Optically anisotropic film

An optically anisotropic film comprising a liquid crystal composition having twisted nematic alignment, characterized in that the helical axis of twisted nematic alignment is substantially parallel to the direction normal to the film plane, and the liquid crystal composition shows a cholesteric phase, contains at least one liquid crystal oligomer selected from the linear or cyclic liquid crystal oligomers (A) such as obtained from reaction of a vinyl monomer having a specific cholesterol group and pentamethylcyclopentasiloxane, and is defined by the fact that when the thickness of the film is represented by d (.mu.m) and the helical pitch of the twisted nematic phase is represented by P (.mu.m), d.gtoreq.3.times.P and P.ltoreq.0.3 .mu.m or P.gtoreq.0.8 .mu.m, wherein when the numbers of the recurring units (I) and (II) in one molecule of the liquid crystal oligomer (A) are supposed to be n and n', respectively, n and n' are each an integer of 1 to 20 and satisfy the relations of 4.ltoreq.n+n'.ltoreq.21 and n: n'=20:1 to 1:20, and the terminal group of the recurring unit (II) of the liquid crystal oligomer (A) is polymerized. This optically anisotropic film is capable of providing a liquid crystal display device with smaller viewing angle dependency of contrast or display color.

BACKGROUND OF THE INVENTION 
The present invention relates to an optically anisotropic film comprising a 
liquid crystal composition which is useful as a member of a retardation 
film used for liquid crystal displays and the like, a process for the 
production thereof, a retardation film comprising said optically 
anisotropic film, and a liquid crystal display device using said film. 
Because of their advantageous features such as low-voltage drive and light 
weight, liquid crystal displays are popularly used for personal computers 
and word processors. Most of the currently available liquid crystal 
displays use nematic liquid crystal and are roughly divided into two types 
according to the working system: birefringence mode type and optical 
rotatory mode type. The liquid crystal displays adopting the birefringence 
mode are mostly of a system using super twisted nematic liquid crystal 
with a twist angle larger than 90.degree. (this type may hereinafter be 
referred to as STN type). STN type liquid crystal displays are relatively 
low in cost and capable of enlarged image display, but they have a 
disadvantage in that as they resort to the birefringence effect for 
display, there takes place yellow or blue tinting of display, making it 
unable to obtain a black-and-white display. 
For realizing black-and-white display with STN type display system, there 
have been proposed a double-layer cell system featuring lamination, on a 
STN cell, of a liquid crystal cell for compensation having the same cell 
gap as the STN cell but opposite thereto in the direction of twist to make 
color compensation (JP-B-63-53528) and a method in which a stretched film 
(retardation film) of polycarbonate or the like is laminated on the STN 
cell to compensate for the retardation. These techniques have partly been 
offered to practical application. However, the double-layer cell system 
has the problems of high cost and heavy and thick panels, while the 
retardation film compensation system, because of use of a uniaxially 
stretched film having no twisted structure, optical rotatory dispersion 
caused by twist of the STN cell is not sufficiently compensated, so that 
this system is incapable of perfect black-and-white display and also 
inferior to the double-layer cell system in contrast. 
As another birefringence mode type display system, there is known a system 
using a cell in which the liquid crystal molecules having negative 
dielectric anisotropy are aligned in the direction orthogonal to the 
substrate (Deformation of Vertically Aligned Phases (DAP) type cell). This 
system, which makes use of a change of retardation by tilting of the 
liquid crystal molecules on application of an electrical voltage, has a 
problem in that because of angle dependency of retardation, the color of 
the displayed image is subject to change according to the viewing angle. 
As a solution to these problems associated with twisted nematic alignment 
of the STN liquid crystal cell, use of a polymer film having twisted 
nematic alignment as a compensation film has been proposed. For producing 
a polymer film having twisted nematic alignment, methods are known in 
which a monomer having a cholesteric phase is polymerized after alignment 
(JP-A-01-222220 and JP-A-03-140921) or a polymeric compound having a 
cholesteric phase is rapidly cooled after alignment to a temperature below 
the glass transition temperature to fix alignment (JP-A-03-87720). 
As an example of the optical rotatory mode type liquid crystal displays, 
there is a type having molecular alignment with 90.degree. twist (this may 
hereinafter be referred to as TN type). This type of liquid crystal 
display has been embodied, for example, in a panel in which each pixel is 
driven by a thin-film transistor or a diode. This TN type still has the 
problem that the image contrast or color is subject to change according to 
the viewing angle as in the case of the birefringence mode type. 
The above problems of the TN type liquid crystal displays are ascribed to 
anisotropy of refractive index of liquid crystal molecules. For minimizing 
angle dependence of contrast or display color of the liquid crystal 
displays due to anisotropy of refractive index of the liquid crystal 
molecules, methods are studied in which a cholesteric liquid crystal cell 
having negative anisotropy of refractive index is placed on a TN cell 
(JP-A-4-346312) or a polymer liquid crystal showing a cholesteric phase is 
rapidly cooled after alignment to the glass transition temperature or 
below to fix alignment (JP-A-5-61039). 
The method using a cholesteric liquid crystal cell for compensation, 
however, is at a disadvantage in that the panel becomes heavy and thick as 
a whole, resulting in an elevated production cost. Also, the method 
involving polymerization of monomers has the problem that since 
temperature dependence of alignment of the monomers is higher than that of 
the polymeric compounds, it is necessary to carry out polymerization 
immediately after the alignment treatment at the same temperature, 
therefore the producing conditions are restricted. Further, in the case of 
the method using a polymeric compound having a cholesteric phase, since 
fixation of the cholesteric phase structure is effected by rapid cooling 
to or below the glass transition temperature, it is necessary to use a 
compound whose glass transition temperature is well higher than room 
temperature, so that a high temperature is required for the alignment 
treatment, which is unfavorable in terms of producing operation and 
economy. Moreover, in case a polymeric compound is used, since its tem- 
perature dependence of refractive anisotropy is lower than the liquid 
crystal, there is produced a difference between refractive anisotropy of 
the compensation film and that of the liquid crystal cell with change of 
the temperature at which the panel is used, resulting in a reduced effect 
of improving viewing angle characteristics. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an optically anisotropic 
film having fixed twisted nematic alignment which contributes to improving 
contrast of STN cell over a wide temperature range; and an optically 
anisotropic film having negative anisotropy of refractive index which can 
compensate viewing angle dependence of contrast and color of TN cell, due 
to its anisotropy of refractive index, over a wide temperature range; and 
an industrial production process thereof; a laminate film comprising said 
optically anisotropic film; and liquid crystal display devices using said 
optically anisotropic film and/or said laminate film, which reduce viewing 
angle dependence of contrast or display color. 
Efforts by the present inventors for eliminating the above prior art 
problems led to a finding that a film formed from a liquid crystal 
composition showing a cholesteric phase and containing a polymeric liquid 
crystal oligomer having a specific structure by properly aligning the 
molecules of the composition and then fixing the twisted nematic structure 
of the composition by polymerization is effective for reducing viewing 
angle dependence of contrast or display color over a wide temperature 
range. 
It was further found that according to the process of the present invention 
there can be obtained an optically anisotropic film containing a 
polymerized liquid crystal oligomer and having twisted nematic alignment 
on a substrate which has been subjected to a horizontal alignment 
treatment, and by using this film, it is possible to obtain a liquid 
crystal display device showing small viewing angle dependence of contrast 
or display color over a wide temperature range. The present invention has 
been attained on the basis of the above finding. 
In accordance with the present invention, there is provided an optically 
anisotropic film comprising a liquid crystal composition having twisted 
nematic alignment, characterized in that the helical axis of twisted 
nematic alignment is substantially parallel to the direction normal to the 
film plane, and said liquid crystal composition shows a cholesteric phase 
and contains at least one liquid crystal oligomer selected from the linear 
or cyclic liquid crystal oligomers (A) having the following recurring 
units (I) and (II) as main structural units, wherein when the numbers of 
the recurring units (I) and (II) in one molecule of said oligomer (A) are 
supposed to be n and n', respectively, n and n' are independently an 
integer of 1 to 20 and satisfy the relations of 4.ltoreq.n+n'.ltoreq.21 
and n: n'=20:1 to 1:20, and the terminal group of the recurring unit (II) 
of the liquid crystal oligomer (A) is polymerized: 
##STR1## 
wherein A is a group represented by the following formula (III) or (IV): 
##STR2## 
wherein, in the formula (III), --Si--O-- is the main chain of the formula 
(I) or (II) and, in the formula (IV), --C--CH.sub.2 -- is the main chain 
of the formula (I) or (II) and COO group is positioned in the side chain 
which is neither R.sub.1 nor R.sub.2 ; when A in the formula (I) is the 
formula (III) and when A in the formula (II) is the formula (III), R.sub.1 
and R.sub.2 represent independently hydrogen, a C.sub.1 -C.sub.6 alkyl 
group or a phenyl group, and when A in the formula (I) is the formula (IV) 
and when A in the formula (II) is the formula (IV), R.sub.1 and R.sub.2 
represent independently hydrogen or a C.sub.1 -C.sub.6 alkyl group; k and 
k' are independently an integer of 2 to 10; m and m' are independently 0 
or 1; Ar.sub.1, Ar.sub.2 and Ar.sub.3 are independently a 1,4-phenylene 
group, a 1,4-cyclohexylene group, a pyridine-2,5-diyl group or a 
pyrimidine-2,5-diyl group; L' is --CH.sub.2 --O--, --O--CH.sub.2 --, 
--COO--, --OCO--, --CH.sub.2 --CH.sub.2 --, --CH.dbd.N--, --N.dbd.CH-- or 
a divalent group represented by the formula: 
##STR3## 
p' is 0 or 1; R is an optically active group; and R' is hydrogen or a 
C.sub.1 -C.sub.5 alkyl group. 
