Liquid crystal composition

A ferroelectric liquid crystal is formed by incorporating, as the main component, a predetermined amount of a ferroelectric liquid crystal having a specific naphthalene ring structure into conventional liquid crystals. This liquid crystal composition has a broad driving temperature range and in particular, can be effectively driven even in a low-temperature region. Moreover, this liquid crystal composition has a high orientation uniformity, a high memory stability, and a high contrast ratio.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a liquid crystal composition. More 
particularly, the present invention relates to a liquid crystal 
composition in which an improved orientation, high contrast, high memory 
stability, and high-speed response are obtained by using a conventional 
liquid crystal and a liquid crystal having a specific naphthalene 
structure as main components, and mixing same at a predetermined ratio. 
Furthermore, the present invention relates to a ferroelectric liquid 
crystal display device comprising the above liquid crystal composition. 
(2) Description of the Related Art 
Since a liquid crystal display element is a plate type element with a small 
power consumption, the liquid crystal display element is widely used as a 
display element of a word processor, a lap top computer or the like. 
Especially, a super twisted nematic (STN) liquid crystal display element 
can be applied to a display having a relatively large display capacity 
such as 640.times.400 dots, and is widely used for personal computers and 
the like. 
Nevertheless, as the display capacity of the liquid crystal display 
increases and applications to word processors or personal computers having 
an enhanced performance are attempted, the application of a conventional 
STN becomes difficult because it has an unsatisfactory display background 
color and viewing field angle. 
As a display system proposed for overcoming these defects of the 
conventional liquid crystal display, a ferroelectric liquid crystal 
display (FLCD) was proposed [N.A. Clark and S.T. Largerwall, J. Applied 
Physics Letters, 36, 899 (1980)]. 
Also U.S. Pat. No. 4,367,927 (Noel A. Clark) discloses "Chiral Smectic Cor 
H Liquid Crystal Electro-Optical Device", wherein a liquid crystal 
electro-optical device comprising: 
a quantity of chiral, at least one of smectic C and smectic H, liquid 
crystal having a plurality of adjacently disposed layers each comprised of 
a plurality of molecules, each molecule having a long axis; 
first and second means, perpendicular to and contiguous with said layers, 
for aligning the long axes of said molecules adjacent to said first and 
second means parallel to said first and second means, said molecules of 
said layers in a bulk of said liquid crystal forming helixes having axes 
perpendicular to said layers, the distance between said first and second 
means being less than the distance at which said helixes form in the 
absence of an electric field, said first and second means causing said 
long axes to assume one of a first and second stable orientation; 
means for applying an electric field to at least a portion of said layers, 
said electric field being parallel to said layers and perpendicular to 
said first and second means, and for reversing the direction of said 
electric field, the strength of said electric field being sufficient to 
shift the long axis of molecules in said portion of said layers from said 
first orientation to said second orientation upon application of said 
electric field in a direction reversed from the direction of said electric 
field previously applied; and 
means for processing light passing through said liquid crystal so that 
light passing through portions of said liquid crystal having said first 
orientation can be distinguished from light passing through portions of 
said liquid crystal having said second orientation, indicated. 
FLCD shows a spontaneous polarization because the polarizations of liquid 
crystal molecules are oriented in one direction, and since this 
spontaneous polarization is reversed when the polarity of the applied 
electric field is reversed, FLCD shows ferroelectric characteristics. 
Accordingly, as the drive method, a method is adopted in which the 
spontaneous polarizations of liquid crystal molecules are oriented in one 
direction by applying a pulse wave form electric field having a negative 
polarity, the display contents are held by the memory effect of the liquid 
crystal, i.e., the property whereby after the orientation of the 
polarizations, the ferroelectric characteristic, that is, the direction of 
the spontaneous polarization, is not changed, and the direction of the 
spontaneous polarization is first changed by an application of an electric 
field having a reverse polarity. Therefore, in FLCD, the memory effect 
must be kept stable, but in the conventional ferroelectric liquid 
crystals, a satisfactory memory effect necessary for stable driving cannot 
be obtained, or if obtained, the response time is long and the 
characteristics of FLCD are not fully utilized. 
A surface stabilized ferroelectric liquid crystal display (SSFLCD) is 
recognized as having a great possibilities with regard to a high 
information content, wide viewing angle, high contrast ratio, and fast 
switching. To realize a high performance SSFLCD, much research has been 
carried out into the development of liquid crystal materials, driving 
methods, and liquid crystal molecular orientations. 
Nevertheless, the SSFLCD is still under development, mainly because the 
layer structure of the SSFLCD is a "chevron" not a "bookshelf" structure, 
and because of this structure, the SSFLCD shows "zig zag defects", 
(Y.Ouchi, J. Lee, H. Takezoe, A. Fukuda, K. Kondo, T. Kitamura and A. 
Mokoh, J.J.A.P. 27 L1993 (1988)), resulting in a poor contrast ratio and 
unstable bistability. 
