U.S. patent application number 11/670545 was filed with the patent office on 2007-06-07 for filament or fibre.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to JACOB M. J. DEN TOONDER, JAN M. KRANS, MICHEL P. B. VAN BRUGGEN, JOHANNES T. A. WILDERBEEK.
Application Number | 20070128437 11/670545 |
Document ID | / |
Family ID | 32607720 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070128437 |
Kind Code |
A1 |
WILDERBEEK; JOHANNES T. A. ;
et al. |
June 7, 2007 |
FILAMENT OR FIBRE
Abstract
A filament or fibre (2) comprising: a first conductive layer
(4); an electro-optically active layer (6); a second conductive
layer (8); wherein the filament or fibre has an off-state and an
on-state, the electro-optically active layer comprising a
combination of an electro-optically active substance and a
polymer.
Inventors: |
WILDERBEEK; JOHANNES T. A.;
(VEGHEL, NL) ; VAN BRUGGEN; MICHEL P. B.;
(HELMOND, NL) ; KRANS; JAN M.; (DEN BOSCH, NL)
; DEN TOONDER; JACOB M. J.; (HELMOND, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
EINDHOVEN
NL
|
Family ID: |
32607720 |
Appl. No.: |
11/670545 |
Filed: |
February 2, 2007 |
Current U.S.
Class: |
428/375 |
Current CPC
Class: |
D01D 5/426 20130101;
D01F 8/00 20130101; C09K 2323/035 20200801; Y10T 428/2933
20150115 |
Class at
Publication: |
428/375 |
International
Class: |
D02G 3/00 20060101
D02G003/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 21, 2004 |
GB |
0411349.4 |
Claims
1. A filament or fibre (2) comprising: a first conductive layer
(4); an electro-optically active layer (6); a second conductive
layer (8); wherein the filament or fibre has an off-state and an
on-state, the electro-optically active layer comprising a
combination of an electro-optically active substance and a
polymer.
2. A filament or fibre according to claim 1 wherein the
electro-optically active substance comprises a liquid crystalline
material.
3. A filament or fibre according to claim 1 wherein the polymer
content is substantially between 0.5 to 40%.
4. A fibre or filament according to claim 1 wherein the
electro-optically active substance comprises ferro-electric
phase.
5. A filament or fibre according to claim 1 wherein the polymer
content is substantially between 30% to 99.8%.
6. A filament or fibre according to claim 1 wherein the polymer
comprises a substantially isotropic polymer phase, and the liquid
crystalline material comprises a dispersed liquid crystalline
phase.
7. A filament or fibre according to claim 5 wherein the liquid
crystalline phase comprises liquid crystalline domains having an
average diameter of approximately 0.5-2 .mu.m.
8. A filament or fibre according to claim 5 wherein the
electro-optically active layer comprises a polymer comprising a
polymer backbone (40) to which mesogenic cores (42) are attached,
and a liquid crystalline solvent.
9. A filament or fibre according to any one of claim 5 where the
liquid crystalline material comprises a liquid crystalline
director, which director is controlled uniaxially.
10. A filament or fibre according claim 5 wherein the liquid
crystalline material comprises a liquid crystalline director, which
director is controlled biaxially.
11. A filament or fibre according to claim 9 further comprising an
alignment layer for enforcing the director control.
12. A filament or fibre according to claim 9 wherein, in one of the
on-state or the off-state, the refractive index of the polymer is
different to that of the liquid crystalline material, for a
predetermined wavelength of incident light.
13. A filament or fibre according to claim 12, wherein in the other
of the on-state or the off-state, the refractive index of the
polymer matches the ordinary refractive index of the liquid
crystalline material.
14. A filament or fibre according to claim 12 wherein the
electro-optically active layer comprises an anisotropic
polymer.
15. A filament or fibre according to claim 12 wherein the
electro-optically active substance comprises material possessing a
smectic phase.
16. A filament or fibre according to claim 12 wherein the
electro-optically active substance comprises material possessing a
chiral nematic phase or cholesteric phase, optionally induced by a
chiral dopant.
17. A filament or fibre according to claim 12 wherein the polymer
is at least partly based on non-covalent, supramolecular
interactions.
18. A fibre or filament (2) according to claim 12 having a
substantially circular cross-section, the first conductive layer
(4) comprising an inner conductive core extending axially along the
filament or fibre, and the second conductive layer (8) comprising
an outer electrode, the electro-optically active layer (6) being
positioned between the inner core and the outer electrode.
19. A fibre or filament according to claim 18 wherein the outer
electrode is at least partially transparent.
20. A fibre or filament according to claim 18 further comprising a
first coating layer completely or partially coating the conductive
core.
21. A fibre or filament according to any one of claim 18 further
comprising a second coating layer positioned between the
electro-optically active layer and the outer electrode.
22. A fibre or a filament according to any one of claim 18 where
the or each coating layer comprises an alignment layer.
23. A fibre or filament according to any one of claim 18 further
comprising one or more metal wires wound around the outer
electrode.
24. A fibre or filament according to any one of claim 18 further
comprising spacers positioned between the inner electrode and the
outer electrode.
