Title: Liquid crystal composition, selectively reflective film and method for producing the same
Abstract: The present invention provides a liquid crystal composition including a liquid crystal compound containing at least one polymerizable group, two or more kinds of photoreactive chiral compounds that undergo isomerization when respectively irradiated with light beams having mutually different wavelengths and exhibit mutually different HTPs after isomerization, and a polymerization initiator; a selectively reflective film produced using the liquid crystal compound; and a method for producing the selectively reflective film. Instead of the two or more kinds of photoreactive chiral compounds, a single photoreactive chiral compound that undergoes reversible isomerization when irradiated with either of two light beams having mutually different wavelengths may be used.
Patent Number: 6,893,585 Issued on 05/17/2005 to Ichihashi
| Inventors:
|
Ichihashi; Mitsuyoshi (Shizuoka-ken, JP)
|
| Assignee:
|
Fuji Photo Film Co., Ltd. (Kanagawa, JP)
|
| Appl. No.:
|
243955 |
| Filed:
|
September 16, 2002 |
Foreign Application Priority Data
| Sep 17, 2001[JP] | 2001-280981 |
| Current U.S. Class: |
252/299.01; 428/1.1 |
| Intern'l Class: |
C09K 019/52 |
| Field of Search: |
428/11,13,131
252/299.01
|
References Cited [Referenced By]
U.S. Patent Documents
| 5668614 | Sep., 1997 | Chien et al.
| |
| 5798057 | Aug., 1998 | Hikmet.
| |
| 5827449 | Oct., 1998 | Hanelt et al.
| |
| 5989461 | Nov., 1999 | Coates et al.
| |
| 5995184 | Nov., 1999 | Chung et al.
| |
| 6099758 | Aug., 2000 | Verrall et al.
| |
| 6117920 | Sep., 2000 | Jolliffe et al.
| |
| 6217955 | Apr., 2001 | Coates et al.
| |
| 6466297 | Oct., 2002 | Goulding et al.
| |
| 6511719 | Jan., 2003 | Farrand.
| |
| 6616990 | Sep., 2003 | Prechtl et al.
| |
| 6645397 | Nov., 2003 | Ichihashi.
| |
| 6669999 | Dec., 2003 | Hsieh et al.
| |
| Foreign Patent Documents |
| 8-245901 | Sep., 1996 | JP.
| |
| 9-133810 | May., 1997 | JP.
| |
| 10-39812 | Feb., 1998 | JP.
| |
| 10-054905 | Feb., 1998 | JP.
| |
| 10-508882 | Sep., 1998 | JP.
| |
| 10-339867 | Dec., 1998 | JP.
| |
| 11-142647 | May., 1999 | JP.
| |
| 11-148080 | Jun., 1999 | JP.
| |
| 11-271529 | Oct., 1999 | JP.
| |
| 2000/-105315 | Apr., 2000 | JP.
| |
| 2000/-226580 | Aug., 2000 | JP.
| |
| WO 0033129 | Jun., 2000 | WO.
| |
| WO 0034808 | Jun., 2000 | WO.
| |
| WO 0034808 | Jun., 2000 | WO.
| |
Primary Examiner: Huff; Mark F.
Assistant Examiner: Sadula; Jennifer R.
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
1. A liquid crystal composition comprising:
a liquid crystal compound containing at least one polymerizable group;
a photoreactive chiral compound that undergoes reversible isomerization when
irradiated with either of two light beams having mutually different wavelengths
and exhibits an HTP after isomerization by each light beam, which HTPs are mutually
different; and
a polymerization initiator,
further comprising a surfactant exerting an excluded volume effect distributed
over an air interface side.
2. A liquid crystal composition comprising:
a liquid crystal compound containing at least one polymerizable group;
two or more kinds of photoreactive chiral compounds that undergo isomerization
when respectively irradiated with light beams having mutually different wavelengths
and exhibit mutually different HTPs alter isomerization; and
a polymerization initiator,
further comprising a surfactant exerting an excluded volume effect distributed
over an air interface side.
3. A liquid crystal composition comprising:
a liquid crystal compound containing at least one polymerizable group;
two or more kinds of photoreactive chiral compounds that undergo isomerization
when respectively irradiated with light beams having mutually different wavelengths
and exhibit mutually different HTPs after isomerization; and
a polymerization initiator,
further comprising a non-photoreactive chiral compound,
further comprising a surfactant exerting an excluded volume effect distributed
over an air interface side.
4. A selectively reflective film produced by polymerizing and curing a liquid
crystal composition comprising a liquid crystal compound containing at least one
polymerizable group; two or more kinds of photoreactive chiral compounds that undergo
isomerization when respectively irradiated with light beams having mutually different
wavelengths and exhibit mutually different HTPs after isomerization; and a polymerization initiator,
wherein the liquid crystal composition further comprises a surfactant exerting
an excluded volume effect distributed over an air interface side.
5. A selectively reflective film produced by polymerizing and curing a liquid
crystal composition comprising a liquid crystal compound containing at least one
polymerizable group; a photoreactive chiral compound that undergoes reversible
isomerization when irradiated with either of two light beams having mutually different
wavelengths and exhibits an HTP after isomerization by each light beam, which HTPs
are mutually different; and a polymerization initiator,
wherein the liquid crystal composition further comprises a surfactant exerting
an excluded volume effect distributed over an air interface side.
