Design And Structure-Property Research Of Physically Crosslinked Elastomer Nanocomposites:Simulation And Experiment

Elastomers are widely used and have become indispensable strategic materials for social development attributed to their outstanding extensibility and recoverability.However,elastomers need to be mechanically reinforced before utilizing.Adding functional nano materials can improve their mechanical properties.The crosslinking process of traditional chemically crosslinked elastomers usually produces toxic and harmful substances and chemically crosslinked elastomers are difficult to be recycled.Therefore,there are many environmental and resource waste problems.Introducing functional nano materials into elastomers to prepare physically crosslinked elastomer nanocomposites can not only enhance their mechanical properties but also bring about self-healing and reprocessing ability.Microstructure and interface interaction strength have significant influences on the macroscopic properties of elastomer nanocomposites,and the physical interaction strength is weaker than that of chemical bonds.Although researchers have improved the mechanical properties of physically crosslinked elastomer composites by increasing the physical interaction strength and density,they still can not meet the requirements of high mechanical property working conditions.In this dissertation,based on different interfacial styles（interaction at interface and interface structures）between functional nano materials and elastomers,the molecular dynamics（MD）simulation method is adopted to design and investigate the elastomer nanocomposites with“brick-mud”structure,penetration structure and hydrogen bond network structure.The structure-property relationship between microstructure and macroscopic properties of the nanocomposites is established.Finally,the simulation results of the elastomer nanocomposites with hydrogen bond network structure are verified by experiments.The main contents and results are as follows.Matrix-free elastomer nanocomposites constructed by grafted elastomer chains（“mud”）and graphene nanosheets（“brick”）with“brick-mud”structure are designed.The effects of interaction strengthε_end-end and chain length L_g on the tensile mechanical properties（stress-strain curve and stress at definite elongation）,the mechanical reinforcement mechanisms,as well as self-healing behavior,are investigated via MD simulations.When the interaction strengthε_end-end is high,the physical network is robust,resulting in good mechanical properties.Attributed to the formation of a better“brick-mud”structure,a longer chain length L_g leads to a better dispersion state,which brings about better mechanical properties.The physical network formed by the functional groups at the end of elastomer chains is the key source of mechanical reinforcement.The clusters formed by the aggregation of these functional groups are gradually destroyed in the deformation process which can dissipate energy.Therefore,the mechanical properties are enhanced.The self-healing efficiency is positively correlated with the self-healing time,but the self-healing efficiency has an upper limit with the extension of self-healing time.Because the activation energy of the end group aggregation process is greater than that of the cluster disaggregation process,the aggregation process is more sensitive to temperature than the disaggregation process,which means that high temperature is beneficial to the self-healing process.Driven by enthalpy,physically crosslinked elastomer nanocomposites with high mechanical properties formed by functionalized linear elastomer chains penetrating the pores of porous fillers（PFs）are innovatively designed.The effects of chain stiffness k,interaction strengthε_center-end and PF weight fraction w on the microstructures and tensile mechanical properties of the nanocomposites are investigated via MD simulations.The penetration structure between elastomer chains and PFs is conducive to dispersing PFs in the elastomer matrix and enhancing the mechanical properties of nanocomposites.This penetration structure is also the key source of mechanical reinforcement.Compared with neat elastomer materials,the elastomer chain size and the glass transition temperature increase while the elastomer chain mobility decreases in nanocomposites.Intermediate k,ε_center-end,and w can bring about the best mechanical property due to the robust penetration structure.Elastomer nanocomposites with hydrogen bond network structure are designed and prepared by introducing zirconium-based metal-organic framework UiO-66 into polyurea elastomer matrix.Firstly,the effects of hydrogen bond network structure on the tensile mechanical properties and reprocessing ability,together with the mechanical reinforcement mechanisms are explored by MD simulations.Then,the nanocomposites are synthesized by experimental methods,and the mechanical properties as well as the structure-property relationship are studied.The simulation conclusions are verified by experiment results.The good dispersion state of UiO-66 nanoparticles（NPs）is beneficial to constructing the hydrogen bond network structure,resulting in better mechanical properties.Intermediate w can lead to a good dispersion state of UiO-66 NPs and hydrogen bond network structure.The results of simulations and experiments show that 8 wt%is the relatively best weight fraction that can obtain the relatively best mechanical properties.Due to the binding effect of the hydrogen bond network formed by the isocyanate parts of elastomer chains and UiO-66 NPs,the glass transition temperature increases while the elastomer chain mobility decreases in nanocomposites.The mechanical reinforcement mechanisms are analyzed from the perspective of energy,the mechanism of nanocomposites resisting outside force can be divided into three stages:（a）when strain is very low,the elastomer chains are rapidly stretched to resist outside force;（b）when the physical crosslinking points are stretched,the physical network begins to resist the outside force and becomes the main source of mechanical reinforcement;（c）when strain is high,the slipping and unentanglement of elastomer chains are the main sources of mechanical reinforcement.In addition,the stress at 500%during the second tensile process can reach 80%of the initial tensile stress,indicating that this kind of material possesses reprocessing ability.