Smectic C liquid crystal composition and a liquid crystal display element

A composition containing a smectic C compound, having a .DELTA..epsilon. of negative value and enabling to effect a high speed response; a ferroelectric chiral smectic C liquid crystal composition using the same; and a ferroelectric liquid crystal display device using the ferroelectric chiral smectic C liquid crystal composition are provided, PA1 which composition containing a smectic C compound comprises at least one compound expressed by the formula (I) ##STR1## wherein R.sup.1 and R.sup.2 each represent different linear alkyl groups of 1 to 9 C, and PA1 at least one compound expressed by the formula (II) ##STR2## wherein R.sup.3 and R.sup.4 each represent the same or different linear alkyl groups of 1 to 18 C and X is H or F.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a smectic C liquid crystal compound and its 
composition. More particularly, it relates to a smectic C liquid crystal 
compound and its composition and a ferroelectric smectic C liquid crystal 
composition using the same and further a light-switching element using the 
composition. 
2. Description of the Related Art 
Liquid crystal compositions have been broadly used as a display element 
material. Most of the current liquid crystal display elements are those of 
TN type display mode, and this display mode utilizes nematic phase. 
The TN type display mode used for the liquid crystal display is roughly 
classified into two modes. One mode is an active matrix mode having 
switching elements fixed to the respective pixels. An example of this mode 
is a mode using a thin film transistor (TFT). The display grade has 
reached a level matching that of CRT (cathod ray tube), but it is 
difficult to bring the picture surface into a large scale and the cost 
therefor is high. 
Another mode is STN (super twisted) mode. The contrast and the visual 
sensation-dependency have been improved as compared with those of 
conventional simple matrix mode, but the display grade has not yet reached 
the level of CRT, but its cost is cheap. These two modes have a merit and 
a demerit when its grade and production cost are taken into consideration. 
A mode expected ten years ago to solve the two problems is a mode of 
ferroelectric liquid crystal (FLC). At present, what is merely called FLC, 
refers to a surface-stabilized ferroelectric liquid crystal (SSFLC). This 
SSFLC was proposed by A. Clark and S. T. Lagerwall (see Applied Physics 
Letters, 36, 899, 1980). Since then, it has been called a liquid crystal 
of the next generation, and its commercialization has been tried by 
appliance makers and material makers, as well as improvement of the 
characteristics and commercialization have been made. 
Because, ferroelectric liquid crystal elements are provided with the 
following characteristics in principle: 
(1) high speed response properties, 
(2) good memory properties, and 
(3) broad viewing angle. 
The above characteristics suggest a possibility of SSFLC into a large 
capacity display and have made SSFLC very attractive. 
However, as the research has advanced, problems to be solved have been 
clarified. 
Among the problems, a stabilized exhibition of memory is the first problem. 
As the causes of the difficulty in the stabilized exhibition of memory, 
non-uniformity of smectic layer structure (for example, twisted alignment, 
chevron structure) and occurrence of the inside electric field considered 
to originate from the excess spontaneous polarization, etc. have been 
considered. 
As a means for exhibiting stabilized memory properties, a method of using a 
ferroelectric liquid crystal composition having a negative dielectric 
anisotropy has been proposed (see Paris Liquid Crystal Conference, p. 217 
(1984)). This method has been referred to as AC stabilizing effect. 
Liquid crystal molecules having a negative .DELTA..epsilon. value have such 
a property that when an electric field is impressed in a vertical 
direction to the electrode in a cell subjected to a homogeneous alignment 
treatment, the molecules are directed in a state parallel to the glass 
substrate (the parallel axis of the molecules is directed vertically to 
the direction of the electric field). When a low frequency electric field 
is impressed, the spontaneous polarization replies to the electric field; 
hence when the direction of the electric field is inverted, the liquid 
crystal molecules, too, follow the inversion and move to another 
stabilized state, where they become a state parallel to the substrate due 
to the effect of .DELTA..epsilon.. Whereas, when a high frequency electric 
field is impressed, the spontaneous polarization cannot follow the 
inversion, but only .DELTA..epsilon. is effected; hence even when the 
direction of the electric field is inverted, the liquid crystal molecules 
do not move and become parallel to the substrate, as they are. This is a 
mechanism of exhibiting the memory properties utilizing an AC stabilizing 
effect. A high contrast is thereby obtained. This concrete example has 
already been reported (see SID '85 digest, p.128, 1985). 
Further, "a method of utilizing a liquid crystal material having a negative 
dielectric anisotropy" has been separately proposed by Surguy et al. (P. 
W. H. Surguy et al., Ferroelectrics, 122, 63, 1991). This technique is a 
promising one for realizing the high contrast, and P. W. Ross, Proc. SID, 
217 (1992) discloses a ferroelectric liquid crystal display employing this 
technique. This ferroelectric liquid crystal display will be described 
below in more detail. 
In the case of a conventional ferroelectic liquid crystal material whose 
dielectric anisotropy is not negative, as the voltage (V) becomes high, 
.tau. (a pulse width necessary for effecting memory) lowers monotonously. 
Whereas, in the case of a ferroelectric liquid crystal material having a 
negative anisotropy, .tau.-V characteristic showing a minimum value 
(.tau.-V min) is obtained. Surguy et al. have reported JOERS/Alvey driving 
method as a driving method utilizing this characteristic. 
The principle of this driving method refers to a method wherein, when a 
voltage of .vertline.Vs-Vd.vertline. is impressed, a memory state of a 
ferroelectric liquid crystal element is switched, and when a voltage of 
.vertline.Vs+Vd.vertline. higher than the above voltage is impressed, and 
when a voltage of .vertline.Vd.vertline. lower than the above voltage is 
impressed, the memory state is not switched. 
Since the ferroelectric liquid crystal material having a negative 
dielectric anisotropy can be applied to a display element utilizing the 
AC-stabilizing effect and the .tau. min, as described above, the above 
material has potentially a possibility of being practically utilizable. 
However, the response speed of the ferroelectric liquid crystal material 
used for the above element utilizing .tau. min is still low. Further, 
Vs+Vd, too, is as high as 57.5 V to 60 V; thus it has not yet reached a 
practical level. According to the report of Ross et al. (P. W. Ross, Proc. 
SID, 217 (1992)), the driving voltage of the ferroelectric liquid crystal 
display prepared for trial is 55 V. As to the cost of the IC driver for 
driving the ferroelectric liquid crystal display, the higher the voltage, 
the higher the cost. Thus, the high driving voltage becomes a serious 
cause of increased cost. In order to prepare a ferroelectric liquid 
crystal display having a relatively low cost, it is necessary to drive it 
using a general-purpose IC driver whose cost is not so high; thus it is 
necessary to suppress the driving voltage down to 40 V or lower. The 
reason for needing a high driving voltage at the present time is that the 
voltage value (V min) in .tau.-V characteristic is high; hence in order to 
drive the display at 40 V or lower, it is necessary to develop a 
ferroelectric liquid crystal material exhibiting a V min of 35 V or lower. 
According to Surguy et al., V min is obtained by the following equation: 
##EQU1## 
In the above equation, E min refers to the minimum value of electric field 
intensity; d refers to cell thickness; Ps refers to spontaneous 
polarization value; .DELTA..epsilon. refers to dielectric anisotropy; and 
.theta. refers to tilt angle. As seen from this equation, in order to make 
the V min a lower voltage, a larger negative dielectric anisotropy and a 
less spontaneous polarization value are required. However, since the 
response speed of the ferroelectric liquid crystal is related to the 
spontaneous polarization value, if the spontaneous polarization value is 
reduced, it is difficult to obtain a high speed response. Accordingly, for 
the liquid crystal material, a low viscosity material having a negative 
dielectric anisotropy is required. 
The present inventors have already filed a patent application directed to a 
ferroelectric liquid crystal composition suitable to drive utilizing the 
AC-stabilizing effect (Japanese patent application laid-open Nos. Hei 
1-168792, Hei 1-306493 and Hei 4-4290). However, the response speed 
thereof has not yet been fully practical. 
The object of the present invention is firstly to provide a smectic C 
liquid crystal composition having a negative .DELTA..epsilon. and making 
the high speed response possible, secondly to provide a ferroelectric 
chiral smectic C liquid crystal composition using the same, and thirdly to 
provide a liquid crystal display element using the above ferroelectric 
chiral smectic C liquid crystal composition and the driving method 
thereof. 
SUMMARY OF THE INVENTION 
The first object of the present invention is achieved by items (1) to (10) 
mentioned below. 
The second object of the present invention is achieved by items (5) to (9) 
mentioned below. 
The third object of the present invention is achieved by items (11) to (16) 
mentioned below. 
(1) A smectic C liquid crystal composition comprising at least one member 
of compounds expressed by the formula (I) 
##STR3## 
wherein R.sup.1 and R.sup.2 represent different linear alkyl groups of 1 
to 9 carbon atoms, and 
at least one member of compounds expressed by the formula (II) 
##STR4## 
wherein R.sup.3 and R.sup.4 represent the same or different linear alkyl 
groups of 1 to 18 carbon atoms and X represents H or F. 
(2) A smectic C liquid crystal composition according to item (1), wherein X 
of the formula (II) is F. 
(3) A smectic C liquid crystal composition according to item (1) or (2), 
wherein the proportion of the compounds expressed by the formula (I) is 5 
to 50% by weight based upon the total weight of the compounds expressed by 
the formula (I) and the compounds expressed by the formula (II). 
(4) A smectic C liquid crystal composition according to either one of items 
(1) to (3), wherein the phase transition range is in the order from the 
higher temperature side, isotropic phase, nematic phase, smectic A phase 
and smectic C phase. 
(5) A ferroelectric chiral smectic C liquid crystal composition, obtained 
by adding at least one of optically active compounds to the smectic C 
liquid crystal composition set forth in either one of items (1) to (4). 
(6) A ferroelectric chiral smectic C liquid crystal composition according 
to item (5), wherein the mixing proportion of said optically active 
compound is 20% by weight or less based upon the weight of said smectic C 
liquid composition. 
(7) A ferroelectric chiral smectic C liquid crystal composition according 
to item (5) or item (6), wherein its .DELTA..epsilon. is negative and its 
absolute value is 2 or more. 
(8) A ferroelectric chiral smectic C liquid crystal composition according 
to either one of item (5) to item 
(7), wherein its spontaneous polarization value is 10 nCcm.sup.-2 or less. 
