Azobenzene-functionalized Polymer Research Articles

Nanoscale Melting of 3D Confined Azopolymers through Tunable Thermoplasmonics.

Phase transitions that are thermally induced by using light at the nanoscale play a vital role in material science. Enhanced optical heating sustained by resonant nanostructures can turn out to be insignificant when a higher thermal conductivity of a heatsink, regardless of the pumping intensity. In this Letter, we demonstrate an approach to control an operating temperature range due to excess heating of a structured heatsink. A design rationale has been performed by using a 2D array of TiN:Si voxels, consisting of stacked TiN and Si pillars. All the TiN nanoheaters responsible for enhanced light absorption at plasmon resonance are of equal size, and the height of the Si pillars varies to control heat localization. A height-dependent temperature rise of the Si pillars is found from Raman thermometry. Herein, for the first time, we have determined the melting temperature of azobenzene-functionalized polymers at the nanoscale using the tunable plasmonic metasurface.

Fabrication of azobenzene-functionalized porous polymers for selective CO2 capture

Many solid adsorbents have been prepared for the CO2 capture. In particular, the photoresponsive adsorbents have attracted extensive interests because of their tunable pore structure and variable responsive behaviors provoked by the external light. However, it is challenging to fabricate the photoresponsive adsorbents featured the big CO2 capacity and high CO2 selectivity. Herein, copolymerized between 4-phenylazobenzoyl chloride, 2,4,6-trichloro-1,3,5-triazine and melamine, a series of azobenzene-functionalized porous polymers (PTM-AZOs) are successfully synthesized. The PTM-AZOs are verified in possession of proper pore structures, large surface area and photoconductive properties through a series of characterization. The PTM-AZO-2 with the trans-isomerization exhibits the best CO2 adsorption amount of 2.7 mmol·g−1 (273 K and 0.1 MPa), while the CO2/N2 selectivity can reach 2459 and 607 on the trans- and cis-isomerization, respectively. The regulatable pore structures controlled by the photoresponsive azobenzene groups affect the CO2 capture performance of the PTM-AZOs.

Triptycene-Derived Photoresponsive Fluorescent Azo-Polymer as Chemosensor for Picric Acid Detection.

Two new triptycene-based azobenzene-functionalized polymers (TBAFPs) have been synthesized using the well-known Pd-catalyzed Sonogashira cross-coupling polycondensation reaction between 2,6-diethynyltriptycene and (meta or para) dibromo-azobenzenes. Enhancement of the fluorescent emission intensity was observed upon trans → cis isomerization of −N=N― linkage in TBAFPs. The cis-lifetime of TBAFP1 is rather long (greater than 2 days). The resulting materials were tested as a potential chemosensor for the detection of picric acid (PA)—a water pollutant as well as chemical constituent of explosives used in warfare. PA was found to interact strongly with TBAFPs, which led to significant quenching of the latter’s fluorescence emission intensities. The binding constants are in the order of 105 M–1. TBAFPs were also able to detect PA in nanomolar concentrations.

Azobenzene-functionalized polymers by ring-opening metathesis polymerization for high dielectric and energy storage performance

Block copolymers with push–pull azobenzene pendants and core–shell nanostructures exhibited high and regulated dielectric constants by photoisomerization of azobenzene groups, low dielectric loss, and high energy density.

Visible light-, pH-, and cyclodextrin-responsive azobenzene functionalized polymeric nanoparticles

Multiple stimuli-responsive nanoparticles that could respond to visible-light, pH and host-guest interactions were fabricated by self-assembly of tetra-ortho-methoxy-substituted azobenzene (mAzo) functionalized poly(dimethylaminoethyl methacrylate) (PDMAEMA) (PDMAEMA-mAzo), which was synthesized by quaternization between the azobenzene derivative (mAzo-Br) and the dimethylaminoethyl units of PDMAEMA. The amphiphilic azobenzene-functionalized polymer (PDMAEMA-mAzo) would self-assemble into a micellar nanoparticle consisting of hydrophobic mAzo groups as a core and hydrophilic PDMAEMA segments as a shell. Under green light irradiation, the azobenzene could isomerize from the trans form to the cis, while the morphology of the nanoparticle changed little and the loaded Nile red would not be released. At acidic condition, the azobenzene mAzo and PDMAEMA could be protonated and the nanoparticle was swollen and the loaded Nile red would be released. When mixed with cyclodextrin (CD), the nanoparticles were dissociated due to the formation of the host-guest complex of CD and azobenzene. Upon green light irradiation, the trans azobenzene changed to the cis form and the host-guest interaction could be disrupted, where the cis-azobenzene-containing polymer would self-assemble into nanoparticles. The dissociation of the nanoparticles upon addition of CD would induce the release of loaded Nile red. The prepared multiple-responsive azobenzene functionalized polymer nanoparticles may offer significant opportunities for diverse applications in the fields of nanotechnology and biotechnology for controlled release as smart nanocarriers.

