Azopolymer Films Research Articles

Surface relief gratings in azo-polymers revisited

We use the concept of vectorial photochemical tweezing to rationalize the experimental observations of surface relief gratings in azo-polymers, e.g., the photochemically induced motion of the polymer in a one-dimensional intensity gradient produced by two-laser beam interference. Vectorial motion of matter occurs when photochemically active, polarization sensitive molecules are photo-selected in a gradient of light intensity. Directional motion is imposed parallel to the gradient vector with an efficiency that depends on the respective orientations of the vectors of light polarization and intensity gradient. Different combinations of polarizations of the interfering beams leading to differing efficiencies of matter motion are revisited and discussed. We show that the magnitude of photoisomerization force dictates the efficiency of the observed matter motion. We also show that the spatial distribution of the photo-moved matter is Gaussian, the height and width of which exhibit an intensity dependence which is predicted by the theory of photochemical vectorial tweezing; both theory and experiments indicate that the photoisomerization force, which acts on thin films of azo-polymers, is in the mN range.

Nature-Inspired Quasicrystal SRG Using Fibonacci Sequences in Photo-Reconfiguration on Azo Polymer Films

In this study, we present the fabrication route of quasicrystal surface relief grating (SRG) of azopolymer. Based on photo-reconfigurable azobenzene property, various morphologies of grating patterns were inscribed on azopolymer thin-films by two-beam coupling light interference lithography (LIL) process. Multiplexing the LIL with different rotation sequence on azopolymers resulted in unusual surface structures including quasicrystal-like SRGs with Fibonacci sequences in rotational LIL. We investigated the effect of rotation sequence on the grating patterns by atomic force microscope, revealing that rotation sequence is a critical parameter determining surface structures of azopolymer films.

Diffraction efficiency calculations of polarization diffraction gratings with surface relief

In this paper, we evaluate the optical response of a stack of two diffraction gratings of equal one-dimensional periodicity. The first one is a surface-relief grating structure; the second, a volume polarization grating. This model is based on our experimental results from polarization holographic recordings in azopolymer films. We used films of commercially available azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly denoted as PAZO. During the recording process, a polarization grating in the volume of the material and a relief grating on the film surface are formed simultaneously. In order to evaluate numerically the optical response of this “hybrid” diffraction structure, we used the rigorous coupled-wave approach (RCWA). It yields stable numerical solutions of Maxwell’s vector equations using the algebraic eigenvalue method.

Polarization holographic gratings in PAZO azopolymer recorded with different recording-beams polarizations

Polarization holographic gratings (PHG) were recorded using a laser emitting a wavelength of 491 nm in thin films of the (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) azopolymer, shortly denoted as PAZO. Thin azopolymer films with various thicknesses were spin-coated on plastic and glass substrates. Four different polarization states of the recording beams were used, and the results compared: a) two vertical linear polarizations, b) horizontal and vertical linear polarizations, c) linear polarizations at +45° and –45° relative to the recording plane, and d) two orthogonal circular polarizations – left- and right-handed (LCP and RCP). The diffraction efficiency in the +1 diffraction order was monitored in real time by a probing laser beam at the wavelength of 635 nm. The results indicate that the highest diffraction efficiency is achieved when recording with orthogonal polarizations – linear at ±45° or left and right circular. This is explained by the ability of the azopolymer material to record the variations in the polarization state of light better than the variations in its intensity. The holographic gratings obtained can be used to enhance the light-extraction efficiency of an OLED device.

Photofluidic Near-Field Mapping of Electric-Field Resonance in Plasmonic Metasurface Assembled with Gold Nanoparticles.

We present a near-field mapping of electric fields from the individual superspherical and ultrasmooth gold nanoparticles (AuNPs) and artificially assembled AuNP nanostructures by measuring the reconfiguration of an azobenzene-containing polymer(azo-polymer) film. Various configurations of AuNPs and the azo-polymer were studied with atomic force microscopy measurements and calculations. The interference was systematically studied with AuNP dimers of various gap distances and different embedding depth in the polymer film. Finally, we successfully demonstrated the interference of standing waves in artificially assembled plasmonic metasurface.

