Azo Monomer Research Articles

High free volume polymers of intrinsic microporosity for efficient solar thermal fuels with enhanced energy storage capacity

Anchoring photoswitches on high free volume polymers is an emerging strategy to prepare high-performance solar thermal fuels (STFs), however, it still lacks a deep understanding about the relationship between free volume and energy storage performance. Herein, a bottom-to-up strategy was developed to prepare azobenzene-based STFs based on polymers of intrinsic microporosity (Azo-PIMs), in which free volumes could be changed by controlling the feeding ratio of co-monomers. Azo-PIM-1 and Azo-PIM-2 were prepared to have 7.41% and 19.07% azo grafting densities, respectively. The fractional free volume (FFV) and specific surface areas of Azo-PIM-1 are 0.17 and 552.8 m2 g−1, respectively, which are higher than that of Azo-PIM-2 (0.15 and 335.5 m2 g−1) due to the lower azo grafting density. The energy densities (EDg) of Azo-PIM-1 and Azo-PIM-2 in cis isomers are 13.39 J g−1 and 36.75 J g−1, and the corresponding storage enthalpy (ΔH) are 88.53 kJ mol−1 and 103. 35 kJ mol−1, respectively. Their ΔH are higher than that of the azo monomer of 83.76 kJ mol−1, demonstrating that PIMs as templates can efficiently increase the stored energy per azo moiety. Meanwhile, the higher ΔH of Azo-PIM-2 than that of Azo-PIM-1 indicates that the reduced free volume can enhance the energy storage capacity by prohibiting the rotation of azo photoisomerization. This work provides a basis for understanding the role of free volume on the energy storage performance of polymer-based STFs.

Synthesis and phase behaviour of a class of comb-like cholesteric azobenzene liquid crystalline polymers

ABSTRACT In this paper, we designed and synthesised two azo liquid crystal monomers (M1 and M2) and a chiral liquid crystal monomer N, followed by side-chain graft copolymerisation with polymethylhydrosiloxane (PMHS) to synthesize comb-like liquid crystalline polymers PN1 and PN2. The structures of all liquid crystal monomers and polymers conform to the molecular designs by FT-IR and 1H NMR. The liquid crystal properties were tested by DSC and POM, and the results showed that M1 and M2 were thermotropic enantiotropic nematic liquid crystals and N was a thermotropic enantiotropic cholesteric liquid crystal. Both the PN1 and PN2 series were amorphous polymers. The PN1 and PN2 series exhibited a typical Grand-jean texture. The glass transition temperature and isotropic temperature of the PN1 and PN2 series decreased first and then increased with the increase of azo monomer M content. And the glass transition temperature and isotropic temperature of the PN1 series were higher than the PN2 series. Both the PN1 and PN2 series had good thermal stability. The mesophase range of the PN1 series was 173.5°C~181.5°C, and the PN2 series was 162.5°C~172.9°C.

AIEgen-Terminated Charge-Switchable Zwitterionic Azo Polymer for Tumor Hypoxia Imaging

For fluorescent probes, getting over the obstacles encountered in an organism is an important prerequisite. A zwitterionic to cationic polymer is an attractive approach to realize long-term circulation and high cellular uptake efficiency at the same time. Here, we reported a hypoxia-activatable zwitterionic to cationic azo polymer for tumor imaging, which was composed of an AIEgen-decorated RAFT agent and an azo monomer unit with a carbonate linkage. The zwitterion group endowed it with great anti-fouling property, avoiding being captured by proteins in the blood stream. Under the hypoxic microenvironment, the azo bond was broken and changed into amine with positive charge, which enhanced the cellular uptake efficiency. Meanwhile, the fluorescence quenching effect was cut off because of the destruction of the azo units. Therefore, the probe achieved a “turn-on” process and could be used as a fluorescent probe for tumor hypoxia imaging.