6. A method for producing a selectively reflective film using a liquid crystal
composition comprising a liquid crystal compound containing at least one polymerizable
group; two or more kinds of photoreactive chiral compounds that undergo isomerization
when respectively irradiated with light beams having mutually different wavelengths
and exhibit mutually different HTPs after isomerization; and a polymerization initiator,
the method comprising the steps of:
preparing the liquid crystal composition, which is capable of displaying selective
reflection of a first color;
coating the liquid crystal composition on a surface of a substrate;
imagewise irradiating light having a first wavelength to which one of the photoreactive
chiral compounds is photosensitive to undergo isomerization, such that selective
reflection of a second color is displayed;
imagewise irradiating light having a second wavelength, which is different from
the first wavelength and to which another of the photoreactive chiral compounds
is photosensitive to undergo isomerization, such that selective reflection of a
third color is displayed; and
polymerizing the liquid crystal compound,
wherein the liquid crystal composition further comprises a surfactant exerting
an excluded volume effect distributed over an air interface side.
7. A method for producing a selectively reflective film using a liquid crystal
composition comprising a liquid crystal compound containing at least one polymerizable
group; a photoreactive chiral compound that undergoes reversible isomerization
when irradiated with either of two light beams having mutually different wavelengths
and exhibits an HTP after isomerization by each light beam, which HTPs are mutually
different; and a polymerization initiator, the method comprising the steps of:
preparing the liquid crystal composition, which is capable of displaying selective
reflection of a first color;
coating the liquid crystal composition on a surface of a substrate;
imagewise irradiating light having a first wavelength to which the photoreactive
chiral compound is photosensitive to undergo isomerization, such that selective
reflection of a second color is displayed;
imagewise irradiating light having a second wavelength, which is different from
the first wavelength and to which the photoreactive chiral compound is photosensitive
to undergo isomerization, such that selective reflection of a third color is displayed;
and
polymerizing the liquid crystal compound,
wherein the liquid crystal composition further comprises a surfactant exerting
an excluded volume effect distributed over an air interface side.
8. The method for producing a selectively reflective film according to claim
7, wherein the surfactant is a nonionic surfactant.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal composition, a selectively
reflective film exhibiting selective reflection of light ranging from the ultraviolet
region to the infrared region for use in a color filter and the like, and a method
for producing the selectively reflective film.
2. Description of the Related Art
In recent years, attention has been drawn to liquid crystal materials, such as
a cholesteric liquid crystal, that have a helical structure and exhibit selective
reflection of a variety of colors depending on a twisting power (twist angle) of
the helical structure. Since such liquid crystal materials are excellent in selective
reflection and color purity of selectively-reflected light, they are widely used
in various applications including optical films, liquid crystal color filters,
recording media and the like.
Color filters (selectively reflective films) used in, for example, color liquid
crystal displays are generally composed of red (R), green (G) and blue (B) pixels
and a black matrix arranged therebetween for improving display contrast. Conventional
color filters are mainly produced by dispersing pigments in a resin or by dyeing
a resin with dyes and their production is ordinarily conducted by spin-coating
a colored resin solution on a glass substrate to provide a colored resist layer
followed by photo-lithographic patterning the resultant resist layer to form color
filter pixels, or by directly printing colored pixels on a substrate.
The printing process described above has a problem in that color filters that
are low in pixel resolution and hence unsuitable for forming detailed image patterns
are produced. The spin-coating process has drawbacks, which cause large material
loss and produce uneven coating when coating is applied to a large-area substrate.
If a color filter is produced by an electro-deposition process, the obtained color
filter has a relatively high resolution and reduced unevenness in colored layers,
but there are drawbacks in that, for example, the production process is complicated
and handling of processing liquids is difficult.
In light of the foregoing, there has been a demand for a method for effectively
and readily producing a color filter having high quality with reduced material loss.
Color filters are required to have capabilities including high transmittance
and high color purity. In recent years, attempts have been made to meet the above-mentioned
requirements, for example, by suitably selecting types of dyes and resins to be
dyed when dyes are used, or by using finely dispersed pigments when pigments are
used. Recently, the level of performance required of color filters with respect
to, for example, transmittance and color purity, are increasingly and extremely
high when the filters are used in liquid crystal display (LCD) panels. Particularly,
it is difficult for the color filters used in reflective LCDs to satisfy all of
the requirements of good paper-white display, good contrast and good color reproducibility.
Since color filters produced in a conventional manner, such as by dyeing a resin
with dyes or dispersing pigments in a resin, are color filters of a light-absorbing
type, color purity improvement obtained by increasing transmittance has almost
reached its limit.
On the other hand, another type of color filter utilizing polarized light, which
is mainly made of a cholesteric liquid crystal, is known. Since this type of color
filter utilizing polarized light reflect light having predetermined wavelengths
and transmit light of other wavelengths, light-utilizing efficiency is very high,
and transmittance and color purity are highly remarkable as compared to color filters
of the light-absorbing type. When such color filters utilizing polarized light
are produced, spin-coating is typically employed to achieve evenness in layer thickness.
However, large material loss is generated through spin-coating, making this production
process disadvantageous in terms of cost.
In order to solve the above-described problems, a photoreactive chiral compound
is effectively used for producing color filter films capable of exhibiting uniformity
in color purity and requiring a reduced number of steps in a production process.
