Note: Descriptions are shown in the official language in which they were submitted.
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This is a divisional application of Canadian patent
application Serial No. 400,415 filed April 2, 1982 and assigned
to Minnesota Mining and Manufacturing Company.
FIELD OF INVENTION
The invention pertains to thermally addressable
~hoL~steric-smeetic li~uid crystal display devices, and more
par~L~ularly to thermally addressed visual display devices which
use a licJht absorption technique to provide a dark image upon a
lighter background.
1~ sACKGRO~ID OF T~IE INVENTION
Heretofore, the ability to fabricate large scale multi-
plaxed liquid crystal displays was very difficult. The difficulty
was primarily due to "cross-talk" effects, and the necessity to
quicky refresh the slowly responding liquid crystal medium. Large
seale multiplexed displays notoriously have had problems with
"cross-talk", i.e., the unwanted sensitizing of partially
seleeted display elements. This problem results ~rom the small
root mean square voltage ratio between the "on" and "off" elements
aehievable in a large scale multiplexed liquid cxystal display.
As displays become larger, a new problem appears. Most
device effects do not have intrinsic storage. The display must
therefore be repeatedly scanned to update; this is often with
typical display effects done at 60 ~z (per frame). The result for
large area matrices is a small duty cycle for each individual row
or column. Most display media only partially respond to small
duty cycle voltage information and the resulting effect is only a
fraction of the dc equivalent voltage. The result is low contrast
or brightness. As the display matrix gets larger, the duty
cycle gets less and less and optical performance gets poorer and
poorer. The result is a very poo.r (below commercial standards)
optical performance as the x-Y matrix gets larger and larger.
These two problems have severly limited the ability
to provide large scale multiplexed displays, and to
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date, no one has produced a device which has high contrast,
wide viewing angle, which is easy -to fabricate, easy to
operate, and which has low cost.
The present invention has developed a low cost,
large scale multiplexed, visual display device that has
resolved -the aforemen-tioned problems, while providing a new
lLquid crystal device having many advantages over -the prior
art.
Whi:Le the presen-t invention is concerned
prlmarily wi-th large scale, thermally addressed multiplexed
devices, its new ligh-t absorbing rnethod is easily
applicable to devices which are not large scale, and which
do not utilize multiplexing. The subject invention is
believed to have wide application in the field of thermally
addressed liquid crystal displays, and is not considered as
being limited to any particular device or system.
DISCUSSION OF RELATED ART
The invention features certain classes of smectic
liquid crystal hosts tha-t have a cholesteric phase upon
heating. A small percentage of pleochroic dye is added to
the material. The display is addressed in a thermal
electric mode. For the "on" elements, the liquid crystal
texture is light absorbing due to the dye which strongly
absorbs incoming light. The "off" elements and the
background have homeotropic smectic A texture, where the
dye exhibits minimum absorption.
The concept of pleochroic dye switching as the
Guest Host effect in nematic liquid crystals, was first
suggested in an article toO G.H. Heilmeier, J.A.
Castellano, and L.A. Zanoni, Mol. Crystals and Liquid
Crystals 8, 293 (1969).
Others have suggested that -the liquid crystal
structure can be twisted nematic, homogeneous, or
homeotropic. Most of these devices using pleochroic dyes
mixed with the liquid crystal material have generally
required external devices such as polarizers or wave plates
to improve the contras-t of the image.
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Dyes of high order parameter in a choles-teric
liquid crystal host were firs-t suggested in an ar-cicle to:
D.L. White, G.N. Taylor, J. of App Phys. 45 4718 (1974).
Displays using this liquid crystal medium have
high con-trast and do not require external polarizers.
These displays have high brigh-tness and a wave viewing
an~le not available with -the field effect twis-t nematic
:LLq~lid crystal d:isplays. Such devices use a cholesteric to
l~tnat:ic transition effec-t with liquid crystal displays.
tO Such clevices ~lse a cholesteric to nematic transition effect
w:i-th l:iquid crystals of positive dielectric anisotropy.
