X-ray microdiffraction study of chain orientation in poly(p-phenylene terephthalamide)

Single fibers of poly(p-phenylene terephthalamide) (PPTA; trade names: Kevlar(29), Kevlar(49), and Kevlar(149)) have been scanned through a 3 mu m diameter X-ray beam and the degree of orientation (f(c)) determined at every point. All samples show a gradient in f(c) values from the skin to the core. The highest degree of orientation was found at the edges of the fibers. This effect is particularly strong for Kevlar(29). In addition, ripples in orientation were observed along the fiber axis for Kevlar(29), but on a smaller scale than the transversal gradient. Upon tensile deformation up to about 3 GPa applied stress, the transversal f(c) anisotropy is gradually reduced and disappears at the highest stress values. Kevlar(29) attains at about 1.5 GPa applied stress the same degree of orientation as Kevlar(49).