Note: Descriptions are shown in the official language in which they were submitted.
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Holographic recording material
The present invention relates to a recording material for a holographic volume
storage medium, its preparation and use for the recording of volume holograms.
S
Holography is a method in which, using the interference between two coherent
light
beams (signal wave and reference wave), objects can be imaged in suitable
storage
materials and those images can be read out again using light (reading beam)
(D. Gabon Nature 151, 454 (1948), N. H. Farath, Advances in Holography, Vol.
3,
Marcel Decker (1977), H. M. Smith, Holographic Recording Materials, Springer
(1977)). By changing the angle between the signal wave and the reference wave
on
the one hand and the holographic storage material on the ether hand, numerous
holograms can be written into the material at one and the same specimen
position
and ultimately can also be read out again individually. The coherent light
source
used is generally the light of a laser. A very wide variety of materials are
described
as the storage material, for example inorganic crystals such as LiNb03 (for
example), organic polymers (for example M. Eich, J. H. Wendorff, Makromol.
Chem., Rapid Commun. 8, 467 (1987), J. H. Wendorff, M. Eich, Mol. Cryst. Liq.
Cryst. 169, 133 (1989)) or photopolymers (Uh-Sock Rhee et al., Applied Optics,
34
(5), 846 (1995)).
However, those materials do not yet meet all the requirements made of a
holographic
recording medium. In particular, the recorded hologram does not have adequate
stability. Multiple recording is generally possible to only a limited extent,
since on
recording a new hologram, the hologram already recorded is overwritten and
therefore erased. That is true especially of inorganic crystals, which are
subjected to
complex heat treatment in order to compensate for those stability problems.
Photopolymers, on the other hand, exhibit the problem of shrinkage, which has
an
adverse effect on the holographic imaging properties.
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Materials having a high degree of stability of the recorded holograms are also
known, for example from EP 0 704 513 and German Application DE-A-19703132,
which has not yet been laid open.
However, the high optical density of those materials does not permit the
preparation
of holographic volume storage media such as are required for the storage of
numerous holograms in a storage material.
Accordingly, there was a need for a material which is suitable for the
preparation of
sufficiently thick holographic volume storage media and which allows numerous
holograms to be stored at one specimen position of the recording material in
such a
manner that they have long-term stability. With the materials known hitherto,
the
storage of numerous holograms one after another at one position led to the
gradual
erasing of the holographically stored information: holograms recorded later
addressed the same molecules as had been used in the building up of holograms
recorded previously, so that the information of earlier holograms was lost
after only
a few further recording processes.
Accordingly, the invention provides a recording material for a holographic
volume
storage medium, containing at least one dye which changes its spatial
arrangement
when a hologram is recorded and, optionally, at least one shape-anisotropic
grouping, characterised in that it permits the recording of two or more
holograms at
one specimen position.
That is preferably effected in that the dye, of which there is at least one,
changes its
spatial arrangement in such a manner that it can no longer be excited by the
electromagnetic radiation, or changes its absorption behaviour, in particular
lowers
its sensitivity to the actinic light, preferably reduces it by from 10 % to
100 %, more
preferably from 50 % to 100 % and most preferably from 90 % to 100 %, in each
case based on the sensitivity prior to recording of the first hologram.
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However, the dye can also reduce its absorption behaviour, in particular its
sensitivity to the actinic light, in that it flips into the direction
perpendicular to the
polarising direction of the actinic light and its molecular longitudinal axis
comes to
lie at an angle with the polarising direction of the actinic light of from
10° to 90°,
preferably from 50° to 90°, more preferably from 75° to
90° and most preferably
from 85° to 90°.
In that manner it is possible successfully to carry out the recording of
several
holograms at one specimen position, that is to say the information of the
early
holograms is not erased completely.
That change in excitation behaviour with respect to electromagnetic radiation
when
the hologram is recorded can be achieved by the dye changing its spatial
arrangement in the polymeric or oligomeric organic, amorphous material.
With materials of that type it is possible, when a hologram is recorded, to
prevent
the holograms already previously recorded in that material from being
unacceptably
diminished, completely damaged or even entirely overwritten.
From the point of view of measurement techniques, an unacceptable attenuation
means that the remaining information can no longer be resolved in relation to
the
background noise.
The information is stored holographically. To that end, two polarised,
coherent
beams are brought to interference on the specimen.
As a result of exposure to that actinic light, the dyes change their spatial
position in
the polymeric or oligomeric layers. Dyes which, on exposure, orient their
molecular
longitudinal axis into the plane spanned by the two recording beams (incident
plane)
can no longer be excited by that light if the polarisation of the light lies
perpendicular to the incident plane. The information (hologram) recorded in
those
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dyes during that recording process is protected against change during
recording of a
subsequent hologram. Dyes which do not come to lie completely perpendicularly
to
the polarising direction of the light but form an angle O other than
90° with that
polarising direction, are addressed further during subsequent hologram
exposures.
However, the probability of those dyes being reoriented and, especially, the
light
sensitivity of the dyes decrease all the more, the closer the angle of the
molecular
longitudinal axis to the 90° position.
The molecular longitudinal axis can be determined, for example, by means of
the
molecular form by molecular modelling (e.g. CERILTSZ).
