Objectives: Polymerization stress reduction is critical to prolonging the clinical performance and lifetime of composite restoratives. Dental resins and composites have been modified by the addition of nanogel polymeric particles as a route to heterogeneous materials with reduced polymerization shrinkage and stress. This study investigates how nanogel modulus and content interact to affect polymer stress development.
Methods: A series of nanogels was synthesized through solution copolymerization of urethane dimethacrylate and either butyl methacrylate (BMA), ethyl methacrylate (EMA) or isobornyl methacrylate (IBMA). Glass transition temperature (Tg) of the nanogels was measured using dynamic mechanical analysis. Nanogels were added to a 70/30 mass ratio of ethoxylated bisphenol-A-dimethacrylate and triethylene glycol dimethacrylate at 10, 25, and 40wt%. Stress development and degree of conversion (DC) were measured simultaneously by coupled tensometer/near-infrared spectroscopy. Flexural modulus was measured using a mechanical testing system.
Results: In all materials, stress decreased with increased nanogel loading. Furthermore, stress decreased with increased nanogel Tg. Delayed onset of significant stress development was observed with increased nanogel content. Decreased stress was obtained for nanogel-containing materials despite DC and modulus results equal to or greater than the control.
Materials | Nanogel Tg (°C) | DC (%) | Stress (MPa)* | Modulus (GPa)* |
Control | - | 71.0±2.7 | 2.2±0.1a,c | 1.3±0.1a |
BMA 10wt% | 53±0.6 | 70.3±3.6 | 2.4±0.1a | 1.5±0.0a,b |
BMA 25wt% | 74.3±0.1 | 1.7±0.2c | 1.6±0.1b | |
BMA 40wt% | 78.3±2.4 | 1.3±0.1b | 1.5±0.1a,b | |
EMA 10wt% | 63±0.7 | 70.2±2.2 | 2.0±0.2c | 1.6±0.0b |
EMA 25wt% | 71.3±0.7 | 1.5±0.2b,c | 1.7±0.0b | |
EMA 40wt% | 77.2±3.4 | 1.2±0.0b | 1.6±0.1b | |
IBMA 10wt% | 94±0.3 | 72.4±0.0 | 1.9±0.1c | 1.6±0.0b |
IBMA 25wt% | 70.7±1.7 | 1.3±0.1b | 1.7±0.1b | |
IBMA 40wt% | 75.6±1.3 | 1.2±0.0b | 1.8±0.0b |
*One-way ANOVA/Tukey pairwise groupings
Conclusion: Stress development during polymerization appears to depend not only on the concentration of nanogel incorporated but also on the structure and properties of the prepolymer. Nanogel additives produced stress reduction of nearly 50% while enhancing both conversion and mechanical properties of dental polymers.
Support: NIH/NIDCR RC1-DE20480