EP0830634A1

EP0830634A1 - Liquid crystal alignment materials and devices

Info

Publication number: EP0830634A1
Application number: EP97907225A
Authority: EP
Inventors: Julian Hollingshurst; John William Goodby; Ian Charles Sage; Guy Peter Bryan-Brown
Original assignee: UK Secretary of State for Defence
Current assignee: Qinetiq Ltd
Priority date: 1996-04-03
Filing date: 1997-03-19
Publication date: 1998-03-25
Also published as: JPH11508063A; GB9607059D0; WO1997038349A1

Abstract

An alignment layer for a liquid crystal device comprises or containing a polymer of structure (I) in which C represents a carbon atom, A is selected from H, Cl, F, CN, CO2R, OCOR, OR, where R is a straight chain or branched alkyl group having 1 to 15 carbon atoms, X and Y are independently selected from the same groups as A, subject to at least one of X and Y on each C atom being selected from F and H, B is a linking group comprising a single bond, or a chain of from 1 to 15 methylene groups in which one or more non-adjacent methylenes may be replaced by O, CO2, OCO, P is a photoactive group selected from cinnamate or anthracene groups substituted with at least one group selected from F, Cl, CN, CF3, OCF3, Br, wherein the interfacial energy of the polymer surface is within 3.0 ergs per square centimetre of the liquid crystal surface energy, and in the range 35 to 50 ergs per square centimetre. A method of providing an alignment layer on a surface of a liquid crystal cell wall includes the step of depositing a layer of a polymer containing at least one polymer as described above on the surface, followed by irradiation with polarised actinic light, and controlling the exposure time and/or intensity of light used to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer. The alignment layer formed as above may be used to provide one or both alignment layers on the two walls of a liquid crystal cell. The cell may incorporate nematic, cholesteric, or a smectic liquid crystal material.

Description

Liquid Crystal Alignment Materials and Devices

This invention relates to materials and methods for achieving alignment of liquid crystal materials on a solid surface, and to devices made using these methods.

Liquid crystal devices commonly comprise a layer of a liquid crystal material contained between two cell walls. These walls carry electrode structures for applying an electric field across the layer so that the layer may be switched between different molecular arrangements, e.g. between light transmitting and light blocking states. Many devices carry surface mounted structures such as thin film transistors; these are used in active matrix displays. Most devices also have a surface alignment treatment on the cell walls which impart a preferred molecular alignment direction to contacting liquid crystal molecules.

An object of this invention is to provide means of achieving a defined surface alignment of a liquid crystal material on a surface, which does not require mechanical rubbing or other physical contact which may damage the surface or structures on it. A further objective of the invention is to provide means by which the pretilt angle and surface anchoring energy may be altered without the need for mechanical rubbing or contact.

It is well known in the art that fabrication of liquid crystal devices which have advantageous performance and low defect densities requires control of the alignment of the liquid crystal at the surfaces of the device. Different types of liquid crystal alignment have been described. Homeotropic alignment refers to an alignment in which the unique optical axis of a liquid crystal phase, especially a nematic liquid crystal phase, is held perpendicular to the adjacent surface. Planar alignment, sometimes referred to as homogeneous alignment, refers to alignment in which the unique optic axis of the liquid crystal phase lies parallel to the adjacent surface. Planar alignment may also impose a direction in which the optic axis of the liquid crystal lies, in the plane of the adjacent surface.

Tilted planar alignment or tilted homogenous alignment refer to alignment in which the liquid crystal unique optic axis lies at an angle, termed the pretilt angle from the plane of the adjacent surface. The pretilt angle may be as small as a fraction of one degree, or as large as several tens of degrees.

Tilted homeotropic alignment refers to an alignment in which the optic axis of the liquid crystal lies tilted away from the normal to the adjacent surface. This deviation is again termed a pretilt angle.

In liquid crystal devices, said alignment geometries are chosen and used in combination to achieve specific optical and electro-optic properties from the device and may be combined in new ways or with new liquid crystalline fluids to provide new types of devices.

Several methods are known in the art by which defined liquid crystal alignment geometries may be achieved. Deposition of a polymer layer, for example a polyimide layer on the surface followed by mechanical rubbing provides a pretilted planar alignment. A disadvantage of this method is that the mechanical rubbing process may cause damage to structures on the surface, for example to the elements of an active switching matrix fabricated on the surface. A further disadvantage is that a static electric charge may be generated during the rubbing process, and may damage the surface or structures on the surface or connected to it. A planar alignment or tilted planar alignment may also be achieved by evaporating a variety of inorganic substances onto the surface from an oblique angle if incidence. A disadvantage of this method is that it requires slow and costly vacuum processing. A further disadvantage is that the resulting evaporated layer may show a high capacity to absorb contaminants onto itself from the environment or from other materials used in construction of the device. These contaminants may adversely affect the operation of the device.

