Time-dependent nanoindentation behavior of high elastic modulus dental resin composites
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Nanoindentation and the viscous-elastic–plastic (VEP) model developed by Oyen and Cook for lightly filled thermoplastic polymer composites were used to characterize the elastic modulus, hardness, and viscoelastic response of a new high elastic modulus dental resin composite. The VEP model was used because loading rate studies indicated a viscous component in the loading/unloading response of our highly filled, thermosetting acrylic resin composites. Increasing the volume fraction of our high modulus filler increased the elastic modulus and hardness and decreased the viscous response in our composites. Coupling the filler and resin matrix with a commercial coupling agent like Metaltite or MPTMS (3-methacryloxypropyltrimethoxysilane) that ionically bonds to the filler and covalently bonds to the matrix decreases the viscous response and increases the hardness of the composite. The coupling agents did not affect the elastic modulus. The ability of the VEP model to predict load–displacement trajectories and the correlation of the elastic modulus and hardness values determined from the VEP model with those from the direct continuous stiffness measurement mode nanoindentation measurements indicate that the VEP model can be extended to highly filled, thermosetting systems. This is valuable since the potential to predict elastic, plastic, and viscous contributions to behavior should be valuable in the design and understanding of future highly filled resin composite systems.
I. INTRODUCTION
Dental resin composites, routinely composed of a mixture of silanized silicate fillers with an acrylic monomer matrix, have served clinically as dental restorations for decades, i.e., filling cavities in teeth, or fixing/joining crowns to the supporting tooth structure. Extensive research has been conducted to understand the microstructural, physical, and mechanical properties of dental resin composites and, ultimately, to overcome the failure problems associated with these properties. Recent clinical and laboratory results have shown that the elastic modulus of supporting resin composite adhesives has direct influence on the flexural failure of all-ceramic dental crowns.1–5 This has focused attention on the elastic modulus of resin composites. Thus, we developed a high elastic modulus resin composite; reinforced by stiff alumina nanoparticles rather than the conventional silicate fillers.6 Lee et al.7–10 have demonstrated that fracture in a top brittle layer was suppressed by substituting higher modulus resin composites for more compliant ones. However, other properties also influence the performance of resin composite adhesives. In particular, creep a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0070 J. Mater. Res., Vol. 25, No. 3, Mar 2010
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associated with viscous flow is of concern. Viscous flow during cyclic loading at oral temperatures causes permanent dimensional changes. This can lead to enhanced stresses on the tensile surface
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