In the no field (off) mode, the dye molecules
follow the helical structure of the host and exhibit strong
light absorp-tion. In the on condition, the dye is in a
homeo-tropic nematic host and the absorption is minimized.
Thus, the display presents a white image against a dark (or
colored) background. A white image against a dark
background is, however, generally not desirable. In
addi-tion, it has been well reported that such a cholesteric
to nematic transition effect cannot be multiplexed above
approximately 5-10 lines and give commercial performance.
This is due to the change in the slope of the contras-t
versus voltage relationship that causes "cross-talk".
Recently, a paper was presented in the 8th
International Liquid Crystal Conference at Kyoto, Japan by
Professor A. Sasaki et al., entitled "Laser Addressed
Liquid Crystal Multifunction Light Valve"; in which he
described a laser addressed projection display utilizing a
liquid crystal of 90:10 mixture of p-p' cyano-octyl biphenyl
and cholesteryl nonanoate. The mixture should have a
cholesteric phase followed by a smectic A phase upon
cooling. However, the display is a projection device -that
derives its image contrast purely through scattering. The
thermal addressing is by a scanning laser beam. No dyes
are used in his material.
Recently, high order parame-ter and light stable
dyes have become available. Devices using these dyes will
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provide viab]e displays for many applications. However,
they have two major drawbacks which may restrict their
application to simple displays of very low information
content only.
S These dye displays are very difficul-t to
mul-tiplex. Even a few rows represen-t a s-tate of the art
development. Large size matrix addressing has been
achieved only by adding external non-linear elemen-ts -to
each display element.
E~'or non-emissive (reflec-tive) displays, a white
image against a dark background is formed. This is
esthetically undesirable and of limited commercial utility.
Techniques to reverse the image contrast to a more pleasing
dark against a light background are available, but the
added complica-tion increases the complexity and cost.
In 1978 C. Tani and I. Ueno discussed the
application of pleochroic dyes to certain smectic liquid
crystals in a scientific paper (Appl. Phys. Let-t. Vol. 33
No. ~, 15 Aug 1978). The authors, however, specifically
teach against the use of the smectic "A" phase as having
utility in the pleochoric dye system: they indicate that i-t
has application only in scattering applica-tions such as in
laser addressed ligh-t valves. They concluded that only
materials having smectic H or possibly B phase structure
have useful properties in combination with pleochroic dyes.
Further, they discuss the utilization of slow cooling as
having utility with pleochroic dyes and tha-t rapid cooling
of the elements is only applicable to light scattering
devices.
The present invention utilizes pleochroic dyes to
produce an absorbing state rather than a scattering state
and uses thermal XY local hea-ting as distinct from the laser
heating as described in other art. Further it u-tillzes
rapid cooling of the element wi-th liquid cyrstals
preferentially of the smectic "A" phase. The last factor
is directly against the teaching of Tani and Ueno, but has
been found to be most effective in this application.
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~ lso, recently, a system has been reported in -the
French Literature, which uses a thermally addressed smectic
"A" crystal medium~ Such a system is described in an
article entitled: MATRIX ADDRESSED S~ECTIC LIQUID CRYSTAL
DISPLAY: M. Hareng, S. Le Berre, R. Hehlen, and JoN~
:Perbet, Thomson-CSF Laboratoire Central de Recherches.
Proceedirlgs Erom Society of Information Display 1980
~on~erence, :L,ate News Paper.
Such a system does not use dyes, and employs a
l~ scatterincJ light technique, ra-ther than a light absorption
technique as described by this invention.
In addition, the system described is embodied in
a very different device than de-tailed in this invention.
Because of the crucial difference of the light scattering
as compared to light absorption, the device can be viewed
only through a projection optical system that results in a
very bulky, power in-tensive system.