The present invention also provides an optically anisotropic film 
comprising a liquid crystal composition having twisted nematic alignment, 
characterized in that the helical axis of twisted nematic alignment is 
substantially parallel to the direction normal to the film plane, and said 
liquid crystal composition shows a cholesteric phase and contains at least 
one liquid crystal oligomer selected from the liquid crystal oligomers (A) 
mentioned above and at least one low-molecular weight compound having a 
polymerizable group, wherein the terminal group of the recurring unit (II) 
of the liquid crystal oligomer and/or the low-molecular weight compound 
having a polymerizable group are polymerized. 
The present invention further provides an optically anisotropic film 
comprising a liquid crystal composition having twisted nematic alignment, 
characterized in that the helical axis of twisted nematic alignment is 
substantially parallel to the direction normal to the film plane, and said 
liquid crystal composition shows a cholesteric phase and contains at least 
one liquid crystal oligomer selected from the liquid crystal oligomers (A) 
mentioned above and at least one liquid crystal oligomer selected from the 
liquid crystal oligomers (B) other than (A), said liquid crystal oligomers 
(B) being selected from the linear or cyclic liquid crystal oligomers 
having the following recurring units (V) and (VI) as main structural 
units, wherein when the numbers of the recurring units (V) and (VI) in one 
molecule of said liquid crystal oligomer (B) are supposed to be n" and 
n'", respectively, n" and n'" are independently an integer of 1 to 20 and 
satisfy the relation of 4.ltoreq.n"+n'".ltoreq.21, and the terminal group 
of the recurring unit (II) of the liquid crystal oligomer (A) and/or the 
terminal group of the recurring unit (VI) of the liquid crystal oligomer 
(B) are polymerized: 
##STR4## 
wherein A, R.sub.1, R.sub.2, k', m', L', Ar.sub.2 , Ar.sub.3 and R' 
represent the same as defined above; k" is an integer of 2 to 10; m" is 0 
or 1; Ar.sub.4 and Ar.sub.5 are independently a 1,4-phenylene group, a 
1,4-cyclohexylene group, a pyridine-2,5-diyl group or a 
pyrimidine-2,5-diyl group; L" is --CH.sub.2 --O--, --O--CH.sub.2 --, 
--COO--, --OCO--, --CH.sub.2 --CH.sub.2 --, --CH.dbd.N--, --N.dbd.CH or a 
divalent group represented by the formula: 
##STR5## 
p" is 0 or 1; and R" is a halogen, a cyano group, a C.sub.1 -C.sub.10 
alkyl group, a C.sub.1 -C.sub.10 alkoxyl group, a C.sub.6 -C.sub.10 aryl 
group or a benzoyl group having a C.sub.1 -C.sub.10 alkyl group or a 
C.sub.1 -C.sub.10 alkoxyl group. 
The present invention further provides an optically anisotropic film 
comprising a liquid crystal composition having twisted nematic alignment, 
characterized in that the helical axis of twisted nematic alignment is 
substantially parallel to the direction normal to the film plante, and 
said liquid crystal composition shows a cholesteric phase and contains at 
least one liquid crystal oligomer selected from the above-mentioned liquid 
crystal oligomers (A), at least one liquid crystal oligomer selected from 
the above-mentioned liquid crystal oligomers (B) and at least one 
low-molecular weight compound having a polymerizable group, wherein the 
terminal group of the recurring unit (II) of the liquid crystal oligomer 
(A) and/or the terminal group of the recurring unit (VI) of the liquid 
crystal oligomer (B) and/or the low-molecular weight compound having a 
polymerizable group are polymerized. 
The present invention also provides a process for producing an optically 
anisotropic film which comprises forming a film of the liquid crystal 
composition described above, heat treating the film so that the helical 
axis of twisted nematic alignment will become substantially parallel to 
the direction normal to the film plane, and then polymerizing the 
polymerizable group of said compound. 
The present invention additionally provides a laminate of an optically 
anisotropic film and a transparent semitransparent substrate having an 
alignment layer on its surface. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention comprehends the following embodiments in its claimed 
scope. 
1! An optically anisotropic film comprising a liquid crystal composition 
having twisted nematic alignment, characterized in that the helical axis 
of twisted nematic alignment is substantially parallel to the direction 
normal to the film plane, and said liquid crystal composition shows a 
cholesteric phase and contains at least one liquid crystal oligomer 
selected from the linear or cyclic liquid crystal oligomers (A) having the 
following recurring units (I) and (II) as main structural units, wherein 
when the numbers of the recurring units (I) and (II) in one molecule of 
the liquid crystal oligomer (A) are supposed to be n and n', respectively, 
n and n' are independently an integer of 1 to 20 and satisfy the relations 
of 4.ltoreq.n+n'.ltoreq.21 and n: n'=20:1 to 1:20, and the terminal group 
of the recurring unit (II) is polymerized: 
##STR6## 
wherein A is a group represented by the following formula (III) or (IV): 
##STR7## 
wherein, in the formula (III), --Si--O-- is a main chain of the formula 
(I) or (II) and, in the formula (IV), --C--CH.sub.2 -- is a main chain of 
the formula (I) or (II), and COO group is positioned in the side chain 
which is neither R.sub.1 nor R.sub.2 ; when A in the formula (I) is the 
formula (III) and when A in the formula (II) is the formula (III), R.sub.1 
and R.sub.2 are independently hydrogen, a C.sub.1 -C.sub.6 alkyl group or 
a phenyl group, and when A in the formula (I) is the formula (IV) and when 
A in the formula (II) is the formula (IV), R.sub.1 and R.sub.2 are 
independently hydrogen or a C.sub.1 -C.sub.6 alkyl group; k and k' are 
independently an integer of 2 to 10; m and m' are independently 0 or 1; 
Ar.sub.1, Ar.sub.2 and Ar.sub.3 are independently a 1,4-phenylene group, a 
1,4-cyclohexylene group, a pyridine-2,5-diyl group or a 
pyrimidine-2,5-diyl group; L' is --CH.sub.2 --O--, --O--CH.sub.2 --, 
--COO--, --OCO--, --CH.sub.2 --CH.sub.2 --, --CH.dbd.N--, --N.dbd.CH-- or 
a divalent group represented by the formula: 
##STR8## 
p' is 0 or 1; R is an optically active group; and R' is hydrogen or a 
C.sub.1 -C.sub.5 alkyl group. 
2! An optically anisotropic film set forth in 1!, wherein said liquid 
crystal composition showing a cholesteric phase is the one in which when 
the thickness of said optically anisotropic film is represented by d 
(.mu.m) and the helical pitch of the choleseteric phase is represented by 
P (.mu.m), d.gtoreq.3.times.P and P.ltoreq.0.3 .mu.m or P.gtoreq.0.8 
.mu.m. 
3! An optically anisotropic film set forth in 1!, wherein the twist angle 
of twisted nematic alignment of said liquid crystal composition falls in a 
range of 70.degree.-300.degree., and said liquid crystal composition shows 
a cholesteric phase with a helical pitch of 0.2 -50 .mu.m. 
4! An optically anisotropic film comprising a liquid crystal composition 
having twisted nematic alignment, characterized in that the helical axis 
of twisted nematic alignment is substantially parallel to the direction 
normal to the film plane, and said liquid crystal composition shows a 
cholesteric phase and contains at least one liquid crystal oligomer 
selected from the liquid crystal oligomers (A) set forth in 1! and at 
least one low-molecular weight compound having a polymerizable group, 
wherein the terminal group of the recurring unit (II) of the liquid 
crystal oligomer (A) and/or the low-molecular weight compound having a 
polymerizable group are polymerized. 
5! An optically anisotropic film set forth in 4!, wherein said liquid 
crystal composition is the one in which when the thickness of said 
optically anisotropic film is represented by d (.mu.m) and the helical 
pitch of the cholesteric phase is represented by P (.mu.m), 
d.gtoreq.3.times.p and P.ltoreq.0.3 .mu.m or P.ltoreq.0.8 .mu.m. 
6! An optically anisotropic film set forth in 4!, characterized in that 
the twist angle of twisted nematic alignment of said liquid crystal 
composition falls in a range of 70.degree.-300.degree., and the helical 
pitch of said liquid crystal composition is 0.2-50 .mu.m. 
7! An optically anisotropic film comprising a liquid crystal composition 
having twisted nematic alignment, characterized in that the helical axis 
of twisted nematic alignment is substantially parallel to the direction 
normal to the film plane, and said liquid crystal composition shows a 
cholesteric phase and contains at least one liquid crystal oligomer 
selected from the above-mentioned liquid crystal oligomers (A) and at 
least one liquid crystal oligomer selected from the liquid crystal 
oligomers (B) other than (A), said liquid oligomers (B) being selected 
from the linear or cyclic liquid crystal oligomers having the following 
recurring units (V) and (VI) as main structural units, wherein when the 
numbers of the recurring units (V) and (VI) in one molecule of said liquid 
crystal oligomer (B) are supposed to be n" and n'", respectively, n" and 
n'" are independently an integer of 0 to 20 and satisfy the relation of 
4.ltoreq.n"+n'".gtoreq.21, and the terminal group of the recurring unit 
(II) of the liquid crystal oligomer (A) and/or the terminal group of the 
recurring unit (VI) of the liquid crystal oligomer (B) are polymerized: 
##STR9## 
wherein A, R.sub.1, R.sub.2, k', m', p', L', Ar.sub.2 , Ar.sub.3 and R' 
represent the same as defined above; k" is an integer of 2 to 10; m" is 0 
or 1; Ar.sub.4 and Ar.sub.5 are independently a 1,4-phenylene group, a 
1,4-cyclohexylene group, a pyridine-2,5-diyl group or a 
pyrimidine-2,5-diyl group; L" is --CH.sub.2 --O--,--0--CH.sub.2 --, 
--COO--, --OCO--, --CH.sub.2 --CH.sub.2 --, --CH.dbd.N--, --N.dbd.CH--or a 
divalent group represented by the formula: 
##STR10## 
p" is 0 or 1; R" is a halogen, a cyano group, a C.sub.1 -C.sub.10 alkyl 
group, a C.sub.1 -C.sub.10 alkoxyl group, a C.sub.6 -C.sub.10 aryl group 
or a benzoyl group having a C.sub.1 -C.sub.10 alkyl group or a C.sub.1 
-C.sub.10 alkoxyl group. 
