IONOMER GLASSES: DESIGN AND CHARACTERIZATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 421

of 1.16 nm, close enough to that of calcium 0.94 nm and therefore, strontium can
be substituted for calcium. Strontium not only can be substituted for calcium in
the glass structure, but also in apatite structures. Solid solution of mixed calcium
strontium hydroxyapatite and pure strontium hydroxyapatite were reported and
produced from aqueous solutions [70]. Hill et al. [69], investigated the infl uence
of substituting strontium for calcium on the structure and the nucleation and crys-
tallisation behaviour of an apatite stoichiometric ionomer glass based on similar
series of glasses as above. 27 Al MAS-NMR spectroscopy showed that the usual
aluminium species present in a calcium glass were replaced by F-Sr(n) and Al-F-
Sr(n) species in the strontium glass and mixed species in a calcium-strontium
glass. But as expected, there was only a little infl uence on the glass structure. Sig-
nifi cant infl uence of the substitution was observed, however, on the nucleation
and crystallisation behaviour of strontium and mixed calcium-strontium glasses.
Replacement of calcium by strontium promoted surface crystallisation of the
apatite phase. Low strontium substitution resulted in a calcium fl uorapatite phase
being formed fi rst, whereas higher strontium substitution resulted in a mixed
calcium/strontium fl uorapatite. Also, strontium substitution resulted in the for-
mation of an anorthite phase instead of the mullite phase at Tp2. The crystallisa-
tion of anorthite was the result in the reduction in quantity of the apatite phase
leaving increased quantities of calcium that subsequently would form anorthite.
In the 100% strontium substituted glass the fi rst phase was as expected pure
strontium fl uorapatite but the second phase was not an anorthite. XRD analysis
gave a peak at about 7.5° 2 theta, which at the time was not identifi ed [69] .
However, further studies revealed that the second phase in the strontium ionomer
glass was indeed a Sr-feldspar phase (SrAl 2 Si 2 O 8 ) most probably Sr - hexacelcian
[71]. Generally, two main changes were observed in the strontium substituted
glasses: strontium hindered apatite formation and inhibited bulk nucleation. The
observed changes could be possibly attributed to the lower lattice energy of
strontium fl uorapatite as well as the possibility that strontium might have sup-
pressed the amorphous phase separation (APS) that was observed in the calcium
ionomer glasses [63].
A whole series of cation substitutions in the above ionomer glasses are being
studied including Mg and Ba cation substitutions. The general observation is that
the effect of cations on the structure and crystallisation of the ionomer glasses is
strongly connected with the size of the cation. In Ba-substituted glasses, for
example, the substitution did not seem to have a strong infl uence in the silicon
and phosphorous environment of the glass. FTIR studies showed, however that
Ba substitution can lead to a lower inter-tetrahedral angle in Si-O-Si and a less
strained glass network [72]. The crystallisation process of the glasses is strongly
infl uenced by the barium substitution; in low barium contents the main phases
are calcium fl uorapatite, mullite and some mixed barium-calcium fl uorapatite,
whereas in high barium contents, there is no fl uorapatite forming but mostly a
barium aluminosilicate phase together with crystalline BaPO 4 [73] . A lot more
attention should be given to the cation substitution as morphologies and conse-
quently the properties of ionomer glasses change.