Characterization of the chemical interactions between 4-MET and enamel by Raman spectroscopy

Objectives. The goal of this investigation was to use Raman spectroscopy to study the chemical interactions between 4-methacryloxyethyl trimellitic acid (4-MET), a hydrolized form of 4-methacryloxyethyl trimellitate anhydride (4-META) and powdered substrates such as bovine enamel powder and synthetic carbonate-containing apatite. Methods. Powdered samples were prepared by immersing them in 4-MET/methyl methacrylate (MMA) solution for 15 min, 3 h and 24 h. They were then dried for spectroscopy. An experimental Raman microprobe equipped with a cryogenically-cooled camera was employed to acquire spectra of the samples. Results. Spectra of samples wetted for 24 h strongly indicate that there is strong ionic interaction between Ca2+ ions on the powdered sample surface and 4-MET. Spectra of the 15-min samples, however, suggest only some degree of hydrogen bonding. The spectrum of the 3-h 4-MET/enamel powder sample showed features intermediate between the two. Significance. Results presented in this paper show that salt formation between 4-MET and dental substrates is not likely to be instantaneous. On the contrary, the rate of such reaction is probably too slow relative to that of polymerization of the co-monomers to be important under clinical conditions. The function of the 4-MET in the 4-MET/MMA resin system appears, therefore, to be wetting the tooth surface through hydrogen bonding, which then allows MMA monomers to diffuse and penetrate deeper through the surfaces. The primary bonding mechanism of this system is more likely due to a micro-mechanical locking mechanism.