Amorphous Azopolymer Research Articles

Polarization-driven reversible actuation in a photo-responsive polymer composite

Light-responsive polymers and especially amorphous azopolymers with intrinsic anisotropic and polarization-dependent deformation photo-response hold great promises for remotely controlled, tunable devices. However, dynamic control requires reversibility characteristics far beyond what is currently obtainable via plastic deformation of such polymers. Here, we embed azopolymer microparticles in a rubbery elastic matrix at high density. In the resulting composite, cumulative deformations are replaced by reversible shape switching – with two reversible degrees of freedom defined uniquely by the writing beam polarization. We quantify the locally induced strains, including small creeping losses, directly by means of a deformation tracking algorithm acting on microscope images of planar substrates. Further, we introduce free-standing 3D actuators able to smoothly undergo multiple configurational changes, including twisting, roll-in, grabbing-like actuation, and even continuous, pivot-less shape rotation, all dictated by a single wavelength laser beam with controlled polarization.

Molecular Dynamics of Azobenzene Polymer with Photoreversible Glass Transition

The azobenzene polymer with photoswitchable glass transition temperatures and reversible solid-to-liquid transitions has attracted much attention in recent years. The different glass transition temperatures of the azopolymer in the trans-state and the cis-state introduce evident dynamical heterogeneity during cis–trans interconversion. The correlation between molecular dynamics and rheology in azopolymers remains elusive. In this work, we studied the molecular dynamics of an unentangled amorphous azopolymer without a liquid crystal phase. We found that the Lodge–Mcleish self-concentration (SC) model can quantitatively describe the composition dependence of the glass transition temperature, revealing the local environment of the cis (or trans) segments. The terminal relaxation behavior can be well predicted by the Rouse model with the homogeneous bead friction coefficient and the heterogeneous bead–spring (HBS) model with varying friction coefficients for cis or trans subchains. The difference between two models lies in the relaxation time at small length scales and its distribution. The HBS model predicts broadening the relaxation time spectrum, whose width varies with temperature and the cis content. This prediction is consistent with the broadening of the glass transition temperature in azopolymers. Then, the models were used to study the kinetics of thermal cis-to-trans isomerization of the azopolymer in the melt, proving to be a first-order kinetic process and further justified by the ultraviolet absorption spectrum.

Intermolecular Interactions and Intramolecular Motions in Photomechanical Effect: Nonlinear Thermo- and Photomechanical Behaviors of Azobenzene-Functionalized Amide-Imide Block Copolymers.

To discern multiple intertwined effects, a set of azobenzene-functionalized amide-imide block copolymers, azo(PA-co-PI)-x, where x is amide-block content, viz., [azoPA] = 25, 50, 75 mol %, was synthesized from 2,2-bis{4-[4-(4-aminophenyldiazenyl)phenoxy]phenyl}propane(azoBPA), 4,4'-oxydibenzoyl chloride (ODBC), and 4,4'-oxydiphthalic anhydride (OPDA). Including homopolymers (azoPA and azoPI), this series of amorphous azopolymers possesses a high glass-transition temperature (Tg > 210 °C) and a modulus (E' ∼ 1.23-2.50 GPa). Their photobending (ca. 23-90°) and photostress (ca. 250-380 kPa) were assessed in the form of cantilevers with a linearly polarized 445 nm light. Nonlinear composition/[azoPA] dependencies of the thermo- and photomechanical properties are correlated. As [azoPA] increases from 0 mol %; Tg, E', photostress, and photobending angle initially decrease to reach four separate minima for azo(PA-co-PI)-50; and then all increase with a higher [azoPA]. The trend considerations of film density, dynamic thermomechanical, Fourier transform infrared (FT-IR), and ultraviolet-visible (UV-vis) measurements implicate that (i) intermolecular association and intramolecular segmental mobility collectively influence the photomechanical outcomes and (ii) two types of hydrogen bonding (HB), namely, amide-amide [HB-AA] and amide-imide [HB-AI] coexist in azo(PA-co-PI)-x copolymers, with [HB-AI] being largely responsible for photomechanical outcomes of azo(PA-co-PI)-x with [azoPA] <40-50 mol %, and [HB-AA] for [azoPA] >40-50 mol %. We hypothesize that the "U-shaped" photomechanical effect apparently stems from the cooperative "unzipping" of H bonds in the [HB-AA]* excited state with H bonds in [HB-AI]* being stabilized by electrostatic interactions inherent in an excited intermolecular complex.

