Electrospinning of polymer melts: Phenomenological observations

Melt electrospinning is an alternative to solution electrospinning, however, melt electrospinning has typically resulted in fibers with diameters of tens of microns. In this paper we demonstrate that polypropylene fibers can be reduced from 35 ± 8 μm in diameter, to 840 ± 190 nm with a viscosity-reducing additive. Melt electrospun blends of poly(ethylene glycol)-block-poly(?-caprolactone) (PEG₄₇-b-PCL₉₅) and poly(?-caprolactone) (PCL) produced fibers with micron-scale diameters (2.0 ± 0.3 μm); this was lowered to 270 ± 100 nm by using the gap method of alignment for collection. The collected melt electrospun fibers often fused together where they touched, allowing the stabilization of relatively thick non-woven felts. The melt electrospun collection also included coiled circles and looped patterns of fibers approximately 150-250 μm in diameter. The polymer jet was visible between the collector and spinneret for particularly significant lengths, and underwent coiling and buckling instabilities close to the collector. The focused deposition of melt electrospun fibers was maintained when multiple jets were observed, with the collections from multiple jets separated by 3.8 ± 0.5 mm for a 5 cm collector gap. The frequent fusion points between melt electrospun fibers, and a reduction in diameter for the gap method of alignment, indicated that the melt electrospun fibers are still slightly molten at collection.     

Comparison of experimental and theoretical transverse elastic modulus of carbon fibers

The transverse elastic modulus of PAN-based carbon fibers as measured by experimental methods, calculated from theoretical equations and analyzed by the finite element method (FEM) is discussed. Raman spectroscopy was the primary method utilized to measure the transverse elastic modulus of carbon fibers in carbon-fiber reinforced plastics (CFRP). A lead oxide (PbO) thin film was deposited on the surface of a CFRP specimen using physical vapor deposition as the pretreatment in order to measure the strains of the carbon fibers and epoxy matrix phases by Raman spectroscopy. Since the relation between the Raman peak wave number of PbO thin films and tensile strain has already been developed, the transverse strain of the carbon fibers could be measured. The transverse strain of the carbon fibers was analyzed using a 2-D FEM model. The transverse modulus of the carbon fibers was determined by fitting the experimental result from Raman spectroscopy to the FEM model. The determined transverse modulus (10.4 GPa) is compared with those experimentally measured by nanoindentation (13.4 GPa), numerically analyzed using 2-D and 3-D FEM models (5.25 GPa and 28.7 GPa, respectively), and theoretically calculated from the Mori-Tanaka, Halpin-Tsai, and Uemura equations (24.8 GPa, 17.4 GPa, and 28.4 GPa, respectively).     

Facile synthesis of hexagonal boron nitride fibers with uniform morphology

Hexagonal boron nitride (h-BN) fibers were synthesized via the polymeric precursor method using boric acid (H₃BO₃) and melamine (C₃H₆N₆) as raw materials. The precursor fibers were synthesized by a water bath and BN fibers were prepared from the precursor at 1600 °C for 3 h in flowing nitrogen atmosphere. The products were characterized by X-ray powder diffraction, Fourier transformation infrared spectroscopy, thermogravimetry and scanning electron microscopy. The results showed that h-BN fibers with uniform morphology were successfully fabricated. The well-synthesized fibers were 1–2 μm in diameter and 200–500 μm in length.     

Electrochemical production of nanopore arrays in a nickel aluminium alloy

Directional solidification of a eutectic is a novel route for the production of nanostructures. This method was applied to the quasibinary NiAl-Re system. Re as the minor phase forms fibers, which are parallel aligned in the NiAl matrix. At a temperature of 1690 ± 10 °C using a thermal gradient of 40 °C cm⁻¹ and a growth rate of 30 mm h⁻¹, the fibers formed had a diameter of about 400 nm. An electrochemical method is presented here that simultaneously passivates the NiAl matrix and selectively electrodissolves the Re. In this manner, it was possible to form an array of nanopores each with the same diameter of 400 nm. The mechanisms behind this procedure, as well as the potential of this method for the production of nanoelectrode arrays or nanofilters are discussed.     

