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

Abstract

Fibers of 21 rigid-rod aromatic polyamides with different substitution patterns at 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 G0 and the axial compressive strength σC were determined for each fiber sample before and after 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 G0 and σC. In general, the results provided by our experiments are consist with this theoretical approach.