A "bookshelf" structure obtained by an oblique deposition technique has 
been reported (M. Johno, A. D. L. Chandani, Y. Ouchi, H. Takezoe, A. 
Fukuda, M. Ichihashi and K. Furukawa, J.J.A.P. 28 L119 (1989)), but a 
bookshelf structure with a rubbed polymer film cell and suitable for a 
mass production of the SSFLCD has not been reported. 
Accordingly, the development of a liquid crystal material maintaining a 
high-speed response characteristic, a characteristic of FLCD, and having a 
satisfactorily stable memory effect is desired in the art. 
A mixture composed mainly of an ester compound and a phenylpyrimidine 
compound is mainly used for the conventional FLCD, but a satisfactorily 
stable memory effect cannot be obtained by any of the known compositions 
of this type. The main reason for this is that, since a compatible Coulomb 
mutual action (interaction) is not effected in the interface between each 
liquid crystal molecule and the substrate, microdomains are formed, or if 
a certain orientation is obtained, many zigzag defects are generated as 
described hereinafter. 
Under this background, the inventors proposed an FLCD in which a high-speed 
response characteristic and a stable memory effect can be manifested by 
using a naphthalene type liquid crystal having a naphthalene ring in the 
core unit portion of the liquid crystal molecule (Unexamined Japanese 
Patent No. TOKKAIHEI 1-101389). The naphthalene type liquid crystal is a 
material which is different from conventional FLCD, in that the generation 
of zigzag defects is not observed, and in which a liquid crystal 
orientation having an excellent uniformity can be manifested, and further, 
the naphthalene type liquid crystal has a much better memory 
characteristic than those of the conventional liquid crystal materials. 
In general, however, the naphthalene type liquid crystal has a high 
viscosity, and an increase of the viscosity is especially conspicuous at 
temperatures lower than 15.degree. C., and therefore, the naphthalene type 
liquid crystal is defective in that the response time is abruptly 
lengthened in a low-temperature range (lower than 10.degree. C.). 
Accordingly, the inventors searched for a liquid crystal material retaining 
the characteristic of conventional liquid crystals, i.e., a relatively low 
viscosity in a low-temperature region, and having the good orientation and 
memory characteristics inherently possessed by the naphthalene type liquid 
crystal. First, a mixture of a conventional phenylpyrimidine type mixed 
liquid crystal with the naphthalene type liquid crystal was examined, and 
as a result, it was confirmed that the compatibility between the 
phenylpyrimidine liquid crystal and the naphthalene type liquid crystal is 
generally poor and the ferroelectric phase-showing temperature range is 
narrowed by the mixing. Accordingly, the inventors tried to obtain a 
stable memory characteristic in a broad ferroelectric phase-showing 
temperature range by mixing various naphthalene type liquid crystals into 
conventional liquid crystals as the base, but it was found that 
naphthalene type liquid crystals differ in their compatibility with the 
conventional liquid crystals according to the difference of the molecular 
structure, and that the intended effects of the present invention can be 
attained by a novel liquid crystal composition formed by combining the 
liquid crystals described below. The present invention is based on this 
finding. 
The research objectives of the present inventors were as follows. 
First, to clarify the relationships between the molecular structure and 
layer structure of the liquid crystal, and then to obtain a bookshelf 
structure; second to investigate dependence of the layer structure on the 
electro-optical properties thereof; and third, to obtain a high contrast 
ratio and stable bistability with the bookshelf layer structure of the 
SSFLCD with a rubbed polymer film. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a liquid crystal 
composition in which an improvement of the orientation of the 
ferroelectric liquid crystal display (elimination of zigzag defects and 
attainment of a high uniformity in the display plane), a stabilization of 
the memory effect, and an improvement of the contrast ratio are 
satisfactorily obtained. 
To realize this object, according to the present invention, the chevron 
structure (see FIG. 2), which is the conventional liquid crystal layer 
structure, is changed to the bookshelf structure (see FIG. 1). 
A secondary object of the present invention is to provide a liquid crystal 
composition having a bookshelf structure and has satisfactory 
characteristics required for liquid crystal materials from the practical 
viewpoint, such as the driving temperature and the liquid crystal 
viscosity. 
A third object of the present invention is to provide a liquid crystal 
composition having a satisfactory spontaneous polarization while obtaining 
the foregoing primary and secondary objects. 
A fourth object of the present invention is to provide a liquid crystal 
display device comprising the above-mentioned liquid crystal composition 
of the present invention. 
