Crystallinity and disorder in textile

Crystallinity and disorder are important structural parameters for understanding relationships between structure and physical properties. Flaws and distortions are the main features that limit the ultimate properties of textile fibers. Some of these crazes, cracks and voids are revealed under the electron microscope, either on the surface or in cross sections stained with heavy metals (J, 2). However, these staining techniques (that reveal the main morphological features) make it much more difficult to determine the degree of distortion of the crystalline fraction. Theoretically, line profile studies permit separation of effects due to crystalline size from those due to structural distortions. However, the lack of peaks in semicrystalline fiber x-ray patterns hinders that approach. 

Nevertheless, when we carry out x-ray crystallinity measurements on textile fibers, we must consider distortions that always affect crystalline material. Even in a completely crystalline material, the scattered x-ray intensity is not located exclusively in the diffraction peaks. That is because the atoms move away from their ideal positions, owing to thermal motion and distortions. Therefore, some of scattered x-rays are distributed over reciprocal space. Because of this distribution, determinations of crystallinity that separate crystalline peaks and background lead to an underestimation of the crystalline fraction of the polymer. In this paper, we attempt to calculate the real crystallinity for textile fibers from apparent values measured on the x-ray pattern. This is done by taking into account the factor of disorder following Ruland s method (3). 

Kevlar is the condensation product of 1,4-diaminobenzene and isophthalic acid. The fibres are highly crystalline and consist of a system of sheets regularly pleated along their long axes and arranged radially (ref. 5). The relatively small amount of disorder in the structure is due to chain termination or defects in the packing of the sheets. The grade chosen for this study was Kevlar 29, a low modulus form available in a variety of textile constructions. 

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