GB2258318A

GB2258318A - Liquid crystal materials.

Info

Publication number: GB2258318A
Application number: GB9115151A
Authority: GB
Inventors: Robert Hamilton Mccloughlin
Original assignee: Scimat Ltd
Current assignee: Scimat Ltd
Priority date: 1991-07-12
Filing date: 1991-07-12
Publication date: 1993-02-03
Also published as: EP0594728A1; WO1993001254A1; JPH06509181A; GB9115151D0

Abstract

A liquid crystal material is incorporated into a porous polymer matrix having generally random pores of internal volume 10-100 cubic microns to provide an article that may be used in e.g. electro-optical devices, for example windows for buildings, whose transmissivity may be adjusted by variation of an applied electric field.

Description

OPTICAL DEVICES, THEIR PRODUCTION AND USE The present invention relates to an article useful as or as part of an optical device, for example as a thermochromic device or as a part of an electro-optical device. It also relates to a method for making the article and to devices in which the article is used.

Various electro-optical devices using liquid crystals are known, and these devices are controlled by an applied electric field to bring about a change, for example, in the extent to which the liquid crystal scatters light or rotates the plane of polarised light. Thermochromic liquid crystal devices are also known in which a change in temperature brings about a change in the colour of the device.

It has been proposed in US Patent No 4048358 to incorporate a dichroic liquid crystal or a non-dichroic liquid crystal containing a dichroic dye into a polypropylene sheet having an open micropore structure in which the individual pores are of elongate form, are all aligned and are slightly larger than the liquid crystal molecules. The sheet is placed between protective sheets of glass or plastics which have electrodes coated on their inner faces to produce a liquid crystal cell, which can be switched from a light scattering to a transparent state.

However, as discussed below, we have failed to produce a change of light scattering with applied voltage in a cell made in accordance with the disclosure of that patent and using a nematic liquid crystal.

A display cell having a liquid crystal material incorporated into a porous matrix and allegedly having a variable opacity with applied electric field is disclosed in US Patent No. 4411495. However, the difference in contrast available is relatively small, it is believed due to the orienting properties of the fibrous structures used, and the matrix material has to be relatively thick.

One problem with which this invention is concerned is to provide an optical device, in which a liquid crystalline material is incorporated into a microporous matrix, which can be incorporated into a liquid crystal cell and can enable the cell to be switched between states of different optical transmissivity, and can operate at relatively low switching voltages. Another problem with which the invention is concerned is to provide an optical device as aforesaid which can be switched between states of different optical transmissivity of relatively low applied field and/or at relatively low frequencies of the applied field.

In one aspect the invention provides an article comprising a liquid crystal material in a microporous polymer matrix in which the pore structure is such as to tend not to align the molecules of the liquid crystal or to produce an alignment which is incomplete and/or readily destroyed.

In another aspect this invention provides an optical device comprising a liquid crystalline material in a matrix having pores which are connected so that the liquid crystalline material can permeate through the matrix, wherein the pores have a low surface area in relation to their volume. Desirably the pores are not oriented in any overall direction relative to the matrix, and desirably they have an internal volume of 10 to 100 cubic microns estimated by inspection in a scanning electron micrograph.

In an alternative aspect the invention provides an optical device comprising liquid crystalline material in a matrix which is a microporous polymer having connected pores so that the liquid crystalline material can diffuse through the polymer, the pores comprising main pores developed by removal of discrete particles of soluble material from the polymer and having connections therebetween, for example micro-cracks, which develop during manufacture.

In a further aspect the invention provides a method of manufacturing an optical device comprising the steps of: extruding a molten blend of a polymer and a filler comprising individual particles whose length is not unduly great in relation to their width so that the particles are not needle-like, the maximum length to width ratio desirably not exceeding about 3:1; optionally remove the filler in whole or in part; and allowing a liquid crystalline material to impregnate the matrix.

In many applications it is desirable to remove the filler wholly or in part before the liquid crystalline material is impregnated into the matrix. In general, the optical contrast for film from which the filler has been fully extracted is better than the contrast available for film in which some of the filler has been left behind. The difference between the two is less pronounced where the polymer is coloured. It may be advantageous to leave some filler behind, primarily to reduce manufacturing cost, and it is a surprising feature that useful contrast can be achieved when filler remains present in the film.

