IONOMER GLASSES: DESIGN AND CHARACTERIZATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 408

silicate glasses, which is the most important type of ionomer glasses used today
for commercial glass ionomer cements.
12.4 DESIGN
The models for glass structures today are all based on the original ideas of
Zachariasen and are grouped under the general term of Random Network Theory,
although Zachariasen ' s fi rst approach did not use the term random but vitreous
network, instead. Zachariasen's rules were formed initially to explain the glass
formation and were not intended to be used as a discussion of structural models.
However, the rules expanded through wide usage and empirical observations into
a set of rules for formulating models of glass structures. Table 12.1 summarises
Zachariasen's rules, stating the conditions for the formation of a continuous 3D
glass network with no indication, however of the degree of long range order,
together with three modifi ed rules for more complex glass systems [18, 19].
The model itself states that glass consists of random assemblies of SiO 4 tetra-
hedra that are linked in the corners to form chains. The simplest composition that
is capable to form a glass network must consist only of such tetrahedra. This type
of structure, however, would be resistant to an acid attack due to its neutral elec-
tron charge. By aluminium inclusion, the behaviour of the glass can be altered as
aluminium provides only three positive charges and therefore, according to
Loewenstein's rules [20], aluminium is forced to take up a four-fold coordination
[AlO 4 ] 5− creating a defi ciency of positive charge in the aluminosilicate network,
which means that a negative surplus charge resides in each SiO 4 tetrahedron
(Figure 12.2). The negative charge can be compensated by cations like Ca 2+ , Na + ,
Mg + or Al 3+ that can play the role of the glass network modifi er.
Loewenstein's rules restrict the formation of the glass network in that, when-
ever two tetrahedra are linked by one oxygen bridge, the centre of only one of
them can be occupied by aluminium, whereas the other centre must be occupied
by silicon or another small ion of four or more electrovalence, like phosphorus.
Similarly, whenever two aluminium ions are neighbours to the same oxygen anion
TABLE 12.1. Zachariasen ' s Rules for Glass Formation (Random Network Theory)
1. Each oxygen atom is linked to no more than two cations
2. The oxygen coordination number of the network cation is small
3. Oxygen polyhedra share only corners and not edges or faces
4. At least three corners of each oxygen polyhedra must be shared in order to form a 3D
network
5. The samples must contain a high percentage of network cations which are surrounded
by oxygen tetrahedral of triangles
6. The tetrahedral or triangles share only corners with each other
7. Some oxygens are linked only to two network cations and do not form further bonds
with any other cations.
[19] .