Materials for engineering, 3rd Edition - (Malestrom)

34
Materials for engineering
Liquid
A
Supercooled
liquid
B
Specific volume
E
C
Glass
F
D
Crystal
T g
T m
Temperature
1.28 Variation in specific volume with temperature for crystalline
materials and glasses.
bind the molecules of the polymer into an amorphous solid or glass. By
analogy with the behaviour of inorganic glasses previously described, above
a certain temperature, known as the glass transition temperature (T g ), thermal
energy causes the polymer molecules to rearrange continuously, which, in
turn, causes the volume of the polymer to increase. As the temperature rises,
a polymer becomes first leathery then rubbery, until eventually it has the
characteristics of a viscous liquid. A curve of specific volume with temperature
again appears as in Fig. 1.28, with an inflection appearing at T g . The actual
value of T g again depends on the rate of temperature change, for example the
lower the cooling rate, the lower the value of T g .
On the level of the molecular structure, the glass transition temperature is
the temperature for a particular polymer at which molecular rotation about
single bonds in the backbone becomes possible. Rotation is thermally activated
and the easier it is, the lower the value of T g for a particular polymer. Thus,
T g increases with increasing strength of secondary bonds between chains, by
cross-linking between chains and by the presence of side-branches. Plasticizers
reduce T g , as they increase the space between chains, increasing chain mobility.
In the following chapter we will review the ways in which the mechanical
properties of engineering materials may be assessed.