Method: Nanogels of increasing hydrophobicity were prepared from 70:30 molar ratios of 1) hydroxyethyl acrylate (HEA):glycerol-1,3-diglycerolate diacrylate (GDD), 2) hydroxyethyl methacrylate (HEMA):BisGMA, or 3) isobornyl methacrylate (IBMA): urethane dimethacrylate (UDMA). Nanogels where dispersed in 60:40 HEMA:BisGMA resin (control) at 0, 20 or 40wt% with varying ethanol solvent concentrations (0-50wt%). Resin samples were characterized for water-induced phase separation potential, reaction kinetics, viscosity and flexural modulus in desiccated and hydrated conditions.
Result: Resin viscosity was markedly higher at 40wt% nanogel loading, where nanoparticle additives are nearly confluent, compared with analogous 20wt% loaded samples. However, nanogel-modified resin viscosities were very sensitive to ethanol content and viscosities near the nanogel-free control resin could be obtained with addition of 12% solvent. For the control resin, conversion rose from 79% without solvent to 98% when 12 or 25% ethanol was present while further increase to 50wt% ethanol decreased conversion to 63%. The maximum polymerization rate was inversely proportional to the solvent content but positively affected by nanogel loading, particularly with the more hydrophobic nanogels. With 50% ethanol, the IBMA/UDMA nanogel-modified resin still reached 93% conversion. Addition of ethanol had a greater effect on water compatibility in the monomeric state than did nanogel addition; however, wet HEMA/BisGMA and IBMA/UDMA nanogel-modified resins containing 50wt% EtOH were 19% and 45% greater in flexural modulus compared to hydrated control samples, respectively.
Conclusion: Adhesive resins modified by nanogel additives, particularly more hydrophobic nanogels, enhanced resin reactivity, conversion and dry/wet polymer modulus values, which all suggest stronger and more stable interfaces for restorations.