Charged Polymer Jet Research Articles

Mass Production of Nanocomposites Using Electrospinning

Many conventional polymer processing technologies for compounding micro-/nanocomposites are known in the field. These methods include the direct use of high shear mixers, roll mixers, Banbury mixers, and extruders. With recent interest in advanced composites with nanoscale fillers, efforts have been made to enhance conventional processing technologies as the imposed input energy is often ineffective at breaching the energy barrier to breakup agglomerated nanofiller structures. Electrospinning is a simple, inexpensive process that can be used to produce continuous fibers from submicron to nanometer diameter scale through an electrically charged polymer jet. In this paper, authors present a rotary electrospinning method developed to produce nanocomposites at mass scale by using simultaneously mechanical and electrical forces with a proprietary apparatus. A case study using silica nanoparticles and silicone rubber matrix is presented to demonstrate the capability of the above method of dispersing nanoparticles in highly viscous polymer matrix materials.

Current characteristics of stable cone–jet in electrohydrodynamic printing process

Electrohydrodynamic printing (EHDP) may produce micro-/nano-patterns for various applications using an external electric field to drive a charged polymer jet. This paper investigates both current characteristics and governing parameters at stable cone–jet mode in EHDP process. Experimental results show that the current increases with increasing applied voltage, flow rate, fluidic conductivity, and even concentration. The convection current Ic dominates the EHDP process of extremely low-conductivity solution by Ic ∝ Q1/2, while the conduction current Io dominates the EHDP process of high-conductivity solution by Io ∝ QV, where Q is the flow rate of solution and V is the applied voltage.

Effects of orthogonal rotating electric fields on electrospinning process

Electrospinning is a nanotechnology process whereby an external electric field is used to accelerate and stretch a charged polymer jet, so as to produce fibers with nanoscale diameters. In quest of a further reduction in the cross section of electrified jets hence of a better control on the morphology of the resulting electrospun fibers, we explore the effects of an external rotating electric field orthogonal to the jet direction. Through intensive particle simulations, it is shown that by a proper tuning of the electric field amplitude and frequency, a reduction of up to a 30% in the aforementioned radius can be obtained, thereby opening new perspectives in the design of future ultra-thin electrospun fibers. Applications can be envisaged in the fields of nanophotonic components as well as for designing new and improved filtration materials.

Simplified modeling of the electrospinning process from the stable jet region to the unstable region for predicting the final nanofiber diameter

ABSTRACTElectrospinning allows the production of ultrafine nanofibers through the stretching of a charged polymer jet with an external electrostatic field. In this study, we derived a simplified and accurate model relating the processing parameters, including the solution volumetric flow rate (Q), the applied electric field (E), and the polymer concentration, to the final fiber diameter. The model takes into consideration the jet behavior starting at the stable region and moving to the bending instability region. We validated the model experimentally by performing the electrospinning process with a polyacrylonitrile/N,N‐dimethylformamide solution with different ranges of concentrations (8–11 wt %), Qs (900–1320 μL/h), and Es (88,889–113,889 V/m). The final fiber diameter was measured with scanning electron microscopy. The model predicted the fiber diameter with a relative error of less than 10%. Moreover, a 30% increase in Q resulted in a 15% increase in the fiber diameter, whereas a 30% increase in E resulted in a 14% decrease in the fiber diameter. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44112.

Oriented nanofibrous membranes for tissue engineering applications: Electrospinning with secondary field control