Porous nano- and microfibrous polymeric membrane material for catalytic support

High surface area is essential for attachment of functional groups, ions, moieties and nanoparticles. Surface area of fibrous membrane can be enhanced by reducing the fiber diameter or producing the porous fibers. Flow properties of the fibrous membrane can be improved by placing the fibers apart in the fibrous network. By electrospinning, it is feasible to produce the fibrous membrane of specific surface area and Darcy permeability higher than 60 m²/g and 1 × 10⁻¹¹ m², respectively. The interconnected irregular shape mesopores (2-50 nm) within the fibers increase the accessible surface area. On the other hand, presence of macropores (pores larger than 50 nm) largely increases the pore volume (porosity) in fibers and helps to reduce the diffusion resistances. Beaded fibers in the membrane can be used to reduce the bulk transport resistances. We developed a method for incorporating the mesopores and macropores in the nano/microfibers made of engineering plastics. To achieve ∼60 m²/g specific surface area by reducing the fiber diameter, one needs to draw the fibers down to 50-60 nm. In present study, 60 m²/g of specific surface area is achieved through the porous fibers of average diameter of 900 nm. A specific surface area result from the porous fiber is much higher than one can achieve by reducing the diameter of fibers.     

Generation of carbon nanofilaments on carbon fibers at 550 °C

Employing a relatively new method, in which carbon structures are grown from fuel rich combustion mixtures using palladium particles as catalyst, multi-scale diameter nanometer - micrometer filament structures were grown from ethylene/oxygen mixtures at 550 °C on commercial PAN micrometer carbon fibers. The filaments formed had a diameter roughly equal to the palladium particle size. At sufficiently high metal loadings (>0.05 wt.%) a bimodal catalyst size distribution formed, hence a bimodal filament size distribution was generated. Relative short, densely spaced nanofilaments (ca. 10 nm diameter), and a slightly less dense layer of larger (ca. 100 nm diameter) faster growing fibers (ca. 10 μm/h) were found to exist together to create a unique multi-scale structure. A protocol was developed such that only nano-scale fibers or a mixture of nano and sub-micron fibers could be produced. No large range order was evident in the filaments. This work demonstrates a unique ability to create a truly ’multi-scale’ carbon structure on the surface of carbon fibers. This fiber structure potentially can enhance composite material strength, ductility and energy absorption characteristics.     

Electrospinning and characterization of highly sulfonated polystyrene fibers

Nanofibers of highly sulfonated (IEC ∼4.5 meq/g) polystyrene (SPS) were successfully electrospun. To accomplish this, the process of electrospinning this difficult-to-spin material was studied in detail. Fiber quality was optimized by manipulating the process and solution variables to fabricate continuous bead-free fibers. Bead-free fibers (average diameter 260 nm) were electrospun from 25 wt% SPS (500 kDa) in DMF at an electrode separation of 10 cm, an applied voltage of 16.5 kV and a flow rate of 0.3 mL/h. With increasing solution concentration, and thereby the solution viscosity, the morphology changed from beads to bead-on-string fibers to continuous cylindrical fibers. Beaded fibers and continuous bead-free fibers of SPS (500 kDa) could be spun at ∼2 C_e and 3.5 C_e, respectively, where C_e is the entanglement concentration determined from solution-viscosity measurements. The onset of formation of beaded fibers coincided with a sharp transition in the scaling of the storage modulus-concentration relationship.     

A photocrosslinkable melt processible acrylonitrile terpolymer as carbon fiber precursor

A novel photocrosslinkable and melt processible terpolymer precursor for carbon fiber has been successfully synthesized and characterized. The terpolymer was synthesized by an efficient emulsion polymerization route and has a typical composition of acrylonitrile/methyl acrylate/acryloyl benzophenone in the mole ratio, 85/14/1. It has been characterized by FTIR, NMR, intrinsic viscosity and GPC molecular weights. The composition of the monomer repeat units in the terpolymer was determined by NMR, and was almost identical to the molar feed ratios of the monomers used for polymerization. The T_g of the terpolymers, were somewhat a function of molecular weight, but were in the range 77-91 °C. The fibers were spun from the terpolymer melts unlike the conventional solution spinning method. The terpolymers when stabilized with boric acid afforded a stable melt for about 30 min at 200-220 °C, which was empirically found to be sufficiently long to spin fibers. The terpolymer with the highest molecular weight (M_n, ∼48,000) was not melt processible, apparently because the melt viscosity was very high and the terpolymer degraded fast. However, terpolymers, which had an intrinsic viscosity <0.6 dL/g (NMP, 25 °C) were invariably melt processible. The initial carbon fibers produced from these terpolymer fibers upon complete carbonization exhibited good mechanical properties for proposed automotive applications; the tensile strength of the best fibers generated thus far was in the range 450-700 MPa with a strain to failure of ∼0.4%. The diameter of the carbon fibers was of the order of 7 μm.     