25. A fibre or filament according to claim 24 wherein the spacers
are formed from a non-conductive material.
26. A fibre or filament according to claim 15 having a
substantially square or rectangular cross-section, the first
conductive layer (18) comprising a bottom electrode, the second
conductive layer (16) comprising a top electrode, and the
electro-optically active layer (14) being positioned between the
bottom and top electrode layers.
27. A method for forming a filament or fibre (2) comprising:
forming a first conductive layer (4); applying an electro-optically
active layer (6) either directly, or indirectly, to the first
conductive layer; applying a second conductive layer (8), either
directly, or indirectly, to the electro-optically active layer,
wherein the electro-optically active layer is formed by: (i)
forming the electro-optically active layer from a homogeneous
system of cross linkable monomers and a non-reactive mesogen, prior
to applying the electro-optically active layer to the first
conductor; (ii) inducing a phase change in the homogeneous
system.
28. A method according to claim 27 wherein the phase change is
induced before application of the second conductive layer.
29. A method according to claim 27 wherein the step of inducing a
phase change comprises heating the filament or fibre.
30. A method for forming a filament or fibre comprising: forming a
first conductive layer; applying an electro-optically active layer
either directly, or indirectly, to the first conductive layer;
applying a second conductive layer, either directly, or indirectly,
to the electro-optically active layer, wherein the
electro-optically active layer is formed by: (i) forming the
electro-optically active layer from a homogeneous system of at
least a polymer and a non-reactive mesogen, in combination with a
common solvent, prior to applying the electro-optically active
layer to the first conductor; (ii) removing of the solvent.
31. A method according to claim 30 wherein the solvent is removed
before application of the second conductive layer.
Description
[0001] This invention relates a fibre or filament, especially one
that is suitable for inclusion in a fabric or garment with the aim
of producing optically detectable effects therein.
[0002] Various methods of producing colour changing, or light
emitting fibres are known.
[0003] One known method is based on the use of an electrolumiphore
material which emits light under the influence of an electric
field. Such a method is described in UK patent application No. GB
2273606 and International patent application No. WO 97/15939. The
electric field used in such methods is created by integrating at
least two electrodes in a fibre.
[0004] Other known methods also make use of specific thermochromic
materials, i.e., materials that change colour under the influence
of a change in temperature. Such a method is disclosed in European
patent publication No. EP 0410415.
[0005] Other known methods have used liquid crystalline material as
electro-optically active material for forming a filament or fibre
adapted to have optically detectable effects.
[0006] A problem associated with liquid crystal based active layers
is the inherit lack of sufficient mechanical stability associated
with liquid crystalline materials. As the liquid crystal layer
constitutes primarily low molecular weight components, the overall
material behaviour is that of a liquid or liquid-like layer. This
greatly complicates the construction and processing of a fibre or
filament. When it is required to apply an overlying second
electrode, the process is extremely cumbersome and problematic.
[0007] In addition, the achievable contrast of pure liquid crystal
based layers is often insufficient, and additional functional
layers are required, such as polarizers or brightness enhancement
layers.
[0008] It is an object of the present invention to provide a
filament or fibre having at least one optical property that is
controllably alterable, and in which the filament or fibre has
improved mechanical stability.
[0009] According to a first aspect of the present invention there
is provided a filament or fibre comprising: [0010] a first
conductive layer; [0011] an electro-optically active layer; [0012]
a second conductive layer; [0013] wherein the filament or fibre has
an off-state and an on-state, the electro-optically active layer
comprising a combination of an electro-optically active substance
and a polymer.
[0014] The electro-optically active layer may comprise flexible
polymers, side-chain liquid crystal polymers, main-chain liquid
crystalline polymers, isotropic or anisotropic network type
structures, of covalent or non-covalent, supramolecular nature, or
dispersed polymer particles.
[0015] The presence of a polymer in the electro-optically active
layer can produce a stabilising effect on the mechanical properties
of the filament or fibre which increases the manufacturing options
and simplifies processing.
[0016] Advantageously, the electro-optically active substance
comprises a liquid crystalline material.
[0017] Liquid crystalline materials of the type generally used for
electro-optical applications have a low molecular weight. By
combining such a material with a high molecular weight polymer, the
properties of the electro-optically active layer will become less
liquid-like and more solid-like.
[0018] The properties of the electro-optically active layer may be
tailored to produce a filament or fibre with appropriate
properties, by using appropriate proportions of polymer and liquid
crystalline material.
[0019] In other words, the polymer forming part of the
electro-optically active layer will change the mechanical material
behaviour of the liquid crystalline material from liquid-like (for
pure low molecular weight liquid crystal materials) to more
solid-like. This results in more mechanically stable behaviour,
thus making the use of liquid crystal based effects more realistic
for, for use in, for example, textile electronics.
[0020] A further advantage of using a combination of an
electro-optically active substance and a polymer to form the
electro-optically active layer in the filament or fibre is that
reduced driving voltages, improved contrast, enhanced viewing
angles (reduced off-axes haze) (see for instance Yang, D.-K.,
Chien, L.-C., Doane, J. W., Appl. Phys. Lett., 60, p. 3102, 1992),
are achievable when compared with filaments or fibres in which the
electro-optically active layer comprises solely liquid crystalline
material.