While the prior art teaches the use of pleochroic
dyes of high order parameter for use in liquid crystal
meclia, i-t should also be noted -that these dyes are used
primarily to enhance the light effects produced by the
thermal phase transition of the media. The invention by
con-trast, relies upon the dye to do most of the light
absorp-tion for the crystal medium, the medium acting as a
vehicle for orienting the dye to develop a light absorbing
stance.
BRIEF DESCRIPTION OF THE INVENTION
The invention relates to a thermally addressed
visual device which provides a dark image against a lighter
background. The device comprises a liquid crystal medium
including at least one cholesteric liquid crystal compound
mixed with at least one coloring agent, generally a
pleochroic dye of high order parameter. The medium has
positive dielectric anisotropy. The medium is thermally
sensitive and has a transition between at least two
thermal phases: the upper thermal phase is a cholesteric
phase and a lower thermal phase is a smectic phase. The
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medium develops two textures in the smectic phase: a
light absorbing tex-ture and a homeo-tropic texture. The
homeotropic texture is developed in portions of the medium
by sensitizing the medium as it passes rapidly from the
upper cholesteric phase to the lower, smec-tic phase. The
light absorbing -texture develops in the unsensi-tized
portions of the medium as i-t goes -through -the transition to
the smectic phase.
The medium is sensi-tized by applying a voltage to
:L0 those portions of the medium -to be addressed. The
acldressed portions develop a substan-tially transparen-t
light state, while -the unaddressed por-tions develop a
substantially light absorbing state. The coloring agent or
dye which is locked within the liquid crystal medium as it
develops its ligh-t absorbing texture in the smectic phase
will absorb most of -the light passing through the medium;
the liquid crystal acting as a vehicle to orient -the dye
molecules into a light absorbing position. Electrodes are
provided to sensitize the medium. They are disposed
adjacent the medium. Heating electrodes are also provided
to heat -the medium to an upper thermal phase. In a
multiplexed device, these electrodes define a matrix of
columns and rows disposed substantially a-t righ-t angles to
each other, and in different planes.
In order to obtain a pleasing direct viewable
display, -the row electrodes are made diffusely reflective
to provide high contrast as well as wide viewing angle.
The reflective electrodes provide for a double pass of
light through the cell enchancing light absorbing.
The liquid crystal medium will generally contain
an alkyl cyano biphenyl compound and will generally have
two thermal transitions: between an isotropic and
choles-teric phase, and between the cholesteric and a
smectic "A" phase.
Many liquid crystal compounds with optically
active terminal groups exhibit cholesteric phase. Some of
them also exhibit one or more smec-tic phases when the
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compounds are cooled down from the cholesteric phase. For
example, a paper published by Joseph A. Castellano, C.S.
Oh, M.T. McCaffray, Mol. Crystal l,iq. Cryst., V.?7 pp 417,
1973, lists 40 Schiff base compounds with -the general
structure:
3 - CH = N - ~ - R
where: R = Oco-(cH2)n - CH3 and C--N
Many compounds wi-th high value of m and n exhibit
a cholesteric phase followed by smec-tric phases upon
cooling. To cite a few examples, we have:
3 - CH = N - ~ -OCO-(CH2)4-CH3
46.3C 74.5C 77C
Crystal -~ ~ Smec-tic II~ -_ ~ Smec-tic I ~ _~ Cholesteric
83.6oc
I so troplc
and C2H5 - CH - (CH2)3 O _ ~ - CH = N - ~ - C = N
CH3
40C 48C 66C
Crystal ---~ Smectic-------~ Cholesteric - ~ Istropic
Although these compounds have the desirable phase
transitions for device application, Schiff bases are
generally not very stable. Also, there are other requirements
that a practical material should have. Thus, typical working
materials are formalized with stable compounds at suitable
composition.
One of the requirements for the host liquid
crystal is -that its dielectric anis-tropy should be strongly
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posi-tive. This is usually obtained by using liquid crys-tal
compounds having C - N as one of the termi.nal groups.
