Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1998-11-17
2002-07-23
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C430S019000, C430S056000, C430S270100, C526S292200, C526S292300, C526S311000, C526S312000, C526S313000, C526S298000
Reexamination Certificate
active
06423799
ABSTRACT:
The present invention relates to a process for the extremely rapid inscription of photo-addressable substrates, substrates prepared for this process, and the use of such substrates in information technology. The invention also relates to photo-addressable side-group polymers in which a high double refraction can be induced by irradiation, so that they are suitable for storing optically available information or for producing passive or optically switchable components.
Photo-addressable polymers are known (Polymers as Electrooptical and Photooptical Active Media, V. P. Shibaev (Editor), Springer Verlag, New York 1995). Particularly suitable for this purpose are side-group polymers, of which the group of copolymers is distinguished by a very wide range of possible variations of the properties. Their particular feature is that their optical properties such as absorption, emission, reflection, double refraction and scattering can be reversibly altered by light induction. Such polymers have a special branched structure: side groups that can absorb electromagnetic radiation and that are joined by parts of molecules acting as “spacers” are located on a linear backbone. Examples of this type are the side-group polymers containing azobenzene groups according to U.S. Pat. No. 5,173,381. These substances are characterised by the ability to exhibit double refraction in a specific direction when irradiated with polarised light. The inscribed double refraction patterns can be rendered visible in polarised light.
It is also known that a locally restricted double refraction whose preferred axis moves with the rotation of the direction of polarisation can be inscribed at an arbitrary position in layers of these polymers using polarised light (K. Anderle, R. Birenheide, M. Eich, J. H. Wendorff, Makromol. Chem., Rapid Commun. 10, 477-483 (1989), J. Stumpe, et al., 20. Freiburger Arbeitstagung Flüssigkristalle 1991). These processes are slow. Although in some cases the beginning of anisotropic behaviour can be detected already after a few seconds' exposure time, as a rule minutes and even hours are necessary in order for the effect to reach its achievable maximum. In this connection the magnitude of the effect is roughly proportional to the time employed. It is a particular feature of optical addressing that the optic axis of the inscribed double refraction is vertical to that of the inscribing polarised light. The simple possibility of being able optically to extinguish inscribed information by rotating the direction of polarisation of the inscribing light is based on this property. Inscription and extinction occur equally rapidly in this case; they are identical processes up to the direction of polarisation of the employed light. This is in contrast to the process of thermal extinction by heating the layer above the glass transition temperature of the polymer, in which all information is extinguished at once.
In data representation, storage and processing, two fundamentally different routes are followed, which may be termed serial and analog. In the analog route all data and information are simultaneously collected and converted. A typical example of this is photography using a silver halide film as an analog recording medium. This case involves exposure of a layer of photo-addressable polymers with polarised light through a master pattern. Since all image points are developed simultaneously, the inscription (development) time is seldom critical in these processes. With serial processes however the items of information are called up in succession. In the case of objects with a very high information density, for example images, potentially very large numbers of these image points have to be inscribed in succession, and the development time is thus the summation of the development times of the individual image points. For this reason a high inscription rate while maintaining a sufficient stability of both the initial state of the non-inscribed regions as well as of the final state of the inscribed regions is important. In both processes the accurate reproduction of gradations of differences in brightness (grey Stages) of the master pattern is also extremely important. Hitherto it has not been possible to solve in a technically satisfactory manner the problem of high inscription rates by the optical route, since in addition to the information transmission rate, further boundary conditions are essential. Such boundary conditions include in particular the stability, the extinguishability, and the ability to distinguish degrees of greyness. There are fundamental reasons for this.
As a general rule, only systems in which no mass but only fields or vectors are changed, can react extremely rapidly to control commands. If a mass is moved, for example in rearrangement processes or chemical reactions, the reaction is orders of magnitude slower and is governed also by the viscosity of the medium. For example, the switching time of the low-viscosity nematic rotational cells is at most in the region of msec, whereas a side-group copolymer takes minutes, often many hours, in order to reach the maximum achievable double refraction.
If the reversibility of the inscribed changes is dispensed with, then the energy density can be chosen arbitrarily and in the limiting case the substrate may be locally destroyed. Such materials are described for example by G. Kämpf in Kirk-Othmer, Encyclopaedia of Chemical Technology, 4th ed., 14, 277-338 (1995). This process, many variants of which are described in the literature, has some disadvantages however. The most important of these is that the process involves a considerable attack on the structure of the substrate, and the hole that is formed is basically unstable. In addition there is always the problem of the vaporised material, which may be deposited anywhere in the apparatus or on the storage medium, and finally very high laser energy densities are required, as a rule>10
7
mJ/m
2
.
Since the preservation of the substrate is at the same time the necessary condition for its re-inscribability, the light intensity cannot be increased arbitrarily; instead, the intensity must remain below the decomposition threshold. The stability of the material thus defines the upper limit of the energy density. The smallest amount of energy that is necessary in order to produce a detectable and stable change in the layer has been measured by Coles (in a polysiloxane) as 4×10
6
mJ/m
2
(C.B. McArdle in Side Chain Liquid Crystal Polymers, Editor. C. B. McArdle, Blackie Publishers, Glasgow 1989, p. 374). Assuming that the minimum energy for polymeric substrates is of the same order of magnitude, it follows, if one wishes to treat the substrate gently, that the side-group polymers can only be inscribed slowly (since the minimum energy is already very close to the destruction energy), and accordingly such polymers are unsuitable for serial storage in real time. This principal disadvantage has up to now prevented the technical use of such polymers, and it is the object of the present invention to alleviate this defect.
We have now surprisingly found that extremely rapidly addressable storage media can be produced from the polymers that are slowly photo-addressable per se if the substrates are irradiated over a large area with a light source suitable for conventional inscription, so that an optical anisotropy is produced. Optical anisotropy means that the rate of propagation of the light in the plane of the layer is dependent on the direction. This produces direction-dependent refraction, so-called double refraction. If the suitably prepared substrates are irradiated for a short time with appropriately intensive light, the double refraction is varied extremely rapidly and permanently, i.e. is reduced or completely extinguished. The degree of residual double refraction can be adjusted according to the light intensity.
Two optical processes are thus involved, which differ in their action:
In a generative first process the layer must first of all become anisotropically double refrac
Berneth Horst
Claussen Uwe
Kostromine Serguei
Neigl Ralf
Vedder Hans-Joachim
Bayer AG
Connolly Bove & Lodge & Hutz LLP
Wu David W.
Zalukaeva Tanya
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