The invention further relates to the use of an optical device aforesaid or made by the aforesaid method as a thermochromic device or as part of an electro-optical device.

Relatively long thin pores have been found to exert an orienting influence on a liquid crystal which hinders the effect of an applied electric field in re-aligning the directors of the individual liquid crystal molecules. A high switching potential of e.g. < 120 volts and a high switching frequency e.g. about 1000 Hz are believed to result. This alignment effect is believed to be worsened when the pores present a relatively large surface area to the liquid~~~crystal material or are of relatively small size.

Advantageously the matrix has pores of largest dimension 6 to 12 microns and smallest dimension 1 to 6 microns the ratio of the largest to smallest dimension being generally in the range of 1:1 to 1:3.

If the pores are too large, poor contrast is achieved, whereas if they are too small then the voltage needed to switch states becomes too large and the liquid crystal tends to self-align within the pore chambers.

Individual pores are advantageously separated by walls of thickness 0.25 to 5 microns and are interconnected by holes to the surface of the matrix. If the wall thickness is too small relative to the wavelength of light, the film exhibits poor contrast. If the walls are too thick, the domains of liquid crystal material are small and the optical properties of the article are controlled by the polyethylene matrix, again giving poor contrast. The ratio of the largest to smallest dimension of the pores is generally in the range 1:1 to 1:2 so that the individual pores have an elliptical or rounded brick-like shape but are not long and needle like. The largest dimension of the pores is on average less than 6 microns. The dimensions of the filler present in the material as extruded mainly determine the pore dimensions in the present article. If desired the fillers used may be a mixture, e.g. a mixture of a readily dissolvable material e.g. lithium carbonate and a difficultly dissolvable material e.g. glass.

A typical matrix may be a film composed of polyethylene, density 0.935 having a small proportion of an acrylic acid grafted polypropylene present, a polyethylene oxide based liquid additive and is melt extruded containing a finely divided crystalline lithium carbonate (Li2CO3) which is of maximum particle size 6 microns and is of monochroic crystals of brick-like shape having dimensions about 1:2:3. The material is preferably melt extruded under conditions such as will minimise the orientation of the polymer and control its crystallinity. Thus the polymer is preferably substantially not stretched after melt extrusion and is quenched to minimise or control its crystallinity very shortly after the film has been extruded. This may be brought about by contacting the polymer with a cooled roller located adjacent to the mouth of the die through which the polymer has been extruded.If desired e.g. for ease of processing the matrix may comprise a mixture of polymers.

The lithium carbonate filler is removed from the extruded film by contact with a suitable aqueous liquid such as hydrochloric acid and gives rise to film having a Swiss cheese-like structure having large flattened spherical pores of size about 6 microns separated by walls about 0.5 to 2 microns thick in which there are relatively small interconnecting holes typically of 0.3 microns size as measured by a Coulter porometer. The porosity of the film is variable depending upon the amount of filler originally present from e.g. < 10% to > 70% by volume. Advantageously from the standpoint of its optical properties, it has a tortuosity of above 1.5:1 and preferably in the range 1.5:1 to 6:1. Tortuosity is the ratio of the thickness of a material to the average distance from one face to the other through its internal pores, and is usually estimated optically.

The film is easily filled with liquid crystal by warming it and allowing the liquid crystal material to permeate through the pores in the surface.

The liquid crystal material used may be cholesteric, nematic, smectic, ferroelectric or a mixture of any of these and may optionally also incorporate a dichroic dye. The liquid crystal materials which have been used in the experiments described below are based on cyanobiphenyl compounds. The film is preferably of a non-polar polymer and in addition to polyethylene, it is envisaged that a polycarbonate, polystyrene, and other polyolefins e.g. polypropylene, may be used.

In addition, there may be used polymers containing polar groups, e.g. polymethylmethacrylate and other acrylates having appropriate optical properties. In general the polymer used should be selected so that its refractive index is compatible with the use to which the article is to be put.

