Retardation film and process for producing the same

Details Retardation film and process for producing the same Retardation film and process for producing the same

2001-12-27

2004-06-01

Robertson, Jeffrey B. (Department: 1712)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

Alignment layer of specified composition

C428S001100, C428S001230, C428S001250, C428S001260, C427S162000, C427S163200, C427S487000, C427S508000, C427S515000

Reexamination Certificate

active

06743487

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a retardation film or a phase difference film prepared by irradiating a layer of a mixture of a photosensitive polymer and a low molecular weight compound with linear polarized ultraviolet rays, ultraviolet rays including a perfectly polarized light component and non-polarized light component or non-polarized ultraviolet rays (hereinafter referred to as exposure according to the need) to give molecular orientation thereby developing a retardation and the direction of the optical axis optionally in the above layer of the mixture, and to a process for producing the retardation film.
2. Description of the Prior Art
The retardation films are films passing linear polarized light components oscillating in the major axis directions orthogonal to each other and having birefringence providing a given retardation between these two components. Such retardation films are also utilized in the fields of liquid crystal displays. Particularly retardation films, of which the optical axis is inclined against a principal surface of the film, specifically, the optical axis is inclined with the principal surface of the film and its normal line, serve to enlarge a viewing angle or angle of visibility of a liquid crystal display as optical compensation films decreasing the visual field dependency of the transmission quantity of light which is caused by the inclination of the molecular arrangement of a liquid crystal cell.
There are several prior art technologies for producing such retardation films.
As one of these technologies, there is a process in which a polymer material such as polycarbonate is stretched to orient a high molecular chain thereby producing a difference between the refractive index in the stretching direction and the refractive index in the direction orthogonal to the stretching direction. However, it is difficult to incline the optical axis against the principal surface of the film by this stretching method because molecules are oriented in the stretching direction.
Also, as a method for developing a retardation by exposure to polarized light, there is a method in which a photosensitive polymer such as polyvinyl cinnamate is irradiated with polarized UV light (JP A-7-138308). However, in this method, anisotropy is developed in a direction orthogonal to the direction of the electric field oscillation of the applied polarized UV light and the optical axis cannot be therefore inclined, so that a viewing angle is enlarged with difficulty.
To solve the foregoing problem, a method is known in which a liquid crystal monomer is aligned and fixed on an alignment layer obtained by irradiation with polarized UV light, rubbing treatment or slating deposition of SiO (JP A-8-15681). A method is also known in which a discotic liquid crystal is arranged on an alignment layer prepared by rubbing or an alignment layer prepared by SiO slanting deposition (each of JP A-7-287119 and JP A-7-287120). Further, a method is known in which a discotic liquid crystal containing a photo polymerization initiator is aligned on a photo alignment layer and this alignment is fixed by irradiation with light (JP A-10-278123). However, in these methods using an alignment layer, the process involving the aligning treatment of the alignment layer and the alignment of a liquid crystal material are complicated, raising the production cost of a retardation film which has a large area and inclined optical axis.
Moreover, as one of other processes for producing a retardation film having inclined optical axis, a method is proposed in which an inorganic dielectric is slant-deposited. This method poses the problems that a large scale apparatus is required to form a deposited film continuously on a lengthy sheet and that the process is complicated. The inventor of the present invention has proposed a process for producing a retardation film with inclined optical axis by exposing a side chain liquid crystalline polymer having photo-sensitivity to polarized light (JP A-10-278123). However, this method develops a large retardation, giving rise to the problems that the cloudiness is increased as the film is thickened.
SUMMARY OF THE INVENTION
The invention provides a retardation film suitable for mass production using a simple process and a process for producing the film.
According to a first aspect of the present invention, there is provided a process for producing a retardation film, the method comprising producing a retardation film by a process including a step of irradiating a mixture of a photosensitive polymer and a low molecular weight compound with light, wherein the ratio z of solubility parameters calculated from the evaporation energy and molecular volume of these photosensitive polymer and low molecular weight compound is as follows: 0.93<z<1.06.
In this production process, the mixture of the photosensitive polymer and low molecular weight compound which are compatible is formed in film and exposed to light, whereby the photosensitive polymer and the low molecular weight compound can be oriented. When this exposure is conducted from a direction inclined with a principal surface of the layer formed of the aforementioned mixture, the layer can be oriented with its optical axis being arbitrarily inclined. Therefore, a retardation film with optical axis inclined in a desired direction is obtained.
The low molecular weight compound has the effect of suppressing cloudiness as far as its amount is proper, but on the contrary, causes an increase in cloudiness and a reduction in orientation property when the amount to be added is excessive. From this point of view, the amount of the low molecular weight compound is preferably 5 wt % to 50 wt % though it depends on the types of the photosensitive polymer or low molecular weight compound and the retardation film can be produced if the low molecular weight compound is added in the amount of 0.1 wt % to 80 wt %.
Here, when the compatibility between the photosensitive polymer and the low molecular weight compound is insufficient, a crystal with a size enough to induce phase separation and scattering of visible light by heating a substrate during film forming or after exposure is generated, causing an increase in cloudiness. In order to suppress the phase separation and the generation of microcrystals, it is necessary to control the compatibility between the polymer and the low molecular weight compound.
As a standard of this compatibility, the solubility parameter (&sgr;) calculated from evaporation energy (&Dgr;E
v
) and molecular volume (V) according to the calculation formula (1) as described in Polymer Engineering and Science, Vol. 7, No.2, 147 (1974) may be utilized for the sake of convenience. It has been found from the result of an experiment that when the ratio z of the solubility parameter (&sgr;) of the polymer to the solubility parameter (&sgr;) of the low molecular weight compound is in the range of 0.93<z<1.06, the phase separation and the generation of microcrystals are efficiently suppressed, whereby the cloudiness of the resulting retardation film can be decreased.
&sgr;=(&Dgr;
E
V
/V
)
½
  (1)
The adoption of the aforementioned production process enables mass production of a retardation film in a simple process. Also, the resulting retardation film has decreased cloudiness and is superior in the effect of enlarging a viewing angle when it is used for a liquid crystal display.
As the above photosensitive polymer to be used as the raw material of the retardation film, a liquid crystal material is preferably used.
In addition, as the photosensitive polymer, those preferably used are photosensitive homopolymers or copolymers which have at least one of structures represented by the following molecular structure 1 to 9 and in which the main chain represented by the molecular structure 10 is a hydrocarbon, acrylate, methacrylate, maleimide, N-phenylmaleimide or siloxane. Also, as the low molecular weight compound, those having a molecular structure re

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