Theoretical And Experimental Studies On The Structure And Properties Of Dental Resin Composites

Dental restoration is a clinical treatment accomplished by removing damaged or decayed teeth and replacing the natural structure with durable materials such as alloys,ceramics,cements,and resin composites.At present,dental resin composites(DRCs)have been researched and used in recent decades due to their excellent mechanical properties,good aesthetics,biocompatibility and convenient clinical operating conditions.Typical DRCs can be considered as a mixture of inorganic fillers,organic resin matrix and silane coupling agent.The constituent structure of DRCs has an important impact on thier properties.However,the basic theoretical research on the structure-properties relationship of DRCs is relatively lacking.The purpose of this paper is to study the relationship between the structure and properties of DRCs by combining theory with experiments to provide a theoretical reference for the development of high-performance DRCs.First,an atomistic cross-linked network model of DRCs with high filler content was established,and the microstructure and properties of the DRCs were explored at the atomic/molecular scale through molecular simulation.The maximum filler loading(MFL)of DRCs with different filler formulations was further predicted by discrete element method(DEM)simulation and packing density model,and the relationship between filler packing structure and properties was explored.Finally,the reinforcement mechanism of nanoparticle clusters(NCs)on the mechanical properties of DRCs was revealed.The main contents and innovations of the paper are as follows:1.The network model construction and properties of high filler content DRCs were explored.An improved multi-step dynamic crosslinking procedure was proposed and firstly applied to construct the reasonable molecular model of high filler content DRCs.The predicted elastic modulus and stress-strain results of DRCs with different nanofiller contents show that the DRCs with a 70 wt% filler content have the most excellent mechanical properties.Furthermore,the dynamic property study indicates that the diffusion coefficient of matrices firstly decreased and then increased with the nanofiller content increasing from 0 to 80 wt%,reaching the lowest value at 70 wt%.The migration of resin matrices can be limited by nanofillers at the content within 70 wt%.However,when the filler content is excessively high(80 wt%),the stability of matrices is destroyed and the fractional free volume of DRCs sharply increases,resulting in a decrease of mechanical properties of DRCs.2.A method based on DEM simulations and experiments was firstly developed to predict the MFL of spherical silica particles at single-level and multi-level filling.The results indicated that the presence of modifier increased the MFL,and the increment changed exponentially with the content of the modifier.Compared with the single-level filling,the addition of secondary fillers was beneficial to increase the MFL,and the increment was affected by the particle size and size ratio.The prediction results show a good agreement with the experiment,which proves a great potential of DEM simulation as a numerical experimental method in studying the MFL,and provides an effective method for the optimization of filler formulations.3.A mathematical model was developed for an efficient prediction of the packing density of inorganic fillers in different sizes and quantities to optimize filler formulations of DRCs with high filler content.The packing density data generated by DEM simulation were used to re-derive the parameters of 3-parameter model.The modifier effect was also induced to develop the modified 3-parameter model.The predicted packing density was validated by simulation and experimental results,and the prediction error is within 1.40%.Furthermore,the modified 3-parameter model was used to effectively predict the packing density of binary and ternary mixes of inorganic fillers to provide a guidance for the design of optimal filler formulation.4.This work pioneers the use of DEM simulations combined with experiments to study the mechanical behavior and reinforcement mechanism of NCs fillers in DRCs.The uniaxial compressive strength(UCS)of NCs-reinforced DRCs have an improvement of 9.58% and 15.02%in comparison with nanoparticles(NPs)and microparticles(MPs),respectively,because of the ability of NCs to deflect cracks and absorb stress through gradual fracturing.By using NCs and NPs as co-fillers,the internal defects of DRCs can be reduced,resulting in a further improvement of UCS of DRCs by 6.21%.Furthermore,the mechanical properties of DRCs can be effectively improved by increasing the strength of NCs or reducing the size of NCs.This study deepens the understanding of relationship between filler structure and mechanical behavior in DRCs at the mesoscale and provides an avenue for the application of DEM simulations in composite materials.