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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a new directionally-linked surface useful
for the alignment of liquid crystals. Methods of preparing this new
surface are also described, as is a preferred method wherein a
polymerizable self-assembled and high resolution-patternable sorbed layer
is polymerized by polarized radiation.
2. Discussion of the Prior Art
Flat panel liquid crystal (LC) display devices typically require substrates
which provide for the uniaxial orientation of liquid crystalline
molecules. Currently, such alignment is achieved by initially spin coating
a polymer (polyamide, polyimide, etc.) on a substrate followed by
mechanical rubbing of the polymer surface with cotton, rabbit fur, etc.
This technique has inherent problems, however, in that it is difficult to
obtain a predictably uniform alignment over large areas. In addition,
rubbing creates both charges and dust. Charges lead to the failure of LC
devices due to, e.g., shorting of conducting surfaces, etc., and dust can
provide defect sites. There is, therefore, a current industrial need for a
surface that promotes uniaxial liquid crystal alignment (i.e., both planar
alignment (an alignment orientation where the long axis [or average
director]of the liquid crystal is not perpendicular to the plane of the
substrate surface), and homeotropic alignment, where liquid crystal
molecules are perpendicular to the substrate surface) without the need to
mechanically rub the surface of a substrate, and for a simple technique
for producing such a surface. Colorless substrates that align liquid
crystalline molecules are also desirable for high contrast applications,
etc.
Current approaches for addressing the problems encountered with rubbed
surfaces include the coating of Langmuir-Blodgett (LB) layers on
substrates, and the polymerization or irradiation of substrates that have
been coated with polymers with polarized light.
In the LB film approach, major difficulties are encountered which have not
yet been overcome: (i) it is very difficult to scale-up an LB process for
manufacturing purposes, and (ii) a useful aligning layer is fabricated
only via the layer-by-layer deposition of monolayers onto a substrate at
the air-water interface of an LB trough. Since the monolayer present at
the air-water interface in an LB trough is not in its thermodynamically
stable state, the aligning layer obtained does not have long term
mechanical and thermal stability.
LB layers also generally contain inhomogeneities or domains of defects
within the plane of the film. This makes it difficult to obtain the
uniaxial alignment of a liquid crystal compound over a large area.
Moreover, LB films are known to have a considerable degeneracy with regard
to the tilt of the LB-forming molecules within the plane of the film. As a
consequence, it is not possible to obtain specific pre-tilt directions at
the substrate surface.
Hercules (U.S. Pat. No. 5,032,009; U.S. Pat. No. 4,974,941; U.S. Pat. No.
5,073,294; Nature, Vol 351, 49 May 1991; Liquid Crystals, Vol. 12, No. 5,
869, 1992; Newsletter of the Int. Liq. Crystl. Soc., ("Liquid Crystals
Today"), Vol. 4, No. 2 1994, all incorporated herein by reference) has
reported the alignment of liquid crystals optionally containing dyes with
polarized light and the preparation of a surface made of an isomerizable
dye which is first dispersed in a polymer and subsequently irradiated with
polarized light. Neither the liquid crystal itself, dye, nor host polymer
is covalently bound to the substrate, and the aligning surface is
unstable: heat and/or subsequent irradiation changes or erases any
initially obtained orientation aligning effect.
Schadt et al (Jpn. J. Appl. Phys., Vol. 31, Pt 1, No. 7, p. 2155 (1992); EP
525,477; EP 525,473 and EP 525,478, all incorporated herein by reference)
has also reported aligning surfaces prepared by the irradiation of
polymers having, attached to the polymer backbone, pendant polymerizable
groups, using polarized light. These surfaces require pre-polymerization,
however, and, like those described above, are not covalently bound to a
substrate surface. Moreover, when polymer layers like those of Schadt and
Gibbons are made thin to lower the driving voltage, pinholes arise which
cause defects and shorts.
