Uniaxially oriented polymer fibers

Figure 11.6. Schematic illustrations of brittle fracture, (a) Idealized limiting case of perfectly uniaxially oriented polymer chains (horizontal lines), with a fracture surface (thick vertical line) resulting from the scission of the chain backbone bonds crossing these chains and perpendicular to them. This limit is approached, but not reached, in fracture transverse to the direction of orientation of highly oriented fibers, (b) Isotropic amorphous polymer with a typical random coil type of chain structure. Much fewer bonds cross the fracture surface (thick vertical line), and therefore much fewer bonds have to break, than for the brittle fracture of a polymer whose chains are perfectly aligned and perpendicular to the fracture surface, (c) Illustration of a defect, such as a tiny dust particle (shown as a filled circle), incorporated into the specimen during fabrication, which can act as a stress concentrator facilitating brittle fracture.

In most cases, structural analysis of polymer crystals is carried out using uniaxially oriented samples (fibers or films). The basic procedures include (1) determination of the fiber period (2) indexing (hkl) diffractions and determining the unit cell parameters (5) determination of the space group symmetry (4) structural analysis and (5) Fourier transforms and syntheses and Patterson functions. The first three aspects of the procedure are discussed here, and the last two aspects are left for further references. 

Finally, as in macro-Raman experiments, orientation-insensitive spectra can also be calculated for spectromicroscopy. A method has been developed recently for uniaxially oriented systems and successfully tested on high-density PE rods stretched to a draw ratio of 13 and on Bombyx mori cocoon silk fibers [65]. This method has been theoretically expanded to biaxial samples using the K2 Raman invariant and has proved to be useful to determine the molecular conformation in various polymer thin films [58]. 

If the orientation is uniaxial (i.e., fiber symmetrical), the strong peaks of polymer materials are, in general, found in specific regions of the pattern. The strong WAXS peaks are found close to the equator7 of the WAXS pattern. Thus it is good practice to let an offset WAXS detector monitor the equator region. 

A few examples of the moduli of systems with simple symmetry will be discussed. Figure 1A illustrates one type of anisotropic system, known as uniaxial orthotropic. The lines in the figure could represent oriented segments of polymer chains, or they could be fibers in a composite material. This uniaxially oriented system has five independent elastic moduli if the lines (or fibers) ara randomly spaced when viewed from the end. Uniaxial systems have six moduli if the ends of the fibers arc packed in a pattern such as cubic or hexagonal packing. The five engineering moduli are il-... 

A nylon fiber has uniaxial orientation in which the polymer chains are parallel to the fiber axis. Is Et greater than ET1 Is Gr/ greater than G, 7 Why ... 

Almost all of these examples involve diffusion of a chemical species measuring diffusion rates has long been a specialty of NMR spectroscopy. The studies of KBr and drawn polyethylene produced unique information in the latter case, the known orientation of the deuterium electric field gradient in C-D bonds is used to determine the orientation, with respect to the magnetic field, of a polymer chain of a uniaxially ordered polyethylene fiber. The real time imaging of the polymerization of methyl methacrylate is very interesting and may represent a major direction for NMR imaging applications to polymer science. 

Study of the crystal structure of polysaccharides, particularly of cellulose, has provided the main use for polarized infrared radiation in connection with carbohydrate spectra. Since this is another technique whereby band assignments can be made, the basic steps involved will be described in a simplified manner with reference to a polymer sample having uniaxial orientation. This is a common type of orientation, characteristic of fibers,... 

The case often applied in polymers and liquid crystals is that when a uniaxial orientation is adopted such as in oriented fibers and a monodomain of the nematic phase. The reciprocal lattice points in such systems are... 

Orientation of styrene-based copolymers is usually carried out at temperatures just above T. Biaxially oriented films and sheet are of particular interest. Such orientation increases tensile properties, flexibility, toughness, and shrinkability. PS produces particularly clear and sparkling film after being oriented biaxially for envelope windows, decoration tapes, etc. Oriented films and sheet of styrene-based polymers are made by the bubble process and by the flat-sheet or tentering process. Fibers and films can be produced by uniaxial orientation (237) (see Film AND SHEETING materials). 

Many fibers composed of natural polymers contain uniaxially oriented anisotropic hollow spaces in which metal particles can crystallize under appropriate conditions. As a consequence, the impregnated fibers contain anisotropic assemblies of metal nanoparticles that induce anisotropic optical properties... 

Orientation A process of drawing or stretching of as-spun synthetic fibers or hot thermoplastic films to orient polymer molecules in the direction of stretching. The fibers are drawn uniaxially and the films are stretched either uniaxially or biaxially (usually longitudinally or longitudinally and transversely, respectively). Oriented fibers and films have enhanced mechanical properties. The films will shrink in the direction of stretching, when reheated to the temperature of stretching. 

Uniaxially Oriented A state of material such as polymeric film or composite characterized by the permanent orientation of its components such as polymer molecules or reinforcing fibers in one direction. The orientation is achieved by a number of different processes, e.g., stretching, and is intended to improve the mechanical properties of the material. 

In the case of X-ray studies, the polymer samples are usually uniaxially oriented and yield fiber diagrams that correspond to single-crystal rotation photographs. Electron diffraction studies utilize single crystals. 

Fig. 4.3 Reflected light micrographs of polished composite specimens show the carbon fibers (white) and their orientation within the polymer matrix. Black regions are voids. In (A) various layers are oriented normal to one another, while in (B) two uniaxially oriented fiber tows are shown.

The inherent properties of CNT assume that the structure is well preserved (large-aspect-ratio and without defects). The first step toward effective reinforcement of polymers using nanofillers is to achieve a uniform dispersion of the fillers within the hosting matrix, and this is also related to the as-synthesized nanocarbon structure. Secondly, effective interfacial interaction and stress transfer between CNT and polymer is essential for improved mechanical properties of the fiber composite. Finally, similar to polymer molecules, the excellent intrinsic mechanical properties of CNT can be fully exploited only if an ideal uniaxial orientation is achieved. Therefore, during the fabrication of polymer/CNT fibers, four key areas need to be addressed and understood in order to successfully control the microstructural development in these composites. These are (i) CNT pristine structure, (ii) CNT dispersion, (iii) polymer—CNT interfacial interaction and (iv) orientation of the filler and matrix molecules (Figure 16.14). 

Drawing products in their solid state (monofilament production, uniaxially oriented film, or biaxi-ally oriented film) maximizes molecular orientation and directional properties. In semicrystalline polymers drawing can lead to additional crystallinity development through alignment of the polymer molecules. The ability to successfully draw and process the polymer into oriented fibers, films, and profiles is often dependent on the quenching conditions used to form the precursor fiber, sheet, or profile. 

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