Frank Ko

Electrostatic fabrication of ultrafine conducting fibers: polyaniline/polyethylene oxide blends

Abstract Ultrafine fibers of polyaniline doped with camphorsulfonic acid (PAn.HCSA) blended with polyethylene oxide (PEO) were prepared by a non-mechanical, electrostatic spinning (“electrospinning”) technique. The morphology and fiber diameter of electrospun polyaniline blend fibers revealed that both the PEO and the PAn.HCSA/PEO blend fibers had a diameter ranging between 950 nm and 2.1 μm, with a generally uniform thickness along the fiber. The UV–visible spectra of these electrospun fibers were similar to those for cast films produced from the same solutions. As expected, the conductivity of the non-woven fiber mat, as measured by the four-point probe method, was slightly lower than that of a cast film, due to the high porosity of the non-woven mat. The rate for the vapor phase de-doping/re-doping of the electrospun fibers is at least one order of magnitude faster than for cast films, stressing the enormous effect an increase in the surface-to-volume ratio, accomplished by electrospinning the material into fibers, can have on the selected chemical properties of polyaniline blends.

Regeneration of Bombyx mori silk by electrospinning—part 1: processing parameters and geometric properties

We studied the effect of electrospinning parameters on the morphology and fiber diameter of regenerated silk from Bombyx mori. Effects of electric field and tip-to-collection plate distances of various silk concentrations in formic acid on fiber uniformity, morphology and diameter were measured. Statistical analysis showed that the silk concentration was the most important parameter in producing uniform cylindrical fibers less than 100 nm in diameter.

Reinforcement and rupture behavior of carbon nanotubes–polymer nanofibers

High-resolution transmission electron microscopy examination of carbon nanotube–polyacrylonitrile composite fibers synthesized by electrospinning was conducted. Both single-wall carbon nanotubes and multi-wall carbon nanotubes have been used to reinforce the polymer fibers. A two-stage rupture behavior of the composite fibers under tension, including crazing of polymer matrix and pull-out of carbon nanotubes, has been observed. Carbon nanotubes reinforce the polymer fibers by hindering crazing extension, reducing stress concentration, and dissipating energy by pullout. Distribution of nanotubes in the polymer matrix and interfacial adhesion between nanotubes and polymers are two major factors to determine the reinforcement effect of carbon nanotubes in polymer fibers.

Mapping critical sites in collagen II for rational design of gene-engineered proteins for cell-supporting materials

Collagen II is the most abundant protein of cartilage and forms a network of fibrils extended by proteoglycans that enables cartilage to resist pressure. The surface of the collagen fibril serves as a platform for the attachment of collagen IX, growth factors, and cells. In this study we examined the mechanism of the interaction of chondrocytes with recombinant versions of procollagen II, in which one of the four blocks of 234 amino acids that define repeating D periods of the collagen triple helix has been deleted. Analysis of the attachment of chondrocytes to collagen II variants with deleted D periods indicated that the collagen II monomer contains randomly distributed sites critical for cell binding. However, as was shown by spreading and migration assays, the D4 period, which is between residues 703 to 936, contains amino acids critical for cell motility. We also showed that binding, spreading, and migration of chondrocytes through three-dimensional nanofibrillar collagenous matrices are controlled by an interaction of the collagen triple helix with beta1 integrins. The results of this study provide a basis for the rational design of a scaffold containing genetically engineered collagen with a high density of specific sites of interaction.

