Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-07
2003-05-27
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06570632
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method by which lyotropic liquid crystals are aligned in a bulk solution as a monodomain liquid crystalline cell.
BACKGROUND ART
A liquid crystal is a state of matter in which molecules exhibit long-range orientational order and wherein long-range positional order is either reduced (one-dimensional positional order in smectic phases) or absent (nematic phases). This intermediate ordering places liquid crystals between crystalline solids (which possess both positional and orientational order) and isotropic fluids (which exhibit no long-range order). Solid crystal or isotropic fluid can be transformed into a liquid crystal by changing temperature (creating a thermotropic liquid crystal) or by using an appropriate diluting solvent to change the concentration of mesomorphic molecules (creating a lyotropic liquid crystal).
Alignment of thermotropic liquid crystals is an active area of current research and development. Usually, the alignment technique is based on a special unidirectional treatment of the plates or substrates that bound the liquid crystalline material. Such techniques are disclosed in U.S. Pat. No. 5,596,434 entitled “Self-Assembled Monolayers For Liquid Crystal Alignment.” The '434 patent discloses that the plates are covered with a polymer (such as polyimide) layer which is mechanically rubbed. The direction of rubbing sets the direction of orientation of the thermotropic liquid crystal, i.e., the director, at the substrate, as a result of anisotropic molecular interactions at the interface. The phenomenon of orienting action between the anisotropic (rubbed, for example) substrate and the liquid crystalline alignment is called “anchoring.” Alignment by surface anchoring is a standard means of alignment in thermotropic liquid crystalline displays. Surfaces are typically treated with a polymer or a surfactant in order to obtain the desired alignment effects. The methods of alignment are well established for thermotropic liquid crystals but are not necessarily applicable to lyotropic liquid crystals because of the differences in the molecular structure between the two classes of liquid crystals.
Lyotropic liquid crystals are more difficult to align than their thermotropic counterparts. The reason is that most lyotropic liquid crystals are based on amphiphilic materials (surfactants) dissolved in water or oil. Amphiphilic molecules have a polar (hydrophilic) head and a non-polar (hydrophobic) aliphatic tail. When surfactant molecules are in contact with a substrate, their amphiphilic nature generally results in a perpendicular orientation of the molecule with respect to the plane of the substrate. Either the polar head or the hydrophobic tail of the molecule is attracted to the bounding plate, which results in the perpendicular alignment of the molecule with respect to the substrate. Perpendicular alignment means that the preferred orientation is the so-called homeotropic alignment, in which the optical axis is perpendicular to bounding plates.
An exemplary prior art lyotropic liquid crystal cell is designated generally by the numeral
10
in FIG.
1
. The cell
10
includes a pair of opposed substrates
12
, which are sealed in a well-known manner, that contain surfactant-based lyotropic liquid crystal material designated generally by the numeral
14
. The material
14
is formed using water
16
as a solvent for biphilic molecules
18
. Each of the liquid crystal molecules
18
possess polar (hydrophilic) parts
20
and apolar (hydrophobic) parts
22
. When water
16
is added to biphilic molecules
18
, such as the cationic surfactant cetylpiridinium chloride [C
21
H
38
ClN], a bilayer
26
forms as the hydrophobic regions coalesce to minimize interaction with the water
16
while enhancing the polar component's interaction with water. The concentration and geometry of the specific molecule define the supramolecular order of the liquid crystal. The molecules can aggregate into lamellae as well as disk-like or rod-like micelles, or, generally, aggregates of anisometric shape. Lyotropic liquid crystals are usually visualized as ordered phases formed by the rod-like surfactant molecules
18
(such as C
21
H
38
ClN molecules) in water. These anisometric aggregates form a nematic, smectic, columnar phase, of either non-chiral or chiral (cholesteric phase) nature. For example, the C
21
H
38
ClN molecules form a stack of lamellae of alternating layers of water and biphilic molecules, thus giving rise to a lamellar smectic A phase. The molecules on average are oriented along the direction schematically shown by a thick vertical arrow
28
called the director n. On average, the surfactant molecules are oriented along the director n. Surfactant molecules and thus the director n orient normally perpendicular to the bounding plates
12
(so-called homeotropic orientation).
There is a special class of lyotropic liquid crystals, called lyotropic chromonic liquid crystals (LCLC). The LCLC family embraces a range of dyes, drugs, nucleic acids, antibiotics, carcinogens, and anti-cancer agents. The molecular and macrostructure of LCLCs are markedly different from that of conventional lyotropic liquid crystals based on amphiphilic rod-like molecules with polar heads and hydrophobic alkyl chain tails, also referred to as surfactants. LCLC molecules are believed to be plank-like rather than rod-like, rigid rather than flexible, aromatic rather than aliphatic. The &pgr;—&pgr; interaction of the aromatic cores is the main mechanism of molecular face-to-face stacking according to Lydon [J. Lydon, Chromonics, in: Handbook of Liquid Crystals (Wiley-VCH, Weinheim, 1998) v. 2B, p. 981 and Current Opin. Col. Inter. Sci. 3, 458 (1998)]. Hydrophilic ionic groups at the periphery of the molecules make the material water-soluble. These materials have become a subject of intensive studies lately as it became clear that they can be used as internal polarizing elements in liquid crystal displays, see T. Sergan et al., Liquid Crystals v. 5, pp. 567-572 (2000) and in the amplification and detection of ligands as disclosed in U.S. Pat. No. 6,171,802. These applications are enhanced by a uniform alignment of LCLC materials with the director in the plane of the cell (or slightly tilted).
It has been disclosed by Ichimura et al that inclusion of a non-ionic surfactant into an LCLC such as disodium chromoglycate results in a material that can be aligned by a photoirradiated polymer which incorporates azobenzene groups. But, inclusion of the surfactant material introduces undesirable impurities which can adversely affect the interaction between the liquid crystal and the ligands. Moreover, only photo-sensitive polymers are shown to orient the liquid crystal material without conclusive reasoning as to how this is achieved.
Reliable techniques to align surfactant-free LCLC materials by means of surface anchoring are not known to be available in current literature. The only known art in this regard uses a bulk action of a strong magnetic field applied to the LCLC cell. However, this field-induced alignment is only temporary as the degenerate (no fixed direction of molecular orientation) orientation returns within tens of minutes once the magnetic field is removed.
SUMMARY OF INVENTION
It is thus an aspect of the present invention to provide alignment of lyotropic liquid crystals in bulk solutions.
It is another aspect of the present invention to provide a pair of opposed substrates with alignment layers imparted with orientations in different directions and, in particular, opposite directions.
It is a further aspect of the present invention to provide bulk alignment, as set forth above, wherein the rubbing of the alignment layers generates stable planar alignment of the lyotropic liquid crystal material. Additionally, planar alignment may be obtained by vapor deposition of the alignment material.
It is yet another aspect of the present invention to provide bulk alignment of the liquid crystal material, as set forth above, wherein the liqu
Ishikawa Tomohiro
Lavrentovich Oleg D.
Kent State University
Ngo Julie
Renner Kenner Greive Bobak Taylor & Weber
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