LIQUID CRYSTAL DEVICES

Abstract: Liquid crystal displays comprising as the alignment layer a polyphenyl polymer.

Description: This invention relates to liquid crystal devices, particularly liquid crystal devices comprising an alignment layer, and to the preparation of such surfaces. The structure of liquid crystal display devices is widely described in the literature (see for example the section "Digital Displays" at pp 724 et seq in "Encyclopaedia of Chemical Technology" 3rd Ed. Kirk Othmer) as also is the use of an alignment layer to orientate the liquid crystal. In conventional liquid crystal devices used for display purposes, the liquid crystal composition is often sandwiched between two alignment layers. Both of the alignment layers are supported on transparent electrodes which in turn are supported on transparent, e.g. glass, cover plates. An important contribution of the alignment layer is that it causes the liquid crystal to orientate at a slight angle (typically 1 or 2° with polyimide) - the so-called "tilt bias angle" - to its surface. The tilt bias angle breaks the symmetry that a zero tilt alignment would give and the director then turns in the direction of the bias when the field is applied. In some cases a tilt bias angle of greater than 5° is required - for example greater than 7°, or even 10°; this is particularly so for 270° twisted nematic liquid crystal devices. As mentioned above, the commonly employed polyimide alignment layers give tilt bias angles of only a very few degrees; higher angles are generally obtained by more expensive means, for example by evaporating silicon monoxide from an oblique source under high vacuum. We have now found that surprisingly polyphenylene polymers are capable of producing alignment layers having certain advantages. Thus, we have found it possible, after appropriate rubbing, to obtain a tilt bias angle of between 5° and 15°, particularly between 7° and 12° and especially between 9° and 11°. The use of polyphenylene polymer as the alignment layer, or as a component thereof, is also advantageous over the silicon monoxide method in that the assembly technique is similar to that employed for polyimide. A further advantage of polyphenylene is its relative transparency, particularly when prepared as described, in the critical visible range 500-700 nm. Accordingly the invention provides a cell for a liquid crystal device comprising at least one alignment layer comprising a polyphenylene polymer. The preferred polymer for use according to the invention is one prepared from a cyclohexadiene. According to a second aspect of the present invention there is provided a process for the preparation of a cell for a liquid crystal display device which process comprises the steps of depositing a layer of a polycylohexadiene on a transparent electrode and then converting the polycyclohexadiene into a polyphenylene. Polycyclohexadienes and polyphenylenes suitable for use in the present invention are more fully described in our published European Patent Specifications Nos 76605 and 107895, the disclosures in which specifications are incorporated herein by way of reference. Preferably the polycyclohexadiene is a poly(1,2-disubstituted cyclohexa-3,5-diene) homo-polymer and more preferably the 1,2-substituents are acoyl, e.g. alkanoyl groups having up to six carbon atoms; or alkyl carbonate groups, i.e. ROCOO-, in which the alkyl group R has up to ten carbon atoms. The polycyclohexadiene is preferably spin-coated onto the transparent electrode although we do not exclude the possibility that other coating techniques including typically those known in the electronics art may be used. Fabrication of the actual devices of the invention may be by conventional techniques which have been widely published, for example, unidirectional rubbing of the polyphenylene layer is conventional and may be carried out by techniques well known in the electronics art, e.g. using rayon or cotton cloth. Other processes such as selection of the liquid crystal, production of the cell, filling and sealing of the cell and provision of electronic control are conventional. Conversion of the polycyclohexadiene layer into the polyphenylene layer may be carried out by the processes disclosed in our aforesaid European Patent Specifications. The thickness of each alignment layer in liquid crystal display devices according to the present invention may vary widely and in any given cell may be the same or different, for example they will usually range from 0.2 or even less to 20 micrometers, typically between 0.5 and 2 micrometers, and usually between 0.7 and 1.5 micrometers. The liquid crystal composition used in the present invention may be any liquid crystal composition which is conventionally used in liquid crystal display devices, e.g. a mixture of cyanobiphenyl or equivalent cyclohexyl molecules of the type The transparent electrodes which are used in the present invention may comprise any electrode material which is conventionally used in liquid crystal display devices. The present invention will be further described by reference to the accompanying Figure 1 which shows, by way of example only, a cell for a liquid crystal display device according to the present invention. In Figure 1, two glass plates 1 and 2, covered wholly or partly by transparent electrodes 3 and 4 of indium-tin oxide and then polyphenylene layers 5 and 6 form, with a spacer 7, a cell into which a liquid crystal composition 8 is injected. In use, the electrodes 1 and 2 are charged to appropriate potentials with consequential alignment of the liquid crystals in the cell. Example 1 Glass plates, each 38 mm x 26 mm x 1.0 mm, were coated with an indium-tin oxide layer by conventional means, and washed in a clean air cabinet with filtered chloroform (AR grade). Each of the plates was covered, on the indium-tin oxide side with a 0.85 ml aliquot of poly(benzene-cis-glycol dimethylcarbonate) solution in N-methyl pyrrolidone (NMP) and spun at 10,000 rpm for 4 minutes in a Headway Research Inc Model 1-EClOlD photo resist spinner to obtain a uniform coating. The relationship between polymer concentration in the NMP dope and coating thickness after drying at 175°C for 30 minutes is indicated in Table 1. The polymer coating was then aromatised to polyphenylene by heating in a nitrogen atmosphere at 10°C/min to 350°C and maintaining this temperature for 4 hours. Cells comprising the alignment layers so prepared were made up in the conventional manner. The liquid crystal tilt bias angle of the polyphenylene coating was then determined by the method of Becker et al (Mol Cryst Liq Cryst 72 , 17-25 (1981). (Typically this comprises rubbing the polymer for a few seconds with a belt of rayon under a contact pressure between 200 and 700 grammes with the cloth travelling at 10 metres per minute. The rubbing work is usually in the range 10³ - 10⁵ J/m². The magnetic null method of Scheffer and Nehring (J App Phys (1977) 48 , 1783-1792) can then be applied to determine the tilt bias angle. Values obtained for plates prepared as described were between 9 and 10°. Example 2 Comparison of light transmission behaviour of polyphenylene and polyimide A series of glass slides (75 mm x 25 mm x 1 mm) were coated with different thicknesses of polyphenylene as described in Example 1. For comparison a similar series of slides were coated with polyimide (Pyralin 2550, DuPont). After curing the U.V. visible absorbance spectra of each slide was determined in the range 350-750 nm using a Pye Unicam PU8800 spectrometer using a glass blank as reference. The absorbance of both sets of coatings as a function of coating thickness of 400, 500, 600 and 700 nm is such that the light absorbance of polyphenylene is less than that of polyimide at 500, 600 and 700 nm but the U.V. cut off of polyphenylene occurs at a higher wavelength than polyimide, so that the order is reversed at 400 nm. Example 3 Plates prepared as described in Example 1 were assembled into a standard 8 micrometers twisted nematic cell the cavity of which was then filled with BDH cyanobiphenyl mixture E7. The cell was examined in a polarizing microscope, when it was seen that with no voltage applied the cell looked uniformly dark between parallel polarisers, indicating alignment of the liquid crystal along the rubbing direction activation with 3 volts AC switching the layer on.

