1. Field of the Invention
The present invention relates to a polymer/cholesteric liquid crystal dispersion which is utilized for display elements, image/information recording elements and spatial light modulators, to a method of producing the dispersion and to a liquid crystal display element utilizing the dispersion.
2. Description of the Related Art
A cholesteric liquid crystal display element has, for example, the characteristics that it has a memory storing ability which can retain a display without any power source, it has the ability to obtain a bright display because no polarizing plate is used, and it enables color displaying without using a color filter. Attention has been therefore focused on such display elements in recent years (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 05-080303).
A cholesteric liquid crystal, in particular, is made of rod-shaped molecules oriented spirally and reflects interference light having a wavelength which corresponds to the spiral pitch (called selective reflection). It therefore has the characteristics that bright color display is possible without using any color filter by designing the spiral pitch to have a length corresponding to the wavelength of a red color, a green color or a blue color.
Cholesteric liquid crystal sealed into a cell constituted by a pair of substrates provided with electrodes is known to take any of three types of oriented states: planar (P) orientation, focal conic (F) orientation and homeotropic (H) orientation as shown in FIG. 8A to FIG. 8C. In the figures, reference numeral 2 represents cholesteric liquid crystal, 21 and 22 represent a pair of substrates, and 11 and 12 represent electrodes. The P orientation is a state in which the spiral axis is oriented substantially perpendicular to the surface of the substrate and provides selective reflection. The F orientation is a state in which the spiral axis is oriented substantially parallel to the surface of the substrate, and light is transmitted in this state. The H orientation is an oriented state that appears when a sufficiently high voltage is applied between the pair of electrodes. In this state, the spiral is loosened, molecules are oriented perpendicular to the surface of the substrate, and light is transmitted. These three oriented states can be switched among each other by applying voltage between the electrodes.
Accordingly, if a light absorber having a color such as a black color is disposed on the backside of the cell, it is possible to obtain a bright display colored with the selective reflection color during the P orientation and a dark display colored with the black color of the light absorber during the F or H orientation. Among the above orientation forms, both the P orientation and the F orientation can exist stably without using any power source. The utilization of this property makes it possible to attain a memory display in which a display is maintained without using any power source.
On the other hand, a structure is known in which a polymer/cholesteric liquid crystal dispersion 4 obtained by dispersing a cholesteric liquid crystal 2 as particles in a polymer 1 is sandwiched between a pair of substrates 21 and 22 having electrodes 11 and 12, as shown in FIG. 9, instead of sealing the cholesteric liquid crystal directly between a pair of substrates having electrodes.
In this case as well, the above display principle may be similarly utilized. The polymer/cholesteric liquid crystal dispersion is more resistant than ordinary liquid crystal cells to stresses applied from the outside. Therefore, the dispersion is not only resistant to the breakdown of a stored image but can also be apparently handled as a solid. As a result, there are advantages in that the polymer/cholesteric liquid crystal dispersion can be handled in, for example, a production process more easily than a liquid cholesteric liquid crystal and can be laminated on other functional films such as an optical conductor.
As shown in FIG. 10, however, the reflection spectrum of the polymer/cholesteric liquid crystal dispersion is largely different from that of a liquid crystal cell, and the polymer/cholesteric liquid crystal dispersion has the following problems: (1) the spectrum of the polymer/cholesteric liquid crystal dispersion at the time of light reflection has significantly larger short-wavelength components than those of a liquid crystal cell, whereby only a color display having low color purity can be obtained and (2) the spectrum at the time of a dark display has large short-wavelength components, whereby only a display having a low contrast is obtained. There is also a problem in that (3) although a liquid crystal cell has a relatively stable reflectance at the time of a dark display (dark reflectance) over time, the polymer/cholesteric liquid crystal dispersion has a strong tendency toward an increase in this reflectance, which is accompanied by a display being made lighter in color over time.
The above problems are characteristics found to be common to several methods of producing the polymer/cholesteric liquid crystal dispersion, such as a cholesteric liquid crystal microcapsule using a gelatin and gum arabic as its wall material, a cholesteric liquid crystal microcapsule using a polyurethane resin as its wall material, and a polymer/cholesteric liquid crystal dispersion obtained by dispersing cholesteric liquid crystals in an aqueous solution of a polyvinyl alcohol resin, followed by drying.
Conventionally, the problems (1) and (2) have been caused by the superpositioning of boundary light scattering caused by a difference in refractive index between the polymer and the liquid crystal and have been considered to be unavoidable in polymer/cholesteric liquid crystal dispersions containing numerous cholesteric liquid crystal droplets in the direction of the film thickness. For this reason, as a measure for solving this problem, a method is disclosed, for example, in which a polymer/cholesteric liquid crystal dispersion is formed so as to contain only one liquid crystal droplet in the direction of the film thickness to decrease the influence of light scattering, in JP-A No. 6-160817.
Although contrast is certainly improved according to this method disclosed in JP-A No. 6-160817, the method has a problem in that the area percentage of the cholesteric liquid crystal is reduced, whereby reflectance is reduced. The problem (3) cannot be explained by interfacial light scattering, and neither the cause of nor a preventive measure for the problem has been known.