Note: Descriptions are shown in the official language in which they were submitted.
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1
POLYIMIDE LCD ALIGNMENT LAYERS
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefit of pending U.S. Provisional Application
Nos. 60/259,162 and 60/259,235, both filed on January 2, 2001.
BACKGROUND OF THE INVENTION
Liquid Crystal Displays (LCD) are currently used for a variety of display
applications, such as watch faces, calculators, computer screens and other
types of
electronic equipment. LCD technology offers widely known advantages over
traditional display technologies such as cathode ray tubes. Among these
advantages are low weight and low power consumption.
Liquid crystal displays, however, have previously been considered to
provide a narrower field of view than traditional display technologies, such
as the
previously mentioned cathode ray tubes.
Liquid crystal displays typically contain a plurality of liquid crystal cells.
Each liquid crystal cell generally contains a liquid crystal material
sandwiched
between two substrates. Located on either side of the liquid crystal material
is a
set of electrodes which are typically indium-tin oxide (ITO) or tin oxide. A
pair of
polarizing filters is located outside of the substrates, with each filter on
an opposite
side of the liquid crystal cell. The polarizers are oriented at right angles
relative
to each other. The orientation of the liquid crystal material in the cell
determines
whether light passes through each polarizer in the absence of external
influence
such as an electric field, thereby giving a transparent appearance, or whether
light
is blocked by one of the polarizers, thereby giving the cell a darkened
appearance.
The orientation of the liquid crystal material is changed by the application
of an
electric field by the electrodes to alter light transmission through the cell.
Typically, the liquid crystal material is aligned such that the cell appears
opaque
or transparent absent an application of an electric field through the
electrodes.
When an electric field is applied to such a cell, the orientation of the
liquid crystal
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material is altered in such a way as to prevent the transmission of light
through the
cell, malting the cell appear darkened.
The orientation of a liquid crystal material at its surface is dependent on
the orientation of material it comes in contact with. It is known to coat the
surface
of a substrate with an agent which influences the orientation of a liquid
crystal
material that comes in contact with the coated substrate. Such coating agents
are
known as alignment layers. Various materials and methods have been used in
establishing an alignment layer of a desired orientation. For example, it is
known
in the art that an alignment layer may comprise anisotropically absorbing
molecules which can be oriented by exposure to polarized light. Inorganic thin
films, such as metal oxide films, which have been deposited on a substrate at
an
oblique angle can also be used as alignment layers as disclosed in U.S. Patent
No.
5,635,197.
It is also known to use a polymeric alignment layer which can be
oriented by means of a mechanical buffing process. In such a process, a
polymer
layer is applied to a substrate and is buffed with a cloth or other fibrous
material.
Liquid crystal material coming into contact with a surface treated in this way
typically aligns itself parallel to the direction of buffing.
Polyimides are frequently used as a polymeric alignment material for
liquid crystal cells and for optical compensator layers including O-plate
compensators. Polyimides generally display good chemical stability and are
easily
deposited on a substrate and rubbed. Polyimides are generally prepared by
contacting a diamine with an acid anhydride, producing a polyamic acid. This
polyamic acid may be coated onto a substrate and heat treated at about
150°-
230°C, converting the polyamic acid to a polyimide. The polyimide film
is then
mechanically rubbed as mentioned above.
Inducing the proper orientation of liquid crystal material is important in
optical compensators. As mentioned above, LCDs frequently have a narrow field
of view. It is frequently desirable to increase this field of view especially
in
applications such as computer displays, anionic displays and televisions. The
viewing zone of an LCD that is not equipped with an optical compensator is
narrow
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because light leaks through the liquid crystal material when viewed at angles
other
than those close to normal relative to the surface of the liquid crystal. Such
light
leakage degrades the image quality and can also cause color shifts in color
LCDs.
Optical compensators have been used to increase the viewable angle of LCDs
without negatively affecting image quality when viewed normal to the surface
of
the LCD. Optical compensators typically take the form of an additional layer
of
liquid crystal material located between a polarizer and the ,viewing area, on
the
outer surface of an LCD. This liquid crystal material may be given a specific
orientation under the influence of an alignment layer material.
