Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR...

The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO₂ by B₂O₃ in the respective phosphosilicate base compositions 24.1Na₂O-23.3CaO-48.6SiO₂-4.0P₂O₅ ("S49") and 24.6Na₂O-26.7CaO-46.1SiO₂-2.6P₂O₅ ("S46"), the latter constituting the "45S5 Bioglass" utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO₄, BO₄, and BO₃ moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P-O-Si and P-O-B^[4] motifs, whose populations were estimated by heteronuclear ³¹P{¹¹B} NMR experimentation. The high Na⁺/Ca²⁺ contents give fragmented glass networks with large amounts of nonbridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO₄ < SiO₄ < BO₃ ≪ PO₄. The BO₄/BO₃ intermixing was examined by double-quantum-single-quantum correlation ¹¹B NMR experiments, which evidenced the presence of all three BO₃-BO₃, BO₃-BO₄, and BO₄-BO₄ connectivities, with B^[3]-O-B^[4] bridges dominating. Notwithstanding that B^[4]-O-B^[4] linkages are disfavored, both NMR spectroscopy and MD simulations established their presence in these modifier-rich BPS glasses, along with non-negligible B^[4]-NBO contacts, at odds with the conventional structural view of borosilicate glasses. We discuss the relative propensities for intermixing of the Si/B/P network formers. Despite the absence of pronounced preferences for Si-O-Si bond formation, the glass models manifest subtle subnanometer-sized structural inhomogeneities, where SiO₄ tetrahedra tend to self-associate into small chain/ring motifs embedded in BO₃/BO₄-dominated domains.