NMR Study on the Adhesion Efficacy of Experimental Phosphonic Acid Monomers

Three experimental self-etching primers—consisting of N-methacryloyl-ω-aminoalkyl phosphonic acid (NMωP) with different methylene chain lengths and N-methacryloyl glycine (NMGly)—were formulated. The influence of methylene chain length in NMωP derivatives on the chemical nature of calcium salts was examined following their application to tooth components. Bond strengths of experimental self-etching primers created with these monomers to enamel and dentin were also investigated. Nuclear magnetic resonance spectroscopy showed that NMωPs decalcified tooth components with formation of calcium salts, which changed from calcium hydrogen phosphonate to calcium phosphonate with increase in methylene chain length within the NMωP structure. Disparity in calcium salt formation was related to increases in bond strength to enamel from 18 to 24 MPa. However, bond strength to dentin remained unchanged (22 MPa). The relative dependency of bond strength on monomer methylene chain length was probably attributable to the sites where these NMωP calcium salts had deposited on the bonding substrates.