Recent research of using poly(ionic liquid) (PIL) particles as the dispersal phase has provided a new strategy to develop a high-performance anhydrous polyelectrolyte-based electrorheological (ER) fluid. However, the working temperature range of the ER fluid of PIL particles is still narrow due to an inherently low glass transfer temperature caused by the plasticization of polyatomic organic counter ions in PILs. In this paper, we develop a new ER system based on cross-linked PIL (C-PIL) particles and demonstrate that crosslinking with a suitable degree only slightly degrades the ER properties but significantly improves the working temperature range of the ER fluid of PIL particles. By using differential scanning calorimetry, rheology, and dielectric spectroscopy, we systematically study the ER properties of C-PILs and their temperature dependence at different crosslinking levels and understand the mechanism behind the improved temperature effect. The results indicate that crosslinking can effectively increase the glass transition temperature of PIL particles and enhance local ion-motion induced interfacial polarization by suppressing the thermally promoted long-range drift of mobile counter ions in the PIL matrix, and this results in the improved temperature effect of the ER fluid of C-PIL particles.
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.
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