New Dental Resin Composites Based On A Starburst Monomer

1.BackgroundDimethacrylate-based chemistries feature extensively as resin monomers in dental resin-based materials due to their superior reactivity,exceptional optical and mechanical attributes.Their application covers across various dental practices,serving as restorative materials,pit and fissure sealants,cavity liners,veneers,crowns,endodontic sealers,and orthodontic devices.Amid these chemistries,bisphenol A glycol dimethacrylate（Bis-GMA）holds prominence in commercial dental resin-based materials.Despite its relatively higher mechanical performance,Bis-GMA is hampered by specific limitations attributable to its rigid phenyl ring,polar hydroxyl groups,and linear structure.Challenges endure,encompassing volumetric shrinkage,inadequate mechanical attributes,and estrogenicity.They may lead to microleakage,secondary caries,restoration fracture and potential health risks.Overcoming these limitations is imperative.The polyurethane-based monomer,diurethane dimethacrylate（UDMA）,has been partially or wholly used to substitute Bis-GMA in numerous commercially accessible dental resin-based materials due to its lower viscosity,improved biosafety,and heightened biocompatibility.However,despite UDMA’s lower viscosity vis-à-vis Bis-GMA,it manifests augmented volumetric shrinkage and reduced mechanical attributes,primarily due to its marked flexibility and absence of a phenol ring within the monomer chain.Presently,challenges persist in developing a novel monomer possessing acceptable viscosity while yielding a cured resin matrix characterized by robust strength,minimal volumetric shrinkage and biocompatibility in the realm of dental resin-based materials.Enhancing performance often entails augmenting monomer branching to achieve low viscosity and limited volumetric shrinkage.Compared to linear molecules,branched molecules exhibit lower viscosity,a more compact configuration and diminished free volume.Terminal multifunctional groups on branched molecules contribute to heightened crosslinking density,pivotal for mechanical properties.Post-curing,the polymers’internal branching structure engenders numerous diminutive“cavities”capable of deformation under pressure,tension,or impact.This deformation mechanism augments material toughness and strength,thereby contributing to overall performance.Starburst structures,which belong to a distinctive group of topological variations,consist of multiple star-branched"arms"connected to crosslinked"cores".Selecting distinct arms and cores permits the attainment of desired physicochemical attributes.The unique topologically branched structures of starburst molecules have a significant impact on their hydrodynamic volumes.As a result,starbrust structures exhibit lower viscosities even when compared to linear analogues with identical molecular weights.Hence,this study proffers an innovative strategy to form a starburst structure via the reaction of pre-synthesized multifunctional core and star-branched arms with a complementary reactive terminus.By harnessing the reactivity of isocyanate groups and the resilience conferred by urethane bonds,the aim is to synthesize an end-functional starburst monomer offering low viscosity,substantially improved mechanical attributes,and curtailed volumetric shrinkage in the resin matrix and evaluate its performance when used as the matrix of dental composites.2.Aim2.1 Investigate the synthesis process of the starburst structured monomer—Nine-armed starburst polyurethane acrylate（NPUA）,and evaluate whether NPUA meets the basic performance requirements as a dental resin monomer in terms of biocompatibility,physicochemical properties,mechanical properties,and clinical handling.2.2 Explore the formulation,physicochemical and mechanical properties of NPUA-based resin matrix,and verify its potential as a resin matrix for dental resin composites.2.3 Construct NPUA-based dental resin composites,investigate their compatibility and interfacial bonding effects with the primary dental filler silicon dioxide（Si O₂）,and evaluate the physicochemical and mechanical properties of the NPUA-based dental resin composites.Improving the interface bonding between the filler and NPUA by hydrofluoric acid etching on the surface of the filler.This research aims to provide new strategies for further improving the mechanical properties and reducing polymerization shrinkage of dental resin restorations.3.Methods3.1 For the high reactivity of isocyanate（-NCO）and alcohol hydroxyl（-OH）groups,a nine-armed starburst structured monomer NPUA was synthesized,which has a triazine core formed through graft copolymerization.The reaction rate and completion degree was monitored using the hydrochloric acid-dibutylamine method and attenuated total reflection ATR-FTIR;the product was separated and purified using chromatography,and the purity of NPUA was determined with High-Performance Liquid Chromatography（HPLC）.Characterize The molecular structure and molecular weight of NPUA was characterized using ATR-FTIR,nuclear magnetic resonance（NMR）spectroscopy,and high-resolution mass spectrometry（HRMS）.3.2 The cell compatibility of NPUA was assessed through CCK-8 cytotoxicity tests and live/dead cell staining.3.3 The rheology,polymerization conversion rate,polymerization shrinkage,flexural strength,flexural modulus,hardness,and thermogravimetric changes of NPUA were evaluated to assess its physicochemical properties,mechanical properties,and thermal stability.The cross-linked network and mechanical properties of