Analyses of surface relief gratings inscription in epoxy‐azo linear and crosslinked polymers

ABSTRACTThis work evaluates the inscription of surface relief gratings (SRG) in a series of epoxy‐azo prepolymers and networks, both based on the same azo chromophore. Variables such as matrix structure (thermoplastic/thermoset), film thickness, exposure time, power of the incident laser beam, and content of azobenzene were analyzed on the final modulation depth of the gratings. The main goal of the article is to achieve the efficient formation of SRGs on crosslinked azopolymer films in a single step, in a reasonable time, and to a significant extent. The progressive erasure of the obtained structures with increasing temperature showed that part of the photoinduced information remained encoded in the material: as the relief partially relaxed above the glass transition temperature of the bulk material (116 °C) and up to 200 °C, its modulation remained in about one third of the initial height, and did not completely disappear. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1542–1552

Probing optical near-fields with photoreactive azo-polymers

In this work, we demonstrate a method to visualize optical near-fields on photosensitive azo-polymer thin films with scanning near-field optical microscopy. A near-field intensity profile is determined in a basis of surface deformations of the azo-polymer thin film exposed to linearly polarized light.

Azo-polymer film twisted to form a helical surface relief by illumination with a circularly polarized Gaussian beam.

A helical surface relief can be created in an azo-polymer film simply by illuminating circularly polarized light with spin angular momentum and without any orbital angular momentum. The helicity of the surface relief is determined by the sign of the spin angular momentum. The illumination of circularly polarized light induces orbital motion of the azo-polymer to shape the helical surface relief as an intermediate form; a subsequent transformation to a non-helical bump-shaped relief with a central peak creates a final form with additional exposure time. The mechanism for the formation of such a helical surface relief was also theoretically analyzed using the formula for the optical radiation force in a homogeneous and isotropic material.

The novel photoresponsive oligomers containing azo derivatives of sulfamerazine for spontaneous surface relief grating inscription

In this work, a novel photoresponsive materials has been successfully developed. We have synthesized the series of azopolymers containing derivatives of sulfamerazine and studied their photochromic properties. The polymers were obtained from methacrylate azomonomers, butyl methacrylate and isobornyl methacrylate by radical polymerization. Spectral properties and photoisomerization kinetic constants were determined for thin transparent films, using UV–Vis spectroscopy. The maximum absorption of the films was observed at 432–440nm. The obtained materials showed ability to reversible trans–cis photoisomerization. Reversibility of the process was confirmed during ellipsometric measurements. The change of the real part of the complex refractive index induced by laser irradiation was between 0.010 and 0.053. Additionally, some properties of described polymers and azobenzene derivatives were calculated and were utilized in order to better understand the differences in the photochromic behaviour of the new materials. Moreover, we determined the suitability of the obtained azopolymers for spontaneous surface relief grating inscription. Spontaneous surface relief grating recording on the thin azopolymer films was carried out using set-up with one laser beam. The spontaneous SRG was successfully recorded in all investigated polymers.

Azopolymer film as an actuator for organizing multiwall carbon nanotubes

In this work we show the feasibility of using an azopolymer as an actuator to induce nano- and microscale movements controlled with light from the far field. We study azopolymers and their interaction with multi-walled carbon nanotubes (MWCNTs) by inducing surface relief gratings (SRG) through optical illumination. Upon different optical treatments, the MWCNTs are organized in the troughs or the crests of a surface relief grating. Large scale organization of MWCNTs has potential in applications such as transparent electronics.

