Note: Descriptions are shown in the official language in which they were submitted.
~6~
The present invention relates generally to liquid
crystal technology, and more specifically to the manufacture
of a new light modulating material consisting of microdrop-
lets of liquid crystal dispersed in a plastic matrix.
In a typical electrically switched liquid crystal
display device, a film of liquid crystal materiàl is sealed
between two glass or plastic plates that are provided with
transparent conducting electrodes. When a voltage is
applied to the electrodes, the orientation of the liquid
crystal is affected, altering the manner in which light is
absorbed by polarizers attached to the cell or by dichroic
dyes dissolved in the liquid crystal.
The cell construction and operation described above
have several disadvantages. In most applications, the
electrodes must be specially treated or prepared to align
the liquid crystal in a particular way at the surface o~
the cell wall. Furthermore, it is difficult to fill and
then seal the liquid crystal in the narrow region between
the plates. Special techniques and ........... 0
. .
materials are required for cell fabrication, particularly
for mass production. Uniformity in cell thickness and
preparation can be a problem and a limitation in large
scale displays. The use of polarizers complicate cell
construction, add to the cost and can limit the cell
contrast or brightness in that they always absorb a
component of incident light. Finally, the response
time of a conventional liquid crystal display can be
marginal for many applications.
lQ Recent display technology has been directed toward
overcoming some of these difficulties and limitations
One approach has been to develop materials which contain
small regions of the liquid crystals with light scattering
properties that can be electrically manipulated. Two
types of such material have been suggested: materials
containing encapsulated liquid crystals and materials
- with open or connected micropores which can be filled
. .
with liquid crystals. Such material would avoid the
sealing problems of the conventional cell and could
make it possible to construct large displays. Furthermore,
polarizers are not required for the operation of this
- type of display, avoiding the limitations that dichronic
polarizers create.
One prior art proposal for encapsulating liquid
crystals, disclosed in French Patent No. 2,13~,537 dated
December 11, 1972, involves forming an aqueous emulsion
of liquid crystal material with an immiscible binder
such as polyvinyl alcohol. The mixture is emulsified
in a high speed blender or the like to form droplets of
the liquid crystal that are encapsulated by the binder.
The encapulated droplets are then coated on a clear
plastic substrate having the usual conducting electrodes.
A similar technique is described in U.S. Patent
No. 4,435,047 issued March 6, 1984.
A prior art proposal involving filling the open or
connected micropores of a plastic sheet with a nematic
or other type of liquid crystal is disclosed in U.S.
Patent No. 4,048,358 issued September 13, 1977.
Disclosure of the Invention
The invention provides a new light modulating material
comprised of droplets of liquid crystal uniformly dispersed
in a clear or light transmitting, flexible plastic sheet
or film, such as a cured epQXy. In comparison to prior
art materials prepared by encapsulation or emulsification
of liquid crystals,-the new material of the invention
is characterized in part by simplicity of preparation,
and cell fabrication, control of droplet size, shape,
spacing and distribution to provide for improved responsive
features.
The new material is capable of being thermallyr
electrically, magnetically and electromagnetically addressed
to cause the material to be reversibly switched between
a li~ht scattering mode and a light transmissive mode.
The material is optically responsive to strain, whereby
under tension it acts as a polarizer that transmits one
component of plane polarized light while scattering the
other component. Curing the material in ~he presence
of an electric or magnetic field also causes it to act
as an electrically addressable polarizer
The light modulating material of the invention is
prepared by dissol~ing liquid crystal in an uncured
resin and then curing the resin so that droplets of
liquid crystal spontaneously form and are uniformly
dispersed throughout the matrix. Preferred liquid
crystals are of the nematic type or those which behave
as nematic type. Optimum results have been obtained
with cyano-biphenyl liquid crystal.
When prepared in the manner described, the liquid
crystal droplets have been observed to be of uniform
)3~
.
- 4
si~e and spacing and to have a diameter ranging upward
from about 0.2 microns depending primarily upon the
curing procedure and the specific materials used.
One suitable epoxy resin is a mixture of epichloro-
hydrin and b-isphenol A (part A) and a curing agent (part
B). Other useful epoxy resins are those that can be
cured by ultra-violet radiation. Preferred polyurethane
resins have high tensile and tear strength. A suitable
polyurethane is a mixture based on toluene diisocyanate,
-10 polyether glycols, methylenebisisoorthochloroaniline,
and various polyols.
The physical principle of operation of the invention
is based on the ability of the birefringent liquid crystal
droplets to scatter or transmit light depending upon
the relationship of the indicies of optical refraction
of the liquid crystal and the resin matrix. In tempera-
r~_', ture responsive displays, the index of refraction of
the liquid crystal in its isotropic phase is matched or
similar to tha~ of the resin so that the material will
transmit incident light, while an index of refraction
in the liquid crystalline phase, usually the extraordinary
index, is mismatched with respect to the refractive
index of the resin so that incident light is scattered
and the material is opaque.
For electrically responsive uses, the ordinary
index of refraction of the liquid crystal should match
or be similar to the refractive index of the resin. In
the absence of an electric field, incident light is
scattered by the extraordinary index of refraction. In
the presence of a field which aligns the liquid crystal's
extraordinary index of refraction (the optic axis of
the liquid crystal droplet) normal to the surfaces of
the material, incident light is transmitted. When the
optic axes of the liquid crystals are aligned, such as
by stretching the material, the component of plane
~6~0;~
polarized incident light perpendicular to the direction
of strain is transmitted, while the other component is
scattered by the extraordinary index of refraction to
achieve a light polarizing effect.
Temperature responsive material can be prepared in
accordance with the invention using nematic, cholesteric
or many different kinds of smectic liquid crystals, as
well as mixtures thereo~ A thermo-optic response at
any particular temperature can be obtained by the use
of a liquid crystal that transforms at that temperature
from the liquid crystalline phase to the isotropic phase.