NPUA after polymerization theoretically analyzed using molecular dynamics.3.4 NPUA-based photo-cured resin matrices was prepared and the effect of the content of the active diluent triethylene glycol dimethacrylate（TEGDMA）on the physicochemical properties of NPUA was studied by measuring the viscosity,polymerization conversion rate,polymerization shrinkage,water absorption,solubility,and water contact angle of the resin system.3.5 The flexural strength,flexural modulus,hardness,fracture mode,and thermal stability of NPUA/TEGDMA photo-cured resins were tested using a universal testing machine,nanoindentation instrument,field-emission scanning electron microscope（FE-SEM）,and thermogravimetric analyzer.The influence of TEGDMA on the mechanical and thermal properties of NPUA was investigated,and the optimal performance formula that meets the requirements of resin matrices based on comprehensive performance was decided.3.6 Based on the optimal resin formula above,the Si O₂/NPUA/TEGDMA resin composites was prepared and their polymerization conversion,polymerization shrinkage and mechanical properties were tested.The Bis-GMA/TEGDMA resin composites with the same content of Si O₂ filler was used as the control group.3.7 The effect of filler/resin matrix interface bonding on the properties of NPUA-based resin composites was investigated by hydrofluoric acid etching of Si O₂.4 Results4.1 Using hexamethylene diisocyanate trimer（HDIT）and pentaerythritol triacrylate（PETA）as raw materials,under the catalysis of dibutyltin dilaurate（DBTL）,the starburst-structured monomer NPUA was successfully synthesized.HPLC analysis showed that the purity of the NPUA monomer was over 97%.Characterization by FTIR,HRMS,and NMR confirmed that the molecular structure and molecular weight of the NPUA monomer were as expected.4.2 The CCK-8 cytotoxicity assay indicated that the median lethal concentration(LC₅₀)of the NPUA monomer to fibroblast cells was between 140-160μg/m L,significantly higher than that of Bis-GMA(LC₅₀ between 40-60μg/m L).Live/dead cell staining with Calcein-AM revealed that at 60μg/m L,the density of live cells with NPUA was markedly higher than in the Bis-GMA group.4.3 The viscosity of the NPUA monomer（158±1 Pa·s）was only 13%of that of Bis-GMA,and the polymerization shrinkage rate（2.5±0.1%）was 53%of Bis-GMA.The polymerization conversion rate after 40 seconds of visible light curing was 23.8%,which is lower than that of Bis-GMA（P<0.5）.The flexural strength,flexural modulus,and hardness of NPUA were 165.34±7.67 MPa,7.04±1.83 GPa,and 0.81±0.24 GPa,respectively,significantly higher than those of Bis-GMA（P<0.05）.Thermogravimetric analysis（TG）showed that the thermal decomposition temperatures at 5%,10%,and 50%mass loss were higher for NPUA than for common dental resin monomers Bis-GMA,UDMA,and TEGDMA.Molecular dynamics and mechanical property simulations indicated that NPUA had less free volume and a higher elastic modulus after cross-linking compared to the Bis-GMA system.4.4 When the content of the active diluent TEGDMA was within 20-50 wt%,polymerization shrinkage rate of the NPUA-based resin were lower than the control Bis-GMA（70 wt%）/TEGDMA（30 wt%）group,and the hydrophobicity was higher（P<0.05）.Additionally,as the TEGDMA content increased,the viscosity and water contact angle of the NPUA-based resin decreased,while the polymerization conversion rate and polymerization shrinkage rate increased.Notably,the NPUA-based resin system with30-40%TEGDMA had a viscosity close to that of the control group,meeting the clinical requirements for resin matrix viscosity.4.5 The flexural strength,flexural modulus,and hardness of the NPUA-based resin were all higher than those of the Bis-GMA-based resin.With an increase in NPUA content,the flexural strength,flexural modulus,and hardness gradually increased and stabilized at 70wt%and above（P<0.05）.FE-SEM images showed that the NPUA-based resin exhibited a ductile fracture interface.TG results indicated that the thermal decomposition temperatures at 5%,10%,and 50%mass loss were higher for NPUA than for the control group.4.6 The polymerization conversion rate of the NPUA-based resin nanocomposites was higher,and the polymerization shrinkage rate was lower than that of the control group（P<0.05）.FE-SEM images revealed that silanized Si O₂ fillers were uniformly distributed within the NPUA-based resin matrix and had good interfacial bonding with the resin matrix.The flexural strength,flexural modulus,and hardness of the NPUA-based resin nanocomposites were significantly higher than those of the Bis-GMA-based group（P<0.05）.4.7 Hydrofluoric acid etching of Si O₂ filler improves the flexural strength,flexural modulus and hardness of NPUA-based resin composites.5.Conclusions5.1 This research work resulted in the synthesis of a rigid-flexible nine-arm starburst monomer NPUA via the grafting-onto approach.Compared to the primary dental resin monomer Bis-GMA,NPUA exhibits reduced viscosity,volumetric shrinkage and superior biocompatibility.5.2 The NPUA-based resin systems outperform the Bis-GMA-based resin system across multiple aspects of volumetric shrinkage,water absorption,water solubility,and hydrophobicity.5.3 NPUA has good compatibility with Si O₂ fillers in dental resin composites.The NPUA monomer has the potential to replace Bis-GMA monomer in traditional dental resin matrices.Hydrofluoric acid etching of Si O₂ fillers can improve the interfacial bonding of NPUA-based resin composite.NPUA-based resin composites meet the needs of clinical dental resin restorations.