U.S. patent application number 12/153059 was filed with the patent office on 2008-10-02 for liquid crystal alignment layer.
This patent application is currently assigned to ZBD Displays Ltd.. Invention is credited to Richard M. Amos, Guy P. Bryan-Brown, John C. Jones, Emma L. Wood.
Application Number | 20080241426 12/153059 |
Document ID | / |
Family ID | 32109248 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080241426 |
Kind Code |
A1 |
Amos; Richard M. ; et
al. |
October 2, 2008 |
Liquid crystal alignment layer
Abstract
This invention relates to a photopolymer mixture capable of use
for a liquid crystal alignment layer and to an alignment layer
formed from such a photopolymer mixture. The mixture comprises at
least two polymerisable materials which are blended together in
proportion so as to give a predetermined, preferably low, surface
energy. A low surface energy, e.g. less than 4.times.10.sup.-2 N/m
can result in an alignment layer which imparts a particular
orientation to liquid crystal molecules with out requiring any post
cure treatments. The polymerisable materials may be monomers,
oligomers or diluents that form long chain molecules when cured and
the mixture may contain additives to lower the overall surface
energy.
Inventors: |
Amos; Richard M.; (Malvern,
GB) ; Bryan-Brown; Guy P.; (Malvern, GB) ;
Jones; John C.; (Malvern, GB) ; Wood; Emma L.;
(Malvern, GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
ZBD Displays Ltd.,
Worcestershire
GB
|
Family ID: |
32109248 |
Appl. No.: |
12/153059 |
Filed: |
May 13, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10531302 |
Apr 14, 2005 |
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PCT/GB2003/004483 |
Oct 17, 2003 |
|
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12153059 |
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Current U.S.
Class: |
428/1.2 ;
349/187 |
Current CPC
Class: |
G02F 1/133742 20210101;
C09K 2323/00 20200801; G02F 1/13378 20130101; C09K 2323/02
20200801; G02F 1/133502 20130101; Y10T 428/31504 20150401; G02F
1/133711 20130101 |
Class at
Publication: |
428/1.2 ;
349/187 |
International
Class: |
G02F 1/1337 20060101
G02F001/1337; G02F 1/13 20060101 G02F001/13 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 2002 |
GB |
0224179.2 |
Nov 7, 2002 |
GB |
0225921.6 |
Claims
1. (canceled)
2. A liquid crystal alignment layer as claimed in claim 19 wherein
the surface energy is such that, in use in a liquid crystal cell,
the photopolymer imparts a predetermined orientation of the liquid
crystal director with respect to the local surface normal.
3. A liquid crystal alignment layer as claimed in claim 2 wherein
the predetermined orientation is a homeotropic orientation.
4. A liquid crystal alignment layer as claimed in claim 27 wherein
the first polymerisable material is an oligomer or diluent.
5. A liquid crystal alignment layer as claimed in claim 4 wherein
the second polymerisable material is a monomer.
6. A liquid crystal alignment layer as claimed in claim 19 wherein
the mixture further comprises an additive.
7. A liquid crystal alignment layer as claimed in claim 6 wherein
the additive reduces the surface energy of the cured mixture.
8. A liquid crystal alignment layer as claimed in claim 7 wherein
the additive is an acrylate or an epoxy functionallised
polydimethyl siloxane material.
9. A liquid crystal alignment layer as claimed in claim 19 wherein
the surface energy is less than 4.times.10.sup.-2 N/m.
10. A liquid crystal alignment layer as claimed in claim 19 wherein
the surface energy of the cured photopolymer has a dispersive
energy of between 12 mN/m and 40 mN/m and a polar energy of between
O mN/m and 15 mN/m.
11. A liquid crystal alignment layer as claimed in claim 19 wherein
the mixture comprises an immiscible component.
12. A liquid crystal alignment layer as claimed in claim 11 wherein
the immiscible component comprises one of the polymerisable
materials.
13. A liquid crystal alignment layer as claimed in claim 11 wherein
the immiscible component comprises a solid particulate.
14. A liquid crystal alignment layer as claimed in claim 19 wherein
the proportion of first material to second material is such so as
to give a predetermined viscosity and refractive index.
15. A liquid crystal alignment layer as claimed in claim 12 wherein
the refractive index of the alignment layer is between 1.35 and
1.80.
16. A liquid crystal alignment layer as claimed in claim 12 wherein
the viscosity of the uncured photopolymer is between 5 centipoise
and 10 poise.
17. A liquid crystal alignment layer as claimed in claim 19 wherein
the a first polymerisable material and the second polymerisable
material together are cured to form a polymeric material having a
predetermined surface energy.
18. (canceled)
19. A liquid crystal alignment layer comprising a polymerised
photopolymer bearing a surface profile wherein the polymerised
photopolymer comprises a cured photopolymer mixture comprising at
least a first polymerisable material and a second polymerisable
material such that the photopolymer is cured in contact with a
master containing the inverse of the surface profile, and in which
the cured photopolymer retains this shape after release from the
master.
20. A liquid crystal alignment layer according to claim 19 wherein
said at least a first polymerisable material and a second
polymerisable material are in proportion such that the mixture has
a predetermined surface energy that enables release from the master
containing the inverse of the surface profile while maintaining the
fidelity of the required profile.
21. (canceled)
22. A liquid crystal cell comprising a liquid crystal material
located between two cell walls wherein at least one of the cells
walls carries an alignment layer according to claim 19.
23. A liquid crystal cell as claimed in claim 22 wherein the
alignment layer has the same apparent refractive index as the
liquid crystal material.
