Ultrahigh‐modulus fibers from rigid and semirigid aromatic polyamides

Abstract

Intrinsic viscosity, concentrated‐solution viscosity, and fiber properties of poly(p‐benzamide) (PBA) and the polyterephthalamide of p‐aminobenzhydrazide (X‐500) are presented. PBA yields high‐modulus fibers from solution spinning of anisotropic dopes, and X‐500 yields high‐modulus fibers from isotropic solutions. Both polymers exhibit a wormlike conformation. However, with a low‐molecular‐weight PBA (10,500) an essentially rigid conformation prevails. The viscosity of both polymers goes through a maximum and a minimum on increasing polymer concentration. However, for PBA the maximum and minimum are observed at low shear stress, and for X‐500 the discontinuity is observed only at high shear rate. The initial modulus of low‐molecular‐weight PBA fibers is unaffected by LiCl concentration but increases with polymer concentration of anisotropic dopes. X‐500 (molecular weight, 41,000) is unable to form an anisotropic solution at rest. The initial modulus of X‐500 is unaffected by LiCl but depends slightly on polymer concentration. Both shear and elongational flow contributions appear to be secondary for truly stiff polymers. Orientation is controlled by polymer concentration, and the modulus of low‐molecular‐weight PBA attains the ultrahigh level even for “as‐spun” fibers. On the contrary, control of shear and elongational parameters is shown to be essential, and feasible, in order to match the as‐spun modulus of PBA, with semirigid polymers unable to form anisotropic solutions at rest.