Volume holography is a promising technology for high-capacity optical data storage. [1,2] When utilizing the entire volume of a medium instead of only its surface, capacities above one terabyte on a memory medium the size of a compact disc are theoretically possible. Recent developments in the field of holographic storage materials and advances in storage techniques and technology are summarized in review articles by Ashley et al. [3] and Hesselink et al. [4] Although there are still material challenges, write-once media seem to be fairly advanced. In search of rewritable media, many studies have been performed on photorefractive crystals and organic glasses. [1,4] Manufacturing single crystals of the required size is very expensive, and a major drawback of the organic photorefractive materials is the requirement of very high electric field strengths. With increasing material thickness, the voltage necessary for creating the field becomes difficult to handle. A different potential class of rewritable media are polymers containing photoaddressable azobenzene chromophores.[5] Many studies have been performed on azobenzene-containing polymer films with thicknesses in the range of 0.5-10 lm. [3,6] However, the preparation of the thick samples required for multiplexing with sufficiently low optical density remains difficult to achieve. Because the absorption coefficient depends on the wavelength, it is in principle possible to tune a laser with an adjustable wavelength (e.g., a dye laser) to the very edge of the absorption band. [7] If writing is to be performed at a wavelength where fixed-wavelength, solid-state lasers are available (515 or 532 nm), however, a concept that allows for diluting the chromophores is required. So far, the best results have been reported by Minabe et al. with a homogeneous polymer blend based on an azobenzene-containing polymer and a similar but nonabsorbing polymer. [8] In a sample of 250 lm thickness, a set of 20 data pages were successfully inscribed; yet, the individual holograms were not completely spatially overlapping. The important issues of stabilizing the inscribed data and rewritability were not addressed. We use a different concept of controlling the optical density, by using block copolymers that contain azobenzene side groups in one block. Block copolymers with less than 15 vol % of one component form spherical microphases with typical diameters in the range of 10-20 nm. Thus, we can macroscopically dilute the chromophores while, at the same time, keeping them in a local confinement. The latter is important for the stability of the inscribed information. In an earlier paper, we have already reported the formation of individual holographic gratings in 10 lm thick films of a material that contained azobenzene and nonabsorbing three-ring mesogenic side groups in the minority phase.[9] In the confined geometry, the presence of a cooperative effect of the side groups was demonstrated. The mesogenic side groups were reoriented together with the chromophores; furthermore, in materia...

Diblock copolymers, consisting of a photoaddressable mesogen‐containing dispersed phase and a polystyrene matrix, have been synthesized and characterized as holographic data storage materials. The photoaddressable phase contains p‐methoxy‐substituted azobenzene side groups and benzoylbiphenyl mesogenic side groups in a statistical distribution. Three‐ring mesogenic groups in combination with photoaddressable azobenzene side groups have been introduced for the first time in block copolymers in order to increase the local refractive‐index difference between illuminated and non‐illuminated volume elements, as well as to improve the stability of the orientation, while, at the same time, decreasing the optical density. To this end, a series of block copolymers with different azobenzene/mesogen ratios have been synthesized. The light‐induced reorientation dynamics of the chromophores and mesogens and the long‐term stability of the orientation have been studied with volume holographic measurements. A remarkable increase in stability of the holographic gratings has been achieved.

Semifluorinated azobenzene liquid‐crystalline side‐chain polymers with different perfluoroalkyl lengths are used to develop surface relief gratings (SRGs). With sufficient fluorinated side‐chain lengths, surface relief gratings could be completely suppressed due to the self‐assembled liquid‐crystalline order at the surface. This indicates that there is a surface‐dependent nature to SRG formation. The SRG formation from the semifluorinated azobenzene liquid‐crystalline side‐chain polymers is compared to methoxy‐terminated azobenzene side‐chain polymers, and results show that the perfluoroalkyl–azobenzene polymers require a longer formation time than the methoxy‐terminated polymers.

The paper presents the synthesis of azobenzene-functionalized block copolymers based on a poly-(methyl methacrylate) (PMMA) segment and an azobenzene-functionalized poly(hydroxyethyl methacrylate) segment, and a basic study of blending these block copolymers with homopolymers is given. Two diblock copolymers, prepared via different routes, were synthesized by a living anionic polymerization followed by a polymer analogous reaction to attach the azobenzene side groups. Self-assembly of the block copolymers resulted in phase-separated morphologies on the nanometer scale. The photoaddressable azobenzene segments are dispersed in the PMMA matrix and locally confined. Special focus is given to the preparation and characterization of block copolymer blends with PMMA homopolymer in order to dilute the phase-separated azobenzene morphology and reduce the optical density while maintaining the confinement. The block copolymer blends were characterized with respect to their morphology and initial holographic experiments were performed.