Polymers for liquid crystal alignment

Details Polymers for liquid crystal alignment Polymers for liquid crystal alignment

1998-02-12

1999-07-27

Wu, Shean C.

Compositions

Liquid crystal compositions

Containing nonchiral aligning agents

349123, 349127, 349191, 428 1, C09K 1956, G02F 11337

Patent

active

059285613

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the alignment of liquid crystals.
In order to exploit certain properties of liquid crystals in some of the numerous devices in which they are used, alignment of the liquid crystal molecules is necessary (for example see J Appl. Phys 74 3111 (1993)).
For example, in a liquid crystal cell containing a twisted nematic or super twisted nematic material, the material is located between two substrates and alignment of the liquid crystal molecules at the surface of each substrate is necessary.
A thin alignment layer which may be organic (eg a polymer) or inorganic (e.g. SiO) is deposited on each surface. After deposition, alignment of an organic layer may be achieved by unidirectional rubbing of the layer using textile or cloth. Alignment of an inorganic layer such as SiO may be achieved by a suitable choice of evaporation direction.
The substrates may be coated with polyimide by a spinner and then baked to form a cured layer of approximately 50 nm thickness. Each layer is then rubbed, substantially unidirectionally, with suitable material and when liquid crystal material is introduced, the molecules in the vicinity of each layer are aligned in the direction of rubbing. Often it is preferable for the molecules to possess a small angle pre-tilt, typically 2-3.degree.. Sometimes higher pre-tilts are required.
Whilst proving reasonably effective in the alignment of liquid crystal materials, the above method is known to introduce mechanical and electrostatic damage to the active layers. A method of aligning the liquid crystal which does not involve physical contact with the active layers is desirable.
The use of poly(vinyl cinnamate) coatings to effect alignment of liquid crystal materials is documented in, for example, Jpn J Appl Phys 31 2155 (1992). Substrates are coated with the polymer and then exposed to plane polarised ultraviolet light. This provides for alignment of liquid crystal material in a direction substantially at right angles to the direction of polarisation of the light.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is reaction scheme for the synthesis of the present polymer;
FIG. 2 is a schematic device of the invention.


DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to this invention, a method of aligning liquid crystal material comprises the steps of: coating formula I: ##STR2## where Y=(CH.sub.2).sub.n, wherein non adjacent CH.sub.2 groups may be replaced by O, CO.sub.2, OCO, OCO.sub.2, or phenylene;
n may have any integral value from 1 to 16;
An=the anthracenyl group: ##STR3## to which Y and X may be connected to at any point: each X is independently selected from halogen, alkyl, alkoxy, CO.sub.2 R, (where R=alkyl, alkenyl, aryl), CN or NO.sub.2 and
Typically the polymers would have the repeat unit: ##STR4## but this is not intended as limiting: for example, other side groups or chains may be present in the polymer (eg the polymer may be a copolymer).
In a preferred embodiment of the invention, the polymer has the repeat unit: ##STR5##
According to a second aspect of the invention, a liquid crystal cell is provided which is characterised by the liquid crystal material contained therein being aligned by the method of the first aspect of the invention.
The primary extinction band in the anthracene derivatives lies close to the 254 nm band available from cheap, easily obtained, mercury discharge lamps. Therefore an economical light source can be used, with high efficiency, leading to short exposure times and consequently to fast throughput in production and low unit costs. The absorption peak in PVCi is very weak at these standard wavelengths, leading either to the use of long exposure times and high production costs or to the use of expensive non-standard lamps to accomplish the photochemical crosslinking.
The short wavelength of the anthracene extinction band also leads to very good photochemical stability of cells made using this alignment agent. Standard LC displays are equipped with a polarising filter which has very high extinction (in all polarisations) at

REFERENCES:
patent: 5298590 (1994-03-01), Isogai et al.
patent: 5350539 (1994-09-01), Mishina et al.
patent: 5443864 (1995-08-01), Takimoto et al.
patent: 5538823 (1996-07-01), Park et al.
WPIDS 92-069187, 1992.
Japanese Journal of Applied Physics, vol. 31, No. 7 Part 01, Jul. 1, 1992, pp. 2155-2164, XP000371722 Schadt M et al: "Surface-Induced Parallel Alignment of Liquid Crystal by Linearly Polymerized Photopolymers" cited in the application see whole document.

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