Synthesis, Spectroscopic, Thermodynamics and Kinetics Analysis Study of Novel Polymers Containing Various Azo Chromophore

Two novel polymers containing azo and ether groups were synthesized by oxidative polycondensation in an aqueous alkaline medium by NaOCI oxidants. The azo dye monomers that were polymerized were synthesized by diazotization of 2-amino-4-chlorophenyl phenyl ether and coupling reaction with 2,7-dihydroxynaphthalene and 1,3-benzenediol. Structures of the synthesized compounds were characterized by spectroscopic studies such as FT-IR, UV-vis, 1H-NMR. Gel permeation chromatography was used to evaluate the molecular weight and molecular weight distribution of the azo polymers. Furthermore, the surface morphology of the azo monomers and polymers were scrutinized by using scanning electron microscope. To investigate the effect of solvent on absorption, the electronic absorption spectra of the synthesized compounds were measured in six solvents with different polarity. The thermal behaviors of the monomers and polymers were identified by the TG, DTG and DTA techniques. In addition, the Coats-Redfern, Horowitz-Metzger and Broido methods for the determination of the kinetic parameters were used in the kinetic analysis of thermal decomposition of the compounds.

Synthesis and Self-Assembly of Multistimulus-Responsive Azobenzene-Containing Diblock Copolymer through RAFT Polymerization.

This paper gathered studies on multistimulus-responsive sensing and self-assembly behavior of a novel amphiphilic diblock copolymer through a two-step reverse addition-fragmentation transfer (RAFT) polymerization technique. N-Isopropylacrylamide (NIPAM) macromolecular chain transfer agent and diblock copolymer (poly(NIPAM-b-Azo)) were discovered to have moderate thermal decomposition temperatures of 351.8 and 370.8 °C, respectively, indicating that their thermal stability was enhanced because of the azobenzene segments incorporated into the block copolymer. The diblock copolymer was determined to exhibit a lower critical solution temperature of 34.4 °C. Poly(NIPAM-b-Azo) demonstrated a higher photoisomerization rate constant (kt = 0.1295 s−1) than the Azo monomer did (kt = 0.088 s−1). When ultraviolet (UV) irradiation was applied, the intensity of fluorescence gradually increased, suggesting that UV irradiation enhanced the fluorescence of self-assembled cis-isomers of azobenzene. Morphological aggregates before and after UV irradiation are shown in scanning electron microscopy (SEM) and dynamic light scattering (DLS) analyses of the diblock copolymer. We employed photoluminescence titrations to reveal that the diblock copolymer was highly sensitive toward Ru3+ and Ba2+, as was indicated by the crown ether acting as a recognition moiety between azobenzene units. Micellar aggregates were formed in the polymer aqueous solution through dissolution; their mean diameters were approximately 205.8 and 364.6 nm at temperatures of 25.0 and 40.0 °C, respectively. Our findings contribute to research on photoresponsive and chemosensory polymer material developments.

Synthesis of an Azobenzene-containing Main-chain Crystalline Polymer and Photodeformation Behaviors of Its Supramolecular Hydrogen-bonded Fibers

The synthesis of a new azobenzene (azo)-containing main-chain crystalline polymer with reactive secondary amino groups in its backbone and photodeformation behaviors of its supramolecular hydrogen-bonded fibers are described. This main-chain azo polymer (namely Azo-MP6) was prepared via first the synthesis of a diacrylate-type azo monomer and its subsequent Michael addition copolymerization with trans-1,4-cyclohexanediamine under a mild reaction condition. Azo-MP6 was found to have a linear main-chain chemical structure instead of a branched one, as verified by comparing its 1H-NMR spectrum with that of the azo polymer prepared via the polymer analogous reaction of Azo-MP6 with acetic anhydride. The thermal stability, phase transition behavior, and photoresponsivity of Azo-MP6 were characterized with TGA, DSC, POM, XRD, and UV-Vis spectroscopy. The experimental results revealed that it had good thermal stability, low glass transition temperature, broad crystalline phase temperature range, and highly reversible photoresponsivity. Physically crosslinked supramolecular hydrogen-bonded fibers with good mechanical properties and a high alignment order of azo mesogens were readily fabricated from Azo-MP6 by using the simple melt spinning method, and they could show “reversible” photoinduced bending under the same UV light irradiation and good anti-fatigue properties.

