GB2185487A - Copolymer having both liquid crystalline and electro-optical properties - Google Patents

Abstract

A copolymer has side groups contributing both liquid crystalline and non-linear electro-optical properties to the copolymer. A suitable copolymer has the structure:- <IMAGE> where n is 2-10 and R is alkoxy or aryl-alkoxy.a

Description

SPECIFICATION Electro-optical material This invention relates to an optical material and in particular to the type of optical material whose refractive index varies non linearly with intensity. Such material are known as non-linear electro-optical materials. One example of such a material is CS2 (carbon disulphide). Another example is 3-methyl-4-nitropyridine-N-oxide (POM). Such materials have useful practical properties. For example they can generate light of a frequency which is a multiple of the frequency of light passing into them. They are also useful in devices employing frequency mixing, parametric amplification, the linear electro-optic (or Pockels) effect, the Kerr effect and optical bistability.

Further information can be obtained from the book "Applied Non-linear Optics" by F. Zernike and J.E. Midwinter published by Wiley 1973.

Inorganic non linear electro-optical materials are mechanically strong, and can therefore be formed into a desired shape such as a thin slice or waveguide.

Single crystal materials are used, in order to ensure an ordered structure within the material.

An ordered structure, is necessary, as many of the non-linear electro-optical effects are dependent on the anisotropy of a non-centrosymmetric crystal structure.

Organic non-linear electro-optical - materials, however, display non-linear electro-optical characteristics to a much greater extent than do inorganic materials, and much work has been carried out investigating these materials.

Current work on this subject is mainly devoted to preparing single crystal organic materials and slicing them, ie, the work is being directed along the same lines as were pursued with inorganic materials.

Although single crystal organic materials have enhanced non-linear electro-optical properties, they are much weaker, mechanically, than inorganic materials, and tend to crumble during polishing. At present, therefore, single crystal inorganic electro-optical materials are extensively used for the applications described above.

The invention arose as a result of considering ways other than the use of single crystals, which would lead to an ordered structure within the material.

The invention provides a polymer containing side groups capable of contributing, when aligned, both liquid crystalline and non linear electro-optical properties to the polymer.

In such a polymer the alignment required to give the polymer its non-linear electro-optical properties can be achieved because of the liquid crystalline properties which allow techniques conventionally used in the field of liquid crystals to be employed to give such alignment. The side groups are preferably attached rotatably (to allow alignment) to positions spaced along a polymeric chain to form a comb polymer. The rotatable connection may be provided by flexible spacers.

Each side group may contribute both the aforementioned liquid crystalline properties and the non-linear electro-optical properties.

In a preferred construction of the polymer, separate side groups are used to give the liquid crystalline properties and the non-linear electro-optical properties respectively. In such a polymer the latter align with the former due to dipole-dipole interaction, which is a well known effect used in guest-host liquid crystal displays.

By employing the invention it is possible to produce a non-linear electro-optical material which, because it is organic has the good non-linear electro-optical properties associated with organic materials, and which, because of its polymeric liquid crystal structure is readily formed into a desired shape such as an aligned thick film.

An electro-optical material made in accordance with this invention may be either a solid or a liquid at room temperature.

If it is a liquid at room temperature, it may readily be spread over a glass surface, to form an aligned thick film by standard techniques used in the liquid crystal display device industry.

If it is a solid at room temperature, it may be heated until it is a liquid, and then formed into an aligned thick film before being allowed to cool back to a solid.

One way in which the invention may be performed will now be described by way of example, with reference to the following diagrams in which: Figure 1 shows a polymer made in accordance with this invention.

Figure 2 illustrates a monomer which has liquid crystalline properties and is used in the production of the polymer of Fig. 1.

Figure 3 illustrates a monomer which has non-linear electro-optical properties and also is used in the production of the polymer of Fig. 1.

Figure 4 shows a first reaction occurring when making the non-linear electro-optical monomer of Fig. 3.

Figure 5 shows a second reaction occurring when making the non-linear electro-optical monomer of Fig. 3.

A comb polymer not necessarily carbon-based, as in the illustration, having non-linear electrooptical properties, made in accordance with this invention, is illustrated in Fig. 1. It comprises a polymeric chain 1, having two types of side groups attached periodically along it. The chain 1 does not have to be carbon based, it could, for example, be based on siloxane [+SiO. One of the side groups 2 is formed from a monomer having liquid crystalline properties, as shown in Fig. 2. The other side group 3 is formed from a monomer having non-linear electro-optical properties, as shown in Fig. 3.

