| Abstract: | Ultra-high-modulus fibers such as Du Pont PRD-49 (initial modulus up to ～1000 g/den) and Monsanto X-500 (initial modulus up to ～600 g/den) are spun from solutions. Both polymers are characterized by a high intrinsic rigidity of individual molecular chains and considerable orientation along the fiber axis. The thermodynamics of solution for rigid and semirigid macromolecules is critically reviewed in order to illustrate conditions under which spontaneous formation of highly oriented fibers is expected. In the case of semirigid polymers, the free energy of (random) mixing pure solvent and parallellized polymer may, according to Flory, become positive for some critical value of a “flexibility parameter.” Formation of an ordered phase for semirigid polymers is not, however, observed by lowering temperature or increasing polymer concentration. In the case of rod-like polymers, still according to Flory, at some critical value of polymer concentration (which decreases with the axial ratio of the macromolecule) the isotropic solution of rods undergoes phase separation with formation of a partly ordered solution. This theoretical prediction is satisfactorily verified by data. While Du Pont fibers are spun from this anisotropic solution, Monsanto's X-500 only yields an isotropic solution at room temperature up to the limit of polymer concentration at which crystallization occurs. This inability of X-500 to form anisotropic solutions at the expected critical concentration is attributed to a partial degree of flexibility. Mechanical properties and orientation of fibers spun from the anisotropic solution appear to be superior to those obtained by spinning from isotropic solution, according to Du Pont's results. When a polymer has a partial degree of flexibility, alteration of physico-chemical variables such as solvent type, solvent composition, temperature, and polymer concentration may still be used in order to increase its rigidity. Theoretical arguments and data supporting this contention are discussed. Moreover, alteration of these variables may also be used to alter the crystallization temperature, allowing formation of the anisotropic solution to occur at a high enough polymer concentration. This expectation was verified in the case of X-500. Finally, the all important role of mechanical orientation of solutions is emphasized. According to Hermans, under high enough shear stress, the difference between the isotropic and the anisotropic solution vanishes. In line with these consideration, drawing techniques are particularly useful in order to achieve almost-perfect orientation and theoretical moduli. |
