Structural transformation of polyacrylonitrile fibers during stabilization and low temperature carbonization

The effect of oxidative stabilization and carbonization processes on the structure, mass and mechanical properties of polyacrylonitrile (PAN) precursor fibers was analyzed. A gradual densification of the fibers occurring from mass loss, decrease in fiber diameter and increase in density were observed after stabilization at a maximum temperature of 255 °C and carbonization at a maximum temperature of 800 °C. The tensile strength and modulus of the fibers were found to decrease after stabilization but then increased after low temperature carbonization. The thermal processing of the precursor fibers affected their mode of failure after tensile loading, changing from a ductile type of failure to a brittle type. The type of failure correlated well with the crystal structure changes in the fibers. Whilst the PAN precursor fiber started to exotherm above 225 °C in air, no prominent exothermic reaction was measured in the carbonized fibers in air up to 430 °C. The aromatization index of stabilized fiber was calculated to be ∼66%, and that of carbonized fiber was ∼99%. FTIR studies indicated that the variation in the chemical structure of the fibers with the stabilization of the fibers. Radial heterogeneity in the stabilized fibers was observed however it was not promoted to the carbonized fibers. Finally, a method to calculate mass retention of PAN precursor fiber after heat treatment was developed, and the calculated percentage mass retained of the precursor fiber after oxidation and carbonization were found to be 81% and 51%, respectively. . This study proposes an effective method to calculate the percentage of mass retained by precursor fibers after stabilization and low temperature carbonization to provide a model for evaluating carbon fiber yield from a given amount of fibers.

Applications of electrospun nanofibers for electronic devices

In recent decades, electrospinning of nanofibers has progressed very rapidly in both scientific and technological aspects, and electrospun nanofibers have shown enormous potential for various applications. In particular, electrospun nanofibers have significantly enhanced the application performance of many electronic devices, such as solar cells, mechanical-to-electric energy harvesters, rechargeable batteries, supercapacitors, sensors, field-effect transistors, diodes, photodetectors, and electrochromic devices. This chapter provides a comprehensive summary on the recent progress in the application of electrospun nanofibers in electronic devices.

Layer-by-layer structured membranes of silk fibroin and polyethylenimine on electrospun silk fibroin nanofibers

Self-assembled silk fibroin (SF)-polyethylenimine (PEI) multilayered films were fabricated on ethanol treated electrospun SF nanofibrous substrates via the electrostatic layer-by-layer (LBL) adsorption. The film coated membranes were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectrophotometer (XPS). The SEM images showed that the multilayers of SF-PEI were formed on the surface of the ethanol treated SF nanofibres. The characteristics such as the fiber shape and porous structure were well maintained as the number of the coated SF-PEI bilayers was less than five. However, obvious adhesive substances and blocked pores were observed on the surface of the fibers as the number of bilayers of SF-PEI increased to six. Furthermore, the obvious core-shell structures were observed by TEM. The thickness of five SF-PEI bilayers was approximately 80nm. Additionally, the XPS results also revealed that the SF-PEI multilayer composite membranes formed. The adsorption mainly depended on a simple electrostatic interaction between the layers of SF and PEI. These SF-PEI multilayer assembled nanofibrous membranes could be a promising material for use as a sensor, gene delivery agent and scaffolds.

The charge effect of cationic surfactants on the elimination of fibre beads in the electrospinning of polystyrene

Polystyrene nanofibres were electrospun with the inclusion of cationic surfactants, dodecyltrimethylammonium bromide (DTAB) or tetrabutylammonium chloride (TBAC), in the polymer solution. A small amount of cationic surfactant effectively stopped the formation of beaded fibres during the electrospinning. The cationic surfactants were also found to improve the solution conductivity, but had no effect on the viscosity. Only DTAB had an effect on the surface tension of the polymer solution, the surface tension decreasing slightly with an increase in the concentration of DTAB.

