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

It has been found that in mixtures of nematic E7 and smectogenic cholesteryl oleyl carbonate (COC) the SmA – N* transition temperature is substantially (by ~20 K) increased, as compared with pure COC, at E7 concentrations around ~40 wt. %. Within the same concentration range, the isotropic transition is preceded by formation of blue phase (BP), with its maximum width of ~3.5 K clearly correlated to the increased thermal stability of the SmA phase. With other cholesterol esters or cyanobiphenyls (cholesteryl nonanoate and 5CB), this effect was either much weaker or not observed. The spectra of selective Bragg reflection of light (BRL) were measured in all characteristic sections of the mesophase temperature range, including the unwinding of the cholesteric helix on cooling towards SmA phase and changes in selective BRL characterising the blue phase. In the latter case, the measured λmax values were shown to be dependent both on the helical twisting power in the cholesteric phase and on the lattice size and orientation in the blue phase. In an attempt to increase the BP temperature stability, ferroelectric nanoparticles of BaTiO3 were added to the studied mixtures. The observed broadening of the BP temperature range was associated with accumulation of nanoparticles at disclination lines.