The process is reversible so that when the temperature
of the material is decreased through the isotropic to
liquid crystalline phase transition, the material switches
from the clear to the opaque state. Thermo-optic devices
that respond to different temperatures can be made usiny
liquid crystals having different isotropic-liquid crys-
talline phase transition temperatures.
As a temperature responsive material, there are
several features of thè invention that differ signif-
icantly from prior art materials and devices and provideimportant advantages. The operation of prior art choles-
teric liquid crystal devices such as disclosed, for
example, in Patent No. 3,872,050, is based upon the
Bragg scattering of light when the temperature dependent
pitch length of the cholesteric helix becomes comparable
to the wavelength of the incident light. The operation
of those prior art devices which depend on phase changes
of liquid crystal material, such as disclosed in Patent
No. 4,279,152, requires altering the ordering of dye
molecules in order to change light absorption properties.
In the material of this invention, the temperature resolu-
tion between the white opaque and clear states is governed
by the width of the isotropic to liquid crystalline
phase transition nd as such is an improvement over the
2~03~
temperature resolution of conventional cholesteric devices
which depend upon the width of the visible spectrum and
the temperature dependence of the pitch length of the
cholesteric helix. Another advantage of the invention
is that visual contrast between the on and off states
is controlled by the contrasting light scattering proper-
ties of the dispersed liquid crystal in the isotropic
phase relative to that in the liquid crystalline phase,
whereas in the cholesteric liquid crystal temperature
- 10 indicators of the prior art, the visual contrast is
governed by the Bragg scattering properties of the twisted
cholesteric material relative to a background substrate.
The present invention also permits the use of a
wide variety of liquid crystals and phases, including
- those which have high thermal stability and lifetime~
The cholesteric liquid crystal indicators of the prior
art have been restricted to cholesteric or chiral mater-
-- - ials which have the appropriate temperature dependence
of the pitch length. Such liquid crystals can have
poor stability so that displays constructed with them
have limited lifetime.
Electro-optic or magnetic-optic responsive material
is prepared using nematic liquid crystal, or mixtures
that incorporate and behave as a nematic. As used herein,
the term "nematic" means nematic liquid crystal and
liquid crystal mixtures having the properti~s of a nematic.
- The dispersed nematic liquid crystal in its resin matrix
is placed between two conducting surfaces, one or both
of which are transparent. When a voltage of the appropriate
magnitude is applied to the conducting surfaces, the
~aterial will switch from a white opaque state to a
clear state. The process is reversible upon removal of
the voltage. If desired, a pleochroic dye can be
incorporated into the liquid crystal to enhance the
visual contrast between the clear and opaque states of
8~3 ~ ~
the electrically responding material. By using a black
dye, for example, the material will appear black in the
opaque state.
As an electrically responsive material, the inven-
tion has features and advantages different from other
known voltage or current responsive materials involving
llquid crystals. With the material of the invention r
an electric field from an a.c. or d.c. voltage source
applied to the conductors on the surfaces of the material
aligns the optic axes of the nematic liquid crystal
droplets and ~he extraordinary index of refraction of
the liquid crystal parallel to the electric field so
that light is transmitted. Upon removal of the applied
~, electric field, the surface interactions between the
resin matrix and the dispersed nematic liquid crystal
rapidly restore the random alignment of the optic axes
to the condition existing prior to the application of
the electric field to achieve light scattering by the
extraordinary index of refraction. The large number
and small size of the dispersed liquid crystal droplets
provide for a large surface area to volume ratio between
the epoxy and the liquid crystal to bring about the
switching effect. An important characteristic of the
invention is that the droplets can be readily shaped to
yield fast switching times. With the material of the
invention, the clear state to the opaque state response
time can be made in approximately 1-10 milliseconds.
When a pleochroic dye is incorporated into the liquid
crystal the principle of operation remains different
from other guest-host displays incorporating pleochroic
dyes in that it is the resin-liquid crystal surface
interactions and the large surface-to-volume ratio of
the dispersed liquid crystal that restores the nematic
directors and hence the guest dye component to its ran-
dom opaque state orientation upon removal of an applied
- 8
electric field. This is in contrast to known "phase
change" dichroic display cells in which a cholesteric
component is added to the liquid crystal to cause or
induce random alignment of the dye in the opaque state.
One specific embodiment of an electrically respons-
ive display cell incorporates a strained sheet or film
prepared in the manner described above. In the presence
of an electric field which aligns the extraordinary
index of refraction normal to the surfaces of the sheet
or film, ùnpolarized incident light is transmitted through
the cell. In the absence of an electric field, the
extraordinary index of refraction is parallel to the
direction of strain, with the result that one component
of plane polarized incident light is transmitted and
the other component is scattered. The switch time of
the stretched material is about 1 millisecond compared
to 10-100 milliseconds with unstretched material. Such
a cell can act as a light switch when incorporated with
a second polarizer.
Another embodimen~ of an electrically responsive
polarizing material may be made with liquid crystal
- droplets in a cured polyurethane film sandwiched between
glass slides. Translating the slides relative to one
another in the opposite directions strains the film.
The strained film scatters light polarized along the
axis of strain and is-transparent to light polarized
perpendicular to the axis of strain. Application of an
electric field will switch the film to a non-polarizing
transmitting state. Unstrained film may be switched
from a non-polarizing scattering state to a non-polarizing
transmitting state by the application of an electric
field.
An important feature of the present invention is
the novel technique of curing the resin with dissolved
liquid crystals in the presence of an applied magnetic
or electric field of sufficient strength to cause the
liquid crystals in the droplets to align. The liquid
crystals within the droplets are aligned during curing.
Once the curing process is complete, the alignment becomes
permanent and persists upon removal of the applied field.
This field-alignment-phenomenon allows the fabrication
of switcha~le light polarizers. A switchable light
polarizer which polarizes light in the absence of an
applied voltage is made by selecting a liquid crystal
with a positive anisotropy in the dielectric as well as
diamagnetic susceptibility~ A film of the liquid crystal
dissolved in resin is cured in the presence of a magnetic
field directed in the plane of the film. When cured,
the optic axes of the liquid crystal droplets are aligned
in the plane of the film. The cured film polarizes
light. Placing the cured film between transparent elect-
rodes and applying a voltage of sufficient strength
causes the polarizing effect to be switched off.
