U.S. patent application number 11/515736 was filed with the patent office on 2007-01-25 for topologically structured polymer coating.
This patent application is currently assigned to ROLIC AG. Invention is credited to Carsten Benecke, Mohammed Ibn-Elhaj, Martin Schadt, Klaus Schmitt, Hubert Seiberle.
Application Number | 20070020404 11/515736 |
Document ID | / |
Family ID | 26316015 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070020404 |
Kind Code |
A1 |
Seiberle; Hubert ; et
al. |
January 25, 2007 |
Topologically structured polymer coating
Abstract
Sub-micron-structured (nanostructured) polymer film or coatings
are made by coating a substrate with a mixture of materials. One of
the materials is removed using a selective solvent, leaving pores
or other nanostructure. The substrate may be grooved, providing a
competing nanostructure. The coating may act as an antireflective
coating, optical retarder, optical diffuser, or orientation
layer.
Inventors: |
Seiberle; Hubert; (Weil am
Rhein, DE) ; Schadt; Martin; (Seltisberg, CH)
; Ibn-Elhaj; Mohammed; (Allschwil, CH) ; Benecke;
Carsten; (Weil am Rhein, DE) ; Schmitt; Klaus;
(Lorrach, DE) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
ROLIC AG
|
Family ID: |
26316015 |
Appl. No.: |
11/515736 |
Filed: |
September 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10110994 |
Apr 19, 2002 |
|
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PCT/CH00/00562 |
Oct 18, 2000 |
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11515736 |
Sep 6, 2006 |
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Current U.S.
Class: |
428/1.2 ;
428/212 |
Current CPC
Class: |
G02B 1/11 20130101; C09K
19/542 20130101; Y10T 428/1005 20150115; G02F 1/1391 20130101; G02B
1/08 20130101; Y10T 428/24521 20150115; Y10T 428/2457 20150115;
B29K 2995/0072 20130101; B29C 41/003 20130101; B82Y 30/00 20130101;
B05D 5/061 20130101; G02B 5/0257 20130101; G02F 1/13378 20130101;
C09K 2323/02 20200801; G02F 1/133711 20130101; C09K 19/56 20130101;
G02B 5/3083 20130101; G02B 5/1847 20130101; Y10T 428/24942
20150115; Y10T 428/24479 20150115 |
Class at
Publication: |
428/001.2 ;
428/212 |
International
Class: |
C09K 19/00 20060101
C09K019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 1999 |
GB |
9924746.2 |
Oct 27, 1999 |
GB |
9925458.3 |
Claims
1-47. (canceled)
48. A method of creating a topologically structured polymer film or
coating on a substrate with a free surface facing away from the
substrate, comprising mixing at least two materials, wherein at
least one material is not cross-linkable and the other material is,
applying the mixture to a substrate by using a coating method,
cross-linking said cross-linkable material, and removing a
substantial portion of the non-crosslinked material, wherein the
cross-linkable material is maintained in an oriented state having a
molecular alignment during crosslinking.
49. A method according to claim 48, wherein the cross-linking of
the other material is effected by exposing it to light, which is
optionally polarized.
50. A method according to claim 48, wherein the cross-linkable
material comprises molecules of a rod-like shape.
51. A method according to claim 48, wherein the cross-linkable
material is liquid crystalline.
52. A method according to claim 48, wherein the non-cross-linkable
material is liquid crystalline.
53. A method according to claim 52, wherein the non-cross-linkable
material is nematic liquid crystalline.
54. A method according to claim 48, wherein the non-cross-linkable
material comprises a surfactant.
55. A method according to claim 48, wherein the orientation is
imposed by an underlying orientation layer.
56. A method according to claim 55, wherein the underlying
orientation layer is a photo-oriented layer.
57. A method according to claim 48, wherein the substrate already
has an aligned structure, and the film or coating is applied
thereto with an orientation which may be at from 0.degree. to
90.degree. to that alignment, or wherein this angle is different in
different parts of the film or coating, optionally pixelwise.
58. A method according to claim 48, wherein the cross-linking and
alignment are induced by linearly polarized UV light.
59. A method according to claim 48, wherein the two materials are
dissolved in respective solvents before mixing, the solvents being
mutually miscible but each being a poor solvent for the other
solute.
60. A method according to claim 48, wherein the cross-linkable
material is present in 1/10 to 30 parts by weight per part of
non-cross-linkable material.
61. A method according to claim 60, wherein the cross-linkable
material is present in 1/4 to 4 parts by weight per part of
non-cross-linkable material.
62. A method according to claim 48, wherein the removal of the
substantial portion of said one of the materials is by using a
solvent inactive towards the other material.
63. A topologically structured polymer film or coating on a
substrate with a free surface facing away from the substrate,
obtained by: mixing at least two materials, wherein at least one
material is not cross-linkable and at least one other material is,
applying the mixture to the substrate by using a coating method,
cross-linking said cross-linkable material, and removing a
substantial portion of the non-crosslinked material, wherein the
cross-linkable material is maintained in an oriented state having a
molecular alignment during crosslinking.
64. A topologically structured polymer film or coating, comprising
a material which has hollows, wherein the material is crosslinked
and wherein the cross-linked material has orientation having
molecular alignment.
65. A film or coating according to claim 64, wherein the
crosslinked material is a liquid crystal polymer.
66. A film or coating according to claim 64, wherein the hollows
have arisen from the presence of another material which was then
removed.
67. A film or coating according to claim 64, wherein the
orientation of the cross-linked material is different in different
parts of the film or coating.
68. A film or coating according to claim 63, wherein the film or
coating comprises hollows at least 99% of which are smaller than
1000 nm in at least one direction parallel to the film or coating
plane.
69. A film or coating according to claim 63, wherein the film or
coating comprises hollows at least 99% of which are smaller than
500 nm in at least one direction parallel to the film or coating
plane.
70. A film or coating according to claim 63, wherein the film or
coating comprises hollows at least 99% of which are smaller than
200 nm in at least one direction parallel to the film or coating
plane.
71. A film or coating according to claim 68, wherein the average
distance between adjacent hollows is less than 500 nm.
72. A film or coating according to claim 68, wherein the hollows
are elongated.
73. A film or coating according to claim 68, wherein the hollows
are of continuously narrowing shape from top to bottom.
74. A film or coating according to claim 63, wherein the film or
coating is optically transparent.
75. A film or coating according to claim 74, wherein the film or
coating is optically birefringent.
76. A film or coating according to claim 75 wherein the film or
coating is optically birefringent such that the effective optical
anisotropy of the film or coating is larger than the anisotropy of
the film or coating material used.
77. A film or coating according to claim 75, wherein the film or
coating is optically birefringent such that the effective no of the
film or coating is lower than no of the film or coating material
used.
78. A film or coating according to claim 63, wherein the film or
coating can act as an optical diffuser.
79. A film or coating according to claim 78, wherein the film or
coating can act as an anisotropic optical diffuser.
80. A film or coating according to claim 63, wherein the film or
coating can act as a polarizer.
81. A film or coating according to claim 63, wherein the film or
coating is over-coated with an additional layer.
82. A film or coating according to claim 78, wherein the film or
coating is at least partly covered by a reflective layer that can
act as a diffusing reflector.
83. A film or coating according to claim 82, wherein the reflective
layer can act as an anisotropic diffusing reflector.
84. A film or coating according to claim 82, wherein the reflective
layer is metallic.
85. A stack or multilayer comprising two or more films or coatings
according to claim 63.
86. A film or coating according to claim 63, or a stack or
multilayer comprising two or more films or coatings according to
claim 63, on a substrate, wherein the substrate is optically
transparent.
