Method of imbibing a component into a liquid crystal composite

A method is disclosed for making a liquid crystal composite including a component such as a pleochroic dye. The component is imbibed into droplets of a liquid crystal material in a containment medium.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to liquid crystal composites suitable for use in 
light valves and methods of making such composites including components 
such as dyes. 
BACKGROUND OF THE INVENTION 
Liquid crystal light valves in which the electro-optically active element 
is a liquid crystal composite are known. The composite comprises plural 
volumes or droplets of a liquid crystal material dispersed, encapsulated, 
embedded, or otherwise contained within a polymer matrix. Exemplary 
disclosures include Fergason, U.S. Pat. No. 4,435,047 (1984) ("Fergason 
'047"); West et al., U.S. Pat. No. 4,685,771 (1987); Pearlman, U.S. Pat. 
No. 4,992,201 (1991); and Dainippon Ink, EP 0,313,053 (1989). These light 
valves may be used in displays and window or privacy panels. 
The prior art also discloses the concept of having a further material 
disposed between the polymer matrix and the liquid crystal material. See, 
for example, Fergason, '047; Fergason et al., U.S. Pat. No. 4,950,052 
(1990) ("Fergason 052"); and Raychem, WO 93/18431 (1993) ("Raychem '431"). 
The purpose of having this further material has been variously stated as 
preserving the integrity of the volumes of liquid crystal material and for 
altering the electro-optical properties of the composite. 
Improved processes for making composites, including an intervening further 
material or materials, are disclosed in copending, commonly-assigned 
applications of Reamey et al., "A Method of Making Liquid Crystal 
Composite", Ser. No. 08/217,581 (Attorney Docket MP1425-US1), filed Mar. 
24, 1994; and Havens et al., "Liquid Crystal Composite and Method of 
Making", Ser. No. 08/217,268, (Attorney Docket MP1497-US1), filed Mar. 24, 
1994; the disclosures of which are hereby incorporated by reference. 
It is desirable in certain applications to include a dye or other component 
in the liquid crystal material of composites including such intervening 
further materials. However, where the intervening material is set in place 
by polymerization, the dye or other component may interfere with the 
polymerization. The present invention provides an effective process for 
making such composites. 
SUMMARY OF THE INVENTION 
There is provided a method of making a liquid crystal composite comprising 
plural volumes of liquid crystal material and a component dispersed in a 
containment medium. The method comprises the steps of forming volumes of a 
liquid crystal material in the containment medium, and then imbibing a 
component into the liquid crystal material in the volumes by placing a 
solution of the component and a liquid crystal material into contact with 
the containment medium. 
The component preferably is a pleochroic dye, but other components 
(including non-pleochroic dyes) may also be introduced into the liquid 
crystal material in this manner. Other components may include interface 
modifiers, twist agents, and additives for lowering the operating field. 
The present invention is also directed to an optical device for producing a 
display. The device comprises a liquid crystal material in a containment 
medium and a pleochroic dye imbibed into the liquid crystal material. The 
device may include electrode means for applying an electric field to 
different portions of the liquid crystal material in the containment 
medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1a and 1b show a light valve 10 made from a liquid crystal composite, 
such as described in Fergason '047. Light valve 10 comprises a liquid 
crystal composite 11 in which droplets or volumes 12 of nematic liquid 
crystal material 13 having a positive dielectric anisotropy are dispersed 
in an encapsulating material 14. A pleochroic or dichroic dye 23 may be 
mixed with liquid crystal material 13 in droplets 12. 
Composite 11 is sandwiched between first and second electrodes 15a and 15b, 
made from a transparent conductor such as indium tin oxide ("ITO"). The 
application or not of a voltage across electrodes 15a and 15b from power 
source 16 is controlled by switch 17, shown in FIG. 1a in the open 
position ("off-state"). As a result, no voltage is impressed across 
composite 11, and the electric field experienced by liquid crystal 
material 13 and dye 23 is effectively zero. Due to surface interactions, 
the liquid crystal molecules preferentially lie with their long axes 
parallel to the curved interface with encapsulating material 14, resulting 
in a generally curvilinear alignment within each droplet. The alignment of 
dye 23 follows the alignment of the liquid crystal molecules. In this 
particular embodiment, encapsulating material 14 also acts as a matrix to 
contain the droplets 12 of liquid crystal material 13 and dye 23. The 
curvilinear axes in different droplets 12 are randomly oriented, as 
symbolized by the differing orientations of the curvilinear patterns. 
Liquid crystal material 13 may have an extraordinary index of refraction 
n.sub.e which is different from the index of refraction n.sub.p of 
encapsulating material 14 and an ordinary index of refraction n.sub.o 
which is substantially the same as n.sub.p. (Herein, two indices of 
refraction are said to be substantially the same, or matched, if they 
differ by less than 0.05, preferably less than 0.02.) Incident light ray 
18 traveling through composite 11 has a high statistical probability of 
encountering at least one interface between encapsulating material 14 and 
liquid crystal material 13 in which the liquid crystal index of refraction 
with which it operatively interacts is n.sub.e. Since n.sub.e is different 
from n.sub.p, there is refraction or scattering of light ray 18, both 
forwardly and backwardly. Additionally, in the off-state, the dye 23 
provides a substantial amount light absorption, causing, depending on the 
dye, composite 11 to produce a colored visual effect. See, e.g., Wiley, 
U.S. Pat. No. 5,206,747 (1993). 
FIG. 1b shows light valve 10 in the on-state, with switch 17 closed. An 
electric field, which is directionally indicated by arrow 19, is applied 
between electrodes 15a and 15b, and across composite 11. Liquid crystal 
material 13, being positively dielectrically anisotropic, aligns parallel 
to the electric field direction. Dye 23, which follows the orientation of 
the liquid crystal molecules, also aligns parallel to the electric field 
direction. (The required voltage is dependent inter alia on the thickness 
of the composite and typically is between 3 and 100 volts.) Further, this 
alignment with the field occurs in each droplet 12, so that there is order 
among the directors from droplet to droplet, as shown symbolically in FIG. 