(9) A ferroelectric chiral smectic C liquid crystal composition according 
to either one of item (5) to item 
(8), wherein said optically active compound is expressed by either one of 
the formulas (III-A) to (III-I): 
##STR5## 
wherein R.sup.5 and R.sup.6 represent the same or different linear or 
branched alkyl group or alkoxy group of 2 to 18 carbon atoms and symbol * 
represents an asymmetric carbon atom. 
(10) A smectic C liquid crystal compound wherein R.sup.1 and R.sup.2 in the 
compounds expressed by the formula (I) set forth in item (1) are those of 
linear alkyl groups each having lengths set forth in the following Table: 
______________________________________ 
R.sup.1 
5 6 6 7 8 8 8 
R.sup.2 
3 2 3 3 2 3 4 
______________________________________ 
(11) A liquid crystal display element wherein a ferroelectric chiral 
smectic C liquid crystal set forth in either one of item (5) to item (9) 
is used. 
(12) A ferroelectric liquid crystal display element according to item (11), 
wherein the direction of the bend of the smectic layer structure of said 
ferroelectric liquid crystal is the same as the pretilt direction of the 
liquid crystal molecules on the interface of the liquid crystal/the 
aligned film. 
(13) A ferroelectric liquid crystal display element according to item (11) 
or (12), wherein the pretilt angle of liquid crystal molecules on the 
interface of the liquid crystal/the aligned film is 10.degree. or less. 
(14) A driving method of a ferroelectric liquid crystal display element 
which comprises a pair of insulating substrates each having an electrode, 
a ferroelectric liquid crystal composition placed between said substrates, 
a driving means for switching the optical axis of liquid crystals by 
selectively impressing a voltage onto said electrodes and a means for 
optically identifying the switching of said optical axis; said liquid 
crystal composition comprising a ferroelectric chiral smectic C liquid 
crystal composition having a bistable state set forth in either one of 
item (5) to (8), said electrodes being provided so that a plurality of 
scanning electrodes and a plurality of signal electrodes are arranged in 
the direction crossing each other, and the region where said scanning 
electrodes are crossed with said signal electrodes being made pixel, 
wherein said pixel is driven so that using voltages V.sub.1, V.sub.2, 
V.sub.3 or V.sub.4 satisfying the following relations, 
EQU 0&lt;V.sub.2 &lt;V.sub.4 
EQU V.sub.2 -V.sub.1 &lt;V.sub.4 -V.sub.3 
when the first pulse voltage V.sub.1 and the succeeding pulse voltage 
V.sub.2, or the first pulse voltage -V.sub.1 and the succeeding pulse 
voltage -V.sub.2 are impressed onto the pixel on a selected scanning 
electrode, the ferroelectric liquid crystal molecules constituting a part 
of said pixel are brought into one stable state or another stable state 
depending upon the polarity of the impressed voltage, without relying on 
the stable state prior to the voltage impression, and when the first pulse 
voltage V.sub.3 and the succeeding second pulse voltage V.sub.4 or the 
first pulse voltage -V.sub.3 and the succeeding second pulse voltage 
-V.sub.4 are impressed onto the same pixel, the stable state of the 
ferroelectric liquid crystal molecules constituting the part of said pixel 
prior to the voltage impression are retained. 
(15) A driving method of the ferroelectric liquid crystal display element 
according to item (14), wherein the ferroelectric liquid crystal in said 
element has a bistable state, and in the characteristic of the pulse 
width-pulse voltage of a monopolar pulse required for rewriting from one 
stable state to another, the pulse voltage affording the minimum value of 
the pulse width is 60 V or less. 
(16) A driving method of a ferroelectric liquid crystal display element 
according to item (14), wherein the ferroelectric liquid crystal in said 
element has a bistable state, and in the characteristic of the pulse 
width-pulse voltage of a monopolar pulse required for rewriting from one 
stable state to another, the pulse voltage affording the minimum value of 
the phase width is 35 V or less.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will be described in more detail. 
A preferable preparation of the liquid crystal compound having different 
alkyl chain lengths, expressed by the formula (I) is shown below. 
4-Alkylbenzohydrazide 1 is preferably obtained by reacting hydrazine with a 
4-alkylbenzoic acid ester. As the reaction solvent used at that time, 
lower alcohols such as methanol, ethanol, etc., hydrocarbons, halogenated 
solvents, etc. are preferably usable. The reaction temperature is 
preferably room temperature to 200.degree. C., more preferably 50.degree. 
C. to 100.degree. C. Further, 1 may be also similarly obtained by reacting 
hydrazine with a 4-alkylbenzoic acid chloride 2, followed by separating 
monohydrazide. 
An N-(4-alkylbenzo)-N'-(4-alkylbenzo)hydrazide 3 is obtained by reacting 1 
with 2. A basic substance is used for seizing hydrogen chloride generated 
in the reaction. Its preferable examples are pyridine, triethylamine, etc. 
As the reaction solvent, THF, toluene, etc. are preferably usable. The 
reaction is carried out preferably at room temperature to 150.degree. C., 
more preferably at 50.degree. to 80.degree. C. 
The objective 2-(4-alkylphenyl)-5-(4-alkylphenyl)-1,3,4-thiadiazole 4 is 
obtained by reacting a sulfurizing agent with dihydrazide 3. Examples of 
the sulfurizing agent preferably used are 
2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphoethane-2,4-disulfide 
(Lawesson's reagent), phosphorus sulfide, single substance of sulfur, etc. 
As the reaction solvent, THF, toluene, etc. are preferably usable. The 
reaction temperature is room temperature to 150.degree. C., preferably 
50.degree. to 80.degree. C. 
##STR6## 
In the compound of the structure expressed by the formula (I) of the 
present invention, a compound having the same alkyl chain strength at both 
ends has been prepared by Dimitrowa et al. (J. Prakt. Chem., 322, 933 
(1980)). Further, it has also been described in Flussige Kristalle in 
Tabellen II (p.359 (1984)) edited by Demus et al. In these documents, only 
compounds having the same alkyl chain length for R.sub.1 and R.sub.2 have 
been described, but compounds having different alkyl chain lengths have 
not been described. According to these, it is described that these 
documents compounds exhibit smectic A phase and nematic phase, but it is 
not described that they exhibit smectic C phase. Further, the compound of 
this structure falls within a compound claim of WO88/08019 
(priority-claiming date: Apr. 16, 1987). Claim 17 of the patent has been 
limited only to branched alkyl. In a document of this applicant (not of 
the inventor), it is described citing the above-mentioned document 
(Flussige Kristalle in Tabellen II edited by Demus et al.), that the 
compound exhibits smectic A phase (Ferroelectrics, Vol. 85 , p.329 
(1988)). Accordingly, it is apparent that they included the compound of 
this structure in the compound claim without practically preparing the 
compound. Namely, the compound of the present invention expressed by the 
formula (I) is not included in WO88/08019. 
Further, in order to confirm that the characteristics of the compound of 
the present invention are different from those of the compound of 
WO88/08019, the present inventors have carried out the following separate 
experiment: 
A composition of compound (I-2) with compound (II-6) and a composition of 
comparative compound A with compound (II-6) were prepared. 
.epsilon..parallel., .epsilon..perp. and .DELTA..epsilon. at T=Tc-10, of 
the respective compositions were measured. The results are shown in the 
following Table 1. From the results, it was found that .DELTA..epsilon. of 
compound (I-2) was larger in terms of negative value, than 
.DELTA..epsilon. of comparative compound A. WO88/08019 mainly claimed a 
compound of the type of comparative compound (A). The result of this 
experiment shows that an important compound was present at a part 
different from that intended by WO88/08019. 
TABLE 1 
__________________________________________________________________________ 
##STR7## 
Concentration of Extrapolating 
Compound compound 10 wt % 
30 wt % 
value 
__________________________________________________________________________ 
##STR8## .epsilon. .parallel. .epsilon. .perp. .DELTA. 
.epsilon. 
4.4 4 7.1 -3.2 
-4.3 
##STR9## .epsilon. .parallel. .epsilon. .perp. .DELTA. 
.epsilon. 
4.6 4.5 -2.7 -2.7 
__________________________________________________________________________ 
The compound expressed by the formula (I) in the present invention has also 
been disclosed in Japanese patent application laid-open No. Hei 4-28787 
which is a patent application directed to composition and element. In this 
patent application, the presence of smectic C phase has been described, 
but as a preparation example, only one kind of compounds wherein R.sup.1 
and R.sup.2 having the same alkyl chain length (R.sup.1 and R.sup.2 : both 
6C) having already been known at that time, has been described. Further, 
as preferable compounds, ten kinds of compounds have been disclosed (among 
these kinds, 4 kinds have the same alkyl chain length). However, these 
compounds belong to one component among three components, and yet they are 
compounds of 10 kinds among those of 209 kinds (since they are expressed 
in terms of linear and branched alkyl groups, the actual number of the 
compounds amount to several tens times the above number); hence it cannot 
be said that they are main components in Japanese patent application 
laid-open No. Hei 4-28787. The alkyl chain lengths of the disclosed 
compounds are summarized as follows. 
______________________________________ 
R.sup.1 
5 5 6 6 8 8 8 10 10 10 
R.sup.2 
5 7 4 6 5 8 9 6 8 10 
______________________________________ 
While these compounds are also used in the compositions of Examples, the 
quantity thereof used is only 14 parts by weight even in the maximum 
quantity. Further, compounds which are mainly used are those wherein 
R.sup.1 and R.sup.2 have the same alkyl chain length, and the description 
of the physical properties, such as phase transition points, etc. is 
directed only to the above-mentioned one kind. In short, the preparation 
method having different alkyl chain lengths in the structure, the 
superiority of compounds having different alkyl chain lengths, difference 
of the physical properties in compounds having different alkyl chain 
strengths, etc. are not described at all. Namely, for the first time, the 
present invention has noted a compound expressed by the formula (I) and 
having different alkyl chain lengths. 
The phase transition series of the compound of the present invention is 
generally in the order from the higher temperature side, of isotropic 
liquid phase, nematic phase and smectic C phase. This applies to even 
compounds having the same R.sup.1 and R.sup.2 However, the melting 
temperature and the temperature range of nematic phase are far different 
depending upon the alkyl chain length. For reference, the phase transition 
temperatures in the case of R.sup.1 having an alkyl chain length of 8 are 
exemplified below. 
