Arrangement of substituted, rigid-rod aramids in the highly-ordered solid state

Fibers of various rigid-rod, fully aromatic polyamides, produced by polycon densation of differently substituted p-phenylenediamines and terephthaloyl- dichlorides, were spun from nematic solution and analyzed with wide-angle X -ray diffraction (WAXD). A post-spin treatment was employed to improve the degree of orientation and crystallinity of the fibers. The dominating cryst al structures were similar to those found in poly(p-phenylene-terephthalami de) (PPTA) and described in the literature, i.e., "Modification I" and "Mod ification II". With the exception of only one case, the investigated fibers do not suffer a structural transformation upon heat treatment comparable t o that found in PPTA. Steric and electronic effects of the ring substitutio n lead in most cases to a small disturbance of the solid-state structure in the fibers. Major structural changes caused by ring substitution are rare. rare. Effects of the ring substitution pattern in the chains on the fiber crystal structure, their structural,older, and their orientational properti es could be observed. While the substitution of the aromatic rings seems to affect the competition between the basic crystal structures "Modification I" and "Modification II" (in analogy to PPTA) and the observed crystal stru ctures agree with the results of detailed atomistic modeling predictions, n o simple pattern seems to exist that would help predict, which candidate cr ystal structure is most stable.