEM micrograph of fracture surface showing the presence of electrospun

    nanofibers in the matrix resin. Reprinted from [16], with permission from Elsevier.

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    their hierarchical structural arrangement possess superior fracture

    toughness values compared to any of the synthetic scaffold mate-

    rials that are currently used [83]. To match the mechanical integ-

    rity 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 colla-

    gen 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 and1.23 nm in width) are self-assembled in an orderly fash-

    ion 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 arrange-

    ment of collagen and bone crystals at the structural level can crit-

    ically affect the mechanical integrity of the whole system [8890].

    The strength of the tissue scaffolds processed using the conven-

    tional techniques lack the architecture design seen in the natural

    composites and hence their mechanical characteristics are drasti-

    cally different from natural bone composites. However, the elec-

    trospinning 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 con-

    trolled 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 Youngs mod-

    ulus 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 tough-

    ness materials and materials with good thermal and electrical con-

    ductivity, very few studies have investigated the use of electrospun

    fibers in composites [1618,91]. 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 previ-

    ous sections, fibers with finer diameters have preferential orienta-

    tion of the polymer chains along the fiber axis. The orientation of

    macromolecules in the fibers improves with the reduction in diam-

    eter, 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 be-

    tween the fibers can lead to an interpenetrated structure when dis-

    persed 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 fibermatrix

    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 orienta-

    tion of the fibers in the polymer matrix. To achieve better rein-

    forcement, 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 be-

    tween fibers and matrix material needs to be controlled such that

    the fibers are capable of deflecting the cracks by fibermatrix inter-

    face 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 elec-trospun 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 fi-

    ber fragments of several 100 nm in length.

    7.3. Conductive fiber composites

    Electrospinning has found applications in developing flexible

    and compliant conductive nanofibers for applications in miniatur-

    ized devices [23,9294]. Researchers seek to develop compliant

    electrodes for electroactive polymer actuators. Use of electrospin-

    ning to produce fibers from conductive polymer matrices can be

    useful for these applications. Moreover, electrospinning can beused 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 dur-

    ing 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 effi-

    ciency, 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 elec-

    trochemical reactions is affected by the surface area of the elec-

    trode, the high surface area of the fibers for the development ofporous electrodes can be exploited.

    Fig. 19. TEMmicrograph of nylon 6,6-CNT fiber. The CNTs areembeddedin thecoreregion of the fiber and are aligned along the fiber axis.

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    Norris et al. [92] fabricated ultrafine electrically conductive

    polyaniline/polyethylene oxide (PAN/PEO) fiber blend using elec-

    trospinning. 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 in-

    creased 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

    Electrospun fibers with micron-sized diameters have found

    extensive functions in filtration applications [23,9698]. Electro-

    spun non-woven fabrics used for filtration provide the advantagesof 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 appli-

    cations in industry and for particle collection in clean rooms. Typ-

    ically, aerosol particles are filtered due to the electrostatic

    attraction that exists between the filter media and aerosol parti-

    cles. Electrospun fibers used in filtration media can improve the

    efficiency of filtration as the static charge used to produce the elec-

    trospun fibers may remain in the fibers and help in the filtration of

    aerosol particles. It is seen that the filtration efficiency of the elec-

    trospun mats is comparable to the commercially available filters

    but the advantage lies in the filter mass which is substantially low-

    er for the former than the latter [23,98].It is well-known that as the surface area of the fibers is in-

    creased, 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 signif-

    icantly improved compared to the larger fibers. For efficient filtra-

    tion, 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 con-

    trolled and can make an impact in high efficiency particulate air

    filtrations.

    7.5. Filler reinforced fiber systems

    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 fabri-

    cating lightweight fibrous composite with unique properties for

    protective clothing and body armor applications [1]. Commonly

    used fillers are carbon nanotubes (CNTs), organoclay, hydroxyapa-

    tite (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 biodegrad-

    able polymer fibers shows potential for bio-medical applications.

    Similarly, CNT inclusion in electrospun fibers enhances the overallstrength and conductivity. Such CNT reinforced fibers are particu-

    larly useful for miniaturized electronic components and load-bear-

    ing 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 demon-

    strated that the high electrostatic forces and shear force experi-

    enced 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 ef-

    fect to the neighboring molecules. Thus, orientation of polymer

    chains during electrospinning and the presence of hard fillers with-

    in 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 melt-

    processing. More interestingly, for electrospun fiber samples, filler

    addition increases the tensile strength. However, filler addition de-

    creases the tensile strength of melt-processed composites (bulk).

    In the electrospun fibers, HAP particles are contained in the nanof-

    Fig. 20. TEM micrograph of electrospun fiber filled with magnetite particles.

    H (Oe)

    -10000 -5000 0 5000 10000

    M(emu/g)

    -30

    -20

    -10

    0

    10

    20

    30

    Magnetic Fiber

    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 en-

    hanced 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 perfor-

    mance 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 pro-

    cessed 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 opti-

    mizing 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 nanofi-

    bers. However, very few studies can be found on the macromolec-

    ular 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 miniatur-

    ized 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 amor-

    phous 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 estab-

    lished due to the higher degree of chain orientation in the formercompared 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 signif-

    icantly increased.

    The heterogeneity in the skin and core regions of the fibers is

    more remarkable when the fibers are reinforced with CNTs. Inclu-

    sion of CNTs in the fiber matrix presents an additional interface for

    surface chain orientation [99]. Therefore, the overall chain orienta-

    tion increases with the CNT loading. These oriented regions are

    stiffer compared to the regions of disoriented chains. Conse-

    quently, 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 andskin 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 fil-

    ler materials to increase the strength of electrospun fibers

    [44,87,99]. However, future work should focus more on the mul-

    ti-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 dis-

    tribution of the fillers are very attractive since the composite is ex-

    pected to display enhanced magnetic-field dependentsuperparamagnetism. Such features are seldom achieved using

    other conventional techniques. We have recently demonstrated

    that magnetic particle electrospun fibers can display superpara-

    magnetism 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 prop-

    erties of the nanoparticles can be utilized to obtain multi-func-

    tional 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.

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