Chemo-mechanical characterization of phase-separated dentin adhesives

Abstract

The premature failure of composite dentin restorations can be traced to the adhesive-dentin (a/d) interface. The continuity, stability and mechanical properties of the dentin adhesive polymer in the interface depend on its composition and morphology. The adhesive is composed of a hydrophilic/hydrophobic monomer pair, which phase separates upon interaction with dentinal water, resulting in variable chemical composition and pore structure of the adhesive in the interface. The phase separation has been quantified using a ternary monomer-water phase diagram. The polymerization of the monomer phases, composition and pore structure of the resulting polymer phases were investigated. A novel micro-poromechanical constitutive model was developed to model the polymer mechanical, swelling and transport behavior while accounting for polymer-water interaction and pore pressure. The model was then used to characterize polymer formulations along the phase boundary. Parametric studies were performed to illustrate flow-dependent mechanical behavior of hydrophilic polymers. Experimental observations showed negligible amount of crosslinker in hydrophilic-rich phases and identified the cut-off point on the phase boundary to achieve significant polymerization. The results also quantified the composition of polymers formed along the phase boundary. Crosslink density and hydrophilicity were identified as the critical parameters which determined the sorption, swelling, diffusion and poromechanical behavior of the polymer phases. These two parameters were found to depend on the phase chemical composition. Effects of external environment and confinement on the flow-dependent mechanical behavior have also been studied parametrically for hydrophilic polymers. The combination of experimental results and micro-poromechanical modeling provide structure-property relationships obtained from first principles, which will aid in adhesive design and analysis of the adhesive-dentin interface performance.