In accordance with one fundamental aspect of the present invention, the 
foregoing objects can be obtained by a liquid crystal composition 
comprising (1) 5 to 60% by weight of at least one member selected from the 
group consisting of compounds represented by the following formula (I): 
##STR1## 
wherein R.sub.1 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.2 represents an alkyl group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, which may be substituted 
with an alkoxy group having 1 to 6 carbon atoms, 
(II) 5 to 60% by weight of at least one member selected from the group 
consisting of compounds represented by the following general formula (II): 
##STR2## 
wherein R.sub.3 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.4 represents an alkyl or alkoxy group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, 
(III) 5 to 60% by weight of at least one member selected from the group 
consisting of compounds represented by the following formula (III): 
##STR3## 
wherein R.sub.5 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.6 represents an alkyl or alkoxy group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, and (IV) 5 to 70% by 
weight of at least one member selected from the group consisting of 
compounds represented by the following formulae (IV-1) through (IV-5): 
##STR4## 
wherein R.sub.8 and R.sub.9 represent an alkyl group having 6 to 16 carbon 
atoms, and R.sub.7 represents an alkyl group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms. 
In accordance with another aspect of the present invention, there is 
provided a liquid crystal composition, which further comprises (V) 5 to 
40% by weight of at least one member selected from the group consisting of 
compounds represented by the following formulae (V-1) and V-2): 
##STR5## 
wherein R.sub.10 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.11 represents an alkyl group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, in addition to the 
foregoing compounds (I) through (IV). 
This liquid crystal composition of the second aspect of the present 
invention has been completed based on the finding that the compound of 
group (V) has a strong effect of expanding the spontaneous polarization, 
and in this liquid crystal composition, by incorporating the compound of 
group (V) into compounds (I) through (IV), the response speed of the 
liquid crystal is further increased within a range at which the viscosity 
of the entire liquid crystal is not increased. 
In accordance with one aspect of the present invention, there is provided a 
ferroelectric liquid crystal display device, comprising: 
A liquid crystal panel, comprising: 
a first transparent substrate; 
a second transparent substrate substantially in parallel with the first 
transparent substrate; 
first transparent electrode means formed on the internal surface of said 
first transparent substrate; 
second transparent electrode means formed on the internal surface of said 
second transparent substrate; 
a spacer between the first and second transparent substrates, said first 
and second transparent substrates and said spacer forming a enclosed 
space, said first and second transparent electrodes each having an 
internal surface facing the enclosed space; 
first liquid crystal orienting means formed on the internal surface of said 
first transparent substrate; 
second liquid crystal orienting means formed on the internal surface of 
said second transparent substrate; 
a ferroelectric liquid crystal having a spontaneous polarization and 
forming a bookshelf smectic-C layer structure, charged in the enclosed 
space; and 
means for causing the liquid crystal at selected portions of the panel to 
have a polarization of the liquid crystal molecules oriented in one 
direction and the liquid crystal at other portions of the panel to have a 
polarization of the liquid crystal molecules oriented in a reverse 
direction thereto, comprising means for applying a pulse wave form 
electric field alternately to the positive and negative sides by applying 
a pulse wave form electric voltage across the first and second electrodes, 
wherein said ferroelectric liquid crystal is a chiral smectic-C liquid 
crystal composition comprising (1) 5 to 60% by weight of at least one 
member selected from the group consisting of compounds represented by the 
following formula (I): 
##STR6## 
wherein R.sub.1 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.2 represents an alkyl group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, which may be substituted 
with an alkoxy group having 1 to 6 carbon atoms, 
(II) 5 to 60% by weight of at least one member selected from the group 
consisting of compounds represented by the following general formula (II): 
##STR7## 
wherein R.sub.3 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.4 represents an alkyl or alkoxy group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, 
(III) 5 to 60% by weight of at least one member selected from the group 
consisting of compounds represented by the following formula (III): 
##STR8## 
wherein R.sub.5 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.6 represents an alkyl or alkoxy group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, 
and (IV) 5 to 70% by weight of at least one member selected from the group 
consisting of compounds represented by the following formulae (IV-1) 
through (IV-5): 
##STR9## 
wherein R.sub.8 and R.sub.9 represent an alkyl group having 6 to 16 carbon 
atoms, and R.sub.7 represents an alkyl group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms. 
In accordance with another aspect of the invention, there is provided 
ferroelectirc liquid crystal display device, which further comprises (V) 5 
to 40% by weight of at least one member selected from the group consisting 
of compounds represented by the following formulae (V-1) and V-2): 
##STR10## 
wherein R.sub.10 represents an alkyl or alkoxy group having 6 to 16 carbon 
atoms, and R.sub.11 represents an alkyl group having at least one 
asymmetric carbon atom and 4 to 13 carbon atoms, in addition to the 
foregoing compounds (I) through (IV). 
In the present ferroelectric liquid crystal display device, the 
above-mentioned bookshelf smectic-C layer structure is perpendicular to 
the first and second substrates. 
Also, in the present ferroelectric liquid crystal display device, the 
above-mentioned first and second liquid crystal orienting means comprise a 
polymer film subjected to the rubbing treatment. 
Further, in the present above-mentioned ferroelectric liquid crystal 
display device, above-mentioned polymer film is formed of polyvinyl 
alcohol (PVA), polyimide, or polyamide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The ferroelectric liquid crystal composition of the present invention 
comprises compounds of groups (I) through (IV) mentioned above, and may 
further comprise a compound of group (V). 