Photoinduced chirality in azopolymer-based nanocomposites with different TiO2 nanoparticles concentrations

After its discovery, photoinduced chirality in azopolymer materials has been studied with growing interest because of its potential applications. According to our previous investigations, it is possible to induce chirality in an amorphous azopolymer with a high linear photoanisotropy by illumination with elliptically polarized light. In the present work, we studied the photoinduced chirality in thin nanocomposite films based on the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt], shortly PAZO, doped with TiO2 nanoparticles with a size of about 20 nm. The thickness of the prepared films was approximately 550 nm and the concentration of the nanoparticles varied from 0 to 3 wt. %. A He-Cd laser beam (λ = 442 nm) was used, with the input ellipticity varying from –1 (left circularly polarized) to +1 (right circularly polarized). The ellipticity was set using a quarter-wave and a half-wave plates. We studied the dependences of the output ellipticity and the angle of self-rotation of the polarization azimuth on the input ellipticity. Based on the experimental data obtained, we could analyze the dependence of the self-rotation of the polarization azimuth on the concentration of the nanoparticles. The results show that small concentrations of nanoparticles lead to an enhancement of the self-rotation, hence, the chirality.

Tunable Surface Patterning of Azopolymer by Vectorial Holography: The Role of Photoanisotropies in the Driving Force.

The capability to pattern polymer surfaces at different length scales is an important goal in different research fields, including display technologies, microelectronics, optics, as well as biorelated and medical science. However, the ability to optically and dynamically manipulate topography is a key feature enabling remote control of associated effects/processes mediated by the surface. Azopolymers are largely investigated to this aim based on their sensitivity to optical fields and reconfigurability capabilities. In this work, surface relief formation induced by polarization patterns on an amorphous azopolymer structurally engineered to have large photoinduced birefringence has been investigated both experimentally and theoretically. Based on the different light polarization patterns, depth and shape of the relief grating can be controlled. An optically induced gradient force model that includes both the spatial distribution and the anisotropy of the material permittivity has been theoretically analyzed. The proposed approach is able to explain the experimental results and to overcome the limitation of existing models.

Probing of the mechanical properties and monitoring of the drying process of azopolymer thin films for optical recording

Azopolymers prepared in the form of thin films are promising candidates as media for optical recording due to their photo-anisotropic properties. The thermo-physical and mechanical properties of the prepared films, as well as the uniformity of their thickness and smoothness of the formed layers are important characteristics for application of these materials. In the present study, thin films of two amorphous and one liquid crystalline azopolymers were characterized and, in the experiment, a commercially available amorphous azopolymer was included as a base for comparison. Atomic force microscopy was applied for surface topography determination for thin film samples of all four azopolymers and roughness parameters as the average roughness and root mean square values are estimated. These values show that amorphous azopolymers are characterized by a high smoothness and a flatter surface while the thin film from the liquid crystalline polymer shows a structure with pronounced topographic features and thus exhibits much higher surface roughness. The conducted depth-sensing indentation of non-exposed thin films prepared by spin-coating on glass substrates allowed to determine mechanical properties such as universal and indentation hardness, elastic modulus and elastic part of indentation work. The creep behavior of the tested materials was also analyzed and the elastic-plastic behavior for all azopolymers was established. The most pronounced plastic behavior was observed for the liquid crystalline one. We obtained different temporal scales of the drying process of solutions of the tested materials in a polymer casting experiment by a quantitative dynamic speckle analysis. Analysis involved statistical processing of correlated sequences of laser speckle patterns, reflected the properties of the respective materials and was supported by the reported mechanical characteristics.

Optical anisotropy induced at five different wavelengths in azopolymer thin films: Kinetics and spectral dependence

Azopolymers are highly efficient materials, able to register the polarization state of light. They have broad range of applications from diffractive optical elements with unique polarization properties to biophysics.We present a study of birefringence kinetics, induced with five different pump lasers with wavelengths 355, 442, 491, 514 and 532 nm in thin films of widely used amorphous azopolymer PAZO – poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfon amido]-1,2-ethanediyl, sodium salt]. During the real-time measurement, the value of the photoinduced birefringence (Δn) is determined at 635 nm, using a DPSS laser. When saturation is reached, the spectrum of the birefringence is measured. Our results indicate that the investigated azopolymer can be used for applications in a wide wavelength range.

On stress – strain responses and photoinduced properties of some azo polymers

Mechanical properties of two amorphous azo polymers were characterized in tensile measurements and correlated with photoinduced behaviour of the materials observed upon irradiation with blue light of moderate intensity. Side-chain azobenzene poly(amide imide)s capable of forming high-quality free-standing layers of a large area and thickness of a few micrometers were chosen for the investigations. High Young's modulus of ca. 2 GPa and yield stress of the order of 30 MPa were found for the materials. Photoinduced birefringence measurements evidenced efficient chromophore orientation in both polymers, while polymer mass transport upon illumination with interference pattern was hardly observed. Nevertheless, subtle periodic changes in the surface topography of the films after irradiation with right- and left-handed circularly polarized beams suggested that opto-mechanical stress reached the value of a few tens of MPa, i.e., the minimum value needed for inducing plastic deformations in the studied azo polymers.