The reorientation of the dyes after exposure to actinic light results, for
example,
from investigations into polarised absorption spectroscopy: A sample
previously
exposed to actinic light is studied between two polarisers in a LJVlVIS
spectrometer
(e.g. CARY 4G, UVNIS spectrometer) in the spectral range of the absorption of
the
dyes. When the sample is rotated about the sample normal, and with the
polarisers in
a suitable position, for example in the crossed condition, the reorientation
of the
dyes follows from the variation in intensity of the extinction as a function
of the
sample angle and as a result can be clearly determined.
For the recording of several holograms there are various multiplex processes,
such as
angle multiplexing, wavelength multiplexing, phase multiplexing, shift
multiplexing, peristrophic multiplexing and others.
A measure of the sensitivity to the actinic light is the holographic
sensitivity. It is
calculated, for example, from the holographic growth curve, that is to say the
development of the diffraction efficiency (= diffracted intensity based on
incident
intensity of the reading laser) as a function of the energy deposited by the
recording
beams. The sensitivity is defined as the gradient of the root of the
diffraction
efficiency according to the deposited energy, standardised to the thickness of
the
storage medium.
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The invention provides a recording material for a holographic volume storage
medium, which at the wavelength of the recording laser has an optical density
< 2,
preferably _< 1, more preferably S 0.3. In that manner it is possible to
ensure that the
actinic light leads to homogeneous transillumination of the entire storage
medium
and a thick hologram can be produced. The optical density can be determined
using
commercial UV/VIS spectrometers (e.g. CARY 4G, UV/VIS spectrometer).
The recording material according to the invention is preferably a material
which has
an irradiated thickness of >_ 0.1 mm, more preferably 0.5 mm, more preferably
still
>_ 1 mm and most preferably not greater than 5 cm.
The grouping that interacts with the electromagnetic radiation is a dye. The
material
according to the invention consequently contains at least one dye. The
electromagnetic radiation is preferably laser light, preferably in the
wavelength
range from 390 to 800 nm, more preferably in the range from 400 to 650 nm,
most
preferably in the range from S 10 to 570 nm.
For the purposes of reading, the recording material is not exposed to two
interfering
beams, as in the case of recording, but only to one beam, the reading beam.
The
wavelength of the reading beam is preferably of longer wavelength than that of
the
signal wave and the reference wave, for example from 70 to 500 nm longer.
However, reading with the wavelength of the recording laser is also possible
and
will be used especially in the case of the commercial use of holographic
volume
storage media. For that purpose, however, the energy of the reading beam is
reduced
during the reading process either by reducing the intensity of exposure or the
exposure time, or by reducing the intensity of exposure and the exposure time.
The optical density of the recording material according to the invention is
adjusted
by the following two parameters
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a) via the molar extinction coefficient of the dye, of which there is at least
one,
and/or
b) via the concentration of the dye, of which there is at least one, in the
polymeric
or oligomeric organic material.
Dyes having low extinction coefficients are, for example, dyes having a non-
polar
and/or only slightly polarisable structure. Such dyes may originate, for
example,
from the classes of the anthraquinone, stilbene, azastilbene, azo or methine
dyes.
Azo dyes are preferred. Special preference is given to azo dyes having an
absorption
maximum of the ~c~* band that is less than or equal to 400 nm, more preferably
less
than 400 nm.
Azo dyes have, for example, the following structure of formula (I)
1 S Xt (R~)m
w
i XZ
wherein
X' and XZ are the same or similar,
R' and RZ each independently of the other represents hydrogen or a non-ionic
substituent, and
m and n each independently of the other represents an integer from 0 to 4,
preferably from 0 to 2.
X' and XZ are the same or similar because they are both bonded via atom types
or
atom groupings which are either the same or of a similar electronic structure.
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The Hammett constants, for example, can be used as a measure of the similarity
of
the electronic structure of the atom types or groupings.
X' and Xz preferably represent -X'~-R3 and XZ'-R4, respectively,
wherein
X'' and Xz' represent a direct bond, -O-, -S-, -(N-RS)-, -C(R6R')-, -(C-0)-,
-(CO-O)-, -(CO-NRS)-, -(SOZ)-, -(SOZ-O)-, -(SOZ-NRS)-, -(C=NR8)- or
-(CNRB-NRS)-,
R3, R4, RS and R8 each independently of the others represents hydrogen, C,- to
Czo-
alkyl, C3 to C,o cycloalkyl, CZ- to Czo alkenyl, C6 to C,o aryl, C,- to
CZO alkyl-(C=O)-, C3- to C,o-cycloalkyl-(C=O)-, CZ- to CZO alkenyl-
(C=O)-, C6 to C,o aryl-(C=O)-, C,- to CZO alkyl-(SOZ)-, C3- to C,o
cycloalkyl-(SOz)-, Cz to CZO alkenyl-(SOz)- or C6 to C,o aryl-(SOZ)-,
or
X'~-R' and Xz~-R4 may represent hydrogen, halogen, cyano, nitro, CF, or CC1,,
R6 and R' each independently of the other represents hydrogen, halogen, C,- to
CZO alkyl, C,- to CZO alkoxy, C,- to C,o cycloalkyl, Cz- to CZO alkenyl
or C6 to C,o aryl.
