The use of liquid crystalline materials in electro-optical devices is, of course, well-known. They exhibit electro-optical properties as a result of the alignment of their molecules when exposed to an electrical field, such as when a thin film is disposed between two electrically conducting plates across which a potential difference is applied.

A composite material formed from a binary emulsion of a thermotropic liquid crystal and a non-liquid crystalline organic compound is disclosed in WO 99/00464. The material is a mutual dispersion of a polymer and the liquid crystal, which contains more than 50% of the polymer component (and therefore less than 50% of the optically active liquid crystal). The polymer is present in the material as generally irregular particles. The material, which is in the form of a solid film, is said to be highly adhesive to glass or plastic supports and is prepared by cross- linking to form a polymer matrix containing the particles.

A blend of a thermotropic liquid crystalline polymer and an organic compound is described in US 4867538. The blend is a mixture of the polymer and the compound. The compound is not in the form of discrete particles. The compositions are solid and no unusual physical properties are ascribed to them.

EP-A-0334176 describes a composition of a polymer matrix that exhibits a non-linear optical response containing a silver colloid. The compositions are formed as films by spin coating from solution and there is no mention of their physical properties.

Electro-optical displays including colloid-disperse systems with light controlling qualities are taught in GB-A-2090011. The electro-optical compositions need to be sealed within a cell before they can be used in the displays.

Sequeria and Hill, Journal of Rheology, Volume 42, pages 203 to 213, 1998 describe the rheological properties of particle suspensions in liquid crystalline media. The suspensions were formed by mixing a low molecular weight thermotropic liquid crystal, N- (4-methoxybenzylidine)- 4-butylaniline, with 31.5 volume % zeolite particles. The physical data given in the paper indicates that thin films made of this suspension do not have sufficient flexural rigidity to support their own weight. Thus, the suspension had a yield stress tus of about 8 Pa at room temperature (25°C) and a storage modulus G'of about 1.5 Pa at the same temperature. There is no mention in the paper of any electro-optical properties of the composition.

Liquid crystalline materials can be difficult to handle and to process. For this reason, as described in GB-A-2090011, for example, they are generally sealed within a cell having rigid walls. There exists a need for an electro-optical composition which can be more readily handled and processed.

The present invention aims to alleviate the problems associated with the processing and handling of switchable electro-optical materials.

The invention is based on the finding that certain compositions containing liquid crystalline materials, which can be considered as being composite materials, can be produced in a form which is a soft solid at room temperature.

According to the invention in its first aspect, there is provided a composition which exhibits electro-optical properties and is a soft solid at 25°C comprising a dispersion of particles in a liquid crystalline material.

It was unexpected that such a composition could be obtained which is a soft solid but retains switchable electro-optical properties. The composition may be a solid which is, at room temperature, sufficiently soft to be moulded into different shapes by hand and sufficiently rigid to be capable of being cut with a knife and to retain its cut shape for at least 10 minutes at room temperature.

The compositions typically have storage moduli G'at 25°C of greater than 1,000 Pa, more preferably 1,000 to 500,000 Pa. The storage modulus G'and the yield stress-cys of the compositions may be determined by conventional methods.

The compositions of the invention, in spite of being soft solids, retain their switchable electro-optical properties. Thus, when a thin film of a composition is placed between a pair of electrodes at room temperature, the film changes from being birefringent when viewed through cross polarisers using an optical microscope to exhibiting no birefringence on

application of a potential difference across the cell. Thus, the liquid crystalline material in the composition becomes homeotropically aligned on application of the potential difference.

The particles used in the composition are preferably of a relatively small size. Desirably, the particles have a size in the range of from lOOnm to 1000 nm, more preferably 200 nm to 800 nm. Preferably, the particles are substantially spherical in shape (ie the particles generally appear to be regular spheres when viewed under a microscope).

If it is desired that the particles should not interfere with the optical properties of the liquid crystalline material, which may not be the case for all applications of the compositions of the invention, the particles may have a refractive index within + 0.05 of one of the refractive indices of the liquid crystalline material.

The particles may be made from any suitable material but are preferably of a polymer. A suitable polymer is poly (methyl methacrylate) (PMMA).

In order to obtain a homogeneous dispersion of the particles in the isotropic phase of the liquid crystalline material, the particles are desirably non-agglomerated. So as to inhibit or prevent agglomeration of the particles, they preferably comprise a stabilising compound on their surface. The stabilising compound may be a polymer, such as a poly (hydroxycarboxylic acid) for example, which is chemically grafted onto the surface of the particles. The polymer is preferably a poly (hydroxy C, 2-C20 carboxylic acid) eg, poly (12-hydroxystearic acid).

Stabilised PMMA particles may be obtained by conventional methods such as, for example, those described in L Antl et al,"The Preparation of Poly (Methyl Methacrylate) latices in Non-aqueous Media", Colloids and Surfaces, Volume 17, pages 67-78 (1986).

The particles are preferably present in the composition in an amount of from 2% to 50% by volume of the composition with the balance being the liquid crystalline material and, optionally, other additives such as agents which stabilise the composition. At levels of particles in the composition below 2 % by volume, the composition tends to lose its advantageous physical properties. At levels of particles above 50% by volume, the physical properties and the switchable electro-optical properties of the compositions are expected to become inferior. More preferred levels of particles in the compositions of the invention are from 2% to 30% by volume, such as from 5 % to 20% by volume for example.

