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

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 Sisingle bondOsingle bondAl linkages and the Q44Al species a silicon with four Sisingle bondOsingle bondAl 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 Alsingle bondOsingle bondPO32− type species indicating local charge compensation of Al3+ and P5+ in the glass structure. 19F MAS-NMR indicated the presence of Fsingle bondCa(n), Alsingle bondFsingle bondCa(n), Fsingle bondSr(n), Alsingle bondFsingle bondSr(n) and Alsingle bondFsingle bondNa(n) species where Fsingle bondM(n) indicates a fluorine surrounded by n next nearest neighbour cations and Alsingle bondFsingle bondM(n) represents a fluorine bonded to aluminium with the metal, M in close proximity charge balancing the tetrahedral AlO3F species. The proportion of Alsingle bondFsingle bondM(n) species increased with increasing fluorine content of the glass and lower non-bridging oxygen contents. There was no evidence of Sisingle bondF 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.