Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties – Part I: NMR

The effect of the variation in phosphate (P2O5) content on the structure of two series of bioactive glasses in the quaternary system SiO2–Na2O–CaO–P2O5 was studied. The first series (I) was a simple substitution of P2O5 for SiO2 keeping the Na2O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate ( PO 4 3 - ) , therefore as P2O5 was added the Na2O and CaO content was varied to provide sufficient Na+ and Ca2+ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P2O5 phase was included (NC′). 31P and 29Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. 31P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P2O5 glass shown to be partially crystalline by X-ray diffraction was heat treated, and the 31P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium–sodium orthophosphate phases and amorphous sodium orthophosphate respectively. 31P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. 29Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q3 species in the glass and a reduction in Q2 with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp 29Si-NMR resonance at −88 ppm, indicating a crystalline Q2 six-membered combeite (Na2O · 2CaO · 3SiO2) silicate-type phase, which was confirmed by powder X-ray diffraction. 29Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment.