DD297030A5 - METHOD FOR REVERSIBLE OPTICAL INFORMATION STORAGE AND LIGHT-INDUCED ORIENTATION OF LIQUID CRYSTALLINE POLYMERS - Google Patents

Abstract

The object of providing a process for the reversible optical storage of information and for light-induced orientation fixed in the polymer on the basis of liquid-crystalline polymers was achieved by adding a homoeotropically oriented film of an azochromophore-containing side chain polymer in the range below the glass transition temperature of the polymer with linear information is written to polarized actinic light. At the irradiated points, a macroscopically uniform reorientation of the liquid-crystalline polymer takes place in a planar or quasi-planar orientation. This state is stably fixed in the glassy state of the polymer and can be read out non-destructively by non-actinic light and quenched by heat. The cycle of writing - reading - erasing can be repeated several times. {Liquid-crystalline, side-chain polymer; azo chromophores; optical storage; Information; reorientation; homoeotropic in planar; below glass transition temperature; linear polarized, actinic write-in light; non actinic reading light}

Description

Field of application of the invention

The method according to the invention relates to the light-induced orientation of liquid-crystalline polymers and to an optically addressable read / write memory based on polymer liquid crystals with information that can be overwritten repeatedly and a permanent memory effect for data processing.

Characteristic of the known state of the art

Commercially available are optical memories that can use only once-written information, Compact Disc Read-On-Iy (CD-ROM), and Writo-Once-Read-Many-Times (fWORM) arrays.

For erasable optical memories, there are approaches based on magneto-optic materials, dye polymers, liquid crystals and intermetallic compounds, which use changes of physical or chemical states (refractive index, crystal structure, phase) by the action of photon energy. (R.A. Bartolini, Optical Engineering 15, 99-108 (1976); J. Hecht, High Technology 7 (8), 43 [1987]).

In the liquid crystal display technology, laser-addressed displays with low-molecular-weight liquid crystals have been known for some years as a display screen or memory (WE Haas et al., J. Elektrochem., Soc., 121, 1667, 1974, K. Ogura, H. Hirabayashi, A. Uejima, K. Nakamura, Jap. J. Appl. Phys. 21,969 [1982]). However, external fields are required to maintain the memory states and, in addition, the high mobility of the resolving power and thus the storage density are considerably limited.

Liquid-crystalline side-chain polymers combine liquid-crystalline properties, in particular their supramolecular order, orientability and the anisotropy of physical properties, with polymer-specific properties, in particular the formation of a glass state. One known way of writing information into thin liquid crystal polymer films is to locally heat to the isotropic state by means of an external heat source, preferably a laser beam, to form light scattering multi domain domains as the spot cools. These macroscopically reoriented regions, which have variations in optical properties, can be frozen in the glassy state of the polymer. This process known as Therme recording is described in DE 3603263 and DE 3603267 (1986). This method is macroscopic in many variants for main and side chain liquid crystalline polymers without or in combination with external electric or magnetic fields

oriented or unoriented prongs and offered variation in the writing temperatures known. The disadvantages of this method are the high laser power required for the writing process and the low resolution. However, such phase transitions can also be achieved by photochemical reactions of photochromic low molecular weight liquid crystals or photochromic liquid crystalline polymers, or by the addition of phosphorous compounds to liquid crystals or liquid crystalline polymers. Thus, the photoinduced change in the geometric shape of molecules, for example, by EZ photoisomerizations of azochromophores, can interfere with the entire liquid crystal system, thereby photochemically reducing the phase transition temperature or inducing a phase transition (Ikeda, Macromol, 23, 36, and Ί2 [19SO)).

Eich and Wendorff {Macromol. Chem. Rapid Commun. 8.59 [19871 and 8,467 [1987]; DE 3623395) describe the erasable optical data storage with photochromic side-chain polymers in the glassy state of the polymers. This laser-induced refractive index change is caused by a combination of photochemical and thermal effects, whereby the photogenerated Z-azobenzene chromophores locally interfere with the liquid-crystalline order of the polymer and lead to a variation of optical property changes. This local perturbation of the mesogenic matrix is retained even when the aza-chromophores relax back to the thermodynamically stable Ε-form. The entire process causes a significant change in the refractive index. This process has the advantage of being at low writing laser intensities and at room temperature in the glassy state of the polymer.

Recently, when irradiating azo compounds with linearly polarized light in the glassy state of amorphous polymers, it has been demonstrated that weak dichroism and weak birefringence can be produced by the principle of angle-dependent photoelection (A. Kozak, G. Williams, Mol. Phys. 87, 106511989 |).

Birenheide was able to show on azochromophore-containing liquid-crystalline side-chain polymers (K. Anderle, R. Birenheide, M. Eich, JH Wendorff, Macromol Chem, Rapid Commun., 10, 477 (1989)) that this process takes place in anisotropic polymers owing to the different distribution function of the orientation the azo chromophore is much more efficient in varying optical properties. Birenlieide, however, demonstrated for this process and these storage media that in the glass state only a reorientation of the azo chromophores takes place as a completely local process, or does not disturb the adjacent side chains of the polymer and thus the liquid-crystalline matrix (K. Anderle, R. Birenheide, JH Wendorff, 19, Freiburger Workshop Liquid Crystals 1990).

