Electrooptical display element

Compounds of the formula I EQU R.sup.1 --A.sup.1 --Z.sup.1 --A--O--R.sup.2 in which PA0 R.sup.1 and R.sup.2, independently of one another, are each alkyl having 1 to 9 C atoms, in which, in addition, one or two CH.sub.2 groups may be replaced by --O--, --CO--, --O--CO--, --CO--O-- and/or --CH.dbd.CH--, where two O atoms are not linked directly to one anther, or one of the two groups R.sup.1 and R.sup.2 is alternatively H, PA0 Z.sup.1 is --CH.sub.2 --O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a single bond, PA0 A.sup.1 is trans-1,4-cyclohexylene in which, in addition one or two nonadjacent CH.sub.2 groups may be replaced by O and/or S, or 1,4-cyclohexenylene, and PA0 A is unsubstituted or fluorine-substituted 1,4-phenylene, can be used as components of liquid-crystalline dielectrics containing at least three liquid-crystalline compounds for shortening the switch-on time T.sub.on of electrooptical display elements based on a TN cell on switching the voltage from a first voltage below or approximately at the threshold voltage to a second voltage on the increase of the electrooptical characteristic line.

The invention relates to electrooptical liquid-crystal display elements (LC 
display elements) based on TN cells having a particularly short switch-on 
time T.sub.on when switched from a first voltage below or approximately at 
the threshold voltage to a second voltage which is on the increase of the 
electrooptical characteristic line. The invention is thus of particular 
importance in high-multiplex LC display elements and in particular in 
TFT-addressed LC display elements having a particularly short switch-on 
time. 
For LC display elements, the properties of nematic or nematic cholesteric 
liquid-crystalline materials to modify their optical properties, such as 
light absorption, light scattering, birefringence, reflectivity or color 
are utilized. At the same time, the function of display elements of this 
type is based, for example, on the phenomena of dynamic scattering and the 
deformation of allied phases, on the guest-host effect, the 
Schadt-Helfrich effect in the twisted cell, the SBE effect or the 
cholesteric-nematic phase transition. 
For industrial use of these effects in electronic components, LC phases are 
required which must satisfy a large number of requirements. Particularly, 
important here are the chemical resistance to moisture, air and physical 
influences, such as heat, radiation in the infra-red, visible and 
ultra-violet region and electrical direct current and alternating current 
fields. Furthermore, a liquid-crystalline mesophase in a suitable 
temperature range, low viscosity, an optical anisotropy matched to the 
layer thickness, low temperature dependence of the threshold voltage, 
steep gradients of the electrooptical characteristic line and adequate 
dissolution capacity for pleochroic dyes are required of LC phases which 
can be used industrially. 
In none of the series of compounds having a liquid-crystalline mesophase 
known hitherto is there a single compound which satisfies all these 
requirements. Generally, therefore, mixtures of two to 25, preferably 
three to 18, compounds are prepared in order to obtain substances which 
can be used as LC phases. However, ideal phases cannot easily be produced 
in this way since components of high melting and clear points frequently 
also confer high viscosity on the mixtures. The switching times of the 
electrooptical display elements produced therewith are thereby altered in 
an undesired manner. 
In order to shorten the switching times, components of low viscosity or 
glass-transition temperatures and/or particularly advantageous elastic 
constants are employed in the LC phases known hitherto. However, these 
phases do not simultaneously satisfy all the abovementioned requirements. 
In particular, their switch-on time T.sub.on is, as before, still too long 
for many applications, in particular in LC display elements in which the 
switchover takes place from a first voltage below or approximately at the 
threshold voltage to a second voltage on the increase of the 
electrooptical characteristic line. 
An essential distinction is made in electrooptical liquid-crystal display 
elements between static drive and multiplex drive. In order to switch on 
in the case of static drive, the voltage is switched from 0 volts to a 
voltage approximately at or above the saturation voltage (=the voltage 
with which 90% of the maximum contrast is obtained). On switching off, the 
voltage is switched back to 0 volts. In electrooptical liquid-crystal 
display elements which are driven in "multiplex operation", the voltage is 
switched from a first voltage V (off) to a second voltage V (on) and back. 
The ratio between these two voltages is determined by the "multiplex 
ratio" and the "bias". The absolute values for the two voltages are given 
by the operating voltage. 