8! An optically anisotropic film set forth in 7!, wherein the liquid 
crystal composition showing a cholesteric phase is characterized by the 
fact that when the thickness of the optically anisotropic film is 
represented by d (.mu.m) and the helical pitch of the colesteric phase is 
represented by P (.mu.m), d.gtoreq.3.times.P and P.ltoreq.0.3 .mu.m or 
P.gtoreq.0.8 .mu.m. 9! An optically anisotropic film set forth in 
7!, wherein the twist angle of twisted nematic alignment of the liquid 
crystal composition showing a cholesteric phase falls in a range of 
70.degree.-300.degree., and said liquid crystal composition shows a 
cholesteric phase whose helical pitch is 0.2-50 .mu.m. 
10! An optically anisotropic film comprising a liquid crystal composition 
having twisted nematic alignment, characterized in that the helical axis 
of twisted nematic alignment is substantially parallel to the direction 
normal to the film plane, and said liquid crystal compsotion contains at 
least one liquid crystal oligomer selected from the liquid crystal 
oligomers (A) set forth in 1!, at least one liquid crystal oligomer 
selected from the liquid crystal oligomers (B) set forth in 7! and at 
least one low-molecular weight compound having a polymerizable group, 
wherein the terminal group of the recurring unit (II) of the liquid 
crystal oligomer (A) and/or the terminal group of the recurring unit (VI) 
of the liquid crystal oligomer (B) and/or the low-molecular weight 
compound having a polymerizable group are polymerized. 
11! An optically anisotropic film set forth in 10!, wherein the liquid 
crystal composition showing a cholesteric phase is characterized by the 
fact that when the thickness of said optically anisotropic film is 
represented by d (.mu.m) and the helical pitch of the cholesteric phase is 
represented by P (.mu.m), d.gtoreq.3.times.P and P.ltoreq.0.3 .mu.m or 
P.gtoreq.0.8 .mu.m. 12! An optically anisotropic film set forth in 10!, 
wherein the twist angle of twisted nematic alignment of the liquid crystal 
composition showing a cholesteric phase falls in a range of 
70.degree.-300.degree., and the helical pitch of said liquid crystal 
composition is 0.2 -50 .mu.m. 
13! An optically anisotropic film set forth in any of 1! to 12!, 
comprising a liquid crystal oligomer having the specified recurring units 
in which R is represented the following formula (VII) or (VIII): 
##STR11## 
wherein R.sub.3 represents --H or the following formula (IX): 
##STR12## 
wherein R.sub.4 represents --H or a methyl group, and R.sub.5 represents 
--H or R.sub.6 (R.sub.6 is a linear or branched C.sub.1 -C.sub.20 alkyl 
group or a linear or branched C.sub.1 -C.sub.20 alkoxy-carbonyl group and 
may have asymmetric carbon when branched). 
14! A process for producing an optically anisotropic tropic film, which 
comprises forming a film of a liquid crystal composition set forth in any 
of 1! to 13!, heat treating the film so that the helical axis of twisted 
nematic alignment will become substantially parallel to the direction 
normal to the film plane, and then polymerizing the polymerizable group of 
said low-molecular weight compound and/or liquid crystal oligomer (A) 
and/or (B). 
15! A laminate of an optically anisotropic film set forth in any of 1! to 
13! and a transparent or semitransparent substrate having an alignment 
layer on its surface. 
16! A laminate set forth in 15!, wherein the substrate is a glass plate 
or a polymer film. 
17! A laminate set forth in 15!, wherein the apparent refractive index of 
the laminate is defined by the following formula (1): 
EQU nX.gtoreq.nY&gt;nZ (1) 
wherein nX and nY represent the maximum value and the minimum value, 
respectively, of the in-plane refractive index of the laminate, and nZ 
represents the refractive index in the thickness direction of the 
laminate. 
18! A liquid crystal display comprising a liquid crystal cell comprising a 
liquid crystal layer held between a pair of substrates provided with 
electrodes, said liquid crystal layer having positive dielectric 
anisotropy and oriented substantially horizontally with a helical axis 
aligned to the substrate when no electrical voltage is applied, at least 
one polarizing film provided on either upper or lower outside, or both 
outsides of said liquid crystal cell, and an optically anisotropic film 
set forth in any of 1! to 13! or a laminate of said optically 
anisotropic film and a substrate set forth in any of 15! to 17!, which 
is disposed between said liquid crystal cell and said polarizing film. 
19! A liquid crystal display comprising a liquid crystal cell comprising a 
liquid crystal layer held between a pair of substrates provided with 
electrodes, said liquid crystal layer having positive dielectric 
anisotropy and oriented substantially horizontally to the substrate when 
no electrical voltage is applied, at least one polarizing film provided on 
either upper or lower outside, or both outsides of said liquid crystal 
cell, and an optically anisotropic film set forth in 2!, 5!, 8!, 11! 
or 13! or a laminate set forth in any of 15! to 17!, which is disposed 
between said liquid crystal cell and said polarizing film. 
20! A liquid crystal display comprising a liquid crystal cell comprising a 
liquid crystal layer held between a pair of substrates provided with 
electrodes, said liquid crystal layer having negative dielectric 
anisotropy and oriented substantially vertically to the substrate when no 
electrical voltage is applied, at least one polarizing film provided on 
either upper or lower outside, or both outsides of said liquid crystal 
cell, and an optically anisotropic film set forth in 2!, 5!, 8!, 11! 
or 13! or a laminate set forth in any of 15! to 17!, which is disposed 
between said liquid crystal cell and said polarizing film. 
21! A liquid crystal display device set forth in 18!, wherein the liquid 
crystal cell is a TN cell with twisted orientation, said optically 
anisotropic film is at least one member selected from those set forth in 
2!, 5!, 8!, 11! and 13!, and said laminate is at least one member 
selected from those set forth in 15! to 17!. 
22! A liquid crystal display device set forth in 18!, wherein the liquid 
crystal cell is a STN cell with twisted orientation, said optically 
anisotropic film is at least one member selected from those set forth in 
3!, 6!, 9!, 12! and 13!, and said laminate is at least one member 
selected from those set forth in 15! to 17!. 
The present invention is described in detail below. 
The liquid crystal oligomer (A) used in the present invention is a side 
chain type liquid crystal oligomer having recurring units (I) and (II). 
The back-bone of the side chain type liquid crystal oligomer is 
constituted by, for example, a poly-l-alkylacrylic acid ester or a 
polysiloxane. Such an oligomer may be of a linear-chain or cyclic 
structure, but the cyclic structure is preferred because of better 
chemical stability. Preferred examples of poly-1-alkylacrylic acid esters 
usable for said purpose are polymethacrylic acid esters and polyacrylic 
acid esters, the former being more preferable. Among these side chain type 
liquid crystal oligomers, those of the polysiloxane basis are preferred. 
There is generally used one in which the group closely associated with 
liquid crystalline property (which group may hereinafter be referred to as 
mesogen group) is bonded to the backbone through a flexible chain (which 
may hereinafter be referred to as spacer). 
The length of the spacer, the type of mesogen group and the degree of 
polymerization of the side chain type liquid crystal oligomers (A) are 
preferably so selected that the transition temperature from liquid crystal 
phase to isotropic phase (which may hereinafter be referred to as liquid 
crystal phase/isotropic phase transition temperature) will become 
200.degree. C. or below, preferably 170.degree. C. or below, more 
preferably 150.degree. C. or below, for facilitating drying with the 
substrate or orientation treatment, although the upper limit temperature 
showing the liquid crystal phase is not specifically defined. The 
transition temperature from crystal phase or glass phase to liquid crystal 
phase of these liquid crystal oligomers is not defined; such transition 
may be carried out at a temperature lower than room temperature. 
The side chain type liquid crystal oligomer used in the present invention 
needs to be oriented to have anisotropy of refractive index, and the 
number of the structural recurring units of the liquid crystal oligomer 
(A) is an important factor for facilitation of the orienting operation. 
When the number of the recurring units is large, the viscosity of the 
oligomer and its liquid crystal transition temperature elevate, 
necessitating a high temperature and a long time for effecting desired 
orientation. When the number of the recurring units is small, orientation 
tends to relax under a room temperature condition. Accordingly, the 
numbers n and n' of the recurring units of the liquid crystal oligomer 
used in the present invention are each independently an integer of 1 to 
20, and they are selected so that n+n'=4 to 21. In view of orientation 
characteristics and fixation of orientation after polymerization, the n:n' 
ratio should fall in a range of 20:1 to 1:20, preferably 5:1 to 1:20. The 
n/n' ratio may be properly adjusted when synthesizing the liquid crystal 
oligomer as described later. 
