Chemistry Reference
In-Depth Information
the influence of high-energy radiation or free radicals released by decomposition
of peroxides in the polymer compound. The main advantage of this cross-linking
is enhanced dimensional stability under load and elevated temperatures.
Polyethylene and poly(vinyl chloride) are classed as thermoplastics, however,
since their major uses hinge on their plasticity when heated.
1.8
Elastomers, Fibers, and Plastics
Polymers can be usefully classified in many ways, such as by source of raw mate-
rials, method of synthesis, end use, and fabrication processes. Some classifications
have already been considered in this chapter. Polymers are grouped by end use in
this section, which brings out an important difference between macromolecules
and other common materials of construction. This is that the chemical structure
and size of a polymeric species may not completely determine the properties of
an article made from such a material. The process whereby the article is made
may also exert an important influence.
The distinction between elastomers, fibers, and plastics is most easily made in
terms of the characteristics of tensile stress
strain curves of representative sam-
ples. The parameters of such curves are nominal stress (force on the specimen
divided by the original cross-sectional area), the corresponding nominal strain
(increase in length divided by original length), and the modulus (slope of the
stress
strain curve). We refer below to the initial modulus, which is this slope
near zero strain.
Generalized stress
strain curves look like those shown in
Fig. 1.2
.For
our present purposes we can ignore the yield phenomenon and the fact that
such curves are functions of testing temperature, speed of elongation, and charac-
teristics of the particular polymer sample. The nominal stress values in this
figure are given in pounds of force per square inch (psi) of unstrained area
(1 psi
10
3
N/m
2
).
Elastomers recover completely and very quickly from great extensions, which
can be up to 1000% or more. Their initial moduli in tension are low, typically
up to a range of about 1000 psi (7 MN/m
2
) but they generally stiffen on stretch-
ing. Within a limited temperature range, the moduli of elastomers increase as the
temperature is raised. (This ideal response may not be observed in the case of
samples of real vulcanized rubbers, as discussed in
Section 4.5
.) If the tempera-
ture is lowered sufficiently, elastomers become stiffer and begin to lose their
rapid recovery properties. They will be glassy and brittle under extremely cold
conditions.
Figure 1.3a
illustrates the response of an elastomer sample to the applica-
tion and removal of a load at different temperatures. The sample here is
assumed to be cross-linked, so that the polymer does not deform permanently
under stress.
6.9
5
3
