Simulated image of electrospun nonwoven web of PVA and corresponding nanofiber diameter distribution

Fiber diameter is the most important characteristic in electrospun nonwoven webs. Understanding how it is influenced by the electrospinning parameters is essential to produce webs with desired characteristics. In this contribution, Direct Tracking method for measuring electrospun fiber diameter is described. To evaluate the accuracy of the technique, a simulation algorithm for generating webs with known characteristics was employed. To verify the applicability of the method on real samples, an electrospun polyvinyl alcohol (PVA) mat, as a representative of real webs, was used. Since the Direct Tracking method uses a binary image as its input, local thresholding was applied to segment the SEM micrograph of the electrospun web. The results indicate that the method could be used successfully for determining fiber diameter in electrospun nonwoven webs.     

Evaluation of electrospun nanofiber pore structure parameters

Nanofibers produced by electrospinning method are widely used for drug delivery, as tissue scaffolding materials and filtration purposes where specific pore characteristics are required. For continued growth in these areas, it is critical that the nanofibers be properly designed for these applications to prevent failure. Most of the current methods only provide an indirect way of determining pore structure parameters and contain inherent disadvantages. In this study, we developed a novel image analysis method for measuring pore characteristics of electrospun nanofiber webs. Five electrospun webs with different pore characteristics were analyzed by this method. The method is direct, very fast, and presents valuable and comprehensive information regarding pore structure parameters of the webs. Two sets of simulated images were generated to study the effects of web density, fiber diameter and its variations on pore characteristics. The results indicated that web density and fiber diameter significantly influence the pore characteristics, whereas the effect of fiber diameter variations was insignificant.     

Distance transform algorithm for measuring nanofiber diameter

This paper describes a new distance transform method used for measuring fiber diameter in electrospun nanofiber webs. In this algorithm, the effect of intersection is eliminated, which brings more accuracy to the measurement. The method is tested by a series of simulated images with known characteristics as well as some real webs obtained from electrospinning of PVA. Our method is compared with the distance transform method. The results obtained by our method were significantly better than the distance transform, indicating that the new method could successfully be used to measure electrospun fiber diameter.     

Synthesis of Porous NiO and ZnO Submicro- and Nanofibers from Electrospun Polymer Fiber Templates

Porous nickel oxide (NiO) and zinc oxide (ZnO) submicro- and nanofibers were synthesized by impregnating electrospun polyacrylonitrile (PAN) fiber templates with corresponding metal nitrate aqueous solutions and subsequent calcination. The diameter of the NiO and ZnO fibers was closely related to that of the template fibers and larger diameters were obtained when using the template fibers with larger diameter. SEM results showed that the NiO and ZnO fibers have a large amount of pores with diameters ranging from 5 nm to 20 nm and 50 nm to 100 nm, respectively. Energy dispersive X-ray (EDX) spectra and X-ray diffraction (XRD) patterns testified that the obtained materials were NiO and ZnO with high purity.     

Photoluminescent electrospun polymeric nanofibers incorporating germanium nanocrystals

The photoluminescent germanium nanocrystals (Ge-NCs) were successfully incorporated into electrospun polymeric nanofiber matrix in order to develop photoluminescent nanofibrous composite web. In the first step, the synthesis of Ge-NCs was achieved by nanosecond pulsed laser ablation of bulk germanium wafer immersed in organic liquid. The size, the structural and the chemical characteristics of Ge-NCs investigated by TEM, XPS, XRD and Raman spectroscopy revealed that the Ge-NCs were highly pure and highly crystalline having spherical shape within 3–20 nm particle size distribution. In the second step, Ge-NCs were mixed with polyvinyl alcohol (PVA) polymer solution, and then, Ge-NC/PVA nanofibers were obtained via electrospinning technique. The electrospinning of Ge-NCs/PVA nanoweb composite structure was successful and bead-free Ge-NCs/PVA nanofibers having average fiber diameter of 185 ± 40 nm were obtained. The STEM analysis of the electrospun Ge-NCs/PVA nanofibers elucidated that the Ge-NCs were distributed homogeneously in the polymeric nanofiber matrix. The UV–Vis absorption and photoluminescence spectroscopy studies indicated the quantum confinement effect of Ge-NCs on the optical properties of the electrospun Ge-NCs/PVA nanoweb.     