Non-ionic substituents are to be understood as being halogen, cyano, nitro, C,-
to
CZO alkyl, C,- to C2o alkoxy, phenoxy, C3- to C,o-cycloalkyl, CZ- to Czo
alkenyl or C6
to C,o aryl, C,- to CZO alkyl-(C=O)-, C6 to C,o aryl-(C=O)-, C,- to C2o alkyl-
(SOZ)-,
C,- to CZO alkyl-(C=O)-O-, C,- to CZO alkyl-(C=O)-NH-, C6 to C,o aryl-(C=O)-NH-
,
C,- to CZO alkyl-O-(C=O)-, C,- to CZO alkyl-NH-(C=O)- or C6- to C,o aryl-NH-
(C=O)-.
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The alkyl, cycloalkyl, alkenyl and aryl radicals may in turn be substituted by
up to
three radicals from the group halogen, cyano, nitro, C,- to CZO alkyl, C,- to
CZO-
alkoxy, C,- to C,o cycloalkyl, Cz to Czo alkenyl or C6 to C,o aryl, and the
alkyl and
alkenyl radicals may be straight-chained or branched.
Halogen is to be understood as being fluorine, chlorine, bromine and iodine,
particularly fluorine and chlorine.
The recording material according to the invention is preferably polymeric or
oligomeric organic, amorphous material, more preferably a side-chain polymer,
also
more preferably a block copolymer and/or a graft polymer.
The principal chains of the side-chain polymer originate from the following
basic
structures: polyacrylate, polymethacrylate, polysiloxane, polyurea,
polyurethane,
polyester or cellulose. Polyacrylate and polymethacrylate are preferred.
The block copolymers consist of a plurality of blocks, of which at least one
type
contains the above-described copolymer systems. The other blocks consist of
non-
functionalised polymer structures which serve to dilute the functional block
in order
to adjust the required optical density. The extent of the functional block is
below the
wavelength of light, preferably in the region of less than 200 nm, more
preferably
less than 100 nm.
The polymerisation of the block copolymers takes place, for example, by means
of
free-radical or anionic polymerisation or by means of other suitable
polymerisation
processes, optionally followed by a polymer-analogous reaction, or by a
combination of those methods. The homogeneity of the systems is in a range
less
than 2.0, preferably less than 1.5. The molecular weight of the block
copolymers
obtained by free-radical polymerisation reaches values in the region of
50,000;
values greater than 100,000 can be adjusted by anionic polymerisation.
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The dyes, especially the azo dyes of formula (I), are bonded to those polymer
structures covalently, generally by way of a spacer. For example, X' (or Xz)
then
represents such a spacer, especially in the meaning X'~-(Q'); T'-S'-
wherein
X'~ is as defined above,
Q' represents -O-, -S-, -(N-RS)-, -C(R6R')-, -(C=O)-, -(CO-O)-, -(CO-NRS)-,
-(SOZ)-, -(SOz-O)-, -(SOZ NRS)-, -(C=NRB)-, -(CNRg-NRS)-, -(CHz)P , P- or
m-C6H4 or a divalent radical of the formula
I ~ ~ ~N.
or ~NJ
i represents an integer from 0 to 4, wherein when i > 1 the individual
radicals
Q' may have different meanings,
T' represents -(CHZ)p , wherein the chain may be interrupted by -O-, -NR9- or
-OSiR'°20-,
S' represents a direct bond, -O-, -S- or -NR9-,
p represents an integer from 2 to 12, preferably from 2 to 8, more preferably
from 2 to 4,
R9 represents hydrogen, methyl, ethyl or propyl,
R'° represents methyl or ethyl, and
RS to R8 are as defined above.
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PCTIEP00101479
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Preferred dye monomers for polyacrylates or -methacrylates then have the
formula (II)
O s Xt, (R1)T
S .T~~Q )' ~ / .,N
N I w
XZ
~R2)~
wherein
R represents hydrogen or methyl, and
the other radicals are as defined above.
Particularly preferred monomers of formula (II) are, for example:
O
O~O ~ I 1"~ CH3
w N
O '~~, N'N I ~ O
O
O
a,
O ~ ~ :N
N ~ ~ O
'o
O
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Y'ISt' '
to °
°~° ~ I
a
O
O~O
U
CH
O
O~O i ~
O
O ~ i :N
N ( ~ O
O NC
~O I i :N
~O N I
CN
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O
N
N
CF3
O
O i ( CI
.O
O_SO ( i N.N
I i
NOZ
O
O H N I w
:N
N
I i :CH3
O
The polymeric or oligomeric organic, amorphous material according to the
invention
may carry in addition to the dyes, for example of formula (I), shape-
anisotropic
groupings. Those groupings, too, are generally bonded to the polymer
structures
covalently via a spacer.
Shape-anisotropic groupings have, for example, the structure of formula (III)
(R")
X3 4
I ,i Y.z
~II)~
wherein
Z represents a radical of formula
X~
I ~R~= (IIIa) or
).
