Laterally substituted ionic liquid crystals and the resulting rheological behavior

A series of quaternary ammonium salts (1a-1e) with different lateral substitution groups in the middle part of the hydrophobic chains were designed and synthesized. Differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction showed that compound 1a formed the smectic T phase at high temperature. Upon increasing the volume of the lateral groups in 1b, 1c and 1d, low melting point ILCs with smectic A phases were observed. However, the liquid crystalline structure was broken in compound 1e with large lateral groups. FT-IR measurements showed that the lateral groups resulted in an increase of the gauche conformation, which implies that the bulky effect of the lateral groups reduced the hydrophobic interactions and prevented close stacking. The phase behavior of compounds 2a-2c carrying an ethyl formate lateral group with various main chain lengths was also studied, and it was found that the clearing-point temperature is strongly dependent on the length of the hydrophobic chains. The rheological properties of the laterally substituted ILCs were further investigated. Typical shear thinning behavior was observed for all the ILCs, based on the shear rate-dependent viscosity curves. However, the viscosity vs. temperature curves showed that the laterally substituted ILCs exhibited biphasic interval behavior, which is different from the conventional ILCs, such as compounds 3a and 3b, where a drastic decrease was observed. The rheological behavior observed in this system is attributed to the slow kinetics caused by the laterally substituted molecular structure.