Claims: A liquid crystal alignment layer comprising a polyphenylene polymer in contact with an electrode component supported upon a substrate. An alignment layer as claimed in Claim 1 characterised in that the polyphenylene polymer is prepared from a cyclohexadiene. An alignment layer as claimed in Claim 1 of thickness from up to 20 micrometers. An alignment layer as claimed in Claim 3 of thickness 0.5 to 2 micrometers. An alignment layer as claimed in any of Claims 1 to 4 and capable of providing a tilt bias angle between 5° and 15°. A cell for a liquid crystal display device comprising an alignment layer as claimed in any one of the preceding claims. A liquid crystal display device comprising a cell as claimed in Claim 6 containing therewithin a suitable liquid crystal composition. A method of preparing a cell for a liquid crystal display device which comprises the steps of forming upon a suitable supported electrode component an alignment layer comprising a polyphenylene polymer. A method as claimed in Claim 8 in which the preparation of the polyphenylene comprises the steps of forming a layer of a polycyclohexadiene upon the electrode component, and aromatising the said polycyclohexadiene to polyphenylene.

Inventors: BARLOW, WILLIAM ANDREW, RAYNES, E. PETER MINISTRY OF DEFENCE
Applicants: IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1985
Main classification: G02F1
IPC codes: C08G61, G02F1