The formation of beaded fibres was attributed to an insufficient stretch of the filaments during the whipping of the jet, due to a low charge density. Adding the cationic surfactants improved the net charge density that enhanced the whipping instability. The jet was stretched under stronger charge repulsion and at a higher speed, resulting in an exhaustion of the bead structure. In addition, a polymer/surfactant interaction was found in the polystyrene–DTAB solution system, while this interaction was not found in the polystyrene–TBAC system. The polymer/surfactant interaction led to the formation of thinner fibres than those formed in the absence of the interaction.

The effects of a non-ionic surfactant, Triton X-405, on the electrospun fibres were also studied. The addition of Triton X-405 did not eliminate the fibre beads, but reduced the bead numbers and changed the morphology. Triton X-405 slightly improved the solution conductivity, and had a minor effect on the surface tension, but no effect on the viscosity.

Using chitosan as a thickener for electrospinning dilute PVA solutions to improve fibre uniformity

Chitosan was added to PVA aqueous solutions as a thickener to improve the electrospinning process. The presence of a small amount of chitosan considerably improved the uniformity of as-spun nanofibres. This improvement is attributed to its significant effect on the solution viscosity and conductivity, with only a slight impact on the surface tension. The concentration of the PVA required to produce bead-free and uniform nanofibres was reduced with the increase in chitosan concentration. The chitosan thickener suppressed the jet break-up and facilitated the jet stretching so that fine and uniform fibres could be electrospun even from a dilute PVA solution.

One-step coating of fluoro-containing silica nanoparticles for universal generation of surface superhydrophobicity

Stable superhydrophobic surfaces with water contact angles over 170 degrees and sliding angles below 7 degrees were produced by simply coating a particulate silica sol solution of co-hydrolysed TEOS/fluorinated alkyl silane with NH₃^.H₂O on various substrates, including textile fabrics (e.g. polyester, wool and cotton), electrospun nanofibre mats, filter papers, glass slides, and silicon wafers.

Needleless-electrospinning of PVA nanofibres

In this paper, we demonstrated that a thin metal disk can be used as nozzle to electrospin PVA nanofibres on a large-scale. With the rotation of a disk covered with a thin layer of electrically charged PVA solution, a large number of fibres were electrospun simultaneously from two sides of the disk and deposited on the electrode collector. The fibre production rate can be as high as 6.0 glhr, which is about 270 times higher than that of a corresponding normal needle based electrospinning system (0.022 g/hr). The effects of applied voltage, the distance between the disk nozzle and collector, and PVA concentration on the fibre morphology were examined. The dependency of fibre diameter on the PV A concentration showed a similar trend to that for a conventional electrospinning system using a syringe needle nozzle, but the diameter distribution was slightly wider for the disk electrospun fibres. The profiles of electric field strength in disk electrospinning showed considerable dependence on the disk thickness, with a thin disk exhibiting similar electric field strength profile to that of a needle electrospinning system.

Disk-electrospinning of PVA nanofibres

In this study, we demonstrated that a thin aluminium disk can be used as nozzle to electrospin PVA nanofibres on a large-scale. A schematic of this electrospinning system and a SEM image of as-spun PVA nanofibers are shown in Figure 1. The lower part of the disk is inside a bath containing the polymer solution, which is connected to a high voltage powder supply. During electrospinning, the disk rotates and picks up a thin layer of electrically charged PVA solution. A large number of fibres are then electrospun simultaneously from two sides of tile disk and deposited on the electrode collector.
With the small prototype unit we used, the fibre production rate can be as high as 6.0 which is about 270 times higher than that of a corresponding normal needle electrospinning system (0.022g/hr). The effects of appliedb voltage, the distance between the disk nozzle and collector, and PVA concentration on the fibre morphology were examined. The dependency of fibre diameter on the PVA concentration showed a similar trend to that for a conventional electrospinning system using a syringe needle nozzle, but the diameter distribution was wider for the disk electrospun fibres in this study.
The profiles of electric field strength in disk electrospinning showed considerable dependence on the disk thickness, with a thin disk exhibiting similar electric field profile to
that of a needle electrospinning system, but a thick disk (cylinder) exhibiting levelled electric field between the disk and the collector. PVA nanofibres electrospun from disk electrospinning were compared to that electrospun from syringe needle and metal cylinder nozzles.