The liquid crystal transition temperature and orientation characteristics 
of the side chain type liquid crystal oligomer (A) are also affected by 
the spacer connecting the mesogen group to the backbone. Too short a 
spacer deteriorates the orientation characteristics of mesogen group while 
too long spacer tends to cause relaxation of orientation. Therefore, as 
spacer, alkylene group or alkyleneoxy group with a carbon number of 2 to 
10 is preferred. C.sub.2 -C.sub.6 alkylene or alkyleneoxy group is 
especially preferred because of easier orientation. For facilitation of 
synthesis, alkyleneoxy group is more preferred. Typical examples of the 
preferred groups are --(CH.sub.2).sub.2 --, --(CH.sub.2).sub.3 --, 
--(CH.sub.2).sub.4 --, --(CH.sub.2).sub.5 --, --(CH.sub.2).sub.6 --, 
--(CH.sub.2).sub.3 --O--, --(CH.sub.2).sub.4 --O--, --(CH.sub.2).sub.5 
--O--and --(CH.sub.2).sub.6 --O--. 
It is industrially advantageous that the optically anisotropic film of the 
present invention has large anisotropy of refractive index, and for this 
reason, mesogen group used in this invention is preferably one which has 
large anisotropy of refractive index. The structures which can provide 
such mesogen group include those of the oligomers composed of the 
recurring units (I) and (II) wherein Ar.sub.1, At.sub.2 and Ar.sub.3 
represent independently 1,4-phenylene group, 1,4-cyclohexylene group, 
pyridine-2,5-diyl group or pyrimidine-2,5-diyl group. More preferably, 
Ar.sub.1, Ar.sub.2 and Ar.sub.3 are independently 1,4-phenylene group, 
pyridine-2,5-diyl group or pyrimidine-2,5-diyl group, most preferably 
1,4-phenylene group. 
The group R in the recurring unit (I) contributes to the development of 
cholesteric phase, so that it is essential that the group R be an 
optically active group, and in view of stabilization of cholesteric phase, 
a group having the following structure is preferred: 
##STR13## 
wherein R.sub.3 represents --H or the following formula: 
##STR14## 
wherein R.sub.4 represents --H or a methyl group, and R.sub.5 represents 
--H or R.sub.6 (R.sub.6 represents a linear or branched C.sub.1 -C.sub.20 
alkyl group or a linear or branched C.sub.1 -C.sub.20 alkoxycarbonyl 
group, and may have asymmetric carbon when branched). 
Examples of the mesogen groups usable for the linear or cyclic liquid 
crystal oligomers of the recurring units (I) are shown in Tables 1 and 2. 
In Table 1, number 1, for example, in the column of "Spacer" indicates that 
in the recurring unit (I), --Ar.sub.1 --R is 
##STR15## 
R.sub.3 is 
##STR16## 
and the spacer --(CH.sub.2).sub.k --(O).sub.m -- is --(CH.sub.2).sub.3 -- 
group (corresponding to the case of k=3 and m=0 in the formula (I)). Other 
numbers in the respective tables have the similar representations. 
3 TABLE 1 
- Spacer 
Ar.sub.1 R R.sub.3 (CH.sub.2).sub.3 (CH.sub.2).sub.4 (CH.sub.2).sub.5 
(CH.sub.2).sub.3 O (CH.sub.2).sub.4 O (CH.sub.2).sub.5 
O 
##STR17## 
##STR18## 
1 2 3 4 5 
6 
" 
##STR19## 
7 8 
9 10 11 12 
" 
##STR20## 
13 14 15 16 17 18 
" 
##STR21## 
19 20 21 22 23 24 
" 
##STR22## 
25 26 27 28 29 30 
3 TABLE 2 
- Spacer 
Ar.sub.1 R R.sub.3 (CH.sub.2).sub.3 (CH.sub.2).sub.4 (CH.sub.2).sub.5 
(CH.sub.2).sub.3 O (CH.sub.2).sub.4 O (CH.sub.2).sub.5 
O 
##STR23## 
##STR24## 
31 32 33 34 35 39 
" 
##STR25## 
37 38 39 40 41 42 
" 
##STR26## 
43 44 45 46 47 48 
" 
##STR27## 
49 50 51 52 53 54 
" 
##STR28## 
55 56 57 58 59 60 
Among these recurring unit structures, those of Nos.1-6 and 31-36 are 
preferred. 
Examples of the divalent groups L' connecting Ar.sub.2 and Ar.sub.3 in the 
recurring unit (II) are --CH.sub.2 --O--, --O--CH.sub.2 --, --COO--, 
--OCO--, --CH.sub.2 --CH.sub.2 --, --CH.dbd.N--, --N.dbd.CH--, a group 
represented by the formula: 
##STR29## 
and a group in which Ar.sub.2 and Ar.sub.3 are directly bonded 
(corresponding to the case of p'=0 in the formula (II)). Preferably the 
connecting group L' is --CH.sub.2 --CH.sub.2 --, --COO--or --OCO--, more 
preferably --COO-- group. 
The terminal group of the recurring unit (II) is a group for fixing 
orientation of the liquid crystal oligomer by polymerization. 
Polymerizable groups usable in this invention are those of the formula 
--OCO--C(R').dbd.CH.sub.2 (R' is hydrogen or C.sub.1 -C.sub.5 alkyl 
group), which include acrylate groups and methacrylate groups. The 
polymerization method of these groups is not specified, but usually 
photopolymerization or thermal polymerization using a radical 
polymerization initiator is employed. Photopolymerization is preferred for 
simplicity of operation and high orientation fixing efficiency. Known 
photopolymerization initiators can be used. 
Examples of the polymerizable mesogen groups usable for the linear or 
cyclic liquid crystal oligomers composed of the recurring units (II) are 
shown in Tables 3 and 4. 
TABLE 3 
__________________________________________________________________________ 
Spacer 
Ar.sub.2 (L').sub.p' Ar.sub.3 
R' (CH.sub.2).sub.3 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
(CH.sub.2).sub.3 O 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
__________________________________________________________________________ 
O 
##STR30## H 61 62 63 64 65 66 
" CH.sub.3 
67 68 69 70 71 72 
##STR31## H 73 74 75 76 77 78 
" CH.sub.3 
79 80 81 82 83 84 
##STR32## H 85 86 87 88 89 90 
" CH.sub.3 
91 92 93 94 95 96 
##STR33## H 97 98 99 100 101 102 
" CH.sub.3 
103 104 105 106 107 108 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
Spacer 
Ar.sub.2 (L').sub.p' Ar.sub.3 
R' (CH.sub.2).sub.3 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
(CH.sub.2).sub.3 O 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
__________________________________________________________________________ 
O 
##STR34## H 109 110 111 112 113 114 
" CH.sub.3 
115 116 117 118 119 120 
##STR35## H 121 122 123 124 125 126 
" CH.sub.3 
127 128 129 130 131 132 
##STR36## H 133 134 135 136 137 138 
" CH.sub.3 
139 140 141 142 143 144 
##STR37## H 145 146 147 148 149 150 
" CH.sub.3 
151 152 153 154 155 156 
__________________________________________________________________________ 
Among these polymerizable mesogen groups, those of Nos. 67-72, 79-84, 
91-96, 139-144 and 151-156 are preferred. Those of Nos. 79-84 are 
especially preferred. Of these mesogen groups, those bonded to the linear 
or cyclic polysiloxane-based backbone are preferred for high orientation 
performance. Those bonded to the cyclic polysiloxane backbone are most 
preferred. 
In the optically anisotropic film of the present invention, in case it has 
negative anisotropy of refractive index which is effectual for 
compensating viewing angle dependence of contrast or display color of TN 
cell due to refractive anisotropy of the liquid crystal molecules in a 
wide temperature range, it is necessary to adjust the helical pitch length 
of the cholesteric phase of the liquid crystal composition to 0.3 .mu.m or 
less or to 0.8 .mu.m or more in order to avoid selective reflection of 
visible light. 
For improving contrast by compensating optical rotatory dispersion due to 
the twisted structure of the liquid crystal cell, the twist angle of 
twisted nematic alignment of the film must be adjusted to stay in a range 
of 70.degree.-300.degree.. For obtaining a film having such a twist angle, 
it is recommended to adjust the helical pitch length of the cholesteric 
phase of the liquid crystal composition to a range of 0.2-50 .mu.m, 
preferably 1-35 .mu.m, more preferably 2-20 .mu.m. When the helical pitch 
length of the cholesteric phase is small, it is necessary to reduce the 
film thickness for adjusting the twist angle of twisted nematic alignment 
to be in a range of 70.degree.-300.degree., which makes it hard to obtain 
a desired retardation. When the pitch length is large, it needs to 
increase the film thickness for realizing a required twist angle, which 
poses an economical problem. 
The helical pitch length of the cholesteric phase of the liquid crystal 
composition can be controlled by adjusting the mesogen group structure of 
the liquid crystal oligomer (A) or by changing the recurring unit (I)/(II) 
ratio. 
For the synthesis of the liquid crystal oligomers (A), the methods 
disclosed in U.S. Pat. Nos. 4,410,570, 4,358,391 and 5,211,877 can be 
employed. More specifically, there can be used a method in which said side 
chain mesogen group is added to the polysiloxane backbone, or a method in 
which an acrylic acid ester or methacrylic acid ester having a mesogen 
group through a flexible spacer is polymerized. 
