NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 78

3. Elastomers or rubbers are characterized as linear polymers with lim-
ited cross-linking. At atmospheric temperatures the secondary bonds
have melted. The cross-linking enables the material to return to its
original shape when unloaded. Three forms of elastomers are polyiso-
prene (natural rubber), polybutadiene (synthetic rubber), and poly-
chloroprene (Neoprene).
4. Natural materials are characterized as being grown in all plant matter.
The primary material of interest is wood, which is composed of cellu-
lose, lignin, and protein.

Other than the natural polymers, the balance of the organic solids are
produced from refining and processing crude oil. In general, these products
are classified as plastics. The properties of these materials are highly vari-
able. Mechanical properties depend on the length of the polymer chains, the
extent of cross-linking, and the type of radical compound. All of these fac-
tors can be controlled and altered in the production process to alter the
material properties.

■ 2.4.1 Polymer Development, Structure,

and Cross-Linking
The physical structure of the polymer chain grossly affects the mechanical
response of plastics. The word polymer literally means multiple “mer”
units. The mer is a base molecule that can be linked together to form the
polymers. Figure 2.21 shows the structure of a simple ethylene molecule
and the development of a polymer. The square boxes are carbon atoms and
the open circles are hydrogen. Forming the polymer requires breaking the
double bond, activating the monomer, and allowing it to link to others to
form a long chain. The end of the chain either links to other chains or a ter-
minator molecule, such as OH. Chains with useful mechanical properties
require at least 500 monomers. The number of monomers in the chain
defines the degree of polymerization; commercial polymers have a degree of
polymerization of to
The complexity of the linear chain polymer is increased by replacing
hydrogen atoms with side groups or radicals, as shown in Figure 2.21. The
radicals or side groups can be aligned on one side of the chain ( isotactic ),
symmetrically on alternate sides of the chain ( sindiotactic ), or in a random
fashion ( atactic) . The radicals can range from simple to complex molecules.
For example, polyvinyl chloride has a Cl radical, polypropylene has a
radical, and polystyrene has The ability of the polymer chains to
stack together is determined by the arrangement of the side chains. The
simple chains can fold together into an orderly arrangement, whereas the
complex side groups prevent stacking, leading to the amorphous nature of
these materials.
More complex linear polymers are formed when two of the hydrogen
atoms are replaced by different radicals. Polymethylmethacrylate, (Plexiglas),
has the radicals

10 3 10 5 .

CH 3

C 6 H 5 .

CH 3 and COOCH 3 . As the complexity of the radicals and the