87. An optical component comprising a substrate bearing a
transparent liquid crystal polymer layer, which has hollows within
its thickness spaced on average less than 500 nm apart, 99% of said
hollows being less than 500 nm across and being at least twice as
long as they are across, which liquid crystal polymer layer is
aligned at a molecular level at 0.degree. to 90.degree. to an
alignment of the hollows.
88. An assembly comprising a layer of alignable molecules in
contact with a film or coating according to claim 63, the molecules
being aligned thereby.
89. An antireflectively coated object comprising a film or coating
according to claim 63.
90. A liquid crystal cell wherein the liquid crystal molecules can
adopt one of at least two different stable alignments in contact
with a wall of the cell, the cell wall comprising a film or coating
according to claim 63.
91. An optical display device comprising a film or coating
according to claim 63 or a stack or multilayer comprising two or
more films or coatings according to claim 63.
92. An element for protection against forgery and/or copying, which
comprises a film or coating according to claim 63 or a stack or
multilayer comprising two or more films or coatings according to
claim 63.
93. A method according to claim 49, wherein the light is UV light,
which is optionally polarized.
94. A method according to claim 56, wherein the underlying
orientation layer is an azo dye or linearly polymerized
photopolymer.
95. An optical component according to claim 87, which is an optical
retarder.
96. An assembly comprising a layer of alignable molecules in
contact with an optical component according to claim 87, the
molecules being aligned thereby.
97. An antireflectively coated object comprising an optical
component according to claim 87.
98. A liquid crystal cell wherein the liquid crystal molecules can
adopt one of at least two different stable alignments in contact
with a wall of the cell, the cell wall comprising an optical
component according to claim 87.
99. An optical display device comprising an optical component
according to claim 87.
100. A film or coating according to claim 64, wherein the film or
coating comprises hollows at least 99% of which are smaller than
1000 nm in at least one direction parallel to the film or coating
plane.
101. A film or coating according to claim 64, wherein the film or
coating comprises hollows at least 99% of which are smaller than
500 nm in at least one direction parallel to the film or coating
plane.
102. A film or coating according to claim 64, wherein the film or
coating comprises hollows at least 99% of which are smaller than
200 nm in at least one direction parallel to the film or coating
plane.
103. A film or coating according to claim 64, wherein the film or
coating comprises hollows and the average distance between adjacent
hollows is less than 500 nm.
104. A film or coating according to claim 64, wherein the film or
coating comprises hollows and the hollows are elongated.
105. A film or coating according to claim 64, wherein the film or
coating comprises hollows and the hollows are of continuously
narrowing shape from top to bottom.
106. A film or coating according to claim 64, wherein the film or
coating is optically transparent.
107. A film or coating according to claim 106, wherein the film or
coating is optically birefringent.
108. A film or coating according to claim 107, wherein the film or
coating is optically birefringent such that the effective optical
anisotropy of the film or coating is larger than the anisotropy of
the film or coating material used.
109. A film or coating according to claim 107, wherein the film or
coating is optically birefringent such that the effective no of the
film or coating is lower than no of the film or coating material
used.
110. A film or coating according to claim 64, wherein the film or
coating can act as an optical diffuser.
111. A film or coating according to claim 110, wherein the film or
coating can act as an anisotropic optical diffuser.
112. A film or coating according to claim 64, wherein the film or
coating can act as a polarizer.
113. A film or coating according to claim 64, wherein the film or
coating is over-coated with an additional layer.
114. A film or coating according to claim 110, wherein the film or
coating is at least partly covered by a reflective layer that can
act as a diffusing reflector.
115. A film or coating according to claim 114, wherein the
reflective layer can act as an anisotropic diffusing reflector.
116. A film or coating according to claim 114, wherein the
reflective layer is metallic.
117. A stack or multilayer comprising two or more films or coatings
according to claim 64.
118. A film or coating according to claim 64, or a stack or
multilayer comprising two or more films or coatings according to
claim 64, on a substrate, wherein the substrate is optically
transparent.
119. An assembly comprising a layer of alignable molecules in
contact with a film or coating according to claim 64, the molecules
being aligned thereby.
120. An antireflectively coated object comprising a film or coating
according to claim 64.
121. A liquid crystal cell wherein the liquid crystal molecules can
adopt one of at least two different stable alignments in contact
with a wall of the cell, the cell wall comprising a film or coating
according to claim 64.
122. An optical display device comprising a film or coating
according to claim 64 or a stack or multilayer comprising two or
more films or coatings according to claim 64.
123. An element for protection against forgery and/or copying,
which comprises a film or coating according to claim 64 or a stack
or multilayer comprising two or more films or coatings according to
claim 64.
Description
[0001] This invention relates to the production of topologically
structured polymer films or coatings. These may find application as
optical films or coatings and more particularly as orientation
layers, optical retardation layers, anti-reflective coatings, and
optical diffusers. The coatings typically exhibit sub-micron
profiles such as pores, grooves, channels or stripes.
[0002] A known isotropic topologically structured polymer coating
is taught by Walheim et al in Science 283 (22 Jan. 1999) 520-2. A
binary polymer blend (of polystyrene and polymethylmethacrylate
dissolved in tetrahydrofuran) is allowed to demix to a controlled
extent during spin coating onto a smooth substrate. By exposing the
coated substrate to cyclohexane, which selectively dissolves
polystyrene, a porous coating of PMMA is obtained. This coating can
be modified by varying the materials and the demixing times, but no
long-range order or patterning of the topological structure can be
imposed on it.
[0003] It would be desirable to have a more versatile film or
coating, with for example, the possibility to generate not only
isotropic topological structure but also anisotropic topological
structure with the capability to pattern said structure. Moreover
this film or coating should exhibit high physical and chemical
stability, and properties which are not critically dependant upon
the demixing time or solvent.
[0004] Such films or coatings could then be used, for example, as
orientation layers (in some cases as bistable orientation layers)
for liquid crystals and liquid crystal displays. Other technical
applications become also feasible such as the production of low
effective refractive index layers usable as antireflective
coatings, or of optically diffusive layers, and the production of
optically anisotropic coatings (especially of liquid crystal
polymers) leading to novel optical components. By using
photo-alignment, photo-patterned grooved coatings are possible.
[0005] According to the present invention, a method of creating a
topologically structured polymer film or coating comprises mixing
at least two materials, applying the mixture to a substrate and
removing at least one of the materials (for example, by using a
solvent inactive towards the other material), characterized in that
the one material is not cross-linkable and at least one other
material is, that other material being cross-linked after
application to the substrate and before use of the solvent.
[0006] A wide variety of substrates can be used, such as glass,
plastics, metals, semiconductors or paper.
[0007] The cross-linking is preferably effected by exposure to
light such as UV light, which may be polarized, and contributes to
a good stability of the film or coating.
[0008] Preferably, the cross-linkable material comprises molecules
of a rod-like (calamitic) shape. Advantageously, the cross-linkable
material and the non-cross-linkable material have chemical
structures that are similar, i.e. with a good compatibility
regarding miscibility. More preferably, the cross-linkable material
is liquid crystalline, and in such a case the non-cross-linkable
material is also preferably mesogenic (liquid crystalline).
[0009] From the mixture it will be understood that a homogeneous
transparent film may be formed and exposed to light, preferably UV
light. During this process, the crosslinkable material is
progressively being crosslinked, and the crosslinked material and
the non-crosslinkable material demix gradually. After crosslinking
has finished, the non-crosslinked material is removed with a
selective solvent (or evaporated upon heating the layers) to create
pores. In this way, it will be seen that layers/films/coatings
having structures (holes, grooves, etc) on a sub-micrometer length
scale can be obtained.