Biopitch-based general purpose carbon fibers: Processing and properties

Eucalyptus tar pitches are generated on a large scale in Brazil as by-products of the charcoal manufacturing industry. They present a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units common to lignin. The low aromaticity (60-70%), high O/C atomic ratios (0.20-0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches. In the present work, eucalyptus tar pitches are evaluated as precursors of general purpose carbon fibers (GPCF) through a four-step process: pitch pre-treatment and melt spinning, and fiber stabilization and carbonization. Homogeneous isotropic fibers with a diameter of 27 μm were obtained. The fibers had an apparent density of 1.84 g/cm³, an electrical resistivity of 2 × 10⁻⁴ Ω m, a tensile strength of 130 MPa, and a tensile modulus of 14 GPa. Although the tensile properties advise against using the produced fibers as structural reinforcement, other properties give rise to different potential applications, as for example in the manufacture of activated carbon fibers or felts for electrical insulation.     

TEM and XPS studies to reveal the presence of cobalt and palladium particles in the inner core of carbon nanofibers

In this paper, the presence of a considerable number of Co (3-4 nm) and Pd (1-4 nm) particles in the inner tube (4-9 nm inner diameter) of carbon nanofibers is demonstrated with TEM and XPS. Oxidation of freshly grown fibers in nitric acid resulted in opening of the inner tube of the fibers and in creating adsorption sites on the internal and external surface of the fibers needed for anchoring of the metal precursors. It is demonstrated that analysis with TEM tilt series is a very powerful tool to locate the actual position of the metal particles, i.e. on the external or internal surface of the fibers. The fraction of metal present in the inner core of the fibers varied from 10-15% for Pd to 28-34% for Co, depending on the synthesis method.     

Orientation and dimensional changes in mesophase pitch-based carbon fibers

The dimensional changes and microstructure evolution of AR-mesophase pitch fibers are reported as a function of heat treatment temperatures (HTTs) that ranged from 300 to 3000 °C. The length measurements for AR fibers indicate that starting from an oxidized state, there is partial relaxation of orientation in the range of 600-900 °C, and the length of the fibers shrinks about 8% relative to that in the stabilized state. Above 900 °C, the fiber length does not decrease; instead, it increases slightly. Thus, fibers heat treated to 2400 °C on average have a length that is bigger than those heat-treated to 900 °C. The small increase in length of fibers heat treated to high temperatures (>900 °C) is likely a consequence of alignment of the graphene layer planes along the fiber axis that can result in an expansion along the longitudinal direction. The small minimum in the length profile at 900 °C corresponds to a small maximum in the misorientation angle of the graphene layer planes measured by wide-angle X-ray diffraction on the fiber bundles. Single filament orientation measurements, using monochromated synchrotron radiation, show a similar maximum misorientation at 600 °C. The fiber microstructure, characterized by scanning electron microscopy, also reveals that the first major change is observed at 900 °C, where a radial texture is observed. At higher HTTs, the development of radial texture is more pronounced and by 2400 °C the graphene-layer planes are seen very clearly.     

Fabrication of high strength PVA/SWCNT composite fibers by gel spinning

High-strength composite fibers were prepared from polyvinyl alcohol (PVA) (Degree of polymerization: 1500) reinforced by single-walled carbon nanotubes (SWCNTs) containing few defects. The SWCNTs were dispersed in a 10 wt.% PVA/dimethylsulfoxide solution using a mechanical homogenizer that reduced the size of SWCNT aggregations to smaller bundles. The macroscopically homogeneous dispersion was extruded into cold methanol to form fibers by gel spinning followed by a hot-drawing. The tensile strength of the well-oriented composite fibers with 0.3 wt.% SWCNTs was 2.2 GPa which is extremely high value among PVA composite fibers ever reported using a commercial grade PVA. The strength of neat PVA fibers prepared by the same procedure was 1.7 GPa. Structural analysis showed that the PVA component in the composite fibers possessed almost the same structure as that of neat PVA fibers. Hence a small amount of SWCNTs straightforward enhanced by 0.5 GPa the tensile strength of PVA fibers. The results of mechanical properties and Raman spectra for the SWCNT composites suggest the relatively good interfacial adhesion of the nanotubes and PVA that improves the load transfer from the polymer matrix to the reinforcing phase.     

