Polyimides pretilt angle

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].

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. 

Fig. 2.13 Chain length dependence of the LC pretilt angle for BPDA and PMDA t) e polyimides. Reproduction by permission from [46]. 

Fig. 2.18 Relation between the pretilt angle of the LC and the inclination angle of the polyimide backbone structure. Reproduction by permission from [54].

Fig. 2.37 Rubbing strength and pretilt angle relations of two types of polyimide ( -A, x-B polyimide) rubbing strength represents the offset height from the standard position.

Fig. 4.7.6 LC molecular configuration of the multi-domain mode formed by two kinds of polyimide with different pretilt angle magnitudes. Part (a) shows the splayed TN configuration and part (b) shows a conventional TN configuration.

Control of pretilt angles by using two kinds of polyimide [14,15]... 

Control and modification of nematic liquid crystal pretilt angles on polyimides. Figure 2, K.-W. Lee, A. Lien, J. Stahis and S.-H. Paek, Japanese Journal of Applied Physics, Part 1, 36, p. 3591 (1997). Reproduced by permission of Institute of Pure and Applied Physics. 

Ban, BS Kim, YB (1999). Surface Free Energy and Pretilt Angle on Rubbed Polyimide Surfaces. Journal of Applied Polymer Science, Vol. 74, No. 2, (Oct), pp. 267-271. 

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