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

Structure parameters of various poly(p-phenylene terephthalamide fibers hav e been investigated using WAXD and correlated with mechanical properties. T he mechanical properties examined were modulus E and strength sigma; the pe rtinent structural parameters include orientation angle phi (300), lattice constants a, b, c, paracrystalline parameter g(H) apparent crystal sizes AC S(110), ACS(200), ACS(001), intensity ratio I-110/ I-200 and transverse cry stallinity X. The parameters, c, g(H) and I-110/I-200 are found to be inter related and to provide indications of nonreversible chain conformational ch anges due to post-treatment. It is concluded that the fiber modulus is dete rmined by the combination of the orientation of the crystallites and the pa racrystalline 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 rela tionship is derived from our proposed morphological model in which crystall ites are: (a) formed from chains have nonlinear conformations; and (b) pack ed with an orientation distribution. The correlation of structure with stre ngth has also been studied. In addition, different types of Kevlar (R) fibe rs, Kevlar (R) 119, Kevlar (R) 29, Kevlar (R) 49 and Kevlar (R) 149 show sl ight, systematic, variations in structure. In particular, all Kevlar (R) fi bers except Kevlar (R) 149 show the forbidden 001 diffraction reflection, w hich has been related to conformational differences. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.