Compensation films discotic

Also, the chemistry and polymerization of discotic monomers has been reviewed [10]. Their current and emerging uses are in optical compensation films for liquid crystal displays, carbon nanostructures, organic electronics, solar cells, light-emitting diodes, and field-effect transistors. 

Polymerizable hexabenzoates of triphenylene 3, 4, and 5 (Scheme 3.3) are the revolutionary materials, which have made the less abundant discotic nematic liquid crystalline materials more ubiquitous in a world with LCDs more than people by acting as the compensation films for widening the viewing angle [28]. Among the polymerizable compounds, the acryloyl derivative 3 is compatible for photopolymerization speed and thermal stability. 

In the on-state of a TN cell, the LC directors in the upper half are reoriented along the rubbing direction with almost no twist and the lower half has a similar structure with the director plane orthogonal to that of the upper half Thus, a uniform phase compensation film, such as a uniaxial a plate, cannot compensate the upper and lower parts simultaneously. Instead, a pair of wide-view films need to be used separately on both sides of the TN LC cell in order to compensate each of the half layers. Fuji Photo has skillfully developed discotic LC films for widening the viewing angle of TN cells. The molecular stractures of the wide-view (WV) discotic material are shown below in Figure 8.4. 

To obtain a comparable viewing angle with VA and IPS, OCB requires more sophisticated optical compensation based on a discotic material [58]. Figure 8.31 shows the compensation schemes for a normally white OCB mode. The fundamental idea is similar to that for TN. The retardation matching between the cell and the optical compensation film is especially important for the OCB mode, partially because the black state of the normally white OCB cell has a finite residual retardation value that must be compensated by an optical film. For example, any retardation fluctuation of the cell or the film is easily noticeable. The OCB system requires a high level of uniformity. And the cell parameters, as well as the film parameters, should be optimized in order to maximize the optical performance. 

When a discotic liquid crystal is sandwiched between two substrates (or exposed to air), the direction of the uniaxial axis can be controlled by alignment layers, external electric fields, and chiral dopants [46,47]. It is therefore possible to develop discotic compensation films with spatially varied uniaxial axis orientations. For example, Fuji Photo Film Co. developed discotic compensation films for TN LCDs. In both the TN display and discotic compensation film, the liquid crystal directors vary in the vertical direction. Each layer of nematic liquid crystal with a certain director orientation is compensated by a layer of discotic liquid crystal with the same director orientation. 

When the Pi cell is used for a display, it requires a compensation film to create the black state. Because of its symmetrical tilt alignment near the substrate, the Pi cell has a biaxial birefringence in the black state. Several approaches were tried to develop a compensation film for the Pi cell. To compensate for the birefringence of the black state by using only one film, biaxiality is required by the film [2]. A combination of an a-plate and c-plate was used [7] to reduce the light leakage in the black state. To compensate for each side of the distributed LC molecules, discotic films were used as the compensation films as shown in Fig. 4.6.4 [4, 5, 8, 9,10]. 

H. Mori, M. Nagai, H. Nakayama, Y. Ito, K. Kamada, K. Arakawa, K. Kawata, Novel optical compensation method based upon a discotic optical compensation film for wide-viewing-angle LCDs, SID 03 Digest, 1058-1061(2003)... 

There are as yet ho accepted standard materials for discotic nematic liquid crystals. It is expected that the birefringence of these materials will be negative, and they have already found application as optical compensation films for liquid crystal displays [23]. In order to illustrate an example of the optical properties of discotic nematic liquid crystals, we give in TABLE 10 measurements of the ordinary and extraordinary refractive indices of hexakis[(4-octylphenyl)ethinyl]benzene (Tm = 100 C). These are taken from [24] and were measured for a wavelength of 589 nm. 

Figure 5.13 Compensation of the dark state in TN mode by discotic negative compensation film...

The polymer discotic material (PDM) developed by Fuji Photo Film has a hybrid alignment, which mimics half of the bend alignment structure of the OCB cell. In contrast to the discotic film developed for TN LCDs, the azimuthal alignment direction of the PDM layer is oriented at 45° to the transmission axis of polarizer, and the in-plane retardation of the PDM layer compensates for the in-plane retardation of the on-state OCB cell. The total in-plane retardation of the PDM layer should be the same as that of the on-state OCB cell so that the voltage-on state becomes black at a voltage lower than 5 Vj s. 

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