TABLE 2 
______________________________________ 
N phase Com- 
temperature 
pound 
R.sup.2 
R.sup.2 
Cr SC N Iso width No. 
______________________________________ 
8 2 70 .multidot. 
93.2 
.multidot. 
139.4 46.2 I-1 
8 3 77 .multidot. 
105.0 
.multidot. 
156.0 51.0 I-2 
8 4 55 .multidot. 
124.6 
.multidot. 
146.8 22.2 I-3 
8 5 56 .multidot. 
139.7 
.multidot. 
155.8 16.1 I-4 
8 6 62 .multidot. 
143.7 
.multidot. 
151.7 8.0 I-5 
8 7 71 .multidot. 
150.1 
.multidot. 
154.8 4.7 I-6 
8 8 80 .multidot. 
147.5 
.multidot. 
149.2 1.7 R-5 
______________________________________ 
In this Table, Cr, SC, N and Iso, respectively represent crystalline phase, 
smectic C phase, nematic phase and isotropic liquid phase, and the 
numerals described below R.sup.1 and R.sup.2 represent the respective 
alkyl chain lengths of R.sup.1 and R.sup.2, and the numerals under Cr, SC, 
N and Iso represent the respective phase transition temperatures. The 
melting points are shown below Cr--Sc. The temperature unit is .degree. C. 
As seen from the above Table, compounds having the same alkyl chain length 
have far higher melting points. Further, the larger the difference between 
the left and right alkyl chain lengths in the formula, the broader the 
temperature width of nematic phase, but this cannot be generally said. 
Further, a compound having a combination of R.sup.1 and R.sup.2 of 8 and 4 
(even when the alkyl chain lengths of R.sup.1 and R.sup.2 are contrary to 
each other, the resulting compounds are the same), and a compound of a 
combination of R.sup.1 and R.sup.2 of 8 and 6, respectively exhibit 
melting points of 55.degree. C. and 56.degree. C. and belong to a class of 
lower melting points as for linear three-ring compound exhibiting smectic 
C phase. 
The preserving temperature of ferroelectric liquid crystals has been said 
to be usually preferably -30.degree. C. to 70.degree. C., and the lower 
the melting point, the better the compound. In short, in the case of 
compounds exhibiting the same smectic C phase, the compounds having a 
lower melting point are better. Further, the phase transition series 
desired for ferroelectric liquid crystals is in the order from the higher 
temperature side, of isotropic liquid phase, cholesteric phase, smectic A 
phase and chiral smectic C phase. In order to realize such a phase 
transition series, the series is easily obtained by adding an optically 
active compound to a smectic C liquid crystal mixture exhibiting such a 
phase transition series. For this purpose, it is necessary to prepare the 
smectic C liquid crystal mixture exhibiting the phase transition series. 
Thus, as the condition required for compounds, the compounds should 
naturally exhibit smectic C phase, and besides it is necessary to exhibit 
nematic phase or smectic A phase. Ideally, it is desired that only one 
kind of compounds exhibits smectic C, nematic, smectic A phases within a 
broad range and has a low melting point. However, as practically such a 
compound is not existent, it is necessary to mix several kinds of 
compounds. The compound expressed by the formula (I) and having different 
alkyl chain lengths has a smectic C phase within a broad range and a broad 
nematic phase and further has a low melting point. 
In order to prepare the above phase transition series by using the compound 
of the formula (I), it is necessary to mix the compound with other 
compounds to cause a smectic A phase to be exhibited. It has been found 
that compound expressed by the formula (II), for example, the following 
compound: 
##STR10## 
is suitable to the above purpose. 
The phase transition points of the compound are as follows: 
##STR11## 
The compound expressed by the formula (II) exhibits smectic C phase at such 
a relatively low temperature, exhibits smectic A phase and has a low 
melting point. 
When a compound expressed by the formula (I) is combined with a compound 
expressed by the formula (II), it is possible to prepare a smectic C 
liquid crystal mixture whose phase transition series is in the order from 
the higher temperature side, of isotropic liquid phase, nematic phase, 
smectic A phase and smectic C phase. 
As the compound of the formula (I), in the case of R.sup.1 =8 in Table 2, a 
compound of R.sup.2 =4 is most preferable, and a compound of R.sup.2 =2 or 
5 is more preferable. Further, a compound of R.sup.2 =3 is also preferable 
if its quantity is a certain one, due to its broad nematic phase. In these 
cases, practically it is preferred to use a mixture obtained by mixing a 
compound of R.sup.2 =4 with compounds of R.sup.2 =2 and R.sup.2 =5 to 
further lower the melting point and further broaden the temperature range 
of nematic phase. In addition, in the case of compound of R.sup.2 =6, the 
melting point lowers, but the temperature range of nematic phase is 
narrow, and in the case of compound of R.sup.2 =7, the melting point is 
high and the temperature range of nematic phase is narrow; hence the 
properties of these compounds are inferior to those of the above 
compounds. Compound of R.sup.2 =8 wherein R.sup.1 and R.sup.2 have the 
same length has a very high melting point and a very narrow temperature 
range of nematic phase; hence the compound is not preferable. 
For comparison, the present inventors prepared a compound wherein R.sup.1 
and R.sup.2 have the same linear alkyl group. Its phase transition points 
are shown in Table 3. 
TABLE 3 
______________________________________ 
N phase 
temper- 
Com- 
ature pound 
R.sup.1 
R.sup.2 
Cr SC N Iso width No. 
______________________________________ 
4 4 105 .multidot. 
.multidot. 
147.2 -- R-1 
5 5 95 .multidot. 
122.5 
.multidot. 
160.0 37.5 R-2 
6 6 84 .multidot. 
135.6 
.multidot. 
152.5 16.9 R-3 
7 7 83 .multidot. 
148.7 
.multidot. 
159.7 11.0 R-4 
8 8 80 .multidot. 
147.5 
.multidot. 
149.2 1.7 R-5 
______________________________________ 
In this Table, Cr, Sc, N and Iso, respectively represent crystalline phase, 
smectic C phase, nematic phase and isotropic phase, and the numerals 
described below R.sup.1 and R.sup.2 represent alkyl chain lengths and the 
numerals below Cr, SC, N and Iso represent the respective phase transition 
temperatures. The melting points are shown below Cr.SC. The temperature 
unit is .degree. C. 
As compared with the compounds of Table 2, it is seen that the above 
compound has a higher melting point and hence it is undesirable. 
As seen from Table 3, when R.sup.1 and R.sup.2 are the same, the melting 
point is high. Hence such a compound is not practical. Further, from Table 
2, it has been found that even in the case where R.sup.1 and R.sup.2 are 
different, there are a compound having a practical melting point and a 
compound having an unpractical melting point. The practically suitable 
alkyl chain length has been found by preparation of compounds for the 
first time; hence it is seen that the above fact cannot be easily 
anticipated from so far known examples. For example, in the above Table, 
in the case of R.sup.1 =4 and R.sup.2 =4, no smectic C phase is exhibited 
and also the melting point is very high. Further, in the case of R.sup.1 
=8 and R.sup.2 =8, smectic C phase is exhibited, but the temperature range 
of nematic phase is only 1.7.degree. C. and also the melting point is 
high. Whereas, in the case of the compound of the present invention 
wherein R.sup.1 =8 and R.sup.2 =4, the smectic C phase and the nematic 
phase are exhibited each within a broad range, and yet the melting point 
is very low. As described above, the compound of the present invention 
cannot be easily anticipated by a person of skill in the art from Japanese 
patent application laid-open No. Hei 4-28787; hence the compound of the 
present invention can be regarded as having an inventive step. 
Next, preferable compounds among the compounds of the present invention 
expressed by the formula (I), and their phase transition temperatures are 
shown in the following Table 4: 
TABLE 4 
______________________________________ 
Particularly preferable compounds of the formula (I) 
N phase 
temper- 
Com- 
ature pound 
R.sup.1 
R.sup.2 
Cr SC N Iso width No. 
______________________________________ 
8 2 70 .multidot. 
93.2 .multidot. 
139.4 46.2 I-1 
8 3 68 .multidot. 
114.2 
.multidot. 
152.6 38.4 I-2 
8 4 55 .multidot. 
124.6 
.multidot. 
146.8 22.2 I-3 
8 5 56 .multidot. 
139.7 
.multidot. 
155.8 16.1 I-4 
5 3 68 .multidot. 
89.3 .multidot. 
164.4 75.1 I-7 
6 2 62 .multidot. 
73.3 .multidot. 
139.8 66.5 I-8 
6 3 55 .multidot. 
96.2 .multidot. 
153.1 58.9 I-9 
______________________________________ 
In the Table, Cr, SC, N and Iso represent crystalline phase, smectic C 
phase, nematic phase and isotropic liquid phase, respectively, and the 
numerals below R.sup.1 and R.sup.2 represent alkyl chain lengths and 
numerals below Cr, SC, N and Iso represent the respective phase transition 
temperatures. The numerals below Cr--SC represent meltihg points. The 
temperature unit is .degree. C. 
Further, besides, combinations of R.sup.1 and R.sup.2 of 7 with 2 and 7 
with 4 are considered to be also preferred. 
Further, usable compounds which are inferior to those of Table 4 and their 
phase transition temperatures are shown in Table 5. 
TABLE 5 
______________________________________ 
Preferable compounds expressed by the formula (I) 
N phase 
temper- 
Com- 
ature pound 
R.sup.1 
R.sup.2 
Cr SC N Iso width No. 
______________________________________ 
8 6 62 .multidot. 
143.7 
.multidot. 
151.7 8.0 I-5 
8 7 71 .multidot. 
150.1 
.multidot. 
154.8 4.7 I-6 
5 2 66 .multidot. 
63.8 .multidot. 
151.1 87.3 I-10 
6 4 75 .multidot. 
116.3 
.multidot. 
149.4 33.1 I-11 
6 5 79 .multidot. 
129.3 
.multidot. 
157.6 28.3 I-12 
7 3 73 .multidot. 
107.6 
.multidot. 
158.3 50.7 I-13 
7 6 72 .multidot. 
142.1 
.multidot. 
155.5 13.4 I-14 
9 3 73 .multidot. 
120.8 
.multidot. 
153.1 32.3 I-15 
9 5 62 .multidot. 
