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Fibers of 21 rigid-rod aromatic polyamides with different substitution patterns and their aromatic rings, produced by polycondensation of functionalized p-phenylenediamine and functionalized terephthaloyl dichloride and spun from nematic solutions as described in the accompanying paper (B. H. Glomm, M. C. Grob, P. Neuenschwander, and U. W. Suter, Macromol. Chem. Phys.) were characterized by the mechanical properties most relevant for compressive failure. In particular, the torsional moduli G 0 and the axial compressive strength σ C were determined for each fiber sample before and efter employing a post-spinning heat treatment optimized to improve the degrees of orientation and the crystallinity of the fibers. The dependence of the measured values on the structural parameters of the respective polymers was studied, leading to the result that the volume of the side-chains of the studied aramids seems to influence the extent of the mechanical anisotropy of the fibers, probably through an effect on the interchain interactions. The relationship between the torsional modulus and the axial compressive strength was scrutinized in the light of the theoretical approach of DeTeresa, Allen, and Farris, and Allen, which suggests the existence of a proportionality between G 0 and σ C . In general, the results provided by our experiments are consist with this theoretical approach.

Fibers of various rigid-rod, fully aromatic polyamides, produced by polycondensation of differently substituted p-phenylenediamines and terephthaloyldichlorides, were spun from nematic solution and analyzed with wide-angle X-ray diffraction (WAXD). A post-spin heat treatment was employed to improve the degree of orientation and crystallinity of the fibers. The dominating crystal structures were similar to those found in poly(p-phenylene-terephthalamide) (PPTA) and described in the literature, i. e., “Modification I” and “Modification II”. With the exception of only one case, the investigated fibers do not suffer a structural transformation upon heat treatment comparable to that found in PPTA. Steric and electronic effects of the ring substitution 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. Effects of the ring substitution pattern in the chains on the fiber crystal structure, their structural order, and their orientational properties 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 structures agree with the results of detailed atomistic modeling predictions, no simple pattern seems to exist that would help predict, which candidate crystal structure is most stable.

Two different, novel approaches to crosslink fully aromatic, rigid-rod aramid chains were studied. First, the new rigid-rod aramid poly[1,4-phenylen-2,5-bis(prop-2-ynyloxy)-terephthalamide] with an inherent viscosity of η inh = 2.74 dL/g was synthesized by low temperature polycondensation of p-phenylendiamine and 2,5-bis(prop-2-ynyloxy)-terephthaloylchloride. The pendant alkinyl moieties allowed thermally induced crosslinking at temperatures higher than 200°C. No weight loss was found due to this treatment, but curing gave rise to the formation of stable radicals. However, no fiber spinning experiments were carried out using this material due to the insufficient stability of the polymer chains against degradation when being dissolved in sulfuric acid. Furthermore, fibers of a rigid rod polyamide containing pyrimidine moieties, produced by polycondensation of bis-silylated 2,5-diaminoprimidine and terephthaloyl dichloride, were spun from nematic solutions. Fibers were crosslinked by complexation with nickel(II)-ions in the swollen state. Both crosslinked and non-crosslinked, otherwise identically processed samples, were characterized by wide-angle X-ray diffraction (WAXD) measurements and mechanical tests. A post-spin heat treatment was employed to improve the low degrees of orientation and crystallinity that the untreated fibers in general showed. The dominating crystal structures of both fiber samples are similar to Modification II of the well characterized fibers from poly(p-phenylene-terephthalamide) (PPTA). The number and size of the morphological defects in the crosslinked fibers was significantly higher than in the non-crosslinked samples. The influence of the annealing on the mechanical fiber properties and the molecular order in the fibers was investigated. The values of all mechanical parameters were considerably lower in the case of the crosslinked fibers, probably due to the collapse of the entire supramolecular order and fiber morphology.

2′,5′-Diamino-4-(dimethylamino)-4′-nitrostilbene was polymerized with terephthaloyl dichloride and 2,6-difluoroterephthaloyl dichloride in a low-temperature solution polycondensation to give two novel, fully aromatic rigid-rod polyamides. Nematic solutions of these polymers were processed into fibers and films that were characterized by wide-angle X-ray diffraction measurements. A post-spin annealing process was employed to enhance the chain orientation in the fibers. The dominating crystal structure was found to be similar to “modification II” of the fibers from poly(p-phenyleneterephthalamide), but the fibers do not suffer a comparable structural transformation upon heat treatment. A corona-discharge poling process gave rise to a remarkable gain in average chain orientation in the films. Again the crystal structure was found to be similar to “modification II”. The aramids investigated in this work represent a new approach to the design of liquid-crystalline rigid-rod polymers, where different mechanisms of orientation can be combined. In the nematic phase, the rigid-rod molecules form highly oriented domains that can be oriented using mechanical processes such as shearing. In addition the stilbene units that are fixed in the polymer backbone with their dipole moments oriented transverse to the main chain can be oriented in an electric field. The combination of both orientation mechanisms seems to cause a synergistic effect, probably since each affects different levels of the polymer microstructure in the solid.