[0010] The non-cross-linkable material may contain a surfactant.
The crosslinkable material may contain an additive such as a
dichroic dye. Usefully, the cross-linkable material may be
maintained in an oriented state during crosslinking, the
orientation being imposed for example by an underlying orientation
layer, which may be a photo-oriented layer such as an azo dye or
linearly polymerized photopolymer, such being cheaper, simpler and
much more versatile than a mechanically produced orientation
layer.
[0011] The formation of topological structures is based on the
control of the demixing (nanophase separation) of two molecularly
mixed components forming a homogeneous phase during crosslinking. A
decisive advantage of this process is that size, shape and
distribution of the topological structures can be adjusted over a
wide range by a suitable choice of the experimental parameters,
which are in particular the kind of solvent(s), the ratio of the
mixture, the concentration in the solvent, and intensity and
duration, wavelength, angle of incidence and state of polarization
of light.
[0012] Particularly, if the chemical structures of the crosslinking
and the non-crosslinking materials are similar, and especially if a
surfactant is added, a molecularly well mixed solution can be
achieved, which additionally ensures that the crosslinking process
is the main cause of demixing.
[0013] A further possibility to enhance the film homogeneity and
the formation of nano-pores is the use, instead of one, of at least
two solvents (Sol1 and Sol2) when preparing the mixture, whereby
for instance Sol2 is a good solvent for the non-crosslinkable
material and a poor solvent for the cross-linked material, e.g.
hexane or ethanol, and Sol1 is a good solvent for the cross-linked
material (may be a good or poor solvent for the non-cross-linkable
material), e.g. ethylester. In this process, it is preferable in a
first step to dissolve the cross-linked and the non-crosslinkable
material in Sol1 and Sol2, respectively. Then the two obtained
solutions are mixed and homogenized. Sol 1 and Sol 2 are preferably
selected to be miscible with each other.
[0014] In a preferred embodiment the present invention provides a
method to fabricate a modulated (almost periodic) topology. For
this, the mixture, preferably comprising a liquid crystalline
material, would be oriented. The orientation would induce
elongation of the pores and lead to the formation of grooves (or
channels, stripes, etc). Size and distribution of the "grooves" can
be varied from a few nm to few .mu.m depending on the preparation
parameters (such as mixture, light irradiation time, temperature .
. . ).
[0015] In principle, any method to orient the mixture can be used,
such as employing a layer that induces an alignment of adjacent
liquid crystal material. Particularly suitable are orientation
layers made by photo-orientation methods (usually using linearly
polarized light), and especially well suited are linearly
photo-polymerized (LPP) orientation layers, also known as
photo-oriented polymer networks (PPN), among other things because
these layers can be easily applied also to non-planar surfaces.
Such methods are for instance disclosed in U.S. Pat. Nos. 4,974,941
Gibbons et al, 5,838,407 Chigrinov et al, and 5,602,661 Schadt et
al. An other possibility would be to use a mixture that itself
contains a photo-orientable component. Examples of this kind of
orientation method can for instance be found in the International
Application PCT/IB99/01001 (ROLIC) filed 3 Jun. 1999.
[0016] Useful multilayer structures (especially in the field of
bistable alignment) can be obtained if the first layer according to
the invention has an aligned topological structure which is
preferably aligned not parallel to the orientation of the
cross-linked material (although parallel is possible for increased
anchoring), the orientation and the alignment preferably being at
right angles to each other. In these cases, the alignment and
crosslinking of the second layer may be induced by linearly
polarized UV light. The UV light illumination can be adjusted such
that the anchoring forces from the alignment and from the
orientation are competitive with each other.
[0017] In the method according to the invention the two materials
may be dissolved in respective solvents before mixing, the solvents
being mutually miscible but each being a poor solvent for the other
solute.
[0018] The cross-linkable material may be present in a proportion
of 1/10 to 30 parts, preferably 1/4 to 4 parts, by weight per part
of non-cross-linkable material, the latter being destined for
dissolution out of the finished coating.
[0019] The invention extends to a topologically structured polymer
film or coating which has been created by a method as set forth
above. In such a film or coating the structure may comprise hollows
at least 99% of which are smaller than 1000 nm, preferably smaller
than 500 nm, and more preferably smaller than 200 nm in at least
one direction parallel to the coating plane. It will be understood
that the hollows may arise from dissolving out the non-cross-linked
material from the coating. The average distance between adjacent
hollows is typically less than 500 nm, and the hollows may be
elongated, which may be understood to be a result of orientation in
the cross-linked material. In most applications, the coating, and
in many also the substrate, are optically transparent.
[0020] Preferably the substrate already has an aligned structure,
achieved for example by employing a layer that induces an alignment
of adjacent liquid crystal material. Particularly suitable are
orientation layers made by photo-orientation methods (usually using
linearly polarized light), and especially well suited are linearly
photo-polymerized (LPP) orientation layers. Also the aligned
structure could alternatively be achieved by rubbing, brushing or
other mechanical means. The said coating is applied thereto with an
orientation which may be at from 0.degree. to 90.degree. to that
alignment, or wherein this angle is different in different parts of
the coating, optionally pixelwise.
[0021] The invention also provides a topologically structured
polymer film or coating on a substrate, comprising, as the coating,
a material which has hollows and/or topological structure,
characterized in that the material is crosslinked. The hollows
and/or structure may be such as could have arisen from the presence
of another material which was then removed.
[0022] The invention also provides an optical retarder comprising
such a film or coating on a substrate, as explained by way of
example later.
[0023] The invention extends to a topologically structured polymer
film or coating which acts as an optical diffuser. In this case,
the holes or grooves as a rule have dimensions in the order of the
wavelength of light.
[0024] The invention also extends to the superimposition of two or
more layers, preferably in the form of a stack or multilayer.
[0025] The invention also provides an optical component, howsoever
made, comprising a substrate bearing a transparent liquid crystal
polymer layer which is aligned at from 0.degree. to 90.degree.
(preferably at 90.degree. for the most interesting competition
between two stable alignments) to the grooves and which has hollows
within its thickness spaced on average less than 500 nm apart, 99%
of said hollows being less than 500 nm across and being at least
twice as long as they are across. Parallel alignment (i.e.
"0.degree.") is possible, for example for high anchoring energy
applications.
[0026] The invention extends to an assembly comprising a layer of
alignable molecules in contact with a coating or optical component
as set forth above, the molecules being aligned thereby.
[0027] The invention further extends to an antireflectively coated
object comprising a coating or component as set forth above and to
a liquid crystal cell wherein the liquid crystal molecules can
adopt one of at least two different stable alignments in contact
with a wall of the cell, the cell wall comprising a coating or
component as set forth above.
[0028] The invention further extends to an optical display device,
preferably a liquid crystal display, comprising a film or coating
as set forth above.
[0029] Furthermore, the invention also provides elements for
protection against forgery and copying.
[0030] Some applications of the invention will be described by way
of illustration.
[0031] Antireflective coatings for the visible spectral region are
required whenever light reflection of a surface is to be reduced or
avoided. An example is the undesirable reflection of the glass of a
display device. The basic principle of such coatings can be
understood in terms of destructive interference between light
reflected from air-film and film-substrate interfaces. Glass or
plastic substrates, for example, require a coating film with a low
effective refractive index n.sub.eff.apprxeq.1.2. However, because
of a lack of suitable low-refractive-index materials, this
requirement cannot be realized with homogeneous single-layer
coatings, and therefore multilayer coatings are usually used.
