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.
Dynamic optical data storage and other applications in the field of optical data processing would be enabled by the development of suitable nonlinear optical materials. In this paper we present an experimental investigation of the orientation photorefractive effect in nematic liquid crystal mixtures with different optical anisotropy and various cell construction layers.
We found a thermally-induced threshold reorientation of the nematic liquid crystal from homeotropical alignment to planar one. The fenomenon was observed in a symmetric cell with inner surfaces covered with fluoro polyvinyl-cinnamate. This threshold reorientation is an anchoring transition of the second order. We suggest that a competition of aligning abilities of flexible polymer fragments and main polymer chains is the basic reason for this phenomenon.
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.
We studied electro-optical characteristics of the liquid crystal (LC) cell with weak planar anchoring of the director with bounding substrates. The weak surface anchoring let us to decrease the cell driving voltage. Moreover, because of the absence of the strong subsurface director deformations, the cell could operate in the Mauguin regime. Using these we proposed a new type of the LC switching mode--the in-plane sliding mode. We realized this mode in the LC cell comprising of one reference substrate with strong director anchoring and one photoaligning material-covered substrate with weak anchoring.
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.
Surface driven reorientation effects in a nematic liquid crystal cell caused by light-induced changes of the anchoring energy were studied. The threshold director reorientation takes place for the perpendicular directions of the reference and light-induced easy axes. The transition was found to be close to the second-order one revealing the critical increase of the director thermal fluctuations in the vicinity of the transition point. The mentioned effects were experimentally studied in cells of liquid crystal 5CB aligned by the photosensitive azo-containing polymer layer.
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