Biomedical Engineering Reference
In-Depth Information
F
O
O
Al
O
Si
O
Figure 12.4.
Schematic illustration showing the structural role
of calcium and fl uorine in ionomer glasses. The calcium ions
disrupt the glass network forming non-bridging oxygens as well
as charge balancing charge defi cient AlO
4
tetrahedra. Fluorine
replaces bridging oxygens and forms non-bridging fl uorines
[24].
O
-
Ca
2+
O
F
O
P
O
Al
O
O
O
reactivity of the glasses resulting in shorter working and setting times but leading
to increased Young's modulus and compressive strength of cements. On the other
hand, fracture toughness of cements is not infl uenced signifi cantly by the addition
of fl uorite in the glass composition [24].
Generally, fl uorine has two major roles in the glass structure. Firstly, it lowers
the refractive index of the glass and enables a match to the polysalt matrix giving
rise to optically translucent cements, a property of particular importance for ante-
rior restorations at the front of the mouth, where the appearance of the cement is
critical and secondly, it results in fl uorine complexes present in the polysalt matrix
that lead to fl uoride release from the set cement that has a caries inhibitory role.
Furthermore, Wood and Hill [28] observed a signifi cant reduction in the glass
transition temperature by increasing the fl uorine content in a series of ionomer
glasses of the composition 2SiO
2
- Al
2
O
3
- (2 - x)CaO - xCaF
2
. The reduction was
attributed to the replacement of BOs by non-bridging fl uorines reducing the
network connectivity and allowing network motion at a lower temperature.
However, recent MAS-NMR (Magic Angle Spinning Nuclear Magnetic Reso-
nance) studies [29] showed that the above explanation was only partly true.
12.5.1 Structural Characterization of Ionomer Glasses by Solid
State MAS-NMR Spectroscopy
Solid state MAS-NMR spectroscopy is a powerful technique to study the struc-
ture of complex amorphous and crystalline solid materials. For nuclei with spin
I
= 1/2, high resolution MAS-NMR spectra allow the observation of narrow lines
by removing effects of dipolar and chemical shift anisotropies. This provides
structural information on the local surroundings of nuclei in different environ-
ments. MAS-NMR of abundant quadrupolar nuclei, on the other hand, can
provide some structural information for complex structures with the analysis of
the isotropic chemical shift, the quadrupolar interaction, and their respective dis-
tributions. The half-integer spins
I
1/2 are subjected to strong quadrupolar inter-
actions among the 2
I
allowed transitions and the central one (+1/2,
>
1/2) is usually
observed. The broadening of the central transition comes primarily from the
second-order quadrupole interaction, which is not completely averaged to zero
by the magic angle spinning (MAS) [30]. This produces an anisotropic line which
shape depends on the site symmetry and width on the magnitude of the electric
−
Search WWH ::
Custom Search