Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-07-03
2002-07-16
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S175000
Reexamination Certificate
active
06421107
ABSTRACT:
The invention relates to a process of preparing a multilayer cholesteric film. The invention further relates to a multilayer film obtainable by the inventive process, and to the use of the inventive process and a film prepared thereby in optical elements such as polarizers, compensators, alignment layers, color filters or holographic elements, in cosmetics and liquid crystal pigments, for decorative and security applications.
BACKGROUND OF THE INVENTION
Cholesteric liquid crystal (CLC) materials, when formed into thin layers with planar alignment, i.e. wherein the cholesteric helix axis is oriented substantially perpendicular to the plane of the layer, exhibit the well-known effect of selective reflection of light, where the wavelength of the reflected light is dependent on the pitch of the cholesteric helix. By using polymerizable CLC materials, the aligned CLC layer can be converted to a coherent polymer film that retains the selective reflection properties of the original material.
CLC polymer films are known in prior art and have been proposed for a variety of uses, for example as broadband or notch polarizers, as color filters in displays or projection systems, and for decorative and security purposes, like the preparation of colored image films or cholesteric pigment flakes.
For some applications, it is desirable to form a multilayer cholesteric film, comprising two or more cholesteric layers e.g. exhibiting different reflection wavelength.
Multilayer cholesteric polymer films have been described in prior art. For example, EP 0 634 674 suggests to prepare a multilayer cholesteric liquid crystal polymer film by bringing together a pair of chiral nematic liquid crystal polymer films, applying pressure and heating the polymers above their glass transition temperature to allow the films to adhere.
Maurer et al., SID 90 Digest, Vol .21, pp. 110 (1990) describes a polarizing color filter obtained by combining several polarizing films with different reflection wavelength. For the preparation of each film, a layer of a CLC side chain polysiloxane comprising chiral and achiral sidegroups is brought between two glass plates and oriented by shearing at high temperatures.
JP 01-133003-A (Sumitomo Chem. Ind.) and JP 08-271731-A (Nitto Denko) disclose polarizing plates that are obtained by lamination of one or more CLC polymer layers onto a quarter wave plate.
However, the methods of preparing multilayer cholesteric films as described in the above documents bear several disadvantages. Thus, it is often very difficult and requires high temperatures to achieve uniform alignment in the CLC polymer layer. For example, Maurer et al. mentions an aligning temperature of 150° C., whereas JP 01-133003-A and JP 08-271731-A mention that temperatures well above the glass temperature of the CLC polymers are required. This is especially disadvantageous when polymers with high glass temperatures, like acrylates, styrenes or methacrylates are used, and is highly unsuitable in particular for mass production.
Furthermore, according to the method of multilayer preparation as described e.g. in JP 01-133003-A, the polymers have to be selected such that the different polymer layers exhibit different glass temperatures. Thus, when laminating and aligning ,e.g., a second layer on top of a first layer, the aligning temperature (and thus the glass temperature) of the second layer has to be lower than the glass temperature of the first layer, so as not to affect the uniform orientation of the first layer, etc. This severely limits the choice of suitable materials and makes the production process more complicated.
Therefore, there is a need for a method to prepare a multilayer cholesteric film in an efficient and cost-effective manner that does not have the above mentioned drawbacks, allows better control of the reflection wavelength and is suitable for mass production.
SUMMARY OF THE INVENTION
One aim of the present invention is to provide a process of preparing a multilayer cholesteric film that fulfills the above requirements. Other aims of the invention are immediately evident to a person skilled in the art from the following description.
A simple way to prepare a multilayer cholesteric film is by first coating and polymerizing a cholesteric layer of one wavelength, and then onto this directly coating and polymerizing a second cholesteric layer, e.g. of a different wavelength.
In this case it is important to ensure that the helical pitch, and thus the reflection wavelength of either layer, does not change in an uncontrollable manner.
In some cases, however, it is advantageous if the variation of the helical pitch can be controlled.
The inventors of the present invention have found that by controlled migration of unpolymerized material between adjacent layers, in particular migration of unpolymerized chiral material from the first, polymerized layer to the second, still unpolymerized layer, it is possible to alter the helical pitch of the second layer, whilst the pitch of the first layer remains substantially unchanged, depending on the extent of polymerization and the crosslinking density of the first layer.
Furthermore, the inventors found that, when preparing a multilayer cholesteric film, it is possible to control the migration of unpolymerized chiral material, and thus to alter the helical pitch in the single cholesteric layers prior to their polymerization, by using a process according to the present invention, which will be described in detail below.
One object of the present invention is a process of preparing a multilayer cholesteric film comprising two or more layers of polymerized cholesteric liquid crystal (CLC) material with planar orientation, wherein
the first layer is prepared by applying a layer of polymerizable CLC material on a substrate, preferably by coating, optionally aligning the CLC material in a planar orientation, and polymerizing the aligned CLC material, for example by means of heat or actinic radiation, and
the second and higher layers are prepared by applying a layer of polymerizable CLC material on a substrate, preferably by coating, optionally aligning the CLC material in a planar orientation, and polymerizing the aligned CLC material, for example by means of heat or actinic radiation, wherein one of the previously polymerized layers serves as substrate,
characterized in that the cholesteric helical pitch of adjacent layers is varied by controlled migration of a non-polymerized chiral material between the layers.
Another object of the present invention is a multilayer cholesteric film obtainable by a process as described above and below.
Another object of the present invention relates to the use of a process as described above and below, and of a multilayer cholesteric film obtainable by this process, in optical elements such as polarizers, compensators, alignment layers, color filters or holographic elements, in cosmetics and liquid crystal pigments, for decorative and security applications.
FIG. 1
depicts the visible light absorption spectrum of two-layer cholesteric films prepared from two polymerizable CLC mixtures with different reflection wavelength in varying order, compared to the spectrum of the one-layer films prepared from the CLC mixtures.
FIG. 2
depicts the visible light absorption spectrum of two-layer cholesteric films prepared from two polymerizable CLC mixtures with different reflection wavelength, wherein the second layer was annealed for various time periods before being polymerized, compared to the spectrum of the one-layer films prepared from the CLC mixtures.
The term ‘layer of cholesteric liquid crystal material with planar orientation’ means a liquid crystal material with a cholesteric phase, i.e. wherein the mesogens are oriented with their main molecular axis in a preferred direction within molecular sublayers, said preferred orientation direction in different sublayers being twisted around a helix axis that is substantially perpendicular to the plane of the layer, i.e. substantially parallel to the layer normal. This definition also includes orien
Greenfield Simon
May Alison Linda
Merck Patent Gesellaschaft
Millen White Zelano & Branigan P.C.
Nguyen Hoan
Sikes William L.
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