A Comparative Study On The Microscale And Macroscale Mechanical Properties Of Dental Resin Composites

Light-cured resin composites are usually composed of dental resin matrix,photoinitiation system and inorganic filler and have become the current mainstream dental restoration materials because of their outstanding characteristics,such as superior esthetics,low cost,environmental-friendly characteristics,simple repair operation,and good biocompatibility.Pure resin materials are disadvantageous in their mechanical properties,such as strength,stiffness,durability,and wear resistance.In this work,the effects of the SiO₂ particles on the mechanical properties of bisphenol A-glycidyl methacrylate（Bis-GMA）/triethylene glycol dimethacrylate（TEGDMA）dental resin composites were studied by combined nanoindentation tests and macroscale tensile tests.Moreover,near-infrared（NIR）spectroscopy,scanning electron microscope（SEM）,and atomic force microscope（AFM）techniques are adopted to characterize the polymerization kinetics,the morphology of fractured sections,and the modulus distribution of the particle–matrix interfaces,respectively and the strengthening mechanism of composite resin was discussed.The main research contents are as follows.（1）Study on micromechanical properties of dental composite resins.The effects of SiO2 content and size on the elastic modulus,energy storage modulus,loss factor and hardness of Bis-GMA/TEGDMA composite resin were studied by the quasi-static test and dynamic test module of nanoindentation test technology.The influence of tip oscillating force frequency on the energy storage modulus,hardness and loss factor of composite resin under dynamic loading was also studied.The results showed that the mechanical properties such as elastic modulus,energy storage modulus,hardness and loss factor of resin composites increased with the increasing of SiO₂ particle filling amounts.When the vibration frequency of the force was taken as a variable,the composite resin exhibited better mechanical properties at high frequencies,but the loss factor decreased with the increasing of force frequency.With the same filling mass ratio,the mechanical properties of the composite resin were enhanced more significantly by smaller SiO₂ particles,but the loss factor of the composite decreased with the decreasing of the particle size.（2）Study on macroscopic mechanical properties of dental composite resins.A universal testing machine was used to study the irreversible deformation,tensile modulus,fracture strength and other macroscopic mechanical properties of the restored composite resin with the filling amount and size of the filled particles.The results showed that after the loading is removed,the irreversible deformation of the specimen which failed to recover gradually decreases with the increasing of the nanoparticle content,and the residual irreversible deformation was little affected by the particle size,while the tensile modulus and fracture strength of the resin composites increased with the increasing of the SiO₂particle filling amount.The effect of small-size silica particles on tensile modulus and fracture strength of composite resin is better than that of large-size particles.（3）Study on strengthening mechanism of dental resin composites.Near-infrared spectroscopy（NIR）,scanning electron microscopy（SEM）and atomic force microscopy（AFM）were used to investigate the strengthening mechanism of resin composites.The results showed that the conversion rate increases first and then decreased with the increasing of particle contents when small-size particles were filled,while the conversion rate decreased with the increasing of particle content when large-size particles were filled,but the overall change was small.The roughness of the fracture surface increased with the increasing of SiO₂ particle content.In addition,we also found the boundary layer whose modulus gradually decreased from the edge of nanoparticles to the resin matrix based on the modulus mapping technology of atomic force microscopy,and compared the composite resin filled with two sizes of particles.It was found that the thickness of the gradient boundary layer did not change with the size of the packed particles.Finite element modeling was adopted to illustrate the role of this gradient boundary layer in alleviating the stress concentration on the filler–matrix interface.The present study validates mechanical reinforcement and provides a potential new insight for understanding the reinforcing mechanism of dental resin composites.