The liquid crystalline material may, for example, be selected from any of the known organic thermotropic liquid crystalline compounds, including cholesterics, for example. The liquid crystalline material may have a smectic phase and/or a nematic phase. Suitable examples of liquid crystalline materials include relatively low molecular mass compounds (ie, having a molecular mass of less than about 1000 Daltons) which have an isotropic-nematic transition temperature higher than about 30°C, preferably in the range of 35°C to 100°C. An example of a class of suitable liquid crystalline compounds are the alkyl cyanobiphenyls, such as, for example 4-n-pentyl-4'-cyanobiphenyl. Preferably, the liquid crystalline material is not polymeric.

In a second aspect, the present invention provides a switchable electro- optical device comprising a composition of the invention and means for optically switching the composition. The device may be a flat panel display.

Means for optically switching liquid crystalline materials in electro-optical devices are, of course, well-known. The conventional means may be used in devices which comprise the compositions of the invention.

The compositions have significant advantages when they are used as the electro-optical material in conventional electro-optical devices. As a result of their properties of being soft solids at room temperature and of being capable of being cut with a knife at room temperature, their handling is greatly improved compared to conventional liquid crystalline materials. Thus, when the compositions of the invention are being processed for use in an electro-optical device, they may simply be cut and moulded to the required size. Thus, the need for handling liquid materials and the provision of sealed cells to contain them can be avoided in preparing an electro-optical device.

As a consequence of the physical properties of the compositions of the invention, they can be used to produce an electro-optical device which comprises an optically switchable display that is flexible. For example, a composition of the invention may be provided as a thin film sandwiched between two flexible sheets which are capable of applying a potential difference across the composition.

In a third aspect, the present invention provides a process for preparing a composition of the invention which comprises forming a homogenous

dispersion of particles in a liquid crystalline material at a temperature above the isotropic-nematic transition temperature of the material and then cooling the dispersion. Typically, the dispersion is cooled to room temperature, although it may be cooled to below room temperature if desired. The homogenous dispersion of the particles may be formed at a temperature of from about 40°C to 60°C, although the actual temperature used in any given case will depend on the particular liquid crystalline material which is used and its isotropic-nematic transition temperature.

Preferably, the dispersion is prepared by a method comprising sonicating and tumbling a mixture of the particles and the liquid crystalline material.

This method may be carried out in a heated vessel, such as an oven. By tumbling, it is meant that the mixture is caused to fall repeatedly under the influence of gravity, preferably during rotation in a rotating vessel. The method allows the formation of a substantially homogenous dispersion of the particles in the liquid crystalline material.

In a fourth aspect, the present invention provides the use of particles for improving the handling of a liquid crystalline material. The handling may be improved by converting the material to a soft solid. Suitable soft solids can be moulded by hand and cut with a knife at room temperature.

Preferably, the particles used in the fourth aspect of the invention are particles used in the composition of the invention which are described above in detail.

The following non-limiting examples illustrate the present invention.

Examples

Sample preparatiott The particles used were nearly-monodisperse polymethylmethacrylate (PMMA) spheres, sterically-stabilised with chemically-grafted poly-12- hydroxystearic acid, radius R=250 nm, initially dispersed in cis-decalin at approximately 30 % volume fraction. The particles were prepared according to the method of Antl et al, Colloids and Surfaces, Volume 17, pages 67-78 (1986). Thus suspended, these particles behave as nearly- perfect hard spheres. The solvent was evaporated off to provide dry particles, which were then redispersed in liquid crystal 4-n-pentyl-4'- cyanobiphenyl (5CB) at 44 2°C by sonicating and slowly tumbling in an oven. At this temperature, pure 5CB is in its isotropic phase (isotropic-nematic transition temperature TIN z 35°C). Particle-liquid crystal composites with particle volume fractions of 5%, 10% and 20% were prepared. In all cases, when the mixture was cooled from about 44°C to room temperature (25°C), a waxy solid resulted.

Rheology To quantify the mechanical properties of the composite, small-amplitude oscillatory viscoelastic measurements were performed at lHz, strain amplitude 2%, and a constant cooling rate of 3.5°C per minute. In the isotropic phase (temperature T>35°C) the sample behaves as a fluid eg storage modulus G'about 0.01 Pa and loss modulus G"about 0.1 Pa for the 5 % particle composite. Close to the pure 5CB isotropic-nematic transition temperature (TIN ---35'C) 35 °C) a sudden increase in both storage and loss moduli was observed, and the sample became a viscoelastic solid eg for a 5% composite the storage modulus G'suddenly increases by four

orders of magnitude to about 100 Pa, gradually increasing to > 1000 Pa as the temperature is further decreased. The low-temperature G'is an increasing function of particle volume fraction , reaching a value of about 105 Pa for the + = 20% composite.

Electro-optical switching Optical cells (10! thick) with ITO electrodes were prepared. The glass surfaces were coated with silicon oxide, for homogenous surface alignment of the liquid crystal. The cells were filled with 10% composites, and observed through crossed polarizers using an optical microscope. The samples appeared birefringent. On application of a voltage of about 2V across the cell, the entire liquid crystal composite becomes homeotropically aligned (ie no birefringence).