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Biomaterials & Bioengineering |
Infrared Spectrometric Study of Acid-degradable Glasses
E.A.P. De Maeyer1,
R.M.H. Verbeeck2 and
C.W.J. Vercruysse
Department of Dental Materials Science, Institute for Biomedical Technologies (IBITECH), Ghent University, De Pintelaan 185 (P8), B-9000 Ghent, Belgium;
1 Senior Research Assistant of the FSR-Flanders (Belgium);
Correspondence: 2 corresponding author, ronald.verbeeck{at}rug.ac.be
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ABSTRACT
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The composition of glasses used in glass-ionomer cements affects their leaching behavior and hence the properties of the cement. The aim of this study was to correlate the composition and leaching behavior of these glasses with their infrared absorption characteristics. The wavenumber of the absorption band of the Si-O asymmetric stretching vibration  AS shifts to a higher value with decreasing content of mono- and bivalent cations in the glass. This effect can be ascribed to the influence of these extraneous ions on the glass network order and connectivity. Preferential leaching of these ions induces an increase of AS and a general modification of the band profile. The results can be correlated with the x-ray diffraction characteristics of the glass.
Key Words: acid-degradable glasses • glass-ionomer cements • infrared spectroscopy • glass composition • leaching
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INTRODUCTION
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Glass-ionomer cements (GIC) set by an acid-base reaction between a polyalkenoic acid and a degradable glass which is basically a SiO2-Al2O3-LF2 glass (with L any bivalent cation) (Wilson and McLean, 1988). In the silica network of these glasses, aluminum partly replaces silicon, inducing negative sites which are compensated for by mono- and/or bivalent cations (e.g., Na+, Ca2+, Sr2+), the "network dwellers". The same cations can also act as network modifiers, disrupting the connectivity of the silica network structure (Shelby, 1997). The primary setting of GIC results from the crosslinking of the polyalkenoic acid with Al3+ and L2+ which are leached from the acid-degradable aluminosilicate glass (Wilson and Mclean, 1988; Nicholson, 1998). A secondary hardening mechanism occurs by the formation of hydrated silicate (phosphate) in the matrix, increasing the material’s strength with time (Wasson and Nicholson, 1993; Matsuya et al., 1996; Nicholson, 1998). Hence, the cement-forming ability of a glass and the properties of the resulting GIC will critically be determined by the acid degradability.
Previous studies demonstrated that the acid degradability and leaching stoichiometry of the glass are significantly affected by its composition (De Maeyer et al., 1998,1999). Moreover, these properties are correlated with the x-ray diffraction (XRD) characteristics of the glass (De Maeyer and Verbeeck, 2001). XRD provides structural information such as medium-range order and interatomic distances. To obtain complementary information on specific molecular entities, we investigated, in the present study, the infrared spectra of some glasses used in commercial GIC before and after acid degradation. Up to now, IR studies on the pure glass component of GIC are lacking.
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MATERIALS & METHODS
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The GIC glasses investigated (Table ) were leached by means of a pH-stat method (De Maeyer et al., 1998). A 0.250-g quantity of GIC glass was degraded at 37 ± 0.1°C in 200 mL 0.01 mol/L acetic acid (pH = 3.4), the pH of which was maintained constant by the controlled addition of 0.1 mol/L HCl. After t min, the suspension was filtered on a 0.22-µm Millipore filter, and the residue was dried at room temperature under vacuum.
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Table. Vibrational Wavenumbers and Composition of Commercial Degradable GIC Glasses Before and After Acid Degradation (N = 2)
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A quantitative parameter representative of the cationic substitutions in the glass before and after leaching was calculated by:
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with (X) the molar content of constituent X in the glass phase (Table ).
FTIR spectra of the glass samples dispersed in CsBr were recorded before and after leaching in the range 400-2000 cm–1 with a resolution of 1 cm–1, by means of a Mattson Galaxy 6030 FTIR spectrophotometer. We determined the exact positions of some specific band maxima by fitting the band with an asymmetric double-sigmoid profile and a horizontal baseline.
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RESULTS
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The absorptions in the region 400-850 cm–1 differ from the general spectra of amorphous silica (Soda, 1961) and vary for the glasses investigated (Fig. 1a). The distinct band near 460 cm–1 can be ascribed to a O-Si-O bending mode  B (Efimov, 1996; Poenar et al., 1996). The wavenumber B of this band hardly varies as a function of the glass chemical composition (Table). A band at  565 cm–1 is due to the Si-O-Si bending vibration (Miller and Lakshmi, 1998), while bands near 635 and 730 cm–1 can be ascribed to symmetric stretchings of Si-O (Efimov, 1996; Miller and Lakshmi, 1998).
