A retardation film is used in an STN (Super Twisted Nematic) liquid crystal display device or the like to solve problems such as color compensation and the expansion of viewing angle. As the material of a retardation film for color compensation has been used a polycarbonate, polyvinyl alcohol, polysulfone, polyether sulfone or amorphous polyolefin. Liquid crystalline polymer and discotic liquid crystals have also been used as the material of a retardation film for the expansion of viewing angle in addition to the above materials.
A vertical alignment liquid crystal display device in which liquid crystals are aligned almost vertically to a substrate when voltage is off has already been used in monitors and TVs due to its high contrast and wide viewing angle. It is described in the 1997 Society for information display international symposium digest of technical papers at pages 845 to 848 that the use of a retardation film is important to obtain a wide viewing angle.
A retardation film made from a polycarbonate homopolymer produced from bisphenol A as a starting material has been widely used in the above STN liquid crystal display device.
However, as especially a vertical alignment liquid crystal display device has higher quality than an STN liquid crystal display device, it has been found that a retardation film made from a polycarbonate material which has been used in the conventional STN liquid crystal display device cannot obtain sufficiently high display quality. That is, the retardation value and the optical axis of a retardation film are changed by stress in the step of joining together a retardation film made from a polycarbonate homopolymer and a polarizer film, stress in the step of joining the laminated polarizer film obtained in the above step to a liquid crystal display device, or the shrinkage stress of a polarizer film which is produced during a durability test at a high temperature or at a high temperature and a high humidity, with the result that the brightness of the screen of the liquid crystal display device becomes nonuniform particularly when black is displayed on the entire screen, thereby deteriorating display quality. The place where this brightness nonuniformity appears which depends on the mode of the liquid crystal display device is around the edges of the four sides of the screen of the liquid crystal display device in most cases. Therefore, this phenomenon will be referred to as “frame phenomenon” and this problem will be referred to as “frame problem” in this description hereinafter.
Cellulose acetate, polyolefin and polycarbonate are known as the material of the retardation film.
However, a retardation film made from cellulose acetate has poor stability of molecular orientation as cellulose acetate has a high water absorption coefficient, thereby making it difficult to use it when a high degree of orientation is required within the plane and to suppress variations in anisotropy within the plane for the same reason. Since a polyolefin having a cyclic skeleton such as a norbornene skeleton has a low photoelastic constant and low intrinsic birefringence, it must be stretched at a high draw ratio to obtain a retardation required for a retardation film. Since a bulky molecular structure such as a norbornene skeleton is employed to obtain a high glass transition point, a retardation film made from the polyolefin has low impact resistance, handling ease and stretchability, easily breaks and often ruptures. Therefore, this film has a lot of problems to be solved when it is produced or used as a retardation film.
Meanwhile, a polycarbonate comprising an aromatic dihydroxy compound (bisphenol) having two aromatic rings through a bond group out of aromatic polycarbonates has appropriate flexibility and a high glass transition point. However, a homopolymer having a bisphenol A skeleton which is widely used in an STN mode has no problem with handling ease and stretchability but does have the above frame problem. Therefore, it is difficult to use it in a vertical alignment liquid crystal display device which must have high quality.
There are many kinds of polycarbonates and there are a large number of examples in which the polycarbonates are used as retardation films. JP-A 7-246661 and JP-A 6-82624 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) propose retardation films made from a polycarbonate comprising a dihydroxy component other than a bisphenol A skeleton.
Polycarbonates are divided into aliphatic and aromatic polycarbonates. In general, aliphatic polycarbonates are not used as the material of a retardation film because they have a low glass transition temperature and poor productivity though they have a low photoelastic constant. One of the causes of the frame phenomenon is that stress generated by the shrinkage of a polarizer spreads to a retardation film through an adhesive layer to change the retardation of the retardation film. Therefore, a retardation film having a lower photoelastic constant is considered to be preferred because a change in retardation caused by stress becomes smaller, which is not a necessary and sufficient condition. Meanwhile, aromatic polycarbonates have high production ease and their glass transition temperatures can be easily raised by the existence of an aromatic ring. As described above, they are actually used as the material of a retardation film but they have a problem that their photoelastic constants are relatively high. Attempts have already been made to reduce the photoelastic constant of an aromatic polycarbonate film, and some homopolymers and copolymers are proposed.
However, in the case of these aromatic polycarbonates, though the reason is not clear, probably due to the existence of an aromatic ring, it is difficult to reduce the photoelastic constants of the aromatic polycarbonates to the level of a commercially available optical film made from a polyolefin having a bulky functional group such as the above norbornene skeleton. That is, although polycarbonates are superior to the above polyolefin in handling ease and moldability, it is difficult to reduce their photoelastic constants while realizing a high glass transition temperature.
JP-A 6-25398 discloses a polycarbonate resin having a high refractive index and low birefringence which comprises a structural unit represented by the following formula (a):
wherein R1 to R4 are each a hydrogen atom, halogen atom, phenyl group or alkyl group having 1 to 3 carbon atoms,and a structural unit represented by the following formula (b):
wherein w is a single bond, alkylidene group, cycloalkylidene group, phenyl substituted alkylidene group, sulfone group, sulfide group or oxide group, R5 and R6 are each a hydrogen atom, halogen atom, phenyl group or alkyl group having 1 to 3 carbon atoms, and m and n are each an integer of 1 to 4, and which contains the structural unit (b) in an amount of 41 to 95 mol %. It is disclosed in Examples of the above publication that polycarbonates (powders) produced by the solution polymerization of 9,9-bis(4-hydroxyphenyl)fluorene and bisphenol A in molar ratios of 85/15 (Example 1), 75/25 (Example 2) and 50/50 (Example 3) are dissolved in methylene chloride to obtain films. However, the publication is silent about uniaxially oriented or biaxially oriented films made from the above polycarbonates and therefore about retardation films composed of these films as well.
JP-A 2001-318232 discloses an optical film which is made from a polycarbonate containing 1 mol % or more of a recurring unit represented by the following formula (c):
and having a glass transition temperature of 160° C. or higher, and which has a heat shrinkage factor when heated at 80° C. for 500 hours of 0.07% or less, a hardness measured by a super microhardness meter of 16 kg/mm2 or more, a thickness of 10 to 200 μm and a retardation (R(550)) at a wavelength of 550 nm satisfying |R(550)|≦20 nm. It is disclosed in Example 7 of the above publication that a polycarbonate copolymer produced by the solution polymerization of 30 mol % of a bisphenol compound represented by the following formula:
and 70 mol % of bisphenol A is dissolved in methylene chloride to obtain a cast film which is then stretched uniaxially to 1.5 times at 196° C. to obtain an optical film having an R(550) of 5.0 nm.