R. Parthasarathy, A. Misra, L. Song, Q. Ye, P. Spencer
Tennessee State University,
Keywords: phase separation, elastic modulus, mechanical softening, free water, bound water, loss modulus, glass transition
Summary:Dentin adhesive systems for composite tooth restorations are composed of hydrophilic/ hydrophobic monomers, solvents and photoinitiators. With the wet-bonding technique, acid-etching is used to remove the dentin’s mineral phase without altering the collagen matrix, and the resulting voids in the water-laden collagen matrix are filled with adhesive and polymerized in situ to create a hybrid layer. The ideal hybrid layer is a polymerized 3D polymer – collagen network which provides a continuous and stable link between the bulk adhesive and subjacent intact dentin. There is substantial evidence that this ideal is not achieved. Commercial dentin adhesives are composed of hydrophilic and hydrophobic monomers, solvents and photoinitiators. The optimal adhesive must possess a hydrophilic/hydrophobic balance so that the adhesive is hydrophilic enough to achieve integration with the wet, demineralized dentin matrix in the monomer state and relatively hydrophobic in the polymer state to avoid swelling and sorption of oral fluids, and maintain its mechanical integrity. The adhesives undergo phase separation, and concomitant compositional change during their application in the wet oral environment; phase separation compromises the quality of the hybrid layer in the adhesive/dentin interface. As a result, bacteria and bacterial by-products infiltrate gaps in the composite/tooth margin, facilitating cariogenic and aciduric bacterial outgrowth, encouraging recurrent decay, pulpal damage, and composite failure. In this work, the adhesive composition in the hybrid layer has been represented using the phase boundaries of a ternary phase diagram for the hydrophobic monomer/hydrophilic monomer/water system. The polymer phases, previously unaccounted for, play an important role in determining the mechanical behavior of the bulk adhesive and the chemo-mechanical properties of the phases are intimately related to the effects produced by differences in the hydrophobic-hydrophilic composition. Our previous work has shown that the mechanical properties of the adhesive not only affect the overall bond or shear strength but also have a profound influence on the load-transfer mechanism at the adhesive/dentin interface and its fatigue life. This work investigates the relationship between composition and chemo-mechanical properties of the polymer phases formed on the water-adhesive phase boundary using quasi-static and dynamic mechanical testing, mass transfer experiments and vibrational spectroscopy. As the composition of the polymer phases varies from hydrophobic-rich to hydrophilic-rich, the amount of the adsorbed water and the nature of polymer-water interaction vary non-linearly, and strongly correlates with the change in elastic moduli and the stress at failure under wet conditions. We have discerned this based on the reduction in OH peaks in the FTIR spectra of D2O-saturated polymer phases caused by H-D exchange. The failure strain, loss modulus and glass transition temperature vary non-monotonically with composition and are explained based upon primary and secondary transitions observed from temperature sweep under dynamic mechanical testing. The structure-property relationships developed in this work aid in the assignment of mechanical properties and choice of suitable constitutive models for polymer phases in the hybrid layer for predictive micromechanical finite element analysis of the adhesive/dentin interface.