Mathematical models for continuous electrospun nanofibers and electrospun nanoporous microspheres

A brief review of mathematical models of electrospinning is given. The nano‐effect and electrospinning dilation are presented to explain how to prepare extremely high strength continuous nanofibers and nanoporous microspheres, respectively. According to the established models, vibration‐electrospinning is introduced to improve electrospinability, Siro‐electrospinning is suggested to mimic the spinning procedure of a spider and magneto‐electrospinning is used to control the instability arising in the electrospinning process. A new theory linked to both classical mechanics and quantum mechanics should be developed to explain certain special phenomena in electrospinning. E‐infinity theory is considered to be a potential theory to deal with quantum‐like properties and nano‐effect on the nanoscale. The emphasis of this brief review is upon the authors' recent work, and the references are not exhaustive. Copyright © 2007 Society of Chemical Industry     

High permittivity nanocomposites fabricated from electrospun polyimide/BaTiO3 hybrid nanofibers

High dielectric permittivity, good mechanical properties, and excellent thermal stability are highly desired for the dielectric materials used in the embedded capacitors and energy‐storage devices. This study reports polyimide (PI)/barium titanate (BaTiO₃) nanocomposites fabricated from electrospun PI/BaTiO₃ hybrid nanofibers. The PI/BaTiO₃ nanocomposites were investigated using Fourier transform infrared spectroscopy, scanning electron microscope, transmission electron microscope, thermal gravimetric analysis, an electromechanical testing machine, a LCR meter and an electric breakdown strength tester. The results showed that BaTiO₃ fillers were uniformly dispersed up to 50 vol% in PI matrix. The dielectric permittivity of the composite (50 vol% BaTiO₃) was 29.66 with a dielectric loss of 0.009 at 1 kHz and room temperature. The dielectric permittivity showed a very small dependence on temperature (up to 150°C) and frequency (100 Hz–100 kHz). The nanocomposites also showed high thermal stability and good mechanical properties. The PI/BaTiO₃ nanocomposites will be a promising candidate for uses in embedded capacitors, especially in high temperature circumstance. POLYM. COMPOS., 37:794–801, 2016. © 2014 Society of Plastics Engineers     

Continuous yarns from electrospun fibers

A technique for making continuous uniaxial fiber bundle yarns from electrospun fibers is described. The technique consists of spinning onto a water reservoir collector and drawing the resulting non-woven web of fibers across the water before collecting the resulting yarn. Yarns from electrospun fibers of poly(vinyl acetate), poly(vinylidene difluoride) and polyacrylonitrile are used to illustrate the process of yarn formation and fiber alignment within the yarn. A theoretical production rate of 180 m of yarn per hour for a single needle electrospinning setup makes the process suitable for lab-scale production of electrospun yarns.     

Splashing needleless electrospinning of nanofibers

This article proposes a new needleless electrospinning apparatus applying the method of splashing polymer solution onto the surface of a metal roller spinneret. When a high voltage is applied, many spinning jets form on the free surface of polymer solutions. Multiple electrified jets undergo strong stretching and bending instability, solvent evaporates, and solidified nanofibers deposit on the collector, as in an ordinary single‐needle electrospinning process. The production of nanofibers is enhanced by 24–45 times comparing with a single‐needle system. And the productivity is easy to scale up. The effects of processing parameters, including solution concentration, applied voltage, distance between spinneret to collector, and rotational speed of the roller spinneret on the morphology of nanofibers are investigated in this article. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Upward needleless electrospinning of multiple nanofibers

A new approach to electrospinning of polymer nanofibers is proposed. A two-layer system, with the lower layer being a ferromagnetic suspension and the upper layer a polymer solution, is subject to a normal magnetic field provided by a permanent magnet or a coil. As a result, steady vertical spikes of magnetic suspension perturbed the interlayer interface, as well as the free surface of the uppermost polymer layer. When a normal electric field is applied in addition to the system, the perturbations of the free surface become sites of jetting directed upward. Multiple electrified jets undergo strong stretching by the electric field and bending instability, solvent evaporates and solidified nanofibers deposit on the upper counter-electrode, as in an ordinary electrospinning process. However, the production rate is shown to be higher.     

