Polymer dispersed liquid crystal displays

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%. 

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

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. 

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. 

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. 

Sheraw, C.D. et al.. Organic thin-fihn transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates, App. Phys. Lett. 80, 1088-1090, 2002. 

C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G. Kane, I. G. Hill, M. S. Hammond, J. Campi, B. K. Greening, J. Prancl, and J. West. Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates. Applied Physics Letters, 80(6) 1088-1090, 2002. 

J. W. Doane, Polymer dispersed liquid crystal displays, Liquid crystals, applications and uses, Vol. 1, Chapter 14, ed. B. Bahadur (World Scientific, Singapore, 1990). 

A. Kanemoto, Y. Matsuki, and Y. Takiguchi, Back scattering enhancement in polymer dispersed liquid crystal display with prism array sheet, Proc. Intnl. Display Research Conf., 183 (1994). 

The synthesis, characterization and mesomorphic behavior of diacrylates based on polymerized liquid crystals are reported. Several types of polymer stabilized liquid crystal display devices were prepared from the dispersions of low concentration of diacrylates in liquid crystals and subsequently the prepolymers were polymerizaton by ultraviolet radation toorm polymer networks. The morphology stupes show that the orientation of polymer networks induced by the surface treatment of the substrate has led to the preferential liquid crystal alignment... 

Jeon Young Jae, Lee Gae Hwang, Jang Jae Eun, et al. Applications of multidirectional reflective light-control films on reflective polymer-dispersed liquid crystal displays for enhancement in image quality at lower viewing angles. Liq. Cryst. 39 no. 11 (2012) 1314-1319. 

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... 

G.P. Crawford, Reflective liquid crystal display materials Liquid crystal polymer dispersions, 27th International SAMPE Technical Conference, pp. 484-496,1995. 

Volume 7 summarizes new trends on liquid crystals, display, and laser materials. The topics include liquid crystals for electro-optic applications, switchable holographic polymer-dispersed liquid crystals, electrochromism and electrochromic materials for displays, materials for solid-state dye lasers, photophysical properties of laser orientational relaxation processes in luminescence, and lasing of dyes and photosensitive materials for holographic recording. 

Figure 5.30 Principle of operation of a polymer-dispersed liquid crystal display. The contours of the liquid crystal droplets in the polymer matrix correspond to the director orientation, which here is dipolar. In the off state, the cell scatters light and appears opaque, due to refractive index variations between the liquid crystal and the polymer. However when an electric field is applied and the liquid crystal directors reorient, the refractive index along the field is matched to that of the matrix and the cell becomes clear...

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). 

For driving matrix liquid crystal display panels, the silicon metal-oxide semiconductor field effect transistor (MOSFET) fabricated on a silicon monolithic wafer has been investigated by several groups [134-150]. The MOS transistor circuit fabrication techniques are well developed and have been used to produce various LSI devices. A dynamic scattering mode, a planar type GH mode or a polymer dispersed (PD) mode are used in these displays because the silicon wafer is intrinsically opaque. The circuit configuration of the panel is essentially the same as that of the p-Si TFT switch matrix addressed liquid crystal display panel as shown its equivalent circuit in Fig. 18(a). 

In addition to the TN and IPS modes in nematic displays, himdreds of other displays methods were invented and tested over the last 30 years. Out of them the bistable nematic development based on surface flexoelectric interactions, the bistable cholesterics displays based on switching between planar and focal conic textures and the polymer dispersed liquid crystal displays found some applications. None of them, however, offer better than a few milliseconds switching time. 

C.C. Bowley, A.K. Fontecchio, G.P. Crawford, J.J. Lin, L. Li, S. Paris, Multiple gratings simultaneously formed in holographic polymer dispersed liquid crystal displays, Appl. Phys. ljett, 76, 523 (2000). 

Applications of microparticles can be found in medicine, biochemistry, colloid chemistry, and aerosol research [48]. Some uses include separation media for chromatographic application, high surface area substrates for immobilized enzymes, standards for calibration, spacers in optical cavities and liquid crystal displays, and three-dimensional microenvironments for cell encapsulation. It should be stressed that even a scaled-up MF synthesis enables generation of a relatively small amount of particles, in comparison with conventional emulsion, dispersions, or suspension polymerizations. Thus, most practical applications of such microbeads should utilize their high-value unique properties, for example, a uniform distribution of sizes and control of morphology, structure, and shape. Therefore, some of the demonstrated applications of polymer microbeads are still in the proof-of-concept stage. 

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