N=the number of "multiplex" lines (1/N=multiplex ratio) 
##EQU1## 
At a multiplex ratio of 1:4 and a bias of 1:3, for example, the 
V(on):V(off) ratio is 1.73. At an operating voltage of 3 volts, V(on) is 
1.73 volts and V(off) is 1.00 volt. 
At a multiplex of 1:16 and a bias of 1:5, the V(on):V(off) ratio is, for 
example, 1.29; at an operating voltage of 5 volts, V(on) is 1.58 volts and 
V(off) is 1.22 volts. 
For a given display element with pre-specified drive, the dielectric is 
generally selected so that the initial voltage V(off) is just below or in 
the region of the "threshold voltage" (=voltage for 10% of the maximum 
contrast) for certain observation angle or for a certain observation angle 
range. The second voltage V(on) is then on the increase of the 
characteristic line above the threshold voltage. 
In display elements which are intended to display various gray steps (for 
example video displays), it is necessary for the brightness to be varied 
as desired in the range from, for example, 10% of maximum brightness up 
to, for example, 90% of maximum brightness by switching the voltage. 
In the case of multiplex drive (in particular also with gray steps) and TFT 
drive, when switching on and switching off or when switching from one gray 
step to another gray step, the voltage is switched from below or in the 
region of the threshold voltage to a second voltage above the threshold 
voltage in the region of the increase in the characteristic line. 
In the case of "static drive", the switch-on times are generally relatively 
short since the switch-on voltage is very much greater than the saturation 
voltage. 
In the case of multiplex drive and TFT drive, in contrast, the switch-on 
times, in particular, are usually relatively long. 
There is thus still a great demand for liquid-crystalline phases having 
high clear points, low melting points and very short switch-on times when 
used in LC display elements in which the voltage is switched from below or 
approximately at the threshold voltage to a second voltage on the increase 
of the electrooptical characteristic line. 
The invention has the object of providing LC display elements which have a 
broad operating temperature range and only have the abovementioned 
disadvantages to a lesser extent, or not at all. 
It has now been found that LC display elements having particularly 
favorable electrooptical properties, in particular having extremely short 
switch-on times T.sub.on when the voltage is switched from a first voltage 
below or approximately at the threshold voltage to a second voltage on the 
increase of the electrooptical characteristic line, are obtained when the 
dielectric contains at least one component of the formula I 
EQU R.sup.1 --A.sup.1 --Z.sup.1 --A--O--R.sup.2 
in which 
R.sup.1 and R.sup.2, independently of one another, are each alkyl having 1 
to 9 C atoms, in which, in addition, one or two CH.sub.2 groups may be 
replaced by --O--, --CO--, --O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
where two O atoms are not linked directly to one another, or one of the 
two groups R.sup.1 and R.sup.2 is alternatively H, 
Z.sup.1 is --CH.sub.2 --O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a 
single bond, 
A.sup.1 is trans-1,4-cyclohexylene in which, in addition, one or two 
nonadjacent CH.sub.2 groups may be replaced by O and/or S, or 
1,4-cyclohexenylene, and 
A is unsubstituted or fluorine-substituted 1,4-phenylene. 
The invention thus relates to a LC display element containing a 
liquid-crystalline phase described above, in particular an electrooptical 
display element based on a TN cell having a particularly short switch-on 
time T.sub.on when the voltage is switched from a first voltage below or 
approximately at the threshold voltage to a second voltage on the increase 
of the electrooptical characteristic line, characterized in that the 
dielectric simultaneously contains at least one component of the formula I 
EQU R.sup.1 --A.sup.1 --Z.sup.1 --A--O--R.sup.2 
in which 
R.sup.1 and R.sup.2, independently of one another, are each alkyl having 1 
to 9 C atoms, in which, in addition, one or two CH.sub.2 groups may be 
replaced by --O--, --CO--, --O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
where two O atoms are not linked directly to one another, or one of the 
two groups R.sup.1 and R.sup.2 is alternatively H, 
Z.sup.1 is --CH.sub.2 --O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a 
single bond, 
A.sup.1 is trans-1,4-cyclohexylene in which, in addition, one or two 
nonadjacent CH.sub.2 groups may be replaced by O and/or S, or 
1,4-cyclohexenylene, and is unsubstituted or fluorine-substituted 
1,4-phenylene, 
and at least one component selected from the compounds of the formulae II 
and III 
EQU R.sup.3 --A.sup.2 --Z.sup.2 --A.sup.3 --A'--R.sup.4 II 
EQU R.sup.3 --A.sup.2 --Z.sup.2 --A'--Z.sup.3 --A.sup.3 --R.sup.4III 
in which 
R.sup.3 is H or alkyl having 1 to 9 C atoms, in which, in addition, one or 
two nonadjacent CH.sub.2 groups may be replaced by --O--, --CO--, 
--O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
R.sup.4 is H, alkyl having 1 to 9 C atoms in which, in addition, one or two 
nonadjacent CH.sub.2 groups may be replaced by --O--, --CO--, --O--CO--, 
--CO--O-- and/or --CH.dbd.CH--, halogen, CN, NCS or N.sub.3, 
Z.sup.2 and Z.sup.3 are each --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, 
--CH.sub.2 O-- --O--CO--, --CO--O--, or a single bond, 
A.sup.2 and A.sup.3 are each trans-1,4-cyclohexylene in which, in addition, 
one or two nonadjacent CH.sub.2 groups may be replaced by O and/or S, 
A' is unsubstituted or fluorine-substituted 1,4-phenylene in which, in 
addition, one or two CH groups may be replaced by N. 