To form a typical liquid crystalline monomer, as illustrated in Fig. 2, the following procedures are carried out: (i) 0.5 mol. (69g.) of 4-hydroxybenzoic acid is dissolved in a mixture of 180 ml ethanol and 75 ml. water, with the addition of 75g. of potassium hydroxide. A trace of potassium iodide is added, and the solution is heated and stirred under reflux while 0.55 mol. (759.) of c!)-chlorohex- anol is added slowly. The mixture is refluxed for about 18 hours, the solvent evaporated off and the solid residue is dissolved in 500 ml. of water. The aqueous solution is washed with ether (three 100 ml. extractions) and then acidified strongly with hydrochloric acid. The precipitate is filtered off and recrystallised from ethanol. The yield is about 50% of the theoretical yield. This procedure produces 4-(co-hydroxy hexyloxy) benzoic acid.

(ii) 0.25 mol. (59.5g) of the 4-(w-hydroxyhexyloxy) benzoic acid is mixed with 150 ml. acrylic acid, 150 ml. benzene, 10g. toluene sulphonic acid and 10 g. hydroquinone and refluxed for about 18 hours in a Dean & Stark apparatus until the calculated amount of water is collected (about 4.5 ml.). The cooled mixture is diluted with about 1 litre of ether, washed with warm water and finally dried over anhydrous sodium sulphate. The solvent is then removed and the product recrystallised from iso-propanol. The yield is about 60%. This produces 4-(co-propenoyloxy-hexyloxy) benzoic acid.

(iii) 0.06 mol. (18g) of 4-(a;-propenoyloxyhexyloxy) is mixed with 2 drops of dimethylformamide and a trace of 2,6-di-tert-butyl-4 methyl phenol and is dissolved in 20 ml. thionyl chloride.

The mixture is stirred for 30 minutes at room temperature, then excess thionyl chloride is removed at reduced pressure, first using a water pump and then under high vacuum, using a rotary pump and cold trap. The product is dissolved in 50 ml. of dry ether and is gradully added to a solution of 0.06 mol. (13.5g.) of 4-hydroxy-4'-methoxy biphenyl and 10 ml. triethylamine in 200 ml dry tetrahydroruan at 0-5"C. After 3 hours, the solvent is removed, and the solid residue dissolved in chloroform, washed with water and dried over anhydrous sodium carbonate, leaving a monomer.

(iv) This monomer is isolated from the chloroform solution and purified by column chromatography on Silica gel 60 (Fluka AG) (ie, proprietry silica gel) using methylene chloride as eluant.

The main zone from the column is isolated by evaporation of the solvent, and has phase transitions k 96 n 258 i. (ie, the crystal/nematic phase change occurs at 96"C, and the nematic/isotropic phase change occurs at 258 C).

The reactions described above, may be summarised as follows: (i) HO-C6H4-COOH+CI-(CH2)6-OH~HO-(CH2)6-O-C6H4-COOH (ii) CH2=CH-COOH+HO-(CH2)6-O-C6H4-COOH vCH2=CH-COO-(CH2)6-O-C6H4-COOH (iii) OH-C5H4-C6H4-OCH3+CH2--CH-COO-(CH2)6-O-C6H4-COOH wCH2=CH-COO-(CH2) 6-O-C6H4-COO-C6H4C6H4-OCH3 The resulting monomer, is a monomer of the type illustrated in Fig. 2.

Referring to Fig. 2, the molecules forming the liquid crystalline monomer referred to generally as 4, may be notionally divided into three groups in order to explain their functions. The group of molecules within the area defined by line 5 is a polymerisable group, and forms part of the polymeric chain. The group of molecules within the area defined by line 6 forms a flexible spacer, and allows movement within the polymer. The group of molecules within the area defined by line 7, is responsible for the liquid crystal characteristics of the monomer.

To form the non-linear electro-optical monomer, the following procedures are carried out.

