On the electrical conductivity in the nematic phase of 4-n-alkyloxybenzoic acids HOBA, OOBA, and NOBA

Abstract

First de Vries /1/ assumed a differentiation of the nematic liquid crystalline phase in a classical one and a nematic phase with smectic C-like order. On the base of X-ray investigations the authors in /2/ showed that in the nematic phase (N) so-called cybotactic groups can exista kind of smectic C (Sc) pretransitional effect. The molecules in the so formed molecular complexes are arranged in layers. The temperature dependence of the elastic constants /3, 4/, electrical conductivity anisotropy 15 to 8/, and dielectric anisotropy /9/ in the nematic phase with smectic C ordering displays anomalies. Their existence can be explained with the forniation of smectic-like molecular complexes.In the present communication we report results on the electrical conductivity dl of 4-n-heptyl-, 4-n-octyl-, and 4-n-nonyloxybenzoic acids (HOBA, OOBA, and NOBA).The phase transition temperatures are as follows: HOBA-C-SC: 365 K, The liquid crystals were oriented planar in a sandwich cell (d = 2 0 p m ) .The semitransparent glass electrodes have a conducting Sn02 layer evaporated with SiO. The sample was placed inside a temperature regulated Mettler hot stage. An ac bridge (f = 1000 Hz) for the electrical conductivity measurements was used. The electric field was smaller than the threshol'd one for electrohydrodynamic instabilities, Fig: 1 represents the experiment'al dl= f(T) curves for HOBA on cooling from an isotropic liquid (curve 1 ) as well as on heating back to the isotropic phase (curve 2 ) with 0.2 K/min rate. A sharp decrease in the electrical conductivity was observed on curve 1 at 390.6 K. A few degrees below this tem-1)boul. Lenin 72,

Surface memory effect in nematics with smectic C short-range order and hydrogen bonded molecules

“…around 120°C for OOBA), the monomers disappear and only cyclic and open dimers remain, the cyclic predominating. In a set of articles [23][24][25][26] we have demonstrated very sharp anomalies exhibited by the temperature dependences of some typical liquid crystal constants at T = T*. We also found that this temperature divides the nematic phase interval for OOBA into two subphases: a hightemperature one N, with the properties of a conventional nematic and a low-temperature one N, with markedly quasi-smectic character.…”

Spontaneous chiral domains in the nematic phase of achiral trans-4-alkylcyclohexanecarboxylic acids

“…Figures 1 (a) and 1 (b) illustrate this with the appearance of order transitions [ 4], which mechanism for the compound trans- 4-hexylcyclo-were described as anchoring transitions driven by hexanecarboxylic acid. Such dimer molecules have a temperature [5,6 ]. core rigid enough for the appearance of a mesophase We decided to investigate by means of optical micro- [1 ].…”

Fan-shaped, toric and spherulitic textures of mesomorphic oxadiazoles

“…Moreover, it is also remarkable the behaviour of the nematic phase, displaying a texture transition. These transitions have been previously observed inside the nematic phase of some mesomorphic thermotropic materials belonging to the families of the alkyloxybenzoic and cyclohexane acids [7][8][9][10][11][12][13]. It is a more or less abrupt change, driven by the temperature, in the nematic texture seen by means of the polarised light microscope.…”