Liquid Crystals. Iam-Choon Khoo

Liquid Crystals - Iam-Choon Khoo


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These and other effects due to the presence of dye molecules or other photosensitive agents in liquid crystals are discussed in more detail in Chapter 8.

      1.4.3. Polymer‐dispersed and Polymer‐stabilized Liquid Crystals

Schematic illustration of alignment of a dichroic dye-doped nematic liquid crystal. Schematic illustration of a polymer-dispersed liquid crystal material.

      There are now many techniques for preparing such polymer‐liquid crystalline composite, including the phase separation and the encapsulation methods [2, 19] for PDLC, and optical holographic interference methods [20–22] for making PDLC photonic crystals and grating [23, 24] have also demonstrated 1‐D polymer/liquid crystal layered structures that exhibit high diffraction efficiency as well as laser emission capabilities.

      Another type of polymer‐liquid crystal “composite” is formed by mixing liquid crystal with monomer and subject the mixture to UV light. The rigid polymer network that results from the curing of the monomer follows the liquid crystal order and is able to extend the temperature range of the mesophase. This procedure has been employed to extend the temperature range of BPLCs from a few degrees to over 60° [25], for example.

      Liquid crystals, particularly nematics, behave physically very much like liquids. Owing to the random scattering of light caused by thermal fluctuations of the anisotropic constituent molecules, bulk unaligned nematic liquid crystals take on a milky appearance. They become crystal clear when confined in thin cells where the director axis of the bulk is controlled and aligned by strong anchoring forces from the cell boundaries that are treated in a variety of ways. Such surface alignment techniques and the underlying science are understandably very complex; the following discussions are intended as an introduction to some common practices.

      1.5.1. Nematic LC Cells Assembly

Schematic illustration of nematic liquid crystal cells.

      Planar alignment can be achieved in many ways. A commonly employed method is to first coat the cell wall with some polymer such as PVA (polyvinyl alcohol) and then rub it unidirectionally with a lens tissue. This process creates elongated stress/strain on the polymer and facilitates the alignment of the long axis of the liquid crystal molecules along the rubbed direction (i.e. on the plane of the cell wall). As a matter of fact, there is a commercial so‐called rubbing machine for preparing planar cell windows. Another method is to deposit silicon oxide obliquely onto the glass slide.

      For preparing a PVA‐coated planar sample in the laboratory, the following technique has proven to be quite reliable. Dissolve chemically pure PVA (which is solid at room temperature) in distilled deionized water at an elevated temperature (near the boiling point) at a concentration of about 0.2%. Dip the cleaned glass slide into the PVA solution at room temperature and slowly withdraw it, thus leaving a film of the solution on the slide. (Alternatively, one could place a small amount of the PVA solution on the slide and spread it into a thin coating.) The coated slide is then dried in an oven, followed by unidirectional rubbing of its surfaces with a lens tissue. The rest of the procedure for cell assembly is the same as that for homeotropic alignment.

      Besides these two standard cell alignments, there are many other variations such as hybrid, twisted, supertwisted, and fingerprint; multi‐domain vertically aligned; and so on. In recent years [17, 27], photo‐alignment of dye‐doped cell window surface has also been shown to be highly effective for imparting the desired liquid crystal director axis arrangement.

      For smectic‐A, the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps


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