Method for broadening bandwidth of cholesteric liquid crystal

#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.

#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.

#The present invention will be described in detail with reference to the drawing showing the preferred embodiment of the present invention, wherein:

#FIG. 1 is an illustrate diagram showing a helix-structured cholesteric liquid crystal with a constant pitch at constant temperature.

#FIG. 2 is an illustrate diagram showing that the pitch of the cholesteric liquid crystal is reduced, or blue-shift, during an exothermic polymerization reaction.

#The present invention discloses a method for preparing cholesteric liquid crystals with increased reflection bandwidth and the cholesteric liquid crystals prepared therefrom. The improved broadband cholesteric liquid crystals so prepared can be advantageously utilized as a circular polarizer in a liquid crystal display.

#As discussed above when a thin layer containing cholesteric liquid crystals is irradiated with a beam of unpolarized light, the part of the light which is compatible with the direction of the helical pitch of the cholesteric liquid crystal layers will be reflected, while the remainder of the light (which is incompatible or having a reversed helical direction of the chiral molecules) is transmitted. The transmitted light is reflected by a mirror formed on the backlight device, causing it to be reverse-polarized. The reflected light than transmits through this optically active layer. Theoretically, 100% of the backlight can be transmitted through the cholesteric reflective polarizer.

#However, the cholesteric reflection occurs only within a spectral band between two wavelengths λ1 and λ2 wherein λ1=p×n1, λ2=p×n2, p is the pitch of the cholesteric molecule, and n1 and n2 are denoted as the ordinary and extraordinary reflective indexes, respectively, of the cholesteric material. The bandwidth Δλ, where the cholesteric reflection occurs, is determined by the equation of Δλ=p×(n1−n2)=p×Δn. 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, Δn, or by mixing different cholesteric liquid crystals with overlapping pitches. Since there is only a limited range within which the amount of Δn can be increased, most researchers are focusing on modifying the pitches of the cholesteric liquid crystals.