Interfacial Polarization and Electroresponsive Electrorheological Effect of Anionic and Cationic Poly(ionic liquids)

Owing to weak ion pair electrostatic interaction and intrinsically hydrophobic nature, poly(ionic liquids) (PILs) have potentiality as anhydrous polyelectrolyte-based electrorheological (ER) smart materials. Up to now, however, only cationic PILs as ER materials have been studied. In this work, we report the first ER materials based on anionic poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide tetraethylammonium] (P[STFSI][N2222]) and conduct a comparative study on electroresponsive ER effect with its cationic counterpart poly[(p-vinylbenzyl)triethylammonium trifluoromethanesulfonamide] (P[VBTEA][TFSI]) having similar structure of component ions. It shows that the ER effect of anionic P[STFSI][N2222] is sound but it is different from that of cationic P[VBTEA][TFSI], though they have a similar structure of ions. At room temperature, the ER effect of P[VBTEA][TFSI] is higher than that of P[STFSI][N2222], while the power consumption of P[STFSI][N2222] is lower than that of P[VBTEA][TFSI] and the working-temperature region of P[STFSI][N2222] is broader than that of P[VBTEA][TFSI]. Dielectric spectroscopy study indicates that compared to P[STFSI][N2222], the stronger ER effect of P[VBTEA][TFSI] is because P[VBTEA][TFSI] has larger intensity and faster rate in interfacial polarization than P[STFSI][N2222]. Raman spectra and activation energy measurements demonstrate that the different polarization characteristics further originate from the fact that compared to N2222+ cation, TFSI- anion as mobile charge can contribute higher charge number and mobility due to its strong plasticization effect. But the plasticization effect also increases the segment relaxation of PILs and thus results in large ion leakage and a narrow working-temperature region of P[VBTEA][TFSI]. This study extends the diversity scope of PILs for ER materials and gives an in-depth understanding about the influence of component ion structure on the interfacial polarization and the ER effect of PILs.