Electrooptical liquid crystal system

Details Electrooptical liquid crystal system Electrooptical liquid crystal system

2002-03-18

2004-01-27

Wu, Shean C (Department: 1756)

Compositions

Liquid crystal compositions

C252S299610, C252S299620, C252S299630, C252S299640, C252S299660, C252S299670, C570S125000, C570S128000

Reexamination Certificate

active

06682661

ABSTRACT:

SUMMARY OF THE INVENTION
The invention relates to an electrooptical liquid system
which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent polymer matrix,
in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix,
which exhibits an electrically switchable transparency which is essentially independent of the polarization of the incident light,
the precursor of the PDLC film of which comprises one or more monomers, oligomers and/or prepolymers and a photoinitiator, and is cured photoradically, and
the liquid crystal mixture of which comprises one or more compounds of the formula I
in which
Z
1
and Z
2
independently of one another, are a
single bond, —CH
2
CH
2
—, —COO—, —OCO—
or —C═C—,

independently of one another, are trans-1,4-cyclohexylene, 1,4-pheny- lene, 2-fluoro-1,4-phenylene, 3-flu- oro-1,4-phenylene, 2,3-difluoro-1,4- phenylene or 3,5-difluoro-1,4-pheny- lene and one of
may also be pyrimidine-2,5-diyl, pyri- dine-2,5-diyl or trans-1,3-dioxane- 2,5-diyl,

X
1
and X
2
independently from one another, are H or F,
Q
is CF
2
, OCF
2
, C
2
F
4
,OC
2
F
4
or a single bond,
Y
is H, F, Cl or CN,
n
is 0, 1 or 2, and
R
is alkyl having up to 13 C atoms, in
which one or two non-adjacent CH
2
groups can also be replaced by —O—
and/or —CH═CH—.
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 being also 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 favoured in case the precursor of the polymer matrix consists of multifunctional acrylates and multifunctional mercaptanes.
In PDLC films, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index n
o
, is selected in such a way that it more or less coincides with the refractive index n
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 &thgr;=0°) now sees an optically isotropic medium and appears transparent.
No polarisers are 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 favourable transmission properties in particular for projection applications, but in addition also for displays having high information content and for further applications.
The liquid crystal mixtures used for producing PDLC systems have to meet a wide range of demands. One of the refractive indices of the liquid crystal mixture is selected such that it matches with the refractive index of the polymer matrix. The term matching of refractive indices used here covers not only the case n
o
(resp. another refractive index of the liquid crystal mixture) ~n
p
, but also the condition n
o
(resp. another refractive index of the liquid crystal mixture)<n
p
which is sometimes chosen to reduce off-axis haze and enlarges the view angle as is described, for example, in EP 0,409,442.
The liquid crystal mixture preferably has a positive dielectrical anisotropy but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures (see, for example, N. A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed.
Furthermore, the liquid crystal mixture should have a high clearing point, a broad nematic range, no smectic phases down to low temperatures and a high stability and should be distinguished by an optical anisotropy &Dgr;n and a flow viscosity &eegr; which can be optimized with respect to the particular application, and by a high electrical anisotropy.
A series of matrix materials and polymerization processes have hitherto been proposed for producing PDLC systems. The PIPS, SIPS and TIPS technologies are described in some detail in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Optics, 157, 1988, 427. The PDLC systems described in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Optics, 157, 1988, 427 are based on an epoxy film, while in EP. 0,272,585 acrylate systems are given. The PDLC system of WO 89/06264 is based on multifunctional acrylates and multifunctional thioles, and Y. Hirai et al., SPIE Vol. 1257, Liquid Crystal Displays and Applications, 1990, p.2 describe PDLC systems the precursor of the polymer matrix of which being based on monomers and oligomers. Further suitable matrix materials are described, for example, in U.S. Pat. No. 3,935,337, WO 91/13126 and in further references.
Electrooptical systems containing PDLC films can be addressed passively or actively. Active driving schemes employing an active matrix having nonlinear addressing elements like, for example, TFT transistors integrated with the image point, are especially useful for displays with high information content.
When the PDLC system is addressed by means of an active matrix, a further far reaching criterion is added to the requirements listed so far which must be fulfilled by the cured polymer and the liquid crystal mixture being embedded in microdroplets. This is related to the fact that each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the so-called holding ratio (HR) which is defined as the ratio of the drop in voltage across an image point in the nonaddressed state and the voltage applied; a process for determining the HR is given, for example, in Rieger, B. et al., Conference Proceeding der Freiburger Arbeitstagung Flüssigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg 1989. Electrooptical systems having a low or relatively low HR show insufficient contrast.
A further serious problem is often that the liquid crystal mixture has insufficient miscibility with the

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