Liquid-crystalline medium

Details Liquid-crystalline medium Liquid-crystalline medium

2002-12-20

2004-10-26

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C252S299670, C349S186000

Reexamination Certificate

active

06808763

ABSTRACT:

The invention relates to a liquid-crystalline medium and to twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN here covers any relatively highly twisted display element having a twist angle with a value of between 160° and 360°, such as, for example, the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays are distinguished compared with standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, associated therewith, better contrast values, and by significantly lower angle dependence of the contrast.
Of particular interest are TN and STN displays having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimized using mostly monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for every application.
In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K
33
/K
11
and relatively small values for &Dgr;∈/∈
1
, where &Dgr;∈ is the dielectric anisotropy and ∈
1
is the dielectric constant perpendicular to the longitudinal molecular axis.
In addition to optimization of the contrast and response times, further important requirements are made of mixtures of this type:
1. broad dip window
2. high long-term chemical stability
3. high electrical resistance
4. low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex STN displays (with a multiplex rate in the region of about 1/400), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about 1/64 and 1/16 respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.
Thus, there continues to be a great demand for TN and STN displays, in particular for medium- and low-multiplex STN displays, having very short response times at the same time as a large working-temperature range, high characteristic-line steepness, good angle dependence of the contrast and low threshold voltage which meet the above-mentioned requirements.
The invention has an object of providing liquid-crystalline media, in particular for TN and STN displays, which do not have the above-mentioned disadvantages or only do so to a lesser extent and at the same time have short response times, in particular at low temperatures, and very good steepnesses. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these and other objects can be achieved if use is made of liquid-crystal mixtures which comprise one or more compounds of the formula A
and at least one compound of the formula B
in which
R
a
and R
bb
are H, an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF
3
or at least monosubstituted (i.e., up to perhalosubstituted) by halogen, it also being possible for one or more CH
2
groups in these radicals to be replaced, in each case independently of one another, by —O—, —S—,
—CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another,
R
b
is an alkenyl radical having from 2 to 7 carbon atoms,
L
1
is H or F, and
x is 0 or 1.
The use of the compounds of the formulae A and B in the mixtures for TN and STN displays according to the invention results in
high steepness of the electro-optical characteristic line,
low temperature dependence of the threshold voltage,
very fast response times, in particular at low temperatures.
The compounds of the formulae A and B significantly shorten, in particular, the response times of TN and STN mixtures while simultaneously increasing the steepness and reducing the temperature dependence of the threshold voltage.
The mixtures according to the invention are furthermore distinguished by the following advantages:
they have low viscosity,
they have a low threshold voltage and operating voltage, and
they effect long shelf lives in the display at low temperatures.
The invention furthermore relates to a liquid-crystal display having
two outer plates, which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5° and 600°,
a nematic liquid-crystal mixture consisting of
a) 15-75% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 25-85% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between −1.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below −1.5, and
d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,
characterized in that component A comprises at least one compound of the formula A
and component B comprises at least one compound of the formula B
in which R
a
, R
b
, R
bb
, x and L
1
are as defined above.
The invention also relates to TN and STN displays, in particular medium- and low-multiplexed STN displays, containing the liquid-crystal mixture according to the invention.
Formula A covers, in particular, compounds of the sub-formulae A-1 to A-4
in which alkyl is a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms, and alkenyl is a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms.
Particular preference is given to mixtures according to the invention which comprise at least one compound of the formula A-1 and/or A-2, particularly preferably in each case at least one compound of the formula A-1 in which L
1
=F.
In the formulae A, R
a
(and alkyl or alkenyl in formulae A-1 t

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