Methods: Nanogels were prepared by solution photopolymerization involving isobornyl methacrylate (IBMA)/urethane dimethacrylate (UDMA) or IBMA/ethoxylated bisphenol-A dimethacrylate (EBDMA) (70:30 mole ratio) with mercaptoethanol (15mol%) as chain transfer agent, which allows subsequent addition of isocyanatoethyl methacrylate that is used to convert inert nanogel particles to reactive macromers. The model bonding agent was composed of BisGMA/HEMA (60:40 mass ratio) and included 12wt% of ethanol to mimic the residual solvent content of an air-thinned solvated adhesive. A bisacylphosphine oxide initiator (Irgacure 819; 0.2wt%) was introduced to promote visible light photopolymerization. Reactive nanogels were incorporated in the adhesive formulation at 25wt% and viscosity was measured with a viscometer. Flexural modulus and strength of photocured 2x2x25mm beam specimens (n=5) were determined in three-point bending mode. Water sorption and solubility was evaluated on disc specimens (n=2) which were taken through two sorption/drying cycles using distilled water.
Results: The addition of the IBMA/UDMA reactive nanogel raised viscosity of the monomeric adhesive composition from 0.12±0.01 to 1.9±0.04 Pa.s while lowering the water sorption and solubility of the polymer by 23 and 18%, respectively. Flexural modulus was improved significantly from 0.45±0.01 to 0.62±0.05 or 0.80±0.07 GPa when either IBMA/UDMA or IBMA/EBDMA nanogels were included. Similarly, flexural strength was increased significantly from 33.8±1.3 to 40.1±2.4 or 44.9±2.7 MPa with the added nanogels based on IBMA//UDMA or IBMA/EBDMA, respectively.
Conclusions: The introduction of reactive, hydrophobic nano-scale polymeric particles in dental adhesives increases viscosity, which may be no issue in fully solvated materials, while reducing water uptake and solubility of the polymers as well as improving mechanical strength properties.
Supported by: NIH/NIDCR DE5R21DE018354 and Septodont/Confi-Dental.