In case of adding mesogen group to the polysiloxane backbone, the reacting 
material having the same structure as the side chain mesogen group of the 
recurring units (I) and (II) and having .omega.-alkenyloxy group producing 
an alkyleneoxy group (spacer) and having unsaturated double bond at the 
terminal is reacted with polysiloxane in the presence of a platinum 
catalyst. In this reaction, it is possible to control the bonding ratio of 
the two types of mesogen groups, i.e., non-polymerizable mesogen groups 
and polymerizable mesogen groups, by adjusting the feed rate of the 
reacting material relative to said mesogen groups. Similarly, for those 
oligomers in which the backbone is an acrylic acid ester or an 
.alpha.-alkylacrylic acid ester, the ratio of the polymerizable mesogen 
groups to the non-polymerizable mesogen groups can be controlled by 
adjusting the monomer feed rate when two types of monomers having the 
corresponding mesogen groups are copolymerized. 
The liquid crystal oligomer obtained in the manner described above is 
preferably one which shows a cholesteric phase. 
Another method of controlling the helical pitch length of the liquid 
crystal composition comprises mixing in the liquid crystal oligomer (A) 
other types of liquid crystal oligomer or a low-molecular weight compound. 
As the other types of liquid crystal oligomer to be mixed with the 
oligomer (A), there can be used, in addition to those selected from the 
liquid crystal oligomers (A), the liquid crystal oligomers (B) having the 
recurring units (V) and (VI) as main structural units. 
The side chain type liquid crystal oligomer used in the present invention 
needs to be oriented so that it will have anisotropy of refractive index, 
and in this case the number of the recurring units of the liquid crystal 
oligomer (B) becomes a key factor in facilitating the orienting operation. 
When the number of the recurring units is large, viscosity of the oligomer 
and its liquid crystal transition temperature are high, necessitating a 
high temperature and a long time for effecting desired orientation. When 
the number of the recurring units is small, orientation may be relaxed 
under a room temperature condition. The numbers n" and n'" of the 
recurring units (V) and (VI) are each independently an integer of 0 to 20, 
and they are selected so that n"+n'"=4 to 21. The n"/n'" ratio may be 
properly selected and can be adjusted when synthesizing the liquid crystal 
oligomer. 
The liquid crystal transition temperature and orientation characteristics 
of the side chain type liquid crystal oligomer (B) are also affected by 
the spacer connecting the mesogen group to the backbone. Too short a 
spacer deteriorates the orientation characteristics of mesogen group while 
too long spacer tends to cause relaxation of orientation. Therefore, as 
spacer, alkylene group or alkyleneoxy group with a carbon number of 2 to 
10 is preferred. C.sub.2 -C.sub.6 alkylene or alkyleneoxy group is 
especially preferred because of easier orientation. For facilitation of 
synthesis, alkyleneoxy group is more preferred. Typical examples of the 
preferred groups are --(CH.sub.2).sub.2 --, --(CH.sub.2).sub.3 --, 
--(CH.sub.2).sub.4 --, --(CH.sub.2).sub.5 --, --(CH.sub.2).sub.6 --, 
--(CH.sub.2).sub.3 --O--, --(CH.sub.2).sub.4 --O--, --(CH.sub.2).sub.5 
--O--and --(CH.sub.2).sub.6 --O--. 
For maximizing anisotropy of refractive index of the obtained optically 
anisotropic film, mesogen group of the liquid crystal oligomer (B) is 
preferably one which has large anisotropy of refractiver index. The 
structures which can provide such mesogen group include those of the 
oligomers composed of the recurring units (V) and (VI) wherein Ar.sub.2, 
Ar.sub.3, Ar.sub.4 and Ar.sub.5 represent independently 1,4-phenylene 
group, 1,4-cyclohexylene group, pyridine-2,5-diyl group or 
pyrimidine-2,5-diyl group. Preferably they represent independently, 
4-phenylene group, pyridine-2,5-diyl group or pyrimidine-2,5-diyl group, 
most preferably 1,4-phenylene group. 
The divalent groups L' and L" connecting Ar.sub.2 -Ar.sub.3 and Ar.sub.4 
-Ar.sub.5 in the formulae of recurring units (V) and (VI) include 
--CH.sub.2 --O--,--O--CH.sub.2 --, --COO--, --OCO--, --CH.sub.2 --CH.sub.2 
--, --CH.dbd.N--, --N.dbd.CH--, the groups represented by the formula: 
##STR38## 
and the groups in which Ar.sub.2 and Ar.sub.3 or Ar.sub.4 and Ar.sub.5 are 
directly bonded. Preferably the connecting groups L' and L" are 
independently --CH.sub.2 --CH.sub.2 --, --COO--or --OCO--, more preferably 
--COO-- group. 
The group R" in the recurring unit (V) influences dielectric anisotropy and 
orientation performance of mesogen group, so that R" is selected from 
halogen, cyano group, C.sub.1 -C.sub.10 alkyl group, C.sub.1 -C.sub.10 
alkoxy group, C.sub.6 -C.sub.10 aryl group and benzoyloxy group having 
C.sub.1 -C.sub.10 alkyl group or C.sub.1 -C.sub.10 alkoxy group, 
preferably cyano group, C.sub.1 -C.sub.10 alkyl group and C.sub.1 
-C.sub.10 alkoxy group, more preferably cyano group, for obtaining a 
liquid crystal oligomer film with high anisotropy of refractive index. 
Examples of the mesogen groups usable for the linear or cyclic liquid 
crystal oligomers having the recurring units (V) are shown in Tables 5, 6, 
7 and 8. 
TABLE 5 
__________________________________________________________________________ 
Spacer 
Ar.sub.4 (L").sub.p" Ar.sub.5 
R" (CH.sub.2).sub.3 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
(CH.sub.2).sub.3 O 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
__________________________________________________________________________ 
O 
##STR39## CN 157 158 159 160 161 162 
" OCH.sub.3 
163 164 165 166 167 168 
" OC.sub.2 H.sub.5 
169 170 171 172 173 174 
" C.sub.6 H.sub.13 
175 176 177 178 179 180 
" CH.sub.3 
181 182 183 184 185 186 
##STR40## CN 187 188 189 190 191 192 
" OCH.sub.3 
193 194 195 196 197 198 
" OC.sub.2 H.sub.5 
199 200 201 202 203 204 
" C.sub.6 H.sub.13 
205 206 207 208 209 210 
" CH.sub.3 
211 212 213 214 215 216 
__________________________________________________________________________ 
TABLE 6 
__________________________________________________________________________ 
Spacer 
Ar.sub.4 (L").sub.p" Ar.sub.5 
R" (CH.sub.2).sub.3 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
(CH.sub.2).sub.3 O 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
__________________________________________________________________________ 
O 
##STR41## CN 217 218 219 220 221 222 
" OCH.sub.3 
223 224 225 226 227 228 
" OC.sub.2 H.sub.5 
229 230 231 232 233 234 
" C.sub.6 H.sub.13 
235 236 237 238 239 240 
" CH.sub.3 
241 242 243 244 245 246 
##STR42## CN 247 248 249 250 251 252 
" OCH.sub.3 
253 254 255 256 257 258 
" OC.sub.2 H.sub.5 
259 260 261 262 263 264 
" C.sub.6 H.sub.13 
265 266 267 268 269 270 
" CH.sub.3 
271 272 273 274 275 276 
__________________________________________________________________________ 
TABLE 7 
__________________________________________________________________________ 
Spacer 
Ar.sub.4 (L").sub.p" Ar.sub.5 
R" (CH.sub.2).sub.3 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
(CH.sub.2).sub.3 O 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
__________________________________________________________________________ 
O 
##STR43## CN 277 278 279 280 281 282 
" OCH.sub.3 
283 284 285 286 287 288 
" OC.sub.2 H.sub.5 
289 290 291 292 293 294 
" C.sub.6 H.sub.13 
295 296 297 298 299 300 
" CH.sub.3 
301 302 303 304 305 306 
##STR44## CN 307 308 309 310 311 312 
" OCH.sub.3 
213 314 315 316 317 318 
" OC.sub.2 H.sub.5 
319 320 321 322 323 324 
" C.sub.6 H.sub.13 
325 326 327 328 329 330 
" CH.sub.3 
331 332 333 334 335 336 
__________________________________________________________________________ 
TABLE 8 
__________________________________________________________________________ 
Spacer 
Ar.sub.4 (L").sub.p" Ar.sub.5 
R" (CH.sub.2).sub.3 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
(CH.sub.2).sub.3 O 
(CH.sub.2).sub.4 
(CH.sub.2).sub.5 
__________________________________________________________________________ 
O 
##STR45## CN 337 338 339 340 341 342 
" OCH.sub.3 
343 344 345 346 347 348 
" OC.sub.2 H.sub.5 
349 350 351 352 353 354 
" C.sub.6 H.sub.13 
355 356 357 358 359 360 
" CH.sub.3 
361 362 363 364 365 366 
##STR46## CN 367 368 369 370 371 372 
" OCH.sub.3 
373 374 375 376 377 378 
" OC.sub.2 H.sub.5 
379 380 381 382 383 384 
" C.sub.6 H.sub.13 
385 386 387 388 389 390 
" CH.sub.3 
391 392 393 394 395 396 
__________________________________________________________________________ 
Among these mesogen groups, those of Nos. 157-162, 187-192, 217-222, 
337-342 and 367-372 having cyano group are preferred. Those of Nos. 
187-192 are especially preferred. Of these mesogen groups, those bonded to 
the polysiloxane-based backbone are preferred for high orientation 
performance. Those bonded to the cyclic siloxane backbone are most 
preferred. 
The terminal group of the recurring unit (VI) is a group for fixing 
orientation of the liquid crystal oligomer by polymerization. 
