Mehdi Afshari

High Performance Fibers Based on Rigid and Flexible Polymers

This chapter covers recent developments in the production of well established high performance fibers such as Kevlar, PBO, Spectra and Dyneema fibers and depicts a new super strong M5. The latter fibers have the modulus of 330GPa and tenacity of 5GPa. DuPont de Nemours is currently developing commercial M5 fibers and yarns. A very interesting monomer namely, 2,5‐dihydroxyterephthalic is used for making poly{2,6‐diimidazo[4,5‐b:4′,5′‐e]pyridinylene‐1,4‐(2,5‐dihydroxy)phenylene} (PIPD). The unique feature of the polymer is that the two hydroxyl groups (on terephthalic acid) can form intermolecular hydrogen bonds and therefore fibrillation, that is often a problem for aramid fibers, is practically eliminated. As a result, M5 fibers have the highest compressive strength among synthetic fibers. Exploratory evaluation of the UV stability of M5 indicated excellent performance in that field. The mechanical properties of the new fiber make it competitive with carbon fiber in most applications ‐ in light, slender, load bearing stiff advanced composite components and structures.

Effect of blend ratio on bulk properties and matrix-fibril morphology of polypropylene/nylon 6 polyblend fibers

Ternary blends of polypropylene (PP), nylon 6 (N6) and polypropylene grafted with maleic anhydride (PP/N6/PP-g-MAH) as compatibilizer with up to 50 wt% of N6 were investigated. PP-g-MAH content was varied from 2.5 to 10%. Blends of the two polymers PP/N6 (80/20) without the compatibilizer were also prepared using an internal batch mixer and studied. The ternary blends showed different rheological properties at low and high shear rates. The difference depended on the amount of N6 dispersed phase. Co-continuous morphology was observed for the blend containing 50% N6. This blend also exhibited higher viscosity at low shear rate and lower viscosity at high shear rates than the value calculated by the simple rule of mixture. At higher shear rates, viscosity was lower than that given by the rule of mixture for all blend ratios. An increase in viscosity was observed in the 80/20 PP/N6 blend after the concentration of the interfacial agent (PP-g-MAH) was increased. Polyblends containing up to 30% N6 could be successfully melt spun into fibers. DSC results showed that dispersed and matrix phases in the fiber maintained crystallinity comparable to or better than the corresponding values found in the neat fibers. The dispersed phase was found to contain fibrils. By using SEM and LSCM analyses we were able to show that the N6 droplets coalesced during melt spinning which led to the development of fibrillar morphology.

The promotion of axon extension in vitro using polymer-templated fibrin scaffolds.

Biomaterial nerve cuffs are a clinical alternative to autografts and allografts as a means to repair segmental peripheral nerve defects. However, existing clinical biomaterial constructs lack true incorporation of physical guidance cues into their design. In both two- and three-dimensional systems, it is known that substrate geometry directly affects rates of axon migration. However, the ability to incorporate these cues into biomaterial scaffolds of sufficient porosity to promote robust nerve regeneration in three-dimensional systems is a challenge. We have developed fibrin constructs fabricated by a sacrificial templating approach, yielding scaffolds with multiple 10-250 μm diameter conduits depending on the diameter of the template fibers. The resulting scaffolds contained numerous, highly aligned conduits, had porosity of ∼ 80%, and showed mechanical properties comparable to native nerve (150-300 kPa Youngs modulus). We studied the effects of the conduit diameters on the rate of axon migration through the scaffold to investigate if manipulation of this geometry could be used to ultimately promote more rapid bridging of the scaffold. All diameters studied led to axon migration, but in contrast to effects of fiber diameters in other systems, the rate of axon migration was independent of conduit diameter in these templated scaffolds. However, aligned conduits did support more rapid axon migration than non-aligned, tortuous controls.

4 – Producing polyamide nanofibers by electrospinning

Publisher Summary Conventional fiber spinning techniques, such as wet spinning, dry spinning, melt spinning, and gel spinning, usually produce polymer fibers with diameters down to the micrometer range. Polymer fibers can be generated from an electrostatically driven jet of polymer solution or polymer melt. This process, known as electrospinning, has received a great deal of attention in the past decade because of its ability to consistently generate polymer fibers that range from 5 to 500 nm in diameter. The parameters that affect the electrospinning process are solution properties, including viscosity, polymer concentration, polymer molecular weight, conductivity, and surface tension; controlled process variables, including hydrostatic pressure in the capillary, electric potential at the tip of needle, emitting electrode polarity, and the distance between the tip of the needle and the collection screen; and ambient parameters, including temperature, humidity, and air velocity in the electrospinning chamber. There are relationships between polymer solution concentration and electric field strength, and the distance between tip and collection screen for achieving a steady jet of the polymer solution.

Production methods for polyolefin fibers

Abstract This chapter discusses various polymerization routes for major polyolefins, such as polyethylenes and polypropylene, with emphasis on various catalysts. Ziegler–Natta catalysts as well as newer metallocee catalysts are reviewed. Major industrial polymerization processes are also discussed. The chapter also describes the most important extrusion methods for polyolefins, which can create a variety of products from processes such as melt spinning, film casting, film blowing, injection molding, blow molding and rotational molding. Melt spinning of polyolefins is described in detail.