Study On The Influence Of Polymer Structure On Dielectric/Viscoelastic Coupling Mechanism

Dielectric polymers are a class of energy storage and conversion materials,whose energy conversion efficiencies are closely related to the dielectric/viscoelastic coupling behavior.Currently,the key issues in preparing high-performance dielectric polymers include:（1）how to achieve high dielectric constants but low losses at the same time;（2）how to reduce viscoelastic hysteresis losses and maintain the appropriate elastic moduli.However,improving the dielectric properties also leads to higher dielectric losses and viscoelastic hysteresis losses.Aiming at the dielectric/viscoelastic coupling properties,the innovative work and results are as follows:（1）A simulation method on the dielectric and viscoelastic properties of polymers has been developed in the framework of density functional theory combined with all-atom molecular dynamics simulations.The important properties such as dielectric/dielectric loss ratios,actuation sensitivities,viscoelastic hysteresis losses,and glass transition temperatures of different systems were evaluated by this method,which provides a theoretical foundation for studying dielectric/viscoelastic coupling performance.（2）The differences in electronic and dipolar polarization of various systems were investigated by taking into account group charge distribution,dipole orientation,and molecular chain conformation.Polyethylene and poly（methyl acrylate）homopolymers were chosen as examples,their electronic permittivities were calculated using the equilibrium electron density distribution;the dipolar permittivities were obtained by the dipole relaxation process.The results indicate that the electronic permittivities of polyethylene and poly（methyl acrylate）are 2.21 and 2.28,respectively;while their dipolar permittivities are 0.14 and 4.65,respectively.In the comparison of polyethylene and poly（methyl acrylate）,it is clear that the dielectric properties of non-polar polymers are mainly generated by electronic polarization;whereas the dielectric properties of polar polymers are produced by a combination of electronic and dipolar contributions.Further computations show that dielectric losses of polyethylene and poly（methyl acrylate）are equal to 0.0063 and 0.054,respectively.Thus,for homopolymers,low dielectric constants or high dielectric losses make them difficult to meet the requirements for high-performance dielectric polymers.（3）The effects of chain structure on the dielectric/viscoelastic coupling were investigated for the poly（ethylene-co-methyl acrylate）system.The polarizabilities of different chain structures were analyzed based on the molecular chain charge distributions,surface electrostatic potentials,HOMO-LUMO energy levels,and dipole relaxation processes.The properties of dielectric constants,dielectric losses,elastic moduli,and glass transition temperatures were calculated by considering charge transfer and space conformation of molecular chains.The results reveal that the alternating structure have the strongest polarization with a dielectric constant equal to5.29,where dipolar polarization accounts for 54%.By comparing the dielectric/dielectric loss ratios,actuation sensitivities,and viscoelastic hysteresis losses of different sequence structures,it is found that the alternating structure exhibits the best comprehensive performance.However,considering the synthetic conditions in the laboratory,the dielectric/dielectric loss ratio and actuation sensitivity of the triblock structure are increased by 8%and 4%,respectively,and the viscoelastic hysteresis loss is reduced by 1%,implying that the triblock structure has a better comprehensive performance.（4）The effects of microphase separation on the dielectric/viscoelastic coupling for polydimethylsiloxane-b-poly（thioether）-b-polydimethylsiloxane copolymer were also investigated.Firstly,the miscibility between the two polymers was evaluated using solubility parameters;then,the microphase separation structures with different morphologies were formed by varying the relative contents and chain sequences.On this basis,charge,dipole,and molecular distributions were calculated.It is shown that the solubility parameters of polydimethylsiloxane and polythioether are equal to 21.55 and14.55（J/m³）^0.5,respectively,implying that they are poorly miscible and possess the ability of microphase separation.When the molar ratio is 1:1,the triblock molecular chains are self-assembled into a layered and ordered structure,resulting in a large number of incompatible interfaces.The interfacial polarization is found to increase the dielectric constant of the triblock structure by 15%in comparison with the alternating structure without microphase separation.However,the interface increases the dielectric loss and elastic modulus,leading to 29%and 7%reductions in the dielectric/dielectric loss ratio and actuation sensitivity of the triblock structure,respectively.Through the investigations,the laws of microphase separation on charge distribution and transport,polymer chain spatial conformation,and dipole orientation ability have been elucidated,and it is clearly pointed out that microphase separation of block copolymers is not conducive to achieving high energy conversion efficiency.（5）A study on the effect of curved interfacial structure on dielectric/viscoelastic coupling was carried out for the composite system of polydimethylsiloxane and SiO₂nanoparticles.Based on the interfacial electronic structure,the work function,surface electrostatic potential,and electron density difference were calculated for exploring the interfacial polarization mechanism.The effect of interfacial curvature on the energy conversion efficiency was evaluated by counting the normal,tangential,and total dielectric constants based on the charge and dipole moment distribution in different interfacial regions.The interfacial glass transition temperatures were obtained by introducing the external field effect.It is shown that the doping of SiO₂ particles into the polydimethylsiloxane matrix results in a mismatch in the electrostatic potential of the two phases at the interface and leads to interfacial polarization.When the sizes of the particles reaches 3.6 nm,the interfacial permittivity increases by 35%,but the dielectric/dielectric loss ratio decreases by 58%.While the strong attraction of the particles restricts the polymer chain movement,the glass transition temperature is increased by 11%,which means that the viscoelastic hysteresis loss will increase.Comparing the normal and tangential dielectric constants,it is found that the normal dielectric constant is reduced by 24%,thus increasing the difficulty of interfacial charge transfer and reducing the energy conversion efficiency.Therefore,when using nanocomposite methods to prepare high-performance dielectrics,particular attention is required to the problem of reduced energy conversion efficiency due to the reduction of normal dielectric constant.