24. A method of making an alignment layer for a liquid crystal
material comprising the steps of: i) taking a photopolymer mixture,
ii) introducing the photopolymer mixture to a substrate, iii)
taking a master bearing an inverse of the desired grating and
impressing it into the photopolymer mixture on the substrate, iv)
curing the photopolymer mixture on the substrate, and v) removing
the master from the cured photopolymer mixture characterised in
that the photopolymer mixture comprises the photopolymer mixture
according to claim 19.
25. A method of making an alignment layer as claimed in claim 24
wherein the photopolymer mixture has a component which has a lower
surface energy than the rest of the mixture and the step of curing
the photopolymer mixture involves curing the mixture such that
there is a greater concentration of the lower surface energy
component at one surface of the cured mixture than the other.
26. A photopolymer mixture comprising a first polymerisable
material and a second polymerisable material, the proportion of
first polymerisable material to second polymerisable material being
such that the photopolymer mixture has a predetermined viscosity
and refractive index.
27. A liquid crystal alignment layer as claimed in claim 19 wherein
the first polymerisable material is a short chain polymerisable
material and the second polymerisable material is a short chain
polymerisable material.
Description
[0001] This invention relates to photopolymers having particular
characteristics, especially to photopolymers useful for forming
alignment gratings for liquid crystal devices and methods for
forming such alignment gratings.
[0002] Alignment layers are well known in the field of liquid
crystal displays. Such alignment layers are provided on the
internal surface of at least one cell wall to provide a
preferential alignment to the liquid crystal material in the cell.
Some alignment layers impart a particular alignment direction due
to the material properties of the layer or the method of formation.
Another type of alignment layer has a particular surface profile
which is designed to impart particular orientations to the liquid
crystal molecules.
[0003] One particular alignment layer is described in U.S. Pat. No.
6,249,332. Here is described a particular surface profile which
allows the liquid crystal device to adopt any one of two stable
bulk liquid crystal alignments. However surface profiles or
gratings are used for a number of different types of liquid crystal
display. As used herein the term grating shall be taken to mean any
periodic or non periodic surface profile which is shaped so as to
impart a particular alignment to liquid crystal molecules. Other
examples of grating aligned liquid crystal devices can be found in
U.S. Pat. No. 5,796,459, U.S. Pat. No. 5,917,570, WO 01/40853, U.S.
Pat. No. 5,764,325, EP0894285 and U.S. Pat. No. 5,754,264.
[0004] One way of forming a suitable grating is to use hard contact
photolithography in a deep UV photoresist material. A UV
photoresist material is taken and either exposed through a mask or
subjected to a particular interference pattern so as to alter the
properties of part of the material in a controlled way. The exposed
material is then treated so as to remove the exposed or unexposed
part depending upon the material and leave the desired grating
structure. Photolithography however is a relatively expensive and
time consuming process.
[0005] Another known method of producing gratings is embossing. A
master is used to imprint another material. For example, standard
hot foil embossing or stamping techniques have been used to produce
grating structures in plastic that are suitable for use as
alignment layers (e.g. Lee et al., Jpn. J. Appl. Phys. Vol. 32
(1993) pp. L1436-L1438). Embossing offers the possibility of
increased throughput of grating fabrication but current techniques
suffer difficulties in accuracy and reproducibility.
[0006] Another embossing method involves embossing into a curable
photopolymer which is then exposed to cure the material before the
master is removed.
[0007] By its very nature the alignment layer has an effect on the
physical, optical, electrical and chemical properties of the cell.
It is therefore an object of this invention to provide an improved
alignment layer material and improved grating formed from such
alignment layer.
[0008] Thus according to the present invention there is provided a
photopolymer mixture for use in producing an alignment layer for a
liquid crystal device characterized in that the photopolymer
mixture comprises at least a first polymerisable material and a
second polymerisable material in proportion such that the mixture
has a predetermined surface energy when cured.
[0009] In the simplest form of embossing, a photopolymer is
introduced onto a substrate and a master is impressed into the
photopolymer. The photopolymer is then exposed to a radiation
source to cure it and the resulting grating removed from the
master. The adhesion properties of the photopolymer are important
to control. In addition since the polymer is in contact with the
liquid crystal when assembled into a cell the chemical, physical,
optical and electrical properties also should be controlled.
[0010] As used herein the term photopolymer mixture means any
mixture which, before curing, exhibits a viscosity which is low
enough for the material to flow and which, on exposure to
appropriate radiation, is cured to form a solidified polymer
material. Thus for the avoidance of doubt the terms does not mean
that the material must be a polymer in its uncured form but just
that it is polymeric when cured. The photopolymer mixture may also
be referred to simply as a photopolymer.
[0011] The term polymerisable material means any material which, on
curing forms a polymeric material and includes monomers, oligomers
and diluent materials, i.e. non polymer materials that have various
functional groups and can bond to polymeric materials or each other
to form long chain molecules.
[0012] Curing may be by optical means, including UV, visible and IR
wavelengths, or by other irradiative means, including X-ray, ionic
or electronic beam and thus photopolymer should be taken to include
materials which are polymerised by any of these methods.
[0013] The photopolymer mixture is arranged to have a surface
energy, when cured, of less than 4.times.10.sup.-2 N/m, preferably
less than 3.5.times.10.sup.-2 N/m and may even be
2.5.times.10.sup.-2 N/m or lower.
[0014] The surface energy of the cured photopolymer is important in
that the surface energy effects not only the release of the cured
film from the master, and also the adhesion to the substrate, but
also in that the surface energy effects the pretilt of the liquid
crystal material in an assembled device, i.e. the orientation of
the liquid crystal director with respect to the local surface.
