Mathematical model for anomalous creep in model dentin adhesives

ABSTRACT We have recently measured creep deformations in a model dentin adhesive under a variety of moisture conditions. These measurements have indicated that the rate of creep and total creep strain increase substantially when dry stored photopolymerized model adhesive is mechanically loaded under changing moisture conditions. Objective: The aim of the study is to develop a mathematical model to explain the creep behavior of a model dentin adhesive under conditions that simulate the wet, oral environment. Methods: A micromechanical approach is applied to model the rate-dependent behavior of dentin adhesives. In this approach, the structural components are modeled as directional rheological elements that undergo water-induced damage. The derived model is used to predict the creep response of model dentin adhesive samples that have been tested under different moisture conditions. Results: The model replicates the observed creep behavior, including the high creep rate and total creep strains obtained in the experiments. For the samples tested under constant moisture conditions, the behavior is found to be linear viscoelastic over a large range of stress-level. The stress range for linear behavior is approximately 30% of the overall failure stress of the adhesive under slow loading (~160MPa for dry case and ~60MPa for submerged samples loaded at 0.0075N/min). For the samples that are stored dry and tested in submerged conditions, the behavior is found to be highly nonlinear even at low stress-levels (~4.5MPa). Conclusions: The high degree of nonlinearity for samples that are stored dry and tested in submerged conditions is traced to the simultaneous water induced damage/softening of structural components and the application of the external load. In the oral environment, the interplay of mechanical loads and water-induced damage can lead to creep-failure of adhesives under function. Supported NIH/NIDCR R01 DE014392, DE014392-08S109