Structural properties and superhydrophobicity of electrospun polypropylene fibers from solution and melt

Isotactic polypropylene (iPP) has successfully been electrospun from both solution and melt using an elevated temperature setup. First, PP nanofibers with two different average diameters (0.8 μm and 9.6 μm) were obtained via electrospinning of iPP in decalin, and the effect of deformation and solidification on the morphological and structural features of the resulting fibers was studied. Secondly, melt electrospun PP fibers with two different average diameters were also fabricated to compare the structures with those of solution electrospun PP fibers. DSC and XRD results show that β form crystals which can increase the impact strength and toughness of electrospun fibers are present in sub-micron scale PP fibers from solution, while fibers from melt mostly show α form crystals. The annealed fibers have changed their morphological forms into α and γ crystal forms. Finally, it is observed that electrospun PP fiber webs both from solution and melt exhibit superhydrophobicity with a water contact angle about 151° which is substantially higher than those of a commercial PP non-woven web and a compression molded PP film, 104° and 112°, respectively. Such superior hydrophobicity was observed for all PP electrospun fibers and it was not altered by the processing scheme (solution or melt) or fiber diameter (sub-micron or micron). Enhanced hydrophobicity of electrospun PP fiber webs contribute to excellent barrier performance without losing permeability when they are applied to protective clothing.     

Optimization of Process-Control Parameters for the Diameter of Electrospun Hydrophilic Polymeric Composite Nanofibers

A composite nanofiber composed of three polymers, namely polyvinyl alcohol/polyvinyl pyrrolidone/polyethylene oxide, is produced. The experiments are constructed using three design of experiment techniques, Taguchi L₉, Taguchi L₂₇, and Screening method. The experiments are verified using the analysis of variance (ANOVA) method and later a mathematical model is developed using the regression method. The impact of electrospun processing parameters, namely applied voltage, flow rate, and working distance, on nanofibers' diameter is measured. The working distance is a significant factor in controlling the size of the fiber diameter, while the applied voltage has the lowest effect on it. As a result of the regression equation, a Genetic algorithm is used to find the optimum variables for the required fiber diameter, which is 156 nm for flow rate = 0.001 mL h⁻¹, voltage = 30 kV, and distance = 200 mm with a 3% difference from the experimental fiber diameter.     

Wire Melt Electrospinning of Thin Polymeric Fibers via Strong Electrostatic Field Gradients

Here, a novel melt electrospinning method to produce few‐micron and nanometer thick fibers is presented, in which a polymer‐coated wire with a sharp tip is used as the polymer source. The polymer coating is melted via Joule heating of the source wire and extracted toward the target via electrostatic forces. The high viscosity and low charge density of polymer melts lower their stretchability in melt. The method relies on confining the Taylor cone and reducing initial jet diameter via concentrated electrostatic fields as a means to reduce the diameter of fibers. As a result, the initial jet diameter and the final fiber diameter are reduced by an order of magnitude of three to ten times, respectively, using wire melt electrospinning compared to syringe‐ and edge‐based electrospinning. The fiber diameter melt electrospun via this novel method is 1.0 ± 0.9 µm, considerably thinner than conventional melt electrospinning techniques. The generation of thin fibers are explained in terms of the electrostatic field around the wire tip, as obtained from finite element analysis (FEA), which controls the size and shape of the melt electrospun jet.     