143.3 
.multidot. 
155.8 12.5 I-16 
7 5 62 .multidot. 
136.7 
.multidot. 
155.9 23.2 I-17 
______________________________________ 
In the Table, Cr, SC, N and Iso represent crystalline phase, smectic C 
phase, nematic phase and isotropic liquid phase, respectively, and the 
numerals below R.sup.1 and R.sup.2 represent the respective alkyl chain 
lengths, and numerals below Cr, SC, N and Iso represent the respective 
phase transition temperatures. The temperature unit is .degree. C. The 
compound expressed by the formula (II) is a compound in the composition 
disclosed in Japanese patent application laid-open No. Hei 2-135278 filed 
by the present applicant, and this compound is used in the smectic C 
mixture for the ferroelectric liquid crystal of the present invention and 
the ferroelectric chiral smectic C liquid crystal composition using the 
same. However, the combination of the compound of the formula (II) with 
the compound of the formula (I) has not been disclosed in the above 
Japanese patent application laid-open No. Hei 2-135278. 
The compound of the formula (II) exhibits smectic C phase at a relatively 
low temperature and also has a relative low melting point. 
Preferable compounds among those expressed by the formula (II) and their 
phase transition temperatures are shown in Table 6 together with compound 
number. 
TABLE 6 
__________________________________________________________________________ 
Preferable compounds expressed by the formula (II) 
R.sup.3 
R.sup.4 
X Cr SB SC SA Iso 
Compound No. 
__________________________________________________________________________ 
8 4 H 33 .multidot. 
57.3 
.multidot. 
66.8 
.multidot. 
69.4 II-1 
9 5 H 43 .multidot. 
65.0 72.4 
.multidot. 
74.5 II-2 
10 
5 H 44 .multidot. 
66.7 70.4 
.multidot. 
74.4 II-3 
7 6 F 25 .multidot. 
40.6 
.multidot. 
50.5 II-4 
7 7 F 33 .multidot. 
40.1 
.multidot. 
50.4 II-5 
7 8 F 26 .multidot. 
46.0 
.multidot. 
53.4 II-6 
7 9 F 38 .multidot. 
45.2 
.multidot. 
63.6 II-7 
8 8 F 35 .multidot. 
49.9 
.multidot. 
54.8 II-8 
8 9 F 47 .multidot. 
50.3 
.multidot. 
55.4 II-9 
8 10 F 44 .multidot. 
53.2 
.multidot. 
56.5 II-10 
9 7 F 35 .multidot. 
45.6 
.multidot. 
57.6 II-11 
9 12 F 46 .multidot. 
57.8 
.multidot. 
62.1 II-12 
10 
9 F 47 .multidot. 
57.8 
.multidot. 
61.4 II-13 
__________________________________________________________________________ 
In this Table, Cr, SB, SC, SA and Iso represent crystalline phase smectic B 
phase, smectic C phase, smectic A phase and isotropic liquid crystal 
phase, respectively, and the numerals below R.sup.3 and R.sup.4 represent 
the alkyl chain lengths thereof and numerals below Cr, SB, SC and Iso 
represent the respective phase transition temperatures. The numerals below 
Cr--SC represent melting points. Temperature unit is .degree. C. 
Next, combinations of the compounds of the formula (I) with those of the 
formula (II) will be described. The composition of the present invention 
appears to fall within the composition claimed in the above WO88/08019. 
However, the compound of the structure of the formula (I) is only one kind 
among 10 kinds of the structure exemplified as preferable ones, and as 
described above, there is no practical preparation example thereof. 
Further, as seen from Examples 7 to 64 of this patent, there is disclosed 
no compound having a low melting temperature and also having broad smectic 
C phase and nematic phase as in the compound of the present invention. The 
formula containing the compound of the structure of the formula (II) 
includes both a pyrimidine core and pyridine core, and is not limited to a 
compound of pyrimidine core as in the present invention. In particular, 
the examples have no combination with pyridine core. Whereas, compound 
suitable to the present invention is only compounds of the formula (II) 
having pyridine core. In short, it cannot be obvious from WO88/08019 that 
a compound of the formula (I) is combined with that of the formula (II), 
thereby obtaining a smectic C liquid crystal mixture suitable to an 
element utilizing .tau. min and a ferroelectric smectic C liquid crystal 
composition by mixing an optically active compound with the above mixture. 
Next, the phase transition temperatures of compound of the formula (I) (NO. 
I-3 of Table 1 ) and compound of the formula (II) (No. II-6 of Table 5) 
are shown in FIG. 1. Further, for comparison, the phase transition 
temperatures of compound of the formula (I) wherein R.sup.1 =6 and R.sup.2 
=6 and compound (No. R-3 of Table 3) and compound of the formula (II) 
(II-6 of Table 6) are shown in FIG. 2. 
In addition, the dotted line in FIG. 1 shows the melting point in the case 
of a combination of a compound (No. R-3) wherein R.sup.1 =6 and R.sup.2 =6 
and a compound of (No. II-6). 
As described above, it can be seen that the composition of the present 
invention (FIG. 1) has a considerable difference in the melting point. 
Thus, the combination of the compounds (I) and (II) of the present 
invention is superior. 
As seen from the above phase diagrams, when the compound of the formula (I) 
and that of the formula (II) of the present invention are combined, it is 
possible to prepare a smectic C liquid crystal mixture having a phase 
transition temperature range in the order from the higher temperature 
side, of isotropic liquid phase, nematic phase, smectic A phase and 
smectic C phase, and yet having a low melting point, and when an optically 
active compound is further added, it is possible to constitute a 
ferroelectric chiral smectic C liquid crystal composition having a phase 
transition temperature range in the order from the higher temperature 
side, of isotropic liquid phase, cholesteric phase, smectic A phase and 
chiral smectic C phase. 
Further, to the above smectic C liquid crystal composition or ferroelectric 
chiral smectic C liquid crystal composition, it is possible to add 
compound other than those of the formula (I) and the formula (II) in order 
to broaden the phase transition temperature ranges of smectic C phase, 
smectic A phase and nematic phase, or to make the melting points lower, as 
far as the characteristics of the above compositions are not notably 
damaged. 
As to the optically active compound used in the present invention, it does 
not matter which one is used, as far as it does not notably damage the 
characteristics when it is combined with the smectic C liquid crystal 
mixture of the present invention, but a compound capable of increasing the 
response speed is preferable. 
Further, since the spontaneous polarization of the composition can be 
adjusted by the quantity of the optically active compound added, a 
compound having a low viscosity is preferable. For example, when the 
spontaneous polarization of the composition is intended to have a definite 
value, the proportion of the optically active compound used is necessarily 
determined by the latent spontaneous polarization (if plural compounds are 
used, it is determined by composition ratio therein). When compositions 
having the same spontaneous polarization are compared with each other, 
taking the tilt angle into account, then the difference between the 
response speeds is determined by the viscosities of the respective 
compositions. When the used smectic C mixtures are the same, the 
viscosities of the respective compositions depend upon the viscosity of 
the optically active compound and its concentration. In short, an 
optically active compound having balanced spontaneous polarization and 
viscosity is preferable. However, it is difficult to prepare the 
respective compositions so as to have all the same spontaneous 
polarizations and all the same tilt angles; thus in order to determine 
whether the optically active compound is suitable or unsuitable, it is 
necessary to practically prepare the compound. 
The structural formulas (III-A) and (III-B) of preferable optically active 
compounds used in the present invention and representative compounds 
thereof are exemplified below. 
##STR12## 
In the above formulas, R.sup.5 and R.sup.6 each represent a linear alkyl 
group or alkoxy group and * represents an asymmetric carbon atom. 
As described above, the viscosity of the optically active compound is an 
important factor. Further, the viscosity of the smectic C liquid crystal 
mixture occupying a large proportion in the ferroelectric chiral smectic C 
liquid crystal composition is a still more important factor. In 
particular, when comparison is made using the same optically active 
compound in the same quantity, the superiority or inferiority in viscosity 
of the smectic C mixture appears notably. This can also be applied to 
comparison of the physical properties of other smectic C liquid crystal 
mixtures. 
Conditions required for the smectic C liquid crystal mixture are those 
wherein the phase transition temperature range is in the order from the 
higher temperature side, of isotropic liquid phase, nematic phase, smectic 
A phase and smectic C phase; the smectic C phase is exhibited in a broad 
range; the melting point is low; the viscosity is low; etc. 
The smectic C liquid crystal mixture obtained by mixing the compound of the 
formula (I) with that of the formula (II) can satisfy all of the 
above-mentioned conditions. 
Next, the ferroelectric liquid crystal display element of the present 
invention will be described referring to FIG. 3. 
FIG. 3 shows a cross-sectional view illustrating the basic constitution of 
the liquid crystal display element using the ferroelectric liquid crystal 
composition of the present invention. This liquid crystal display element 
consists basically of a pair of insulating substrates 1 and 2 having 
electroconductive films 3 and 4 as electrodes; a ferroelectric liquid 
crystal composition 8 interposed between the substrates 1 and 2; a driving 
means (not shown) for switching the optical axis of liquid crystal by 
selectively impressing a voltage onto the above electrodes; and a 
polarizing plate 9 as a means for optically identifying the above 
switching of the optical axis. Further, in this figure, 5 refers to an 
insulating film, 6 refers to an alignment-controlling film and 7 refers to 
a sealing material. As the insulating substrates 1 and 2, a 
light-transmitting substrate is used, and a glass substrate is usually 
used. On the insulating substrates 1 and 2, InO.sub.3, SnO.sub.2, ITO 
(Indium-Tin oxide) or the like is coated according to CVD (Chemical Vapor 
Deposition) or a sputtering method, to form electrodes 3 and 4 having a 
definite pattern. The film thickness of the transparent electrodes is 
preferably 50 to 200 nm. 
On the transparent electrodes 3 and 4, an insulating film 5 having a film 
thickness of 50 to 200 nm is formed. As the insulating film 5, inorganic 
thin films such as those of SiO.sub.2, SiNx, Al.sub.2 O.sub.3, Ta.sub.2 
O.sub.5, etc., organic thin films such as those of polyimide resin, 
photoresist resin, polymer liquid crystals, etc. can be used. In the case 
where the insulating film 5 is inorganic, the film can be formed according 
to deposition method, sputtering method, CVD method, solution-coating 
method, etc. Further, in the case where the film is organic, a solution 
dissolving an organic substance or its precursor is coated according to 
spinnercoating method, immersion-coating method, screen-printing method, 
roll-coating method, etc., followed by forming the film by curing under 
appropriate curing conditions (such as heating, light-irradiation, etc.), 
or it is possible to form the film according to deposition method, 
sputtering method, CVD method, LB (Langmuir-Blodgett) method, etc. This 
insulating film 5 may be omitted. 