Influence of a Crosslinker Containing an Azo Group on the Actuation Properties of a Photoactuating LCE System.

Photoactuating liquid crystalline elastomers (LCE) are promising candidates for an application as artificial muscles in microdevices. In this work, we demonstrate that by optimizing (1) the illumination conditions and (2) the mixture of azo monomer and azo crosslinker, thick films of an all-azo LCE can be prepared, which show a strong length change without bending during photoactuation. This becomes possible by working with white light (about 440 nm), whose absorption is low, leading to a large penetration depth. By adding an azo crosslinker to a previously prepared system, several improvements of the actuation properties—like a stronger photoactuation at lower operational temperatures—could be achieved. In addition, films of different crosslinker concentrations and thicknesses were produced by photopolymerization at varying temperatures within a magnetic field, and their thermo- and photoresponsive behavior was investigated. An extraordinarily strong maximal thermal actuation of 46% and—by exposure to white light at 70 °C—a photoresponsive change in length of up to 40% in just about 13 s could be obtained. Even densely crosslinked samples were still able to photoactuate remarkably. Isothermal back-deformation could either be achieved by irradiation with red light (7 min) or by keeping the film in the dark (13 min).

Effect of Photoinduced Size Changes on Protein Refolding and Transport Abilities of Soft Nanotubes.

Self-assembly of azobenzene-modified amphiphiles (Glyn Azo, n=1-3) in water at room temperature in the presence of a protein produced nanotubes with the protein encapsulated in the channels. The Gly2 Azo nanotubes (7 nm internal diameter [i.d.]) promoted refolding of some encapsulated proteins, whereas the Gly3 Azo nanotubes (13 nm i.d.) promoted protein aggregation. Although the 20 nm i.d. channels of the Gly1 Azo nanotubes were too large to influence the encapsulated proteins, narrowing of the i.d. to 1 nm by trans-to-cis photoisomerization of the azobenzene units of the Gly1 Azo monomers packed in the solid bilayer membranes led to a squeezing out of the proteins into the bulk solution and simultaneously enhanced their refolding ratios. In contrast, photoinduced transformation of the Gly2 Azo nanotubes to short nanorings (<40 nm) with a large i.d. (28 nm) provided no further refolding assistance. We thus demonstrate that pertubation by the solid bilayer membrane wall of the nanotubes is important to accelerate refolding of the denatured proteins during their transport in the narrow nanotube channels.

Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties

This study describes the synthesis of an azobenzene-containing diblock copolymer, poly(StO50-b-Azo7), by using the macro-chain transfer agent StO macro-CTA, and employing sequential reverse addition-fragmentation transfer (RAFT) polymerization. We studied the effects of azobenzene (azo) unit on E/Z photoisomerization, the phase transition temperature, and self-assembly behavior of diblock copolymer, and evaluated the characteristic time involved in the decay process of the photoisomerization kinetics of diblock copolymer. Diblock copolymer poly(StO50-b-Azo7) exhibited moderate thermal stability, with thermal decomposition temperature of 5% weight loss at approximately 340.9 °C, suggesting that the enhancement of the thermal stability was attributed to the incorporation of azo segments into block copolymer. The diblock copolymer showed lower E–Z photoisomerization rates (T1 = 68.0 s) compared with azo monomer (T1 = 10.95 s). Gradually adding water to the tetrahydrofuran (THF) solution of poly(StO50-b-Azo7) produced spherical micelles. Spherical aggregates of poly(StO50-b-Azo7) were obtained (mean diameter = approximately 181.4 nm) by diluting the polymer disperse in a mixture of THF/H2O (water content = 10 vol%), and are shown in TEM images of the diblock copolymer. The results of this study contribute to research on the development of photoresponsive polymer materials.

