Azobenzene Side-chain Polymer Research Articles

Induction of Highly Ordered Liquid Crystalline Phase of an Azobenzene Side Chain Polymer by Contact with 4'-Pentyl-4-cyanobiphenyl: An In Situ Study.

The orientation of liquid crystal (LC) molecules is significantly governed by solid interfaces and free surfaces, and a variety of functional materials have been developed using these properties. Although LC materials are already in industrial use, particularly for LC display panels, various studies have been conducted in recent years to better grasp the interface behavior of LC molecules. In this work, we succeeded in in situ observations of induction of higher ordered LC phases at the interface between a side-chain LC azobenzene polymer film with a thickness of ∼400 nm and a low-molecular-mass nematic LC, 4'-pentyl-4-cyanobiphenyl of 35 μm thickness, using small-angle X-ray scattering measurements and polarized optical microscopy. It is revealed that the two different mesogens cooperatively form hybrid higher ordered smectic LC phases probably through weak electron transfer immediately after interfacial contact. The induction process consists of three stages in terms of dynamic structure evolutions. Upon UV irradiation, the hybrid smectic LC structure diminished. This study provides new insights into the behavior of LC molecules near the alignment film on the solid substrate.

The increase in diffraction efficiency of an azobenzene side-chain polymer using imidazolium and ammonium ionic liquids

The advancement of the information age has intensified the focus on photosensitive materials for information storage devices. To develop new photosensitive two azobenzene side-chain polymers i.e., poly(E)-3-(4-((4-nitrophenyl)diazenyl)phenoxy)propyl acrylate (polymer-1) and poly(E)-3-(4-((2-methoxy-4-nitrophenyl)diazenyl)phenoxy)propyl acrylate (polymer-2), were developed, and their diffraction efficiency was evaluated. The impact of ionic liquids (ILs) on the diffraction efficiency was evaluated by combining the polymers with imidazolium and ammonium families of ILs such as 1-butyl-3-methylimidazolium bromide [Bmim]Br, 1-ethyl-3-methyl-imidazolium-bromide [Emim]Br (imidazolium ILs), and triethylammonium methanesulfonate [TMEAS] (ammonium IL). The molecular interaction of both azobenzene side-chain polymers with the ILs was evaluated before the diffraction efficiency studies by employing UV–vis, FT-IR, and confocal Raman spectroscopies. The spectroscopic studies revealed the interaction of the polymers with the imidazolium and ammonium ILs. The mean diffraction efficiency of polymers-1 and −2 were ∼0.05 and ∼0.022%, respectively. After the addition of the ILs, the diffraction efficiency increased. The highest diffraction efficiency was achieved with the polymer-2 + [Emim]Br system of 3.5% and polymer-2 + TEMS combination of 4.03%. Therefore, although the diffraction efficiency of polymer-1 was higher than that of polymer-2, after adding the ILs, the diffraction efficiency of polymer-2 surpassed that of the polymer-1 + ILs system.

Reversible Controlling the Supramolecular Chirality of Side Chain Azobenzene Polymers: Chiral Induction and Modulation.

Chirality represents a fundamental structure in nature and the induction and reversible modulation of supramolecular chirality with feasible techniques is of great value in the design of new chiroptical smart materials. Herein, two kinds of azobenzene side-chain polymers (without spacer: Azo-PMA0; with 6 spacers: Azo-PMA6) are synthesized, the length of spacer and azobenzene chromophores play a vital role in chirality transfer and modulation. The supramolecular chiral arrangement of Azo-PMA0 (amorphous phase) can be completely controlled and reversibly modulated over multiple cycles by 450nm circularly polarized light driven by the supramolecular interaction between azo groups of polymer chains, with an absorption dissymmetry factor (g) value of 0.0019. The chiroptical properties of Azo-PMA6 (liquid crystal state) can also be reversibly modulated by UV light and thermal annealing treatment during trans-cis isomerization of azo chromophore, with the g-value changes from 0-0.038. The successful construction of reversible chiral induction and modulation based on side chain azobenzene polymers may pave the way for designing photo-switchable functional materials.

