Liquid crystal and polymer dispersions

Simoni F 1997 Nonlinear Optical Properties of Liquid Crystals and Polymer-Dispersed Liquid Crystals (Singapore World Scientific)... 

F. Simoni, Nonlinear optical properties of liquid crystals and polymer dispersed liquid crystals World Scientific, Singapore 1997. 

Chilaya, G., Hank, G., Koswig, H.D., Sikhamlidze, D. Electric-field controlled color effect in cholesteric liquid crystals and polymer-dispersed cholesteric liquid crystals. J. Appl. Phys. 80, 1907-1909 (1996)... 

The introduction of dispersions of liquid cryst s and polymers has grown into a broad class of materials important in electrically controll, scattering-based light shutters and bistable reflective displays. In liquid crystal and polymer dispersions the weight of polymers used can be varied from 80% to as low as 0.5%, depending on the application and type of polymer used. The systems containing a polymer of 20% or higher has been extensively studied are referred to as polymer dispersed liquid crystds (PDLC). (i) Currently, the systems of most interest are those with polymer concentrations less than 10%. 

Liquid Crystal and Polymer Dispersions in a Microgravity Environment... 

Liquid crystal and polymer dispersions are fabricated using thermally-induced phase separation (TIPS), solvent-induced phase separation (SIPS), or Polymerization-induced phase separation (PIPSX/I)- For TIPS, a homogeneous mixture of a low-molecular weight liquid crystal and thermoplastic polymer is cooled below the critical phase separation temperature to induce phase separation into liquid crystal rich and polymer rich domains. The morphological properties (domain size, number of domains per unit volume, and the composition of the domains) depend primarily on the choice of liquid crystal and thermoplastic polymer, the initial weight fraction of liquid crystal in die initial mixture, and the rate of cooling. 

F. Simoni, Non-linear optical properties of liquid crystals and polymers dispersed liquid crystals - series on liquid crystals, Vol. 2, pp. 217-250, World Scientific, Singapore. (1997). 

Khoo, I. C., Y. Z. Williams, B. Lewis, and T. Mallouk. 2005. Photorefractive CdSe and gold nanowire-doped liquid crystals and polymer-dispersed-liquid-crystal photonic crystals. Mol. Cryst Liq. Cryst. 446 233-244. 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

SIPS is the proper method when the polymer matrix has a melting temperature higher than that of the liquid crystal or when the TIPS method cannot be applied. An important parameter in controlling the size and uniform dispersion of the LC droplets is the difference between the solubility parameter of the low molecular weight liquid crystal and polymer matrix (Chen and Shanks 2007). For small solubility differences in the two components, there is a more uniform distribution of the LC droplets into the polymer matrix. 

Polymer-dispersed chiral liquid crystals and polymer stabilized chiral liquid crystals are very promising materials for flat panel display applications allowing us to make thin, adapted to plastic, low-power consumption, lightweight displays particularly useful for numerous portable applications. The application potential of these materials has driven several basic scientific studies in this area. The particular area of interest addressed in this chapter is how confinement can modify the macroscopic and microscopic ordering of chiral nematics. Even in one of the simplest confined systems, where a chiral... 

Khoo, 1. C., Kan Chen, and Y. Wilhams. 2006. Orientational photorefractive effect in undoped and CdSe nano-rods doped nematic liquid crystal Bulk and interface contributions. J. Sel. Top. Quantum Electron 12(3) 443 50 see also Khoo, I. C., Yana Zhang Wilhams, B. Lewis, and T. Mallouk. 2005. Photorefractive CdSe and gold nanowire-doped hquid crystals and polymer-dispersed-liquid-crystal photonic crystals. Mol. Cryst Liq. Cryst. 446 233-244. 

Figure 1.20 Encapsulation of microdroplets of liquid crystals in ORMOSIL matrices results in materials with better transparency and thermal stability than polymer-dispersed liquid crystals. Gel-glass dispersed liquid crystal device switched between the OFF and ON state (thickness 10 pm, 4 x 2 cm, Fp p = 90V). (Reproduced from ref. 45, with permission.)...

For all but spherically symmetrical molecules, van der Waals forces are anisotropic. The polarizabihties of most molecules are different in different molecular directions because the response of electrons in a bond to an external field will usually be anisotropic. A consequence of this effect is that the dispersion force between two molecules will depend on their relative molecular orientation. In nonpolar liquids, the effect is of minor importance because the molecules are essentially free to tumble and attain whatever orientation is energetically favorable. However, in sohds, hquid crystals, and polar media, the effect can be important in determining the relative fixed orientation between molecules, thereby affecting or controlling specific conformations of polymers or proteins in solution, critical transition temperatures in liquid crystals and membranes, and so on. Repulsive forces in polar molecules are also orientation dependent, and are often of greater importance in controlling conformations and orientations. 

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