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~ j (R,s)
(IIIb)
wherein
A represents O, S or N-C,- to C4 alkyl,
X' represents -X3'-(QZ)~-TZ-SZ-,
X4 represents X°'-R",
X3' and X4' each independently of the other represents a direct bond, -O-, -S-
,
-(N-RS)-, -C(R6R')-, -(C=O)-, -(CO-O)-, -(CO-NRS)-, -(SOZ)-, -(Sp2 O)-,
-(SOZ-NRS)-, -(C=NR8)- or -(CNRe-NRS)-,
R5, Re and R'3 are each independently of the others hydrogen, C,- to CZO
alkyl, C3- to
C,o cycloalkyl, CZ to CZa alkenyl, C6 to C,o aryl, C,- to CZO alkyl-(C=O)-,
C,- to C,o cycloalkyl-(C=O)-, Cz- to CZO alkenyl-(C=O)-; C6- to C,o aryl-
(C=O)-, C,- to C2o alkyl-(SOZ)-, C3- to C,a cycloalkyl-(SOZ)-, CZ to CZO-
alkenyl-(SOZ)- or C6 to C,o aryl-(SOZ)-, or
X4'-R'3 may represent hydrogen, halogen, cyano, nitro, CF3 or CCl3,
R6 and R' each independently of the other represents hydrogen, halogen, C,- to
CZO-
alkyl, C,- to CZO alkoxy, C,- to C,o cycloalkyl, Cz- to CZO alkenyl or C6 to
C,o aryl,
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Y represents a single bond, -COO-, OCO-, -CONH-, -NHCO-, -CON(CH3)-,
-N(CH3)CO-, -O-, -NH- or -N(CHj)-,
R", R'z, R'S each independently of the others represents hydrogen, halogen,
cyano,
nitro, C,- to C2o alkyl, C,- to Czo alkoxy, phenoxy, C3- to C,o cycloalkyl, CZ-
to CZO alkenyl or C6 to C,o aryl, C,- to Czo alkyl-(C=O)-, C6 to C,o aryl-
(C=O)-, C,- to Czo alkyl-(SOZ)-, C,- to Cza alkyl-(C=O)-O-, C,- to CZO alkyl-
(C=O)-NH-, C6 to C,a aryl-(C=O)-NH-, C,- to CZO alkyl-O-(C=O)-, C,- to
CZO alkyl-NH-(C=O)- or C6 to C,o aryl-NH-(C=O)-,
q, r and s each independently of the others represents an integer from 0 to 4,
preferably from 0 to 2,
' QZ represents -O-, -S-, -(N-RS)-, -C(R6R')-, -(C=O)-, -(CO-O)-, -(CO-NRS)-,
-(SOz)-, -(SOz-O-)-, -(S02-NRS)-, -(C=NR8)-, -(CNRB-NRS)-, -(CHZ)p , p- or
m-C6H4- or a divalent radical of the formula
w w
/ or ~
j represents an integer from 0 to 4, wherein when j > 1 the individual
radicals
Q' may have different meanings,
TZ represents -(CHZ)p , wherein the chain may be interrupted by -O-, -NR9- or
-OSiR'°20-,
SZ represents a direct bond, -O-, -S- or -NR9-,
p represents an integer from 2 to 12, preferably from 2 to 8, more preferably
from 2 to 4,
R9 represents hydrogen, methyl, ethyl or propyl, and
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R'° represents methyl or ethyl.
Preferred monomers having such shape-anisotropic groupings for polyacrylates
or
-methacrylates then have the formula (IV)
O (R )o
3'
S T (Q~
i Y.z
(IV),
wherein
R represents hydrogen or methyl, and
the other radicals are as defined above.
Particularly preferred shape-anisotropic monomers of formula (IV) are, for
example:
O
O~O / I
O ~
O
O
O~O /
a,
O ~~ y
O
O
O
W
O
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The alkyl, cycloalkyl, alkenyl and aryl radicals may in turn be substituted by
up to
three radicals from the group halogen, cyano, nitro, C,- to C2o alkyl, C,- to
Czo
alkoxy, C3- to C,o cycloalkyl, Cz- to C2o alkenyl or C6 to C,a aryl, and the
alkyl and
alkenyl radicals may be straight-chained or branched.
Halogen is to be understood as being fluorine, chlorine, bromine and iodine,
particularly fluorine and chlorine.
In addition to those functional units, the oligomers or polymers according to
the
invention may also contain units which serve mainly to lower the percentage
content
of functional units, especially of dye units. In addition to that function,
they may
also be responsible for other properties of the oligomers or polymers, for
example
the glass transition temperature, liquid crystallinity, film-forming property,
etc..
1 S For polyacrylates or -methacrylates, such monomers are acrylic or
methacrylic acid
esters of formula (V)
R
O O
R'~
(V),
wherein
R represents hydrogen or methyl, and
R'" represents optionally branched C,- to C2o alkyl or a radical containing at
least
one further acryl unit.
Polyacrylates and polymethacrylates according to the invention then contain as
repeating units preferably those of formula (VI), preferably those of fonmulae
(VI)
and (VII) or of formulae (VI) and (VIII) or those of formulae (VI), (VII) and
(VIII)
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R
HzC O
R
S\ ~ O
T
~ , HZC
~QvX,. ~ S \ z
T
i (Qz)~ R
O
\ ~R~~ X HzC
N:N m ~ lR") ~ .Ru .
(R~)~ ~ tar) ~nr) (vrrr)
to w ( Y~z
X2
and and
wherein the radicals are as defined above.
It is also possible for there to be present several of the repeating units of
formula (VI) and/or of the repeating units of formula (VII) and/or (VIII).
The relative proportions of VI, VII and VIII are as desired. The concentration
of VI,
depending on the absorption coefficient of VI, is preferably from 0.1 to 100
%,
based on the mixture in question. The ratio of VI to VII is from 100:0 to
1:99,
preferably from 100:0 to 30:70, more preferably from 100:0 to 50:50.
