Selective stabilisation of blue phase liquid crystal induced by distinctive geometric structure of additives

We successfully stabilised liquid crystalline blue phases (BPs) by introducing two suitable additives with different geometric molecular structures, linear-shaped cobalt oleate complex (Co-OL) or tetrahedral-shaped tetraoctadecylammonium bromide (TODAB), into a liquid crystal (mixture of 4ʹ-pentyl-4-biphenylcarbonitrile, JC-1041XX and chiral dopant). The BPs temperature range and phase sequence depending on the addition amount and shape of additives were systematically investigated to determine the optimal concentration and shape dependency required to achieve a stabilising effect for BPs. From the polarising optical microscope results, the BPs temperature range for all of the samples with additives was not only broadened but also shifted to room temperature compared to that of BPs without additives. The widest BPs temperature range was increased to 15.3°C by the addition of 3 wt% Co-OL. According to the UV-vis reflection spectroscopy results, the Co-OL has a more significant stabilisation effect on BP I than on BP II, and the widest BP I range increases to 11.0°C. On the other hand, TODAB is effective for BP II stabilisation with the broadest BP II range reaching 1.8°C. These selective stabilisation effects are attributed to the specific shape of additives that closely match the structures of the disclination lines of the BPs.