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

doi:10.1016/j.jnoncrysol.2006.12.001

Journal of Non-Crystalline Solids

Volume 353, Issue 3, 1 March 2007, Pages 237-243

https://doi.org/10.1016/j.jnoncrysol.2006.12.001 Get rights and content

Abstract

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.

Introduction

Apatite based materials are popular as biomaterials, especially in orthopaedic and dental applications for hard tissue restoration. There are many studies on bioactive glass-ceramics intended for use as biomaterials [1]. Apatite–mullite glass-ceramics based on SiO₂–Al₂O₃–P₂O₅–CaO–CaF₂ glasses have been developed by Hill and co-workers [2], [3]. The glasses crystallized to form fluorapatite (Ca₅(PO₄)₃F) and mullite (3Al₂O₃2SiO₂) with an appropriate heat treatment. As both of the crystals interlock on crystallization, the glass-ceramics exhibit high fracture toughness values. In a previous study [2], the effect of fluoride content of the glass on the nucleation and crystallization behavior was studied by means of differential scanning calorimetry (DSC) and X-ray diffraction analysis. In the absence of fluoride, the glass crystallized to form tricalcium phosphate (Ca₃(PO₄)₂) and anorthite (CaO · Al₂O₃ · 2SiO₂), while fluorapatite and mullite crystallized with increasing fluoride contents. It is not clear why the addition of fluoride caused fluorapatite and mullite formation, but the crystallization process is expected to be dependent on the glass structure. A better grasp of the glass structure would aid understanding of this process. The mechanism of apatite crystal nucleation in these glass-ceramics is thought to occur as a result of prior amorphous phase separation (APS) or glass-in-glass phase separation. This view is supported by the optimum nucleation temperatures being close to the experimentally determined glass transition temperatures and the presence of two loss peaks in dynamic mechanical thermal analysis experiments on nucleated glasses [4]. The two loss peaks are thought to correspond to two glass transition temperatures corresponding to the two glass phases present. One phase is thought to be calcium, phosphate and fluoride rich, that subsequently crystallizes to fluorapatite and the other phase is thought to be aluminium and silicon rich, that eventually crystallizes to mullite. Related glasses containing alkali metal that also crystallize to fluorapatite have been studied by Hoeland et al. [5], [6], [7]. These glasses show marked evidence of APS on examination by transmission electron microscopy. The APS step in these latter glasses is thought to be a result of their alkali metal content, which is thought to aid the phase separation process by disrupting the glass network.

There are several studies on the structure of alumino-silicate glasses containing fluoride [8], [9], [10], [11], [12]. In a ¹⁹F NMR study Zeng and Stebbins [8] showed that fluoride with one Al and several Ca atom neighbours was a predominant species in an anorthite glass containing fluoride. It was also reported that fluoride additions to alumino-silicate glasses resulted in fivefold and sixfold coordinated Al species in the glass [9]. Maeda et al. [10] reported that most of the fluoride is attached to Al and Ca rather than Si in SiO₂–Al₂O₃–CaO–CaF₂ glasses. There are limited reports on the structure of multi-component glasses containing fluoride, used for glass-ceramics [13], [14], whereas there are no reports on multi-component glasses containing fluoride and used as the basis for glass ionomer dental cements. Glass ionomer cements are systems that set via an acid–base reaction between an ion leachable glass powder and an aqueous mixture of acid such as polyacrylic acid. In the present study, the results of ²⁷Al, ²⁹Si, ³¹P and ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) for 4.5SiO₂–3Al₂O₃–1.5P₂O₅–(5−z)CaO–zCaF₂ glasses with z = 0–3 are reported. The aim of the study is to elucidate the effect of fluoride content on the glass structure.

Section snippets

Preparation of the glasses

The glasses were based on 4.5SiO₂–3Al₂O₃–1.5P₂O₅–(5−z)CaO–zCaF₂ composition with z = 0–3. The glass compositions studied did not evolve any significant amount of silicon tetrafluoride during melting. The glasses were designed to contain sufficient amounts of calcium and phosphate ions to locally charge balance the aluminium atoms so that they remain in a fourfold coordination state. There is a strong evidence to suggest that charge deficient ${AlO}_{4}^{-}$ groups are locally charge balanced by ${PO}_{4}^{3 -}$

²⁹Si MAS NMR spectra

Fig. 1 shows ²⁹Si MAS-NMR spectra of the glasses with various fluoride contents. The silicon spectrum shows a large broad peak at −90 ppm that shifts slightly towards the negative direction with increasing fluoride content. The small change in the chemical shift of the glasses is quite reasonable because the glass compositions studied have a constant ratio of network forming cations (Si⁴⁺, Al³⁺ and P⁵⁺) to network modifying cation (Ca²⁺). Engelhardt et al. [17] described that the chemical shift

Conclusion

²⁷Al MAS-NMR spectra showed an asymmetric large broad peak at around 50 ppm for all glasses. With increasing fluoride content, two new peaks appeared: a shoulder peak and a small peak at 30 ppm and −5 ppm, respectively. These two peaks were assigned to five and sixfold coordinated aluminium, respectively. The ²⁹Si MAS-NMR spectra for the glasses showed a large broad peak at −90 ppm that shifted slightly towards the negative direction with increasing fluoride content. The fluoride addition caused an

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The FTIR spectra of the BG composites exhibited split bands at approximately 560 and 600 cm⁻¹ corresponding to a apatite formation in the surface or on the surface under all immersion conditions. SEM showed the presence of a reacted layer of glass particles in the composite surface and the presence of a surface layer of apatite in AS7.
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Published by Elsevier B.V.

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

Abstract

Introduction

Section snippets

Preparation of the glasses

29Si MAS NMR spectra

Conclusion

Clin. Mater.

J. Non-Cryst. Solids

Sol. State Nucl. Mag. Res.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Solid State Chem.

J. Non-Cryst. Solids

Geochim. Cosmochim. Acta

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Am. Ceram. Soc.

J. Mater. Sci.: Mater. Med.

²⁹Si MAS NMR spectra