A switchable polarizer which polarizes light in
the presence of an applied field is made by selecting
liquid crystals having a positive dielectric anisotropy
and curing a solution of the liquid crystal in resin in
an a.c. field created by applying voltage to conducting
surfaces on the curing film. When cured, the optic
axes of the liquid crystal droplets are aligned in a
direction normal to the film surface. The cured film
is clear and non-polarizing. Application of an electric
or magnetic field in the plane of the film causes the
film to switch to a polarizing state.
Optically switchable polarizers can be fabricated
by curing a liquid crystal-resin solution in the presence
of an applied field which causes the optic axis of the
liquid crystal droplets to align normal to the film
surface. The cured film is clear and non-polarizing~
A high intensity electromagnetic source will cause the
~.2~
10
optic axes of the droplets to reorient so that the film
becomes opaque and light scattering
Display materials with improved light scattering
properties can be fabricated with films containing drop-
lets distorted by a compressive strain applied to thefilm. The distortion aligns the extraordinary index of
the refraction of the liquid crystals parallel to the
surface of the film, but random in the plane of the
film. In the absence o~ an applied field, this film
scatters light and appears opaque; it can be switched
to a clear state by the application of an a.c. voltage
of sufficient strength to switch the optic axes of the
liquid crystals to a direction normal to the surface.
A display formed from this material would be expected
to have improved contrast over displays made with spherical
droplets in that the index mismatch is maximized in
such f ilms in the scattering state. A compressive strain
can be conveniently made by suitable adjustment of the
curing temperature such that contrasting thermal expan-
sivities between the cell walls and the resin-liquid
crystal material induces the compressive strain at
operating temperatures.
In accordance with the foregoing, one feature of
the invention is the provision of a new light modulating
material comprising microdroplets of liquid crystal
dispersed in a light transmissive, solid sheet or film
of a cured resin, the liquid crystal having been dissolved
in the uncured resin and the microdroplets having been
formed spontaneously upon curing.
Another feature of the invention is the provision
of a temperature responsive, light modulating material
as described in the previous paragraph which is further
characterized in that the liquid crystal has an optical
index of refraction in the isotropic phase that is similar
to the refracti~e index of the resin and an index of
.
.
refraction in the liquid crystalline phase that is mis-
matched with the refractive index of the resin.
Still another feture of the invention is the provision
of an electrically responsive, light modulating material
and device characterized by a solid sheet or film of
light transmissive, cured resin containing dispersed
microdroplets of nematic liquid crystal having an ordinary
index of refraction similar to the refractive index of
the resin so that light is transmitted in the presence
of an electric field and having its other index of re-
fraction such that incident light is scattered in theabsence of the field, the liquid crystal being soluble
in the uncured resin and the microdroplets being spon-
taneously formed during curing.
A further feature of the invention is the provision
of a light polarizing material comprising microdroplets
of liquid crystal dispersed in a sheet or film of a
light transmissive cured resin, the liquid crystal having
an ordinary index of refraction similar to the refractive
index of the resin and being soluble in the uncured
resin with the microdroplets being spontaneously formed
during curing, nd the material being strained to align
the optical axes of the microdroplets in the direction
of strain.
An additional feature of the invention is the pro-
vision of an electrically addressed, light polarizing
device incorporating the material described in the pre-
vious paragraph which is further characterized in that
the liquid crystal is a nematic having an ordinary index
of refraction similar to the refractive index of the
resin, whereby light is transmitted through the device
unpolarized in the presence of an electric field.
Another feature of the invention is the provision
of a light modulating material comprising a light trans-
missive plastic containing dispersed microdroplets of
~ ~ ~ 8~
nematic liquid crystal having an ordinary index of re-
fraction similar to the refractive index of the plastic,
the material being characterized in that the optical
axes of the microdroplets are aligned in one direction
According to a preferred method of manufacture, the
material is made by curing the resin in a field that is
effective to align the optical axes of the microdroplets
during curing..
In one specific embodiment o~ the invention, the
material described in the previous paragraph is made so
that the optical axes.are normal to.the plane of the
sheet, whereby incident light is transmitted unpolarized.
In the presence of an electric, magnetic or electro-
magnetic field, the Qptical axes are oriented in another
direction effective to scatter at least one component
of incident light. In another specific embodiment, the
-- . material described in the previous paragraph is prepared
so that the optical axes are aligned in one direction
in the plane of the sheet, whereby the material acts as
a light polarizer, and whereby the optical axes are
aligned normal to the plane of the sheet in the presence
- of a field so that light is transmitted unpolarized.
Still other features and advantages and a full
understanding of the invention will become apparent to
those skilled in the art from the following description
of the best modes and the accompanying drawings.
Brief Description of the Drawinqs
Figure 1 is a fragmentary, schematic illustration
in cross-section of a resin-liquid crystal material in
which the liquid crystal droplets are in the isotropic
phase and the material is in a clear state.
Figure 2 is an illustration similar to Figure 1
except that the liquid crystal droplets are in the liquid
crystalline phase and the material is in an opaq~e state.
()3
13
Figure 3 is a fragmentary, schematic view in cross-
section of the resin-liquid crystal material incorporated
in an electrically actuated device.
Figure 4 is a fragmentary, schematic illustration
S in cross-section of the resin-liquid crystal material
in a stretched condition.
Figure 5(a) is a fragmentary, schematic illustration
in cross-section of the resin-liquid crystal fllm cured
in an applied field directed in the plane of the film.
- 10 Figure 5(b) is fragmentary, schematic illustration
in cross-section of the film of Figure 5(a) incorporated
in an electrically actuated device.
Figure 6(a) is an illustration similar to Figure
5(a) showing a resin-liquid crystal film cured in a
field normal to the plane of the film.
Figure 6(b) is a schematic illustration of the
film of Figure 6(a) in an applied field directed in the
plane of the film.
