Nematics surface alignment

A twisted nematic display consists of a top electrode and bottom electrode, typically spaced 10 pim apart, whose surface alignments are perpendicular. The director, which is parallel to the surface, therefore rotates by 90° from top to bottom. Light enters the device at the top through a... 

Y. Kawanishi, T. Tamaki, M. Sakuragi, T. Seki, Y. Suzuki, and K. Ichimura, Photochemical induction and modulation of nematic homogeneous alignment by the polarization photochro-mism of surface azobenzenes, Langmuir 8, 2601-2604 (1992). 

The electrode surfaces that are in contact with the liquid crystal matrix are treated by a layer in order to align the liquid crystal molecules in a particular direction. This alignment layer consists of a polymer that is brushed in a certain direction. There are several principles in how to do the alignment. In a twisted nematic device, the surface alignment directions at the two electrodes are perpendicular to each other. For this reason, the... 

Figure 10.17 (a) The birefringence of an oriented one-comb PLC (polyacrylate obtained by photopolymerization of a surface aligned MLC) as a function of temperature. The various phases (K, crystalline S, smectic N, nematic and I, isotropic) are shown in the graph, (b) The order parameter of the same polymer (open symbol) in its nematic state as a function of reduced temperature, T/T., where Tj is the isotropization temperature. Data for the monomer (filled symbol) are shown in the same graph, ((a) and (b) from data of Broer et al [111].)... 

In LCDs, liquid crystals are usually sandwiched between two substrates. A certain alignment of the liquid crystal at the surface of the substrates is usually necessary in order for a display to operate properly. Bistable nematic liquid crystals can be created by using surface alignment layers. They are divided into two categories zenithal bistable TN and azimuthal TN. 

As shown in Fig. 5, there are two principal modes of surface alignment (a) planar (or homogeneous) and (b) homeotropic. Although subtle changes may be induced in threshold voltages by pretilting the surface molecules, the type of surface alignment forms the basis for orientation dependent optical effects in nematic polymers. 

The simplest display application of a nematic polymer liquid crystal, using polarized light, involves the splay Freedericksz transition shown in Fig. 5(a) for a material with positive Ac and planar surface alignment. The device is constructed using two polarizers crossed with each other, placed above and below the cell, with their polarization direction at 45° to the director of the planar texture. Above a threshold voltage given by... 

Sheng P (1976) Phase transition in surface-aligned nematic films. Phys Rev Lett 37 1059... 

The nature of the electrochemical reactions in a cell, especially with d.c. drive, depends amongst other factors on the molecular constitution of the nematic substances, on impurities in the liquid crystal and on the conductivity and orientation additives [84]. Even the electrodes and substrates, their contamination and the drive voltage and its duty cycle can also affect these reactions. The primary failure mechanisms are breakdown of the surface alignment, formation of bubbles, discoloration of the liquid crystal layer or a narrowing of the liquid crystalline temperature range. 

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