Chemistry Reference
In-Depth Information
Load applied
(a)
Load removed
Time
(b)
Elastic recovery
Delayed recovery
Permanent
deformation
Elastic
deformation
Time
(c)
Time
FIGURE 1.3
Deformation of various polymer types when stress is applied and unloaded. (a)
Crosslinked ideal elastomer. (b) Fiber. (c) Amorphous plastic.
semicrystalline polyethylene. Their strain recovery behavior is variable, but the
elastic component is generally much less significant than in the case of fibers
(
Fig. 1.3c
). Increased temperatures result in lower stiffness and greater elongation
at break.
Some chemical species can be used both as fibers and as plastics. The fiber-
making process involves alignment of polymer molecules in the fiber direction.
This increases the tensile strength and stiffness and reduces the elongation at
break. Thus, typical poly(hexamethylene adipamide) (nylon-66, structure 1-6)
fibers have tensile strengths around 100,000 psi (700 MN/m
2
) and elongate about
25% before breaking. The same polymer yields moldings with tensile strengths
around 10,000 psi (70 MN/m
2
) and breaking elongations near 100%. The macro-
molecules in such articles are randomly aligned and much less extended.
Synthetic fibers are generally made from polymers whose chemical composi-
tion and geometry enhance intermolecular attractive forces and crystallization.
A certain degree of moisture affinity is also desirable for wearer comfort in textile