Polymerizable groups usable in this invention are those of the formula 
--OCO--C(R').dbd.CH.sub.2 (R' is hydrogen or C.sub.1 -C.sub.5 alkyl 
group), which include acrylate groups and methacrylate groups. The 
polymerization method of these groups is not specified, but usually 
photopolymerization or thermal polymerization using a radical 
polymerization initiator is employed. Photopolymerization is preferred for 
ease of operation and high orientation fixing efficiency. Known 
photopolymerization initiators can be used. 
As the polymerizable mesogen groups used for the linear or cyclic liquid 
crystal oligomers having the recurring units (VI), those shown in Tables 3 
and 4 can be employed. 
Among the polymerizable mesogen groups shown in Tables 3 and 4, those of 
Nos. 67-72, 79-84, 91-96, 139-144 and 151-156 having methacrylate group 
are preferred, and those of Nos. 79-84 are especially preferred. Of these 
mesogen groups, those bonded to the linear or cyclic polysiloxane-based 
backbone are preferred as they give good properties to the subject 
oligomers, and those bonded to the cyclic polysiloxane backbone are 
especially preferred. 
For the synthesis of these liquid crystal oligomers (B), there can be 
employed the same methods as used for the synthesis of liquid crystal 
oligomers (A) mentioned above. 
The liquid crystal oligomer (B) obtained in the manner described above is 
preferably one which shows the nematic phase. In case the helical pitch 
length of the cholesteric phase is controlled by mixing a low-molecular 
weight compound with the liquid crystal oligomer (A) or a mixture of 
liquid crystal oligomers (A) and (B), the liquid crystal composition is 
polymerized after orientation, so that compatibility after polymerization 
becomes an important factor for the successful production of said film. 
For this reason, the low-molecular weight compound used in this process is 
preferably one having a polymerizable group. It is also desirable that the 
polymerizable low-molecular weight compound to be added to the liquid 
crystal oligomer is of a structure which shows a liquid crystal phase for 
uniform mixing. 
Some examples of the polymerizable low-molecular weight compounds usable in 
the present invention are shown below by structural formula. It should be 
noted that these compounds are shown merely for the purpose of 
exemplification and not intended to limit the compounds usable in this 
invention. 
##STR47## 
In the above formulae, a is an integer of 1 to 10, b is an integer of 0 to 
10, and R is a linear or branched C.sub.1 -C.sub.10 alkyl or alkoxy group. 
The group R may be optically active when branched. 
The amount of the low-molecular weight compound added is preferably 0 to 50 
wt%, more preferably 0 to 40 wt%, based on the liquid crystal composition. 
When the compound is added in excess of 50 wt%, film forming properties of 
the composition and/or orientation stability may be impaired. 
The process for producing an optically anisotropic film according to the 
present invention comprises the steps of forming a film of a liquid 
crystal composition, heat treating the formed film so that the helical 
axis of twisted nematic alignment will become substantially parallel to 
the direction normal to the film plane, and then polymerizing the 
polymerizable group of low-molecular weight compound and/or liquid crystal 
oligomers (A) and/or (B). The method of forming a film of a liquid crystal 
composition is not critical in this invention, but usually a film is 
formed on a smooth substrate. The optically anisotropic film formed on the 
substrate may be peeled from the substrate and attached to another 
substrate or it may be used with the substrate attached thereto. The 
substrate used in the present invention is preferably transparent or 
semitransparent, and the usable substrates include inorganic substrates 
such as glass, and polymer films. Inorganic substrates include transparent 
or semitransparent glass plates, outer side of glass plate used for liquid 
crystal cell, and plates of inorganic compounds such as oxides or 
fluorides of Si, Al, Mg, Zr or the like, and ceramics. 
Polymer films include the films of polycarbonates, polysulfone, 
polyarylates, polyether sulfone, cellulose diacetate, cellulose 
triacetate, polystyrene, ethylene-vinyl alcohol copolymer, polyethylene 
terephthalate, polyethylene naphthalate and the like. Of these polymers, 
polycarbonates, polyarylates, polysulfone, cellulose triacetate, 
polyethylene terephthalate and polystyrene are preferred. 
Thickness of the polymer film used as substrate is not specified, but it is 
preferably 0.8-500 .mu.m, more preferably 10-300 .mu.m, even more 
preferably 40-200 .mu.m. 
For producing such a polymer film, various molding methods such as solvent 
casting, extrusion molding, press molding, etc., can be employed. 
In case of using a polymer film as substrate, it is recommended to select 
appropriate one in consideration of the heat treatment temperature so as 
to avoid the troubles that tend to occur in carrying out the heat 
treatment of the liquid crystal oligomer described later, such as 
deformation of the substrate which may take place when the heat treatment 
temperature exceeds the glass transition temperature of the substrate used 
or the flow temperature of the substrate in case the substrate contains 
additives. 
In production of the optically anisotropic film according to the present 
invention, the liquid crystal composition is oriented so that the helical 
axis of twisted nematic alignment will become substantially parallel to 
the direction normal to the film plane. For realizing such twisted nematic 
alignment, usually a horizontal alignment treatment is applied on the 
substrate surface. 
Known methods such as rubbing or oblique evaporation can be applied for 
horizontal alignment treatment, rubbing being preferred for industrial 
practice of the treatment. 
When rubbing is employed, the substrate with no liquid crystal oligomer 
film formed yet thereon is directly rubbed or rubbing is applied after an 
alignment layer has formed on the substrate. The latter method is 
preferred for stability of alignment. 
Known materials capable of horizontally aligning the liquid crystal 
molecules, such as polyimides, polyamides and polyvinyl alcohol, can be 
used for forming the alignment layer. 
The alignment layer can be formed by employing a suitable coating method 
such as roll coating, gravure coating, bar coating, spin coating, spray 
coating, printing, dipping, etc. As for the thickness of the alignment 
layer, usually the layer can perform its normal aligning function when its 
thickness is 0.01 .mu.m or greater, but the layer workability is 
deteriorated when the layer is too thick, so that the layer thickness is 
preferably in a range of 0.01 to 5.0 .mu.m, more preferably 0.02 to 3.0 
.mu.m. 
The thus formed alignment layer is subjected to after-treatments such as 
drying and curing according to the type of the layer and then rubbed. A 
known method can be employed for rubbing in this invention. For instance, 
in case of using a rubbing roller, no specific restrictions are imposed on 
the material of the roller, amount of penetration of the roller into the 
layer, moving rate of the roller relative to the substrate, frequency of 
rubbing, etc., and the optimal conditions can be selected in consideration 
of the type of the alignment layer, the type of the liquid crystal 
oligomer and other factors. 
For oblique evaporation, there can be used, for instance, a method using an 
oblique deposit of an inorganic material. The inorganic material used in 
this method is preferably one which shows a prismatic or a columnar growth 
during evaporation. Examples of such inorganic materials include SiO, 
SiO.sub.2, SiO.sub.x (1&lt;x&lt;2), MgO, MgOy (0&lt;y&lt;1), MgF.sub.2, Pt, ZnO, 
MOO.sub.3, WO.sub.3, Ta.sub.2 O.sub.5, SnO.sub.2, CeO.sub.2, LiNbO.sub.3, 
LiTaO.sub.3, ZrO.sub.2, Bi.sub.2 O.sub.3, TiZrO.sub.4, HfO.sub.2 and the 
like. Of these materials, SiO, SiO.sub.2, SiOx (1&lt;x&lt;2), MgO, MgOy (0&lt;y&lt;1), 
MgF.sub.2, Pt and ZnO are preferred, and SiO, SiO.sub.2 and SiOx (1&lt;x&lt;2) 
are most preferred. 
There are available various methods for oblique evaporation such as 
evaporation by resistance heating, evaporation by electron beam heating 
and sputtering. Of these methods, evaporation by electron beam heating and 
sputtering are preferred for evaporating a high-melting point inorganic 
material. The degree of vacuum for evaporation is not specified, but the 
upper limit pressure is decided in view of uniformity of the evaporation 
film while the lower limit pressure is decided in view of productivity. 
Specifically, the degree of vacuum used for evaporation is usually in a 
range of 1 Torr to 5.times.10.sup.-6 Torr. 
The evaporation rate of the inorganic material is preferably in a range of 
0.01 to 10 nm/sec, more preferably 0.1 to 5 nm/sec, because an evaporation 
rate below the above-defined range leads to a poor productivity while an 
evaporation rate above said range is detrimental to uniformity of the 
evaporation film. 
As for the thickness of the evaporated film of inorganic material, good 
alignment can not be obtained when the film thickness is thin and 
productivity is lowered when the film thickness is thick, so that the film 
thickness is usually in a range of 0.01 to 1,000 .mu.m, preferably 0.05 to 
100 .mu.m, more preferably 0.1 to 5 .mu.m. 
Then a film of a liquid crystal composition is formed on the substrate 
which has undergone said horizontal alignment treatment. The film can be 
formed by coating a liquid crystal composition on the substrate in the 
state of solution or in the state of isotropic phase. Coating in the state 
of solution is preferred. Ordinary coating methods such as roll coating, 
gravure coating, bar coating, spin coating, spray coating, printing, 
dipping, etc., can be used. 
Thickness of the liquid crystal composition film is preferably 0.1 to 20 
.mu.m, more preferably 0.5 to 10 .mu.m, even more preferably 1 to 7 .mu.m. 
When the film thickness is less than 0.1 .mu.m, the film may fail to 
develop its optical properties to a satisfactory degree. A film thickness 
exceeding 20 .mu.m is undesirable in economical terms. 