Ideally the surface energy of the cured photopolymer is arranged
such that when used as an alignment grating the liquid crystal
material adopts the desired orientation. For instance using a
photopolymer having a relatively low surface energy at the surface
which contacts the liquid crystal material can ensure that in use
the liquid crystal material adopts a homeotropic orientation where
the liquid crystal director is locally normal to the surface.
Typical grating structures where such orientation would be useful
would be a zenithally bistable (ZBD) device as described in U.S.
Pat. No. 6,249,332. Other devices where an alignment layer with a
substantially normal orientation of the liquid crystal director to
the local surface would be useful would be those described in
EP0894285, a grating aligned Vertically Aligned Nematic (VAN)
device or a grating aligned Hybrid Aligned Nematic (HAN)
structure.
[0015] Therefore the surface energy of the photopolymer when cured
is conveniently a dispersive energy of 12 mN/m-40 mN/m, even more
preferably 25 mN/m-32 mN/m. The polar energy is preferably 0.0
mN/m-15 mN/m, more preferably 0.0 mN/m-10.0 mN/m, most preferably
0.0 mN/m-2.0 mN/m.
[0016] Preferably the refractive index of the photopolymer mixture
is within the range 1.35 to 1.80, more preferably within the range
1.45 to 1.55. When assembled into a liquid crystal device an
alignment layer formed as a grating forms an interface with the
liquid crystal material which has a varying profile. The alignment
grating may therefore effect the optical quality of the assembled
cell. Preferably therefore the refractive index of the photopolymer
material forming the alignment layer is substantially the same as
the refractive index of the liquid crystal material with which it
is to be used. The skilled person will appreciate that the liquid
crystal material will exhibit two refractive indexes. The alignment
layer is therefore preferably chosen to be substantially equal to
one of the refractive indexes or some intermediate value.
[0017] The alignment layer should further have minimal optimal
absorption within the visible spectrum, i.e. in the wavelength
range of about 400 to 650 nm, so as to maximise brightness of the
assembled display. The optical absorption is preferably less than
0.5 per micron and even more preferably less than 0.01 per micron
within the visible band, for instance as measured at a wavelength
of 400 nm.
[0018] Following curing the photopolymer must adhere well to the
substrate material to which it is applied and yet, when used to
form embossed gratings, must release cleanly from the grating
master. Therefore the photopolymer is preferably adapted so as to
have a higher adhesion to the substrate than the master. One way of
achieving this is to ensure that the photopolymer has incompatible
chemical groups to the material from which the master is formed.
The photopolymer may additionally comprise an additive to aid
release from the master.
[0019] The photopolymer mixture may comprise an additive to modify
the surface energy of the cured photopolymer. The additive may
comprise a silicone or alternatively perfluorinated or hydrocarbon
alkyl chain monomers may be used. Silicones can be used to reduce
surface energy and modify release from various surfaces. Suitable
additives can include, for example, an acrylate or epoxy
functionallized polydimethyl siloxane (PD MS) material, Alkyl (8-30
carbon chain) acrylates/epoxies/vinyls and fluoro- or
perfluoro-alkyl (3-30 carbons) acrylates/epoxies.
[0020] For photopolymers which are to be used to produce embossed
alignment gratings the viscosity is important in that the viscosity
of the uncured photopolymer determines the thickness of the
embossed film for a given set of embossing parameters, such as
pressure and speed, and ultimately determines the throughput of the
embossing step, which affects manufacturing cost. Preferably the
viscosity of the uncured photopolymer mixture is within the range 5
cP (centipoise) to 10 P (Poise), more preferably in the range 40
cP-500 cP or less than 200 cP.
[0021] Photopolymers generally shrink on curing. Shrinkage however
reduces the amplitude of the embossed grating compared to that of
the master, may be non-uniform across the layer and may distort the
shape of the grating grooves. Preferably therefore the degree of
shrinkage on cure is less than 20%, more preferably less than 10%
and in some cases less than 3%.
[0022] The photopolymer preferably has a polymerisation speed such
that an exposure dose of less than 2 J/cm.sup.2 is required more
preferably less than 100 mJ/cm.sup.2.
[0023] The cured photopolymer film should be non-soluble in the
liquid crystal material and also preferably is non-soluble in
isopropanol and acetone as the film typically has to survive
several solvent washes.
[0024] It has been known that in use ions in the alignment layer
may leech into the liquid crystal material affecting electrical
switching characteristics. The photopolymer should therefore have a
minimal ionic content. Metallo-organic compounds could be added to
the photopolymer to prevent ions leeching. The metallo-organic
compounds could effectively `cage` any ions within the
photopolymer.
[0025] In use the electrodes supplying voltage to the liquid
crystal cell supply voltage across the alignment layer as well. Any
drop in voltage across the alignment layer results in the required
driving voltages to be increased to compensate. It is therefore
preferable that the photopolymer has a high dielectric constant
when cured so as to reduce voltage drop. Preferably the relative
permittivity of the cured photopolymer is in the range of 2-50,
more preferably 4-20.
[0026] At least one of the two short chain polymerisable materials
may be a monomer and at least one of the short chain polymerisable
materials may be an oligomer. Monomers content decreases viscosity
of the photopolymer and increases shrinkage. Oligomer content
decreases shrinkage but increases viscosity and to a large extent
controls the physical properties. Therefore careful control of the
proportion of oligomer to monomer can control the viscosity and
shrinkage of the material. Choice of non-ionic materials with a
high dielectric constant allows gratings which do not detract from
the switching properties of the liquid crystal cells. Monomer and
oligomer materials are available in a range of refractive indexes
and again the proportion of each can be chosen to provide a
refractive index which substantially matches that of the liquid
crystal material being used. One or both of the polymerisable
materials may be diluent materials, i.e. materials that are not
polymeric themselves but may have one or more functional groups and
join with another polymerisable material or from long chain
molecules.
