Liquid-crystalline nematic organosiloxanes which can be...

Details Liquid-crystalline nematic organosiloxanes which can be... Liquid-crystalline nematic organosiloxanes which can be...

1999-07-20

2001-10-09

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From silicon reactant having at least one...

C528S031000, C528S026000, C528S025000, C428S001520, C428S001230, C252S299610, C252S299620, C252S299630, C252S299680

Reexamination Certificate

active

06300454

ABSTRACT:

TECHNICAL FIELD
The present invention relates to organosiloxanes containing at least two mesogenic side groups, to their preparation, and to their use for the production of optically anisotropic layers.
BACKGROUND ART
Low-molecular-weight nematic compounds, which are widely used in liquid-crystal displays (LCDs), are distinguished by good alignability and switchability in an electric field. However, their aligned structure is not particularly stable without an electric field. With increasing warming, the nematic alignment decreases until finally an ordered structure is no longer present. By contrast, some applications, for example polarizing filters and retardation films, require materials whose nematic alignment is retained over a broad temperature range. In many cases, it is also preferred for the aligned structure to be fixed in a solid film instead of, as in the case of low-molecular-weight nematic compounds, for a liquid substance to be enclosed between substrates.
A known way of fixing liquid-crystalline properties is the binding of the liquid crystals into polymer networks, for example by subsequent crosslinking of aligned liquid-crystalline side-chain polymers. However, liquid-crystalline side-chain polymers are generally more difficult to align than low-molecular-weight liquid crystals owing to their higher viscosity, which generally increases with increasing molecular weight. Cholesteric siloxanes are examples of liquid-crystalline side-chain polymers which can be easily aligned . U.S. Pat. No. 5,211,877 describes easily aligned compounds based on cyclic organosiloxanes having side chains containing cholesterol and methacryloyl groups. However, replacement of cholesterol by a compound containing two phenyl radicals only induces a material having a smectic SA phase and secondary amounts of a more highly ordered SB phase. This material is difficult to align and is therefore unsuitable for the proposed applications.
U.S. Pat. No. 5,610,258 discloses that according to the art, neither organosiloxanes having various types of side groups, each containing two aromatic or cycloaliphatic radicals, nor organosiloxanes having identical mesogenic side groups each containing more than two aromatic or cycloaliphatic radicals, are easy to align and at the same time exhibit a broad nematic phase. In contrast, the patent discloses cyclic organosiloxanes having at least three different mesogenic side groups per organosiloxane, where at least two mesogenic side groups per organosiloxane contain at least three aromatic, heteroaromatic and/or cycloaliphatic radicals. These organosiloxanes are said to be readily alignable side-chain polymers having a broad nematic phase. The synthesis of the compounds described proceeds via a large number of separate steps and is too complex for an industrial process. Production of optically anisotropic layers from these materials, to be complete within an acceptable time in production terms, produces only hazy, optically anisotropic layers.
SUMMARY OF INVENTION
The object of the present invention is to provide compounds which are simple to synthesize on a large scale, can easily be converted into a state with nematic alignment, and which can be used to produce clear, optically anisotropic layers by subsequent crosslinking.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to nematic organosiloxanes (F) having at least two mesogenic side groups, of which at most one has three nuclei and all the others have two nuclei, where the term “nucleus” (and its plural “nuclei”) is taken to mean a six-membered ring or a bicyclic structure consisting of six-membered rings, and where at least one of the mesogenic side groups contains a methacryloyl or acryloyl group.