Electrospinning of Technical Lignins for the Production of Fibrous Networks

Abstract Electrospinning is an effective strategy to produce micron and sub-micron diameter fibrous networks from a variety of polymeric systems. Using seven different technical lignins the effect of lignin structure on fiber formation by electrospinning was studied. Surprisingly, none of the technical lignins could be electrospun into continuous fibers, although beaded fiber formation was observed for the softwood Kraft lignin system at high concentration (>50 wt%). However, the addition of poly(ethylene oxide) dramatically affected the electrospinning behavior and fiber formation. For all of the technical lignins a clear transition from electrospray or beaded fibers to uniform fibers was observed upon addition of poly(ethylene oxide); the lignin concentration dependent on poly(ethylene oxide) content. In all of the systems a linear increase in fiber diameter with increasing lignin concentration was observed. At the same concentration, the various lignin solutions had varying viscosities and different electrospinning behavior, that is, fiber diameter and ability to form uniform fibers, suggesting lignin specific structures and intermolecular interactions are influencing solution properties and electrospinning behavior. In fact, specific viscosity versus concentration plots reveal scaling exponents’, η ∼ c7.4–7.8 consistent with a branched polymer participating in intermolecular interactions such as hydrogen bonding or association complexes.

Analysis of multiaxial warp-knit preforms for composite reinforcement

Abstract Three-dimensional structures of multiaxial warp-knit (MWK) fabrics have been recently developed for multidirectional reinforcement of composites. Multilayers of linear yarns are assembled in warp (0 °), weft (90 °), and bias (±θ) directions to provide in-plane reinforcement in specific directions, and they are stitched together by knitting yarns to provide structural integrity and through-the-thickness reinforcement. Based on the experimental observations, the unit cell geometry of the MWK fabric is identified along with idealized cross-sectional shapes of insertion and stitch yarns. A geometric model is developed relating the geometric parameters and process variables. Expressing fiber volume fraction and fiber orientation in terms of structural and processing parameters, this geometric model provides a basis for the establishment of process windows for the MWK fabric preforms as well as for the prediction of the mechanical behavior of the MWK reinforced composites.

Unit cell geometry of 3-D braided structures

The traditional approach used in modeling of composites reinforced by three-dimensional (3-D) braids is to assume a simple unit cell geometry of a 3-D braided structure with known fiber volume fraction and orientation. In this article, we first ex amine 3-D braiding methods in the light of braid structures, followed by the development of geometric models for 3-D braids using a unit cell approach. The unit cell geometry of 3-D braids is identified and the relationship of structural parameters such as yarn orienta tion angle and fiber volume fraction with the key processing parameters established. The limiting geometry has been computed by establishing the point at which yarns jam against each other. Using this factor makes it possible to identify the complete range of allowable geometric arrangements for 3-D braided preforms. This identified unit cell geometry can be translated to mechanical models which relate the geometrical properties of fabric pre forms to the mechanical responses of composite systems.

Microfabricated formaldehyde gas sensors.

Formaldehyde is a volatile organic compound that is widely used in textiles, paper, wood composites, and household materials. Formaldehyde will continuously outgas from manufactured wood products such as furniture, with adverse health effects resulting from prolonged low-level exposure. New, microfabricated sensors for formaldehyde have been developed to meet the need for portable, low-power gas detection. This paper reviews recent work including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, and nanowire-based sensors. This paper also investigates the promise of polymer-based sensors for low-temperature, low-power operation.

Sandwich-fabric panels as spacers in a constrained layer structural damping application

Sandwich-fabric panels can provide for an alternative spacer material in a constrained layer damping configuration. Constraining layer configurations with sandwich-fabric spacers can be a weight efficient replacement for full composite constraining layers, if the shear stiffness of the rubber used is not too high. It seems that in any case the use of sandwich-fabric spacers can lead to a more cost-effective damping treatment. To predict the damping of multilayer materials a strain energy method was used. The damping of multilayer beams could be accurately modelled by calculating the distribution of strain energies in the structure with the help of finite elements and knowledge of the loss factors of the individual layers (as a function of frequency).

Numerical modeling of an electrostatically driven liquid meniscus in the cone–jet mode

Abstract A numerical model has been developed for an electrostatically driven liquid meniscus for a dielectric fluid. The model is able to calculate the shape of the liquid cone and the resulting jet, the velocity fields inside the liquid cone–jet, the electric fields in and outside the cone–jet, and the surface charge density at the liquid surface. The mathematical formulas with proper boundary conditions for the relevant physical processes are described in detail. The equations of continuity, momentum and electric potential are solved numerically with an iterative procedure developed for the model. The results of the present model fit well with experimental observations of the cone shape and jet formation.