Photoactive pentaerythritol derivatives and orientation layers

Details Photoactive pentaerythritol derivatives and orientation layers Photoactive pentaerythritol derivatives and orientation layers

2001-11-21

2004-02-17

Pyon, Harold (Department: 1772)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

Alignment layer of specified composition

C349S124000, C524S380000, C528S424000

Reexamination Certificate

active

06692797

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention describes materials and methods for achieving alignment of liquid crystal materials on a substrate surface and devices fabricated using these methods and materials.
Liquid crystal display devices (LCDs) or light shutters generally comprise a layer of liquid crystalline material between two solid substrates to form a cell. These substrates are generally coated with a conducting material, such as Indium/Tin Oxide (ITO) to form electrodes or electrode patterns. An electric field applied across the cell or between the electrodes switches the liquid crystal between different molecular arrangements or states. Thus the light transmission through the cell can be modulated depending on the cell configuration, the type of liquid crystalline material, the presence of polarisers, etc. A preferred molecular alignment direction and pretilt angle (&thgr;) is imparted by an alignment layer on top of the electrodes and in contact with the liquid crystalline material.
It is well known in the art that fabrication of liquid crystal devices which have advantageous performance and low defect densities requires control of the alignment of the liquid crystalline material at the surfaces of the device. Different types of liquid crystal alignment have been described. Homeotropic alignment refers to an alignment in which the unique optical axis of a liquid crystal phase is held perpendicular to the adjacent surface.
Planar alignment, sometimes referred to as homogeneous alignment, refers to alignment in which the unique optic axis of the liquid crystal phase lies parallel to the adjacent surface. Planar alignment may also impose a direction in which the optic axis of the liquid crystal lies in the plane of the adjacent surface.
Tilted planar alignment or tilted homogeneous alignment refer to alignment in which the liquid crystal unique optic axis lies at an angle, termed the pretilt angle (&thgr;) from the plane of the adjacent surface. The pretilt angle may be as small as a fraction of one degree or as large as several tens of degrees.
Tilted homeotropic alignment refers to an alignment in which the optic axis of the liquid crystal lies tilted away from the normal to the adjacent surface. This deviation is again termed a pretilt angle.
In liquid crystal devices, said alignment geometries are chosen and used in combination to achieve specific optical and electro-optic properties from the device and may be combined in new ways or with new liquid crystalline mixtures to provide new types of devices.
Several methods are known in the art by which defined liquid crystal alignment may be achieved. Deposition of a polymer layer, for example a polyimide layer, on the substrate surface followed by mechanical rubbing provides a pretilted planar alignment. A planar alignment or tilted planar alignment may also be achieved by evaporating a variety of inorganic substances, for example SiO
x
, onto the surface from an oblique angle of incidence. A disadvantage of this method is that it requires slow and costly vacuum processing. A further disadvantage is that the resulting evaporated layer may show a high capacity to absorb contaminants onto itself from the environment or from other materials used in fabrication of the device.
A homeotropic alignment can be obtained by depositing a surfactant, for example a quatemary ammonium salt, onto the surface from solution in a suitable solvent. A disadvantage of this treatment is that the resistivity of the liquid crystal device may be lowered by the surfactant and the resulting alignment may also show poor stability.
Structured alignment patterns of subpixel size and above can be achieved by illumination of a polymer layer containing photochemically orientable dyes or photochemically dimerisable and/or isomerisable molecules, as described, for example, in EP-A-0445629. A disadvantage of this method is that the solubility of the dye molecules in the polymer matrix is limited and the chemical and photochemical stability over time is insufficient.
Another method for achieving structured non-contact orientation is the photodimerisation of polymers incorporating photodimerisable groups, such as cinnamate or coumarin derivatives, as described, for example, in Jpn. J. Appl. Phys., Vol., 31, 2155 (1995) and EP-A-9410699.0. A disadvantage of these materials is the polydispersity of the materials produced by polymerisation. This requires, for example, different solution concentrations for spin coating depending on the average molecular weights of the polymers which can not be determined with any great accuracy and which are often not reproducible from one batch to another. This can give rise to unreproducible alignment as well as also requiring repeated purification cycles of the polymer product in order to remove unreacted monomer and oligomers. The attachment of low molar mass photoreactive units to monodispersed polymer backbones can lead to polymers with unreacted sites, which can give rise to dielectric breakdown of cells containing such materials. This is especially important for active matrix devices.
An object of this invention is to provide means of achieving a defined surface alignment of a liquid crystalline material on a substrate surface, which does not require mechanical rubbing or other methods of physical contact which may damage the surface or structures on the surface. This is especially important for active matrix displays based on the use of surface mounted thin film transistors. Static electricity or dust caused by mechanical rubbing or buffing polymer layers, such as polyimide or polyamide, in order to induce a unidirectional alignment due to microgrooves can cause defects in thin film transistors and lead to dielectric breakdown. Such alignment layers also suffer from the disadvantage that the microgrooves possess inherent defects themselves, which can result in random phase distortion and light scattering. This impacts detrimentally on the optical appearance of the displays or the efficiency of the light shutters. Additionally, mechanical buffing does not allow locally oriented regions of the surface to be aligned with different azimuthal angles. This is a substantial drawback since sub-pixelisation can lead to higher contrast and an improved optical efficiency.
SUMMARY OF THE INVENTION
According to this invention materials are provided of Formula I:
where
X
1-8
are each independently selected from: H, halogen, CN, OH, straight or branched chain alkyl having from 1 to 16 carbon atoms, where one or more non-adjacent CH
2
groups may be substituted by CH(CN), CH(CF
3
), CHF, CHCl, CH(CH
3
);
S
1-8
are spacer units;
PG
1-4
are photopolymerisable/dimerisable groups
m
1
, m
2
, m
3
, and m
4
are each independently selected from the integers 1 and 0;
A
1-8
are each independently selected from the aromatic rings:
where ~ indicates a sigma bond between part of the molecule shown in formula I and a carbon atom at any position in one of the aromatic rings;
and where the CH groups present in the aromatic rings may each be independently substituted by C(CN), C(CF
3
), C-halogen, C(CH
3
), CR, where R is selected from straight or branched chain alkyl and may include from 1 to 8 carbon atoms and including where one or more non-adjacent CH
2
groups may be substituted by CH(CN), CH(CF
3
), CHF, CHCl, CH(CH
3
).
Preferably the spacer groups S
1-4
are, independently of one another, selected from groups having the general formula:
L
1
—(CH
2
)
n
—L
2
where: n=1 to 30, where each CH
2
group present in the chain linking L
1
and L
2
may be independently substituted by CH(CN), CH(CF
3
), CHF, CHCl, CH(CH
3
), L
1
and L
2
are independently selected from: single covalent bond, O, COO, OOC, CH
2
O, and OCH
2
. More preferably S
1-4
are independently selected from oxycarbonylalkanoyloxy, oxyalkoxy, oxycarbonylalkoxy, oxyalkanoyloxy, oxycarbonylphenoxyalkanoyloxy, oxyalkoxyalkyl containing from 1-16 carbon atoms.
In a preferred embodiment spacer groups S
5-8
are each independently selected from: COO, OOC, C≡C,

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