Highly sensitive updatable green hologram recording polymer with photoisomerizable azobenzene with highly birefringent acetylene as the side chain

To develop highly sensitive updatable hologram materials, an azobenzene monomer was synthesized herein that supports effective photoisomerization under visible green beam irradiation. Furthermore, an acetylene group was introduced via Sonogashira coupling for higher birefringence. Polymerization was performed on the synthesized azobenzene monomer and on the methyl methacrylate (MMA) monomer, which serves as the skeleton of the film. The structures of all molecules were characterized by nuclear magnetic resonance (NMR) spectroscopy, and UV-Visible absorption spectroscopy was performed to analyze their optical properties. The holographic writing and rewriting capabilities of the newly synthesized azobenzene side-chain polymer were characterized by using a green laser as a writing beam, which showed that a high diffraction efficiency was reliably achievable under low-energy laser irradiation when evaluated with a four-wave-mixing optical scheme. In this study, high birefringent acetylene group containing azobenzene-based polymers were synthesized as materials for rewritable green-laser holograms, and their properties were evaluated. Birefringence change of polymer, which has an absorption wavelength suitable for green laser (532 nm), is 0.018. The diffraction efficiency of polymer was 72% under low-energy irradiation (7 J/cm2). These results show that the hologram recording material made in this study with high birefringent azobenzene are stable, without undergoing decomposition even when repeated recording and rewriting were performed.

Polarization modulation by means of tunable polarization gratings in an azobenzene side-chain liquid-crystalline polymer film.

Polarization modulation was achieved by means of a new type of grating recorded with two 532nm beams at varying polarization angles in an azobenzene side-chain liquid-crystalline polymer film through light regulation instead of voltage control. In contrast to conventional polarization holographic gratings, the polarization state of the ± first-order diffracted beams of the recorded gratings depended strongly on angles between polarization directions of two recording beams, while the polarization state of the second-order diffraction remained unchanged. With the polarization angle changing from 0 to 90deg, the ± first-order diffraction efficiency increased from 5.15% to 10.53%. Diffraction properties of the recorded gratings were attributed to the combination of polarization holographic gratings and amplitude gratings based on the calculation of Jones matrices and polarization holographic theory.

Synthesis and optical properties of azobenzene side chain polymers derived from the bifunctional fumaric acid and itaconic acid

Three type polymers containing the same azobenzene side chain group with different backbone structures, poly(DMAz6Fm), poly(DMAz6It) and poly(MAz6Ac) derived respectively from fumaric acid, itaconic acid and acrylic acid, were synthesized. The dependence of photo-response properties on the polymeric architecture was investigated for the solutions and films. By irradiation of the linearly polarized light (436 nm), uniaxial orientations of the thin films were induced and anisotropic properties were investigated by measurements of the order parameters and birefringence values.

Photoinduced In-Plane Motions of Azobenzene Mesogens Affected by the Flexibility of Underlying Amorphous Chains

Photoinduced in-plane motions are explored for a series of surface-grafted diblock copolymers comprising of an amorphous chain block and a liquid crystalline (LC) azobenzene (Az) side chain polymer block. In this architecture, amorphous polymers with different glass transition temperature (Tg) and the LC Az polymer are stepwisely grafted on substrate surfaces by the surface initiated atom transfer radical polymerization. Amorphous polymers employed are poly(hexyl methacrylate) (PHMA), poly(butyl methacrylate) (PBMA), and poly(methyl methacrylate) (PMMA) with Tg of −20, +27, and +110 °C, respectively. The extent of photoinduced in-plane anisotropy of the LC Az layer by linearly polarized light irradiation at optimal temperatures are in the order of PHMA > PBMA > PMMA which coincides with that of lowering Tg. Thus, the photoinduced motions of the LC Az blocks are coupled with the dynamic nature of the underlying amorphous chain anchored to the substrate. The structural information obtained by the grazing incidence angle X-ray diffraction measurements further shows the important role of the amorphous polymer layer in the ordering and photoalignment of LC mesogens on the top.

