Anhydrous Electrorheological Effect And Its Temperature Dependence Of Crosslinked Poly(Ionic Liquid)s

Electrorheological fluid(ERF)is a kind of soft smart material whose rheological properties can be rapidly and reversibly adjusted by an electric field.It has potential applications in many fields such as intelligent control,intelligent manufacturing,health care,energy,robot and so on.However,insufficient ER performances limit the practical applications of ERF.Polyelectrolyte is considered to be one of the most promising ER materials due to its low density,good softness and high ER effect.However,the conventional polyelectrolytes need to absorb a little water to activate the ER effect,the problems of high leakage current,thermal instability and dielectric breakdown caused by water also influence the stability of the materials and restricted the real applications.Recently,our group has invented a new polyelectrolyte-based ERF on the basis of poly(ionic liquid)(PIL)particles.Different from the conventional polyelectrolytes,the PILs are composed of large size fluorinated polyatomic component ions.The component ions can endorse the PIL ER particles with weak ion pair electrostatic interaction and intrinsically hydrophobic nature.As a result,the PILs can have enough concentration of mobile ions to induce strong interfacial polarization and ER effect in the dry glassy state.In particular,because of the intrinsically hydrophobic nature,the PIL-based ERF is not sensitive to moisture.Therefore,PILs are providing novel platforms to develop new-generation anhydrous polyelectrolyte-based ERF with versatile performances.However,the polyatomic fluorinated counterions also have plasticizing function,which inevitably reduces the glass transition temperature(T_g)of PIL particles.As a result,the working temperature range of the PIL-based ERF is limited because the PIL particles easily soften and even aggregate at the temperature above T_g due to segment relaxation.To solve the problem,in this dissertation,we have firstly understand the critical influence of the glassy polymer matrix on the anhydrous ER effect of PILs and then proposed a new crosslinking method to improve the working temperature range of PIL-based ER materials with large size organic counterions.By changing the crosslinked spacer and the size of the counterions,we have studied the influence of crosslinked structure on the ER effect and the reasons of the improvement of temperature dependence of ER effect of crosslinked PILs,and preliminarily established the structure-property relationship of crosslinked PILs.Finally,based on the structure-property relationship and the understanding of the origin of the improvement of the temperature dependence of the ER effect of crosslinked PILs,we have prepared a kind of crosslinked PIL-based ER materials with high ER effect and wide working temperature range.The main works of this paper include:1.The mechanism behind anhydrous ER effect of PILs has been studied by a comparable investigation of ER effect of PILs with its monomer counterpartWe have prepared the poly[[2-(methacryloyloxy)ethyl]trimethylammonium hexafluorophosphate](P[MTMA][PF₆])by free radical polymerization and conduct a comparable study on ER effect with its monomer counterpart([MTMA][PF₆]).The morphology,structure and the thermal properties of the samples have been characterized by SEM,NMR,XRD,TGA and DSC,and the ER effect at different temperatures has been measured by rheometer under electric fields.It has found that PILs show a much higher ER effect than the corresponding monomer counterpart.The dielectric analysis has indicated that,compared with the crystalline structure of monomer counterpart,the glassy polymer matrix in PILs not only provides lower activation energy to the dissociation and motion of ions but also limit the solubility of dissociated ions into the carrier liquid,which results to strong interfacial polarization and strong anhydrous ER effect of PILs.2.A crosslinking method has been proposed to improve the working temperature range of ER effect of PILsWe have prepared crosslinked PILs by free radical copolymerization of divinylbenzene(DVB)and[VBTMA][TFSI]with different weight ratio.It has found that with the increase of the crosslinker,the T_g of PILs shifts towards higher temperatures.The ER results have shown that with the increase of the crosslinker,the ER effect of PILs at room temperature decreases slightly,but the working temperature range broadens gradually and the leaking current density also decreases,the ER efficiency increases.The dielectric analysis has indicated that,although cross-linking enhances the activation energy of ionic movement and decreases the rate of interfacial polarization induced by the local motion of the ions,the cross-linking significantly restricts the segment relaxation of PILs and improves the thermal stability of PILs.This should be the reason of improvement of the working temperature range of ER effect of PILs.3.The origin of the improvement of the temperature dependence of the EReffect of crosslinked PILs has been studied by investigating the ER effect of self-crosslinked PILs with different lengths of alkyl spacerWe have designed and prepared a series of divinyl imidazole self-crosslinked PILs(P[C_nDVIM][TFSI])with different lengths of alkyl spacer.The ER results have shown that with the decrease of the length of alkyl spacer,the crosslinked polymer network becomes dense and the number density of mobile ions increases,but the ER effect of self-crosslinked PILs decreases and the working temperature range is broadened.The dielectric spectra and WAXS analysis have indicated that the change of the number of mobile ions after crosslinking is not the key factor affecting the ER effect of the crosslinked PILs.The density change of the crosslinked network is the critical factor to affect the ER effect and its temperature dependence of the crosslinked PILs because the density change of the crosslinked network strongly influences the activation energy of ion motion and the interfacial polarization induced by the local motion of ions.4.The origin of the improvement of the temperature dependence of the ER effect of crosslinked PILs has been deeply understand by investigating the ER effect of self-crosslinked PILs containing types of counterionsWe have further designed and synthesized a series of divinyl imidazole type self-crosslinked PILs with different counter anions(P[C₂DVIM][X],X=tetrafluoroborate(BF₄^-),PF₆^-,trifluoromethane sulfonate(Tf O^-),TFSI^-)by free radical polymerization.The ER results have shown that all of the self-crosslinked PILs containing different counterions show a high ER effect and wide working temperature range,but the type and the size of the counterions have an important effect on the ER effect and the temperature dependence of P[C₂DVIM][X].With the increase in the size of the counterions,the ER effect decreases.However,different from the linear PIL system,P[C₂DVIM][X]contains inorganic counterions show a higher ER effect than P[C₂DVIM][X]with organic counter anions,and the temperature dependence is much different.By combining the dielectric spectra analysis and WAXS analysis,we have found that the mesh size of the crosslinked network directly influences the local movement of dissociated anions and the interfacial polarization induced by the local motion of mobile ions,and this should be responsible for the fact that the self-crosslinked PILs show different dependence on the size and type of counterions from the linear PILs.5.On the basis of understanding the origin of the improvement of the temperature dependence of the ER effect of crosslinked PILs,we have prepared crosslinked PILs with high ER effect and wide working temperature rangeOn the basis of understanding the origin of the improvement of the temperature dependence of the ER effect of crosslinked PILs,we finally have designed and prepared a kind of diethylene benzylammonium self-crosslinked PILs(P[BVDEA][PF₆]).The ER results have shown that the crosslinked PILs show a high ER effect and a wide working temperature range.The yield stress of P[BVDEA][PF₆]ERF is 1.6 times to that of the linear P[VBTMA][PF₆]ERF and the P[BVDEA][PF₆]ERF can still show high ER effect at high temperature of 120?.