The liquid crystal composition of the present invention will now be 
described with reference to the layer structure formed by the liquid 
crystal. 
Note, in all of the compounds of groups (I) through (V), groups R.sub.1 
through R.sub.11 may be either linear or branched. 
Generally, in a display using a ferroelectric liquid crystal, the layer 
structure formed by the liquid crystal is ideally vertical to the 
substrate, as reported by Clark et al (see FIG. 1, in which reference 
numeral 1. represents the substrate and reference numeral 2 represents the 
layer structure). This structure is called a "bookshelf structure". In the 
display using a liquid crystal having this structure, the orientation is 
improved, a high contrast is obtained, the memory stability is improved, 
and a high-response characteristic is obtained. 
In the conventional ferroelectric liquid crystal, however, it is known 
that, in practice, the layer is bent in a &lt;-shaped form, as shown in FIG. 
2 (in which reference numeral 1 represents the substrate and reference 
numeral 2 represents the layer structure). In the &lt;-shaped bent structure 
(chevron structure), the inherent electro-optical switching of the 
ferroelectric liquid crystal does not occur, and the layer structure per 
se is unstable, and accordingly, a practical application of the liquid 
crystal to a display device is difficult. In the chevron structure, since 
the layer is bent, the polarizations of respective liquid crystal 
molecules cancel one another out, with the result that the macro 
spontaneous polarization is reduced and the response is delayed. Moreover, 
since the directions of respective molecules differ from one another, the 
polarization becomes unstable in the memory state and the memory 
characteristic is poor. Furthermore, where the bent parts of &lt;-shaped 
layers are contiguous to each other (see FIG. 2), a domain wall called a 
disclination portion is generated because the state of the advance of 
light is changed at the boundary of the orientations of liquid crystal 
molecules. This disclination portion appears as a display defect (zigzag 
defect) which causes a reduction of the contrast. 
Accordingly, the structure in which the layer is vertical to the substrate 
(bookshelf structure) is indispensable for realizing the inherent display 
of the ferroelectric liquid crystal. The present invention relates to a 
novel liquid crystal material in which the bookshelf structure can be 
constructed. 
The liquid crystal composition of the present invention consists of a 
mixture comprising compounds of groups (I) through (IV) as mentioned 
above. 
The compound of group (I) is a liquid crystal essential for the 
construction of the bookshelf structure. 
The compound of group (II) is also a liquid crystal essential for the 
construction of the bookshelf structure, and this liquid crystal is 
characterized in that the liquid crystal phase transition temperature is 
relatively broad on the higher-temperature side. 
Further, the compound of group (III) is a liquid crystal essential for the 
construction of the bookshelf structure, and this liquid crystal is 
characterized in that the viscosity is relatively low. 
The compound of group (IV) is used in combination with the compounds of 
groups (I), (II) and (III), and the main function of this compound is to 
expand the ferroelectric liquid crystal phase temperature range (driving 
temperature range). 
Namely, to obtain the intended effects of constructing the bookshelf 
structure and obtaining a high contrast and a stable memory 
characteristic, the liquid crystal compounds of groups (I), (II) and (III) 
are indispensable. 
In the liquid crystal composition of the present invention, the liquid 
crystal compound of group (IV) must be incorporated into the liquid 
crystal compounds of groups (I), (II) and (III), to maintain the 
characteristics of the liquid crystal at practically satisfactory levels. 
Namely, although a highest effect of stabilizing the bookshelf structure 
is obtained when the composition is composed solely of the liquid crystal 
compounds of groups (I), (II) and (III), the compound of group (IV) is 
incorporated to realize a practically satisfactory driving temperature 
range and liquid crystal viscosity and to impart a satisfactory 
performance to a liquid crystal display. 
In general, the bookshelf structure can be constructed most easily in the 
compound of group (I). Therefore, in principle, use of the compound of 
group (I) in a large amount is preferable. Nevertheless, since the 
viscosity of the naphthalene type liquid crystal is high, a practically 
applicable composition is formed by the incorporation of the compound of 
group (IV). The compound of group (II) is a liquid crystal which is 
especially effective for expanding the driving temperature range toward 
the high-temperature side. Among naphthalene type liquid crystals, the 
compound of group (III) has a low viscosity and is effectively used when 
an extreme reduction of the viscosity is desired. In practice, since the 
viscosity can be reduced to some extent by mixing the compounds of groups 
(I), (II) and (III), it is possible to mix all or parts of the compounds 
of groups (I), (II), (III) and (IV) and use the mixture. 
Preferably, the liquid crystal compounds of groups (I), (II) and (III) are 
used in amounts as large as possible, as long as the viscosity and driving 
temperature range are practically satisfactory. Namely, each of the 
compounds of groups (I), (II) and (III) is preferably used in an amount of 
5 to 60% by weight, with the balance being the compound of group (IV). 