Study the nonlinear optical property of pull/push type azo dye-doped polymer using 633nm He–Ne laser

Azo dyes with different pull/push substituents are selected as photosensitive molecules. Guest–host amorphous azo polymer samples are prepared by doping these azo dyes directly in poly(methyl methacrylate) polymeric matrices. Nonlinear optical properties of these pull/push type azo dye-doped polymers are investigated using the Z-scan experiment. The influence of the pull/push substituents and the azo dye concentration on the nonlinear optical property is studied.

Birefringence improvement in azopolymer doped with MFI zeolite nanoparticles

AbstractHybrid organic/inorganic materials based on combination of polymers and inorganic nanoparticles (NP) attract considerable attention due to their advantageous electrical, optical, or mechanical properties. Recently it was reported that doping photopolymers with nanoparticles allows to achieve near 100% net diffraction efficiency in case of conventional holographic recording. Thus, we have synthesized novel organic/inorganic composite materials by incorporating MFI (Mordenite Framework Inverted) type zeolite nanoparticles in an amorphous side-chain azopolymer. A considerable improvement of the photoresponse in thin films of these composite materials has been observed compared to the non-doped samples - nearly 25% increase of the saturated value of the birefringence.Moreover the photoinduced birefringence is stable in time which allows these materials to be used as media for diffractive optical elements with high efficiency and unique polarization properties.

Azopolymer‐based micro‐ and nanopatterning for photonic applications

ABSTRACTAzopolymers comprise a unique materials platform, in which the photoisomerization reaction of azobenzene molecules can induce substantial material motions at molecular, mesoscopic, and even macroscopic length scales. In particular, amorphous azopolymer films can form stable surface relief patterns upon exposure to interfering light. This allows obtaining large‐area periodic micro‐ and nanostructures in a remarkably simple way. Herein, recent progress in the development of azopolymer‐based surface‐patterning techniques for photonic applications is reviewed. Starting with a thin azopolymer layer, one can create a variety of photonic elements, such as diffraction gratings, microlens arrays, plasmonic sensors, antireflection coatings, and nanostructured light‐polarization converters, either by using the azopolymer surface patterns themselves as optical elements or by utilizing them to microstructure or nanostructure other materials. Both of these domains are covered, with the aim of triggering further research in this fascinating field of science and technology that is far from being harnessed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 163–182

Smart Materials: Supramolecular Chiral Structures: Smart Polymer Organization Guided by 2D Polarization Light Patterns (Adv. Funct. Mater. 14/2012)

On page 2964 Gabriella Cipparrone and co-workers report on the supramolecular chiral structuring of an amorphous azo-polymer guided by 2D polarization light patterns. Periodic chiral microstructures with spiral- or ribbon-like shape and identical or opposite helicity are demonstrated. They exhibit high stability and complete reconfigurability. These results provide an alternative approach to design a new class of materials with periodic chiral arrangements that have potential advantages for applications in smart functional devices.

Pure two-dimensional polarization patterns for holographic recording

Two-dimensional (2D) polarization patterns are achieved by the interference of two pairs of beams with perpendicular planes of incidence and orthogonal polarizations (i.e. linear or circular). In both cases, imposing a phase shift of π/2 between consecutive beams contains the amplitude modulation of the optical field in the superposition region and, thus, pure 2D polarization patterns are created. The recording of these interference fields in a polarization-sensitive material, namely an amorphous azopolymer, creates reconfigurable 2D periodic microstructures with peculiar diffraction properties.

Opposite photo-induced deformations in azobenzene-containing polymers with different molecular architecture: Molecular dynamics study

Photo-induced deformations in azobenzene-containing polymers (azo-polymers) are central to a number of applications, such as optical storage and fabrication of diffractive elements. The microscopic nature of the underlying opto-mechanical coupling is yet not clear. In this study, we address the experimental finding that the scenario of the effects depends on molecular architecture of the used azo-polymer. Typically, opposite deformations in respect to the direction of light polarization are observed for liquid crystalline and amorphous azo-polymers. In this study, we undertake molecular dynamics simulations of two different models that mimic these two types of azo-polymers. We employ hybrid force field modeling and consider only trans-isomers of azobenzene, represented as Gay-Berne sites. The effect of illumination on the orientation of the chromophores is considered on the level of orientational hole burning and emphasis is given to the resulting deformation of the polymer matrix. We reproduce deformations of opposite sign for the two models being considered here and discuss the relevant microscopic mechanisms in both cases.

