Azopolymer Films Research Articles

Efficient All-Optical Molecule-Plasmon Modulation Based on T-shape Single Slit

Efficient all-optical molecule-plasmon modulation is experimentally demonstrated by employing a compact T-shape single slit on a metal film coated with an azopolymer film, in which the azobenzene molecules can be reoriented by a pump beam. In the T-shape single slit, the transmission spectra exhibit periodic behaviors and are quite sensitive to variations of the refractive index of the azopolymer in the groove. Under a pump beam, the azobenzene molecules are reoriented, so the SPPs in the groove feel a refractive index quite different from that of the originally isotropic azopolymer with randomly orientations. This leads to a high modulation depth of about 53 % (3.3 dB) and a phase variation of >π experimentally.

Photoinduced orientation in nunchaku-like azo molecular glass studied by birefringence characterization and FT-IR spectroscopy

Photoinduced orientation of amorphous azo materials based on nunchaku-like molecules was studied by birefringence characterization, polarized infrared spectroscopy, and 2D-IR correlation spectroscopy. The nunchaku-like molecules (AZBP-CA and AZBP-CN) contained both an azo chromophore and a rigid biphenylene unit linked with the flexible spacer. Upon irradiation with a linearly polarized Ar+ laser beam (488 nm), the birefringence growth and unique relaxation behavior were investigated for solid thin films of the materials. The results show that the high birefringence of AZBP-CA upon the light irradiation is related with the hydrogen-bonding in the material as indicated by IR spectroscopy. The nunchaku-like molecule films show the unique relaxation behavior when switching off the exciting laser, i.e. the photoinduced birefringence further increases instead of partial decay, which is typically observed for most azo polymer films. The amount of the post-irradiation birefringence growth is positively correlated with the illumination time during the writing process. Polarized infrared spectroscopic investigation indicates that the orientations of the azo chromophore and biphenylene unit are strongly correlated with each other. As revealed by the two-dimensional infrared correlation spectroscopy, the azo chromophore and biphenylene unit move cooperatively almost at the same time. The flexible spacer of the molecules prefers to align in the same direction as the azo chromophore after the light irradiation, which suggests that the photoinduced orientation of the azo chromophore transfers through the flexible spacer to the biphenylene unit.

Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination

When an azobenzene-containing polymer film is exposed to non-uniform illumination, a light-induced mass migration process may be induced, leading to the formation of relief patterns on the polymer-free surface. Despite many years of research effort, several aspects of this phenomenon remain poorly understood. Here we report the appearance of spiral-shaped relief patterns on the polymer film under the illumination of focused Laguerre–Gauss beams with helical wavefronts and an optical vortex at their axis. The induced spiral reliefs are sensitive to the vortex topological charge and to the wavefront handedness. These findings are unexpected because the doughnut-shaped intensity profile of Laguerre–Gauss beams contains no information about the wavefront handedness. We propose a model that explains the main features of this phenomenon through the surface-mediated interference of the longitudinal and transverse components of the optical field. These results may find applications in optical nanolithography and optical-field nanoimaging.

Spontaneous nano pattern formation under pulsed exposure: relaxation kinetics study

Formation of spontaneous surface relief grating under pulsed exposure on the surface of an azo polymer film has been investigated. We did experiments in different repetition rates and pulse widths by use an electro mechanical chopper and a diode-pumped solid-state laser with wavelength of 532 nm as pump beam. Incidence of pump beam, has been set normal to the surface of azo-polymer film. Spontaneous surface relief grating with grating pitch of ~1,250 nm was formed. For low repetition rate, relaxation due to cis-trans isomerization becomes dominant and growth rate becomes slower. A threshold for spontaneous pattern formation was found. In addition to an increase in the diffraction intensity during illumination, anomalous variation of diffraction intensity by switching on and off the laser was observed. By switching off the recording beam sharp enhancement occurred. By switching on the recording beam, after a sharp decrease in the intensity of diffracted beam, it increased further under illumination. Anomalous variations, strongly depends on polarization of the probe beam and the irradiation time for the formation of spontaneous SRG. Sharp enhancement has been attributed to cis-trans thermal isomerization and sharp decrease has been attributed to isomeration in whole surface under illumination.

