Homeotropic displays

The textures in homeotropic lamellar phases of lecithin are studied in lecithin-water phases by polarizing microscopy and in dried phases by electron microscopy. In the former, we observe the La phase (the chains are liquid, the polar heads disordered)—the texture displays classical FriedeVs oily streaks, which we interpret as clusters of parallel dislocations whose core is split in two disclinations of opposite sign, with a transversal instability of the confocal domain type. In the latter case, the nature of the lamellar phase is less understood. However, the elementary defects (negative staining) are quenched from the La phase they are dislocations or Grandjean terraces, where the same transversal instability can occur. We also observed dislocations with an extended core these defects seem typical of the phase in the electron microscope. 

The static homeotropically aligned guest-host nematic mixture (plus guest dye and chiral dopant) is optically transparent and, therefore, the display appears colourless in the non-activated state. The lower limit of the pitch for a given cell gap, before a twisted nematic structure becomes energetically more favoured than the homeotropic nematic structure, is determined by the djp ratio ... 

The first nematic guest-host prototype nematic GH-LCD reported by Heilme-ier and Zanoni " contained methyl red (157) as the dichroic dye dissolved in 4-butoxybenzoic acid as the nematic liquid crystal host. Other hosts investigated later included 4-methoxycinnamic acid and 4-ethoxy-4-aminoben-zonitrile (28), see Table 3.4. The melting point of these three single components is very high. Therefore, prototype GH-LCDs had to be operated and evaluated at very high temperatures. Thermal decomposition of the mixtures led sequentially to lower contrast, homeotropic orientation due to decomposition products and finally device breakdown. However, these initial experiments were sufficient to demonstrate the feasibility of this display type. 

A more sophisticated retardation plate is a film consisting of the perpendicular arranged combination of two optical uniaxial materials, e g., uniaxially stretched polycarbonate foil with a homeotropic oriented LC siloxane layer [18]. The purpose of such a foil is the improvement of the optical properties of LC displays, especially the viewing angle dependence of the contrast. 

To demonstrate the effect of such a combination a three-layer system between crossed polarizers may be considered The first picture was calculated as model for a LC display operating by change of A by application of an electric field, the second is the combination with a color-compensating foil and the third is the additional application of a homeotropic oriented layer of a LC siloxane. 

Fig. 5.8 A photoaddressed and multiswitchable cholesteirc LC display of 6.0 wt% chiral dopant 8 in LC host E7 in a 5 pm thick homeotropic aligned cell (1.5 in. x 1.5 in.). Top schematic cholesteric textures Middle demonstration of an image Bottom crossed polarized textures. Reproduced with permission from [76]. a Planar state, b Focal conic state, c Planar state. Copyright 2011 John Wiley Sons...

Liquid crystalline triphenylene hexacarboxyUc esters 15 have been recently synthesized and the columnar phase has been observed to align homeotropically on different substrate surfaces [82]. The alignment behavior has been studied by polarizing optical microscopy and X-ray diffraction techniques. POM shows a dark view whereas optical microscopy image displayed a dendritic texture typical of columnar hexagonal assembly. The hexagonal columnar phase displays... 

Fig. 7.18. Scanning electron microscopy pictnres of actual surface gratings (right) and a schematic illustration of the switching between the homeotropic state C and the low-tilt (planar) state D (left). For a symmetric grating (top) the (bulk) director of the planar state is parallel to the cell plane while for a blazed grating (bottom) it is pretilted. Modified from Jones, reproduced with kind permission of the Society for Information Display.

The cholesteric texture occurring with perpendicular boundary orientation, on the other hand, is not a uniform texture. Under the microscope the entire field of view is filled with right-and left-handed spirals. This cholesteric texture is known as the scroll texture and appears very much like an end-on view of a bundle of rolled-up scrolls. Capacitance measurements show that the cholesteric helical axis in this texture is still predominantly perpendicular to the plane of the layer. An exact analysis of the structure of this texture, even for the case of equal elastic constants, seems difficult to achieve. Important from the applications point of view is that the scroll texture is adopted without disclinations immediately after a display element is turned off and that this structure is essentially nonscattering. Homeotropic boundary orientation therefore solves the problem of display after-images. 

The smectic A is an untilted phase in which the mass density wave is parallel to the director. The cost in free energy of buckling the layers into saddle-shaped deformations is low, with the result that it is relatively easy to construct devices that show bistability between a scattering focal conic director configuration in which the layers are buckled and a clear homeotropic configuration in which the director is perpendicular to the cell walls and the layers parallel to the walls. Transitions between these two textures have been exploited in laser-written projection displays and in both thermo-optic and electrooptic matrix displays. The various mechanisms employed are summarized in Fig. 12. 

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