Induced smectic ordering and blue phase formation in mixtures of cyanobiphenyls and cholesterol esters

Abstract

It has been found that in mixtures of nematic E7 and smectogenic cholesteryl oleyl carbonate (COC) the S$_A$-N* transition temperature is substantially (by ~20 K) increased, as compared with pure COC, at E7 concentrations around ~40%. Within the same concentration range, the isotropic transition is preceded by formation of blue phase, with its maximum width of ~3.5 K clearly correlated to the increased thermal stability of the S$_A$ phase. With other cholesterol esters or cyanobiphenyls (cholesteryl nonanoate, 5CB), this effect was either much weaker or not observed. No enhancement of smectic phase was noted when E7 was replaced by N$_{tb}$-forming nematic mixture based on CB7CB with two cyanobiphenyl moieties. Selective Bragg reflection of light (BRL) spectra were measured in all three temperature regions, including the unwinding of the cholesteric helix on cooling towards S$_A$ phase and characteristic selective BRL changes in the blue phase. In the latter case, the measured {\lambda}$_{max}$ values were dependent both on the helical twisting power in the cholesteric phase and on the lattice size and orientation in the blue phase. Also considered were the effects of ferroelectric BaTiO$_3$ nanoparticles accumulated at disclination lines upon the blue phase thermal stability.