Liquid Crystals. Iam-Choon Khoo

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Figure 1.18. Schematic depiction of a polymer‐dispersed liquid crystal material: (a) in the absence of an external alignment field, highly scattered state; (b) when an external alignment field is on, transparent state.

      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.

1.5. LIQUID CRYSTAL CELLS FABRICATION

      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

For nematics, two commonly used alignments are the so‐called homeotropic and homogeneous (or planar) alignments, as shown in Figure 1.19a and b, respectively. To create homeotropic alignment, the cell walls are treated with a surfactant such as HTAB (hexadecyl trimethyl ammonium bromide) [26]. These surfactants are basically soaps whose molecules tend to anchor themselves perpendicular to the wall and thus impart homeotropic alignment to the bulk liquid crystal molecules.

Industrial processing of liquid crystals‐based devices or display screens, including various transparent conductive coating of the cell windows for electro‐optical applications, is understandably more elaborate, often held as proprietary information or patents. In the laboratory, a quick and effective way to make a homeotropic nematic liquid crystal is as follows: dissolve 1 part of HTAB in 50 parts of distilled deionized water by volume. Clean two glass slides (or other optical flats appropriate for the spectral region of interest). Dip the slides in the HTAB solution and slowly withdraw them. This effectively introduces a coating of HTAB molecules on the glass slides. The glass slides should then be dried in an oven or by other means. To prepare the nematic liquid crystal sample, place a spacer (Mylar or some nonreactive plastic) of desirable dimension and thinness on one of the slides. Fill the inner spacer with the nematic liquid crystal (which should be heated to the isotropic phase), and immediately place the second slide on top and secure the two slides with a clip. Once assembled, the sample should be left alone, and it will slowly (in a few minutes) settle into a clear homeotropically aligned state.