Finally, Ichimura (Abstract from the Tawiguchi Conference, (Japan), 1994,
incorporated herein by reference) has used polarized light to orient
polymers bearing side chain azobenzenes. The polymers were applied to
substrates using an LB technique and a spin-coating technique, and showed
alignment of liquid crystals in contact therewith. However, these films,
like those of Schadt and Gibbons, are not bound to the substrate surface,
and they suffer from the general drawbacks discussed above for such films
as well as those discussed regarding LB films.
Thus, there remains a need for a tough new, non-rubbed, non-LB based
alignment surface which can be easily produced and used in liquid crystal
devices which require alignment layers. The present invention provides
such a surface.
OBJECTS OF THE INVENTION
Accordingly, it is one object of the present invention to provide a surface
for the uniaxial planar alignment, homeotropic alignment, etc., of liquid
crystalline molecules in contact therewith, the pre-tilt angle .theta. of
aligned liquid crystals varying from
0.degree..ltoreq..theta..ltoreq.90.degree. (measured as the angle the long
axis (director) of the liquid crystalline molecule makes with the surface
plane of the substrate).
It is another object of the present invention to provide liquid crystal
display devices, spatial light modulators, phase modulators, non-linear
optical devices, etc., comprising substrates which provide uniaxial planar
alignment, homeotropic alignment, etc., of liquid crystalline molecules in
contact therewith.
It is another object of the present invention to provide a simple method
for producing a surface that provides for the uniaxial planar alignment,
homeotropic alignment, etc., of liquid crystalline molecules in contact
therewith.
It is another object of the present invention to provide an alignment
surface for liquid crystals that is colored or colorless, that provides
for the control of the pretilt angle .theta. of liquid crystal molecules
in contact therewith from 0.degree. to 90.degree., and which provides
uniaxial planar alignment (meaning that for all .theta.s other than
0.degree. and 90.degree. all or substantially all of the liquid
crystalline molecules in contact with the surface tilt in the same
direction.
These and other objects, which will become apparent after a review of the
following detailed description, have all been achieved by the inventors'
discovery of a surface containing anisotropic chemical and, it is thought,
geometric features which guide liquid crystalline molecules in contact
therewith in preferred orientations and with tilt angles .theta. of from
0.degree. to 90.degree..
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying drawings, wherein:
FIG. 1 is a schematic representation of one process of effecting
chemisorption of silane molecules onto a surface. A substrate is dipped in
a solution containing silane molecules. The silanes covalently attach to
the surface;
FIG. 2 shows the structures of non-directionally polymerized hydrophobic
silanes referred to in the specification. When chemisorbed on a surface,
these silanes promote a perpendicular (homeotropic) alignment of LC
molecules;
FIG. 3 shows the structures of some hydrophilic silanes investigated for LC
alignment. All have a --NH2 (amine) group at the end of the molecule and a
trialkozy silyl group at the other end;
FIG. 4 shows the optical textures obtained with E-63 on an ITO substrate
treated with different hydrophilic silanes;
FIG. 5 is a schematic representation of one embodiment of the different
steps involved in the invention alignment process, Step 2: optional
modification of the chemisorbed silane (DETA, EDA, APS or ABTE) layer and
attachment of the photopolymerizable group (cinnamoyl chloride); Step 3:
Irradiation by polarized UV light and formation of oligomer pairs
(.beta.-truxiamides);
FIG. 6 is the UV absorption spectra of the chemisorbed silane layer after
attachment of the cinnamoyl chloride unit. (a) DETA, (b) EDA and (c) ABTE.
FIG. 7 is the UV absorption spectra of the chemisorbed silane layer before
and after photopolymerization.
