Interfacial Polarization and Electrorheology of Suspensions Containing Monodispersed Ellipsoidal Poly(ionic liquid) Particles

The shape parameters of dispersed phase particles, such as the aspect ratio, have a significant influence on the performance of composite and suspension materials. However, due to the difficulty in preparing uniform dielectric particles having the same individual particle volume and chemical structure but different aspect ratios, the influence of the aspect ratio on the electrorheological (ER) effect of particle suspensions remains controversial. In this article, we prepared uniform ellipsoidal poly(ionic liquid) (PIL) particles with different aspect ratios via a thermal stretching method and investigated the influence of aspect ratio on the ER effect of suspensions containing ellipsoidal PIL particles. Because the ellipsoidal PIL particles are obtained by uniaxially stretching the identical uniform PIL microspheres, they are prolate ellipsoids of revolution (spheroids) having the same individual particle volume, density, and chemical structure and uniform size distribution and shape. In particular, the PIL particles are not hydrophilic because they have polyatomic fluorine-containing counterions. Thus, we remove the influences of volume, density, chemical structure, size and shape distribution of particles, and external water on the ER effect, leaving the aspect ratio as the sole factor to affect the ER effect. Under electric fields, we found that, unlike the early conclusion, the ER effect of PIL particle suspensions first decreases and then increases with the increase in aspect ratio. Dielectric spectroscopy analysis and microscopic ER structure observation showed that the difference in the dielectric intensity and the relaxation time of the two interfacial polarizations along the long axis and short axis of low-aspect-ratio ellipsoidal PIL particles is not significant, which leads low-aspect-ratio ellipsoidal particles to orientate randomly along the direction of the electric field and form short and thin chain-like ER structures, and this is the reason why their ER effect is lower compared to their spherical counterparts. However, high-aspect-ratio ellipsoidal PIL particles exhibit significantly increased polarization intensity along the long axis, which leads them to orient with their long axis along the direction of the electric field and form a thick, network-like ER structure at even low particle volume fraction, and this is why their ER effect significantly increases. Additionally, we found that the settling velocity of the low-aspect-ratio ellipsoidal PIL particle suspension with a moderate volume fraction is even faster than that of the spherical counterpart, while that of high-aspect-ratio particle suspensions is significantly reduced. The results of this paper give clear clarification about the influence of aspect ratio on the ER effect of particle suspensions.