Figure 7(a) is a fragmentary schematic illustration
in elevation of a resin-liquid crystal film in a stressed
condition such that the optic axes are randomly oriented
in the plane of the film.
Figure 7(b) is a fragmentary, schematic illustration
in cross-section of the film of Figure 7(a) incorporated
in an electrically actuated device.
Best Modes for CarrYinq Out the Invention
Referring now to the drawings, Figure 1 illustrates
a preferred display material of the invention consisting
of cured, clear resin matrix 10 which contains droplets
11 of liquid crystal. As shown in Figure 1, the liquid
crystal component is at a temperature such that it is
in a clear isotropic phase. The liquid crystal is
selected so that its optical index of refraction in the
isotropic phase, ni, has a value similar to that of the
clear resin, ns, so that light, as at Ior incident upon
2~30;3~
1~
the material will pas~ readily through it unscattered,
as at IT. The material in the condition illustrated in
Figure 1 is referred to as being in the clear state.
Figure 2 illustrates the same material except that
the liquid crystal component 11' is in a liquid crystal-
line phase. The liquid crystalline phase can be a nematic,
cholesteric or smectic phase or mixtures thereof. When
in the liguid crystalline phase, the index of optical
refraction, i.e., the extraordinary index of refraction
ne, is different from that of the isotropic phase and
that of the resin-matrix 10 so that incident light, as
at Ior will be scattered by the liquid crystal, as at
Is. The mismatch between the index of refraction of
the resin matrix, ns, and that of the liquid crystal,
ne, and the light scattering properties of the liquid
crystal droplets cause the material to scatter light.
The material in the opaque state illustrated in Figure
2 appears as a white opaque texture. The resin matrix
containing the liquid crystal switches from a clear
state to a white opaque state as the temperature is
decreased to change the liquid crystal from the isotropic
to the liquid crystalline phase.
Figure 3 illustrates an electro-optic responsive
device 15 capable of being reversibly switched between
the opaque and clear states. A clear resin 16 containing
dispersed microdroplets 17 of liquid crystal having an
ordinary index of refraction nO similar to the refractive
index of the resin is sandwiched between electrical
conductors 18, one or both of which are transparent. A
voltage source 19 is connected to the conductors 18 by
a switch 20 having off and on positions 21, 22 respectively.
As illustrated in ~igure 5, when an electric field is
applied across the resin-liquid crystal material by
closing the switch 20, the material appears in the light
transmissive or clear state. The application of the
~X~80
electric field has the effect of aligning the extraordinary
index of refraction ne of the liquid crystal in a direction
normal to the surface of the film thereby allowing incident
light as at Io to pass through the display device 15
unscattered to emerge as at IT. When the voltage source
19 is disconnected by placing the switch 20 in its off
position 21, surface interactions at the droplet wall
between the liquid crystal and the resin return the
droplets to their random orientation as illustrated in
Figure 2 so that the resin-liquid crystal material of
the device appears as a white opaque texture.
Figure 4 illustrates the optical response that is
obtained when the displa~ material of the invention is
stretched by creating a mechanical strain, as indicated
by the arrows 30. The cured, clear resin is designated
by reference numeral 32 and the microdroplets of liquld
crystal that are elongated in the direction of stretch
are designated by reference numeral 34. The liquid
crystal can be nematic, smectic or cholesteric or a
mixture thereof in the liquid crystalline phase. Preferably,
the ordinary index of refraction nO f the liquid crystal
is similar to the refractive index ns of the resin.
Stretching of the material results in a distortion
of the liquid crystal droplet. The spherical droplet
adopts an elliptical shape with the long axis of the
ellipse parallel to the direction of stretch. This
distortion of the droplet results in the liquid crystal
within the droplet aligning itself with the long axis
of the ellipse. The result is that upon stretching all
liquid crystal droplets will have their optic axes and
hence their extraordinary index of refraction, ne,
aligned with the direction of stretch. Unpolarized
incident light, as at Io~ will have components which
are parallel to the stretch direction and parallel to
the optic axes of the droplets. These components will
.
6~
16
experience a large difference between the refractive
index of the liquid crystal droplet, ne, and that of
the surrounding resin, ns, and will be scattered.
Components of the incident light in a direction orthogonal
to the direction of stretch will encounter a refractive
index within droplet that is similar to the resin and
will pass through the film unaffected. The film therefore
acts as a light polarizer. In addition to the polarizing
effect, it has been found that the application of mechanical
stress to the liquid crystal-resin material embodied in
an elec~ro-optic cell as shown in Figure 3 decreases
the switch time between the field-on and field-off states.
Figure 5(a) illustrates a scattering polarizer 50
that is obtained when the material of the invention is
cured in the presence of a magnetic or electric field,
as indicated by the arrow 52. The cured, clear resin
- is designated by reference numeral 54 and the droplets
of liquid crystal having their extraordinary index of
refraction ne aligned in one direction in the plane of
the film are designated by reference numeral 56. The
liquid crystal has a positive anisotropy in the dielectric
and diamagnetic susceptibility. When cured in the presence
of an a.c. electric field or magnetic field of sufficient
strength to orient the liquid crystal droplets during
the curing process, the droplets will retain that orien-
tation upon removal of the field. The film will serve
as a light polarizer similar to that of the stretched
film described above in connection with Figure 4.
Components of incident unpolarized light, as at
Io~ which are parallel to the direction of alignment
OL the extraordinary index of refraction will be scattered,
as at Is, due to the mismatch between the index of refrac-
tion of the resin nS and the aligned optical axes ne f
the liquid crystal droplets. Incident light polarized
orthogonal to the direction of alignment will encounter
3~
17
no difference between the ordinary index of refraction
nO of the droplet 56 and that of the resin ns and will
be transmitted polarized as at IT~ The scattering
polarizer 50 of Figure 5(a), as well as the stretched
film of Figure 4, can be switched to a non-polarizing
light transmissive state in a-manner shown in Figure
5(b~, where the resin 54 containing the droplets as at
56' is sandwiched between electrical conductors 58, both
of which are preferably transparent. A voltage source
60 is connected to the conductors 58. The application
of the electric field has the effect of aligning the
extraordinary index of refraction normal to the surface
of the film thereby allowing incident light to pass
through unpolarized. When the voltage source 60 is
disconnected by placing the switch 62 in its off position
the droplets relax to their equilibrium orientation
illustrated in Figure 4 and Figure 5(a) and the display
material again polarizes incident light.
