Dual Electric Field Induced Alignment of Electrospun Nanofibers

A dual‐field method is described to produce aligned fibers by electrospinning. By applying a secondary electric field perpendicular to the primary field, control over the orientation of the fibers on the collector is obtained. The dual‐field approach is used in conjunction with dual electrodes and a rotating collector. Both approaches were used to electrospin poly(lactic acid) fibers under conditions intended to produce fibers with a diameter greater then 400 nm to aid observation by optical microscopy and digital photography. Digital photography was used to visualize a large area of aligned fiber and image analysis software was used to quantify the degree of alignment. Fibers aligned with the aid of a dual field showed better alignment than those from a single electric field under otherwise identical conditions.

     

Fabrication of nanofibers by a modified air-jet electrospinning method

A modified air-jet electrospinning (MAE) setup was demonstrated for contributing to the large-scale nanofibers production. With this single nozzle air-jet electrospinning device, the productivity of nanofibers can be increased more than forty times as compared with using the single-needle electrospinning (SNE) setup. When compared with other needle-less electrospinning setups, the benefits of this setup include ability to keep stable concentration of electrospun solution and to produce more uniform and thinner fibers, controlling of the jets formed speed and position, higher throughput, lower critical voltage, easier assembling, simpler operation, and so on. Four different parts of the fiber generator were, respectively, charged as electrospun electrodes to produce fibers. The distributions of the electric field with different electrodes were simulated and investigated for explaining the experimental results including the fibers productivity, the deposition area of nanofiber mats, as well as the surface morphology of the fibers. When the whole nozzle was charged, as compared with charging other electrodes, the MAE system produced thinner fibers with larger standard deviation on a much larger scale. By reduction of charged area, the received fibers presented lower productivity and thicker diameter with lower standard deviation. Especially, when a half of the nozzle was charged, the deposition area of nanofiber mats was larger than charging other electrodes. Besides, when a half of the nozzle was charged, the influences of electrospinning parameters such as applied voltage, collecting distance and the flow rate of air on nanofibers morphology were also investigated. Furthermore, based on this spinning unit, multi-nozzle air-jet electrospinning setup can be designed for larger production of nanofibers.     

Impact of substrate geometry on electrospun fiber deposition and alignment

Aligned, uniform fiber matrixes are highly desirable in numerous engineering and physical science applications. Here, modified electrospinning (ES) deposition substrates (paired and in parallel) are explored to achieve rapid preparation of multiple topographies. Three ES substrates with well‐defined geometries (rectangular, concave, and E‐shaped) were investigated (arranged in parallel) for their impact on fiber size, morphology, orientation, and cell behavior. The results indicate fiber alignment and orientation can be improved and modulated based on the substrate geometry. In addition, altering the interdistance space between various parallel substrates has a clear impact on fiber diameter size and alignment (random, aligned, and perpendicular orientation). Electric field simulations based on substrate geometries show greater probable regions of aligned electric field vectors and distribution, which indicates the most likely deposition attributes of electrospun PCL fibers. Fibrous PCL membranes were biocompatible, and cell growth and guidance were along the fiber path, with evidence of branching at intersecting fibers for multiaxial fibrous topographies. These findings show that the substrate geometry can be optimized to effectively assemble multiaxial layered and well‐aligned fibers in a controlled fashion, which is ideal to support several application developments dependent on fiber topography, integrity, and morphology. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44823.     

Effects of an external electric field on the collection efficiency of air filters: Filtration mechanisms with an external e-field

An external electric field was applied on the filter to improve its collection efficiency, and the collection efficiencies of the different filters under various conditions were evaluated. Dominant electrical filtration mechanisms for each condition were investigated using experimental and theoretical approaches. Four types of air filters were used as test filters: a charged fiber filter, a low-grade filter with 50% collection efficiency in the most penetration particle size (MPPS) zone, and two high-grade filters with more than 95% collection efficiency in the MPPS zone. Three different particle charge states—neutralized, single-charged and uncharged—were considered. For neutralized particles, the external electric field led to a 14.5%p. and 2.5%p. increase in the collection efficiencies of the low-grade filter and charged fiber filter, respectively. With the electric field, the collection efficiency of the low-grade filter increased by 30%p. for single-charged particles. The electric field also affected the collection efficiencies of the charged filter and high-grade filters, but the effect was not significant. For uncharged particles, the electric field did not lead to a remarkable increase in the collection efficiencies of any of the filters. Through experimental and theoretical analysis, it was found that the polarization force imposed on the charged fiber was the dominant factor for the charged fiber filter regardless of application of the external electric field. The Coulombic force imposed on the electric field was the dominant factor for the low-grade filter, while both the Coulombic and the polarization forces affected the collection efficiency of the high-grade filter.