FIG. 8 shows the liquid crystal alignment obtained with a plain glass cell
having a directionally polymerized surface thereon. Both directionally
polymerized and unpolymerized regions are shown. The LC molecules are
aligned (dark region) in a uniform planar orientation in the polymerized
regions;
FIG. 9 shows the liquid crystal alignment in a twisted nematic (TN) cell
comprising directionally linked surfaces coated on substrates. The base
substrates consist of bare ITO glass. Both directionally linked and
non-linked coated regions are shown. The liquid crystalline (LC) molecules
are aligned in a uniform planar orientation in the directionally linked
regions;
FIG. 10 shows the liquid crystal alignment in a twisted nematic cell
comprising directionally linked surfaces coated on substrates. Base
ITO-coated glass substrates are first coated with a passivation layer of
SiO.sub.2 which is 690.ANG. thick. Both directionally linked and nonlinked
regions are shown. The LC molecules are aligned in a uniform planar
orientation in the polymerized regions;
FIG. 11 shows the electrooptic characteristics of a TN cell with a
directionally linked coating on the substrates. The underlying substrates
consist of bare ITO glass;
FIG. 12 shows the electrooptic characteristics of a TN cell with a
directionally linked coating on the substrates. The underlying substrates
consist of passivated ITO;
FIG. 13 shows some of the many alternative directionally linkable groups
useful in the present surface and method. X represents an absorbable,
adsorbable, chemisorbable, etc., end of the molecule. The spacer here is a
single bond.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present inventors have discovered that an anisotropic "sorbed" (meaning
chemisorbed, adsorbed, absorbed, etc.) directionally linked layer produces
a surface having superior alignment properties. Their discovery includes
the formation of layers on a substrate surface by chemisorption,
optionally followed by chemical modification of the chemisorbed layer by
attachment of a directionally linkable group thereto, and, finally,
creation of an anisotropic surface by directional linking using, e.g.,
polarized radiation. This process creates an anisotropic surface aligning
layer that provides for the uniform planar alignment of LC molecules
without any rubbing.
The invention alignment surface is preferably a directionally-linked layer
of molecules adsorbed, absorbed or chemically bonded to a substrate. By
"layer" the inventors mean more than one molecule, and the invention is
not limited to a monolayer, a continuous layer, etc. Covalent bond
formation of the layer molecules to the substrate is preferred (i.e.,
chemisorption). For example, an --OH group on the surface of a glass
substrate provides a site for the attachment of a chemisorbable molecule.
Chemisorption therefore, as used herein provides an aligning layer with
long-term thermal and mechanical stability. See FIG. 1 for the
chemisorption of silanes.
In particular, the present invention alignment surface comprises a
directionally linked layer with anisotropic chemical (i.e., molecular)
and, it is believed, geometric features. These features are provided by a
sorbed (i.e., an adsorbed, absorbed, preferably chemisorbed, etc.) layer
or layers which comprise one or more compounds having the following
general formula:
[X].sub.m --[S].sub.n --[P].sub.o 1
where X is a chemical functional group capable of adsorption, absorption or
chemisorption to a surface or substrate, S is a spacer and P is a
directionally linkable group.
X is a group that sorbs to surfaces, particularly surfaces and substrates
used in devices requiring aligned liquid crystals (e.g., glass, ITO-coated
glass, polymer surfaces, diamond surfaces, microvoid-containing materials,
silicone wafers optionally comprising a predetermined pattern, silicon
wafers patterned after adsorption, absorption or chemisorption of the
above-described compounds, etc.). Of course, substrates other than those
currently used in liquid crystalline devices may also be coated with the
compounds described above. Examples of useful surfaces include gold,
silver, copper, mirror surfaces, MgF.sub.2, chromium, platinum, palladium,
mica, aluminum oxide, aluminum, amorphous hydrogenated silica, gallium
arsenide, polysilicon, sulfides including cadmium sulfide, seleides,
silver bromide films, oxidized metal surfaces, metal surfaces, plastic
(polymer) surfaces, etc.
