Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2002-07-16
2004-12-14
Hampton Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S310000, C528S322000, C528S335000, C528S345000, C528S353000, C522S149000, C522S164000, C522S165000, C428S001260, C428S001270, C428S473500, C349S183000, C349S193000
Reexamination Certificate
active
06831148
ABSTRACT:
The present invention relates to new photoactive polymers based on polyimides, polyamic acids and esters thereof and their use as orientation layers for liquid crystals and in the construction of unstructured and structured optical elements and multi-layer systems
The successful functioning of a Liquid Crystal Device relies upon the ability of the LC molecules within that device to adopt and maintain an alignment imposed upon them. Alignment of the LC molecules is achieved by use of an orientation layer which defines a direction of orientation for the LC molecules of the device with the result that the longitudinal axes of the molecules become aligned with the direction of orientation defined by the orientation layer. In addition to this directional alignment, the orientation layer is also able to impart to the LC molecules an angle of tilt so that the molecules align themselves at an angle to the surface of the orientation layer rather than lying parallel hereto.
Tilt angles of between 1° and 15° are usual for Nematic LCDs. Some electro-optical effects used for liquid crystal displays (LCD) however require alignment layers with very high pretilt angles. Vertically aligned nematic (VAN) LCDs for instance require pretilt angles between 85° and 90°, measured from the surface plane. In the case of hybrid aligned nematic (HAN) LCDs, the pretilt angle at one of the substrates has to be in the above range, whereas the tilt angle at the other substrate is low (typically 0-10°).
Methods of preparing structured and unstructured orientation layers are well known to a skilled person. Customarily used uniaxially rubbed polymer orientation layers such as, for example, polyimides however impact a series of disadvantages like dust generation during rubbing process, destruction of thin film transistors and lack of structuring. The rubbing process consequently does not allow the production of structured layers Orientation layers in which the direction of orientation can be predetermined by irradiation with polarised light have been known for some time. It is by that means possible to avoid the problems inherent in the rubbing process In addition, it is possible to provide areas having different orientation and thus to structure the orientation layer as described for example in
Jpn. J. Appl. Phys
. 31 (1992), 2155-64 (Schadt et al.). In that process the dimerisation of polymer-bonded photoreactive cinnamic acid groups induced by irradiation with linearly polarised light is employed leading to an anisotropic polymer network. Those photo-oriented polymer networks can be used wherever structured or unstructured liquid crystal orientation layers are required. In addition to their use in LCDs, such orientation layers can also be used, for example, in the production of so-called hybrid layers as exemplified in European Patent Applications EP-A-0611981, EP-A-0689084 and EP-A-0753785 (all F. Hoffmann-La Roche A G). Using those hybrid layers of photostructured orientation polymers and crosslinkable low molecular weight liquid crystals it is possible to realise optical elements such as, for example, non-absorptive colour filters, linear and circular polarisers, optical delay layers and so on.
EP-A-0611786 and WO-A-96/10049 (both F. Hoffmann-La Roche A G) as well as EP-A-0763552 (Rolic A G), describe cinnamic acid polymers that are suitable in principle for the synthesis of such anisotropically crosslinked, photostructured orientation layers for liquid crystals. In the case of the compounds described in EP-A-0763552 and WO-A-96/10049, on irradiation with linearly polarised light it is possible, in addition to inducing the desired orientation, simultaneously to induce an angle of tilt. It is thus possible to produce layers having structuring in respect of surface orientation and angle of tilt.
The above photostructured orientation layers have the disadvantage, however, that for certain applications, especially for use in TFT displays, they result in adjacent liquid crystal mixture having an insufficient electrical resistance value. In TFT displays, too low a resistance value of the liquid crystal medium leads to an inadequate “holding ratio”, which is a measure of the voltage drop in the display after the voltage has been switched off. Low holding ratio values, however, bring about undesirable changes in brightness and contrast over time and thus result in unstable graduations of the grey tones.
Recently photoreactive materials for orientation layers with improved holding ratios were described in WO-A-99/49360 (Rolic A G), JP-A-10-195296, JP-A-10-232400 (both Samsung Electron Devices Co., Ltd), WO-A-99/15576 (Rolic AG) and WO-A-99/51662 (Kanegafuchi Kagaku Kogyo K K). In WO-A-99/49360, JP-A-10-195296 and JP-A-10-232400 blends of polymeric compounds containing photoreactive polymers on the one hand and polyimides on the other hand are proposed. A disadvantage of such blends is their limited miscibility. Low contents of photoreactive polymers however lead to a loss of orienting properties and consequently to a reduced contrast ratio of liquid crystal layers to be oriented whereas a reduced polyimide content results in insufficient holding ratios. In WO-A-99/15576 and WO-A-99/51662 polyimides incorporating photoreactive cinnamate groups in their side chains are described. WO-A-99/15576 discloses photoactive polymers which contain as side-chains photocrosslinkable groups of the following formula:
A typical monomer unit is 3,5-diaminobenzoic acid 6-[2-methoxy-4-(2-methoxycarbonylvinyl)phenoxy]hexyl ester:
The cinnamic acid derivatives disclosed in WO-A-99/15576 are linked to the polyimide backbone such that the photoreactive groups are pointing away from the backbone.
WO-A-99/51662 discloses photoactive polymers having a cinnamic skeletal structure. A typical monomer disclosed is of the following formula:
The polyimide compositions are said to combine the photoreactivity and thermal reactivity characteristic of the cinnamic acid skeletal structure. There is no teaching of the improvement of the orientation of liquid crystals.
Consequently stable photoalignable materials for high pretilt angles with sufficiently high holding ratios are not known so far. The problem underlying the invention was, therefore, to find photoreactive polymers that, when irradiated with polarised light, result in the production of stable, high-resolution orientation patterns having very high angle of tilt and at the same time result in sufficiently high holding ratios in the adjacent liquid crystal medium.
Surprisingly, it has now been found that polyimides, incorporating cinnamic acid derivatives in their side chains in such a way, that the cinnamic acid groups are linked to the polyimide backbone via the carboxylic group by means of a flexible spacer, perfectly fulfil the above requirements. The illumination of those compounds, using linearly polarised light, results in excellent orientation of the liquid crystals, in a sufficiently high holding ratio of the liquid crystal medium and simultaneously in an appreciable increase of the tilt angle up to 90°.
A first aspect of the present invention therefore provides photoactive polymers from the class of polyimides polyamide acids and esters thereof, characterised in that they comprise as side-chains photocrosslinkable groups of the general formula I:
wherein the broken line indicates the point of linkage to the polyimide main chain and wherein:
A represents pyrimidine-2,5-diyl, pyridine-2,5-diyl, 2,5-thiophenylene, 2,5-furanylene, 1,4- or 2,6-naphthylene, or phenylene; which is optionally substituted by a group selected from fluorine, chlorine, cyano or by a C
1-18
cyclic, straight-chain or branched alkyl residue, which is optionally substituted by a single cyano group or by one or more halogen atoms and in which one or more non-adjacent alkyl —CH
2
— groups are optionally replaced by a group Q;
B is a straight-chain or branched alkyl residue which is unsubstituted, mono-substituted by cyano or halogeno, or poly-substituted by halogeno, having 3 to 18 carbon atoms, wherei
Buchecker Richard
Marck Guy
Muller Olivier
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hampton Hightower P.
Rolic AG
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