Multi-Level Structure And Electromechanical Properties Of Dielectric Elastomer Materials:A Molecular Dynamics Simulation Study

Dielectric elastomer materials have broad prospects in the fields of artificial muscles,micro-robots,and energy collectors.However,at present,it is still difficult to grasp the fine structures of the materials,and the interaction between mechanical and electrical properties is still unclear.Given this,acrylate-based and silicone-based dielectric elastomers are taken as the research objects,and molecular dynamics simulation combined with quantum chemical calculation is used to investigate the structures of molecular chain,composite,interface,locality,and properties such as dielectric,mechanical,electromechanical coupling and electromechanical stability.The main results are as follows:（1）A set of theoretical calculation methods that can describe the microstructure and electromechanical properties of the materials are constructed considering the coupling effect of dielectric/mechanical properties.The effects of polar molecular chain structure and pre-stretching on the electromechanical properties of the system are investigated using the butyl acrylate/glycidyl methacrylate copolymer as an example.As the content of glycidyl methacrylate increases,the orientation polarization ability and the dielectric constant increase,the dielectric loss increases and then decreases,the modulus increases,and the permanent deformation and hysteresis loss decrease.The electromechanical coupling analysis shows that when glycidyl methacrylate is about 22%,the dielectric/loss ratio of the system increases to55.5,the electrical sensitivity factor increases to 0.82,and the driving strain reaches 0.16,so the comprehensive performance is the best.With the increase of pre-strain,the dielectric constant changes from isotropic to anisotropic,showing an increasing trend followed by a decreasing trend.It is found that the dielectric constant is the highest at 200%strain,and the comprehensive performance is improved the most.Therefore,it is necessary to consider not only the molecular chain structure but also the appropriate pre-stretching in the process of material synthesis and device design.（2）The nanocomposite system of Si O₂/poly（methyl acrylate）is constructed.The effect of Si O₂ surface adsorption energy on the electromechanical properties of acrylate-based dielectric elastomers is investigated under the condition of infinite dilution.Through the analysis of static structure,electronic structure,interfacial polarization,and interaction,it is clear that chain conformation reconstruction and charge rearrangement promote the improvement of interface polarization ability and interface dielectric response.When the adsorption energy is weak,the polymer chains are less affected by the nanoparticles;the interfacial polarization ability is weak;the improvement of the dielectric properties is small.Too high adsorption energy will lead to the anchoring of molecular chains,which will inhibit the ability of charge transfer and dipole moment reorientation,weaken the interfacial polarization,and reduce the comprehensive dielectric efficiency and mechanical-electrical coupling performance.When the adsorption energy is about 100 k J·mol^-1·nm^-2,the chain conformational reconstruction and dipole activity are relatively balanced.Thus,the dielectric constant can be increased by 77%and the dielectric/loss ratio by 42%,with excellent overall performance.（3）Based on the surface energy control of Si O₂,the effect of particle concentration on the electromechanical properties of Si O₂/acrylate-based dielectric nanocomposites is investigated.It is shown that the dielectric constant is highly sensitive to the concentration of Si O₂ filler.That is,with the concentration increases,the dielectric constant increases and then decreases,but the dielectric ratio increases monotonically.When the filler concentration is low,the interface dielectric response is weak,and the improvement of dielectric constant,dielectric loss,and electrical sensitivity factor is small.If Si O₂ is overloaded,it will cause interface overlap and excessive accumulation of molecular chains.The conformational transformation ability and dipole activity are reduced,so the dielectric constant is decreased.When the concentration of Si O₂ is about 7 vol%,the interface has a proper dielectric response.The dielectric constant is 5.5,the dielectric ratio can reach 58,and the electrical sensitivity factor is 1.35,so the comprehensive performance is the best.Therefore,in the interface design of nanocomposites,the surface adsorption should be controlled to maximize the dielectric response,and the filler concentration should be controlled to avoid interface overlap.（4）Taking the silicone rubber-based dielectric elastomer material as an example,the local electric field model is adopted to evaluate the relevant local structure transition of the dielectric film from the initial flat state to the dielectric breakdown based on realistic boundary conditions.According to the changes in free volume,local elastic modulus,dipole orientation,chain end ratio,and nucleation point,it is found that the Maxwell stress forms an equilibrium with elastic stress at lower electric field strength.with electric field increases to the failure strength,the chain end leads to low modulus,high dipole orientation,and concentrated stress in a region.Then it goes through the stages of cavity embryo formation,metastable critical nucleation,hole nucleation barrier crossing,and stable hole emergence,which is a typical thermodynamic first-order phase transition behavior.Where the stable cavities gradually increase and merge into clusters,eventually forming a stable conducting channel.The electrons accelerate along the channel,then electromechanical instability occurs,accompanied by the change termination of dipole orientation and free volume.In contrast,pre-stretching makes chain orientation degree increase,chain-end effect decrease,critical nucleation volume increase,hole nucleation is difficult,and electromechanical stability is improved.Therefore,in the synthesis and design of dielectric elastomers,defects such as chain ends should be avoided,and the degree of chain orientation should be improved by pre-stretching.In summary,by discussing the response relationship between the multi-level microstructure and comprehensive electromechanical of dielectric elastomer materials,we understand the nature of molecular chain structure affecting mechanical-electrical enhancement,clarify the direction of interfacial regulation of polymer nanocomposite systems,and elucidate the evolution mechanism of electromechanical instability and dielectric failure.Such results provide a scientific basis for the design and preparation of long-life high-performance dielectric elastomer materials.