Biomedical Engineering Reference
In-Depth Information
A systematic study of fl uorine containing ionomer glasses by Stamboulis
and Hill [29, 33-36] using MAS-NMR spectroscopy gave a lot of information on
the structural role of fl uorine in these glasses. Various authors proposed in the
past that fl uorine was present either as fl uorite clusters [36] or bound to silicon as
Si-F [37] or, as reported recently [32, 38-43], bound to aluminium as Al-F type
species. Hill et al. [42] suggested that Si-F bonds were not present in a glass
composition based on 2SiO
2
- Al
2
O
3
- CaO - CaF
2
but instead fl uorine was bound to
Al as Al-F rather than being present as CaF
2
clusters. Kohn et al. [38] and Zeng
and Stebbins [40] found evidence of the presence of aluminium in fi ve - and
six-fold coordination in fl uoro-aluminosilicate glasses but no evidence for the
presence of Si-F bonds for the specifi c glass compositions. However, they
concluded that in fl uoro-alumino-silicate glasses the formation of Si-F bonds if
present could be signifi cant. The latter probably depends strongly on the glass
composition as well as the fl uorine content. Specifi cally, Stamboulis et al. [35] in a
recent study in glasses of the general composition 4.5SiO
2
- 3Al
2
O
3
- 1.5P
2
O
5
- (5 - x)
CaO - xCaF
2
where
x
= 0 - 3, reported that Si - F - Ca(n) species were clearly present
in the high fl uorine containing glasses. Figure 12.5 shows the
19
F MAS - NMR
spectra of the above series of glasses. The lower fl uoride containing glasses
(LG120 and LG115) showed a peak with two maxima at
150 ppm sug-
gesting that fl uoride was preferentially associated with Ca i.e., F-Ca(n) and with
both calcium and aluminium i.e., Al - F - Ca(n). The higher fl uoride containing
glasses LG26, LG96 and LG99 with
x
= 2.0, 2.4 and 3, respectively, showed the
formation of an additional peak at
−
90 and
−
−
125 ppm that increased in intensity with
increasing fl uoride content.
Zeng and Stebbins [40] had reported that in fl uoro - alumino - silicates a signal
from Si-F-Ca(n) groups could occur in the range of
123.4 to 134.5 ppm. There-
fore, it is possible that in low fl uoride containing glasses, the Si-F-Ca(n) species
could be present in small amounts but could be largely hidden by overlapping
peaks. On the other hand, high fl uorine containing glasses showed clearly that
Si - F - Ca(n) species increased signifi cantly with fl uorine content. Similar glasses
based on the composition 2SiO
2
- Al
2
O
3
- (2 - x)CaO - xCaF
2
were studied by
Stamboulis et al. [29] and the
19
F MAS-NMR spectra showed two peaks at
−
−
150
and
90 ppm attributed to Al - F - Ca(n) and F - Ca(n) species, respectively. There
was no evidence of any Si-F-Ca(n) species present even at high fl uoride contents
(Figure 12.6). In all glasses however, it was observed that Al-F-Ca(n) species
increased with fl uoride content.
Figure 12.7 shows the relative proportion of four- Al(IV), fi ve - Al(V) and six -
Al(VI) fold coordinated aluminium as a function of fl uoride content in a series of
glasses based on the composition 4.5SiO
2
- 3Al
2
O
3
- 1.5P
2
O
5
- (5 - x)CaO - xCaF
2
.
It is worth noticing that the proportion of Al(IV) decreased linearly with
fl uoride content of the glass, whereas the proportion of Al(V) increased with fl uo-
ride content up to
x
= 1 and was almost kept constant between
x
= 1 and 3, but the
proportion of Al(VI) increased with higher fl uoride contents (
x
−
1). The peaks
corresponding to both Al(IV) and Al(VI) in the
27
Al MAS - NMR spectra (Figure
12.8) shifted slightly to a more negative direction with increasing fl uoride content
≥
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