61519-05-9

Measurements of the complex dielectric permittivity for the isotropic, nematic and smectic phases (SA and SC) of the 4-n-pentylphenyl-4-n-octyloxythiobenzoate (8S5) have been performed in the radio frequency range and at one microwave frequency of 9.38 GHz.The dielectric anisotropy of 8S5 is positive in both the radio and microwave frequency regions.However, in the smectic C phase there is a change of sign of the dielectric anisotropy which might be connected with dipole-dipole correlations.The low frequency relaxation process exhibits a Debye-type behaviour in all liquid-crystalline phases.The temperature dependence of the dielectric relaxation time will be discussed in terms of the Arrhenius law.

A variety of homologs of 4-alkylbenzenethio-4 prime -alkyl or alkoxybenzoates in which the 4 prime -alkoxy group is C//7-C//1//4, the 4 prime -alkyl group is C//7-C//1//4, the 4 prime -alkyl group is C//7-C//1//2 and the 4-alkyl group is C//5-C//8 or C//1//0 were prepared and their mesomorphic properties studied by microscopy, DSC and X-ray diffraction. A comparison is made of the mesomorphic properties in the thio- and oxy-esters and the effect on mesomorphic properties of replacing the ether oxygen atom in the ester linkage with a sulfur atom. The types of smectic phases observed are discussed; two apparently new biaxial smectic phases were found in the akoxy-alkyl series. Relationships between mesomorphic and thermodynamic properties with molecular structure are discussed.