DE2548925A1 - METHOD OF ALIGNMENT OF LIQUID CRYSTALS - Google Patents

Description

Dipl.-Ing. Egon Prince Dr. Gertrud Hauser D - eooo Manchen 6o, October 30, 1975 Dipl.-Ing. Gottfried Leiser Em, br _g . "T, _O B. 19

Patents 2 54892 5

Telegram ": labyrinth Munich

Telephone: 8315 10

Telex: 5212 226 pthl d Postschedckoitto: Munich 117078-800 Bank: Deubdie Bank, Munich 66A) SOOO

THOMSON - CSP

173, vol. Haussmann

75008 PARIS / France

Our reference: Q? 1874

Process for aligning liquid crystals

The invention relates to methods for aligning liquid crystals and their application to "helical nematic" type display cells. As is well known, in the display cells have a liquid crystal film between the flat and parallel surfaces of two transparent ones Electrodes attached. If it is a liquid crystal in mesomorphic phase with nematic or smectic Structure, you can. the elongated molecules of the liquid running parallel to the electrode planes Give orientations in the case of the so-called "planar" orientation. In the case of the helical structure, which represents a special case of planar alignment, the molecular axes remain perpendicular to a common one Direction, however, appear for one in this

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Direction looking observer as regularly displaced in a predetermined sense. This gives structure the liquid crystal has an easily observable ability to rotate in polarized light. This ability to rotate disappears if you apply an electrical pressure to the structure, which in the case of liquid crystals with positive dielectric anisotropy runs along it which must be oriented perpendicular to the molecular axes.

In the following, the side with a transparent conductive layer is used as the inner surface of the electrodes (e.g. made of tin oxide) covered thin glass plate, which is covered with the liquid crystal when assembling the Cell should come into contact.

Various methods are known to give the molecules of a liquid crystal a more or less permanent joint Alignment parallel to the electrodes impart. In particular, the following are mentioned:

Previously rubbing the inner surface of the electrode to create scratches or grooves parallel to a common direction; To improve the stability you have to use an abrasive (especially diamond with a binding agent), which leaves traces which can partly destroy the conductive layer of the thin glass plate during cleaning of the electrodes by means of ultrasound. There are also friction methods that do not produce scratches; However, then the orientation of the molecules due to material transfer the friction body, for example is a sulfonate-treated paper piece, _v on the rubbed surface

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causes. An acceptable result can only be achieved with frequent, e.g. 100 to 200, rubbing of the Surface.

The vacuum deposition of a thin silicon oxide (Si O) layer while maintaining a very small angle of incidence; this second, technically very difficult to implement Method not only requires expensive apparatus, but also does not give a very stable alignment of the molecules of liquid crystals.

The invention overcomes the disadvantages of those described above Methods.

The principle of the method according to the invention is based on two essential points that distinguish the process from the known:

1. Use of organic polymers with functional groups that have a parallel alignment of the Molecules of the liquid crystal can cause, these polymers being insoluble in the liquid crystal are and are completely wetted by this; that allows, starting from one on the surface an electrode spread out polymer layer, the achievement of an even distribution of the molecules of the liquid crystal in a layer adjacent to the electrode;

"2. Alignment of the molecules of the polymer along a common direction in the plane of the polymer layer by a mechanical treatment that results in a few, even a single, straight-line displacements of a soft rubbing body is limited, causing an orientation of the molecules of the liquid crystal in a the layer adjacent to the electrode ensured.

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The method according to the invention for aligning liquid crystals is characterized by at least the two following procedural steps:

a) Deposition of an organic polymer layer on the inner surface of the liquid crystal cells certain electrodes, the organic polymer having the following three properties:

- Insolubility in the actual liquid crystal;

- complete wettability by the liquid crystal;

- the ability to bring about a parallel alignment of the molecules of the liquid crystal

b) rubbing the layer with a body sliding thereon without scratching the layer, moving in a predetermined direction.

The following is the principle of the known methods for applying a polymer layer to a substrate repeated.

1. Deposition by electrical discharge in a gas phase

In this method, a plasma is generated from monomer vapor at very low pressure in a device two metal plates between which the electrical discharge potential is applied. The ions of the plasma preferentially deposit on one of the plates: on

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the electrode of the cell to be treated is attached to this plate. Under these conditions the deposited exists Layer of long-chain molecules (polymer formed "in situ") whose axis is perpendicular to the electrode surface runs.

