Page 80 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 43

C

Force B

D
A

Elongation
FIGURE 2.18 Elongation of an elastomer as a function of applied force, stress, where A is the original
“relaxed” state, B is the movement to full extension, C is the point at which the elastomer “breaks,” and D is
the force necessary to pull two separate pieces of elastomer apart.

of chains must be low. The cohesive energy forces between chains should be low permitting rapid,
easy expansion. In its extended state a chain should exhibit high tensile strength, whereas at low
extensions it should have a low tensile strength. Cross-linked vinyl polymers often meet the desired
property requirements. The material, after deformation, should return to its original shape because
of the cross-linking. This property is often referred to as an elastic “memory.” Figure 2.18 illus-
trates force versus elongation for a typical elastomer. As the elastomer is pulled, the largely random
chain segments become “stretched out” forming microcrystalline domains resulting in a decreased
entropy or increased order. Eventually, most of the chains are part of these microcrystalline domains
resulting in further elongation requiring much increased force (stress). This microcrystallinization,
physical cross-linking, also confers to the elastomer a greater brittleness, eventually resulting in the
rubber breaking as additional stress is applied.
Fiber properties include high tensile strength and high modulus (high stress for small strains).
These properties can be obtained from high molecular symmetry and high cohesive energies
between chains, both requiring a fairly high degree of polymer crystallinity. Fibers are normally
linear and drawn (oriented) in one direction, producing higher mechanical properties in that direc-
tion. Typical condensation polymers, such as polyesters and nylons, often exhibit these properties.
o
If the fiber is to be ironed, its T should be above 200 C, and if it is to be drawn from the melt, its T

g g
o
should be below 300 C. Branching and cross-linking are undesirable since they disrupt crystalline
formation, even though a small amount of cross-linking may increase some physical properties,
if effected after the material is drawn and processed. Permanent press garments often have some
cross-linking, ensuring a “remembrance” of the “permanent press.”
Products with properties intermediate between elastomers and fibers are grouped together under

the heading “plastics.” Plastics typically have some flexibility and have dimensional stability, that
is, they act as somewhat flexible solids. Many polymers can act as members of two of these three

categories depending on the treatment of the material. Thus, nylon-66 is fibrous in behavior when it

is treated so that the chains have good alignment and are stretched to both increase this alignment
and to form fibers. Nylon-66 is plastic when it has less alignment, that is, is more amorphous, and

used as a bulk material rather than as a fiber. Polyesters also can be either fibers or plastics under
the same conditions as given for nylon-66. Other materials, such as PVC and siloxanes, can be pro-
cessed to act as plastics or elastomers.

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