Degree of Conversion, Density, partition coefficients and Density of Leachables 60 

Table 3-1 gives the degree of conversion (DC), polymerized adhesive density, and partition coefficient of water and density of leachables in polymerized adhesive. The DC of all the polymerized dentin adhesives was found to be in the range 87% to nearly 100%. As the free radical polymerization proceeds, microgels of polymer form with certain amount of dissolved monomer and water within them and are separated from the remaining monomer-water mixture.

Further propagation of the polymerization reaction is limited by the diffusion of the reactive species from the monomer-water mixture into the microgel, which depends on the viscosity of the resin. The presence of water in the monomer resin decreases the viscosity, thereby increasing the propagation of the polymerization reaction and the degree of conversion.

The densities of all the dentin adhesives were close to 1.2 g/cm3. The LogP values have been calculated as described earlier, and vary from 1.07 to 1.80. They are used as an indicator of hydrophilicity. The solubility values range from 0.00 to 4.82%. They are found to be uncorrelated to the degree of conversion values. However, a weak trend of decrease in solubility with LogP can be observed. This is consistent with the fact that more hydrophilic formulations are also more likely to leach out. Uniform samples were obtained as shown by the small standard deviation of all measured quantities.

3.3.2 Mechanical Properties

Figure 3-2 shows a comparison of the elastic moduli of the polymers in dry, wet and rubbery elastic conditions. The elastic moduli under dry and wet conditions are measured at room temperature (25ºC), while the rubbery modulus is measured at the glass transition temperature. The elastic moduli in both dry and wet conditions show an increasing trend with the percentage of BisGMA in the monomer formulation. Under dry conditions, the elastic modulus increases non-linearly with BisGMA content. Under wet conditions, the modulus is negligibly small up to about 20% BisGMA and follows a sigmoidal relationship with the BisGMA content in the polymer. The rubbery modulus also increases with BisGMA content. For up to about 20% BisGMA content, we observe that the rubbery modulus is in the neighborhood of the modulus

In the dry condition, the strong hydrogen bonding between HEMA and BisGMA polymer segments is responsible for the elastic stiffness. The plasticizing effect of water is responsible for lowering the elastic modulus in the wet condition. The polymer-solvent interaction responsible for plasticization is closely related to the water solubility in the monomer. Dissolution of HEMA monomer in water is energetically feasible and takes place spontaneously by breaking of HEMA-HEMA hydrogen bonds and formation of HEMA-water hydrogen bonds. However, dissolution of BisGMA in water is not energetically feasible because of the hydrophobic effect caused by the bulky aromatic groups and the large non-polar part of the BisGMA molecule. In the wet condition, when the percentage of BisGMA is below 20, the adhesive behaves like an elastomer because the BisGMA segments are too far from each other to interact via hydrogen bonding. However, at BisGMA contents greater than 20%, the BisGMA segments interact with each other through hydrogen bonds which are not spontaneously plasticized by water due to the hydrophobic effect. Due to the presence of these BisGMA- BisGMA hydrogen bonds, the behavior of the polymer in the wet condition is much stiffer than that predicted by rubbery elasticity. Correlations between wet moduli and solubility parameters have been obtained for dentin adhesives (Hosaka, Tagami et al. 2007). As the BisGMA content increases further, the polymer approaches a network structure.

Figure 3-3 (a) and (b) show the apparent stress strain behavior in both dry and wet conditions of the polymer formulations along the water-adhesive phase boundary. The increasing yield stress with BisGMA content is because of increasing density of hydrogen bonds as well as covalent crosslinks. In the dry condition, we observe that the polymers with an intermediate quantity of BisGMA show significant plastic deformation at yield for the formulations; they experience ductile failure. The polymers with either very low or very high percentage of BisGMA

experience brittle failure. Figure 3-3(c) shows the variation of apparent failure strain with BisGMA in the dry condition. The apparent failure strain increases, peaks and then falls with increase in BisGMA content. Typically, glassy polymers fail by a combination of mechanisms: bond breaking, which is typically associated with brittle failure and microshear band formation, which is typically associated with ductile failure and takes place by a viscous flow process (Haward and Young 1997). Bond breakage appears to be the primary mode of failure between HEMA segments, as indicated by the relatively brittle failure for the polymers with low BisGMA contents. For very high BisGMA contents, the scope for viscous flow is very less because of decreased free volume and high covalent crosslink density; hence the mode of failure is brittle. The variation of apparent failure strain with BisGMA content is similar to that of loss modulus with BisGMA (see section 3.4). The loss modulus also represents the extent of energy absorbed by viscous flow; thus it is correlated to the extent of ductile failure. From Figure 3-3 (e), we observe two transitions, one as a shoulder in the loss tangent graph, and the other at a much higher temperature around 160ºC. The significance of the shoulder increases with increase in BisGMA concentration. We hypothesize that the inter-molecular hydrogen bonds between HEMA segments, which control the behavior at low BisGMA concentrations are well below their glass transition (~160ºC) at room temperature. Thus, we observe highly brittle failure at low BisGMA concentrations. On the other hand, the second transition is observed in polymers with higher BisGMA concentrations. This transition is much closer to room temperature; hence is strongly associated with viscous flow at yield as opposed to the glass transition associated with the hydrogen bonds. Therefore, the corresponding polymer samples exhibit ductile failure. The relation between the yield stress and transition processes have been studied earlier for other

Figure 3-3 (d) shows the variation of apparent failure strain with BisGMA in the wet condition. In the wet state, the HEMA-HEMA hydrogen bonds have been dissolved or plasticized by water and their contribution to the yield stress is lost, thus decreasing its magnitude. As the BisGMA content decreases, the stress-strain behavior increasingly becomes like that of an elastomer at low BisGMA contents. At higher BisGMA contents, the viscous flow phenomenon is still present due to the hydrogen bonding between BisGMA segments; for HB80 onwards, the viscous flow is negligible, the polymer chains are completely mobile and the behavior is close to that of an elastomer.