Polymerizable liquid crystals

Details Polymerizable liquid crystals Polymerizable liquid crystals

2003-10-06

2004-09-21

Huff, Mark F. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299640, C252S299650, C252S299600, C252S299660, C252S299670, C252S299680

Reexamination Certificate

active

06793986

ABSTRACT:

The present invention relates to novel polymerizable liquid-crystalline compounds, to a process for their preparation, to compositions comprising these compounds, and to coating compositions and pigments based on these compounds for various applications.
Aligned, low molecular weight liquid crystals can be permanently fixed by UV polymerization, since UV polymerization is so fast that relaxation of the aligned liquid crystals is not possible. When crosslinkable cholesteric liquid crystals or crosslinkable mixtures of nematic liquid crystals and chiral dopants are used, UV polymerization leads to cholesteric networks exhibiting the characteristic optical properties of a cholesteric mesophase. It is of particular importance that the formation of such networks stabilizes the color flop effect, i.e. the property of a cholesteric liquid crystal to appear in a different color depending on the viewing angle. This significantly simplifies the preparation of cholesteric special-effect or color flop pigments.
Cholesteric pigments are platelet-shaped particles exhibiting shape anisotropy which are prepared from the photocrosslinkable, cholesteric starting mixture via a plurality of process steps. Said starting mixture has to be converted, in its mesophase, into an aligned film and fixed by subsequent UV polymerization. At the same time, this step determines the thickness of the platelet-shaped particles. The resulting cholesteric network then has to be ground to pigment particles in another process step.
Cholesteric color flop pigments of various compositions are already known. Siloxane-based color flop pigments are described, for example, in EP-A-0 601 483. Said pigments were prepared by crosslinking cyclic siloxanes having both chiral and mesogenic side groups via acrylate or methacrylate groups on the mesogenic side groups and processing to pigments.
WO-A-97/27252 describes color flop pigments obtainable by polymerizing mixtures comprising a chiral liquid-crystalline polymerizable monomer, an achiral liquid-crystalline polymerizable monomer and a chiral compound, and a polymeric binder and/or a monomeric, polymerizable compound and/or a dispersion auxiliary. WO-A-99/11733 describes an improved process for preparing color flop pigments and numerous different types of crosslinkable cholesteric mixtures. Suitable achiral liquid-crystalline polymerizable monomers have the general formula
Z—Y—A—Y—M—Y—A—Y—Z
where M is a mesogenic group, A is a spacer group, Y is one of various bridging groups and Z is a polymerizable end group. Preferred radicals Z are acrylate radicals. Each monomer preferably has two polymerizable groups Z.
EP-A-0 675 186 describes photocrosslinkable liquid crystals comprising four crosslinkable groups per molecule. The compounds have the following general formula
where the mesogen is a linear trinuclear group, B is a bridge linking the two mesogens and each R is a crosslinkable radical. The mesogen preferably comprises three bridged p-phenylene groups, and the crosslinkable radicals R are in each case terminal in p position. Said compounds are used for preparing optical components. Their suitability for preparing crosslinked cholesteric special-effect pigments was not investigated. Said compounds have the disadvantage that the crosslinking density per mesogen unit which can be achieved is low.
It is an object of the present invention to provide improved crosslinkable achiral liquid-crystalline monomers, in particular those which make it possible to prepare liquid-crystalline polymers having relatively high crosslinking density.
We have found that, surprisingly, this object is achieved by providing liquid-crystalline compounds of the formula (I)
where
A
1
and A
2
are identical or different and are each a crosslinkable group;