Template‐Based Synthesis of Aluminum Nitride Hollow Nanofibers Via Plasma‐Enhanced Atomic Layer Deposition

Aluminum nitride (AlN) hollow nanofibers were synthesized via plasma‐enhanced atomic layer deposition using sacrificial electrospun polymeric nanofiber templates having different average fiber diameters (~70, ~330, and ~740 nm). Depositions were carried out at 200°C using trimethylaluminum and ammonia precursors. AlN‐coated nanofibers were calcined subsequently at 500°C for 2 h to remove the sacrificial polymeric nanofiber template. SEM studies have shown that there is a critical wall thickness value depending on the template's average fiber diameter for AlN hollow nanofibers to preserve their shapes after the template has been removed by calcination. Best morphologies were observed for AlN hollow nanofibers prepared by depositing 800 cycles (corresponding to ~69 nm) on nanofiber templates having ~330 nm average fiber diameter. TEM images indicated uniform wall thicknesses of ~65 nm along the fiber axes for samples prepared using templates having ~70 and ~330 nm average fiber diameters. Synthesized AlN hollow nanofibers were polycrystalline with a hexagonal crystal structure as determined by high‐resolution TEM and selected area electron diffraction. Chemical compositions of coated and calcined samples were studied using X‐ray photoelectron spectroscopy (XPS). High‐resolution XPS spectra confirmed the presence of AlN.     

Optimization of the electrospinning conditions for preparation of nanofibers from polyvinylacetate (PVAc) in ethanol solvent

In order to fabricate polyvinylacetate (PVAc) fiber by electrospinning, we have been examined electrospun polyvinylacetate (PVAc) under various conditions after dissolving it in ethanol solution. As the concentration of spinning solution increased, the diameter of the electrospun PVAc fiber increased. At the concentration lower than 10 wt.%, beads were formed while over the 25 wt.%, distinct fiber was not observed. At the tip-collector distance (TCD) of 7.5 cm or less, the jet of spinning solution was unstable and the fiber diameter decreased. On the other hand, at the TCD of 10 cm or more, the strength of electric field became too weak and the fiber diameter increased. As the flow rate of spinning solution increased, the fiber diameter increased and at the flow rate of 300 μl/min or more, it increased sharply. For 15 wt.% PVAc, the fiber diameter decreased as the applied voltage increased. At a high-applied voltage, however, charge acceleration caused the spinning solution not to be separated and thus the fiber diameter increased. As a result of dissolving PVAc in ethanol and electrospinning it in the following conditions, a fiber with the diameter of about 700 nm was spun: the concentration of 15 wt.%, the TCD of 10 cm, the spinning solution flow rate of 100 μl/min, and the applied voltage of 15 kV.     

High-flux microfiltration filters based on electrospun polyvinylalcohol nanofibrous membranes

A novel class of high-flux microfiltration filters consisting of an electrospun nanofibrous membrane and a conventional non-woven microfibrous support is being presented. The nanofibrous non-woven layer was fabricated by electrospinning of polyvinylalcohol (PVA) directly onto the microfibrous support and then followed by chemical cross-linking with glutaraldehyde (GA) in acetone. By altering the processing parameters, such as the applied voltage and the distance between the spinneret and the collector, as well as the concentration of PVA solution, electrospun PVA membranes with an average fiber diameter of 100 ± 19 nm were obtained. Characterizations revealed that the mean pore size of the electrospun PVA membranes ranged from 0.30 μm to 0.21 μm with the electrospun PVA membrane thickness varying from 10 m to 100 μm. Due to the high porosity, microfiltration filters based on these electrospun membranes showed 3–7 times higher pure water flux than the Millipore GSWP 0.22 μm membrane. The nanofibrous PVA membranes with an average thickness of 20 μm could successfully reject more than 98% of the polycarboxylate microsphere particles with a diameter of 0.209 ± 0.011 μm, and still maintain 1.5–6 times higher permeate flux than that of the Millipore GSWP 0.22 μm membrane.     