Reversible Photoinduced Switching of Permeability in a Cast Non‐Porous Film Comprising Azobenzene Liquid Crystalline Polymer

Permeation characteristics of an azobenzene-containing liquid crystalline (LC) non-porous film are investigated using a metallic corrosion method. Thin films (300 nm) are fabricated by the solution casting of an azobenzene side-chain LC polymer on freshly polished carbon steel coupons. Coated coupons are treated under the following conditions: a) gradual annealing at a cooling rate lower than 1 °C · min(-1) from 150 °C (above its Tg ) to room temperature, and b) irradiation at 465 nm (20 mW · cm(-2) ) with either circularly polarized light (CPL) or non-polarized light (NPL). The morphology of these films is characterized using X-ray diffraction, polarized optical microscopy, and transmission measurements. The results suggest that the annealing treatment resulted in the formation of a polydomain structure consisting of locally ordered small smectic domains that lack mutual orientation. Ordered micro domains are surrounded by disordered phases. CPL and NPL irradiation generates a monodomain orientated structure and an isotropic liquid crystal glass, respectively. The permeability of these non-porous films treated by CPL, NPL, and annealing are found to be 6.14 × 10(-4) , 1.92 × 10(-2) , and 1.56 × 10(-3) cm(3) · m(-2) · d(-1) . An orientation-dependent structure model is constructed to explain the permeation phenomenon, considering the ordered phase is impermeable, only the disordered phase is accessible to penetrating molecules. Fast switching of gas permeation is demonstrated by alternative irradiation of the film with CPL and NPL, which results in an approximately 30-fold difference in the permeability of the non-porous film.

Two-dimensional polarization holographic recordings in azobenzene liquid-crystalline polymer thin films

Two-dimensional （2D） polarization holographic recordings were accomplished in azobenzene side-chain liquid-crystalline polymer thin films. The 2D polarization grating makes up of two orthogonal 1D polarization gratings, which were written at the same place by two orthogonally linearly polarized 532 nm beams. It is found that the diffraction efficiency of 2D grating is lower than that of 1D grating, and the 2D grating acts as the integration of its componential 1D gratings. Furthermore, we find that the polarization dependency of diffraction efficiency and the polarization conversions arise from the collaboration of linear and circular birefringence.

Inverse Relaxation of Photoinduced Birefringence in a Liquid-Crystalline Azobenzene Side-Chain Polymer

Photoinduced birefringence in a liquid-crystalline azobenzene side-chain polymer is investigated. It is observed that the birefringence does not show any decay but increases after switching off the pump light at room temperature. The magnitude of the birefringence relaxation is found to depend on the exposure dose of the pump light. A discussion about the mechanism of the inverse relaxation of birefringence is presented.

Image storage based on circular-polarization holography in an azobenzene side-chain liquid-crystalline polymer

Image storage was investigated in a liquid-crystalline azobenzene polymer film based on circular-polarization holographic recording. An image was stored in pure polarization holograms using two orthogonal circularly polarized 532 nm beams and was reconstructed with a 633 nm beam. The polarization holograms showed high stability and a high diffraction efficiency of 31.8%. Moreover, the polarization holograms could convert arbitrary probe polarization state into circular polarization, and the diffraction efficiency was dependent on the probe polarization state, which was favorable for controlling the reconstructed images.

Polarization holographic gratings in an azobenzene side-chain liquid-crystalline polymer

Polarization holographic recording was investigated in a liquid-crystalline azobenzene side-chain polymer. A highly stable polarization holographic grating with the diffraction efficiency of 12.6% was written in the polymer film by two orthogonally linearly polarized 532 nm beams with equal intensities of 150 mW/cm2. The polarization grating was studied with a 633 nm probe beam by surveying the conversion of the polarization state of the probe beam and measuring the dependence of diffraction efficiency on the polarization direction of the probe beam. It was found that the ± first-order diffraction efficiency and the polarization of the diffracted wave depended strongly on the polarization of the readout wave. The unusual diffraction properties of the recorded grating were attributed to the presence of the circular photoinduced birefringence.

Orientational order of 4′- n-pentyl-4-cyanobiphenyl molecules deposited on azobenzene monolayers

Using optical second harmonic generation (SHG) and polarized absorption (PA) measurements, the n-th orientational order parameters S n ( n = 1, 2, 3) of 4′- n-pentyl-4-cyanobiphenyl (5CB) films were determined whilst 5CB molecules were deposited onto silica substrate coated with a trans- or cis-form 6Az10PVA (azobenzene side-chain polymer) monolayer by evaporation method. The experimental results clearly show that the orientational order of 5CB film depends on the form of surface azobenzene monolayer, especially at the beginning of 5CB deposition. It was also found that the orientational order saturates when monolayer of 5CB film is deposited on the azobenzene monolayer.

