Wilhelm Steinmann

Spinnability and Characteristics of Polyvinylidene Fluoride (PVDF)-based Bicomponent Fibers with a Carbon Nanotube (CNT) Modified Polypropylene Core for Piezoelectric Applications

This research explains the melt spinning of bicomponent fibers, consisting of a conductive polypropylene (PP) core and a piezoelectric sheath (polyvinylidene fluoride). Previously analyzed piezoelectric capabilities of polyvinylidene fluoride (PVDF) are to be exploited in sensor filaments. The PP compound contains a 10 wt % carbon nanotubes (CNTs) and 2 wt % sodium stearate (NaSt). The sodium stearate is added to lower the viscosity of the melt. The compound constitutes the fiber core that is conductive due to a percolation CNT network. The PVDF sheath’s piezoelectric effect is based on the formation of an all-trans conformation β phase, caused by draw-winding of the fibers. The core and sheath materials, as well as the bicomponent fibers, are characterized through different analytical methods. These include wide-angle X-ray diffraction (WAXD) to analyze crucial parameters for the development of a crystalline β phase. The distribution of CNTs in the polymer matrix, which affects the conductivity of the core, was investigated by transmission electron microscopy (TEM). Thermal characterization is carried out by conventional differential scanning calorimetry (DSC). Optical microscopy is used to determine the fibers’ diameter regularity (core and sheath). The materials’ viscosity is determined by rheometry. Eventually, an LCR tester is used to determine the core’s specific resistance.

Modification of the mechanical properties of polyamide 6 multifilaments in high-speed melt spinning with nano silicates

In the last few years research activities have been focused on the modification of fiber properties with nano-scaled additives. One of the most important fields of research is the alteration of mechanical properties such as the tenacity and the specific breaking load. In this study, we determined the influence of nano-phyllosilicates on the drawability of polyamide 6 multifilament yarns. It was first demonstrated that the drawability of the fibers drastically increased in an industrially relevant high-speed melt spinning process. Structural properties of the material are identified by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Changes in the crystalline properties as well as in the alignment of the silicates are compared with the stress–strain curves of the fibers, and a molecular mechanism for the drawing process is derived from these experiments. In a first step, a significant phase transition in the crystalline structure unaffected by the silicates occurs for low draw ratios (DRs). Beyond this point, where unmodified fibers start to break, a gliding between the silicate layers takes place, which is responsible for an extended drawability of the fibers. This mechanism leads to new possibilities for fiber processing, which can be used to research ultra-fine filaments in future studies.

Essential Properties of Fibres for Composite Applications

In this chapter, essential properties of fibres required for fabricating good performance composites are presented. Fundamental aspects of these properties and principles of their characterization techniques are discussed. Firstly, the geometrical aspects of fibres (for short and endless fibres) are described. The second part deals with the different types of structures in fibres with respect to molecular orientation mostly responsible for anisotropy of fibres. In the third part, the mechanical properties (especially tensile properties) and failure mechanisms for different types of fibres are discussed and correlated to the structure of the fibres. The following part is concerned with the surface of fibres, which is responsible for the interaction of the fibres with the matrix material in composites and has a large influence on the wetting behavior and adhesion to matrix materials. In the last parts, further physical properties (heat capacity, thermal conductivity, thermomechanical properties and electrical conductivity) and the durability of fibres are described.

Synthetic Fibres for Composite Applications

This chapter gives the details of various synthetic fibres (both organic and inorganic such as glass, carbon, aramides, polyolefins, ceramic fibres, etc.) used to reinforce composite materials for conventional as well as very high-tech applications. Production and properties of these fibres and also the most common applications in fibre reinforced composites are included in this chapter.

Extrusion of CNT-modified Polymers with Low Viscosity - Influence of Crystallization and CNT Orientation on the Electrical Properties

Several polymers were modified with multiwalled carbon nanotubes (CNT) to study the influences of the crystallization in the polymeric matrix and of the CNT orientation during extrusion on the electrical conductivity. Experiments were carried out with common semi-crystalline polymers (polypropylene, polyethylene, polyamide 6) and compared to an amorphous polymer (ethylene vinyl acetate). All polymers were grades with low viscosity, so that the CNT could be oriented well during extrusion. For all materials, the percolation threshold was determined, and the lowest value of 3% was found in polypropylene. The percolation threshold was correlated to the degree of crystallinity of the matrix polymers, so that crystallites could be seen as an excluded volume for CNT. The crystallization itself was analyzed by differential scanning calorimetry (DSC), whereby nucleation effects and changes in the crystallization temperature were found. The shear rate during extrusion had a large influence on the electrical conductivity. This effect was analyzed by transmission electron microscopy (TEM), with which the orientation of CNT in the direction of extrusion was visualized and differences between the polymer matrices were explained.

Novel Melt-Spun Polymer-Optical Poly(methyl methacrylate) Fibers Studied by Small-Angle X-ray Scattering

The structural properties of novel melt-spun polymer optical fibers (POFs) are investigated by small-angle X-ray scattering. The amorphous PMMA POFs were subjected to a rapid cooling in a water quench right after extrusion in order to obtain a radial refractive index profile. Four fiber samples were investigated with small-angle X-ray scattering (SAXS). The resulting distance-distribution functions obtained from the respective equatorial and meridional SAXS data exhibit a real-space correlation peak indicative of periodic cross-sectional and axial variations in the scattering density contrast. Simple model calculations demonstrate how the structural information contained particularly in the equatorial distance distribution function can be interpreted. The respective results are qualitatively verified for one of the fiber samples by comparison of the model curve with the measured SAXS data. Eventually, the study confirms that the cross-sectional variation of the (scattering-) density is the main reason for the formation of radial refractive-index profiles in the POFs.