Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1997-05-23
2001-01-09
Wu, Shean C. (Department: 1756)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C252S299500, C252S299610, C349S036000, C349S176000, C349S182000
Reexamination Certificate
active
06172720
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a chiral nematic liquid crystal material for use in a display device and includes an additive that lowers the viscosity of the material.
BACKGROUND OF THE INVENTION
Stabilized cholesteric liquid crystals have high viscosity due to the presence of a large amount of chiral dopants and a corresponding strong twisting nature inherent in the chiral material. High viscosity, however, can undesirably increase the response time of these materials when used in electrooptic devices.
Materials have been added to nematic liquid crystals for reducing their viscosity. Nematic liquid crystal materials have drastically different properties than cholesteric liquid crystal materials. For example, cholesteric materials exhibit weakly scattering focal conic and reflecting twisted planar states which are stable in the absence of a field. One cannot predict the effect that additives for lowering the viscosity of nematic liquid crystals may have in a chiral nematic liquid crystal material.
The textures of chiral nematic materials make predicting their behavior difficult compared to nematics. The liquid crystal molecules of chiral nematic materials have directors that are helically oriented, which makes it difficult to conduct experiments such as measurements of dielectric constant and transmission. In view of the difficulty in measuring properties of cholesteric liquid crystal materials and the unpredictability in formulating compositions of such materials, the need remains for additives that can lower the viscosity of a chiral nematic liquid crystal material for use in a display device.
SUMMARY OF THE INVENTION
The present invention is directed in general to a liquid crystal material for a display device which comprises a first chiral nematic liquid crystal component and a second component that exhibits no liquid crystalline phase at any temperature. The second component is present in an amount effective to reduce the bulk viscosity of the material at room temperature by at least about 26% and, more preferably, by at least about 46%. Alternatively, the second component, which substantially lowers the viscosity of the liquid crystal material, is used in an amount of at least about 5% by weight based upon the total weight of the liquid crystal material and, more preferably, in an amount of at least about 10% by weight. The material preferably has positive dielectric anisotropy. Display devices that include the present material exhibit greatly improved properties including higher contrast ratios, shorter pulse widths and lower voltages.
In particular, the second component is achiral and has a molecular weight not exceeding about 205 grams/mole. The material has a bulk viscosity that is less than about 82 centipoise, which may be obtained using the second component in an amount of at least about 10% by weight based upon the total weight of the material. The second component is preferably used in an amount ranging from about 5 to about 20% by weight based upon the total weight of the material and, more preferably, in an amount ranging from greater than 8% to about 15% by weight. Amounts of all chemical components herein are in % by weight based upon the total weight of the liquid crystal material, unless otherwise indicated.
A preferred embodiment of the present invention is directed to a display device comprising a liquid crystal material including the first chiral nematic liquid crystal component and the second component which exhibits no liquid crystalline phase at any temperature. The second component reduces the bulk viscosity of the material by at least about 26%. Cell wall structure cooperates with the material to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures. The display device has a monochromatic reflective contrast ratio of at least about 30:1. The device for applying an electric field may transform the material to a homeotropic state by applying a pulse of a width less than about 6 milliseconds and a voltage not greater than about 45 volts. The display is preferably operated in a reflective mode using frontlighting.
As used in this disclosure, a pulse train is applied to the liquid crystal material, the pulse train comprising at least one AC pulse generated while the voltage is on. A pulse is defined in this disclosure as one period of an electromagnetic wave. The pulses that are used in the present invention are preferably square wave pulses. The pulse width referred to herein refers to the width of the entire pulse train.
Consequently, a liquid crystal display device having a chiral nematic material without the second component of the invention may be addressed by applying a pulse of a given voltage and width PW
std
effective to transform the material to a homeotropic state. A method of the invention for increasing switching speed employs the inventive liquid crystal material including the first chiral nematic liquid crystal component and the second component that exhibits no liquid crystalline phase at any temperature. The liquid crystal material cooperates with the cell wall structure to form focal conic and twisted planar textures that are stable in the absence of a field. The second component is present in an amount effective to transform the liquid crystal to the homeotropic state with a pulse width PW
imp
that is at least about 50% less than the pulse width PW
std
at the given voltage.
In particular, the second component is preferably present in an amount greater than 8% by weight based upon the total weight of the material and, more preferably, in an amount of at least 10% by weight. The pulse width PW
std
may be about 15 milliseconds. In contrast, the pulse width PW
imp
attained according to the present invention is preferably not greater than about 6 milliseconds and even not greater than about 4 milliseconds. The pulse width PW
imp
is preferably at least about 70% less than the pulse width PW
std
.
The second component for lowering the viscosity of the material may be comprised of a compound having the following general formula I:
wherein R
1
is selected from the group consisting of an alkyl group and an alkoxy group each having from 1 to 9 carbon atoms.
More preferably, the second component of formula I may have the following formula:
wherein A comprises any group that renders the second component polar and soluble in the first component. In this regard, group A preferably comprises a cyano group. When group A is a cyano group the alkyl or alkoxy group of R
1
preferably does not have seven carbon atoms. If group A is a cyano group and the alkyl or alkoxy group of R
1
has seven carbon atoms, the second component is preferably present in an amount of greater than 10% by weight based upon the total weight of the liquid crystal material.
Preferred substituted phenyl compounds that satisfy the general formula I comprise the following: 1-phenylpentane, 1-phenylhexane, 1-cyclohexylbenzene, p-cyanopentylbenzene, and 4-phenylbutyronitrile.
Alternatively, the second component for lowering the viscosity of the material may be comprised of a compound having the following formula II:
R
2—
B II.
wherein R
2
is selected from the group consisting of an alkyl group and an alkoxy group each having from 1 to 11 carbon atoms and B comprises any group that renders the second component polar and soluble in the first chiral nematic liquid crystal component. A preferred second component having a formula that satisfies formula II is undecyl cyanide.
The second component in the case of both formulas I and II preferably has a molecular weight not exceeding about 205 grams/mole. Groups A and B are selected from the group consisting of an alkyl group, an alkoxy group and a cyano group.
The viscosity lowering additive of formula I is preferably comprised of a monocyclic aromatic compound such as benzene. However, those skilled in the art
Khan Asad Aziz
Laskos, Jr. Stanley Joseph
Stefanov Michael E.
Kent Displays Incorporated
Watts Hoffman Fisher & Heinke
Wu Shean C.
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