Polyaniline–nylon 6 composite nanowires prepared by emulsion polymerization and electrospinning process

Polyaniline (PANI) nanoparticles doped with the dodecylbenzene sulfonic acid (DBSA) were prepared and these nanoparticles were electrospun with nylon 6 as matrix material into fiber web. Depending on the contents and concentrations of PANI and nylon 6, either nylon 6 nanofibers (～96 nm) or PANI‐nylon 6 composite nanofibers (～12 nm) were obtained. The electrical conductivity of PANI(DBSA)–nylon 6 electrospun fiber web was lower than that of PANI(DBSA)‐nylon 6 film because of the low crystallinity of the PANI(DBSA)–nylon 6 composite electrospun fiber web. However, it showed that the PANI–nylon 6 composite nanofibers would have applications as the nanowires for connecting the microelectromechanical system (MEMS). © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1277–1286, 2006     

Large-scale synthesis of hollow carbon fibers with ultra-large diameter by thermally controlled pyrolysis

Hollow carbon fibers (HCF) with ultra-large diameter have been synthesized and the versatility to convert them into the corresponding carbon-based composites has been demonstrated. The hollow carbon fibers were fabricated by thermal controlled carbonization of electrospun polyacrylonitrile fibers. For the existence of inorganic silica shell during pyrolysis, heat release will be blocked at the boundary, driving the polyacrylonitrile precursor fiber to form hollow structure. The diameter of the as-prepared hollow carbon fibers can exceed 150 nm. Sol-gel-derived Fe₃O₄ nanoparticles can grow on the outer-surface and the inner-surface of hollow carbon fibers. The microwave absorption performance of ternary HCF@Fe₃O₄@PPy composite is testified and the values of reflection loss exceeding −10 dB can be obtained in the frequency of 3.3-11.3 GHz. The large diameter of hollow carbon fibers can have inner and outer interfaces in the corresponding composites, which make them great potential for a variety of applications in future.     

Effect of air blowing on the morphology and nanofiber properties of blowing‐assisted electrospun polycarbonates

Polycarbonate (PC) nanofibers are prepared using the air blowing‐assisted electrospinning process. The effects of air blowing pressure and PC solution concentration on the physical properties of fibers and the filtration performance of the nanofiber web are investigated. The air blowing‐assisted electrospinning process produces fewer beads and smaller nanofiber diameters compared with those obtained without air blowing. Uniform PC nanofibers with an average fiber diameter of about 0.170 μm are obtained using an applied voltage of 40 kV, an air blowing pressure of 0.3 MPa, a PC solution concentration of 16%, and a tip‐to‐collection‐screen distance (TCD) of 25 cm. The filtration efficiency improvement of the air blowing‐assisted electrospun web can be attributed to the narrow distribution of fiber diameter and small mean flow pore size of the electrospun web. Performance results show that the air blowing‐assisted electrospinning process can be applied to produce PC nanofiber mats with high‐quality filtration. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Electrospin of polysulfone in N,N’-dimethyl acetamide solutions

Nanofibers of polysulfone (PSU) were prepared by electro-spinning from 10∼20 wt.% PSU solutions in N,N’-dimethyl acetamide (DMAc) mixed with 0.0∼0.1 wt.% LiCl. With increasing PSU concentration, the morphology of fibers electrospun were bead, mixture of bead-fiber and fiber, and smooth fibers when PSU concentration was 10, 12–15, and 18–20 wt.%, respectively. The bead sizes decreased and fiber diameters increased as PSU concentration was increased. The fiber diameter decreased with increases of the LiCl concentration and the distance from spinneret to collection plate. The fiber diameter also decreased with decreasing solution feeding rate. The fiber diameter distribution electrospun from 20 wt.% PSU solutions was much broader than those electrospun from 18 wt.% PSU solution. For 18 wt.% PSU solution, the average fiber diameter (AFD) decreased when the applied voltage V was increased from 7 to 12 kV. However, for 20 wt.% PSU solutions, the AFD increased when V was increased from 7 to 12 kV. The different morphology of fibers electrospun from 18 and 20 wt.% PSU solutions was attributed to the much higher viscosity of 20 wt.% PSU solution than 18 wt.% PSU solution.     

