Structure–property relation in poly(p-phenylene terephthalamide) (PPTA) fibers

Structure parameters of various poly(p-phenylene terephthalamide fibers have been investigated using WAXD and correlated with mechanical properties. The mechanical properties examined were modulus E and strength σ; the pertinent structural parameters include orientation angle φ200, lattice constants a, b, c, paracrystalline parameter gII,apparent crystal sizes ACS110, ACS200, ACS00l, intensity ratio I110/I200 and transverse crystallinity X. The parameters, c, gII and I110/I200 are found to be interrelated and to provide indications of nonreversible chain conformational changes due to post-treatment. It is concluded that the fiber modulus is determined by the combination of the orientation of the crystallites and the paracrystalline parameter through the following equation:(10)1/Ef=(1/E0+D1gII2)+A〈sin2φ〉in which Ef is the fiber modulus; gII the paracrystalline parameter; φ the orientation angle; and E0, D1 and A the material constants. This relationship is derived from our proposed morphological model in which crystallites are: (a) formed from chains have nonlinear conformations; and (b) packed with an orientation distribution. The correlation of structure with strength has also been studied. In addition, different types of Kevlar® fibers, Kevlar® 119, Kevlar® 29, Kevlar® 49 and Kevlar® 149 show slight, systematic, variations in structure. In particular, all Kevlar® fibers except Kevlar® 149 show the forbidden 001 diffraction reflection, which has been related to conformational differences.