[0032] As an alternative to multilayers, it is known that porous
films can be used to reduce reflections at optical wavelengths.
(Many others have studied this so-called "moth-eye" effect.) To a
first approximation, provided the pore size is less than
.lamda..sub.0/4 (.lamda..sub.0 is the wavelength in the free
space), the layer will appear as a continuous film with an
effective refractive index given by an average over the film.
Therefore, the challenge is to maximize the volume fraction of the
pores in order to obtain very low n.sub.eff. It is the grading of
the apparent refractive index that gives rise to the reduction in
the reflection coefficient. Based on this idea different approaches
have been developed including sol-gel method, surfaces patterned
with sub-micrometre gratings, superposition of sub-micro particles
and the use of polymer blends.
[0033] The invention provides a method to produce nano-porous
crosslinked films using a simple process applicable to planar and
especially also to non-planar surfaces. This process is based on
the control of the phase separation of molecularly mixed compounds
on the nanometre length scale. Depending on the preparation
parameters, size, shape and volume fraction of pores can be
accommodated. Using ellipsometric measurements we found that the
effective refractive index (n.sub.eff) of such layers can also be
adjusted and decreased down to 1.2 or lower. Therefore the
reflection coefficient off glasses can be efficiently reduced.
[0034] A further advantage is the usually from top to bottom
continuously narrowing shape of the pores/hollows (cf FIG. 3c
described in more detail later), because such "gradient hollows"
have the effect that the refractive index changes continuously over
the depth of the hollows, which in turn makes effective
antireflection possible over a broad spectral region with only one
layer.
[0035] Furthermore, if layers having an elongated, grooved
structure according to a special embodiment of the invention as
described above are used, the elongated structure also affects the
effective refractive index and makes it direction-dependent. Such
layers can therefore also be used as antireflection layers that are
sensitive to the angle of incidence.
[0036] Orientation layers: It is known that grooved surfaces can
produce uniform and parallel alignment of liquid crystals due to
anisotropic surface interactions. It is believed that this is
because the periodic topology of mechanically grooved substrates
minimises the elastic deformation energy of the liquid crystals by
forcing the director to align along the grooves. Grooves are
traditionally produced by rubbing (glasses plates, polymer layers,
. . . ), unidirectional polishing, tangential evaporation,
tangential ion-beam etching of oxides, or formation of a grating on
the substrate. Although mechanical alignment is relatively simple,
the techniques used to produce the grooves are, however, difficult
to implement, involve expensive equipment, and are relatively slow.
Furthermore, brushing (rubbing) processes have several inherent
disadvantages such as the generation of dust particles and static
surface charges. Another limitation is that mechanical alignment is
a large-scale process and therefore not adapted to generate local
variations (patterns) of the surface director, certainly not on the
pixel scale.
[0037] Films or coatings of crosslinked layers made according to
the invention, oriented as described above, show oriented grooves
and therefore may be used as orientation layers for an adjacent
liquid crystal material. It is believed that the alignment effect
on liquid crystals is caused by the topology of the crosslinked
layer, quite similar in effect to a brushed orientation layer.
[0038] Mixed (bistable) anchoring: The invention furthermore makes
possible a mixed alignment, i.e. an orientation layer that at any
given spot possesses two (or more) different orientation directions
at the same time. Such an orientation layer may for instance be
used to generate a bistable (or multistable) liquid crystal
anchoring system.
[0039] For this, anchoring competition between two (or more)
superposed layers is used. One is a topologically structured film
or coating according to the present invention, which tends to
impart a liquid crystal alignment through its topology
(nano-grooves) by mechanisms described above. The other is a
photo-oriented orientation layer, preferably a linearly polymerized
photopolymer (LPP) film, on top of the topologically structured
film or coating, inducing a competing liquid crystal alignment in a
direction different from the anchoring direction of the
topologically structured film or coating, for instance
perpendicular to it.
[0040] Depending on the thicknesses of the two superposed layers,
the size of the "grooves" (A, A.sub.eff and .lamda., see FIG. 4)
and the light irradiation direction, it is possible to find a range
of parameters which gives rise to bistable anchoring.
[0041] Use as optical retarders: It is known that for a layer with
modulated surface profile, such as linear gratings or porous films
with an inclined columnar structure formed in vapour deposition
processes at an oblique angle, the apparent dielectric constant
(also called the relative permittivity) as a function of depth will
be different for light polarized parallel or perpendicular to the
grating. Such films can be birefringent and have been proposed for
construction of phase retardation plates.
[0042] Since the present invention provides a method to produce
topologically structured liquid crystal polymer (LCP) layers, it
also gives a new possibility to make special optical retarders. For
such modulated surface profile LCPs there is, in addition to the
intrinsic anisotropy of the LCP material (.DELTA.n.sub.LCP), a
geometric contribution (.DELTA.n.sub.grooves) to the apparent
optical anisotropy (or correspondingly to the apparent dielectric
constant anisotropy). This will modify the overall apparent
anisotropy of the LCP layer (often one can approximately think of
.DELTA.n.sub.eff.apprxeq.".DELTA.n.sub.LCP+.DELTA.n.sub.grooves").
Depending on the volume fraction of the grooves, it is possible to
increase .DELTA.n.sub.eff up to 50% or even more. The modification
to the polarization of the transmitted ray is particularly of use
in the fabrication of optical components such as retarders,
waveplates, etc, where in this way a high .DELTA.n can be achieved
from lower .DELTA.n materials.
[0043] Use as optical diffusers: Optical films with different
refractive indices along certain film axes may have a substantial
diffuse component to their reflective and transmissive properties.
Known examples are stretched polymers filled with inorganic
inclusions not small compared to the wavelength of light, stretched
polymer dispersed liquid crystals (PDLCs) or polymer blends. The
mismatch in the refractive index along a particular axis has the
effect that incident light polarized along that axis will be
substantially scattered. In contrast, incident light polarized
along an axis in which the refractive indices are matched will be
transmitted or reflected with a lesser degree of scattering.
[0044] Suitably adapted embodiments of the invention can be used
also in this application field. The invention provides
topologically structured polymer films or coatings with
controllable dimensions and refractive indices along certain film
axes. Typically, the topologically structured (LCP) layers possess
a quite large refractive index mismatch of about 0.5 (i.e. the
difference in refractive indices between the holes or grooves (air)
and the polymer matrix). Therefore, with dimensions of the holes or
grooves in the order of the wavelength of light, a substantial
diffuse reflection is attained. In general, the size of the
structures should be less than several wavelengths in at least one
direction if diffuse reflection is desired. By suitably choosing
the values of parameters such as the size (with respect to
wavelength within the film) and shape (interfacial geometry or
topology) of the holes or grooves, their volume fraction, the film
thickness, and consequently the degree of the refractive index
mismatch, a desired degree of diffuse reflection and total
transmission of light--in a defined direction--is achievable.
Furthermore, the value of the refractive index mismatch can be
additionally controlled by varying the birefringence of the (LCP)
material used.
[0045] It is furthermore possible to orient (as described above)
such films or coatings and thereby control the optical properties,
with the additional possibility of patterning. This allows to
manipulate the refractive index mismatch to achieve desirable
degrees of diffuse and specular reflection and transmission along a
defined viewing direction and/or viewing cone. Furthermore, by
tailoring the geometry of the grooves and/or the patterns, also the
distribution of scattered light can be influenced.
