Role of interfacial entropy in the command-surface effect

Abstract

The paper presents a study of the evolution of the tilt angle with time in ''command-surface'' liquid crystal cells under irradiation with UV light. In these cells, the liquid crystal orientation can be switched via a photoinduced isomerization of azo moieties attached to the surface. The trans and the cis state induce homeotropic and homogeneous alignment, respectively. Polymeric systems assembled with the Langmuir-Blodgett ͑LB͒ technique were employed. Not only homeotropic and homogeneous alignment, but also tilted alignment is found. The azimuthal direction of tilt is given by the dipping direction during LB transfer. In order to study the underlying microscopic mechanisms, we investigated the switching kinetics, performed in situ evanescent UV-visible spectroscopy, and measured the anchoring energy. The material parameters of both the command layer and the liquid crystal were systematically varied. The well defined azimuthal direction of tilt is ascribed to packing anisotropies in the command layer induced during LB transfer. The polar tilt angle is determined by the competition between interfacial entropy, on the one hand, which favors mixing and thereby homeotropic alignment, and a polar interaction, on the other hand, favoring homogeneous alignment.

Modeling the Interface Region of Command Surface 2. Spectroscopic Evaluations of Azobenzene/Liquid Crystal Hybrid Langmuir−Blodgett Films under Illumination

“…Büchel et al 17 showed, with an LB layer composed of a polyglutamate having an Az side group, that visible light irradiation induces the orientational change of LC molecule without a threshold. This is contradictory to the present observation.…”

“…11,[14][15][16][17][18][19] The trans and cis forms of the surface Az layer generally induce homeotropic and homogeneous or planer alignment, respectively. In the same manner, the 6Az10-PVA/5CB hybrid LB films investigated here exhibit reversible changes in molecular tilt upon alternative UV and visible light irradiation.…”

Three-Dimensional Manipulation of an Azo Polymer Liquid Crystal with Unpolarized Light

“…The induced alignment was also stable on further irradiation with 436 nm unpolarized light at the same incident angle, in contrast to the results reported in the literature, in which the tilt angle was controlled by the amount of exposure energy. [10,11] Removal of the induced alignment was easily achieved by irradiation with unpolarized light at 366 nm, where a photo-chemical phase transition occurs. [1] The alignment could be reinduced on irradiation with unpolarized light at 436 nm.…”

Photocontrolled Alignment of Chiral Nematic Liquid Crystals

“…[3,4] These processes coupled with isomeric configurations and conformations of the azobenzene have also been shown to play a crucial role in the observed photoinduced anchoring transitions of nematic LCs. [9,10] The pretilt angle is defined here as the angle between the surface and the average tilt direction of the LC director. [9,10] The pretilt angle is defined here as the angle between the surface and the average tilt direction of the LC director.…”