Characterisation of fluorine containing glasses by 19F, 27Al, 29Si and 31P MAS-NMR spectroscopy
Introduction
The composition and structure of the fluoro-alumino-silicate glasses used to form glass polyalkenoate (ionomer) cements is critical to cement formation. It has been established that in simple CaOAl2O3SiO2 glasses that the Al:Si ratio of the glass plays a critical role in determining the reactivity of the glass and subsequent cement properties.1, 2 However, in more complex glasses containing fluorine and phosphate the Al:Si ratio is much less important.3 Very little is known about the structural role of fluorine and phosphate in ionomer glasses. In many phospho-alumino-silicate glasses the P5+ cation is known to locally charge balance charge deficient AlO4− tetrahedra forming AlOP bonds.4 An understanding of the structural role of fluorine has significant implications for both the degradation kinetics of the glass and for subsequent fluoride release and caries inhibition. Early studies of ionomer glasses1, 2 suggested that fluorine formed SiF and AlF bonds forming SiO3F and AlO3F tetrahedra and thus it was assumed that fluorine replaces bridging oxygens by non-bridging fluorines in the glass structure. Kohn et al.5 found evidence for AlF bonds as well as coordinations states of five, Al(V) and six, Al(VI) for aluminium in a sodium fluor-alumino-silicate glass and suggested the presence of AlF5 and AlF6 species being present. More recently Zeng and Stebbins 6 using 19F MAS-NMR have found evidence for AlFCa(n) and FCa(n) where n represents the number of Ca next nearest neighbours in a calcium fluoro-alumino-silicate glass of the following composition: 2SiO2Al2O30.5CaO0.5CaF2. Stebbins et al.7 also found evidence for five and six coordinated aluminium using 27Al MAS-NMR. In other fluoro-silicate glasses have found evidence for fluorine complexing the cations of highest charge to size ratio and have suggested that this is a general feature of such glasses.8 Maeda et al.9 reported that most of the fluorine is attached to Al and Ca rather than Si in SiO2Al2O3CaOCaF2 glasses.
This paper will review the structure of both model fluoro-alumino-silicate glasses used for experimental glass polyalkenoate cements using solid state magic angle nuclear magnetic resonance spectroscopy (MAS-NMR). MAS-NMR is a powerful technique for probing the structure of amorphous glasses and provides information on the local environment of selected species and their next nearest neighbours.
The results for the model glasses will be used to analyse and interpret the spectra of glasses that form the basis of commercial cements such as Fujii IX and Ketac Molar.
Section snippets
Materials and methods
The following series of glasses have been studied:
- A
Series – 2SiO2–Al2O32-XCaOXCaF2 – a phosphorus free series where oxygen is replaced by fluorine.
- B
Series – 4.5SiO2–3Al2O31.5P2O55-XCaOXCaF2 – a phosphorus containing series where oxygen is replaced by fluorine and the ratio of Ca:P is 1.67 corresponding to that in apatite, Ca5(PO4)3OH/F.
- C
Series – 4.5SiO2–3Al2O31.5P2O53-YCaO2-ZCaF2 YSrOYSrF2 – a series where Sr is substituted for Ca in a glass of the B Series. Strontium is the commonest species used
Results
The results for the NMR spectra of the A series glasses are shown in Fig. 1, Fig. 2, Fig. 3. The results for the B Series and C series glasses are given in Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9. The results for the D series glasses in Fig. 10, Fig. 11. The spectra for the commercial glasses are shown in Fig. 12, Fig. 13, Fig. 14.
All the 29Si spectra showed a single peak with a chemical shift between −89 and −110 ppm.
The high field 27Al spectra of the low fluorine content glasses of the
Discussion
Engelhardt et al.13 proposed that the chemical shift of Si in alumino-silicate glasses is dependent on both number of bridging oxygen (m) per SiO4 unit and number of aluminium (n) connected by oxygen with the SiO4 unit. When a structural unit of the alumino-silicate glass network is expressed as Si(OSi)m−n(OAl−)n(O−)4−m, (Qm(nAl), 4 ≥ m ≥ n ≥ 0), the chemical shift increases with decreasing m or increasing n. The chemical shift of the 29Si in the A series glasses based on 2SiO2–Al2O32-XCaOXCaF2
Conclusions
All the glasses studied have silicon in Q33Al and Q44Al type environments. There is no evidence for the presence of SiF bonds in any of the glasses studied. Fluorine exists as FCa(n), FSr(n), AlFCa(n) AlFSr(n) and AlFNa(n) species in ionomer glasses. Substituting Sr for Ca has virtually no influence on the structure of the glass. The AlF(M)n species are favoured at the expense of FM(n) species by higher fluorine contents in the glass.
Phosphorus preferentially locally charge balances aluminium
Acknowledgement
The authors would like to thank the EU DGXII for funding ULTRASET No.: G5RD-CT-2001-00475.
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