Uniaxially oriented fibers

The fibers were annealed at 200 C for 5 min. in N2, wound parallel on a sample holder and flat plate X-ray photographs taken by exposing the fibers for 1 h (WAXS) to a beam collimated at 90° to the fiber axis, uatorial diffractometer traces of uniaxially oriented fibers were taken uring slit collimation. WAXS patterns were obtained on polaroid film and the difoctometer traces with a Siemens X-ray system. Nickel filtered CuKa radiation was used. The X-ray unit was operated at 30 KV and 20 mA. The sample to film distance was 71.4 mm. 

The mechanical properties of particulate-filled composites are generally isotropic that is, they are invariant with direction provided there is a good dispersion of the fillers. On the other hand, fiber-filled composites are typically anisotropic. In general, fibers are usually oriented either uniaxially or randomly in a plane. In this case, the composite has maximum modulus and strength values in the direction of fiber orientation. For uniaxially oriented fibers. Young s modulus, measured in the orientation direction (longitudinal modulus, El) is given by Equation 9.1 ... 

Apparently this model in its present form does not predict a fibrillated fracture morphology caused by shear failure, as has been clearly observed for the PpPTA, PBO and PBT fibers. Knoff noticed the similarity between the tensile failure morphology of PpPTA fibers and that of a uniaxially oriented fiber-reinforced composite. The latter fails in tension via matrix shear failure initiated at the fiber ends. This made him to conclude that, if the shear forces at a discontinuity exceed the shear strength of the bond between the fibrils, the fiber tensile strength should be proportional to the fiber shear strength [182]. 

Uniaxially oriented fiber systems with fibers randomly spaced when viewed from an end cross section are anisotropic materials. For continuous ( long ) fiber composites... 

Uniaxially orientated fiber composite material has a high modulus in only one direction. To obtain composite materials with good mechanical properties in at least two or three directions, the fiber should have random orientation. Figure 2.13 shows the Young s moduli 2D and 3D and shear moduli Gid (jjp of the composite material, which has an In-plate random orientation and three-dimensional random orientation. 

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.

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]. 

Isotropization in the Case of Fiber Symmetry. If methods for the analysis of isotropic data shall be applied to scattering patterns with uniaxial orientation, the corresponding isotropic intensity must be computed. By carrying out this integration (the solid-angle average in reciprocal space) the information content of the fiber pattern is reduced. One should consider to apply an analysis of the longitudinal and the transversal structure (cf. Sect. 8.4.3). 

The multidimensional chord distribution function (CDF) for oriented materials (in particular useful for the study of materials with uniaxial orientation, i.e., fibers) (Sect. 8.5.5)... 

Figure 9.4. The orientation of structural entities (rods) in space with respect to the (vertical) principal axis and the values of for, the uniaxial orientation parameter (Hermans orientation function) for (a) fiber orientation, (b) isotropy, (c) film orientation...

As are the other multipole-expansion coefficients, the uniaxial orientation parameter is computed from Eq. (9.6). For materials with fiber symmetry the relation simplifies4 and... 

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-... 

The most common examples of uniaxially oriented materials include fibers, films, and sheets hot-stretched in one direction and composites containing fibers all aligned in one direction. Some injection-molded objects are also primarily uniaxially oriented, but most injection-molded objects have a complex anisotropy that varies from point to point and is a combination of uniaxial and biaxial orientation. 

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 ... 

Therefore, uniaxially oriented samples should be prepared for this purpose, which give so-called fiber pattern in X-ray diffraction. The diffraction intensities from the PPX specimen of P-form, which had been elongated 6 times at 285°C, were measured by an ordinary photographic method. The reflections were indexed on the basis of the lattice constants a=ft=2.052nm, c(chain axis)=0.655nm, a=P=90°, and y=120°. Inseparable reflections were used in the lump in the computation by the least square method. 

The structural anisotropy in crystalline or structurally ordered BPDA-PFMB films was studied in this laboratory with wide-angle X-ray diffraction (WAXD) methods. In brief, WAXD experiments were designed to examine both the reflection and transmission modes of thin-fihn samples. In addition, uniaxially oriented polyimide fiber WAXD patterns were obtained to aid in the identification of the film structure. The film WAXD pattern obtained from the reflection mode corresponded well to the fiber pattern scanned along the equatorial direction (Figure 16.3), " which indicates that the reflection mode pattern represents the (hkQ) diffractions. On the other hand, as shown in Figure 16.4, the (001) diffractions were predominant in the film WAXD pattern obtained via the transmission mode. This pattern corresponded to the fiber pattern scanned along the meridian direction. These experimental observations clearly indicate that the c-axes of the crystals are preferentially oriented parallel to the film surface however, within the film, they are randomly oriented. 4.2 5 j( should be pointed out that the WAXD experiments are only sensitive to crystalline or ordered structures in polyimide films. They do not provide any information on the amorphous regions. 

The simplest case to consider is that where the fluorescent molecules are rigidly oriented uniaxially as could be the case for a highly oriented fiber which had been dyed with a fluorescent substance. Suppose we excite the fluorescence with linearly polarized light with the electric vector... 

Actually in the case of uniaxially oriented dye molecules unpolarized exciting light could also be used to give the same result since the emission axis of the molecule coincides with the axis of the fiber. This technique... 

The fibers spun from nematic phase systems and solvent composition 25.0/75.0 exhibited twice the tenacity and modulus of those extruded from the system of the solvent composition 29.3/70.7. The two fiber types showed significant differences in birefringence, and in the appearance of their external surfaces and fracture surfaces. The comparable moduli of the fiber spun from a nematic solution with those of Fortisan may result from high uniaxial orientation of cellulose molecules in the former fiber. 

Figure 13. X-ray diffraction patterns of PDHS crystallized by slow cooling from 180 and then uniaxially oriented at room temperature (vertical fiber axis) (a) pattern recorded at 30 °C and (b) pattern recorded at 100 °C. (Reproduced from reference 26. Copyright 1986 American Chemical Society.)...

Fig. 9 Wide angle X-ray diffraction of uniaxially oriented isotactic polypropylene rod. (A) Beam perpendicular to the fiber axis. (B) Beam parallel to the fiber axis. Fiber patterns and full scans.

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