[0021] These improvements are known from the electro-optical
characterization of conventional liquid crystal display
applications. Switching voltages of twisted nematic (TN) devices of
the order of 2-3 V can for instance be lowered to approximately 1 V
when stabilized with 2% polymer. See for instance Bos, P. J.,
Rahman, J., Doane, J. W., SID Dig. Tech. Pap., 24, p. 877, 1993.
Similar findings are observed for super twisted nematic (STN)
devices.
[0022] In addition, it is possible to reduce or eliminate defects
such as stripe deformations in 270.degree. super twisted nematic
liquid crystals. See for instance Bos, P. J., Fredley, D., Li, J.,
Rahman, J. in "Liquid crystals in complex geometries. Formed by
polymer and porous networks", Crawford, G. P., Zumer, S. (Eds.),
Chapter 13, Taylor & Francis, London, 1996.
[0023] The liquid crystalline material may comprise any suitable
liquid crystal or mixture of liquid crystals such as liquid
crystals used for TN or STN configurations.
[0024] The combination of an electro-optically active substance,
such as a liquid crystalline substance, and a polymer to form the
electro-optically active layer in the filament or fibre, can
consist of a homogeneous or inhomogeneous mixture, depending on the
ratio of the constituents and the manufacturing conditions, of
which the polymerisation conditions can be an important aspect.
[0025] Advantageously, the electro-optically active layer comprises
a polymer content of between 0.5 to 40%.
[0026] Preferably, the electro-optically active layer comprises a
polymer stabilized ferro-electric or anti-ferro-electric liquid
crystalline material. Such a layer may be used to create extremely
fast switching filaments or fibres with typical switching rates in
the range from 200 ms to 20 ms.
[0027] In another preferred embodiment of the present invention the
electro-optically active material comprises a polymer stabilized
chiral, nematic or cholesteric fibre. Fibres incorporating such an
electro-optically active layer may also include colours that can be
accurately tuned. Chiral nematic or cholesteric liquid crystals
show a polarization-selective reflection provided the wavelength of
the circularly polarized incoming light fulfils the reflection
condition: .lamda.= np
[0028] where .lamda. is the wavelength of the reflected light, n is
the average refractive index of the liquid crystal, and p is the
pitch length of the helix of the liquid crystal director. One
handedness of the incoming circularly polarized light is absorbed
and reflected, whereas the other handedness is transmitted,
provided a monolithically aligned liquid crystal is used (e.g.
having a Grandjean or fingerprint texture). The exact color can be
tuned by the choice of materials, e.g. the choice of liquid
crystal, and for instance the polymerisation conditions,
determining the effective pitch of the chiral nematic phase.
[0029] Conveniently the ratio of polymer to liquid crystal in the
electro-optically active layer may be any suitable ratio, but
preferably is in the region of 30-99.8%, more preferably 50-80%.
Such polymer systems are known as polymer dispersed liquid crystal
(PDLC) systems.
[0030] When the polymer system is a PDLC system, the polymer
preferably comprises a substantially continuous isotropic polymer
phase, and the liquid crystalline material comprises a dispersed
liquid crystalline phase.
[0031] Advantageously, the liquid crystalline phase comprises
droplets or domains, having an average diameter in the range 0.3-3
.mu.m, preferably in the range 1-2 .mu.m, containing liquid
crystalline material. Usually, a modal dispersion in/of the
diameter of the droplets or domains is observed.
[0032] Conveniently, the liquid crystalline phase is randomly
aligned, and of nematic nature, although in principle other liquid
crystalline phases, such as chiral nematic, smectic, or discotic
phases can be used too. In the off-state of the filament or fibre,
there is a mismatch between the isotropic refractive index of the
continuous polymeric phase, and that of the randomly aligned
dispersed liquid crystalline phase. Because of this, and the micron
sized domain size of the liquid crystalline phase, light scattering
will occur, resulting in a white layer.
[0033] Upon application of a voltage inducing the on-state, the
director of the dispersed nematic liquid crystalline droplets will
orient parallel to the electric field, provided the liquid
crystalline phase has a positive dielectric anisotropy. If the
materials are chosen such that the refractive index of the polymer
phase matches the ordinary refractive index of the liquid
crystalline dispersed phase, no effective refractive index mismatch
is experienced, and the layer will appear transparent. A
particularly well-suited material combination is for instance, the
NOA 65/E7 system, that can be obtained from Norland (Cranbury,
N.J., U.S.A.) and Merck (Darmstadt, Germany), respectively. The
liquid crystal E7 (see FIG. 4) is actually a eutectic mixture
consisting of 50.6% 4'-pentylcyanobiphenyl, 25.2%
4'-heptylcyanobiphenyl, 17.8% 4'-octyloxycyanobiphenyl, and 6.4%
4'-pentylcyanoterphenyl (see Wilderbeek et al., Advanced Materials,
15(12), p. 985-988, 2003).