Since each of the compounds of groups (I), (II) and (III) has a high 
viscosity, if the amount of any of the compounds of groups (I), (II) and 
(III) exceeds 60% by weight, the response time becomes too long and the 
liquid crystal composition cannot be practically used. If the amount of 
any of the compounds of groups (I), (II) and (III) is smaller than 5% by 
weight, the effect of constructing the bookshelf structure is not 
satisfactory. 
In the compound of group (I) constituting the liquid crystal composition of 
the present invention, R.sub.1 is preferably an alkyl group having six to 
sixteen carbon atoms or an alkoxy group having six to sixteen carbon atoms 
and especially preferably an alkyl group having 8 to 14 carbon atoms or an 
alkoxy group having 8 to 14 carbon atoms. R.sub.2 is preferably an alkyl 
group having at least one asymmetric carbon atom and 4 to 13 carbon atoms, 
especially 5 to 9 carbon atoms, which may be substituted with an alkoxy 
group having 1 to 6 carbon atoms. 
In the compound of group (II), R.sub.3 is preferably an alkyl group having 
6 to 16 carbon atoms or an alkoxy group having 6 to 16 carbon atoms, and 
especially preferably an alkyl group having 8 to 14 carbon atoms or an 
alkoxy group having 8 to 14 carbon atoms. R.sub.4 is preferably an alkyl 
or alkoxy group having at least one asymmetric carbon atom and 4 to 13 
carbon atoms, especially 5 to 9 carbon atoms. 
In the compound of formula (III), R.sub.5 is preferably an alkyl group 
having 6 to 16 carbon atoms or an alkoxy group having 6 to 16 carbon 
atoms, and especially preferably an alkyl group having 8 to 14 carbon 
atoms or an alkoxy group having 8 to 14 carbon atoms. R.sub.6 is 
preferably an alkyl or alkoxy group having at least one asymmetric carbon 
atom and 4 to 13 carbon atoms, especially 5 to 9 carbon atoms. 
In the compound of group (IV), R.sub.8 and R.sub.9 each represent an alkyl 
group having preferably 6 to 16 carbon atoms, especially preferably 8 to 
12 carbon atoms. R.sub.7 represents an alkyl group having at least one 
asymmetric carbon atom and preferably 4 to 13 carbon atoms, especially 
preferably 5 to 9 carbon atoms. 
Note, the halogen atom as the element to be bonded to the asymmetric carbon 
atom is, for example, chlorine or bromine. 
In the compound of group (V), R.sub.10 is preferably an alkyl group having 
6 to 16 carbon atoms or an alkoxy group having 6 to 16 carbon atoms and 
especially preferably an alkyl group having 8 to 14 carbon atoms or an 
alkoxy group having 8 to 14 carbon atoms. R.sub.11 represents an alkyl 
group having at least one asymmetric carbon atom and having preferably 4 
to 13 carbon atoms, especially preferably 5 to 9 carbon atoms. 
Preferable amounts of the compounds of groups (I) through (V) are now 
described. The preferable amount of the compound of group (I) is 10 to 50% 
by weight, the preferable amount of the compound of group (II) is 10 to 30% 
by weight, the preferable amount of the compound of group (III) is 5 to 30% 
by weight, the preferable amount of the compound of group (IV) is 10 to 60% 
by weight, and the preferable amount of the compound of group (V) is 20 to 
40% by weight. 
The liquid crystal of group (IV) is used in combination with the liquid 
crystals of groups (I) through (III) or with the liquid crystals of groups 
(I) through (III) and (V) and exerts a function of expanding the 
ferroelectric liquid crystal phase temperature range. Accordingly, the 
mixing ratio of the liquid crystal of group (IV) is changed according to 
the desired ferroelectric liquid crystal phase temperature range. Since a 
temperature range of from about -10.degree. C. to about 60.degree. C. is 
generally required, the amount added of the liquid crystal of group (IV) 
should be adjusted to 5 to 70% by weight according to the characteristics 
of the liquid crystal mixture of the liquid crystals of groups (I) through 
(III) or (I) through (III) and (V). If the amount of the liquid crystal of 
group (IV) is smaller than 5% by weight, the effect of expanding the 
liquid crystal phase temperature range, especially on the low-temperature 
side, is poor, and if the amount of the liquid crystal of group (IV) is 
larger than 70% by weight, it becomes impossible to maintain the bookshelf 
structure. 
The liquid crystal of group (V) is used in an amount of 5 to 40% by weight, 
preferably 20 to 40% by weight, mainly for obtaining a sufficient 
spontaneous polarization. If the amount of the liquid crystal of group (V) 
is smaller than 5% by weight, a satisfactory effect of increasing the 
spontaneous polarization cannot be obtained, and if the amount of the 
liquid crystal of group (V) is larger than 40% by weight, the viscosity of 
the liquid crystal composition becomes too high and it is difficult to 
maintain the bookshelf structure. 
The present invention will now be described in detail with reference to the 
following preferred examples, that by no means limit the scope of the 
invention. 
EXAMPLE 1 
A liquid crystal composition (hereinafter referred to a "composition 1") 
was obtained by mixing the following compounds of groups (I) through (IV) 
in the amounts described below. 