O-plate compensation films, or O-plate compensators, are one type of
optical compensator. O-plate compensators generally minimize reversal of gray
levels and improve overall gray scale stability. O-plate compensators have
been
previously described as comprising a positive birefringent material which has
a
principle optic axis oriented at an oblique angle relative to the surface of
the liquid
crystal layer. An oblique angle includes any angle between 0° and
90°. In previous
O-plate compensators, this angle has been provided in various ways. For
example,
U.S. Patent No. 5,619,352 describes an O-plate compensator which includes an
alignment layer, a liquid crystal pretilt layer, and a liquid crystal
compensator
layer. The described O-plate compensator depends on the liquid crystal pre-
tilt
layer to provide an adequate pre-tilt angle for the liquid crystal compensator
layer
because the alignment layer produces only a 1° to 10° liquid
crystal pretilt angle
at the alignment layer/liquid crystal pre-tilt layer interface. The described
O-plate
compensator therefore depends on multiple layers of liquid crystal material to
provide an adequate angle of orientation of the liquid crystal material. A
similar O-
plate compensator is also described in U.S. Patent No. 5,956,734 and PCT
Application No. WO 96/10770. The use of high pre-tilt alignment layers is also
known in LCDs known as pi-cells.
It should be appreciated that the term "pre-tilt angle" has frequently
been used in the prior art to describe a final angle provided by a combination
of
an alignment layer and a liquid crystal layer. Heretofore, no single polyimide
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alignment layer for a liquid crystal layer has provided a pre-tilt angle
greater than
about 15°.
Therefore, there is a need for a polyimide alignment layer material
which can provide a high, uniform pre-tilt angle.
SUMMARY OF THE INVENTION
In general, the present invention provides a polyimide comprising a
reaction product of at least one dianhydride and at least one diamine, wherein
the
at least one diamine contains a mesogenic group, with the proviso that when
the
at least one dianhydride is 2,2'-bis-(3,4-dicarboxyphenyl)-1,1,1,3,3,3-
hexafluoropropane dianhydride (6FDA) or dibromo-biphenyltetracarboxylic
dianhydride, the at least one diamine is not
HEN O ~ NHS
A A
I I
~CH2~ 6 ~CH2~ 6
O O
25 CN CN
wherein A is selected from the group consisting of O and COO.
The present invention also provides a method for inducing a
predetermined orientation of a liquid crystal material, the method comprising
applying an alignment layer material to a substrate, and buffing the alignment
layer material, thereby providing an alignment layer with a pre-tilt angle,
wherein
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ua.ss8 5
the alignment layer material is a reaction product of at least one dianhydride
and
at least one diamine, and wherein the at least one diamine contains a pendent
mesogenic group.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Figure 1 is a schematic summary of a method of preparing a mesogenic
group of the present invention.
Figure 2 is a graph showing the pre-tilt angles of the polyimides
6FDA/C6CNand6FDA/C6CN(ether) after heat treatmentatvarioustemperatures.
Figure 3 is a graph showing the pre-tilt angles of the polyimides
6FDA/C6CN and 6FDA/C6BP after heat treatment at various temperatures.
Figure 4 is a graph showing the pre-tilt angles of the polyimides
6FDA/C6CN and 6FDA/CllCN after heat treatment at various temperatures.
o Figure 5 is a graph showing the pre-tilt angles of polyimides containing
varying amounts of diamines with mesogenic pendent groups (C6BP) and
diamines with perfluorinated carbon pendent groups (PFMB).
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed toward polyimides which can be used
to prepare liquid crystal display alignment layers for a liquid crystal
device, such
as an optical compensator, for example. The polyimides of the present
invention
contain mesogenic substituents and may optionally include functional
substituents.
