Imidazolium-based polymerized ionic liquid crystals containing fluorinated cholesteryl mesogens^†

A series of polymerized ionic liquid crystals (PILCs) bearing fluorinated cholesteryl mesogens were synthesized in this work, which include polymerized imidazolium bromides (PIBs) and polymerized imidazolium hexafluorophosphates (PIHs). The PIBs were synthesized using alkyl bromine-containing polysiloxanes and 1-butyl-1H-imidazole, and the PIHs were synthesized by anion metathesis reaction using the corresponding PIBs and KPF₆. The chemical structures, liquid crystalline (LC) properties, and electrorheological (ER) effect of these PILCs were characterized by use of various experimental techniques. All the PILCs showed smectic A mesophase on heating and cooling cycles. The smectic layer structure of these PILCs are originated from the rigid fluorinated cholesteryl mesogens and the flexible moieties in the LC phase, but the ion pairs (imidazolium cations–PF₆⁻, Im⁺–PF₆⁻; or imidazolium cations–Br⁻, Im⁺–Br⁻) can disperse in the polysiloxane matrix and expand the d-spacing in the smectic layers. The PIHs show lower T_g and T_i than the corresponding precursor PIBs, which is due to the larger ion volume of Im⁺–PF₆⁻ for PIHs than that of Im⁺–Br⁻ for PIBs. A series of 40 V% ER fluids were prepared by mixing the PILCs with polydimethylsiloxane (PDMS), and the ER behaviors were studied. All the PILC/PDMS fluids showed ER effect, and the PIH/PDMS fluids show a little greater ER effect than the PIB/PDMS fluids. The PILC droplets in the ER fluids become deformed owing to both the orientation of fluorinated cholesteryl mesogens and the suppression of ionic migration when a DC electric field was applied, resulting in the occurrence of ER effect. Copyright © 2015 John Wiley & Sons, Ltd.