##STR11## 
EXAMPLE 2 
A liquid crystal composition (hereinafter referred to as "composition 2") 
was obtained by mixing the following compounds of groups (I) through (IV) 
in the amounts shown below. 
##STR12## 
EXAMPLE 3 
A liquid crystal composition (hereinafter referred to as "composition 3") 
was obtained by mixing the following compounds of groups (I) through (IV) 
in the amounts described below. 
##STR13## 
EXAMPLE 4 
A liquid crystal composition (hereinafter referred to as "composition 4") 
was obtained by mixing the following compounds of groups (I) through (V) 
in the amounts described below. 
##STR14## 
EXAMPLE 5 
A liquid crystal composition (hereinafter referred to as "composition 5") 
was obtained by mixing the following compounds of groups (I) through (V) 
in the amounts described below. 
##STR15## 
EXAMPLE 6 
A liquid crystal composition (hereinafter referred to as "composition 6") 
was obtained by mixing the following compounds of groups (I) through (V) 
in the amounts described below. 
##STR16## 
EXAMPLE 7 
A liquid crystal composition (hereinafter referred to as "composition 7") 
was obtained by mixing the following compounds of groups (I) through (V) 
in the amounts described below. 
##STR17## 
In the following comparative examples, liquid crystal compositions lacking 
one or more of the indispensable components of the liquid crystal 
composition of the present invention were prepared. 
COMATIVE EXAMPLE 1 
A liquid crystal composition (hereinafter referred to as "composition 8") 
was prepared according to the following recipe, without using the compound 
of group (II). 
##STR18## 
COMATIVE EXAMPLE 2 
A liquid crystal composition (hereinafter referred to as "composition 9") 
was prepared according to the following recipe, without using the 
compounds of groups (II) and (III). 
##STR19## 
COMATIVE EXAMPLE 3 
A liquid crystal composition (hereinafter referred to as "composition 10") 
was prepared according to the following recipe, without using the 
compounds of groups (II) through (IV). 
##STR20## 
COMATIVE EXAMPLE 4 
A liquid crystal composition (hereinafter referred to a "composition 11") 
was prepared by using only the compounds of group (I), without using the 
compounds of groups (II) through (IV). 
##STR21## 
EXAMPLE 8 
Evaluation Panel 
With respect to each of the liquid crystal compositions obtained in the 
foregoing examples and comparative examples, the display characteristics 
were evaluated by using the following evaluation panel. A transparent 
electrode-attached glass substrate having a size of 50 mm .times. 60 mm 
and a thickness of 1.1 mm was used, and PVA (polyvinyl alcohol) was used 
as the liquid crystal-orienting film. The PVA film had been subjected to 
the rubbing treatment, and the panel gap was adjusted to 2.0 .mu.m. A 
liquid crystal composition shown in Table 1 was sealed in this panel and 
the electro-optical response characteristics thereof were evaluated. 
A liquid crystal display device according to the present invention is 
constructed as follows. Ferroelectric liquid crystal composition according 
to the present invention is charged in the enclosed space formed between 
first and second transparent substrates substantially in parallel each 
other, and a spacer inserted between the substrates. 
Transparent electrodes are formed on the internal surface of the substrate 
spatially and in the line, and each transparent electrode formed on each 
substrate is aligned perpendicularly to the other, to thereby form a 
matrix addressed display. 
Each of the terminals of the first switching means is electrically 
connected to one of the first transparent electrodes, and each of the 
terminals of the second switching means is electrically connected to one 
of the second transparent electrodes. The terminal of the first switching 
means and the terminal of the second switching means are connected to 
opposite terminals of a voltage source. The first second switching means 
include means for applying the voltage from the voltage source across any 
of the first transparent electrodes and any of the second transparent 
electrodes. 
Evaluation Items 
(1) Each evaluation panel was observed in the cross Nicol state by a 
microscope, to examine the uniformity of the orientation of liquid crystal 
molecules. Namely, the presence or absence of zigzag defects and the change 
of the light transmission according to the place were checked. The change 
of the light transmission according to the place is shown as the in-plane 
uniformity in Table 1. The in-plane uniformity was evaluated according to 
the following procedures. 
Namely, the display plane (transparent electrode matrix) was divided into 
5.times.5 (=25) spots. The light transmission at each spot was compared 
with the mean value of the light transmissions at the respective spots, 
and the ratio (%) of the difference between the light transmission at each 
spot and the mean value to the mean value was calculated. 
The memory stability (memory retention ratio) was evaluated according to 
the following procedures. 
Namely, pulse waves having a length of 400 .mu.s and a wave height of 15 V 
were applied to each panel, and the light transmission after 0.5 second 
was compared with the light transmission at the time of the application of 
the pulse, and the ratio (%) of the light transmission after 0.5 second to 
the light transmission at the time the application of the pulse was 
calculated. The higher the light transmission after 0.5 second, the 
greater the memory stability. 