[0034] Examples of further materials and useful combinations are
for instance extensively described in Drzaic, P. S., "Liquid
crystal dispersions", World Scientific, Singapore, 1995. Due to the
aligned director in the on-state, an off-axis refractive mismatch
of the refractive indices will exist, resulting in an
angle-dependent hazy appearance.
[0035] Alternatively, the polymer materials and liquid crystalline
materials can be chosen such that their respective refractive
indices, i.e. the isotropic polymer refractive index and the
ordinary refractive index of the dispersed liquid crystalline
phase, match in the on-state, when an electric field is present. In
this case, the on-state is the transparent state, and the off-state
is the opaque state. Other combinations, such as matching of the
isotropic polymer refractive index with the extraordinary
refractive index of the dispersed liquid crystalline phase, or
using liquid crystalline materials with negative dielectric
anisotropy, are also possible.
[0036] Alternatively, the electro-optically active layer comprises
an anisotropic gel comprising a polymer having a polymer backbone
to which mesogenic cores are attached, and a liquid crystalline
substance.
[0037] Preferably, the fibre or filament has a substantially
circular cross section, the first conductive layer comprising an
inner conductive core extending axially along the filament or
fibre, and the second conductive layer comprising an outer
electrode, the electro-optically active layer being positioned
between the inner core and the outer electrode.
[0038] Advantageously, the outer electrode is at least partially
transparent.
[0039] Alternatively, the fibre or filament has a substantially
square or rectangular cross section, the first conductive layer
comprising a bottom electrode, the second conductive layer
comprising a top electrode, and the electro-optically actively
layer being positioned between the bottom and the top electrode
layers.
[0040] According to a second aspect of the present invention there
is provided a method for forming a filament or fibre
comprising:
[0041] forming a first conductive layer;
[0042] applying an electro-optically active layer either directly,
or indirectly, to the first conductive layer;
[0043] applying a second conductive layer, either directly, or
indirectly, to the electro-optically active layer, wherein the
electro-optically active layer is formed by:
[0044] (i) forming the electro-optically active layer from a
homogeneous system of cross linkable monomers and a non-reactive
mesogen, prior to applying the electro-optically active layer to
the first conductor;
[0045] (ii) inducing a phase change in the homogeneous system.
[0046] The phase change in the homogeneous system may take place
either before or after application of the second conductive
layer.
[0047] Preferably the step of inducing a phase change comprises
illuminating or heating the filament or fibre.
[0048] According to a third aspect of the present invention there
is provided a method for forming a filament or fibre
comprising:
[0049] forming a first conductive layer;
[0050] applying an electro-optically active layer either directly,
or indirectly, to the first conductive layer;
[0051] applying a second conductive layer, either directly, or
indirectly, to the electro-optically active layer, wherein the
electro-optically active layer is formed by:
[0052] (i) forming the electro-optically active layer from a
homogeneous system of at least a polymer and a non-reactive
mesogen, in combination with a common solvent, prior to applying
the electro-optically active layer to the first conductor;
[0053] (ii) removing of the solvent.
[0054] The solvent may be removed before application of the second
conductive layer. This results in a heterogeneous system.
[0055] Alternatively, the solvent may be removed after application
of the second conductive layer. This results in a homogeneous
system.
[0056] Optionally, the method comprises the additional step of
heating the homogeneous or heterogeneous system.
[0057] The invention will now be further described by way of
example only with reference to the accompanying drawings in
which:
[0058] FIGS. 1a and 1b are schematic representations of a fibre
according to a first embodiment of the invention.
[0059] FIG. 2 is a schematic representation of a fibre according to
a second embodiment of the present invention;
[0060] FIGS. 3a and 3b are schematic representations of a polymer
dispersed liquid crystal optical element suitable for forming the
electro-optically active layer forming a fibre according to the
present invention;
[0061] FIG. 4 shows the chemical composition of a non-reactive
liquid crystalline mixture E7;
[0062] FIG. 5 is a schematic representation of a set up for a
continuous manufacturing process for manufacturing a fibre or
filament according to the present invention;
[0063] FIGS. 6a and 6b are schematic representations of an
isotropic gel suitable for forming the electro-optically active
layer forming a filament or fibre according to the present
invention;
[0064] FIG. 7 shows the chemical composition of C3M and 5CB
polymers suitable for use in the present invention.
[0065] Referring to FIGS. 1a and 1b, fibres, according to a first
embodiment of the present invention are shown schematically.
[0066] FIG. 1a shows a fibre 2 comprising a central conductive core
4 extending axially along the fibre. The core is surrounded by an
electro-optically active layer 6 which in turn is surrounded by an
outer electrode 8. The fibre 2 further comprises a protective layer
10.
[0067] It is to be understood that a conductive core according to
the present invention is designated generally by the reference
numeral 4. Although the conductive core may directly consist of a
conductive metal wire, the conductive core 4 may also comprise an
elongate core, preferably formed from an electrically insulating
material, the core having a core axis, and covered by an
electrically conductive material. The electrically conductive
material may be fabricated in several ways, by using thin layer
deposition techniques, lithographic methods, X-ray lithography,
particle beams and other non-lithographic techniques.