1b. When the liquid crystal and dye molecules are aligned in this manner, 
the liquid crystal index of refraction with which incident light ray 18 
operatively interacts is n.sub.o. Because n.sub.o is substantially the 
same as n.sub.p, there is no scattering at the liquid 
crystal-encapsulating material interface. As a result, ray 18 is 
transmitted through composite 11, which now appears transparent. 
Transmission rates of at least 50%, and preferably on the order of 70% or 
higher may be attained. 
In another configuration of composite 11, the birefringence of the liquid 
crystal material may be relatively low, and the ordinary and extraordinary 
indices of refraction of the liquid crystal are matched closely, if not 
identically, to that of the encapsulating material 14. Thus, refraction 
and scattering at the interfaces between the liquid crystal material and 
the encapsulating medium are minimized. However, the pleochroic dye in the 
liquid crystal material provides controlled attenuation of light by 
absorption as a function of whether an electric field is applied to the 
droplets 12 and of the magnitude of the field. The dye absorbs light in 
both the off-state and the on-state. The degree of light absorption, 
however, is significantly less in the on-state. This configuration is 
described in Fergason, U.S. Pat. No. 4,556,289 (1985). 
The electro-optical performance (e.g., switching voltage, off-state 
scattering, switching speed, and hysteresis) of light valve 10 is 
dependent on the nature of the surface interactions between encapsulating 
material 14 and liquid crystal material 13. An encapsulating material 
which is desirable in respect of characteristics such as mechanical 
properties, ability to protect against environmental contaminants, UV 
stability, etc., may be undesirable in respect of its surface interactions 
with the liquid crystal material, for example causing the switching speed 
to be too slow or the switching voltage to be too high. Thus, it is 
desirable to be able to divorce the surface interactions from the other 
characteristics of the encapsulating material. 
FIGS. 2a-2b (where numerals repeated from FIG. 1a-1b denote like elements) 
show a light valve 20 of the present invention in which this objective is 
achieved. Light valve 20 comprises a liquid crystal composite 21. The 
liquid crystal composite includes liquid crystal material 13 and dye 23 
which is first surrounded by an interfacial material 22a and then by an 
encapsulating material 22b, and finally by a matrix material 22c. The 
encapsulating material serves an encapsulating function only and the 
matrix function is served by the matrix material. Light valve 20 may have 
a colored appearance in the off-state (FIG. 2a) and be transparent in the 
on-state (FIG. 2b), for the reasons given above. 
Liquid crystal material 13 and dye 23 in droplets 12 are separated from 
encapsulating material 22b by interfacial material 22a. Thus, the surface 
interactions affecting the alignment of liquid crystal material 13 and dye 
23 are predominantly with interfacial material 22a and not with 
encapsulating material 22b. Interfacial material 22a may be selected on 
the basis of its interactions with the liquid crystal material and dye. 
The encapsulating material 22b may be selected on the basis of its 
mechanical, optical, or other properties. For example, the encapsulating 
material has to stabilize the emulsion of liquid crystal in a carrier 
medium where an emulsion process is used. In this way, the necessity to 
compromise in respect of one set or another of properties may be avoided. 
Matching of n.sub.o of the liquid crystal material with the index of 
refraction n.sub.p of the interfacial material is important only if the 
thickness of the layer of interfacial material is comparable to the 
wavelength of light. Generally the thickness is less than approximately 
100 nm, much less than the wavelengths of 400 to 700 nm for visible light, 
so that the matching of the indices of refraction is normally not 
necessary. However, where the layer of interfacial material is thick or 
where minimizing on-state haze is an objective (e.g., in window 
applications), matching of the indices of refraction is desirable. 
In order to obtain the advantages of the present invention, it is not 
necessary that interfacial material 22a completely separates encapsulating 
material 22b from liquid crystal material 13 and dye 23. It is sufficient 
that interfacial material 22a at least partially separates the latter two 
materials, so that the switching characteristics (speed, voltage, 
hysteresis, etc.) of light valve 20 are characteristic of an interfacial 
material-liquid crystal material interface and not of an encapsulating 
material-liquid crystal material interface. Preferably, interfacial 
material 22a effectively separates encapsulating material 22b and liquid 
crystal material 13, by which is meant that the interfaces of liquid 
crystal material 13 are primarily with interfacial material 22a and not 
with encapsulating material 22b. 
In the foregoing figures, the droplets, capsules or volumes 12 of liquid 
crystal material 13 and dye 23 have been shown as having a spherical shape 
as a matter of convenience. Other shapes are possible, for example oblate 
spheroids, irregular shapes, or dumbbell-like shapes in which two or more 
droplets are connected by channels. Also, the thickness of the layer of 
interfacial material 22a and the size of droplets 12 have been greatly 
exaggerated for clarity. 
The liquid crystal composites of the present invention provide low voltage, 
high voltage-holding displays with good optical performance, as discussed 
further below in connection with the examples provided herein. 
In accordance with the present invention, one may emulsify the liquid 
crystal material, the encapsulating material, and the interfacial material 
(or a precursor thereof) in a carrier medium to form an intermediate in 
which the liquid crystal material and interfacial material (or precursor 
thereof) are contained within the encapsulating material; cool to separate 
the interfacial material (or precursor) and deposit it between the 
encapsulating material and the liquid crystal material; where an 
interfacial material precursor was used, cure the precursor (e.g., 
photochemically); separate the carrier medium for example by 
centrifugation, to form capsules or pellets in which the liquid crystal 
material is successively surrounded by interfacial material and 
encapsulating material. The use of a centrifuge may, in some instances, be 
unnecessary. However, extensive centrifuging generally results in lower 
operating voltages, as the breadth of the droplet size distribution is 
decreased. 