Polyimides may be schematically represented by the structure
O O
- II 11
'/C\ASC\ B N/
N.C/ ~Ci \
II II n
O O
wherein A is one or more residues from an acid dianhydride group and B is one
or
more residues from a diamine compound and n is a positive number. It has been
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UA.388
known that the properties of the polyimide may be altered by varying the
components "A" and "B" as listed above. However, the use of polyimides
containing mesogenic substituents to prepare high pre-tilt alignment layers
has not
been previously known. In the present invention, mesogenic groups are
contributed
to the structure of a polyimide by the diamine component. Any acid dianhydride
useful in the synthesis of polyimides may be utilized in the present
invention. Such
acid dianhydrides are commercially available.
As mentioned above, polyimide polymers are prepared from diamines
containing pendent mesogenic groups. In one particular example, the diamine
contains a backbone portion, a methylene spacer, a linking group, and a
pendent
mesogenic group. The pendent mesogenic group is attached to the methylene
spacer, the methylene spacer is attached to the linking group, and the linking
group is attached to the backbone portion. The linking group is selected from
the
group consisting of an ester and an ether. In another embodiment, suitable
diamines are represented by formulas I and II below.
/(CHz)~ O -RZ
R1 R3
H2N ~ ~ . NH2 (I)
R3 R1
R2 O-(CHI)
H~,N NHS,
R1
I (II)
(CH2)x
R2
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In formulas I and II, R1 is an ester or ether linlung group, R2 is a
mesogenic group or a functional group as defined below, and x is a positive
number. In formula I, Rg is hydrogen or a halogen. In one embodiment, x is
between 6 and 18. In another example, x is between 6 and 11. In one particular
example, x is 6. In another example, R3 is bromine.
Mesogenic groups are groups with a rod-like molecular structure. That
is, mesogenic groups, or simply mesogens, are groups with a length to width
ratio
of at least 5:1. Functional groups are those groups which allow one polyimide
molecule to react with another molecule. Among preferred functional groups are
groups which permit the crosslinking of polyimide molecules within a layer.
Especially preferred functional groups include molecules which allow the
photopolymerization of polyimide molecules, such as acrylate and methacrylate
groups. Suitable diamines include those containing a substituent selected from
the
group of compounds containing one or more of the subunits represented by
formulas III, IV, V, and VI.
O GH3
C-C=CH2 (III)
O O
O CN
O X O
c-o O o-c O R4 (vI)
so
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In formula VI, X may be hydrogen or an organic group having from 1 to 20
carbon
atoms, and Rq. may be an organic group selected from the group consisting of
esters, ethers, groups containing a methylene subunit, groups containing a
crosslinking subunit and groups containing a combination of any of these
subunits.
Groups containing acrylate or methacrylate subunits may be crosslinlced such
as
by photopolymerization, for example. In one example, X is an organic group
containing between 1 and 16 carbon atoms. In another example, X is an organic
group containing between 1 and 12 carbon atoms. In still another example, X is
a
methyl group. In yet another example, when the at least one dianhydride is
2,2'-
bis-(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride ordibromo-
biphenyltetracarboxylic dianhydride, the substituent is not represented by
formula
V.
By way of example and not of limitation, a mesogenic group shown in
VI may be synthesized by the following method as described below with
reference
to Figure 1. Ethyl 4-hydroxybenzoic ester is alkylated with 6-chlorohexanol to
produce intermediate (1) . Intermediate (1) was used in two different
reactions. In
the first reaction, the hydroxyl group of intermediate (1) was protected using
3,4-
dihydro-2-pyran (DHP) forming intermediate (2) . Intermediate (2) was then
hydrolyzed to form a THP-benzoic acid derivative (4) . In the second reaction,
intermediate (1) was hydrolyzed to generate a hydroxy terminated benzoic acid
(3). The hydroxy terminated benzoic acid (3) is contacted with CH2CH2COC1 in
an organic solvent to form a intermediate (5). Tert-butyl dimethylsilyl
chloride
(TBDMS) was used to protect methyl hydroquinone. The major isomer was isolated
by chromatography and then reacted with the THP-benzoic acid derivative (4)
forming intermediate (6). This reaction product was then selectively
deprotected
using tert-butylamonium fluoride (TBAF) in tetrahydrofuran (THF) to form
intermediate (7) . Intermediate (7) was esterified with intermediate (5)
forming
ester (8) and the resulting ester was deprotected with AMBERLYSTRlS in a
mixture of methanol and THF, forming mesogenic group (9) . Various components
used in this synthesis method may be varied to affect the composition of the
resulting diamine without undue experimentation.
