Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Reexamination Certificate
1999-04-26
2002-07-02
Wu, Shean C. (Department: 1756)
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
C252S299670
Reexamination Certificate
active
06413448
ABSTRACT:
BACKGROUND OF THE INVENTION
Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Development of a flat panel display device capable of high quality color/gray scale output is an important technological goal. Liquid crystal displays have a number of unique useful characteristics, including low voltage and low power of operation. In such displays, a thin layer of liquid crystal material is placed between glass plates and the optical properties of small domains in the layer are controlled by the application of electric fields with high spatial resolution. This gives a device called a spatial light modulator (SLM), which is an array of pixels which either block or transmit light. The current generation of LCDs utilize the supertwisted nematic cell for displays with high contrast but limited off-axis viewability. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes a preferred orientation in an applied electric field. However, since the coupling to an applied electric field by this mechanism is rather weak, the electro-optical response time of liquid crystal based displays may be too slow for many potential applications such as in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens. Fast optical response times become increasingly important for applications to larger area display devices. Insufficient nonlinearity of liquid crystal based displays can also impose limitations for many potential applications.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N. A. Clark and S. T. Lagerwalll (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
Ferroelectric liquid crystals (FLCs) are fluids possessing thermodynamically stable polar order. As the liquid crystal cools from a normal isotropic liquid (I) to a crystalline state (X), it passes through a series of mesogenic phases of increasing molecular order. Some of these phases include the smectic A (A or S
A
), and smectic C (C, SC, or S
C
), or chiral smectic C (C*). Only the smectic C phase possesses the thermodynamically stable polar order necessary to exhibit a net dipole moment. In the smectic C phase the molecules self-assemble into layers, with the long axis of the molecules coherently tilted with respect to the layer normal. The single polar axis of the phase is normal to the tilt plane. For most such FLCs, a spontaneous macroscopic dipole density or spontaneous ferroelectric polarization (P) along the polar axis is easily measurable.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess this spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Meyer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the chiral molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. Fast switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity. The necessary switching speed to achieve a fall color display with temporal gray-scale is about 6&mgr; sec.
Birefringence is given by the following equation:
&Dgr;
n=n
e
−n
o
where n
e
is the index of refraction along the extraordinary axis of a birefringent material (parallel to the optical axis) and n
o
is the index of refraction along the ordinary axis (perpendicular to the optical axis). Many compounds of the present invention have improved solubility in FLC mixtures containing such compounds, and improved melting temperatures of FLC mixtures containing such compounds. Many compounds of the present invention confer to FLC mixtures containing them decreased viscosity and improved tilt angle.
Another important material characteristic is the birefringence. The birefringence of a compound or composition is the difference in refractive indices between different orientations of the LC.
Birefringence refers to the property of a liquid crystal to interact more strongly with light along one LC axis than along another LC axis. Most LCs are made of a core with extensive electron delocalization, to which one or two tails may be attached to help orient the molecules, give a dipole moment or polarization, or confer other desirable properties on the molecule. Typical LCs are rod-shaped with the majority of the &pgr; electron delocalization along the long or extraordinary axis (also referred to as the director). As a consequence, the extraordinary axis of LCs have a higher index of refraction than the ordinary axis, so the birefringence (An) is positive. Birefringence of a liquid crystal at a given wavelength is given by:
Δ
n
=
G
(
T
)
λ
2
λ
*
2
λ
2
-
λ
*
2
′
where &Dgr;n is the birefringence at a given wavelength, G is a constant, T is the temperature, &lgr; is the wavelength, &lgr;* is the mean resonance frequency which can be calculated using the spectrum of a material, or its birefringence at several wavelengths. See, e.g., S.-T. W (1986) Phys. Rev. A 33:1270; S.-T. W (1987) Opt. Eng. 26:120; S.-T. W, C.-S. W (1989) J. Appl. Phys. 66:5297; S.-T. W et al. (1993) Opt. Eng. 32:1775. As the wavelength of interest moves away from &lgr;*, the birefringence decreases asymptotically until in the infrared, the birefringence is relatively constant (except in the near-IR portion of the spectrum).
The optimum thickness of the FLC film when used as a half-wave plate (the half-wave thickness d
1/2
) in the device depends on the birefringence (&Dgr;n) of the material and light wavelength (&lgr;) according to the following equation:
d
1
/
2
=
λ
2
Δ
n
Optimal thickness of a FLC film is achieved when the contrast is maximized and true color transmission is exhibited.
Some of the materials with the lowest birefringence currently available possess birefringence around 0.20. This corresponds to a thickness of the FLC for visible light modulation of about 2 &mgr;m gm. The use of thinner devices is limited by manufacturing techniques and material characteristics. Manufacturing techniques for large area, very thin devices are expensive and difficult to implement. When thin LC cells are used, small variances in cell thickness can have a significant effect on the cell's optical properties. For example, a 0.1 &mgr;m varian
Chen Xin-Hua
More Kundalika M.
Thurmes William
Wand Michael
Displaytech, Inc.
Greenlee Winner & Sullivan, P.C.
Wu Shean C.
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