[0068] The electrode material can be either inorganic or organic
and includes, but is not limited to, indium tin oxide, gold,
silver, copper, platinum, and their derivatives, and conductive or
semi-conductive oligomers or polymers, e.g. polyaniline and
thiophene derivatives such a poly(3,4-ethylenedioxythiophene): PEDT
or PEDOT.
[0069] Optionally, these oligomers or polymers may contain
additives to optimise the electrical and thermal conductivity, and
enhance the lifetime.
[0070] In preferred embodiments, the core is substantially
cylindrical in shape and may be formed from a non-conductive
flexible polymer fibre. Examples of suitable polymer fibres
include, but are not limited to, polyesters, polyamides,
polyacrylics, polypropylenes, vinyl-based polymers, wool, silk,
flax, hemp, linen, jute, rayon-based fibres, cellulose
acetate-based fibres and cotton.
[0071] An advantage of using polymer fibres is that they are
readily available and have mechanical properties which can be
adapted to suit the particular fibre requirement e.g. in terms of
strength and flexibility. This is to be contrasted with conductive
metal wires which have only a limited range of mechanical
properties.
[0072] It is furthermore to be understood that the conductive core
as described above, may optionally also include one or more
additional coatings, overlaying the electrically conductive
material. The primary function of this coating is preferably to
protect the electrodes, since these are by nature very fine and
delicate. However, the coating may also perform a secondary
function which includes, but is not limited to, an adhesion layer,
a barrier layer, a sealing or covering layer, a UV shielding layer,
a polarizing layer, a brightness enhancing or perception
improvement layer, a colouration layer, an additional conductive or
semi-conductive electrode layer, a channelling layer, a dielectric
layer or any combinations thereof.
[0073] The fibre shown in FIG. 1b has a ribbon-like or flat fibre
layout. This fibre 12 comprises an electro-optically active layer
14 surrounded by first and second electrode layers 16, 18.
[0074] These basic configurations may have further layers added as
appropriate, as shown in FIG. 2, and the fibre 2, 12 may not always
have a protective layer 10.
[0075] FIG. 2 illustrates a fibre 20 comprising a central
conductive core 22, an electro-optically active layer 24, an outer
electrode layer 26 and a protective layer 28. The fibre 20
comprises a first alignment layer 30 positioned between the central
core 22 and the electro-optically active layer 24, and a second
alignment layer 32 positioned between the electro-optically active
layer 24 and the outer electrode 26.
[0076] Again, it is to be understood that a conductive core
according to the present invention may directly consist of a
conductive metal wire, but the conductive core 22 may also comprise
a non-conductive core, preferably formed from an electrically
insulating material, that is covered by an electrically conductive
material.
[0077] The fibre 20 further comprises a functional layer 34 which
will be described in more detail herein below.
[0078] It is to be understood that the alignment layers and the
functional layers are not essential to all embodiments of the
present invention. The nature of the electro-optically active layer
will determine the structure of the fibre.
[0079] For instance, alignment layers are required in those systems
where the orientation in a preferred direction is essential to the
functioning of the electro-optical layer. For example, it may be
preferred to induce a well-defined twist of the liquid crystalline
director in a TN or STN device by aligning the liquid crystals at
the boundaries of the electro-optically active layer. As these
switching principles are based on the modulation of the
polarization of the incident light, usually at least one
polarization layer is required to make the effect visible.
Furthermore, additional requirements may hold for these specific
systems, such as the control over the retardation, d.DELTA.n, where
d is the thickness of the electro-optically active layer and
.DELTA.n is the birefringence of the liquid crystalline phase (see
for instance Gooch, C. H., Tarry, H. A., Electronics Letters, 10,
p. 2, 1974, and Gooch, C. H., Tarry, H. A., J. Phys. D: Appl.
Phys., 8, p. 1575, 1975.
[0080] In addition, further features may be present in the fibre,
for example, thin metal wires may be wound around the outer
electrode, which wires act as an electrical shunt. Spacers may be
included to define the thickness of electro-optically active layer.
The spacer means are preferably formed from a non-conductive
material, such as glass or polystyrene, and may be in the form of,
for example, elongate wires or substantially spherical beads of
specific size, or thin continuous filaments wound around the core
electrode. The wound filaments are thus situated between the core
electrode and the outer electrode and define the spacing between
them.
[0081] The electro-optically active layer forming part of the fibre
or filament of the present invention, is a combination of a polymer
and the electro-optically active substance such as liquid
crystalline material.
[0082] A fibre of the type shown in FIG. 2 is formed by coating an
electrically conductive fibre core using conventional methods such
as dip coating, spray coating, vapour deposition, ink-jet printing,
micro-contact printing or sputtering, with a liquid crystal
alignment layer such as a polyimide derivative, or a photoalignment
layer.
[0083] Examples of alignment layers are extensively described in
the literature, see for instance Cognard, J., Mol. Cryst. Liq.
Cryst. Suppl. Ser., 1, p. 1-77, 1982. Non-limitative examples are
polyimide layers, photoorientable layers, such as coumarin-based or
cinnamate-based polymers or layers consisting of surfactants. Also,
mechanical interaction with the fibre core may induce the preferred
alignment of the liquid crystals.
