Characterisation of fluorine containing glasses by ¹⁹F, ²⁷Al, ²⁹Si and ³¹P MAS-NMR spectroscopy

Abstract 

Objective

The aim of this study is to characterise a range of model and commercially available glasses used to form glass (ionomer) polyalkenoate cements.

Methods

A range of model fluoro-alumino-silicate glasses that form the basis of glass (ionomer) polyalkenoate cements and five commercial glasses have been characterised by ²⁹Si, ²⁷Al, ³¹P and ¹⁹F Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR).

Results

The ²⁹Si spectra indicate a predominantly Q³3Al and Q⁴4Al structure where the Q³3Al species represents a silicon with one non-bridging oxygen and three SiOAl linkages and the Q⁴4Al species a silicon with four SiOAl bonds. Aluminium was found in predominantly four coordinate sites, but glasses with high fluorine contents showed an increasing proportion of five and six coordinate aluminium. In phosphate containing glasses the phosphorus was present as AlOPO₃²⁻ type species indicating local charge compensation of Al³⁺ and P⁵⁺ in the glass structure.

¹⁹F MAS-NMR indicated the presence of FCa(n), AlFCa(n), FSr(n), AlFSr(n) and AlFNa(n) species where FM(n) indicates a fluorine surrounded by n next nearest neighbour cations and AlFM(n) represents a fluorine bonded to aluminium with the metal, M in close proximity charge balancing the tetrahedral AlO₃F species. The proportion of AlFM(n) species increased with increasing fluorine content of the glass and lower non-bridging oxygen contents. There was no evidence of SiF bonds in any of the glasses.

Conclusions

The local structure of the phosphate containing glasses with regard to fluorine, calcium, strontium and phosphate is similar to that of fluorapatite the mineral phase of tooth. This may explain the ease with which these glasses crystallize to fluorapatites and the recently observed mineralization of glass polyalkenoate cements found in vivo.