Pretilt

Note 3 Surface pretilt is the deviation angle of the director away from the surface. It is used to control the threshold voltage and affects viewing angles. 

Therefore, switch-off times are independent of the field strength and directly dependent on material parameters, such as viscosity coefficients and elastic constants, and the cell configuration. Therefore, they are often three or four orders of magnitude larger than the switch-on times. However, sophisticated addressing techniques can produce much shorter combined response times ( on + off The nematic director should be inclined, e.g. 1° pretilt,... 

A standard TN-LCD consists of a nematic liquid crystal mixture of positive dielectric anisotropy contained in a cell with an alignment layer on both substrate surfaces, usually rubbed polyimide, crossed polarisers and a cell gap of 5- 0fim, see Figure 3.7. The nematic director is aligned parallel to the direction of rubbing in the azimuthal plane of the device. The alignment layer induces a small pretilt angle (6 1-3°) of the director in the zenithal plane. The... 

The major difference between the configuration of the OMI sandwich cell and other STN-LCDs is that the optical path difference (5 = And 1 pm) is much lower. There is no requirement for a significant pretilt (0 < 0 < 5°), the twist angle of the chiral nematic layer is lower (180°), the front polariser is parallel to the nematic director (a = 0°) and the polariser and analyser are crossed (P = 90°). The 180° twist gives rise to strong interference between the two elliptically polarised rays. If the optical path difference is small, e.g. 0.4 m, a bright, white, non-dispersive off-state is produced. The chiral nematic mixture should be of positive dielectric anisotropy, low birefringence and exhibit a low cell gap to pitch ratio dip 0.3). 

This general description is typical for STN-LCDs with a twist angle 240° < O < 270°, pretilt angle 5° < 0 < 10° and a cell gap, d 5 pm. The steepness of the electro-optical curve, y, should be as low as possible in order to optimise the number of lines to be addressed ... 

The cell contains a nematic mixture with a twist of 270° and homogeneous alignment with a high pretilt angle (0), see Figure 3.16. The nematic mixture is composed of one or several dichroic dyes, a chiral dopant and a nematic host of low birefringence. 

For polystyrene, on the other hand, optimal parallel alignment of the respective tt systems is achieved for the liquid crystal oriented perpendicular to the rubbing direction (Fig. 6.9D). In this orientation, the backwards tilt of the polystyrene phenyl rings with inspect to the rubbing direction does not cause any out-of-plane directional asymmetry along the perpendicular axis of the liquid crystal orientation. Consequently no pretilt of the oriented liquid crystals is expected, as it is indeed observed. 

Fig. 6.14. Ion beam incidence angular dependence of the liquid crystal pretilt angle (3 and the molecular tilt angle 7 of the polymer segment distribution at the film surface for polyimide (top) and amorphous carbon (bottom). As predicted by the alignment model the liquid crystal pretilt angle / follows the molecular tilt angle 7. The line is a fit to y 0) using a model that assumes finite, but different cross sections for breaking of phenyl rings oriented along or perpendicular to the ion beam direction [35].

The LC molecules orient on the alignment layer in a predefined in-plane and out-of-plane direction. The out-of-plane (polar) orientation is called the pretilt angle. The directional orientations and pretilt angles on both surfaces in combination with a chiral dopant added to the LC mixture determine the twist direction of the LC molecules in an LCD. Most commonly left-handed dope molecules that cause a left-handed twist sense are added to the LC mixture. Both the in-plane and out-of-plane directions are important to obtain the proper switching behaviour upon driving the display. 

Fig. 6.5. On rubbed polyimide films (A) liquid crystals orient parallel to the rubbing direction with an upwards tilt with respect to the rubbing direction. Note that this pretilt angle f3 is defined as the average out-of-plane tilt angle of the liquid crystal rods in the bulk of the liquid crystal ensemble. This may differ from the angle of the first monolayer, for which P is indicated here for simplicity. On rubbed polystyrene (B) liquid crystals orient perpendicular to the rubbing direction without any out-of-plane tilt angle.

Fig. 6.10. (A) Liquid crystals align on rubbed and ion beam irradiated polyimide surfaces along the treatment direction, but with opposite pretilt angles. (B) The respective polarization dependences possess the same overall orientation, but opposite shifts with respect to a = 0° within the plane parallel to the rubbing direction (solid squares). This is in agreement with the presented alignment model, as the derived molecular distribution factors illustrate (C).

Comparing the spectra in the right panel, which characterize the molecular tilt angle, one finds the same polarization dependence for the two ion beam irradiated materials, which is opposite to the one of the rubbed polyimide film. Hence, a downwards liquid crystal pretilt angle is expected for both ion beam treated surfaces. Again, since the overall shape and the tt intensities and their dichroism is comparable for the two ion beam irradiated films, liquid crystals ai e expected to exhibit a technologically sufficient pretilt angle on an ion beam irradiated amorphous carbon layer. 

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