Figure 6(a) illustrates a film 70 that is obtained
when the material of the invention is cured in the
presence of a.c. electric or magnetic field as indicated
by the arrow 72. The cured, clear resin is desi~nated
by the reference numeral 74. The droplets of liquid
crystal having their extraordinary index of refraction
ne aligned normal to the surface of the film are
designated by the numeral 76. The liquid crystal has a
positive dielectric or diamagnetic anisotropy. Incident
light as at Io encounters no difference between the
extraordinary index of refraction ne f the droplet and
the refractive index nS of the resin and is transmitted
unscattered as at IT. The film appears clear. Figure
6(b) illustrates the film of Figure 6~a) when subjected
to an applied field in the plane of the film as indicated
by the arrow 80. The field mzy be a magnetic field, an
electric field or an electromagnetic field such as that
6~3~32
18
created by a high intensity light source. The application
of the field has the effect of aligning the extraordinary
index of refraction ne in the plane of film. Components
of unpolarized incident light in a direction parallel
to the direction of alignment of the extraordinary index
of refraction will encounter a mismatch between the
extraordinary index of refraction of the droplet and
the index of the resin and will be scattered. Com-
ponents of unpolarized incident light in a direction
orthogonal to the direction of alignment of the extraord-
inary index of refraction will encounter no such mismatch
and will pass through the device as polarized light, as
at IT-
Figure 7(a) illustrates a film 90 that is obtained
when the material of the invention is strained by asquee2ing which occurs when the material is cured at a
. tem~erature such that there is a mismatch between the
thermal expansivity of the resin 92 and that of con-
straining cell walls 94. The disk-like droplets of
liquid crystal having their extraordinary index of refrac-
tion aligned parallel to the surface of the film but
random in the plane of the film are designated by the
numeral 96. Incident light will be scattered due to
the mismatch between the extraordinary index of refraction
of the droplets 96 and the resin 92, and the device
will appear opaque. The disk-like droplets of Figure
7(a) will have an enhanced scattering effect as compared
to the spherical droplets of Figure 2, since ne lies in
the plane of the film for all the droplets 96~ Figure
7(b) shows the resin 92 containing droplets 96' sandwiched
between conducting electrodes 94'. The dielectric
anisotropy of the liquid crystals is positive and appli-
cation of a voltage across electrodes 94' causes the
extraordinary inde~ of refraction to align in a direction
normal to the film surfaces. Light incident upon the
19
device will detect no difference between the indices of
refraction of the droplets 95' and the resin 92 and
will be transmitted unscattered resulting in a clear
device. Such films exhibit improved display contrast
over films with spherical droplets.
The material described in conjunction with Figures
1-7 may be prepared by mixing together an uncured resin
such as epoxy or polyurethane and the liquid crystal.
Addition of the preferred 'cyanobiphenyl liquid crystals
-lQ to an uncured resin appears to result in a true solution
of liquid crystals in the resin. Prior to curing, the
liquid crystals in the solution do not appear to scat~er
incident light and the solution appears clear.
The cyanobiphenyl li'quid crystal readily dissolves
15 ' in uncured resin so that only gentle mixing is necessary
to form a homogeneous solution. In order to remove air
bubbles which may appear during mixing, the solution
can either be centrifuged or placed in an evacuation
chamber prior to curing.
As the liquid crystal-resin solution is cured, the
resin begins to solidify. As solidification of the
resin occurs, the liquid crystal molecules become immis-
cible in the resin and a~gregate into droplets. When
the resin is fully solidified, pockets or droplets of
liquid crystals in the liquid crystalline phase are
found entrapped within'the solid phase. The droplets
appear to be uniformly dispersed throughout the solid
and spherical in shape The size, shape and spacing of
the droplets depend upon a number of factors such as
the temperature at which the resin is cured, the types
of resin and liquid crystal material used, the relative
amounts of those materials, and the manner and rate of
curingr The materials of Figures 1-7 also can be prepared
using epoxy resins cured by ultraviolet radiation and
t'he like.
Thermo-optic materials that respond to different
temperatures are easily prepared using li~uid crystals
with different isotropic-liquid crystalline phase trans-
ition temperatures. They can also be prepared using
mixtures of different liquid crystal materials. With
currently existing nematic liquid crystals, it is pos-
sible to obtain a liquid crystal with an isotropic-
nematic phase transition at any temperature within the
range from -30C to 250C.
The cured resin containing the dispersed liquid
crystal microdroplets is normally a flexible solid and
~can be cut or cast into films or large articles. The
thermo-optic material has applicability in high resolu-
tion, high visual contrast thermometers or temperature
1~ indicators which can, for example, be used in medical
or other technologies, cold food packaging, refrigeration,
ice detection on road surfaces, and medical thermograms
for the detection of breast cancer, the location of the
placenta, etc. The thermo-optic material also can be
used in thermally addressed high contrast, wide viewing
angle, flat panel displays. Such displays can be elect-
rically addressed by a resistant or Joule-Thomson effect
- device to locally change the temperature of the material.
The material can also be addressed by a high intensity
light beam to locally heat the material surface.
Electro-optic displays and polarizing devices as
illustrated in Figures 3, 5(b) and 7(b) can be constructed
by deposition or painting of transparent conductive
coatings on the surfaces of the material or by curing
the resin while sandwiched between two plates containing
transparent conductive coatings. The visual contrast
between the opaque and clear states can be enhanced by
a suitable background such as one which is dark or lighted.