[0084] The use of a polyimide alignment layer can be advantageous
as the rubbing conventionally required for inducing the desired
alignment can be directly accomplished via the manufacturing
method. However, mechanical rubbing introduces defects and is a
source of electrostatic discharge and dust.
[0085] Preferably, photoalignment layers are used, as the alignment
can be induced by illumination which is a non-contact method (see
for instance Schadt, M. et al., Nature, 381, p. 212, 1996, and
Wilderbeek et al., Advanced Materials, 15(12), p. 985, 2003).
[0086] The liquid crystal alignment layer is conventionally
finalized using heat curing or UV-irradiation and effects a
pre-tilt angle of 3 to 4.degree.. This pre-tilt is required to
lower the threshold voltage for switching, and to control the
rotation direction of the liquid crystals, thus reducing for
instance the formation of disclinations.
[0087] The fibre is subsequently coated by applying an
electro-optically active layer either directly, or indirectly, to
the first conductive layer.
[0088] The electro-optically active layer can be formed using
several procedures:
[0089] (i) directly, by applying an inhomogeneous polymer/LC system
directly to the fibre. Usually, the rheology of such a system is
paste-like, allowing for the practical deposition on the fibre.
[0090] (ii) indirectly, by applying a homogeneous polymer/LC system
directly to the fibre, using a suitable common solvent to the
polymer and mesogen. Upon removal of the solvent, e.g. by
evaporation or curing, the final morphology is established as a
coating on the fibre.
[0091] (iii) Indirectly, by in-situ generation, using an initially
homogeneous mixture of crosslinkable monomers and a non-reactive
mesogen. After application of the mixture on the fibre, phase
separation is induced either
[0092] a. Thermally
[0093] b. By (photo-)chemical means.
[0094] Optionally, a second alignment layer may be applied to the
electro-optically active layer. A second electrode is then applied
to the second alignment layer, or directly to the electro-optically
active layer.
[0095] The second electrode may be applied to the electro-optically
active layer before, or after the layer has been formed.
[0096] Usually, the fibre, or stack is covered by a protective
cover layer to protect the electro-optic substance 6 and to provide
additional stability and support in the fibre 2 or 12. Preferably
the protective cover is formed from a non-conductive material and
is at least partially transparent to light. Conveniently, the
protective cover is formed from a flexible polymer.
[0097] Various polymers/liquid crystal composites may be used. One
such composite is a polymer dispersed liquid crystal system (PDLC)
containing a relatively high, for example, 50 to 80% polymer
content. Such a system comprises a continuous isotropic polymer
phase and a dispersed low molecular weight micron sized liquid
crystalline phase.
[0098] Referring to FIGS. 3a and 3b, such a system is shown
initially in the off-state in FIG. 3a and then in the on-state in
FIG. 3b. The system comprises an isotropic polymer phase 36 and a
dispersed low molecular weight micron sized liquid crystalline
phase 38.
[0099] In the off-state shown in FIG. 3a, there is a mismatch
between the isotropic refractive index of the continuous polymeric
phase, and that of the randomly aligned dispersed liquid crystal
phase. Because of this, and the micron sized domain size, light
scattering will occur, resulting a white layer.
[0100] Preferably, the liquid crystalline phase is a nematic phase,
although in principle other liquid crystalline phases, such as
chiral nematic, smectic, or discotic phases can be used too.
[0101] Upon application of a voltage in the on-state as shown in
FIG. 3b, the director of the dispersed nematic liquid crystalline
droplets 38 will orient parallel to the electric field, provided
the nematic liquid crystal has a positive dielectric
anisotropy.
[0102] If the materials are chosen such that the refractive index
of the polymer 36 matches the ordinary refractive index of the
dispersed liquid crystalline phase 38, no effective refractive
index mismatch is experienced and the layer will appear
transparent.
[0103] A particular well-suited material combination illustrated in
FIG. 4 is for instance the NOA 65/E7 system, that can be obtained
from Norland (Cranbury, N.J., U.S.A.) and Merck (Darmstadt,
Germany), respectively. The liquid crystal E7 is actually a
eutectic mixture consisting of 50.6% 4'-pentylcyanobiphenyl, 25.2%
4'-heptylcyanobiphenyl, 17.8% 4'-octyloxycyanobiphenyl, and 6.4%
4'-pentylcyanoterphenyl (see Wilderbeek et al., Advanced Materials,
15(12), p. 985-988, 2003). Another example consists of the epoxy
EPON 828 (Shell Chemical Co.), the curing agent Capcure 3800
(Miller-Stephenson Chemical Co.), and the liquid crystal E7. Yet
another example consists of polymethylmethacrylate (PMMA) and the
liquid crystal E7, using the common solvent chloroform. Examples of
further materials and useful combinations are for instance
extensively described in Drzaic, P. S., "Liquid crystal
dispersions", World Scientific, Singapore, 1995. Due to the aligned
director in the on-state, shown in FIG. 3b, an off axis refractive
mismatch of the refractive indices will exist, resulting in an
angle dependent hazy appearance. This type of polymer/liquid
crystal composite system may be generated in situ, as described
above, by inducing a phase separation from an initially homogeneous
system of crosslinkable monomers and a non-reactive mesogen. The
phase separation is either induced thermally, by evaporation of a
co-solvent, or by chemical or photochemical means.