Carbon fiber from natural biopolymer Bombyx mori silk fibroin with iodine treatment

Carbon fibers were prepared from silk fibers after an iodine treatment and the carbon yield, fiber morphology, structure and mechanical properties were investigated. A single or multi-step carbonization process was used for the preparation. In the single step process, silk fibroin (SF) fibers were heated from 25 to 800 °C with a heating rate of 5 °C min⁻¹ under Ar atmosphere. However, the carbon fiber obtained was partially melted and was too fragile to handle. For better performance, SF fibers were treated with iodine vapor at 100 °C for 12 h and untreated and iodinated SF fibers were heated from 25 to 800 °C by a multi-step carbonization process, which was defined based on the optimum thermal degradation rate of silk. In this multi-step process, the carbon fibers obtained from iodinated SF were structurally intact and stable in appearance, and the carbon yield achieved was ca. 36 wt.%, much higher than the value for untreated SF. X-ray diffraction, Raman spectroscopy and transmission electron microscopic observation revealed that the obtained carbon fibers from both untreated and iodinated SFs had a basically amorphous structure. The strength of carbon fibers prepared from iodinated SF using the multi-step carbonization was considerably increased compared to that of untreated SF. According to viscoelastic measurement, by heating above 280 °C the iodine introduced intermolecular cross-linking of the SF, and its melt flow was inhibited which produced a higher yield and better performance of the carbon fiber.     

The production of submicron diameter carbon fibers by the electrospinning of lignin

Lignin fibers with and without platinum were synthesized in a single step by electrospinning of lignin/ethanol/platinum acetyl acetonate and lignin/ethanol solutions, respectively. The fibers obtained were stabilized in air at low temperature to avoid fiber fusion during the subsequent carbonization process. The effect of the carbonization temperature (600-1000 °C) on surface chemistry, morphology, textural properties, and oxidation resistance of the final carbon fibers was studied. The carbonization process decreased the oxygen content of the fibers, increasing the carbon and surface platinum proportion and producing a well developed microporous structure. Carbon fibers with and without platinum with apparent surface areas of 1178 and 1195 m²/g, respectively, and micropore volumes of around 0.52 cm³/g were obtained. The diameter of the carbon fibers obtained is in the range of 400 nm to 1 μm. Carbon fibers with surface platinum of 0.6% in weight were obtained. The carbon fibers with and without platinum showed high oxidation resistance despite their highly developed porous structure.     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

Crosslinking of PET through solid state functionalization with alkoxysilane derivatives

A new way for crosslinking poly(ethylene terephthalate) (PET) films and fibers is described using solid state functionalization of the PET end groups (alcohol and acid) with two reagents, respectively, 3-isocyanatopropyltriethoxysilane (IPTESI) or/and 3-glycidoxypropyltrimethoxysilane (GOPTMSI). This functionalization is then followed by hydrolysis-condensation reactions of PET-alkoxysilane end groups leading to the PET crosslinking.First of all, the functionalization reactions were investigated on model compounds by ¹H NMR spectroscopy in a range of temperature 80-160 °C. Furthermore, the diffusion of reagents in solid PET, depending on the initial degree of PET crystallinity, was characterized in the same temperature range through the variation of sample mass. On the other hand, this method allowed us to determine the diffusion coefficients and the solubility of the reagents in solid PET at different temperatures and initial crystallinity degrees.End groups functionalized PET films and fibers by alkoxysilane were then crosslinked by immersion of the samples in hot water. The crosslinking density was characterized by measuring the insoluble fraction of PET in good solvent constituted by a mixture of trifluoroacetic acid and dichloromethane (50/50 vol.). An insoluble fraction close to 70% was obtained by the functionalization treatment of amorphous PET film (8% crystallinity) by a mixture of GOPTMSI + IPTESI (50/50 M) at 155 °C for 1 h followed by hydrolysis-condensation reactions at 80 °C for 72 h. Thermomechanical and thermal properties of films and fibers were observed and found to be considerably enhanced in comparison to the untreated samples. The tensile properties of these partially crosslinked samples were maintained up to 320 °C.     

Structure and dynamic mechanical properties of poly(ethylene terephthalate-co-4,4′-bibenzoate) fibers

Poly(ethylene terephthalate-co-4,4′-bibenzoate) (PETBB) fibers containing 5, 15, 35, 45, 55, and 65 mol% bibenzoate (BB) were melt spun. Fiber structure has been determined using wide angle X-ray diffraction, birefringence, and FTIR spectroscopy. When drawn to their respective maximum draw ratios, the structures and properties of high BB containing fibers (PETBB45, 55 and 65) are significantly different than those of PET and low BB containing fibers (PETBB5, 15, and 35). For example, 90% of the ethylene glycol units in high BB containing fibers are in the trans conformation, while only 80% of these units are in trans conformation in PET and low BB containing fibers. Overall orientation of the high BB containing fibers is higher (orientation factor f > 0.85) than those of PET and low BB containing fibers (f < 0.6). Orientation of the crystalline regions is quite high (f_cr ∼ 0.95) for both groups of fibers, while orientation of the amorphous regions (f_am) of high BB containing fibers is higher (∼0.8) than those of the PET and low BB containing fibers (∼0.4). High BB containing fibers exhibit much higher storage modulus and modulus retention with temperature than low BB containing fibers. Glass transition temperature determined from the dynamic loss tangent peak decreased with increasing BB content, while this transition completely disappeared in the high BB containing fibers. The magnitude of the secondary transition, observed at about −50 °C, decreased with increasing BB content. Another secondary transition, not observed in PET, was observed at about 70 °C in high BB containing fibers. These dynamic mechanical results have been rationalized in terms of the observed structural parameters.     