On the insulating film 5, an alignment-controlling film 6 having a film 
thickness of 10 to 100 nm is formed. In the case where the insulating film 
5 is omitted as described above, the alignment-controlling film is formed 
directly on electroconductive films 3 and 4. As the alignment-controlling 
film 6, an inorganic or organic film may be used. For the inorganic 
alignment-controlling film, silicon oxide or the like is usable, and as a 
film-forming method, known methods are usable, such as oblique deposition 
method, rotating deposition method, etc. In the case of the organic 
alignment-controlling film, polyamide, polyvinyl alcohol, polyimide, etc. 
are usable. The film is usually rubbed. Further, in the case where polymer 
liquid crystal or LB film is used, alignment by way of magnetic field, 
alignment according to spacer edge method, etc. are applicable. Further, 
the film can be formed with SiO.sub.2, SiOx, etc. according to deposition 
method, sputtering method, CVD method, etc., followed by rubbing. Then, 
two insulating substrates 1 and 2 are laminated onto each other by the 
medium of a sealer 7, followed by filling a ferroelectric liquid crystal 
composition 8 to make a ferroelectric liquid crystal element. 
As the ferroelectric liquid crystal composition 8, those described in the 
above items (5) to (9) are preferably used. 
In FIG. 3, the ferroelectric liquid crystal element has been described as 
regards a switching element of one pixel, but the ferroelectric liquid 
crystal element of the present invention is also applicable to a display 
device for a large capacity matrix, and in this case, as shown in a plan 
view of FIG. 4, there is used a typical device wherein the electrodes of 
the upper and lower substrates 1 and 2 are combined in the form of a 
matrix type. FIG. 5 is a figure for illustrating C1 alignment and C2 
alignment in the ferroelectric liquid crystal display element of FIG. 4. 
A preferable alignment-treating method for the above ferroelectric liquid 
crystal display element is a rubbing method. The rubbing method mainly 
includes parallel rubbing, anti-parallel rubbing, one side rubbing, etc. 
Parallel rubbing refers to a method wherein the upper and lower substrates 
are rubbed and the rubbing directions are parallel. Anti-parallel rubbing 
refers to a method wherein the upper and lower substrates are rubbed and 
the rubbing directions are anti-parallel. One side rubbing refers to a 
method wherein only one side substrate of the upper and lower substrates 
is rubbed. 
The most preferred method of obtaining a uniform alignment in the present 
invention is a method wherein the cell treated by parallel rubbing 
treatment is combined with a ferroelectric liquid crystal having an INAC 
morphoric range. In this case, a helical structure is present in the 
nematic phase, but since the aligning direction of molecules is controlled 
from both the sides of the upper and lower substrates, a uniform alignment 
is liable to be obtained in the nematic phase, and when the temperatures 
are descending from the state down to smectic A phase and then chiral 
smectic C phase, alignment uniform in the normal direction to a layer is 
easily obtained. 
However, even in the ferroelectric liquid crystal element of parallel 
rubbing, the aligning state formed in the chiral smectic C is never only 
one. The causes for which the state does not become uniform, are two. 
One of the two causes is directed to the bend of smectic layer. It is well 
known that the ferroelectric liquid crystal cell exhibits a bent layer 
structure (chevron structure), and as shown in FIG. 5, two domains can be 
present. Kobe et al. have named them C1 and C2 in relation to pretilt. 
Another cause is directed to uniform (U) and twist (T). "Uniform" is an 
alignment exhibiting extinction site and "twist" is an alignment 
exhibiting no extinction site. Koden et al. have reported that three 
alignments of C1U (C1-uniform), C1T (C1-twist) and C2 have been obtained 
in the ferroelectric liquid crystal cell of parallel rubbing using a high 
pretilt aligning film (M. Koden et al., Jpn. J. appl. phys., 30, L1823 
(1991)). The present inventors further have made research in more detail, 
and as a result, have confirmed that there are 4 aligning states of C1U, 
C1T, C2U and C2T in the ferroelectric liquid crystal cell of parallel 
rubbing. FIG. 6 shows the molecular alignment in these aligning states. 
Four aligning states obtained in the ferroelectric liquid crystal cell 
having a negative dielectric anisotropy have been compared, and as a 
result, the present inventors have found the following facts. Since C1U 
and C1T alignments are difficultly switched, driving is difficult. 
Further, C1T alignment has no extinction site; hence, even if the 
alignment is switched, good contrast cannot be obtained. Whereas, C2U 
alignment affords a good switching characteristic and contrast, and while 
C2T alignment does not exhibit extinction properties at the time of 
impressing no electric field, but when the liquid crystal material has a 
negative dielectric anisotropy, the above C2T alignment exhibits 
extinction properties as in the case of uniform alignment, at the time of 
impressing a suitable bias voltage; hence a good switching characteristic 
and contrast are obtained even by C2T alignment. 
The appearance of C1 and C2 alignments is related to a pretilt angle, and 
C2 state can occur within a pretilt angle of 0.degree. to 15.degree.. When 
the pretilt angle is high, C2 state is only one state showing the 
extinction site, and this is rather preferred as reported by Koden et al. 
However, there is a tendency that as the pretilt angle increases, the 
alignment is liable to become rather C1 alignment than C2 alignment; hence 
the tilt angle is preferably 10.degree. or less. 
Next, the driving method will be described. Of course, Joers/Alvey driving 
method according to the driving waveform A as shown in FIG. 7 can be used, 
but the driving method according to the driving waveform (B) as shown in 
FIG. 8 is also considered. These driving methods are a partially 
rewritable driving method and are preferable for preparing a display 
having a large display Capacity of e.g. 2,000.times.2,000 lines, using the 
above ferroelectric liquid crystal element. In the case of driving 
waveform (B), the waveforms of voltages impressed to pixel are expressed 
by (a) to (d), and since .tau.s obtained by impressing the voltages of the 
waveforms (b) to (d) in the case of non-rewriting are equal, the 
quantities of transmitted light are equal. Hence, a good display having no 
flicker is obtained. Further, Malvern driving method (WO92/02925 (PCT)) 
whose example is a driving waveform (C) as shown in FIG. 9, is a method 
wherein the main pulse width can be varied so as to have an optional 
length, as compared with Joers/Alvey's driving method according to a 
driving waveform (A) wherein an 0 V part of one time slot and a main pulse 
part which is not an 0 V of one time slot are used as shown in FIG. 10. 
Hence, the above method is one of preferable methods since the timing of 
impressing the voltage is lapped between the electrodes whereby the line 
address time can be shortened. 
Including the above driving method, a driving method for a ferroelectric 
liquid crystal material having a .tau.-V characteristic wherein the pulse 
width .tau. exhibits a minimum value is characterized as follows. 
In these driving inethods, voltages V.sub.1, V.sub.2, V.sub.3 or V.sub.4 
having the following relations are impressed to a pixel, 
EQU 0&lt;V.sub.2 &lt;V.sub.4 
EQU V.sub.2 -V.sub.1 &lt;V.sub.4 -V.sub.3 
and the pixel is driven so that when the first pulse voltage V.sub.1 and 
the succeeding pulse voltage V.sub.2, or the first pulse voltage -V.sub.1 
and the succeeding pulse voltage -V.sub.2 are impressed onto a part of the 
pixel on a selected scanning electrode, the ferroelectric liquid crystal 
molecules are brought into one stable state or another stable state 
depending upon the polarity of the impressed voltage, whatever the stable 
state prior to the voltage impression, and when the first pulse voltage 
V.sub.3 and the succeeding second pulse voltage V.sub.4 or the first pulse 
voltage -V.sub.3 and the succeeding second pulse voltage -V.sub.4 are 
impressed onto the part of the same pixel, the stable state of the 
ferroelectric liquid crystal molecules prior to the voltage impression is 
retained. 
Namely, in the initial 2 time slots of a selection period, as compared with 
the waveform applied for rewriting, a waveform applied for retention has a 
higher second pulse voltage and a larger difference between the first 
pulse voltage and the second pulse voltage. For example, as to such 
voltages, V.sub.1, V.sub.2, V.sub.3 and V.sub.4, 
EQU V.sub.1 =V.sub.d, V.sub.2 =V.sub.s -V.sub.d, V.sub.3 =V.sub.d and V.sub.4 
=V.sub.s +V.sup.d, 
in the driving waveform (A) shown in FIG. 7; 
EQU V.sub.1 =0, V.sub.2 =V.sub.s -V.sub.d, V.sub.3 =-V.sub.d and V.sub.4 
=V.sub.s +V.sub.d 
in the driving waveform (B) shown in FIG. 8; and 
EQU V.sub.1 =V.sub.d, V.sub.2 =V.sub.s -V.sub.d, V.sub.3 =-V.sub.d and V.sub.4 
=V.sub.s +V.sub.d, 
in the driving waveform (A) shown in FIG. 9. 
In the .tau.-V characteristic of ferroelectric liquid crystal element, the 
voltage V min affording the minimum value .tau. min of the pulse width 
.tau. is directly related to the maximum value of the voltage impressed at 
the time of driving. In view of the pressure resistance of the driving 
circuit used for driving, a ferroelectric liquid crystal material having a 
V min of 60 V or lower is preferred, and in order to use a driving circuit 
using a general-purpose IC driver, the material having a V min of 35 V or 
lower is preferred. Further, in the driving of a ferroelectric liquid 
crystal material having a .tau.-V characteristic exhibiting a minimum 
value of pulse width .tau., by optionally forming a region having 
different driving characteristics in the pixel, for example according to a 
process of modifying the element structure such as cell gap, electrode 
shape, etc., it is possible to use a waveform applied to rewriting at a 
specified part in the pixel, as a waveform applied to retention at other 
part in the same pixel, or to use a waveform applied to retention at a 
specified part in the pixel, as a waveform applied to rewriting at other 
part in the same pixel. Thus, it is possible to carry out graduation 
display. 
