# Characterization of Heat-Polymerized Monomer Formulations for Dental Infiltrated Ceramic Networks
## 3 Results and Discussion
### Mechanical Properties  
Figure 5a,b shows the elastic modulus and hardness of the monomer formulations cured at different temperatures and weight percentages of TEGDMA, respectively. There is a clear inverse correlation with the increase of TEGDMA in hardness, elastic modulus, and fracture toughness. The highest values for elastic modulus were seen in BisGMA:TEGDMA 8:2 (3.03 GPa) cured at 120 \({}^{\circ}\)C, and the lowest was UDMA:TEGDMA 1.9 (0.63 GPa) cured at 100 \({}^{\circ}\)C. The temperature did not affect UDMA:TEGDMA formulations. The highest hardness values were seen in the BisGMA:TEGDMA 9:1 formulation (249.07 \(\pm\) 8.47 MPa), and the lowest in UDMA:TEGDMA 1:9 (33.81 \(\pm\) 13.88 MPa). TEGDMA, with its low molecular weight, increases the flexibility of the polymer with an increase in weight ratio. This can be explained by the TEGMDA molecule's ability to coil under load and rotate about its ether linkages [37]. These properties are essential when considering dental restorations. Elastic properties and hardness should be as close as possible to the properties of our human teeth to replicate and establish the biomechanical functions and demands. A high hardness and elastic modulus can damage and abrade the opposing teeth, while the inverse would damage the restoration. Comparable damage tolerance is needed to absorb the loading energy and provide the essential durability and robustness for clinical success. In the case of high strength ceramics of alumina and zirconia, these strong and incredibly stiff ceramics can be made more compliant with the addition of monomers in PICN materials. Relative success in property matching has been seen in commercial dental PICN materials [4; 48]. [PERSON] et al. used similar formulations for infiltration of silica-based ceramic networks [47]. They measured an elastic modulus for the final PICN material of 18.7 GPa for BisGMA:TEGDMA (5:5) and 18.9 GPa for UDMA:TEGDMA (8:2) formulations, both values closely matching the dentin elasticity. Figure 5c shows fracture toughness of the monomer formulations cured at different temperatures and weight percentages of TEGDMA. Basically, the fracture toughness was found considerably higher for UDMA- versus BisGMA-based formulations. Fracture toughness linearly decreased with increasing TEGDMA content. The highest values were seen in UDMA:TEGDMA formulations which ranged from 0.262 \(\pm\) 0.078 MPa\(\cdot\)m\({}^{0.5}\) (1:9) to 0.931 \(\pm\) 0.159 MPa\(\cdot\)m\({}^{0.5}\) (9:1). UDMA cured at 100 \({}^{\circ}\)C was 0.756 \(\pm\) 0.082 MPa\(\cdot\)m\({}^{0.5}\), and BisGMA:TEGDMA formulations ranged from 0.179 \(\pm\) 0.043 MPa\(\cdot\)m\({}^{0.5}\) (1:9) to 0.683 \(\pm\) 0.113 MPa\(\cdot\)m\({}^{0.5}\) (9:1). Fracture toughness is the ability of a material to resist crack growth from a pre-existing crack. In terms of clinical relevance, this mechanical property offers greater understanding of failure over elasticity. A resin formulation for infiltration purposes should hence exhibit a high fracture toughness with preference for UDMA-based mixtures at higher UDMA:TEGDMA ratios. Clinical fractures often originate from existing flaws, and fracture toughness gives us a measure of the load needed to propagate the crack. The interpenetrating nature of PICN materials distinctly separates both material constituents; therefore, any advancing cracks are strongly subject to a variety of toughening mechanisms. When materials are effective in arresting developing cracks and absorb enough crack advancing forces, an increased input of energy is required to further propagate the crack, leading to a rising resistance curve (R-curve). Fracture toughness of a material is contributed to by intrinsic toughening mechanisms such as the inherent plasticity of the material working ahead of the crack tip [49]. As polymers are covalently bonded networks, at a critical shear strain, molecules are susceptible of sliding with respect to each other [50].