One example of a workable cholesteric liquid
crystal comprises a mixture of X, Y and Z materials, each
having a percentage by weight in an approximate range of:
40 to 60 of X; 30 to 50 of Y; and 5 to :L5 o:E Z;
~espect:ively, where:
- ~ L7 ~ ~ ~ C --N;
10 21 ~ C = N; and
2 5 ICH CH2 ~ C - N.
C~13
More particularly, the aforemen-tioned mixture can comprise:
X jS rC8H17 - ~ ~ CN 50.5% by weigh-t
Y iS ~ C10H21 ~ ~ CN 41.4%
Z iS ~C2H5 - ICH - CH2 ~ CN 8.1%
CH3
Which has a phase transition as follows:
34.5C 40.7C
Crystal -~ Smectic~____ Cholesteric -~.Isotropic
In a liquid crystal which has a smectic phase
followed by a nematic phase, good display performance
requires the temperature range of the nematic phase to be
narrow. With cholesteric materials, however, the temper-
ature range of the cholesteric phase does not necessarily
have to be narrow.
To the host material, a high order parameter
pleochroic dye or dye mixture is added in a range of approx-
imately 0.5 to 3 percent by weight of the total compositin.
More particularly, about 1% by weight of a purpledye have the Eormula:
N ~ - N = N - ~ N = N - ~ - N = N ~ N
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g
is added -to the above cholesteric liquid crystal medium.
This dye is sold by E.M. Laboratories, Elmsford, N.Y.
In operating the device, -the liquid crystal
medium is passed rapidly through i-ts thermal transi-tion
from its upper cholesteric phase -to its lower thermal,
smectic phase. The transition must be accomplished
reasonably rapidly, hence rapid thermal pulses are used
l.hat heat the liquid crystal locally but do not significantly
heat the surrounding glass. Hence -the natural cooling
:L0 per:iod lmmediately following the passage of the heat pulse
is also rapid and hence the liquid crys-tal medium passes
through -the nematic phase rapidly. This greatly enhances
the optical eEfect and results in greater contrast.
Certain portions of the medium are sensitized.
These portions define the background of the medium. These
sensitized portions develop the substantially light trans-
paren-t state when the medium passes in to the smectic thermal
phase. The remaining unsensitized portions of -the medium
develop a light absorbing state. When light (generally
ambien-t) is passed through the medium, the unsensi-tized
portions absorb the light to provide a dark image upon -the
ligh-ter sensi-tzed background. The addressed portions of
the medium may be sensitized in a chronological sequency.
When the liquid crystal material cools down
either from the isotropic state through the cholesteric
state to the smectic state, or from cholesteric to smectic
state, the texture obtained in the smec-tic state depends on
the cooling rate, surface alignment, the pitch of the
cholesteric molecules and some other factors. The materials
best suitable for this new device have molecular pitch in the
1- 3 ~m region. Most display devices have perpendicular
alignment on both glass surfaces. This type of alignment
is not absolutely necessary for this new display.
When the cooling rate is slow (for example less
than 500C/min) we have two cases:
(1) Cooling from the isotropic phase through a
narrow (approximately 10C or less) choles-
teric phase: a clear homeotropic texture is
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obtained.
(2) Cooling from the cholesteric phase to smectic
phase: a scattering SA texture is obtained if the
cooling rate is up to 100C/min. region with slower
cooling rate, the clear homeotropic te~ture is
obtained.
With fast cooling rate corresponding to the actual
displ~ operation (up to 250,000C/min.), scattering textures are
always obtained.
The scattering state obtained with fast cooling in the
cholesteric to smectic system has finer structure as compared to
those obtained with nematic to smectic system. When a pleochroic
dye is added to the material, the scatteri~g state becomes a light
absorbing state. Because of its finer structure, the color is
very deep.
The present invention and that of copending application
Serial No. 400,415 will now be described in greater detail with
reference to the accompanying drawings in which:
Figure 1 is a perspective, exploded, schematic view of a
visual device made in accordance with the invention;
Figure 2 is a plan schematic view of the device shown in
Figure 1, illustrating how an image can be formed in the
cholesteric-smectic liquid crystal medium by a multiplexing
technique;
Figure 3 is a graphical illustration of the chronological
sequencing of the row and column electric waveforms of the device
depicted in Figure 1.