An emulsion may be prepared by rapidly agitating a mixture of liquid 
crystal material, interfacial material (or precursor thereof), 
encapsulating material, and a carrier medium, typically water. Optionally, 
an emulsifier, wetting agent, or other surface active agent may be added. 
Suitable emulsification techniques are disclosed in Fergason '047, 
Fergason '052, Raychern '431, and Andrews et al., U.S. Pat. No. 5,202,063 
(1993), the disclosures of which are incorporated herein by reference. 
The capsules may then be dispersed in a medium in which a matrix material 
(or precursor thereof) is present. This emulsion can then be coated onto 
electrode-coated substrate 15b and allowed to dry, cure, solidify, etc., 
to form a film 31 (see FIG. 3a). The matrix material is thus caused to set 
around the capsules to form a liquid crystal composite. By "set," it is 
meant that the matrix material hardens into a continuous resinous phase 
capable of containing dispersed therein plural volumes of liquid crystal 
material, with intervening layers of encapsulating and interfacial 
material. The matrix material may set by evaporation of a solvent or a 
carrier medium such as water, or by the polymerization of a precursor 
monomer. 
The emulsion is usually dried for over one hour at room temperature so 
water and other volatiles may be removed. In some cases, the dried 
emulsion may be stored for weeks before imbibition takes place. Film 31 
includes the matrix material and capsules 12a of the liquid crystal 
material successively surrounded by the interfacial material and the 
encapsulating material. At this stage, the dye is not present in the 
liquid crystal material. 
Thereafter, in a preferred embodiment, as shown in FIGS. 3b-3d, a liquid 
crystal material 33 having a dye dissolved therein (or some other 
component as discussed below) is placed directly into contact with an 
exposed surface 31a of undyed film 31. This solution may contain between 
about 0.1 and 10% dye and more preferably between about 0.5 and 5% dye. 
The liquid crystal material in solution with the dye may be different from 
the liquid crystal material in the capsules of the undyed film (see 
Example VI). 
The liquid crystal material including the dye is separated from composite 
film 31 at its periphery by spacers 32. The function of the spacers is 
simply to maintain contact between the film 31 and the liquid crystal 
containing dye 33, while preventing direct contact of the film 31 with the 
substrate 34 on which the liquid crystal/dye mixture resides. 
Under selected conditions of time and temperature, the dye diffuses or 
imbibes into the capsules or droplets 12a of liquid crystal material to 
form liquid crystal composite 21, which includes dye 23 (see also FIG. 
2b). The residual dyed liquid crystal material 33 is then separated from 
liquid crystal composite 21, and any excess dyed liquid crystal material 
33 is removed from surface 21a of liquid crystal composite 21, by exposing 
that surface to a nitrogen stream. The excess dyed liquid crystal material 
may also be removed by a deionized (DI) water wash, or by gently rolling 
or squeeging-off the dyed liquid crystal material on exposed surface 21a 
of liquid crystal composite 21. Thereafter, an electrode-coated substrate 
15a may be laminated onto surface 21a to form a light valve. 
The temperature and duration at which imbibing takes place affects the 
cosmetic and electro-optical performance of a liquid crystal device. If 
the temperature is too low and contact time too short, little dye 
transfers into the film. On the other hand, if the temperature is too high 
or the contact time too long, device performance, such as contrast ratio, 
is adversely impacted. The time of imbibition may be between about 0.1 and 
160 hours, but preferably between about 0.5 and 6 hours. From a processing 
point of view, short times are more desirable. 
The temperature at which imbibition takes place may be set at room 
temperature, about 20.degree. C., to about 150.degree. C. The preferred 
temperature is between about 20.degree. and 90.degree. C. If the 
temperature is too low, imbibition occurs slowly. If the temperature is 
too high, degradation of liquid crystal and dye can occur. 
The use of a nitrogen stream is the most effective method for removing 
excess liquid crystal and dye. Washing with DI water can produce cosmetic 
defects on the composite's surface which can be seen in the on-state of a 
liquid crystal device. Water washing also produces higher operating 
fields, lower hysteresis, and faster switching speed. 
The imbibition method of the present invention may also be used to 
introduce other components, other than a dye, into the liquid crystal 
material. Examples of such other components include twist agents, 
interface modifiers and additives for lowering the operating field. The 
implementation is analogous to the imbibition of dye as described above. 
In some cases, the additive, interface modifier, or twist agent, for 
example, will interfere with emulsion formation, interface agent curing or 
coating. In these cases, the imbibition process can be used to introduce 
these materials subsequent to these processes. Additives for lowering the 
operating field may be those described in Raychem, WO 93/18431 (1993), the 
disclosure of which is incorporated by reference. Such additives include 
ethylene oxide copolymers, propylene oxide copolymers, diols such as 
Surfynol.TM. 104, phenolic compounds, silane coupling agents, and 
acrylates or methacrylates. Interface modifiers or agents can be anionic, 
cationic or non-ionic surfactants and block copolymers. Twist agents are 
chiral materials which lead to a twisting of the liquid crystal directors 
within the droplet, such as CB-15 (E. Merck), 
##STR1## 
and other cholesterol derivatives. The component should be soluble in the 
liquid crystal carrier at a level which at which the component is active. 
The level needed is usually less than 10% by weight. 
It should be understood that in the context of the present invention the 
encapsulating material and the matrix material may not be the same 
material. Also the method of the present invention may be used to 
introduce a dye into liquid crystal volumes in a film wherein the 
encapsulating material acts as a matrix to contain droplets of liquid 
crystal material and dye, and an interfacial material separates the liquid 
crystal material and the encapsulating material (see Example III below). 
Such a film is disclosed in above-mentioned application Ser. No. 
08/217,268. 