This separation method is particularly common in the German Offenlegungsschrift 24 49 652 described. In the present invention, the orientation of the molecules of the layer during stage "b" is modified so that one finally has an alignment parallel to the electrode plane receives.

2. Separation by the thermal effect from the gas phase

In this method, a monomeric vapor stream is established onto a substrate placed in a constant temperature oven. The steam flow should be "laminar", i.e. have a uniform velocity and constant incidence on the substrate surface. The monomer polymerizes as a result of the action of heat on contact with the substrate.

This separation method is described in ER-OS 73.46955. In this case, the incidence of the steam flow is grazing. In the present invention the idea can be different, since in the course of stage "b" the orientation of the molecules is modified in such a way that it runs parallel to the electrode plane.

3. Deposition by transferring a homogeneous thin film onto the substrate, followed by thermal polymerization

It is known to apply a thin and homogeneous (e.g.

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5 "to 50 Angstroms thick) layer of one in the liquid to form insoluble monomers or prepolymer (weakly polymerized polymer). Such a layer can be transferred to the two main surfaces of a substrate previously immersed under the layer. To this For this purpose, the substrate is very slowly (10 cm / sec.) Across the layer while taking the same (which the Substrate wetted) "pulled". For this purpose one uses a mechanical device with a slow and regular Piston lowering system (thanks to a viscous liquid acting as a buffer), which in the Anglo-Saxon Literature is referred to as a "dash-pot". In such a device the substrate actually remains stationary, while the tub is slowly lowered so that a process equivalent to pulling takes place. TJm des To be safe transition of a layer with a homogeneous thickness, the device should by a so-called "floating barrier" · System to be completed, which a constant lateral pressure on the water surface Layer exerts and prevents expansion of the same during "pulling".

After the transfer, the film, i.e. the layer of the monomer or the prepolymer, becomes effective polymerized by heat in an oven.

4 · Deposition by drawing a substrate immersed in a solution of prepolymer

This method, which is analogous to the method described above, "in which a device of the" dash pot "type is also used for Application enables the production of thick layers (several thousand angstroms).

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It is followed by a thermal polymerization, which increases the degree of polymerization of the layer deposited on the substrate.

The polymers mentioned in the examples below can be deposited by at least one of the methods described above. Because of their chemical Nature becomes its parallel to the separation area Orientation favors. This alignment is through one or more rubbing (in principle, the alignment takes place with a single transition of the sliding body) causes, whereby the alignment takes place in a predetermined direction and one rubs the layer with a body, either with so-called "optical quality" paper (such as is used to clean optical glasses) or covered with very soft fabric, rubs.

In addition, the polymers mentioned are in the liquid crystal insoluble, which is practically guaranteed when a certain degree of polymerization is exceeded: The The number of monomeric molecules linked on average in the chains of the polymer exceeds, for example 100.

Finally, the polymers mentioned remain transparent regardless of their degree of polymerization.

example 1

The monomer is paraxylene, the usable polymer of which is sold under the trade name "Parylene".

The second method of deposition is used (polymerization in gas phase due to the effect of heat).

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The thickness of the layer can reach several microns.

The molecules are aligned with a friction body that exerts a pressure of around 50 g / cm.

Excellent results are obtained with the following liquid crystal: 4-pentyl-4'-cyanobiphenyl.

A "helical nematic" type display cell is obtained by making two depositions (one on each electrode) and aligning the molecules in two perpendicular directions.

Example 2

The monomer is methyl methacrylate, which after the first above-mentioned method is polymerized (discharge in gas phase).

The thickness of the layer reaches 500 angstroms.

Excellent results are obtained using the same method of aligning the molecules and under Use of the same plush crystal as in the first one Example. So-called "helical" type cells are made in the same way.

Example 3

The monomer is methylsiloxane, which is deposited by the third method (transfer to the substrate 4by pulling a! Film).

The polymerization takes place by heat treatment at 100 ° 0 for one hour.

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This method gave a layer thickness of 50 angstroms.

The results obtained are when the same treatments are applied as in the first example and "when used." of the same liquid crystals excellent.

Example 4

A solution of polystyrene in divinylbenzene is used and the polymer is separated according to method no. 4 away.

The thickness of the layer reaches a few thousand * angstroms.

