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

Fibers of 21 rigid-rod aromatic polyamides with different substitution patt erns at their aromatic rings, produced by polycondensation of functionalize d p-phenylenediamine and functionalized terephthaloyl dichloride and spun f rom nematic solutions as described in the accompanying paper (B. H. Glomm, M. C. Grob, P. Neuenschwander, and U. W. Suter, Macromol. Chem. Phys.), wer e characterized by the mechanical properties most relevant for compressive failure. In particular, the torsional moduli G(0) and the axial compressive strength sigma(C) were determined for each fiber sample before and after e mploying a post-spinning heat treatment optimized to improve the degrees of orientation and the crystallinity of the fibers. The dependence of the mea sured values on the structural parameters of the respective polymers was st udied, leading to the result that the volume of the side-chains of the stud ied 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 stren gth was scrutinized in the light of the theoretical approach of DeTeresa, A llen, and Farris, and Allen, which suggests the existence of a proportional ity between G(0) and sigma(C). In general, the results provided by our expe riments are consist with this theoretical approach.