Strong carbon nanofibers from electrospun polyacrylonitrile

Strong carbon nanofibers with diameters between 150 nm and 500 nm and lengths of the order of centimeters were realized from electrospun polyacrylonitrile (PAN). Their tensile strength reached a maximum at 1400 °C carbonization temperature, while the elastic modulus increased monotonically until 1700 °C. For most carbonization temperatures, both properties increased with reduced nanofiber diameter. The tensile strength and the elastic modulus, measured from individual nanofibers carbonized at 1400 °C, averaged 3.5 ± 0.6 GPa and 172 ± 40 GPa, respectively, while some nanofibers reached 2% ultimate strain and strengths over 4.5 GPa. The average tensile strength and elastic modulus of carbon nanofibers produced at 1400 °C were six and three times higher than in previous reports, respectively. These high mechanical property values were achieved for optimum electrospinning parameters yielding strong PAN nanofibers, and optimum stabilization and carbonization temperatures, which resulted in smooth carbon nanofiber surfaces and homogeneous nanofiber cross-sections, as opposed to a previously reported core–shell structure. Turbostratic carbon crystallites with average thickness increasing from 3 to 8 layers between 800 °C and 1700 °C improved the elastic modulus and the tensile strength but their large size, discontinuous form, and random orientation reduced the tensile strength at carbonization temperatures higher than 1400 °C.     

Temperature-induced changes in morphology and microstructure of electrospun mullite nanofibers fabricated from a hybrid mullite sol

Mullite nanofibers with small diameter and high surface area are an ideal candidate as the reinforcements in composite materials, and have promising applications in the fields of catalysis, filtration, thermal storage and so forth. In this work, electrospun mullite nanofibers were successfully synthesized using a hybrid mullite sol. The morphology and microstructure of fibers calcined at different temperatures were investigated. The morphology of fibers synthesized at 900 °C is porous with coarse surface, and after crystallization it becomes compact with smooth surface. The densities of fibers increase with the increasing temperatures. At 1200 °C the surface of fibers becomes coarse again, as a result of the grain growth of mullite. The crystallization path of fibers was revealed that the Al-rich mullite (4Al₂O₃·SiO₂) together with amorphous silica formed at 1000 °C, changed into mullite with higher silica contents as temperature further increased, and finally transformed into a stable 3Al₂O₃·2SiO₂ phase at 1200 °C. During this crystallization process, the flow of amorphous silica phase and the formation of mullite crystal structure benefit the densification of fibers, leading to the resultant fibers with fine and compact microstructure. The present findings can provide a guideline for the preparation of the promising high-mechanical mullite nanofibers and the synthesized nanofibers display great potential as reinforcements in structural ceramic composites.     

Needle and needleless electrospinning for nanofibers

A metallic needle is most often used in conventional electrospinning, where a point‐plate electric field with nonuniform distribution is formed in single‐needle electrospinning (SNE). Low flow rate in SNE has restricted the application of electrospinning on an industrial scale. Multiple needles have been introduced to enhance the flow rate. However, multiple needles make the electric field distribution much more complex. To resolve this problem, alternative electrospinning setups with more uniform electric field have to be developed. Flat spinnerets have been demonstrated to replace the needle in SNE setups. The operating diagrams for flat spinneret electrospinning (FSE) were determined and differed significantly from those for SNE. Nanofibers produced by FSE were more uniform than those from SNE. These differences were explained by the differences in electric fields simulated using finite element analysis (FEA). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

静电纺纳米纤维的应用

综述了静电纺纳米纤维在保护性服用材料、传感器、过滤防护材料、高分子纳米模板、纳米复合改性材料、航空航天等方面的应用;详述了在生物医用材料方面的应用;展望了静电纺丝纳米纤维的发展前景;指出应继续研发具有特殊性能的静电纺纳米纤维新产品,扩大其应用领域,最终实现成果产业化。     

Properties of carbon nanofibers prepared from electrospun polyimide

A solution of a polyimide (PI, Matrimid® 5218) in dimethylacetamide was electrospun, and carbonization of the electrospun nonwoven fabrics produced carbon nanofiber fabrics. The effects of iron(III) acetylacetonate (AAI) on carbonization and the resulting morphology were also investigated. The carbonization behavior of the nonwoven fabrics was examined by X‐ray diffraction and Raman spectroscopy. AAI promoted carbonization of the nonwoven fabrics and increased the carbon yield. Addition of 3 wt % AAI increased the crystal dimension of electrospun PI nanofibers from 1.06 to 4.18 nm and decreased the integrated intensity ratio from 3.37 to 1.83 when heat treated at 1200°C. Scanning electron microscopy images of the carbonized nonwoven fabrics showed that AAI remained as particles within the fibers after carbonization. In addition, transmission electron microscopy observations revealed that turbostratic‐oriented graphite layers were observed around the particles even at 1200°C, which have been reported only on carbonization of rigid‐chain solvent insoluble PI materials under tension. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97:165–170, 2005     

Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns

The fiber spinning technique of electrospinning was optimized in order to prepare unidirectional aligned, structurally oriented, and mechanically useful carbon precursor fibers with diameters in the nanoscale range. The fiber spinning velocity and fiber draw ratio was measured to be between 140 and 160 m/s and 1:300,000, respectively, for fibers spun from 10 wt% polyacrylonitrile (PAN) solutions with dimethylformamide (DMF). A high-speed, rotating target was used to collect unidirectional tows of PAN fibers. Aligned and (+) birefringent fibers with diameters between 0.27 and 0.29 μm (FESEM) were collected from electrospinning 15 wt% PAN in DMF solutions at 16 kV onto a target rotating with a surface velocity between 3.5 and 12.3 m/s. Dichroism measurements (Polarized FTIR) of the nitrile-stretching vibration show an increase in the molecular orientation with take-up speed. Wide angle X-ray diffraction patterns (WAXD) show equatorial arcs from the reflection at and (1120) reflection at A maximum chain orientation parameter of 0.23 was determined for fibers collected between 8.1 and 9.8 m/s. Twisted yarns of highly aligned PAN nanofibers with twist angles between 1.1 and 16.8° were prepared. The ultimate strength and modulus of the twisted yarns increase with increasing angle of twist to a maximum of 162±8.5 MPa and 5.9±0.3 GPa, respectively, at an angle of 9.3°.     

Graphitic carbon nanofibers developed from bundles of aligned electrospun polyacrylonitrile nanofibers containing phosphoric acid

Graphitic carbon nanofibers (GCNFs) with diameters of approximately 300 nm were developed using bundles of aligned electrospun polyacrylonitrile (PAN) nanofibers containing phosphoric acid (PA) as the innovative precursors through thermal treatments of stabilization, carbonization, and graphitization. The morphological, structural, and mechanical properties of GCNFs were systematically characterized and/or evaluated. The GCNFs made from the electrospun PAN precursor nanofibers containing 1.5 wt.% of PA exhibited mechanical strength that was 62.3% higher than that of the GCNFs made from the precursor nanofibers without PA. The molecules of PA in the electrospun PAN precursor nanofibers initiated the cyclization and induced the aromatization during stabilization, as indicated by the FT-IR and TGA results. The stabilized PAN nanofibers possessed regularly oriented ladder structures, which facilitated the further formation of ordered graphitic structures in GCNFs during carbonization and graphitization, as indicated by the TEM, XRD, and Raman results.     

Strong electrospun nanometer-diameter polyacrylonitrile carbon fiber yarns

Nanometer-diameter electrospun PAN yarns with different degrees of alignment were stabilized and carbonized and their properties evaluated. The parameters varied during production included draw ratio, temperature, heating rate and time in order to determine the ideal oxidation and carbonization conditions. The 10% PAN carbon yarn made at 16 kV and 9.8 m s⁻¹ take-up velocity had the highest ultimate strength of approximately 1 GPa while maintaining constant length conditions during the process. The ultimate strength increased to approximately 1.7 GPa after the yarns were impregnated with an adhesive to make nano-sized unidirectional composite samples. The ultimate strength of PAN yarns generally increased with increasing take-up velocity because of the increase of molecular orientation.     

Toughened electrospun nanofibers from crosslinked elastomer‐thermoplastic blends

A crosslink‐able elastomeric polyester urethane (PEU) was blended with a thermoplastic, polyacrylonitrile (PAN), and electrospun into nanofibers. The effects of the PEU/PAN ratio and the crosslinking reaction on the morphology and the tensile properties of the as‐spun fiber mats were investigated. With the same overall polymer concentration (9 wt %), the nanofiber containing higher composition of PEU shows a slight decrease in the average fiber diameter, but the tensile strength, the elongation at break and tensile modulus of the nanofiber mats are all improved. These tensile properties are further enhanced by slight crosslinking of the PEU component within the nanofibers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Crosslinking of the electrospun gelatin nanofibers

