Azobenzene-containing diblock copolymers are a promising material class for holographic data storage. They have many
advantages, yet, a problem are their long writing times. Low-molecular-weight glasses containing azobenzene moieties
are also photo-addressable and develop holographic gratings much faster. By blending molecular glass formers with
diblock copolymers, materials for holographic data storage with distinctly improved properties can be obtained. The
writing times of holographic gratings in the blends decrease with increasing content of the molecular glass former;
nevertheless, the gratings are still stable.
In recent years the developments in computer technology have drastically accelerated. In order to handle the ever
growing amount of data to be stored, an increase in the storage capacities of mass storage media is necessary. A
promising approach is holography, where the storage capacity is greatly increased by using the entire volume instead of
only the surface of the medium.
Write-once media, which are mainly based on photopolymer systems, seem to be fairly advanced. For a rewriteable
media, only few systems can meet the strict requirements for holographic data storage materials. Photoaddressable
azobenzene-based polymer systems are the most promising candidates, however the photo-physical sensitivity of these
materials has to be further increased.
Low molecular-weight organic glasses with azobenzene moieties can also be used for reversible inscription of
holographic volume gratings. They exhibit a faster response time than a comparable photoaddressable polymer due to a
lack of chain entanglements. A new photochromic molecular glass with optimized physical and photo-physical
properties is investigated as blending materials to improve the photochromic response of photoaddressable polymers. By
doping this photochromic molecular glass into photoaddressable polymers we are able to combine the high stability of
polymer systems with the fast response of molecular glasses, thus creating a system which has the advantages of both
material classes.
This paper reports on a new series of photochromic azobenzene-containing molecular glasses which are investigated as
blending materials to improve the photochromic response of photoaddressable polymers. In order to identify potential
candidates for blending, a variety of molecular glasses are synthesized and screened with respect to their photochromic
response. Various end groups at the azobenzene chromophore and different core compounds are employed to obtain high
thermal stability and good glass-forming properties as well as high sensitivity and refractive-index modulation. The best
combination of structural variations is chosen to tailor a photoaddressable material with optimized physical and
photophysical properties. By doping this photochromic molecular glass former into a photoaddressable polymer we are
able to combine the high stability of polymer systems with the faster response of molecular glasses, thus creating a
system which has the advantages of both material classes. With this approach we achieve an increase of the low
photochromic sensitivity of the polymer, which is the biggest problem of these materials. In order to obtain a better
understanding of the influence of the photochromic molecular glass former on the photochromic response in a polymer
system we conduct holographic experiments on a concentration series of the glass former in an inert polymer matrix. By
inscribing the holographic gratings at elevated temperatures we are able to increase the sensitivity of our molecular glass
further.
The formation of phase and surface relief gratings in low-molecular-weight organic glasses containing azobenzene
moieties has been studied with holographic methods. Advantages of this class of materials are the simple synthesis, the
perfectly amorphous phase, and the possibility of blending them with polymers. Surface relief gratings are formed very
efficiently in molecular glasses, and this process can be explained by the gradient force model. Heights up to 610 nm
were measured; the temporal evolution of the diffraction efficiency could be reproduced in computer simulations. For
technical applications, the surface relief gratings can easily be duplicated by replica molding. Since surface gratings are
detrimental to holographic data storage at high densities, it is also possible to suppress their formation by using proper
polarizations of the writing beams. Reorientation of the azobenzene groups in the bulk of the glasses and angular
multiplexing was demonstrated and the thermal stability of the corresponding phase gratings was studied. Different
combinations of molecular cores and substituents at the azobenzene moieties were investigated to find the best systems
which yield a high sensitivity and fast grating build-up.
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