Thermally crosslinked rigid‐rod aramids, 2. Fiber spinning and fiber properties

Abstract

Fibers of three rigid-rod aromatic copolymers, produced by polycondensation of 2,2,6,6-tetraoxo-I ,3,5,7-tetrahydro-2,6-dithia-s-indacene-4,8-diamine (DSDA), p-phenylenedianiine 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@-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 timedependent, probably due to the generation of stable radicals during heat treatment. The radicals may give rise to chain scission.Poly(pphenyleneterephtha1amide) (PFTA), as a typical representative for rigid-rod polymers, has a tensile strength of 3,4 GPa compared to a compressive strength in the range of 400 MPa2).Different authors have described the typical fiber buckling or formation of kinkbands caused by axial compressive failure3-'). Recent work on the structural and morphological anisotropy of the aramid fibers suggested a stress coupling phenomenon that leads to a dependence of the axial compressive strength on the shear modulus2). The shear modulus in turn has quantitatively been explained in terms of the local, molecular structure for PFTA. The crystal structure is well known3S9-'I). On the molecular level, the fibers consist of hydrogen-bonded planes parallel to the fiber axis. Two adjacent planes are connected only by comparatively weak van der Waals interactions. Rutledge and Suter 13) have calculated the different shear moduli in and perpendicular to these planes by detailed atomistic simulations. This computational approach gives rise to the assumption that a gliding of two adjacent Part I: cf. ref. 14). b,

An investigation of novel approaches in order to provide crosslinked fully aromatic polyamide chains

“…The stiff polymer with its limited chain mobility prevents the radicals from recombining. A similar effect was found for other thermally crosslinked aramids [25][26][27][28] . In those cases the radicals were found to be responsible for decreasing fiber properties.…”

“…1) was spun to fibers. The fibers were prepared by the dryjet wet spinning process 27,40,[41][42][43][44][45][46]53) with equipment constructed for the spinning of small amounts of polymer. The apparatus and the spinning procedure have been described for experiments with PPTA 40,46,53) .…”

“…The first class of said copolymers could be crosslinked without any significant mass loss in the solid state by brief exposure to 400 8C in nitrogen, and fiber spinning experiments processing these copolymers were also reported [22][23][24] . In the case of the latter copolymer class, both fiber spinning experiments from sulfuric acid and characterization of monofilaments before and after thermal crosslinking were described 26,27) . Other authors tried to crosslink by thermal cleavage of halogens 28) or by a c-radiation treatment [29][30][31] .…”

“…The polymer was spun into fibers by a dry-jet wet spinning process described elsewhere 27,40,[41][42][43][44][45][46] . The fiber properties and crystal structures were investigated after a post-spin heat treatment (annealing) under argon.…”

“…The influence of such crosslinking by complexation on the fiber packing was investigated by further WAXD measurements in the same way as described in more detail in a previous study on thermally crosslinked rigid rod aramids [25][26][27] .…”