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BACKGROUND OF THE INVENTION
This invention relates to the transformation of liquid crystalline
materials; and, more particularly, to the formation of a mixed Grandjean
and focal-conic texture of the cholesteric mesophase.
Many chemical compounds and mixtures have been found to exhibit physical
characteristics some of which are typically associated with liquids and
others which are typically unique to solid crystals. Liquid crystalline
substances typically exhibit the mechanical characteristics, such as
viscosities, which are ordinarily associated with liquids. In addition,
the optical scattering and transmission characteristics of liquid
crystalline substances are similar to those characteristics which are
typically unique to solid crystals. Because of these similarities to both
the liquid and crystalline forms of matter, the name "liquid crystals" has
become generic to substances exhibiting these dual properties.
The molecular structure of liquid crystals helps to explain their dual
characteristics. In liquid or fluid substances, the molecules are
typically randomly distributed and oriented throughout the mass.
Conversely, in crystalline solids the molecules are typically rigidly
oriented and arranged in a specific crystalline structure. Liquid crystals
resemble solid crystals in that the molecules of the liquid crystalline
substance are regularly oriented in a fashion analogous to but less
extensive than the molecular orientation and structure in a crystalline
solid. It has been observed that many substances exhibit liquid
crystalline characteristics only in a relatively narrow temperature range;
below this temperature range the substances appear only as crystalline
solids, and above the temperature range they appear only as liquids.
Liquid crystals are known to appear in at least three different forms: the
smectic, nematic, and cholesteric forms. These structural forms are
sometimes referred to as mesophases, thereby indicating that they are
states of matter intermediate between the liquid and crystalline states.
In the smectic structure the molecules are arranged in layers with their
major axes approximately parallel to each other and approximately normal
to the planes of said layers. Within a given layer the molecules may be
organized in uniform rows, or randomly distributed throughout the layer,
but in either case the major axes are still approximately normal to the
plane of the layer. The attractive forces between layers are relatively
weak so that the layers are free to move in relation to each other,
thereby providing the smectic liquid crystalline substance with the
mechanical properties of a planar or two-dimensional, soap-like fluid.
In the nematic structure, the major axes of the molecules lie approximately
parallel to each other, but the molecules are not organized into definite
layers as in the smectic structure.
In the cholesteric structure, the molecules are believed to be arranged in
definite layers as in the smectic structure; however, within a given
layer, the molecules are believed to be arranged with their major axes
approximately parallel in a fashion resembling the structure of nematic
liquid crystals. Because the major axes of the molecules in the
cholesteric structure are believed to be parallel to the planes of the
layers, the molecular layers are very thin. The cholesteric liquid
crystalline structure typically have molecules which are derivatives of
cholesterol or which are shaped very similarly to molecules of
cholesterol. Because of the shape of the cholesteric molecule, in the
cholesteric structure the direction of the major axes of the molecules in
each of the aforementioned thin layers is displaced slightly from the
direction of the major molecular axes in the adjacent molecular layers.
When compared to a hypothetical straight line axis passing through a
cholesteric liquid crystalline substance and perpendicular to the
molecular planes within said substance, the angular displacement of the
direction of the molecular axes within each adjacent molecular layer
traces out a helical path around the hypothetical straight line axis.
Cholesteric liquid crystals are known to exhibit various observable
textures. For example, cholesteric liquid crystals may adopt a
homeotropic, a focal-conic, or a Grandjean plane texture as modifications
of the cholesteric mesophase itself, as described, for example, in Gray G.
W., "Molecular Structure in the Properties of Liquid Crystals", Academic
Press, London, 1962, pages 39-54. The application of an electric field to
a liquid crystalline material in the Grandjean texture of the cholesteric
mesophase to transform the material from the Grandjean texture to the
focal-conic texture of the cholesteric mesophase is known. See, for
example, U.S. Pat. No. 3,704,056 to Wysocki et al.
The application of an electrical field to a liquid crystalline material in
the cholesteric mesophase to transform the material from the cholesteric
mesophase to the nematic mesophase is known. See, for example, U.S. Pat.
No. 3,652,148.
Phase transforming a liquid crystalline material from the Grandjean texture
of the cholesteric mesophase to the homeotropic texture of the nematic
mesophase by an applied electrical field and then abruptly switching off
the applied electrical field to cause reversion of the liquid crystalline
material from the nematic mesophase to the Grandjean texture of the
cholesteric mesophase is reported in "Electric Field Induced Texture
Changes in Certain Nematic/Cholesteric Liquid Crystal Mixtures", W.
