Andrey Dyadyusha

An Oblique Orientation of Nematic Liquid Crystals on a Photosensitive Aligning Polymer

Abstract The problem of an oblique orientation of a liquid crystal in cells with light induced easy orientation axis is considered. Theoretical and experimental studies of the director distribution in a cell with a photosensitive orientant, fluorinated polyvinylcinnamate polymer film, are performed. The two exposure technique for liquid crystal oblique orientation and the condition for a new anchoring transition in a cell with a photosensitive orientant are also presented.

Oblique photoalignment of a nematic liquid crystal by modified polyvinylcinnamates

The polyvinylcinnamate modified by the partial substitution of side chains for stearoyl fragments has been investigated. An oblique liquid crystal alignment with pretilt angle values controlled by the concentration of the stearoyl groups is obtained. The double degeneracy of an easy axis induced by polarized-light-irradiation technique is also verified.

Non-monotonic exposure dependence of the pretilt angle and surface polarity of the photo-orientant F-PVCN

We report on the first observation of a non-monotonic UV exposure dependence of the pretilt angle of a liquid crystal on a photoaligning surface. We found this non-monotonic dependence on fluorinated poly(vinyl cinnamate) (PVCN-F) surface while non-fluorinated PVCN revealed a monotonic decrease of the pretilt angle with exposure. These dependences correlate with light-induced changes of the polarity of PVCN-F and PVCN surfaces. The obtained data point to an important role of the surface polarity and the light-induced changes in mechanisms of pretilt angle generation on photoaligning materials.

Peculiarity of photo-aligning of liquid crystals and accompanying effects

Review of our results of the steadies of the distribution of the director of a nematic liquid crystal in a cell with photosensitive aligning layers is presented. It is shown that a doubly degenerate easy orientation axis is induced on the nematic-orientant interface. It was also found that the degeneracy of the easy axis orientation could be removed by NLC molecules flow during LC cell filling and an oblique liquid crystal orientation is realized. The theoretical and experimental investigations of the reorientation of the director toward the light-induced easy axis caused by light-induced adsorption of dye molecules are presented. Threshold and nonthreshold effects are considered.

Orientation of a nematic liquid crystal on a soft photoaligning surface

Orientation of a liquid crystal on an aligning surface is characterized by the direction of an easy axis, e and anchoring energy, W. Traditional rubbed aligning layers provide a rigid surface and a strong anchoring of LC (W>10-1 erg cm-2). Therefore, magnetic or electric field usually does not reorient director on the aligning surface, and both of these parameters remain constant during the application of the field to the LC cell. Novel aligning materials, such as photoaligning and brush- like orientants possess very soft surface, containing flexible groups and usually provide much smaller anchoring energy (W<10-2 erg cm-3). In this case the reorientation of the director on the aligning surface is essential. In this report we show that the surface director reorientation can result in the realignment of flexible fragments of a weak anchoring photoaligning surface, i.e. in the modification of the aligning surface. Therefore, both the easy axis and the anchoring energy of the soft aligning layer can be changed during the application of external field to LC cell. The effect of the aligning surface modification induced by director reorientation on a soft surface of polyvinylcinnamate (PVCN) was found in a cell filled with LC 5CB. Namely, we observed that application of electric field results in the producing of a transient pretilt angle of 5CB on the PVCN surface, which relaxed to the initial state after switching off the external field during about 10 min. To describe the obtained results we propose to consider the interface containing LC molecules and flexible fragments of the aligning layer as an LC with effectively high orientation viscosity (gamma) s. Anchoring energy and easy axis of the LC corresponds to its equilibrium state. In the studied case the value (gamma) s approximately =5 P is appeared to be in fifty times greater than the value (gamma) 1 of 5CB.

Photoinduced liquid crystal anchoring transition on polysiloxane-based aligning surface

We report a novel azimuthal anchoring transition of nematic liquid crystal on a polysiloxane-cinnamate treated surface under the UV irradiation. It is found, that the direction of the light-induced easy axis strongly depends on the irradiation time. Under short exposure times the orienting surface provides an orientation of LC parallel to the direction of the uV light polarization. Longer times give the orthogonal to the UV polarization orientation of LC. There is region of exposure times that give a bistable anchoring conditions and domains with different directions of the director orientation. We propose a phenomenological model of this anchoring transition and surface bistability based on the competition between trans-cis isomerization of the side fragments of the polysiloxane-cinnamate material and cross-linking photoreaction of the transform of side fragments resulting in orthogonal molecular easy axes.