Electro-optic displays that are black or colored
in the opaque state can be constr~cted by the addition
21
of a dichroic dye to the liquid crystal. For example,
an electro-optic display as in Figures 3 or 7~b) which
is black when no voltage is applied but white when a
voltage is applied can be constructed by the use of
nematic liquid crystal containing a black pleochroic
dye. Such a li~uid crystal when cured in a clear resin
and placed on a white back~round can achieve a white on
black display.
A scattering polari~er film as illustrated in Figure
4 can be used as a strain monitor. The direction of
strain placed on the film controls the direction in
which the film polarizes. A change in the direction or
magnitude of the applied strain results in a change in
the direction or degree of polarization. The change of
polarization can be monitored by viewing the film through
a polarized lens.
Electrically addressable scattering polarizers can
be fabricated by stretching or by curing in the presence
of a field as illustrated in Figures 4 and 5(b), respect~
ively. A voltage of sufficient magnitude applied to
transparent conductors on one of these films aligns the
optic axes of the droplets normal to the surface of the
film and causes the polarizin~ effect to be switched
off and the film to appear transparent. Such a material
is useful in display windows or other devices where it
is desirable to switch the polarizing effect off and
on.
Optically switchable materials can be fabricated
with films prepared by curing in the presence of an
applied field or by straining so that the extraordinary
index of refraction, ne, is aligned normal to the surface
/ 'of the film. Such a film is clear and transmits light
/ at normal light intensities. Incident light of sufficiently
/ ~ ~ high intensity will cause the liquid crystals to reorient
/ ~ so that the optic axes of the droplets ~s ~witched to a
22
direction in the plane of the film. This film will
scatter light and appear opaque. The film acts as a
non-linear optical device to be used as a protective
coating to a high intensity electromagnetic source or
as a device in optical computing~ A non-linear optical
response is also possible in which high intensity incident
light changes the value of the refractive indices of
the liquid crysta~ relative to the index of the resin.
The best modes-of the invention are further illus-
trated and described by the following specific examples.
EXAMPLE I
A high contrast temperature responsive materialwas prepared using-a two-component epoxy material sold
under the trade designation Bostik 7575 by Emhart Chemi-
.,.~.
~ cal Group/ Bostik Division and the liquid crystal. Part
- A of the epoxy resin was an equimolar mixture of bisphenol
A and epichlorohydrin. Part B was a fatty polyamine
curing agent. The liquid crystal ~available as E-8
from EM Industries) was a mixture consisting of (by
weight): 4'n-pentyl-4'~cyanobiphenyl (5CB), 43 wt%;
4'-n-propoxy-4-cyanobiphenyl (30CB), 17 wt%; 4'-n-pentoxy-
4-cyanobiphenyl (50CB) 13 wt%; 4'-n-octyloxy-4-
cyanobiphenyl (80CB) 17 wt% and 4'-n-pentyl-4-cyano-
terphenyl (5CT) 10 wt%.
Part A and part B of- the epoxy resin and the liquid
crystal were mixed in equal proportions by voiume accord-
ing to the prescription 33-1/3~ part A, 33-1/3% part B
and 33-1/3% liquid crystals. All three components were
mixed together by gentle stirring for three minutes to
~ h ~Og~neO~s
form a ~h3~g4~4a~s solution. The solution was then cen-
trifuged for 1 minute to remove bubbles incorporated in
the stirring process. Samples were prepared by spreading
the uncured material with a uniform thickness on glass
pla'es. After curing for 48 hours, the samples that
~ tr~cle ~ncl~k
~6~
23
were about 200 microns thick had a pure white opaque
texture (opaque state). Samples having a film thickness
between 10 and 200 miçrons were also white in appearance,
but were less opaque. The films were peeled from the
glass surfaces to yield a solid flexible material. When
these films were heated to the nematic-isotropic phase
- transition temperature near 80C, they abruptly become
clear or transparent Iclear state). The films remained
clear at temperatures above 80C and returned to a pure
opaque condition when-cooled below 80C. The contrast
between the op~que and clear states depended on the
film thickness. Thickness of 200 ~ 100 microns showed
high visual contrast between the opa~ue and clear states.
The nematic isotropic transition temperature exhibited
~y the clear and opaque states of the film was very
near the nematic-isotropic transition temperature of
the liquid crystal prior to disperison in the epoxy
resin.
ExAMæLE II
An electrically responsive device using the same
material described in Example I was constructed. In
this example, the uncured mixture of Example I after
centrifuging was sandwiched between two glass slides
having indium oxide conductive coatings on their sur-
faces adjacent to the mixture. An insulating spacer
- ~ (Teflon tape) was used between the glass slides to con-
i~ trol the film thickness to approximately 75 microns.The film had a pure white opaque texture after 24 hours
of curing (opaque state). When an a.c. voltage of 100
volts was applied to the conducting surfaces of the
glass plates, the material turned clear (clear state).
A film thickness of less than 10 microns showed less
visual contrast between the clear and opaque states,
and also required a smaller switching voltage. A dark
~ ~rQ~ ~ark
24
or reflective background on the display was found to
improve the on-off visual contrast.
A sample of area 2.0 cm2 with an applied voltage
of 100 volts was observed to draw 5x10-8A in the clear
state giving a driving power of 5x10-6 watts.
EXAMP LE I I I
An electrically responsive guest-host device that
was an opaque blue in the opaque state and clear in the
-Lo clear state was constructed by the addition of a blue
dye to the liquid crystal mixture. The blue dye was 1-
~p-n-butylphenylamino)-4-hydroxyanthraquinone. It was
added to the liquid crystal mixture of Example 1, accord-
ing to the proportion 1.5% by weight blue dye to 98~5%
by weight liquid crystal. This mixture ~as then mixed
with part A and part B of the epoxy resin of Example 1
in the proportion (by volume) 33-1/3% part A, 33-1/3~
part B and 33-1/3~ blue dye and liquid crystal. As in
Example II, the material was allowed to cure between
two glass slides with conductive surface coatings. In
this example, a larger visual contrast between the clear
and opaque states was obtained with smaller film thick-
ness and hence lower voltages were applied to the con-
ducting surfaces. A display having a thickness of about
10 microns was found to be driven into the clear state
with an applied voltage of 25 volts.
