Compositions – Liquid crystal compositions
Patent
1993-08-13
1996-03-12
Wu, Shean
Compositions
Liquid crystal compositions
25229961, 25229963, 25229964, 25229966, 359103, C09K 1952, G02F 113
Patent
active
054983657
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an electrooptical liquid crystal system crystal mixture being dispersed in the 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 independent of the polarization of the incident light.
BACKGROUND OF THE INVENTION
In PDLC films which are 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, 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 elec-trodes, 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 required for operating PDLC systems, as a result of which these systems have high transmission. PDLC systems provided with active matrix addressing have been proposed on the basis of these favorable transmission properties in particular for projection applications, but in addition also for displays having high information content and for further applications.
A series of matrix materials and polymerization processes have hitherto been proposed for producing PDLC systems. In the so called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is firstly 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., the Swiss cheese morphology is distinctly favored if the precursor of the matrix is a thiolene system, i.e. contains a thiol- and an ene-component.
PDLC systems the precursor of the matrix of which is purely based on ene-type compounds, are described in EP 0,272,585 and in Y.Hirai et al., SPIE Vol. 1257, Liquid Crystal Displays and Applications, 1990, p. 6. EP 0,272,585 favors systems with a high oligomer concentration of 15-70% and Hirai describes PDLC films the precursor of the matrix of which consists of monofunctional acrylates and acrylate oligomers in varying ratios. Both precursors, however, tend to disadvantageously form PDLC film with a morphology which is different from the "Swiss cheese" morphology.
PDLC systems, another complication is that the liquid crystal mixture usually tends to dissolve into the polymer matrix to a lesser or higher degree. In the polymer matrix, the liquid crystal act as an isotropic material exhibiting
REFERENCES:
patent: 5242616 (1993-09-01), Finkenzehler et al.
patent: 5323251 (1994-06-01), Coates et al.
patent: 5356557 (1994-10-01), Jubb et al.
Hirai et al., "Phase diagram and phase separation in lc/prepolymer mixture", SPIE, Liquid Crystal Displays and Applications, vol. 1257 (1990), pp. 2-8.
Coates David
Nolan Patrick
Merck Patent Gesellschaft mit beschrankter Haftung
Wu Shean
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