Furthermore, the contrast ratio was evaluated based on the ratio of the 
light transmission in the "on" state and the light transmission in the 
"off" state at the time of the memory, i.e., after 0.5 second. 
(2) Evaluation of Driving Temperature Range of Ferroelectric Liquid Crystal 
The ferroelectric liquid crystal display is different from the STN liquid 
crystal display in that not only the temperature range showing the liquid 
crystal state but also the temperature range where in a reversal of the 
polarization is possible, is important. Namely, if the reversal of the 
spontaneous polarization is not sufficient, the contrast and memory effect 
are not satisfactory, and the characteristics of the liquid crystal as the 
display element are poor. 
Accordingly, the practical driving temperature range of the ferroelectric 
liquid crystal is an important factor in the evaluation of the liquid 
crystal composition. In the present example, the driving temperature range 
was determined according to the following conditions. 
PVA was coated on a transparent electroconductive film-attached glass 
substrate having a size of 60 mm .times. 50 mm and a thickness of 1.1 mm, 
the rubbing treatment was carried out, and each liquid crystal composition 
was sealed into a cell sandwiching the substrate using an SiO.sub.2 sphere 
having an average diameter of 1.6 .mu.m as the spacer, to thereby 
fabricate a panel. The panel gap was 2.0 to 2.2 .mu.m. 
The panel was placed in a constant-temperature thermostat tank and the 
electric-optical characteristics were measured by using an He-Ne laser as 
the light source. The driving was effected by applying pulse waves having 
a length of 400 .mu.s at intervals of 0.5 second on the positive and 
negative sides alternately. The wave height was 15 V (.+-.15 V). The 
temperature range wherein the contrast ratio of the electro-optical 
response was at least 5/1 at the time of driving was designated as the 
driving temperature range. 
The viscosity was measured at 25.degree. C., by the triangular wave method. 
The results of the measurements of the respective items are shown in Table 
1. 
TABLE 1 
__________________________________________________________________________ 
In-Plane Memory 
Driving 
Composition Uniformity 
Contrast 
Retention 
Temperature 
Viscosity 
No. Zigzag Defects 
(%) Ratio 
Ratio (%) 
Range (.degree.C.) 
(mPa .multidot. s, 25.degree. 
__________________________________________________________________________ 
C.) 
1 not found 
&lt;5 25 97 10-42 453 
2 " &lt;5 20 93 10-40 530 
4 " &lt;5 22 95 5.43 431 
6 " &lt;5 22 98 5-42 420 
7 " &lt;5 25 97 4-41 410 
8 " &lt;5 17 88 15-30 895 
9 " &lt;5 18 90 14-27 90 
10 " -8 15 92 35-40 480 
11 " -8 12 93 29-38 530 
conventional 
found 15-25 1 0 5-45 250 
liquid 
crystal* 
__________________________________________________________________________ 
Note 
*phenylpyrimidine type mixed liquid crystal 
Note, the viscosity of the liquid crystal composition (composition 9) of 
Comparative Example 2 is low. This is because, since the driving 
temperature range is narrow and the measurement temperature of 25.degree. 
C. is close to the smectic phase-showing temperature, the viscosity is 
low. From the practical viewpoint, preferably the driving temperature 
range is at least from 10.degree. to 40.degree. C. and the viscosity at 
25.degree. C. is 400 to 550 mPa.s. 
Each of the compositions of Examples 1, 2, 4, 6 and 7 satisfies these 
requirements. 
From the results shown in Table 1, it is understood that the liquid crystal 
composition of the present invention is superior to the comparative liquid 
crystal compositions in all of the contrast ratio, the memory retention 
ratio, and the driving temperature range. 
EXAMPLE 9 
In the present example, it was determined whether or not the liquid crystal 
composition of the present invention actually had the bookshelf structure. 
In general, the layer spacing in the layer structure of a ferroelectric 
liquid crystal is scores of .ANG., typically, thirty .ANG., and therefore, 
the structure can be confirmed by a usual X-ray diffractometry. Since an 
X-ray peak corresponding to the layer spacing is obtained by the 
transmission X-ray diffractometry, in the bookshelf structure shown in 
FIG. 1, a peak is obtained only when an X-ray is incident at an angle 
corresponding to the Bragg angle (2dsin.theta.=n.lambda.). Therefore, a 
single peak is obtained in the case of the bookshelf structure. 
In contrast, in the case of the chevron structure shown in FIG. 2, peaks 
are obtained at Bragg angles corresponding to both "/" and " " in the 
layer bent in the &lt;-shaped form; i.e., double peaks are observed. 
Note, double peaks are observed in conventional liquid crystals, as shown 
in Yukio Ouchi et al, Japanese Journal of Applied Physics, 27, 11, L1993 
L1995. (An example taught in this reference is shown in FIG. 3.) 
As apparent from FIG. 4, in the liquid crystal composition (composition 1) 
of the present invention, a single peak is obtained in the X-ray 
diffraction pattern, and accordingly, it is confirmed that the liquid 
crystal composition of the present invention has the bookshelf structure. 