Electrospinning is an electrical field driven method to produce polymer fibre membranes by deposition of a charged polymer jet onto a grounded collector. Fibre alignment within these mats is usually achieved by a fast collector movement, which is not feasible for all collector geometries, such as small diameter tubes or free-form moulds. The aim of this study was to evaluate the use of charged deflector plates to apply a dynamic, alternating electrical field perpendicular to the spinning direction, in order to directly control the fibre trajectory. Different field signal types, deflector plate voltages and deflection frequency ranges have been investigated. 210 poly(ɛ)caprolactone (PCL) membranes were electrospun. SEM images of each membrane were analysed using ImageJ. Main fibre diameter and orientation, as well as the degree of fibre alignment, were calculated, while a subset of the spun scaffolds were tested for their tensile properties. Higher deflector plate voltage amplitude resulted in a better fibre alignment. The best alignment was observed in a low deflection frequency range from 2 to 10 Hz. Mean main fibre direction was 87±18°, relative to the deflection axis, while fibre alignment had only a minor effect on the average fibre diameter. Young′s modulus and yield stress increased with the ratio of the parallel fibre component. The feasibility of the described method to achieve fibre alignment was demonstrated. However, the main fibre direction is not aligned with the deflection axis, but consistently perpendicular to it, which is also reflected in the tensile properties of spun samples.

Dynamics of an electrically charged polymer jet

Electro-hydrodynamic equations describing the behavior of a charged polymer jet are analyzed by analytical methods and scaling approach. A FENE-P constitutive equation is employed to describe the viscoelastic properties of a conducting polymer liquid. Effects of the electric field, the flow rate, and the material parameters on the jet dynamics are investigated. Four different regimes are examined. In particular, a regime in which the electric current is linearly proportional to the electric field and independent on the flow rate and a regime in which the electric current is linearly proportional to the flow rate and independent on the electric field are identified. An operating window limiting the region of a stable cone-jet mode is also considered.

Electrospinning Fundamentals: Optimizing Solution and Apparatus Parameters

Electrospun nanofiber scaffolds have been shown to accelerate the maturation, improve the growth, and direct the migration of cells in vitro. Electrospinning is a process in which a charged polymer jet is collected on a grounded collector; a rapidly rotating collector results in aligned nanofibers while stationary collectors result in randomly oriented fiber mats. The polymer jet is formed when an applied electrostatic charge overcomes the surface tension of the solution. There is a minimum concentration for a given polymer, termed the critical entanglement concentration, below which a stable jet cannot be achieved and no nanofibers will form - although nanoparticles may be achieved (electrospray). A stable jet has two domains, a streaming segment and a whipping segment. While the whipping jet is usually invisible to the naked eye, the streaming segment is often visible under appropriate lighting conditions. Observing the length, thickness, consistency and movement of the stream is useful to predict the alignment and morphology of the nanofibers being formed. A short, non-uniform, inconsistent, and/or oscillating stream is indicative of a variety of problems, including poor fiber alignment, beading, splattering, and curlicue or wavy patterns. The stream can be optimized by adjusting the composition of the solution and the configuration of the electrospinning apparatus, thus optimizing the alignment and morphology of the fibers being produced. In this protocol, we present a procedure for setting up a basic electrospinning apparatus, empirically approximating the critical entanglement concentration of a polymer solution and optimizing the electrospinning process. In addition, we discuss some common problems and troubleshooting techniques.

Electrospinning Fundamentals: Optimizing Solution and Apparatus Parameters

Electrospun nanofiber scaffolds have been shown to accelerate the maturation, improve the growth, and direct the migration of cells in vitro. Electrospinning is a process in which a charged polymer jet is collected on a grounded collector; a rapidly rotating collector results in aligned nanofibers while stationary collectors result in randomly oriented fiber mats. The polymer jet is formed when an applied electrostatic charge overcomes the surface tension of the solution. There is a minimum concentration for a given polymer, termed the critical entanglement concentration, below which a stable jet cannot be achieved and no nanofibers will form - although nanoparticles may be achieved (electrospray). A stable jet has two domains, a streaming segment and a whipping segment. While the whipping jet is usually invisible to the naked eye, the streaming segment is often visible under appropriate lighting conditions. Observing the length, thickness, consistency and movement of the stream is useful to predict the alignment and morphology of the nanofibers being formed. A short, non-uniform, inconsistent, and/or oscillating stream is indicative of a variety of problems, including poor fiber alignment, beading, splattering, and curlicue or wavy patterns. The stream can be optimized by adjusting the composition of the solution and the configuration of the electrospinning apparatus, thus optimizing the alignment and morphology of the fibers being produced. In this protocol, we present a procedure for setting up a basic electrospinning apparatus, empirically approximating the critical entanglement concentration of a polymer solution and optimizing the electrospinning process. In addition, we discuss some common problems and troubleshooting techniques.

