Biaxial films having controlled refractive indexes along the directions of three axes are useful in the optical field utilizing polarized light. In particular, in the field of liquid crystal display, such films permitting precise control of polarized light are of a great importance.
In the case of preparing such optically biaxial film, it is a popular process to obtain the film by biaxially stretching a film obtained from a polymer (see, for example, JP-A-2-264905). In the case of obtaining a biaxial film by the biaxially stretching technique, the refractive indexes along the directions of three axes can be controlled to desired levels with comparative ease because the refractive indexes can be controlled through draw ratio.
On the other hand, it has recently been proposed to obtain a biaxial film by using a biaxial liquid crystal (see, for example, JP-A-2002-6138 and JP-A-2002-174730). In comparison with biaxially stretched films which have conventionally been often used, the biaxial film using the biaxial liquid crystal has a merit of reducing its thickness so much that using the biaxial liquid crystal for the biaxial film is a useful means for reducing the thickness or weight of a device.
However, in the case of preparing a biaxial film by using a biaxial liquid crystal, there arises a problem that refractive indexes of the film along the directions of three axes can not arbitrarily be controlled. This is attributed to that the refractive indexes of a resultant biaxial film along the directions of three axes are almost decisively determined by the refractive indexes of the liquid crystal phase-developing compound (liquid crystal compound) along the directions of three axes. That is, in order to adjust the refractive indexes of the biaxial film along the directions of three axes to desired levels, it is the only method to synthesize a liquid crystal compound having desired refractive indexes. However, in comparison with the number of compounds developing uniaxial liquid crystal, the number of compounds capable of developing biaxial liquid crystal is so small (see, for example, D. Demus, J. Goodby, et al., Handbook of Liquid Crystals Vol. 2B; Low Molecular Weight Liquid Crystals II, WILEY-VCH p. 933-943) that it has been extremely difficult to arbitrarily control the refractive indexes along the directions of three axes.