A homeotropic alignment can be obtained by depositing a surfactant, for example, a quaternary ammonium salt onto the surface from solution in a suitable solvent. A disadvantage of this treatment is that the resistivity of the liquid crystal device may be lowered by the surfactant, and the resulting alignment may also show poor stability.

Materials for use in so-called orientation films in Liquid Crystal Displays are disclosed in UK Patent Application GB 2281977 A and German Patent Application DE 4417409 A1

According to the present invention there is provided a photoactive polymer or polymer mixture comprising or containing at least one polymer of structure

BP in which C represents a carbon atom, A is selected from H, Cl, F, CN, CO₂R, OCOR, OR, where R is a straight chain or branched alkyl group having 1 to 15 carbon atoms, X and Y are independently selected from the same groups as A, subject to at least one of X and Y on each C atom being selected from F and H B is a linking group comprising a single bond, or a chain of from 1 to 15 methylene groups in which one or more non-adjacent methylenes may be replaced by O, C0₂, OCO,

P is a photoactive group selected from cinnamate or anthracene groups substituted with at least one group selected from F, Cl, CN, CF₃, OCF_3l Br. Preferably the total interfacial energy of the polymer surface is within 3.0 erg per square centimetre of the liquid crystal surface energy, and in the range 35 to 50 ergs per square centimetre.

n is conventionally known as the number of repeat units and may typically be in the range 4-1000. Preferably n is in the range 20-300. a is 1.

The polymer used may be a single polymer or a mixture of polymers. In the case of a mixture of polymers, one or more polymers of structure 1 may be mixed together or may be mixed with other photoactive polymers in order to adjust the interfacial energy of the polymer surface. The types of polymer included in the current invention may be any of the known types of polymer including homo and co polymers.

The interfacial energy of the surface may be measured by examining the contact angles made between the surface and reference liquids, according to the method described by Shohei Naemura, PhD Thesis, University of Kyoto, 1982.

Suitable polymers include derivatives of poly(vinyl cinnamate), poly(2- cinnamoyloxyethylacrylate) and poly(2-(anthracene-5-carboxy)ethyl) acrylate. According to an aspect of this invention a method of providing an alignment layer on a surface of a liquid crystal cell wall includes the step of depositing a layer of a polymer containing at least one polymer of structure I on the surface, followed by exposure to actinic light, and controlling the exposure time and/or intensity of light used to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer.

According to an aspect of this invention a liquid crystal device comprises a layer of a liquid crystal material contained between two cell walls both carrying electrodes structures and surface treated to provide an alignment to liquid crystal molecules;

CHARACTERISED IN THAT:

the surface treatment is a layer of the polymer of structure 1 and exposed to actinic light.

The invention will now be described by way of example only with reference to the accompanying drawing of which:

Figure 1 is a plan view of a liquid crystal device; and

Figure 2 is a cross sectional view of Figure 1.

The device of Figures 1 , 2 comprises a liquid crystal cell 1 formed by two cell walls 2, 3 spaced typically 1 to 15μm apart by a spacer ring 4 to contain a layer 5 of a liquid crystal material. The inside faces of both walls 2, 3 are coated with a indium tin oxide layer 6, 7 forming transparent electrodes. The electrodes may be of sheet like form covering the complete wall, or formed into e.g. strip electrodes to provide an array of addressable electrode intersections. The walls are also coated with an aligning layer 8, 9 of polymer. These layers provide both an alignment direction, indicated by A, and A_b as orthogonally arranged. If the material 5 is nematic then the device may be the known twisted nematic device. In this case polarisers 10, 11 are used to distinguish between the device voltage ON and OFF states.

The liquid crystal material may be nematic, cholesteric, or smectic material. The device may be used as a display device, e.g. displaying alpha numeric information, or an x,y matrix displaying information. Alternatively the device may operate as a shutter to modulate light transmission, e.g. as a spatial light modulator, or as a privacy window.

The alignment layer may be produced as described, by way of example only, in the following examples:

Example 1 :

Poly(vinyl 4-chlorocinnamate) was prepared by reaction of 4-chloroacryloyl chloride with poly(vinyl alcohol) in pyridine at room temperature overnight. The product was recovered by removal of the pyridine on a rotary evaporator, washed with water, and purified by successive dissolution in dichloromethane followed by precipitation from ethyl alcohol until the colour was pale yellow.