[0046] Topologically structured polymer films or coatings according
to the invention, which have optically diffusing properties, can be
utilized to make a variety of optical devices. Examples are
(polarizing) diffusers and reflectors, as well as special
configurations for the improvement of the performance of liquid
crystal displays, particularly with respect to brightness, viewing
characteristics, and parallax. The films or coatings can be used
as--isotropic or anisotropic--diffusers inside various display
types, and as--isotropic or anisotropic--front scattering films
or--isotropic or anisotropic--diffusing reflectors for reflective
liquid crystal displays. As reflective polarizers, they are
particularly useful to increase the contrast, to reduce the glare,
or to enhance the polarization of light. Advantageously, they can
also be used as electrodes (optionally pixelwise). Furthermore,
because of their topological structure, they can be used as
orienting layers for liquid crystals or other materials to be
oriented. A further application are authentication elements (e.g.
to safeguard banknotes, credit cards, securities, identity cards
and the like against forgery). They can for instance be highly
reflective only for certain viewing angles thus providing a tilting
effect to the observer, and with a specific pattern even a sign or
picture is possible.
[0047] By controlling the amount of orientation or by writing with
laser light during the cross-linking process, the invention
provides also a possibility to make periodic structures, for
instance optical gratings. In addition, combinations of laser
written periodic structures and alignment produced (patterned)
structures are feasible. Advantageously, in this way new
topologically anisotropic structured coatings can be made, having
optical properties combining both scattering and diffraction
effects.
[0048] The highly scattering direction with respect to the grooves
direction depends on several parameters such as the anisotropy of
the LCP used (with respect to both the magnitude and sign). For
example, anisotropic structured LCP coatings fabricated using LCPs
with positive anisotropy (.DELTA.n), aligned along the grooves
direction, scatter the incident light in the direction parallel to
the grooves direction strongly due to the larger index mismatch,
while the components of the incident light in the direction
perpendicular to the grooves will transmit through the film with
less disturbance. As a result, the unpolarized light will be
partially polarized after passing through the film. Extinction
ratio of such coatings can have values of about 2 or higher, and
even higher extinction ratios can be achieved by adjusting the film
thickness, the volume fraction and size of the grooves, and the
degree of index match and mismatch through the use of materials
with a well defined birefringence and/or the use of dyes with
specific absorption axis with respect to the groove's direction. By
adjusting the period and the length of the grooves, the coating can
also be made anti-reflective in one direction and scattering in
another direction. This enables the fabrication of low loss
polarizers with high extinction ratio useful for
polarization-sensitive optical elements. The scattering and/or
polarizing properties can be further improved using the
superposition of two or more layers, for instance by coating a
substrate on both sides, by multilayers, or a stack of layers.
Depending on the desired property (brightness, viewing angle,
polarization efficiency, etc.) the topological structure of each
layer can be made similar or not, with the grooves directions in
the successive layers equal to or different from each other.
Additionally, an--isotropic or an anisotropic--anti-reflection
coating may be used to reach high values of transmission.
[0049] The topologically structured polymer films or coatings
according to the invention can be over-coated, without destroying
the topography, with various materials such as polymers, metals,
dielectrics, etc. Furthermore, special over-coatings can be used to
manipulate the topological or optical properties of the final
coating. Coatings covered with a metal layer, for example, can be
used to fabricate diffusing reflectors or reflection polarizers.
Other optical elements, such as UV filters or UV polarizers and
infrared polarizers, become also feasible.
[0050] In another aspect of the invention, the topologically
structured films or coatings can be used as a master to make
replica of isotropic or anisotropic topological structures of
various materials such as metals, pre-polymers, polymers,
dielectrics, etc. For this purpose, the desired material (e.g.
aluminium) will be applied onto the film or coating, optionally
further handled (e.g. heated, exposed to light, coated, etc.) to
achieve specific performances, and then separated from the
underlying film or coating.
[0051] It is a further advantage of the films or coatings according
to the invention that they can be easily coated to a variety of
devices or substrates such as paper, plastic, metal, dielectric,
etc., and that it is also possible to transfer them from one
substrate to another. For the transfer, for example known hot
stamping techniques are suitable, using a commercially available
transfer film as a substrate.
[0052] After transferring a film or coating according to the
invention, it can advantageously be coated on it's underside with
another material.
[0053] The invention will now be described by way of example with
reference to the accompanying drawings, in which:
[0054] FIG. 1 gives atomic force microscope (AFM) images of three
nano-porous anti-reflection LCP films being coatings according to
the invention. These are transparent with low effective refractive
index and different pore size lower than the visible wavelength:
(a) pores with an average diameter of 200 nm and a height of, 90
nm, (b) pores with an average diameter of 180 nm and a height of
120 nm, and (c) pores with an average diameter of 100 nm and a
height of 50 nm.
[0055] FIG. 2 (a) shows the variation of the effective refractive
index, n.sub.eff, as a function of the wavelength The open circles
correspond to the layer from Example 3 (FIG. 1c) and the solid
squares correspond to the layer from Example 1 (FIG. 1a). FIG. 2(b)
shows light transmission versus wavelength of an anti-reflective
layer from Example 2 (FIG. 1b). The glass substrate used was coated
on one side with the anti-reflective layer. The figure demonstrates
a relatively broad-band anti-reflective LCP layer with almost zero
% reflection (since 96% transmission is achieved for one coated
glass side).
[0056] FIG. 3 (a) is an example of an atomic force microscope image
of the obtained nano-grooves from Example 4 and FIG. 3(b) is a
corresponding sketch of the nanogrooved structure; the double-arrow
indicates the aligning direction of the LPP layer. The values of
the period .lamda. and the height A of the grooves can be tuned.
FIG. 3(c) is an AFM cross-sectional profile of the LCP layer along
the trace indicated as a black line in FIG. 3(a).
[0057] FIG. 4 (a) is a schematic view of an LCP-LPP layer used to
obtain a mixed alignment, showing a grooved LCP layer with an
aligning direction along the x axis, covered by an LPP layer with
an aligning direction along the y axis, wherein the LPP layer had
partially filled the LCP grooves and reduced their height from A=40
nm to A.sub.eff=10 nm. FIG. 4(b) shows a mixed alignment observed
between crossed polarisers (from Example 6), two different regions
being observed having two different colours corresponding to the
two anchoring directions, these colours becoming inverted between
these two regions when the sample is rotated by 45.degree..
[0058] FIG. 5 (a) is a plot of the ordinary (n.sub.o) and
extraordinary (n.sub.e) refractive indices of the layers from
Example 7 according to wavelength, as determined from ellipsometry
measurement, FIG. 5(b) shows a comparison of the optical anisotropy
(.DELTA.n=n.sub.e-n.sub.o) of the nano-grooved LCP layer from
Example 7 (squares) with that of a corresponding LCP layer without
grooves (circles), according to wavelength, and FIG. 5(c) is a plot
of light transmission versus viewing angle of s-polarized light
(solid squares) and p-polarized light (open triangles) of a glass
slide coated on one side with an anti-reflective LCP layer from
Example 7.
[0059] FIG. 6 (a) is an optical microscopy photograph taken between
crossed polarisers of an optically patterned nano-grooved LCP layer
from Example 8, with two aligning directions a.sub.1 and a.sub.2 in
adjacent pixels of size 100.times.100 .mu.m; FIG. 6(b) is the
corresponding atomic force image taken in region a.sub.2; FIG. 6(d)
is that taken in region a.sub.1 and FIG. 6(c) shows the interface
between these two regions.