Model of optically induced memory in amorphous azopolymers

A phenomenological model of the light — azopolymer radiation interaction is proposed in which chromophores interact with the polymer matrix due to the mutual orientational diffusion. It is established that for some amorphous azopolymers, the model satisfactorily describes the photoinduced dichroism dynamics both at the stage of light action and relaxation, and allows the estimate of the main optical parameters of chromophores (absorption cross section and quantum yields of trans and cis isomerisations, times of the spontaneous decay of cis isomers) to be obtained by the experimentally measured dichroism. The comparison of the results of model calculations with the experimental ones shows that only a part of trans isomers participating in trans — cis isomerisation contributes to the dichroism.

All-optical switching using supramolecular chiral structures in azopolymers

The self-rotation on the polarization azimuth and the formation of a supramolecular chiral structure when an elliptically polarized light beam propagates through an amorphous azopolymer film are used to implement a two-wavelength chiroptical switch. The macrostructures generated by each excitation beam have different pitches, which produce different rotation angles of the polarization ellipse when an arbitrary probe beam is used. In the proposed device, one of the excitation beams is taken as a probe. The value and the sign of the rotation angle of the polarization ellipse of the probe beam when the second beam is illuminating the sample mainly depend on the relationship between their input states of polarization.

Unusual surface reliefs from photoinduced creeping and aggregation behavior of azopolymer

We report the spontaneous formation of unusual surface reliefs, in which two sets of sinusoidal gratings were hierarchically structured, merely by single-step holographic inscription on amorphous azopolymer films. By monitoring of growth behavior of surface reliefs during holographic inscription, we found that the formation of additional grating is caused by the creeping and resulting aggregation of dome structures. Our direct observation of creeping and aggregation behavior is expected to contribute to enhancing the understanding of unusual surface reliefs, and also in fabricating complex surface reliefs.

Formation mechanism of surface relief structures on amorphous azopolymer films

The electromagnetic force from two interfering light beams on isotropic materials is derived for surface relief grating formation processes. The Navier-Stokes equation is solved for the electromagnetic force induced motion inside the ''photoplasticized'' amorphous azopolymer film. The predicted inscription rates and surface velocities in surface relief grating formation processes are consistent with the experimental evidences, indicating that the surface relief gratings on amorphous azopolymer films are formed by the electromagnetic force on the materials, under the condition of ''photoplasticization'' through sufficient trans-cis-trans isomerization cycling.

Photoinduced linear and/or circular birefringences from light propagation through amorphous or smectic azopolymer films

The self-induced rotation of the azimuth of elliptically polarized light passing through birefringent azopolymer thin films is investigated. The experiments were carried out on thin films of the amorphous p(DR1M-co-MMA) and p(DR1M) azopolymer samples and of the p(6MAN) derivative in its glassy and liquid-crystalline phases. In fact, using various controlled input light ellipticities, linear birefringence (LB) and/or circular birefringence (CB) measurements were performed separately and in conjunction with polarization analyses of the transmitted pump beam. According to a general theoretical analysis based on Jones’ matrix formalism, it is thus shown that the induced rotation angle through the films depends mainly on the ellipticity of the input light, on the generated LB level and, to a lesser extent, on the CB photoinduced in the liquid-crystalline phase. In the latter case, it is concluded that irradiation with circularly polarized light does induce a chiral arrangement in the polymer film, although the photosensitive chromophores do not contain any optically active group.

Time dependent analysis of the formation of a half-period surface relief grating on amorphous azopolymer films

The formation of a surface relief grating (SRG) on amorphous copolymer films containing azobenzene chromophores in side chain positions is conducted using two linearly polarized interfering beams with orthogonal polarization directions (p+s). Time dependent analyses of the diffraction efficiency were carried out by measuring simultaneously the transmitted zero order (I(0)) as well as the first (I(±1)) and second (I(±2)) diffracted orders. Two main contributions to the diffraction efficiency are evidenced: an anisotropic (birefringence) part coming from the normal period grating plus another anisotropic and surface relief part (SRG) due to the half-period grating. The origin of the weak amplitude SRG stems from the interference of particular I(±1) diffracted orders, whose resulting polarization and angular Bragg’s conditions are in accordance with the existence of a double frequency modulation. Using the matricial products of Jones, theoretical expressions of transmitted and diffracted orders are thus derived. Extracted values of the phase shifts and, therefore, of the birefringence (Δn,Δn′) and surface relief amplitude (2Δd) are then compared to data from optical and topographic atomic force microscopy measurements. Time dependent calculations of these relevant parameters as well as analyses of the polarization states of transmitted and diffracted orders are in good agreement with the proposed model.