Civil Engineering Reference
In-Depth Information
However, many synthetic macromolecules have a relatively simple struc-
ture being formed by a chemical process called polymerization, which
consists in joining together repeating (
poly
) small units (
mers
) to form a
macromolecule called a
polymer
.
Polymers with long linear chains and a high degree of symmetry are
called
linear
(e.g., thermoplastics), and those with side branches are called
branched
polymers. If the macromolecules are highly interconnected
through chemical cross-linking of linear polymers by means of chemical
reactions, they become a three-dimensional network (e.g., thermosets).
They are, therefore, called three-dimensional polymers. The mechanical
behavior of cross-linked molecules is different from those without cross-
linking. Examples of polymer types are as follows:
•
Linear polymer, e.g., poly(vinyl chloride)
%
(CH
2
)
n
%
CHCl
•
Branched polymer, e.g., natural rubber (polyisoprene)
%
C
Η
=
CH
%
C
=
CH
2
2
|
C
Η
3
•
Network polymers, e.g., silicones, epoxies, and urethane
foams
-
R
O
|
|
%
Si
%
O
%
Si
%
O
%
|
O
silicone
|
%
Si
%
O
%
|
R