[0060] FIG. 7 (a) is a schematic representation of an optically
patterned (grooved) LCP layer from Example 10, with two aligning
directions a.sub.1 (parallel to the i axis) and a.sub.2 (making an
angle of 45.degree. with respect to the i axis) in adjacent pixels
as indicated; XYZ represent the laboratory frame axes (with Z the
view axis), and ijk are the substrate frame axes (with k normal to
the substrate); .theta.i, .theta.j and .theta.k are the angles
between i and X axes, j and Y axes, and k and Z axes, respectively;
the angle (.alpha.) define the illumination direction, with respect
to the YZ plane. FIG. 7(b) shows two photographs illustrating the
viewing angle dependence of the reflected light from an optically
patterned "grooved" LCP layer prepared as described in Example 10,
with a polarizer (polarization axis parallel to Y) placed between
the substrate and the camera; the bright areas in the left image
correspond to region a.sub.2, and those in the right picture to
region a.sub.1.
[0061] FIG. 8 (a) shows various photographs illustrating the
illumination and viewing angle dependence of the reflected light
from an optically patterned "grooved" LCP layer from Example 11;
FIG. 8(b) are corresponding atomic force microscopy images
(10.times.10 .mu.m scan) of the coating from Example 11 taken in
region a.sub.1 (left) and that taken in region a.sub.2 (right).
EXAMPLE 1
Making a Layer Being a Coating According to the Invention
[0062] A mixture Mix1 was prepared containing an LCP pre-material
(photocrosslinkable liquid crystal pre-polymers) and a
non-photocrosslinkable nematic liquid crystal material. The LCP
components, denoted Monomer 1, Monomer 2 and Monomer 3 are
crosslinkable diacrylate monomers, and the non-crosslinkable
component, denoted 5CAPO2, is a non-crosslinkable nematic liquid
crystal monomer. ##STR1##
[0063] Mix1 was made from:
[0064] 58.9 wt %=52.6 mg of Monomer 1
[0065] 13.0 wt %=11.6 mg of Monomer 2
[0066] 4.3 wt %=3.8 mg of Monomer 3
[0067] 9.3 wt %=8.3 mg of 5CAPO2
[0068] 12.1 wt %=10.8 mg of Ethanol
[0069] 1.2 wt %=1.1 mg of a photoinitiator (Irgacure (trade mark)
369 by CIBA), and
[0070] 1.2 wt %=1.1 mg of BHT (butyl hydroxy toluene) as an
inhibitor.
[0071] A 1.5 wt % solution of mixture Mix1 was made in ethyl
acetate, then homogenized with moderate stirring for 15 min using
ultrasonic vibrations (with a Sonifier (trade mark) "W-250" digital
by BRANSON Ultrasonics Corporation), and filtered trough a 0.2
.mu.m filter.
[0072] The solution was then thinly spin coated at 1000 rpm onto
glass plates. The plates were warmed for 1 minute at 50.degree. C.
on a hotplate. The layers were then irradiated for 5 minutes under
nitrogen at room temperature by isotropic (not-polarized) light
from a mercury lamp at an intensity in the ultraviolet of 4.1
mW/cm.sup.2 to crosslink the LCP monomers. The layers were
optically transparent. Finally, the layers were rinsed with ethyl
acetate to remove the non-crosslinked material. After this
procedure, the layers were still transparent. The total thickness
of the resulting anti-reflection LCP layer was about 90 nm. Using
contact-mode Atomic-Force-Microscopy (AFM), the layers were found
to contain nano-pores (FIG. 1-a). The pores have an average
diameter of about 200 nm and an average height of about 90 nm. The
effective refractive index of the resulting anti-reflection LCP
layers was evaluated, at optical wavelengths between 400 nm and
1000 nm, using a Variable Angle Spectroscopic Ellipsometer (V. A.
S. E. from J. A. Woolam Co., Inc. Research & Instrumentation,
Lincoln, Neb. USA). An example of the results is shown in FIG. 2-a
(solid squares).
EXAMPLE 2
Varying the Component Ratio
[0073] A mixture Mix2 was made from:
[0074] 29.6 wt %=35.5 mg of Monomer 1
[0075] 6.8 wt %=8.1 mg of Monomer 2
[0076] 2.2 wt %=2.7 mg of Monomer 3
[0077] 35.9 wt %=43.1 mg of 5CAPO2
[0078] 24.3 wt %=29.1 mg of Ethanol
[0079] 0.6 wt %=0.68 mg of a photoinitiator (Irgacure (trade mark)
369 by CIBA), and
[0080] 0.6 wt %=0.68 mg of BHT (butyl hydroxy toluene) as an
inhibitor.
[0081] A 7.8 wt % solution of the mixture Mix2 was made in ethyl
acetate, then homogenized with moderate stirring for 15 min using
ultrasonic vibrations (with a Sonifier (trade mark) "W-250" digital
by BRANSON Ultrasonics Corporation), and filtered trough a 0.2
.mu.m filter. The solution was then thinly spin coated at 3000 rpm
onto glass plates. The layers were then irradiated for 5 minutes
under nitrogen at room temperature using isotropic (non-polarized)
light from a mercury lamp at an intensity in the ultraviolet of 4.5
mW/cm.sup.2 to crosslink the LCP monomers. The layers were
optically transparent. Finally, the layers were rinsed with a
solvent to remove the non cross-linked material. After this
procedure, the layers were still transparent. The total thickness
of the resulting anti-reflection LCP layer was about 120 nm. Using
contact-mode AFM, the layers were found to contain nano-pores. The
pores have an average diameter of about 180 nm and an average
height of about 120 nm (see FIG. 1-b). The transmission of the
resulting nano-porous LCP layer was evaluated, at optical
wavelengths between 400 nm and 1000 nm, using a Variable Angle
Spectroscopic Ellipsometer (V. A. S. E. firm J. A. Woolam Co., Inc.
Research & Instrumentation, Lincoln, Neb. USA). The results
showed the high-performance anti-reflection of the LCP layers with
zero % reflection in a wide wavelength region of the visible light
(cf FIG. 2-b).
EXAMPLE 3
Again Varying the Component Ratio
[0082] A mixture Mix3 was made from:
[0083] 20.2 wt %=35.9 mg of Monomer 1
[0084] 3.8 wt %=6.7 mg of Monomer 2
[0085] 1.3 wt %=2.3 mg of Monomer 3
[0086] 18.2 wt %=32.3 mg of 5CAPO2
[0087] 55.3 wt %=98.4 mg of Ethanol
[0088] 0.62 wt %=1.1 mg of a photoinitiator (Irgacure (trade mark)
369 by CIBA), and
[0089] 0.62 wt %=1.1 mg of BHT (butyl hydroxy toluene) as an
inhibitor.
[0090] A 4.7 wt % solution of the mixture Mix3 was made in ethyl
acetate, then homogenized with moderate stirring for 30 min at
50.degree. C., and filtered through a 0.2 .mu.m filter. The
solution was then thinly spin coated at 1000 rpm onto glass plates.
The layers were then irradiated for 5 minutes under nitrogen at
room temperature using isotropic (non-polarized) light from a
mercury lamp at an intensity in the ultraviolet of 4.5 mW/cm.sup.2.
After this irradiation, the LCP monomers had become cross-linked.
The layers were optically transparent. Finally, the layers were
rinsed with ethyl acetate to remove the non cross-linked material.
After this procedure, the layers were still transparent The total
thickness of the resulting anti-reflection LCP layer was about 120
nm. Using contact-mode AFM, the layers were found to contain
nano-pores. The pores have an average diameter of about 100 nm and
an average height of about 50 nm (see FIG. 1-c).