Greubel et al., Vol. 24, Molecular Crystals and Liquid Crystals, pages
103-111, 1973. Therein, it is also reported on page 105 that if the liquid
crystalline material is only partly brought into the electrical field
induced nematic mesophase by increasing the field very slowly, then upon
abruptly switching off the applied electrical field, only the areas with
nematic alignment will return to the planar (Grandjean) texture. The other
parts of the initially Grandjean texture which were not transformed into
the nematic mesophase by the slowly applied electrical field shows the
focal-conic texture of the cholesteric mesophase.
We have now discovered that a liquid crystalline material transformed by an
applied electrical field into the nematic mesophase from either the
focal-conic or Grandjean texture of the cholesteric mesophase can be
transformed into a mixture of the focal-conic texture and the Grandjean
texture of the cholesteric mesophase by decreasing the amplitude of the
applied electrical field over a period of time effective to form the
mixture. This discovery differs from that reported in the Greubel et al.
article. In that article their end result of focal-conic region and
Grandjean texture region as reported on page 105 of the article is not a
mixture of two textures but two regions of different textures which come
from two sources: the focal-conic texture comes from the initially
Grandjean texture which did not transform into the nematic mesophase
during slow application of the electrical field; and, the Grandjean
texture in the mixture comes from the portion of the liquid crystalline
material which did transform into the nematic mesophase. These two
regional textures are formed by abruptly switching off the applied
electrical field. On the other hand, our discovery is that liquid
crystalline material initially in either the focal-conic or Grandjean
texture of the cholesteric mesophase and transformed into the nematic
mesophase can be transformed from the nematic mesophase into a mixture of
the focal-conic and Grandjean textures of the cholesteric mesophase by a
controlled decrease of the magnitude of the applied electrical field. Only
the regions transformed into the neumatic mesophase are transformed into
the focal-conic and Grandjean texture mixture. The regions of the
Grandjean or focal-conic texture of the initial cholesteric mesophase
which were not transformed into the nematic mesophase remain in their
initial focal-conic or Grandjean texture.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a novel method for
forming a mixture of the Grandjean and focal-conic textures of the
cholesteric mesophase.
It is a further object of this invention to provide a method of forming a
mixture of the Grandjean and focal-conic textures of the cholesteric
mesophase from a liquid crystalline material in the nematic mesophase.
These and other objects of the present invention are provided in accordance
with the practice of the present invention by applying an electrical field
to a liquid crystalline material in either the Grandjean or focal-conic
texture of the cholesteric mesophase, the applied electrical field being
within the cholesteric to nematic electrical field range of said liquid
crystalline material, to transform the liquid crystalline material into
the nematic mesophase; and then decreasing the magnitude of the applied
electrical field over a period of time effective to transform the liquid
crystalline material from the nematic mesophase to the mixture of the
Grandjean and focal-conic textures of the cholesteric mesophase. Typical
suitable times for field removal range from about 10 to about 200
milli-seconds.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The first portion of the practice of the present invention, i.e., applying
an electrical field to a liquid crystalline material in either the
Grandjean or focal-conic texture of the cholesteric mesophase to transform
the material into the nematic mesophase, the applied electrical field
being within the cholesteric to nematic electrical field range of said
liquid crystalline material, is amply described in U.S. Pat. No.
3,652,148, hereby expressly incorporated by reference. While that patent
is generically directed to the broader invention of transforming an
optically negative liquid crystalline material into an optically positive
liquid crystalline material by applying an electrical field across the
material, the lists of liquid crystalline materials and mixtures thereof,
the description of cell fabrication, the electrical field strengths
required for phase transformation, etc., are described in detail in U.S.