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Figures 4a and 4b show a schematic view of two different
light modulating textures developed in the cholesteric liquid
crystal medium of the device of Figure 1, when the medium passes
rapidly into its smectic phase Figure 4a depicts a homeotropic,
s~tbstantially light transparent texture, and Figure 4b illustrates
a ~ub~tantially light abso.rbing texture.
_t'~ ED D~SCRI-PTION OF THE INVENTION
Generally speaking, thîs invention relates to new methods,
compositions, and visual devices utilizing the thermal addressing
of cholesteric smectic liquid crystal media. The visual devices
of this invention feature a
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highly contrasted dark image on a lighter background.
Where the devices of the invention are multiplexed,
they are capable of being multiplexed up to a large number
of rows.
This invention provides new displays that incor-
porate pleochroic dyes of high order parameter lnto a
smectic A liquid crystal material tha-t has a cholesteric
phase upon heating. By us:ing a thermal electric addressing
techn:ique described hereinafter, this display has m~jor
advantages over the previously known dye switching displays.
A cholesteric liquid cr~stal with positive
dielectric aniso-tropy can develop a homeotropic texture
under the influence of an electric field. ~ homeotropic
smectic A phase is formed, if the material is rapidly
cooled through the phase transition. The homeotropic SA
phase is clear or transparent and shows very little color
(colorless) with dissolved pleochroic dye. Without an
electric field, a light absorbing -texture is formed in the
medium. Thus, by controlling -the electric field across the
liquid crystal layer during the cholesteric to smectic A
phase transi-tion, one can create at his will, either a
colored sta-te or a non-colored state. Once these states
are formed, they are stable until erased by heating into
the istropic or coholesteric phase again.
Although the above description assumes that the
material is heated into -the isotropic state, it is noted
that this is no-t absolutely necessary. In reality, only
heating to the cholesteric state is needed. Also, due to
the different physical mechanisms of forming the colored
scattering state, the temperature range of the cholesteric
state does not necessarily have to be narrow to ensure a
good display performance.
The smectic A phase can be aligned homeotropically,
as shown in Figure 4a, if the surface of the display is
treated with materials such as Lecithin. In -this structure,
the material is transparent.
There are two forms of thermally addressed
smectic A displays. One type uses a scanning laser beam to
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address the display elements. The o-ther -type is x, y
ma-trix addressed. The row electrodes are heated
sequen-tially with electric current and the display is
written by applying voltages on the columns. During the
writing process, only the dots associated -to the row where
the heating current has just been removed are aEfec-ted. In
oLher words, only -the dots where the liquid crystal
mal:t.~l.Lcl.L is cooling to -the smec-tic state respond to -the
wrLting pu.Lses on the column electrodes.
:lO As the :liquid crystal material cools rapidly
throuyh -t:he cholesteric phase -to the smectic phase, i-t can
Eorm two difEerent textures. Wi-th a voltage applied on the
column, the liquid crystal material is switched -to a
homeo-tropic state during the cholesteric phase and assumes
-the transparent homeotropic smectic A texture after cooling
is completed. Without the applied voltage, a light
absorbing -texture is developed instead. Thus, the dots
associa-ted with a cooling row electrode can be written into
a -txansparent state or a light absorbing sta-te by applying
or not applying voltages on the columns. The
choles-teric-smec-tic ma-terial used in the invention display
device has positive dielectric aniso-tropy. The transition
must be accomplished reasonably rapidly, hence rapid
thermal pulses are used that heat the liquid crystal
locally but do not significantly heat the surrounding
glass. Hence the natural cooling period immediately
following the passage of the heat pulse is also rapid and
hence the liquid crystal medium passes through the
cholesteric phase rapidly. This greatly enhances the
optical efEect and results in greater contrast.