When a liquid crystal composition containing a photoreactive chiral compound is
patternwise irradiated with light having wavelengths to which the photoreactive
chiral compound is photosensitive, the chiral compound causes a reaction, which
progresses depending on the intensity of irradiated energy to induce a change in
helical pitch (twist angle of the helix) of she liquid crystal compound. Through
this process, desired selective color reflection can easily be obtained for each
pixel merely by conducting patterning exposure using varied light quantities. This
process for producing color filters is advantageous in that patterning exposure
may be conducted only once using a photomask having a different light transmittance.
Thus, by conducting imagewise patterning exposure and subsequently fixing the
cholesteric liquid crystal compound, a film capable of functioning as a color filter
can be formed. This production process may be applied to an optical film, image
recording, and the like.
When the liquid crystal composition is exposed to light of wavelengths to which
the photoreactive chiral compound is photosensitive, selective reflection of light
changes, for example, from B (blue) through G (green) to R (red), depending on
the amount of the irradiated light. As shown in FIG. 2, when light of the color
G (green) is reflected, a width (a
2) of an amount of irradiated light
is small, whereby the green light becomes bluish or yellowish if irregular exposure
occurs at the time of irradiating light. Accordingly, it has been difficult to
produce color filters exhibiting color uniformity.
SUMMARY OF THE INVENTION
In view of the above-described problems of the prior art, it is an object of
the
present invention to provide a selectively reflective film excellent in color uniformity,
a liquid crystal composition from which the selectively reflective film can be
produced, and a method for readily producing the selectively reflective film.
A first aspect of the invention is a liquid crystal composition comprising: a
liquid
crystal compound containing at least one polymerizable group; two or more kinds
of photoreactive chiral compounds that undergo isomerization when respectively
irradiated with light beams having mutually different wavelengths and exhibit mutually
different HTPs (twisting power) after isomerization; and a polymerization initiator.
A second aspect of the invention is a liquid crystal composition comprising: a
liquid crystal compound containing at least one polymerizable group; a photoreactive
chiral compound that undergoes reversible isomerization when irradiated with either
of two light beams having mutually different wavelengths and exhibits an HTP after
isomerization by each light beam, which HTPs are mutually different; and a polymerization initiator.
A third aspect of the invention is a selectively reflective film produced by
polymerizing
and hardening a liquid crystal composition comprising a liquid crystal compound
containing at least one polymerizable group; two or more kinds of photoreactive
chiral compounds that undergo isomerization when respectively irradiated with light
beams having mutually different wavelengths and exhibit mutually different HTPs
after isomerization; and a polymerization initiator.
A fourth aspect of the invention is a selectively reflective film produced by
polymerizing
and hardening a liquid crystal composition comprising a liquid crystal compound
containing at least one polymerizable group; a photoreactive chiral compound that
undergoes reversible isomerization when irradiated with either of two light beams
having mutually different wavelengths and exhibits an HTP after isomerization by
each light beam, which HTPs are mutually different; and a polymerization initiator.
A fifth aspect of the invention is a method for producing a selectively reflective
film using a liquid crystal composition comprising a liquid crystal compound containing
at least one polymerizable group; two or more kinds of photoreactive chiral compounds
that undergo isomerization when respectively irradiated with light beams having
mutually different wavelengths and exhibit mutually different HTPs after isomerization;
and a polymerization initiator, the method comprising the steps of: preparing the
liquid crystal composition, which is capable of displaying selective reflection
of a first color; coating the liquid crystal composition on a surface of a substrate;
imagewise irradiating light having a first wavelength to which one of the photoreactive
chiral compounds is photosensitive to undergo isomerization, such that selective
reflection of a second color is displayed; imagewise irradiating light having a
second wavelength, which is different from the first wavelength and to which another
of the photoreactive chiral compounds is photosensitive to undergo isomerization,
such that selective reflection of a third color is displayed; and polymerizing
the liquid crystal compound.
A sixth aspect of the invention is a method for producing a selectively reflective
film using a liquid crystal composition comprising a liquid crystal compound containing
at least one polymerizable group; a photoreactive chiral compound that undergoes
reversible isomerization when irradiated with either of two light beams having
mutually different wavelengths and exhibits an HTP after isomerization by each
light beam, which HTPs are mutually different; and a polymerization initiator,
the method comprising the steps of: preparing the liquid crystal composition, which
is capable of displaying selective reflection of a first color; coating the liquid
crystal composition on a surface of a substrate; imagewise irradiating light having
a first wavelength to which the photoreactive chiral compound is photosensitive
to undergo isomerization, such that selective reflection of a second color is displayed;
imagewise irradiating light having a second wavelength, which is different from
the first wavelength and to which the photoreactive chiral compound is photosensitive
to undergo isomerization, such that selective reflection of a third color is displayed;
and polymerizing the liquid crystal compound.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1I are schematic drawings showing a part of a process for
producing a liquid crystal color filter of the present invention.
FIG. 2 is a graph showing a relationship between amounts of light irradiated
on a liquid crystal composition and a selectively reflected wavelength.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, a liquid crystal composition, a selectively reflective
film and a method for producing the selectively reflective film according to the
present invention are described.
<Liquid Crystal Composition>
The liquid crystal composition according to the first aspect of the invention
comprises a liquid crystal compound containing at least one polymerizable group,
two or more kinds of photoreactive chiral compounds that undergo isomerization
when respectively irradiated with light beams having mutually different wavelengths
and exhibit mutually different HTPs after isomerization and a polymerization initiator,
as well as additional components as necessary. Each of the components are described hereinafter.