The dyes of formula (I) and the dye monomers of formula (II) exhibit a
principal
absorption band (~-n* band) in the short-wave range and a secondary absorption
band (n-~* band) in the longer wave range. The molar extinction coefficient s
of that
n-~* band is in the range of from 400 to 5000 * 10' cm2/mol. At an assumed dye
molar mass of 400 g/mol, oligomers or polymers at an irradiated thickness of
0.1 mm have an optical density _< 2, if they contain from < 1.6 % (for s =
5000) to
< 20 % (for s = 400) of such dyes.
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The polymers and oligomers according to the invention preferably have glass
transition temperatures Tg of at least 40°C. The glass transition
temperature can be
determined, for example, according to B. Vollmer, Grundri~i der
Makromolekularen
Chemie, p. 406-410, Springer-Verlag, Heidelberg 1962.
The polymers and oligomers according to the invention have a molecular weight,
determined as the weight average, of from 5000 to 2,000,000, preferably from
8000
to 1,500,000, determined by gel permeation chromatography (calibrated with
polystyrene).
Graft polymers are prepared by the free-radical binding of dye monomers of
formula
(II) and, optionally, additionally of shape-anisotropic monomers of formula
(IV)
and/or, optionally, additionally of monomers of formula (V) to oligomeric or
polymeric base systems. Such base systems may be polymers of very different
kinds,
for example polystyrene, poly(meth)acrylates, starch, cellulose, peptides. The
free-
radical binding can be carried out by irradiation with light or by the use of
reagents
producing radicals, for example tert-butyl hydroperoxide, dibenzoyl peroxide,
azodiisobutyronitrile, hydrogen peroxide/iron(II) salts.
As a result of the structure of the polymers and oligomers, the intermolecular
interactions of the structural elements of formula (VI) with one another or of
formulae (VI) and (VII) with one another are so adjusted that the formation of
liquid-crystalline order states is suppressed and optically isotropic,
transparent non-
scattering films, foils, plates or cuboids can be produced. On the other hand,
the
intermolecular interactions are nevertheless sufficiently strong that, on
irradiation
with light, a photochemically induced, cooperative, directed reorientation
process of
the photochromic and non-photochromic side groups is brought about.
There preferably occur between the side groups of the repeating units of
formula (VI) or between those of formulae (VI) and (VII) interaction forces
which
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are sufficient for the photo-induced change in the configuration of the side
groups of
formula (VI) to bring about a unidirectional - so-called cooperative -
reorientation of
the other side groups ((VI) and/or (VII)).
In the optically isotropic amorphous photochrornic polymers, extremely high
values
of the optical anisotropy can be induced (0n to 0.4).
As a result of the influence of actinic light, order states in the polymers or
oligomers
are generated and modified and the optical properties are thus modulated.
The light used is polarised light whose wavelength lies in the range of the
absorption
band, preferably in the range of the long-wave n-~* band, of the repeating
units of
formula (VI).
The polymers and oligomers can be prepared by processes known in the
literature,
for example in accordance with DD 276 297, DE-A 3 808 430, Makromolekulare
Chemie 187, 1327-1334 (1984), SU 887 574, Europ. Polym. 18, 561 (1982) and
Liq.
Cryst. 2, 195 (1987).
The preparation of films, foils, plates and cuboids is possible without the
necessity
for complex orientation processes using external fields and/or surface
effects. They
can be applied to substrates by spin coating, immersion, pouring or other
coating
processes which are readily controllable from a technological point of view,
can be
brought between two transparent plates by pressing or flowing, or can simply
be
prepared as a self supporting material by pouring or extrusion. Such films,
foils,
plates and cuboids can also be prepared from liquid-crystalline polymers or
oligomers which contain structural elements in the described sense, by sudden
cooling, that is to say by a cooling rate of > 100 K/min., or by rapid removal
of the
solvent.
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Preference is given to a process for preparing the holographic volume storage
medium in which there is contained a step according to a conventional
injection-
moulding process in the range up to 300°C, preferably up to
220°C, more preferably
180°C.
S
The layer thickness is >_ 0.1 mm, preferably >_ 0.5 mm, more preferably >_ 1
mm. A
particularly preferred preparation process for layers in the millimetre range
is the
injection-moulding process. In that process, the polymer melt is forced
through a
nozzle into a shaping holder, from which it can be removed after cooling.
A preferred method of preparing the recording material or the polymer
according to
the invention contains a process wherein at least one monomer is polymerised
without further solvent, the polymerisation preferably being free-radical
polymerisation and, more preferably, being initiated by free-radical
initiators and/or
I1V light and/or thermally.
The process is carried out at temperatures of from 20°C to
200°C, preferably from
40°C to 150°C, more preferably from 50°C to 100°C
and most preferably of about
60°C.
In a particular embodiment, AIBN is used as the free-radical initiator.
It has frequently been found to be advantageous to use concomitantly a
further,
preferably liquid monomer. Such monomers are to be understood as being
monomers which are liquid at the reaction temperatures, which monomers are
preferably olefinically unsaturated monomers, more preferably based on acrylic
acid
and methacrylic acid, most preferably methyl methacrylate.
The content of monomers of formula (II) in the copolymers is preferably from
0.1 to
99.9 wt.%, more preferably from 0.1 to SO wt.%, more preferably still from 0.1
to 5
wt.% and, most preferably, from 0.5 to 2 wt.%.
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The method of holographic data storage is described, for example, in LASER
FOCUS WORLD, NOVEMBER 1996, page 81 ff.
