NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 81

separation between them, resulting in a volume that is greater than
required for tightly packed, motionless molecules. This excess volume is
termed the free volume. As the material cools, the motion of the molecules
is reduced and the viscosity increases. At a sufficiently low temperature,
the molecules are no longer free to rearrange; thus, their position is fixed
and the free volume becomes zero. This is the glass transition temperature.
Below this temperature, the secondary bonds bind the material into an
amorphous solid; above this temperature, the material behaves in an elas-
tic manner. Figure 2.23 illustrates the concept of melting and glass transi-
tion temperatures, and The glass transition temperature of Plexiglass
is 100°C; at room temperature it is a brittle solid. Above it becomes
leathery and then rubbery. for natural rubber is it is flexible at
all atmospheric temperatures. However, when frozen, say in liquid nitro-
gen, it becomes a brittle solid.

T m T g .

T g ,

T g - 70°C;

■ 2.4.3 Mechanical Properties

The mechanical behavior of polymers is directly related to the degree of ori-
entation of the molecules and the amount of cross-linking by covalent
bonds. The modulus of a polymer is the average of the stiffness of the bonds.

T m

Temperature

(a)

T m
(b)

Temperature

Glass
transition

Free volume

FIGURE 2.23 Melting point and glass
transition temperatures: (a) perfect crystalliza-
tion, (b) imperfect crystallization, and (c) glass
formation.

T g T m

(c)

Temperature