Polycarbonates and other polymer materials are utilized in optical data storage media, such as compact disks. In optical data storage media, it is critical that polycarbonate resins have good performance characteristics such as transparency, low water affinity, good processibility, good heat resistance and low birefringence. High birefringence is particularly undesirable in high density optical data storage media.
Improvements in optical data storage media, including increased data storage density, are highly desirable, and achievement of such improvements is expected to improve well established and new computer technology such as read only, write once, rewritable CDs digital versatile disks (DVDs) and magneto-optical (MO) disks.
In the case of CD-ROM technology, the information to be read is imprinted directly into a moldable, transparent plastic material, such as bisphenol A (BPA) polycarbonate. The information is stored in the form of shallow pits embossed in a polymer surface. The surface is coated with a reflective metallic film, and the digital information, represented by the position and length of the pits, is read optically with a focused low power (5 mW) laser beam. The user can only extract information (digital data) from the disk without changing or adding any data. Thus, it is possible to "read" but not to "write" or "erase" information.
The operating principle in a WORM drive is to use a focused laser beam (20-40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance. These changes can take various forms: "hole burning" is the removal of material, typically a thin film of tellurium, by evaporation, melting or spalling (sometimes referred to as laser ablation); bubble or pit formation involves deformation of the surface, usually of a polymer overcoat of a metal reflector.
Although the CD-ROM and WORM formats have been successfully developed and are well suited for particular applications, the computer industry is focusing on erasable media for optical storage (EODs). There are two types of EODs: phase change (PC) and magneto-optic (MO). In MO storage, a bit of information is stored as a .about.1 .mu.m diameter magnetic domain, which has its magnetization either up or down. The information can be read by monitoring the rotation of the plane polarization of light reflected from the surface of the magnetic film. This rotation, called the Magneto-Optic Kerr Effect (MOKE) is typically less than 0.5 degrees. The materials for MO storage are generally amorphous alloys of the rare earth and transition metals.
Amorphous materials have a distinct advantage in MO storage as they do not suffer from "grain noise", spurious variations in the plane of polarization of reflected light caused by randomness in the orientation of grains in a polycrystalline film. Bits are written by heating above the Curie point, T.sub.c, and cooling in the presence of a magnetic field, a process known as thermomagnetic writing. In the phase-change technology, information is stored in regions that are different phases, typically amorphous and crystalline. These films are usually alloys or compounds of tellurium which can be quenched into the amorphous state by melting and rapidly cooling. The film is initially crystallized by heating it above the crystallization temperature. In most of these materials, the crystallization temperature is close to the glass transition temperature. When the film is heated with a short, high power focused laser pulse, the film can be melted and quenched to the amorphous state. The amorphized spot can represent a digital "1" or a bit of information. The information is read by scanning it with the same laser, set at a lower power, and monitoring the reflectivity.
In the case of WORM and EOD technology, the recording layer is separated from the environment by a transparent, non-interfering shielding layer. Materials selected for such "read through" optical data storage applications must have outstanding physical properties, such as moldability, ductility, a level of robustness compatible with popular use, resistance to deformation when exposed to high heat or high humidity, either alone or in combination. The materials should also interfere minimally with the passage of laser light through the medium when information is being retrieved from or added to the storage device.
As data storage densities are increased in optical data storage media to accommodate newer technologies, such as digital versatile disks (DVD) and higher density data disks for short or long term data archives, the design requirements for the transparent plastic component of the optical data storage devices have become increasingly stringent. In many of these applications, previously employed polycarbonate materials, such as BPA polycarbonate materials, are inadequate. Materials displaying lower birefringence at current, and in the future progressively shorter "reading and writing" wavelengths have been the object of intense efforts in the field of optical data storage devices.
Low birefringence alone will not satisfy all of the design requirements for the use of a material in optical data storage media. High transparency, heat resistance, low water absorption, ductility, high purity and few inhomogeneities or particulates are also required. Currently employed materials are found to be lacking in one or more of these characteristics, and new materials are required in order to achieve higher data storage densities in optical data storage media. In addition, new materials possessing improved optical properties are anticipated to be of general utility in the production of other optical articles, such as lenses, gratings, beam splitters and the like.
Birefringence in an article molded from a polymeric material is related to orientation and deformation of its constituent polymer chains. Birefringence has several sources, including the structure and physical properties of the polymer material, the degree of molecular orientation in the polymer material and thermal stresses in the processed polymer material. For example, the birefringence of a molded optical article is determined, in part, by the molecular structure of its constituent polymer and the processing conditions, such as the forces applied during mold filling and cooling, used in its fabrication which can create thermal stresses and orientation of the polymer chains.