Monolithic, Hierarchical Surface Reliefs by Holographic Photofluidization of Azopolymer Arrays: Direct Visualization of Polymeric Flows

AbstractHierarchically ordered, monolithic surface reliefs have attracted a great deal of interest due to their applications in advanced photonics and interface sciences. While many impressive achievements in fabrication of such surface reliefs have been made over the last decade, all established methods are still restricted by a number of factors, such as limited control of structural features, inherently induced structural defects, impractically low throughput, and technical barriers caused by mechanical contact. Herein, a deterministic and scalable fabrication of hierarchically ordered, monolithic surface reliefs by holographic photofluidization of azopolymer line arrays is demonstrated. In particular, it is shown that the structural features of monolithic surface reliefs including shapes and modulation heights can be deterministically tunable by adjusting the polarization and irradiation time of the holographic interference pattern. Moreover, by a direct visualization of azopolymeric flow according to the light polarization, a long‐standing question about the origin of surface‐relief‐grating formation on azopolymer film is addressed in terms of polymeric flows. Finally, as proof of concept for the practical application of the obtained hierarchical surface reliefs, dependence of wetting properties on modulating height is demonstrated.

Self-Structured Surface Patterns on Epoxy-Based Azo Polymer Films Induced by Laser Light Irradiation

In this study, two series of epoxy-based azo polymers with high chromophore density were synthesized and self-structured surface pattern formation on the polymer films was studied by laser light irradiation under different conditions. To synthesize the azo polymers, two epoxy-based precursor polymers (PEP-AN and PEP-35AN) were prepared by step polymerizations of N,N-di(epoxypropyl)aniline and N,N-di(epoxypropyl)-3,5-dimethylaniline with aniline and 3,5-dimethylaniline, respectively. The azo polymers were obtained through postpolymerization azo-coupling reactions between the precursor polymers and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline. The epoxy-based precursor polymers and azo polymers were characterized by using 1H NMR, FT-IR, UV–vis, and DSC analyses. The self-structured surface pattern formation on films of the two series of azo polymers was studied by irradiating the polymer films with a normal-incident laser beam at two different waveleng...

Synthesis of azobenzene-functionalized star polymers via RAFT and their photoresponsive properties

Azobenzene-functionalized star polymers were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. First, azobenzene-functionalized linear macro chain transfer agents (Macro-CTA) were synthesized by RAFT polymerization of 6-[4-(4′-Methoxyphenylazo)phenoxy]hexylmethacrylate (MAz6Mc) using 2-(2′-cyanopropyl)dithiobenzoate (CPDB) as RAFT agent in presence of AIBN as initiator in anisole. Subsequently, star azopolymers were synthesized by polymerization of a difunctional azomonomer, BMA2Az, with resultant Macro-CTA in presence of AIBN as initiator in anisole. Star azopolymers were characterized by GPC and spectroscopic methods. Thermal properties of star azopolymers were determined by DSC and TMA. Molecular weight versus conversion and molecular weight versus polymerization time attest to living polymerization characteristics. Photoisomerization behaviors of star azopolymers were studied by irradiation of both UV and visible light. Surface relief gratings were inscribed on star azopolymer films upon exposure to an interference pattern of (RCP + RCP) Ar + laser. A diffraction efficiency of 20% was obtained by exposure of Star-8 K(2.6 K) polymer film to an (RCP + RCP) Ar + laser for about 30 min. Surface relief grating structures were investigated by AFM and polarized optical microscopy.

Azo-polymers modified with nucleobases and their interactions with DNA molecules

The photo-fluidization process which is specific for azo-materials opens a new perspective for their use in the field of molecules nano manipulation at the surface of the azo polymer films. This is possible considering that in the case of the UV irradiation from a polarized laser source the azo material has an unidirectional flow. Here, we investigated the structuring phenomena occurring on the surface of the azo-polysiloxanes films modified with nucleobases, upon UV irradiation. Measurements of topography and adhesive forces between polymeric substrates and a hydrophilic probe have been done by atomic force microscopy (AFM). The response of the material upon irradiation has been investigated also by using UV−VIS spectroscopy. This method allowed us to draw the photo-isomerization and relaxation curves. Also, preliminary tests were conducted to determine the capacity of the film surface to immobilize DNA molecules.