Copyright © 2017 American Association for Aerosol Research     

Rapid prototyping of a miniaturized Electrospinning setup for the production of polymer nanofibers

Fabrication of polymeric micro/nanofibers with controllable size, density, orientation, and composition is required for their translation into functional devices and materials. Electrospinning (ES) is a frequently used fiber fabrication technique, where ES parameters such as the applied electric field strength, architecture of the setup, and solution composition are manipulated to control the fiber properties. Here, we present a bench‐top method for fabricating miniaturized, integrated, and highly tunable ES setups based on shrinkable polymer substrates. We show that using a combination of numerical modeling and controlling different parameters in the ES setup, including the spinneret to collector distance, and spinneret and collector designs, it is possible to tune the density, alignment, and orientation of electrospun fibers. In this way, we have produced 300–600 nm wide poly(ethylene oxide) fibers arranged as nonwoven mats on planar electrodes, aligned fibers on electrode edges, and individual suspended fibers spanning gaps between collector electrodes. The ability to rapidly prototype ES setups should enable us to study the effects of spinneret–collector configurations on fiber morphology, distribution, and conformation and to aid in the development of miniaturized ES setups designed to serve specific applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40629.     

The effect of fiber orientation on the toughening of short fiber‐reinforced polymers

The effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well‐aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull‐out (including snubbing), fiber breakage, fiber–matrix debonding, and localized matrix‐yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull‐out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2740–2751, 2003     

Effects of porosity,fiber size,and layering sequence on sound absorption performance of needle‐punched nonwovens

The relationships between the material parameters, i.e., the fiber fineness, porosity, areal density, layering sequence, and airflow resistivity with the normal‐incidence sound absorption coefficient of nonwoven composites consisting of three layers have been studied. The monofiber or multifiber needle‐punched nonwovens included poly(lactic acid) (PLA), polypropylene (PP), glass fiber, and hemp fibers. Air flow resistivity was statistically modeled and was found to increase with decreasing fiber size and nonwoven porosity. The former models developed for glass fiber mats in the literature were found to be inconsistent with the air flow resistance of the nonwovens reported below. The effects of the layering sequence on air flow resistivity and sound absorption were obtained. It was found that when the layer including reinforcement fibers, i.e., hemp or glass fiber, faced the air flow/sound source, the air flow resistance and the absorption coefficient were higher than the case when the layer including reinforcement fibers was farthest from the air flow/sound source. The difference was more pronounced if there was a greater difference between the resistivity values of the constituent layers of the nonwoven composite. Sound absorption coefficient was statistically modeled in terms of air flow resistivity and frequency. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Directed assembly of carbon nanocones into wires with an epoxy coating in thin films by a combination of electric field alignment and subsequent pyrolysis

We report on a nanoparticle manipulation method for assembling carbon nanocones and disks (CNCs) into molecular wires that create a regular two-dimensional network inside thin films. This technique includes electric field induced assembly and electric field orientation (dielectrophoresis), curing and pyrolysis. First, CNCs are dispersed near to the individual particle scale in a two component epoxy adhesive containing phenolic resin. Second, a thin layer (?10 μm) of this dispersion is spread onto interdigitated metal electrodes (spacing between 10 and 100 μm) on a glass substrate (area of several cm²). CNC wires are assembled and aligned by an alternating electric field (∼1 kHz, ∼1 kV/cm) yielding an epoxy film with uniaxially aligned CNC pathways (diameter 1–5 μm) in-plane. Third, the aligned film is cured by heating, which leads to a solid film where the wire alignment is maintained within the cross-linked polymer matrix. Finally, most of the cured epoxy is removed from in between the CNC wires by further heating (pyrolysis), which results in a network of aligned, separated wires with a CNC interior and polymer covering. This procedure provides a general concept for forming aligned and stable networks of CNC wires over large surfaces.     

Alignment and optimization of nylon 6 nanofibers by electrospinning

Nonwoven electrospun nylon 6 nanofibers produced with formic acid under different concentrations have been examined. The effects of the solution properties, electric field, and spinneret‐to‐collection distance on the fiber uniformity, morphology, and average diameter have been established. The optimum polymer solution concentration (20 wt %), applied voltage (15 kV), and spinning distance (8 cm) have been found to make uniform nylon 6 fibers. A simple technique that can produce a bundle of aligned electrospun fibers suspended between two grounded disks is described. Alignment and stretching of the fibers are derived by the electrostatic interactions between the positive electrode on the spinneret and the grounded disks. The gap between the disks and the collection time have been varied to systematically study the degree of alignment and the density of the collected nylon 6 fibers. The number of the distributed fibers in the bundle can be controlled by the alteration of the deposition time, the voltage, and the width of the gap. Scanning electron microscopy images have indicated a greater degree of fiber alignment with increasing disk gaps and collection times. The article also provides a comprehensive review of the design of various mechanisms for nanofiber alignment. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