Modelling of electrospinning process at various electric fields

In electrospinning process nanostructured solid fibres are produced from charged polymer jet oriented at external electrostatic field. Materials such as polymer, composites and tissue scaffold fibres have been fabricated with electrospinning techniques. The typical obstacle of electrospun fibre production is the whipping instability, resulting from the chaotic oscillation of polymer jet. In this Letter, feasibility of suppressing the whipping instability is analysed and explained using a mathematical model based on the usage of secondary electrostatic field created by finite length hollow cylinder. It is shown that the three-dimensional path of polymer jet calculated by computer simulations focus the characteristic spot size of the deposited electrospun fibre to a smaller diameter, which is coherent to the experimental observations.

Electrospinning of Continuous Aligning Yarns with a ‘Funnel’ Target

AbstractA new electrospinning set‐up is described for continuous aligning of poly(L‐lactide) yarn. It comprises a slowly rotating grounded ‘funnel’ target and a winder placed right up to the funnel. A charged polymer jet is ejected from a nozzle. The electrospun fibers are first accumulated on the mouth plane of the funnel to form a web. The web is then pulled upward and guided to a winder on which twisted fibers are continuously wound as a bundle. Each filament of the bundle is slanted at about 45° relative to the collecting direction, and its average diameter is 6.0 ± 1.9 µm. The drawn fiber shows improved tensile properties as a result of the increased molecular orientation. The morphologies of the twisted fibers before and after drawing are compared.magnified image

Fabrication of Patterned Fluorescence Images in Electrospun Microfibers

Recently, generation of patterned fluorescence images based on the “precursor approach” has gained much attention. The concept of the ‘precursor approach’ is to use different electronic properties between the protected and unprotected forms. For example, a dye molecule is nonfluorescent when the key functional group of the dye molecule is protected with a protecting group. If the protecting group is removed under photoinduced chemical transformation, the fluorescence can be regenerated, allowing patterned fluorescent images in the polymer film by selective removal of the protecting group in the exposed areas. The “precursor approach” allows rapid and costeffective generation of patterned images in one step, without the need for additional wet developing processes. We have previously reported that the fluorescent quinizarin molecule can be readily converted to a nonfluorescent protected molecule t-BocQ by introduction of acid-labile tert-butoxycarbonyl (t-Boc) groups to the quinizarin hydroxyl moieties (Scheme 1). When the t-Boc groups were selectively removed by photogenerated acids, yellow fluorescent patterns were appeared in the UV exposed areas. Thus, the nonfluorescent t-BocQ precursor molecule was transformed to unprotected quinizarin by the photoinduced chemical transformation. The vast majorities of the fluorescence patterns generated based on the “precursor approach” have been fabricated in polymer film. During the course toward the development of fiber-based chemosensor systems, we have discovered a new strategy for the fabrication of patterned fluorescence images in microfibers. The key strategy and procedure employed for the fluorescence patterning is schematically presented in Figure 1. A viscous chloroform solution containing the precursor molecule t-BocQ (1.3 wt%), poly(methyl methacrylate) (PMMA) (19.6 wt%), and a photoacid generator (PAG), triphenylsulfonium triflate (0.6 wt%) is placed in a syringe. A high voltage (18 kV) is then applied to the metal syringe needle causing ejection of a charged polymer jet from the polymer solution. The microfibers are collected on the surface of a grounded aluminum plate (15 cm from the needle). A scanning electron microscope (SEM) image confirms the formation of microfibers (Figure 1). Photomasked UV irradiation of the electrospun fiber should generate strong acids from the PAG in the exposed areas. The photochemically produced strong acids should promote catalytic deprotection of the t-Boc groups from the precursor molecules, allowing patterned fluorescence images in the polymer fibers. In order to investigate the feasibility of fluorescence patterning, the polymer fiber mats obtained as described above were irradiated with 254 nm UV light through a photomask for 2 min. The UV-treated fiber mats were then placed on a hot plate (100 C) for 1 min for a post-exposure