Systematic parameter study for ultra-fine fiber fabrication via electrospinning process

Processing parameters effects on the morphology such as fiber diameter and its uniformity of electrospun polymer nanofibers was investigated. A processing map summarized effects of solutions properties and processing conditions on the electrospun nanofiber morphology was obtained. Polymer concentration, its molecular weight, electrical conductivity of solvents were found as dominant parameters to control the morphology. Based on the systematic parameter study, electrospun PLLA fibers as small as 9 nm were successfully produced.     

Ultrafine electrospun nanofiber created from cross-linked polyimide solution

This paper reports a novel study focused on the preparation of non-beaded ultrafine uniform nanofibers with a narrow fiber diameter distribution from fluorinated polyimide by electrospinning. Increasing the viscosity of the polymer solutions by the addition of hexamethylenediamine (HMDA) as a cross-linking agent can result in electrospun nanofibers with a smaller diameter. The cross-linking reaction between the polyimide and HMDA was examined by FTIR-ATR and GPC under different cross-linking times. The ultrafine uniform nanofibers were achieved with the use of HMDA. Finally, we successfully prepared the non-beaded ultrafine uniform nanofibers within a narrow nano-range (27 ± 5 nm).     

Mechanical properties of individual electrospun PA 6(3)T fibers and their variation with fiber diameter

Young’s modulus and yield strength of individual electrospun fibers of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) have been obtained in uniaxial extension. The Young’s modulus is found to exhibit values in excess of the isotropic bulk value, and to increase with decreasing fiber diameter for fibers with diameter less than roughly 500 nm. The yield stress is also found to increase with decreasing fiber diameter. These trends are shown to correlate with increasing molecular level orientation within the fibers with decreasing fiber diameter. Using Ward’s aggregate model, the correlation between molecular orientation and fiber modulus can be explained, and reasonable determinations of the elastic constants of the molecular unit are obtained.     

Fabrication of ultrafine fibers of poly(γ-glutamic acid) and its derivative by electrospinning

Poly(γ-glutamic acid) (PGA) was successfully electrospun by the addition of poly(ethylene glycol) (PEG) and Triton X-100 in its aqueous solution to produce the PGA non-woven mat of the ultrafine fibers. The average fiber diameter was in the range between 200 nm and 2 μm. The fiber mat was quickly soluble in water due to the large surface area of the fibers. The electrospinning of PGA butyl ester with the esterification degree of 61% in 1,1,1,3,3,3-hexafluoro-2-propanol gave the water-insoluble nanofiber mat.     

Characteristics of Nylon 6 nanofilter for removing ultra fine particles

Electrospinning is a fabrication process that uses an electric field to make polymer nanofibers. Nanofibers have a large specific surface area and a small pore size; these are good properties for filtration applications. In this paper, the filtration characteristics of a Nylon 6 nanofilter made by electrospun nanofibers are tested as a function of the fiber diameter. Nanofilter media with diameters in the range of 100–730 nm can be produced in optimized conditions. The pressure drop of a Nylon 6 nanofilter linearly increases with the increasing face velocity. An electrospun Nylon 6 filter (mean fiber diameter: 100 nm) shows a much lower pressure drop performance relative to the commercial HEPA filter media when the filtration efficiency of the Nylon 6 nanofilter and the HEPA filter are over 99.98% with test particles of 0.02–1.0 μm in diameter. The pressure drop at 5 cm/s of the face velocity is measured as 27 mmAq for the Nylon 6 nanofilter media, and 37.1 mmAq for the HEPA filter media. The particle size with minimum efficiency decreases with the decreasing fiber diameter. And the minimum efficiency becomes greater as the fiber diameter is decreased.