Anna Sobolewska

Synthesis, characterization, and study of photoinduced optical anisotropy in polyimides containing side azobenzene units.

In this paper, novel processable aromatic polymers with imide rings and attached as side-chain azobenzene units are presented. Polymers differ in the chemical structures of chromophores and polymer backbones. Azopolymers were obtained by a two-step synthetic approach. This includes the preparation of a precursor poly(esterimide) and poly(etherimide) with pendant phenolic hydroxyl groups, followed by the covalent bonding of NLO chromophores onto the polyimide backbone by the Mitsunobu reaction. The degree of functionalization of polymers was estimated by UV-vis spectroscopy. Polymers were characterized and evaluated by FT-IR, (1)H NMR, X-ray, UV-vis, DSC, and TGA methods. The synthesized polymers exhibited glass transition temperatures in the range of 167-228 degrees C, thermal stability with decomposition temperatures in the range of 275-446 degrees C, and excellent solubilities in common organic solvents. The light-induced optical anisotropy was studied in obtained azopolymers with the help of a holographic grating recording technique. Two polarization geometries were applied for the grating inscription s-s and p-p. The influence of the polarization geometry on the diffraction efficiency dynamics and on the depth of the surface modulation was not observed, which is different from results reported in the literature. Surface relief gratings, which appeared after the light exposure, were observed by atomic force microscopy. Additionally, the optical anisotropy in poly(esterimide)s was investigated by photoinduced birefringence measurements. For the first time, in polyimide with covalently bonded azobenzene derivatives, the high photoinduced birefringence (Delta n = 0.01) was measured.

Polarization Dependence of Holographic Grating Recording in Azobenzene-Functionalized Polymers Monitored by Visible and Infrared Light

The holographic grating recording process in thin films of amorphous azobenzene-functionalized polymers has been widely reported in the literature. In spite of the many reports, little is known about the mechanisms responsible for different temporal behaviors of the diffraction efficiency during long recording times. Here, we report on experimental studies of the diffraction efficiency changes during the holographic diffraction grating recording process in photochromic polymer. The gratings were inscribed for four different polarization combinations of the recording beams: s-s, p-p, s-p and right to left circular polarization (RCP-LCP) employing the degenerate two-wave mixing technique. The grating recording process was simultaneously monitored by three different wavelengths: 514.5 nm (writing) and 632.8 and 904 nm (reading). The temporal evolution of the diffraction efficiency (for all polarization configurations and for each wavelengths) was precisely fitted within the model, which assumes simultaneous formation of the absorption grating and three coupling phase gratings shifted by 0 or pi with respect to each other. Two of the phase gratings originate from the refractive index grating changes in the bulk (volume) of a material and the third one from the surface relief modulation. The model enabled us to extract relevant parameters for each grating such as the build-up time constant, its final amplitude, and the phase shifts between phase gratings. Performed studies and the discussion of results revealed the main differences in the diffraction grating formation process for s-s, p-p, s-p, and RCP-LCP polarization configurations.

On the inscription of period and half-period surface relief gratings in azobenzene-functionalized polymers.

Laser-light-induced surface relief grating inscription was carried out in the newly synthesized azobenzene-functionalized poly(amide-imide)s having the same main- and side-chain structures but different substituents in the azobenzene groups. The gratings were inscribed employing the two-wave mixing technique with linearly polarized laser beams. Three different polarization configurations were used: s-s, p-p, and s-p. The relatively deep surface relief gratings of period Lambda were formed for the case of s-s and p-p polarizations, whereas the s-p inscription resulted in the half-period grating (Lambda/2) with the weak surface modulation. The origin of the formation of Lambda/2 structure for s-p configuration results from the interference between zeroth- and first-order beams scattered on the polarization refractive index grating and having the same polarization. On the basis of this idea, we presented the simple kinetic model predicting and modeling the half-period grating formation with its temporal evolution. The proposed model is consistent with the experimental findings.