The method of the present invention may also be used to imbibe a dye into 
volumes of liquid crystal material in a film made by an emulsion process 
but not including the interfacial material. That is, this method may be 
used to imbibe a dye into liquid crystal volumes in a film like that 
disclosed in above-mentioned application Ser. No. 08/217,581, which 
includes matrix and o encapsulating materials but not an interfacial 
material. 
The present invention may also be used to imbibe a dye into volumes of 
liquid crystal material surrounded by only an encapsulated material. Such 
a film is disclosed in Fergason '047 and shown in FIGS. 1a and 1b. 
Additionally, a dye may be introduced into liquid crystal volumes in a film 
wherein one material provides both the interfacial and matrix material 
functions, and an encapsulating material per se is not used. Such a film 
may be made by a phase separation process (see Example IV below). A film 
made by a phase separation process may be thought of as including only a 
matrix material. A phase separation process is described in West et al., 
U.S. Pat. No. 4,685,771 (1987), which is hereby incorporated by reference. 
The method of the present invention may be used to imbibe a pleochroic dye 
into a liquid crystal material in a containment medium. The containment 
medium may comprise an encapsulating material, a matrix material, a 
combination of encapsulating and matrix materials, a combination of 
interfacial and encapsulating materials, or a combination of interfacial, 
encapsulating and matrix materials, all as described above. The 
containment medium, in whatever form it may take, induces a distorted 
alignment of the liquid crystal material and dye in the absence of a 
prescribed input such as an electrical field. An ordered alignment is 
produced when an electrical field is applied across the liquid crystal 
material and dye in the containment medium. Light may then be transmitted 
through the liquid crystal composite. 
Suitable encapsulating materials include poly(vinyl alcohol), poly(vinyl 
pyrrolidone), poly(ethylene glycol), poly(acrylic acid) and its 
copolymers, poly(hydroxy acrylate), cellulose derivatives, epoxies, 
silicones, acrylates, polyesters, styrene-acrylic acid-acrylate 
terpolymers, and mixtures thereof. A combination of an aqueous carrier 
medium and an encapsulating material which is soluble or colloidally 
dispersible in the aqueous carrier medium is particularly preferred. 
Although surface active agents may be employed, it is generally preferred 
that the encapsulating material be capable of forming capsules containing 
the liquid crystal material without their addition. In such cases, the 
encapsulating material itself should have good surface active properties 
(i.e., be a good emulsifier). A class of polymers-having such 
characteristics are amphiphilic polymers containing both hydrophilic and 
lipophilic segments. Examples of this class include partially hydrolyzed 
poly(vinyl acetates) (e.g., Airvol.TM. 205 from Air Products), 
ethylene-acrylic copolymers (e.g., Adcote.TM. from Dow Chemical), and 
styrene-acrylic acid acrylate terpolymers (e.g., Joncryl.TM. from S. C. 
Johnson). 
As noted above, one may initially form the emulsion not in the presence of 
the interfacial material, but a precursor thereof, which may eventually be 
polymerized to form the interfacial material. Phase separation between the 
liquid crystal material and the interfacial material precursor may be 
effected by solvent removal or temperature change as described above. 
Thereafter, the interfacial material precursor is converted to the 
interfacial material by polymerization. Polymerization of the interfacial 
material precursor may be initiated by heating (where phase separation is 
effected by solvent removal) or, preferably, photochemically, for example 
by irradiation with UV light. Since the interfacial material's solubility 
characteristics will be different from those of the interfacial material 
precursor, it may not be necessary, where temperature change methods are 
used, to do the emulsification at a temperature above the ordinary service 
temperature of the final composite. As used herein, "polymerizing" and 
"polymerization" may include the reaction of the interfacial material (or 
its precursor) with the encapsulating material to fix the interfacial 
material between the liquid crystal material and the encapsulating 
material. 
Suitable interfacial material precursors include mono- or difunctional 
acrylates, mono- or difunctional methacrylates, epoxies (for example, 
those cured with thiols, amines or alcohols), isocyanates (for example, 
those cured with alcohols or amines), and silanes. Precursors with 
branched alkyl units, for example 2-ethylhexyl acrylate, are preferred. 
Suitable interfacial materials are the corresponding polymers and oligomers 
derived from the above-listed precursors, namely acrylates, methacrylates, 
epoxies, polyurethanes, polyureas, siloxanes, vinyl polymers, and mixtures 
thereof. 
Suitable matrix materials include polyurethane, poly(vinyl alcohol), 
epoxies, poly(vinyl pyrrolidone), poly(ethylene glycol), poly(acrylic 
acid) and its copolymers, poly(hydroxy acrylate), cellulose derivatives, 
silicones, acrylates, polyesters, styrene-acrylic acid-acrylate 
terpolymers, and mixtures thereof. Various combinations of these materials 
can be used to form the matrix. For instance, in a preferred embodiment, 
the matrix may comprise a 50:50 blend of poly(vinyl alcohol) and 
polyurethane. 
Various dichroic or pleochroic dyes may be used in the method of the 
present invention. Exemplary dye materials are black dichroic mixtures 
such as MGG1 dye mixture, as described below. Azo, anthraquinone, and 
perylene dyes may be used. 
A preferred combination of interfacial material, encapsulating material, 
and matrix material is poly(2-ethylhexyl acrylate), poly(vinyl alcohol), 
and a 50:50 blend of poly(vinyl alcohol) and polyurethane, respectively. A 
black pleochroic dye blend is preferred. Most applications want an 
"on-off" shutter requiring a black off-state. A black dye blend may be 
obtained by mixing at least three dyes, as described in the examples. Such 
composites were found to have especially low operating fields, low 
field-off transmission, wide operational temperature ranges, and good 
voltage-holding performances. 
It can be advantageous to crosslink, physically entangle molecular chains, 
or otherwise ensure that the encapsulating material is fixed in place, so 
that displacement by the matrix material is minimized. 