A Straightforward Protocol for the Highly Efficient Preparation of Main-Chain Azo Polymers Directly from Bisnitroaromatic Compounds by the Photocatalytic Process

A novel strategy for the synthesis of main-chain Azo polymers directly from bisnitroaromatic compounds by the photocatalytic process has been achieved under mild conditions. This approach avoids the tedious synthesis of Azo monomers and proceeds with a high monomer conversion (∼100%) and excellent selectivity (∼100%) but without generating a significant amount of inorganic wastes and impurities (Cu2+ or azoxy groups) existing in main-chain Azo polymers compared to previous methods. The polymerization was monitored by UV–vis, FT-IR, and MALDI-TOF-MS, indicating that the reaction process proceeded with formation of the azoxy repeating units from the corresponding bisnitroaromatic monomers and the reduction of corresponding azoxy repeating units to the azo repeating units. Furthermore, the recyclable heterogeneous photocatalysts represent an attractive green process for production of main-chain Azo polymers.

Photo- and pH-sensitive azo-containing cationic waterborne polyurethane

In this study, a novel photo- and pH-sensitive cationic waterborne polyurethane containing side chain aromatic azo groups (Azo-CWPU) was synthesized by step-growth addition polymerization from polyether polylol, isophorone diisocyanate, N-methyl diethanolamine and a new aromatic azo-containing monomer, 2,2′-(4-((4-ethylphenyl)diazenyl)phenylazanediyl)diethanol (EDPD). Azo-CWPU showed a significant hypsochromic contrast to the azo monomer and rapid changes in UV–Vis absorption spectra at different irradiation times. Azo-CWPU dispersion displays bright yellow, orange and red color with the change of pH values. Regarding the orange dispersion, we assume that there is a mixture of the azoic form and the hydrazone form. Compared to cationic waterborne polyurethane dispersion without aromatic azo groups (CWPU), Azo-CWPU dispersion with smaller hydrodynamics radius and more narrow distribution shows good thermodynamic properties of polyurethane.

Synthesis of well-defined easily crosslinkable azobenzene side-chain liquid crystalline polymers via reversible addition–fragmentation chain transfer polymerization and photomechanical properties of their post-crosslinked fibers

The synthesis of a series of well-defined easily crosslinkable azobenzene (azo) side-chain liquid crystalline polyacrylates (including both homopolymers with an N-hydroxysuccinimide carboxylate-substituted azo mesogen and copolymers with both the above crosslinkable azo mesogen and a non-crosslinkable azo mesogen) via reversible addition–fragmentation chain transfer (RAFT) polymerization and photomechanical properties of their post-crosslinked fibers are described. The RAFT polymerizations of the azo monomers proved to be living and well-controlled, as revealed by their linear kinetic plots as well as the linear increase of the molecular weights of the azo polymers with monomer conversions and their low molar-mass dispersities (Đ⩽1.17). All the obtained azo polymers exhibited high thermal stability and relatively low glass transition temperatures, but only the azo homopolymers with longer flexible spacers and azo copolymers showed liquid crystallinity. In addition, the crosslinked azo polymer fibers with easily controlled diameters and high alignment order were obtained by the first fabrication of the uncrosslinked azo polymer fibers by the simple melt spinning method and their subsequent crosslinking with a difunctional primary amine under mild conditions. The contents of the crosslinkable azo mesogen in the polymers had significant influence on the photomechanical properties of their post-crosslinked fibers, and those with appropriate crosslinking densities could exhibit fast and reversible photoinduced bending and unbending behaviors and excellent photodeformation fatigue resistance even at close to room temperature.

A carboxylic azo monomer and its homopolymer: synthesis, self-organization and fluorescence behaviour in solution

The fluorescence of a carboxylic azo monomer and its homopolymer was studied in solution, which can be controlled by solvent polarity, pH value and photoirradiation.

Synthesis of Reactive Azobenzene Main-Chain Liquid Crystalline Polymers via Michael Addition Polymerization and Photomechanical Effects of Their Supramolecular Hydrogen-Bonded Fibers