EXAMPLE IV
A temperature responsive film was made using the
liquid crystal mixture of Example I and a two-component
fast curing epoxy resin (trade designation EP0-TEK~ 02)
consisting of bisphenol A resin, part A, and an aliphatic
curing agent (part B). The epoxy resin and liquid crystal
were mixed in the proportion (by volume): 25% part A,
25% part B, and 50~ liquid crystal. The film preparation
k
trclc~ r
- ~ \
~ 3v
~5
procedure used was identical to that of Example I. After
a curing time of two days the film had an opaque white
texture at temperatures below the liquid crystal isotropic-
nematic transition temperature (80~C~, but was clear
above that temperature.
~XAMPLE V
A mechanical stress and temperature responsive
material with light polarizing properties was prepared
by the dispersion of-the liguid crystal 4'-octyl-4-
cyanobiphenyl (available as K-24 from EM Industries) by
mixing it with a two-component epoxy resin in the pro-
portion (by volume~: 33-1/3% part A, 33-1/3% part B,
and 33-1/3~ liguid crystal. The epoxy resin consisted
f an equimolar mixture of bisphenol A and epichlorohy-
drin (part A), and a fatty polyamide curing agent, part
- B, purchased from Bostik Division, Milano, Italy. Two
samples were prepared, one using R-24 as the liquid
crystal and another using a mixture (by volume) of 75~
K-24 and 25% anisylidene-p-butylaniline. The stirring
procedure of the mixture was identical to that of Example
I. A film of thickness about 50 microns was prepared
by letting the mixture cure between a microscope glass
slide and a plastic cover slip. Once the mixture was
cured, the plastic cover slip was easily removed so the
film could be easil~peeled off the glass substrate. A
uniformly flat, ~n~ ~nd flexible material was obtained
that was opague at room temperature.
Upon stretching this film unidirectionally, it
became more transparent. The light passing through the
stretched film was observed to be linearly polarized in
a direction perpendicular to the stretching direction.
Upon heating the material to a temperature whereby the
liquid crystal was in the isotropic phase, the material
became clear and no polarization was observed either in
~ 8~
26
the free or in the stretched condition. Instead of a
stretch, a shear or simply unidirectional pressure served
to produce the same polarizing effect.
EXAMPLE VI
An electro-responsive cell with light polarizing
properties was prepared by mixing the following substances
in the following order: Epoxy part B, 32.5~ (by weight);
nematic liquid crystal, 33.5~; spacer material, 0.7~;
Epoxy part A, 33.3%. The epoxy was the same as that
described in Example V~ The nematic liquid crystal
(available as E-7 from EM Industries) was a mixture of
(by weight) 4'-n-pentyl-4-cyano~biphenyl (5CB), 51%;
4'-n-heptyl-4-cyano-biphenyl (7CB), 21%; 4'-n-octoxy-4-
cyano-biphenyl, 16%; and 4l-n-pentyl-4-cyano-terphenyl,
12~. The spacer material was a powder with a particle
- ~ size of 26 um (sùpplied as Alufrit~-PS-26 by the Atomergic
-'; Chemicals Corporation). While the mixture of liquid
crystal and epoxy was still in its uncured, fluid state
it was placed between two glass slides with transparent
conductive coatings to which a voltage could be applied.
The material was then cured for five days at -24C.
Upon subsequent warming to room temperature, the disparate
coefficients of expansion of the glass slides and the
epoxy resin matrix caused the matrix and the droplets
of the liquid crystals dispersed in the matrix to be
strained so that light passing through the material was
linearly polarized as evidenced by the extinction of
light viewed through a crossed polarizing lens. A volt-
age of 30 volts a.c. was then applied to the conductivecoatings and the responsive material was switched to a
state in which transmitted light was only faintly polar-
ized.
c2~ tr c~ de , n clr ~
~L2~
EXAMPLE VII
An electro-responsive cell was constructed from a
flexible epoxy resin-liquid crystal sheet stretched and
sandwiched between two transparent conducting surfaces.
The flexible epoxy resin-liquid crystal sheet was pre-
pared by first mixin~ liguid crystal with epoxy Part B,
then adding Part A in 1:1:1 proportions. The liquid
crystal was the same as in Example IV. Part A was an
equimolar mixture of bisphenol A and epichlorhydrin;
r P~rttBI~was a chemical curing agent (both available from
Division, Milan, Italy). The epoxy resin-li~uid
crystal mixture was allowed to cure between two plexiglass
sheets spaced about 50um apart. After curing for one
day, the resultant opaque white flexible sheet was
lS removed from the plexiglass. The sheet was stretched
by about 5 to 10% in one direction and sandwiched between
two glass slides. Each slide was coated on one side
with a transparent conductive coating. The sandwich
was constructed so that the conductive coatings faced
the stretched sheet.- A linearly polarizing film was
oriented with the sandwiched, stretched sheet to achieve
maximum extinction of transmitted light and then attached
to the sandwich.
Another cell was constructed as described, except
that the opaque white flexible sheet was not stretched
before being sandwiched between the conductive glass
slides.
When a voltage of 200 volts was applied to the
cell with stretched material, the response time between
opaque and clear states was in the order of one milli-
second. The cell with unstretched material responded
in 25-40 milliseconds.
28
~XAMPLE VIII
A dispersion of liquid crystals in a flexible solid
epoxy matrix was made by curin~ an epoxy~ uid crystal
formulation by ultraviolet light. The epoxy formulation
was a mixture of 3.8 grams of resin (Shell brand EPON~
resin 828), 0.4 grams of W activated epoxy curative
(3M brand FC-50~) and 0.9 grams of trimethylene glycol.
