TEMPERATURE INDEPENDENT THRESHOLD VOLTAGE FOR AN ELECTROOPTIC EFFECT

The threshold voltage of the cholesteric-nematic phase change effect in general decreases considerably with increasing temperature. The temperature dependence for the threshold voltage can be compensated by using an induced cholesteric phase with a helix pitch decreasing sufficiently with increasing temperature. The appropriate temperature dependence of the pitch can be obtained if the nematic phase is doped with two chiral compounds which induce opposite sign of the helix screw sense when used separately. The efficacy of this method was investigated for the two nematic mixtures E 8 and S 1132 to which the two chiral compounds CB 15 and S 811 had been added, the induced pitch values being 3-5 mu m. The threshold voltage measured in samples of 23. 5 mu m and 13 mu m thickness using homoeotropic boundary conditions are found to be constant within plus or minus 3% over a temperature range of 50 K.