Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 33
(Fig. 1.1(a)) which is highly stable and which can link with other tetrahedra by
the sharing of an oxygen atom. Unlike organic polymers, the molecules are not
constrained to form linear chains, but form three-dimensional random networks
(Fig. 1.27). The network has a high degree of mobility at high temperature, to
form a liquid which, in the case of pure silica, has a high viscosity.
This problem is overcome in commercial glasses by introducing other
metal oxides, usually Na 2 O and CaO, which have the effect of breaking up
the network. These network modifiers reduce the cross-linking between the
tetrahedra, making the glass much more easily worked at high temperature.
The way the volume of a given mass of this material changes as it is
cooled is shown in Fig. 1.28. At A, the material is a normal liquid: if it
crystallizes on cooling, then B represents the freezing point at which there is
a sharp decrease in the volume to C, after which the crystalline material will
continue to shrink as the temperature falls, but at a slower rate, to D. In the
case of a glass, which does not crystallize as it cools, shrinkage will occur
along AE (Fig. 1.28). At a particular temperature, depending upon the rate of
cooling, the rate of contraction slows to that along EF, whose slope is similar
to that of CD found in the crystalline material. The temperature at which the
rate of contraction changes is known as the glass transition temperature
(T g ), and its value depends on the rate of cooling of the glass, being lower at
slower cooling rates.
The glass transition temperature in organic polymers
In a polymer that does not crystallize, at low temperatures, secondary bonds
1.27 Silica tetrahedra in a random network to form a glass.