NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 79

CH 2 CH 2

Carbon
Hydrogen

Activated
monomer

Activated
polymer

Te r minator

Polymer

Radical or
side chain

Isotactic

Sindiotactic

Atactic

FIGURE 2.21 Polymer structures.

substitution for hydrogen increase, it becomes more difficult to form regular
patterns.
In thermoplastics, formed from linear polymers, the structure of the mol-
ecules is a blend of amorphous and crystal structure. When there are few side
groups, an ordered structure is produced [Figure 2.22(a)]. As the number of
side groups increases, the structure becomes increasingly random and cross-
links develop. These structures are shown in Figure 2.22(b). Thermosets are
formed from polyfunctional monomers. They are formed in a condensation
reaction; in essence, the reaction bonds two chains together. Since the chains
are formed from polyfunctional crystals, they have an amorphous structure
with extensive cross-linking. Elastomers are formed with linear chains that
have a limited number of cross-links.

■ 2.4.2 Melting and Glass Transition Temperature

The reaction of polymers to temperature depends on the degree to which
the material has crystallized. Highly ordered polymers have a fairly well-
defined transition between elastic and viscous behavior. As the percent of
crystallization decreases, the melting point is not well defined. However,
the point at which these polymers transition to a glass phase is well
defined. At elevated temperatures, the motion of the molecules forces a