Section 19.5.4. Fibre preparation

Natural fibre specimens may require only the selection of fibres or regions of fibres in which the polymers are well oriented, but many other fibres must be made in the laboratory, and orientation can often be improved by a wide variety of laboratory procedures.

Orientation often requires controlled conditions of relative humidity and temperature during the preparation of the fibre; in many cases, these conditions must be maintained during data collection. In some cases, tension must be applied to the fibres; in an increasing number of cases, magnetic fields have been found to improve orientation. Fibres may be drawn directly from concentrated polymer solutions or made by stretching gels, using weights to stretch strips of polymer films cast on Teflon blocks or applying radial heating while forming polymer films (Arnott, Guss et al., 1974; Chandrasekaran, Radha, Lee & Zhang, 1994). A drop of concentrated polymer solution may simply be dried while suspended between two supports. Magnetic fields have dramatically improved the orientation in dried fibres of polymers having significant dipole moments (Torbet, 1987).

Oriented sols, generally enclosed in glass capillaries, are usually made using shearing forces, either by moving the sol in the capillary (Gregory & Holmes, 1965) or by centrifugation (Cohen et al., 1971). Again, magnetic fields can greatly improve orientation, sometimes in combination with centrifugation (Yamashita, Suzuki, & Namba, 1998).

Any of these stretching or orienting processes might facilitate the growth of long microcrystallites along the fibre axis. Crystallization in general and lateral organization in particular are achieved primarily by careful choice of solution conditions, including solvent, pH, additives, relative humidity and temperature. In both crystalline and noncrystalline specimens, annealing processes are often important to both crystallization and orientation.