Photocrosslinkable polyimides

Details Photocrosslinkable polyimides Photocrosslinkable polyimides

2000-03-23

2002-01-22

Berman, Susan W. (Department: 1711)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

Alignment layer of specified composition

C428S001270, C430S020000, C430S287100, C349S183000, C349S193000, C522S149000, C522S164000, C528S335000, C528S345000

Reexamination Certificate

active

06340506

ABSTRACT:

The invention relates to novel crosslinkable, photoactive polymers from the class of polyimides, polyamide acids and esters thereof, and to their use as orientation layers for liquid crystals and in the construction of unstructured and structured optical elements and multi-layer systems.
In (electro-optical) liquid crystal devices the orientation layer is of particular importance. It serves to ensure a uniform and trouble-free alignment of the longitudinal axes of the molecules.
For the orientation of liquid crystal molecules in liquid crystal displays (LCDs) it is customary to use uniaxially rubbed polymer orientation layers, such as, for example, polyimide. The direction of rubbing in that process gives the direction of orientation. Especially for use in LCDs, however, in addition to giving the direction of orientation it is also necessary for an angle of tilt to be imparted by the orientation layer. That angle can be produced on the mentioned polymer surfaces likewise by the rubbing process. When a liquid crystal is brought into contact with such a surface, the liquid crystal molecules do not lie parallel to the surface but at an angle thereto, that is to say the angle of tilt is transferred to the liquid crystal. The size of the angle of tilt is determined both by rubbing parameters, such as, for example, rate of advance and contact pressure, and by the chemical structure of the polymer. For the production of liquid crystal displays, angles of tilt of between 1° and 15° are required, depending upon the type of display. The larger angles of tilt are required especially for supertwisted nematic (STN) LCDs in order to avoid the formation of so-called fingerprint textures. In TN and TFT-TN LCDs, the direction of rotation and of tilting is defined by the angle of tilt, so that “reverse twist” and “reverse tilt” phenomena are prevented. Whereas, in the switched-off state, reverse twist results in areas having the incorrect direction of rotation (which manifests itself optically in the display having a spotty appearance), reverse tilt becomes particularly apparent when the LCD is switched on, causing very troublesome optical effects as a result of the liquid crystals tilting in different directions. Reverse twist can be prevented by doping the liquid crystal mixture with a chiral doping agent. To suppress reverse tilt there has hitherto been no alternative to the use of orientation layers having an angle of tilt.
The above-described orientation by rubbing is associated with some serious disadvantages, however, that can have a marked effect on the optical quality of the liquid crystal displays. For example, the rubbing process produces dust which can result in optical defects in the display. At the same time the polymer layer becomes electro-statically charged, which may, for example in the case of thin film transistor.(TFT)-TN LCDs, result in the destruction of the underlying thin layer transistors. For those reasons the yield of displays that are free of optical defects has not been optimum in LCD production hitherto.
A further disadvantage of the rubbing process is that it is not possible to produce structured orientation layers in a simple manner, since local variation of the direction of orientation is not possible in the rubbing process. Rubbing is therefore used chiefly to produce layers having uniform alignment over a large area. Structured orientation layers are of great interest, however, in many areas of display technology and integrated optics. For example, they can be used in improving the viewing angle dependency of twisted nematic (TN) LCDs.
To improve viewing angle dependency,. LCDs having pixels that alternate in respect of angle of tilt were proposed some time ago (e.g. EP-A-0 683 418). That proposal was based on the realisation that the angles of tilt of polyimide layers change at sites which have been irradiated beforehand with laser light. That change is based on the destruction of some of the imide bonds at the site of irradiation, for which purpose comparatively short wavelengths and high light intensities are required. Using that procedure it is possible in principle to produce orientation layers having domains with different angles of tilt. The disadvantages lie, however, in the high irradiation energy required for the purpose and the comparatively long irradiation time. In addition, the orientation of the longitudinal axes of the molecules in the plane of the layer is produced, as before, by rubbing, so that the above-mentioned problems arising in the rubbing process cannot be eliminated. In particular, that procedure cannot be used to produce structured surface orientations.
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 directions of orientation and thus to structure the orientation layer.
One possible method for the structured orientation of liquid crystals utilises the isomerisation capacity of certain dye molecules to induce a preferred direction photo-chemically by irradiation with polarised light of a suitable wavelength. This is achieved, for example, by mixing a dye into an orientation polymer, which is then irradiated with polarised light. Such a guest/host system is described, for example, in U.S. Pat. No. 4,974,941. In that system, azobenzenes are mixed into polyimide orientation layers and then irradiated with polarised light. Liquid crystals that are in contact with the surface of a layer so irradiated become oriented in accordance with that preferred direction. That orientation process is reversible, that is to say by further irradiation of the layer with light of a second direction of polarisation it is possible for the direction of orientation that has already been established to be changed round again. Since that reorientation process can be repeated as often as desired, orientation layers on that basis are less suitable for use in LCDs.
Another possible method of producing high-resolution orientation patterns in liquid crystalline layers is described in
Jpn. J. Appl. Phys.
, Vol. 31 (1992), 2155. In that process the dimerisation of polymer-bonded photoreactive cinnamic acid groups induced by irradiation with linearly polarised light is employed for the structured orientation of liquid crystals. Unlike the reversible orientation procedure described above, in the case of the photostructurable orientation layers described in
Jpn. J. Appl. Phys.
, Vol. 31 (1992), 2155 an anisotropic polymer network is synthesised. 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-0 611 981, EP-A-0 689 084, EP-A-0 689 065 and EP-A-0 753 785. 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-0 611 786, EP-A-0 763 552 and WO-A-96/10049 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-0 763 552 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 orientation layers formed from the polymers described in EP-A-0 611 786, EP-A-0 763 552 and WO-A-96/10049 have the disadvantage, however, that for certain applications, especially

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