In addition to the silanes depicted in FIG. 1, other suitable X groups
include any chemical functional group capable of sorption to the
above-described surfaces. Chemical groups capable of covalently bonding to
the substrate surface (chemisorption) are preferred. Examples include any
group making an Si--O bond with a surface hydroxyl group including for
example, --SiR.sub.2 OH where R is C.sub.1 -C.sub.10 alkoxy,
--Si(OH).sub.3, and mono/di/trialkoxy silanes. Other examples of X include
a carboxyl (COO) group; phosphorus-containing groups, a thiol group; an
alcohol group; a carbonyl group; a (meth)acrylate group; titanates;
zirconates; a thiocyanate group; a (meth)acrylic acid group; an isocyanate
group; an isothiocyanate group; an acyl cyanate group; an acyl thiocyanate
group; etc., each X group being chosen so as to sorb to the desired
substrate, preferably chemisorb. Where it is possible, the X group may be
chiral (for example, a trialkoxysilane group having different alkoxy
group).
It is also possible that a single layer-forming molecule of formula 1 can
contain two or more of the above chemically functional X groups, and that
one or more than one are used for sorption to the surface. Thus, n in the
above formula can be an integer greater than 1, and is preferably from
1-4, most preferably 1 or 2. Further, X groups on different layer-forming
molecules can bond to one another while others sorb to the substrate. See
FIG. 1 where oligomers form through X groups.
S in the above formula is a spacer group. In addition to the spacer groups
described in FIG. 3 (hydrocarbon chains and hydrocarbon chains interrupted
by one or two NH groups) other spacer groups may be used. Suitable spacer
groups include any chemical moiety that separates X from P and that does
not prevent X and P from performing their functions. For example, suitable
spacer groups include a single bond and a linear C.sub.1 -C.sub.30 or
branched C.sub.3 -C.sub.30 alkyl group each optionally interrupted, when
there are at least two carbon atoms, by one or more aromatic groups,
peptide groups, heterocyclic groups, NH, NR where R is a C.sub.1 -C.sub.18
hydrocarbon group, O, S, COO, oxygenated sulfur, i.e., SO.sub.n where n is
1-4, CO, phosphorous, oxygenated phosphorous such as phosphine, phosphate,
phosphite, etc. group which are preferably non-adjacent. The spacer groups
may be optionally substituted, for example, with hydroxyl, nitro, halogen,
those substituents listed below, etc. Other suitable S groups include a
C.sub.4 -C.sub.70 aromatic group optionally substituted with alkyl,
hydroxyl, nitro, halogen, etc., groups, a C.sub.3 -C.sub.30 heterocyclic
group optionally substituted with alkyl, hydroxyl, nitro, halogen, etc.,
groups, and a saturated or partially unsaturated C.sub.3 -C.sub.30 cyclic
hydrocarbon group optionally substituted with hydroxyl, halogen, nitro,
etc., groups and including substituted and unsubstituted steroids like
cholesterol, etc. It is stressed that any spacer which links X and P and
does not negate their functions may be used.
The spacer of the present invention may be chiral. Preferably the spacer
separates X from P by from 1 to 1000 angstroms, preferably 2 to 70
angstroms, most preferably 3 to 30 angstroms, including 5, 10, 15, 20 and
25 angstroms and all ranges therebetween, and is chemically tailored to
provide a desired pretilt angle .theta. of preferably uniaxially oriented
liquid crystal molecules in contact with the invention surfaces of from
0.degree..ltoreq..theta..ltoreq.90.degree. (i.e., pure planar to
homeotropic alignment).
A layer-forming molecule of formula 1 can bear more than one spacer group
and preferably has as many spacers as there are directionally linkable
groups. Thus, n is an integer of 1 or more, preferably 1-4, most
preferably 1 or 2. n preferably is equal to or less than o (the number of
P groups).
P in the above formula is a group capable of directionally linking to
another P group. Directional linking includes dimerization,
oligomerization, polymerization, photoreactions including insertions,
isomerizations, Norish I and II reactions, etc. wherein at least two P
groups are anisotropically linked. Groups capable of directional charge
transfer interactions, ionic bonding, hydrogen bonding etc. are also
included. P groups encompass all functional groups capable of being
directionally (i.e., anisotropically) linked to another, preferably to a
close-by or neighboring P group.