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.

Effect of secondary relaxation transitions on photo-induced anisotropy in glassy azobenzene-functionalized polymers

The β-relaxation transition in side-chain azobenzene-functionalized amorphous polymers can be directly found from the temperature behavior of infrared dichroism.

Determination of the Glass Transition Temperature of Freestanding and Supported Azo-Polymer Thin Films by Thermal Assisted Atomic Force Microscopy

In this paper we introduce and apply the method for determination of the glass transition temperature of the sub-100 nm thick freestanding and supported polymer films based on thermally assisted atomic force microscopy (AFM). In proposed approach changes of the phase of an oscillating AFM cantilever are used to determine glass transition temperature. An anomalous decrease of the glass transition temperature for both free-standing and supported azobenzene-functionalized polymer thin films is shown.

Light Propagation and Photoactuation in Densely Cross-Linked Azobenzene-Functionalized Liquid-Crystalline Polymers: Contribution of Host and Concerted Isomerism

The photomechanical behavior of azobenzene-functionalized liquid-crystalline polymers (azo-LCPs) is closely related to UV light propagation. Here, we report the effect of light absorption by the LC host and the concerted isomerism of azobenzene on this property. In situ measurements of light absorptivity and computer simulations revealed that the light propagation of a liquid-crystalline polymer is affected by the UV absorption of the LC host under the same concerted isomerism of azobenzene. The lower UV light absorption of the LC host results in deeper UV light propagation and a sharper isomerization profile, which eventually induces faster bending when compared to the higher UV light absorption of the LC host. On the other hand, increasing the azobenzene concentration in the polymer greatly decreases the speed of light propagation, which is regarded as one of the factors inhibiting further increases in the bending speed of azo-LCP films.

Near-field Raman dichroism of azo-polymers exposed to nanoscale dc electrical and optical poling.

Azobenzene-functionalized polymer films are functional materials, where the (planar vs. homeotropic) orientation of azo-dyes can be used for storing data. In order to characterize the nanoscale 3D orientation of the pigments in sub-10 nm thick polymer films we use two complementary techniques: polarization-controlled tip-enhanced Raman scattering (TERS) microscopy and contact scanning capacity microscopy. We demonstrate that the homeotropic and planar orientations of the azo-dyes are produced by applying a local dc electrical field and a resonant longitudinal optical near-field, respectively. For a non-destructive probe of the azo-dye orientation we apply a non-resonant optical near-field and compare the intensities of the Raman-active vibrational modes. We show that near-field Raman dichroism, a characteristic similar to the absorption dichroism used in far-field optics, can be a quantitative indicator of the 3D molecular orientation of the azo-dye at the nanoscale. This study directly benefits the further development of photochromic near-field optical memory that can lead to ultrahigh density information storage.

TERS microscopy as a probe for visualizing the orientation of chromophores embedded in the glassy polymers

Photo-induced effects on azobenzene-functionalized polymer thin films under exposure to strongly focused higher laser modes are investigated. Tip-enhanced Raman scattering microscopy provides a possibility for probing and visualizing the orientation of anisotropic chromophores embedded in the glassy environment at room temperature.

Impact of Backbone Rigidity on the Photomechanical Response of Glassy, Azobenzene-Functionalized Polyimides

Azobenzene-functionalized polyimide materials can directly transduce light into mechanical force. Here, we examine the impact of polymer backbone rigidity on the photomechanical response in a series of linear, azobenzene-functionalized polymers. The rigidity of the backbone was varied by the polymerization of five dianhydride monomers with a newly synthesized diamine (azoBPA-diamine). The azobenzene-functionalized linear polymers exhibit glass transition temperatures (Tg) ranging from 276 to 307 °C and maintain excellent thermal stability. The photomechanical response of these materials was characterized by photoinduced cantilever bending as well as direct measurement of photogenerated stress upon exposure to linearly polarized, 445 nm light. Increasing the rigidity of the polymer backbone increases the magnitude of stress that is generated but decreases the angle of cantilever deflection.

Generic stochastic Monte Carlo model of the photoinduced mass transport in azo-polymers and fine structure of Surface Relief Gratings

We study theoretically the possible origin of a double-peak fine structure of Surface Relief Gratings in azo-functionalized poly(etherimide) reported recently in experiments. To improve the statistics of experimental data additional measurements were done. For the theoretical analysis we develop a stochastic Monte Carlo model for photoinduced mass transport in azobenzene-functionalized polymer matrix. The long sought-after transport of polymer chains from bright to dark places of the illumination pattern is demonstrated and characterized, various scenarios for the intertwined processes of build-up of density and SRG gratings are examined. Model predicts that for some azo-functionalized materials double-peak SRG maxima can develop in the permanent, quasi-permanent or transient regimes. Available experimental data are interpreted in terms of model's predictions.

