Section 19.5.2. Types of fibres

Fibres fall into essentially two classes with respect to the degree of ordering of the polymer chains. Within each class, there are varying degrees of disorder; furthermore, many fibres exhibit properties intermediate between those of the two ideal classes.

In noncrystalline fibres, the polymers are parallel to each other, but their positions and orientations are otherwise uncorrelated. Diffraction patterns from these fibres are confined to layer lines (Fig. 19.5.2.1a) because of the repeating nature of the polymer helix, but are otherwise continuous and correspond to the cylindrical average of the Fourier transform of a single particle.

Figure 19.5.2.1| top | pdf | X-ray diffraction patterns showing (a) continuous intensity on layer lines from an oriented nucleic acid fibre and (b) Bragg reflections from an oriented and polycrystalline polysaccharide fibre.

In polycrystalline fibres, the polymers form fully ordered microcrystallites, and each fibre consists of many such microcrystallites, randomly oriented about the fibre axis. In diffraction patterns from polycrystalline fibres, the layer lines are sampled to form discrete reflections (Fig. 19.5.2.1b); the diffraction pattern is the cylindrical average of a single-crystal diffraction pattern and is, in fact, equivalent to the diffraction pattern that would be obtained from a rotating single crystal.

Polycrystalline fibres may be disordered in various ways. For example, the helical polymers may be subject to rotational or translational disorder, and this disorder may be partial (a small number of alternative packings for each particle) or complete (for example, completely random rotational particle orientations). Rotational disorder may be coupled to translational disorder (screw disorder). The resulting diffraction patterns may contain both discrete reflections and continuous diffraction along layer lines; depending upon the type of disorder, the discrete reflections may be confined to the equator (layer line zero) or the low-resolution part of the pattern, or they may be dispersed throughout the pattern. Variations in diffraction effects due to different types of disorder have been discussed by Arnott (1980) and Stroud & Millane (1995).

Fibres are also subject to orientational disorder. The polymer helices in noncrystalline fibres and the microcrystallites in crystalline fibres are not perfectly aligned to the fibre axis; the deviation from parallelism is called the disorientation of the fibre. Disorientation causes the reflections from crystalline fibres and the diffracted intensity from noncrystalline fibres to be spread into arcs (Debye–Scherrer arcs).