When the optical device of the invention is part of an electro-optical device it is desirable that the material can be switched between a light blocking and a light transmissive state. In order to obtain the best contrast the liquid crystal material should have the same refractive index as the matrix when all its directors have been aligned. For example, polyethylene was initially chosen because its refractive index, 1.52, matched that of nematic liquid crystal mixtures E7 and ZLI1132 supplied by Merck Industrial Chemicals, Poole, Dorset, United Kingdom.Of these two liquid crystals, the E7 mixture gave better results because it had a relatively high optical anisoptropy of 0.2246 and a better match of its refractive index of 1.5216 to the refractive index of the polyethylene, the filler which may be an inorganic material which is originally present in the microporous matrix being assumed to be substantially removed. However, the filler may be wholly or partly retained and may have the effect of modifying the refractive index of the microporous matrix e.g. so that it has the same value as a particular selected liquid crystal material.

When an article according to the invention is used as part of an electro-optical device for altering the effect of the liquid crystal present in the article on incident light depending on whether or not an electric field is applied across the article, it is desirable that the electrical resistance (DC and/or AC resistance) is generally similar to that of the liquid crystal material. A typical polyethylene film has a DC resistivity of about 1017 ohm cm whereas the DC resistivity of the liquid crystal materials is about 10 11ohms cm. The incorporation into the polyethylene of a small amount e.g. 1 to 20% by weight on the weight of the polyethylene of a copolymer containing grafted acid groups can improve the properties of the resulting optical device either alone or where another resistivity modifying agent is present.The mechanism for this reduction in resistance is not at present known, but possible mechanisms could be the formation of lithium salts of the acrylic acid groups, the attachment to the acrylic acid group of the polyethylene oxide based additive or simply absorption of water.

The filler present in the extruded composition can be an organic or an inorganic material, and good results have been obtained with inorganic crystalline materials, for example lithium carbonate and calcium carbonate. Desirably the particles of for example lithium carbonate have a maximum size of about 6 microns and particles whose size is less than 2 microns are desirably removed.

The present optical device is easy to fabricate, can be fabricated from an essentially pure polymer and is stable to light and heat. Because of its pore structure it exhibits good switching properties of the liquid crystal between its states. Migration of low molecular weight materials e.g. dichroic dyes present in a liquid crystal material into the polymer matrix has not been observed. This enables the liquid crystal material to be dyed with a dichroic dye providing a first colour and the polymer to be dyed with a second colour so that an optical device having different coloured states can be manufactured. The filler can be left in the polymer, for example either to support its structure or to modify its index of refraction.

The films can be used in the manufacture of a wide variety of liquid crystal based optical devices e.g.

windows for houses, offices etc or switchable light transmissivity where the ability to respond to 50 or 60 Hz alternating fields is a particular advantage. It may also find use e.g. in the manufacture of display screens and devices, particularly those of relatively large area.

The invention further comprises a liquid crystal cell in which an article as aforesaid is placed between layers, at least one of which is transparent and which have electrodes on surfaces in contact with the article. The layers may be of glass or a transparent plastics and the cell may if desired further include crossed polarisers to improve contrast.

The following drawings accompany this application: Figure 1 is a diagrammatic view of apparatus for making a microporous matrix for use in an optical device according to the invention; and Figures 2 and 3 are micrographs showing different microporous materials.

The invention is illustrated in the following Examples in which all the liquid crystalline materials and dyes were obtained from Merck.

Example 1 Liquid crystal in a polyethylene matrix which has been rolled and stretched A mix of 550 grams of lithium carbonate, maximum particle size 6 microns, ground and air classified in an Alpine Fluidised Bed Opposed Jet Mill Type 200 AFG, was blended with 4500 grams of Sclair 8405, a linear polyethylene density 0.937 produced by Dupont of Canada primarily for rotational blow moulding, and 47 grams of lithium stearate by mixing for 2 minutes in a Papenmeier high speed mixer. The mixed materials were then melt blended in a twin screw continuous melt compounding line, Baker Perkins MPV50, extruded as strands through a multi strand extrusion die, air cooled and converted to small pellets in a Betol pelletiser.

Referring to Figure 1, this material was then remelted in a 3 inch 25LD John Brown Plastics Extruder 10, fitted with a 30 inch wide Flexilip film die 12.