The liquid crystal was the same as that described in
Example VI. A solution was made by first mixing 0.3
grams of the epoxy formulation with 0.1 grams of the
liquid crystal. The solution was then cured for thirty
minutes under an ultraviolet lamp. The cured material
appeared an opaque white, but when heated tuxned clear
at the nematic liquid crystal isotropic transition
temperatures, thereby acting as a thermally responsive
light switch. The flexible solid, opaque dispersion
material also became partially clear upon stretching~
The transmissive light throu~h the stretched material
was observed to be linearly polarized and would extinguish
light when a crossed polarizer was placed either in
front or behind the stretched material. The material
only needed to be stretched 5-10~ of its original length
to show polarization effects. A slight compression or
other mechanical distortion also showed polarization
effects. When viewed through a cross polarizer the
material acted as a mechanically responsive light switch.
- EXAMPLE IX
A scattering polarizer was made using the same
liquid crystal described in Example VII and Bostik,
parts A and B (Bostik S.p.A., Milan, Italy). The ratio
of Bostik parts B and A was 1:0.94, respectively; a
mixture of parts A and B and liquid crystal was made
using 33 wt~ of E-7. 0.1 wt.% spacer material was added
3s to the mixture. The spacer material was a powder with
~ 1~r~c~e ~ark
29
a particle size of 26 um (supplied as Alufrit PS-26 by
the Atomergic Chemicals Corporation). The mixture was
stirred and centrifuged several times to achieve a homo-
geneous and gas-free solution which was then sandwiched
between two conducting glass plates ten minutes after
mixing the various components. The resulting 26 um
film was placed in a 47 kGauss magnetic field in a direc-
tion containing the plane of the film (henceforth called
the direction of cure) and left there for 41 hours at
15C. After removal from the field and cooling to room
temperature, the resulting solid film was observed to
be opaque when viewed with a linear absorption polarizing
filter whose polarization was parallel to the direction
of cure. If the polarization of the filter was turned
perpendicular to the direction of cure, the film appeared
transparent.
` `- The polarizing properties of the film were further
me2sured using polarizing light from a high intensity
light source at normal incidence. The ratio of the
intensity of the transmitted light when the beam was
polarized perpendicular to the direction of cure to the
intensity of the transmitted light when the beam was
polarized in a direction parallel to the direction of
cure was measured to be 30. Upon application of an
electric field in a direction normal to the film the
material switched to a non-polarizing (transmitting)
state. The response time was less than 0.3 milliseconds;
the time required for the film to relax back to the
polarizing state was less than 3.0 milliseconds. The
intensity of the light transmitted by the film was reduced
upon switching from the non-polarizing to the polarizing
state. This reduction was 2 orders of magnitude in the
case of incident light polarized in the direction of
curing, but was only about 3 fold for incident light
polarized in a direction perpendicular to the direction
of curing.
EXAMPLE X
A polarizer similar to that of Example I was made
except that a mixture of 67~ E-20 (43.96~ 4'-n-pentyl
4'-cyanobiphenyl; 40.78~ 4'-n-heptyl-4'-cyanobiphenyl;
9.22~ 4'-n-octyloxy-4-cyanobiphenyl; 6.05% 4'-n-pentyl-
4-cyanoterphenyl; BDH Chemicals, Ltd.) and 33~ lOCB
(4'-methoxy-4-cyanobiphenyl), deuterated on the methoxy
position, was used instead of E-7. This film showed
~the same polarizing properties as in Example I. A bulk
sample with the same composition was cured in an NMR
(Nuclear Magnetic Resonance) glass tube under the same
conditions as Example I. Deuterium nuclear magnetic
resonance spectra of this sample were taken at tempera-
tures between 10C and 45C and with the directions of
cure-oriented both along the static magnetic field and
perpendicular to it. Deuterium spectral patterns showed
that the liquid crystal molecules preferred an average
orientation such that the long molecular axis was along
the direction of the magnetic field during the curing
process.
EXAMPLE XI
Two films were made with the same compositions as
in Example IX and were cured in an electric field at
9C for 43 hours. During the curing process an a.c.
voltage of 100 v, oscillating at a frequency of 1 KHz
was applied to transparent conductors on the surfaces
of one of the films. The other film was cured without
the presence of an applied electric field. Following
the curing process, the films were examined for their
optical properties. At room temperature the film cured
in the electric field was more transparent than the
^~ ~
~68~)3~
31
film cured without the elecric field. This demonstrated
that the application of an a.c. electric field during
the curing process locked in the orientation of the
optic axes of the droplets in the cured medium.
EXAMPLE XII
A scattering polarizer was made by usin~ E-7 as
j ` the liquid crystal and Conuthane Tu50A, parts A and B
(Conap Inc., Buffalo, New York), as the polyurethane.
Part A is a prepolymer formed from the reaction of an
excess of toluene diisocyanate and polyether glycols,
and part B is a mixture of 4-4'-methylenebisisoortho-
chloroaniline and various polyols. Parts A and B were
mixed in the ratio of 1:0.94 respectively. A mixture
of 35~ E-7 and 65% parts A and B was made. To this was
added 26 um Alufrit spacers as in Example IX. The sample
was ~entrifuged to remove gas bubbles. A 26 um film
was made by sandwiching the mixture between conductive
glass plates. -The resulting sandwich was cured overnight
at 65C. Upon curing, droplets formed resulting in a
device which was opaque and scattering at room temperature.
A slight strain on the film resulted in the polarization
of light passing through the film. When viewed with a
linear absorption polarizin~ filter the device appeared
opague when the direction of polarization of the filter
and the direction of ~he applied strain were aligned;
the device appeared transparent when the directions
were orthogonal. The device may be switched electrically.
Approximately 26 volts must be applied across a film 26
um thick in order to completely switch from the scattering
to clear state. In both the stressed and relaxed states,
the application of an electric field caused the device
to respond in about 4 milliseconds. The relaxation
time of the device is hi~hly dependent on the stress.
c~ 7'r~C~e ~a.~l~
~ );3;~
A strained device relaxed in about 5 milliseconds, whereas
an unstrained device required 18 milliseconds to relax.
Many modifications and variations of the invention
will be apparent to those skilled in the art in light
of the foregoing detailed disclosure. Therefore, it is
to be understood that, within the scope of the appended
claims, the invention can be practiced otherwise than
as specifically shown and described
