Rationale for specific aim 3: Mechanical Modeling 12 

As explained in specific aim 2, the mechanical properties of the polymer phases in the hybrid layer are time and rate dependent. In order to quantify the material properties that govern this time and rate dependency, a constitutive model incorporating the features of polymer matrix viscoelasticity, flow-dependent viscoelasticity and influence of water sorption on polymer matrix properties becomes necessary. Though constitutive models that incorporate poroelasticity and fiber network distributions have been developed (Huang, Mow et al. 2001, Nia, Han et al. 2012), they are either focused on the poromechanical behavior of saturated, swollen hydrogels (Biot 1941, Hoang and Abousleiman 2009, Hoang and Abousleiman 2012, Wang and Hong 2012) or on the diffusion properties of polymers (Anderson and Quinn 1974, Pusch and Walch 1982, Reinhart and Peppas 1984, Lustig and Peppas 1987, Colton, Satterfield et al. 2004). Further, the models focused on the development of poromechanical behavior do not explicitly consider the effect of chemical structure. For modeling dentin adhesives, a single model accounting for the diffusion, swelling, dry to wet transition and resulting poromechanical behavior is necessary. The various polymer phases in this study, in particular, the hydrophilic-rich phases can be considered as free-swollen, crosslinked networks. Therefore, the material model for these polymers has been constructed which computes the macro-scale poromechanical behavior by superposition of polymer matrix with interstitial water. The polymer-polymer and polymer- water interactions are explicitly incorporated in the model using parameters. This model has been used by our group to explain the rate dependent behavior of soft tissue such as condylar cartilage (Parthasarathy, Misra et al. 2011).

1.4 Summary (General):

The project is aimed at characterization and chemo-mechanical modeling of polymers formed from saturated/oversaturated mixtures of dentin adhesive monomers with water. The following steps are carried out: 1. Development of a phase diagram for the monomer components and water based on chemometric analysis using FTIR spectra, 2. Determination of the morphology, pore structure and pore size distribution of relevant polymer phases using 3D micro X-Ray computer tomography, 3. Qualitative study of the chemical composition and polymer-phase distribution using micro-Raman imaging analysis, 4. Development of mechanical model by incorporation of homogenized polymer matrix into a poro-mechanics framework. 5. Estimation of chemo-poro- mechanical properties along the adhesive-water phase boundary using swelling equilibrium, 6. Estimation of the diffusion coefficient of water in the polymer phases using mass change experiments and 7. Simulation of monotonic and creep experiments under wet and dry conditions using calibrated chemo-poro-mechanical model. The results of the characterization will provide the basis for the determination of the model parameters porosity, density, local constants and diffusion coefficients.

1.5 Relevance:

In composite resin restorations, the weak link is the hybrid layer which is a composite of demineralized collagen and phase-separated dentin adhesive. The longevity of the composite restoration depends on the physicochemical and mechanical properties of the hybrid layer, and in turn, on the various polymer phases in the hybrid layer. The methodology described in this dissertation is applied to characterize these dentin adhesive polymer phases formed in the

Such results can help to choose adhesives which will improve the life of composite restorations, potentially avoiding the need for repeated restoration replacement, which decreases the need for a more complex restoration and risk of eventual tooth loss.

Control Adhesive Formulation

Standard methacrylate-based adhesive system of 45/55 wt% 2- hydroxyethyl methacrylate (HEMA)/ 2, 2-bis[4-(2-hydroxy-3- methacryloxypropoxy)phenyl]-propane (BisGMA)

Photoinitiator Systems

Different photo-initiator systems are tested based on camphorquinone (CQ), ethyl-4-(dimethylamino) benzoate (EDMAB) and diphenyliodonium hexafluorophosphaste (DPIHP) Polymer

Phases

Four categories of polymer systems - a) neat adhesive resin formed without the presence of water, b) hydrophobic-rich phase formed after phase separation in water, c) hydrophilic-rich Table 1-1 Nomenclature of adhesive resin monomers and polymers

Sample

Monomer

abbreviation*

Polymer abbreviation*

Neat adhesive with HEMA to BisGMA

in the mass ratio X:(100-X)

HBX-NRM HBX-NRP

Neat adhesive with water percent at

miscibility limit

HBX-PB HB-RPP

Oversaturated mixture of neat adhesive

with W mass percent of water

HBX-WM HBX-WP

Hydrophilic-rich phase oversaturated

mixture

HBX-WM-APM HBX-WM-APP

Hydrophobic-rich phase oversaturated

mixture HBX-WM-RPM HBX-WM-RPM

Abbreviation for photoinitiator system % Composition by Massa CQ EDMAB DPIHP QTX 2PI 0.5 0.5 0 0 3PI 0.5 0.5 0.5 0 4PI 0.25 0.5 0.5 0.25

Table 1-3 List of photoinitiator formulations consisting of camphorquinone (CQ), ethyl-4- (dimethylamino) benzoate (EDMAB), diphenyliodonium hexafluorophosphate (DPIHP) and 3- (3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-2-hydroxypropyl] trimethyl ammonium chloride (QTX)

Figure 1-1 Schematic of a tooth restored with composite restoration. This figure depicts the relationship of the dentin-adhesive bond to the composite restoration (Misra, Spencer et al. 2005, Singh, Misra et al. 2011)

Figure 1-2. Scanning electron micrograph of demineralized dentin matrix (which is primarily type I collagen). The holes that are distributed throughout the matrix represent the dentinal tubules. Under clinical conditions, pulpal fluid is pulsing through the dentinal tubules (Misra, Spencer et al. 2005)

Figure 1-4. Illustration representing adhesive phase separation and water entrapment at the dentin/adhesive interface

Select composition of experimental monomer mixture (B-2) Determine miscibility Is miscibility comparable/superior to control? NO

Study phase separation

Polymerization behavior satisf actory? NO SPECIFIC AIM 1 CHARACTERIZATION OF MONOMER PROPERTIES SPECIFIC AIM 2 CHARACTERIZATION OF POLYMER PROPERTIES SPECIFIC AIM 3 MECHANICAL MODELING Hydrophilic-rich phase Hydrophobic-rich phase Multi-phase NO

Change initiator system

YES

Identif y phases to characterize

Chemical composition X-Ray Micro - CT Raman- Spectroscopy Mass Change experiment Dif f usion coef f icients Pore structure & porosity Hydrophilic-rich phase Hydrophobic- rich phase Multi- phase Multi- phase Mechanical Testing Mechanical Modeling Fiber Network

Homogenization Local Fiber Law

Poromechanics Model Fluid Transport Model Final Model Creep Monotonic

Conf ined Unconf ined

Drained Undrained

Wet Dry

Experimental Results

2 CHARACTERIZATION OF DENTIN ADHESIVE SYSTEM USING A TERNARY PHASE DIAGRAM

Due to limited penetration and phase separation, commercial dentin adhesives consisting of a hydrophobic/hydrophilic monomer pair such as BisGMA /HEMA undergo phase separation and compositional change in adhesive dentin interfaces upon interaction with water and dentinal fluid. Consequently, the adhesive in the restoration varies in composition and morphology throughout the interface, thus affecting its chemical and mechanical properties, which influence its ability to transfer mechanical load and offer protection to exposed collagen. We have modeled the variability in composition of the adhesive monomer using a ternary adhesive-water phase diagram. We also determine the polymerization of the adhesive along the phase boundary, and study the nature of adhesive polymer polymerized in the presence of water. The results quantify the variability in adhesive monomer composition, and its effect on the resulting adhesive polymer. This type of investigation provides a thorough understanding of the composition and properties of the adhesive in the adhesive dentin interface.