The film thickness also needs to be set in consideration of the twist angle 
required for an optically anisotropic film and the helical pitch length of 
the liquid crystal composition used. In an optically anisotropic film 
having negative anisotropy of refractive index effective for compensating 
angle dependence of contrast or display color due to refractive anisotropy 
of the liquid crystal cell over a wide temperature range, it is desirable 
that the film thickness is more than 3 times, preferably more than 5 
times, even more preferably more than 10 times the helical pitch length of 
cholesteric phase of the liquid crystal composition. When the film 
thickness is small, there may undesirably take place leakage of light in 
the case of placing a sample between orthogonally positioned polarizing 
plates due to optical rotation of the cholesteric orientation. This leads 
to a reduction of contrast when the film is adapted in a liquid crystal 
cell. In an optically anisotropic film designed to improve contrast by 
compensating optical rotatory dispersion due to the twisted structure of 
the STN liquid crystal cell, it is desirable to make the film thickness 
smaller than the cholesteric helical pitch. 
Then a heat treatment of liquid crystal composition is carried out. This 
heat treatment is preferably conducted in a temperature range defined by 
(Tg+30).ltoreq.T.sub.heat .ltoreq.(T.sub.soft -30), preferably 
(Tg+40).ltoreq.T.sub.heat .ltoreq.(T.sub.soft -40) wherein T.sub.heat 
(.degree. C.) is heat treatment temperature, Tg (.degree. C.) is 
transition temperature from crystal phase or glass phase to liquid crystal 
phase of the liquid crystal composition, and T.sub.soft (.degree. C.) is 
the temperature which causes deformation of the substrate or the alignment 
film. Specifically, the heat treatment is preferably carried out in a 
temperature range of 60.degree.-200.degree.C. in view of ease of the 
operation. 
The period of heat treatment is also not critical, but since too short a 
time can not provide sufficient twisted nematic alignment while too long a 
time is unfavorable in economical terms, the heat treatment time is 
preferably 0.2 minutes to 20 hours, more preferably one minute to one 
hour. 
The above heat treatment allows the liquid crystal composition to have 
twisted nematic alignment with the helical axis disposed parallel to the 
direction normal to the film plane. The heating and cooling rates in the 
heat treatment are not specified. 
Next, the liquid crystal composition is polymerized for fixing twisted 
nematic alignment. As the polymerization method, since it is necessary to 
carry out polymerization while maintaining said alignment, 
photopolymerization, radiation polymerization using .gamma.-rays, etc., or 
thermal polymerization is recommended. Known polymerization initiators can 
be used for photopolymerization and thermal polymerization. Of these 
polymerization methods, photopolymerization and thermal polymerization are 
preferred because of simple process, photopolymerization being most 
preferred for good retention of alignment. 
The intensity of irradiation light applied for photopolymerization may be 
properly decided depending on the film thickness and the type of the 
liquid crystal oligomer used, but usually it is recommended to apply light 
of an intensity of 0.01 to 5.0 J/cm.sup.2, preferably 0.1 to 3.0 
J/cm.sup.2. When the applied light intensity is below 0.01 J/cm.sup.2, the 
reaction of the polymerizable group may be incomplete. Use of light with a 
higher intensity than 5.0 J/cm.sup.2 poses an economical problem. 
In a liquid crystal display device provided according to the present 
invention, at least one optically anisotropic film of the present 
invention, or a laminate of said optically anisotropic film and a 
substrate, is disposed between a liquid crystal cell comprising a liquid 
crystal layer held between a pair of substrates provided with electrodes, 
said liquid crystal layer including liquid crystal molecules having 
positive anisotropy of dielectric constant and oriented substantially 
horizontally with a helical axis aligned vertically to the substrate when 
no electrical voltage is applied, and at least one polarizing film 
disposed on either upper or lower outside, or both outsides of said cell. 
Examples of the liquid crystal cell comprising a liquid crystal layer 
oriented substantially horizontally with a helical axis aligned vertically 
to the substrate when no electrical voltage is applied, which can be 
adapted in a liquid crystal display device of the present invention, are a 
TN type liquid crystal cell with a twist angle of approximately 90.degree. 
and an STN type liquid crystal cell with a twist angle of 
180.degree.-300.degree.. The twist angle of the liquid crystal cell can be 
adjusted by the amount of the twisting agent added to the liquid crystal 
composition or by the alignment treatment of the upper and lower 
substrates. In the liquid crystal display device of the present invention, 
the way of disposition of the polarizing plate and the optically 
anisotropic film of the present invention is not specified, and they can 
be appropriately disposed according to the required product properties. 
Also, in the liquid display device of the present invention, said 
optically anisotropic film may be disposed on one side alone of the liquid 
crystal cell or on both sides thereof as far as the film is positioned 
between the polarizing plate and the liquid crystal cell. 
In another liquid crystal display device provided according to the present 
invention, at least one optically anisotropic film of the present 
invention, or a laminate of said optically anisotropic film and a 
substrate, is disposed between a liquid crystal cell comprising a liquid 
crystal layer held between a pair of substrates provided with electrodes, 
said liquid crystal layer including liquid crystal molecules having 
positive anisotropy of dielectric constant and oriented substantially 
horizontally to the substrate when no electrical voltage is applied, and 
at least one polarizing film disposed on either upper or lower outside, or 
both outsides of said cell. Examples of the liquid crystal cell comprising 
a liquid crystal layer having positive anisotropy of dielectric constant 
and oriented substantially horizontally to the substrate when no 
electrical voltage is applied, which can be adapted in a liquid crystal 
device of the present invention. In the liquid crystal display device of 
the present invention, the way of disposition of the polarizing plate and 
the optically anisotropic film of the present invention is not specified, 
and they can be appropriately disposed in accordance with the required 
product properties. Also, in said liquid crystal display device, the 
optically anisotropic film may be provided on one side alone of the liquid 
crystal cell or on both sides thereof as far as the film is positioned 
between the polarizing plate and the liquid crystal cell. 
In still another liquid crystal display device provided according to the 
present invention, at least one optically anisotropic film of the present 
invention, or a laminate of said optically anisotropic film and a 
substrate, is disposed between a liquid crystal cell comprising a liquid 
crystal layer held between a pair of substrates provided with electrodes, 
said liquid crystal layer including liquid crystal molecules having 
negative anisotropy of dielectric constant and oriented substantially 
vertically to the substrate when no electrical voltage is applied, and at 
least one polarizing film disposed on either upper or lower outside, or 
both outsides of said cell. The liquid crystal cell comprising a liquid 
crystal layer held between a pair of substrates provided with electrodes, 
having negative anisotropy of dielectric constant and oriented 
substantially vertically to the substrate when no electrical voltage is 
applied, which can be adapted in the liquid crystal display device of the 
present invention, can be produced by placing the liquid crystal 
composition between a pair of substrates which have been subjected to a 
vertical alignment treatment. In the liquid crystal display device of the 
present invention, the way of disposition of the polarizing film and the 
optically anisotropic film of the present invention is not specified, and 
they can be appropriately disposed according to the required product 
properties. Also, in said liquid crystal display device, said optically 
anisotropic film may be provided on one side alone of the liquid crystal 
cell or on both sides thereof as far as the film is positioned between the 
polarizing plate and the liquid crystal cell.

The present invention is explained in more detail in the following 
Examples; however, it should be recognized that the scope of the present 
invention is not restricted to these Examples. 
The phase transition temperature of the liquid crystal oligomers was 
determined by polarization microscopical observation and measurement by a 
differential scanning calorimeter (DSC). 
Negative anisotropy of refractive index of the obtained optically 
anisotropic film was confirmed by zero retardation in the plane vertical 
to the normal line of the film and increase of retardation with slanting 
of the film from the horizontal plane. 
Retardation of the film was determined by Senarmont's method using a 
polarization microscope with light of 546 nm. 
The twist angle of the obtained optically anisotropic film was determined 
by applying linearly polarized light to the optically anisotropic film and 
determining the direction of oscillation of the emitted linearly polarized 
light (using MCPD-1000 mfd. by Otsuka Denshi KK). The phase difference 
.DELTA.n.multidot.d (.DELTA.n is apparent anisotropy of refractive index 
of the film and d is film thickness)! of the optically anisotropic film 
was determined by analyzing the transmitted light spectrum obtained by 
using a cell gap measuring device (TFM-120AFT mfd. by Orc Seisakusho 
Ltd.). 
EXAMPLE 1 
A polyimide alignment layer was formed on a cleaned glass substrate by spin 
coating and heat treated at 200.degree. C. for 3 hours. The formed 
alignment layer was approximately 0.02 .mu.m thick. This alignment layer 
was rubbed by using a rubbing machine. 
A 7: 3 mixture of the vinyl monomers of the following formulae (1) and (2) 
was reacted with pentamethylcyclopentasiloxane in the same way as 
described in JP-B-63-41400 to obtain a cyclic pentasiloxane liquid crystal 
oligomer: 
##STR48## 
The glass transition temperature and the liquid crystal phase/isotropic 
phase transition temperature of the obtained liquid crystal oligomer were 
14.degree. C. and 114.degree. C, respectively, and the oligomer showed a 
cholesteric phase in the temperature range of 14-114C. As a result of 
extrapolation from the measured value of selective reflection wavelength 
of the composition consisting of said liquid crystal oligomer and nematic 
liquid crystal, the selective reflection wavelength of the liquid crystal 
oligomer alone was determined to be 280 nm. From this wavelength, the 
helical pitch of the cholesteric phase of said liquid crystal oligomer is 
given as 0.2 .mu.m. 
