Laterally Substituted Ionic Liquid Crystals And The Resulting Rheology Behavior

Ionic liquids have attracted a lot of attention owing to their negligible vapor pressure, highion conductivity and wide electrochemical windows. As a kind of special ionic liquids,ionic liquid crystals combine the inherent features of ionic liquids with the supramolecularself-organization of liquid crystals, and can be used as anisotropic ion conductors, orderedreaction media and templates for synthesizing nano materials. However, someshortcomings, such as relatively high melting points and viscosity, are also combined fromboth sides, and become the obstacles for industrial applications. Besides pursuinglow-melting ILCs with low viscosity, of particular interest is to bridge the correlationbetween molecular topology and physical properties, as well as the mesophase behavior.In nonionic liquid crystals, detailed and systematic work by Weissflog and Demus hasshown that the introduction of lateral substituent groups to rod-like mesogens will lead toan evident decrease in both of the melting and clearing point, which can be quite differentafter the introduction of strong electrostatic interaction. In ILCs, the2-position substitutionof cations, such as imidazolium and bipyridinium have been reported to lead to lowertransition temperature or the disappearance of liquid crystalline properties. However,systematic investigation about the effect of substituents in hydrophobic chains of ILCs hasrarely been reported, which should prevent intermolecular close stacking effectively,enhance the molecular freedom of motion, and further lower the melting point, withoutlimiting the cation structures.In this work, we set out to investigate the correlation between the lateral substituent groupsand the phase behavior of ILCs, intending to obtain some enlightenment in thestructure-property relationships of ILCs. A series of ionic compounds with different lateralgroups on hydrophobic chains were designed and synthesized. A detailed investigation showed that the length of the lateral groups was reflected regularly in the phase transitiontemperatures and liquid crystal structures, and the lateral groups with appropriate sterichindrance did decrease the melting point effectively. However, unlike non-ionic liquidcrystals, the liquid crystalline properties were no longer maintained with a relatively longlateral chain. It’s noteworthy that the existence of lateral groups has led to evident slowkinetics of phase transitions from POM, DSC and XRD results. Another three compoundswith similar molecular structure but different chain length were synthesized, exhibitingliquid crystalline properties at relatively low temperature as expected. The results indicatethat the introduction of appropriate steric hindrance into the middle part of hydrophobicchains of ionic compounds can be effective in depressing the melting point of ILCs.The rheological properties have been proved to be very important in plenty of applications,e.g. spinning, ion transport, solvents, lubricants and response time. Although systematicwork has been devoted to nonionic liquid crystals, liquid crystal polymers, lyotropic liquidcrystals and ionic liquids, little work has been done in ILCs until a recent report about theshear thinning behavior. The rheology behavior of ILCs can be quite different from othersystems, due to the existence of the stabilization force of electrostatic interaction, andabsence of solvents and possibly the rigid mesogenic units. Thus we investigated therheological properties, including the steady state flow and temperature dependence of thecompounds of the above compounds. To obtain new understanding in thestructure-property relationships, two compounds with simple molecular structures,1-octadecyl-3-methylimidazolium hexafluorophosphate and1-octadecyl-2.3-dimethy-imidazolium hexafluorophosphate were used for comparison. All of the compoundsshowed typical shear thinning at low shear rates, but varied at higher shear rates.Interestingly, the temperature-dependent viscosity traces showed a biphasic area at aroundthe clearing point for all of the laterally substituted compounds, but not for compounds3aand3b, where there is only a direct and drastic decrease at the clearing point. The previousliteratures and experimental data lead to the explanation that this discrepancy is greatlycorrelated to the slow kinetics caused by the lateral groups, which elongated the transitionof the clearing point, and consequently, this dynamic interval was observed.