Nano related research in fibres and textiles

With the increasing hype surrounding what nanotechnology can actually deliver, research emphasis in this area needs to be placed on how nanotechnology can bring tangible benefits to existing industries and ordinary consumers. This paper gives selected examples of real world applications of nano-structured materials, including nano fibrous and particulate materials. It reviews recent research into nano-structured surface coating of textile substrates for enhanced functionalities, and the development of fine and uniform nanofibres for advanced applications. Emphasis has been placed on relevant research activities in the Centre for Material and Fibre Innovation at Deakin University, Australia. In the nano-structured surface coating area, several examples of enhancing fabric performance and functionality are provided, including silica coating for photochromic textiles, superhydrophobic surface coating and transparent ZnO coating to reduce colour fading of textiles exposed to UV radiation. In the nanofibre area, these activities include: elimination of beaded fibres without increasing the average diameter of the electrospun nanofibres, electrospinning of side-by-side bi-component nanofibres, new insight into the evolution of fibre morphology in electrospinning and the electrospinning technology itself.

Carbon nanotube polymer composites and nanofibres

This study examined how carbon nanotubes (CNTs) in electrospun polymeric nanofibres influenced the polymer morphology, and how polymer morphology change induced by different post-electrospinning treatments influenced CNT-polymer interaction and nanofibre properties. The results showed that both the polymer structure and morphology played important roles in determining the composite and nanofibre properties.

Large-scale electrospinning of polymer nanofibers using needleless nozzle

In this study, we have demonstrated that a rotating metal wire coil can be used as a nozzle to electrospin nanofibers on a large-scale. Without using any needles, the rotating wire coil, partially immersed in a polymer solution reservoir, can pick up a thin layer of charged polymer solution and generate a large number of nanofibers from the wire surface simultaneously. This arrangement significantly increases the nanofiber productivity. The fiber productivity was found to be determined by the coil dimensions, applied voltage and polymer concentration. The dependency of fiber diameter on the polymer concentration showed a similar trend to that for a conventional electrospinning system using a syringe needle nozzle, but the coil electrospun fibers were thinner with narrower diameter distribution. The profiles of electric field strength in the coil electrospinning was calculated and showed concentrated electric field intensity on the wire surface.

Recent developments in electrospinning of nanofibers and nanofiber yarns

Electropsipinning is a simple, but efficient and versatile, technology to produce polymeric nanofibers for diverse applications in both textile and non-textile areas. In this paper, recent research developments in electrospinning and electrospun nanofibers, especially thaose from the Centre for Material and Fiber Innovation, Deakin University, are introduced. Important findings on needleless mass-electrospinning and direct electrospinning of highly-twisted continuous manfiber yarns are presented.

Synthesis and characterization of silver nanoparticles and titanium oxide nanofibers : toward multifibrous nanocomposites

A new method was investigated to produce new multiscale fibrous nanocomposites comprised of titanium oxide (TiO₂) nanofibers and silver (Ag) nanoparticles (NPs). The process involved electrospinning TiO₂ precursor solution containing colloidal solution of Ag NPs, and organic solvent (dimethyl-n′n-formamide) to fabricate a porous, nonwoven, free-standing nanofiber mesh. Postprocess heating of the electrospun nanofibers entailed calcination in air environment at 500°C for 3 h. Microemulsion processing was used to generate NPs of Ag in a monodispersed distribution throughout the colloidal solution. X-ray diffraction data were consistent with the anatase phase of TiO2, while transmission electron microscopy and hydrogen desorption measurements revealed a very porous microstructure. It was demonstrated that NP colloidal stability is solvent dependent. It is anticipated that incorporation of metal particles in nanofibers will lead to enhanced photocurrent generation, when used in functional devices.