Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-01-18
2004-11-09
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000, C252S299010
Reexamination Certificate
active
06816218
ABSTRACT:
FIELD OF THE INVENTION
This work aims to find a suitable homeotropic alignment method for orthogonal liquid crystals, especially for nematic and smectic A liquid crystal mixtures on plastic substrates.
A preferred application is the orientation of reactive liquid crystal mixtures for the preparation of anisotropic polymeric films.
The reactive liquids crystal are dissolved in a suitable solvent such as toluene and coated as a thin film onto a flexible plastic substrate. When the solvent evaporates and the reactive liquid crystal remains, the liquid crystal has to align instantly, so that high speed roll to roll coating of the plastic substrate is possible. This is achieved by the method described hereafter. The reactive liquid crystal is subsequently polymerised using UV light, which is acting upon a photoinitiator mixed into the reactive liquid crystal, or alternatively by heat, which in turn is acting upon a thermal initiator mixed into the reactive liquid crystal. A polymer film having oriented constituent molecules i.e. monomer molecules is thus produced. It is well known that photopolymerisation of aligned reactive liquid crystals gives rise to polymer films having molecules aligned in the same way as the monomer reactive liquid crystal, thus freezing or stabilizing the director orientation of the liquid crystal and preserving it's anisotropic properties. Such films are characterized by and useful for their optical properties. They are uniaxial, positive birefringent films.
BACKGROUND OF THE INVENTION
Homeotropic alignment of a liquid crystal occurs when the long axes of the molecules of the liquid crystal phase are, on average, essentially normal to the substrate upon which they form a thin film. Very few materials spontaneously align in this way and thus some kind of aligning agent is required to produce this desired alignment. Orthogonal liquid crystal phases like nematic liquid crystal phases and orthogonal smectic phases (e.g. smectic A and smectic B) can be aligned in this way. An Overview about orientation technologies for liquid crystals is given e.g. in J. Cognard, Molecular Crystals Liquid Crystals 78 Supplement 1 (1981) pp 1-77 and in JA. Castellano MCLC, 94 (1983) pp 33-41.
Conventional aligning agents for liquid crystals are designed to be effective on glass substrates, some examples of such conventional organic aligning agents are lecithin, trichloro- and trimethoxypropyl silane, hexadecyl trimethyl ammonium halides and alkyl carboxylato monochromium salts. In each case a very small amount (of typically less than 1%) of the active component is dissolved in a suitable volatile solvent and spread onto the substrate e.g. by spin coating or other well known methods, when the solvent has been evaporated, a thin film of the organic aligning agent remains on the substrate. These materials are characterized by having a polar end group which is assumed to be attracted to the polar glass surface and a long alkyl chain which orientates normal to the glass surface. In some cases a chemical reaction is essential i.e. in the case of chlorosilanes which react with hydroxyl groups on the glass surface. Spontaneous homeotropic alignment of liquid crystals occurs on such surfaces.
Inorganic aligning agents can also be used. For example it is well known that evaporation at normal incidence of SiO
2
or MgF
2
onto a substrate of glass can be used to produce homeotropic alignment of liquid crystals.
This patent application, however, relates to the alignment of orthogonal liquid crystal phases especially of nematic and of smectic A liquid crystals on a plastic substrate. The plastic substrates are preferably flexible substrates and may be plastic films or also anisotropic liquid crystal polymer films. It was found that the conventional aligning agents are not effective for the alignment of liquid crystals on such plastic substrates. Typically and almost generally either no alignment or only very poor alignment is exhibited. It is suspected that the surface of the plastic substrate has much less affinity for the polar end groups of the aligning agents.
In contrast to the conventionally used organic aligning agents, inorganic aligning agents such as typically used on glass surfaces e.g. SiO
2
have been found to provide good homeotropic alignment when sputter coated or evaporated onto plastic substrates. It was found, however, that TiO
2
-coated substrates do not align so well and are more difficult to wet with the usual coating solvents. One of the problems with these types of coatings which is common to SiO
2
, TiO
2
and others, is that they tend to have a significant surface roughness which prevents the polymer film being released from the aligning layer after polymerisation. This could be attributed to the well known fact that adhesion is very much enhanced by using microscopically rough surfaces compared to smoother surfaces. As these coatings are usually deposited onto a high temperature stable substrate such as polyethylene terephtalate (PET) which, due to the stretching processes used in its manufacture, is birefringent the aligned polymer film must be removed from the substrate before the optical properties of the coated polymer itself are accessible.
SUMMARY OF THE INVENTION
This invention relates to a technique which can be used to quickly align orthogonal liquid crystals especially nematic and smectic A liquid crystals on plastic substrates and which is applicable to roll to roll coating of plastic substrates. It was found that certain organic and inorganic surfactants can be used to provide homeotropic alignment on plastic substrates. This will be explained in the following.
One type of such organic surfactants is the family of perfluoroalkyl sulphonates represented by the surfactant FC 431 (tradename of commercial product obtainable from the 3M company, USA) which is a liquid dissolved in a solvent. When added to the reactive liquid crystal (20% solids), in toluene at a concentration of from 0.1 to 0.5% and preferred at a concentration of from 0.7 to 0.4% it promotes homeotropic alignment on plastic surfaces such as triacetyl cellulose (TAC), (available from the Lonza company in Switzerland) or polyethylene terephthalate (PET), (available from the ICI company in England under the tradename of Melinex 401). To improve the alignment, corona discharge (CD) treatment of the substrate has been found to be helpful. This possibly could be attributed to the CD treatment increasing the polarity of the surface by the formation of hydroxyl and carbonyl groups. However, the alignment process even with a CD treatment is rather slow, taking some seconds after the solvent has evaporated, which has a detrimental effect on the speed at which any coating process can proceed and consequently is not acceptable. The aligning agent has to migrate from the bulk to the surface which appears to have little attraction for it. Thus the alignment process is both weak and slow.
A further topic of the present invention is to induce this type of surfactant to align faster whilst also making it into a form that is suitable for roll to roll coating applications. As the surfactant is a liquid and thus it is practically not possible to coat it onto the surface of the substrate if the substrate is in the form of a roll, unless two or more coating stations are available. However, surprisingly if the FC431 is embedded in a UV curable polymer C3 film which is coated onto the substrate and subsequently UV cured, the resultant polymer film with the surfactant retains its alignment capabilities and the alignment is now much faster and thus allowing a highly desirable greater coating speed.
We found that surfactants can be fixed on the surface e.g. by making e.g. a mixture of 5% surfactant in an acrylate monomer mixture such as a mixture of hexanediol diacrylate and ethylhexylacrylate and 4% Irgacure 907 and coating this onto the PET (or TAC) film and then photocuring the mixture. This provides a faster alignment.
The active polymerizable component can be a monomer or a mixture of monomers or a mixture
Coates David
Dickson Colum
Joicey David
Parri Owain Llyr
Scott John
Chowdhury Tarifur R.
Merck Patent GmbH
Millen White Zelano & Branigan P.C.
LandOfFree
Homeotropically aligned liquid crystal layer and process for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Homeotropically aligned liquid crystal layer and process for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Homeotropically aligned liquid crystal layer and process for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3280961