B2O3/SiO2 substitution effect on structure and properties of Na2O–CaO–SrO–P2O5–SiO2 bioactive glasses from molecular dynamics simulations

Abstract

The effect of B₂O₃/SiO₂ substitution in SrO-containing 55S4.3 bioactive glasses on glass structure and properties, such as ionic diffusion and glass transition temperature, was investigated by combining experiments and molecular dynamics simulations with newly developed potentials. Both short-range (such as bond length and bond angle) and medium-range (such as polyhedral connection and ring size distribution) structures were determined as a function of glass composition. The simulation results were used to explain the experimental results for glass properties such as glass transition temperature and bioactivity. The fraction of bridging oxygen increased linearly with increasing B₂O₃ content, resulting in an increase in overall glass network connectivity. Ion diffusion behavior was found to be sensitive to changes in glass composition and the trend of the change with the level of substitution is also temperature dependent. The differential scanning calorimetry (DSC) results show a decrease in glass transition temperature (T_g) with increasing B₂O₃ content. This is explained by the increase in ion diffusion coefficient and decrease in ion diffusion energy barrier in glass melts, as suggested by high-temperature range (above T_g) ion diffusion calculations as B₂O₃/SiO₂ substitution increases. In the low-temperature range (below T_g), the E_a for modifier ions increased with B₂O₃/SiO₂ substitution, which can be explained by the increase in glass network connectivity. Vibrational density of states (VDOS) were calculated and show spectral feature changes as a result of the substitution. The change in bioactivity with B₂O₃/SiO₂ substitution is discussed with the change in pH value and release of boric acid into the solution.