The invention furthermore relates to an appropriate display element whose 
dielectric additionally contains at least one component of the formula IV 
EQU R.sup.3 --A.sup.2 --Z.sup.2 --A.sup.3 --(Z.sup.4 --A.sup.4).sub.m --Z.sup.3 
--A'--(Z.sup.5 --A.sup.5).sub.n --R.sup.4 IV 
in which 
Z.sup.4 and Z.sup.5 are each --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, 
--CH.sub.2 O--, --O--CO--, --CO--O-- or a single bond, 
A.sup.4 and A.sup.5 are each trans-1,4-cyclohexylene or unsubstituted or 
fluorine-substituted 1,4-phenylene in which, in addition, one or two CH 
groups may be replaced by N, 
m and n are each 0 or 1, 
and R.sup.3, R.sup.4, A.sup.2, A.sup.3, A, Z.sup.2 and Z.sup.3 have the 
meaning given in claim 4, and an appropriate electrooptical display 
element which is operated in the region of the first transmission minimum 
according to Gooch-Tarry. 
The invention furthermore relates to the use of compounds of the formula I 
EQU R.sup.1 --A.sup.1 --Z.sup.1 --A--O--R.sup.2 
in which 
R.sup.1 and R.sup.2, independently of one another, are each alkyl having 1 
to 9 C atoms, in which, in addition, one or two CH.sub.2 groups may be 
replaced by --O--, --CO--, --O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
where two O atoms are not linked directly to one another, or one of the 
two groups R.sup.1 and R.sup.2 is alternatively H, 
Z.sup.1 is --CH.sub.2 --O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a 
single bond, 
A.sup.1 is trans-1,4-cyclohexylene in which, in addition one or two 
nonadjacent CH.sub.2 groups may be replaced by O and/or S, or 
1,4-cyclohexenylene, and 
A is unsubstituted or fluorine-substituted 1,4-phenylene, 
as components of liquid-crystalline dielectrics containing at least three 
liquid-crystalline compounds, for shortening the switch-on time T.sub.on 
of electrooptical display elements based on a TN cell when the voltage is 
switched from a first voltage below or approximately at the threshold 
voltage to a second voltage on the increase of the electrooptical 
characteristic line, and to a process for shortening the switch-on time 
T.sub.on of electrooptical display elements based on a TN cell when the 
voltage is switched from a first voltage below or approximately at the 
threshold voltage to a second voltage on the increase of the 
electrooptical characteristic line, characterized in that the dielectric 
contains at least one component of the formula I 
EQU R.sup.1 --A.sup.1 --Z.sup.1 --A--O--R.sup.2 
in which 
R.sup.1 and R.sup.2, independently of one another, are each alkyl having 1 
to 9 C atoms, in which, in addition, one or two CH.sub.2 groups may be 
replaced by --O--, --CO--, --O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
where two O atoms are not linked directly to one another, or one of the 
two groups R.sup.1 and R.sup.2 is alternatively H, 
Z.sup.1 is --CH.sub.2 --O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a 
single bond, 
A.sup.1 is trans-1,4-cyclohexylene in which, in addition one or two 
nonadjacent CH.sub.2 groups may be replaced by O and/or S, or 
1,4-cyclohexenylene, and 
A is unsubstituted or fluorine-substituted 1,4-phenylene. 
