Average orientation, crystalline regions

The sound velocity in a fiber, and the sonic modulus calculated therefrom, are related to molecular orientation (De Vries ). As shown by Moseley ), the sonic modulus is independent of the crystallinity at temperatures well below the T (which means that the inter- and intramolecular force constants controlling fiber stiffness are not measurably different for crystalline and amorphous regions at these temperatures). An orientation parameter a, calculated from the sonic modulus, is therefore taken as a measure for the average orientation of all molecules in the sample, regardless of the degree of crystallinity. The parameter is called the total orientation , as contrasted to crystalline and amorphous orientation, from X-ray data. 

The orientation of the crystalline region is measured by x-ray diffraction techniques. The orientation in the less-ordered amorphous region is determined by x-ray, infrared measurements, sonic modulus technique [183], or by separating the crystalline and amorphous contributions to the fiber birefringence [79]. The average orientation of the crystalline regions is specified by the Hermans Stein orientation functions ... 

The elastic moduli of an extruded PLC are largely determined by its molecular orientation, so we first consider the orientation parameters calculated from the observed meridional X-ray reflections. For liquid crystalline copolyesters and copolyesteramides, the sharpness of these reflections gives an indication of the coherence length, a value corresponding to 10 monomers being typical. Therefore, the orientation parameter obtained from the meridional reflection is not specific to the crystalline regions but instead reflects the average orientation of all the chains in the polymer. 

The density method is undoubtedly employed most frequently for the determination of fibre crystallinity. Needless to say, the fibre must be free of voids and a correction for the presence of Ti02 delustrant must be used. As already mentioned, the unit cell dimensions (and hence the density of the crystalline regions) are often influenced by the crystallization conditions. Similarly, the density of the noncrystalline regions may be influenced by their orientation. Consequently, realistic values of crystallinity can be obtained only when average densities of crystalline and noncrystalline portions are determined independently for each specimen. ... 

---