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Mesogenic group (9) may be contacted with dinitro diphenic acid
followed by tin (II) chloride reduction in ethanol to form a diamine of
formula I.
Such a diamine may be used to prepare a polyimide of the present invention.
Mesogens within the class encompassed by V may be synthesized in the
following manner. 4-cyano-4'-hydroxybiphenyl may be contacted with an c~-
bromoalkanol in an SN2 reaction in refluxing acetone over 3-4 days. This
results
in the production of a 4-(c.~-hydroxyalkoxy)-4'-cyanobiphenyl compound which
may be further purified by recrystalization from ethanol. Alternatively, 4-
cyano-4'-
hydroxybiphenyl may be contacted with an cx,c~ alkanediol in a Mitsunobu
reaction
to form a 4-(c~-hydroxyalkoxy)-4'-cyanobiphenyl compound. The product of the
Mitsunobu reaction may be purified by flash chromatography. Mesogens within
the
class encompassed by IV may be synthesized by similar methods, by starting
with
a hydroxybiphenyl compound instead of 4-cyano-4'-hydroxybiphenyl.
These mesogens may be used to produce mesogen-containing diamine
compounds of formula I by coupling the mesogen with a dinitro diphenic acid
using the standard dicyclohexylearbodiimide (DCC)/DMAP procedure to produce
a dinitro intermediate compound. Alternatively, dinitro diphenic acid may be
converted to 4,4'-dinitro-2,2'-biphenyl-carbonyl chloride by refluxing with
thionyl
chloride. The mesogen may be contacted with 4,4'-dinitro-2,2'-biphenyl-
carbonyl
chloride in an organic solvent such as triethylamine or methylene chloride to
produce a dinitro intermediate. The dinitro intermediate may be reduced to
form
a diamine by stannous chloride reduction or by reduction using hydrazine in an
organic solvent at 80 °C.
Mesogens of the present invention may also be coupled to brominated
biphenylcarboxylic acids to produced brominated diamines of formula I.
Cyanuric
acid is contacted with bromine and the resulting compound is used to brominate
4,4'-dinitro-2,2'-biphenyl-carboxylic acid yielding 6,6'-dibromo-2,2'-
biphenylcarboxylic acid. This brominated carboxylic acid may be coupled with a
mesogen and reduced as described above to produce a brominated diamine.
Diamines of formula II may be synthesized by the following technique.
3,5-dinitrobenzoic acid is esterified with n-octadecanol to afford n-octadecyl
3,5-
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dinitrobenzoate using DCC as a dehydration agent in dichloroethane. The
dinitrobenzoate is reduced to n-octadecyl 3,5-diaminobenzoate using hydrazine
as
a reducing agent. By substituting other alcohols for n-octadecanol, the value
of x
in formula II may be varied.
Diamines of the present invention may be purified by chromatography
on deactivated silica gel and subsequent r ecrystalization. Purified diamines
may
then be contacted with acid dianhydrides to produce polyimides. The synthesis
of
polyimides is known in the art. See for example, "Synthesis and
Characterization
of Aromatic Polyesters and Polyimides Containing Mesogenic Pendent Groups,"
PhD dissertation of Shyh-Yeu Wang, The University of Akron, December,1995, the
disclosure of which is herein incorporated by reference. Briefly summarized,
polyimide precursors may be synthesized from dianhydrides and diamines by
either
a 2-step or a l-step method. In the 2-step method, a soluble polyimide
precursor,
i.e., a polyamic acid, is prepared by the reaction of dianhydrate and diamine
in a
polar aprotic solvent at room temperature. The polyimide precursor is
cyclodehydrated to form the corresponding polyimide either by thermal or
chemical methods. The 2-step method gives high molecular weight polyimides if
the diamine is highly reactive. However, when the diamine contains electron
withdrawing groups such as CFg, CN and NO~,, for example, the reactivity of
the
diamine is reduced and low molecular weight products result. When such
electron
withdrawing groups are present, the 1-step method is preferred. In the 1-step
method, polymerization is carried out by heating the dianhydride and diamine
at
1S0°-220°C in high boiling solvents, such as m-cresol and p-
chlorophenol for
example, in the presence of a tertiary amine catalyst. Under these conditions,
polymerization and imidization occur essentially simultaneously. The water
generated from imidization is continuously removed, such as by distillation
for
example.
