Structural variations of bioactive glasses obtained by different synthesis routes

Two bioactive glasses with different chemical compositions (mol%) 46.2SiO2-26.9CaO-24.3Na2O-2.6P2O5 (45S5) and 40SiO2-54CaO-6P2O5 (A2) were synthesized by the use of sol-gel and melt-quenching techniques. The effect of synthesis method on glass structure was investigated using X-ray diffraction, FTIR, Raman, XPS, 29Si and 31P MAS-NMR spectroscopic methods. The results show that the synthesis route has significant influence on the glass structure. Both melt-derived A2 and 45S5 glasses exhibit fully amorphous structure, while gel-derived ones, stabilised at 700 °C, reveal the presence of crystalline silicate and phosphate phases. Gel-derived glasses exhibit more polymerized structure compared to melt-quenched ones. Phosphorus is present in the orthophosphate type environment (Q0) together with some pyrophosphate (Q1) species and it does not take part in the formation of Si-O-P bonds. This indicates that phosphorus acts as a glass structure modifier and forms phosphate-rich phase separated from a silica-rich one. The theoretically predicted network connectivity is consistent with the experimental determination only for melt-derived glasses, assuming silicon as the only network former.