Laterally substituted curable liquid crystals

Details Laterally substituted curable liquid crystals Laterally substituted curable liquid crystals

2001-08-16

2004-01-13

Huff, Mark F. (Department: 1756)

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

C252S299610, C252S299630, C560S061000, C560S085000, C428S001400

Reexamination Certificate

active

06676851

ABSTRACT:

The present invention relates to laterally substituted curable Liquid Crystals (LCPs) having mesogenic properties or properties which cause these LCPs to be compatible with a mesogenic molecular architecture. In particular the present invention relates to laterally substituted curable Liquid Crystals (LCPs) having a low melting point and good alignment properties and the use of such LCPs in the preparation of substantially uniform or patterned films in which the orientation of the LCP molecules in the plane and relative to the plane of the substrate can be controlled.
Films prepared from curable Liquid Crystals (LCP films) are well known to a skilled person and are used in the preparation of optical and electro-optical devices. These films are generally manufactured by using coating techniques such as spin coating. This involves casting an organic solution of a cross-linkable LCP or LCP mixture onto a substrate provided with an orientation layer. The organic solvent is subsequently removed to give a well orientated, solvent free mesogenic LCP layer. This mesogenic LCP layer may be cross-linked to give a LCP film. The thickness of the LCP film depends upon the viscosity and therefore the concentration of the organic solution of the polymerisable LCP mixture used in the coating process. The uniformity of the film formed depends upon the ability of the LCPs to form homogeneous layers free of tilt domains as well as the stability of the LCP mixture during the coating and cross-linking processes. By the term “tilt domains” it should be understood to mean regions within the LCP film in which the long axis of the LCP molecules form tilt angles with the substrate plane which are of the same size, but have opposite directions.
A problem with known LCPs, especially those having high clearing and melting points, is that they are not able to form mixtures that remain stable during both the coating and cross-linking processes. These prior art LCP mixtures tend to be characterised by a poor solubility in organic solvents; a tendency for the components of the mixture to separate from one another; and a tendency to crystallise. Although attempts have been made to solve these problems by, for example, preparing LCPs with lower melting points, the ability of these prior art LCPs to align with the tilt direction imposed on the film tends to be poor. Such poorly aligned films tend to be characterised by a low contrast ratio.
A further problem associated with existing LCP materials is the formation of tilt domains and disclinations during the preparation of LCP films. By the term disclination it should be understood to mean borderlines of neighbouring tilt domains where LCP molecules of opposite tilt angles are adjacent. These tilt domains and disclinations result in both a disturbance in the uniform appearance of the film and an inhomogeneous optical performance.
The aforementioned problems are of particular relevance if photooriented and photopatterned orientation layers are used for the orientation of LCPs. This so called linear photopolymerisation (LPP) technology (
Nature,
381, p. 212 (1996) allows the production of not only uniform but also structured (photopatterned) orientation layers. If such structured orientation layers are used for the orientation of LCPs, the LCP molecules should adapt the information given by the orientation layer with respect to the direction of alignment and the tilt angle in each single pixel individually.
There is, therefore, a need for a new LCP material that may be used in the preparation of LCP mixtures and layers, which significantly reduces the aforementioned disadvantages and which is especially suitable when applied to LPP orientation layers. The present invention addresses that need.
A first aspect of the invention provides a compound of formula (I)
wherein
G
1
and G
2
independently represent a polymerisable mesogenic residue;
X represents a group selected from —CH
2
—, —O—, —CO—, —COO—, —OOC—, —CONR′—, —OCOO—, —OCONR′;
Sp represents a group of the formula —(CH
2
)p— in which p is an integer of 1 to 18 and in which one or two non adjacent —CH
2
— groups are optionally replaced by —CH═CH—; or in which one or two —CH
2
— groups are optionally replaced by one or two groups selected from the group consisting —O—, —CO—, —COO—, —OOC—, —CONR′—, —OCOO—, —OCONR′— with the proviso that firstly the spacer group does not contain two adjacent heteroatoms and secondly when X is —CH
2
—, p can also have a value of 0;
Q represents a a polar group selected from —CN, —COR, —COOR, —OCOR, —CONR′R, —NR′COR, —OCOOR, —OCONR′R, —NR′COOR, F, Cl, —CF
3
, —OCF
3
or —OR or a cyclic group which is unsubstituted or optionally substituted by a group selected from a lower alkyl, lower alkenyl, lower alkoxy, lower alkenyloxy, halogen, —CN, —COR″, —COOR″, —OCOR″, —CONR′R″, —NR′COR″, —OCOOR″, —OCONR′R″, —NR′COOR″, —CF
3
, and —OCF
3
; where
R represents hydrogen, a lower alkyl, a lower alkenyl or a cyclic group as defined above; and
R′ is hydrogen, a lower alkyl or a lower alkenyl group
R″ represents a lower alkyl or a lower alkenyl group.
The compounds of the invention have been found to have lower melting points compared to the compounds of the prior art. They have also been found to be more miscible with other components of the liquid crystal mixtures of which they form a part. have a reduced tendency to crystallise from and little effect on the clearing points of such mixtures. In addition they exhibit improved alignment abilities compared to the compounds of the prior art.
Laterally substituted mesogenic compounds are known from WO 95/24454, WO 95/24455, U.S. Pat. Nos. 5,650,534, 5,593,617, 5,567,347 and 5,707,544. However, many of these compounds are not suitable for preparing LCP films and networks that are substantially free of tilt domains. Others exhibit high melting points, higher viscosities (U.S. Pat. No. 5,567,347), lower clearing points (U.S. Pat. No. 5,593,617), poor solubility and/or poor orientation properties. It has been found that by using the compounds of the present invention it is possible to control the orientation or alignment of LCPs or LCP mixtures in the plane of the substrate, to form a tilt angle relative to the plane and to suppress the formation of tilt domains in the mesogenic layers and films formed. The compounds of the invention may therefore be used in the preparation of high contrast optical or electro-optical devices.
The compounds of the invention have also been found to be highly miscible with other LCP compounds over a broad range of concentrations. These compounds and mixtures containing them are extremely soluble in organic solvents. These properties mean that it is possible to prepare coating solutions having a concentration and viscosity that can be controlled over a wide range. Consequently, the thickness of the LCP layers formed using these coating solutions can be readily controlled.
The compounds of the invention are further characterised by relatively low melting points and clearing points that are generally above room temperature. Therefore, during the formation of LCP films or networks using the compounds of the invention or mixtures thereof, spontaneous crystallisation does not tend to occur. This property (otherwise known as supercooling) greatly facilitates the formation of LCP films and networks free of defects. This further means that it is also possible to reduce the number of liquid crystal components used in the manufacture of LCP mixture.
The polymerisable mesogenic residues G
1
and G
2
may be the same or different, but are preferably the same.
The group X is preferably selected from —CH
2
—, —O—, —COO— and —OOC—.
The spacer group Sp may be optionally substituted by one or more fluorine atoms. Groups in which there are no substituent groups present are preferred. It is especially preferred that the integer p has a value of from 1 to 11 and that no more than one —CH
2
— group is replace

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