Catalytic growth of macroscopic carbon nanofiber bodies with high bulk density and high mechanical strength

Carbon nanofibers (CNF) are non-microporous graphitic materials with a high surface area (100-200 m²/g), high purity and tunable surface chemistry. Therefore the material has a high potential for use as catalyst support. However, in some instances it is claimed that the low density and low mechanical strength of the macroscopic particles hamper their application. In this study we show that the bulk density and mechanical strength of CNF bodies can be tuned to values comparable to that of commercial fluid-bed and fixed-bed catalysts. The fibers were prepared by the chemical decomposition of CO/H₂ over Ni/SiO₂ catalysts. The resulting fibers bodies (1.2 μm) were replicates of the Ni/SiO₂ bodies (0.5 μm) from which they were grown. The bulk density of CNF bodies crucially depended on the metal loading in the growth catalyst. Over 5 wt% Ni/SiO₂ low density bodies (0.4 g/ml) are obtained while 20 wt% Ni/SiO₂ leads to bulk densities up to 0.9 g/ml with a bulk crushing strength of 1.2 MPa. The 20 wt% catalysts grow fibers with diameters of ∼22 nm, which grow irregularly in space, resulting in a higher entanglement and a concomitant higher density and strength as compared to the thinner fibers (∼12 nm) grown from 5 wt% Ni/SiO₂.     

Exfoliation of nitric acid intercalated carbon fibers: effects of heat-treatment temperature of pristine carbon fibers and electrolyte concentration on the exfoliation behavior

Effect of heat treatment temperature of mesophase pitch-based carbon fibers on the exfoliation behavior of derived intercalation compounds with nitric acid was studied. Carbon fibers heat-treated above 2500 °C gave intercalation compounds with mass increase of more than 80 mass% and resulted in a marked exfoliation by a rapid heating to 1000 °C, where no memory of original single fiber was observed. On those below 2000 °C, on the other hand, their residue compounds showed mass increase less than 80 mass% and the appearance after exfoliation at 1000 °C was similar to the original single fiber. On 1150 °C-treated carbon fibers, mass increase was only 13 mass% and no evidence of intercalation was detected even after electrolysis and, as a consequence, the formation of only small fissures along their fiber axis was observed, with no apparent exfoliation. The dependence on electrolyte concentration was also examined on 3000 °C-treated carbon fibers.     

Release characteristics of four model drugs from drug-loaded electrospun cellulose acetate fiber mats

Ultra-fine fiber mats of cellulose acetate (CA; M_w ≈ 30?000 Da; degree of acetyl substitution ≈ 2.4) containing four different types of model drugs, i.e., naproxen (NAP), indomethacin (IND), ibuprofen (IBU), and sulindac (SUL), were successfully prepared by electrospinning from 16% w/v CA solutions in 2:1 v/v acetone/N,N-dimethylacetamide (DMAc). The amount of the drugs in the solutions was fixed at 20 wt.% based on the weight of CA powder. The morphology of the drug-loaded electrospun (e-spun) CA fiber mats was smooth, with the average diameters of these fibers ranging between 263 and 297 nm. No presence of the drug aggregates of any kind was observed on the surfaces of these fibers, suggesting that the drugs were encapsulated well within the fibers. After submersion in the acetate buffer solution at 37 °C for 24 h, the drug-loaded e-spun CA fiber mats swelled particularly well (i.e., 570-630%), while the corresponding solvent-cast film counterparts did not. The release characteristics of the model drugs from both the drug-loaded CA fiber mats and the drug-loaded as-cast CA films were carried out by the total immersion method in the acetate buffer solution at 37 °C. At any given immersion time point, the release of the drugs from the drug-loaded e-spun CA fiber mats was greater than that from the corresponding as-cast films. The maximum release of the drugs from both the drug-loaded fiber mats and films could be ranked as follows: NAP > IBU > IND > SUL.