The present invention, however, must be carefully
distinguished from other similar systems wherein a
scattering texture rather than a ligh-t absorbing texture is
developed in the smectic material. Displays developing the
scattering texture are generally not suitable for direct
viewing, and are often used only in projection systems.
Tlle optical contrast developed by a scattering
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texture against a transparent texture is simiular to those
obtained with -the dynamic scattering effect. Under many
commonly encoun-tered illumination conditions, it will not
give a pleasing, high contrast image.
The situation becomes quite difEerent, however,
when a pleochroic dye of high order parameter is introduced
:ln to the smectic A material. The dye becomes locked into
the l.iquid crys-tal, and assumes the orientation of the
:L:iquid crystal molecules. The dye molecules in the
1~ scatkering texture of the host absorb light strongly,
I:ransEorming the normal scat-tering tex-ture into a light
absorbing texture, either deeply colored or dark, as shown
in Figure ~b. In the homeotropic smectic texture, the dye
molecules have minimum absorption, since they do not absorb
ligh-t incident upon the edge of their molecular structure.
This texture, therefore, develops a transparent background.
This results in a high contrast display that is suitable
for direct viewing. No external polarizers are required.
The addressing technique is substan-tially the same as
smectic displays without the dye.
Now referring to Figure 1, an exploded view of
a -typical multiplexed, visual display device 10, is
illustrated. The device comprises a cholesteric-smectic
liquid crystal medium 11 containing the pleochroic dye,
which material is disposed between two glass substrate
plates 12 and 13, respectively. The top substrate plate 12
supports a plurality of column electrodes Cl, C2, C3, etc~,
which make up one half of the x y matrix for addressing the
liquid crystal material 11. The column electrodes are made
of electrically conductive, light transparent material such
as indium tin oxide~ which can be vacuum deposited on the
glass plate 12.
The bottom plate 13 supports a plurality of row
electrodes rl, r2, r3, etc., which make up -the remaining
half of the x y matrix. The row electrodes are
electrically conductive and are made diffusely reflective
with material such as silver or aluminum. The row
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electrodes are designed to be diffusely reElec-tive in order
to provide good display image with wide viewing angle.
The liquid crystal medium 11 is generally sealed
between the two substra-te plates 12 and 13 wi-th the
electrodes in contac-t on either side. Light (generally
ambient) is passed -through (arrow 18) the glass composite,
as shown.
The physical operation of -this display 10 can
best be illustra-ted wi-th a simple example of a 5 x 7 matrix
displaying a charac-ter "A", as shown in Figure 2. The rows
of the matrix are -tied together at one end to the common
16 and are sequentially heated by applying electric pulses
to the other ends 17. In time zone 0, (see Figure 3) row 1
is heated such -tha-t the liquid crystal material over the
row 1 electrode rl is in the isotropic or cholesteric
sta-te. In time zone 1, row 2 elec-trode r2 is hea-ted. In
-the meantime, row 1 rapidly cools down and the dots
associated with it are written by applying electric vol-tage
on the column electrodes. In this example, electrodes C
and C5 have voltage applied such that the do-ts rlcl and
rlc5 will be in the transparent state. C2, C3, C~ have no
voltage applied, and the dots rlc2, rlc3, rlc4 have a
colored light absorbing texture. During time zone 2, row
3, electrode r3 is heated and row 2 cools down, and the
voltage on the columns assume the values corresponding to
the "on" and "off" pattern of dots associated to row 2.
The entire waveform for displaying a character l'A", is
shown in Figure 2~
The colored light absorbing texture associated to
the "on" dots is metastable and has long relaxation time
generally over a few months. This texture can be
automatically erased by hea-ting the row during rewriting of
the display. The light absorbing texture is not affected
by the writing voltage applied on the column electrodes.
This assures that "cross-talk" will not be a problem, and
makes possible a large scale matrix display.
