Low Tg Nanogels as Reactive Additives in Polymeric Dental Materials

Objective: Prior studies with high glass transition temperature (Tg) nanogel polymeric particles have shown their potential to dramatically reduce photopolymerization shrinkage stresses in dental resins. The aim of this study was to examine how lower Tg nanogel additives affect polymer formation as well as the physical and mechanical properties of a dimethacrylate-based resin.

Method: The nanogels were synthesized through solution copolymerization of either isodecyl methacrylate (IDMA) or isobornyl methacrylate (IBMA) with bisphenol-A-ethoxylate diacrylate (BisEA). They were added at 5, 10, or 25 wt% to a 70/30 mass ratio of ethoxylated bisphenol-A-dimethacrylate and triethylene glycol dimethacrylate (control). The nanogel Tg was characterized (dynamic mechanical analysis) along with evaluation of resin reaction kinetics/conversion (near-infrared spectroscopy), mechanical properties (three-point bending on a universal testing device) and polymerization stress (tensometer).

Result: The nanogel compositions provided Tg values of -46°C and -24°C for IDMA/BisEA and IBMA/BisEA, respectively. The table below shows conversion, stress, and mechanical properties for each material. Compared to the control, degree of conversion increased by as much as 20%. Modulus values increased by as much as 38% with 5wt% nanogel additives, but at the 25wt% level, mechanical properties dropped by up to 40%.

Material	Conversion (%)	Stress (MPa)	Modulus (GPa)	Flex. Strength (MPa)
Control	72.0±2.2	2.3±0.1	1.3±0.1	61.7±2.9
IDMA/BisEA 5%	79.9±0.6	2.6±0.2	1.4±0.0	57.8±2.9
IDMA/ BisEA 10%	81.2±0.8	2.5±0.1	1.3±0.1	55.7±3.0
IDMA/ BisEA 25%	85.3±0.4	1.8±0.1	0.8±0.0	37.0±1.4
IBMA/ BisEA 5%	77.0±0.7	2.7±0.0	1.8±0.1	73.4±3.1
IBMA/ BisEA 10%	78.9±1.1	2.5±0.0	1.7±0.0	70.3±2.2
IBMA/ BisEA 25%	82.6±1.6	1.9±0.1	1.6±0.0	62.2±0.5

Conclusion: Our results suggest that low Tg nanogels can be used at relatively low concentrations to improve local conversion and mechanical properties of dental resins, without substantially affecting stress development.