Structural and Setting reaction studies of Glass Polyalkenoate Cements by MAS- NMR Spectrocopy
Creators: name: Zainuddin, Norhazlin
Publisher: name: Imperial College London
Year: 2009

description: Glass polyalkenoate cements (GPCs) are produced from a reaction between fluoroaluminosilicate glass powder with poly(acrylic acid) (PAA). These cements are widely used in dentistry as adhesive and tooth restorative materials. During the setting reaction, the Al(IV) changes its coordination number to Al(VI) and crosslink to PAA chains. In the present work, the structure of the glass and the setting reaction of GPCs were studied by 29Si, 31P, 19F and 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The influences of alumina content as well as mixed cation such as Ca/Na and Ca/Sr on the glass structure were studied. The Q structure of the glass changed with the alumina content as the number of neighbouring Al in SiO4 network decreased. The decrease in alumina content also resulted in more disrupted glasses and a higher tendency to undergo amorphous separation and crystallization because there are more Ca cation available to form NBO and F-Ca(n). Furthermore, the influence of substituting sodium for calcium was investigated. The Q structure of the glass was not affected with the addition of Na. It was found that Na penetrates the phosphorus and fluorine environments and forms Na/Ca pyrophosphate and F-Ca(2)/Na(1) and Al-F-Na(n) species. The effect of Sr substituting for Ca was also studied. It was suggested that Sr may have a little influence on the glass structure due to their similarity in charge to size ratio. The Sr may slightly affect the phosphorus environment due to the difference in field strength of Ca and Sr. In this study, the setting reaction of GPCs was followed by 29Si, 31P, 19F and 27Al MAS-NMR spectroscopy. All GPCs continue to have a pyrophosphate environment of Q1 Al-O-PO3 3- regardless of the original glass composition. This suggests that the Al(IV) from the Al-O-P bonds are not released from the glass network, thus are not available to crosslink to PAA chains. Therefore, the phosphate content is important for controlling the working and the setting time of GPCs. The conversion of Al(IV) to Al(VI) was determined from the 27Al MAS-NMR data and the Al(VI):Al(IV) + Al(V) ratio was calculated in order to follow the setting reaction of the GPCs. The addition of fluorine in the GPCs was found to enhance the acid attack during the setting reaction. In contrast, it was suggested that Na delays the setting reaction by forming Na polysalts and hence, disrupting the crosslinking of Al3+ to PAA. Although the presence of Sr has little influence on the structure of the glass, the conversion of Al(IV) to Al(VI) is lower for Sr and mixed Ca/Sr cements than Ca cement. The difference is not yet fully understood, but it is suggested that it may be due to the larger size of Sr ions. A commercial GPC, Carbomer® containing fluoroapatite (FAP) designed to remineralise in the mouth was also studied. This study showed the involvement of the FAP component in the setting reaction and hence, the amount of FAP available for promoting the remineralisation process decreases. The Al(V) is present in all GPCs regardless of the setting time. In addition, the participation of Al(VI) from the glass in the setting reaction is not clear because both Al(VI) in the glass and in the cement matrix have very similar chemical shift. The present work suggests that there are two sites of Al(IV) after the setting reaction, which are attributed to the Al(IV) in the original glass and in the degraded layer of the glass. The reduction of Al-F-Na(n) and Al-F-Ca(n) species after the setting reaction and the unavailability of Al-O-P bonds for acid attack suggest that ion exchange between cations (Na and Sr) with protons from PAA may take place during the setting reaction. All GPCs show similar fluorine environments after setting. It was suggested that fluorine maybe bonded to Al in higher coordination states than four. Finally, there is a reconstruction of the silicate network during the setting reaction, as a result of the release of Al3+ and other ions from the glass network. It is assumed that the reconstruction of the silicate network depends on the release rate of the Al(IV). It is hoped that the improved structural understanding of the glass and the setting reaction of the GPCs obtained during this study will lead to the design of new GPCs for specific applications.