EXAMPLE 4
Preparation of an LCP Layer According to the Invention on an
Orientation Layer, Leading to a Modulated Profile "Nano-Grooved"
Topological Structure
[0091] A 2% solution of the photo-orienting material JP 265
(commercially available from CIBA), which is a linearly
photo-polymerizable polymer (LPP), in cyclopentanone was thinly
spin coated at 3000 rpm onto glass plates. The plates were warmed
for 10 minutes at 180.degree. C. on a hotplate. The resulting layer
had a thickness of about 60 nm and was then irradiated for 30
seconds at room temperature with linearly polarized UV light from a
200 W mercury high pressure lamp. The polariser was a film
polariser HNP'B by Polaroid. The wavelength of the light was
further limited by a UV filter WG295 (Schott) and a bandpass filter
UG11 (Schott). The intensity of the UV light at the plate was
determined as 1 mW/cm.sup.2.
[0092] The thus prepared orientation layer was then spin coated
with the solution from Example 3 using the same experimental
treatment. The resulting layers were transparent before and after
rinsing with the solvent. The total LCP film thickness was about
120 nm. Using contact-mode AFM it was found that instead of
substantially round pores nano-grooves (or -channels) are formed,
which are elongated in the direction of the underlying LPP
orientation layer (which is in this case also the direction of the
polarization of UV light which had irradiated the LPP layer). The
grooves had an average period (.lamda.) of about 100 nm and an
average height (A) of about 40 nm (cf FIG. 3).
EXAMPLE 5
Use of the Grooved LCP Layer of Example 4 as an Orientation Layer
in an LCD Cell
[0093] With the coated sides facing inwards, two plates from
Example 4 were assembled into a TN-LCD cell with a twist angle of
90.degree. (and a parallel-sided cell with a twist angle of
0.degree., respectively), using glass spheres of 5 .mu.m thickness
as spacer. The cell was filled with a nematic liquid crystal
mixture MLC 12000-00 (Merck) at a temperature slightly above the
nematic-isotropic transition temperature (89.degree. C.), and
slowly cooled. Observation of the prepared cells between crossed
polarisers showed that the liquid crystal is uniformly oriented.
Using a polarization microscope, the orientation direction which
the coated plates had imposed on the nematic mixture was
established to be substantially parallel to the "grooves"
direction.
EXAMPLE 6
Making a Hybrid (Mixed) Orientation Layer
[0094] A 1% solution of the photo-orienting LPP material JP 265
(from CIBA) in cyclopentanone was thinly spin coated at 4000 rpm
onto the coated side of the plates from Example 4, i.e. on an LCP
layer having a grooved topology with an average height (A) of the
grooves of about 40 nm. After the coating, AFM investigation showed
that the photo-orienting layer (LPP layer) covered the LCP layer
completely, as sketched in FIG. 4-a, and that the nano-grooves
still existed. The result was an LCP-LPP layer whose grooves had an
average period (.lamda.) of about 100 nm and an average effective
height (A.sub.eff) of 10 nm.
[0095] The plates were warmed to 180.degree. C. for 10 min, then
irradiated at room temperature with linearly polarized UV light
from a 200 W mercury high pressure lamp. The polariser was a film
polariser HNP'B by Polaroid. The wavelength of the light was
further limited by a UV filter WG295 (Schott) and a bandpass filter
UG11 (Schott). The intensity of the UV light at the plate was
determined as 1 mW/cm.sup.2. In order to demonstrate the aligning
competition between the two superposed layers, we used the fact
that the aligning effect of the photo-orienting LPP increases with
increasing duration of irradiation. Therefore, the layers were
subdivided into different sections, each one irradiated for a fixed
period of time ranging from 3 seconds to 10 minutes. The direction
of polarization of light was laying in the plane perpendicular to
the plates and to that of the "grooves" direction as sketched in
FIG. 4-a
[0096] With the coated sides facing inwards, the LCP-LPP coated
plate was assembled into a TN-LCD cell using as the second plate a
plate that has been coated with a single orientation layer of the
photo-orienting material JP 265 (prepared according to the
procedure given in Example 4) and glass spheres of 5 .mu.m
thickness as spacers. The anchoring direction of the second plate
was fixed at an angle of 45.degree. with respect to the direction
of the grooves of the first plate. The cell was filled with a
nematic liquid crystal mixture MLC 12000-00 (Merck) at a
temperature slightly above the nematic-isotropic transition
temperature (89.degree. C.), and slowly cooled.
[0097] Observation of the prepared cells between crossed polarisers
showed that the nematic liquid crystal in the cell is uniformly
oriented over all the different sections of the cell corresponding
to different UV irradiation time. Using a polarization microscope,
it was found that the orientation direction of the liquid crystal
was parallel to the direction of the grooves of the LCP-LPP coated
plate in sections where the UV illumination time was short (less
than 25 seconds). For sections where the UV illumination time was
long (longer than 5 min), the orientation direction of the liquid
crystal was established to be parallel to the orientation direction
of the LPP layer superposed to the grooved LCP layer, i.e. the
orientation of the liquid crystal was perpendicular to the
direction of the grooves.
[0098] Observation under cross-polarisers using a polarising
microscope showed that for sections where the UV illumination time
was of medium duration (between 25 seconds and 5 minutes), the cell
contained both orientations corresponding to two different colours
(see FIG. 4-b). By rotating the cell, it was possible to pass from
one colour (orientation) to the other by rotation through an angle
of 45.degree.. Thus, the aligning competition, if properly
adjusted, can be used to generate bistable anchoring of liquid
crystals.
EXAMPLE 7
Making an Optical Retardation Layer with Artificially High
Effective Anisotropy
[0099] A mixture Mix4 was made from:
[0100] 17.0 wt %=36.2 mg of Monomer 1
[0101] 3.2 wt %=6.8 mg of Monomer 2
[0102] 1.1 wt %=2.3 mg of Monomer 3
[0103] 19.2 wt %=40.8 mg of 5CAPO2
[0104] 58.5 wt %=124.3 mg of Ethanol
[0105] 0.5 wt %=1.05 mg of a photoinitiator Irgacure (trade mark)
369 by CIBA), and
[0106] 0.5 wt %=1.01 mg of BHT (butyl hydroxy toluene) as an
inhibitor.
[0107] A 6.7 wt % solution of mixture Mix4 was made in ethyl
acetate, then homogenized with moderate stirring for 30 min at
50.degree. C., and filtered trough a 0.2 .mu.m filter.
[0108] Example 4 was repeated using the above solution. The layers
were transparent before and after rinsing with the solvent. The
total LCP film thickness is about 110 nm. AFM investigations showed
the formation of "nano-grooves", which had an average period
(.lamda.) of about 150 nm and an average height (A) of about 75
nm.
[0109] Using a tilt compensator and a polarization microscope, the
layer was found to have an optical retardation of about 19 nm,
which corresponds to an effective optical anisotropy of about 0.17.
It was also established that the orientation of the optical axis
all over the layer was parallel to the orientation direction of the
LPP layer.
[0110] This finding was further confirmed with ellipsometric
measurements. FIG. 5 shows an example of the results obtained. FIG.
5-a gives the ordinary (n.sub.o) and extraordinary (n.sub.e)
refractive indices versus wavelength of the layers from the present
example; FIG. 5-b gives in squares the corresponding optical
anisotropy .DELTA.n=n.sub.e-n.sub.o, and by way of comparison in
circles the optical anisotropy of a corresponding LCP layer without
grooves.
[0111] In addition, FIG. 5-c shows light transmission versus view
angle of s-polarized light (solid squares) and p-polarized light
(open triangles) of a glass slide coated on one side with an
anti-reflective LCP layer according to the present example.
