Polar Alkenyls: Physical Properties and Correlations with Molecular Structure of New Nematic Liquid Crystalsf

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Abstract
The elastic constants k11, k22, k33 , the bulk and rotational viscosities, the birefringence, the static dielectric and electro-optical properties in TN-LCDs of new, positive dielectric and electro-optical properties in TN-LCDs of new, positive dielectric nematic liquid crystals, namely polar alkenyls, are reported. The molecules comprise a double bond in their hydrocarbon side chain as well as different rigid cores, namely directly- and ethane linked PCH-cores as well as heterocyclic (dioxane) cores. Despite their dielectric and conformational similarity, large variations of the elastic constants depending on the position of the alkenyl double bond are shown to result in alkenyls with PCH cores. This leads to variations of k33/k22 and k = k11 + k 33-2k22)/4 among different alkenyls with the same chain length of almost a factor of 2 at constant reduced temperature (Tc-10°C). From molecular modelling follow rod-shaped van der Waals conformations with constant length/width ratio L/ W for all directly linked alkenyls. Therefore, and due to k33/k11 α constant, nematic hard-rod models neglecting molecule-specific interactions and implying k33/k22 α L/W are qualitatively shown to be inadequate to explain the elastic ratios of nematics. Despite up to 30% larger optical anisotropics the response times of some alkenyls in TN-LCDs are ∼30% shorter than those of PCHs. This result is not primarily due to the low rotational viscosity γ1 of alkenyls but rather due to their low visco-elastic ratios γ1 /k. The fast response and the wide range of elastic properties render the stable polar alkenyls applicable for highly multiplexed as well as fast responding LCDs.