All-optical fabrication of ellipsoidal caps on azobenzene functional polymers

We have fabricated an azobenzene (azo) polymer microellipsoidal cap array of hexagonal symmetry with high-power laser ablation and subsequent single-beam-induced mass migration. High-power interference with polarization-controlled triple beams is utilized to inscribe a circular bump array directly on the surface of the azo polymer film. The produced circular cap array is exposed to the linearly polarized beam, and the caps are stretched along the polarization direction of the irradiating beam. A model of gradient force emerged by the interaction of the polarized beam and azo polymer is constructed to explain the mechanism of such polarization-induced microscale shape deformation.

Sensing Diffraction Gratings of Antigen‐Responsive Hydrogel for Human Immunoglobulin‐G Detection

Sensing diffraction gratings of antigen-responsive hydrogel were developed for human immunoglobulin-G (H-IgG) detection. The IgG-sensing gratings were fabricated by soft lithographic duplication of photoinduced surface relief gratings on azo polymer films to the hydrogel surfaces. The quantitative detection of H-IgG was achieved by detecting modulation of the diffraction efficiency when the gratings were exposed to antigen solutions. The gratings showed high sensitivity and specificity for the H-IgG detection. The detection could be performed at a reasonably fast rate with good reusability. In the process, no fluorophore labeling or additional signal enhancement was required.

Glucose sensing through diffraction grating of hydrogel bearing phenylboronic acid groups

A new glucose sensing strategy has been developed by using diffraction-gratings of a stimuli-responsive hydrogel bearing phenylboronic acid groups. The sensing gratings were fabricated through soft-lithographic duplication of photoinduced surface-relief-gratings (SRGs) on azo polymer films to the hydrogel surfaces. For the preparation, elastomeric stamps were obtained by casting PDMS prepolymer against the azo polymer masters and curing properly. The hydrogel gratings containing phenylboronic acid groups were shaped by contact printing with the PDMS stamps and a specially prepared “ink”, which was a precursor solution curable by UV light irradiation. The gratings showed a fast and highly sensitive response to glucose in buffer solutions as read out by the increase of the diffraction efficiency (DE/DE 0). The saturated DE/DE 0 value was linearly correlated to the glucose concentration of the solutions. The gratings showed ability for the continuous detection and were durable for repeated applications. The sensing ability of the gratings was stable when kept in the neutral buffer at room temperature for more than one month. Owing to the versatile nature, this approach can be further explored to prepare various grating sensors for the saccharide recognition and detection.

Realization of submicrometer structures by a confocal system on azopolymer films containing photoluminescent chromophores

The mass migration phenomenon occurring on the free surface of azobenzene-containing polymers illuminated by light of appropriate wavelength is employed to pattern polymeric films constituted by an azopolymer doped with photoluminescent chromophore. Different topographical features are obtained by adjusting the laser scanning parameters, in particular the laser polarization direction. A detailed microscopic analysis of the patterned samples, based on atomic force microscopy and spectral confocal imaging, evidences the photobleaching of the emitting chromophore in the exposed regions, whereas the emitting features of the chromophore are preserved by the mass migration process. Our results are important for the design of nanostructured light emitting photonic devices based on azopolymers.

Polymer diffraction gratings on stimuli-responsive hydrogel surfaces: Soft-lithographic fabrication and optical sensing properties

In this work, a soft-lithographic contact printing approach was developed to fabricate polymer diffraction gratings on stimuli-responsive hydrogel surfaces. For the fabrication, azo polymer films with surface-relief-gratings (SRGs) on surfaces were used as masters, which were inscribed by irradiating solid films of BP-AZ-CA with interfering Ar + laser beams. Elastomeric stamps were prepared by replica molding of poly(dimethylsiloxane) (PDMS) prepolymer against the masters and curing properly. One-dimensional (1D) and two-dimensional (2D) gratings were fabricated on the surfaces of pH-sensitive hydrogel plates through soft-lithographic contact printing. Results show that the swellings of the hydrogel in buffer solutions are sensitively reflected by the variation of the gratings’ diffraction parameter. The pH of the buffer solutions shows a significant influence on the rate and degree of the diffraction parameter variation. Through the hydrogel volume change in response to external stimuli, the diffraction gratings can optically sense the variation of solution pH, which can be extended to sense the ionic strength and other analytes in solutions. Such a transduction mechanism is useful for optically detecting the external stimuli or molecular recognition events. The sensitive grating can also be used as the smart optical filters and other optical devices.