Pat. No. 3,652,148. These need not be elaborated upon herein. Furthermore,
a whole host of liquid crystalline materials and mixtures thereof
typically suitable for providing a liquid crystalline material in the
Grandjean or focal-conic textures of the cholesteric mesophase are listed
in U.S. Pat. Nos. 3,704,056 and 3,680,950 hereby expressly incorporated by
reference. Other typical suitable liquid crystalline materials in the
Grandjean texture of the cholesteric mesophase comprise: a mixture of
about 1 part by weight cholesteryl oleyl carbonate and about 2 parts by
weight p' -pentyl-p-cyanobiphenyl; methoxybenzylidene-butylaniline(MBBA);
about 10 parts by weight TN-100 (Hoffmann LaRoche) and about 4 parts by
weight cholesteryl oleyl carbonate and about 95% by weight K15
(4-cyano-4'-n-pentyl biphenyl) and about 5% by weight cholesterol
chloride.
Once the Grandjean or focal-conic texture of the cholesteric mesophase has
been transformed into the nematic mesophase by an application of an
electrical field within the cholesteric-to-nematic electrical field range
of the liquid crystalline material, the magnitude of the applied
electrical field is decreased over a period of time effective to transform
the liquid crystalline material from the nematic mesophase into the
mixture of the Grandjean and focal-conic textures of the cholesteric
mesophase. The period of time is greater than an abrupt switching off of
the applied electrical field and less than the electrical field removal
time period for the liquid crystalline material to become completely
focal-conic. That is, we have discovered that if the applied electrical
field amplitude is decreased over a sufficiently long period of time,
greater than about 200 milli-seconds the liquid crystalline material in
the nematic mesophase will transform completely into the focal-conic
texture of the cholesteric mesophase. The aforementioned Greubel et al.
article reports that the abrupt switching off of the applied electrical
field causes the nematic mesophase to transform completely into the
Grandjean texture of the cholesteric mesophase. Thus, the period of time
over which the applied electrical field is to be reduced in accordance
with the practice of the present invention in order to achieve a
transformation from the nematic mesophase into a mixture of the Grandjean
texture and focal-conic texture of cholesteric mesophase, is inbetween the
abrupt turn off of Greubel et al. and the very slow turn off resulting in
the complete transformation from nematic mesophase to focal-conic texture
of the cholesteric mesophase.
This period of time to be utilized in decreasing the applied electrical
field in accordance with the practice of the present invention, varies
from material to material. However, utilizing our reported discovery, one
skilled in the art can readily find the appropriate period of time with
which to practice the present invention for any given material simply by
phase transforming a liquid crystalline material from the Grandjean
texture of the cholesteric mesophase into the nematic mesophase by
applying an electrical field within the cholesteric-to-nematic electrical
field range of the liquid crystalline material; and, then, decreasing the
magnitude of the applied electrical field over various periods of time at
least two of which do not result in a mixture of the Grandjean and
focal-conic textures, one of the two period of times yielding complete
transformation into the Grandjean texture of the cholesteric mesophase and
the other period of time resulting in the complete transformation into the
focal-conic texture of the cholesteric mesophase. Then, one skilled in the
art should readily appreciate that the period of time over which the
applied electrical field is to be decreased in accordance with the
practice of this invention is one lying between these two extremes. A
typically suitable range is from about 10 to about 200 milli-seconds.
Furthermore, the period of time to be utilized can be varied between these
two extremes to obtain varying relative amounts of the Grandjean and
focal-conic textures in the mixture. At the lower range of the time period
between these two extremes, the Grandjean texture predominates; whereas,
at the upper range of the time period between these two extremes, the
focal-conic texture predominates.
Variations in the relative amounts of each texture in the mixture of
Grandjean and focal-conic textures will result in variations in brightness
and contrast of the resulting image. The various electro-optic cells used
for imaging in U.S. Pat. No. 3,652,148 can be employed in the practice of
the present invention to provide photoconductor address, electron beam
address, X-Y matrix address, shaped electrode address, shaped liquid
crystalline layer imaging, etc. Enhanced brightness and contrast in
imaging is provided by selectively varying the relative amounts of
Grandjean and focal-conic textures in the mixture resulting from the
practice of the present invention.
The following examples further specifically define the present invention
with respect to forming a mixture of the Grandjean and focal-conic
textures. The parts and percentages are by weight unless otherwise
indicated. The examples below are intended to illustrate various preferred
embodiments of the present invention.
EXAMPLE I
A mixture of about 95% by weight K15 (4-cyano-4'-n-pentyl biphenyl)
available from British Drug House and about 5% by weight cholesterol
chloride is placed at a thickness of about 10 microns between two
electrodes. Each electrode comprises a glass slide overcoated with a layer
of indium oxide over which silicon monoxide is deposited in accordance
with the Janning technique. This technique is reported in "Thin Film
Surface Orientation For Liquid Crystals", J. Appl. Phys. Letter, Vol. 21,
No. 4, August 15, 1972. This silicon monoxide is deposited at an angle of
60.degree. from the normal to the glass slide.
