Page 401 - Handbook of Properties of Textile and Technical Fibres

374 Handbook of Properties of Textile and Technical Fibres

CO CO
HN HN

2 4 6
1 3 5 OC NH OC NH

CO CO
HN HN
PA 6 α form

CO
CO HN
CO HN
HN

5 3
6 4 1 1 3 5 NH OC NH
2 2 4 6 NH OC
OC
CO
CO HN
CO HN
HN
PA 66 α form
Figure 12.8 Hydrogen bonds of the a forms of PA 6 and PA 66.

utilization. Crystalline and noncrystalline phases and their characteristics, such as size
distribution, overall content, and orientation have major roles. Fiber morphology plays
an important role in tensile properties of polyamide fibers. During deformation, various
structural units are responsible for load transfer or resistance against deformation. These
units are in fact changed as well and the result is yielding when intermolecular barriers
to segmental rearrangements are overcome. For undrawn fibers, the strain softening
appears after the yield point, i.e., reduction in stress to a level corresponding to plastic
flow. At higher strains, the stress increases again as the chain molecules orientate, in a
process known as “strain hardening.” The balance of strain softening and strain
hardening is important for toughness. At only very small strains, are the response elastic
and structural phases not substantially changed.
Crystallinity, crystal size and species, molecular orientation, and amorphous
structures are among the most important elements affecting basic tensile characteristics
such as fiber tenacity, modulus, and elongation at break (Aharoni, 1997).
Technical polyamide fibers generally have high crystallinity, large crystal size, and
highmolecularorientationinbothcrystallineandamorphousphases(Najafietal.,2017b).
Polyamides are characterized by a capability to form hydrogen bonds between
the polymeric chains. Hydrogen bonds promote rapid crystallization during the
spinning process, hindering further chain mobility and molecular orientation for
obtaining enhanced tensile properties. The amide groups can form strong hydrogen