# Characterization of Heat-Polymerized Monomer Formulations for Dental Infiltrated Ceramic Networks
###### Abstract  
(1) Objectives: This work examined properties of dental monomer formulations of an aromatic dimethacrylate (BisGMA), aliphatic urethane dimethacrylate (UDMA), and triethylene glycol dimethacrylate (TEGDMA). The monomers were combined in different ratio formulations and heat-polymerized containing the initiator benzoyl peroxide (BPO) specifically for the purpose of infiltration into polymer-infiltrated composite structures. (2) Methods: The monomers were combined in different weight ratios and underwent rheological analysis (viscosity and temperature dependence), degree of conversion, and mechanical properties (elastic modulus, hardness, fracture toughness). (3) Results: Rheological properties showed Newtonian behavior for monomers with a large dependence on temperature. The addition of BPO allowed for gelation in the range of 72.0-75.9 \({}^{\circ}\)C. Degree of conversion was found between 74% and 87% DC, unaffected by an increase of TEGDMA (up to 70 wt%). Elastic modulus, hardness, and fracture toughness were inversely proportional to an increase in TEGDMA. Elastic modulus and hardness were found slightly increased for UDMA versus BisGMA formulations, while fracture toughness ranged between 0.26 and 0.93 MPa-m\({}^{0.5}\) for UDMA- and 0.18 and 0.68 MPa-m\({}^{0.5}\) for BisGMA-based formulations. (4) Significance: Heat-polymerization allows for greater range of monomer formulations based on viscosity and degree of conversion when selecting for infiltrated composite structures. Therefore, selection should be based on mechanical properties. The measured data for fracture toughness combined with the reduced viscosity at higher UDMA:TEGDMA ratios favor such formulations over BisGMA:TEGDMA mixtures. methods: theory - methods: numerical  
The conventional dental base monomers in composite formulations include Bisphenol A diglycidyl ether dimethacrylate (BisGMA), urethane dimethacrylate (UDMA), and triethyleneglycol dimethacrylate (TEGDMA) [21], the last of which is used as a diluent and therefore dictates viscosity based on ratio formulations. The chemical structures of the monomers are shown in Figure 1.  
BisGMA is advantageous in that it has a high molecular weight and presents lower polymerization shrinkage. The stiff Bisphenol A core and two pendant hydroxyl groups allow for strong hydrogen bonds to be formed and have the largest and lowest concentration of double bonds [22]. On the other hand, UDMA has a flexible aliphatic core with two urethane links [23]. It is smaller than BisGMA and has a higher concentration of double bonds. TEGDMA is the smallest in size, has a high concentration of double bonds, but in turn has the greatest shrinkage. These three monomers in differing formulations comprise the majority of resin matrices used in dental composites. For photo-polymerized resins under ambient temperatures, the double bond conversion is rapid [24]. As the resin turns glassy and vitrifies, mobility is reduced, continued monomer conversion is hindered, and reaction rate slows [25; 26]. Mobility is greatly reduced to the extent that photo-polymerization will always lead to incomplete conversion. Photo-polymerized composites are guided by principles to maximize the degree of conversion as insufficient conversion could lead to color instability and material degradation [27; 28; 29; 30; 31]. Conversely, heat polymerization as used by PICN materials allows the consistent input requirements of heat energy to continue the reaction which results in a higher degree of conversion. Numerous studies have reported on mechanical property characterizations of photo-polymerized unfilled dental resins, including degree of conversion [32; 33; 34], elastic modulus [35], hardness [36; 37; 34], and fracture toughness [38]. In comparison, few studies report on mechanical properties of heat-polymerized unfilled dental resins. [PERSON] [39] and colleagues found significant differences in strength between photo-polymerized and heat-polymerized UDMA, with significant increases with increased temperature and pressure.