Molecular tilt direction in a slightly tilted homeotropic aligned liquid crystal cell

Abstract

The influence of hardness and topology of substrate surface on the pretilt angle were investigated for a slightly tilted homeotropic alignment achieved by the rubbing technique. On a soft surface, the grooves are produced which align in the same direction as the rubbing direction while the liquid crystal molecules tilt in the direction opposite to the rubbing direction. On a hard surface, no grooves are observed and the molecules tilt in the same direction as the rubbing direction. These results were applied to the fabrication of a two-domain vertically aligned nematic liquid crystal display.

Pretilt Angle of Liquid Crystals and Liquid-Crystal Alignment on Microgrooved Polyimide Surfaces Fabricated by Soft Embossing Method

“…Recently, the control of pretilt angle of liquid-crystal molecules has been intensively investigated. [5][6][7][8][9][10][11][12][13][14][15][16] It was pointed out that the rubbing process induces polar functional groups and repeating units to reorient out of the plane of the alignment film and nonpolar aliphatic side chains to partially reorient toward the bulk of the alignment film. [5][6][7]15 These studies demonstrate that the pretilt angle of liquid-crystal molecules increases along with the rubbing strength.…”

“…In addition to the uniform alignment of liquid-crystal molecules, an appropriate pretilt angle of liquid-crystal molecules is necessary for the twisted nematic liquid-crystal displays to prevent from reverse tilt disclinations upon exerting an external electric field. Recently, the control of pretilt angle of liquid-crystal molecules has been intensively investigated. − It was pointed out that the rubbing process induces polar functional groups and repeating units to reorient out of the plane of the alignment film and nonpolar aliphatic side chains to partially reorient toward the bulk of the alignment film. − , These studies demonstrate that the pretilt angle of liquid-crystal molecules increases along with the rubbing strength. − The pretilt angle can also be increased by simply introducing long, linear alkyl side chains or other nonpolar groups to the polyimide alignment layers. ,− On the other hand, applying the surface treatment of UV exposure or O 2 plasma to the polyimide alignment layers would increase the surface polarity and decrease the pretilt angle of liquid-crystal molecules. ,, All these previous studies implied that the pretilt angle is strongly related to the surface polarity. In this study, the advancing contact angle of water is used as an index of the surface polarity.…”

Control of liquid crystal pretilt angles by using organic/inorganic hybrid interpenetrating networks

“…New techniques based on surface-induced alignment have been proposed to produce intermediate pretilt angle. These include mechanical rubbing of polyimide having a long alkyl side chain [3][4][5], nanostructured surface based on random distribution of vertical and horizontal polyimide domains [6][7][8][9][10], ion beam exposure [11,12], photoirradiation of the photosensitive alignment layer containing azobenzene [13], nanotexture formation by an atomic force microscope local oxidation [14], use of dual alignment layer [15,16], and nanoparticles-doped planar polyimide film [17]. However, there remain several technical difficulties to be overcome such as high cost and complexity of the manufacturing process, the contact problem resulting from the rubbing process of polyimide.…”

Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces

“…More recently, interest has grown in obtaining LC orientation that is tilted (often referred to as "pretilt") by a significant polar angle from the normal to the surface. 7,8 A particularly attractive method to achieve this controllable and robust pretilt involves rubbing of polyimide that contains a long alkyl side chain. 9,10 Until now the rubbinginduced orientations of the different moieties were inferred from the resulting liquid crystal alignment.…”