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

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

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).

The 29Si spectra indicate a predominantly Q33Al and Q44Al structure where the Q33Al species represents a silicon with one non-bridging oxygen and three Si-O-Al linkages and the Q44Al species a silicon with four Si-O-Al 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 Al-O-PO3(2-) type species indicating local charge compensation of Al3+ and P5+ in the glass structure. 19F MAS-NMR indicated the presence of F-Ca(n), Al-F-Ca(n), F-Sr(n), Al-F-Sr(n) and Al-F-Na(n) species where F-M(n) indicates a fluorine surrounded by n next nearest neighbour cations and Al-F-M(n) represents a fluorine bonded to aluminium with the metal, M in close proximity charge balancing the tetrahedral AlO3F species. The proportion of Al-F-M(n) species increased with increasing fluorine content of the glass and lower non-bridging oxygen contents. There was no evidence of Si-F bonds in any of the glasses.

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.