[0104] During the course of these processes, phase separation into
polymer-rich and polymer-poor regions will occur, and the final
morphology can be accurately tuned, depending on the proper process
conditions.
[0105] An advantage of such a system is the resulting mechanical
stability. The overall characteristics of the electro-optically
active layer are that of a solid-like material.
[0106] In addition, polarizers and alignment layers are not
required as the switching principle of a PDLC is based on
scattering, rather than modulation of the polarization of the
incident light. Thus, the exact alignment of the liquid crystals at
the boundaries of the electro-optically active layer is not
required. In fact, in the off-state, the mesogenic molecules adopt
a random director profile that varies from one droplet or domain to
the other droplet or domain.
[0107] Furthermore, the concept is suited for production in a
continuous process. The phase separation kinetics can be very fast,
and phase separation can be achieved within minutes to several
seconds, allowing for reel-to-reel fabrication.
[0108] FIG. 5 shows a schematic set-up for a continuous
manufacturing process for a fibre or filament according to the
present invention. A fibre 52 is drawn through a fluid containing
reservoir 54 from reel 62 to reel 64, via rollers 66, 68. The fibre
is subsequently coated with the mixture. Formation of the desired
morphology can be realized via the methods described herein.
Optionally, the morphology may be established using the
illumination sources 58 situated after the fluid reservoir.
[0109] Optionally, additional reservoirs (not shown) may be present
before or after the reservoir 54, for instance to apply different
coatings, such as a second electrode, cover layer, alignment
layers, adhesion promotion layers, wetting layers, polarizers,
brightness enhancement layers, before the fibre is wound up
again.
[0110] A screen may 56 be used to shield the material present in
the fluid reservoir 54 from the light coming from the one or more
illumination sources 58 present, such as UV light sources, in order
to prevent premature induced chemical or physical changes, such as
phase separation and/or polymerisation and/or precipitation and/or
degradation, of the material present in the reservoir. A small
opening 60 in the screen enables the transport of the fibre from
reel 62 to reel 64. Although the screen 56 shown in FIG. 5 has a
flat and rectangular layout, the actual shape may differ as long as
its shape fulfils the role of shielding the contents of the
reservoir 54 from the light coming from the lighting sources 58.
For example, a small diaphragm may be used directly at the edge of
the fluid reservoir.
[0111] The illumination sources 58 can be of various types, but
preferably emit or radiate light with a wavelength in the visible
to UV region. UV-light sources are particularly appropriate, and
for instance medium or high pressure mercury light sources may be
used. Optionally, the heat that is produced by these type of lamps
can be blocked by placing an infrared screen (not shown), that is
transparent to the wavelength required to induce the desired phase
change of the electro-optical layer, in between the light source
and the fibre 52.
[0112] The fibre 52 is transported from its origin, preferably from
reel 62 to its destination, or reel 64 with a velocity v (m s-1).
The velocity of the fibre is determined by the angular velocity w
(rpm or rad s-1) of the reels, as imposed by for instance an
electrical motor (not shown).
[0113] Preferably, the entire set-up or parts of the set-up can be
placed in an enclosure (not shown) that enables control over the
environment with respect to the gas conditions. For instance, it
may be advantageous to process and/or illuminate the fibre and/or
electro-optical layer in an inert atmosphere, such as nitrogen,
helium or argon or mixtures thereof, or to process and/or
illuminate the fibre and/or electro-optical layer in a pressurized
environment different from atmospheric condition (e.g. vacuum,
reduced pressure, high pressure).
[0114] Good voltage transmission characteristics can be obtained
ranging from approximately 1V per micron to 0.5V per micron, across
the electro-optically active layer.
[0115] The electro-optically active layer described above is suited
to produce reflective fibres, as the degree of front and back
scattering can be accurately tuned by the processing methods as
described herein, and as for instance described by Cornelissen, H.
J. et al., Proceedings of the 17th International Display Research
Conference, Toronto (Canada), p. 144, 1997.
[0116] Optionally, dyes can be added to the polymer/liquid crystal
composite in order to produce colour changes in the fibre.
EXAMPLE 1
[0117] A flexible polyester foil (polyethyleneterephtalate) coated
with a thin conductive layer of poly(3,4-ethylenedioxythiophene)
was covered with a reactive mixture consisting of 60% w/w of
multifunctional reactive monomers (NOA65, Norland, Cranbury, N.J.,
U.S.A.) and 40% w/w of a eutectic liquid crystalline mixture (E7,
Merck, a mixture consisting of cyanobiphenyls and one
cyanoterphenyl, as specified herein). The layer thickness was tuned
accurately by using spacers with well-defined thickness by
spincoating. Upon irradiation with UV-light, the polymer dispersed
liquid crystal is formed. A second electrode, e.g., a second
polyester foil with conductive coating, can be applied before or
after irradiation. The resulting flexible foil or ribbon-like fibre
can be switched between a scattering state and a transparent state
with a moderate voltage (10-40V).