Note, in FIGS. 3 and 4, .alpha. indicates the rotation angle of the liquid 
crystal cell to the incident direction of X-rays, which corresponds to the 
leaning angle of the liquid crystal layer to the substrate. 
As apparent from the foregoing description, according to the present 
invention, a liquid crystal composition is formed by adding a 
predetermined amount of a ferroelectric liquid crystal having a specific 
structure to conventional liquid crystals, so that the liquid crystal 
composition has the bookshelf structure. Accordingly, in the liquid 
crystal composition of the present invention, the viscosity is relatively 
low even in a low-temperature region and driving is possible in this 
low-temperature region. Furthermore, the liquid crystal composition of the 
present invention has a very good orientation uniformity as the liquid 
crystal and further, has a high memory stability. 
The inventors used a liquid crystal molecular orientation using a mixture 
of a naphthalene system ferroelectric liquid crystal: 
##STR22## 
which showed a high degree of flatness in the core structure of the liquid 
crystal molecule, and obtained a zigzag defect-free alignment with a 
rubbed polyvinyl alcohol cell with a 2 .mu.m gap. The inventors also used 
a liquid crystal molecular orientation using a mixture of a naphthalene 
system and phenylpyrimidine system ferroelectric liquid crystals, and 
obtained a zigzag defect-free alignment with the same type of cell. 
The inventors investigated the temperature dependence of the smectic layer 
structure of these cells, by X-ray diffraction experiment, and the results 
are shown in FIG. 5 and 6. These results are quite different from 
previously reported X-ray diffraction experiment results. (T. P. Rieker, 
N. A. Clark, G. S. Smith, D. S. Parmer, E. B. Sirota and C. R. Safinya, 
Phys. Rev. Lett. 59 2658 (1987)). 
FIG. 5 shows a chevron structure, but unlike the usual smectic liquid 
crystals, the layer tilt angle increases near the S.sub.A to S.sub.C * 
transition temperature, then decreases with a lowering of the temperature. 
This temperature dependence of the layer tilt angle does not coincide with 
the temperature dependence of the molecular tilt angle shown in FIG. 7 and 
this lack of coincidence suggests that the interaction between the 
naphthalene system liquid crystal molecule and the substrate surface is 
strong enough to anchor the liquid crystal molecule near the substrate 
with a very low pretilt. This strongly anchored liquid crystal molecule 
could possibly decrease the layer tilt of the chevron structure to make 
bookshelf structure. 
Another possibility exits in that the molecular axes of the naphthalene 
system liquid crystal are not straight, making the molecular rotation of 
the naphthalene system liquid crystal different from the conventional 
liquid crystal molecular rotation. This rotational difference too might 
induce a bookshelf structure. 
FIG. 6 shows a bookshelf structure. As shown in the FIG. 6, the smectic 
layer is almost perpendicular to the substrates. These layer structures 
made it possible to clarify the relationship between the electro-optical 
properties and the smectic layer structure. FIG. 8 shows that the optical 
switching time is proportional to the applied electric field strength in 
the region where the smectic layer tilt angle is small. When the layer 
tilt is large, i.e., a chevron structure, the optical switching time is 
not proportional to the applied voltage. Therefore, a comparison of FIGS. 
7 and 8 indicates that the obtained bookshelf structure is a almost 
identical to Clark and Lagerwall's model. 
The bistability of the SSFLCD is also obviously related to the layer 
structure, as shown in FIG. 9. In FIG. 9 "memory stability" means the 
ratio between the light transmittance 0.5 seconds after removing the drive 
voltage and the transmittance when the drive voltage is applied. FIG. 9 
shows that a smaller tilt angle produces a more stable memory effect. 
In the course of those investigations, the inventors clarified that the 
bookshelf structure has superior optical switching and memory properties, 
and further clarified that the naphthalene system material gives us a 
bookshelf layer structure with a rubbed polymer film cell. 
In 1980, Clark and Lagerwall reported some advantages of the surface 
stabilized ferroelectric liquid crystal cells with the bookshelf smectic 
layer structure model, and since then much research has been carried out. 
Nevertheless, the bookshelf structure proposed by Clark and Lagerwall with 
rubbed polymer cells has never been reported, and the naphthalene system 
material is the first ferroelectric liquid crystal to realize the 
bookshelf structure with rubbed polymer cells. 
Due to the realization of a bookshelf structure and a low layer tilt 
chevron structure by the naphthalene system liquid crystals, the inventors 
obtained the following four effective items. 
(1) An investigation can be made into the detailed relationship between the 
smectic layer structure and the electro-optical properties. 
(2) An investigation also can be made into the temperature dependence of 
the bookshelf layer structure. 
(3) The zigzag defects-free alignment of the S.sub.C * liquid crystal cells 
can provide a high contrast black and white display. 
(4) A bookshelf smectic layer structure is obtained with a rubbed polymer 
cell and is suitable for mass productions.