A new and efficient strategy for obtaining a series of reactive azobenzene (azo)-containing main-chain liquid crystalline polymers (LCPs) is described, which involves the first design and synthesis of acrylate-type azo monomers with different length of flexible spacers and an amino end-group (in its trifluoroacetate salt form) and their subsequent Michael addition polymerization under mild reaction conditions. The resulting polymers showed rather high thermal stability, relatively low glass transition temperatures, a broad temperature range of smectic C liquid crystalline phase, and reversible photoresponsive behavior. The presence of secondary amino groups in the backbones of these azo main-chain LCPs not only made them highly reactive precursors for various new functional linear and cross-linked azo LCPs but also led to the formation of hydrogen-bonding interactions among their polymer chains. Supramolecular hydrogen-bonding cross-linked LCP fibers were directly fabricated by using the simple melt spinning method, which proved to have a high order of mesogen along the fiber axis and exhibit good mechanical properties, fast and reversible photoinduced bending and unbending behaviors, and large photoinduced stress (240 kPa) at close to ambient temperature as well as excellent photodeformation fatigue resistance.

Azo-containing polyurethanes with nonlinear-optical properties

The specific features of nonlinear-optical properties of polymers and the principles of the molecular design of macromolecules containing chromophore groups are considered. Approaches to the design of hydroxyl-containing azo monomers and polyurethanes with nonlinear-optical properties formed on their basis are generalized. The classification of azo-containing polyurethanes in terms of their chemical structure and method of synthesis is suggested, and the structure and nonlinear optical properties of these polymers are discussed.

Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 3: Control of Properties of Self-Assembled Multilayer Thin Films

Novel azobenzene polyelectrolytes have been used to fabricate biocompatible self-assembled multilayer (SAMU) thin films of variable absorbance, thickness, organization, and morphology. The prepared SAMU films are useful for directed cell growth, and this application relies directly on control of contact and surface energy, and requires the ability to tune the surface characteristics which are critical to their development. The azo polyelectrolytes employed here were similar in their degree of polymerization and repeat unit composition of acrylic acid monomer and azo monomers, and only differ from each other due to the presence of different substituted head R-groups present on the p-position of the aromatic ring of the azo chromophores. Possession of characteristics of both the self-assembly due to acrylic acid groups, and photoswitchability of the azo monomer enable the azobenzene functionalized polyelectrolytes to exhibit novel photo-reversible applications. The azo polyelectrolytes with the substituted R-group pairs of shorter-ionized hydrophilic COOH and SO3H, shorter-non-ionized hydrophobic H and OC2H5, and larger-nonionized hydrophobic octyl C8H17 and C8F17 were used as polyanions and counter charge PDAC used as polycation to fabricate the layer-by-layer SAMU films onto glass and silicon substrates. The fabricated SAMU films were also characterized by various techniques. The UV absorption maxima, λmax p of the SAMU films move to lower wavelength relative to solution to exhibit a blue shift for the hydrophobic R-groups, while this behaviour was not observed for the hydrophilic R-groups. Similarly, the thickness, organization, morphology and other properties of the thin films were found to be dependent on the type of substituted R-groups of the azo polyelectrolytes due to the inter-related factors of ionization, hydrophobicity/hydrophilicity, solubility, and aggregation of azo PEL in the dipping solutions used for fabrication of the SAMU films. Understanding and controlling the adsorption characteristics of azo multilayer thin–film of switchable functionalities are vital to explore their potential for the development and application of new devices in diverse areas of biosensor, drug delivery systems, on-chip microscale chemical process and microfluidics systems.

Synthesis and mesomorphic properties of different liquid crystal methacrylic azo monomers with ‘de Vries’-like behaviour

The synthesis and characterisation of four azo aromatic monomers, possessing two aliphatic tails, one with a methacrylate group and six methylenic units, and the other oxyhexyl or oxydodecyl, was carried out. The aliphatic tails are attached to an aromatic system consisting of two phenyl rings linked through an azo group. Two of then possess a free hydroxyl group in one of the phenyl rings. The materials were characterised by polarised light microscopy, differential thermal analysis and X-ray diffraction. In the cooling cycle, M6OA8 exhibits a smectic A (SmA) phase throughout the liquid crystalline range, whereas M6A12 develops SmA and SmC mesophases; other monomers show a phase transition from SmA to a ‘de Vries’-like phase. The photoisomerisation of the chromophores was studied in solution by irradiation at different wavelengths.