Immediately on exiting from the die the material was rolled between the closing nip of two 12 inch diameter chilled polished steel rollers 14, 16, internally heated to 65 0C and rotating at a surface speed of 4 metres/minute. The film was then reheated by passing it between a rubber nip roller 20 and a heated 12 inch diameter polished steel roller 18, internally heated to 1140C with a surface speed of 4.4 metres/ minute.

The film was stretched lengthwise by pulling it away from the heated roller 18 into a nip between a 8 inch rubber roller 22 and a 12 inch steel roller 24 shown cooled to < 25 0C, and rotating at surface speed of 24 metres/ minute and wound onto a winder 26. The Li2CO3 was then removed from the film by passing it through a bath containing a 16% solution of HC1. The film was then washed in water, dewatered by passing it through a bath of isopropanol and dried with warm air.

The final material was in appearance a white opaque film, 14 microns thick with a density of 0.47 gram/cm3.

Electron micrographs show that it had a highly orientated fibrous structure between two continuous surfaces, punctuated with small pores.

Samples of this film were impregnated with a liquid crystal material, Type E7 marketed by B D H, a division of Merck, by heating the liquid crystal to 70 0C and soaking the polymer film in it. After removing the excess liquid crystal material by wiping the film with a tissue the film was placed between two pieces of indium tin oxide (ITO) coated glass. It was found when a potential of 145 volts at 1000 Hz was applied between the conductive ITO layers that the film became more transparent. This effect was less noticible at lower frequencies although it was noted that the light scattering effect of the structure changed at 100 Hz but it is thought that the high alignment force between the liquid crystal and the highly orientated polymer cause the crystal to follow the AC wave form.Also the contrast between the on and off states was very low due to this alignment.

Figure 2 is an electronmicrograph of the film from which the elongated pore structure is apparent.

Example 2 Liquid crystal in a polyethylene matrix which has been rolled but not stretched A porous film was made as described in Example 1 except during the extrusion stage no reheating or stretching of the polymer web occured. The extruder RPM and roller speeds were adjusted to produce a film of the same dimensions as Example 1.

After testing in the same way as in Example 1, this film was found to become transparent at a lower voltage, 120 volts, and lower frequency, 50 Hz. The contrast between the on and off state was marginally greater than that of Example 1.

Scanning electronmicrographs showed that the film had a highly orientated structure with some internal pores in the order of 1-3 micron diameter separated by thin, < 1 micron, polymer walls.

Example 3 Liquid crystal in a polyethylene matrix which has been neither rolled nor stretched A porous film was made as described in Example 1 except that during the extrusion stage the roller nip 14,16 immediately after the 30 inch die 12 was open and no rolling, reheating or post extrusion stretching of the polymer web was employed. Orientation of the elongate lithium carbonate crystals towards the plane of the film was therefore minimised, tending to produce pores having no overall orientation relative to the film. Speeds of the extruder screw and rollers were adjusted to obtain film having approximately the same dimensions as in previous examples. Testing as described in Example 1 revealed a more opaque material in the off state and a lower potential required, 80 volts to obtain a transparent structure.

Optical switching characteristics were determin-ed by observation and were confirmed by the measurement of the change in AC impedence with applied voltage due to the dielectric anisotropy of the liquid crystal material. Scanning electron micrographs (Figure 3) revealed that the film had a porous structure composed of randomly orientated flattened pores, approximately 4 microns long interconnected through their thin walls by very small holes. Porosity of both surfaces was low consisting of round pores mainly < 1 micron diameter.

Example 4 Use as matrix of medium density polyethylene In a compound made as Example 1 the polymer component was replaced by Lotrex LG 0410, a linear medium density 0.939, film grade polyethylene with a MF1 of 4, manufactured by Enimont. When made into a porous film as in Example 3 and tested as in Example 1 it was seen to change from a opaque material to a transparent material when 90 volts 50 Hz was applied. This was verified by electrical measurement as in Example 3.

Scanning electron micrograph examination showed that the film had a structure very similar to that of Example 3.