The erase-writing process for this display is
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z
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very fast. Generally, less than a 100 ~ second writing
-time can be achieved. If the display is refreshed at fR
-times per second, the total number of rows that can be
multiplexed will be
M =
R T
where ~r - the tlme required -to write the row.
W.ith :ER = 30 her-tz, which is similar to the rate
oE a conventional CRT, and T = 100 ~ sec., we have n = 333
rows. Thus, the display can be multiplexed up -to a rather
large number of rows.
In prac-tical display driving, the heating pulse
can be applied over several time zones before the cooling
and wri-ting cycle. This lowers the voltage requiremen-t for
-the heating pulses. However, the heating pulse should be
IS short enough to avoid heat spreading to the neighboring
rows and to minimize glass heating that inhibits rapid
cooling.
A high contrast is achieved for the colored or
black image due to -the light absorbing character of the dye
material vis-a-vis the transparent background.
The contrast is further improved by the diffusely
reElective nature of the row electrodes, which provide a
double light pass back through (arrow 15) the medium 11,
wherein the unaddressed dye molecules in the light
absorbing state (image) can absorb more light as compared
to the addressed transparent background.
The medium 11 is depicted in the sensitized
(addressed) homeotropic phase in Figure 4a, and is shown in
the unaddressed light absorbing phase in Figure 4b. Light
(arrow 20) entering the homeotropic material of Figure 4a,
passes between the liquid crystal molecules 21. The dye
molecules 22 are not light absorbing in this phase, because
they are locked in the crystal to confront the light rays
upon their edge, as shown.
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However, in the light absorbing phae, -the dye
molecules 22 are locked in the crystal molecules 21 in a
randomly angled pat-texn, as shown in Figure 4b. In this
phase, the dye molecules 22 will strongly absorb the
impinging light rays 20 to produce an intensely colored or
dark image.
The crystal liquid medium 11 can be comprised o:E
at least one al.kyl cyano biphenyl compound.
More particularly, the liquid crystal will be
comprised of a mixture of cyano biphenyl compounds of the
:Eollowing formulas:
X is C8H17 ~ ~ CN 50.5~ by weight
Y is CloH21 _ ~ ~ CN 41.4
Z is C2H5 - ICH CH2 ~ CN 8.1
CH3
One example of a workable cholesteric liquid
crystal comprises a mixture of X, Y, and Z materials, each
having a percentage by weight in an approximate ran~e of:
40 to 60 of X; 30 to 50 of Y; and 5 to 15 of Z;
respectively, where:
X is C8H17 ~ - C - N;
Y is CloH21 * ~ C N; and
Z is C2H5 - tCH ~ ~ - C a N.
CH3
More particularly, the aforementioned mixture can
comprise:
X is C8H17 ~ ~ CN 50.5~ by weight
Y is C10H21 - ~ CN 41.4%
Z is C2H5 - ~CH - CH2 ~ CN 8.1
CH3
7~;~2
-17-
Which has a phase transition as follows:
34.5C 40.7C
Crystal - ~ Smectic ~ Cholesteric~ -~ Istropic
In a liquid crystal which has a smectic phase
followed by a nematic phase, good display performance,
requires the temperature range of the nematic phase to be
narrow. With cholesteric materials, however, the
temperature range of the cholesteric phase does not
necessar:ily have to be narrow.
To the host ma-terial, a high order parameter
pleochroic dye or dye mixture is added in a range of
approximately 0.5 to 3 percent by weight of the total
composi-tion.
More particularly, about 1% by weight of a purple
dye having the formula:
N ~ - N = N ~ N = N ~ N = N ~ N
is added to the above cholesteric liquid crystal medium.
This dye is sold by E.M. Laboratories, Elmsford, N.Y.
While the medium generally features pleochroic
dyes of high order parameter, it is also contemplated that
other coloring agents such as:
C4Hg ~ N = N ~ N = N ~ N
may also provide reasonable image contrast.
Having thus described the invention, what is
desired to be protected by Letters Patent is presented by
the following appended claims:
~ . ~