Photoinduced Heating of Freestanding Azo-Polymer Thin Films Monitored by Scanning Thermal Microscopy

Despite numerous attempts to capture a temperature rise in azobenzene-functionalized polymer thin films exposed to laser irradiation, so far no attention has been paid to direct temperature measurements. Here, we characterize a photoinduced heating of freestanding thin films using nanoscale resolution scanning thermal microscopy. The polymer films under study are composed of epoxy-based oligomers with chemically attached nitroazobenzene chromophores. A temperature change of 1.7 K only is observed when an 800 nm thick film is subject to resonant 532 nm illumination with a modest intensity of 25 mW/cm2. A freestanding and glass substrate supported 20 nm azo-polymer films exhibit an anomalous depression of the glass transition temperature by approximately 80 and 70 K, respectively, that is probed with thermally assisted atomic force microscopy. Our results show that the photoinduced heating can negatively affect an ordered state of the azo dyes within the polymer ultrathin film (<100 nm) at room temperature.

Polarization holographic recording in thin films of pure azopolymer and azopolymer based hybrid materials

Recently, a birefringence enhancement effect was observed in azopolymers doped with various nanoparticles. The paper presents comparison between the parameters of polarization holographic gratings recorded in a pure azopolymer PAZO (Poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) and in a hybrid PAZO-based organic/inorganic material with incorporated ZnO nanoparticles of size less than 50 nm. Laser emitting at 491 nm is used for the holographic recording. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The time dependence of the diffraction efficiency is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that both the diffraction efficiency and the height of the surface relief for the hybrid samples are enhanced with respect to the pure azopolymer films.

Near-field optical mapping of single gold nano particles using photo-induced polymer movement of azo-polymers

In this study, polymer movement was induced in azo-polymer films by optical near-fields generated in the vicinity of single gold nano particles (GNPs) to visualize near-field distribution with a spatial resolution beyond the diffraction limit of light. A linearly polarized (Ex) laser beam was irradiated into GNPs to excite local surface plasmon resonance that enhanced the near-field around the GNPs. The findings indicated that different GNP diameters (that is, 50nm and 80nm) resulted in different deformation patterns on the films. The results were compared with theoretical calculations of near-field distributions, and the observations revealed that the deformation patterns were dependent on the ratio between Ex and Ey wherein each possessed a different field distribution.

Towards understanding photomigration: Insights from atomistic simulations of azopolymer films explicitly including light-induced isomerization dynamics.

The light-induced surface modification of a thin film of poly-(disperse orange-3-methylmethacrylate) is investigated computationally using atomistic molecular dynamics simulations specifically tailored to include photoisomerization dynamics. For a model surface consisting of a periodic pattern of alternating irradiated and dark spots, it is shown that repeated photoisomerization in the irradiated areas initially leads to a local temperature increase and a raised surface profile accompanied by a migration of molecules away from the bright spots. After switching off the light source and letting the system cool down, this leads to an inversion of the surface profile, i.e., dips in the bright spots and bumps in the dark spots. To separate the effect of photoisomerization from the pure heating effect, a second simulation is performed in which no photoisomerization is allowed to occur in the bright spots, but the equivalent amount of energy is introduced there locally in the form of heat. This also leads to a raised surface in these areas; however, no outward migration of molecules is observed and the surface pattern practically vanishes when the system is subsequently cooled back to room temperature.

Photoswitching of glass transition temperatures of azobenzene-containing polymers induces reversible solid-to-liquid transitions.

The development of polymers with switchable glass transition temperatures (Tg) can address scientific challenges such as the healing of cracks in high-Tg polymers and the processing of hard polymers at room temperature without using plasticizing solvents. Here, we demonstrate that light can switch the Tg of azobenzene-containing polymers (azopolymers) and induce reversible solid-to-liquid transitions of the polymers. The azobenzene groups in the polymers exhibit reversible cis-trans photoisomerization abilities. Trans azopolymers are solids with Tg above room temperature, whereas cis azopolymers are liquids with Tg below room temperature. Because of the photoinduced solid-to-liquid transitions of these polymers, light can reduce the surface roughness of azopolymer films by almost 600%, repeatedly heal cracks in azopolymers, and control the adhesion of azopolymers for transfer printing. The photoswitching of Tg provides a new strategy for designing healable polymers with high Tg and allows for control over the mechanical properties of polymers with high spatiotemporal resolution.

