Influence of strontium substitution on structure and crystallisation of Bioglass® 45S5

Abstract

The changes in the structure of 45S5 Bioglass® and its heat-treated versions upon SrO substitution were investigated by solid-state multinuclear magic angle spinning nuclear magnetic resonance (MAS-NMR), X-ray diffraction and differential scanning calorimetry. A series of the melt-derived glasses with SrO gradually substituted for calcium (0, 25, 50, 75 and 100%) on a molar basis was synthesised. The glass transition temperature, T_g, and crystallisation temperature decreased upon SrO substitution. Glasses with 75 and 100% of SrO substitution showed a minor crystalline phase. ³¹P MAS-NMR showed that phosphorus is present as orthophosphate in the entire glass series; ²⁹Si MAS-NMR data showed a predominantly Q² silicate network. A small step change in the position of the ³¹P and ²⁹Si MAS-NMR signals found between the glasses with 25 and 50% of SrO substitution is attributed due to an increase in the ring-type fraction of the Q² silicate glass over the mainly chain topology in the original 45S5 glass. On heat treatment combeite (Na₂Ca₂Si₃O₉) and combeite-type (Na₂CaSi₂O₆) silicate phases were identified. For glasses with increasing SrO contents the equivalent Sr²⁺ substituted phases were formed (viz. Na₂Sr₂Si₃O₉ and Na₂SrSi₂O₆) above 600 °C. The polymerisation degree of phosphorus increased and in addition to orthophosphate amorphous pyro- and meta-phosphate species were observed on heat treatment between 600 and 650 °C, the amount of which reduced with SrO substitution. Above 800 °C, changes in ³¹P MAS-NMR spectra suggested that several phosphate containing phases crystallised and the fraction of the remaining glass was below the detection limit. In contrast to the SrO containing glasses, in the original 45S5 composition a significant fraction of phosphorus remained amorphous even after heat treatment at temperatures above 800 °C.