The Structure,Polarization And Anhydrous Electrorheological Effect Of Anionic Poly (Ionic Liquid)s Containing Highly Delocalized Fluoride Ion

Electrorheological fluid（ERF）is one kind of smart soft material whose rheological property can be quickly and reversibly controlled by an electric field.It has important application potential in shock absorption and noise reduction,rehabilitation appliances,precision polishing,microfluidic control and robots.However,the insufficient performance has restricted the practical application of ERF.Based on the weak anion-cation association and hydrophobic property of the fluoride ion-containing poly（ionic liquid）s（PILs）,our research group developed a new type of polyelectrolyte ER system,which exhibits strong interface polarization and anhydrous ER effect.When the large size of fluorinic ions（e.g.TFSI^-）with high charge delocalization is used as mobile counterions,it can further reduce the anion-cation association energy,promote ion dissociation and interfacial polarization,and thereby bring about higher ER effect.However,the plasticization effect of TFSI^-is easy to reduce the glass transition temperature（_g）of PILs and result in large current leakage and narrow working temperature range of ERF.Therefore,how to overcome the shortcomings of excessive plasticization while taking advantage of the high delocalization effect of TFSI^-is of great significance for the development of PILs-based ER materials with superior performance.To solve this problem,this thesis developed a new ERF based on anionic PILs wich containing tethered TFSI^-on the polymer backbone.This tethered TFSI^-can not only effectively restrict the plasticization effect,but also ensure the delocalization effect of TFSI^-.By comparison with the cationic PILs with a similar polymer backbone but TFSI^-as mobile untethered counterions,it was demonstrated that the anionic PILs containing tethered TFSI^-can significantly reduce the leaking current density and widen the operating temperature range while the ER effect is not significantly reduced.By changing the size and geometry of mobile counter ions,the structure-polarization-ER property relationship was systematically studied to guide the material design of anionic PILs-based ERF with high performance.In order to promote the application of PILs-based ERF,we also developed a Pickering emulsion polymerization method to prepare PILs/inorganic composite ER particles.The composite particles well combined high ER performance PILs with high hardness inorganic nanoparticles and showed high temperature stability,whihch provided a candidate for the novel PILs-based ER polishing particles.The main works are summarized as follows:1.Preparation,polarization and ER effect of anionic PILs containing highly delocalized TFSI^-To maintain high delocalization effect of TFSI^-but effectively limit its plasticization effect,we developed a new hydrophobic anionic PIL ER material of poly[p-styrenesulfonyl-（trifluoromethylsulfonyl）imide][tetraethylammonium]（P[STFSI][N2222]）that contains tethered TFSI^-on the polymer backbone.The ER measurement showed that,compared with the ERF containing cationic PILs（poly[（p-vinylbenzyltriethyl）ammonium][bis（trifluoromethanesulphonyl）imide]（P[VBTEA][TFSI]））with same backbone but inverse ion pair structure,the current density of ERF of P[STFSI][N2222]is significantly reduced and the operating temperature range is significantly widened,while the ER effect is only slightly reduced.By combining DSC,dielectric spectroscopy,Raman spectroscopy analysis and DFT calculation,it was found that after TFSI^-was fixed on the polymer backbone,the plasticization of TFSI^-can be significantly reduced while high delocalization effect is still maintained.This is the reason of the decreased current density and broadened temperature range of P[STFSI][N2222]ERF.2.Influence of size of mobile countercations on the microstructure,polarization and ER property of anionic PILsIn order to understand the structure-rheological property relationship of the anionic PILs having highly delocalized TFSI^-,we synthesized a series of poly[p-styrenesulfonyl（trifluoromethylsulfonyl）imide][tetraalkylammonium]（P[STFSI][Nnnnn],n=1,2 and 3）with different size of quaternary ammonium cations as mobile counterions.The influence of countercation size on the dielectric polarization and ER effect of anionic PILs and the mechanism behind the influence were studied by combining rheology,dielectric spectroscopy,Raman spectroscopy,X-ray scattering and DFT calculation.The results showed that the size of quaternary ammonium cation has a non-monotonic influence on dielectric polarization and ER effect and P[STFSI][N2222]has the best polarization performance and the strongest ER effect.Raman spectroscopy analysis and activation energy measurement showed that,compared with the smaller N1111⁺and larger N3333⁺,the moderately sized N2222⁺as a countercation can provide a higher number of mobile ions and a lower activation energy for ion motion.DFT calculation and X-ray scattering characterization further demonstrated that the influence of the quaternary ammonium cation on the aggregation state structure of PILs is an important reason for the change of the number of mobile ions and the activation energy of ion motion with the size of mobile countercations.The low matrix elastic potential of uniform amorphous P[STFSI][N2222]can lead to lower ion motion activation energy for its high interfacial polarization and ER effect.3.Influence of geometry of mobile countercations on the conductivity,polarization and ER effect of anionic PILsUsingdifferentgeometriesofcations（Z=tetraethylammonium ion（N2222⁺）,1-propyl-1-methyl pyrrolidinium（PMPyrr⁺）,1-propyl-pyridinium（PPyri⁺）and1-propyl-3-methyl imidazolium（PMIm⁺））as mobile counterions,we further synthesized a series of poly[p-styrenesul-fonyl（trifluoromethylsulfonyl）imide]based anionic PILs（P[STFSI][Z]）.The influence of geometry of countercations on the interfacial polarization and rheological effect of anionic PILs containing tethered TFSI^-and the mechanism behind the influence were studied by rheology,dielectric spectroscopy,Raman spectroscopy and DFT calculation.The results showed that as the geometry of cations changes from tetrahedron to plane,the polarization properties of anionic PILs gradually improve and the ER effect gradually increases.Through activation energy measurement,DFT calculation and Raman spectroscopy analysis,it was found that as the geometry of cations changes from tetrahedron to plane,the P[STFSI][Z]gradually softens,the ion mobility and conductivity gradually increases,the polarization rate gradually increases and thus,the ER effect increases.When the planar cations are used as counterions,it can effectively improve the ER effect of P[STFSI][Z],especially at low temperature.4.Pickering emulsion polymerization,structure and property of PILs/Si O2 composite ER particlesIn order to promote the application of PILs-based ERF,we developed one kind of composite particles with both electrorheological properties and high hardness-poly[2-（methacryloyloxy）ethyltrimethylammonium][bis（triuoromethanesulfonyl）im ide]/Si O₂（P[MTMA][TFSI]/Si O₂）by Pickering emulsion polymerization.The composite structure was confirmed by scanning electron microscope（SEM）,transmission electron microscope（TEM）and energy dispersive X-ray detector（EDX）.It was found by ER measurement that the introduction of nano-Si O₂ particles can significantly reduce the leakage current density and improve the temperature stability of rheological property.It was found by dielectric spectra measurement that Si O₂ can act as cross-linking points to control the transport of counterion in the PIL matrix and inhibit the thermal activation of segment relaxation.This is the reason why the composite ER particles show low leakage current density and high temperature stability.The composite particles combined the high hardness of inorganic nanoparticles and the excellent ER property of PILs,which provides a candidate for the development of electric field-assisted polishing materials in the future.