Experiments Predominantly with Fiber

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. 

There is significant government, university, and industrial testing experience with nonstandard methods for determining fracture of ceramics and ceramic composites. For fiber-reinforced composites, this experience has been obtained predominantly at room temperature, although significant strides have been made in the past five years in elevated temperature testing. 

The peaks A and B in both spectra clearly originate from the noncrystalline domain in the samples, indicating that the noncrystalline domain reported from X-ray diffraction has a relatively ordered structure rather than a randomly distributed one. This conclusion derived from NMR is also supported by the conclusion that the noncrystalline domain is highly oriented in aromatic polyamides on the basis of X-ray diffraction studies [30, 32], The relatively broad peaks show a wider distribution for each 0nh value compared with peak C, which is also reasonable. The chemical shieldings of the peaks A and B are almost the same between PMIA and P4M-MPTA. This indicates that the local structure in the noncrystalline domain is similar for these polyamide fibers. It has been reported that the fraction of noncrystalline domain in the P4M-MPTA sample is higher than in the PMIA sample. The increase in the fraction of the noncrystalline domain comes predominantly from the contribution of peak A, i.e., the structure with 0nh = 31-42°, which is derived from the solid-state NMR experiment. 

On the basis on the carried out structural investigations of amorphous PET fibers simultaneous heat - mechanically modified at isothermal conditions and constant strain stress values it can be make the following conclusions The mechanical strain force applied simultaneously with the linear heating of the studied PET yarns affects significantly the deformation behaviour and samples crystallization kinetics. Moreover in contrast to the results obtained in the first experiment, all of the so treated specimens are partially crystaUine. The role of the tensile stress in the adjustment of the interacting processes of the fluid like deformation and stress-induced crystallization clearly reveals in the ultimate samples deformation. At stress values from 1.56 MPa to 2.16 MPa predominates the fluid like fibers extension, while the further stress increasing leads to the earlier crystallization start and thereby to decrease of the final fibers length. 

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