Example 5 Use of a polypropylene matrix In an extended film made as in Example 1 the polymer component was replaced by a polypropylene homopolyer, Appryl grade 3030FN1. The ratio of polymer to Li2CO3 was changed to 100 parts polymer to 175 parts Li2 CO3 by weight. Methods of compounding and extrusion were the same as in Example 1 except for high process equipment temperature setting and the nip roll and pull roll speeds being 3 metre/minute and 9 metre/minute respectively. A 40 micron thick film was obtained after the Li2CO3 extraction process which when tested as described in Example 1 was seen to change in transparency at 160 volts. This was confirmed by measurement of changes of AC impedence with applied voltage as in Example 3.

Example 6 Use of calcium carbonate as extractable component A porous film 32 micron thick was made as per Example 3 except that the Li2CO3 component was replaced by OMYA BLH, a commercially available calcium carbonate with a mean particle size of 5 microns. The ratio of polymer to the filler was adjusted to accommodate differences in density between lithium and calcium carbonates.

Samples tested as in Example 1 but with the E7 liquid crystal replaced by ZL1 1132, also marketed by BDH, were observed to become more transparent when subjected to a field. In the unenergised state this material was very opaque due to the high degree of scattering of incident light by its structure.

Example 7 Use of a matrix of high density polyethylene A porous film 31 microns thick was made as in Example 6 except that the polymer Sclair 8405 was replaced by a high density polyethylene Stamylex 9119F, density 0.964, manufactured by Dutch State Mines (DSM).

Samples tested as in Example 6 were observed to become more transparent in an electric field than they are in a no-field-state. When viewed through a polarised filter this material was found to have a degree of optical orientation in both the on and off field states. The contrast between the switched states was relatively poor.

Example 8 Impregnation of matrix without removal of filler A thin film 11 micron thick was made as in Example 3 from a polymer/filler mix with a ratio, by weight, of 810 parts Lotrex LG0410 to 610 parts of Li2CO3 with a maximum particle size of 6 micron. Prior to testing as in Example 1 this film did not have the Li2CO3 removed.

After impregnation with a liquid crystal, E7 supplied by BDH, this film was tested as in Example 1 and was observed to become optically clear at a potential of 100 volts.

Example 9 Impregnation of matrix with partial removal of filler A thin film made as described in Example 8 from which 70% of the Li2CO3 was removed by the method described in Example 1 was observed to become optically clear when a potential of 70 volts was applied to it. This material was also more opaque in the off state than that described in Example 8.

Example 10 Use of a dichroic dye A porous film made using the formulation and method described in Example 8 was, after extraction of the Li2CO3 impregnated with a liquid crystal 2L11132 containing 2% of a blue dichroic dye, D102 supplied by BDH. Then resulting material was tested in the same manner as previously described Examples, and the film was observed to change from a deep blue in the off state to a water white clear film in the on state. The absence of visible dye in the switched state indicates that dye does not significantly migrate from the liquid crystal into the polymer where it would give rise to a permanent colouration.

Example 11 Adjustment of electrical properties of the matrix to those of the liquid crystal A porous 14 micron thick film was made as described in Example 3 from a formulation consisting of, byweight, 500 parts Lotrex LG 0410, 615 parts 6 micron Li2CO3 9.3 parts lithium stearate and 50 parts Polybond 1003, an acrylic acid grafted polypropylene homopolymer manufactured by B P Chemicals.

A sample of this film was impregnated with a liquid crystal material, ZL1 1132, and its optical properties tested as in Example 3. The sample was observed to become transparent at 100 volts 50 Hz which was confirmed by A.C resistivity measurements. It was also noted that the change in AC resistivity due to the dielectric anisotropy of the liquid crystal present was greater than that observed 6 in samples containing no Polybond - 1.4 x 10 ohms versus 0.4 x 106 ohms. The AC impedence of the polymer matrix with Polybond present more closely matches the AC impedence of the liquid crystal material, resulting in improved sensitivity to the electric field used to switch the liquid crystal device.

Example 12 Adjustment of electrical properties of the polymer matrix to those of the matrix A porous 13 micron thick film was made as described in Example 3 from a formulation consisting of 400 parts Sclair 8405, 600 parts 6 micron Li2CO3, 9.3 parts lithium stearate, 40 parts Polybond 1003 and 20 parts of Triton X-100, a commercially available surfactant.