On recording a hologram, the above-described polymer films are irradiated by
two
coherent laser beams of a wavelength that brings about the required light-
induced
reorientations. One beam, the object beam, contains the optical information to
be
stored, for example the variation in intensity resulting from the passage of a
light
beam through a two-dimensional, chequered pixel structure (data page). In
principle,
however, light that is diffracted, scattered or reflected by any two- or three-
dimensional object can be used as the object beam. On the storage medium, the
object beam is brought to interference with the second laser beam, the
reference
beam, which is generally a flat or circular wave. The resulting interference
pattern is
imprinted in the storage medium as modulation of the optical constants
(refractive
index and/or absorption coefficient). That modulation permeates the entire
irradiated
area, especially the thickness of the storage medium. If the object beam is
then
blocked off and the medium is exposed solely to the reference beam, the
modulated
storage medium acts as a kind of diffraction grating for the reference beam.
The
intensity distribution resulting from the diffraction corresponds to the
intensity
distribution which originated from the object to be stored, so that it is no
longer
possible to distinguish whether the light comes from the object itself or
whether it
results on account of diffraction of the reference beam.
For the storing of different holograms at one specimen position, various
multiplex
processes are used: wavelength multiplexing, shift multiplexing, phase
multiplexing,
peristrophic multiplexing and/or angle multiplexing and/or others. In the case
of
angle multiplexing, the angle between the storage medium, in which a hologram
has
been stored under the relevant angles, and the reference beam is changed. From
a
certain angle change, the original hologram disappears (Bragg mismatch): the
incident reference beam can no longer be deflected by the storage medium to
reconstruct the object. The angle from which that occurs depends on the
thickness of
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the storage medium (and on the modulation of the optical constants produced in
the
medium): the thicker the medium, the smaller the angle by which the reference
beam
must be changed.
A further hologram can be recorded in that new angle configuration. The
reading of
that hologram again takes place precisely in the angle configuration between
the
storage medium and the reference beam in which it was recorded.
By gradually changing the angle between the medium and the recording beams, it
is
therefore possible to record a plurality of holograms in the same place on the
storage
medium.
The polymer systems described in the present patent have the great advantage
that,
when a subsequent hologram is recorded, the information deposited in the
storage
medium relating to the previous holograms is not erased and that more than 5
holograms, preferably more than 50, more preferably more than 100, more
preferably still more than 500 and most preferably more than 1000 holograms
can be
recorded at one place on the storage medium. The objects to be stored are data
pages
produced by transmission of a liquid crystal display. Those data pages have
256 x
256 pixels, preferably 512 x 512 pixels, more preferably 1024 x 1024 data
pixels.
The invention also provides a recording material for a holographic volume
storage
medium consisting of a polymeric or oligomeric organic, amorphous material
which
contains at least one grouping that interacts with electromagnetic radiation
and,
optionally, at least one shape-anisotropic grouping, characterised in that it
has an
optical density S 2, preferably <_ 1, more preferably <_ 0.3. The recording
material can
be used for storing data in the form of an unsupported film or, preferably, in
a
multilayer structure. The multilayer structure is, for example, a sandwich in
which
the actual recording medium is surrounded by at least one substrate. The
substrate
may be transparent media having high optical quality, for example glass
plates,
quartz plates or plates of polycarbonate. High optical quality is to be
understood as
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meaning that the scattering efficiency, that is to say the quotient between
light
scattered at that sandwich and the incident light, is not less than 10-4,
preferably not
less than 10-5, especially not less than 10-6. In order to determine that
quotient, the
sample can be exposed to the beam of an HeNe laser. Detection is by means of a
S CCD camera.
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Examples
Example 1
Preparation of monomers:
O
0~ O
N ~ ~ N H
N N
p
a) 4-(2-Hydroxyethyloxy)benzoic acid
138 g of p-hydroxybenzoic acid and 0.5 g of KI are placed in 350 ml of
ethanol, with
stirnng. A solution of 150 g of KOH in 150 ml of water is added dropwise. 88.6
g of
ethylenechlorohydrin are added dropwise at 30-60°C in the course of 30
minutes.
The reaction mixture is stirred under reflux for 15 hours. The solvent is then
distilled
off completely first under normal pressure and then in vacuo. The residue is
dissolved in 1 litre of water and acidified using HCI. The precipitate is
filtered off
with suction and recrystallised from 1.8 litres of water. The product is dried
and
recrystallised twice from ethanol. The yield is 46 g (25 % of the theoretical
yield).
M.p. 179.5°C.
b) 4-(2-Methacryloyloxyethyloxy)benzoic acid
45 g of 4-(2-hydroxyethyloxy)benzoic acid, 180 ml of methacrylic acid, 10 g of
p
toluenesulfonic acid and 10 g of hydroquinone are heated at reflux in 150 ml
of
chloroform, with stirring. The water that forms during the reaction is
separated off in
a water separator. The reaction mixture is diluted with 150 ml of chloroform,
washed
several times using 100 ml of water each time, and dried over NazSO,. The
drying
agent is filtered off and .the chloroform is distilled off to two thirds in a
rotary
evaporator. The product precipitates, is filtered off with suction and is
recrystallised
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twice from isopropanol. The yield is 28 g (45 % of the theoretical yield).
M.p.