[0027] Additives may be added to the mixture in order to control
the surface energy and can also modify the viscosity of the uncured
photopolymer. The additives may comprise materials which are not
polymerisable, including photo-initiators and stabilisers.
[0028] The photopolymer may also comprise immiscible components,
either in solid or liquid form, or both, i.e. components which do
not mix into the rest of the photopolymer mixture. This may help
control the mechanical, electrical or optical properties of the
photo-polymer layer. One advantage that the inventors of the
present invention have found to be particularly useful from the
inclusion of an immiscible component is the effect on domain
nucleated devices, such as the ZBD of U.S. Pat. No. 6,249,332, The
immiscible components give rise to nucleation and domain pinning
sites in a similar fashion to that described in WO98/04953 for a
ferroelectric liquid crystal. The effect of this on a ZBD device is
to increase the operating range for addressing the two stable
states, improving mechanical stability of a displayed image and the
operable temperature range and reducing unwanted growth of the
opposite state after the device has been latched into the desired
state. The immiscible component may comprise polymerisable
materials, for example alkyl with perfluoro, ketones or epoxies,
insulating solid particulate materials, for example silicones or
silicates, or conducting solid particulates such as C60 balls and
tubes, ITO or metal particulates.
[0029] Nucleation and domain pinning sites may also be produced in
the photopolymer by controlled polymerisation, for example using a
multi-step curing process. An example of such a method would use a
first step in which polymerisation occurs earlier and more rapidly
in certain parts of the photopolymer, followed by a second step
where the bulk of the sample is cured when in contact with a master
grating, say a shaped shim. For example, the first cure can be made
to introduce particulate of cured photopolymer in an uncured
matrix, for example by exposing through a mask containing random
transmissive dots. The layer is then embossed using a shim of the
required shape and is exposed uniformly over the required area to
cure the polymer. After removal of the shim (e.g. de-lamination)
the resulting grating has a roughened structure wherein the
lengthscale of the local roughness is typically lower than that
that can be produced in the original photolithography used to
create the shim.
[0030] Adding an immiscible component destroys the uniformity of
the photopolymer when cured resulting in parts of the alignment
layer having different properties. This can offer a number of
advantages. For instance a local difference in electric
permittivity would, in use, create local field hotspots for
nucleation of a switched state.
[0031] The surface affinity properties would also change. Where the
photopolymer is treated with a surfactant after curing the density
of the surfactant may then vary over a microscopic length scale.
The surface roughness induced by having immiscible components would
provide topographic pinning sites for domains in a bistable cell on
length scales far below the pitch of the grating, i.e. less than
100 nm.
[0032] There is also the possibility of changing the optical
properties of the alignment layer using immiscible components which
offers the potential for changing white balance, diffuser and
grating combinations. Of course the immiscible component could be
index matched to the photopolymer matrix and liquid crystal
material if preferred to take advantage of the ability of index
matching the alignment layer and liquid crystal material.
[0033] Advantageously the photopolymer mixture may be adapted such
that on curing it forms an alignment layer having a surface energy
which varies from the surface designed to contact the liquid
crystal material to the surface in contact with the substrate. For
instance it may be desired that that surface that contacts the
liquid crystal material has a low surface energy so as to impart a
homeotropic orientation to the liquid crystal materials in use
whereas to ensure good adhesion with the substrate the surface
energy of the other surface is higher.
[0034] One way of achieving this is for the photopolymer mixture to
comprise a component having a low surface energy and vary the
concentration of this component throughout the thickness of the
cured layer.
[0035] If the photopolymer material is embossed prior to curing to
form an alignment grating and the master used for embossing has a
low surface energy then the low surface energy component will tend
to migrate to the embossed surface to minimise the interfacial free
energy. If the photopolymer is cured after this migration has
occurred to at least a partial extent the cured film will have a
concentration gradient of low surface energy material and hence
different surface energies at its different faces.
[0036] Alternatively a multi step cure process could be used. For
example, the photopolymer may be formed from a series of
polymerisable components that form the majority of the formulation
that cure at a first wavelength at a given rate. The photopolymer
may also contain a second set of components that lead to a low
surface energy that cure at a second wavelength and rate. The
components are designed so that the photopolymer strongly absorbs
one of the two wavelengths.
[0037] For example, a photopolymer is formulated according to the
present invention with higher surface-energy components that
polymerise when exposed to wavelength .lamda.1 and lower
surface-energy components that polymerise when exposed to
wavelength .lamda.2, such that the higher surface-energy components
(which form the majority of the formulation) strongly absorb
.lamda.2. This photopolymer is initially flexo-printed onto the
glass substrate comprising an electrode. A shim, for example formed
from a plastic sheet, with the inverse of the required surface
relief pattern and formed from a material that transmits both
wavelengths .lamda.1 and .lamda.2 is then pressed into the
photopolymer. The photopolymer being liquid, flows around the shim
to form the required shape. The photopolymer is then exposed to
illumination of .lamda.2 through the shim for a given duration.
This begins to cross link the minority, low surface energy
components of the photopolymer mixture, leading to a higher
concentration of these components at the top surface (ie the
surface in contact with the shim), provided that this first
wavelength is sufficiently absorbed strongly by the majority
(higher surface energy) components of the photopolymer, i.e. as
.mu.2 cannot penetrate far into the photopolymer it is only at the
top surface that it will cause polymerisation of the low surface
energy component and this localised polymerisation will draw more
low energy component out of the bulk liquid. This process may be
helped using a post-exposure bake, in which the temperature of the
photopolymer mixture is raised to allow further diffusion of the
low surface energy components to the upper surface. Following this
procedure, the substrate is exposed to .mu.1 to cure the remainder
of the photopolymer. Once fully cured, the shim is removed and may
be ready for use as a homeotropic grating liquid crystal alignment
layer, perhaps after an additional bake (eg at 150.degree. C. for 1
hour).