PHOTO-INDUCED DICHROISM AND SURFACE-RELIEF-GRATINGS OF HYPERBRANCHED AZO POLYMERS SYNTHESIZED BY AZO-COUPLING REACTION

A novel hyperbranched azo precursor polymer containing electron donor-acceptor type azobenzene chromophores was synthesized by step-growth polymerization of an AB_2 monomer,di[2-(N-ethylanilino)ethyl]-5-aminoisophthalate,through azo coupling reaction.The hyperbranched precursor polymer was then reacted with the diazonium salts of 4-amino benzonitril to introduce another kind of donor-acceptor azo chromophores in the peripheral groups.The structure and properties of the azo polymers were characterized by the spectroscopic methods and thermal analysis.The photo-induced dichroism of the hyperbranched polymer was studied and the orientation order parameter of the polymer was estimated to be 0.063.Thin films of the hyperbranched azo polymers were used to fabricate surface-relief-gratings(SRGs) by exposing the thin films to an interference pattern of Ar~+ laser beam at modest intensities(150 mW/cm~2).The azobenzene chromophore introduced by post-azo-coupling reaction in the para-positions of the terminal azobenzene units shows a significant effect on the SRG inscription rate.The hyperbranched azo polymers can potentially be used for applications such as reversible optical data storage,photoswitching,sensors,and other photo-driven devices.

Molecular-level interactions of an azopolymer and poly(dodecylmethacrylate) in mixed Langmuir and Langmuir–Blodgett films for optical storage

The applicability of azopolymers in optical storage can be extended through the use of nanostructured films produced with the Langmuir–Blodgett (LB) technique, but the film properties need to be optimized since these polymers generally do not form stable Langmuir films to be transferred onto solid substrates. Here, photoinduced birefringence was investigated for mixed Langmuir–Blodgett films from the homopolymers 4-[ N-ethyl- N-(2-methacryloxyethyl)]-4′-nitroazobenzene (HPDR1-MA) and poly(dodecylmethacrylate) (HPDod-MA). The interactions between these polymers were studied in Langmuir and LB films. Surface pressure–area isotherms pointed to molecular-level interactions for proportions of 51 mf%, 41 mf% and 31 mf% of HPDR1-MA. Phase segregation was not apparent in the BAM images, in which the morphology of the blend film was clearly different from that of the Langmuir films of neat homopolymers. Through PM-IRRAS, we noted that the interaction between the azopolymer and HPDod-MA affected the orientation of carbonyl groups. Strong interactions for the mixture with 41 mf% of poly(dodecylmethacrylate) led to stable Langmuir films that were transferred onto solid supports as LB films. The photoinduced birefringence of 101-layer mixed LB films show features that make these films useful for optical storage, with the advantage of short writing times in comparison to other azopolymer films.

Ag nanoparticle/azopolymer nanocomposites: In situ synthesis, microstructure, rewritable optically induced birefringence and optical recording

Ag nanoparticle/azopolymer nanocomposites are prepared with controlled concentration of Ag nanoparticles by in situ reduction of Ag(I) β-diketone complexes in an azopolymer matrix. The nanocomposites form an organic–inorganic network-like structure by interactions between the azopolymer matrix and the Ag nanoparticles. The Ag/azopolymer nanocomposites are homogeneous and highly transparent even when the content of Ag is as high as 5.6 wt%. Birefringence of the azopolymer without Ag and the nanocomposites can be optically induced and erased. However, after 5 cycles of optically writing–erasing processes, 9.7% decrease of the birefringence is observed for the azopolymer film without Ag and only 2.9% decrease of the birefringence is observed for the nanocomposite film with 5.6 wt% Ag, showing the stability of photo-induced birefringence of the nanocomposite is improved by introducing Ag nanoparticles into the azopolymer matrix. Relatively high concentration of Ag nanoparticles does not strongly hinder the mobility of azobenzene groups during photoisomerization and one-photon recording. In two-photon optical recording experiments, an image on the nanocomposite with 5.6 wt% Ag can be observed with writing power as low as 6 mW and no such an image can be observed in the azopolymer without Ag under the same two-photon recording condition.