It has surprisingly been found that the pre-tilt angle of the alignment
layer may be altered by varying the composition of various substituents of
mesogen-containing polyimides. For example, the linking group R1 in formulas
I and II greatly influences the pre-tilt angle generated by the resulting
polyimide.
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When R1 is an ester group the resulting polyimide has a greater pre-tilt angle
than
when R1 is an ether group. It has also been determined that a cyano-
substituted
biphenyl mesogenic group (formula V, for example) gives a polyimide that
exhibits
a slightly higher pre-tilt angle than a polyimide containing a non-substituted
biphenyl mesogenic group (formula IV).
An alignment layer material made from the polyimide of the present
invention provides a high pre-tilt angle. In one embodiment, the alignment
layer
provides a pre-tilt angle between about 5° and about 90°.
Preferably, the alignment
layer provides a pre-tilt angle between about 10° and about 80°.
More preferably,
the polyimide layer provides a pre-tilt angle between about 20° and
about 80°. In
one particular example, the polyimide layer provides a pre-tilt angle between
about 40° and about 70°. It will be appreciated that the pre-
tilt angle described
herein relates to the use of one single alignment layer with one single liquid
crystal
layer and not a plurality of layers to obtain the above mentioned angles. A
greater
thickness of liquid crystal material is not required.
It has also been determined that the dianhydride used to synthesize a
mesogen-containing polyimide also affects the pre-tilt angle. For example, the
dianhydride 2,2'-bis-(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane
dianhydride (6FDA) provides a polyimide with a greater pre-tile angle than a
similar polyimide based on 3,3',4,4"-biphenyltetracarboxylic dianhydride
(BPDA).
The present invention is not limited to the dianhydrides 6FDA and BPDA. Any
acid
dianhydride suitable for generating traditional polyimides for alignment
layers may
also be used in the polyimide of the present invention. Among other acceptable
acid dianhydrides are 2,2'-bis[4-(3,4 dicarboxyphenoxy)phenyl]propane
dianhydride (BisA-DA), pyromellitic diahydride (PMDA), dibromo-
biphenyltetracarboxylic dianhydride, 3,6-diphenylpyromellitic dianhydride,
3, 6-bis (trifluon omethyl) pyromellitic dianhydride, 3, 6-bis (methyl)
pyromellitic
dianhydride, 3,6-diidopyromellitic dianhydride, 3,6-dibromopyromellitic
dianhydride, 3,6-dichloropyromellitic dianhydride, 3,3',4,4'-
benzophenonetetracarboxylic acid dianhydride, 2,3,3',4'-
benzophenonetetracarboxylic acid dianhydride, 2,2',3,3'-benzophenone
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tetracarboxylic acid dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride,
bis(2,5,6-trifluoro-3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-
dicarboxyphenyl)propane dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride
(4,4'-oxydiphthalic anhydride), bis(3,4-dicarboxyphenyl)sulfone dianhydride,
(3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride),
4,4'-[4,4'-isopropylidene-di(p-phenyleneoxy)]bis(phthalic anhydride),
N,N-(3,4-dicarboxyphenyl)-N-methylamine dianhydride,
bis (3,4-dicarboxyphenyl) diethylsilane dianhydride; naphthalene
tetracarboxylic
acid dianhydrides such as 2,3,6,7- and 1,2,5,6-naphthalene-tetracarboxylic
acid
dianhydride, 2,6-dichloronaphthalene-1,4,5,5-tetracarboxylic acid dianhydride;
or
heterocyclie aromatic tetracarboxylic acid dianhydrides such as
thiophene-2,3,4,5-tetracarboxylic acid dianhydride, pyrazine-
2,3,5,6-tetracarboxylic acid dianhydride and pyridine-2,3,5,6-tetracarboxylic
acid
dianhydride.