(Liquid Crystal compound)
The liquid crystal compounds may be appropriately selected from low molecular
weight liquid crystal compounds, high molecular weight liquid crystal compounds
and polymerizable liquid crystal compounds having anisotropy of a refractive index
Δn of 0.05 to 0.40. Among them, a nematic liquid crystal compound is particularly
preferable. These liquid crystal compounds may be aligned by using, for example,
an aligned substrate which has undergone aligning treatment such as rubbing while
the liquid crystal compound is in a molten and liquid crystal state. If the liquid
crystal state is fixed by transforming the state into a solid phase, cooling, polymerization
or the like may be carried out.
Specific examples of the liquid crystal compound include the following compounds.
In the present invention, however, the liquid crystal compounds are not limited
thereto.
##STR1##
##STR2##
In the formulae shown above, n represents an integer of 1 to 1,000.
Also usable as preferable examples are compounds similar to the compounds exemplified
above, in which linking groups of aromatic rings are replaced by any group having
the following structure.
##STR3##
—N═N—
##STR4##
Among the compounds listed above, liquid crystal compounds carrying in the
molecule at least one polymerizable or crosslinking group are preferable from the
viewpoints of securing sufficient curability and heat resistance of the layer.
The content of the liquid crystal compound is preferably 30 to 99.9% by mass,
and more preferably 50 to 95% by mass relative to the total solid content of the
liquid crystal composition. If the content is less than 30% by mass, alignment
may be insufficient to fail to achieve selective reflection of desired colors.
(Photoreactive Chiral Compound)
In the invention, two or more kinds of photoreactive chiral compounds are used
which undergo isomerization when respectively irradiated with light beams having
mutually different wavelengths and exhibit mutually different HTPs. For example,
a photoreactive chiral compound which is photosensitive to a shorter wavelength
of about 313 nm and another photoreactive chiral compound which is photosensitive
to a longer wavelength of above 365 nm are simultaneously used. Since the respective
photoreactive chiral compounds exhibit different HTPs after isomerization, if the
liquid crystal composition has been prepared beforehand to display selective reflection
of G (green) light, and light of a shorter wavelength and light of a longer wavelength
are successively irradiated, then the liquid crystal composition displays selective
reflection of R (red) light and B (blue) light, respectively. In this case, the
two kinds of photoreactive chiral compounds having mutually opposite senses are
used. If necessary, a non-photoreactive chiral compound (described later) may be
included in the composition.
In order to prepare a liquid crystal composition capable of displaying selective
reflection of a specified color, calculation is conducted from the values of HTP
each obtained with one photoreactive chiral compound which is photosensitive to
a shorter wavelength and the other photoreactive chiral compound which is photosensitive
to a longer wavelength, both before and after UV irradiation (after isomerization).
For example, suppose that a liquid crystal composition developing B color (reflecting
light of a wavelength of 450 nm) is irradiated with light having a wavelength of
365 nm to reflect light of the color G (reflecting light of a wavelength of 530
nm), and further irradiated with light having a wavelength of 313 nm to reflect
light of the color R (reflecting light of a wavelength of 640 nm). Incidentally,
both of the photoreactive chiral compounds have the same rotational sense. Further,
suppose that the one photoreactive chiral compound to undergo isomerization when
irradiated with light having a wavelength of 365 nm has HTPs, before and after
isomerization, of 40 and 20, respectively; while the other photoreactive chiral
compound to undergo isomerization when irradiated with light having a wavelength
of 313 nm has HTPs, before and after isomerization, of 30 and 10, respectively.
Then, helical pitches to reflect each of BGR colors are calculated and from the
obtained values, a change in a reciprocal number of the helical pitches (μm)
of the composition is calculated at the time when reflected light is changed from
B to G and from B to R, respectively. In this case, if an average refractive index
is 1.5, changes in a reciprocal number at the time of changing the light of colors
from B to G and from B to G are about 0.5 and 1.5, respectively. Thus, the amount
of the one chiral compound necessary to undergo isomerization when irradiated with
light having a wavelength of 365 nm is calculated to be 2.5% by mass (=0.5×100/(40-20)),
and the amount of the other chiral compound necessary to undergo isomerization
when irradiated with light having a wavelength of 313 nm is calculated to be 5.0%
by mass (=1.0×100/(30-10)). Since in this case the selectively reflected wavelength
of B color becomes too long, a non-photoreactive chiral compound having the same
sense is added to the composition for adjustment. If the non-photoreactive chiral
compound has an HTP of 20, it is enough to add the non-photoreactive chiral compound
at about 4% by mass. Incidentally, calculated values only serve as a rough estimate
so that some adjustment may be needed when applying the values practically.