In addition to the cinnamoyl groups described in FIG. 5, any other suitable
directionally linkable groups may be used including those depicted in FIG.
13 and any other group capable of directional linking to a neighboring P
group by means of polarized radiation, electric polymerization, heat,
surface manipulation with a scanning tunneling microscope, atomic force
microscope, etc. These groups can directionally link in any
fashion--meaning that anisotropic dimer formation, oligomer formation,
polymer formation, charge transfer complexation, ionic attraction, etc.
all produce useful surfaces providing LC alignment. In addition, the P-P'
directional link may be chiral. Directional linking of P groups is
preferably effected by circularly or elliptically polarized UV light. A
preferred embodiment of the present invention is a surface coated with a
chemisorbed layer of [X].sub.m --[S].sub.n --[P].sub.o molecules which
have been polymerized with polarized UV radiation to form mostly dimers.
Preferred sorbable molecules according to the invention have m=n=o=1.
However, since a layer-forming molecule of the present invention can bear
more than one P group, o may be an integer greater than 1. While each
spacer preferably has one P group, more than one P group may be present on
each spacer. Thus o is preferably greater than or equal to n and is
preferably an integer of from 1-4.
When one layer-forming molecule of formula 1 has two or more P groups the P
groups on a single molecule may be directionally linked to one another, or
some or all of the several P groups on one molecule may be directionally
linked to P groups on other molecules. With two or more P groups on a
single molecule a combination of these effects can be used. For example,
two P groups on one molecule can directionally link, followed by further
directional linking with a P group on a neighboring molecule. By
"neighboring" any molecule with a P group that reacts with the subject P
group is meant.
The invention [X].sub.m --[S].sub.n --[P].sub.o compounds are prepared by
simple organic reactions well known to and within the skill of the
ordinary artisan and explained in, e.g., Introduction to Organic
Chemistry, A. Streitwieser and C. Heathcock, Macmillan, 1976; Reagents for
Organic Synthesis, Fieser and Fieser, John Wiley and Sons, 1967 and
succeeding volumes; Survey of Organic Syntheses, John Wiley and Sons, Vols
I and II, 1970; and Advanced Organic Chemistry, March, Wiley, 1985, all
incorporated herein by reference.
The invention anisotropic surfaces may be made as thin as one layer of
molecules of formula 1 or several multiples thereof. Any thickness is
acceptable. A preferable thickness is 3 to 3000 angstroms. The invention
surfaces can be prepared by processes including the electric poling,
magnetic field, solvent flow, etc., alignment of [X].sub.m --[S].sub.n
--[P].sub.o molecules followed by directional linking with heat, light,
chemical activation, etc., and are not limited to those produced by the
process described below. Those surfaces produced by the below-described
process are preferred.
The preferred process of the present invention comprises two or three
steps: (i) formation of a sorbed, preferably chemisorbed, layer or layers
of [X].sub.m --[S].sub.n --[P].sub.o molecules on a substrate (or
[X].sub.n [S].sub.m or [X].sub.n molecules if desired), (ii) optional
chemical modification of the sorbed molecules to provide at least one
spacer group S and at least one directionally linkable group P therein if
none is present prior to sorption and (iii) directional linking using any
of the methods described above, preferably using polarized radiation,
particularly polarized UV radiation. Step (ii) can be omitted if the
material used in step (i) already comprises a spacer and directionally
linkable group.
