Page 424 - Handbook of Properties of Textile and Technical Fibres

The chemistry, manufacture, and tensile behavior of polyamide fibers 397

Core
Crack
Skin propagation
path

Figure 12.32 Crack propagation path in typical polymeric fiber skin (Kausch, 1985).

relaxation processes in crystalline and noncrystalline regions. Lim et al. (1989)
confirmed that the existence of an anisotropic phase in a polymer is critical to the for-
mation of fibrils. They also discussed how an anisotropic material possesses intrinsic
low entropy characteristics, which are reminiscent of fibrillar structures. Cook and
Gordon (Atkins and Mai, 1985; Cook and Gordon, 1964) determined the stress
components at the crack tip in an anisotropic solid and confirmed that the crack prop-
agates in the direction that displays the least material strength.
A typical crack propagation path of an oriented polymer, provided by Kausch
(1985), is shown in Fig. 12.32.
Polyamide fibers are characterized as a transversely isotropic polymer. In essence,
as molecular orientation in the fiber direction increases, initial modulus in the axial
direction increases dramatically. The converse to this statement also holds true, in
that the initial modulus measured in the transverse direction decreases as the molecular
orientation increases along the fiber axis (Steven, 1995). The tensile modulus, when
measured perpendicular to the chain direction, is considerably lower because of the
weak secondary van der Waals’ bonds between the chains.
Hadley et al (Ward and Hadley, 1993; Hadley et al., 1969) determined the five
elastic constants for oriented filaments of PA 66, where the orientation was determined
in terms of draw ratio and optical birefringence. Further studies showed that X-ray
diffraction measurements were also pertinent to the determination of mechanical
anisotropy. Murthy et al. (1995) examined the effects of drawing and annealing on
anisotropy in PA 6 fibers using X-ray diffraction and other methods. In agreement
with the results from Ward and Hadley (1993), they confirmed that the distinct rami-
fications of the drawing process increases crystallinity, crystalline perfection, and
molecular orientation. In addition, they determined that the anisotropy in the amor-
phous region increases with increasing draw ratio.