Structural characterization of ionomer glasses by multinuclear solid state MAS-NMR spectroscopy

In the present study we report the results of 29Si, 27Al, 31P and 19F magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) of 4.5SiO2–3Al2O3–1.5P2O5–(5−z)CaO–zCaF2 glasses with z = 0–3 to elucidate the effect of fluoride content on the glass structure. The 29Si MAS-NMR spectra gave a chemical shift of about −90 ppm corresponding to Q3(3Al) and Q4(3Al). The 27Al MAS-NMR showed a large broad central peak around 50 ppm, which is assigned to four-coordinated Al linked via oxygen to P. A shoulder around 30 ppm and a small peak at about 0 to −10 ppm appeared in the 27Al MAS-NMR spectra of the glasses on increasing the fluoride content assigned to five-coordinated and six-coordinated Al species, respectively. The 31P MAS-NMR spectra indicated the presence of Al–O–P bonds. The 31P chemical shift decreased with increasing fluoride content as a result of calcium being complexed with fluoride. This resulted in a reduction of the number of available cations to charge balance non bridging oxygens in phosphorus and an increase in the number of Al–O–P bonds being formed, instead. The 19F spectra indicated the presence of Al–F–Ca(n) and F–Ca(n) species in all the glasses containing fluoride as well as an additional Si–F–Ca(n) species in the glasses with higher fluoride content.