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 sigma; the pertinent structural parameters include orientation angle phi (300), lattice constants a, b, c, paracrystalline parameter g(H) apparent crystal sizes ACS(110), ACS(200), ACS(001), intensity ratio I-110/ I-200 and transverse crystallinity X. The parameters, c, g(H) and I-110/I-200 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: 1/E-f = (1/E-0 + D(1)g(H)(2)) + A(sin(2) phi) (10) in which E-f is the fiber modulus; g(H) the paracrystalline parameter; phi the orientation angle; and E-0, D-1 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 (R) fibers, Kevlar (R) 119, Kevlar (R) 29, Kevlar (R) 49 and Kevlar (R) 149 show slight, systematic, variations in structure. In particular, all Kevlar (R) fibers except Kevlar (R) 149 show the forbidden 001 diffraction reflection, which has been related to conformational differences. (C) 2001 Elsevier Science Ltd. All rights reserved.