The three possible steps involved in the invention process are shown
schematically in FIGS. 1 and 5. First a sorbed (preferably chemisorbed)
layer or layers of molecules bearing no P group is placed on the surface
of a substrate as a foundation, which is then rendered photosensitive by
attachment of a P group (in this case, a UV chromophore). Finally, an
anisotropic surface is created by photopolymerization with polarized UV
light wherein the polymerizable groups of neighboring (presumably
adjacent) molecules are bonded together, the direction of the
polymerization being dictated by the polarization direction of the
radiation. UV radiation, IR radiation, visible radiation etc., are
included. The final result is a surface containing anisotropic molecular
and, it is thought, geometric features whose direction dictates the
direction of orientation for the long axis, or average director, of the
liquid crystal molecules. Preferred methods of carrying out the invention
steps are described below.
(i) Substrate preparation: The first stage of the invention sorption,
preferably chemisorption, process preferably includes preparation of the
substrate surface. This procedure applies to glass, ITO-coated glass,
silicon wafers, etc. and is simply the cleaning of the substrate surface.
In one preferred embodiment the substrate is sonicated twice in
chloroform. Then the substrate is washed in 1:1 HCl/methanol for 30
minutes (a step that is omitted for ITO-coated glass), followed by
3.times. rinsing with distilled water. It is then washed in concentrated
H.sub.2 SO.sub.4 for 30 minutes and again rinsed three times with
distilled water. The substrate is heated in distilled water to
80.degree.-100.degree. C. for about 5 minutes then cooled. After such
treatment the substrate is ready for chemisorption, adsorption,
absorption, etc. of compounds of general formula [X].sub.m --[S].sub.n
--[P].sub.o or [X].sub.m --[S].sub.n or [X].sub.m if no polymerizable
group or spacer is present.
(ii) Attachment of the sorbed layer--A solution for adsorption, absorption,
chemisorption, etc. may be prepared or other art known methods of sorption
may be used. A typical preferred chemisorption solution contains 1% by
volume of the desired chemisorbable material, 5% by volume distilled water
and 94% by volume of a 1 mM acetic acid solution in methanol. The
substrate to be treated is dipped in this solution and allowed to stand
for 5-15 minutes at room temperature. The solution is then poured out, and
the substrate rinsed 3.times. with fresh methanol. The substrate may then
be baked at 120.degree. C. for 15 minutes to dry. The chemisorbable
molecules become chemically bonded to the surface: i.e., the molecules are
covalently bonded thereto. The chemisorbed layer is then ready for
chromophore attachment, if no chromophore is present in the material in
the first place. The layer can be sorbed in a particular pattern.
(iii) Attachment of the spacer and directionally linkable group to the
initially sorbed layer:
In this step (optional since the molecule absorbed, adsorbed, chemisorbed
to the surface in (ii) above may already possess a spacer and a
polymerizable group) a directionally linkable group and, if desired, a
spacer other than a single bond is attached to the sorbed layer. The
substrate with the sorbed layer is immersed in a solution containing a
compound with, e.g., a spacer bonded to a directionally linkable group and
a chemically reactive group for reaction with the initially sorbed layer,
like cinnamoyl chloride. Acetonitrile can be used as the solvent. The
substrate is then allowed to stand in the solution for one hour in the
dark. This ensures attachment of the spacer and, e.g., polymerizable
chromophore to the sorbed layer. In four specific embodiments using APS,
ABTE, EDA and DETA, cinnamoyl chloride chemical modification was
ascertained by the UV absorption peak seen at 275 nm (see, e.g., FIG. 6).
The directionally linkable group, etc., can be provided in a pattern if so
desired.
(iv) Directional linking:
The last step of the invention process is directional linking, preferably
directional dimerization, oligomerization, polymerization, etc.
Directional oligomerization, dimerization and polymerization are
preferred. In one preferred embodiment a substrate with a
photopolymerizable chemisorbed layer is irradiated with polarized UV
radiation to obtain an anisotropic surface. A typical dosage of UV
radiation is about 3 J/cm.sup.2 for about 15 minutes. As the skilled
artisan knows, the time varies with lamp intensity, closeness of the
substrate to the lamp, etc. This treatment leads to a photo-induced
reaction between photopolymerizable or photodimerizable groups of,
presumably, adjacent chemisorbed molecules, forming thereby a cyclobutane
ring (in the case of cinnamic acid polymerizable derivatives). See FIG. 5.
