Pretilt angle direction

Microgroove surface control of pretilt angle direction... 

Microgroove alignment layers which can control the pretilt angle direction (Fig. 3.4.5) have also been proposed [9]. 

Fig. 3.4.5 The structure of a microgroove alignment layer to control pretilt angle direction.

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... 

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).

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