Mueller imaging polarimetry of holographic polarization gratings inscribed in azopolymer films.

Three types of polarization gratings have been recorded in azopolymer films by the symmetrical superposition of different orthogonal pairs of polarized beams. The inscribed holographic elements have been analyzed microscopically in a Mueller polarimeter in order to image the optical anisotropies photoinduced in the film. In the most of cases, the spatial modulation of diattenuation, birefringence, and optical rotation reproduced quite well previous results reported in the literature. Nevertheless, in the particular case of coherent superposition of p- and s-polarized beams, the spatial frequency for optical rotation (related to the Stokes parameter V) was different from the one observed in linear anisotropy (related to the Stokes parameter U). It is shown by theory and experiment that, in the polarized field used to record this polarization grating, the fourth-Stokes parameter changes sign, which implies a change in circular polarization handedness, practically once between two adjacent maxima.

Spatio‐Temporal Control of Dynamic Topographic Patterns on Azopolymers for Cell Culture Applications

Cell response to exogenous cues is the result of a complex integration of multiple biochemical/biophysical signals, which might occur simultaneously and might be characterized by specific spatial and temporal patterns. Among these signals, surface topography plays an important role in affecting cell functions and fate. However, the current understanding of the interplay between cells and topography relies on static environment. Here the intrinsic light‐responsive properties of azopolymers and the versatility of laser‐based confocal microscope technique is exploited, aiming to induce spatio‐temporal dynamic topographic changes in situ during cell culture. Diverse patterns can be designed on cell‐populated azopolymer films with high control on time, space, and on‐off signal modification. The technique proposed in this study enables the development of synthetic platforms that finely control cell orientation and migration both in time and space. The results may pave the way to unravel complex processes involved in cell‐topography interactions, thus allowing to define the spatio‐temporal features that most effectively influence cell functions.

Instantaneous photoinduced patterning of an azopolymer colloidal nanosphere assembly

Colloidal azopolymer nanospheres assembled on a glass substrate were exposed to a single collimated laser beam. The combination of photo-fluidic elongation of the spherical colloids and light induced self-organization of the azopolymer film allows the quasi-instantaneous growth of a large amplitude surface relief grating. Pre-structuration of the sample with the nanosphere assembly supports faster creation of the spontaneous pattern. Confinement into the nanospheres provides exceptionally large modulation amplitude of the spontaneous relief. The method is amenable to any kind of photoactive azo-materials.

Synthesis and Characterization of Photoprocessable Lignin-Based Azo Polymer

Lignin-based azo polymer-bearing pseudo-stilbene-type azo chromophores have been synthesized by a post-azo coupling reaction between the modified alkali lignin and diazonium salts of 4-aminobenzoic acid in DMF with high yield. The polymer synthesized was characterized by using spectroscopic methods. Significant dichroism and surface relief patterns could be photoinscribed on the prepared azo polymer films by using appropriate laser beams. Self-assembly of the lignin-based polymer in selective solvents (THF/H2O) can give uniform colloidal spheres, which can be elongated along the polarization direction of the irradiation light. This novel synthesized lignin-based azo polymer could potentially be used for applications such as reversible optical data storage, photoswitching, sensors, and other photo-driven devices, which provide a simple strategy for value-added utilization of lignin biomass resources.

Fabrication of fluorescent surface relief patterns using AIE polymer through a soft lithographic approach

A new aggregation-induced emission (AIE) active polymer (PS-TPE) with high tetraphenylethene (TPE) loading density was synthesized. The synthesized polymer showed significant AIE properties, good solubility and high thermal stability. Soft-lithographic contact printing process by using photoinduced surface relief structures on azo polymer film as masters and duplicated PDMS elastomer as stamps was used to fabricate fluorescent PS-TPE patterns. Various fluorescent structures with high contrast including surface relief gratings, periodically dotted patterns, and quasi-crystal structures can be easily fabricated through this approach. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1838–1845