The followings are specific examples of photoreactive chiral compounds that are
photosensitive to a shorter wavelength of about 313 nm, however, these examples
are not intended to limit the invention. To the right of each compound there is
shown a sense of twist, an HTP, a peak absorption wavelength and a solvent used
for measuring wavelengths thereof. The wavelength at which the photoreactive chiral
compound isomerizes is almost equal to the peak absorption wavelength.
| |
|
##STR5##
|
Counter clockwise (CCW) twist, HTP (27) 302 nm, EtOAc |
|
##STR6##
|
Clockwise (CW) twist, HTP (44) 309 nm, EtOAc |
|
##STR7##
|
CW twist, HTP (33) 277 nm, EtOAc |
|
##STR8##
|
CW twist, HTP (58) 311 nm, EtOAc |
|
##STR9##
|
CCW twist, HTP (42) 276 nm, EtOAc |
|
##STR10##
|
CCW twist, HTP (43) 308 nm, EtOAc |
|
##STR11##
|
CCW twist, HTP (41) 309 nm, EtOAc |
|
##STR12##
|
CCW twist, HTP (48) 291 nm, EtOAc |
|
##STR13##
|
CW twist, HTP (57) 312 nm, EtOAc |
|
##STR14##
|
CCW twist, HTP (25) 274 nm, EtOAc |
|
##STR15##
|
CCW twist, HTP (18) 293 nmm EtOAc |
|
##STR16##
|
CW twist, HTP (40) 306 nm, EtOAc |
| |
The followings are specific examples of photoreactive chiral compounds that are
photosensitive to a longer wavelength of about 365 nm.
| |
|
##STR17##
|
CCW twist, HTP (30) 312 nm, EtOAc |
|
##STR18##
|
CCW twist, HTP (18) 292 nm, EtOAc |
|
##STR19##
|
CCW twist, HTP (23) 279 nm, CHCl3 |
|
##STR20##
|
CCW twist, HTP (21) 357 nm, EtOAc |
|
##STR21##
|
CW twist, HTP (40) 330 nm, CHCl3 |
|
##STR22##
|
CW twist, HTP (56) 347 nm, CHCl3 |
|
##STR23##
|
CW twist, HTP (14 or more) 331 nm, EtOAc |
|
##STR24##
|
CW twist, HTP (17 or more) 349 nm, CHCl3 |
|
##STR25##
|
CW twist, HTP (34) 337 nm, CHCl3 |
|
##STR26##
|
CW twist, HTP (63) 337 nm, CHCl3 |
|
##STR27##
|
CW twist, HTP (35) 330 nm, CHCl3 |
|
##STR28##
|
CW twist, HTP (44) 353 nm, CHCl3 |
|
##STR29##
|
CW twist, HTP (41) 388 nm, CHCl3 |
|
##STR30##
|
CW twist, HTP (45) 373 nm, CHCl3 |
|
##STR31##
|
mw = 6000, CW twist, HTP (29) |
|
##STR32##
|
mw = 12000, CW twist HTP (15) |
| |
(Non-photoreactive Chiral Compound)
In the invention, it is preferable to include a non-photoreactive chiral compound,
together with the above-described photoreactive chiral compounds, to make the liquid
crystal composition display selective reflection of a desired color. From the viewpoints
of improving color hues and color purity of the liquid crystal compound, preferable
examples of the non-photoreactive chiral compound include isomannide, catechin,
isosorbide, fenchone and carvone. Additional examples include chiral compounds
described, for example, in Japanese Patent Application Laid-Open (JP-A) No. 2000-44451,
Japanese National Publication No. 10-509726, WO 98/00428, Japanese National Publication
Nos. 2000-506873 and 9-506088
, Liquid Crystals 1996, 21, 327 and
Liquid
Crystals 1998, 24, 219.
The content of the chiral compound is preferably 5 to 30% by mass relative to
the total solid content of the liquid crystal composition.
(Polymerization Initiator)
If a polymerization reaction is utilized for the liquid crystal compound having
a polymerizable group to fix the helical structure after the twisting power has
been changed by irradiating the liquid crystal with light, a polymerization initiator
is added to the composition. The polymerization initiator can suitably be selected
from conventionally known compounds that are photoreactive and thermoreactive.
Among others, photo-polymerization initiators capable of accelerating a reaction
by light irradiation are particularly preferable. In order to fix the helical structure
after the desired helical structure has been formed, the polymerization reaction
of the liquid crystal composition is preferably allowed to proceed rapidly.
Examples of the photo-polymerization initiator suitably selected from conventionally
known initiators include p-methoxyphenyl-2,4-bis(trichloromethyl)-s-triazine, 2-(p-butoxystyryl)
-5-trichloromethyl-1,3,4-oxadiazole, 9-phenylacridine, 9,10-dimethylbenzphenazine,
benzophenon/Michler's ketone, hexaarylbiimidazole/mercaptobenzimidazole, benzyldimethylketal,
thioxanthone/amine, triarylsulfonium hexafluorophosphate, bisacylphosphine oxides
such as bis-(2,4,6-trimethylbenzoyl)phenylphosphine oxide described in JP-A No.
10-29997 and acylphosphine oxides such as those described in DE4230555 by Lucirin TPO.
The polymerization initiators having a different spectrally sensitive range from
that of the photoreactive chiral compounds (described later) are preferably chosen.
Here, "having a different spectrally sensitive range" refers to that their central
photosensitive wavelengths do not overlap each other, and alignment of the liquid
crystal is not altered at the time of imagewise exposure and polymerization for
hardening, to an extent that image displaying property and color purity may not
be impaired. In order to avoid overlapping of the central photosensitive wavelengths,
a band pass filter or the like is used to control the wavelength of light irradiated,
in addition to suitably selecting the molecular structures of the both compounds.
The content of the polymerization initiator is preferably 0.1 to 20% by mass,
and more preferably 0.5 to 5% by mass relative to the total solid content of the
liquid crystal composition. If the content is less than 0.1% by mass, curability
at the time of irradiating light may be lowered, occasionally requiring a prolonged
time for hardening. If the content exceeds 20% by mass, light transmittance in
the ultraviolet-visible region may be decreased.