The strong band between 1000 and 1200 cm–1 in the spectra of Fig. 1a is characteristic of the asymmetric stretching vibration AS of Si-O (Soda, 1961; Efimov, 1996). The asymmetry toward higher wavenumbers indicates that the band consists of several components, which correspond to different asymmetric stretching modes and to differences in the vibrations of bridging (Si-O-Si) and non-bridging (terminal Si-O– groups) oxygens (Efimov, 1996). The latter are induced by the incorporation of the network modifiers. The position of maximum absorption of the global band contour AS varies between 1007 and 1056 cm–1 before leaching (Table).
The acid degradation hardly changes B and increases AS for all glasses (Table). The IR band profiles of VBG, KSG, PBG, and KMG are hardly affected by leaching, whereas for KFG, VMG, PFG, CFSG, and DYG, they change significantly (Fig. 1b). For the latter glasses, an absorption band near 800 cm–1 appears, and the intensity of the band near 730 cm–1 decreases. Moreover, the AS band is narrowed, and two shoulders near 1180 and 950 cm–1 appear. A distinct band near 1640 cm–1 appears in all spectra after leaching.
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DISCUSSION
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The symmetric stretching of Si-O normally occurring near 800 cm–1 shifts to 730 cm–1 due to the presence of aluminum (Farmer et al., 1979). However, a further interpretation of the absorptions in the region 500-850 cm–1 is difficult, since crystalline constituents (such as Al2O3, metal fluorides, TiO2 ...) also contribute to absorptions in this region (Dorsey, 1968; Svehla, 1976; Matsuya et al., 1984; Miller and Lakshmi, 1998), and extraneous ions incorporated into the glass phase can induce specific absorptions as well (Farmer et al., 1979; Poenar et al., 1996).
The fact that B hardly varies as a function of glass composition and is hardly affected by leaching indicates that the bending vibration is relatively insensitive to the incorporation of extraneous ions into the silica network. Only KSG exhibits a relatively low B value, possibly due to the interference of a TiO2 absorption at lower wavenumber (Miller and Lakshmi, 1998).
In contrast, the change of AS with the glass composition indicates that the energy of this vibration mode is significantly affected by the glass composition. The increase in AS indicates that the distortions in the arrangement of the SiO4 tetrahedra and/or within the tetrahedra increase (Poenar et al., 1996). AS—and hence the network distortion—increases with increasing P content in phosphosilicate glasses (Poenar et al., 1996). In contrast, AS decreases with increasing Al/Si ratio of silicate glasses, indicating that Al diminishes the distortions in the silica tetrahedra and/or network (Farmer et al., 1979). Miller and Lakshmi (1998) also report a "weakening of the Si framework" resulting from the "substitutional insertion" of Al, Ti, or Zr, as reflected by a decrease of AS.
In view of the composition of the GIC glasses of the present study (Al/Si >> P/Si; De Maeyer et al., 1998), it can be expected that the effect of Al on AS will dominate the effect of P. However, a correlation between the Al/Si ratio and AS is lacking (Table). This is in line with previous observations that the XRD characteristics are predominantly affected not by the Al content of the glass, but by its content of mono- and bivalent cations, as reflected by B (De Maeyer and Verbeeck, 2001). Since part of the Na+ and L2+ ions acts as dweller and compensates for the negative charges created by an Al3+ Si4+ substitution, the Al content in the silica glass network is indirectly taken into account in B. The other part of mono- and bivalent cations acts as modifier. The corresponding change in the network connectivity induces the splitting of the AS band as well as a shift of the band maximum with increasing B (Fig. 2a). This agrees with the decrease of AS with increasing Na content observed for Na-silicate glasses (Efimov, 1996).
The glasses whose IR spectra are not significantly affected by the acid degradation (VBG, KSG, PBG, and KMG) dissolve nearly stoichiometrically (De Maeyer et al., 1999). Hence, their mean Si-O bond environments are left unchanged, and relatively high amounts of Si are leached from the glass. In corresponding GIC, this Si can diffuse in the polysalt matrix and induce a secondary hardening by repolymerization into hydrated silicate.