Effects of polymer media on electrospun mesoporous titania nanofibers

Titania nanofibers were fabricated via a polymer-assisted electrospinning technique. A mixture of titanium tetraisopropoxide and poly(vinyl pyrrolidone) in an alcohol medium utilized as the sol–gel solution was ejected through a needle under a strong electrical field. Accompanied by solvent evaporation and sol–gel solidification, a highly charged polymer jet induces the formation of polymer/metal oxide nanofibers before reaching the ground electrode. The sintering process was carried out afterward to encourage the crystallization and phase transformation of titania, as well as the removal of polymer domains. Electrospinning processes and sintered titania nanofibers exhibited a strong relationship in the presence of polymer media. Process parameters and the characterizations of nanoconstructed titania, including fiber morphologies and mesoporous crystal structures, were investigated and discussed.

Electrical conductivity of a single electrospun fiber of poly(methyl methacrylate) and multiwalled carbon nanotube nanocomposite

Electrospinning produces polymeric fibers with diameters in the range of 10μm–10nm by accelerating a charged polymer jet under a high electric field. We report the preparation of conducting nanocomposite fibers of poly(methyl methacrylate) (PMMA) and multiwalled carbon nanotubes (MWCNTs) by electrospinning. The fibers obtained are long and well aligned. The carbon nanotubes are found to be oriented along the fiber axis. The room temperature dc electrical conductivity of a single fiber with MWCNT (0.05% w/w) shows about a ten orders of magnitude improvement from the pure PMMA. The conductivity increases with MWCNT concentration.

Stretching of a straight electrically charged viscoelastic jet

A charged polymer jet may be accelerated and stretched by an external electric field, and this process is relevant to electrospinning for making nanofibers. The stretching of an electrified jet is governed by the interplay among electrostatics, fluid mechanics and rheology, and the role of viscoelasticity has not been systematically explored before. This paper presents a slender-body theory for the stretching of a straight charged jet of Giesekus fluid. Results show strain-hardening as the most influential rheological property. It causes the tensile force to rise at the start, which enhances stretching of the jet. Further downstream, however, the higher elongational viscosity tends to suppress jet stretching. In the end, strain-hardening leads to thicker fibers. This confirms the main result of a previous study using empirical rheological models. The behavior of the electrically driven jet forms an interesting contrast to that in conventional fiber spinning.

The stretching of an electrified non-Newtonian jet: A model for electrospinning

Electrospinning uses an external electrostatic field to accelerate and stretch a charged polymer jet, and may produce ultrafine “nanofibers.” Many polymers have been successfully electrospun in the laboratory. Recently Hohman et al. [Phys. Fluids, 13, 2201 (2001)] proposed an electrohydrodynamic model for electrospinning Newtonian jets. A problem arises, however, with the boundary condition at the nozzle. Unless the initial surface charge density is zero or very small, the jet bulges out upon exiting the nozzle in a “ballooning instability,” which never occurs in reality. In this paper, we will first describe a slightly different Newtonian model that avoids the instability. Well-behaved solutions are produced that are insensitive to the initial charge density, except inside a tiny “boundary layer” at the nozzle. Then a non-Newtonian viscosity function is introduced into the model and the effects of extension thinning and thickening are explored. Results show two distinct regimes of stretching. For a “mildly stretched” jet, the axial tensile force in the fiber resists stretching, so that extension thinning promotes stretching and thickening hinders stretching. For a “severely stretched” jet, on the other hand, the tensile force enhances stretching at the beginning of the jet and suppresses it farther downstream. The effects of extensional viscosity then depend on the competition between the upstream and downstream dynamics. Finally, we use an empirical correlation to simulate strain hardening typical of polymeric liquids. This generally steepens the axial gradient of the tensile stress. Stretching is more pronounced at the beginning but weakens later, and ultimately thicker fibers are produced because of strain hardening.