Yu. Kurioz

Measurement of Azimuthal Anchoring Energy of Nematic Liquid Crystal on Photoaligning Polymer Surface

Abstract A method to determine the surface azimuthal anchoring energy of a nematic liquid crystal is proposed. The technique implies the measurement of the director deviation on the cell substrate as a function of strength and direction of the applied magnetic field. As an example, the dependence of the azimuthal anchoring coefficient on the exposure time is measured at the interface between the nematic K 15 and polyvinylcinnamate film exposed by UV light. The analogous measurements performed in a wedge cell show that the method with magnetic field is more precise.

Orientation of a Liquid Crystal on a Soft Photoaligning Surface

In this paper, we present experimental and theoretical results dealing with soft anchoring i.e. anchoring that can be changed due to the influence of the adjacent LC bulk reoriented by strong external field. Such a change means that the magnitude forces responsible for the structure of the surface (easy axis) are less than or equal to forces of the interaction between LC molecules.

Highly Sensitive Photoaligning Materials on a Base of Cellulose-Cinnamates

We report on development and characterization of new photoaligning polymers based on cellulose main chain and side fragments based on cinnamic acid containing alkenyl groups. The materials possess excellent photosensitivity, rather strong anchoring energy, low sticking parameter and can be considered as a promising candidate for LCD application, especially for plastic LCDs.

Surface driven transition in a nematic liquid crystal cell

Surface driven reorientation effects in a nematic liquid crystal cell caused by light-induced changes of the anchoring parameters were studied. Theoretical consideration of one-dimensional flat distributions of the director has shown that the director can undergo threshold reorientation between hybrid, homeotropic, and planar alignments as the anchoring energy varies continuously. The threshold reorientation takes place when the reference and light-induced easy axes are perpendicular. In the one-elastic-constant approximation the light-induced transition was found to be of second order as shown by a critical increase of the director thermal fluctuations in the vicinity of the transition point. These effects were experimentally studied in the cells containing 5CB liquid crystal aligned by the photosensitive azo-containing polymer layer.

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.

Light-Induced Anchoring Transitions and Bistable Nematic Alignment 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 a 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 isomerisation of the side fragments of the polysiloxane-cinnamate material and cross-linking photoreaction of the trans-form of side fragments resulting in orthogonal molecular easy axes.

P-173: Newly Developed Cellulose-Based Photopolymer with High Anchoring Energy and Low-Image-Sticking

We report on development and characterization of new photoaligning polymers based on cellulose main chain and side fragments based on cinnamic acid containing alkenyl group. The aligning properties of the photopolymers such as photosensitivity, pretilt angle, anchoring and image sticking characteristics are described.

Mechanically and Light Induced Anchoring of Liquid Crystal on Polyimide Film

Abstract The interplay between the effects of mechanical rubbing and UV light induced deimidization on polyimide films has been studied. Exposure to depolarized UV light of rubbing PI film was found to suppress effective anchoring energy. Polarized light led to the alteration of the orientational distribution of PI molecules obtained by rubbing i.e to the change of both anchoring energy and easy axis direction. A simple model based on the selective UV light absorption with consequent deimidization of PI molecules describes the UV-initiated processes on the rubbed polyimide layers but contradicts the experimental data obtained for non-rubbed layers. The results obtained show that UV exposure can be effectively used to control anchoring parameters.

Director modulation of nematic liquid crystal on photosensitive chalcogenide surface

ABSTRACT We report the first observation of light-induced director patterns formation in a planar liquid crystal (LC) cell with photosensitive chalcogenide film As20Se80. The angular patterns appear under the irradiation of the chalcogenide surface by Gaussian laser beam through the LC layer was observed. The experimental results are explained in terms of the transfer of heat from the chalcogenide film to the LC after the light absorption in the chalcogenide layer. Light-induced heating causes a change of the birefringence of the LC and, as a result, spatially modulated polarization state in the plane of the chalcogenide layer. Since the chalcogenide film is a photoaligning material, the spatial modulation of the light polarization leads to a formation of the spatially modulated easy orientation axis and the ring structure of the director in the cell. The model of liquid crystal gauss-shape prism was proposed also. We believe that this effect can be applied for development of different optical and electro-optical elements, such as beam shapers, micro-lenses, zone plates, etc.

Sticking effect of liquid crystal on photosensitive aligning materials

The sticking effect on photoaligning surfaces was investigated. We demonstrated that additional irradiation of photoaligning polymers with cinnamoil side groups with not-polarized UV-light strongly decreases their sticking parameter. We associate this effect with cross-linking of the flexible side-groups by UV light and, as a consequence, with light-induced strengthening of the photoaligning surface. Restriction of mobility of the flexible groups on the photoaligning surface (surface strengthening) resulted in depressing the sticking effect. The method of the decrease of the sticking effect by the light-induced strengthening is rather general, and it can be applied for any photoaligning materials undergoing a light-induced cross-linking of polymer fragments. For fluoro polyvinyl-cinnamate the light-induced strengthening allowed us to get the record value of the sticking parameter, S0 = 0.2%, which is better than traditional rubbed polyimide surfaces provide. Such a value of the sticking parameter along with other aligning characteristics allows considering fluoro polyvinyl-cinnamate as very prospective material for modern liquid crystal display technologies.