Liquid crystal compounds, liquid crystal medium and liquid...

Details Liquid crystal compounds, liquid crystal medium and liquid... Liquid crystal compounds, liquid crystal medium and liquid...

2001-02-02

2003-05-27

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299660, C558S017000, C570S127000, C430S020000

Reexamination Certificate

active

06569505

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to liquid crystal compounds, liquid crystal media and to liquid crystal displays comprising these media, in particular to displays of the OCB type and to displays of the composite systems type like PDLCs and most particular amongst these to holographic PDLCs.
PROBLEM TO BE SOLVED AND STATE OF THE ART
L
iquid
C
rystal
D
isplays (LCDs) are widely used to display information. Electro-optical modes employed are e.g. the
t
wisted
n
ematic (TN)-, the
s
uper
t
wisted
n
ematic (STN)- and the
e
lectrically
c
ontrolled
b
irefringence (ECB)-mode with their various modifications, as well as others. Besides these modes, which all do use an electrical field, which is substantially perpendicular to the substrates, respectively to the liquid crystal layer, there are also electro-optical modes employing an electrical field substantially parallel to the substrates, respectively the liquid crystal layer like e.g. the
i
n-
p
lane
s
witching (IPS)-mode (compare e.g. DE 40 00 451 and EP 0 588 568).
Besides the various different modes using the liquid crystal medium as such, oriented on surfaces, which typically are pre-treated to achieve uniform alignment of the liquid crystal material, there are applications using composite systems of liquid crystal materials of low molecular weight together with polymeric materials such as e.g.
p
olymer
d
ispersed
l
iquid
c
rystal (PDLC)-,
n
ematic
c
urvilinearily
a
ligned
p
hase (NCAP)- and
p
olymer
n
etwork (PN)-systems, as disclosed for example in WO 91/05 029. These composite systems typically use an electrical field substantially perpendicular to the composite layer. LCDs are used for direct view displays, as well as for projection type displays. Besides these applications LCDs, especially LCDs comprising composite systems like PDLCs and in particular so called holographic PDLC (HPDLC) systems are used in practical applications. HPDLCs are described e.g. in Date, Takeuchi, Tanaka, and Kato, Journal of the SID 7/1 (1999), p. 17 to 22, which is incorporated by reference. These HPDLC displays are generating three bright colors, preferably primary colors, utilizing Bragg reflection. This technique results in excellent bright colors as it does neither need polarizers, nor color filters. A single layer of the periodic structure of polymer and liquid crystal controls the reflection of one particular color. To realize three primary colors consequently three layers, one for each color are required. Each of the three layers has to be addressed independently. This requires three sets of HPDLC films, each with corresponding electrodes. This large number of layers and corresponding electrodes, which is difficult to realize with a good yield in mass production, can beneficiously be reduced when the “two-frequency” drive method is applied.
For composite systems a high An of the liquid crystal used is required in order to achieve an efficiently scattering state and to realize a good contrast. Though there have been proposed PDLC-systems with liquid crystal mixtures with low An to improve the so called off axis haze, the predominant problem in most cases is to achieve sufficient contrast in the first place. This is especially the case for PDLC-systems, which are disclosed e.g. in Date, Takeuchi, Tanaka, and Kanto, Journal of the SID 7/1 (1999), p. 17-22. The liquid crystals available typically are characterized by &Dgr;n values of up to 0.280 or even up to 0.29. This upper limit, however, is still insufficiently low for many applications. Further it has so far only been achieved accepting various compromises with respect to the other properties of the liquid crystal mixtures used. The most typical undesired trade-offs are an insufficiently high clearing point, an unfavourably narrow nematic phase range, a rather high temperature for the lower end of the stability of the nematic phase, too low dielectric anisotropy and hence too high operating voltages, unfavourable elastic constants and last not least too high viscosity values or combinations thereof. Good compatibility with the precursors of the polymers of the composite systems and easy phase separation during the formation of the composite systems are obvious prerequisites for liquid crystals for such applications.
Another promising electro-optical mode used in LCDs is the
o
ptically
c
ompensated
b
ent (OCB) mode. This mode is described e.g. in Yamaguchi et al., “Wide-Viewing-Angle Display Mode for the Active-Matrix LCD Using Bend-Alignment Liquid-Crystal Cell”, SID 93, Digest, p. 277 (1993).
This mode is very promising. It is particularly well suited for direct view applications, as it is characterized by a favourable viewing angle dependence. Also the response times are quite short. However for video rate response for the display of changing grey shades the response time still needs to be improved. Compared to a conventional TN display in an OCB displays the amount of deformation of the director is much smaller. Whereas in a TN display the director is oriented almost parallel to the substrates in the unpowered state and changes its direction to almost perpendicular to the substrates upon application of the driving voltage, in an OCB display the director orientation changes to the same final orientation, but starts from an already almost homeotropic bent starting configuraton. Thus, a higher birefringence of the liquid crystal media used is required.
Liquid crystal compounds with a terminal phenyl ring bearing a terminal isothiocyanate group and two F-atoms in ortho position thereto are known from DE 40 27 869.7.
Liquid crystal mixtures consisting predominantly or even entirely of terminally cyano substituted biphenyls and terphenyls are as a rule characterized by suitable high &Dgr;&egr; values, but have limited values of &Dgr;n and already are likely to show insufficient stability at low temperatures, i.e. in most cases either formation of a smectic phase and or crystallisation. Liquid crystal mixtures using large quantities of halogenated tolane compounds with three phenyl rings, which are almost dielectrically neutral, are disclosed, e.g. in the European Patent Application No. EP 99111782.1 are characterized by comparatively low &Dgr;&egr; values which are not suitable for most applications and often even show severe problems with respect to the stability of the nematic phase at low temperatures.
Thus, there is a significant need for liquid crystal media with suitable properties for practical applications such as a wide nematic phase range, low viscosities, appropriate optical anisotropy &Dgr;n according to the display mode used especially a suitably high &Dgr;n for OCBs and for composite systems like PDLCs and for the latter in particular with suitably large good compatibility with polymer precursors for the composite systems.
SUMMARY OF THE INVENTION
Surprisingly, it now has been found that liquid crystal media with high An especially useful for composite systems can be realized which do not exhibit these drawbacks of the materials of the prior art or at least do exhibit them to a significantly lesser degree.
These improved liquid crystal media according to the instant application are realized by using at least two components: a first liquid crystal component (called component A) comprising compounds of formula I, which are strongly dielectrically positive compounds with very high values of &Dgr;n
wherein
R
1
is n-alkyl, n-alkoxy with 1 to 10 C-atoms, preferably 1 to 7 C-atoms, more preferably 2 to 5 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms, preferably with 2 to 5 C-atoms or CN, NCS, halogen, preferably F, Cl, halogenated alkyl, halogenated alkenyl or halogenated alkoxy, preferably mono-, di- or oligo-fluorinated alkyl, mono-, di- or oligo-fluorinated alkenyl or mono-, di- or oligo-fluorinated alkoxy, F, Cl, CF
3
or OCF
3
, CF
3
OCF
2
H or OCF
3
,
each, independently of each other, are
Z
11
and Z
12
each, are independently of each other, a single bond or trans —CH═CH—, in case n
1
is 0, Z
12
is a sin

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