the radicals X are identical or different, preferably identical, and are each a single bond, —O—, —S—, —C═N—, —O—CO—, —CO—O—, —O—CO—O—, —CO—NR—, —NR—CO—, —NR—, —O—CO—NR, —NR—CO—O—, —CH
2
—O— or —NR—CO—NR, in which R is H or C
1
-C
4
-alkyl; and
M is a mesogenic group.
The compounds of the formula I according to the invention are notable for the fact that they are capable of forming a liquid-crystalline phase and stabilize this phase particularly well and permanently owing to their increased crosslinkable group content.
The present invention preferably provides compounds of the general formula I, where A
2
is ortho to A
1
at each occurrence.
Preference is likewise given to compounds of the general formula I where A
1
and A
2
are each, independently of one another, a group of the formula
Z—X—(Sp)
n
—
where
Z is a crosslinkable radical;
X is as defined above;
Sp is a spacer having from 1 to 30 carbon atoms, in which the carbon chain may be interrupted by ether oxygen, thioether sulfur or nonadjacent imino or C
1
-C
4
-alkylimino groups; and
n is 0 or 1.
A
1
and A
2
are preferably identical.
According to a preferred embodiment Z is selected from:
where the radicals R are each, independently of one another, C
1
-C
4
-alkyl, for example methyl, ethyl, n- or i-propyl or n-, i- or t-butyl.
According to another preferred embodiment Sp is selected from:
—(CH
2
)
p
—, —(CH
2
CH
2
O)
m
CH
2
CH
2
—, —CH
2
CH
2
SCH
2
CH
2
—, —CH
2
CH
2
NHCH
2
CH
2
—,
m is from 1 to 3 and p is from 1 to 12.
According to another preferred embodiment M is selected from groups of the general formula II:
where
X is as defined above, and
Q is substituted or unsubstituted alkylene, such as linear or branched C
1
-C
12
-alkylene, or a substituted or unsubstituted aromatic bridging group.
Preferred aromatic bridging groups are selected from
and substituted analogs thereof. Substituted analogs of said bridging groups can carry from 1 to 4 identical or different substituents per aromatic ring, preferably one or two substituents per ring or per bridging group. Suitable substituents are selected from C
1
-C
4
-alkyl as defined above, nitro, halogen, such as F, Cl, Br, I, phenyl or C
1
-C
4
-alkoxy, the alkyl radical being defined as above.
The present invention likewise provides a process for preparing compounds of the general formula I, which comprises reacting a compound of the formula III
where A
1
and A
2
are as defined above and X′ is a reactive side group, with a mesogen compound of the general formula IV
X″—M—X″  (IV)
where M is as defined above and X″ is a reactive side group, X′ and X″ being selected such that they are capable of forming group X.
The present invention provides in particular a process in which a mesogen of the formula IV where X″ is OH is reacted with a compound of the formula III where X′ is —COOH or —COHal, where Hal=F, Cl, Br or I.
The present invention furthermore provides a composition comprising at least one compound of the formula I and, if desired, further components selected from cholesteric, crosslinkable or noncrosslinkable groups, inorganic pigments, colorants, photoinitiators, flow control agents, UV stabilizers, binders and polymerizable or nonpolymerizable diluents or carriers.
Preferred cholesteric compounds are, for example, chiral compounds of the general formulae Xa, b, c and d
(Z—X
5
)
n
Ch  (Xa),
(Z—X
2
—Sp—X
5
)
n
Ch  (Xb),
(P
1
—X
5
)
n
Ch  (Xc)
(Z—X
2
—Sp—X
3
—M—X
4
)
n
X  (Xd),
where
Z is as defined above,
Sp is a spacer as defined above,
X
2
, X
3
and X
4
are each, independently of one another, a chemical single bond, —O—, —S—, —O—CO—, —CO—O—, —O—CO—O—, —CO—NR—, —NR—CO—, —O—CO—NR—, —NR—CO—O— or —NR—CO—NR, where at least one of the groups X
3
and X
4
is —O—CO—O—, —O—CO—NR—, —NR—CO—O— or —NR—CO—NR— and R is C
1
-C
4
-alkyl;
X
5
is as defined for X
2
, X
3
and X
4
or is —CH
2
—O—, —O—CH
2
—, —CH═N—, —N═CH— or —N≡N—,
M is a mesogenic group as defined above,
P
1
is a radical selected from hydrogen, C
1
-C
30
-alkyl, C
1
-C
30
-acyl, C
3
-C
8
-cycloalkyl, unsubstituted or substituted by one to three C
1
-C
6
-alkyl, and where the carbon chain of the alkyl, acyl and cycloalkyl radicals may be interrupte

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