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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.

Fibers of three rigid-rod aromatic copolymers, produced by polycondensation of 2,2,6,6-tetraoxo-1,3,5,7-tetrahydro-2,6-dithia-s-indacene-4,8-diamine (DSDA), p-phenylenediamine and terephthaloyl dichloride, were spun from nematic solutions and characterized by wideangle X-ray-diffraction (WAXD) measurements and mechanical tests. A post-spin heat treatment was employed to improve the low degrees of orientation and crystallinity exhibited generally by the untreated fibers. The dominating crystal structures of the copolymers are similar to “modification I” of the well known fibers from poly(p-phenyleneterephthalamide) (PPTA). Thermally induced crosslinking of the fibers in the solid state was performed simultaneously with post-spin treatment. The influence of the annealing conditions on the mechanical fiber properties and the molecular order in the fibers was also investigated. All mechanical parameters were time-dependent, probably due to the generation of stable radicals during heat treatment. The radicals may give rise to chain scission.

Fibers of three constitutionally isomeric rigid-rod polymers, produced by polycondensation of 2,6-dichloro-p-phenylenediamine and terephthaloyl dichloride, were spun from nematic solutions and 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 showed. The dominating crystal structure of constitutionally ordered head-to-head/tail-to-tail fibers seems to be very similar to “Modification II” of the fibers from poly(p-phenyleneterephthalamide) (PPTA), but the fibers do not suffer a comparable structural transformation upon heat treatment. Influences of the constitutional regularity in the chains on the crystal structure, structural order and mechanical properties of the fibers could be observed. Both the observed crystal structure and the experimentally determined mechanical properties agree well with the results of detailed atomistic modeling predictions.