Surprisingly, and in contrast to the current state of knowledge, these organosiloxanes (F) have liquid-crystalline phases over a broad temperature range, and good nematic alignability, enabling optically anisotropic layers to be easily produced therefrom.
Suitable nuclei are six-membered homocyclic or heterocyclic rings, such as, for example, phenyl rings, cyclohexyl rings, pyran rings, pyridine rings, pyrimidine rings or dioxane rings, and bicyclic systems consisting of six-membered rings, such as naphthalene systems. This list is exemplary and not limiting.
In the nematic organosiloxanes (F), the proportion of mesogenic side groups containing methacryloyl or acryloyl groups is preferably at least 10 mol %, particularly preferably at least 40 mol % based on the total mol of mesogenic side groups. The methacryloyl or acryloyl groups on the mesogenic side groups are preferably terminal.
The nematic organosiloxanes (F) are preferably those which contain at least two identical or different units of the general formula (1)
[MR
1
p
SiO
q/2
]  (1)
in which, independently of one another in each case, either p=1 and q=2 or p=2 and q=1,
R
1
is a C
1
- to C
10
-hydrocarbon radical, such as the methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl or n-decyl radical, or the phenyl radical, and in which
M is a mesogenic side group of the general formula (2)
R
2
—X
1
—(A
1
—X
2
)
a
—R
3
—A
2
  (2)
 where, in the above formula (2),
R
2
is a radical of the formula (CH
2
)
n
, where n is an integer having a value of from 2 to 10 and in which one or more methylene units which are not adjacent to one another may be replaced by oxygen atoms or dimethylsilyl radicals,
R
3
is a chemical bond or a radical of the formula (CH
2
)
m
, where m is an integer having a value of from 1 to 12, in which one or more methylene units which are not adjacent to one another may be replaced by oxygen atoms or dimethylsilyl radicals,
X
1
is a chemical bond or is selected from divalent radicals from the group consisting of —O—, —COO— and —OCO—,
X
2
are identical or different linking groups which are selected, independently of one another, from the group consisting of a chemical bond and divalent radicals —COO—, —OCO—, —CH,CH
2
—, —CH═N—, —N═CH—, —N═N—, —CONH—, —HN(O)C—, —C≡C—, —CH═CH—, —N═N(O)— and —N(O)═N—,
A
1
are identical or different nuclei which are selected, independently of one another, from divalent radicals 1,4-phenylene, 1,4-cyclohexylene, 2,5-pyridinylene, 2,5-pyranylene, 2,5-pyrimidinylene, 5,2-pyrimidinylene, 2,5-(1,3-dioxanylene), 5,2(1,3-dioxanylene), 2,6-naphthylidene, 2,7-naphthylidene and 1,4-naphthylidene,
A
2
is an end group which is selected from the group consisting of hydrogen atoms, halogen atoms, hydroxyl, phenyl, acryloyloxy, (meth)acryloyloxy, (meth)acrylethyleneoxy, cyclohexyl and C
1
- to C
10
-alkenyl radicals, in which one or more methylene units which are not adjacent to one another may be replaced by oxygen atoms or dimethylsilyl radicals, and
a can adopt a value of 2 or 3.
Besides the units of the general formula (1), the organosiloxanes (F) can preferably contain units of the formulae R
1
2
Si and R
1
3
Si. Preferred end groups are trimethylsilyl groups.
Particularly preferred organosiloxanes (F) of the general formula (1) are substituted cyclotetrasiloxanes, cyclopentasiloxanes, tetramethyldisiloxanes, and linear polymethylsiloxanes, preferably containing from 2 to 15 silicon atoms, and having trimethylsilyl groups as end groups. Particularly preferred radicals R
1
are methyl radicals.
Organosiloxanes (F) in which one mesogenic side group has three nuclei are particularly suitable for the production of optically anisotropic layers, since the use of trinuclear mesogens causes an increase in the clearing point and contributes toward stabilization of the nematic phase.
It has been found that the introduction of the —COO— or —OCO— group as radical X
2
promotes the formation of a homogeneous nematic phase without the interfering presence of crystalline components or other phases.
Particularly preferred organosiloxanes (F) contain at least two units of the general formula (1) containing mesogenic side groups of the general formula (3)
where R
2
, X
2
, R
3
and A
2
are a

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