The above discussions have been in the context of nematic liquid crystals 
having a positive dielectric anisotropy, but other types of liquid 
crystals may be encapsulated by the method of this invention. One may 
apply the techniques of this invention to liquid crystal composites in 
which the liquid crystal material is a chiral nematic (also known as 
cholesteric) one, such as disclosed in Crooker et al., U.S. Pat. No. 
5,200,845 (1993), and copending, commonly-assigned application of Jones, 
"Chiral Nematic Liquid Crystal Composition and Devices Comprising the 
Same," Ser. No. 08/139,382, filed Oct. 18, 1993 (Attorney Docket No. 
MP1495-US1). Also, composites in which the liquid crystal material is a 
smectic, as disclosed in Pearlman et al., U.S. Pat. No. 5,216,530 (1993), 
are contemplated. 
The practice of this invention may be further understood by reference to 
the examples below, which are provided by way of illustration and not of 
limitation. All relative amounts are by weight unless indicated otherwise. 
The electro-optical performance of liquid crystal devices of the present 
invention are provided in the tables associated with the examples. The 
following general procedures were used in making these measurements. 
Optical measurements were obtained with f/0 collection optics and a 
collimated 550.+-.40 nm light source. For each test, T.sub.on is the 
maximum transmission in the presence of a voltage, T.sub.off is the 
percent transmission in the absence of an applied voltage, and E.sub.90 is 
the field (in volt per micron (V/.mu.m)) required to turn a device on to 
90% of the difference between T.sub.on and T.sub.off. In order to measure 
T.sub.on and E.sub.90, samples were stepped up and down in voltage (25 
steps up/25 steps down, 0.7 sec/step) to a relatively high field 
(typically 8-10 V/.mu.m). The value T.sub.90 is given by the equation: 
T.sub.90 =0.9(T.sub.n -T.sub.off)+T.sub.off. The applied field needed to 
reach T.sub.90 on the up curve is E.sub.90 (the up curve being the % T/V 
curve obtained with increasing voltage). E.sub.90 is substantially 
independent of sample thickness. The corresponding operating voltage 
V.sub.90 is thickness-dependent and has units of volts. V.sub.90 is 
obtained by multiplying E.sub.90 by the thickness (t) in microns of the 
liquid crystal structure (V.sub.90 =t.multidot.E.sub.90). 
The switching speed of a device is a measure of the time for a film of 
encapsulated liquid crystal material to turn on or off with the 
application or removal of a voltage. One way to measure switching speed is 
to monitor the optical response of the film while applying and then 
removing the voltage. Switching speeds were obtained by giving a sample a 
1 sec, 33.3 Hz square wave signal at E.sub.90. The time it takes a device 
to go from 10% to 90% of its final response, when the voltage is applied 
may be referred to as the "rise time", while the time for the device to 
drop from 90% to 10% of its response, upon removal of the voltage, may be 
referred to as the "fall time". The measured switching speeds depend on 
the voltage applied. For displays that show moving graphics, it is 
desirable to have rise and fall times of less than about 50 msec. If the 
switching speeds are much slower, blurting of the moving image results. 
For "frame-sequential" displays, faster rise and fall times, e.g., less 
than about 15 msec, are desired to obtain good color purity. 
The voltage holding ratio (VHR) is defined as the percentage of the 
originally applied voltage that remains at the end of a 15 msec hold time. 
VHR was measured by applying a series of alternating polarity voltage 
pulses to the devices. The pulses were 30-300 .mu.m sec in duration and 
were applied every 15 msec. During the 15 msec hold time, the device was 
held in open circuit and the decay of the applied voltage across the 
device was monitored. The VHR measurement was taken at "steady state", 
which for most devices tested was obtained after 20 pulses. Larger values 
of VHR are more desirable. The VHR measurement was normally performed at 
or above E.sub.90. Displays of the present invention preferably have a VHR 
that is at least 50%, more preferably at least 80%, and most preferably at 
least 90%. 
A device may show hysteresis in its optical response--the optical response 
of a device at a given voltage depends on whether the device reached the 
given voltage from a previously higher or lower voltage. Many displays are 
designed such that a given electrical signal (voltage) should correspond 
to a desired optical response. Hysteresis degrades the ability of the 
device to accurately reach that desired optical response. This would have 
the effect of lowering the number of gray levels in a high resolution 
display. One way to measure hysteresis is to ramp the voltage applied to 
the device up and then down to compare optical response curves. The 
greater the difference between the up and down curves, the greater the 
hysteresis. The hysteresis value for a device would depend strongly on the 
time and voltages used in the test. In most applications, it is desired to 
have the hysteresis as low as possible: less than 20% difference, with 
less than 6% preferred. 
EXAMPLE I 
Into a vial was weighed 8.4922 g of liquid crystal TL216 (EM Industries) 
and 1.5519 g of acrylate mixture PN393 (EM Industries). This mixture was 
stirred until clear, then 9.8508 g of it was added into a beaker. To this 
beaker was added 10.9622 g of a 10% w/w aqueous solution PVA (Airvol.TM. 
205) and 6.5545 g of water. This solution was mixed to yield an emulsion 
with a mean volume diameter of 1.80 .mu.m as determined by Coulter 
counter. The emulsion was degassed overnight, and then cooled at about 
0.degree. C. for 30 mid prior to curing with an ultraviolet (UV) light 
source at 12 mW/cm.sup.2 for 5 min. The cured emulsion was then poured 
into a tube and centrifuged in a multi-step process. The supernatant was 
decanted leaving a pellet of centrifuged emulsion at the base of the tube. 
The pellet was determined to have 17.56% water by drying a portion of the 
pellet overnight at 60.degree. C. Into a vial was added 0.800 g of pellet 
and 1.0387 g of a 50/50% solution of PVA (Airvol.TM. 205) and Neorez.TM. 