The cholesteric to nematic transition voltage threshold for the mixture
used is about 10 volts. This voltage is applied between the two electrodes
and the initially Grandjean texture of the cholesteric mesophase is
transformed into the nematic mesophase. The entire layer of liquid
crystalline material is transformed into the nematic mesophase.
The applied voltage of about 10 volts is decreased in magnitude to about 0
within a time period of about 1 micro-second. The entire layer of liquid
crystalline material assumes the Grandjean texture of the cholesteric
mesophase. The applied voltage of about 10 volts is decreased in magnitude
to about 0 over periods of time varying up to about 10 micro-seconds. In
each case, the entire layer of liquid crystalline material assumes the
Grandjean texture.
The applied voltage of about 10 volts is decreased in amplitude to about 0
volts over various time periods ranging from about 10 milli-seconds to
about 200 milli-seconds. The entire liquid crystalline layer becomes a
mixture of the Grandjean and focal-conic textures of the cholesteric
mesophase.
The applied voltage of about 10 volts is reduced in amplitude to about 0
over varying time periods greater than about 200 milli-seconds. In each
case, the entire liquid crystalline layer assumes the focal-conic texture
of the cholesteric mesophase.
EXAMPLE II
Example I is repeated except that the indium oxide coatings on the glass
slides are shaped in the form of a letter "X".
When the applied about 10 volts is decreased in magnitude over a period of
time between about 10 to about 200 milli-seconds, the region of the liquid
crystalline layer corresponding to the shaped electrodes in the form of a
"X" is a mixture of the focal-conic and Grandjean textures of the
cholesteric mesophase. The "Background" region of the liquid crystalline
layer; i.e. that region of the layer outside the boundaries of the shaped
electrodes exhibits the Grandjean texture of the cholesteric mesophase.
When the applied voltage of about 10 volts is decreased in magnitude to
about 0 over a time period varying from about 1 to about 10 micro-seconds,
the entire layer of liquid crystalline material, including that
corresponding to the shaped electrodes, exhibits the Grandjean texture.
When the applied about 10 volts is decreased in magnitude to about 0 over
periods of time greater than about 200 milli-seconds, the region of the
liquid crystalline layer corresponding to the shaped electrodes exhibits
the focal-conic texture of the cholesteric mesophase and the background
region of the liquid crystalline layer exhibits the Grandjean texture of
the cholesteric mesophase.
In preferred embodiments of the present invention, as in the Examples, the
liquid crystalline material initially provided in either the Grandjean
texture or the focal-conic texture of the cholesteric mesophase, comprises
a mixture of a nematic and an optically active material. The optically
active material can be either mesomorphic or non-mesomorphic, as is
well-known in the art. See, for example, U.S. Pat. No. 3,909,114, hereby
expressly incorporated by reference, at Columns 5 and 6 thereof, for a
listing of a whole host of nematic and mesomorphic and non-mesomorphic
optically active materials which can be utilized. The combination of a
nematic liquid crystalline material and an optically active material is
particularly preferred to provide enhanced storage of the mixture of
Grandjean and focal-conic textures provided in accordance with the
practice of the present invention. Since the voltage applied to transform
the initially Grandjean or focal-conic texture of the cholesteric
mesophase into the nematic mesophase is decreased in magnitude over a
period of time in accordancce with the practice of the present invention,
it will be appreciated that when the applied voltage is reduced in
magnitude to about 0, the liquid crystalline layer must have "memory" or
"storage" in order for the resulting Grandjean and focal-conic texture
mixture to have persistence after the about zero applied voltage is
reached. Liquid crystalline materials having the structure of the
cholesteric mesophase exhibit this persistence or storage. Therefore, a
cholesteric liquid crystalline material or a nematic liquid crystalline
material and an optically active material can be employed to achieve this
persistence or storage.
While the present invention has been described with respect to preferred
embodiments, it will be appreciated, upon a reading of the present
disclosure, that other modifications and variations may be made without
departing from the spirit of the present invention. These are intended to
be included within the scope of the present invention.
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