Compendium Supplement Seopodent May 2010 : Page 2
Figure 1 Structures of the dimer acid dimethacrylate monomers. the degree of conversion of the resin phase during com- posite curing, reductions in conversion as a means to limit shrinkage should not be considered a practical option. The shrinkage coefficient is specific to the type of monomer chemistry, and for most methacrylate-based monomers, it is fixed.2,3 The development of the silorane resin chemistry by 3M™ ESPE™,4 as well as the use of thiol-ene step-growth polymerization in dental resins,5,6 are examples of materials that offer lower molar shrinkage coefficients as compared to methacrylate mon omers. However, reliance on alternative reaction mechanisms to achieve a reduced shrinkage coeffi- cient may also necessitate revised bonding and filler surface treatment procedures, as well as introduce unconventional polymerization behavior. This suggests opportunities for the development of new methacrylate-based monomers with low initial reactive group concentrations as a means to low- shrinkage dental resins. OBJECTIVE The aim of this study was to formulate and evaluate novel dental resins based on C36 dimer acid core-structured di- methacrylate monomers, which in turn are derived from a natural source of linoleic acid. A straightforward characterization 2 of conversion, polymerization shrinkage, and polymerization stress was the initial basis of this work; however, this study was expanded to include an examination of the effect of heterogeneous polymer structure on the evolution of po- lymerization shrinkage and stress. MATERIALS AND METHODS The dimer acid-derived monomers—DADMA I, II, and III (Figure 1)—were prepared as previously described.7 The DADMA monomers were formulated in various com- positions with UDMA and/or ethoxylated bisphenol A dimethacrylate (Bis-EMA). A control resin composed of 2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl] propane (Bis-GMA) and TEGDMA at a 7:3 mass ratio was used. All the commercial monomers were donated by Esstech Inc (www.esstechinc.com). Camphorquinone (0.3 wt%) (Sigma-Aldrich®, www.sigmaaldrich.com) and ethyl 4-dimethylaminobenzoate (0.8 wt%) (Sigma-Aldrich) were used as the photoinitiator system, which was activated by 60 secs of visible light exposure at 500 mW/cm2 from a dental curing unit (VIP™, Bisco, www.bisco.com). Real-time photopolymerization kinetics was moni- tored by near-infrared spectroscopy (Nexus 670, Thermo Compendium—Volume 31 (Special Issue 2)
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