(Polymerizable Monomer)
The liquid crystal composition of the invention may further include a polymerizable
monomer. If the polymerizable monomer is included in the liquid crystal composition,
the monomer serves to fix the helical structure (to display selective reflection)
of the liquid crystal after the twisting power of the liquid crystal has been changed
by light irradiation and a distribution of selectively reflected wavelengths has
been established (patterning), whereby the strength of the fixed liquid crystal
composition can be further increased. However, the polymerizable monomer needs
not always be included if the nematic liquid crystal compound has an unsaturated
bond within a molecule.
As the polymerizable monomer, a monomer having an ethylenically unsaturated bond
may be exemplified. Specific examples thereof include polyfunctional monomers such
as pentaerythritol tetracrylate and dipentaerythritol hexacrylate.
Specific examples of the monomer having an ethylenically unsaturated bond
are shown below. However, these examples are not limiting the present invention.
##STR33##
The content of the polymerizable monomer is preferably 0.5 to 50% by mass relative
to the total solid content of the liquid crystal composition. If the content is
less than 0.5% by mass, sufficient curability may not be obtained. If the content
exceeds 50% by mass, the polymerizable monomer may interfere alignment of the liquid
crystal molecules, occasionally leading to insufficient color development.
(Air Interface Alignment Agent)
In the invention, it is preferable to include in the liquid crystal composition
a surfactant which exerts an excluded volume effect distributed over an air interface
side (hereinafter referred to as an "air interface alignment agent"). If an air
interface alignment agent is included, the agent serves to three-dimensionally
control an alignment state at a surface of the layer interfacing with air, when
a liquid crystal composition is applied in a state of a coating solution. Particularly
when the agent is applied in a cholesteric liquid crystal phase, light of selectively
reflected wavelength having higher color purity can be obtained.
An air interface alignment agent is a surfactant exerting an excluded volume
effect.
As used herein, exerting an excluded volume effect means control of aligning liquid
crystal (molecules) at the air interface side, that is, three-dimensional control
of a spatial alignment state of a liquid crystal at a layer surface interfacing
with air when a layer including a liquid crystal composition is formed by application
of coating. In more detail, this term means control of pre-tilt angles of liquid
crystal molecules at the air interface side.
The requirements for a preferable molecular structure of an air interface alignment
agent are to have a flexible hydrophobic moiety and a moiety having at least one
ring unit and a structural stiffness (hereinafter referred to as a stiff moiety).
The flexible hydrophobic moiety can be either a perfluoro chain or a long alkyl
chain depending on the kind of a liquid crystal compound used. Since a hydrophobic
moiety is flexible, the hydrophobic moiety can effectively be located on the air side.
An air interface alignment agent may be of a short molecular chain having a molecular
weight of the order of several hundreds or of the polymer or the oligomer consisting
of the short molecular chain. Furthermore, there may be included a polymerizable
functional group in the molecule of the agent depending on the use purposes.
When such an air interface alignment agent is used, a flexible hydrophobic moiety
of an air interface alignment agent is arranged to the air interface; and at the
same time, a stiff moiety is arranged to a liquid crystal molecule orientation,
shaped to be flat and positioned in parallel to the air interface, to thereby permit
alignment of liquid crystal molecules parallel to the air interface.
On the other hand, if a stiff moiety is oriented in a direction perpendicular
to the air interface, liquid crystal molecules can be aligned in a direction perpendicular
to the air interface.
Specifically, a nonionic surfactant is preferably used and the following
compounds may be exemplified.
##STR34##
##STR35##
##STR36##
The addition amount of an air interface alignment agent is preferably a quantity
to cover a surface of the air interface side of a layer including a liquid crystal
composition by one molecule, and preferably of from 0.05 to 5 mass % and more preferably
of from 0. 1 to 1.0 mass % relative to the total solid content of the liquid crystal
composition. When the addition amount is less than 0.05 mass %, the effects of
the agent are not always exerted, while when the addition amount is in excess of
5 mass %, an air interface alignment agent itself occasionally causes an association,
resulting in phase separation from liquid crystal.
If an air interface alignment agent is used, a surface tension can be decreased.
Other kinds of surfactants than the air interface alignment agent can be used together
with this agent for the purpose of further reducing a surface tension and improving coatability.
(Additional Components)
As additional components, the following may be included in the composition: a
binder resin, a solvent, a surfactant, a polymerization inhibitor, a thickening
agent, a dye, a pigment, an ultraviolet absorbent, a gelling agent and so on.
Examples of the binder resin include polystyrene compounds such as polystyrene
and poly-α-methylstyrene; cellulose resins such as methylcellulose, ethylcellulose
and acetylcellulose; acidic cellulose derivatives having, as its side chain, a
carboxylic group; acetal resins such as polyvinyl formal and polyvinyl butyral;
and methacrylic acid copolymer, acrylic acid copolymer, itaconic acid copolymer,
crotonic acid copolymer, maleic acid copolymer and partially-esterified maleic
acid copolymer described in JP-A Nos. 59-44615, Japanese Patent Application Publication
(JP-B) Nos. 54-34327, 58-12577 and 54-25957, JP-A Nos. 59-53836 and 59-71048.
