Liquid Crystals. Iam-Choon Khoo

Liquid Crystals - Iam-Choon Khoo


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rel="nofollow" href="#fb3_img_img_f88d5888-93ec-5b05-bbaf-ec0848371fba.png" alt="upper D Subscript slash slash Baseline equals 1.95 times 1 0 Superscript negative 3 Baseline c m squared slash normal s upper D Subscript up-tack Baseline equals 1.2 times 1 0 Superscript negative 3 Baseline c m squared slash normal s"/> Elastic constants:
Viscosity coefficients: (at 20 °C)StartLayout 1st Row gamma 1 left-parenthesis Rotational Viscosity right-parenthesis equals 283 m upper P a dot normal s 2nd Row eta left-parenthesis Splay Viscosity right-parenthesis equals 41 m m squared slash normal s a t 20 degree normal upper C semicolon eta left-parenthesis Twist Viscosity right-parenthesis equals 37 m m squared slash normal s a t 20 degree normal upper C EndLayout

      The absorption coefficient α in the UV (~0.2 μm) regime is on the order of 103 cm−1; in the visible (~0.5 μm) regime, α ≪100 cm−1; in the near‐IR (~3–5 μm and ~9–12 μm) regime, α ~ 102 cm−1. There are, of course, large variations among the thousands of liquid crystals that have been synthesized. It is possible to identify liquid crystals with the desirable absorption/transparency for a wavelength of interest. For example, the terahertz and microwave regime, [5, 6] have identified/synthesized liquid crystals (LC’s) with relatively low absorption coefficient α ~ 100 cm−1; low absorption is important since applications in such long‐wavelength regimes require much thicker interaction length (cell thickness).

      There are several distinct types of liquid crystals: lyotropic, polymeric, thermotropic, and discotic. These materials exhibit liquid crystalline properties as a function of different physical parameters and environments such as temperature, molecular constituents’ structure, and concentration.

      1.3.1. Lyotropic Liquid Crystals

      Lyotropic liquid crystals are obtained when an appropriate concentration of material is dissolved in some solvent. The most common systems are those formed by water and amphiphilic molecules (molecules that possess a hydrophilic part that interacts strongly with water and a hydrophobic part that is water insoluble) such as soaps, detergents, and lipids. Here the most important variable controlling the existence of the liquid crystalline phase is the amount of solvent (or concentration). There are quite a number of phases observed in such water‐amphiphilic systems, as the composition and temperature are varied; some appear as spherical micelles, and others possess ordered structures with 1‐, 2‐, or 3‐D positional order.

Schematic illustration of chemical structure and cartoon representation of sodium dodecyl sulfate (soap) forming micelles.

      1.3.2. Polymeric Liquid Crystals

Schematic illustration of three different types of polymeric liquid crystals.

      1.3.3. Thermotropic Liquid Crystals: Smectic, Nematic, Cholesteric, and Blue‐phase Liquid Crystals

      Although the molecular structures of thermotropic liquid crystals are quite complicated, they are often represented as “rigid rods” that interact with one another to form distinctive ordered structures (or phases) as a function of ascending temperature: crystals, smectic, nematic, cholesteric (including blue‐phase), and the isotropic liquid phase. In smectic liquid crystals, there are several subclassifications in accordance with the positional and directional arrangement of the molecules.

      As explained in greater detail in the following chapters, these mesophases are defined and characterized by many physical parameters such as long‐ and short‐range order, orientational distribution functions, and so on. Here we continue to use the rigid‐rod model and pictorially describe these phases in terms of their molecular arrangement.


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