The invention finally relates to a dielectric for use in an electrooptical 
display element based on a TN cell having a particularly short switch-on 
time T.sub.on when the voltage is switched from a first voltage below or 
approximately at the threshold voltage to a second voltage on the increase 
of the electrooptical characteristic line, characterized in that it 
simultaneously contains one component of the formula I 
EQU R.sup.1 --A.sup.1 --Z.sup.1 --A--O--R.sup.2 
in which 
R.sup.1 and R.sup.2, independently of one another, are each alkyl having 1 
to 9 C atoms, in which, in addition, one or two CH.sub.2 groups may be 
replaced by --O--, --CO--, --O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
where two O atoms are not linked directly to one another, or one of the 
two groups R.sup.1 and R.sup.2 is alternatively H, 
Z.sup.1 is --CH.sub.2 --O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a 
single bond, 
A.sup.1 is trans-1,4-cyclohexylene in which, in addition one or two 
nonadjacent CH.sub.2 groups may be replaced by O and/or S, or 
1,4-cyclohexenylene, and 
A is unsubstituted or fluorine-substituted 1,4-phenylene, and at least one 
component selected from the compounds of the formulae II and III 
EQU R.sup.3 --A.sup.2 --Z.sup.2 --A.sup.3 --Z.sup.3 --A'--R.sup.4II 
EQU R.sup.3 --A.sup.2 --Z.sup.2 --A'--Z.sup.3 --A.sup.3 --R.sup.4III 
in which 
R.sup.3 is H or alkyl having 1 to 9 C atoms, in which, in addition, one or 
two nonadjacent CH.sub.2 groups may be replaced by --O--, --CO--, 
--O--CO--, --CO--O-- and/or --CH.dbd.CH--, 
R.sup.4 is H, alkyl having 1 to 9 C atoms in which, in addition, one or two 
nonadjacent CH.sub.2 groups may be replaced by --O--, --CO--, --O--CO--, 
--CO--O-- and/or --CH.dbd.CH--, halogen, CN, NCS or N.sub.3, 
Z.sup.2 and Z.sup.3 are each --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, 
--CH.sub.2 O-- --O--CO--, --CO--O--, or a single bond, 
A.sup.2 and A.sup.3 are each trans-1,4-cyclohexylene in which, in addition, 
one or two nonadjacent CH.sub.2 groups may be replaced by O and/or S, 
A' is unsubstituted or fluorine-substituted 1,4-phenylene in which, in 
addition, one or two CH groups may be replaced by N, 
and a corresponding dielectric containing at least one component of the 
formula IV 
EQU R.sup.3 --A.sup.2 --Z.sup.2 --A.sup.3 --(Z.sup.4 --A.sup.4).sub.m --Z.sup.3 
--A'--(Z.sup.5 --A.sup.5).sub.n --R.sup.4 IV 
in which 
Z.sup.4 and Z.sup.5 are each --CH.sub.2 CH.sub.2 --, --OCH.sub.2 --, 
--CH.sub.2 O--, --O--CO--, --CO--O-- or a single bond, 
A.sup.4 and A.sup.5 are each trans-1,4-cyclohexylene or unsubstituted or 
fluorine-substituted 1,4-phenylene in which, in addition, one or two CH 
groups may be replaced by N, 
m and n are each 0 or 1, 
and R.sup.3, R.sup.4, A.sup.2, A.sup.3, A, Z.sup.2 and Z.sup.3 have the 
meaning given in claim 4. 
The dielectrics may also contain further additives known to those skilled 
in the art and described in the literature. For example, 0-15% of 
pleochroic dyes, furthermore conductive salts, for example 
ethyldimethyldodecylammonium 4-hexoxybenzoate, tetrabutylammonium 
tetraphenylborate or complex salts of crown ethers (compare, for example, 
Haller et al., Mol. Cryst. Liq. Cryst. Volume 24, pages 249-258, (1973)) 
for improving the conductivity, or substances for modifying the dielectric 
anisotropy, the viscosity and/or the orientation of the nematic phases can 
be added. Such substances are described, for example, in DE-OS 2,209,127, 
2,240,864, 2,321,632, 2,338,281, 2,450,088, 2,637,430 and 2,853,728. 
