Flat element with a cholesteric liquid crystal structure

Details Flat element with a cholesteric liquid crystal structure Flat element with a cholesteric liquid crystal structure

1999-03-31

2003-07-22

Dudek, James (Department: 2871)

Liquid crystal cells, elements and systems

Cell containing liquid crystal of specific composition

In cholesteric phase

C349S183000, C349S175000

Reexamination Certificate

active

06597426

ABSTRACT:

BACKGROUND OF THE INVENTION
As is known for media with shape anisotropy, heating may result in liquid crystalline phases, called mesophases. The individual phases differ by the spatial arrangement of the molecular centers on the one hand, and by the molecular arrangement in respect of the long axes on the other hand (G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis Horwood Limited, Chichester 1974). The nematic liquid crystalline phase is distinguished by only one orientation long-range order existing through parallel arrangement of the long axes of the molecule. Provided that the molecules forming the nematic phase are chiral, the result is a cholesteric phase in which the long axes of the molecules form a helical superstructure perpendicular thereto (H. Baessler, Festkörperprobleme XI, 1971). The chiral moiety may either be present in the liquid crystalline molecule itself or be added as doping substance to the nematic phase, inducing the cholesteric phase. This phenomenon was first investigated on cholesterol derivatives (for example H. Baessler, M. M. Labes, J. Chem. Phys., 52, (1970) 631; H. Baessler, T. M. Laronge, M. M. Labes, J. Chem. Phys., 51, (1969) 799; H. Finkelmann, H. Stegemeyer, Z. Naturforschg. 28a, (1973) 799; H. Stegemeyer, K. J. Mainusch, Naturwiss., 58, (1971) 599, H. Finkelmann, H. Stegemeyer, Ber. Bunsenges. Phys. Chem. 78, (1974)) 869.
The cholesteric phase has remarkable optical properties: a high optical rotation and a pronounced circular dichroism which arises due to selective reflection of circularly polarized light within the cholesteric layer. The colors which are apparently different depending on the angle of view depend on the pitch of the helical superstructure, which in turn depends on the twisting ability of the chiral component. In this connection it is possible to alter the pitch, and thus the wavelength range of the selectively reflected light, of a cholesteric layer in particular by changing the concentration of a chiral doping substance. Cholesteric systems of this type provide interesting possibilities for practical application. Thus, it is possible by incorporating chiral moieties into mesogenic acrylic esters and orienting in the cholesteric phase, e.g. after photopolymerization, to prepare a stable, colored network, although the concentration of chiral component therein cannot then be changed (G. Galli, M. Laus, A. Angelon, Makromol. Chemie, 187, (1986) 289). It is possible by admixing noncrosslinkable chiral compounds with nematic acrylic esters and by photopolymerization to prepare a colored polymer which still contains large amounts of soluble components (I. Heyndricks, D. J. Broer, Mol. Cryst. Liq. Cryst. 203, (1991) 113). It is furthermore possible, by random hydrosilylation of mixtures of cholesterol derivatives and acrylate-containing mesogens with defined cyclic siloxanes and subsequent photopolymerization to obtain a cholesteric network in which the chiral component may comprise up to 50% of the material employed; however, these polymers still contain distinct amounts of soluble materials (F. H. Kreuzer, R. Maurer, Ch. Müller-Rees, J. Stohrer, Presentation No. 7, 22nd Meeting on Liquid Crystals, Freiburg, 1993).
DE-A 35 35 547 describes a process in which a mixture of cholesterol-containing monoacrylates can be converted by photopolymerization into cholesteric layers. However, the total amount of chiral component in the mixture is about 94%. Although the mechanical stability of such a material, as pure side-chain polymer, is not very great, the stability can be increased only by highly crosslinking diluents.