Novel Azobenzene-Functionalized Polyelectrolytes of Different Substituted Head Groups 1: Synthesis, Characterization and Absorption Spectroscopy Studies

A new class of a series of amphiphilic polyelectrolytes functionalized with azobenzene chromophores have been synthesized and thoroughly characterized by various techniques. A facile two stage strategy is developed, and first involved the preparation of a precursor base polymer, designated as P(APEA), by the free-radical copolymerization of the monomers of acrylic acid (AA) and 2-(phenylethylamino)ethyl acrylate (PEA). In the second step, precursor PAPEA polymers are reacted and post-polymer modified with the diazonium salts to synthesize azobenzene polyelectrolyte, PAPEA-R with different substituted R-groups present on the para position of the aromatic ring of the azo chromophores. The PAPEA-R polyelectrolytes are same in their degree of polymerization and repeat unit compositions of AA and PEA monomers, and only differ from each other by the type of R-groups. The copolymers were classified on the basis of the characteristics of their R-groups into the hydrophilic-ionizeable smaller pair of ─SO3H and ─COOH, the hydrophobic-non-ionizeable smaller pair of ─ H and ─OC2H5, and the hydrophobic-non-ionizeable larger octyl pair of ─C8H17 and ─ C8F17. The prepared copolymers are also characterized by NMR spectroscopy for structure, GPC for molecular weight, and UV-Visible spectroscopy for absorption determination. In DMF solvent, approximately similar absorption maxima, (λmax) values were observed for azo chromophore-containing monomers and after incorporating these into their corresponding polymers structures. However, for the self-assembled multilayer thin films, λmax moved to lower wavelengths to exhibit a blue shift with hydrophobic R-group of ─C8H17, while this behaviour was not observed for hydrophilic R-group of ─COOH. The shift in λmax is found to be highly dependent on the type of substituted R-group, and attributed to aggregation of hydrophobic azo chromophores in DMF:H2O mixture employed for self-assembly. The presence of the ionizeable AA and light-sensitive azo-chromophore functionalized PEA monomers in the PAPEA-R polyelectrolytes impart self-assembling and photoswitchable characteristics, respectively. Through understanding and controlling the solubility and complex solution aggregation behaviour of the different substituted R-groups of azo PEL, their adsorption, thickness, morphology, wetting, molecular-control, and photoresponsiveness can be tailored to enhance the capabilities of the self-assembled multilayer film process in diversified areas of microfluidics, sensing, and controlled release.

Synthesis of Random Copolymers Poly (methylmethacrylate‐co‐azo monomer) by ATRP‐AGET

AbstractThe synthesis of the azo molecule 1‐(2‐(4‐nitrophenyl) diazenyl) naphtalen‐2‐ol which has been functionalized with a methacryloxyl fraction is now reported. This azo monomer was copolymerized with methyl methacrylate (MMA) by ATRP where the active species is prepared “in situ” after the reduction reaction of the metal complex Cu (II) ‐ HMTETA by tin 2‐ethylhexanoate in 2‐butanone as solvent. Experimental conditions for the controlled homopolymerization of MMA were established. By adjusting the amount of reducing agent, a good correlation between theoretical and experimental molecular weight was obtained. Such conditions were also employed for the random copolymerization of MMA with the synthesized azo monomer.

Photochromic Block Copolymer Poly(styrene‐b‐azo monomer) by ATRP

AbstractPhotoactive monomers [(E)‐1(4‐methacryloxyphenyl)‐2‐(nitrophenyl) diazene] (azo 1) and (E)‐1‐(4‐methacryloxyethyl‐phenyl)‐2‐phenyl‐diazene (azo2) were synthesized. The azo 1 monomer showed poor reactivity under the evaluated polymerization method. The Azo 2 monomer was used in the block copolymerization with polystyrene by the normal ATRP methodology. Several polystyrene macroinitiators were prepared with molecular weights of 10, 20 and 30 k g/mol. The end functionality was increased by the addition of 0.1 equivalents of copper II chloride. Block copolymers in 30% yield and low polydispersity were synthesized by ATRP from the polystyrene macroinitiators and the azo monomer. The block copolymer exhibits good photochromic activity.