The band profile of glasses that dissolve non-stoichiometrically (KFG, VMG, PFG, CFSG, and DYG; De Maeyer et al., 1999) changes significantly after leaching (Fig. 1b). The appearance of the band near 800 cm–1 and the decrease of the band intensity near 730 cm–1 are related to the preferential leaching of Al and other extraneous ions from the silica network, resulting in a shift of the symmetric Si-O stretching vibration band (Farmer et al., 1979). The changes in the band profile between 900 and 1200 cm–1 agree with the evolution of the band contour reported by Matsuya et al.(1984,1996) when the GIC sets. The absorptions near 1180 cm–1 and between 1018 and 1073 cm–1 both originate from the asymmetric Si-O stretching, but their attribution to the Si-O-Si bridge and the Si-O– terminal group is still unclear (Efimov, 1996). The band near 950 cm–1 is assigned to the O-H deformation vibration of silanol groups (Soda, 1961; Farmer et al., 1979; Matsuya et al. 1984,1996; Miller and Lakshmi, 1998).
The pronounced band near 1640 cm–1 that appears after leaching is caused by the bending vibration of water (Svehla, 1976; Davis and Tomozawa, 1996). Its intensity is higher for the glasses exhibiting significant modifications of the Si-O band profiles (Fig. 1). This water is probably included in the sample, since it apparently cannot be removed by drying. The absorptions near 950 and 1640 cm–1 indicate that a silica gel is formed upon acid degradation: Si-OH groups ( 950 cm–1) formed after preferential leaching of the metals will partially repolymerize with the inclusion of water ( 1640 cm–1). Since, in the present study, pure GIC glasses without polyacid component were investigated, it can be concluded that this silica gel phase is formed as a surface layer on the glass particles and not as a hydrated silica matrix phase interpenetrating the metal polyacrylate matrix (Nicholson, 1998). Such a surface layer can hinder/prevent further degradation, as evidenced by the decrease in the dissolution rate of these glasses with time (De Maeyer et al., 1998).
The increase of AS upon leaching is related to a decrease of B (Table, Fig. 2a). This effect is most pronounced for glasses that dissolve extensively and not stoichiometrically, such as DYG (De Maeyer et al., 1999). The decrease of B, the increase of AS, and the narrowing of the global band upon leaching reflect the preferential leaching of Al3+, L2+, and Na+. This leaves a less-substituted silica network with a more limited range of Si-O bond environments in which Si-O absorbs at a higher wavenumber.
The scattering of the experimental points with respect to the best-fitting (B, AS) line calculated by a weighted regression (Fig. 2a) is due to experimental errors on AS and B, to different relative silanol/repolymerized silica contents in the glasses after leaching, and to the fact that B is only an approximative parameter (De Maeyer and Verbeeck, 2001). A possible amorphous phase separation occurring during glass preparation, as suggested by Hill et al.(1992), could also contribute to the latter.
The present observations can be related to the results of a previous XRD analysis, where B was found to be correlated with the position of the major amorphous diffraction band <2 > (the so-called first sharp diffraction peak) and hence with the mean size of the characteristic polyhedron of medium-range order, as well as with the Si-Si distance and the Si-O-Si angle in the glass (De Maeyer and Verbeeck, 2001). More specifically, it was suggested that an increase in B is related to a decrease in the mean Si-Si distance and Si-O-Si angle in the SiO4 tetrahedra. Since the medium range order ( <2>) and the molecular arrangement ( AS) are highly correlated (Fig. 2b), the present results then indicate that a decrease in the Si-O-Si angle could be caused by a decrease in the distortions in the (arrangement of the) SiO4 tetrahedra. The latter is the result of the disruption of the network connectivity upon incorporation of mono- and bivalent cations into the silica network ( increasing B) and results in lower AS values.
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ACKNOWLEDGMENTS
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This study was supported by grants from the National Fund for Scientific Research (Belgium) and the "Bijzonder Onderzoeksfonds" of Ghent University, which are gratefully acknowledged. The authors thank the GIC manufacturers for providing the GIC glasses.
Received for publication October 23, 2000.
Revision received January 31, 2002.
Accepted for publication May 28, 2002.
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Journal of Dental Research, Vol. 81, No. 8,
552-555 (2002)
DOI: 10.1177/154405910208100810
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ABSTRACT
INTRODUCTION
) and after (
) leaching.