Addition of nanogel prepolymer particles to dental monomers has been used to reduce polymerization shrinkage and stress. Objective: Physical isolation of photoinitiator (PI) within nanogel particles dispersed in and swollen by dental monomers can spatially localize initial polymerization to potentially delay macrogelation and vitrification while enhancing nanogel-based stress reduction. Methods: Nanogels were prepared by thermal polymerization from the following comonomers: isobornyl methacrylate, ethoxylated bisphenol-A-dimethacrylate and a urethane methacrylate-functionalized product of 2-hydroxy-2-methylpropiophenone (PI) in molar ratios of either 69:30:1 (A) or 60:30:10 (B). These photoactive nanogels were added to triethylene glycol dimethacrylate (TEGDMA) in concentrations ranging between 0.1 and 50wt%. The formulations, containing no other photoinitiator, were UV-photocured for 10min while simultaneously monitoring reaction kinetics (near-infrared) and stress development (tensometer).
Results: Addition of an analogous non-photoactive nanogel to TEGDMA, which contained homogeneously distributed free PI, at nanogel contents of 0, 20 and 40wt% produced uniform conversion results of 65.8(3.5), 68.4(0.8) and 67.3(3.1)% with stress values of 3.38(0.18), 2.49(0.07) and 1.85(0.06)MPa, respectively.
| Nanogel wt% in TEGDMA | Conversion, % | Stress, MPa |
Nanogel A 1mol% PI | 5 | 57.3(2.6) | 1.02(0.22) |
10 | 63.3(1.0) | 1.68(0.19) | |
25 | 72.7(0.1) | 2.03(0.03) | |
50 | 78.4(3.8) | 1.80(0.08) | |
Nanogel B 10mol% PI | 0.1 | 51.0(3.6) | 0.54(0.37) |
0.5 | 60.3(1.0) | 1.39(0.17) | |
1 | 64.1(2.0) | 1.62(0.31) | |
5 | 73.4(1.6) | 2.84(0.23) | |
10 | 78.9(1.1) | 3.49(0.03) | |
25 | 84.7(1.4) | 3.29(0.09) | |
50 | 93.4(4.8) | 2.94(0.01) |
Conclusions: Reaction rate and final conversion results were directly related to the photoactive nanogel content as well as the concentration of the tethered PI within the nanogel. High conversion was obtained with the 10mol% PI nanogel at 10wt% or more, with most examples showing conversion higher than possible with free PI. Non-photoactive nanogels decrease stress due to decreased functional group concentration. In the photo-active nanogels, stress increased with conversion until a threshold nanogel content, where it decreased despite significant increased conversion resulting from high localized initiator concentration. Supported by: RC1-DE020480