[0038] It will be apparent to one skilled in the art that this
method may be designed such that exposure is applied through the
bottom substrate to give the same desired concentration gradient,
or using separate exposures through the shim and the substrate.
[0039] Such a concentration gradient may also be created by
combining thermal and UV curing mechanisms. For example, the
majority of the fixable polymer could be formed from epoxy monomers
which polymerise after baking when in the presence suitable thermal
acid generators. To this is added an low-energy monomer that
polymerises when exposed to a suitable UV wavelength (e.g. an
acrylate monomer and photo-initiator sensitive to 365 nm). To
ensure strong absorption by the bulk of the monomer, a suitable dye
(such as Alizarin Yellow GG) may also be added to the polymer
mixture. An example process is as follows. The fixable-polymer is
flexo-printed onto the substrate to the required thickness and the
shim pressed into contact to ensure the correct shape is imposed
into the material. The fixable polymer is exposed to 365 nm of an
appropriate dosage, through the structured shim for a suitable
duration (typically 20 seconds). This causes cross-linking of the
lower energy components (e.g. the acrylated surfactant monomers) at
the upper surface in contact with the shim. Whilst still in contact
with the shim, the polymer is then baked to fully cure the
remaining material before removing the shim.
[0040] The grating coated substrate is then ready for use as a
zenithally bistable surface without further treatment. This
dramatically reduces the number of steps in a commercial process
since surface coating and selective removal are then no longer
required. Note also that an additional advantage of creating the
phase separation of the lower energy components is that it helps
physical separation of the shim. For example, this may allow a
plastic shim to be used on several occasions without physical
damage occurring in the de-lamination process.
[0041] Another route to create such a concentration gradient is to
use components in the fixable polymer that cure at different rates.
For example, if the lower surface-energy components that form the
majority of the fixable polymer cure at a faster rate to the low
surface energy additive(s). The concentration gradient may be
controlled such that all the profiled surface has substantially the
same surface energy or the surface energy could be varied along the
profile.
[0042] The surface energy must be less than 4.times.10.sup.12 N/m,
preferably less than 3.5.times.10.sup.-2 N/m, with good behaviour
having been observed by the present inventors with surface energies
of about 2.5.times.10.sup.-2 N/m or lower.
[0043] In another aspect of the invention there is provided a
photopolymer mixture comprising a first component and a second
component together being capable of cured to form a polymeric
material having a predetermined surface energy. Thus the mixture of
two components which together can polymerise to form a long chain
molecule can have the same advantages as described with respect to
the first aspect of the invention.
[0044] In another aspect of the invention there is provided an
alignment layer for a liquid crystal material, the alignment layer
having a surface profile and comprising a polymerised photopolymer
characterised in that the photopolymer is the photopolymer
according to the first or second aspects of the invention.
[0045] The alignment layer may be disposed on a substrate and have
a surface energy at the surface in contact with the substrate which
is higher than the surface energy on the surface which will contact
the liquid crystal material. As mentioned a low surface energy at
the liquid crystal side of the alignment layer can lead to an
alignment layer with the right alignment properties but a
relatively higher surface energy is preferred at the substrate side
to provide good adhesion.
[0046] In a further aspect of the invention there is provided a
method of making an alignment layer for a liquid crystal material
comprising the steps of;
i) taking a photopolymer, ii) introducing the photopolymer to a
substrate, iii) taking a master bearing an inverse of the desired
grating and impressing it into the photopolymer on the substrate,
iv) curing the photopolymer on the substrate, and v) removing the
master from the cured photopolymer characterised in that the
photopolymer comprises the photopolymer according to the first
aspect of the invention.
[0047] The viscosity and refractive index are preferably in the
ranges described with reference to the first aspect of the
invention.
[0048] Conveniently the first short chain material is an oligomer
or diluent. The second short chain material may be a monomer or
diluent. The photopolymer may also comprise an additive, the
proportion or additive and first and second polymerisable material
being such that the photopolymer has a predetermined surface
energy. Preferably the surface energy is such so as to impart, in
use, a particular alignment to a liquid crystal material. That
alignment is preferably a homeotropic alignment.
[0049] In yet another aspect of the invention there is provided a
liquid crystal cell comprising a liquid crystal material located
between two cell walls, at least one cell wall bearing an alignment
layer according to the previous aspect of the invention. Ideally
the refractive index of the alignment layer is substantially equal
to the refractive index of the liquid crystal material.
[0050] The invention will now be described by way of example only
with reference to the following drawings of which;
[0051] FIG. 1 illustrates an apparatus for embossing a photopolymer
on a glass substrate using a carrier film as a master,
[0052] FIG. 2 shows the detail of how the carrier film and
photopolymer bearing substrate are brought together,
[0053] FIG. 3 shows an apparatus for embossing a photopolymer on a
glass substrate using a flexible shim,
[0054] FIG. 4 shows two possible polarisation orientations for a
ZBD cell,
[0055] FIG. 5 shows the refractive index as a function of
concentration for two actilane monomers,
[0056] FIG. 6 shows an examples of a silicon acrylate that could be
used as a suitable additive,
[0057] FIG. 7 shows a schematic of a ZBD cell with an embossed
grating,
[0058] FIG. 8 shows the voltage to switch between the two stable
states as a function of bipolar pulse width for a ZBD cell With an
embossed grating.
