Fine particles covered by cholesteric liquid crystal

Details Fine particles covered by cholesteric liquid crystal Fine particles covered by cholesteric liquid crystal

2000-10-16

2002-04-23

Yoon, Tae H. (Department: 1714)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With charge transferring layer of specified composition

C428S407000, C428S480000, C427S212000, C252S299010, C523S205000

Reexamination Certificate

active

06376029

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to fine particles coated substantially throughout the whole surfaces thereof with a thin film of a cholesteric liquid crystal having a selective reflection for a circularly polarized light of a specific wavelength.
PRIOR ART
In a cholesteric liquid crystal, interior liquid crystal molecules thereof are aligned in a regularly twisted state so as to describe a spiral in a film thickness direction, having a unique optical property based on the fact that the helical axis thereof is parallel to the film thickness direction, which optical property is a selective reflection of circularly polarized light in a specific wavelength band. Utilizing this property,.various applications have been developed such as, for example, application to an optical use in which only the right or left circularly polarized light is taken out from non-polarized light and an ornamental use which utilizes the phenomenon that reflected/transmitted light is colored because the action in question is limited to a specific wavelength band. Industrially, a cholesteric liquid crystal film obtained by forming a cholesteric liquid crystal into a thin film has so far been mainly manufactured and used in various fields because it is superior in its handleability and processability.
However, the use of the cholesteric liquid crystal in the form of a film inevitably encounters a limit in its industrial application. For development to a further industrial application, the production of fine particles having the selective reflection of circularly polarized light caused by cholesteric liquid crystal are being tried.
As a conventional method for producing fine particles having a selective reflection of circularly polarized light there is proposed, for example in Japanese Patent Laid Open No. 220350/1994, a method wherein a cholesteric liquid crystal is formed into a thin film and this cholesteric liquid crystalline film is pulverized into fine particles. However, it is only a direction perpendicular to the helical axis of cholesteric orientation that the selective reflection of cholesteric liquid crystal is observed, while in a horizontal section there is not obtained the selective reflection. Thus, the selective reflection surface is limited. Besides, since pulverization is performed for obtaining fine particles, a selective reflection surface is rendered uneven, resulting in an increase of scattered light. Thus, the fine particles obtained by the above conventional method are very weak in the selective reflection characteristic of cholesteric liquid crystal and also poor in visibility.
OBJECT OF THE INVENTION
Taking note of the point that the above conventional problems of fine particles having a selective reflection of circularly polarized light are attributable to the smallness of a region which exhibits. the circularly polarized light selectivity on the surfaces of the fine particles, and having made earnest studies, we found out that fine particles having a strong selective reflection of circularly polarized light and superior in reflected light visibility can be obtained not by merely pulverizing a cholesteric liquid crystal film but by coating the surfaces of fine particles as nucleus with a cholesteric liquid crystal having a selective reflection of circularly polarized light.
SUMMARY OF THE INVENTION
The present invention, in the first aspect thereof, resides in fine particles comprising non-liquid crystalline fine particles as nucleus and a thin cholesteric liquid crystal layer formed on the fine particle surfaces, the thin cholesteric liquid crystal layer exhibiting a selective reflection of circularly polarized light substantially in all directions of the fine particles.
The present invention, in the second aspect thereof, resides in a method for producing the above fine particles, which method comprises dispersing non-crystalline organic or inorganic fine particles in a cholesteric liquid crystal solution, removing a solvent used, appealing in a temperature region in which the liquid crystal takes a cholesteric phase and subsequent cooling.
MODE FOR CARRYING OUT THE INVENTION
The present invention will be described in detail hereinunder.
The cholesteric liquid crystal used in the present invention is not specially limited insofar as it forms a cholesteric oriention in the state of liquid crystal. Particularly, a cholesteric liquid crystalline polymer is preferred in view of its easiness of handling and of thin film formation. As the cholesteric liquid crystalline polymer, one exhibiting a cholesteric alignmentability and capable of being fixed easily in its aligned state is preferred.
Usually, for fixing the cholesteric alignment stably it is particularly preferred, when viewed in the phase sequence of liquid crystal, that there be no crystal phase in a lower temperature range than cholesteric phase. If a crystal phase is present, an inevitable passage through the crystal phase at the time of cooling for alignment fixing results in destruction of the cholesteric orientation once obtained. Therefore, as the cholesteric liquid crystalline polymer used in the present invention, one not only having a satisfactory cholesteric orientability based on an interfacial effect but also assuming a glassy state at a temperature lower than cholesteric phase, i.e., below the liquid crystal transition point of the polymer.
As examples of such a cholesteric liquid crystalline polymer are mentioned cholesteric liquid crystalline polymer compositions each comprising a nematic liquid crystalline polymer and an optically active component, and cholesteric liquid crystalline polymers each having an optically active group in the molecule.
As examples of nematic liquid crystalline polymers are mentioned main chain type liquid crystalline polymers such as polyesters, polyimides, polyamides, polycarbonates, and polyester-imides, and side chain type liquid crystalline polymers such as polyacrylates, polymethacrylates, polymalonates, and polysiloxanes. Above all, liquid crystalline polyesters are preferred in point of their easiness of preparation, transparency, easiness of alignment and fixing, and glass transition point.
As typical examples of optically active components used for imparting twist to the above nematic liquid crystalline polyesters and for the formation of a cholesteric alignment, there are mentioned optically active low-molecular compounds or compositions. Any low-molecular compound or composition is employable in the present invention insofar as it has an optically active group. From the standpoint of compatibility with the liquid crystalline polyester, however, optically active liquid crystalline low-molecular compounds or compositions are preferred.
Optically active high-molecular compounds or compositions are also mentioned as examples of optically active components. Any high-molecular compound or composition is employable if only it contains an optically active group in the molecule. From the standpoint of compatibility with the above nematic liquid crystalline polyester, however, optically active liquid crystalline high-molecular compounds or compositions are preferred. Examples are polyacrylates, polymethacrylates, polymalonates, polysiloxanes, polyesters, polyamides, polyester-amides, polycarbonates, polypeptides, cellulose, and compositions containing these liquid crystalline polymers as a main component. Particularly, aromatic, optically active polyesters are preferred from the standpoint of compatibility with the above nematic liquid crystalline polyester.
As examples of cholesteric liquid crystalline polymers having an optically active group in the molecule are mentioned those having an optically active group in the polymer main chain such as polyesters, polyimides, polyamides, polycarbonates, and polyester-imides, as well as those having an optically active group in the polyme side chain such as polyacrylates, polymethacrylates, polymalonates, and polysiloxanes. Above all, optically active liquid crystalline polyesters are preferred in point of eas

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