Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2001-04-25
2003-09-16
Huff, Mark F. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299010, C252S299600
Reexamination Certificate
active
06620466
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display device comprising a novel liquid crystal microstructure formed by a method controlling the microstructure, and a method for forming a novel colloidal liquid crystal composite (CLCC). The present invention also relates to a novel electro-optical device using the novel colloidal liquid crystal composite and a method for forming the electro-optical device.
2. Description of the Related Art
Recently, liquid crystals have been used in various applications and their role in the display field has become more and more important. The applications of the liquid crystals include, for example, computer display screens, wristwatches, architectural windows, privacy windows, automobile windows, automobile sunroofs, switching devices for optical systems, projection display devices, reflective display devices, hand-held paging devices, cellular phones, laptop computers, television screens including car-mounted television screens, automotive displays including radio, dashboard, and on-board navigation systems, helmet displays such as “heads-up” displays, cockpit displays, imaging devices, virtual reality devices, simulation devices, electronic gating devices, diffraction gratings, and calculators. The liquid crystal devices used in the above applications may be monochromatic or polychromatic.
Liquid crystal molecules have generally a rod-like or a disk-like shape, and their physical properties are intermediate those of a crystalline solid and those of an amorphous liquid. The liquid crystal molecules comprise, generally, substituted biphenyl or triphenyl functional groups, in which one of the phenyl groups is separated by a spacer group. Liquid crystal types include, for example, twisted nematic, super twisted nematic, cholesteric, ferroelectric liquid crystals. Liquid crystal devices (LCD) may be assembled by any of the liquid crystal types described above.
Alternatively, another type of display utilizing a liquid crystal dispersed within a polymer matrix in which the liquid crystals create micro-domains (i.e., a polymer dispersed liquid crystal display (PDLC) is known in the art. PDLCs are often used in the form of thin films less than about 200 &mgr;m in thickness, and typically from 2 &mgr;m to 50 &mgr;m. It is known that light transmittance of PDLCs varies with reflective indices of materials and the angle of the reflection varies with respect to both the wavelength of light and temperature.
Although liquid crystals exhibit excellent availability as a display device and an electro-optical device as described above, developments for the liquid crystals are still continued so as to improve their physical properties for overcoming some deficiencies. The conventional liquid crystal devices have a narrow viewing angle. In addition, conventional PDLCs have undesirably high switching voltage resulting in higher driving power, and require additional apparatus for production thereof, which incurs an additional production cost and capital investment.
Several conventional types of display devices and the electro-optical devices have been proposed in relation to liquid crystals having dual-domain structures.
Dual Domain Twisted Nematic Device
It is known that a viewing angle of the twisted nematic display device is asymmetric, and its contrast becomes poor when viewed at certain angles. Many attempts to overcome the above problem have been made by creating a dual-domain structure in the liquid crystal, in which the liquid crystal molecules are aligned with different directions in adjacent domains at each display pixel. The above structure has been realized by various techniques, all of which disadvantageously require an additional process to form two alignment surfaces.
FIG. 1
depicts a schematic cell structure having the above architecture and a schematic production process for obtaining the above structure. The structure shown in
FIG. 1
is a dual-domain twisted nematic device whose pixel is made up of two subpixels. As shown in
FIG. 1
, the simplest way to obtain the above structure is by first rubbing alignment layers coated onto the substrate, patterning photolithographically such portions that need to be masked, rubbing the unmasked portion in a different direction, and then removing the mask pattern. Recently, a special photo-alignment material has become available and the alignment may be controlled by the polarization of irradiated light and any pattern can be created without the masking process, when the light is patterned (E. Hoffman et al. SID'98 Digest, p. 734, 1998).
The top and bottom substrates are positioned at a right-angled alignment, and the two subpixels have different tilt angles. The subpixels, therefore, have 90 degree twist structures with right and left handednesses. Due to the symmetry of the two twist structures, the pixel exhibits a symmetric viewing angle at any bias level, and hence the anisotropy of the viewing angle may be reduced.
However, the cost and time required for obtaining the dual domain structure described above by using these techniques hinder versatile commercialization.
Cholestric Texture Device
In addition to nematic liquid crystal technologies, there are other devices that take advantage of optical properties of cholesteric liquid crystals. Cholesteric liquid crystals show two states when sandwiched between two substrates (i.e. a planar state that exhibits reflection over a narrow band of wavelengths and a focal conic state which scatters light weakly). These two states are shown in FIGS.
2
(
a
) and
2
(
b
). Bi-stable director distributions respectively correspond to the planar state in FIG.
2
(
a
) and a focal conic state in FIG.
2
(
b
) in the cholesteric device. The bi-stable operation between these two states must be secured by either a polymer network formed in the liquid crystal or surface treatments. The former is called a “polymer-stabilized cholesteric texture” (D. K. Yang et. al., “Control of reflectivity and bistability in displays using cholesteric liquid crystals”, J. Appl. Phys. Vol. 76, No. 2, pp.1331-1333) and devices based on these systems are known and marketed by Kent Display.
However, a drawback of this technique is that an additional UV curing process is required to obtain a polymer network. Recently a new kind of the cholesteric texture modes has been presented (U.S. Pat. No. 5,956,113 to G. P. Crawford et. al. incorporated herein by reference) in which a small amount of commercial inorganic silica particles is blended in a host cholesteric liquid crystal, and the aggregates of the particles due to the hydrogen bond develop network structure in the host.
The presence of molecular interaction between the particles and liquid crystal molecules may contribute to promote bi-stability of the device. This technique is prominent in its simple processabilty. However, there is no visible cellular morphology nor remark to the mechanical properties of the liquid crystal composites, or to whether such compositions are allowed to be poured and processed as typical liquid crystal materials. The low viscosity of the composite will limit processing conditions under which the layer of liquid crystal composites may be formed on the substrate.
White-Taylor Device
This device utilizes a small amount of dye compound dissolved in a host liquid crystal material and provides a reflective display (White and Taylor, J. Appl. Phys., Vol. 45, p. 4718, 1974). A typical construction of a White-Taylor device is shown in FIGS.
3
(
a
) and
3
(
b
). Usually, a White-Taylor device has a 300 degree twist configuration provided by a polyimide coating and a rubbing processes. In FIG.
3
(
a
), light is absorbed in a bias-off state by the liquid crystals, whereas the incident light passes through in a bias-on the state in FIG.
3
(
b
). The curved arrows in FIG.
3
(
a
) indicate the helical structure of the molecules in the bias-off state. The device appears dark under the bias-off state because the mo
Crain Jason
Nakamura Hajime
Huff Mark F.
McGinn & Gibb PLLC
Sadula Jennifer R.
Trepp, Esq. Robert M.
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