967 polyurethane (from ICI Resins). The mixture was stirred gently with a 
spatula and filtered through a 3 .mu.m membrane. 
The emulsion was then coated onto an ITO-glass substrate and was allowed to 
dry. The coating was then placed into direct contact with a solution of 
TL216 liquid crystal (EM Industries) containing 3% MGG1 dye (consisting of 
27% SI486, 27% M618 (both from Mitsui Toatsu Chemicals), and 46% GX874 
(from Nippon Kankoh Shikiso Kenkyusho)). The liquid crystal material and 
dye were supported by 1 mil spacers. The contact was maintained for about 
4 hr at a temperature of about 50.degree. C. The coating and dyed liquid 
crystal material were then separated, and the excess dye and liquid 
crystal material were removed by a nitrogen stream. A second ITO glass 
substrate was laminated onto the now-dyed coating. 
The resulting device was characterized for electro-optical performance as a 
function of temperature (Table 1). It showed remarkably flat 
electro-optical behavior from 5.degree. to 55.degree. C., with a room 
temperature E.sub.90 of 0.80 V/.mu.m. 
TABLE 1 
______________________________________ 
Tempe- Rise Fall 
rature T.sub.off 
T.sub.on 
V.sub.90 
E.sub.90 
Time Time VHR 
(.degree.C.) 
(%) (%) (V) (V/.mu.m) 
(msec) 
(msec) 
(%) 
______________________________________ 
5 14.06 68.9 6.1 0.71 272 299 94.3 
15 14.02 66.9 6.6 0.77 103 147 95.9 
25 14.12 65.8 6.9 0.80 60 93 96.3 
35 14.15 65.1 7.0 0.81 44 71 96.4 
45 14.54 61.9 7.3 0.85 25 45 95.8 
55 15.08 60.4 7.4 0.86 17 35 94.3 
______________________________________ 
(Sample thickness was 8.6 .mu.m in each instance.) 
EXAMPLE II 
Into a vial was weighed 12.00 g of liquid crystal TL205 (EM Industries), 
2.3529 g of acrylate mixture PN393, and 0.0471 g of 
1,1,1-trimethylolpropane trimethacrylate ("TMPTMA," from Polysciences). To 
this beaker was added 16.00 g of a 10% w/w aqueous solution PVA 
(Airvol.TM. 205) and 9.60 g of water. This solution was mixed to yield an 
emulsion with a mean volume diameter of 2.0 .mu.m (Coulter counter). The 
emulsion was degassed overnight, and then cooled at about 0.degree. C. for 
30 rain prior to curing with an UV light source at 12 mW/cm.sup.2 for 5 
min. The cured emulsion was then poured into a tube and centrifuged 
(13,500 rpm for 70 min). The supernatant was decanted leaving a pellet of 
emulsion at the base of the tube. The pellet was determined to have 20% 
water by drying a portion thereof overnight. Into a vial as added 0.7776 g 
of pellet and 0.9262 g of a 6.34% w/w aqueous solution of Joncryl.TM. 77 
copolymer. The mixture was stirred gently with a spatula and filtered 
through a 5 .mu.m membrane. Into another vial was added 0.8624 g of pellet 
and 1.0264 g of a 6.32% w/w solution of Joncryl.TM. 74 copolymer. The 
mixture was stirred gently with a spatula and filtered through a 5 .mu.m 
membrane. 
Both emulsions were then coated onto ITO glass substrates and were allowed 
to dry. The coatings were then placed into direct contact with a solution 
of TL205 liquid crystal containing 3% MGG1 dye. The liquid crystal 
material and dye were supported by 1 mil spacers. The contact was 
maintained for about 4 hours at a temperature of about 50.degree. C. The 
coating and dyed liquid crystal material were then separated, and the 
excess dye and liquid crystal material were removed by a nitrogen stream. 
A second ITO glass substrate was laminated onto the now-dyed coatings. 
Electro-optical data for these devices appear in Table 2. Also included is 
a comparative device worked up in accordance with this example in a 50:50 
blend of PVA (Airvol.TM. 205) and Neorez.TM. 967 polyurethane (from ICI 
Resins). 
TABLE 2 
__________________________________________________________________________ 
Thick- Rise Fall 
ness 
T.sub.off 
T.sub.on 
V.sub.90 
E.sub.90 
Time Time 
VHR 
Sample 
(.mu.m) 
(%) 
(%) 
(V) (V/.mu.m) 
(msec) 
(msec) 
(%) 
__________________________________________________________________________ 
50:50 Blend 
7.6 18.50 
66.3 
3.5 0.46 173.3 
776.5 
94.6 
Joncryl 74 
7.9 16.05 
65.4 
5.2 0.66 91 457 90.5 
Joncryl 77 
6.4 23.44 
69.9 
3.9 0.61 107 492 84.0 
__________________________________________________________________________ 
EXAMPLE III 
Into a vial was weighed 2.1499 g of liquid crystal TL205, 0.4210 g of 
acrylate mixture PN393, and 0.0084 g of TMPTMA. This mixture was stirred 
until clear, then 2.4 g of it was added into a beaker. To this beaker was 
added 3.23 g of a 40% w/w solution of Neorez.TM. 967 polyurethane in 3.6 g 
of water. This solution was mixed to yield an emulsion with a mean volume 
diameter of 3.0 .mu.m (Coulter couriter). The emulsion was degassed 
overnight, and then cooled at about 0.degree. C. for 30 min prior to 
curing with an UV light source at 12 mW/cm.sup.2 for 5 min. The mixture 
was filtered through a 5 .mu.m membrane. 
The emulsion was then coated onto an ITO glass substrate, and it was 
allowed to dry for more than 1 hr. The coating was then placed into direct 
contact with a solution of TL205 liquid crystal containing 3% MGG1 dye. 