The observed birefringence of a disk is therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains. Specifically, the observed birefringence is typically a function of the intrinsic birefringence and the birefringence introduced upon molding articles, such as optical disks. The observed birefringence of an optical disk is typically quantified using a measurement termed "vertical birefringence" or VBR, which is described more fully below. For CD's, DVD's and their R (recordable), RW (recordable and rewritable) analogues that rely on reflection differences between the land and grooves the birefringence in the plane of the disk (IBR) is important to control. On the other hand for MO (mageneto-optical disks), which relies on the Kerr rotation of the incident electric field for a read-out signal, both IBR and vertical birefringence are important to control. The in-plane and vertical birefringences are properties of the molded disk and defined as follows: EQU IBR=t(n.sub.r -n.sub..theta.) EQU VBR=(n.sub.r -n.sub.z),
n.sub.r, n.sub..theta. and n.sub.z are the average refractive indices in the three cylindrical, orthogonal directions at a given radial position and "t" is the disk thickness. These birefringences in an article molded from a polymeric material are related to:
(a) residual melt (orientational) and glassy (distortional) stresses due to the molding process, and PA0 (b) polarizability anisotropy of the molecule which is characterized in terms of the stress-optical behavior in the melt and the glassy state. PA0 2-(3-hydroxyphenyl)-2-(4-hydroxyphenyl)propane; PA0 2-(3-hydroxyphenyl)-2-(4-hydroxy-3-methylphenyl)propane; PA0 2-(3-hydroxy-4-methylphenyl)-2-(4-hydroxyphenyl)propane; PA0 2-(3-hydroxy-4-methylphenyl)-2-(4-hydroxy-3-methylphenyl)propane; PA0 2-(3-hydroxyphenyl)-2-(4-hydroxyphenyl)butane; PA0 2-(3-hydroxyphenyl)-2-(4-hydroxy-3-methylphenyl)butane; PA0 2-(3-hydroxyphenyl)-2-(4-hydroxy-3-ethylphenyl)butane; PA0 2-(3-hydroxyphenyl)-2-(3,5-dimethyl-4-hydroxyphenyl)propane; PA0 2-(3-hydroxyphenyl)-2-(3,5-dimethyl-4-hydroxyphenyl)butane.
The former (a) depends on the processing parameters in the injection molding process as well as the molecular weight of the material used to mold the article. After choosing the lowest possible molecular weight (constrained by ductility), the processing conditions give us only limited room to lower the birefringence. Real leaps in obtaining lower birefringence are possible mainly by designing molecules with low polarization anisotropy in the melt and glassy state (factor (b) above). These are characterized by the stress optical coefficients in the melt (C.sub.m) and the glassy state (C.sub.g), respectively. The relationship between C.sub.m, C.sub.g and birefringence may be expressed as follows: EQU .DELTA.n=C.sub.m.times..DELTA..sigma..sub.m (1) EQU .DELTA.n=C.sub.g.times..DELTA..sigma..sub.g (2),
where .DELTA. n is the measured birefringence and .DELTA..sigma..sub.m and .DELTA..sigma..sub.g are the applied stresses in the melt and glassy states, respectively. In the search for improved materials for use in optical articles, C.sub.m and C.sub.g are especially useful since they require minimal amounts of material and are relatively insensitive to uncontrolled measurement parameters or sample preparation methods, whereas measurements of VBR and IBR require significantly larger amounts of material and is dependent upon the molding conditions. It has been found that materials possessing low absolute values of C.sub.g and C.sub.m show lower VBR and IBR assuming that the flow-ductility balance allows synthesis of molecular weights low enough to have similar orientational (melt) and glassy stresses during the injection molding process. Therefore, in efforts aimed at developing improved optical quality, widespread use of C.sub.g and C.sub.m measurements is made in order to rank potential candidates for such applications and to compare them with previously discovered materials.
In applications requiring higher storage density, the properties of low birefringence and low water absorption in the polymer material from which the optical article is fabricated become even more critical. In order to achieve higher data storage density, low birefringence is necessary so as to minimally interfere with the laser beam as it passes through the optical article, for example a compact disk.
Another critical property needed for high data storage density applications is disk flatness. It is known that excessive moisture absorption results in disk skewing which in turn leads to reduced reliability during information recording and retrieval. Since the bulk of the disk is comprised of the polymer material, the flatness of the disk depends on the low water absorption of the polymeric material. In order to produce high quality disks through injection molding, the polymer, such as polycarbonate should be easily processed.
German Application DE 3935639 discloses blends of m,p-BPA homopolycarbonate with tetramethyl-BPA. The present invention does not relate to blends of these materials.
German Application DE 3908037 discloses blends of m,p BPA hompolycarbonate with other polycarbonates. The present invention does not relate to blends comprising m,p-BPA homopolycarbonate.
Japanese Application 01031824 discloses copolymers of m,p BPA, BPA and diphenyl carbonate for applications requiring transparency and hardness. The present invention does not relate to these materials.
U.S. Pat. No. 5,152,811 relates to semi-permeable gas separation membranes based upon uncrosslinked polymers containing meta, para-bisphenol moieties.
There exists a need for compositions having good optical properties and good processibility and which are suitable for use in high density optical recording media.