Photoinduced Reorientation Process and Nonlinear Optical Properties of Ag Nanoparticle Doped Azo Polymer Films

Azobenzene polymer films doped with and without Ag nanoparticles are prepared. The photoinduced reorientation process is investigated by using an Nd:YVO4 pump beam at 532 nm and a low semiconductor laser beam at 650 nm. The reorientation rate of azo polymer films is enhanced in the presence of Ag nanoparticles, and the rate of the azo polymer film with Ag concentration of 2.2 μg/ml is larger than that of the azo polymer films with Ag concentrations of 1.1 μg/ml and 4.4μg/ml. The third-order nonlinear optical properties of the Ag/azo composite film are obtained by the Z-scan technique at a wavelength of 532 nm, and the measured nonlinear refractive index is 9.258 × 10−9 esu. It is shown that the main mechanisms involved in the large nonlinear optical responses come from the local field enhancement of Ag nanoparticles and the nonlinear effect of the azo polymer matrix.

Preparation and characterization of grafted polyethylene based azo-polymer films

A series of grafted azo-polymers was prepared from commercial low density polyethylene thin plates (PE). Polyethylene was reacted in the presence of acryloyl chloride using gamma irradiation to give precursor grafted polymers. These materials were esterified in the presence of six different commercial azo-dyes: (E)-2-(ethyl(4-((4-nitrophenyl)diazenyl)phenyl)amino)ethanol (Disperse Red-1, DR-1), (E)-2-((4-((2-chloro-4-nitrophenyl)diazenyl)phenyl)(ethyl)amino)ethanol (Disperse Red-13, DR-13), (E)-2,2′-(4-((4-nitrophenyl)diazenyl)phenylazanediyl)diethanol (Disperse Red-19, DR-19), (E)-4-((4-nitrophenyl)diazenyl)aniline (Disperse Orange-3, DO-3), 4-((E)-(4-((E)-phenyldiazenyl)naphthalen-1-yl)diazenyl)phenol (Disperse Orange-13, DO-13) and 2-methyl-4-((E)-(4-((E)-phenyldiazenyl)phenyl)diazenyl)phenol (Disperse Yellow-7, DY-7) to give the expected grafted azo-polymer films. The obtained polymers were fully characterized; their thermal, optical properties and morphology were studied. In particular, the influence of the irradiation conditions and the incorporated azo-dye on the polymer properties is discussed.

Effects of absorption overlap on the modulation of the polarized fluorescence of Eu(DBM)3Phen-doped azobenzene polymer films

Two azopolymers, poly(4-(4′-nitrophenylazo)phenoxy)hexyl methacrylate (PNAzoPOH) and poly(6-(N-methyl-N-(4-(4′-nitrophenylazo)phenyl)amino)hexyl methacrylate) (PNAzoPAH), bearing strong push–pull azobenzene chromophores, have been designed and synthesized. Azopolymer films doped with a europium complex, tris(dibenzoylmethanido)(o-phenanthroline)europium(III) (Eu(DBM)3Phen), were prepared by a drop-casting method. UV-vis spectra indicated that the absorption peak of Eu(DBM)3Phen at 365 nm overlaps with that of PNAzoPOH, rather than that of PNAzoPAH. Polarized fluorescence from both of the oriented films was observed to depend on the absorption overlap between the complex and the azopolymer. When excited by unpolarized light at 365 nm, the polarization ratio R of Eu(DBM)3Phen-doped PNAzoPOH film shows an obvious decreasing trend with increasing irradiation time, and the ratio R of Eu(DBM)3Phen-doped PNAzoPAH film has a relative stable value of about 1.47. When excited by polarized light at 365 nm, the florescence intensities from the two films show different dependences upon the angle between the direction of orientation of the film and the direction of polarization of the excitation light: for Eu(DBM)3Phen-doped PNAzoPOH film, the strength of the polarized fluorescence depended on the direction of polarization of the excitation light; for Eu(DBM)3Phen-doped PNAzoPAH film, no such dependence was observed. These different results arise due to differences between the azopolymers in terms of the allocation of energy between the oriented azobenzene groups and the complex.

Optical and Liquid Crystalline Properties of New Alkyl-Substituted Azopolymers

In the present work, we report the synthesis, the mesomorphic behavior and the laser-induced optical characteristics of side-chain liquid crystalline azopolymers without a spacer, but substituted with a hexyloxy group. The azopolymers were studied by DSC, POM and XRD. Results have revealed that the monomer exhibits a nematic phase, whereas polymers develop a double layered SmC phase. Thin films of azopolymers exhibited good response to a laser beam irradiation. Surface relief gratings were recorded with the interference pattern produced by a 413 nm circularly polarized laser beam.