The viscosity of fiber suspensions at low fiber volume fractions

The viscosities of suspensions of glass fibers in an aqueous solution of sucrose have been studied by use of a capillary viscometer. In the aligned condition in the capillary, the viscosity depends little on shear rate within the range studied or on fiber length, but increases with increasing volume fraction of the fibers. The entrance effect was found to depend strongly on fiber volume fraction and fiber length: this indicates that the suspensions are relatively resistant to flow during the initial stages while alignment takes place.     

Unipolar Charging of Fine and Ultra-Fine Particles Using Carbon Fiber Ionizers

A simple and novel unipolar charger using carbon fiber ionizers was developed to effectively charge fine and ultra-fine aerosol particles without the generation of ozone. The particle penetration in the charger was investigated for non-charged, neutralized, and singly charged particles in the size range of 20–200 nm. Particle loss and the intrinsic, exit and extrinsic charging efficiencies of fine and ultra-fine particles were also investigated for non-charged particles at different applied voltages to the charger. Particle penetrations in the charger were nearly 100% for particles larger than 20 nm, irrespective of the initial particle charging state. Particle losses in the charger could be decreased by decreasing the applied voltage to the charger from 4.0 kV to 2.3 kV. The intrinsic charging efficiencies were proportionally increased with the applied voltage, whereas the exit charging efficiencies were almost independent of the applied voltage. Therefore, the extrinsic charging efficiency of the charger becomes higher for the lower applied voltage (2.3 kV), at which about 60% of 20 nm particles were charged. Little (less than 4 ppb) to no ozone was generated under all operation conditions. It can be concluded that the newly developed unipolar charger using carbon fiber ionizers can charge fine and ultra-fine particles at least as effectively as currently available unipolar chargers, but with the major advantage of negligible ozone generation, a highly desirable feature if the charged particles are to be used for chemical or biological analysis.     

Electrospinning of poly(hydroxybutyrate‐co‐hydroxyvalerate) fibrous tissue engineering scaffolds in two different electric fields

Poly(hydroxybutyrate‐co‐hydroxyvalerate) (PHBV) was electrospun into ultrafine fibrous nonwoven mats. Different from the conventional electrospinning process, which involves a positively charged conductive needle and a grounded fiber collector (i.e., positive voltage (PV) electrospinning), pseudo‐negative voltage (NV) electrospinning, which adopted a setup such that the needle was grounded and the fiber collector was positively charged, was investigated for making ultrafine PHBV fibers. For pseudo‐NV electrospinning, the effects of various electrospinning parameters on fiber morphology and diameter were assessed systematically. The average diameters of PHBV fibers electrospun via pseudo‐NVs were compared with those of PHBV fibers electrospun via PVs. With either PV electrospinning or pseudo‐NV electrospinning, the average diameters of electrospun fibers ranged between 500 nm and 4 μm, and they could be controlled by varying the electrospinning parameters. The scientific significance and technological implication of fiber formation by PV electrospinning and pseudo‐NV electrospinning in the field of tissue engineering were discussed. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Polymere hochleistungs-faserverbundwerkstoffe

Advanced polymer-matrix composites are defined as materials consisting of continuous high strength, high modulus fibers, aligned and embedded in a polymeric matrix. Due to their low density they exhibit specific mechanical properties (i.e., strength/density and stiffness/density) that considerably exceed those of high strength metals, which demonstrates their great potential for lightweight structural components. The primary reinforcement materials are various types of carbon fibers, aramid fibers and glass fibers. While these are commonly combined with epoxy resins, many new matrix systems with distinct property profiles, including heat resistant thermoplastics, have been developed recently. This paper presents an overview of this rapidly developing field covering principles of composite-property improvement, synthesis and processing of the constituent materials, and processing and manufacturing techniques to combine fibers and matrix to produce a composite laminate or part. Several mechanical composite properties critical for structural design and performance are discussed in terms of the contribution of and interaction between the fiber and matrix constituent.     

