Study On Properties Of Electrorheological Fluids And Electrorheological Elastomers Based On Ionic Liquid-modified Titanium Nanoparticles

Electrorheological materials（ER）are a kind of intelligent soft materials composed of polarized micro or nanoparticles dispersed in an insulating medium.Their rheological properties can be quickly and reversibly controlled by applying an external electric field.According to the different ER materials of dispersive medium,they can be divided into electrorheological fluids（ERF）and electrorheological elastomers（ERE）.Under the action of the external electric field,the particles of ERF with insulating liquid as the dispersion medium attract each other and form a chain column structure along the direction of the electric field.It is observed that the ERF transforms from a liquid to a solid,and the viscosity and yield stress of the system increase instantaneously.The matrix of ERE is an elastomer,in which the dispersed polarizable particles attract each other under the external electric field,resulting in a rapid and reversible response to the system modulus and damping factor.The structures and properties of dispersed phase particles are the main factor affecting the properties of ER materials.However,nano-scale dispersed phase particles with high ER effects often increase the zero-field viscosity of ERF greatly,which is not conducive to its application.On the other hand,for ERE,in addition to the influence of dispersed phase particles,the dielectric properties and modulus of the matrix are also important factors affecting the ER effect of the system.Traditional dispersed phase particles are difficult to exhibit high ER effect in elastic matrix,and ERE based on silicone rubber has low mechanical properties,which cannot meet the needs of engineering applications.Therefore,a comprehensive study of the influence of the structure of dispersed phase particles and elastomer matrix on the ER properties is the key to solving the problems faced by ERE.To solve the above problems,titanium dioxide（TiO₂）with a high dielectric constant is selected as the dispersed phase particle in this study,which is chemically modified with ionic liquid（IL）to further improve its electrical conductivity and polarizability,thus improving its response to electric field,and ensuring its stability in a wide temperature range and settling stability.Polyurethane（PU）elastomer with flexible structural tunability was selected as the matrix of ERE.The influence of chemical structures of TiO₂ particles surface and PU matrix on the ER properties was studied.The specific research content of this paper is as follows:Based on the excellent stability,structural designability,and polarizability of IL,the IL（1,3-disulfonic imidazole chloride,[Dsim]Cl）with disulfonic acid group was designed and synthesized,and the IL modified TiO₂ionogel particles（TiO₂-[Dsim]Cl）were prepared.The structural analysis showed that[Dsim]Cl was adsorbate on Ti atom by electrostatic action.Therefore,the addition of[Dsim]Cl limited the condensation of the precursor.It thus reduced the size of the ionogel particles,and[Dsim]Cl was mainly distributed in the surface layer of the particles,which helped to improve the interface polarizability of the particles.ER properties and dielectric analysis showed that the addition of[Dsim]Cl can significantly improve the conductivity and dielectric strength of TiO₂-[Dsim]Cl ERF and shorten the relaxation time,which is related to the amount of[Dsim]Cl.Under the electric field strength of 4.0 k V/mm,the yield stress of TiO₂-[Dsim]Cl ERF（15 vol%）can reach 5.2 k Pa,which was much higher than 1.2 k Pa of TiO₂.In addition,adding[Dsim]Cl can significantly improve the temperature stability and sedimentation resistance of the TiO₂-[Dsim]Cl ERF.TiO₂-[Dsim]Cl ERF had relatively stable yield stress in the range of 5～85℃,and the sedimentation,which was still 98%after standing at room temperature for 30 days.Due to the high electrical conductivity of IL,it is easy to cause high leakage current density of TiO₂-[Dsim]Cl ERF.To solve this problem,acetic acid（AA）was selected as the surface modifier,and TiO₂-[Dsim]Cl-AA nanoparticles were synthesized by adding AA in the process of precipitation preparation of TiO₂-[Dsim]Cl.The structural analysis showed that the carboxyl group in AA with Ti atom through the bidentate ligand interaction formed a Ti-O-C bond,which exposed the methyl group to the outermost layer of the particle and reduced the surface conductivity of the particle to a certain extent.ER analysis showed that the addition of AA significantly reduced the leakage current density of ERF while ensuring that TiO₂-[Dsim]Cl-AA had relatively high yield stress.The yield stress of TiO₂-[Dsim]Cl-AA（25 vol%）can reach 128 k Pa,while the leakage current density and zero-field viscosity were only 60μA/cm²and 7.1 Pa·s.In addition,the ERF containing TiO₂-[Dsim]Cl-AA still had excellent anti-sedimentation stability.Aiming at the problem that ERF is easy to leak in practical application,ERE research based on the above particles is further carried out.Different from previous studies,which mainly used silicone rubber as the matrix,PU was used as the matrix to fill TiO₂-[Dsim]Cl and TiO₂-[Dsim]Cl-AA ionogel particles,respectively,to explore the effects of particle concentration（10-30 wt%）and particle surface chemical structure on the ER properties.The results shows that the TiO₂-[Dsim]Cl-AA particles showed better affinity with PU matrix due to the presence of methyl group,which made it had better dispersion in PU matrix.The structural analysis showed that the addition of particles destroys the degree of microphase separation in PU,which helped to improve the polarizability of ERE under electric field.When the content of nanoparticles reached 20 wt%,the elastomer exhibited an obvious ER effect.Under the electric field of 3.0 k V/mm,the storage modulus of the ERE with TiO₂-[Dsim]Cl-AA（20 wt%）can reach 1.05 MPa.The ERE with TiO₂-[Dsim]Cl（20 wt%）showed the highest ER efficiency of 247%.In addition,the surface of TiO₂-[Dsim]Cl particles contained many polarizable groups,which made PU-based ERE exhibit obvious electro-strictive effect under external electric field,and the thickness deformation was 14%under 3.0 k V/mm.To study the effect of matrix structure on the ER properties further,PU with polydimethylsiloxane（PDMS）as the soft segment was synthesized as the ERE matrix,and TiO₂-[Dsim]Cl nanoparticles as the dispersed phase of ERE.The molecular weight of PDMS（M_n=1000,2000,3000）was applied to investigate the influence of dielectric constant and ER efficiency of ERE.The structure analysis showed that the addition of TiO₂-[Dsim]Cl nanoparticles reduced the crystallization capacity and microphase separation degree of PU.Dielectric and ER analysis showed that the dielectric constant of PU-PDMS was related投the M_n of PDMS,when the M_n of PDMS is 2000,the elastomer had a suitable dielectric constant.The storage modulus of PU-PDMS can reach up to 0.9 MPa at the electric field strength of 3.0 k V/mm,and the ER efficiency was 251%.This indicated that the permittivity of the PU matrix had a significant effect on the ER efficiency of ERE.By adjusting the dielectric constant of the matrix,effective regulation of its ER efficiency can be achieved.