In addition, in the description of the present invention, as one embodiment 
of a very preferable method utilizing the ferroelectric liquid crystal 
display element of the present invention, parallel rubbing, C2 alignment, 
a specified driving method, etc. have been mentioned, but, of course, the 
present invention should not be construed to be limited thereto and it 
goes,without saying that the above example is applicable to other types of 
ferroelectric liquid crystal display elements and driving methods. 
Next, applicability of the present invention to liquid crystal display 
element utilizing .tau. min will be described. In a simple system wherein 
the layer structure has been supposed to be a book shelf structure, the 
following equation comes into existence (see Liquid crystals 6, No. 3, 
p.341 (1989)): 
EQU E min=P.sub.s /(3.sup.1/z .multidot..epsilon..sub.0 
.multidot..DELTA..epsilon..multidot.sin.sup.2 .theta.) 
wherein E min represents a voltage in a pulse width of minimum value, Ps 
represents spontaneous polarization, .epsilon..sub.o represents vacuum 
dielectric constant, .DELTA..epsilon. represents dielectric anisotropy and 
.theta. represents tilt angle. According to this equation, in the case 
where a practical voltage, e.g. E min is 40 V or lower, when a liquid 
crystal material having a .DELTA..epsilon. of -2 is used and a cell of 2 
.mu.m is used, the spontaneous polarization should be 7 nC/m.sup.2 or less 
(Ferroelectrics, vol. 122, p. 63 (1991)). Since a practical layer 
structure is mostly a chevron structure, it is impossible to apply this, 
as it is, but it is possible to use it as an estimation. What is known 
from this equation is that the less the Ps and the larger the 
.DELTA..epsilon. and .theta., the lower the E min. 
The pulse width of the minimum value (.EPSILON. min) is inversely 
proportional to the square of spontaneous polarization (Ferroelectrics, 
vol. 122, p. 63 (1991)). 
The .DELTA..epsilon. of the mixture system. of compound (I-3) and compound 
(II-6) of the present invention at room temperature is shown in FIG. 11. 
When .DELTA..epsilon. is extrapolated based upon FIG. 11, .DELTA..epsilon. 
of compound (I-3) is -6, and that of compound (II-6) is -2.5. Within a 
practical mixing ratio, it is considered that the .DELTA..epsilon. of the 
smectic C liquid crystal mixture of the present invention is within a 
range of about -2.5 to -5. While the .DELTA..epsilon. may be also varied 
depending upon the .DELTA..epsilon. of an optically active compound to be 
mixed, it is considered that the above value is sufficiently large in 
terms of negative value, when it is used for the element utilizing .tau. 
min. As to the value of the spontaneous polarization prepared using it, 
when a cell of an E min of 40 V and 2 .mu.m is utilized, referring to the 
above equation, if the tilt angle is 20.degree., it is seen that the 
spontaneous polarization can be made 9 to 18 nC/cm.sup.2, and .tau. min 
can be shortened as much. However, since .tau. min itself is not related 
only to the spontaneous polarization, but as there are factors such as 
viscosity, etc., anticipation is impossible. In short, even when the 
compositions have the same .DELTA..epsilon., .tau. min varies depending 
upon the viscosity, tilt angle, etc. of the compositions. 
.tau.-V curve was prepared using "CS-3000" of a trade name of product made 
by Chisso Corporation. As a result, even when 60 V was impressed to a cell 
of 2 .mu.m at 25.degree. C., .tau. min was not observed. .tau. min should 
appear in the vicinity of 44 V according to calculation. "CS-3000" has a 
.DELTA..epsilon. of -2.7, a tilt angle of 26.degree. and a spontaneous 
polarization of 17.5 nCcm.sup.-2. Like this, it was found that there were 
factors other than .DELTA..epsilon., tilt angle and spontaneous 
polarization; thus, it is practically difficult to anticipate E min and 
.tau. min. 
The smectic C liquid crystal composition of the present invention and the 
ferroelectric smectic C liquid crystal composition obtained by mixing an 
optically active compound with the above composition are not only suitable 
to an element utilizing .tau. min or AC stabilizing effect, but also they 
are usable to conventional SSFLC element. 
Effectiveness of the Invention 
When the compound expressed by the formula (I), of the present invention is 
mixed with the compound expressed by the formula (II) of the present 
invention, in a definite proportion, a smectic C liquid crystal 
composition suitable to an element utilizing .tau. min or AC stabilizing 
effect can be obtained, and further when an optically active compound is 
mixed with the composition, a ferroelectric liquid crystal composition 
suitable to the above element and a liquid crystal display element using 
the composition can be obtained. 
EXAMPLE 
The present invention will be described in more detail by way of 
Preparation examples and Examples, but it should not be construed to be 
limited thereto. 
Various measurements in the present invention were carried out according to 
the following methods: 
The phase transition temperature was measured by placing a sample on a 
slide glass, followed by covering it with a cover glass, placing the 
resulting material on a hot plate, elevating the temperature at 1.degree. 
C./min. and observing under a polarizing microscope. 
The melting point was measured by using a differential scanning 
calorimetric analysis (DSC) and elevating the temperature at 1.degree. 
C./min. 
The spontaneous polarization (Ps) was measured according to Sawyer-Tower 
method. 
The tilt angle (.theta.) was measured by impressing a sufficiently high 
electric field of a critical electric field or higher, to a homogeneously 
aligned cell, to extinguish the helical structure, and further reversing 
the polarity, to obtain the rotation angle (corresponding to 2 ) at 
extinction site under crossed nicols. 
The dielectric anisotropy (.DELTA..epsilon.) was measured by filling the 
respective compositions in a cell having the capacity measured in advance, 
provided with electrodes having a vertically aligning agent coated 
thereonto and having a distance of 2 .mu.m between the electrodes, and in 
a cell having the capacity measured in advance, and provided with 
electrodes subjected to homogeneously aligning treatment and having a 
distance of 2 .mu.m between the electrodes, followed by measuring and 
calculating the capacities of the respective cells by means of LCR meter 
and at 1 V and 10 KHz at 25.degree. C. 
The response time (.tau.) was measured by filling the respective 
compositions in a cell subjected to aligning treatment and having a 
distance of 2 .mu.m between electrodes, impressing a square wave of Vpp=20 
V and 100 Hz and observing the change in the intensity of transmitted 
light at that time. 
The voltage (E min) in the pulse width of minimum value and the pulse width 
(.tau. min) of the minimum value were determined by filling the respective 
compositions in a cell provided with electrodes subjected to aligning 
treatment and having a distance of 2 .mu.m between the electrodes, 
determining a relationship between .tau. and V to prepare a .tau.-V curve 
and observing the resulting curve. In this case, the pulse distance was 
made 100 times the pulse width (.tau.) (100.tau.) (Ferroelectrics, vol. 
122, p. 63 (1991)). 
Any of the measurements of the above Ps, .theta., .tau. and .tau. min were 
carried out at 25.degree. C., and the designation of the compounds used as 
the component compounds of the liquid crystal compositions in the 
below-mentioned Examples and Comparative examples was made under the 
above-mentioned compound numbers. 
Preparation example 1 
Preparation of 2-(4-octylphenyl)-5-(4-ethylphenyl)-1,3,4-thiazole (a 
compound of the formula (I) wherein R.sup.1 is octyl, and R.sup.2 is 
ethyl) 
2-(4-octylphenyl)-5-(4-ethylphenyl)-1,3,4-thiazole was prepared according 
to the following steps 1, 2 and 3. 
##STR13## 
Step 1: Preparation of 4-octylbenzohydrazide THF (100 ml) was added to 
4-octylbenzoic acid ethyl ester (100 g), followed by dropwise adding 
hydrated bydrazine (80 g), heating the mixture under reflux for 2 hours, 
adding ice water after completion of the reaction, filtering off deposited 
crystals, washing with water and recrystallizing from ethanol to obtain 
72.9 g (m.p.: 100.1.degree. to 101.2.degree. C.). 
Step 2: Preparation of N-(4-octylbenzo)-N'-(4-ethylbenzo)dihydrazide 
THF (20 ml) and pyridine (4 ml) were added to the hydrazide (5.0 g) 
obtained at the step 1, followed by dropwise adding 4-ethylbenzoic acid 
chloride (4.3 g), heating the mixture under reflux for 5 hours, adding 
aqueous NaCl after completion of the reaction, filtering off deposited 
crystals and recrystallizing from ethanol, to obtain the objective 
compound (3.6 g). 
Step 3: Preparation of 
2-(4-octylphenyl)-5-(4-ethylphenyl)-1,3,4-thiadiazole 
THF (30 ml) and 
2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphoethane-2,4-disulfide 
(Lawesson's reagent) (3.7 g) were added to the dihydrazide (3.6 g) 
obtained at the step 2, followed by heating the mixture under reflux for 4 
hours, adding water after completion of the reaction, extracting with 
toluene, washing the organic layer with NaOH, sodium thiosulfate and 
water, drying over anhydrous MgSO.sub.4, distilling off the solvent, 
purifying the residue according to column chromatography (with toluene) 
and recrystallizing from ethanol, to obtain the objective compound (1.8 
g). 
Phase transition temperature: 
______________________________________ 
Cr 70.degree. C. SC 93.2.degree. C. N 139.4.degree. C. 
______________________________________ 
Iso 
Example 1 
A smectic C liquid crystal mixture (a) having the following composition was 
prepared: 
______________________________________ 
Compound No. (I-2) 5 wt. % 
Compound No. (I-3) 10 wt. % 
Compound No. (I-4) 15 wt. % 
Compound No. (I-9) 10 wt. % 
Compound No. (II-5) 20 wt. % 
Compound No. (II-6) 20 wt. % 
Compound No. (II-7) 15 wt. % 
Compound No. (II-8) 5 wt. % 
______________________________________ 
The phase transition temperatures and the dielectric anisotropy 
(.DELTA..epsilon.) of the above composition (a) are shown below. 
Cr -17.degree. C. SC 71.5.degree. C. SA 76.0.degree. C. N 84.2.degree. C. 
