Thermoassisted Photorecording in Chiral Smectic Azodye Copolymers
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Thermoassisted Photorecording in Chiral Smectic Azodye Copolymers Mikhail V. Kozlovsky, Wolfgang Haase, and Matthias Rehahn1 Inst. of Physical Chemistry, Darmstadt University of Technology, Petersenstr. 20, 64287 Darmstadt, Germany 1 German Plastics Institute, Schlossgartenstrasse 6, 64289 Darmstadt Germany ABSTRACT Illumination of photochromic liquid-crystalline (PCLC) copolymer films with polarized light during the mesophase formation has been studied. Both thick pressed and thin spin-coated, the films show prominent increase in photoinduced orientation, as compared with conventional isothermal photorecording. Two methods of the thermoassisted recording have been suggested, one of those with simultaneous cooling of the film and another one after a shock heating and quenching. The photoinduced birefringence, ∆nind, and the order parameter of photochromic groups, Sazo, have been measured. The thermally assisted recording produces holographic gratings with high diffraction efficiency, η, up to 20-30%. INTRODUCTION Photochromic polymer materials based on azobenzene dyes have drawn a remarkable attention during the last decade because of their possible applications in modern technologies of data storage and processing. Among these materials, the PCLC side chain copolymers are distinguished for the co-operative reorientation of mesogenic pendant groups along with the azodye moieties, this increases the degree of photoinduced orientation and its stability [1-6]. However, disadvantage of PCLC polymers would be the spontaneous formation of turbid, scattering polydomain textures. Preparation of transparent monodomain films requires special efforts, and such films possess macroscopic orientation which must be overcome during the photorecording. For that reason, mostly the sub-µm thick spin-coated films are used. Of peculiar interest is therefore an unusual phase state reported recently for some chiral side chain polymers, including azodye PCLC copolymers [7–9]. The "isotropic smectic" phase, IsoSm*, possesses an ordered layer structure and a helical superstructure but appears visually transparent and non-birefringent. An extra short range TGB-like structure has been suggested for the mesophase [10]. This allows for easy preparation of 20-30 µm thick IsoSm* films which are transparent in a considerable part of the visible wavelength range (λ > 430 nm). Such films show reasonably high phase retardation, ∆Φ, despite of moderate photoinduced birefringence, ∆nind. However, the orientation in IsoSm* PCLC copolymer films produced by the isothermal illumination at room temperature shows poor stability (no longer than several hours) [11]. Here we report our studies on the illumination of the IsoSm* PCLC copolymer films with polarized light during the mesophase formation, certainly at non-equilibrium conditions. The degree of order for photochromic azobenzene moieties has been measured, as well as the photoinduced birefringence. The holographic grating recording is also discussed. EXPERIMENTAL Two series of the copolymers have been st
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