Compositions – Liquid crystal compositions
Reissue Patent
2000-05-09
2002-04-16
Wu, Shean C. (Department: 1756)
Compositions
Liquid crystal compositions
C430S019000, C430S020000, C349S183000
Reissue Patent
active
RE037658
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the preparation of
of
a new type of information and storage material based upon the chiral optical effects in an amorphous polymer matrix.
PRIOR ART
As one part of a program supported over several years by the National Science Foundation and the Petroleum Research Fund, I have investigated the possibility of forming molecular composites of polyisocyanates with random coil polymers. Such materials have been suggested as ideal for mechanical reinforcement because of the rigid character of the polyisocyanates. See, Eisenbach, C. D.; Hoffman, J; Fischer, K. Macromol Rapid Commun. 1994, 15, 117. In addition, attention is directed to U.S. Pat. No. 5,459,192, and references cited therein, where I disclosed thermally reversible gels comprising liquid solvents wherein the solvent was converted into a thermally reversible gel upon the addition of a rigid polymer, preferably, a liquid crystal forming polymer.
In two attempts at forming such blends reported in the literature only kinetic trapping was possible with phase separation evident, and therefore I undertook synthetic work to incorporate hydrogen bonding groups in the side chains to enhance the attractive interactions between the polyisocyanate and the coil matrix. This was successful with three different polyisocyanates incorporating ether, ester and ketone side chain functions and led to a series of materials with interesting potential for further study of mechanical properties, glass transitions and phase boundaries. See Khatri, C. A.; Vaidya, M. M.; Levon, K,; Green, M. M., Macromolecules, 1995, 28, 4719.
The molecular composites formed from the side chain adapted polyisocyanates with a copolymer of styrene and vinyl phenol showed the expected dependence of the glass transition temperature, Tg, on the composition of the blend. Optical microscopy experiments supported the absence of phase separation while infrared studies demonstrated the expected hydrogen bonding interactions. One of these examples with an ester side chain, benzyl butyrate isocyanate (poly-BBI) exhibiting the effect of composition on the glass transition temperature.
In the microscopy work on the blends there was no evidence of birefringence
.
,
consistent with the thermodynamic mixing, but also emphasizing the fact that these are solid solutions with isotropic properties. This fact allows the possibility, as in all isotropic liquid solutions, of observing molecular optical activity properties.
The polyisocyanates form a stiff helical conformation. In long chains stereoblocks of left and right handed helices are separated by rarely occurring helix reversals which form a kink or bend in the chain. The helix reversals are rapidly mobile in solution allowing interconversion of the equally probable mirror image helical stereoblocks. These polymers
,
which are therefore optically inactive
,
may be converted to highly optically active polymers by incorporating structural elements which favor one or the other helical sense.
Accordingly, the chiral optical properties of these isocyanates are strongly dependent on the conformation or shape of the polymer. This is generally true for optically active polymers, i.e., non-racemic chiral polymers, as has been known since the some early work on optically active stereoregular polymers and continues to be presently observed. Since conformation must always depend on temperature, this means that the chiral optical properties of polymers also strongly depend on temperature. In solution, this dependence is reversible but the character of polymers to form solid solutions (blends) and amorphous states with glass and rubber properties offers an opportunity to control the optical activity properties in a way not possible with liquid solutions. In the glassy state conformational changes are severely restricted and therefore optical activity cannot change. Above the glass transition conformational motions are allowed and one can expect behavior parallel to that in liquid solutions where chiral optical properties are diminished as a continuous function of temperature, the temperature increase causing increasing populations of compensating conformations.
With the above in mind, a discussion of optical storage systems is now in order. Optical data storage can be divided into two types: optical disk and holographic. In the former, bits of information are read written onto circular tracks of a rotating disk using a focused laser been. The disk substrate consists of a material whose optical properties can be altered when illuminated by light from an intense writing beam: a weaker read beam then probes the state of the medium. In current technology, the write beam ablates, melts or photochemically alters the medium, and the read beam detects changes in the reflectivity of the substrate.
In digital holographic storage, a string of
its
bits
is
store
stored
as an image consisting of a two dimensional array of light and dark squares, using a spatial light modulator. A hologram of this image is recorded as an interference pattern in a photorefractive crystal such as LiNbOg
4
. Spatial and rotational multiplexing allow high storage densities. Illuminating the crystal with a reference beam allows the reconstruction of the image, which is then read out with a COD array. A practice multiple-page system based on this technology
ha
has
recently been demonstrated.
In both technologies there is the need for better materials whose optical properties are temporarily or permanently altered by a writing beam. Existing photorefractive crystals are expensive and difficult to grow with reproducible optical properties. Some effort has gone into the development of photorefractive polymers based on doped, photoconducting polymers such as poly(N-vinylcarbazole). These generally require the application of an external poling electric field in order to operate.
It is therefore a primary object of this invention is to create a new type of information storage material based on chiral optical effects in an amorphous polymer matrix.
More particularly, it is an object of
ibis
this
invention to provide an amorphous solid sample of a chiral
no-racemic
non-
racemic
polymer where the optical activity depends on polymer conformation or shape and heat to a selected temperature above the Tg
.
,
wherein the optical activity achieved above Tg will reflect said selected temperature, followed by quenching, wherein the optical activity will be stored in a state where it cannot change.
It is then still a further object of this invention to heat the amorphous polymer with said store optical activity to a temperature near the Tg, whereupon the optical activity will lose its memory of the former heating and change to a value consistent with the Tg, wherein the material will then be available to be heated again to store new information.
Furthermore, it is an object of this invention to use, as a heating unit a pixel which gains energy using, for example, laser diodes and dyes, wherein an information storage system is developed in which information is available in an analog manner that is continuously tracking the energy input and is not “zero-one” but rather any pixel can assume a large number of states (optical activity) as in a holographic system.
Finally, it as a more specific object of this invention to use, as the vehicle for the information storage system described above, a polyisocyanate polymer. Although other polymers whose optical activity depends upon conformation are contemplated, including even polymer which contain a side-chain an optically active group which can racemize (lose its optical activity) when undergoing a conformational motion which is not possible below Tg.
SUMMARY OF THE INVENTION
An optical information storage material which can reversibly store said information comprising a polymeric material with a dependence of optical activity on temperature characterized in that the optical activity is substantially invariant at temperatures below Tg of said polymer, and variant at
a
te
Pokotylo John C.
Polytechnic University
Straub & Polotylo
Wu Shean C.
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