Contactless, photoinitiated snap-through in azobenzene-functionalized polymers

Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report orders-of-magnitude enhancement in actuation rates (approaching 10(2) mm/s) and powers (as much as 1 kW/m(3)). The contactless, ultra-fast actuation is observed at irradiation intensities <<100 mW/cm(2). Due to the bistability and symmetry of the snap-through, reversible and bidirectional actuation is demonstrated. A model is developed to elucidate the underlying mechanics of the snap-through, specifically focusing on isolating the role of sample geometry, mechanical properties of the materials, and photomechanical strain. Using light to trigger contactless, ultrafast actuation in an otherwise passive structure is a potentially versatile tool to use in mechanical design at the micro-, meso-, and millimeter scales as actuators, as well as switches that can be triggered from large standoff distances, impulse generators for microvehicles, microfluidic valves and mixers in laboratory-on-chip devices, and adaptive optical elements.

Liquid‐Crystal Polymers: Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks (Adv. Mater. 41/2013)

Light can induce rich and diverse topographical transformations of initially flat films composed of photoresponsive, azobenzene-functionalized liquid-crystalline polymer networks. The N-fold symmetry of the topography of the photoinduced response of defect-containing azo-LCN films is examined by T. J. White and co-workers on page 5880. It is found to be inherently coupled to the strength of the defect, confirming that the director profile emanating from the topological defect governs the behavior. The topography of a film subsumed with a +10 defect resting on water after exposure to light is shown here.

Photolithographic periodic patterning of gold using azobenzene-functionalized polymers

Metal micro- and nanostructures for optical and electronic applications are typically fabricated by means of interferometric optical or electron-beam lithographies using conventional photo- or electron-beam resists. In this work, we report on fabrication of periodic nanostructures of gold by exploiting photoinduced surface-relief gratings in an azobenzene-functionalized polymer film as masks for reactive ion etching of the metal. The proposed technique provides a convenient, fast and flexible alternative to photoresist-based lithography for fabricating metal nanostructures of large surface area. Owing to the fact that the azo-polymer is sensitive to the polarization rather than the intensity modulation of the exposing light, the technique is particularly suitable for patterning highly reflective surfaces.

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.

Azobenzene Photoisomerization under High External Pressures: Testing the Strength of a Light-Activated Molecular Muscle

The photoinduced isomerization and thermal back relaxation of an azobenzene-functionalized polymer poly(disperse red 1 acrylate) were investigated at increasing external pressures up to 1.5 GPa inside a diamond-anvil spectroscopic cell. The thermal cis-trans isomerization was monitored by laser pump-probe spectroscopy, which demonstrated an increase in the half-life of the isomerization process with increasing pressure. Additionally, the cis content of the photostationary state gradually decreased as a function of pressure, with complete arrest of the trans-cis photoisomerization above 1.5 GPa. The fact that the photoswitching behavior however could still be observed beyond 1 GPa is remarkable and is effectively a measure of the strength of the azobenzene chromophore as an artificial muscle. The changes in the Raman shifts of both trans- and cis-azobenzene were also investigated from ambient pressure up to 4 GPa, and no discontinuities were observed in the pressure vs wavenumber plots indicating no change in phase. The cis-trans photoisomerization of azobenzene was shown however to still be inducible at all the pressures investigated, confirming the suitability of these molecules for high-efficiency light actuation.

Simple Two-Photon Inscription of Surface Relief Gratings with Azobenzene Functionalized Polymer

Two-photon fabrication of surface relief gratings on azobenzene functionalized polymer films was carried out using femtosecond laser pulses at 800 nm. The two-photon inscriptions were achieved by simply employing a relief grating as a phase mask. The relief patterns were successfully recorded with the phase mask and the near-infrared laser pulses. The period of the grating is the same as that of the mask. Polarization dependent formation of the grating was observed.

Optical Interference Lithography Using Azobenzene‐Functionalized Polymers for Micro‐ and Nanopatterning of Silicon

Light-induced mass transport in azobenzene-functionalized polymers is exploited in optical interference lithography to fabricate large-area, periodic 1D and 2D silicon nanostructures. The demonstrated technique is a fast, reliable, and cost-effective alternative to conventional photoresist-based methods of nano- and microfabrication. Potential applications of the technique range from optics and photonics to functional materials and coatings.