Polymer dispersed liquid-crystal

Polymer-dispersed liquid crystals (PDLCs) are made up of nematic liquid crystals dispersed in a solid continuous polymer matrix. These are prepared by mixing a reactive monomer into a non-polymerisable LC medium and then polymerising the reactive monomer to create a polymer matrix, at the same time capturing the LCs as dispersed droplets, greater than 1 pm in diameter, i.e. the wavelength of visible light.3 -33 

Holographic optical elements can also be made by the preparation of polymer-dispersed liquid crystals using twin lasers in transmission holographic photopolymerisation (see section 5.4.2). They have also been made using photorefractive composites of polymer dispersed liquid crystals (see section 5.6.3). 

An alternative approach is to add gelling agents to LCs. The gelling materials that have been nsed range from low MW materials to dendrimers. Potential applications inclnde large area displays and electro-optical switches.  

Polymer-dispersed liquid crystals were introduced many years ago. There are now several techniques for preparing such composite liquid crystalline materials, including the phase separation and the encapsulation methods.More recently, optical holographic interference methods have been employed successfully in making polymer-dispersed liquid crystal photonic crystals (regular array of materials of different refractive indices). Caputo et al. and Strangi et al. have also demonstrated one-dimensional (ID) polymer/liquid crystal layered structures that exhibit high diffraction efficiency as well as laser emission capabilities. 

In polymer dispersed liquid crystals (PDLCs), the concentrations of polymer and liquid crystal are comparable. Liquid crystals form isolated droplets embedded in polymers. We will consider first the liquid crystal director configuration inside droplets, and then the electro-optical properties of PDLCs, as weU as their applications. 

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C2.2.4.5 POLYMER DISPERSED LIQUID CRYSTAL (PDLC) DISPLAYS... 

Figure C2.2.14. Principle of operation of a polymer-dispersed liquid crystal display. The contours of the liquid...

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

Similar behaviour has been observed by Emerson and Zannoni [112] in their simulations of polymer dispersed liquid crystal droplets where the solid... 

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.)...

Refraction of light Polymer dispersed liquid crystals (PDLC), gel and polymer network displays... 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

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... 

The polymer-dispersed liquid crystal was used as a modulating medium in optically controlled modulators instead of the liquid crystal [261-264], The sandwiched structure from polyimide photosensitive film and the polymer dispersed liquid crystal film - i.e. the optically controlled solid state modulator -had the characteristics presented in Fig. 36 [261]. Contrast ratio 35 1, response time > 400 ps, decay time 80 ms and sensitivity 5 x 10 sJcm 2 were obtained. 

Fig. 36. Contrast ratio (/), switch on (2) and switch off (3) times versus voltages for modulator with polymer dispersed liquid crystal. Photocon-ductor-polyimide film. Pulse regime [261]...

Bhargava, R., Wang, S.Q. and Koenig, J. L. (1999) Studying polymer dispersed liquid crystal formation by FTIR spectroscopy 1. monitoring curing reactions. Macromolecules, 32(26), 8982-88. 

Higgins, D.A., J.E. Hall, and A. Xie. 2005. Optical microscopy studies of dynamic within individual polymer-dispersed liquid crystal droplets. Acc. Chem. Res. 38 137-145. 

Mach, P. Rodriquez, S.J. Nortrup, R. Wiltzius, P. and Rogers, J.A. (2001) Monolithically integrated flexible display of polymer-dispersed liquid crystal driven by rubber-stamped organic thin-film transistors. Appl. Phys. Lett., 78, 3592-3594. 

Sheraw, C.D. Zhou, L. Juang, J.R. Gundlach, D.J. Jackson, T.N. Kane, M.G. Hill, I.G. Hammond, M.S. Campi, J. Greening, B.K. Francl, J. and West, J. (2002) Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates. Appl. Phys. Lett., 80, 1088-1090. 

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]. 

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