(i) 0.5 mol (78g) 9-decen-1-ol is treated with 60g. phosphorus tribromide in 500 ml. dry ether at -40"C, for 1 hour in the presence of 50g. pyridine. The mixture is then poured into 1 litre of water at room temperature, shaken well and the ether layer is separated. Two further 100 ml.

ether extractions are made and the ether solution is then shaken with hydrochloric acid solution to extract excess pyridine. The ether extracts are finally washed with water, separated and dried over anhydrous sodium sulphate. The solvent is then removed by evaporation to give the final oily product.

0.2 mol. (449) of the product is mixed with 0.1 mol. (19g) 4-methyl picoline and heated at 120 C for 4 hours. The solid product which is N-(9-decen)4-methyl pridinium bromide is recrystallised from ethanol. The overall yield is about 60%.

(ii) 0.1 mol. (31 g) of the product obtained in (i) is mixed with 15g. p-hydroxy benzaldehyde in 400 ml. ethanol, and 59. of piperidine. The mixture is refluxed for 3 hours under nitrogen, to give a deep red solution, then some of the solvent is removed by distillation. 25 ml. 0.880 ammonia, is added to the concentrated solution, and the mixture is allowed to evaporate at room temperature in a fume cupboard. The crystalline product is redissolved in ethanol, filtered and the solution is allowed to crystallise again. The yield is about 60%, and the product is (1-(9 decen)-4-[4'-oxocyclhexadienylidene]-ethylidene- 1-4, dihydropyridene ie, 9-decenyl merocyanine.

The reactions described above, are summarised in Figs. 4 and 5. The resulting monomer, is of the type illustrated in Fig. 3.

Referring to Fig. 3, the molecules forming the non-linear electro-optical monomer, referred to generally as 8 may be notionally divided into three groups in order to explain their functions.

The group of molecules within the area defined by the line 9 is a polymerisable group, and forms part of the polymeric chain. The group within the area defined by the line 10 forms a flexible spacer, and the group within the area defined by the line 11 is responsible for the nonlinear electro-optical characteristics of the monomer.

The two monomers 4 and 8 are used for the production of the liquid crystal polymer, as described below. Because of the intense absorption of the merocyanine dye structure, a relatively low proportion of monomer 8 is used for practical co-polymers which are required for optical transmission. A recommended range of concentration is within the range 1 to 10% of monomer 8.

In a typical preparation to give a polymer with 3% of merocyanine side chains, a mixture of 5g. monomer 4 and 0.19. monomer 8 is dissolved in 50 ml. tetrahydrofuran. 1 mol. % azoisobutylnitrile is added as initiator and the polymerisation takes place at 55"C for 10 hours under nitrogen. The polymer is precipitated by pouring the final mixture into 150 ml. ether at 0 C. The product is redissolved in chloroform and again reprecipitated into cold ether, filtered off and dried in a vacuum oven at 50"C.

The resulting polymer is a solid and may be used in non-linear electro-optical applications. The illustrated embodiment of the invention is believed to constitute a considerable advance over known non-linear electro-optical materials since it has enhanced non-linear electro-optical properties, and, because of the inherent liquid crystalline properties of the polymer, an ordered thick film can be obtained by standard techniques used in the display device industry, to provide an aligned thick film.

The film may be formed by melting the polymer onto a substrate of glass or lithium niobate having a coating of silicon oxide which has been obliquely evaporated at an angle of 5 to the surface. Sufficient polymer is used to form a film which is 1 to two micrometers thick. The melting takes place at a temperature which is a few degrees above the glass to nematic transition point. The liquid crystal monomers align with the substrate surface orientation layer and cause the non-linear electro-optical monomers to also align. The polymer is then cooled gradually to below the glass transition temperature where the anisotropic structure is stable.

However, other electro-optical side chains may be used and there may be variations in the structure of the polymer, without departing from the scope of the invention. For example, instead of there being two types of side groups on the polymeric chain, one having liquid crystalline properties, and one having non-linear electro-optical properties, there may be just one type of side group having both liquid crystalline and non-linear electro-optical properties. Another variation in the structure of the polymer, would be to have more than two types of side groups, which jointly give the polymer the required characteristics.

Claims (3)

1. A liquid crystalline comb polymer comprising a polymeric chain with side groups attached periodically along it which side groups, in association with the chain, define a material which has liquid crystalline properties, as well as non-linear electro-optical properties.

2. A polymer containing both liquid crystalline and non-linear electro-optical side groups.

3. A polymer substantially as described with reference to Fig. 1 and substantially as illustrated therein.