Substituents in the diamine component other than the mesogenic group
also affect the pre-tilt angle of the resulting polyimide. When a diamine
component contains only mesogenic substituents, a polyimide with a greater pre-
tilt angle results compared to similar polyimides containing a diamine
component
which contain bromine substituents as well as mesogenic substituents.
The pre-tilt angle of a polyimide can also be varied by co-polymerization
of a mixture of diamines with a dianhydride. In such an example, a diamine
containing pendent mesogen groups may be mixed with a diamine containing
perfluorinated carbon atoms and polymerized with a diamine such as 6FDA.
Finally, it has been found that increasing the length of a methylene
spacer gives a higher pre-tilt angle at lower heat treatment temperatures. The
pre-
tilt angle yielded by such polyimide used as an alignment layer, however,
decreases
rapidly as the heat treatment temperatures increase.
In order to demonstrate the practice of the present invention, polyimides
containing pendant mesogen groups contributed by a diamine were synthesized
and tested for the pre-tilt angle they produced. In the following examples one
or
more diamines and an acid dianhydride were polymerized in refluxing m-cresol,
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UA.388 13
1-chloronapthalene or o-dichlorobenzene containing isoquinoline at 180 -
200°C.
A nitrogen purge was used to remove the water. The polymers were isolated by
precipitation in methanol and dried under reduced pressure at about
200°C for 6-8
hours. The resulting polyimides were dissolved in an organic solvent such as
cyclopentanone and N-methylpyrrolidone (NMP) at 1.5 weight percent and
filtered
through 1.0 ~cm filters. Alignment layers were formed by spin coating on an
indium-tin oxide (ITO) glass substrate at 2,000 rpm. The layers were heat
treated
at 150°C, 200°C, 225°C, or 250°C before mechanical
rubbing. The rubbing was
carried out on a LCBM4 liquid crystal buffing machine. Liquid crystal displays
were
then constructed via a standard procedure. ZLI2293 liquid crystal molecules
(available from Merck) were added to the cells at room temperature. The
pretilt
angles provided by the polyimides were determined by either the crystal
rotating
method or the magnetic null method. The composition of the various polyimides
tested, an abbreviation of each polyimide, the pre-tilt angle provided by each
polyimide and comments regarding the observed uniformity of the alignment
layer
are summarized in Tables 1-4. A prior art polyimide (6FDA-PFMB) is also
ineluded
in Table 1 for comparison purposes. Pre-tilt angles after heat treatment at
225°C
are listed in Table 1. In Table 2, pre-tilt angles were measured after heat .
treatment at 250°C, except as noted otherwise. In Table 3, pre-tilt
angles were
measured after heat treatment at 200°C, except as noted otherwise, and
pre-tilt
angles after heat treatment at 150°C are listed in Table 4.
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UA.388 1q,
w ~ w
r, ,.,
U
.N
0
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O O
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UA.388 15
w w
..~ . r, . r,
>~ >~ s~
0 0 0 0
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r-1 N
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o=uf~o
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o=' u=o =o q y
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o= \ =o O o
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n
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o-cr z o n
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O
CA 02433747 2003-07-02
WO 02/054140 PCT/US02/00058
UA.388 1(
p O O
0 0
N ~ d'
r~
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V
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n ~
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z E
o-u~ ~ -o ~ s iz~ °s
p-V cp a
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U. O O ~ r~ ~ O-U ~ O
h, U.,~
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CA 02433747 2003-07-02
WO 02/054140 PCT/US02/00058
UA.388 2 ~
As mentioned above and as indicated by the data in Table l, a number
of different factors affect the pre-tilt angle provided by the polyimide of
the present
invention. The type of linkage between the mesogen and the aromatic portion of
the diamine is one such factor. Fox example, 6FDA/C6CN provides a pre-tilt
angle
of 42°, while 6FDA/C6CN(ether) provides a pre-tilt angle of 20°.
