Fibers are long, narrow, ribbon-like structures, having a different chain orientation than that present in lamellas. Fibers are usually composed of smaller units, fibrils, microfibrils and nanofibrils, oriented in the same direction.

   Fibrillar structures can be formed

  Fibers can be prepared from crystallizable polymers with flexible chains, as well as from those with completely stiff or semi-stiff chain, such as polybenzothiazole and polybenzamide, respectively.

  Depending on the method of preparation, fibers from flexible chain polymers can contain extended noncrystalline portions of chains and chain folds, or they can have perfect or near-perfect order, as shown schematically in Figure 18.

  A change from a spherulitic morphology to a fibrous morphology can be brought about by the application of a tensile stress above Tg for the polymer, a process called cold drawing; fibrous samples prepared in this way contain chain folds and noncrystalline extended chain portions, as in Figure 18.

Fig. 18. Fibrillar crystallization : a. cold-drawn melt crystallized, B. ultradrawn solution crystallized.