As other examples of the binder resin, there may be exemplified a homopolymer
of an acrylic acid alkyl ester and a homopolymer of methacrylic acid alkyl ester
in which an alkyl group may preferably be a methyl, ethyl, n-propyl, n-butyl, iso-butyl,
n-hexyl, cyclohexyl, 2-ethylhexyl or the like group.
Besides, the binder resin may be a polymer having a hydroxyl group to which
is added an acid anhydride, benzyl(meth)acrylate/(methacrylic acid homopolymer)acrylic
acid copolymer, a multiple copolymer of benzyl(meth)acrylate/(meth)acrylic acid/another
monomer, or the like.
The addition amount of the binder resin in the liquid crystal composition is
preferably from 0 to 50% by mass, and more preferably from 0 to 30% by mass. If
the amount is more than 50% by mass, alignment of the cholesteric liquid crystal
compound may sometimes become insufficient.
In the liquid crystal composition of the invention, it is particularly preferable
to use a surfactant, in combination with the photoreactive chiral agent and the
liquid crystal compound, preferably a nematic liquid crystal compound. As the surfactant,
a surfactant exerting an excluded volume effect is preferably used. Here, "exerting
an excluded volume effect" means that the surfactant serves to three-dimensionally
control a spatial alignment state at the surface of the layer interfacing with
air. Specifically, a nonionic surfactant is preferable and suitably selected for
use from the conventionally known nonionic surfactants.
The polymerization inhibitor may be added to the composition to improve storability.
Examples of the polymerization inhibitor include hydroquinone, hydroquinone monomethyl
ether, phenothiazine, benzoquinone, and the derivatives thereof. The addition amount
of the polymerization inhibitor is preferably from 0 to 10% by mass, more preferably
from 0 to 5% by mass, relative to the content of the polymerizable monomer.
The liquid crystal composition may be prepared by dissolving or dispersing the
aforementioned respective components in a suitable solvent and formed into an arbitrary
shape, or disposed onto a support or the like for use. Examples of the solvent
include 2-butanone, cyclohexanone, methylene chloride and chloroform.
Next, the liquid crystal composition according to the second aspect of the
invention is described. The liquid crystal composition according to the second
aspect is the same as that according to the first aspect, except that the composition
comprises a single kind of photoreactive chiral compound.
The photoreactive chiral compound in the composition according to the second
aspect is a compound that undergoes reversible isomerization when irradiated with
either of two light beams having mutually different wavelengths and exhibits an
HTP after isomerization by each light beam, which HTPs are mutually different.
For example, the photoreactive chiral compound shown below isomerizes to a compound
having an HTP of 136 when irradiated with light having a wavelength of 366 nm,
which latter compound reversibly isomerizes to the former compound having an HTP
of 28 when irradiated with light of 495 nm.
##STR37##
The followings are specific examples of the photoreactive chiral compound used
in the second aspect, however, these examples are not intended to limit the invention.
| |
|
##STR38##
|
CCW twist HTP 19 → 49 mw = 592.68 (42 → 107) 313 nm, 405 nm
Br form |
|
##STR39##
|
366 nm, 495 nm |
|
##STR40##
|
CW twist HTP (55) 358 nm, EtOAc |
|
##STR41##
|
405 nm, 580 nm or more |
|
##STR42##
|
450 nm, 570 nm |
|
##STR43##
|
300 nm, 250 nm |
| |
<Change in Helical Structure of Liquid Crystal>
As described above, the liquid crystal composition of the invention comprises
the photoreactive chiral compound, and when the composition is patternwise irradiated
with light beams each having different wavelengths, the twisting power of the liquid
crystal is altered to form the regions in which respective helical structures have
different twisting degrees (twisting power; HTP).
Particularly in case where the liquid crystal phase is a cholesteric
liquid crystal phase, selective color reflection can arbitrarily be obtained by
changing the twisting power of the liquid crystal. If a changing ratio of the twisting
power (changing ratio of a twist) is large, a range of selective color reflection
displayed by the liquid crystal is wide, whereby a broad range of selectively reflected
wavelengths including three primary colors (B, G, R) can be obtained.
Specifically, selective color reflection can be achieved in the following manner.
When the liquid crystal composition of the invention is irradiated with light
having wavelengths to which the photoreactive chiral compound included therein
is photosensitive, the compound responds to the wavelength of the irradiated light
and isomerizes to induce a change in the helical structure (twist angle) of the
liquid crystal, and this structural change allows selective reflection of different
colors to thereby form an imagewise pattern (patterning). Thus, by irradiating
light beams of varied wavelengths for respective desired regions, a plurality of
colors can be reflected depending on the used wavelengths. And by irradiating light
through an imagewise formed photomask having different transmissive areas, colored
regions selectively reflecting different colors can be formed.
When a liquid crystal color filter, an optical film (described later) or the
like is produced, patterning exposure is conducted as described above by imagewise
irradiating light having wavelengths to which the photoreactive chiral compound
is photosensitive, followed by additional irradiation to cause photo-polymerization
of the polymerizable groups for curing the liquid crystal composition to thereby
fix the helical structure of the liquid crystal, such that selective reflection
of desired colors can be displayed. This process will be described in detail later.
As the light source for use in the irradiation, the light sources which emit
UV
light are preferably used from the viewpoints of having high energy and of causing
rapid structure change and a swift polymerization reaction of the liquid crystal
compound. Preferable examples of the light source include a high-pressure mercury
lamp, a metal halide lamp and an Hg—Xe lamp. The light source preferably
has a function of varying light quantities.
