Compositions – Liquid crystal compositions
Patent
1992-09-30
1995-01-31
Wu, Shean
Compositions
Liquid crystal compositions
428 1, 359 73, 359 75, 359 76, 359 96, C09K 1952, G02F 113
Patent
active
053856902
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The invention relates to liquid-crystalline elastomers or thermoset materials having a fixed, anisotropic network structure, and to processes for their preparation.
Liquid-crystalline elastomers are polymer networks which can be prepared, for example, by linking the polymer chains of liquid-crystalline side-chain polymers to one another by means of bifunctional molecules. In these materials, although the chain segments and the mesogenic groups are mobile above the glass transition temperature, the material as such retains its shape stability as a consequence of the crosslinking. In the mechanically unloaded state, the alignment of the nematic director of the mesogenic groups is macroscopically nonuniform, and the elastomer appears opaque. However, if a sample of an elastomeric material, such as, for example, an elastomer film, is uniaxially stretched above the glass transition temperature, the directors of the mesogenic groups align themselves parallel to the direction of the tensile stress. The sample becomes transparent and corresponds in its optical properties to a monocrystal of the same dimension. If the load on the elastomer sample is removed, it returns to the unordered, opaque state due to its elasticity.
Macroscopic organic monocrystals of this type are of extremely great interest for applications, for example, in integrated optics and can furthermore be used, for example, for achieving Kerr cells, Pockels cells, frequency doublers or dichroic filters.
In order to preserve the monocrystal state, it has been proposed, for example, to rapidly cool the aligned elastomer to below the glass transition temperature with an applied mechanical stress (J. Schatzle, H. Finkelmann, Mol. Cryst. Liq. Cryst., 142 (1987) 85). However, this process has the disadvantage that the service temperature range of these "frozen" monocrystals is limited at the top by the glass transition temperature.
JP 02-074924 has proposed generating films having a monodomain structure by aligning liquid-crystalline polymers having a molecular weight of between 1,000 and 10,000 on a substrate plate provided with an alignment layer, and crosslinking the aligned polymers in an applied magnetic field. JP 02-047628 proposes electrostatically aligning and crosslinking ferroelectric liquid-crystalline polymers. Another variant (JP 01-297431) proposes carrying out the alignment of the film on a water surface, and subsequently carrying out the crosslinking thereon.
It is common to all these processes that the degree of order of the films which can be achieved does not satisfy all demands and that the films have too many defect structures. Furthermore, only relatively thin films can be obtained by the conventional processes.
An object of the present invention comprises providing 3-dimensionally crosslinked liquid-crystalline polymers having a fixed, anisotropic network structure which do not have the disadvantages observed in the case of conventional materials or only do so to a lesser extent. A further object of the present invention was to indicate a process for the preparation of these improved organic monocrystals. Further objects of the present invention are revealed to a person skilled in the art by the detailed description of the invention which follows.
It has been found that these objects can be achieved by the novel process and by the provision of the novel elastomers or thermoset materials having a fixed, anisotropic network structure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates preparation of a liquid-crystalline elastomer containing free functional groups;
FIG. 2 illustrates fixing of an anisotropic network structure;
FIGS. 3a and 3b illustrate, respectively, DSC diagrams for the unaligned elastomer of Example 1a and the aligned elastomer monocrystal of Example 1c;
FIG. 4a illustrates an X-ray diffraction pattern for a film produced in accordance with Examples 1a-c;
FIG. 4b illustrates impingement of X-rays on a sample perpendicular to the tensile stress direction;
FIG. 5 illustrates an elastome
REFERENCES:
patent: 5039208 (1991-08-01), Ohmishi et al.
patent: 5098975 (1992-03-01), Omelis et al.
patent: 5164111 (1992-11-01), Dorsch et al.
patent: 5308535 (1994-05-01), Schenble et al.
"Liquid Crystal Side Chem polymers", Finkelmann Liquid Crystals Polymers II/III pp. 155-159. Editor. Gordon, 1984.
Finkelmann Heino
Kupfer Jurgen
Finkelmann Heino
Wu Shean
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