# Characterization of Heat-Polymerized Monomer Formulations for Dental Infiltrated Ceramic Networks
## 3 Results and Discussion
### Mechanical Properties
Having a low elastic modulus increases the amount of work done during flexure and in turn increases the strain energy release rate. These energy consuming processes inevitably increase the plastic zone around the crack tip leading to increased toughness. Conversely, extrinsic toughening mechanisms work outside of the inherent material property and inflict energy consuming processes working behind the crack tip such as crack bridging. The increasing R-curve is seen in interpenetrating networks of metals and ceramics where the metal acts as the compliant phase, and its ductility leads to bridging effects  
Figure 5: Mechanical properties of monomer formulations cured at different temperatures. Elastic modulus (**a**), hardness (**b**), and fracture toughness (**c**). behind the crack tip [51]. Ceramics themselves are brittle in nature, meaning that at a certain load, cracks occur catastrophically. In the case of dental PICN materials, the monomer resin acts as the compliant phase and would be responsible for extrinsic toughening mechanisms if any were to exist. To our knowledge, significant rising R-curve behavior has not been seen in commercial dental PICN materials, but that is not to say that it is an impossible feat. With a slow enough loading rate, localized deformation leading to crazing will form in glassy polymers. With crack advancement, fibrils are left at the trailing edge [50]. Interpenetrating composites outside of dentistry using ceramic and polymers have indicated the potential of bridging polymer fibrils or ligaments using 6-Nylon infiltrated into porous hydroxyapatite leading to a rising R-curve [48; 52]. Fibril bridging effects were also seen in calcium phosphate scaffolds with Poly (e-caprolactone) [53]. Going forward with experimental dental PICN materials, increasing the fracture toughness and inducing the favored R-curve effect in dental PICN materials require a proper understanding of the resin matrix and if the possibility of polymer fibrils exists. It is difficult to compare the property differences between studies using dental monomer formulations. The composition and polymerization method can result in markedly different properties. However, based on information as outlined in Table 1, monomer compositions can be tailored to specific properties and applications. Selection of heat-polymerized monomer formulations must consider many variables. The infiltration of monomers relies on an appropriate viscosity; however, the temperamental response to temperature demonstrate that ideal viscosity can be achieved in most monomer formulations. With the addition of chemical initiator BPO, the monomer formulations need to achieve temperatures of at least 72.0-75.9 \({}^{\circ}\)C to reach gelation and begin the reaction. By keeping the heating procedure as a constant variable, it was demonstrated that the degree of conversion is also unaffected by monomer formulations. These factors are important to consider with photo-polymerized composites but become inconsequential with heat-polymerized monomers. Instead, a pivotal consideration for resin matrix selection is assigned to the mechanical performance. It becomes clear how the relative concentration of monomers affects the contribution to mechanical performance. Increasing the TEGDMA ratio reduces elastic modulus, hardness, and fracture toughness. This could be due to the stronger hydrogen bonds in BisGMA and UDMA restricting the sliding of the molecules relative to each other over TEGDMA, with a pronounced effect due to concentration. The importance for the selection of appropriate resin monomer formulations with specific chemical and physical properties is not just limited to infiltration purposes. The use of similar formulations with comparable relations between various properties has already been applied to 3D printing technologies. A study by [PERSON] et al. has shown the performance of BisGMA:TEGDMA (4:6) formulations for infiltration of 3D-printed zirconia (3Y-TZP) networks [54].