The existence of these .beta.-truxiamide pairs was ascertained by UV
absorption spectra: photopolymerization resulting in the dimer pair
formation results in a drastic reduction in the peak at 275 nm and an
increase in UV absorbance at 193 nm due to the presence of cyclobutane
rings (FIG. 7). Patterning can occur by linking using a pattern of light
passed through a mask, etc.
While not being bound by a particular theory, formation of the
directionally linked surface is believed to result in a highly anisotropic
surface. The polymerization structure, or perhaps more correctly in the
case of cinnamic acid derivatives, the directionally dimerized structure,
is believed to be oriented in a single preferred direction, dictated by
the direction of the polarization of the light used to effect directional
linking. The invention process thus creates a permanent, bound anisotropic
surface layer on a substrate by a process which does not involve any
rubbing or any guest-host interaction. The invention surface is stable to,
e.g., heat and light, maintaining orientation of liquid crystalline
molecules in contact therewith even after exposure to high temperature or
UV radiation for extended periods. Depending on the compound of formula 1
used, the invention surface can be colorless or colored, showing
absorption in the visible range of from 0% to 100%.
EXAMPLES
Example 1
Trichlorosilanes depicted in FIG. 2 containing different lengths of
saturated hydrocarbon chains were covalently attached to glass surfaces by
dipping the glass in a solution containing the silanes. The solution was
5% water, 94% 1 mm solution of acetic acid in MeOH and 1% (vol/vol/vol) of
silane. The resulting layers were hydrophobic with water contact angles in
the range of 75.degree.-90.degree..
The alignment of a commercially available room temperature nematic liquid
crystal (E-63 from Merck) containing a mixture of alkyl cyanobiphenyls and
having the following LC transition temperatures
K-8.degree.C-N-84.degree.C-I in contact with the chemisorbed layers of
these hydrophobic silanes was investigated, and the orientation obtained
was homeotropic (i.e., perpendicular to the substrate surface). Liquid
crystal mixture E7 from Merck containing --C.sub.5, --C.sub.7 and
--OC.sub.8 substituted cyanobiphenyls and --C.sub.5 substituted
cyanotriphenyl can also be used, as can any material, composition, etc.
exhibiting anisotropy, preferably liquid crystallinity.
Example 2
Experiment 1 was repeated with the exception that the silanes studied were
4-aminobutyltriethoxy silane (or ABTE), N-(2-aminoethyl)-3-aminopropyl
trimethoxy silane (or EDA) trimethoxysilylpropyldiethylenetriamine (or
DETA) and 3-aminopropyl trimethoxy silane (or APS). See FIG. 3. All four
silanes have a polar --NH.sub.2 group at the free end of the molecule.
This group modified the hydrophilicity of the substrate surface after
attachment of the molecule. EDA and DETA contain, in addition to the
terminal amine group at the end, one or two additional amine groups as
bridging groups linking the hydrocarbon chains. This permits the variation
of the degree of hydrophilicity and/or the strength of the dipolar
interactions.
The four amine silanes were chemisorbed on ITO-coated glass surfaces, and a
common sandwich cell was made using liquid crystal mixture E-63 from
Merck. Both glass surfaces of the cell in contact with the LC had aligning
chemisorbed layers. The cells were examined on a rotating stage of a
microscope with a light source. The LC cell was placed between crossed
polarizers within the microscope.
FIG. 4 shows photographs of the textures exhibited by E-63 under crossed
polarizers. In the top row of photographs the average director of the LC
is at an angle of 45.degree. to the polarizer or analyzer while in the
bottom row of photographs, the director is parallel to either of them.
Acceptable planar alignment was seen for all chemisorbed layers.