A sample of this film was impregnated with a liquid crystal material, ZL1 1132, and its optical properties tested as in Example 3. The sample was observed to become transparent at 90 volts 50 Hz which was confirmed by impedence measurements. This material also exhibited the large change in AC resistivity noted in Example 11. It was also observed that changes in transparency of the material could be seen at very low potentials < 4.5 volts. The DC component of the polymer's resistivity closely matched the DC component of the resisivity of the liquid crystal material, resulting in a further improvement in field sensitivity.

Example 13 Use of a dye-containing liquid crystal material in a matrix containing filler A thin film 13 micron thick was made as in Example 12 except that the Li2CO3 filler was not extracted. It was impregnated with a dyed liquid crystal material, ZL1 1132 + 2% D102, and tested as in Example 12. It was observed that the film changed from a blue partically transparent to a water white material on the application of 100 volts 50 Hz.

Example 14 Manufacture of inks Fibres 100 micron diameter were made by melt extruding a polymer compound, formulation by weight 500 parts Sclair 8405 and 825 parts OMYA BLH calcium carbonate, through a multi hole strand dye. The fibres were reduced to small particles, after which the calcium carbonate was removed with hydrochloric acid to render the particles porous.

The porous particles were washed in water and IPA were dried and were impregnated with a melted thermochromic liquid crystal, TM216 supplied by BDH. Ater the excess crystal had been removed the particles were dispersed in a water based polyvinyl acetate adhesive, Emultex 592 manufactured by Harco Limited and spread thinly onto a length of black polyethylene film.

After drying, the solution had formed a tough flexible layer which when heated exhibited the thermochromic colour range of the original liquid crystal.

It is believed that an electro-chromic device can be made using a modified version of the ink in which the thermochromic liquid crystal is replaced e.g. by a nematic or cholesteric liquid crystal.

Example 15 Thermo-chronic liquid crystal film A porous film made as in Example 4 was impregnated with a thermochromic liquid crystal, TM216 and attached to a piece of black polyethylene film. When heated this film exhibited the full colour range of the TM216 liquid crystal and was comparable in colour quality as a uncovered layer of the crystal. This finding was unexpected because such a wide range of colour is not observed when some other forms of porous polymer are impregnated with thermochromic liquid crystal material.

Example 16 Matrix impregnated with a curable polymer A porous film manufactured as described in Example 3 was impregnated with a solution comprising of, by weight, 10 parts of W curable acrylate based potting compound, marketed by R S Components and 90 parts acetone. After allowing the acetone to evaporate the film was exposed to a high power W source to cure the retained acrylate material. Examination revealed that the porous film structure was relatively uniformly coated with a layer of acrylate and had a translucent brittle structure.

The film was impregnated with a liquid crystal material and tested as in Example 3. It switched to a transparent state at 120 volts 50 Hz.

Example 17 Matrix of Celgard 2400 A sample of Celgard 2400, a microporous polypropylene manufactured by Hoescht Celanese, was impregnated with a liquid crystal material, E7 manufactured by BDH and tested as in Example 3. No change in transparency was discerned when a potential of 100 V was applied across the sample, principally because the Celgard was transparent in both its ON and OFF states.

Example 18 Matrix of Fibrous Cellulose Acetate A sample of a microporous cellulose acetate made by Sartorius which was a 0.2 micrometre filter disc 130 micrometres thick was impregnated with ZLI 1132 and tested as in Example 3. It was observed at 120 V that there was little change in the transparency of the material.

Claims

CLAIMS:

1. An article comprising a liquid crystalline material in a microporous matrix in which pores are connected so that the liquid crystalline material can permeate through the matrix, wherein the pores have a low surface area in relation to their volume, are generally randomly oriented and have an internal volume of 10 to 100 cubic microns estimated by inspection in a scanning electron micrograph.

2. An article according to claim 1, wherein the matrix has pores of largest dimension 6 to 12 microns and smallest dimension 1 to 6 microns, the ratio of largest to smallest dimension being generally in the range 1:1 to 1:3.

3. An article according to claim 2, wherein the individual pores are separated by walls of thickness 0.25 to 5 microns and are interconnected by holes to the surface of the matrix.