[0059] Embossing of liquid crystal alignment layers offers a
simpler and higher throughput method of manufacture than existing
methods. A master grating is used to emboss a photopolymer, such as
a UV curable photopolymer, which is printed or nip fed onto a
substrate which may be glass. The master grating may be formed on a
nickel shim. Alternatively another film may have been imprinted to
act as a carrier film bearing the master grating. Pressure is
applied to the carrier film or shim such that the photopolymer
flows and forms a film of around 1-1.5 .mu.m. The photopolymer is
exposed to UV light which causes it to solidify. The carrier film
or shim is then removed from the glass leaving behind the textured
polymer film.
[0060] An apparatus for embossing a photopolymer onto a glass
substrate using a carrier film is shown in FIG. 1. The glass
substrate 2 is loaded from the left as shown in the figure. The
photopolymer is either pre-printed on the substrate 2 or may be
applied prior to the substrate reaching rollers 10. A carrier film
master 4 is stored on roll 6. The carrier film has a protective
covering which is removed onto roll 8 to expose the master grating.
The carrier film is brought into contact with the substrate 2 and
pressure applied thereto by rollers 10. This is shown in greater
detail in FIG. 2 which illustrates the principle. The substrate 2
bearing the photopolymer is brought into contact with the carrier
film 4 (in reality the carrier film 4 would wrap partly around top
roller 10a. The carrier film 4 and glass substrate 2 pass through
the rollers 10 and pressure is applied. The rollers may be a hard
rubber and the pressure could result from the elasticity of the
rollers or metal rollers could be used and a certain amount of
pressure applied.
[0061] Referring back to FIG. 1 the pressed carrier film and
substrate are passed through a UV lamp 12 for curing. After curing
the carrier film is removed from the substrate and taken onto roll
14.
[0062] FIG. 3 shows an alternative embossing apparatus where a
flexible nickel shim 16 bearing a master is pressed onto a
photopolymer bearing substrate 18 and pressure applied via moveable
roller 20 before the photopolymer is cured by UV lamp 22.
[0063] As mentioned in such embossing methods the photopolymer
should adhere well to the glass, or ITO if an electrode is laid
down on the glass, yet release cleanly from the carrier film or
shim. In addition as the polymer is in contact with the liquid
crystal material within the assembled cell the chemical, physical,
optical and electrical properties are crucial properties to
control. Important properties include [0064] Release from the
carrier film/shim but good adhesion to the substrate such as
glass/ITO or plastic/ITO, suitable plastic including, for example,
PES (Polyethersulphone) and PET (polyethylterapthalate). [0065]
Refractive index [0066] Optical absorption/coloration [0067]
Surface Energy [0068] Ability to be post treated with a surfactant
(e.g. silane or alcohol treatment) [0069] Viscosity [0070]
Polymerization speed [0071] Shrinkage [0072] Ionic content [0073]
Dielectric permittivity [0074] Low odour and toxicity
[0075] Whilst some photopolymers are known that may for instance
have the same refractive index as a liquid crystal material to be
used they do not posses-low viscosity and the correct surface
energy to align the liquid crystal material. The present invention
resides partly in the realisation that these properties can be
controlled by blending combinations of monomers, oligomers,
diluents and additives to control the properties of the composite
polymer. Monomer content decreases viscosity but increases
shrinkage. Oligomer content decreases shrinkage but increases
viscosity and to a large extent controls the physical properties.
All are readily available in a variety of refractive indexes. The
mix can also be arranged to be optically clear and show little or
no absorption in the visible region of the spectrum.
[0076] In order to test refractive index matching a sample was
prepared and compared with a grating produced using a photoresist
method.
[0077] A master was made by first coating an ITO coated glass
substrate with photoresist UVIII, a deep UV photoresist. This was
achieved by spin coating at 1100 rpm in a gyroset spin coater.
After a suitable soft bake to remove solvent it was placed in hard
contact with a chrome on glass mask and exposed to collimated UV
light for 9 seconds. The chrome on glass mask consisted of 0.35
micron chrome lines with a repeat period of 1 micron. After an
activation bake the resist was developed and washed with de-ionised
water. It was then exposed in an EPROM eraser and baked in a vacuum
oven for 2.5 hours at 173 degrees. The surface was then treated
with a fluorinated polymer called CYTOP. This was spin coated at
3000 rpm at a dilution of 1:3 in perfluorotributylamine. The
substrate had a further 1 hour bake at 160 degrees.
[0078] The master formed was a grating suitable to imprint a
zenithal bistable grating to the photopolymer as described in U.S.
Pat. No. 6,249,332. Thus the assembled cell was a Zenithal Bistable
Liquid Crystal Device.
[0079] The substrate consisted of an ITO coated 0.55 mm thick glass
substrate. This was cleaned with solvents and placed in a UV Ozone
chamber for 10 minute to render the surface high energy. Small
drops of photopolymer were placed on the substrate and the master
placed on top. The laminate was then passed between a pair of soft
rollers. The shore hardness of the rollers was shore hardness `D`
(for example D65). The compression of the rollers was 1.8 mm and
the speed was 2 mm/sec. The laminate was then cured under a UV lamp
and separation was achieved by peeling away the master with a
blade. Cells were made by treating the photopolymer and assembling
against a rubbed poly-imide substrate. They were filled with liquid
crystal material MLC6204-000.
[0080] In order to maintain maximum brightness of the display the
optical absorption of the grating polymer should be minimised. In
addition it is desirable to reduce/remove diffraction effects in
the reflected (undiffracted beam). In the assembled cell two
configurations are possible for the orientation of the polariser
which is adjacent the grating surface. These two configurations are
shown in FIG. 4.
