Polymer dispersed liquid crystal devices

Kim Dae Soo, Yeo Dong Bin, Heo Kyong Chan, et al. Color polymer dispersed liquid crystal device with colored core-shell structure. Mol. Cryst. Liq. Cryst. 613 no. 1 (2015) 39-44. 

Doane, J. W., Polymer dispersed liquid crystal devices, in Liquid Crystals, Applications and Uses, Vol. 1, Bahadur, B., Ed., World Scientific, Singapore, 1990, Chap. 14. 

Coates, D., Normal and reverse mode polymer dispersed liquid crystal devices, Displays, 14, 94, 1993. 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

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.)...

The modulators mentioned above contained liquid crystal molecules. An entirely solid state device is naturally preferable in practice. The polymer-dispersed liquid crystal materials may be prospective counterparts of the modulator. One of the modes of operation of such thin films are as follows. The... 

ICP thin films were used as driving electrodes for polymer-Dispersed Liquid-Crystals (PDLC) display devices. Liquid-crystalline-based display devices, which are commonly made of a liquid-crystal compound sandwiched between two substrates coated with a conducting layer of indium tin oxide (ITO), whose substitution with ICP electrodes could improve the optical and mechanical properties of the display devices. On the way to all-organic displays, PDLC sandwiched between two plastic substrates coated with ICP layers are promising devices for paper-like displays for electronic books which require flexibility, lightness, and low-power consumption. The electro-optical characteristics (transmission properties, drive voltages and switching times) of the PDLC devices depend on the nature of the ICP substrate used [13]. 

Polymer-dispersed liquid crystals are the next generation of display materials. Initial research has outlined the basic principles of their operation The PDLC films combine the properties of plastics and liquid crystals producing display devices impossible with conventional materials. They wfil be used in applications ranging from architectural glass to projection TV and optical computing. 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

Polymer Dispersed Liquid Crystals (PDLCs) are widely researched materials of commercial importance in the electro-optical device industry as their light transmission can be changed by the application of an electric field. The refractive index of these rod-shaped liquid crystals along and perpendicular to their major axis is... 

The third section of the book addresses recent research efforts in making polymer-dispersed liquid crystals consisting of nematic or cholesteric low- molecular-mass liquid crystals in flexible-chain polymers or LCPs. These S3rstems are prepared by polymerization of reactive monomers in the presence of Uquid ciystads stabilized by flexible-chain polymer or LCP. Special attention has been paid to the use of these materials in display and electro-optical devices. 

Tremendous activity [33] has characterized the field of research on polymer-dispersed liquid crystals (PDLCs) which are potentially useful for a variety of electrooptical applications including light shutters and switchable windows, displays, and other devices. These materials consist of micrometer-size nematic droplets dispersed in a polymer matrix and their optical response is based on the electrically controlled light-scattering properties of the droplets. 

Polymer-dispersed liquid crystals (PDLCs) is a relatively new class of promising material for many applications such as, switchable windows, display devices, infrared shutters, angular-discriminating filters, thermoelectrooptic switches, memories, gas-flow sensors, optical sensors, and optical gratings etc. These materials are examples of combined application of polymers and liquid crystals and command the attention of the display industry as well as the researchers. These consist of LC droplets which are dispersed in a polymer matrix. These tiny droplet characteristics are responsible... 

Fig. 5.14. A polymer dispersed liquid-crystal (PDLC) device consisting of a microdispersion of a low molecular weight nematic fluid (MLC) in a conventional transparent polymer host matrix sandwiched between thin coats of transparent, conducting tin oxide. On the left is shown the off state with a refractive-index mismatch between the dispersion and the host that scatters incident light. On the right is shown how an external electric held aligns the director of the nematic matching the refractive indices of the dispersion and the host, yielding an optically transparent medium.

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. 

On polymerizing the monomer, the liquid crystal comes out of solution, forming droplets dispersed in the polymeric matrix. The size of the droplets depends on polymerization rate, ratio of liquid crystal to monomer, and evaporation of solvent in solvated systems. This techique is considered in more detail in the chapter devoted to display devices in the discussion of polymer dispersed liquid crystal displays (PDLCs). 

Polymer dispersed liquid crystal (PDLC) devices usually contain N phases as the liquid crystal material [71] and are used in vision products [72], e.g., privacy windows, projection displays [73], and direct view displays [74, 75]. Cholesteric liquid crystals have also been used [76]. All these devices relax back to the original ground state when the field is removed. Ideally such films consist of dfoplets of liquid crystal in a polymer matrix the reverse situation (reverse phase) consists of a liquid crystal continuum with polymer balls dispersed within it. The latter films are not desirable, because they do not provide reversible electrooptic effects. 

Moreover, polymer dispersed liquid crystals (PDLC) have been extensively studied as promising candidates new materials for application in the field of thermo- and electro-optical devices, such as optic shutters, smart windows, optical sensors, memories and flexible display devices. 

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