The present invention relates to a liquid crystalline polymer film useful as a high function optical element, a process for producing the same, and utilization thereof.
A polymer film having birefringence property is anisotropic in refractive index and functions as an optical element which controls the state of polarization. It has various industrial uses, including a phase contrast film for improving the image quality of a liquid crystal display, a half-wave plate and a quarter-wave plate.
Most of the conventional polymer films having birefringence property have been obtained by stretching films such as polycarbonate films for example. However, a limit has been encountered in improving their performance because the means for controlling their physical properties such as refractive index and birefringence is only the stretching operation which is substantially a processing technique. Particularly, as to controlling a three-dimensional refractive index structure for which there has been an increasing demand recently, a limit is encountered.
On the other hand, an optical material using a iquid crystalline polymer possesses great characteristics; for example, they have a greater birefringence property than that of stretched films, and it is possible to freely control its optical properties such as refractive index or birefringence because the orientation behavior of the liquid crystalline polymer is rich in variety. Particularly, a liquid crystalline polymer which exhibits twisted nematic orientation is a more useful material which possesses a novel ability of rotating the direction of polarization of light because it is possible to effect a molecular twisted orientation which cannot be obtained by a stretched polymer film, and thus the said liquid crystalline polymer is a very useful material.
When such a liquid crystalline polymer is oriented on a substrate, and in the case of a liquid crystal which exhibits a nematic orientation property, there is obtained a homogeneous orientation wherein the orienting direction (hereinafter referred to as the "director") of liquid crystalline molecules is approximately parallel to the substrate or a homeotropic orientation wherein the director is approximately perpendicular to the substrate. Further, in the case of a liquid crystal which exhibits a chiral nematic orientation property, there is obtained an orientation having a spiral structure while making a homogeneous orientation. Besides, the control of refractive indices distribution and that of the spiral structure can be done easily by controlling the structure of the liquid crystal molecules. By these structural controls it is made possible to develop various optical properties.
Thus, the liquid crystalline polymer possesses excellent features not found in other polymers in point of variety of structure and the capability of freely controlling optical properties which is based on such structural variety. But even the liquid crystalline polymer is not almighty, but there is a limit. More particularly, in any orientation behavior, the director of liquid crystal orientation can only exhibit either orientation nearly parallel to a substrate or orientation nearly perpendicular to the substrate. Heretofore, there has been no example of having realized a so-called tilt orientation with liquid crystal molecules tilted from a plane of a substrate.
The present inventors hit upon the idea that if there could be realized a twisted tilt orientation at a certain angle, not 0.degree., between the director of liquid crystal molecules and the plane of a substrate, the capability of freely controlling refractive indices, especially refractive indices in three-dimensional directions, would be enhanced even in comparison with an optical material using a stretched polymer film or a liquid crystalline polymer film obtained by fixing the orientation of an ordinary liquid crystalline polymer exhibiting a twisted nematic orientation. Having made subsequent earnest studies for the purpose of improving the performance or developing a novel feature of an optical material using a liquid crystalline polymer film, the present inventors at last accomplished the present invention.
Accordingly, it is the object of the present invention to provide a liquid crystalline polymer film having a novel feature such as the capability of freely controlling refractive indices in three-dimensional directions, as well as a process for producing the same and the use thereof.