[0081] In the configuration shown in FIG. 4a the input polarisation
samples both the extraordinary and ordinary refractive indexes
(n.sub.e and n.sub.o) of the liquid crystal which cannot both be
matched to the refractive index of the grating polymer (n.sub.p).
Hence the grating/liquid crystal interface can never be optically
buried and diffraction will always exist. In the configuration
shown in FIG. 4b the input polarisation samples only n.sub.o of the
liquid crystal and so the grating diffraction can be removed
completely (at normal incidence) when n.sub.p=n.sub.o. Furthermore
this matching condition is retained for both the defect and
non-defect states. For off axis viewing, the diffraction will start
to appear in the plane parallel the groove direction but will
remain zero for the orthogonal plane.
[0082] Reflectivity data was taken on an Eldim Ezcontrast 160R
machine. Two cells were examined, one where the grating was made in
photoresist, the other where the grating was made by embossing into
a silicone hard coat material (GE Silicones UVHC 8556). The
reflectivity data is shown in table 1 and the refractive index in
table 2.
TABLE-US-00001 TABLE 1 Back Grating Reflec- Con- Front Polariser
Polariser polymer Mode tivity trast HEG1425DUHCARS TDF UVIII O
0.315 14.1 HEG1425DUHCARS TDF Embossed O 0.330 >20
TABLE-US-00002 TABLE 2 Material Name Material Type n (589 nm)
Shipley UVIII Polyhydroxylstyrene photoresist 1.641 GE UVHC8556
Acrylate silicone photopolymer 1.514
[0083] The refractive index of the liquid crystal material
MLC6204-000 is 1.504. The results above show that the reflectivity
and contrast is improved for the embossed cell due to the fact that
the refractive index is closer to n.sub.o compared to resist. This
decreases diffractive losses within the cell.
[0084] The refractive index can be controlled by mixing, for
example, two components that have different refractive indexes. The
refractive index of the mixture is a linear weighting of the
percentage of each material. For example, uncured monomer actilane
420 has n=1.537, and uncured actilane 425, n=1.457. A blend of 59%
actilane 420 and 41% actilane 425 would have a refractive index of
1.504. FIG. 5 shows the refractive index of the mixture as a
function of composition. The actual refractive index of the cured
photopolymer may vary slightly, for instance it may be
approximately 1% higher but this may depend on the extent of
shrinkage. The viscosity of the mixture can be controlled in a
similar fashion (different functional dependence on
concentration).
[0085] The viscosity determines the thickness of the embossed film
for a given set of embossing parameters such as pressure and speed
and ultimately determines the throughput of the embossing step
which affects manufacturing cost. The viscosity is therefore chosen
ideally to be between 40 and 500 centipoise.
[0086] The photopolymer must exhibit excellent release from a
carrier film or a flexible shim (nickel or polymer) but show
excellent adhesion to glass and ITO. This is critical to achieving
a good fidelity copy of surface profile from the carrier film or
shim on the glass substrate. There are two (or more) approaches.
The first is to ensure that the carrier film or shim has a low
surface energy (to form a non stick coating). The second is to
ensure that the photopolymer and the carrier film or shim have
incompatible chemical groups at their surface. Likely carrier film
or shim material are;
Polycarbonate
Polypropylene
Polyethylene
Polyester
PMMA
Nickel
[0087] Hot foil polymer
UV Lacquers
[0088] Silicones can be used as an additive to reduce viscosity and
modify release from various surfaces. They can be, for example, an
acrylate or epoxy functionallized polydimethyl siloxane (PDMS)
material. If used as part of a blend with monofunctional or
bifunctional acrylate monomer, for example, they migrate to the
interface and cause the cured film to be lower in surface
energy.
[0089] One example of a suitable silicone acrylate is shown in FIG.
6.
[0090] The surface energy of the photopolymer is important. Grating
shape does provide alignment of the liquid crystal material but it
is also necessary for the alignment layer to provide either a
planar alignment or a particular homeotropic alignment to the
liquid crystal material. In a particularly advantageous arrangement
of ZBD cell it is desired to have homeotropic alignment of the
liquid crystal material adjacent the alignment layer. The surface
energy of the grating is important in effecting how the liquid
crystal material aligns and a homeotropic alignment requires a
relatively low energy. Having the alignment layer with inherently
the right surface energy means that no post processing of the
alignment layer is required. This reduces the number of
manufacturing steps and thus increases efficiency. Further with no
need to add a surfactant the problems of uneven spread of
surfactant on an uneven surface are avoided and the grating shape
is preserved exactly.
[0091] As the skilled person will be aware the surface energy can
be measured in terms of the polar and dispersive components.
Examples of correct polar and dispersive components are;
.gamma..sub.p=8.6 mN/m and .gamma..sub.d=15.7 mN/m
.gamma..sub.p=0.8 mN/m and .gamma..sub.d=25.1 mN/m
.gamma..sub.p=9.9 mN/m and .gamma..sub.d=14.8 mN/m
[0092] FIG. 7 shows a schematic of a zenithal bistable liquid
crystal cell. Two glass substrates 30, 32 each carry ITO electrode
34, 36. One substrate 30 also carries an embossed grating 38.
Liquid crystal material 40 is introduced into the cell gap. In
operation a voltage is supplied across the liquid crystal material
40 by the electrodes 34, 36. Appropriate voltage pulses can cause
the liquid crystal material to adopt any of the two stable states
which can then be maintained in the absence of power. The voltage
required to address a liquid crystal cell is important for
virtually all liquid crystal cells. Bistable cells are often used
in portable appliances however to preserve battery life due to the
fact that they do not need constant addressing. In some
applications power consumption is critical and it is desired to
have low addressing voltages.
