Degrees of crystallite orientation

The quantitative assessment of the degree of crystallite orientation by x-ray examination is not free of ambiguity. From a comparative analysis [23] in which results obtained from the consideration of (105) and from three different variations of equatorial reflection were compared, the conclusion was that the first procedure can lead to underrated results, i.e., to the underestimation of the orientation. However, it can be assumed that this does not result from an incorrect procedure, but from ignoring the fact that the adjacent (105) reflex can overlap. The absence of the plate effect of the orientation is characteristic of the orientation of crystallites in PET fibers. The evidence of this absence is the nearly identical azimuthal intensity distributions of the diffracted radiation in the reflexes originating from different families of lattice planes. The lack of the plate effect of orientation in the case of PET fiber stretching has to do with the rod mechanism of the crystallite orientation. 

The degrees of orientation for the crystallites in the injection-molded specimens of PECs were determined from the azimuthal breadth at half-maximum intensity in the strongest equatorial reflection by X-ray diffraction. Figure 6 showed the degrees of crystallite orientation of the as-molded and the annealed specimens. 

It could be seen that the degrees of crystallite orientation increased by thermal treatment. This would result from a development in the overall order of crystallite along the flow direction. 

It is probable that varying degrees of ordering of chains exist in a cellulosic material and that a sharp differentiation of crystalline and non-crystalline celluloses may not be feasible or even possible. Theoretically, the lateral surfaces of crystallites are amorphous but may have far less importance in determining such properties as strength, flexibility and extensibility than the non-crystalline cellulose which supplies continuity of structure in the direction of crystallite orientation. Yet properties like moisture absorption and swelling may be more dependent upon the amount of cellulose which exceeds a certain degree of disorder (permeability) than upon location. The definition of crystallinity may, therefore, be made ultimately in terms of practical objectives. 

Peak intensities. If the material exhibits preferred orientation (i.e. a nonrandom distribution of orientations of the crystallites within a powder), the relative intensities of peaks in the powder XRD pattern will deviate from the intrinsic relative intensities that are characteristic of the crystal structure, and hence the powder XRD patterns recorded for two samples of the same material but exhibiting different degrees of preferred orientation may appear substantially different. This issue is particularly pertinent in comparing an experimental powder XRD pattern with a simulated powder XRD pattern for a known crystal structure, as there are implicitly no effects due to preferred orientation in the latter case. 

Theoretical Explanation. The major factors of the thermal expansion coefficient of calcined coke are the degree of preferred orientation of the crystallites and void structure (12-14). For example, the thermal expansion coefficient is low for needle coke because it is strongly affected by the preferred orientation of its crystallites. 

The structure and properties of pyrolytic deposits are influenced by a number of factors, of which temperature is the most important. The size of the crystallites and their degree of preferred orientation increases with temperature, whilst the interlayer spacing decreases [30,31]. Typical values for the density, crystallite diameter and crystallite height are given in... 

This periodicity of about 500 nm has been reported for banded textures observed in both the thermotropic copolyesters and the aramids [407, 430, 439-445]. The lateral banded textures exhibited by some of the thermotropes and the pleated sheet textures exhibited by some of the aramids are observed only in materials which have a poorer degree of molecular orientation. The more highly oriented thermotropic and lyotropic fibers do not exhibit these textures. For instance, heat treated Vectran fibers do not exhibit any lateral banding. Likewise, Kevlar 149 exhibits a higher tensile modulus than Kevlar 49 [448], nearly 80-90% of theoretical predicted values. This is consistent with increased crystallinity and crystallite size, and without a pleated sheet structure. The relationship of the optical... 

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