Example 3
APS, ABTE, EDA and DETA layers were chemisorbed on plane glass according to
the process described above and cinnamoyl groups were provided thereon as
polymerizable group P according to (iii) above. The layers were
directionally polymerized as described above with UV light. In order to
compare the alignment obtained in the directionally polymerized (DP)
treated region with that in a non-polymerized region, a mask was used to
directionally polymerize only part of the chemisorbed layer. A 10 .mu.m
thick cell was prepared with E-63 sandwiched between two directionally
linked surface-treated substrates. All observations were made between
crossed polarizers and the cell was mounted on the rotating stage of a
polarizing microscope.
The LC molecules were found to be aligned extremely well in a uniform
planar configuration in the invention surface region while there was
hardly any alignment in the unpolymerized regions, the line of demarcation
being very sharp. FIG. 8 shows photographs for two positions of the
sample, i.e., when the LC director is at 45.degree. or parallel to the
axis of the polarizer/analyzer. The DP region is uniformly dark in the
latter case while it is uniformly bright in the former case. The quality
of the alignment was so good that even under high magnification few
defects were seen in the invention surface region.
The particular set-up used was either a Nikon Optiphot Polarizing
Microscope with a 100 W white light source and 12 V DC power supply, a
Nikon photodiode, a Melles Griot amplifier (for the optical signal) and a
Keithley digital multimeter (199 system DMM) receiver or an Olympus BH-2
Polarizing Microscope with a 100 W white light source, 12 V DC power
supply, UDT photodiode, and UDT Optometer model 5370 with built-in
amplifier. Both set-ups used a Wavetek model 395 synthesized arbitrary
waveform generator and Trek model 50/750 amplifier to apply the electric
field to the samples. In all cases, excellent contrast ratios were
obtained with the invention surface.
Example 4
Alignment surfaces were prepared in accordance with Example 1 on bare ITO
coated glass surfaces. A cell was made with E-63 in a twisted nematic
configuration so that the LC was in direct contact on both sides with DP
treated substrates mounted such that the direction of polymerization for
the two substrates was orthogonal to each other. This resulted in a
twisted nematic (TN) cell with good alignment, as seen in FIG. 9. The
Figure shows the cell between parallel and between crossed polarizers.
Once again, the invention treated region gave a uniform planar alignment
while the untreated region showed no alignment. It was noted, however,
that the quality of alignment on ITO was, in general, not as perfect as it
was on plain glass. This is believed to be due to inherent surface
inhomogeneities on the ITO surface.
Example 5
ITO coated glass overcoated with a SiO.sub.2 layer was studied in the same
fashion as in Example 1. These substrates are typical of those used by the
display industry. ITO glass (225 ohms/square surface resistance) coated
with 690 .ANG. of SiO.sub.2 (purchased from Donnely Corporation) was used.
The passivated ITO was subjected to treatment under exactly the same
conditions as in Example 3 using APS with a cinnamoyl group. A sample cell
with the directionally linked treated passivated ITO surfaces was prepared
in a TN configuration. Excellent alignment was obtained in the invention
surface regions (FIG. 10). In fact, the aligned regions were defect-free
even when observed with increased magnifying power. The contrast ratio was
very high and comparable to that of commercial TN cells of the same
thickness. While contrast ratios vary with the equipment used to measure
them, we obtained ratios >9 and as high as about 33.
Example 6
X-S-P molecules (n=m=o=1) are chemisorbed on aluminum oxide as in Example
1. P here is acrylate, S is C.sub.10 alkyl, and X is carboxyl. Directional
polymerization as in Example 3 is effected.
Example 7
X-S-P molecules (n=m=o=1) are chemisorbed on a gold surface by dissolving
the molecules in solvent and applying the solution to the gold surface for
24 hours. X here is a thiol group, S is a parasubstituted bicyclohexyl
group and P is a styrenyl group. Directional polymerization as in Example
3 is effected.
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