Distinctly Different Electroresponsive Electrorheological Effect in Low-Molecular-Weight and Polymerized Ionic Liquids: Rheological and Dielectric Relaxation Studies

Abstract

Owing to the robust and tunable properties compared to molecular compounds, ionic liquids (ILs) and polymeric ionic liquids (PILs) are very suitable substances to obtain available smart materials with high stability and strong stimulus-responsive characteristic. In this paper, electroresponsive smart electrorheological (ER) effects in low-molecular-weight and polymeric ionic liquids, such as poly­[[2-(methacryloyloxy)­ethyl]­trimethylammonium hexafluorophosphate] (P­[MTMA]­[PF6]) are investigated and compared by a combination of rheology and dielectric relaxation spectroscopy. It is found that although P­[MTMA]­[PF6] and [MTMA]­[PF6] have same constituent ions, their ER effect is distinctly different. Under electric fields, the ER effect of P­[MTMA]­[PF6] exceeds that of the low-molecular-weight monomeric counterpart by 2-fold of magnitude. As the temperature increases, the ER effect of P­[MTMA]­[PF6] remains in a stable level in wide temperature region, whereas that of [MTMA]­[PF6] shows strong temperature dependence. By dielectric relaxation spectroscopy, it can clarify that the different ER effect is attributed to the different polarization characteristics induced by cation/anion pair in different substance circumstances. Compared to low-molecular-weight crystal [MTMA]­[PF6], the slow interfacial polarization with a good temperature dependency related to strong ER effect in wide temperature region is easier to occur in P­[MTMA]­[PF6] because its amorphous polymeric matrix circumstance not only provides lower activation energy for the dissociation and diffusion of ions, but also limits the dissolution of ions in carrier liquid. The result not only deepens the understanding about mechanism behind the electroresponsive ER effects of ILs and PILs but also highlights the potential of PILs as high-performance stimuli-responsive ER materials compared to the low-molecular-weight monomeric counterpart.

Influence of Tethered Ions on Electric Polarization and Electrorheological Property of Polymerized Ionic Liquids

“…First, three PILs were milled and sieved into particles of 5–15 μm. Then, the density of particles was measured via the method reported in our previous paper [ 12 ]. Simply, the particles were added into the pycnometer (5.0 mL) containing silicone oil and the pycnometer was placed in an ultrasonic cleaning bath and connected to a vacuum pump.…”

“…Different from conventional polyelectrolyte particles whose ER property needs to activate by absorbing moisture, the dry PIL particles have strong ER property. This is because, compared to the conventional polyelectrolytes, the ion-pair interaction in PIL particles is weaker due to large and delocalized nature of polyatomic fluorinated constituent ions and, thus, the untethered ions are easy to dissociate and move to induce interfacial polarization [ 12 ]. Furthermore, the hydrophobic nature of fluorinated constituent ions also makes PIL-based ERF to be insensitive to moisture.…”

Electrorheological Fluids of GO/Graphene-Based Nanoplates

“…Under the DC electric field, they found that composite particles with matching electrical properties had more stable ER properties. When the relaxation time of dispersed particles is in the range of 1.6 × 10 −3 –1.6 × 10 −6 s [ 24 , 100 , 101 , 102 ], the particles have an appropriate polarization rate. Under the combined action of electric field and shear field, the particles can produce a sufficient polarization rate and stable interaction force between particles.…”

The Electric Field Responses of Inorganic Ionogels and Poly(ionic liquid)s

“…For ILs, the structures and properties of anion can affect the non-electrostatic interaction between ILs [ 123 ]. Among the anions of ILs, [TFSI] − is an hydrophobic one, whose large size and weak electrostatic interaction with cations lead to relatively large ion mobility and wide use in electrolytes [ 48 , 124 ]. Whereas it may be not the same in ER fluids.…”