EXAMPLE 8
Optically Patterned "Nano-Grooved" LCP Layer
[0112] A 2% solution of a photo-orienting material (JP 265 from
CIBA) in cyclopentanone was thinly spin coated at 3000 rpm onto
glass plates. The plates were warmed for 10 minutes at 180.degree.
C. on a hotplate. The resulting LPP layer had a thickness of about
60 nm and was then irradiated, in a first step, via a photo mask
(100 .mu.m.times.100 .mu.m squares) with linearly polarized
UV-light for 4 minutes. In this procedure the direction of
polarization of light lay in the plane perpendicular to the plates
(aligning direction al). In a second step, after rotation of the
direction of the linear polarization by 45.degree., the mask was
removed and the layer irradiated for 30 seconds (aligning direction
a.sub.2). This results in a photo-patterned LPP layer with two
different aligning directions a.sub.1 and a.sub.2.
[0113] Then, a 3.8 wt % solution of mixture Mix3 (see Example
3)--made in ethyl acetate, homogenized with moderate stirring for
30 min at 50.degree. C., and filtered through a 0.2 .mu.m
filter--was thinly spin coated at 1000 rpm onto the photo-patterned
LPP layer. The obtained layers were irradiated for 5 minutes under
nitrogen at room temperature using isotropic (non-polarized) light
from a mercury lamp at an intensity in the ultraviolet of 4.5
mW/cm.sup.2. After this irradiation, the LCP monomers had become
cross-linked. The layers were optically transparent. Finally, the
layers were rinsed with a solvent to remove the non cross-linked
material. After this procedure, the layers were still transparent.
The total thickness of the resulting anti-reflection LCP layer is
about 100 nm.
[0114] Using a polarization microscope, it was established that the
orientation of the optical axis of the LCP layer was parallel to
the two aligning directions (a.sub.1 and a.sub.2) of the patterned
LPP layer. FIG. 6 shows an example of the obtained results. Using
contact-mode AFM, the layers were found to contain "nano-grooves"
along the aligning directions, a.sub.1 and a.sub.2, in adjacent
squares. The grooves have an average period .lamda. of about 150 nm
and an average height A of about 70 nm.
EXAMPLE 9
Making an LCP Film with a "Grooved" Topological Structure Leading
to an Anisotropic Diffuser
[0115] A mixture Mix5 was made from:
[0116] 52.6 wt % of Monomer 1
[0117] 9.9 wt % of Monomer 2
[0118] 3.3 wt % of Monomer 3
[0119] 32.8 wt % of 5CAPO2
[0120] 0.7 wt % of a photoinitiator (Irgacure (trade mark) 369 by
CIBA), and
[0121] 0.7 wt % of BHT (butyl hydroxy toluene) as an inhibitor.
[0122] A 16 wt % solution of mixture (Mix5) was made in a mixture
of 9 parts buthyl acetate and 1 part ethanol, then homogenized with
moderate sting for 5 min using ultrasonic vibrations (with a
Sonifier (trade mark) "W-250" digital by BRANSON Ultrasonics
Corporation), and filtered trough a 0.2 .mu.m filter.
[0123] The solution was then thinly spin coated at 800 rpm onto an
LPP orientation layer (of about 60 nm thickness) prepared using the
photo-orienting material JP 265. The plates were warmed for 1
minute at 50.degree. C. on a hotplate. The layers were then
irradiated for 2 minutes under nitrogen at room temperature by
isotropic (non-polarized) light from a mercury lamp at an intensity
in the ultraviolet of 4.5 mW/cm.sup.2 to cross-link the LCP
monomers. The layers were optically transparent. Finally, the
layers were rinsed with ethanol to remove the non cross-linked
material. The total thickness of the resulting LCP layer is about
400 nm. Using contact-mode AFM it was found that grooves (or
channels) are formed, which are elongated in the direction of the
underlying LPP orientation layer. The grooves had an average period
(.lamda.) of about 900 nm and an average height (A) of about 70 nm.
The layers were optically diffusing at certain viewing angles with
respect to the grooves direction.
[0124] The transmission (reflectivity) of such coatings was
evaluated at optical wavelength of 550 nm, using a Variable Angle
Spectroscopic Ellipsometer (V. A. S. E. from J. A. Woolam Co.) at
various viewing angles with respect to the grooves direction. The
transmission of the coatings reaches values of about 90% for the
crossed transmission (i.e. transmission measured with the
orientation direction of the LCP layer perpendicular to the
direction of polarized light), and about 50% for the parallel
transmission (i.e. transmission measured with the orientation
direction of the LCP layer parallel to the direction of polarized
light). This imparts a strong directional dependence of the
reflecting appearance to the layer. The extinction ratio of such
layers can reach values of about 2.
EXAMPLE 10
Making an Optically Patterned Anisotropic Diffuser
[0125] A 2% solution of the photo-orienting material JP 265 in
cyclopentanon was thinly spin coated at 3000 rpm onto glass plates.
The plates were warmed for 10 minutes at 180.degree. C. on a
hotplate. The resulting layer had a thickness of about 60 nm. It
was then irradiated, in a first step, via a photo mask (see FIG.
7-b, the smallest square corresponds to 2000 .mu.m.times.2000
.mu.m) with linearly polarized UV-light for 4 minutes. In this
procedure the direction of polarization of light lay in the plane
perpendicular to the plates (aligning direction al, see FIG. 7-a).
In a second step, after rotation of the direction of linear
polarization by 45.degree., the mask was removed and the layer
irradiated for 30 seconds (aligning direction a.sub.2). This
results in a photo-patterned LPP layer with two different aligning
direction a.sub.1 and a.sub.2.
[0126] Then, the solution of mixture Mix5 (see Example 9) was
thinly spin coated at 800 rpm onto the photo-patterned LPP layer.
The obtained layers were irradiated for 2 minutes under nitrogen at
room temperature using isotropic (non-polarized). light from a
mercury lamp at an intensity in the ultraviolet of 4.5 mW/cm.sup.2.
After this irradiation, the LCP monomers had become cross-linked.
The layers were optically transparent. Finally, the layers were
rinsed with ethanol to remove the non cross-linked material. The
total thickness of the resulting reflection LCP layer is about 400
nm. Using contact-mode AFM, the layers were found to contain
"grooves" with two different directions in the adjacent pixels as
imposed by the orientation layer. The grooves have an average
period of about 900 nm and an average height of about 70 nm.
Optical investigations of the layers showed that the adjacent
pixels have two different orientation directions shifted with an
angle of 45.degree. as imposed by the double UV-illumination. It
also showed that adjacent pixels have different reflection (or
transmission), which also depend on viewing angle with respect to
the grooves direction (see FIG. 7).
EXAMPLE 11
Making an Optically Patterned Diffusing Reflector
[0127] A thin aluminium (Al) layer (of about 80 nm thickness) was
evaporated onto an optically patterned "grooved" LCP layer prepared
according to Example 10. After evaporation, AFM investigations
showed that the evaporated aluminium layer has covered the LCP
layer completely, and that the grooves still existed (see FIG.
8-b). The result was an LCP/aluminium modulated coating with
grooves of an average period (.lamda.) of about 1.0 .mu.m and an
average height of about 70 nm. The grooves in adjacent pixels have
two different orientation directions shifted with an angle of
45.degree. as imposed by the double UV-illumination. Optical
investigations of the coating showed that the adjacent pixels
reflect light in a different way depending on illumination and
viewing angles. Typical examples are shown in FIG. 8-a.
* * * * *