Gelatin (Gt) nanofibers have been prepared by using an electrospinning process in a previous study. In order to improve their water-resistant ability and thermomechnical performance for potential biomedical applications, in this article, the electrospun gelatin nanofibers were crosslinked with saturated glutaraldehyde (GTA) vapor at room temperature. An exposure of this nanofibrous material in the GTA vapor for 3 days generated a crosslinking extent sufficient to preserve the fibrous morphology tested by soaking in 37 °C warm water. On the other hand, the crosslinking also led to improved thermostability and substantial enhancement in mechanical properties. The denaturation temperature corresponding to the helix to coil transition of the air-dried samples increased by about 11 °C and the tensile strength and modulus were nearly 10 times higher than those of the as-electrospun gelatin fibers. Furthermore, cytotoxicity was evaluated based on a cell proliferation study by culturing human dermal fibroblasts (HDFs) on the crosslinked gelatin fibrous scaffolds for 1, 3, 5 and 7 days. It was found cell expansion took place and almost linearly increased during the course of whole period of the cell culture. The initial inhibition of cell expansion on the crosslinked gelatin fibrous substrate suggested some cytotoxic effect of the residual GTA on the cells.     

Structural studies of electrospun cellulose nanofibers

Non-woven mats of submicron-sized cellulose fibers (250-750 nm in diameter) have been obtained by electrospinning of cellulose solutions. Cellulose are directly dissolved in two solvent systems: (a) lithium chloride (LiCl)/N,N-dimethyl acetamide (DMAc) and (b) N-methylmorpholine oxide (NMMO)/water, and the effects of (i) solvent system, (ii) the degree of polymerization of cellulose, (iii) spinning conditions, and (iv) post-spinning treatment such as coagulation with water on the miscrostructure of electrospun fibers are investigated. The scanning electron microscope (SEM) images of electrospun cellulose fibers show that applying coagulation with water right after the collection of fibers is necessary to obtain submicron scale, dry and stable cellulose fibers for both solvent systems. X-ray diffraction studies reveal that cellulose fibers obtained from LiCl/DMAc are mostly amorphous, whereas the degree of crystallinity of cellulose fibers from NMMO/water can be controlled by various process conditions including spinning temperature, flow rate, and distance between the nozzle and collector. Finally, electrospun cellulose fibers are oxidized by HNO₃/H₃PO₄ and NaNO₂, and the degradation characteristics of oxidized cellulose fibers under physiological conditions are presented.     

Spontaneous core-sheath formation in electrospun nanofibers

In recent research, electrospun nanofibers (NFs) from polymer solutions containing metal salts were used to produce high-temperature superconducting ceramic (HTSC) NFs through pyrolysis. In this research, the production of phase separated nanostructures inside NFs spun from polymer solutions containing metal salts was investigated. The metal salts were expected to be preferentially driven into one of the phases (the amorphous phase in a semi-crystalline polymer and the more hydrophilic phase in a triblock copolymer) and yield nanostructured ceramics upon pyrolysis. Surprisingly, the electrospun NFs exhibited the spontaneous formation of a core-sheath structure. The metal-atom-rich core exhibited a 10 nm scale structure while no such structure was observed in the metal-atom-poor sheath. Both the repulsion of metal atoms by the positive surface charge and the exclusion of the metal atoms from the crystallizing front that moves inward from the surface were shown to contribute to the spontaneous formation of the core-sheath structure.     

静电纺丝制备纳米纤维的进展及应用

简述了静电纺丝的制备原理和影响静电纺丝纤维成形的主要工艺因素;介绍了静电纺丝法制备高分子聚合物、生物大分子、无机物纳米纤维的最新进展,以及这些纳米纤维在过滤、传感器、超疏水性材料、生物医用功能材料、纳米模板等领域的应用;指出静电纺丝制备纳米连续长丝技术亟待发展。     

Shape memory polyurethane-based electrospun yarns for thermo-responsive actuation

Electrospun nanofibrous yarns of shape memory polyurethane (SMPU)-based nanofibers were successfully prepared. The electrospun yarns were analyzed to assess the dependence of mechanical and shape memory properties on the yarn twist angle. The yarn with a 60° twist angle has high compactness and density, leading to increased tensile strength, elastic modulus, and strain energy. In addition, this yarn shows a significant improvement in the shape memory recovery stress compared with the non-twisted SMPU nanofibers. Moreover, thermal stimuli allowed for the 60° twisted yarn to lift a load that is 103 times heavier than itself. This yarn had a shape recovery stress of 0.61 MPa and generated a 7.95 mJ recovery energy. The results suggest the electrospun yarns could be used as actuators and sensing devices in the medical and biological fields.