The liquid crystal material and dye were supported by 1 mil spacers. The 
contact was maintained for about 3 hr at a temperature of about 50.degree. 
C. The coating and dyed liquid crystal material were then separated, and 
the excess dye and liquid crystal material were removed by a nitrogen 
stream. A second ITO glass substrate was laminated onto the now-dyed 
coating. Table 3 summarizes the electro-optical performance. 
TABLE 3 
______________________________________ 
Thick- Rise Fall 
ness T.sub.off 
T.sub.on 
V.sub.90 
E.sub.90 
Time Time VHR 
(.mu.m) 
(%) (%) (V) (V/.mu.m) 
(msec) 
(msec) 
(%) 
______________________________________ 
6.0 44.31 76.6 13.1 2.18 12 62 94.0 
______________________________________ 
EXAMPLE IV 
Into a vial was weighed 0.4066 g liquid crystal TL205 and 0.1017 g of 
acrylate mixture PN393. Epostar 10 .mu.m glass spacers were added to the 
homogeneous solution. Several drops were placed on a 43 mil ITO-coated 
glass substrate. A piece of 7 mil ITO-coated Mylar poly(ethylene 
terephthalate) ("PET") was used as the top piece. In order to maintain 
flatness, the Mylar PET was temporarily affixed to a glass substrate using 
water. The top piece was lowered onto the liquid crystal/acrylate solution 
so that the Mylar PET was in contact with the solution. The device was 
cured at 10 mW/cm.sup.2 for 5 min at about 15.degree. C. The sample was 
allowed to equilibrate at 15.degree. C. for 5 min prior to UV exposure. 
The Mylar PET was removed. The sample was placed face down on 1 mil 
spacers on a 50.degree. C. hot plate. A solution of TL205 liquid crystal 
containing 3% MGG1 dye was capillary filled onto the sample. The device 
was allowed to soak for about 3 hr at 50.degree. C. Excess dye/liquid 
crystal was blown off with nitrogen. The sample was laminated with an 
etched substrate for electro-optical characterization (Table 4). This 
example illustrates imbibition into a film made by the phase separation 
(PIPS) method. 
TABLE 4 
__________________________________________________________________________ 
Thick- Rise Fall 
ness 
T.sub.off 
T.sub.on 
V.sub.90 
E.sub.90 
Time Time 
VHR 
Sample 
(.mu.m) 
(%) 
(%) 
(V) (V/.mu.m) 
(msec) 
(msec) 
(%) 
__________________________________________________________________________ 
TL205/ 
11.3 
8.97 
55.5 
12.0 
1.06 38.2 142.3 
91.19 
PN393 
__________________________________________________________________________ 
EXAMPLE V 
Several imbibed samples were laminated to complementary metal-oxide 
semiconductor chips (CMOS), on which various video signals were applied. 
The materials used followed the same general recipe; a typical example is 
listed below. 
Into a vial was weighed 50.8 g of liquid crystal TL205, 10.113 g of 
acrylate mixture PN393, and 0.2023 g of TMPTMA. This mixture was stirred 
until clear, then 59.5 g of it was added into a beaker. To this beaker was 
added 66.11 g of a 10% w/w aqueous solution PVA (Airvol.TM. 205) and 
39.665 g of water. This mixture was mixed to yield an emulsion with a mean 
volume diameter of 1.82 .mu.m as determined by Coulter counter. The 
emulsion was degassed overnight, and then cooled at about 0.degree. C. for 
30 min prior to curing with an UV light source at 11 mW/cm.sup.2 for 5 
min. The cured emulsion was then poured into a tube and centrifuged in a 
multi-step process. The supernatant was decanted leaving a pellet of 
centrifuged emulsion at the base of the tube. The pellet was determined to 
have 18.45% water by drying a portion thereof overnight. Into a vial was 
added 9.5 g of pellet and 14.7606 g of a 50/50% solution of PVA 
(Airvol.TM. 205) and Neorez.TM. 967 polyurethane [from ICI Resins]). To 
this vial was also added 8.0 g of a 1.0% solution of an oligomeric coating 
aid of the structure 
##STR2## 
where the degree of oligomerization x is about 7.2. This and other coating 
aids are described in copending, commonly-assigned application of Lau, 
entitled "Amphiphilic Telomers", Ser. No. 08/222,149 (Attorney Docket No. 
MP1499-US1), filed Mar. 31, 1994, and which disclosure is hereby 
incorporated by reference. The mixture was stirred gently with a spatula 
and filtered through a 5 .mu.m membrane. 
The emulsion was then coated onto an ITO glass substrate and allowed to 
dry. The coating was then placed into direct contact with a solution of 
TL205 liquid crystal containing 3% MGG1 dye. The liquid crystal material 
and dye were supported by 1 mil spacers. The contact was maintained for 
about 4 hours at a temperature of about 50.degree. C. The coating and dyed 
liquid crystal material were then separated, and the excess dye and liquid 
crystal material were removed by a nitrogen stream. A reflective CMOS 
wafer was laminated onto the now-dyed coating. The resulting device was 
then driven with various checkerboard patterns applied to the CMOS chip, 
showing good contrast at reasonably low voltages. 
Usually, the liquid crystal material in which the dye is dissolved is the 
same as liquid crystal material in the undyed film. However, this is not a 
necessary feature of the present invention. For instance, following the 
procedure of Example VI, a black dye in a liquid crystal material was 
imbibed into a film including droplets of the TL205 liquid crystal 
material. 