This liquid crystal oligomer was dissolved in toluene to a concentration of 
40%, and then Irgacure 907 (produced by Ciba Geigy AG) was mixed in the 
solution as photopolymerization initiator in an amount of 2.0 wt% based on 
the liquid crystal oligomer. This solution was spin coated on a glass 
substrate having a polyimide alignment layer. The obtained liquid crystal 
oligomer film was cloudy and polarization microscopical observation 
thereof showed that the film was not aligned at all. 
This liquid crystal oligomer film was heated at 80.degree. C. for 5 minutes 
and then irradiated with ultraviolet light from a high pressure mercury 
lamp at a cumulative irradiation dose of 0.2 J/cm.sup.2. 
Thickness of the thus obtained polymerized liquid crystal oligomer film was 
6 .mu.m, which is 30 times the helical pitch of the cholesteric phase, and 
no tinting by selective reflection of the visible light was observed. This 
polymerized liquid crystal oligomer film became optically extinct when 
placed under crossed nicols, and retardation was almost zero. Retardation 
after a tilt of 50.degree. was 46 nm. 
EXAMPLE 2 
A polyvinyl alcohol alignment layer was formed on a cleaned glass substrate 
by spin coating and heat treated at 100.degree. C. for one hour. The 
obtained alignment layer was approximately 0.05 .mu.m thick. This 
alignment layer was rubbed by using a rubbing machine. 
A vinyl monomer of the following formula (3) was reacted with 
pentamethylcyclopentasiloxane in the same way as described in 
JP-B-63-41400 to obtain a cyclic pentasiloxane liquid crystal oligomer: 
##STR49## 
The obtained liquid crystal oligomer had a glass transition temperature of 
20.degree. C. and a liquid crystal phase/isotropic phase transition 
temperature of 97.degree. C. and showed the nematic phase at 
20.degree.-97.degree.C. 
A 5:95 (by weight) mixture of this liquid crystal oligomer and the liquid 
crystal oligomer used in Example 1 was dissolved in toluene to a solid 
concentration of 30 wt%, and then Irgacure 907 (produced by Ciba Geigy AG) 
was mixed in the solution as photopolymerization initiator in an amount of 
2.0 wt% based on the liquid crystal oligomer. This solution was spin 
coated on a glass substrate having a polyvinyl alcohol alignment layer. 
Polarization microscopical observation of the obtained liquid crystal 
oligomer confirmed that the film had the cholesteric phase. Also, this 
liquid crystal oligomer film was cloudy, and when observed under a 
polarization microscope, it was found that the film was not aligned at 
all. 
This liquid crystal oligomer film was gradually cooled from the temperature 
at which the film assumed the isotropic phase, and after alignment, the 
film was irradiated with ultraviolet light from a high pressure mercury 
lamp at a cumulative irradiation dose of 0.2 J/cm.sup.2. 
The obtained polymerized liquid crystal oligomer film was 3 .mu.m thick, 
and no tinting by selective reflection of the visible light was observed. 
This polymerized liquid crystal oligomer film became optically extinct 
when placed under crossed nicols, and retardation was substantially zero. 
It showed retardation of 21 nm when tilted 50.degree.. 
EXAMPLE 3 
A polyvinyl alcohol alignment layer was formed on a cleaned glass substrate 
by spin coating and heat treated at 100.degree. C. for one hour. The 
obtained alignment layer was approximately 0.05 .mu.m thick. This 
alignment layer was rubbed by a rubbing machine. 
A 1:1 mixture of the vinyl monomer (2) used in Example 1 and the vinyl 
monomer (3) used in Example 2 was reacted with 
pentamethylcyclopentasiloxane in the same manner as described in 
JP-B-63-41400 to obtain a cyclic pentasiloxane liquid crystal oligomer. 
The glass transition temperature and the liquid crystal phase/isotropic 
phase transition temperature of this liquid crystal oligomer were 
19.degree. C. and 118.degree. C., respectively, and it showed the smectic 
phase at 19.degree.-118.degree. C. 
A 10:90 (by weight) mixture of this liquid crystal oligomer and the liquid 
crystal oligomer used in Example 1 was dissolved in toluene to a solid 
concentration of 30 wt%, and then Irgacure 907 (produced by Ciba Geigy AG) 
was mixed in the solution as photopolymerization initiator in an amount of 
2.0 wt% based on the liquid crystal oligomer. This solution was spin 
coated on a glass substrate having a polyvinyl alcohol alignment layer. 
Polarization microscopical observation of the obtained liquid crystal 
oligomer film confirmed cholesteric phase of the film. Also, this liquid 
crystal oligomer film was cloudy, and when observed under a polarization 
microscope, the film was found not aligned at all. 
Then this liquid crystal oligomer film was gradually cooled from the 
temperature at which the film assumed isotropic phase, and after 
alignment, the film was irradiated with ultraviolet light from a high 
pressure mercury lamp at a cumulative irradiation dose of 0.2 J/cm.sup.2. 
The obtained polymerized liquid crystal oligomer film was 4 .mu.m thick, 
and no tinting by selective reflection of the visible light was observed. 
This polymerized liquid crystal oligomer film became optically extinct 
when placed under crossed nicols, and retardation was substantially zero. 
The film showed retardation of 15 nm when tilted 50.degree.. 
A TN liquid crystal panel with small viewing angle dependence of contrast 
can be obtained by placing an optically anisotropic film obtained in 
Examples 1-3 between a TN liquid crystal cell and a polarizing film. 
EXAMPLE 4 
A polyvinyl alcohol alignment layer was formed on a cleaned glass substrate 
by spin coating and heat treated at 100.degree. C. for one hour. Thickness 
of the obtained alignment layer was approximately 0.05 .mu.m. This 
alignment layer was rubbed by a rubbing machine. 
A 3:97 (by weight) mixture of the cyclic pentasiloxane liquid crystal 
oligomer (A) obtained in Example 1 and the cyclic pentasiloxane liquid 
crystal oligomer (B) obtained in Example 2 was dissolved in toluene to a 
solid concentration of 30 wt%, and then Irgacure 907 (produced by Ciba 
Geigy AG) was mixed in the solution as photopolymerization initiator in an 
amount of 2.0 wt% based on the liquid crystal oligomer. This solution was 
spin coated on a glass substrate having a polyvinyl alcohol alignment 
layer. 
Polarization microscopical observation of the obtained liquid crystal 
oligomer film confirmed that the film assumed cholesteric phase. Also, 
this liquid crystal oligomer film was cloudy, and when observed under a 
polarization microscope, the film was not aligned at all. 
This liquid crystal oligomer film was heated to the temperature at which 
the film assumed isotropic phase, and then gradually cooled for alignment. 
Thereafter, the film was irradiated with ultraviolet light from a high 
pressure mercury lamp at a cumulative irradiation dose of 0.2 J/cm.sup.2. 
Thickness of the obtained polymerized liquid crystal oligomer film was 3.0 
m, its twist angle was 243.degree., and .DELTA.n.multidot.d at room 
temperature was 0.748 .mu.m. 
.DELTA.n.multidot.d of this optically anisotropic film measured at 
55.degree. C. was 0.646 .mu.m, which was 86% of that at room temperature. 
EXAMPLE 5 
The procedure of Example 4 was carried out except that the liquid crystal 
oligomer (A) : (B) mixing ratio (by weight) was changed to 4:96 to produce 
a liquid crystal oligomer film. Thickness of the obtained polymerized 
liquid crystal oligomer film was 3.4 .mu.m, its twist angle was 
220.degree. and .DELTA.n.multidot.d at room temperature was 0.980 .mu.m. 
.DELTA.n.multidot.d of the obtained optically anisotropic film at 
55.degree. C. was 0.906 .mu.m, which was 92% of that at room temperature. 
Comparative Example 1 
Temperature dependency of .DELTA.n.multidot.d of a uniaxially oriented 
retardation film having the optical axis in the film plane and composed of 
a polycarbonate having positive anisotropy of refractive index 
(Sumikalight SEF-400426 mfd. by Sumitomo Chemical Co., Ltd.; 
.DELTA.n.multidot.d=380 nm; viewing angle: 39.degree.) was measured. There 
was observed substantially no change between .DELTA.n.multidot.d at room 
temperature and that at 55.degree. C. 
EXAMPLE 6 
The procedure of Example 4 was followed except the liquid crystal oligomer 
(A):(B) ratio (by weight) was changed to 5:95 to obtain a liquid crystal 
oligomer film. Thickness of the obtained polymerized liquid crystal 
oligomer film was 2.8 .mu.m, its twist angle was 213.degree. and 
.DELTA.n.multidot.d at room temperature was 0.925 .mu.m. 
An STN liquid crystal cell having excellent viewing angle characteristics 
and contrast can be obtained by placing said polymerized liquid crystal 
oligomer film between an STN liquid crystal cell and a polarizing plate. 
Since temperature dependence of .DELTA.n.multidot.d of this optically 
anisotropic film is similar to that of STN liquid crystal cell, it is 
possible to improve the viewing angle characteristics and contrast at high 
temperatures. 
According to the present invention, it is possible to obtain a retardation 
plate having a wide viewing angle by using an optically anisotropic film 
having negative anisotropy of refractive index or twisted nematic 
alignment. 
Also, by applying this retardation plate to TN type or STN type liquid 
crystal displays, it is possible to markedly improve the display 
characteristics, especially viewing angle characteristics, of the liquid 
crystal display devices. Further, in the optically anisotropic film of the 
present invention, because of similarity to liquid crystal panel in 
temperature dependence of refractive anisotropy, it is possible to improve 
the viewing angle characteristics of the liquid crystal cell over a wide 
temperature range.