146°C.
c) 4-(2-Methacryloyloxyethyloxy)benzoic acid chloride
25 g of 4-(2-methacryloyloxyethyloxy)benzoic acid, 80 ml of lhionyl chloride
and
0.5 ml of DMF are stirred at room temperature for 30 minutes. Excess thionyl
chloride is then distilled off first under a moderate vacuum and then under a
high
vacuum. The acid chloride formed in a virtually quantitative yield then slowly
crystallises out at room temperature.
Elemental analysis: C"H,3CI04 (268.7)
talc.: C 58.11; H 4.88; Cl 13.19;
found: C 58.00; H 4.90; CI 13.20.
d) 4-Pivalinoylamino-4'-aminoazobenzene
36 g of 4,4'-diaminoazobenzene and 62 g of triethylamine are placed in 400 ml
of
THF. A solution of 23.2 g of pivalic acid chloride in 100 ml of THF is slowly
added
dropwise. After 2 hours' stirring at room temperature, water is added to the
reaction
mixture. The precipitate is filtered off and dried. 42 g of the product are
obtained.
Further purification is carried out by chromatography (silica gel;
toluene/ethyl
acetate 1:1). The yield is 8 g. M.p. 230°C.
e) 4-Pivalinoylamino-4'-[p-(2-methacryloyloxyethyloxy)benzoylamino]azobenzene
1 g of 4-pivalinoylamino-4'-aminoazobenzene is placed in 10 ml of N-methyl-2-
pyrrolidone (NMP) at 50°C and added to a solution of 1 g of 4-(2-
methacryloyloxyethyloxy)benzoic acid in 1 ml of NMP at 50°C. The
reaction
mixture is stirred at that temperature for one hour and is cooled, and 200 ml
of water
are added. The precipitate is filtered off and stirred in 30 ml of methanol at
room
temperature; the mother liquor is filtered off and drying is carried out in
vacuo. The
yield is 1.2 g. M.p. 194°C. ~,",ax = 378 nm (DMF); E = 37000
I/(mol*cm).
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Example 2
a) 32 g of N-benzoyl-p-phenylenediamine were placed in a mixture of 210 ml of
glacial acetic acid, 75 ml of propionic acid and 31 ml of concentrated
S hydrochloric acid at 3-5°C. 50 g of nitrosylsulfuric acid (approx. 40
%) were
added dropwise at that temperature over a period of one hour.
b) 16 g of m-toluidine were dissolved in 130 ml of glacial acetic acid. At 0-
5°C, the
diazotisation from a) was added dropwise thereto over a period of 2 hours. The
mixture was stirred overnight at room temperature. The resulting dye was
filtered
off with suction and suspended in 550 ml of water. The pH was raised to 8.4
using soda. The dye was filtered off with suction again, washed with
isopropanol
and dried. 27 g (54.4 % of the theoretical yield) of the dye of the formula
i
,N
N I
H3C ~ NH2
were obtained. UV/VIS in dimethylformamide: ~,",ax = 416 nm.
c) 5 g of the dye from b) were dissolved in 20 ml of N-methylpyrrolidone at
50°C.
3.5 g of the acid chloride of the formula
O
CI ,
i O'~O
O
were added. The mixture was stirred for 1.5 hours at 50°C. Finally, 20
ml of
water were added and the resulting dye was filtered off with suction. It was
stirred
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with 50 ml of isopropanol, filtered off with suction and dried. 6.2 g (73.4 %
of the
theoretical yield) of the dye monomer of the formula
a
s o ~ ~ -.N
N ~ O
i O~O
v
were obtained. W/VIS in dimethylformamide: ~.",~x = 386 nm.
The dye monomers of the following Table were prepared analogously.
R~'
Rs:
O
O s~
'R
O
Example Rs~ Rsz Rss Tsa - Rss ",'-'Rse
2 CH, H H H p H 378 nm
3 CH3 CH3 H H ~ ~ ~ H 374 nm
4 H H CH, H ~ ~ I H 386 nm
5 CH3 CH3 H H a ~ ' CZHs 371 nm
6 CH, H CH3 H CHs ~ H 395 nm
,N ~
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Example of graft polymers
8.7 g of the starch Perfectamyl A 4692 (86.3 %) from Avebe, Foxhol, NL, were
dissolved in 60 ml of water at 86°C. A mixture of 1.5 g of a 1 wt.%
aqueous FeSO,
solution and 6.1 g of a 3 wt.% aqueous HzOz solution were added thereto.
Stirring
was carried out for 15 minutes at 86°C. Then, at that temperature, a
solution of 1.4 g
of the dye monomer of the formula
O
~N
O H3C ~ / ,N
N
_CH3
O
in 12.5 g of methyl methacrylate and 4.1 g of a 3 wt.% aqueous HZOz solution
were
simultaneously added dropwise over a period of 90 minutes. After a further 15
minutes at that temperature, 0.105 g of tert-butyl hydroperoxide was added and
stirnng was carried out for a further one hour at 86°C. The fine yellow
dispersion
was filtered through a 100 pm polyamide filter.
The dispersion was diluted 1:10 with water, applied to a glass plate and
dried. The
transparent pale yellow film on the glass plate was irradiated foi 10 minutes
with
polarised light, KL 500 cold light lamp from Schott (spot diameter 6 mm).
Between
crossed polarisers, the irradiated spot could be seen brightly in dark
surroundings.
