Method: A series of nanogels were synthesized using free-radical solution copolymerization of either isobornyl methacrylate (IBMA)/urethane dimethacrylate (UDMA), ethoxylated o-phenylphenol acrylate (ALEN)/9,9-bis[4-(2-acryloyloxyethyl)phenyl]fluorene (ABPEF), or vinylcarbazole (VC)/ABPEF. Mercaptoethanol and dodecanethiol were used as chain-transfer agents and the nanogels were refunctionalized with methacrylate groups via isocyanatoethyl methacrylate addition to the mercaptoethanol-derived chain ends. Nanogel additives were homogeneously dispersed at various loading levels into a bisphenol A glycidyl methacrylate/triethylene glycol dimethacrylate (1:1 mass ratio) resin. Silane-treated barium glass and fumed silica fillers were added at 60 and 5 wt%, respectively, to the nanogel-modified resin to create composite formulations. Measurements included bulk nanogel refractive index (refractometer), reaction kinetics (near-IR spectroscopy), visible light transmission (UV-Vis spectroscopy), and polymerization stress (tensometer).
Result: Bulk refractive indices of the nanogels were 1.508±0.001 (IBMA/UDMA), 1.607±0.001 (ALEN/ABPEF), and 1.649±0.002 (VC/ABPEF). Regardless of nanogel refractive index, the modified resins were optically transparent. Addition of nanogel (30 wt% ALEN/ABPEF relative to resin) resulted in as much as a 1.5-fold increase in transmitted light intensity in the polymer phase while reducing the change in light transmission during polymerization to 1.8±0.0 as compared to 10.8±0.7 in the unmodified control composite. Polymerization stress was reduced by up to 35% in samples with 30 wt% nanogel while maintaining high levels of conversion.
Conclusion: Nanogels can be used as a tool both to alter optical properties and to significantly reduce polymerization stress in composite materials. By selective modification of the synthetic recipe, nanogels can be designed to achieve a specific refractive index and thus can be used as a means to tune composite esthetic properties, which potentially offers greater depth of cure as well as greater versatility in filler selection.