EXAMPLE 2
[0118] A conductive core fibre (copper, fibre diameter 120 .mu.m)
was coated by passing the fibre horizontally through a reservoir
containing a mixture of 60% w/w reactive multifunctional monomers
(NOA65, Norland, Cranbury, N.J., U.S.A.) and 40% w/w of a eutectic
liquid crystalline mixture (E7 Merck, a mixture consisting of
cyanobiphenyls and one cyanoterphenyl, as specified herein). The
reservoir was 4.0 mm in diameter and 10.0 mm in length. The
relative intended thickness of the coating or the ratio of the
coating and the conductive fibre radius (e/b) was controlled by the
following parameters: [0119] The speed v at which the conductive
fibre travels through the reservoir (which is in turn adjusted by
the power supplied to the motor) [0120] The capillary number Ca: Ca
= .eta. v .gamma. ##EQU1##
[0121] (where .eta. is the viscosity, and .gamma. is the surface
tension of the uncured material).
[0122] For an uncured coating of 10 .mu.m thickness with a
viscosity of 0.5 Pas and a surface tension of 0.037 N/m, the speed
was 3.0 mm/s. Curing occurred with medium pressure mercury lamps,
situated directly after the fluid reservoir.
[0123] A polymer/liquid crystalline material composite may also be
formed using anisotropic monomers rather than isotropic
monomers.
[0124] Such systems may be produced by photopolymerisation of small
amounts of anisotropic monomers in the presence of a non-reactive
low molecular weight liquid crystalline solvent. Typically,
acrylates, methacrylates, or epoxides are used for the anisotropic
monomers, and a well-described example consists of the reactive
mesogenic acrylate monomer benzoic acid,
4-[3-[(oxo-2-propenyl)oxy]propoxy]-,2-methyl-1,4-phenylene ester
(C3M) and the non-reactive liquid crystal 5CB (4'-pentyl,
[1,1-biphenyl]-4-carbonitrile), as shown in FIG. 7. See for
instance Hikmet, R. A. M. in "Liquid crystals in complex
geometries. Formed by polymer and porous networks", Crawford, G.
P., Zumer, S. (Eds.), Chapter 3, Taylor & Francis, London,
1996, and Wilderbeek et al., Jpn. J. Appl. Phys., Part 1, 41 (4A),
p. 2128, 2002.
[0125] Photopolymerisation of initially homogeneous mixtures of an
anisotropic monomers and low molecular weight liquid crystalline
solvents produces phase separation of the liquid crystalline
polymeric structure into polymer-rich and polymer-poor phases.
Depending on the molecular structure of the monomer used, the
formed polymers are either side-chain or chemically crosslinked
structures, both consisting of a polymer backbone to which
mesogenic cores are attached. Such polymers are known as
anisotropic gels or plasticized liquid crystalline networks and are
schematically illustrated in FIGS. 6a and 6b described herein
below.
[0126] The alignment direction of the mesogens in the network
reflects the initial alignment of the mixture. In this way, a
planarly (horizontally, in the plane of the fibre) or
homeotropically (vertically, perpendicular to the plane of the
fibre) oriented network can be created. The initial alignment is
dictated by interfacial interactions between the LC mixture and
alignment layers such as the before described examples of rubbed
polyimide or photoalignment layers.
[0127] FIGS. 6a and 6b show schematically an anisotropic gel
initially in the off-state in FIG. 6a, and then in the on-state in
FIG. 6b. The anisotropic gel comprises polymer chains 40 with
mesogenic side-chains 42, and non-reactive mesogens 44.
[0128] In the off-state, when no electrical field is applied, the
inert liquid crystal solvent molecules are aligned with the
mesogenic units of the network. Consequently, due to the refractive
index match between the mesogenic units of the network and those of
the inert liquid crystal solvent molecules, incident light is not
scattered, and the system will appear transparent.
[0129] However, in the presence of an electrical field, the liquid
crystal solvent molecules will reorientate along the field lines.
Light scattering will occur due to the induced domain formation,
and the resulting refractive index mismatch, and the system will
become opaque.
[0130] Fibres incorporating such electro-optically active layers do
not require polarizers, as the switching principle is based on the
induced scattering resulting from the refractive index mismatch in
the on-state, rather than modulation of the polarization state of
the incident light, and can produce fast switching fibres, as the
mesogenic units in the polymer network provide the internal
director field that forms the driving force for relaxation to the
aligned state in the field-off condition. The fibres have good
mechanical stability, and there is almost no viewing angle
dependency due to the refractive index matching between the low
molecular weight LC component and the mesogenic moieties of the
network.
[0131] Alignment layers are, however, required when using such an
electro-optically active substance, since the alignment direction
of the mesogens in the network reflects the initial alignment of
the mixture, which in turn is dictated by the interfacial
interactions between the LC mixture and alignment layers.
[0132] The term "polymer" as used hereinabove, should be understood
to include also the term "oligomer".
* * * * *