The individual components of the formula I, II, III and IV of the 
liquid-crystalline phases according to the invention are either known or 
can easily be derived from the prior art by the relevant person skilled in 
the art since they are based on standard processes described in the 
literature. 
Corresponding compounds are described, for example, in German Patent 
Applications 2,636,684, 2,800,553, 2,933,563, 2,933,611, 2,948,836, 
3,001,661, 3,102,017, 3,149,139, 3,206,269, 3,139,130, 3,223,637, 
3,211,601, 3,317,597, 3,317,921, 3,339,218, 3,401,320, 3,410,734 and 
3,545,345, in European Patent Applications 0,019,665, 0,058,512, 
0,062,470, 0,084,194, 0,084,974, 0,090,671, 0,094,198, 0,099,099, 
0,102,047, 0,119,756, 0,122,389, 0,129,177, 0,167,912 and 0,168,683, in 
the International Patent Application WO 85/04874, in Japanese Patent 
Applications 58-126,821-A, 58-177,989-A, 60-161,941-A, 60-161,957-A, 
60-222,458-A and 60-239,470-A and in U.S. Pat. No. 4,439,015. 
Surprisingly, it has been shown that the dielectrics according to the 
invention, in particular the combination of components of the formulae I 
and II and/or III, have significantly shorter switching times (in 
particular significantly shorter switch-on times) under the specific drive 
conditions mentioned (that is to say, for example, in TFT or 
high-multiplex displays) than other dielectrics without this specific 
substance combination, even if relevant physical properties and 
measurement conditions (rotational viscosity, elastic constants, the 
product d.times..DELTA.n of the layer thickness and the optical 
anisotropy, and the drive voltage) are virtually identical. 
The better switching (switch-on) times of the LC display elements according 
to the invention are caused not only by better viscosities or more 
suitable elastic constants. The reason is very probably above all the type 
of interactions between the LC molecules and the display surfaces. In 
particular, it can be assumed that the mixtures according to the invention 
have a greater tilt angle than other comparable mixtures and the switch-on 
time especially is therefore significantly shorter. 
If other compounds which do not meet the criteria mentioned (for example 
molecules which contain exclusively cyclohexane rings) are added to 
dielectrics of this type, the switching times are impaired. 
In the compounds of the formula I, R.sup.1 and R.sup.2 are, independently 
of one another, preferably alkyl in which, in addition, a CH.sub.2 group 
(which is not linked directly to --O-- or --A.sup.1 --) may be replaced by 
--O-- or --CH.dbd.CH--. Straight-chain alkyl groups having 1 to 5 C atoms 
are particularly preferred. Compounds of the formula I in which R.sup.2 is 
alkanoyl are furthermore preferred. R.sup.2 is then preferably 
straight-chain alkanoyl having 2 to 5 C atoms. Z is preferably --CH.sub.2 
CH.sub.2 -- or a single bond. In particularly preferred compounds, Z is a 
single bond. A.sup.1 is preferably trans-1,4-cyclohexylene. A is 
preferably 1,4-phenylene. Particularly preferred components of the formula 
I are those of the subformulae Ia, Ib, Ic and Id: 
##STR1## 
Compounds of the subformula Ia are particularly preferred. 
The dielectrics of the display elements according to the invention 
preferably contain 10 to 80%, in particular 20 to 70%, of the total 
content of compounds of the formula I. A further preferred total content 
of compounds of the formula I is 35 to 69%. 
In the compounds of the formulae II and III, A.sup.2 and A.sup.3, 
independently of one another, are each preferably trans-1,4-cyclohexylene 
or trans-1,3-dioxane-2,5-diyl, particularly preferably 
trans-1,4-cyclohexylene. R.sup.3 is preferably straight-chain alkyl, 
preferably having 2 to 7 C atoms, in which, in addition, one CH.sub.2 
group may be replaced by --O-- or --CH.dbd.CH--. n-Alkyl is particularly 
preferred. R.sup.4 is preferably straight-chain alkyl, preferably having 2 
to 7 C atoms, in which, in addition, one CH.sub.2 group may be replaced by 
--O-- or --CH.dbd.CH--, or is halogen, CN, NCS or N.sub.3. Particularly 
preferred meanings for R.sup.4 are n-alkyl, n-alkoxy, F, CN and NCS. 
Z.sup.2 and Z.sup.3 are preferably each, independently of one another, 
--CH.sub.2 CH.sub.2 --, --CO--O--, --O--CO-- or a single bond. At least 
one of the groups Z.sup.2 and Z.sup.3 is preferably a single bond. 