EXAMPLE VI 
An open-faced film was made as described in Example II with 50:50 PVA 
(Airvol.TM. 205) and NeoRez R967 as the matrix material. The film was 
stored in a box at room temperature for more than 2 weeks. The film was 
inverted and placed in contact with E37 (Merck Ltd.) liquid crystal 
containing 3% of MGG1 dichroic dye mixture at 60.degree. C. for 18 hr. The 
excess liquid crystal was blown off with nitrogen and a second ITO-coated 
glass substrate was laminated onto the film. The electro-optical 
performance of the film was determined. The T.sub.off was 22%, indicating 
significant dye absorption. The V.sub.90 was 4.7 Volts. When the voltage 
was removed from the sample, the sample remained partly on (52% 
transmission). The sample returned to the original T.sub.off of 22% if 
heated to about 50.degree. C. This behavior was repeatable. The VHR for 
the film was 79% at V.sub.90 (4.7V) and 96% at 30V. 
Yet another approach is to cause the dye to diffuse from one set of liquid 
crystal droplets to another. This approach is described in Example VII. 
EXAMPLE VII 
This example involves a standard acrylate-containing undyed emulsion 
blended with a large particle dyed aqueous emulsion. 
Into a vial was weighed 9.231 g of liquid crystal TL205, 1.81 g of acrylate 
mixture PN393, and 0.0362 g of TMPTMA. This mixture was stirred until 
clear, then 10.2763 g of it was added into a beaker. To this beaker was 
added 11.686 g of a 9.77% w/w aqueous solution PVA (Airvol.TM. 205) and 
6.58 g of water. This solution was mixed to yield an emulsion with a mean 
volume diameter of 1.85 .mu.m as determined by Coulter counter. The 
emulsion was degassed overnight, and then cooled at about 0.degree. C. for 
30 min prior to curing with an UV light source at 4 mW/cm.sup.2 for 30 
min. The cured emulsion was then poured into a tube and centrifuged. The 
supernatant was decanted leaving a pellet of emulsion at the base of the 
tube. The pellet was determined to have 21.7% water by drying a portion 
thereof overnight. Into a vial was added 1.3207 g of pellet and 1.5218 g 
of a 6.3% w/w solution of Neorez.TM. 967 polyurethane. The mixture was 
stirred gently with a spatula and allowed to sit. 
For the dyed emulsion, 2.0149 g of a 7% solution of MGG1 dye in TL205 
liquid crystal was weighed into a beaker. To this beaker was added 4.0549 
g of a 10% w/w aqueous solution PVA (Airvol.TM. 205. This solution was 
mixed to yield an emulsion with a mean volume diameter of 2.8 .mu.m as 
determined by Coulter counter. 
Into a new beaker, 2.6335 g of the acrylate:containing undyed emulsion was 
combined with 0.6964 g of the dyed emulsion; the mixture was stirred 
gently with a spatula and filtered through an 8 .mu.m membrane. The 
emulsion was then coated onto an ITO-coated Mylar PET substrate and 
allowed to dry for 1 hour. Another piece of ITO-coated Mylar PET was then 
laminated to the top of the coating. Electro-optical data was collected 
after device formation, as well as after aging, for about 691 hr at 
60.degree. C. When compared to a device consisting of just the dyed 
emulsion (the control sample in Table 5), it was found that the operating 
voltage is substantially reduced by blending with the acrylate-containing 
emulsion. 
TABLE 5 
__________________________________________________________________________ 
Thick- Rise Fall 
ness 
T.sub.off 
T.sub.on 
V.sub.90 
E.sub.90 
Time Time 
VHR 
Sample 
(.mu.m) 
(%) 
(%) 
(V) (V/.mu.m) 
(msec) 
(msec) 
(%) 
__________________________________________________________________________ 
Control 
8.8 
1.82 
30.4 
126.0 
14.3 4.3 126 80.6 
Initial 
12.0 
13.49 
58.6 
55.8 
4.65 1.7 491 96.4 
Blend 
After 691 
12.0 
11.57 
60.2 
39.3 
3.28 3 301 94.7 
hr at 60.degree. C. 
Blend 
__________________________________________________________________________ 
Liquid crystal displays used for displaying high information content and 
motion such as videos often contain "active matrix panels" as electronic 
drivers for providing the voltage signal to the liquid crystal composite. 
For displays operated via active matrix drive, it is desirable to have 
liquid crystal composites that have good contrast as well as high 
brightness at low drive voltages, and which also are highly resistive in 
order to maintain the voltage supplied by the active matrix panel. The 
present invention provides a means of obtaining good contrast, high 
brightness, low voltage, high resistivity liquid crystal composites for 
use with active matrix drive panels. 
One of the substrates 15a or 15b can be a substrate which provides s 
different electrical signals to different portions (picture elements or 
pels) of the display. This substrate, which is sometimes referred to as 
the driver, provides the ability to display patterns by having portions of 
the liquid crystal composite of the display at various levels of 
transmission. The driver can be a patterned electrode, or it can be an 
"active matrix panel". An active matrix panel has an active electronic 
element, e.g., a transistor, at each picture element. The active matrix 
panel can be either transmissive, e.g., a thin film transistor array (TFT) 
on glass, or non-transmissive, e.g., a CMOS wafer. 
The present invention provides for, among other things, the introduction of 
a dye into droplets of liquid crystal material after an interfacial 
material is cured. Thus, the dye does not interfere with the curing or 
polymerization of the interfacial material. A liquid crystal material 
including the dye is used as a carrier for introduction of the dye into 
the droplets or capsules of the liquid crystal material. The resultant 
composite provides a device with good contrast ratios and low operating 
voltages. 
The foregoing detailed description of the invention includes passages which 
are chiefly or exclusively concerned with particular parts or aspects of 
the invention. It is to be understood that this is for clarity and 
convenience, that a particular feature may be relevant in more than just 
the passage in which it is disclosed, and that the disclosure herein 
includes all the appropriate combinations of information found in the 
different passages. Similarly, although the various figures and 
descriptions thereof relate to specific embodiments of the invention, it 
is to be understood that where a specific feature is disclosed in the 
context of a particular figure, such feature can also be used, to the 
extent appropriate, in the context of another figure, in combination with 
another feature, or in the invention in general.