Example 3
Preparation of holographic materials by block polymerisation
A solution of 1 mol% of a monomer of formula (II) or of Example 2 and 0.052 g
of
2,2'-azoisobutyric acid nitrile in 10 g of methyl methacrylate was rinsed with
dry
argon in a glass ampoule for 30 minutes. The ampoule was closed with a rubber
stopper and tempered for 7 days at 60°C. A transparent polymer cylinder
was
obtained. The polymer cylinder could be isolated by breaking the ampoule and
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removing the broken glass. Further storage for 2 weeks at 60°C served
to remove the
methyl methacrylate residues and relieve the stresses within the polymer
block.
The PAP cylinder so obtained was cut into plates having a diameter of 17 mm
and a
S thickness of 1.9 mm in a precision engineering workshop and was then
polished.
The plates have the optical densities according to the invention at the
important
wavelengths: OD(532 nm) and OD(568 nm).
In an analogous manner, copolymer having an azo dye content of 10 mol% is
prepared. In an analogous manner, copolymer is prepared having a content of
the
other monomers of 1 mol% and a methyl methacrylate content of 99 mol%.
Example 4
A polymer from Example 3 is applied to a glass substrate having a thickness of
150 p.m by means of spin coating from a solution. The layer thickness of the
measuring point located at the centre of the substrate is 600 nm. The level of
the
refractive index n of the polymer layer is determined for the three spatial
directions
x, y (layer plane) and z (layer normal) by the prism coupling method. To that
end,
the base of a prism is brought into close contact with the polymer layer. The
angles
at which the polarised light of a laser couples into the layer and passes
through it in
the manner of a waveguide give information about its refractive index at the
wavelength of light. Each coupling in becomes clear as a signal drop on a
detector in
reflection.
When the polarisation of the laser is chosen perpendicular to the incident
plane (s-
polarisation), the refractive index in the polarisation direction can be
determined.
According to the orientation of the substrate, the values for nx and ny can be
determined. The index of the substrate which has the lower refractive index,
the
index of the prism and the laser wavelength (~, = 633 nm) enter into the
calculations.
In the case of polarisation in the incident plane (p-polarisation), the value
for nZ can
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be determined. To that end, one of the two spatial directions x or y must
coincide
with the incident plane. Additionally, the value of the refractive index of
the
direction so chosen (nx or ny) enters into the calculation.
S The refractive indices nX, ny and nz are determined on the sample before,
during and
after several exposures and cancellation processes. Exposure is effected by
irradiating the polymer layer with laser light of wavelength ~, = 514 nm in
perpendicular incidence. The light intensity is 200 mW/cmz. The light is
polarised
linearly in the x direction. Cancellation of the orientation anisotropy so
induced in
the xy plane takes place on polarisation in the y direction.
That can readily be verified by measuring the refractive indices n,~, n~, and
nz at ~, _
633 nm. To that end, the sample is measured, for example, in the untreated
state,
a$er 200 seconds' exposure, after 500 seconds' exposure and after 5000
seconds'
exposure.
Further information is obtained by measuring the refractive indices nx, n~,
and nz at
~. = 633 nm after the first cancellation and the second cancellation, as well
as after a
second or further exposure for 5000 seconds.
The level of the refractive index of each spatial direction is a measure of
the mean
number of chromophores oriented in that direction, because it correlates with
the
inducible polarisation and that is composed principally of the high molecular
polarisabilities along each molecular axis. Since nx and n~, are originally
identical,
there is a macroscopically isotropic distribution in the xy plane. The smaller
value
for nz indicates the planar molecular orientation, resulting from the
preparation
process. The first exposure leads gradually to an orientation distribution
with a
reduced number of chromophores lying in the x direction. The depletion in that
direction takes place in the statistical mean in equal parts in favour of the
other two
spatial directions y and z, to be read off from the increasing values of n~,
and nz. A
double refraction ny nx in the film plane can be cancelled again almost
completely.
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The number of chromophores oriented in the z direction increases, however,
with
each further exposure or cancellation procedure. As a result, those are no
longer
available to a further (subsequent) recording process.
S Example 5
A polymer of a monomer according to Example 2 is present in the form of a
granulate. It is applied to a glass substrate and heated to approximately
180°C. At
that temperature, the polymer melts. On the glass substrate there are located
spacers,
for example of mylar film or glass fibres, and a further cover glass. Using
that glass-
polymer-glass sandwich, layers in the range of from 20 to 1000 pm are
produced.
Example 6
1 S A 500 pm thick polymer film, prepared by the process of Example 5, is
studied in a
holographic structure. An SHG Nd:YAG laser (532 nm) is used as the recording
source. In the beam path of the object beam there lies a Spatial Light
Modulator,
which produces a data mask of 1024 x 1024 pixels. The intensity ratio of the
reference beam to the object beam is 7:1, the total power density falling on
the
sample is 200 mW/cm2. A hologram is recorded by superposing the reference beam
and the object beam, which are polarised perpendicularly to the incident
planes and
which fall onto the sample at an angle of 40° relative to each other
and expose the
sample for 30 seconds; the hologram is subsequently read out by exposure
solely
with the reference beam (exposure time 10 milliseconds). By changing the angle
of
the reference beam by 0.25°, the Bragg condition is broken and the
original
hologram is no longer visible. A new hologram is recorded under that new angle
configuration. The process is repeated 100 times. After each recording
process, there
are read out in addition to the hologram just recorded also all previously
recorded
holograms by adjusting the corresponding reference angle. Even after
completion of
the 100 recording processes, the information in all the holograms is retained.