The orientation of liquid-crystalline polymers by electrical and magnetic fields as well as by mechanical forces and surface effects in the shearing or inflow of liquid-crystalline polymers are introduced methods. High laser intensity allows liquid-crystalline polymers to orient mesogenic units locally in the optical field above the glass transition temperature in an anisotropic phase by a noninear optical effect, the optical induced Fredericks transition (YR Shen, Philos. Trans. R. Soc London A, 313, 327 (1984); M. Eich, JM Wendorff, H. Ringsdorf, HW Schmidt, Macromol Chem. 186, 2639 [1985]).

Endres recently reported the light-induced reorientation of the optical axis of solid-liquid dye copolymers in the viscoelastic region of planar layers by polarized light (B. Endres, M. Krieg, J. Omeis, N. Rau, Polymers and Licht, Bad Neuheim 1990 ).

Object of the invention

The aim of the invention is an erasable optical memory based on liquid-crystalline polymers, which has a virtually unlimited long-term stability below the glass transition temperature of the polymer, with an arbitrarily high number of * write-read-erase cycles.

Explanation of the essence of the invention

It is an object of the invention to provide on the basis of liquid-crystalline polymers a method for their light-induced orientation and for the reversible optical storage of information which are erasably fixed in the polymer.

According to the invention, the charge is each dissolved in that in a homeotropically oriented film of an azochromophore-containing liquid-crystalline side-chain polymers in the region below the glass transition temperature of the polymer pointwise or areal with linearly polarized or unpolarized actinisohem light information is written.

The information storage is carried out using cells consisting of two glass plates, between which there is a layer of the oriented liquid-crystalline side-chain polymer.

In each enrollment process, P.otoselection of the azo groups proceeding via e.c. thermotomy switches a light-induced reorientation of the entire matrix of the liquid-crystalline polymer. This feels up to an orientation of the photocurrent and mesogenic side groups perpendicular to the polarization plane of the write-in light. Thus, a macroscopically uniform, homeotropically oriented layer is reoriented at the irradiated points into a planar or quasiplanar layer.

The information stored in the storage medium is stably fixed in Glaszus.and the polymer and can be read pointwise or area without destruction of the stored information by longer-wave, linearly polarized, non-aklinisches reading light.

The deletion of the stored information or orientation of the liquid-crystalline matrix is effected by heat pointwise or surface. By thermal quenching of the information, the homeotropic starting state of the storage medium is restored by itself in subsequent atmospheres from the isotropic melt.

According to the described writing, reading, Lisch process is repeatedly repeatable.

As a polymeric storage medium will; liquid-crystalline homopolymers or copolymers, preferably polysiloxanes, polyacrylates or polymethacrylates, with azochromophores and mesogenone side groups (preferably phenylbenzoate, phenylbenzamide and biphenyl groups) known per se in copolymers m <t, e.g. Poly (4-methacryloyloxyhexylenoxy-benzoic acid ^ '- methoxyanilide-co-methacryloyloxyhexylenoxy-azobenzene).

Light sources are used to write in. "<> In the area of the absorption of the azo chromosome (s) emit light. The information is punctiform with a lamp, possibly in combination with polarizing elements, or a laser,

holographically or using a lithographic mask surface inscribed.

Pixels of different orientation are written in and read out when the writing and reading processes

with linearly polarized light of different polarization direction in alignment in the preferred direction of the storage medium.

By light-induced orientation, macroscopically uniformly homeotropically oriented layers become photochromic

liquid-crystalline polymers produced planar or quasiplanar oriented layers. Using linearly polarized light, planar oriented layers with a preferred direction are obtained.

The orientation of the azochromophores and the mesogenic side chains is successively changed by the photoselection

and takes place until the orientation of the matrix perpendicular to the plane of polarization of the write-in light, whereby intermediate stages have been set to: to reach a stationary state,

Jas information storage method according to the invention leads through the system-specific combination of Azochromophores with mesogenic side chains upon irradiation with linearly polarized light to a photochemical

induced, cooperative physical Umoriontierungsprozeß, the macroscopically uniform orientation of

Photochromic and mesogenic side groups and the efficient change of the anisotropy of optical properties such as

Dichroism and birefringence switch. The information content of every single pixel and ί amit the written information and overall storage density

is increased by pixels of different polarization directions, so that an information storage with three parameters per pixel is realized.

The information is long-term stable at room temperature and also remains with thermal back reaction of the Z-Azochromophoren

receive. There is a decoupling between light - induced photoproduct and the changed orientation of the

Azochromophores and the mesogenic side chains based memory state, so that the thermal stability of the Photoproducts play no role. embodiments The invention will be explained in more detail in exemplary embodiments. Embodiment 1 The memory cell used consists of two glass substrates with an orientation-promoting organic Layer are coated and which are joined together by means of a spacer so that a cell thickness of 15pm results. The cell is reacted with the isotropic phase heated polymer poly (4-methacryloyloxyhexyleneoxybenzoic acid 4'-

methoxyanilide-co-methacryloyloxyhexyleneoxy-azobenzene) in a recipient. After filling, the cell is cooled rapidly to room temperature, thereby freezing the resulting homeotropic orientation.