These polyimides
differ from each other only in the type of linkage between the mesogen and the
aromatic group: It is apparent, therefore that the ester linkage of 6FDA/C6CN
provides a greater pre-tilt angle than the ether linleage of 6FDA/C6CN(ether).
As also shown in Table 1, the use of a cyano-substituted mesogen gives
a polyimide which provides a slightly higher pre-tilt angle than a non-
substituted
mesogen. For example, the pre-tilt angle provided by 6FDA/C6CN is greater than
the pre-tilt angle provided by 6FDA-C6Biph. These compounds differ only in the
substitution of the biphenyl portion of the mesogens of each polyimide.
The dianhydride used in the polyimides of the present invention also
affect the pre-tilt angle. For example, 6-FDA provides a greater pre-tilt
angle than
BPDA when linleed to BrC6CN. 6FDA-BrC6CN provides a pre-tilt angle of
20°, while
BPDA-BrC6CN provides a pre-tilt angle of 1.5°.
Substituents in the diamine component other than the mesogenic group
also affect the pre-tilt angle of the resulting polyimide. A diamine component
that
contains only mesogenic substituents, such as 6FDA/C6CN, provides a polyimide
with a greater pre-tilt angle than a similar polyimide such as 6FDA-BrC6CN,
which
contains a diamine component having bromine substituents as well as mesogenic
substituents. The pre-tilt angle of 6FDA/C6CN is 42°, while the pre-
tilt angle of
6FDA-BrC6CN is 20°. These polyimides differ only in the presence of
bromine
substituents on the diamine portion of 6FDA-BrC6CN.
As the data,in Table 1-4 indicate, heat treatment temperature influences
the pre-tilt angle provided by the polyimide. The pre-tilt angles of 6FDA/C6CN
and
6FDAJC6CN(ether) after heat treatment at 150°C, 175°C,
200°C, and 225°C are
compared graphically in Figure 2. The pre-tilt angles of 6FDA/C6CN and
6FDA/C6BP are compared in Figure 3 and the pre-tilt angles of 6FDA/C6CN and
6FDA/C11CN are compared in Figure 4 after similar heat treatments. The pre-
tilt
angles provided by 6FDA/C6CN, 6FDA/C6CN(ether), and 6FDA/C6BP are
CA 02433747 2003-07-02
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UA.388 28
relatively similar over the heat treatment temperatures tested as shown in
Tables
1 and 2. The pre-tilt angle provided by 6FDA/C11CN, however decreases as the
heat treatment temperature increases, as seen in Figure 5. The pre-tilt angle
provided by 6FDA/CllCN is about 90° at a heat treatment temperature of
150°C.
At a heat treatment temperature of 200°, the pre-tilt angle drops to
about 40°.
A mixture of diamines may also be used to synthesize a polyimide for use
as an alignment layer. By altering the composition of the polyimide, the pre-
tilt
angle can be varied. Figure 5 is a graph showing the pre-tilt angles of
polyimides
containing diamines with mesogenic pendent groups (C6BP), diamines with
perfluorinated carbon pendent groups (PFMB), or mixtures thereof. Figure 5
illustrates that the pre-tilt angle provided by a polyimide obtaining its
diamine
component only from PFMB is 1.5°. The pre-tilt angle of a polyimide
obtaining its
diamine component from a mixture of PFMB and C6BP increases as the percentage
of C6BP increases relative to PFMB. When the polyimide is solely C6BP-based,
the
pre-tilt angle increases to about 40°.
Based upon the foregoing disclosure, it should nov~r be apparent that the
polyimide alignment layers of the present invention will carry out the objects
set
forth hereinabove. It is, therefore, to be understood that any variations
evident fall
within the scope of the claimed invention and thus, the selection of specific
component elements can be determined without departing from the spirit of the
invention herein disclosed and described.
