Paper white PDLC system

Details Paper white PDLC system Paper white PDLC system

1995-02-24

1997-02-18

Sikes, William L.

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

G02F 11333

Patent

active

056046120

DESCRIPTION:

BRIEF SUMMARY
The invention relates to an electrooptical liquid crystal system which between 2 electrode layers contains a PDLC fill comprising a liquid crystal mixture being dispersed in the form of microdroplets in an optically isotropic, transparent polymer matrix, in which one of the refractive indices of the liquid crystal liquid mixture is matched to the refractive index of the polymer matrix, and the transmission of which can be varied by applying an electrical voltage.
The invention further relates to a PDLC fill with enhanced backscattering.
The preparation of PDLC (=polymer dispersed liquid crystal) fills 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 homogeneously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization, and one can differentiate between radical PIPS, in particular photoradical PIPS, and non-radical (for example, thermally induced) PIPS. 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 being also methods to produce PDLC fills.
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 rims via a step mechanism, and in WO 89/06264 it is pointed out that the step mechanism is favoured 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. Generally, relatively small microdroplets cause relatively high threshold voltage V.sub.th, but relative short switching times t.sub.on and t.sub.off as is discussed, for example, in U.S. Pat. No. 4,673,255. 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.M of the cured polymeric matrix. It must be differentiated between the refractive index n.sub.p of the cured pure polymer (no liquid crystal incorporated) and the refractive index n.sub.M of the cured polymer matrix containing liquid crystal microdroplets; n.sub.M generally deviates from n.sub.p due to liq

REFERENCES:
patent: 4944576 (1990-07-01), Lacker et al.
patent: 5206747 (1993-04-01), Wiley et al.

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