Besides the nematic and cholesteric networks described above, also known are smectic networks which are prepared in particular by photopolymerization of smectic liquid crystalline materials in the smectic liquid crystalline phase. The materials used for this are, as a rule, symmetrical liquid crystalline bisacrylates as described by, for example, D. J. Broer and R. A. M. Hikmet, Makromol. Chem. 190, (1989) 3201-3215. However, these materials have very high clearing points of >120° C. so that there is a risk of thermal polymerization. Piezoelectric properties can be obtained by admixing chiral materials when an S
c
phase is present (R. A. M. Hikmet, Macromolecules 25, 1992, 5759).
The publication by H. Körner and C. K. Ober in Polymer Materials, Science and Engineering, 73 (1995) 456-457 discloses, for example, liquid crystalline cyanates which are thermosetting. Furthermore, Progress in Polymer Science 18 (1993) 899-945, authors E. E. Barclay and C. K. Ober, likewise discloses corresonding liquid crystalline compounds which have epoxides as reactive groups.
Thermally crosslinkable cholesteric liquid crystalline systems have not hitherto been described.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to sheet-like structures obtainable by thermal curing and having a crosslinked cholesteric liquid crystalline ordered structure.
DETAILED DESCRIPTION OF THE INVENTION
The sheet-like structures according to the invention have a superstructure like that of cholesteric liquid crystals. Either the superstructure is present even before the crosslinking, or it is formed during the crosslinking. It is produced
a) from chiral nematic liquid crystalline compounds,
b) from a nematic and a chiral liquid crystalline compound,
c) from a nematic liquid crystalline and a chiral non-liquid crystalline compound or
d) from a compound which is not nematic but undergoes a transition during the thermal curing into a nematic liquid crystalline structure, and a chiral compound.
Examples of suitable chiral compounds in this connection are the compounds described in German Patent Application P 19520660.6, with those disclosed in claim
5
being emphasized. These are compounds of the general structure
(Z—Y
1
—A—Y
2
—M—Y
3
)
n
X  I
in which the variables have the following meanings:
A spacer,
M mesogenic groups,
Y
1
, Y
2
, Y
3
chemical bonds or the groups —O—; —S—; —CO—O—; —O—CO-—; —O—CO—O—; —CO—N(R)— or —N(R)—CO—,
R hydrogen or C
1
-C
4
—alkyl groups,
X a radical of the formula
n 2 to 6 and
Z
a) in at least one case a radical having an isocyanate, isothiocyanate, cyanate, thiirane, aziridine, carboxyl, hydroxyl or amino group and
b) the other radicals are hydrogen or unreactive radicals, where the radicals
L are, independently of one another, C
1
-C
4
—alkyl or —alkoxy, halogen, —CO—OR, —O—CO—R, —CO—NH—R or —NH—CO—R, and
the radicals Z, Y
1
, Y
2
, Y
3
, A and M, can, because they are present n times in I, be identical or different.
Examples of individual chiral compounds are:
where R is OH, OCN, ONC or
The nematic compounds necessary according to a) to c) must be selected from the large number of known thermally crosslinkable structures, it being necessary to take account of the following aspects:
1. The nematic liquid crystalline compounds should have a sufficiently wide phase range.
2. Miscibility with chiral components mentioned in b) and c) must be ensured.
3. Good miscibility with other thermally crosslinkable liquid crystals is desirable to reduce the crystallization temperature and increase the clearing point.
4. The temperature at which the curing is carried out should be as low as possible, a favorable range being from 80 to 200° C., preferably 80 to 130° C.
The following compounds which substantially meet these criteria may be mentioned by way of example:
Combination of compounds A and B permits the melting points to be reduced by comparison with use of the individual components. The same applies to components C and D. Components A and B are cured, as is known, using amines which are added in stoichiometric amount. It is advantageous in this case to use structurally similar amines, preferably diamines, such as
Since the amine component is, as a rule, not a liquid crystal, the overall system must be inherently balanced so that, on curing, a liquid crystalline system is produced or retained.
Details of the composition of such systems may be found in the examples in which, unless noted ot

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