Iso .DELTA..epsilon.: -4.6 
Example 2 
Optically active compounds were mixed with the smectic C liquid crystal 
mixture (a) of Example 1 in the following proportions, to prepare a 
ferroelectric liquid crystal composition (b): 
______________________________________ 
Mixture (a) 97 wt. % 
Compound No. (III-1) 2 wt. % 
Compound No. (III-4) 1 wt. % 
______________________________________ 
The phase transition temperatures, the spontaneous. polarization (Ps), the 
tilt angle (.theta.), the response time (.tau.) in the case where 20 V was 
impressed, the voltage (E min) in the pulse width of minimum value and the 
pulse width (.tau. min) of the minimum value, of the above composition (b) 
are shown below. 
Phase transition temperatures 
______________________________________ 
Cr -28.degree. C. SC* 73.0.degree. C. SA 76.2.degree. C. N 
85.9.degree. C. Iso 
Ps 4.2 nCcm.sup.-2 
.theta. 32.6.degree. 
.DELTA..epsilon. 
-4.6 
.tau. 85 .mu.sec 
E min 21 V/.mu.m 
.tau. min 50 .mu.sec 
______________________________________ 
Comparative Example 1 
A smectic C liquid crystal mixture (c) was prepared wherein the optically 
active compound of Example 1 of Japanese patent application laid-open No. 
Hei 4-4290 filed by the present inventors was excluded and the residual 
components were mixed in the following proportions. 
##STR14## 
The phase transition temperatures and the dielectric anisotropy 
(.DELTA..epsilon.) of the above composition (c) are shown below. 
Cr -12.degree. C. SC 68.2.degree. C. SA 73.0.degree. C. N 76.5.degree. C. 
Iso .DELTA..epsilon.: -3.5 
Comparative Example 2 
The following optically active compounds were mixed with the smectic C 
liquid crystal mixture (c) of Comparative example 1 to prepare a 
ferroelectric liquid crystal composition (d): 
______________________________________ 
Mixture (c) 97 wt. % 
Compound No. (III-1) 2 wt. % 
Compound No. (III-4) 1 wt. % 
______________________________________ 
The phase transition temperatures, the spontaneous polarization (Ps), the 
tilt angle (.theta.), the response time (.EPSILON.) in the case where 20 V 
was impressed, the voltage (E min) in the pulse width of minimum value and 
the pulse width (.tau. min) of the minimum value; of the above composition 
(d) are shown below. 
Phase transition temperatures: 
______________________________________ 
Cr -14.degree. C. SC* 68.2.degree. C. SA 71.4.degree. C. N 
75.9.degree. C. Iso 
Ps 4.7 nCcm.sup.-2 
.theta. 27.0.degree. C. 
.DELTA..epsilon. 
-3.4 
.tau. 250 .mu.sec 
E min 30 V/.mu.m 
.tau. min 128 .mu.sec 
______________________________________ 
When Example 2 is compared with Comparative example 2, the effectiveness of 
the present invention is evident. 
Example 3 
An optically active compounds was mixed with the smectic C liquid crystal 
mixture (a) of Example 1 in the following proportion, to prepare a 
ferroelectric liquid crystal composition (e): 
______________________________________ 
Mixture (a) 98 wt. % 
Compound No. (III-2) 2 wt. % 
______________________________________ 
The phase transition temperatures, the spontaneous polarization (Ps), the 
tilt angle (.theta.), the response time (.tau.) in the case where 20 V was 
impressed, the voltage (E min) in the pulse width of minimum value and the 
pulse width (.tau. min) of the minimum value; of the above composition (d) 
are shown below. 
Phase transition temperatures: 
______________________________________ 
Cr -24.degree. C. SC* 72.5.degree. C. SA 78.0.degree. C. N* 
84.2.degree. C. Iso 
Ps 14.0 nCcm.sup.-2 
.theta. 32.3.degree. 
.DELTA..epsilon. 
-5.0 
.tau. 80 .mu.sec 
E min 17 V/.mu.m 
.tau. min 43 .mu.sec 
______________________________________ 
Example 4 
A smectic C liquid crystal mixture (f) having the following composition was 
prepared. 
______________________________________ 
Compound No. (I-2) 5 wt. % 
Compound No. (I-3) 10 wt. % 
Compound No. (I-4) 10 wt. % 
Compound No. (I-9) 5 wt. % 
Compound No. (I-13) 5 wt. % 
Compound No. (II-5) 20 wt. % 
Compound No. (II-6) 20 wt. % 
Compound No. (II-7) 20 wt. % 
Compound No. (II-8) 5 wt. % 
______________________________________ 
The phase transition temperatures and the dielectric anisotropy 
(.DELTA..epsilon.) of the above composition (f) are shown below. 
Phase transition temperature: 
______________________________________ 
Cr -16.degree. C. SC 71.0.degree. C. SA 75.1.degree. C. N 80.6.degree. C. 
Iso .DELTA..epsilon.: -3.7 
______________________________________ 
Example 5 
Optically active compounds were mixed with the smectic C liquid crystal 
mixture (f) of Example 4 in the following proportions, to prepare a 
ferroelectric liquid crystal composition (g): 
______________________________________ 
Mixture (f) 88 wt. % 
Compound No. (III-1) 6 wt. % 
Compound No. (III-4) 6 wt. % 
______________________________________ 
The phase transition temperatures, the spontaneous polarization (Ps), the 
tilt angle (.theta.), the response time (.tau.) in the case where 20 V was 
impressed, the voltage (E min) in the pulse width of minimum value and the 
pulse width (.tau. min) of the minimum value, of the above composition (g) 
are shown below. 
Phase transition temperatures: 
______________________________________ 
Cr SC* 66.1.degree. C. SA 70.1.degree. C. N* 75.6.degree. C. Iso 
Ps 10.4 nCcm.sup.-2 
.theta. 29.2.degree. 
.DELTA..epsilon. 
-4.6 
.tau. 90 .mu.sec 
E min 17 V/.mu.m 
.tau. min 50 .mu.sec 
______________________________________ 
Example 6 
A smectic C liquid crystal mixture (h) having the following composition was 
prepared. 
______________________________________ 
Compound No. (I-3) 2 wt. % 
Compound No. (I-9) 8 wt. % 
Compound No. (II-5) 30 wt. % 
Compound No. (II-6) 30 wt. % 
Compound No. (II-7) 30 wt. % 
______________________________________ 
The phase transition temperatures and the dielectric anisotropy 
(.DELTA..epsilon.) of the above composition (h) are shown below. 
______________________________________ 
Cr -5.degree. C. SC 49.6.degree. C. SA 59.3.degree. C. N 60.4.degree. C. 
Iso .DELTA..epsilon.: -3.5 
______________________________________ 
Example 7 
The following optically active compounds were mixed with the smectic C 
liquid crystal mixture (h) of Example 6 in the following proportions, to 
prepare a ferroelectric liquid crystal composition (i): 
______________________________________ 
Mixture (h) 97 wt. % 
Compound No. (III-1) 2 wt. % 
Compound No. (III-4) 1 wt. % 
______________________________________ 
The phase transition temperatures, the spontaneous polarization (Ps), the 
tilt angle (.theta.), the response time (.tau.) in the case where 20 V was 
impressed, the voltage (E min) in the pulse width of minimum value and the 
pulse width (.tau. min) of the minimum value, of the above composition (i) 
are shown below. 
Phase transition temperatures: 
______________________________________ 
Cr -7.degree. C. SC* 49.1.degree. C. SA 57.8.degree. C. N* 59.2.degree. 
C. Iso 
Ps 3.8 nCcm.sup.-2 
.theta. 23.1.degree. 
.DELTA..epsilon. 
-4.2 
.tau. 65 .mu.sec 
E min 13 V/.mu.m 
.tau. min 30 .mu.sec 
______________________________________ 
Example 8 
A smectic C liquid crystal mixture (j) having the following composition was 
prepared. 
______________________________________ 
Compound No. (I-2) 10 wt. % 
Compound No. (I-4) 15 wt. % 
Compound No. (I-9) 20 wt. % 
Compound No. (II-5) 18 wt. % 
Compound No. (II-6) 18 wt. % 
Compound No. (II-7) 19 wt. % 
______________________________________ 
The phase transition temperatures and the dielectric anisotropy 
(.DELTA..epsilon.) of the above composition (j) are shown below. 
Phase transition temperature: 
______________________________________ 
Cr -10.degree. C. SC 69.5.degree. C. SA 82.5.degree. C. N 89.2.degree. C. 
Iso .DELTA..epsilon.: -5.3 
______________________________________ 
Example 9 
The following optically active compounds were mixed with the smectic C 
liquid crystal mixture (j) of Example 8 in the following proportions, to 
prepare a ferroelectric liquid crystal composition (k): 
______________________________________ 
Mixture (j) 95 wt. % 
Compound No. (III-1) 3 wt. % 
Compound No. (III-4) 2 wt. % 
______________________________________ 
The phase transition temperatures, the spontaneous polarization (Ps), the 
tilt angle (.theta.), the response time (.tau.) in the case where 20 V was 
impressed, the voltage (E min) in the pulse width of minimum value and the 
pulse width (.tau. min) of the minimum value, of the above composition (k) 
are shown below. 
Phase transition temperatures: 
______________________________________ 
Cr -15.degree. C. SC* 67.3.degree. C. SA 85.4.degree. C. N* 90.9.degree. 
C. Iso 
Ps 5.7 nCcm.sup.-2 
.theta. 26.2.degree. 
.DELTA..epsilon. 
-5.1 
.tau. 63 .mu.sec 
E min 15 V/.mu.m 
.tau. min 35 .mu.sec 
______________________________________ 
Example 10 
A smectic C liquid crystal mixture (1) having the following composition was 
prepared. 
______________________________________ 
Compound No. (I-3) 8 wt. % 
Compound No. (I-9) 32 wt. % 
Compound No. (II-2) 30 wt. % 
Compound No. (II-3) 30 wt. % 
______________________________________ 
The phase transition temperatures and the dielectric anisotropy 
(.DELTA..epsilon.) of the above composition (1) are shown below. 
______________________________________ 
Cr 8.degree. C. SC 80.3.degree. C. SA 86.9.degree. C. N 94.2.degree. C. 
Iso .DELTA..epsilon.: -3.9 
______________________________________ 
Example 11 
A smectic C liquid crystal mixture (m) having the following composition was 
prepared. 
