Abstract
Filler particle functionalization with thiourethane oligomers has been shown to increase fracture toughness and decrease polymerization stress in dental composites, though the mechanism is poorly understood. The aim of this study was to systematically characterize the effect of the type of filler surface functionalization on the physicochemical properties of experimental resin composites containing fillers of different size and volume fraction. Barium glass fillers (1, 3 and 10 µm) were functionalized with 2 wt% thiourethane-silane (TU-Sil) synthesized de novo and characterized by thermogravimetric analysis. Fillers treated with 3-(Trimethoxysilyl)propyl methacrylate (MA-Sil) and with no surface treatment (No-Sil) served as controls. Fillers (50, 60 and 70 wt%) were incorporated into BisGMA-UDMA-TEGDMA (5:3:2) containing camphorquinone/ethyl-4-dimethylaminobenzoate (0.2/0.8 wt%) and 0.2 wt% di-tert-butyl hydroxytoluene. The functionalized particles were characterized by thermogravimetric analysis and a representative group was tagged with methacrylated rhodamine B and analyzed by confocal laser scanning microscopy. Polymerization kinetics were assessed by near-IR spectroscopy. Polymerization stress was tested in a cantilever system, and fracture toughness was assessed with single edge-notched beams. Fracture surfaces were characterized by SEM. Data were analyzed with ANOVA/Tukey's test (α = 0.05). The grafting of thiourethane oligomer onto the surface of the filler particles led to reductions in polymerization stress ranging between 41 and 54%, without affecting the viscosity of the composite. Fracture toughness increased on average by 35% for composites with the experimental fillers compared with the traditional methacrylate-silanized groups. SEM and confocal analyses demonstrate that the coverage of the filler surface was not homogeneous and varied with the size of the filler. The average silane layer for the 1 µm particle functionalized with the thiourethane was 206 nm, much thicker than reported for traditional silanes. In summary, this study systematically characterized the silane layer and established structure–property relationships for methacrylate and thiourethane silane-containing materials. The results demonstrate that significant stress reductions and fracture toughness increases are obtained by judiciously tailoring the organic–inorganic interface in dental composites.
Highlights
Filler particle functionalization with thiourethane oligomers has been shown to increase fracture toughness and decrease polymerization stress in dental composites, though the mechanism is poorly understood
The thiourethane (TU) oligomer was obtained by a click reaction in solution of pentaerythritol tetra-3-mercaptopropionate (PETMP), 1,3-bis(1-isocyanato-1-methylethyl)benzene (BDI), and 3-(triethoxysilyl)propyl isocyanate (2.5:1:1 mol, respectively), as previously described in detail[6]
Filler particles were functionalized with the TU silane described above or 3-(Trimethoxysilyl)propyl methacrylate, which were mixed at 2 wt% in an ethanol:millipore water solution (80:20 vol%) acidified by glacial acetic acid
Summary
Filler particle functionalization with thiourethane oligomers has been shown to increase fracture toughness and decrease polymerization stress in dental composites, though the mechanism is poorly understood. One recent publication using this approach demonstrated up to 35% reduction in polymerization stress for the resultant composite, accompanied by significantly improved mechanical p roperties[7] This is especially promising because it may allow for more homogenous distribution of the thiourethane oligomer into the material, while minimizing the impact on the final viscosity of the c omposite[7]. One hypothesis is that the formation of low Tg polymer brush structures on the surface of the filler are able to deform to compensate for the change in free volume inherent in the polymerization r eaction[9,10] This strategy targets the interface between filler and matrix, one area where stress concentration is more m arked[11]
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