Method for broadening bandwidth of cholesteric liquid crystal

Details Method for broadening bandwidth of cholesteric liquid crystal Method for broadening bandwidth of cholesteric liquid crystal

2001-05-30

2003-12-30

Huff, Mark F. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C252S299010, C349S175000, C349S179000

Reexamination Certificate

active

06669999

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved broadband cholesteric liquid crystals for use as circular reflective polarizers. More specifically, the present invention relates to a novel method to increase the bandwidth of circular reflective polarizers utilizing a broadband cholesteric liquid crystal, which can be advantageously utilized in making liquid crystal displays including, for example, super twisted nematic liquid displays. Unlike many of the prior art methods, the method disclosed in the present invention can be relatively easy and inexpensive to implement.
BACKGROUND OF THE INVENTION
Liquid crystal displays (LCD) are gaining increased popularity in the consumer electronics market. Because they occupy a substantially smaller desktop space than the traditional CRTs, LCD monitors have the potential of replacing the traditional CRT-based monitors, if the price gap can be reduced and the brightness of an LCD monitor can be improved. The brightness problem is associated with the fact that the current generations of LCDs utilize linear polarizers, which absorb light in the optical path. At the present time, even the most advanced liquid crystal displays show a reduced brightness caused by the linear polarizers that are used in the displays. Typically, in a backlit display, the commonly used dichoric linear polarizers can absorb more than 50-60% of the light intensity from the backlight. Since the backlight component typically consumes the greatest extent of electric power relative to other components in a portable computer, improvement in the amount of light absorption will greatly enhance the battery life.
Recently, circular reflective polarizers (CRP), which are also simply called “reflective polarizers”, have been developed which can more efficiently convert unpolarized light into polarized light. Circular reflective polarizers, which can provide the same function as linear dichloric polarizers, usually comprise a layer of a chiral liquid crystalline material, such as a cholesteric liquid crystal, that exhibits a helically twisted molecular structure and further exhibits a planar alignment. If unpolarized light is incident upon a CRP, the portion of the light with the same handedness (i.e., same right-handedness or left-handedness) with the CRP will be reflected as circularly polarized light, while the other portions will be transmitted. By utilizing a properly designed mirror, the reflected light will have its handedness reversed which is then redirected into and passes through the CRP. Thus, in theory, one hundred percent of the a given unpolarized light can be converted into circularly polarized light using a circular reflective polarizer.
Cholesteric liquid crystals, or cholesteric LC, are a type of optically active liquid crystalline materials which have a helical arrangement of the molecular directors from layer to layer. Cholesteric liquid crystals are usually utilized in the form of a thin optically active layer between two parallel substrates in such a way that the axis of the helix is perpendicular to the substrate surfaces. Conforming to the definition of a circular polarizer, if such a thin layer containing the cholesteric liquid crystals is irradiated with a beam of unpolarized light, the part of the light which has the same handedness as the chiral molecules (which can be either right-handed or left-handed helixes) will be reflected, while the remainder of the light (which has the opposite handedness as the chiral molecules) is transmitted. The reflected light is reversed in its handedness by a mirror provided on the backlight device. The reflected light, which originally had the same handedness but now with opposite handedness, then transmits through this optically active layer this time around. As discussed previously, theoretically, 100% of the backlight can be transmitted through the cholesteric liquid crystals.
However, the cholesteric reflection occurs only within a spectral band between two wavelengths &lgr;
1
and &lgr;
2:
&lgr;
1
=p×n
1
and
&lgr;
2
=p×n
2
where p is the pitch of the cholesteric molecule, and n
1
and n
2
are denoted as the ordinary and extraordinary reflective index of the material. And the bandwidth &Dgr;&lgr;, where the cholesteric reflection occurs, is determined by the following equation:
&Dgr;&lgr;=
p×
(
n
1
−n
2
)=
p×&Dgr;n
In order to increase the bandwidth of the cholesteric liquid crystals, one must either choose a material with a large difference in the refractive indices (or higher birefringence), &Dgr;n, or by mixing different cholesteric liquid crystals with overlapping pitches. Since there is only a limited range within which the amount of &Dgr;n can be increased, most researchers are focusing on modifying the pitches of the cholesteric liquid crystals.
U.S. Pat. No. 5,506,704 (WO 9,602,016) discloses a broadband cholesteric polarizer comprising an optically active layer of a polymer material. The polymer material has a cholesteric order and a molecular helix, wherein the pitch of the molecular helix varies with a difference between a maximum pitch and a minimum pitch of at least 100 nm. The cholesteric polarizer is formed by mixing chiral and nematogenic monomers, each having a different reactivity, between two parallel substrates. An actinic radiation is applied in accordance with a radiation profile whose intensity varies over the layer, so that the mixture is polymerized to an optically active layer of polymer material having a cholesteric order.
U.S. Pat. No. 5,691,789 discloses a broadband, reflective circular polarizer. The polarizers are made from a film of a polymer having a cholesteric order, in which a second liquid crystal material, which is a nematic liquid crystal, is distributed in a non-linear fashion across the thickness of the film in a plurality of liquid crystal-rich and liquid crystal-depleted sites in the polymer. The inventors claim that the resulting polarizers have bandwidth approaching 2000 nm and reflect circularly polarized light which is either left-handed or right-handed depending on the final spiral structure of the materials utilized. The circular polarizer the '789 invention is fabricated by mixing commercially available polymers having a cholesteric order, a second liquid crystal material and a photoinitiator at a temperature which maintains the mixture in a liquid crystalline state. The polymer and the liquid crystal are present in a ratio by weight of 1:2, for example, and the photoinitiator is present in an amount of 0.6% by weight in the mixture. The heated mixture is then subjected to actinic radiation for a time and at an intensity sufficient to polymerize the polymer or the liquid crystal material or both. Under all polymerizing conditions the segregation rate of the liquid crystal material has to be greater than the polymerization rate of the material being polymerized. The non-reactive liquid crystal segregates and diffuses, during polymerization, to sites of enlarged pitch in the polymer material from sites of shrunken pitch in the polymer material. Because the actinic light has an exponential intensity distribution due to light attenuation by light absorption, polymerization occurs in a non-linear fashion resulting in a non-linear distribution of the polymer and the liquid crystal material across the film.
U.S. Pat. No. 6,099,758 discloses a broadband reflective polarizer that comprises a layer of a polymerized mesogenic material exhibiting a helically twisted molecular structure with planar alignment. The reflective polarizer in the '758 patent is obtained by a process wherein a mixture of a chiral polymerizable mesogenic material comprising a) at least one achiral polymerizable mesogenic compound, b) at least one chiral compound that can in addition be polymerizable and/or mesogenic, c) a polymerization initiator, is coated on a substrate or between two substrates in form of a layer, aligned in a planar orientation so that the axis of the molecular helix extends transversely to t

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