Cholesteric encapsulated

In the above applications, cholesteric liquid crystals need to be sealed between two glass plates or in the form of micro encapsulates. However, cholesteric liquid crystalline polymers can easily form thin films or be coated on substrates. 

In general, the term liquid crystal is used to describe an intermediate phase between liquid and solid occurring in some organic compounds. The phase of liquid crystal can be divided into two mesophases smectic and nematic. Nematic liquid crystals can be further divided as chiral nematic or archiral nematic. In chiral nematic liquid crystals, sterol-related compounds are called cholesteric, and non-sterol-based compounds are termed chiral nematic. For heat transfer applications, encapsulated forms of chiral nematic [71] or the composite liquid crystal sheets of the cholesteric type [72] are commonly used. Recently, the application of micro-encapsulated liquid crystals has become more popular in heat transfer measurements because of the fast response and easy paintbrush or spray application to the test surface. 

The method to manufacture cost-effective displays is to use proper processes for mass production or on a large scale. One example is a roll-to-roll process which needs a flexible substrate to be coated or printed display materials. Here, an important issue is to provide a monolayer of display materials. In order to supply this mono-layer, a variety of methods have been tried for encapsulated cholesteric liquid crystals by Kent Display, microencapsulated electrophoretic... 

There are two classic approaches for encapsulation emulsification [17] and phase separation [18]. The main difference between the two methods depends on how to make and process encapsulated droplets. In the emulsification method, water is used as a solvent to dissolve a polymer and to form a viscous solution, and cholesteric liquid crystals are mixed with the aqueous solution. By a shearing device like a propeller blade, small droplets of micrometer scale are formed and finally emulsified. The resulting emulsion is then printed on a plastic film and dried by evaporation of water. The disadvantage of this method is the broad size... 

The pitch of a cholesteric may vary quite rapidly with temperature, particularly if the phase diagram of the material contains a cholesteric to smectic A transition just below the temperature range of interest since the pitch in this case diverges to infinity as the smectic A phase is approached. Thus, the color of the selective reflection can be used a s a sensitive indicator of temperature. Material for this purpose is incorporated into plastic films with a black backing to absorb the unreflected polarization, or it is encapsulated to apply as a paint and used for a variety of medical, engineering, and consumer applications. 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

The encapsulated cholesteric liquid crystals are suitable for flexible displays with plastic substrates. They have much higher viscosities than pure cholesteric liquid crystals and can be coated on substrates in roll-to-roll process [71,72]. The polymers used for the encapsulation have good adhesion to the substrates and can make the materials self-adhesive to sustain the cell thickness. Furthermore, the encapsulated Ch liquid crystals can no longer flow when squeezed, which solves the image-erasing problem in displays from pure cholesteric liquid crystals where squeezing causes the hquid crystal to flow and to be switched to the planar state. 

T. Schneider, F. Nicholson, A. Kahn, and J. W. Doane, Flexible encapsulated cholesteric LCDs by polymerization induced phase separation, SID Inti Symp. Digest Tech. Papers, 36, 1568-1571 (2005). 

The second group of LC composites - encapsulated polymeric films are usually based on the cholesteric liquid crystals... 

The encapsulation process for ChLC is mainly attributed to its transport and optical properties [15]. Firstly, since viscosity of pure ChLC is close to that of water, its fluidity prevents ChLC from being coated on flexible substrates. Secondly, when a cholesteric liquid crystal is pressed, the flow generated inside makes the displayed image erase. Therefore, droplet dispersions by encapsulation act as a protector for its bi-stability and optical properties. The additional advantage is that encapsulated cholesteric liquid crystals are self-sealing the materials confined to the droplets cannot flow through an interface of the droplets. 

Other techniques of encapsulating liquid crystalline materials exist, and find application in areas apart from thermography. In particular, the methods used in the fabrication of polymer dispersed liquid crystal N and Sm display devices can be readily applied to cholesteric materials. Polymer dispersed displays are fabricated by dissolving the liquid crystal in the monomeric form of the continuous medium of the final device. 

Churchill D, Cartmell JV (1971a) Radiation sensitive display device containing encapsulated cholesteric liquid crystals. US Patent 3,578,844... 

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