With the same treatment and use of the same Liquid crystals as in the first example give excellent results.

Example 5

A polymer of gamma-glyeidoxy-propyltrimethoxysilane is used: CH ₂ O = CH - CH ₂ O - CH ₂ - CH ₂ - CH ₂ -Si (0

A thin layer (50 to 500 Angstroms) of organic polymer, e.g. by the following method:

The monomer is previously in a mixture of methanol and water (4 volumes of methanol to 1 volume of water) in a ratio of 1 to 200 parts by weight. Of the The pH of the solution is brought to 4.0 by the gradual addition of acetic acid. The latter catalyzes the hydrolysis the methoxy groups of the monomer.

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The ones to be treated, intended for pluid-crystal cells Electrodes are then immersed in the above solution and held for about an hour. then they are pulled out and then dried for two hours at a temperature of 80 ° C under vacuum. The polymerization is finally completed by leaving the electrodes under a neutral atmosphere for one hour keeps in a 120 ° C oven.

The organic polymer thus obtained is in the usual organic solvents and especially insoluble in a number of crystalline liquids. this applies to the biphenyls, the ship's view bases Azoxy compounds and the esters.

The thin polymer layer has good wettability, which is an indispensable condition for the major axis of the liquid crystal molecules to be parallel align with the main surfaces of the electrodes. The measured critical surface tension is approximately 42 dynes / cm, which is higher than the critical surface tension of numerous liquid crystals (30 dynes / cm).

For example, excellent results have been obtained, particularly in the case of the "helical display cells" nematic "type with the following liquid crystals:

Biphenyls, para-nitriles ;

under the trade name "PRIlTOEiEOlT LG 360" as Liquid crystal product sold. -

In addition to the advantages indicated above, the invention enables the simplified production of nematic

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helical structures; if you have a 3Liquid crystal droplets between two in vertical Directionally oriented electrodes includes, one receives immediately and in a permanent manner for this Cell type characteristic arrangement in the form of a screw.

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Claims (1)

Claims yMethod for aligning liquid crystals, marked through at least the following two process stages: a) Deposition of a layer of organic polymer on the inner surface of the electrodes for liquid crystal cells, where the organic polymer has the following three properties: Insolubility in the actual liquid crystal; complete wettability by the liquid crystal; the ability to align the molecules of the liquid crystal in parallel through the polymerizatraolecules cause, b) rubbing the layer in a predetermined direction with a body that glides scratch-free on the layer. 2. The method according to claim 1, characterized in that the rubbing in step b) is based on a single displacement of the Body b. Is restricted. 3. The method according to claim 1, characterized in that the organic polymer by polymerization of one of the the following monomers is obtained: Para-xylene,
Methyl methacrylate,
Methylsiloxane. 0-9 820/0763 4 ·. Method according to claim 1, characterized in that the organic polymer is a copolymers of styrene and divinylbenzene. 5. The method according to claim 1, characterized in that the smoothing under a constant pressure of some tens of g / cm takes place and that the displacement of the sliding body is a few centimeters. 6. The method according to claim 1, characterized in that the organic polymer consists of vaporous monomers is formed by generating an electrical discharge. 7. The method according to claim 1, characterized in that the organic polymer is formed by applying a stream of monomeric vapor to the placed in an oven Electrode aligns. 8. The method according to claim 1, characterized in that for the deposition of the organic polymer on the Electrode on this one on a liquid surface formed monomers is transferred, which subsequently is polymerized by heat treatment. 9. The method according to claim 1, characterized in that the deposit on the electrode is formed by immersing the electrode in a solution of prepolymer and subsequent heat polymerization of the layer obtained . 10 · The method according to claim 1, characterized in that the deposition of a layer of organic polymer on an electrode to be treated by 609 820/07 63 2548975 a) Preparation of a solution of gamma-grlycidoxy-propyltriraethoxysilane in a mixture of methanol and water, gradually increasing the pH of the Solution; b) immersing the electrode to be treated in the solution and vacuum drying at 80 ° C; c) The electrode dried in this way is heated in a drying oven at 120 ° C. for one hour. 11. A liquid crystal device characterized in that it has been aligned according to any one of claims 1 to 10. 12. Obtained by the method according to one of claims 1 to 10, with a nematic helical ■ liquid crystal operated display cell. 609820/0763