Microstructure of a poled surface-relief grating and its electro-optic response

The microstructure of a poled surface-relief grating of an azobenzene side-chain polymer film is investigated. From the in situ monitoring of diffraction efficiency and second-harmonic generation, as well as dynamic-contact electrostatic force microscopy images, the point group symmetry of C2nu is identified. The polarization-dependent electro-optic measurement of the reflected diffraction order further confirms the optically biaxial structure of the poled surface-relief grating.

Liquid crystal alignment by Holographic surface relief grating inscribed on azo-polymer film

We found that the surface relief grating(SRG)made the nematic liquid crystal(LC)align along the grating direction. The SRGs were photo-fabricated on a liquid crystalline azobenzene side-chain polymer film by the conventional holographic method. The twisted nematic liquid crystal(TNLC)cell was prepared using two SRG substrates or two rubbed polyimide substrates as alignment layers, and the LC alignment was investigated by measuring the transmittance of a 90°-TNLC cell located between crossed polarizers. The LC aligning process was studied in-situ using the hybrid LC cell with the rubbed polyimide substrate and the SRG substrate as the reference and the test alignment layers, respectively. The in-situ transmission data show that the topological effect by the SRG formation and the intermolecular interaction effect by the azo chromophore alignment compete with each other.

Ultrafast photoinduced anisotropy and optical switching in azobenzene sidechain polymers

Photoinduced anisotropic and optical switching properties have been studied in two kinds of azobenzene sidechain polymers, a disazobenzene sidechain polymer (DAP–PGMA) and a monoazobenzene sidechain polymer P(MMA–co–MAZ). Picosecond optical switching dynamics have been observed in both DAP–PGMA and P(MMA–co–MAZ) films. The rise time of the optical switching in DAP–PGMA film is about 40 ps, which is much faster than that of P(MMA–co–MAZ) film (∼100 ps). DAP–PGMA shows enhanced photoanisotropy and larger birefringence (∼7.04×10−4) than that of P(MMA–co–MAZ) (∼1.34×10−4). The influence of the azobenzene sidechain groups on the photoanisotropic dynamics is discussed.

Description of photoinduced anisotropy in azobenzene side-chain polyesters

Photoinduced anisotropy in azobenzene side-chain polymers are characterized in two ways, by numerical solution of the proper rate-equations and by approximate analytical expression. Feasibility of these methods was investigated by comparison with results of polarization holographic experiments. Agreement with experimental results proves usefulness of both descriptions for different applications.

Length-scale dependence of surface relief gratings in azobenzene side-chain polymers

Polarized irradiation of azobenzene side-chain polymers yields intriguing surface relief structures. Up to now, the physical mechanism of this process is unclear. We report on a study aimed at clarifying the intrinsic length scales of the substantial mass transport which evolves during laser irradiation. We vary the period of an incident intensity grating by more than an order of magnitude, and investigate the resulting relief amplitudes with atomic force microscopy.

Bicolor surface reliefs in azobenzene side-chain polymers

Holographic studies in azobenzene side-chain polymers reveal light-induced periodic surface reliefs at temperatures far below the glass transition temperature. We performed experiments with two-color holography on an azobenzene polymer. An intensive and linear polarized homogeneous blue pump beam illuminated the sample, and two red beams-outside the trans-azobenzene absorption-were used to inscribe the grating. A surface relief was obtained only with the pump beam on and only if its polarization was perpendicular to the writing beams. The findings are discussed in the frame of a recent mean-field model.

Multifarious Photoinduced Morphologies in Monomolecular Films of Azobenzene Side Chain Polymer on Mica

Monomolecular films of a poly(vinyl alcohol) derivative having an azobenzene (Az) side chain (6Az10-PVA) transferred onto mica by the Langmuir−Blodgett technique were observed by atomic force microscopy in the noncontact mode. Multifarious features of the film morphology after the thermal and photostimulated processes were revealed, which were strongly dependent on the humidity of the atmosphere, the lateral packing density of the monolayer, and the light exposure dose. In a humidified atmosphere, a low-density film (1.2 nm2 Az-1) showed a network morphologythat exhibited a two-dimensional expansion upon irradiation with UV (365 nm) light (trans → cis Az isomerization). The films having the higher densities (0.4 or 0.3 nm2 Az-1) gave spongy-shaped or partially collapsed films and indicated a formation of three-dimensional protrusions on UV light illumination. In comparison with the spectroscopic data, these morphological changes could be related with the proceeding of the Az photoreaction. The morphologic...