Study on the effect of inorganic salts on the alignment of electrospun fiber

Well‐aligned and highly ordered architectures are always required in many fields, such as tissue engineering, electronics, and preparation of composite materials. In this study, electrospun mats with well‐aligned fibers and various fiber assemblies were successfully fabricated by electrospinning of poly(vinylbutyral) (PVB)/inorganic salt solution under the optimal salt condition. Then, the effect of inorganic salts on the degree of electrospun fiber alignment was comprehensively investigated, and the results indicated that the viscosity and conductivity of the solutions were the key factors influencing the degree of fiber alignment. It was expected that this simple and feasible method could be helpful for the fabrication of the well‐aligned electrospun fibers and various fiber assemblies. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The effect of the interface/interphase on fiber composite properties

The interfacial region between fibers and matrix in fiber composites governs the transfer of forces between the relatively weak and compliant matrix and the reinforcing fibers. An effective interphase can ensure that the mechanical properties of the composite reflect the high strength and modulus of the fibers. Although composites can be made with the expected strengths and moduli, it is not entirely clear why this is achieved: Tests with critical composites, i.e., those containing very short aligned fibers, do not show the expected stress-strain behavior. This paper examines the effect of an interphase having a shear modulus that is less than that of the matrix. It is found that to explain the Young's moduli of the short fiber composites, the interphase must have a very low modulus indeed; i.e., a few kPa at most. In addition, the strength results can be accounted for only if we assume that the short lengths of fiber used in the experiments had higher strengths than anticipated. Although agreement between experiments and theory is thus not very good, the small amount of experimental evidence available indicates a need for further systematic experiments on critical (i.e. short aligned fiber) composites before firm conclusions are drawn.     

挤出机头内流场对玻纤取向和分布的影响

本文分析了平直和发散两种挤出机头内的流场,推得发散流道内熔体周向拉伸应变速率的表达式;研究了两种挤出流率下由这两种机头挤出的制品壁内玻纤的取向和分布,并通过流道内的剪切和周向拉伸应变速率,对玻纤取向和分布的形成机理进行解释。结果表明：经平直机头挤出的制品内,玻纤在剪切作用下基本沿流动方向排列。发散机头内熔体受剪切和周向拉伸的共同作用,使制品壁厚方向形成了“表层-次表层-芯层-次表层-表层”的五层结构,并首次发现芯层呈“W”形排列。玻纤的排列不仅受流动过程中的应变影响,更取决于应变速率的大小。     

Size Characterization of Asbestos Fibers by Means of Electrostatic Alignment and Light-Scattering Techniques

Crocidolite asbestos fibers suspended in light mineral oil were aligned by means of an electrostatic field of 1500 V / cm. The maximum fiber concentration was ～8 μg / mL. The light scattered by the suspended fibers, with the incident laser beam normal to the fiber axis, was recorded by two probes: one rotating about the fiber axis and another rotating about the incident beam near the forward direction, i.e., 17.5° from the incident beam. The Rayleigh-Debye-Gans theory was utilized in order to determine the modal diameter and modal aspect ratio of assumed logarithmic normal distribution functions in both the fiber diameter and the fiber aspect ratio. Multiple scattering effects were assumed to be negligible.     

Fabrication of Glass Matrix Composites by Tape Casting

A method has been developed to fabricate borosilicate glass matrix composites reinforced with monofilament SiC fibers by tape casting. Green matrix tapes are laminated with fiber mats of a uniform fiber spacing. The resulting laminate is sintered at 710°C to >98% relative density and HIP-consolidated to full density. The final specimens contain a high volume fraction of fibers (>35 vol%) in a uniform array. A variation of this technique can be used to mount "microcomposites" (i.e., coated fibers) in a glass matrix to facilitate fabrication of push-out test specimens.     

Control of electrospun mat width through the use of parallel auxiliary electrodes

Electrospinning offers a versatile way to produce one-dimensional micrometer or nanometer materials; however, electrospun fibers are typically collected in a random orientation limiting their applications. In the present study, we have expanded upon a technique used to align fibers for control of the fiber distribution during the spinning process through the use of auxiliary counter electrodes. The electrostatic force imposed by the auxiliary electrodes provides a converged electric field, which affords control over the distribution of the fibers on the rotating collector surface. Experimental results demonstrate that the width of electrospun mats can be decreased dramatically when parallel auxiliary electrodes are employed at the collector. There was no apparent difference in the average diameters of the electrospun fibers as a result of the additional auxiliary electrodes, but the fiber distribution density in terms of mat width was greatly improved. Thus, the use of auxiliary counter electrodes at the rotating collectors provides a viable method of converging and controlling the deposition of electrospun fibers.     

熔体静电纺丝中电极结构对电场和纤维的影响

采用Ansys数值模拟方法,建立了电场分析的有限元模型,研究了电极结构不同时的电场分布及其对纤维直径的影响,以及电压大小对纤维直径的影响。并进行了实验对照分析,发现电极结构包括圆板电极和圆环电极影响熔体静电纺丝的电场分布,但场强最大都出现在喷嘴处,并随接收距离的增大成不同趋势减小,但中空电极能集聚电场,稳定场强,获得更细的纤维。