<Fixing of Helical Structure of Liquid Crystal>
As described above, by irradiating the photoreactive chiral compound with light
having specified wavelengths, the twisting power of the co-existing liquid crystal
may be altered to cause a change in the helical structure. The liquid crystal composition
of the invention comprises a liquid crystal compound having polymerizable groups,
a photo-polymerization initiator and at least one photoreactive chiral compound.
By effecting polymerization or cross-linking of the liquid crystal compound, the
altered helical structure can be fixed, and further strength of the fixed liquid
crystal composition can be enhanced.
It is preferable to use the photo-polymerization initiator and the photoreactive
chiral compound whose spectral sensitive range is different with each other.
Specifically, selective color reflection can be achieved in the following manner.
First, similarly to the patterning exposure described in the section of "Change
in Helical Structure of Liquid Crystal" above, the liquid crystal composition of
the invention is irradiated with light having wavelengths to which the photoreactive
compound included therein is photosensitive. By this irradiation, the co-existing
photoreactive compound responds to the wavelength of irradiated light and isomerizes
to induce a change in the helical structure, leading to formation of an imagewise
pattern (patterning). After this patterning, another irradiation is conducted using
another light beam having wavelengths to which the photo-polymerization initiator
is photosensitive. By this irradiation, the photo-polymerization initiator responds
to the wavelength and causes polymerization of the liquid crystal compound, thereby
fixing the composition and maintaining the altered helical structure. Prior to
this step, an additional step of, e.g., nitrogen replacement, may be performed.
In case where the spectrally sensitive range of the photoreactive chiral agent
does not overlap with that of the photo-polymerization initiator, irradiation for
changing the HTP and irradiation for photo-polymerization do not affect each other.
Thus, when imagewise exposure is conducted to induce a change in HTPs, photo-polymerization
of the liquid crystal composition does not progress, whereby patterning can be
achieved with an intended HTP changing ratio. Further, when the photo-polymerization
is conducted to fix the helical structure, the photoreactive compound does not
respond to light irradiation, whereby the changed HTP pattern thus formed can securely
be fixed.
When a liquid crystal color filter, an optical film (described later) or the
like is produced, patterning exposure is conducted as described above by imagewise
irradiating light having wavelengths to which the photoreactive compound is photosensitive,
followed by additional irradiation to cause photo-polymerization of the polymerizable
group for curing the liquid crystal composition to thereby fix the helical structure
of the liquid crystal, such that selective reflection of desired colors can be
displayed. This process will be described in detail later.
The light source for use in the irradiation is the same as those exemplified
in the section of "Change in Helical Structure of Liquid Crystal" above.
Although the above-described process utilizes polymerization for fixing
the liquid crystal composition, fixing may be effected by, for example, cross-linking
through vulcanization or causing glass phase transition.
<Selectively Reflective Film>
A selectively reflective film of the invention comprises the above-described
liquid
crystal composition of the invention. The selectively reflective film can be produced
by patternwise irradiating the liquid crystal composition with light having suitable
wavelengths for achieving purposes described in the section "Change in Helical
Structure of Liquid Crystal" or "Fixing of Helical Structure of Liquid Crystal" above.
Through explaining a production process, the selectively reflective film
of the invention is described hereinafter in detail.
The selectively reflective film of the invention is produced from the liquid
crystal composition of the invention.
In the method for producing the selectively reflective film, the liquid crystal
composition is prepared such that the composition displays selective reflection
of a first color, then the liquid crystal composition is coated on a substrate,
and thereafter the liquid crystal composition is imagewise irradiated with light
having a first wavelength to display selective reflection of a second color. Then,
the liquid crystal composition is imagewise irradiated with light having a second
wavelength, which is different from the first wavelength, to display selective
reflection of a third color. Thereafter, the liquid crystal composition is further
irradiated with light having a range of wavelengths to which the polymerization
initiator is spectrally sensitive to cause photo-polymerization and hardening (hereinafter,
this step may be referred to as an "irradiating step"). Further, depending on a
suitably selected embodiment for producing a selectively reflective film, the method
may further comprise steps of aligning the liquid crystal composition at a surface
thereof (aligning step), transferring a liquid crystal layer by adhering and removing
a transfer material (transferring step), coating a nematic liquid crystal composition
to form a liquid crystal layer (coating step) and the like.
The following is a specific embodiment illustrating a production process, which
comprises the irradiating step described above.
Irradiating Step
In the irradiating step, light irradiation is conducted for both patterning and
fixing (polymerization and hardening) the liquid crystal compound. That is, light
beams having the first and the second wavelengths to which the photoreactive chiral
compound is highly photosensitive are imagewise irradiated for patterning, followed
by irradiating another light having wavelengths to which the polymerization initiator
is highly photosensitive to cause polymerization for fixing the helical structure
of the liquid crystal compound, such that desired selective color reflection is displayed.
When the liquid crystal composition is irradiated with light having a first
wavelength, the photoreactive chiral compound included therein responds to the
wavelength and causes a change in the helical structure of the liquid crystal,
and this structural change allows selective reflection of different colors, to
thereby form an imagewise pattern. Thus, by irradiating light beams of varied wavelengths
for respective desired regions, a plurality of colors can be obtained corresponding
to the used wav