4. An article as claimed in any preceding claim, wherein the tortuosity of the pores is above 1.5:1.

5. An article as claimed in claim 4, wherein the tortuosity of the pores is 1.5 - 6:1.

6. An article according to any of claims 2 to 5, wherein the largest dimension of the pores is on average less than 6 microns.

7. An article according to any preceding claim, wherein the liquid crystal is cholesteric, smectic, nematic, ferroelectric or a mixture of any of these and optionally also incorporates a dichroic dye.

8. An article as claimed in any preceding claim, wherein the microporous matrix contains a dye or pigment.

9. An article according to any preceding claim, wherein the liquid crystalline material and the polymer matrix have substantially the same refractive index.

10. An article according to claim 9, wherein the liquid crystalline material and the matrix have similar resistivity.

11. An article according to claim 10, wherein the matrix comprises a polymer containing ionic or ionisable groups.

12. An article according to any preceding claim, wherein the matrix has as its major component a non-polar polymer.

13. An article according to claim 12, wherein the polymer is polyethylene.

14. An article according to claim 12, wherein the polymer is polystyrene or polypropylene.

15. An article according to any of claims 1 to 11, wherein the matrix comprises as major component an acrylate.

16. An article according to any preceding claim, wherein the matrix comprises a mixture of polymers.

17. An article according to any preceding claim, in the form of a film of thickness 5 to 50 microns.

18. An article according to claim 17, wherein the film is of thickness about 25 microns.

19. An article according to any of claims 1 to 15, wherein the matrix is in the form of individual small particles containing liquid crystalline material.

20. An article according to claim 19 in the form of an ink.

21. An article according to claim 19 in the form of a film formed by deposition on a substrate of an ink containing the particles.

22. An article comprising a liquid crystalline material in a matrix which is a microporous polymer having connected pores so that the liquid crystalline material can diffuse through the polymer, the pores comprising main pores developed by removal of discrete particles of soluble material from the polymer and smaller connections between pores, the connections arising during manufacture of the material.

23. A method of manufacturing an optical device comprising the steps of: extruding a molten blend of a polymer and a filler, said filler comprising individual particle whose length to width ratio does not exceed about 3 to 1; and allowing a liquid crystalline material to impregnate the matrix.

24. A method according to claim 23, wherein the filler is at least partly removed before the liquid crystal material is impregnated into the matrix.

25. A method according to claim 24, wherein the filler is a mixture of a dissolvable and a nondissolvable material.

26. A method according to claim 24, wherein the filler is substantially completely removed before the liquid crystal material is impregnated into the matrix.

27. A method according to claim 24, wherein the filler is not removed before the liquid crystal is impregnated into the matrix.

28. A method according to claim 27, wherein the matrix further comprises a polymerisable material which is impregnated into the matrix and polymerised in situ.

29. A method according to any of claims 24 to 28, wherein the filler comprises an inorganic crystalline material.

30. A method according to claim 23, wherein the filler comprises lithium carbonate or calcium carbonate.

31. A method according to any of claims 23 to 30, wherein the extruded polymer is allowed to solidify under conditions which substantially minimise orientation of the filler.

32. A method according to any of claims 23 to 31, wherein the extruded mixture is rapidly cooled so that the resulting polymer is of limited crystallinity.

33. A process as claimed in any of claims 23 to 32 wherein the blend of molten polymer and particles is extruded as a sheet.

34. A method as claimed in any of claims 23 to 32 wherein the blend of polymer and particles is extruded as fibres which are then reduced to small particles.

35. A method as claimed in claim 34 comprising the further step of dispersing the porous particles in an aqueous, organic or aqueous/organic liquid.

36. Use of an article as claimed in any of claims 1 to 22 or made by the method of claims 23 to 35 as a temperature indicator or in an electro-optical device.

37. An electro-optical device comprising an article as claimed in any of claims 1 to 22 or made by the method of any of claims 23 to 35 in the form of a cell in which the device is sandwiched between electrode carrying sheets, at least one of which is transparent.

38. A device as claimed in claim 37, in which the sheets are of glass or of a transparent plastics.

39. A device as claimed in claim 38, in which the sheets are coated with indium-tin oxide. -40. A device as claimed in any of claims 37 to 39, which is a window for fitting into a building.