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

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.