[0093] As can be seen from FIG. 7 when a voltage is applied between
the electrodes 34, 36 some voltage will drop across the liquid
crystal material 40 and some across the grating 38. A certain
voltage will be required to switch the liquid crystal material
depending on the cell design. Any voltage dropped across the
grating simply adds to the voltage which must be applied across the
electrodes.
[0094] There are two ways to minimize the voltage dropped across
the grating. First the total polymer thickness of the grating layer
should be minimized. This is controlled by ensuring an appropriate
viscosity of the photopolymer and a careful choice of speed and
pressure used during the embossing step. Second the dielectric
permittivity should be as high as possible.
[0095] Ideally the photopolymer should exhibit little shrinkage,
i.e. less than 10% or even 3% in some cases. Shrinkage reduces the
amplitude of the embossed copy compared to that of the shim. This
can be accounted for by making the grating amplitude larger on the
shim. However the degree of shrinkage must be uniform and
repeatable over the area of the grating. Shrinkage will also
distort the shape of the grating grooves. Again so long as the
distortion is uniform and repeatable it can be accounted for by
careful design of the shim. Ideally though the shape should be a
faithful reproduction of the shim (or carrier film master).
Monomers tend to have low viscosity but high shrinkage, conversely
oligomers have higher viscosity and lower volume shrinkage. In some
instances release from the master grating is aided by some
shrinkage.
[0096] The required polymerisation speed is relatively low compared
to standard reel-to-reel processes. A 14 by 16 inch area of film 1
micron thick can be illuminated for several seconds to achieve
cure. Further the photopolymer system may be epoxy, vinyl or
acrylate based.
[0097] The film also has to survive several solvent washes.
Typically the solvents are IPA, acetone and the liquid crystal
material.
[0098] The electrical switching of the liquid crystal in the device
is sensitive to ions leeching from the photopolymer. Ionic content
of the photopolymer should be minimised or the photopolymer blend
be adapted such that ion leeching can be minimised by suitable post
processing such as baking.
EXAMPLE
[0099] As an example a ZBD cell was constructed using an alignment
layer formed from a photopolymer blend. The photopolymer blend was
made with the following constituents:
TABLE-US-00003 Akzo Nobel Actilane 420 66.2% Akzo Nobel Actilane
425 28.3% MBF photoinitiator 0.5% Octadecylacrylate 5.0%
[0100] The solution was stirred and heated to aid mixing. Meanwhile
a master grating surface was made in the following manner: [0101]
1. Shipley UVIII was spin coated onto 0.55 mm thick ITO glass to a
thickness of 1.0 microns and baked for 60 s at 130.degree. C.
[0102] 2. The layer was exposed to UV radiation through a chrome on
glass mask in hard contact and developed in Shipley CD 26 to reveal
a 1 .mu.m pitch grating with the groove troughs fully developed
out. [0103] 3. The grating was then exposed to DUV (254 nm) to
stabilise the grooves followed by a vacuum bake at 170.degree. C.
to fully crosslink the resist. [0104] 4. The grating was then
coated in Asahi Glass Cytop CTX 809A diluted 1:3 in CT Solv 180
(Asahi glass). This film was spin coated at 300 rpm for 30 seconds
in a closed lid spin coater. [0105] 5. The substrate was baked at
100.degree. C., 60 seconds on a hotplate followed by 1 hour at
180.degree. C. in an oven to fully cure the Cytop layer which acts
as a release layer.
[0106] The photopolymer blend was then filtered through a 0.2 .mu.m
filter and was applied drop-wise along the edge of a clean piece of
ITO coated glass. The master grating was laminated face down on
this glass and the pair were compressed between rollers in order to
thin down the photopolymer layer.
[0107] The photopolymer within the laminate was cured using 2.4
J/cm.sup.2 of 365 nm radiation. After curing the laminate was split
apart using a razor blade to reveal a replica grating in the cured
photopolymer layer. The pressure and speed during the lamination
process was adjusted so that the photopolymer thickness from the
ITO underlayer to the bottom of the grating grooves was 0.21
.mu.m.
[0108] Next the photopolymer replica was baked at 180.degree. C.
for 1 hour and then rinsed in isopropanol before being dried at
100.degree. C. for 10 minutes. These extra processes had the effect
of further curing the polymer while also reducing the
concentration/mobility of ionic content.
[0109] A flat region of the photopolymer replica was used to carry
out a liquid contact angle study in order to deduce the polymer
surface energy. By measuring both water drops and dilodomethane
drops, the surface energies were calculated as .gamma..sub.p=0.68
J/m.sup.2, .gamma..sub.d=32.3 J/m.sup.2.
[0110] The replica grating surface was used to make a 4.5 .mu.m
spaced liquid crystal cell by constructing this surface opposite a
surface of ITO glass coated with flat layer of JSR JALS 2021 (which
gives a monostable perpendicular alignment of the liquid crystal).
The cell was then filled with Merck MLC 6204-000 liquid crystal and
heated into the isotropic phase to ensure full wetting of the
grating surface. Cooling of the cell revealed that the grating
surface made of the photopolymer formulation did indeed induce a
perpendicular alignment condition of the liquid crystal.
[0111] Electrodes were attached to the ITO on each of the cell
surfaces to allow electric field to be applied between the plates.
Application of alternating bipolar pulses was found to allow
bistable switching between the two states (VAN and HAN) which exist
on this suitably designed grating surface. FIG. 8 shows the voltage
required to switch the cell in between the two bistable states as a
function of bipolar pulse width in the manner described in patent
GB 2318422.
[0112] Thus it can be seen that the blending of appropriate
photopolymer components allows a grating to be produced that
produces a zenithally bistable cell and allows switching between
stables states without requiring any additional surface
coatings.
* * * * *