Example 2:

The following polymers were prepared analogously:

Poly(vinyl 2-chlorocinnamate)

Poly(vinyl 2,4-dichlorocinnamate)

Poly(vinyl 4-fluorocinnamate)

Poly(vinyl 4-trifluoromethylcinnamate)

Poly(2-(1 -chloroanthracen-5-carboxy)ethyl) acrylate

Poly(2-(2-chloroanthracen-5-carboxy)ethyl) acrylate Example 3

A solution of poly(vinyl 4-chlorocinnamate) was dissolved in propylene glycol methyl ether acetate at a concentration of 6%. The solution was deposited by spinning onto an indium tin oxide coated glass substrate at 3000rpm for 6 seconds. Samples of the substrate were exposed to linearly polarised light from a helium cadmium laser source providing radiation at a wavelength of 325nm. After exposure, the samples were assembled into cells and these were filled with liquid crystal mixture E7 (available from Merck). The tilt angle in each cell was measured by determining the symmetry point of the optical interference pattern which resulted from rotating the cell in a beam of light of wavelength 633nm, between crossed polarising filters. The results obtained are tabulated below:

Exposure J/m² Pretilt angle (degrees)

0.25 20

0.5 48

1.2 65

3.5 80

Example 4

A solution of 3% poly(vinyl 4-chlorocinnamate) and 3% poly(vinyl cinnamate) in propylene glycol methyl ether acetate was deposited onto a indium tin oxide coated glass substrate by spin coating at 3000rpm for 6 seconds. Substrate samples were removed and irradiated as in example 3, and their pretilt angles measured as for example 3. The values of pretilt found are tabulated below:

Exposure J/m² Pretilt angle (degrees)

0 13

0.6 18

1.2 20

6 49 Example 5

A liquid crystal cell was constructed having one wall surface coated with the alignment polymer of example 3, exposed to 0 5J/m² of 325nm linear polarised light, liquid crystal molecules align perpendicular to the polarisation direction The other wall surface was coated with Probimide 32, a polyimide commercially available from Ciba-Geigy chemicals, and mechanically rubbed, liquid crystal molecules align along the rubbing direction The cell was assembled with the alignment direction on the two walls mutually perpendicular The walls were spaced apart using plastic bead spacers by a distance of 6μm The cell was filled with the commercially available liquid crystal mixture ZLI-2293 doped with 0 8% S-811 , both materials obtained from E Merck, Darmstadt The resulting electro-optic response showed a steep switching characteristic with no intrusion of an undesirable stripe characteristic

Claims

We claim:

1. An alignment layer for a liquid crystal device comprising or containing a polymer of structure

BP in which C represents a carbon atom, A is selected from H, Cl, F, CN, C0₂R, OCOR, OR, where R is a straight chain or branched alkyl group having 1 to 15 carbon atoms, X and Y are independently selected from the same groups as A, subject to at least one of X and Y on each C atom being selected from F and H B is a linking group comprising a single bond, or a chain of from 1 to 15 methylene groups in which one or more non-adjacent methylenes may be replaced by O, CO₂, OCO,

P is a photoactive group selected from cinnamate or anthracene groups substituted with at least one group selected from F, Cl, CN,

2. A method of providing an alignment layer on a surface of a liquid crystal cell wall including the step of depositing a layer of a polymer containing at least one polymer as claimed in claim 1 on the surface, followed by irradiation with polarised actinic light, and controlling the exposure time and/or intensity of light used to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer.

3. The method of claim 2 wherein the irradiation is incident from an angle oblique to the surface.

4. The method of claim 2 wherein the irradiation is performed with eliptically polarised radiation

5. The method of claim 2 wherein the polarised radiation is concentrated into an area smaller than the wall being treated, and is scanned over said wall or defined areas thereof

6. A liquid crystal device comprising a layer of a liquid crystal material contained between cell walls carrying electrodes, characterised in that at least one wall carries the alignment layer of Claim 1.

7. The device of claim 6 wherein the interfacial energy of the polymer surface is within 3.0 erg per square centimetre of the liquid crystal surface energy, and in the range 35 to 50 ergs per square centimetre.

8. The device of claim 6 wherein the liquid crystal material is a nematic or a chiral nematic material.

9. The device of claim 6 wherein the liquid crystal material is a smectic C material.

10. An alignment layer of claim 1 wherein n is in the range 4-1000 and a is 1.