Important variations on this theme include removal of the spinnerette from the wet spinning coagulation bath (allowing separate control of spinning and coagulation temperatures) necessary for the spinning of many lyotropic polymer systems (aramid, polyphenylene polybisbenzoxazole [PBO]) (dry jet wet spinning); and the spinning of low-concentration solution to minimize chain entanglements in the resulting fiber (gel spinning). Depending on the conditions chosen, filaments may be molecularly oriented or disoriented as spun. Additional orientation may be imparted through "drawing" processes where the molecules comprising the fibers are elongated at a rate faster than their relaxation time. Heat may be applied to yarns to increase morphological stability through controlled crystallization and/or molecular relaxation processes. Finally, chemistry through the application of heat, pressure (vacuum), or reactants may be utilized as a separate step, or in conjunction with the process steps, to convert "precursor" fibers to the desired final composition. In the ensuing portions of this chapter, processing technologies that are key to the formation of important high-performance fibers critically analyzed.

The technical concepts behind the processing of ultrahigh-strength, (200-500 Ksi), ultrahigh-modulus (greater than 7 Msi) polymeric organic fibers differ from those for inorganic and metallic fibers and are related to the one-dimensional nature of polymeric molecular chains. To make a fiber that takes maximum advantage of the strength of the interatomic forces (covalent bonding) in the polymer, the molecules must be extended and oriented parallel to the axis of the fiber.

The commercial processes for high-strength, high-modulus fibers are based on two physical concepts: melt or dry jet wet spinning from a nematic liquid crystalline phase in which the already rod-like molecules are uniaxially ordered, and melt or gel spinning and drawing of conventional, random-coil polymers under conditions that permit extremely high elongational forces (high draw ratios) to mechanically elongate and orient the component molecules (cf. Figure 3.1).

The resulting highly oriented morphologies yield moduli that approach the theoretical. However, the relatively high strengths achieved are far (10x) from theory, offering potential for further development. These fibers are highly anisotropic, with lateral tensile properties up to two orders of magnitude lower than axial. This results in some inherent performance losses, most notably in compressive properties.

Front Matter (R1-R16)

Executive Summary, Conclusions, and Recommendations (1-8)

1 High-Performance Synthetic Fibers for Composites (9-20)

2 High-Performance Fiber Materials: Applications, Needs, and Opportunities (21-48)

3 Fiber-Forming Processes: Current and Potential Methods (49-102)

4 Important Issues in Fiber Science/Technology (103-108)

5 Important Policy Issues (109-118)

Glossary of Terms (119-126)

Appendix (127-129)