Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field

Abstract

Biaxiality in the nematic phase for a liquid crystalline tetrapode made up of organo-siloxanes mesogens is investigated using polarized infrared spectroscopy. An ordering of the minor director for the homeotropically aligned sample is found to depend on the amplitude of the in-plane electric field. On increasing the in-plane electric field, the minor director, lying initially along the rubbing direction, rotates to the direction of the applied field. The scalar order parameters of the second rank tensor are found to depend significantly on the strength of the electric field. A most significant increase is found in the nematic order parameter and in the parameter that characterizes the phase biaxiality.

Surface alignment, anchoring transitions, optical properties, and topological defects in the thermotropic nematic phase of organo-siloxane tetrapodes

“…We did not notice any biaxiality of the dielectric tensor when measuring the permittivity in the planar cells (which would have resulted in two different values of 3 t ). To some extent, this result correlates with the previous studies by Merkel et al 51 in which the biaxial dielectric anisotropy in the direction perpendicular to the main director was determined to be very weak, only about (À0.011). Such a small value of dielectric anisotropy does not provide clear evidence of N b behaviour, as compared to other possible mechanisms, such as surface tilt, surface inhomogeneities or even surface anchoring transition that we observe in the tetrapode nematic.…”

“…1) with a molecular structure that resembles a tetrapode. [45][46][47][48][49][50][51][52] Originally, 45 the phase sequence for a material with this structure was determined as N b (37 C)N u (47 C)I, where I stands for the isotropic phase. Later on, Figueirinhas et al 46 presented the phase sequence of a mixture with a nematic deuterated probe as T g (À30 C)N b (0 C)N u (47 C)I, here T g is the glass transition temperature, based on extensive solid state NMR investigations, with the results being in line with those reported in ref.…”

Properties of the broad-range nematic phase of a laterally linked H-shaped liquid crystal dimer

“…[43,[59][60][61] The state of the art of biaxiality in bent-core nematics and in other shape LCs have been reviewer recently. [62,63] Other molecular geometries, such as mesogens with four flexible chains, [64,65] tetrapode derivatives [66][67][68][69][70][71] different from the one explored in Ref. [50] also show b N features that have not been disputed so far.…”

Molecular biaxiality determines the helical structure – infrared measurements of the molecular order in the nematic twist-bend phase of difluoro terphenyl dimer

“…Such conditions can be achieved through the anchoring effect or by an external electric field. This has been carried out previously for tetrapodes [48] and a bent-core system [49]. Fig.…”