A' is preferably 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, 
pyridine-2,5-diyl or pyrimidine-2,5-diyl. Particularly preferred meanings 
for A' are 1,4-phenylene and 2- or 3-fluoro-1,4-phenylene. 
Particularly preferred components of the formula II are those of the 
subformulae IIa to IIv: 
##STR2## 
Of these, those of the subformula IIa, IIc, IIj, IIl, IIm, IIn, IIo, IIp, 
IIq, IIr and IIs are preferred. Those of the subformulae IIo and IIp are 
particularly preferred. 
Particularly preferred components of the formula III are those of the 
subformulae IIIa, IIIb and IIIc: 
##STR3## 
Of these, those of the subformulae IIIa and IIIc, in particular IIIa, are 
preferred. 
Besides components of the formula I, preferred dielectrics contain at least 
one component of the formula II. 
The dielectrics preferably contain 5 to 50%, in particular 10 to 40%, of 
the total content of compounds of the formulae II and III. A further 
preferred total content is 20 to 35%. 
In the compounds of the formula IV, Z.sup.4 and Z.sup.5 are each, 
independently of one another, preferably --CH.sub.2 CH.sub.2 --, 
--CO--O--, --O--CO-- or a single bond. A.sup.4 and A.sup.5 are preferably 
trans-1,4-cyclohexylene or 1,4-phenylene. R.sup.3, R.sup.4, A.sup.2, 
A.sup.3, A', Z.sup.2 and Z.sup.3 have the meanings indicated as preferred 
in the formulae II and III. (m+n) is preferably 1. 
Particularly preferred compounds of the formula IV are those of the 
subformulae IVa to IVk: 
##STR4## 
Of these, those of the subformulae IVa, IVb, IVg, IVh, IVj and IVK (sic), 
in particular those of the subformula IVa, are particularly preferred. 
The dielectrics preferably contain 0 to 40%, in particular 5 to 30%, of the 
total content of compounds of the formula IV. 
The content in the dielectrics of compounds which are decidedly positive 
dielectrically is advantageously 10 to 50%, particularly preferably 20 to 
40%. In principle, any customary dielectrically positive components can be 
employed. The choice of the most highly suitable compounds depends, above 
all, on the threshold voltage and optical anisotropy desired. 
Dielectrically positive compounds of the formulae II, III and/or IV can be 
chosen and/or other customary compounds, for example those of the formulae 
Va to Ve below: 
##STR5## 
The examples below are intended to illustrate the invention without 
representing a limitation. 
The symbols have the following meanings: 
______________________________________ 
S-N smectic-nematic phase transition temperature, 
in degrees Celsius, 
c.p. clear point, in degrees Celsius, 
visc. viscosity at 20.degree. (mPa .multidot. s), 
thresh. threshold voltage of a TN cell at 20.degree., observa- 
tion angle 0.degree. (vertical) and a contrast of 10%. 
T.sub.on, 
switch-on or switch-off time in msec. 
T.sub.off 
K.sub.3 /K.sub.1 
ratio between the elastic constants of bending 
and stretching 
n birefringence 
______________________________________ 
Above and below all temperatures are given in .degree.C. The percentages 
are by weight. 
Procedure for measuring the switching time: 
If the switching times of liquid crystals in displays are to be measured, 
the voltage, in the case of so-called static drive, is switched from 0 
volt to a voltage which is, for example, approximately twice to 3 times 
the threshold voltage. If the threshold is, for example, 2.0 volts, the 
switching time can be measured on switching from 0 volts to 4-6 volts and 
back. Under these conditions, the switch-on times are usually somewhat 
shorter than the switch-off times. 
In the case of multiplex drive, however, the voltage is switched from a 
first voltage below or approximately at the threshold to a second voltage 
on the increase of the characteristic line. This applies, in particular, 
in the case of relatively high multiplex ratios and in TFT displays. Under 
these conditions, the switch-on times, in particular, are very long. 
In order to simulate these conditions, the voltage in our measurements is 
switched from a fixed initial voltage of exactly 1.00 volt to a voltage at 
which 40% of the maximum contrast is achieved (when observed vertically). 
The investigations were carried out on mixtures for the "first minimum" 
according to Gooch and Tarry. The product of the layer thickness and the 
birefringence was therefore in the region of about 0.5.