Biomedical Engineering Reference
In-Depth Information
PO
4
3-
tetrahedron
P
2
O
7
4-
pyrosphosphate
group
modifier cations
(Ca
2
+
, Na
+
...)
Figure 4.5
Schematic of invert glass structure.
O
O
O
O
P
O
P
+
H
2
O
P
+
OH
HO
P
Figure 4.6
Schematic of structural water causing cleavage of P-O-P linkages.
cations and phosphate groups, which is why they are often referred to
as
invert
glasses.
As described above, the structure of phosphate glasses depends on their
phosphate (P
2
O
5
) content and on the content of network modifier oxides,
such as CaO or Na
2
O. However, water can also disrupt the phosphate
network, thereby acting like a network modifier. Water, for example
from the atmosphere, can disrupt POP linkages, creating POH groups
instead (Figure 4.6). This reaction occurs readily for vitreous P
2
O
5
and most ultraphosphate glasses; however, it can also affect actual
chain lengths in metaphosphate glasses, resulting in shorter phosphate
chains than predicted based on the composition. With decreasing
chain length, phosphate glasses are less prone to water attack and
subsequent chain scission, and are therefore more stable when exposed
to atmospheric humidity.
Glass properties, such as crystallisation tendency, mechanical prop-
erties or stability against hydrolytic attack, depend not only on the
phosphate content (and the phosphate glass structure) but also on
the size and the charge (i.e. the charge-to-size ratio) of the network
