Retarders and circular polarizers

Details Retarders and circular polarizers Retarders and circular polarizers

2003-03-11

2004-11-30

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C428S001300, C428S001100, C349S098000, C349S117000, C349S121000

Reexamination Certificate

active

06824838

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to retarders (preferably a phase retardation film) that are useful as &lgr;/4 plates in reflective liquid crystal display devices, pickups for writing on optical discs or anti-reflection coatings; circular polarizers using said retarders; and preparation processes thereof. Particularly, the present invention relates to retarders comprising a long transparent substrate and optically anisotropic layers having a laminate structure of rod-like liquid crystals formed thereon by coating; circular polarizers that can be prepared by laminating such a retarder to a polarizer in a roll-to-roll manner; and preparation processes thereof.
DESCRIPTION OF RELATED ART
&lgr;/4 plates find great many applications and have already been practically used. However, most of &lgr;/4 plates achieve &lgr;/4 only at a specific wavelength though they are called &lgr;/4 plates. JP-A 1998-68816 and JP-A 1998-90521 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) disclose retarders obtained by laminating two optically anisotropic polymer films. In the retarder described in JP-A 1998-68816, a quarter-wave plate forming a birefringence light with a phase difference of a quarter wavelength and a half-wave plate forming a birefringence light with a phase difference of a half wavelength are laminated with their optic axes being crossed. In the retarder described in JP-A 1998-90521, at least two retarders having a retardation value of 160-320 nm are laminated at an angle with their slow axes being neither parallel nor orthogonal to each other. The retarders described in both documents specifically having laminate structures of two polymer films. Both documents explain that &lgr;/4 can be achieved in a wide wavelength region with this arrangement. However, the preparation processes of the retarders described in JP-A 1998-68816 and JP-A 1998-90521 require cutting two polymer films at a predetermined angle and laminating the resulting chips in order to control the optical directions (optic axes or slow axes) of the two polymer films. If retarders are to be prepared by laminating chips, the process becomes complex and other disadvantages occur such as liability to quality failure due to misalignment, decrease in yield, increase in cost and liability to deterioration due to contamination. Moreover, it is difficult to strictly control the retardation value in polymer films.
On the other hand, techniques for more easily providing a broadband &ngr;/4 plate by including at least two optically anisotropic layers containing a liquid crystal compound are disclosed in JP-A2001-4837, JP-A2001-21720 and JP-A 2000-206331. Especially, the technique disclosed in JP-A 2001-4837, in which the same liquid crystal molecules can be used, is also attractive in terms of production costs.
During the preparation of a retarder by laminating optically anisotropic layers containing a liquid crystal compound, it is important to control the alignment of the liquid crystal compound. For example, optically anisotropic layers can be formed of homogenously aligned rod-like liquid crystal compounds with respect to a substrate. If a layer containing a rod-like liquid crystal compound is formed on an alignment layer subjected to a rubbing treatment, the rod-like liquid crystal compound is normally homogenously aligned with the long axis being in the rubbing direction. However, our careful studies revealed that the accuracy of liquid crystal alignment decreased when a conventional alignment layer is used for an alignment of liquid crystal molecules in a direction of the rubbing axis to form the liquid crystal molecular layer disclosed in JP-A 2001-4837. It was also found that many optical defects occurred with some types of rod-like liquid crystal compounds or with some types of liquid crystal compositions.
SUMMARY OF THE INVENTION
In view of the problems described above, the present invention aims to provide a retarder (preferably a phase retardation film) that can perform in a broad wavelength range (visible wavelength range) and can be easily and stably prepared in a thin-layer form. The present invention also aims to provide a circular polarizer that can perform in a broad wavelength range (visible wavelength range) and can be easily and stably prepared in a thin-layer form. The present invention also aims to provide a process by which a retarder and a circular polarizer performing well in a broad wavelength range (visible wavelength range) and capable of being in a thin-layer form can be prepared by stably controlling the alignment of the molecules of the liquid crystal compound. The present invention also aims to provide a novel technique for controlling the alignment of rod-like liquid crystals.
As a result of careful studies about retarders having laminate structures of &lgr;/4 liquid crystal layers and &lgr;/2 liquid crystal layers, we found that when an alignment layer which allows liquid crystal molecules to be aligned along the rubbing axis is used, the alignment of the liquid crystal molecules induced by the alignment layer is not stabilized though the rod-like liquid crystal compound should be homogenously aligned at a large azimuthal angle with respect to the longitudinal direction of the transparent substrate to form a &lgr;/2 liquid crystal layer. We also found that liquid crystal molecules are aligned in various directions near free surfaces to bring about optical defects in the liquid crystal molecules though the rod-like liquid crystals in the &lgr;/2 liquid crystal layer and &lgr;/4 liquid crystal layer should be homogenously aligned with high accuracy. Further profound studies based on these findings led us to accomplish the present invention.
In one aspect, the present invention provides a retarder (preferably a phase retardation film) comprising:
a long transparent substrate, and up on the substrate,
a first optically anisotropic layer formed from a homogenously aligned rod-like liquid crystal compound and having a phase shift substantially equal to n measured at a wavelength of 550 nm;
a second optically anisotropic layer formed form a homogenously aligned rod-like liquid crystal compound and having a phase shift substantially equal to &pgr;/2 measured at a wavelength of 550 nm; and
a first alignment layer having a rubbing axis defining the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the first optically anisotropic layer;
wherein the angle between the slow axis in the plane of the first optically anisotropic layer and the longitudinal direction of the transparent substrate is substantially 75°, the angle between the slow axis in the plane of the second optically anisotropic layer and the longitudinal direction of the transparent substrate is substantially 15°, the angle between the slow axis in the plane of the second optically anisotropic layer and the slow axis in the plane of the first optically anisotropic layer is substantially 60° and the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the first optically anisotropic layer is substantially orthogonal to the rubbing axis of the first alignment layer.
As preferred, there are provided the retarder comprising a second alignment layer having a rubbing axis defining the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the second optically anisotropic layer, wherein the angle between the rubbing axis of the first alignment layer and the longitudinal direction of the transparent substrate is substantially −15° and the angle between the rubbing axis of the second alignment,layer and the longitudinal direction of the transparent substrate is substantially 15°; the retarder wherein the first alignment layer is formed of at least one copolymer having at least one of repeating units represented by any one of formulae (I) to (III) below and at least one of repeating units represented by formula (IV) below:
wherein R
1
to R
3
independently represent a

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