Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1992-12-16
1994-06-21
Sikes, William L.
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359103, 25229901, G02F 113
Patent
active
053232518
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electrooptical liquid crystal system crystal mixture being dispersed in form of microdroplets in an optically isotropic, transparent polymer matrix, matched to the refractive index of the polymer matrix, and compared with the other state, essentially independent of the polarization of the incident light.
2. Description of the Related Art
The preparation of PDLC (=polymer dispersed liquid crystal) films is described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585. In the so-called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is first homogenously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization. Differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427) both also being methods to produce PDLC films.
The process of preparation must be controlled very carefully in order to obtain systems with good electrooptical properties. F. G. Yamagishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications, 1989, p.24 differentiate between a "Swiss cheese" and a "polymer ball" morphology. In the latter one, the polymer matrix consists of small polymer particles or "balls" being connected or merging into each other while in the Swiss cheese system, the polymer matrix is continuous and exhibits well defined, more or less spherical voids containing the liquid crystal. The Swiss cheese morphology is preferred because it exhibits a reversible electrooptical characteristic line while the polymer ball system shows a distinct hysteresis generally leading to a drastic deterioration of the electrooptical characteristic line when comparing the virgin and the second run.
According to Yamagishi et al., loc. cit., the Swiss cheese morphology is promoted in case the polymerization reaction runs via a step mechanism, and in WO 89/06264 it is pointed out that the step mechanism is favored in case the precursor of the polymer matrix consists of multifunctional acrylates and multifunctional mercaptanes. Another important point which can be influenced by a careful selection of process parameters such as curing temperature, relative concentration and choice of the components of the prepolymer etc., is the mean size of the microdroplets. The mean droplet size generally influences the threshold voltage, the switching times, the on-state transmission and other properties of the PDLC film. Experimental methods for influencing the average droplet size are described, for example, in U.S. Pat. No. 4,673,255 and in J. L. West, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt., 157, 1988, 427. In U.S. Pat. No. 4,673,255, average drop diameters between 0.1 .mu.m and 8 .mu.m are given, while, for example, a matrix which is based on a glass monolith has pores having a diameter between 15 and 2,000 .ANG.. For the mesh width of the network of PN systems, a preferred range between 0.5 and 2 .mu.m is given in EP 0,313,053.
In PDLC films, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index n.sub.o, is selected in such a way that it more or less coincides with the refractive index n.sub.p of the polymeric matrix. If no voltage is applied to the electrodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.
On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Normally incident light (viewing angle .theta.=0.degree.) now sees an optically isotropic medium and appears transparent.
No polarizers are required for operating PDLC systems, as a result of which these systems have high tra
REFERENCES:
patent: 5011624 (1991-04-01), Yamagishi et al.
patent: 5021188 (1991-06-01), Vaz et al.
Yamagishi et al., "Morphological Control in Polymer-Dispersed Liquid Crystal Film Matrices", SPIE, vol. 1080 (1989), pp. 24-27.
Hirai et al., "Phase Diagram and Phase Separation in LC/Prepolymer Mixture", SPIE vol. 1257 (1990), pp. 2-8.
Coates David
Nolan Patrick
Mai Huy
Merck Patent Gesellschaft mit beschrankter Haftung
Sikes William L.
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