Conoscopic investigations clearly show a large-scale and uniform homeotropic orientation of the

liquid crystalline side chain polymer throughout the cell.

To write information, the cell is encoded using a mask or one as desired

Injecting information deflected point-shaped beam with linearly polarized nktinischem light of a tungsten lamp with the low intensity of I = 1.27 10 ~ ⁸ Einstein per minute in a Wellonlängenbereich between 400 and 500nm irradiated to saturation. This is done macroscopically uniformly at the irradiated sites at room temperature

Reorienting the liquid-crystalline side chains in the planar arrangement with the by the polarization plane of the Einschreiblichtes predetermined orientation direction. The reorientation ^; tion is stable in the glassy state of the liquid crystalline Side-chain polymer fixed. The written information may be with non-actinic, linearly polarized, polychromatic light of one wavelength

greater than 550ηm be read.

When heat is applied, the inscribed information and induced orientation are deleted. When cooling down quickly from the

isotropic melt again sets the macroscopically uniform homeotropic starting device of the cell in liquid crystalline polymer layer.

This process of reversible optical information storage is several 100 cycles without system fatigue

repeatable.

Embodiment 2 Is a cell described in 1 with a homeotropic orientation at room temperature with unpolarized

irradiated actinic light of the same wavelength range, there is a change in the optical anisotropy and a

Reorientation of the liquid-crystalline side chains from the homeotropic to a planar orientation without setting a Preferred direction. This written information is read out with linearly polarized polychromatic light. She is stable in the Glass state of the polymer fixed. Upon heat input, the written information and induced, planar alignment of the side chains is erased. At the

rapid cooling from the isotropic melt again stiffens the macroscopically uniform homeotropic

Initial orientation of the present in the cell liquid crystalline polymer layer. This process is repeatable for several 100 cycles without system fatigue. Execution Example 3 Will the cell described under 1 pixel by pixel with linearly polarized actinic light of different polarization directions

irradiated, pixels of different preferred orientations are obtained. Conoscopic studies show that in a cell with previously macroscopically uniform homeotropic orientation after pixel-wise irradiation with continuous

Changing the oscillation plane of the actinic light in an angular range of 360 °, the side chains always a planar

or quasi-planar orientation perpendicular to the plane of oscillation of the linearly polarized actinic Einschreiblichteseinnehmen. All polarization directions are equal.

The writing process takes place at room temperature. The storage conditions are long-term stable in the glassy state of the polymer

fixed and read out with non-actinic, linearly polarized, polychromatic light.

This is a reversible optical information storage with three parameters (x-y coordinates and orientation) of the Pixel realized. Exemplary embodiment 4

If the cell described under 1 is irradiated with a laser at a wavelength of 454 nm, in addition to the planarization of the side chains, an additional thermal melting of the exposed area takes place. These changes are long-term stable in the glass state of the liquid-crystalline polymer and can be read with non-actinic, linearly polarized, polychromatic light.

Claims (6)

1. A method for reversible optical information storage and light-induced orientation of liquid crystalline polymers, characterized in that a homeotropically oriented film of a located below the glass transition temperature azochromophore liquid crystal side chain polymer is used as a storage medium that for writing an information storage medium pointwise or areal with linearly polarized or unpolarized actinic light is irradiated, wherein at the irradiated points of the storage medium a macroscopic fixed in the polymer in the planar or quasiplanar oriented type with a predetermined by the polarization of Einschreiblichtes orientation direction that for non-destructive reading of the inscribed information storage medium with linearly polarized not actinic light longer wavelength is irradiated spotwise or areal and that the storage medium for deleting the written information is heated pointwise or areal, whereby the homeotropic initial state of the storage medium sets again upon cooling. 2. The method according to claim 1, characterized in that the cycle of writing - read delete - reorganization is repeated several times. 3. The method according to claim 1, characterized in that the write-in and read process is carried out with linearly polarized light of different polarization direction and pixels of different orientation at different locations of the film are written and read, so that an information storage with three parameters. 4. The method according to claim 1, characterized in that the storage medium is irradiated areally with actinic light of a polarization direction and thus planar or quasiplanar oriented layers are prepared with eim preferential orientation. 5. The method according to claim 1, c idurch characterized in that are used as the liquid-crystalline side-chain polymer polysiloxanes, polyacrylates or polymethacrylates, in copolymers in combination with mesogenic side groups known per se, preferably Phenylbenzoat-, Phenylbenzamid- and biphenyl groups. 6. The method according to claim 1 and 5, characterized in that is used as azochromophorhaltiges liquid-crystalline side-chain polymer poly (4-methacryloyloxyhexylenoxy-benzoic acid-4'-methoxyanilide-co-methacryloyloxyhexylenoxy-azobenzene).