______________________________________ 
Compound No. (I-13) 10 wt. % 
Compound No. (I-17) 5 wt. % 
Compound No. (I-2) 5 wt. % 
Compound No. (I-3) 10 wt. % 
Compound No. (I-4) 10 wt. % 
Compound No. (II-5) 20 wt. % 
Compound No. (II-6) 20 wt. % 
Compound No. (II-8) 5 wt. % 
Compound No. (II-7) 15 wt. % 
Cr -16.degree. C. SC 74.degree. C. SA 78.degree. C. N 86.degree. C. 
Iso 
______________________________________ 
Example 12 
Using the liquid crystal composition m of Example 11 and optically active 
compounds shown in Table 7, liquid crystal compositions n and o having 
compositions shown in Table 8 were prepared. The phase transition 
temperatures of the prepared compositions are shown in Table 9. 
TABLE 7 
__________________________________________________________________________ 
Structures of chiral compounds 
__________________________________________________________________________ 
Compound 10 
##STR15## 
Compound 11 
##STR16## 
Compound 12 
##STR17## 
__________________________________________________________________________ 
TABLE 8 
______________________________________ 
Composition of compositions n and o 
Content (wt. %) 
No. Composition n 
Composition o 
______________________________________ 
Composition m 
96 96 
Compound 10 2 2 
Compound 11 2 
Compound 12 2 
______________________________________ 
TABLE 9 
______________________________________ 
Phase transition temperatures of composition n and o (.degree.C.) 
No. Cr SC SA N Iso 
______________________________________ 
Composition n -16 68 75 83 
Composition o -16 70 76 83 
______________________________________ 
Example 13 
Transparent electrodes consisting of ITO of 100 nm were formed on two glass 
substrates, followed by forming an insulating film consisting of SiO.sub.2 
of 100 nm on the transparent electrodes, coating a polyimide aligning film 
PSI-A-SP07 having a film thickness of 50 nm on the insulating film and 
carrying out rubbing treatment. The two glass substrates were laminated in 
a cell thickness of 50 .mu.m so that the rubbing directions could be 
anti-parallel, followed by filling a nematic liquid crystal E-8 (made by 
Merck Co., Ltd.). 
The pretilt angle of the aligning film PSI-A-SP07 was measured according to 
magnetic field capacity method (K. Suzuki, K. Toriyama and A. Fukuhara, 
Appl. Phys. Lett. 33, 561 (1987), to give 5.degree.. 
Example 14 
Transparent electrodes consisting of ITO of 100 nm were formed on two glass 
substrates, followed by forming an insulating film consisting of SiO.sub.2 
of 100 nm on the transparent electrodes, coating a polyimide aligning film 
PSI-A-SP07 in a film thickness of 50 nm on the insulating film and 
carrying out rubbing treatment. Next, the two glass substrates were 
laminated in a cell thickness of 2 .mu.m so that the rubbing directions 
could be parallel, followed by filling the ferroelectric liquid crystal 
composition prepared in Example 3, once heating the cell at a temperature 
at which the liquid crystal composition was changed to an isotropic 
liquid, and then cooling it down to room temperature at a rate of 
1.degree. C./min., to obtain a ferroelectric liquid crystal element having 
C2 alignment over the whole surface in pixels. 
This ferroelectric liquid crystal element was placed between two 
perpendicularly crossed polarizing sheets and a voltage was impressed, to 
evaluate its characteristics. The evaluation conditions and the resulting 
characteristics are shown in Table 10. Further, the .tau.-V characteristic 
of this ferroelectric liquid crystal element was evaluated. The results 
are shown in FIGS. 12.and 13. As apparent from these figures, V min was 
obtained at 35 V and 30 V. 
TABLE 10 
__________________________________________________________________________ 
Characteristics of compositions n and o 
Memory 
Response Spontaneous 
Memory 
Tilt pulse speed/.mu.s .tau.-V 
polarization 
Composition 
C2T/% angle/.degree. 
angle/.degree. 
width/.mu.s 
0-50 
0-90 
10-90 
Vmin/V 
/nC/cm.sup.2 
__________________________________________________________________________ 
Composition n 
100 16 27 190 64 100 62 35 -6 
Composition o 
100 15 27 220 68 105 66 30 -4 
__________________________________________________________________________ 
Measuring temperature: 30.degree. C. 
C2T: Ratio of C2T alignment in the optical field of microscope 
Memory pulse width: Pulse width of phase-stable pulse of .+-.5 V/.mu.m 
exhibiting a bistable switching 
Response speed: Times at which the transmitted lights of 0-50, 0-90, 10-90% 
were changed in the case where a square wave of .+-.5 V/.mu.m and 250 Hz 
was impressed. 
V min: Minimum value of voltage at .tau.-V characteristic 
Example 15 
Using the ferrolectric liquid crystal element prepared in Example 5 and 
using the driving waveform shown in FIGS. 7, 8 and 9, the element was 
driven. Driving conditions and the driving results are shown in Tables 11 
and 12. Switching was possible at a driving voltage of 40 V or lower and a 
good contrast was obtained. 
TABLE 11 
______________________________________ 
Driving characteristics of composition n (30.degree. C.) 
Non- Line 
Driving Pulse Selected selected 
Bias address 
waveform 
width voltage voltage voltage 
time 
______________________________________ 
A 20 .mu.s 25.0 V 40.0 V 7.5 V 80 .mu.s 
B 7 .mu.s 26.0 V 40.0 V 7.0 V 98 .mu.s 
C 11 .mu.s 21.0 V 40.0 V 9.5 V 44 .mu.s 
______________________________________ 
TABLE 12 
______________________________________ 
Driving characteristics of composition o (30.degree. C.) 
Non- Line 
Driving Pulse Selected selected 
Bias address 
waveform 
width voltage voltage voltage 
time 
______________________________________ 
A 28 .mu.s 25.0 V 40.0 V 7.5 V 
112 .mu.s 
B 9 .mu.s 27.0 V 40.0 V 6.5 V 
126 .mu.s 
C 18 .mu.s 20.0 V 40.0 V 10.0 V 
72 .mu.s 
______________________________________ 
Comparative Example 1 
Transparent electrodes consisting of ITO of 100 nm were formed on two glass 
substrates, followed by forming an insulating film consisting of SiO.sub.2 
of 100 nm on the transparent electrodes, coating a polyimide film, aligned 
PSI-A-SP07, in a film thickness of 50 nm on the insulating film and 
carrying out rubbing treatment. Next, these two glass substrates were 
laminated in a cell thickness of 2 .mu.m so that the rubbing directions 
could be parallel, followed by filling ferroelectric liquid crystal 
compositions made by Merck Co., Ltd. (SCE8 and ZLI-5014-000)shown in Table 
13, once heating the cell at a temperature at which the liquid crystal 
compositions were converted into isotropic liquid, and then cooling down 
to room temperature at a rate of 1.degree. C./min, to obtain a 
ferroelectric liquid crystal element having C2 alignment over the total 
surface in pixel. 
TABLE 13 
______________________________________ 
Phase transition temperature of ferroelectric liquid crystal 
composition (.degree.C.) 
No. C.sub.r SC SA N I.sub.Iso 
______________________________________ 
SCE8 .multidot. 
&lt;-20 .multidot. 
58 .multidot. 
78 .multidot. 
98 .multidot. 
ZLI-5014-000 
.multidot. 
&lt;-10 .multidot. 
64 .multidot. 
68 .multidot. 
70 .multidot. 
______________________________________ 
This ferroelectric liquid crystal element was placed between two 
perpendicularly crossed polarizing sheets, followed by impressing a 
voltage to evaluate its characteristics. The evaluation conditions and the 
resulting characteristics are shown in Table 14. 
TABLE 14 
__________________________________________________________________________ 
Various characteristics of ferroelectric liquid crystal compositions 
Memory 
Response Spontaneous 
Memory 
Tilt pulse speed/.mu.s .tau.-V 
polarization 
No. C2T/% angle/.degree. 
angle/.degree. 
width/.mu.s 
0-50 
0-90 
10-90 
Vmin/V 
/nC/cm.sup.2 
__________________________________________________________________________ 
SCE8 100 13 18 400 156 268 193 45 +7 
ZLI-5014-000 
100 13 21 &gt;5000 87 236 210 50 -3 
__________________________________________________________________________ 
Measured temperature: 30.degree. C. 
C2T: Proportion of C2T alignment within the field of vision of microscope 
Memory pulse width: Pulse width of phase-stable pulse of .+-.5 V/.mu.m 
exhibiting a bistable switching 
Response speed: Times at which the transmitted lights of 0-50, 0-90 and 
10-90% were changed in the case where a square wave of .+-.5 V/.mu.m and 
250 Hz was impressed 
V min: Minimum value of voltage at .tau.-V characteristic 
Further, the .tau.-V characteristic of this ferroelectric liquid crystal 
element was evaluated. The results are shown in FIGS. 14 and 15. .tau.-V 
min was obtained, but the value of V min was 45 V and 50 V. 
Comparative Example 2 
Driving was carried out using the ferroelectric liquid crystal element 
prepared in Comparative example 1 and using the driving waveforms shown in 
FIGS. 7, 8 and 9. The driving conditions and the driving results are shown 
in, Tables 15 and 16. For the driving voltage, 50 V was required. 
TABLE 15 
______________________________________ 
Driving characteristics of SCE8 (30.degree. C.) 
Non- Line 
Driving Pulse Selected selected 
Bias address 
waveform 
width voltage voltage voltage 
time 
______________________________________ 
A 37 .mu.s 40.0 V 50.0 V 5.0 V 
148 .mu.s 
B 15 .mu.s 40.0 V 50.0 V 5.0 V 
210 .mu.s 
C 22 .mu.s 30.0 V 50.0 V 10.0 V 
88 .mu.s 
______________________________________ 
TABLE 16 
______________________________________ 
Driving characteristics of ZLI-5014-000 (30.degree. C.) 
Non- Line 
Driving Pulse Selected selected 
Bias address 
waveform 
width voltage voltage voltage 
time 
______________________________________ 
A 39 .mu.s 30.0 V 50.0 V 10.0 V 
156 .mu.s 
B 15 .mu.s 30.0 V 50.0 V 10.0 V 
210 .mu.s 
C 14 .mu.s 25.0 V 50.0 V 12.5 V 
56 .mu.s 
______________________________________ 
As seen from the above Examples, the present invention can provide a large 
capacity ferroelectric liquid crystal display element having a high speed 
response and a high contrast, by driving under a low voltage of 40 V or 
lower.