We present a systematic study of thermal diffusivity in the nematic liquid crystalline phase of 4‐4′‐di‐(n‐alkoxy) azoxy benzenes and several other materials. The data allow to separate the respective contributions of the rigid central core and of the flexible aliphatic chains. For the component of the diffusivity tensor parallel to the long molecular axis, it is found that they differ by as much as a factor of 6 in favor of the core contribution. Combining the present observations with our earlier results, we are now able to draw general conclusions on the thermal diffusivity in a nematic and smetic A, B, and C phases. We have also developed two semiquantitative models based, one on a static network of distributed resistances, and the other on an extension of the Eyring kinetic model for the thermal conductivity of simple liquids. Only the dynamic model yields a satisfactory description of our data. The distance over which the thermal energy is not transferred instantaneously upon collision between two neighboring molecules is found to correspond to the aliphatic chain length (calculated for a freely rotating chain). Using this formalism, a p r i o r i calculations of the thermal diffusivities can be performed, to better than 20%, for any rodlike liquid crystalline material.

Measurements of the force as a function of distance between two solids separated by a liquid crystal film give information on the structure of the film. We report such measurements for two molecularly smooth surfaces of mica separated by the nematic liquid crystal 4'-n-pentyl 4-cyanobiphenyl (5CB) in both the planar and homeotropic orientations at room temperature. The force is determined by measuring the deflection of a spring supporting one of the mica pieces, while an optical technique is used to measure the film thickness to an accuracy of ± (0.1-0.2) nm. The technique also allows the refractive indices of the nematic to be measured, and hence a determination of the average density and order parameter of the liquid crystal film as a function of its thickness. Three distinct forces were measured, each reflecting a type of ordering of the liquid crystal near the mica surfaces. The first one results from elastic déformation in the liquid crystal ; it was only observed in a twisted planar sample where the 5CB molecules are oriented in different directions at the two mica surfaces. The second, measured in both the planar and homeotropic orientations, is attributed to an enhanced order parameter near the surfaces. Both of these are monotonic repulsive forces measurable below 80 nm. Finally, there is a short-range force which oscillates as a function of thickness, up to about six molecular layers, between attraction and repulsion. This results from ordering of the molecules in layers adjacent to the smooth solid surface. It is observed in both the planar and homeotropic orientations, and also in isotropic liquids.

The interfacial orientations of liquid crystal molecules deposited on different anisotropic solid substrates have been studied. The free energy of adhesion, critical surface tension, and polarity of these systems have been found. The polar contributions to the adhesion free energy of liquid crystal-solid systems have been calculated taking into account the liquid crystal orientation. We have also evaluated the order of magnitude of the anisotropy for the dispersion forces.