Three gratings coupling during the holographic grating recording process in azobenzene-functionalized polymer

The authors present a mechanism explaining complex diffraction efficiency dynamics of the holographic grating recording process in azobenzene-functionalized polymer. The mechanism assumes simultaneous formation of three coupling phase gratings with different recording time constants and constant, equal to 0 or π, phase shifts between them. Two of gratings originate from the refractive index changes in the bulk of material and the third one from the surface relief modulation. Diffraction efficiency dynamics curves calculated using the proposed model are in a very good agreement with the experimental results.

Dual photorecording on cholesteric azobenzene-containing LC polymer films using helix pitch phototuning and holographic grating recording

For the first time the possibility to perform dual recording in cholesteric LC polymer films was demonstrated by means of helix pitch photo-tuning and holographic grating recording, as far as these two photo-processes take place under the exposure to light with different wavelengths. For this purpose the cholesteric mixture of the nematic azobenzene-containing copolymer doped with the chiral-photochromic dopant was prepared and the photo-optical properties of thin planarly-oriented films of this mixture were investigated. It was found that the holographic grating recording over ∼1 min allows high diffraction efficiency values to be reached but does not disrupt the supramolecular helical structure that makes possible a two independent images recording.

Photo- and thermal-processing of azobenzene-containing star-shaped liquid crystals

A new class of star-shaped, liquid crystalline, low-molecular weight compounds functionalized with photochromic azobenzene and mesogenic groups was investigated in terms of light-induced anisotropy. The behaviour of the materials under the action of light with simultaneous or subsequent thermal treatment was examined with respect to the induction of anisotropy. The unconventional UV light treatment prior to the irradiation with linearly polarized light allowed induction of very high values of anisotropy (D = 0.77) at room temperature. Moreover, the simultaneous action of light and temperature led to the induction of higher values of dichroism in comparison with anisotropy generated by the standard procedure. Subsequent thermal treatment led to dewetting and the formation of 3D macroscopic stripe- and dome-like structures for one of the investigated compounds. Despite photoinduction of anisotropy by a single beam, the formation of polarization and surface relief gratings by two-beam interference pattern was also investigated.

Surface relief grating in azo-polymer obtained for s-s polarization configuration of the writing beams

The paper reports on the formation of the surface relief gratings in thin films of the azobenzene-functionalized polymer. We evidence that the efficiency of the formation process of the surface relief gratings strongly depends on the length of the main polymer chains in the case of the s-s configuration. The explanation of this relationship is given and is based on the revealing of the differences in the ability of the motion of the main chain which is strictly related to its length.

Biphotonic photochromic reaction results in an increase in the efficiency of the holographic recording process in an azo polymer.

Holographic grating recording in azobenzene-based polymers using a single coherent light of a specific wavelength (monophotonic recording) is the basis of the majority of the applications of this type of material. Here, we report a new approach to holographic recording that uses two light sources having different wavelengths during the recording process (biphotonic recording). The efficiency of the recording process was improved significantly compared to that of the monophotonic recording. The results, by presenting a simple way to increase the efficiency of the recording process, have important implications for the applications of azobenzene-based materials. They indicate how the performance of the already proposed devices can be improved, and they can open a new way to developing applications of this class of materials.

On the long time holographic grating recording process in azo-polymer

The authors report on a complex diffraction efficiency dynamics of the holographic grating recording process in azobenzene-functionalized polymer for a long recording time. The nonexponential dynamics was explain by the mechanism which assumes (i) simultaneous formation of three coupling phase gratings with constant, equal 0 or π, phase shifts between them and (ii) occurrence of gratings erasure processes when the recording time is long. Based on these assumptions and applying the Bessel function, the diffraction efficiency dynamics was calculated.

Origin of the oscillations in the self-diffracted signal in degenerate-two wave mixing experiment in azo-polymer

The authors report on temporal oscillation in the first-order self-diffracted signal observed during the holographic grating recording in the azobenezne-functionalized polymer in the degenerate two-wave mixing experiment. Explanation of the oscillating character of the diffracted signal is given based on the assumption of appearance of symmetrical phase shift between the light intensity pattern and the diffraction grating. Simulations of the experimental results were performed based on simple theoretical considerations and they have shown a good agreement with the experimental data.